sensors Review

Recent Advances of MEMS Resonators for Lorentz Force Based Magnetic Field Sensors: Design, Applications and Challenges Agustín Leobardo Herrera-May 1, *, Juan Carlos Soler-Balcazar 2 , Héctor Vázquez-Leal 3 , Jaime Martínez-Castillo 1 , Marco Osvaldo Vigueras-Zuñiga 2 and Luz Antonio Aguilera-Cortés 4 1 2 3 4

*

Micro and Nanotechnology Research Center, Universidad Veracruzana, Calzada Ruiz Cortines 455, Boca del Río, Veracruz 94294, Mexico; [email protected] Engineering Faculty, Universidad Veracruzana, Calzada Ruiz Cortines 455, Boca del Río, Veracruz 94294, Mexico; [email protected] (J.C.S.-B.); [email protected] (M.O.V.-Z.) Electronic Instrumentation Faculty, Universidad Veracruzana, Cto. Gonzálo Aguirre Beltran S/N, Xalapa, Veracruz 91000, Mexico; [email protected] Departamento de Ingeniería Mecánica, DICIS, Universidad de Guanajuato, Carr. Salamanca-Valle de Santiago km 3.5+1.8 km, Palo Blanco, Salamanca, Guanajuato 36885, Mexico; [email protected] Correspondence: [email protected]; Tel.: +52-229-775-2000 (ext. 11956)

Academic Editor: Stephane Evoy Received: 23 May 2016; Accepted: 12 August 2016; Published: 24 August 2016

Abstract: Microelectromechanical systems (MEMS) resonators have allowed the development of magnetic field sensors with potential applications such as biomedicine, automotive industry, navigation systems, space satellites, telecommunications and non-destructive testing. We present a review of recent magnetic field sensors based on MEMS resonators, which operate with Lorentz force. These sensors have a compact structure, wide measurement range, low energy consumption, high sensitivity and suitable performance. The design methodology, simulation tools, damping sources, sensing techniques and future applications of magnetic field sensors are discussed. The design process is fundamental in achieving correct selection of the operation principle, sensing technique, materials, fabrication process and readout systems of the sensors. In addition, the description of the main sensing systems and challenges of the MEMS sensors are discussed. To develop the best devices, researches of their mechanical reliability, vacuum packaging, design optimization and temperature compensation circuits are needed. Future applications will require multifunctional sensors for monitoring several physical parameters (e.g., magnetic field, acceleration, angular ratio, humidity, temperature and gases). Keywords: Lorentz force; magnetic field sensor; MEMS; resonators; sensing technique

1. Introduction Microelectromechanical systems (MEMS) allow the development of devices that are composed by mechanical and electrical components with a feature size in the micrometer-scale. MEMS devices include signal acquisition and processing, actuators and control mechanisms [1]. These devices provide the following advantages: small size, low power consumption, high sensitivity and reduced fabrication cost [2]. Recently, several researchers [3–8] have designed MEMS magnetic field sensors for potential applications such as biomedical, telecommunications, navigation and non-destructive testing. Most of these sensors include resonators that operate with the Lorentz force, which is generated by the interaction between an external magnetic field and an electrical current. It causes deformations of the resonators, which are increased at resonance. These deformations can be measured using capacitive, Sensors 2016, 16, 1359; doi:10.3390/s16091359

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piezoresistive and optical sensing techniques. Thus, MEMS sensors can have better resolution than Hall effect and search coil sensors [9]. On the other hand, a superconducting quantum interference device (SQUID) is expensive and requires a sophisticated infrastructure [10]. Other conventional devices include the anisotropic magnetoresistive (AMR) and fluxgate sensors. Auto-calibration systems are necessary for AMR sensors, which are saturated with small magnetic fields (close to few mT) [11,12]. Otherwise, fluxgate sensors need a complex fabrication of their magnetic core and coils [13]. MEMS magnetic field sensors use a bias source and a signal processing system. For instance, Dominguez-Nicolás et al. [4] designed a signal conditioning system, implemented in a printed circuit board (PCB), for a MEMS magnetic field sensor. It obtains the sensor response in voltage or current mode for a magnetic field range from −150 µT to +150 µT. The sensor employs a virtual instrument design through Labview software to visualize the 4–20 mA output signal. For industrial applications, the sensor response is processed with a data acquisition card and a programmable logic controller (PLC). Thus, MEMS sensors could commercially compete against conventional magnetic field sensors in a wide variety of applications. 2. Design and Fabrication Design stage is very important in determining the suitable structural configuration, materials, operation principle and sensing technique of MEMS magnetic field sensors. In this stage, MEMS designers can use analytical and numerical methods to predict the performance of MEMS sensors. Designers must consider the rules of the fabrication process to avoid mistakes that affect the performance of the sensors. To obtain reliable designs, designers must have a high level of fabrication and packaging knowledge. In the design stage, simulation and modeling tools are necessary to predict the behavior of MEMS sensors. These tools assist designers in selecting the best fabrication process and materials for the sensors. To develop commercialized MEMS sensors, designers must satisfy the following criteria: (1) optimal design; (2) packaging design; (3) reliable materials properties and standard fabrication process; (4) suitable design and simulation tools; (5) reduction of electronic noise and parasitic capacitances; (6) reliable signal processing systems and; (7) reliability testing. The selection of the sensors fabrication process must consider several factors such as materials, operation principle, dimensions, operating environmental conditions, signal conditioning processes and sensing techniques [14]. The design rules of each fabrication process must be considered during the sensors design stage. For this design, several MEMS design tools can be used including MEMSCAP™ [15], coventorWare™ [16], IntelliSuite™ [17] and Sandia Ultra-planar Multi-level MEMS Technology (SUMMiT V) [18]. These design tools have modules to create the sensor layout and check the design rules, as well as simulating the steps of the micromachining process. These advantages may reduce the work time related to the sensor’s design. In addition, the suitable design of a sensor depends on the designer’s experience in using efficient methodology processes in the selection of better materials and micromachining processes. Figure 1 depicts the layout of a magnetic field sensor based on the SUMMiT V process [19]. Designers could employ the following methodology to design and fabricate MEMS magnetic field sensors: (1)

Application specification. In this stage, designers must know all the technical information of the sensor application such as magnetic field range, environmental conditions, response time and resolution. (2) Conceptual design. This stage includes several conceptual designs of magnetic field sensors to satisfy the requirements of the applications. Next, these designs are evaluated to select the best conceptual design. (3) Detailed design. Analytical and numerical methods are used to predict the sensor’s performance. These designs must incorporate the sensor packaging, which depends of different factors such as materials, operation pressure, cost, sensing technique and environmental conditions. High temperatures can be generated during the sensor packaging, affecting its behavior. For this,

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element finite method (FEM) models can be used to estimate the packaging temperature effect on For this, element finite method (FEM) models can be used to estimate the packaging thebehavior. sensor behavior. temperature effect Designers on the sensor behavior. (4) Fabrication process. identify the sensor requirements and characteristics of potential (4) Fabrication process. Designers identify sensorthe requirements and characteristics of potential fabrication processes. Next, the designerthe selects best fabrication process or develops a new fabrication processes. Next, the designer selects the best fabrication process or develops a new process, in which must consider the materials, etching steps, design rules, technical limitations process, in which must consider the materials, etching steps, design rules, technical limitations and cost. and cost. (5) (5)Material properties verification. Test structures can be fabricated on the same sensor wafer to Material properties verification. Test structures can be fabricated on the same sensor wafer to measure several materials sensorssuch suchasasYoung’s Young’s modulus, facture strength, measure several materialsproperties properties of of the the sensors modulus, facture strength, thermal conductivity, electrical resistivity and dielectric constant. thermal conductivity, electrical resistivity and dielectric constant.

(a)

(b) Figure 1. Layout of a magnetic field sensor with a polysilicon resonator, which is based on SUMMiT

Figure 1. Layout of a magnetic field sensor with a polysilicon resonator, which is based on SUMMiT V V process: (a) 3D; and (b) 2D view of the main mechanical components [19]. Reprinted with process: (a) 3D; and (b) 2D view of the main mechanical components [19]. Reprinted with permission permission from [19]. Copyright© 2013, Bentham Science Publishers. from [19]. Copyright©2013, Bentham Science Publishers.

Generally, the fabrication of MEMS magnetic field sensors can be realized using bulk or surface micromachining processes. These processes use silicon as their can main to its important Generally, the fabrication of MEMS magnetic field sensors bematerial realizeddue using bulk or surface mechanical and electrical properties. For instance, silicon has minimum mechanical hysteresis and micromachining processes. These processes use silicon as their main material due to its important large rupture stress close to 1 GPa. In addition, silicon doped with phosphorus or boron can improve mechanical and electrical properties. For instance, silicon has minimum mechanical hysteresis and itsrupture electricalstress properties. large close to 1 GPa. In addition, silicon doped with phosphorus or boron can improve

its electrical properties.

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The bulk micromachining process employs wet and dry etching techniques to fabricate features The bulk micromachining process employs wet and dry etching techniques to fabricate features in materials through isotropic and anisotropic etching, respectively [20]. Wet etching is a chemical in materials through isotropic and anisotropic etching, respectively [20]. Wet etching is a chemical process that can reach an anisotropic directional etching in crystalline materials (e.g., silicon) process that can reach an anisotropic directional etching in crystalline materials (e.g., silicon) although, although, it can get an isotropic etching in an amorphous material (e.g., silicon dioxide). For it can get an isotropic etching in an amorphous material (e.g., silicon dioxide). For instance, potassium instance, potassium hydroxide (KOH) is a directional wet etchant for crystalline silicon, which hydroxide (KOH) is a directional wet etchant for crystalline silicon, which etches 100 times faster on the etches 100 times faster on the (100) plane than on the (111) plane (see Figure 2). KOH etches very (100) plane than on the (111) plane (see Figure 2). KOH etches very slowly the silicon nitride or silicon slowly the silicon nitride or silicon dioxide, which can be used as etch masks. In wet etching, the dioxide, which can be used as etch masks. In wet etching, the etched depth depends of several variables etched depth depends of several variables such as etched time, temperature, chemical agitation and such as etched time, temperature, chemical agitation and solution concentration. The disadvantages of solution concentration. The disadvantages of the wet etching can be overcome using dry etching the wet etching can be overcome using dry etching (e.g., plasma etching). Dry etching has advantages (e.g., plasma etching). Dry etching has advantages such as anisotropic etch, repeatable process and it such as anisotropic etch, repeatable process and it is easy to start and stop the etched process. is easy to start and stop the etched process. Plasma etching uses a flux of ions, electrons, radicals and Plasma etching uses a flux of ions, electrons, radicals and neutral particles to etch the material neutral particles to etch the material surface. It includes reactive ion etching (RIE), deep reactive ion surface. It includes reactive ion etching (RIE), deep reactive ion etching (DRIE) and high-density etching (DRIE) and high-density plasma etching (HDP). Figure 3 shows a SEM image of two plasma etching (HDP). Figure 3 shows a SEM image of two magnetic field sensors composed by magnetic field sensors composed by silicon resonators with piezoresistive sensing, which were silicon resonators with piezoresistive sensing, which were fabricated using a bulk micromachining fabricated using a bulk micromachining process. These sensors were designed by researchers at process. These sensors were designed by researchers at Micro and Nanotechnology Research Center Micro and Nanotechnology Research Center (MICRONA-UV, Boca del Río, Mexico) into (MICRONA-UV, Boca del Río, Mexico) into collaboration with Microelectronics Institute of Barcelona collaboration with Microelectronics Institute of Barcelona (IMB-CNM, CSIC, Bellaterra, Spain). (IMB-CNM, CSIC, Bellaterra, Spain). Surface micromachining process is a fabrication process of MEMS devices that uses the Surface micromachining process is a fabrication process of MEMS devices that uses the deposition, deposition, patterning and etching of different materials layers on a substrate [20]. Commonly, these patterning and etching of different materials layers on a substrate [20]. Commonly, these layers are layers are employed as structural and sacrificial layers. The sacrificial layers protect the structural employed as structural and sacrificial layers. The sacrificial layers protect the structural layers during layers during the etching process and define the mechanical structure of the MEMS devices. Finally, the etching process and define the mechanical structure of the MEMS devices. Finally, sacrificial sacrificial layers are removed using specific chemical substances. Figure 4 shows a polysilicon layers are removed using specific chemical substances. Figure 4 shows a polysilicon resonator of resonator of a magnetic field sensor, which is fabricated using a surface micromachining process. It a magnetic field sensor, which is fabricated using a surface micromachining process. It was designed was designed by researchers at Micro and Nanotechnology Research Center (MICRONA-UV) into by researchers at Micro and Nanotechnology Research Center (MICRONA-UV) into collaboration with collaboration with Sandia National Laboratories. Sandia National Laboratories.

