SCANNING VOL. 36, 632–639 (2014) © Wiley Periodicals, Inc.
Mechanically Stable Tuning Fork Sensor With High Quality Factor for the Atomic Force Microscope KWANGYOON KIM, JUN-YOUNG PARK, K.B. KIM, NAESUNG LEE,
AND
YONGHO SEO
Faculty of Nanotechnology and Advanced Material Engineering, HMC, and GRI, Sejong University, Seoul, Korea
Summary: A quartz tuning fork was used instead of cantilever as a force sensor for the atomic force microscope. A tungsten tip was made by electrochemical etching from a wire of 50 mm diameter. In order to have mechanical stability of the tuning fork, it was attached on an alumina plate. The tungsten tip was attached on the inside end of a prong of a tuning fork. The phase shift was used as a feedback signal to control the distance between the tip and sample, and the amplitude was kept constant using a lock-in amplifier and a homemade automatic gain controller. Due to the mechanical stability, the sensor shows a high quality factor (103), and the image quality obtained with this sensor was equivalent to that of the cantilever-based AFM. SCANNING 36:632–639, 2014. © 2014 Wiley Periodicals, Inc. Key words: tuning fork sensor, atomic force microscope (AFM), scanning probe microscopy (SPM), quartz tuning fork
Introduction Cantilevers have been mostly used as force sensors for atomic force microscopes (AFM) (Binnig et al., ’86). In order to detect the motion of the cantilever, the optical deflection technique (Putman et al., ’92) is adopted using a laser beam and a position sensitive photo-diode. The optical deflection technique is widely used not only for
Contract grant sponsor: Ministry of Science; Contract grant number: 2013R1A1A1A05005298; Contract grant sponsor: Ministry of Education; Contract grant number: 2010-0020207. Address for reprints: Yongho Seo, Faculty of Nanotechnology and Advanced Material Engineering, HMC, and GRI, Sejong University, Seoul 143-747, Korea E-mail: [email protected] Received 7 May 2014; revised 1 August 2014; Accepted with revision 26 August 2014 DOI: 10.1002/sca.21169 Published online 17 September 2014 in Wiley Online Library (wileyonlinelibrary.com).
topographic measurement, but also for almost all other scanning probe microscopies (SPM), including electrostatic force microscopy, magnetic force microscopy, and near-field scanning optical microscopy. However, there are disadvantages to using the optical deflection technique, particularly for measurements in dark environments and at low temperatures. To overcome these problems of the optical deflection technique, SPM adopting a quartz tuning fork (QTF) as a force sensor was developed, and has been applied to almost all kinds of SPM (Karrai and Grober, ’95; Edwards et al., ’97; Giessibl, ’98; Seo et al., 2002, 2003; An et al., 2014a,b). First of all, the QTF has a high Q-value (quality factor), which is an essential feature to achieve good force sensitivity and high resolution images (Jahng et al., 2007; Oiko et al., 2014). As a representative example of QTF, qPlus sensor (Giessibl, ’98, 2000; Kim et al., 2014), which was developed by Giessibl, has a Qvalue of a few thousands in ambient pressure. The qPlus sensor has a unique design; one prong of the tuning fork is attached to a hard material and the other prong has free motion to sense the force. An ultra-high resolution AFM image with a sub-atomic scale, even better than the scanning tunneling microscopic (STM) image, was achieved using the qPlus sensor (Giessibl et al., 2000). Moreover, QTF needs no laser source as it operates only electrically as a sensor, and a variety of experiments can be performed compared with the conventional SPM. For example, photocurrent imaging in nanometer scale or near field optics in dark conditions is possible. Also, SPM measurements in a high vacuum and at low temperatures are available using QTF (Seo et al., 2005). In the case of conventional SPM, many optical units are not compatible in a vacuum or at low temperatures. Not only the QTF is compatible in a vacuum, but its Q-value is enhanced up to 105. Also, the power dissipation is low enough to be used at ultra-low temperatures (Rychen et al., ’99). Because of these advantages, the QTF sensor has been used widely in SPM field and has been linked to commercialization (Akiyama et al., 2010). In this paper, we suggest a method of fabricating a QTF based AFM sensor including the tip etching and mounting, designing of the sensor holder, and electronic phase detection for
K. Kim et al.: Mechanically Stable Tuning Fork Sensor
feedback control, in order to facilitate the use of QTF based AFM for industrial purposes.
Materials and Methods Fabrication of Tungsten Tip
The electrochemical etching method was applied to make a tungsten (W) tip from W-wire of 50 mm diameter. As the tip should be mounted at the end of a prong of QTF, a low mass of tip is important not to reduce the Q-value of the tuning fork, and that is the reason why thin wire was chosen. Conventional methods fabricating the W-tip for STM are well known (Ibe et al., ’90; Ekvall et al., ’99). Electrochemical etching is composed of two electrodes and an electrolyte, where one of the electrodes (anode) is the tungsten wire. By applying an external voltage, a redox reaction occurs at the electrodes, and the tungsten wire narrows until it becomes a sharp tip. The chemical reaction can be described by equations (Ekvall et al., ’99; Kulakov et al., 2009) Cathode : 6H2 O þ 6e ! 3H2 ðgÞ þ 6OH Anode : WðsÞ þ 8OH ! WO2 4 þ 4H2 O þ 6e
Overall : WðsÞ þ 2OH þ 2H2 O ! WO4 2 þ 3H2 ðgÞ:
ð1Þ In etching tungsten wire, KOH solution (2 M) was used as an electrolyte and platinum wire was used as a cathode. In order not to disturb the liquid near the tungsten wire, which is excited by the H2 gas bubble generated at the cathode (Ekvall et al., ’99), the two electrodes were separated by a plastic plate. To make the tip sharp enough to image samples with high resolution, the optimum condition of etching was explored. The external voltages were applied in both AC (Nam et al., ’95) and DC (Liu et al., ’97) modes to find which was preferable. In each case, 30 tips were made
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and inspected by a scanning electron microscope (SEM). Representative SEM images of the tips made in AC and DC modes are shown in Figure 1(a, b), respectively. Not only is the tip in ac mode blunt, but it also looks as if it is contaminated by other materials. Meanwhile, the tip made in DC mode looks very sharp without residue. As the etching process is very sensitive to the other effects, the quality of the tips was statistically evaluated. The sharp tips having radius less than 50 nm were selected as desirable. The result was that 6 sharp tips were fabricated in AC mode while 24 sharp tips in DC mode, as shown in Figure 1(c). There is an issue whether the DC or AC voltage is effective in the electrolyte dissolving of tungsten (Nam et al., ’95). The etching processes are quite complicated, typically involving redox reaction and H2 gas bubble formation, as well as diffusion of anions and cations (Nam et al., ’95). From our result, it is supposed that the oxidized tungsten was accumulated at the end of the tip by an electrophoresis effect due to the AC field. The tungsten wire was cut into 2 mm long before the etching, and the half of it (1 mm) was submerged into the etchant. We attempted to shut off the etching at the exact moment when the wire was divided into two pieces. At that time, the tip was supposed to be its sharpest. By monitoring current while etching it electrochemically, the disconnecting moment when the current dropped abruptly was able to be observed. However, after the abrupt current dropping, lagging current was found and disappeared in less than a second. Due to the lagging current, the tip can be blunt, which is called over-etching (Ju et al., 2009). In order to prevent over-etching, a shutoff circuit was manufactured, as shown in Figure 2. Our circuit consisted of a high-speed comparator, a buffer, and a relay. This shut-off circuit stopped the etching with a 1 ms ‘shut-off’ time, once the current dropped below a set point. This fast termination of the etching current improved the tip sharpness (McKendry et al., 2008). The etching current was divided into two resistors (0.5 and
Fig 1. Tungsten tips were fabricated by electrochemical etching. (a) The tip made in ac shows a blunt apex with a 160 nm radius, and some residues were attached to the tip. (b) The other tip made in dc looks much sharper and no residues were found. (c) Numbers of successful tips (radius < 50 nm) among 30 samples made in ac and dc, respectively, are compared in a histogram.
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Fig 2. Diagram for shut-off circuit using a solid-state relay. The response time was improved by adapting an optoelectronic relay.