Figure 2. SEM image of a die reverse-side with two silicon structures which were etched using the KOH in bulk micromachining process. (Courtesy of A. L. Herrera-May, Herrera-May, Universidad UniversidadVeracruzana). Veracruzana).

Planar fabrication fabrication methods the microelectronic industry be used in surface methods of theofmicroelectronic industry can be usedcan in surface micromachining micromachining process. instance, this process tools of the microelectronic industry the for process. For instance, thisFor process employs tools ofemploys the microelectronic industry for depositing depositing thesacrificial structurallayers, and sacrificial as well as the patterning and Thus, etching of layers. Thus, structural and as well aslayers, the patterning and etching of layers. devices structures devices structures areetching obtained through etching of the structural layers, which are anchored to the are obtained through of the structural layers, which are anchored to the substrate and others substrate and others structural layers. In the deposition process of structural and sacrificial layers,

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structural layers. In the deposition process of structural and sacrificial layers, residual stress gradients residual stress gradients gradients can be generated generated on the structural structural layers, which can can affect affect the performance can be generated on the can structural layers,on which can affectlayers, the performance of thethe MEMS sensors. residual stress be the which performance of the the stress MEMSgradients sensors. are These stress gradients areconditions caused by bysuch deposition conditions such as high high These caused bygradients deposition as high conditions temperature values and of MEMS sensors. These stress are caused deposition such as temperature values and materials layers with different thermal dilation coefficients. The residual materials layers with different thermal dilation coefficients. The residual stress gradients can be temperature values and materials layers with different thermal dilation coefficients. The residual stress gradients can be decreased using post-deposition annealing steps. decreased using post-deposition annealing steps. stress gradients can be decreased using post-deposition annealing steps.

Figure 3. 3. SEM SEM image image of of two two magnetic magnetic field field sensors sensors formed formed by by silicon silicon resonators, resonators, which which are are fabricated fabricated Figure micromachining process. process. (Courtesy (Courtesy of of A. A. L. L. Herrera-May, Herrera-May,Universidad UniversidadVeracruzana). Veracruzana). using using aa bulk bulk micromachining micromachining process. (Courtesy of A. L. Herrera-May, Universidad Veracruzana).

Figure 4. 4. SEM SEM image image of of magnetic magnetic field field sensor sensor composed composed by by aa polysilicon polysilicon resonator, resonator, which which is is Figure Figure 4. SEM image of magnetic field sensor composed by a polysilicon resonator, which is fabricated fabricated using using aa surface surface micromachining micromachining process. process. (Courtesy of of A. A. L. L. Herrera-May, Herrera-May, Universidad Universidad fabricated using a surface micromachining process. (Courtesy of (Courtesy A. L. Herrera-May, Universidad Veracruzana). Veracruzana). Veracruzana).

2.1. Performance of MEMS 2.1. Performance Performance of of MEMS MEMS Magnetic Magnetic Field Field Sensors Sensors 2.1. Magnetic Field Sensors MEMS magnetic field field sensors sensorsuse useresonators resonatorstotoincrease increasetheir theirsensitivity sensitivity due large strains MEMS magnetic magnetic due to to large strains of MEMS field sensors use resonators to increase their sensitivity due to large strains of of the sensor structure. The sensor structure operates at resonance through Lorentz forces or the sensor structure. The sensor structure operates at resonance through Lorentz forces or the sensor structure. The sensor structure operates at resonance through Lorentz forces or electrostatic forces forces whose whose displacements displacements are are related related to to the the magnitude magnitude of of the the applied applied magnetic magnetic field. field. electrostatic These displacements can be detected using piezoresistive, capacitive or optical sensing techniques. These displacements can be detected using piezoresistive, capacitive or optical sensing techniques. For instance, instance, Figure Figure 5a,b 5a,b depicts depicts the the design design of of aa magnetic magnetic field field sensor sensor which which contains contains aa resonator resonator For

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and piezoresistive sensing [21]. This sensor is formed by a rectangular loop of silicon beams, an 6 of 25 a Wheatstone bridge, as shown in Figure 6. This sensor exploits the Lorentz force (FL) that is generated by the interaction of the electrical current with an external magnetic field (Bx) parallel forces to the sensor electrostatic whose structure: displacements are related to the magnitude of the applied magnetic field.

Sensors 2016, 16, 1359 and aluminum loop

These displacements can be detected using piezoresistive, capacitive or optical sensing techniques. (1) For instance, Figure 5a,b depicts the design of a magnetic field sensor which contains a resonator and piezoresistive sensing [21]. This sensor is formed by a rectangular loop of silicon beams, an aluminum withand a Wheatstone bridge, as shown in Figure 6. This sensor exploits the Lorentz force (FL ) that is loop generated by the interaction of the electrical current with an external magnetic field (Bx ) parallel to the sensor structure: (2) FL = I Al Bx Lz (1)

where Lz is width of the rectangular loop, ω and IRMS are circular frequency and root-mean-square with √ (RMS) of the sinusoidal electrical current Al) supplied to the aluminum loop, respectively. I Al(I= 2IRMS sin(ωt) (2) The sensor structure has a deflection and strain generated by the Lorentz force, causing a where is width of the rectangular loop, ω and IRMS are circular frequency and root-mean-square changeLz(△R i) of the initial resistance (Ri) of two p-type piezoresistors: (RMS) of the sinusoidal electrical current (IAl ) supplied to the aluminum loop, respectively. The sensor structure has a deflection and strain generated by the Lorentz force, causing a change (3) (4Ri ) of the initial resistance (Ri ) of two p-type piezoresistors: where πl is longitudinal piezoresistive coefficient, εl isQlongitudinal strain of the piezoresistors under ∆Ri = πl Eε (3) l T Ri static load, E is Young’s modulus of the piezoresistor material and QT is total quality factor of the resonant structure. where π l is longitudinal piezoresistive coefficient, εl is longitudinal strain of the piezoresistors under output voltage modulus (Vo) of theof Wheatstone bridge material changes due of the piezoresistors staticThe load, E is Young’s the piezoresistor and to QTvariation is total quality factor of the resistance: resonant structure. The output voltage (Vo ) of the Wheatstone bridge changes due to variation of the piezoresistors resistance: (4) 1 Vo = πl Eε l Ri Vi (4) 2 where Vi is input voltage applied to the Wheatstone bridge. whereThe Vi issensor input voltage applied the Wheatstone sensitivity (S) istoobtained as the bridge. ratio between the output voltage (Vo) of the The sensor sensitivity (S) is obtained as thex):ratio between the output voltage (∆Vo ) of the Wheatstone bridge to the magnetic field shift (△B Wheatstone bridge to the magnetic field shift (∆Bx ): S=

(a)

∆Vo ∆Bx

(5) (5)

(b)

Figure5.5. 3D 3Dschematic schematic view view of of the the (a) (a)upper upper and and (b) (b)bottom bottomdesign design of of magnetic magnetic field field sensor, sensor,which which Figure contains a silicon resonator, an aluminum loop and a Wheatstone bridge [21]. Reprinted with contains a silicon resonator, an aluminum loop and a Wheatstone bridge [21]. Reprinted with permission from [21]. Copyright© 2011, Elsevier B.V. permission from [21]. Copyright©2011, Elsevier B.V.

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Figure 6. 3D 3D schematic schematicview viewofofthe theperformance performanceofofa aMEMS MEMSmagnetic magneticfield fieldsensor sensorformed formed byby a arectangular rectangularloop loopofofsilicon siliconbeams, beams,an analuminum aluminumloop loopand andpiezoresistive piezoresistive sensing sensing [21]. Reprinted Reprinted with permission from [21]. [21]. Copyright©2011, Copyright© 2011, Elsevier B.V. B.V.

2.2. Quality Factor qualityfactor factor a resonator affects its sensitivity and resolution. factor can be The quality of aofresonator affects its sensitivity and resolution. This factorThis can be determined determined asthe thetotal ratioenergy of the stored total energy stored(E ins )resonator (Es) lost to the lost per cycle C) as the ratio of in resonator to the energy perenergy cycle (E by(E the C ) caused caused bysources: the damping sources: damping 2πEs Q= (6) Ec For small displacements of the resonator, the quality factor is related with the damping ratio(6) (ζ): 1 (7) For small displacements of the resonator, Q the=quality factor is related with the damping ratio (ζ): 2ζ The main damping sources of MEMS resonators are the following: fluid damping, support (7) damping and thermoelastic damping. Fluid damping is due to energy loss to a surrounding fluid and its value is affected by parameters such as the viscosity and pressure of the fluid, resonator The main damping sources of MEMS resonators are the following: fluid damping, support size, vibration mode and separation of the resonator with respect to the adjacent surfaces [22]. damping and thermoelastic damping. Fluid damping is due to energy loss to a surrounding fluid This damping decreases when the fluid pressure is reduced to vacuum, which increases the resonator and its value is affected by parameters such as the viscosity and pressure of the fluid, resonator size, motions, the sensitivity and resolution of the sensor. Therefore, a vacuum packaging can improve vibration mode and separation of the resonator with respect to the adjacent surfaces [22]. This the performance of magnetic field sensors. The fluid damping is the most dominant source of energy damping decreases when the fluid pressure is reduced to vacuum, which increases the resonator dissipation for resonators operating at ambient pressure. motions, the sensitivity and resolution of the sensor. Therefore, a vacuum packaging can improve Support damping is generated by the vibration energy dissipation in the supports of resonators. the performance of magnetic field sensors. The fluid damping is the most dominant source of energy These supports absorb part of the vibration energy of a resonator, which depends of the support type dissipation for resonators operating at ambient pressure. and dimensions, as well as the vibration mode of the sensor structure [23]. Support damping is generated by the vibration energy dissipation in the supports of resonators. Thermoelastic damping of a resonator is due to the oscillating temperature gradient generated These supports absorb part of the vibration energy of a resonator, which depends of the support during the resonator vibration [24]. This temperature gradient causes thermal energy loss that can be type and dimensions, as well as the vibration mode of the sensor structure [23]. a dominant damping when the resonator operates at vacuum pressure. Thermoelastic damping of a resonator is due to the oscillating temperature gradient generated Total quality factor (QT ) of a resonator can be determined considering different damping sources. during the resonator vibration [24]. This temperature gradient causes thermal energy loss that can be a dominant damping when the resonator1 operates 1 at 1vacuum 1 pressure. = can + be determined + (8) Total quality factor (QT) of a resonator considering different damping QT Qf Qs Qt sources. where Qf , Qs , and Qt are quality factors associated with the fluid damping, support damping and thermoelastic damping, respectively. (8) 2.3. Sensing Techniques MEMS magnetic field sensors can employ different sensing techniques such as capacitive, optical, where Qf, Qs, and Qt are quality factors associated with the fluid damping, support damping and or piezoresistive. These techniques allow the conversion of magnetic field signals into electrical or thermoelastic damping, respectively.