50 V) connected to a real ground and a virtual ground (input of a current amplified), respectively. In order to be used in both AC and DC modes, a selection switch (1) was inserted to determine whether the signal passes through a RMS-to-DC converter, or not, depending on the modes. The current was compared with a set point, and the resultant triggering signal activated the relay. While a voltage of 4 VDC was applied between the anode and cathode using a power supply the current was
fed into a current-to-voltage converter. The converted signal was buffered and connected into the comparator. Simultaneously, the signal was monitored by an oscilloscope. As the abrupt current drops were mostly found between 4 and 0.5 mA, and the set points input into the comparator were selected as 3, 2, and 1 mA. In each case, the SEM image of a representative tip is shown in Figure 3. When the set point was 3 mA (Fig. 3 (a)), the etching time was insufficient and the apex was
Fig 3. SEM images of tips with different shut off voltages: (a) 3 mA, (b) 2 mA, and (c) 1 mA. While panel (a) shows under-etching and (c) over-etching, panel (b) shows desirable result with a suitable set point.
K. Kim et al.: Mechanically Stable Tuning Fork Sensor
too long. When it was 1 mA (Fig. 3(c)), over etching occurred, and the tip became blunt. Case 2 mA (Fig. 3 (b)) exhibited the best result in terms of shape and sharpness of the tip. The tip apex radius was estimated less than 20 nm. Based on these results, the tip for the QTF sensor was fabricated.
Fabrication of QTF Sensor
The etched tip was mounted on an inside surface of a prong of QTF using an instant glue (cyanoacrylate adhesive), as shown in Figure 4(a). The location of the tip was chosen between two prongs, that is, near the center part of the QTF to improve compatibility for conventional cantilever based AFM and tip-enhanced Raman spectroscopy. Gold electrodes were deposited on an alumina plate using an evaporator. As an adhesion layer, Ti was deposited first with 5 nm thickness, and Au 30 nm was done successively. Instant glue was used to attach the QTF to the alumina plate in a stable manner, and then the electrodes on the QTF and alumina plate were connected by a conductive epoxy (silver paste), as shown in Figure 4(b). As the electrodes in the holding part of the
Fig 4. (a) Etched tip was attached at the end of a prong of QTF. (b) A QTF was mounted on an alumina plate with Ti/Au electrodes. (c) Sensor was located above a sample mounted on a tube scanner. The tuning fork excitation direction was parallel to the sample surface, and the tip can be observed from the top of the microscope.
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2
QTF has very small areas (0.1 mm ), the interconnection of the electrodes was a very delicate process not to cause a short-circuit, which was performed under a microscope. Figure 4(c) shows how the QTF was located in the AFM, where the tuning fork was excited in horizontal direction, so that the tip had shear vibration. It was challenging to attach the tiny tip to the end of the prong of the QTF while keeping the high Q-value of the QTF. For that reason, other groups attached the wire before it was etched (Kulawik et al., 2003), or thick wires were used. Scotch tape was used to hold the tip, and its position was adjusted by an xyz translator while being monitored by an optical microscope.
Signal Detection and Feedback Control
The amplitude change and phase shift reflect the interaction between the tip and sample, which were measured by a lock in amplifier (LIA). At first, the
Fig 5. Schematic diagram for overall QTF based AFM system. The excitation signal from a function generator was fed into an AGC and connected into one electrode of the QTF. The induced signal from the other electrode was detected by a lock-in amplifier. From the phase shift, a proper z-position was controlled by a PI controller.
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Fig 6. Circuit diagram of AGC. AGC is composed of a PI controller and multiplier. The amplitude of the QTF is detected by a lock-in amplifier, and the set point was subtracted from it. The resultant signal was integrated and multiplied by the excitation signal. There are two switches to adjust the polarity.