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2.3. Sensing Techniques Sensors 2016, 16, 1359

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MEMS magnetic field sensors can employ different sensing techniques such as capacitive, optical, or piezoresistive. These techniques allow the conversion of magnetic field signals into optical signals, respectively. In addition, MEMS sensorsMEMS need signal conditioning systems with low electrical or optical signals, respectively. In addition, sensors need signal conditioning electronic noise parasitic capacitances. In the following paragraphs, several examples of MEMS systems with lowand electronic noise and parasitic capacitances. In the following paragraphs, several magneticoffield sensors with different sensing are discussed. examples MEMS magnetic field sensors withtechniques different sensing techniques are discussed. Figure7 7shows showsthe theschematic schematicview viewofofthe themain main components a MEMS magnetic field sensor, Figure components ofof a MEMS magnetic field sensor, which uses a piezoresistive sensing through a Wheatstone bridge with four p-type piezoresistors [25]. which uses a piezoresistive through a Wheatstone bridge with four p-type piezoresistors 3 3 TheThe sensor structure is formed with awith silicon plate (400 150 × 15 µm ) supported by fourby bending [25]. sensor structure is formed a silicon plate×(400 × 150 × 15 μm ) supported four beams beams (130 ×(130 12 × 15×µm ). 3The plate interaction between betweenthe thesinusoidal sinusoidal bending × 12 15 3μm ). The plateoscillates oscillatesdue dueto to the the interaction electricalcurrent currentand andmagnetic magneticfield fieldparallel paralleltotothe theplate. plate.This Thisoscillation oscillationgenerates generatesa abending bendingmotion motion electrical (136.52kHz) kHz)ofofthe thebeams, beams, which contain two piezoresistors (40 1 3µm It alters the resistance (136.52 which contain two piezoresistors (40 × 8××81× μm ). It3 ). alters the resistance of of two piezoresistors, modifying the output voltage ofWheatstone the Wheatstone bridge. the sensor has two piezoresistors, modifying the output voltage of the bridge. Thus,Thus, the sensor has an an electrical output response for monitoring magnetic at atmospheric pressure. This has sensor electrical output response for monitoring magnetic fields fields at atmospheric pressure. This sensor a has a quality ofatmospheric 842 at atmospheric pressure, sensitivity of 403 ·T−1 , theoretical resolution quality factor offactor 842 at pressure, sensitivity of 403 mV· T−1,mV theoretical resolution of 143 1/2 and power −1/2,nT ofHz 143 ·Hz−1/2 , noise theoretical of−1/257.5 ·Hz−consumption consumption close technical to 10 mW. nT· theoretical of 57.5noise nV·Hz andnV power close to 10 mW. These These technical of the are MEMS sensorfor are monitoring adequate forresidual monitoring residual magnetic parameters of theparameters MEMS sensor adequate magnetic fields into fields into ferromagnetic tubes.this However, this sensora registered a voltage and a electrical non-lineal ferromagnetic tubes. However, sensor registered voltage offset and aoffset non-lineal electrical response. response.

(b) (a) Figure Figure 7. (a) 7. Schematic view view of a ofMEMS magnetic field piezoresistive sensing sensingthrough through a (a) Schematic a MEMS magnetic fieldsensor sensorwith with (b) (b) piezoresistive Wheatstone bridge bridge composed by four p-type piezoresistors with permission permissionfrom from a Wheatstone composed by four p-type piezoresistors[25]. [25].Reprinted Reprinted with [25].[25]. Copyright© 2015, IOP Publishing. Copyright©2015, IOP Publishing.

Mehdizadeh et al. [26] designed a MEMS magnetic field sensor formed by a dual-plate silicon Mehdizadeh et al. [26] designed a MEMS magnetic field sensor formed by a dual-plate silicon resonator (10 μm thickness) with a gold trace (10 μm width and 200 nm thickness) on one of its two resonator (10 µm thickness) with a gold trace (10 µm width and 200 nm thickness) on one of its plates (see Figure 8). Two narrow beams in the middle of the resonator connecting the two silicon two plates (see Figure 8). Two narrow beams in the middle of the resonator connecting the two plates have behave as piezoresistors. It is due to the periodic tensile and compressive stress when the silicon plates have behave as piezoresistors. It is due to the periodic tensile and compressive stress resonator oscillates in-plane vibration mode. This Lorentz force-based sensor is fabricated using a when the resonator oscillates in-plane vibration mode. This Lorentz force-based sensor is fabricated low-resistivity n-type silicon-on-insulator (SOI) substrate. The quality factor of this resonator has using a low-resistivity n-type silicon-on-insulator (SOI) substrate. The quality factor of this resonator amplification from 1140 to 16,900 at atmospheric pressure. This sensor improves its sensitivity by has amplification from 1140 to 16,900 at atmospheric pressure. This sensor improves its sensitivity increasing the resonator vibration amplitude. The sensor has a sensitivity of 262 mV·T−1 in−1air, a by increasing the resonator vibration amplitude. The sensor has a sensitivity of 262 mV·T in air, resonant frequency of 2.6 MHz and a quality factor of 16,900. Nevertheless, the MEMS sensor a resonant frequency of 2.6 MHz and a quality factor of 16,900. Nevertheless, the MEMS sensor requires requires more studies of the effects of temperature on its performance. more studies of the effects of temperature on its performance.

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(a)

(b)

Figure8.8.SEM SEMimage imageofofthe the(a) (a)main mainelements elementsofofa aMEMS MEMSmagnetic magneticfield fieldsensor sensorwith withpiezoresistive piezoresistive Figure readout, which is composed by a dual-plate silicon resonator with a gold trace and piezoresistors; (b) readout, which is composed by a dual-plate silicon resonator with a gold trace and piezoresistors; Electrical connections of the magnetic field sensor [26]. Reprinted with permission from [26]. (b) Electrical connections of the magnetic field sensor [26]. Reprinted with permission from [26]. Copyright© 2014,IEEE. IEEE. Copyright©2014,

A MEMS magnetic field device (see Figure 9) with a simple resonator and linear electrical A MEMS magnetic field device (see Figure 9) with a simple resonator and linear electrical response response was presented by Herrera-May et al. [27]. It is formed by a perforate plate (472 × 300 ×3 15 was 3presented by Herrera-May et al. [27]. 3 It is formed by a perforate plate (4723 × 300 × 15 µm ), μm ), four flexural beams (18 × 15 × 15 μm ), two support beams (60 × 36 × 15 μm ) and a Wheatstone four flexural beams (18 × 15 × 15 µm3 ), two support beams (60 × 36 × 15 µm3 ) and a Wheatstone bridge with four p-type piezoresistors. The device exploits the Lorentz force and operates at its bridge with four p-type piezoresistors. The device exploits the Lorentz force and operates at its seesaw seesaw resonant frequency (100.7 kHz) without vacuum packaging. A standard bulk resonant frequency (100.7 kHz) without vacuum packaging. A standard bulk micromachining process micromachining process and SOI wafers are used to fabricate the device, as shown in Figure 10. The and SOI wafers are used to fabricate the device, as shown in Figure 10. The dynamic range of the dynamic range of the device can be adjusted by modifying the excitation electrical current, keeping device can be adjusted by modifying the excitation electrical current, keeping its linear electrical its linear electrical response. It has a quality factor of 419.6, a sensitivity of 230 mV·T−1, a resolution of response. It has a quality factor of 419.6, a sensitivity of 230 mV·T−1 , a resolution of 2.5 µT and 2.5 μT and a power consumption of 12 mW. Due to the advantages of this sensor, it could be a power consumption of 12 mW. Due to the advantages of this sensor, it could be employed in employed in applications of non-destructive magnetic testing to detect flaws and corrosion of applications of non-destructive magnetic testing to detect flaws and corrosion of ferromagnetic ferromagnetic materials. For this application, the sensor needs reliability studies of its behavior materials. For this application, the sensor needs reliability studies of its behavior under different under different conditions of temperature, moisture, and fatigue. conditions of temperature, moisture, and fatigue. Figure 11 depicts a schematic view of a MEMS magnetic field sensor with capacitive readout Figure 11 depicts a schematic view of a MEMS magnetic field sensor with capacitive readout technique [28]. This sensor detects magnetic field with orthogonal direction (z-axis along) to surface technique [28]. This sensor detects magnetic field with orthogonal direction (z-axis along) to surface of of the resonant structure. It consists of a set of fixed stators and a shuttle suspended with two thin the resonant structure. It consists of a set of fixed stators and a shuttle suspended with two thin beams beams (see Figure 11), which forms two differential parallel-plate sensing capacitors C1 and C2. A (see Figure 11), which forms two differential parallel-plate sensing capacitors C1 and C2 . A Lorentz Lorentz force is generated on two thin beams caused by the interaction between the magnetic field force is generated on two thin beams caused by the interaction between the magnetic field and and ac electrical current flowing through the beams with the sensor resonant frequency. This force ac electrical current flowing through the beams with the sensor resonant frequency. This force has has an orthogonal direction to the plane of both magnetic field and ac current, causing a an orthogonal direction to the plane of both magnetic field and ac current, causing a displacement of the displacement of the beams and parallel plates. This displacement is detected through the differential beams and parallel plates. This displacement is detected through the differential capacitance variation capacitance variation between the parallel plates and fixed stators (see Figure 12). The sensor between the parallel plates and fixed stators (see Figure 12). The sensor sensitivity is measured as the sensitivity is measured as the differential capacitance shift per variation of magnetic field. This differential capacitance shift per variation of magnetic field. This sensor has an overall sensitivity of sensor has an overall sensitivity of 150 μV·μT−1 at 250 μA of peak driving current,−a1/2 theoretical noise − 1 150 µV·µT at 250 of peak driving current, a −1theoretical noise of 557.2 µV·Hz , a resolution −1/2,µA −1/2, a quality of 557.2 μV· Hz a resolution of 520 nT· mA · Hz factor around 328, a resonant of 520 nT·mA−1 ·Hz−1/2 , a quality factor around 328, a resonant frequency of 28.3 kHz and a typical frequency of 28.3 kHz and a typical industrial packaging. The technical characteristics of this sensor industrial packaging. The technical characteristics of this sensor are suitable for consumer applications are suitable for consumer applications (e.g., digital compass, dead reckoning, heading, map (e.g., digital compass, dead reckoning, heading, map rotation), although this sensor only detects the rotation), although this sensor only detects the components of the magnetic field in one direction components of the magnetic field in one direction and its performance depends on the temperature. and its performance depends on the temperature. This sensor could include a temperature device, This sensor could include a temperature device, embedded on the same MEMS readout electronic, embedded on the same MEMS readout electronic, for post-acquisition compensation of temperature for post-acquisition compensation of temperature alterations. Therefore, this sensor requires initial alterations. Therefore, this sensor requires initial calibration and temperature compensation. calibration and temperature compensation.

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Figure 9. Schematic Figure 9. Schematic view view of of the the main main components components of of aaa MEMS MEMS magnetic magnetic field field device device designed designed by by Figure 9. 9. Schematic Schematic view of the main components of MEMS magnetic field device designed by Figure view of the main components of a MEMS magnetic field device designed by Herrera-May et al. al. [27]. [27]. Reprinted with permission from from [27]. [27]. Copyright©2015, Copyright© 2015, Elsevier Elsevier B.V. B.V. Herrera-May et Reprinted with permission Herrera-May et et al. al. [27]. [27]. Reprinted Reprinted with with permission permission from from [27]. [27]. Copyright© Copyright© 2015, 2015, Elsevier Elsevier B.V. B.V. Herrera-May

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Figure 10. SEM image of a MEMS magnetic field device with piezoresistive sensing: (a) Silicon Figure 10. 10. SEM SEM image of MEMS magnetic magnetic field field device device with piezoresistive piezoresistive sensing: sensing: (a) (a) Silicon Silicon Figure image aaa MEMS SEM image of ofloop; MEMS magnetic resonator and aluminum (b) Wheatstone bridge with with four piezoresistors [27]. Reprinted with resonator and and aluminum aluminum loop; loop; (b) (b) Wheatstone Wheatstone bridge bridge with with four four piezoresistors piezoresistors [27]. [27]. Reprinted Reprinted with with resonator resonator and aluminum loop; Reprinted with permission from [27]. Copyright© 2015, Elsevier B.V. permission from from [27]. [27]. Copyright© Copyright© 2015, Elsevier B.V. permission 2015, Elsevier B.V. Copyright©2015, B.V.

Figure 11. Schematic view of a MEMS magnetic field sensor formed with parallel plates, fixed Figure 11. Schematic Schematic view view of MEMS magnetic field sensor formed with with parallel parallel plates, plates, fixed fixed Figure magnetic field sensor formed view of of aaabyMEMS MEMS magnetic fieldsensor sensoruses formed with parallel plates, fixed stators, 11. and Schematic a shuttle supported two thin beams. This the Lorentz force, which causes stators, and and aa shuttle shuttle supported supported by by two two thin thin beams. beams. This sensor uses the Lorentz force, which causes causes stators, sensor uses force, which This sensor uses the the Lorentz Lorentz force, which causes a displacement of itssupported resonant structure measuredThis through differential capacitors [28]. Reprinted displacementof ofits itsresonant resonant structure measured through differential capacitors [28]. Reprinted aa displacement displacement of its resonant structure measured through differential capacitors [28]. Reprinted structure 2013, measured with permission from [28]. Copyright© IEEE.through differential capacitors [28]. Reprinted with with permission permission from [28]. Copyright© Copyright© 2013, 2013, IEEE. with [28]. permission from from [28]. Copyright©2013, IEEE.IEEE.

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Figure of the mainmain components of a MEMS magneticmagnetic field sensor with capacitive Figure 12. 12.Microphotography Microphotography of the components of a MEMS field sensor with sensing [28]. Reprinted permission from [28]. Copyright©2013, IEEE.2013, IEEE. capacitive sensing [28]. with Reprinted with permission from [28]. Copyright©

Zhang et et al. Zhang al. [29] [29] developed developed aa silicon silicon micromechanical micromechanicalmagnetic magneticfield fieldsensor sensorbased basedonona tuning fork (DETF) resonator, as as shown in Figure 13. 13. ThisThis silicon resonator has two adouble-ended double-ended tuning fork (DETF) resonator, shown in Figure silicon resonator has 2) clamped 2 parallel beams (600 × 8 μm to two doubly fixed crossbars and two sets of comb drive two parallel beams (600 × 8 µm ) clamped to two doubly fixed crossbars and two sets of comb drive electrodes joined joined to to each each parallel parallel beam. beam. The The first first set set of of comb comb drive drive electrodes electrodes operates operates as as actuation actuation electrodes mechanismand andthe thesecond second of electrodes a sensing mechanism. The DEFT resonator is mechanism setset of electrodes actsacts as a as sensing mechanism. The DEFT resonator is driven driven using an electrostatic force that activates its in-plane (46.8 kHz) and anti-phase (49.3 kHz) using an electrostatic force that activates its in-plane (46.8 kHz) and anti-phase (49.3 kHz) vibration vibration modes13 (Figures 13 This and 14). This sensor measures the resonant shift the modes (Figures and 14). sensor measures the resonant frequencyfrequency shift using theusing Lorentz Lorentz force. This force is generated by the interaction of an out-of-plane magnetic field and dc force. This force is generated by the interaction of an out-of-plane magnetic field and dc electrical electrical current flowing through the two crossbars. The Lorentz force modifies the stiffness of the current flowing through the two crossbars. The Lorentz force modifies the stiffness of the two parallel two parallel which resonant function ofmagnetic the applied magnetic field. beams, whichbeams, modifies the modifies resonant the frequency infrequency function ofinthe applied field. This sensor This sensor was fabricated using SOI wafers during a standard bulk micromachining process. Using was fabricated using SOI wafers during a standard bulk micromachining process. Using a resonator a resonator of 10-thickness, field sensor reaches of and 215.74 ppm/T and of 10-thickness, the magnetic the fieldmagnetic sensor reaches sensitivities of sensitivities 215.74 ppm/T 203.71 ppm/T 203.71 ppm/T for and the in-phase anti-phase and in-phase vibration mode, respectively. anti-phase and for the anti-phase vibration mode, respectively. For anti-phase andFor in-phase vibration in-phase vibration mode, the resonator has a quality factor of 100,000 and 42,000, respectively. mode, the resonator has a quality factor of 100,000 and 42,000, respectively. However, the dc current However,tothe currentincreases suppliedthe to crossbars the crossbars increasescausing the crossbars temperature, frequency causing a supplied thedc crossbars temperature, a thermally-induced thermally-induced frequency shift. This effect is minimized by reducing the resistance across the shift. This effect is minimized by reducing the resistance across the crossbars through depositing crossbars through depositing a metal stack of chromium and gold along the crossbars. This sensor a metal stack of chromium and gold along the crossbars. This sensor needs a vacuum packaging and needsresearches a vacuum on packaging and moreofresearches the dependence the sensor performance more the dependence the sensoronperformance withof respect to the Joule effect with and respect to the Joule effect and damping sources. damping sources.

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Figure 13. 13. Schematic DETF silicon silicon resonator, resonator, including including the the Figure Schematic of of aa magnetic magnetic field field sensor sensor formed formed by by aa DETF biasing configuration configuration with with aa transimpedance transimpedance amplifier amplifier (TIA) (TIA) at at the the sensing sensing port. port. The right side side biasing The right depicts the the anti-phase anti-phasevibration vibrationmode modeofofthe theDETF DETF resonator and a cross section schematic view of depicts resonator and a cross section schematic view of the the sensor [29]. Reprinted with permission from [29]. Copyright© 2014, Elsevier B.V. sensor [29]. Reprinted with permission from [29]. Copyright©2014, Elsevier B.V.

Figure Figure 14. 14. Schematic of a magnetic field sensor formed by a DETF silicon resonator, which considers the transimpedance amplifier amplifier (TIA) (TIA) at at the the sensing sensing port. port. The right the bias bias configuration configuration with with aa transimpedance right side side shows Reprinted with with permission permission from from [29]. [29]. shows the the in-phase in-phase vibration mode of the DETF resonator [29]. Reprinted Copyright©2014, B.V. Copyright© 2014, Elsevier B.V. Elsevier

Li etetal.al.[30] designed a magnetic field sensor composed of a flexural resonator × 680 Li [30] designed a magnetic field sensor composed of abeam flexural beam(1200 resonator 3), which is 3 × 40 μm then coupled to current-carrying silicon beams through a microleverage (1200 × 680 × 40 µm ), which is then coupled to current-carrying silicon beams through This resonator This usesresonator electrostatic and capacitive through comb amechanism. microleverage mechanism. usesactuation electrostatic actuation and sensing capacitive sensing30 through fingers each side of the flexural as beam, shownas inshown Figure in 15.Figure The sensor detects thedetects magnetic 30 combonfingers on each side of thebeam, flexural 15. The sensor the field using the resonant frequency shift due to the Lorentz force, which operates as axial load the magnetic field using the resonant frequency shift due to the Lorentz force, which operates ason axial resonator (Figure 16). The microleverage mechanism amplifies the tension produced by the Lorentz load on the resonator (Figure 16). The microleverage mechanism amplifies the tension produced force, increasing the sensor’s sensitivity by a factor of 42 with respect to the same design without this by the Lorentz force, increasing the sensor’s sensitivity by a factor of 42 with respect to the same mechanism. The sensor uses vacuum packaging obtained by using eutetic bonding between the two design without this mechanism. The sensor uses vacuum packaging obtained by using eutetic wafers. The sensorthe obtains sensitivity 6687 ppm/(mA· a noise floor of 0.5 ppm·Hz−1/2 , a aquality bonding between two awafers. Theofsensor obtains a T), sensitivity of 6687 ppm/(mA ·T), noise − 1/2 factor of 540 and a resonant frequency of 21.9 kHz. The sensor’s sensitivity improves when the floor of 0.5 ppm·Hz , a quality factor of 540 and a resonant frequency of 21.9 kHz. The sensor’s quality factor increases. With this, the sensor could be used for compass applications. This sensor sensitivity improves when the quality factor increases. With this, the sensor could be used for compass uses a large silicon beam uses (500 a× large 40 × silicon 3 μm3) beam subject to × flexural mechanical applications. This sensor (500 40 × 3loads, µm3 ) which subjectneeds to flexural loads, reliability testing. which needs mechanical reliability testing.

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Figure 15. SEM image of magnetic field sensor (left) and its simulated vibration mode (right) [30]. Figure 15. 15. SEM image of magnetic field sensor (left) and its simulated vibration mode Figure mode (right) (right) [30]. [30]. Reprinted with permission from [30]. Copyright© 2015, IEEE. Reprinted with with permission permission from from [30]. [30]. Copyright©2015, Copyright© 2015, IEEE. IEEE. Reprinted

Figure 16. Operation principle of a magnetic field sensor with microleverage mechanism. A magnetic Figure 16. Operation principle of a magnetic field sensor with microleverage mechanism. A magnetic Figure Operation principle a magnetic field (I sensor with microleverage mechanism. A magnetic field (B)16. interacts with the DC of excitation current ) to generate a Lorentz force, which operates as field (B) interacts with the DC excitation current (IBB) to generate a Lorentz force, which operates as field (B) interacts with the DC excitation current (I B ) to generate a Lorentz force, which operates as axial load on the flexural beam resonator. This force is mechanically amplified by the microleverage axial load on the flexural beam resonator. This force is mechanically amplified by the microleverage axial load on the flexural beam resonator. This force is mechanically amplified by the microleverage mechanism [30]. Reprinted with permission from [30]. Copyright©2015, IEEE. mechanism [30]. Reprinted with permission from [30]. Copyright© 2015, IEEE. mechanism [30]. Reprinted with permission from [30]. Copyright© 2015, IEEE.