resonance frequency and Q-value of the QTF sensor were measured by sweeping the frequency in the range of 31–33 kHz of the excitation signal. Once the resonance frequency was found, the excitation signal was fixed at that frequency, and the distance between the tip and sample was swept to investigate the force-vs.distance behavior. From the force–distance curve, the sensitivity of the QTF sensor was estimated. A schematic diagram for the overall feedback circuit is shown in Figure 5. A homemade automatic gain controller (AGC) (Couturier et al., 2003) was developed, basically composed of a proportional-integration (PI) controller and a multiplier. The circuit diagram of AGC is as shown in Figure 6. In the PI controller part, a set point was subtracted from the amplitude detected by the LIA, and then the output signal was integrated with a time constant (0.1–10 ms). The final output was multiplied by a sinusoidal signal from a function generator, and then it was fed into an electrode of QTF to be excited. The resultant excitation voltage was in the range of 50–100 mV. An analog device (AD633) was used as a multiplier, and two switches were added to adjust the polarity of the signal and to obtain the negative feedback control.
In the imaging mode, the amplitude was controlled by the AGC, which keeps the vibration amplitude constant, and the phase signal was used for z-directional feedback control to keep a constant force between the tip and sample. With the feedback control, the sample was raster-scanned, and a topography image was obtained. For the z-directional control, another PI controller was used. The circuit diagram of the analog PI controller was reported in our previous publication (Lee et al., 2012).
Results and Discussion Characteristics of QTF Sensor
Frequency spectrum of a representative QTF senor shows a high Q-value (103) as shown in Figure 7, where (a) shows the amplitude and (b) the phase. The original resonance frequency was 32,768 Hz, but it was shifted to 32,280 Hz, and the peak shape was deformed. This frequency shift and deformation are primarily related with mass the loading effect of the tip on the tuning fork. The mass loading effect can be approximated by (Seo et al., 2007)
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Fig 7. (a) Amplitude and (b) phase versus frequency spectra are shown. The full width of half maximum was about 20 Hz, and the Q-value was about 1,600. (c) Phase versus distance curve was measured by z-scan. Sharp signal change after contact was found and there was no noticeable hysteresis between approaching (black) and retracting (red) data.
Df Dm ffi ; f m
ð2Þ
where Df is the frequency shift, Dm is loaded mass of the tungsten tip, and m is effective mass of two prongs. The tip mass, Dm, is estimated as 130 mg (50 times smaller than m) considering its dimension, and the result of Equation (2) is approximately fit to the measured frequency shift. The deformation of the resonance line shape is due to asymmetric geometry as the mass of the tip was loaded on only one of two prongs. In order to obtain higher sensitivity of the QTF sensor, a lower mass of the tip is required. The phase spectrum (Fig. 7(b)) shows a single-peak instead of a typical resonance curve. This problem is attributed to distortion of the simple harmonic oscillator behavior due to additional stray capacitance (An et al., 2013), as well as the unbalanced mass. Most sensors fabricated have Q-values in the range of 300–5,000. In spite of the spectrum distortion problem,
sustained high Q-values guaranteed a high sensitivity of the sensors. In order to evaluate the sensors, the force– distance curves were measured as shown in Figure 7(c). There is no conspicuous hysteresis between approach (black) and retract (red) processes, due to mechanical stability of the sensor. A sharp increment after the contact point means high sensitivity, which results from the high Q-value of the sensor. Slight dips near the contact point were found in these curves, which represent the attractive forces (King et al., 2001). The attractive force could be either van der Waals interaction or meniscus force due to adsorbed water layer.