Aditi and fabricated a MEMS magnetic field sensor (see Figure 17) by anodic andGopal Gopal[31] [31] fabricated a MEMS magnetic field sensor (see Figure 17) bybonding anodic Aditi and Gopal [31] fabricated a MEMS magnetic field sensor (see Figure 17) by anodic technique using SOIusing and glass Thiswafers. sensor This has asensor xylophone of highly doped silicon bonding technique SOI wafers. and glass has aresonator xylophone resonator of highly bonding technique using SOI and glass wafers. This sensor has a xylophone resonator of highly without a metalwithout top electrode. fabrication has the advantages a low temperature doped silicon a metalThe topdevice electrode. The device fabrication hasfollowing: the advantages following: a doped◦silicon without a metal top electrode. The device fabrication has the advantages following: a (low ≤400 C) process, reliable, repeatable, reduced lithography steps and the ability control the gap temperature (≤400 °C) process, reliable, repeatable, reduced lithography stepsto and the ability to low temperature (≤400 °C) process, reliable, repeatable, reduced lithography steps and the ability to between thegap electrodes. addition, the In device uses athe vacuum (1200packaging Pa) at die level control the betweenIn the electrodes. addition, device packaging uses a vacuum (1200which Pa) at control the gap between the electrodes. In addition, the device uses a vacuum packaging (1200 Pa) at is through an anodic bonding process.bonding The xylophone the Lorentz forcethe to Lorentz operate dieobtained level which is obtained through an anodic process. exploits The xylophone exploits die level which is obtained through an anodic bonding process. The xylophone exploits the Lorentz its first resonant mode (108.75 kHz) with a quality factor of 180. The xylophone is electromagnetically force to operate its first resonant mode (108.75 kHz) with a quality factor of 180. The xylophone is force to operate its first resonant mode (108.75 kHz) with a quality factor of 180. The xylophone is actuated, causing displacements that aredisplacements electrostatically sensed by capacitive method 18). electromagnetically actuated, causing that are electrostatically sensed(see by Figure capacitive electromagnetically actuated, causing displacements that are electrostatically sensed by capacitive The device behaves as a parallel plate capacitor, in which a capacitive sensing circuit converts the method (see Figure 18). The device behaves as a parallel plate capacitor, in which a capacitive method (see Figure 18). The device behaves as a parallel plate capacitor, in which a capacitive capacitance shift to an electrical signal. Itshift has to a power consumption ofhas 0.45amW andconsumption a resolution of sensing circuit converts the capacitance an electrical signal. It power sensing circuit the capacitance shift to an electrical signal. It has a power consumption of −1/2 . converts 215 ·Hzand sensor can be used for−1/2 non-destructive magnetic testing of ferromagnetic materials. 0.45nT mW aThis resolution of 215 nT·Hz . This sensor can be used for non-destructive magnetic 0.45 mW and a resolution of 215 nT·Hz−1/2. This sensor can be used for non-destructive magnetic In addition, with an improvement in the vacuum pressure (about 10 Pa), this sensor could be employed testing of ferromagnetic materials. In addition, with an improvement in the vacuum pressure (about testing of ferromagnetic materials. In addition, with an improvement in the vacuum pressure (about as compass in electronic andasgadgets. Forin a constant field, the device 10 aPa), this sensor could bedevices employed a compass electronicmagnetic devices and gadgets. For registered a constant 10 Pa), this sensor could be employed as a compass in electronic devices and gadgets. For a constant amagnetic non-linear displacement when the input displacement current magnitude overcomes µA.current field, the devicevariation registered a non-linear variation when the 400 input magnetic field, the device registered a non-linear displacement variation when the input current magnitude overcomes the 400 μA. magnitude overcomes the 400 μA.

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Figure 17. xylophone beams, Figure 17. 17. SEM SEM image image of of aaa silicon silicon xylophone xylophone supported supported by by four four beams, beams, which which take take advantage advantage of of Figure SEM image of silicon supported by four which take advantage of capacitive sensing to detect magnetic field [31]. Reprinted with permission from [31]. capacitivesensing sensing to detect magnetic [31]. with Reprinted withfrom permission from [31]. capacitive to detect magnetic field [31].field Reprinted permission [31]. Copyright©2016, Copyright© 2016, International Copyright© 2016,Springer Springer International PublishingAG. AG. Springer International Publishing AG. Publishing

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Figure Figure 18. 18. Schematic Schematic of of the: the: (a) (a) operation operation principle principle of of aa xylophone xylophone resonator; resonator; and and (b) (b) its its electrical electrical Figure 18. Schematic of the: (a) operation principle of a xylophone resonator; and (b) its electrical connections for capacitive sensing [31]. Reprinted with permission from [31]. Copyright© 2016, connections for capacitive sensing [31]. Reprinted with permission from [31]. Copyright© 2016, connections for capacitive sensing [31]. Reprinted with permission from [31]. Copyright©2016, Springer Springer International Publishing AG. Springer International Publishing AG. International Publishing AG.

Vasquez Vasquez and and Judy Judy [32] [32] designed designed aa zero-power zero-power magnetic magnetic field field sensor sensor that that is is integrated integrated with with aa Vasquez and Judy [32] designed a zero-power magnetic field sensor that is integrated with micromachined micromachined corner-cube corner-cube reflector reflector (CCR), (CCR), aa commercially commercially available available diode diode laser laser and and aa a micromachined corner-cube reflector (CCR),The a commercially available diode laser and a photodetector photodetector photodetector array, array, as as shown shown in in Figure Figure 19. 19. The sensor sensor is is composed composed of of aa torsional torsional polysilicon polysilicon beam beam array, as shown in Figure 19. The sensor is composed of a torsional polysilicon beamplate. joined to joined joined to to aa polysilicon polysilicon plate plate and and aa permanent permanent magnet magnet (CoNi (CoNi layer) layer) is is deposited deposited on on the the plate. This This a polysilicon plate around and a permanent magnet (CoNi layer) is deposited oncomposed the plate.by This magnet can magnet can rotate the beam axis. On the other hand, the CCR is an orthogonal magnet can rotate around the beam axis. On the other hand, the CCR is composed by an orthogonal rotate of around the beam axis. On the other hand, the CCR is composed by an orthogonal array of two array two fixed polysilicon mirrors and a flexible mirror connected to the torsional beam. Thus, array of two fixed polysilicon mirrors and a flexible mirror connected to the torsional beam. Thus, fixed polysilicon mirrors and a flexible mirror connected to the torsional beam. CCR Thus, the flexible the the flexible flexible mirror mirror is is coupled coupled to to the the magnet magnet rotation, rotation, as as shown shown in in Figure Figure 20. 20. The The CCR is is fabricated fabricated mirrormulti-user is coupled to the magnet rotation, as shown in Figure 20. The CCRfield is fabricated using multi-user using using multi-user MEMS MEMS processes processes (MUMPs) (MUMPs) [33]. [33]. When When the the magnetic magnetic field is is zero, zero, the the three three mirrors mirrors 2) 2) of MEMS processes (MUMPs) [33]. When the magnetic field is zero, the three mirrors (500 × 500 µm (500 × 500 μm the CCR are perpendicular to one another. An alteration of the magnetic (500 × 500 μm2) of the CCR are perpendicular to one another. An alteration of the magnetic field field of the CCRa are perpendicular to one another. An alteration of theproportional magnetic field produces a rotation produces produces a rotation rotation of of the the magnet magnet and and flexible flexible mirror, mirror, which which is is proportional to to the the magnetic magnetic field. field. of the amagnet and flexible mirror, which is proportional to thetwo magnetic field. Then, a diode laser is Then, Then, a diode diode laser laser is is used used to to interrogate interrogate the the CCR, CCR, splitting splitting in in two beams beams the the reflected reflected optical optical beam. beam. used to interrogate the CCR, splitting in two beams the reflected optical beam. The displacements of The The displacements displacements of of these these optical optical beams beams are are measured measured through through aa photodetector photodetector array. array. This This sensor sensor these optical beams power are measured through and a photodetector array. This sensor does not require power does not require consumption it can continuously operate in extremely does not require power consumption and it can continuously operate in extremely harsh harsh consumption and it as canindustrial, continuously operateand in extremely harsh environments such as industrial, environments aerospace the sector. can detect environments such such as industrial, aerospace and the automotive automotive sector. This This sensor sensor can detect aa aerospace field automotive sector. sensor detect a magnetic from 1030 µT to 7540 µT magnetic from 1030 7540 μT an uncertainty of at optical-interrogation magnetic and fieldthe from 1030 μT μT to to 7540This μT with with ancan uncertainty of 113 113 μT μTfield at 1-m 1-m optical-interrogation with anTo uncertainty 113sensor’s µT at 1-m optical-interrogation range. To improve thebesensor’s performance range. improve performance the curvature could and range. To improveofthe the sensor’s performance the mirror mirror curvature could be decreased decreased and the the the mirror curvature could be decreased and the mirror’s size and magnet volume increased, as well as mirror’s size and magnet volume increased, as well as attaching the mirrors directly to the magnet. mirror’s size and magnet volume increased, as well as attaching the mirrors directly to the magnet. attaching thehas mirrors directlyapplication to the magnet. This sensor has asystems potential application in wireless This aa potential in sensing that operate hostile This sensor sensor has potential application in wireless wireless sensing systems that operate in in harsh harsh or orsensing hostile systems that operate in harsh or hostile locations that do not need to provide sensor-node energy. locations that do not need to provide sensor-node energy. However, more studies on the effects locations that do not need to provide sensor-node energy. However, more studies on the effects of of However, studies the effects of etch on the sensor sensitivity should be performed. etch on sensor sensitivity should be performed. etch holes holesmore on the the sensoron sensitivity should beholes performed.

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Figure 19. SEM image of a MEMS magnetic field sensor integrated with a CCR: (a) before; (b) after Figure image magnetic field sensor integrated with aa CCR: CCR: (a) (a) before; before; (b) (b) after after Figure19. 19.SEM SEMReprinted imageofofawith aMEMS MEMS magnetic field sensor integrated assembly [32]. permission from [32]. Copyright© 2007,with IEEE. assembly assembly[32]. [32].Reprinted Reprintedwith withpermission permissionfrom from[32]. [32]. Copyright©2007, Copyright© 2007, IEEE. IEEE.

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Figure 20. Schematic of a MEMS magnetic field sensor composed of a CCR: (a) without; (b) with an Figure20. 20. Schematic of In magnetic sensor composed a CCR: (a) without; (b) torsion with an external magnetic field. addition, a close field up view of the couplingofbetween the(a) mirror and Figure Schematic of aa MEMS MEMS magnetic field sensor composed of a CCR: without; (b) with external magnetic field. In addition, a close up view of the coupling between the mirror and torsion beam is shown [32]. Reprinted with permission from [32]. Copyright© 2007, IEEE. an external magnetic field. In addition, a close up view of the coupling between the mirror and torsion beamisisshown shown[32]. [32].Reprinted Reprintedwith withpermission permissionfrom from[32]. [32]. Copyright©2007, Copyright© 2007, IEEE. IEEE. beam

Park et al. [34] designed a magnetic field sensor formed by a silicon resonator and compact laser Park et system. al. [34] designed a magnetic field sensor formed by a diode siliconfor resonator and compact laser positioning This system has a photodetector and laser monitoring the angular Park et al. [34] designed a magnetic field sensor formed by a silicon resonator and compact laser positioning system. This system hasmirror a photodetector diode for monitoringofthe displacement of a current biased membrane.and Thelaser resonator is composed a angular silicon positioning system. This system has a photodetector and laser diode for thea 3angular displacement of ×a 3000 current mirror membrane. The resonator is ×monitoring composed membrane (3000 × 12biased μm3) coated with an aluminum layer (2500 2500 × 0.8ofμm ).silicon The displacement of a current biased mirror membrane. The resonator is composed of a silicon membrane 3) coated with an aluminum layer 3). The 3 membrane is (3000 × 3000 × 12 μm (2500 × 2500 × 0.8 μm supported by two torsional springs (2100 × 100 × 12 μm ), in which an aluminum wire 3 ). The membrane (3000 × 3000is×supported 12 µm3 ) coated an aluminum layer (2500 ×Figure 2500 µman 3), × membrane by thickness) twowith torsional springs (2100 × 100 ×in12 μm in 0.8 which aluminum (30 μm width and 0.8 μm is deposited, as shown 21. The sensor exploits wire the 3 ), in which an aluminum wire (30 µm isLorentz supported two torsional springs (2100 × motion 100 ×as12 µm (30 μm width 0.8 μm thickness) is deposited, shown in Figure 21.measured The sensor exploits the forcebyinand order to generate a rotational of the mirror that is with the laser width andforce 0.8system. µm thickness) is deposited, as shown inofFigure 21. exploits the Lorentz Lorentz in order generate a rotational motion the mirror thatsensor is measured with the laser positioning Totoincrease the sensitivity of the sensor, the The mirror oscillates at resonance force in order to generate rotational of theofmirror thatapplied is measured the laser positioning system. To aincrease themotion sensitivity the sensor, the magnetic mirrorwith oscillates at positioning resonance (torsional vibration mode) at atmospheric pressure. Thus, the field is related to the system. To increase the sensitivity of the sensor, the mirror oscillates at resonance (torsional (torsional vibration at atmospheric pressure. magneticpressure, field is related to the displacements of themode) mirror. For a coil bias currentThus, of 50 the mAapplied at atmospheric thevibration sensor mode) at atmospheric Thus, applied magnetic field is related to the displacements displacements of the pressure. mirror. For a −1 coil current of 50 mA atmospheric pressure, sensor registers a sensitivity of 62 mV· μT , the a bias resonant frequency of at 364 Hz, a quality factor the of 116, aof −1 −1/2 the mirror. For a coil bias current of 50 mA at atmospheric pressure, the sensor registers a sensitivity registers a sensitivity of 62 mV· μT , a resonant frequency of 364 Hz, a quality factor of 116, resolution of 0.4 nT with a bandwidth of 53 mHz and noise floor level of 1.78 nT·Hz . This sensora −1 , a resonant frequency of 364 Hz, a quality factor of 116, a resolution−1/2 ofcan 62 detect mV·µTaof 0.4 nT with resolution 0.4 nT with bandwidth of 53of mHz and noise floor levelNevertheless, of 1.78 nT·Hz This sensor magnetic fielda between a range nanoTeslas and Teslas. theof.variation of − 1/2 a the bandwidth 53 mHz and noise floor level ofof1.78 nTand ·Hzsprings . This sensor detect the a magnetic field sensor due to heating ofathe membrane should be can studied. can detect behavior aof magnetic field between range nanoTeslas and Teslas. Nevertheless, variation of between a range of nanoTeslas and Teslas. Nevertheless, variation ofbe thestudied. sensor behavior due to the sensor behavior due to heating of the membrane and the springs should heating of the membrane and springs should be studied.

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Figure 21. (a) Microscope image of a MEMS magnetic field sensor with optical readout; and (b) Figure 21. 21.principle (a)(a)Microscope of[34]. aofMEMS magnetic sensor with optical and2016, (b) operation of the image sensor withfield permission from [34]. readout; Copyright© Figure Microscope image aReprinted MEMS magnetic field sensor with optical readout; operation principle of the sensor [34]. Reprinted with permission from [34]. Copyright© 2016, Elsevier B.V. and (b) operation principle of the sensor [34]. Reprinted with permission from [34]. Copyright©2016, Elsevier B.V. Elsevier B.V.

3. Potential Applications 3. Potential Applications 3. Potential Applications Magnetic field sensors based on MEMS resonators have potential applications such as Magnetic field sensors based on MEMS resonators have potential applications such as telecommunications, military, based industrial, automotive, biomedical consumer electronic products. Magnetic field sensors on MEMS resonators haveand potential applications such as telecommunications, military, industrial, automotive, biomedical and consumer electronic products. This is due to the important advantages of MEMS sensors, which include small size, lightweight, telecommunications, military, industrial, automotive, biomedical and consumer electronic products. This is due to the important advantages of MEMS sensors, which include small size, lightweight, low power consumer, wide advantages dynamic range, compact signal conditioning and high These This is due to the important of MEMS sensors, which include small size,sensitivity. lightweight, low low power consumer, wide dynamic range, compact signal conditioning and high sensitivity. These sensorsconsumer, could detect or range, residual stresses of ferromagnetic materials using non-destructive power wide flaws dynamic compact signal conditioning and high sensitivity. These sensors sensors could detect flaws or residual stresses of ferromagnetic materials using non-destructive testingdetect such flaws as Eddy current stresses inspection and the magnetic memory method (MMM) [35–37]. could or residual of ferromagnetic materials using non-destructive testing Eddy such testing such as Eddy current inspection and the magnetic memory method (MMM) [35–37]. Eddy currents are generated on the surface of the ferromagnetic material when a variable magnetic as Eddy current inspection and the magnetic memory method (MMM) [35–37]. Eddy currentsfield are currents are generated on the surface of the ferromagnetic material when a variable magnetic field interacts with this surface. The flaws of material the ferromagnetic material alter theinteracts Eddy with currents, generated on the surface of the ferromagnetic when a variable magnetic field this interacts with this surface. The flaws of the ferromagnetic material alter the Eddy currents, modifying field in relation to the sizethe of the flaws. For instance, an array of magnetic surface. Thetheir flawsmagnetic of the ferromagnetic material alter currents, modifying field modifying their magnetic field in relation to the size of Eddy the flaws. For instance, antheir arraymagnetic of magnetic field sensors could detect flaws ofFor an instance, oil pipeline (see Figure 22) using thesensors Eddy currents technique in relation to the size of the flaws. an array of magnetic field could detect flaws field sensors could detect flaws of an oil pipeline (see Figure 22) using the Eddy currents technique [36]. the other(see hand, MMM detect residual stress of ferromagnetic materials through of an On oil Figure 22)can using theflaws Eddyor [36]. On the other hand, MMM [36]. Onpipeline the other hand, MMM can detect flaws orcurrents residualtechnique stress of ferromagnetic materials through the detect variations ofortheir residual magnetic field. Thus, magnetic fieldthe sensors couldofmeasure these can flaws residual stress of ferromagnetic materials through variations their residual the variations of their residual magnetic field. Thus, magnetic field sensors could measure these modifications ofThus, a magnetic field related to the size measure of the flaws, ormodifications magnitude ofof thea residual stress. magnetic field. magnetic field sensors could these magnetic field modifications of a magnetic field related to the size of the flaws, or magnitude of the residual stress. Lara-Castro et al. [37] developed a portable signal conditioning system of a magnetic field sensor related to the et size thedeveloped flaws, or magnitude the residual stress.system Lara-Castro et al. [37]field developed Lara-Castro al.of[37] a portable of signal conditioning of a magnetic sensor Figure 23), which could detect residual magnetic field of ferromagnetic materials using the a(see portable signal conditioning system of a magnetic field sensor (see Figure 23), which could detect (see Figure 23), which could detect residual magnetic field of ferromagnetic materials using the MMM. residual magnetic field of ferromagnetic materials using the MMM. MMM.

Figure 22. Design of a flaws inspection system in oil pipeline using Eddy current technique [36]. Figure pipeline using using Eddy Eddy current current technique technique[36]. [36]. Figure22. 22. Design Design of of aa flaws flaws inspection inspection system in oil pipeline Reprinted with permission from [36]. Copyright©2011, InTech. Reprinted InTech. Reprintedwith withpermission permission from from [36]. [36]. Copyright© Copyright© 2011, InTech.

Domínguez-Nicolas employing MEMS Domí nguez-Nicolas et et al. al. [38] [38] developed developed aa respiratory respiratory magnetogram Domí nguez-Nicolas al. [38] developed respiratory magnetogram magnetogram employing employing aaa MEMS MEMS magnetic field sensor with piezoresistive sensing and silicon resonator Figure 24). This device magnetic field sensor with piezoresistive sensing and silicon (see Figure 24). This device magnetic field sensor with piezoresistive silicon resonator (see Figure 24). This device measuredthe themagnetic magneticfield fieldduring duringthe therespiratory activity of measured These researchers registered the measured the magnetic field during the respiratory activity of rats. rats. These These researchers researchersregistered registeredthe the magnetogramand andelectromyogram electromyogram of of the thoracic cavity of aa rat magnetogram rat during during its its respiration, respiration,as asshown showninin

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magnetogram and electromyogram of the thoracic cavity of a rat during its respiration, as shown in Figure 25. 25. For For this this magnetogram, magnetogram, Juarez-Aguirre Juarez-Aguirre et et al. al. [39] [39] designed designed aa digital digital signal signal processing processing Figure Figure 25. For this magnetogram, Juarez-Aguirre et al. [39] designed a digital signal processing through through virtual instrumentation. A future application of this magnetogram could be used for for through virtual instrumentation. A future application of this magnetogram could be used virtual instrumentation. A future application of this magnetogram could be used for monitoring the monitoring the health of some organs of the thoracic cavity. For instance, healthy organs of the monitoring the organs health of organs cavity. of the thoracic cavity.healthy For instance, organs of the health of some of some the thoracic For instance, organs healthy of the thoracic cavity thoracic cavity cavity (e.g., (e.g., heart) heart) could could generate generate magnetic magnetic fields fields in in aa determined determined range; range; unhealthy unhealthy organs organs thoracic (e.g., heart) could generate magnetic fields in a determined range; unhealthy organs however, could however, could could cause cause abnormal abnormal magnetic fields. fields. More research research related related to to magnetic magnetic fields fields emitted emitted by by however, cause abnormal magnetic fields.magnetic More researchMore related to magnetic fields emitted by healthy and healthy and and unhealthy unhealthy organs of of the thoracic thoracic cavity cavity is is required. required. In addition, addition, Tapia Tapia et et al. al. [40] [40] built built an an healthy unhealthy organs of theorgans thoracic the cavity is required. In addition,InTapia et al. [40] built an electronic electronic neuron (FitzHugh-Nagumo) to generate controlled spike-like magnetic fields, which were electronic neuron (FitzHugh-Nagumo) generate controlled spike-likefields, magnetic fields, which were neuron (FitzHugh-Nagumo) to generatetocontrolled spike-like magnetic which were measured measured with with aa MEMS MEMS sensor. sensor. In In future future applications, applications, this this sensor sensor could could be be improved improved to to detect detect measured with a MEMS sensor. In future applications, this sensor could be improved to detect spiking activity of spiking activity of of neurons or or muscle muscle cells. cells. spiking neuronsactivity or muscle neurons cells.

Figure 23. 23. Printed Printed circuit circuit board board (PCB) (PCB) for aa portable portable signal conditioning system of MEMS sensor, Figure (PCB) for portable signal signal conditioning conditioning system system of of aaa MEMS MEMS sensor, sensor, which could be used for monitoring residual magnetic field of ferromagnetic materials [37]. couldbebe used monitoring residual magnetic of ferromagnetic [37]. which could used for for monitoring residual magnetic field offield ferromagnetic materials materials [37]. Reprinted Reprinted with permission from [37]. Copyright© 2016, Springer International Publishing AG. Reprinted with permission from [37]. Copyright© 2016,International Springer International AG. with permission from [37]. Copyright©2016, Springer PublishingPublishing AG.

Figure 24. SEMimage imageof MEMSmagnetic magneticfield fieldsensor sensorwith withpotential potentialapplication applicationto obtain SEM image ofofaaaMEMS MEMS magnetic field sensor with potential application totoobtain obtain Figure 24. SEM aa a respiratory magnetogram [38]. Reprinted with permission from Copyright©2013, Ivyspring respiratory magnetogram [38]. Reprinted with permission from [38]. Copyright© 2013, respiratory magnetogram [38]. Reprinted with permission from [38]. Copyright© 2013, Ivyspring International Publisher. Publisher. International Publisher.

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(a)

(b)

Figure 25. set up sensor for monitoring the Figure 25. 25. (a) (a) Diagram Diagram of of an an experimental experimental set set up up of of aaa MEMS MEMS magnetic magnetic field field sensor sensor for for monitoring monitoring the the Figure (a) Diagram of an experimental of MEMS magnetic field respiratory and cardiac activity of a rat; (b) Electromyogram of the thoracic muscles and respiratory and cardiac activity of a rat; (b) Electromyogram of the thoracic muscles and respiratory and cardiac activity of a rat; (b) Electromyogram of the thoracic muscles and magnetogram magnetogram of thoracic during the of rat magnetogram of the theduring thoracic cavity during the respiratory respiratory activity of aawith rat [38]. [38]. Reprinted Reprinted with of the thoracic cavity thecavity respiratory activity of a rat [38].activity Reprinted permission fromwith [38]. permission from [38]. Copyright© 2013, Ivyspring International Publisher. permission from [38]. Copyright© 2013, Ivyspring International Publisher. Copyright©2013, Ivyspring International Publisher.

Other future applications of MEMS sensors include the micro-, nano- and pico-satellites. Other future applications of MEMS sensors include the micro-, nano- and pico-satellites. Miniaturization of satellites allows for the reduction of their launch costs, which can be achieved Miniaturization of satellites allows for the reduction of their launch costs, which can be achieved using using sensors of small size, reliable, low energy consumption and high sensitivity. For instance, sensors of small size, reliable, low energy consumption and high sensitivity. For instance, satellites must satellites must measure small magnetic fields during their space missions. For this application, Lamy measure small magnetic fields during their space missions. For this application, Lamy et al. [41] and et al. [41] and Ranvier et al. [42] designed MEMS sensors composed of polysilicon-xylophone bars Ranvier et al. [42] designed MEMS sensors composed of polysilicon-xylophone bars and a capacitive and a capacitive sensing. In addition, space satellites need inertial measurement units (IMUs) sensing. In addition, space satellites need inertial measurement units (IMUs) formed by magnetic field formed by magnetic field sensors, accelerometers and gyroscopes. These IMUs could be fabricated sensors, accelerometers and gyroscopes. These IMUs could be fabricated on a single chip to decrease the on a single chip to decrease the power consumption and electronic noise. Other IMUs applications power consumption and electronic noise. Other IMUs applications will include the navigation systems will include the navigation systems of ships, trains, military and civil aviation and unmanned of ships, trains, military and civil aviation and unmanned operated vehicles [43–45]. Laghi et al. [46] operated vehicles [43–45]. Laghi et al. [46] fabricated a torsional MEMS sensor through surface fabricated a torsional MEMS sensor through surface micromachining for monitoring in-plane magnetic micromachining for monitoring in-plane magnetic field, as shown in Figure 26. This sensor operates field, as shown in Figure 26. This sensor operates with the Lorentz force and capacitive sensing. with the Lorentz force and capacitive sensing. This device has the following technical parameters: This device has the following technical parameters: size of 282 × 1095 µm2 , vacuum packaging of size of 282 × 1095 μm22, vacuum packaging of 0.35 mbar, resonant frequency of 19.95 kHz, quality 0.35 mbar, resonant frequency of 19.95 kHz, quality factor of 2500, a sensitivity of 0.85 V·mT−1 and −1 and a detection limit of 120 nT·mA·Hz−1/2 −1/2. factor of 2500, a sensitivity of 0.85 V·mT−1 a detection limit of 120 nT·mA·Hz−1/2 .

Figure 26. IMUs Figure 26. 26. (a) (a) SEM image image of of aa MEMS MEMS magnetic magnetic field field sensor sensor with with capacitive capacitive sensing sensing for for IMUs IMUs Figure (a) SEM applications; (b) Top view; (c) the of sensor [46]. applications;(b) (b)Top Top view; and (c) cross-section cross-section of the operating operating principle of the the sensor [46]. applications; view; andand (c) cross-section of theof operating principleprinciple of the sensor [46]. Reprinted Reprinted with from 2015, B. Reprinted with permission permission from [46]. [46]. Copyright© Copyright© 2015, Elsevier B. V. V. with permission from [46]. Copyright©2015, Elsevier B.Elsevier V.

Trafficdetection detectionsystems systems could measure the speed andofsize of vehicles considering MEMS Traffic could measure the speed and size vehicles considering MEMS devices, devices, areinlocated parallel alongside the road These devices have separation a constant which arewhich located parallelinalongside the road [36]. These[36]. devices will have awill constant separation distance and will detect the variation of Earth’s magnetic field generated by the motion of the vehicles, as shown in Figure 27. These changes can be measured at different times (t11 and t22)

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the variation of Earth’s magnetic field generated by the motion of the vehicles, 19 of 25 as shown in Figure 27. These changes can be measured at different times (t1 and t2 ) using the MEMS using theThus, MEMS devices. Thus, vehicle speed will determined theseparation ratio of the devices devices. vehicle speed will be determined usingbethe ratio of theusing devices distance to separation distancet1 to difference t1 and t2. Thewith vehicle size will change be related with the the time difference andthe t2 . time The vehicle size will be related the magnitude of the Earth’s magnitude change of the Earth’s magnetic field. magnetic field.

Figure 27. view of traffic detection systemsystem formed formed by MEMS signal and processing 27.Schematic Schematic view of traffic detection bydevices MEMSand devices signal [36]. Reprinted permission from [36]. Copyright@2011, Intech. processing [36].with Reprinted with permission from [36]. Copyright@2011, Intech.

4. 4. Comparisons Comparisonsand andChallenges Challenges MEMS force-based magnetic sensors offer offer several several advantages advantages MEMS resonators resonators used used in in Lorentz Lorentz force-based magnetic field field sensors with respect to to conventional conventional sensors, sensors, allowing allowing for for their their use use in in future future applications. applications. The with respect The Lorentz Lorentz force-based sensors provide a wide measurement range by changing the magnitude of the excitation force-based sensors provide a wide measurement range by changing the magnitude of the excitation current. current. In In addition, addition, vacuum vacuum packaging packaging reduces reduces the the air air damping, damping, increasing increasing the the quality quality factor factor and and sensitivity ofthe theMEMS MEMS resonators. These sensors low energy consumption and have a sensitivity of resonators. These sensors needneed a lowaenergy consumption and have a compact compact can bethrough fabricated throughmicromachining a standard micromachining process.sensors Also, structure,structure, which canwhich be fabricated a standard process. Also, MEMS MEMS use different sensing techniques (e.g., piezoresistive, optical, or magnetic capacitive) to can use sensors differentcan sensing techniques (e.g., piezoresistive, optical, or capacitive) to detect field. detect magnetic field. Each technique presents specific benefits that are employed for the designers Each technique presents specific benefits that are employed for the designers to develop the best sensor to the best sensorFor forexample, a specificpiezoresistive application. For example, piezoresistive sensing is adequate fordevelop a specific application. sensing is adequate for the bulk micromachining for the bulk micromachining process systems. and simple signal systems. With thesesignal systems, an process and simple signal processing With theseprocessing systems, an electrical output related electrical output signal related with the magnetic field is obtained. The piezoresistive sensing with the magnetic field is obtained. The piezoresistive sensing registers a voltage offset and requires registers a voltage offset and requires compensation circuits. isOn the other hand, temperature compensation circuits. On temperature the other hand, capacitive sensing mostly used in the capacitive sensing is mostly used and in the superficial micromachining it converts the superficial micromachining process it converts the applied magnetic process field intoand an electrical output applied magnetic field electricaldependence output signal. This technique has oflittle temperature signal. This technique hasinto little an temperature and allows the fabrication electronic circuits dependence and allows the fabrication of electronic circuits on the same chip of the magnetic field on the same chip of the magnetic field sensor. It permits the reduction of the device size and parasitic sensor. It permits the reduction of the device size and parasitic capacitances. Generally, the sensors capacitances. Generally, the sensors with capacitive sensing have high air damping at atmospheric with capacitive sensing air damping at atmospheric pressure; therefore theyoptical need vacuum pressure; therefore theyhave needhigh vacuum packaging to increase their sensitivity. Finally, sensing packaging to increase their sensitivity. Finally, optical sensing has electromagnetic has immunity to electromagnetic interference (EMI) and demands lessimmunity electronic to circuitry than both interference (EMI) and demands less electronic circuitry bothcan capacitive piezoresistive capacitive and piezoresistive sensing. Sensors with opticalthan sensing operate and in hostile or harsh sensing. Sensors with optical sensing can operate in hostile or harsh environments without environments without providing energy for the sensor. The superficial and bulk micromachining providing energy for the The superficial andNevertheless, bulk micromachining are suitable for processes are suitable forsensor. this sensing technique. all these processes sensing techniques have this sensing technique. Nevertheless, all these sensing techniques have problems with the heating of problems with the heating of the sensor structure due to the Joule effect, generating thermal stress and the sensor structure due to the More Joule studies effect, generating thermal stress displacements of the displacements of the resonators. about the modifications in and the behavior of the MEMS resonators. More studies about the modifications in the behavior of the MEMS resonators due to the resonators due to the Joule effect must be performed. In addition, the mechanical reliability of the Joule effect must be performed. In addition, the mechanical reliability of the resonators must be resonators must be studied to ensure the best performance of the MEMS sensors. studied to ensure the best performance of the MEMS sensors. Table 1 depicts some characteristics of magnetic field sensors composed by resonators and fabricated with micromachining process. Recently, MEMS magnetic field devices have been developed with important advantages for future commercial markets. To guarantee a reliable

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Table 1 depicts some characteristics of magnetic field sensors composed by resonators and fabricated micromachining process. Recently, MEMS magnetic field devices have been developed Sensors 2016,with 16, 1359 20 of 25 with important advantages for future commercial markets. To guarantee a reliable performance of performance theseresearches devices, more mustrespect be made with respect to vacuum packaging, these devices,ofmore must researches be made with to vacuum packaging, dependence on dependence on changes humidity and temperature, as well as a reduction ofnoise. voltage offset and changes of humidity and of temperature, as well as a reduction of voltage offset and With respect noise. With respect to the reduction of electronic noise, Minotti aetmagnetic al. [47] developed a magnetic to the reduction of electronic noise, Minotti et al. [47] developed field sensor (see Figurefield 28) sensor (see Figure 28) with a custom integrated readout circuit containing a capacitive-sensing with a custom integrated readout circuit containing a capacitive-sensing front-end, a mixer and a mixer and ademodulation. low-pass filterThis forsensor signalhas demodulation. This sensor has accelerations tuning fork afront-end, low-pass filter for signal tuning fork geometry to reject geometry to reject accelerations and vibrations and multiple loops to increase its sensitivity. It and vibrations and multiple loops to increase its sensitivity. It operates in off-resonance mode to operates inboth off-resonance to overcome both between bandwidth and resolution and overcome trade-offs mode between bandwidth andtrade-offs resolution and long-term stability. The sensor is long-term stability. The sensor is fabricated using the thick epitaxial layer for microactuators and fabricated using the thick epitaxial layer for microactuators and accelerometers (ThELMA) process accelerometers (ThELMA) process from CMOS STMicroelectronics [28]. A 0.35-μm CMOS process from from STMicroelectronics [28]. A 0.35-µm process from AustriaMicroSystem (AMS) is used to AustriaMicroSystem (AMS) is In used to fabricate the integrated circuit. In29) addition, theconsumption overall system fabricate the integrated circuit. addition, the overall system (see Figure has a low of −1·mA−1, − 1 − 1 (see Figure 29) has a low consumption of power close to 775 μW, a sensitivity of 0.75 zF· nT power close to 775 µW, a sensitivity of 0.75 zF·nT ·mA , and packaging pressures of about 0.75 mbar. and packaging pressures of aboutranges 0.75 mbar. it can reach large ranges which up to ±2.4 mT, Thus, it can reach large full-scale up toThus, ±2.4 mT, adjusting the full-scale driving current, exceeds adjusting the driving current, which exceeds the full-scale of conventional devices such as the Hall the full-scale of conventional devices such as the Hall effect and anisotropic magneto-resistance effect and anisotropic magneto-resistance (AMR) [48]. (AMR) [48].

Figure 28. 28. SEM SEM image image of of diamond-shaped diamond-shaped tuning fork, including 10 metal metal coils coils deposited deposited over over the the Figure springs and parallel plate (PP) cells. These cells are employed for capacitive readout and tuning [47]. tuning [47]. Reprinted with with permission permission from from [47]. [47]. Copyright@2015, Copyright@2015, IEEE. Reprinted

Reliability studies thethe devices behavior under different environments and Reliability studies are arerequired requiredtotoknow know devices behavior under different environments operation conditions. In addition, studies of the main damping sources of resonators must be and operation conditions. In addition, studies of the main damping sources of resonators must be considered ininthe the design phase of devices [49,50]. Designers use thesetostudies to obtain considered design phase of devices [49,50]. Designers can usecan these studies obtain resonators resonators with minimum damping, which will improve their quality factor and resolution. Vacuum with minimum damping, which will improve their quality factor and resolution. Vacuum packaging packaging can decrease the air damping at resonators, increasing their performance. can decrease the air damping at resonators, increasing their performance. Future applicationsFuture need applications need multifunctional sensors forphysical monitoring several (e.g., physical parameters (e.g., multifunctional sensors for monitoring several parameters acceleration, magnetic acceleration, magnetic field, angular ratio, humidity, temperature and gases). For this, MEMS field, angular ratio, humidity, temperature and gases). For this, MEMS technology will allow technology will of allow the integration of a different sensorsFuture on a single chip. Future sources as the integration different sensors on single chip. energy sources asenergy microgenerators microgenerators incorporated into the devices could provide them with their own power, incorporated into the devices could provide them with their own power, therefore eliminatingtherefore batteries. eliminating batteries.ofAlso, more studies of magnetic be made to improve Also, more studies magnetic stochastic resonancestochastic could beresonance made to could improve the detection of the detection of magnetic fields. This stochastic resonance applied to magnetic field sensors could magnetic fields. This stochastic resonance applied to magnetic field sensors could help to increase help dynamic to increase their dynamic range and [51]. For this reason, sensors should consider their range and sensitivity [51]. Forsensitivity this reason, sensors should consider stochastic-resonance stochastic-resonance coils on the same chip, in which these coils could be fabricated with metallic coils on the same chip, in which these coils could be fabricated with metallic coils around the active coilsof around the active area of the sensors. area the sensors.

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Table 1. Main characteristics of recent MEMS magnetic field sensors based on Lorentz force. Magnetic Field Sensor

Resonator Size (µm × µm)

Resonant Frequency (kHz)

Quality Factor

Herrera-May et al. [25] Mehdizadeh et al. [26] Herrera-May et al. [27] Zhang et al. [29] Langfelder et al. [28] Li et al. [30] Laghi et al. [46] Minotti et al. [47] Park et al. [34]

400 × 150 500 × 1400 472 × 300 600 × 800 89 × 868 1200 × 680 282 × 1095 1700 × 750 3000 × 3000

136.52 2550 100.7 49.3 28.3 21.9 19.95 20 0.36

842 16,900 419.6 100,000 327.9 540 2,500 460 116

Noise 57.5

nV·Hz−1/2

* — ** 37.1 nV·Hz−1/2 * — ** 557.2 µV·Hz−1/2 * 0.5 ppm·Hz−1/2 — ** 30 zF·Hz−1/2 1.78 nT·Hz−1/2

* Theoretical data; ** Data do not available in the literature.

Detection Limit 143

nT·Hz−1/2

* —** 161 nT·Hz−1/2 * — ** 520 nT·mA·Hz−1/2 — ** 120 nT·mA·Hz−1/2 40 nT·mA·Hz−1/2 0.4 nT with BW = 53 mHz

Sensitivity 403 mV·T−1 262 mV·T−1 230 mV·T−1 215.74 ppm·T−1 150 µV·µT−1 6687 ppm·mA−1 ·T−1 0.85 V·mT−1 0.75 zF·nT−1 ·mA−1 62 mV·µT−1

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Figure 29. 29. Photography Figure Photography showing showingthe thestaked stakedMEMS MEMSand andASIC ASICdies, dies,which whichare arewire-bonded wire-bondedonona socket carrier located on the biasing PCB board [47]. Reprinted with permission a socket carrier located on the biasing PCB board [47]. Reprinted with permission from from [47]. [47]. Copyright@2015, IEEE. Copyright@2015, IEEE.

5. Conclusions 5. Conclusions Magnetic field sensors formed by MEMS resonators have important advantages in respect to Magnetic field sensors formed by MEMS resonators have important advantages in respect conventional devices, allowing for their implementation in future applications. Most of these sensors to conventional devices, allowing for their implementation in future applications. Most of these use silicon resonators that exploit the Lorentz force and different sensing techniques such as sensors use silicon resonators that exploit the Lorentz force and different sensing techniques such as piezoresistive, optical, or capacitive. The piezoresistive sensing is a simple technique with an easy piezoresistive, optical, or capacitive. The piezoresistive sensing is a simple technique with an easy fabrication process; although it can have voltage offset and temperature dependence. Capacitive fabrication process; although it can have voltage offset and temperature dependence. Capacitive sensing has little temperature dependence but suffers from parasitic capacitances, which can be sensing has little temperature dependence but suffers from parasitic capacitances, which can be decreased with monolithic fabrication. Optical readout offers immunity to electromagnetic decreased with monolithic fabrication. Optical readout offers immunity to electromagnetic interference interference (EMI) and decreases the electronic components. The main fabrication processes include (EMI) and decreases the electronic components. The main fabrication processes include surface and surface and bulk micromachining, considering materials such as silicon, polysilicon, silicon dioxide, bulk micromachining, considering materials such as silicon, polysilicon, silicon dioxide, aluminum aluminum and gold. However, MEMS devices need more reliability research in order to predict their and gold. However, MEMS devices need more reliability research in order to predict their performance performance under different environments and operation conditions. Future works must consider under different environments and operation conditions. Future works must consider the reduction the reduction of damping and electronic noise, as well as the integration of different sensors on a of damping and electronic noise, as well as the integration of different sensors on a single chip for single chip for monitoring several physical parameters. monitoring several physical parameters. Acknowledgments: This This work work was was partially partially supported supported by by Sandia Sandia National National Laboratory’s Laboratory’s University University Alliance Alliance Acknowledgments: PRODEP “Estudio “Estudio de de Dispositivos Dispositivos Program, FORDECYT-CONACYT through Grant 115976, and projects PRODEP Electrónicos y Electromecánicos Electromecánicos con Potencial Potencial Aplicación en Fisiología Fisiología yy Optoelectrónica” Optoelectrónica” and and “Sistema “Sistema Electrónico Residual de de Estructuras Ferromagnéticas”. TheThe authors would like Electrónico de deMedición Mediciónde deCampo CampoMagnético Magnético Residual Estructuras Ferromagnéticas”. authors would to thank Eduard Figueras from IMB-CNM (CSIC) for his collaboration into the fabrication of MEMS magnetometers like to thank Eduard Figueras from IMB-CNM (CSIC) for his collaboration into the fabrication of MEMS and Fernando Bravo-Barrera from LAPEM for his assistance with the SEM images. magnetometers and Fernando Bravo-Barrera from LAPEM for his assistance with the SEM images. Author Contributions: Agustín Leobardo Herrera-May, Juan Carlos Soler-Balcazar and Héctor Vázquez-Leal wrote Author Contributions: Agustí n Leobardo Herrera-May, Juan Carlos Soler-Balcazar and Héctor Vázquez-Leal the sections of introduction, design and fabrication. Jaime Martínez-Castillo, Marco Osvaldo Vigueras-Zuñiga and Luz Antonio Aguilera-Cortés wrote the sections potential applications, and challenges. wrote the sections of introduction, design of and fabrication. Jaime comparisons Martínez-Castillo, Marco Osvaldo Vigueras-Zuñiga andThe Luzauthors Antonio Aguilera-Cortés wrote the sections of potential applications, comparisons Conflicts of Interest: declare no conflict of interest. and challenges.

References Conflicts of Interest: The authors declare no conflict of interest. 1. Senturia, S.D. Microsystem Design; Kluwer Academic Publishers: New York, NY, USA, 2002. References 2. Allen, J.M. Mechanical System Design; Taylor & Francis: Boca Raton, FL, USA, 2005. 3. Acevedo-Mijangos, J.; Soler-Balcázar, C.; Vazquez-Leal, H.; Martínez-Castillo, J.; Herrera-May, 1. Senturia, S.D. Microsystem Design; Kluwer Academic Publishers: New York, NY, USA, 2002. A.L. Design and modeling of a novelSystem microsensor detect& magnetic two orthogonal directions. Microsyst. Technol. 2. Allen, J.M. Mechanical Design;toTaylor Francis:fields Boca in Raton, FL, USA, 2005. 2013, 19, 1897–1912. [CrossRef] 3. Acevedo-Mijangos, J.; Soler-Balcázar, C.; Vazquez-Leal, H.; Martínez-Castillo, J.; Herrera-May, A.L.

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