Topography Measured by QTF-AFM
As a reference sample, the surface of the data recording layer of a compact disk (CD) was scanned by conventional cantilever based AFM (a) and developed tuning fork based AFM (b) as shown in Figure 8. The digital versatile disk (DVD) surface also was scanned by
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Fig 8. Topographic images of data bits in a CD were measured by (a) a cantilever based AFM and (b) QTF based AFM. High quality AFM images of (c) DVD surface and (d) a standard grating sample were obtained by the QTF sensor
the QTF based AFM (Fig. 8(c)). For a standard CD (DVD), each pit is approximately 200 (160) nm deep by 500 (320) nm wide, and the pitch is about 1.6 (0.74) mm. As the line profiles show parabolic curves in the pits, the surface is supposedly covered by a polymer film as a protecting layer. The images (b, c) obtained by the QTF based AFM shows more detailed features, while cantilever based AFM image (a) shows a smooth surface without substructures. This can be explained by the fact that the QTF sensor has a high Q-value and the excitation amplitude was controlled at a low level (20 nm). The background noise in Figure 8(b)
indicates the mechanical vibration because the antivibration apparatus in the homemade AFM was not installed well, but it is irrelevant to the performance of the QTF sensor. A topography of a standard test sample with a step height 1 mm and pitch 3 mm (TGX 01, MIKROMASCH USA, Lady’s Island, SC, USA) was measured using QTF based AFM, as shown in Figure 8 (d). The line profile shows clear edges of the steps confirming that the tip was sharp and the vibration amplitude was low enough to scan steep slopes. On the other hand, critical disadvantage of QTF sensor is the slow response time, and the scanning time (30 min) in
K. Kim et al.: Mechanically Stable Tuning Fork Sensor
these measurements was much longer than that of the cantilever based AFM. The response time t of the amplitude change for a resonator is known as t ¼ 2Q=v0 , while t of the frequency change is as short as a single oscillation period (Seo and Jhe, 2008). However, it is not yet clear to us what determines the time constant t of the phase shift. We expected a fast response of the QTF as the phase shift was used as a feedback signal, but the result exhibited that the fast feedback loop caused instability of the control. Therefore, the scanning time was not significantly reduced.
Conclusion There are important advantages in the QTF based AFM, compared with cantilever based AFM. Particularly, the QTF based AFM has the potential to achieve high-resolution images by reducing the excitation amplitude, as it has a high Q-value. We fabricated a QTF based sensor with a design compatible with the conventional cantilever based AFM. In particular, a tiny tungsten tip was etched and mounted on the QTF without the loss of high Q-value. Topographic images obtained by using the QTF sensor showed greater detailed structures than that using the conventional AFM, due to low excitation amplitude.
References Akiyama T, de Rooij NF, Staufer U, et al. 2010. Implementation and characterization of a quartz tuning fork based probe consisted of discrete resonators for dynamic mode atomic force microscopy. Rev Sci Instrum 81:063706–063706-8. An S, Lee K, Kim B, et al. 2013. Compensation of stray capacitance of the quartz tuning fork for a quantitative force spectroscopy. Curr Appl Phys 13:1899–1905. An S, Lee K, Kim B, et al. 2014a. Nanopipette combined with quartz tuning fork-atomic force microscope for force spectroscopy/microscopy and liquid delivery-based nanofabrication. Rev Sci Instrum 85:033702–033702-7. An S, Sung B, Noh H, et al. 2014b. Position-resolved surface characterization and nanofabrication using an optical microscope combined with a nanopipette/quartz tuning fork atomic force microscope. Nano-Micro Lett 6:70–79. Binnig G, Quate CF, Gerber C. 1986. Atomic force microscope. Phys Rev Lett 56:930–933. Couturier G, Boisgard R, Nony L, Aime JP. 2003. Noncontact atomic force microscopy: stability criterion and dynamical responses of the shift of frequency and damping signal. Rev Sci Instrum 74:2726–2734. Edwards H, Taylor L, Duncan W, Melmed AJ. 1997. Fast, highresolution atomic force microscopy using a quartz tuning fork as actuator and sensor. J Appl Phys 82:980–984. Ekvall I, Wahlstrom E, Claesson D, Olin H, Olsson E. 1999. Preparation and characterization of electrochemically etched W tips for STM. Meas Sci Technol 10:11–18. Giessibl FJ. 1998. High-speed force sensor for force microscopy and profilometry utilizing a quartz tuning fork. Appl Phys Lett 73:3956–3958.
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Giessibl FJ. 2000. Atomic resolution on Si(111)–(77) by noncontact atomic force microscopy with a force sensor based on a quartz tuning fork. Appl Phys Lett 76:1470–1472. Giessibl FJ, Hembacher S, Bielefeldt H, Mannhart J. 2000. Subatomic features on the silicon (111)–(77) surface observed by atomic force microscopy. Science 289:422–425. Ibe JP, Bey J, Brandow SL, et al. 1990. On the electrochemical etching of tips for scanning tunneling microscopy. J Vacuum Sci Technol A 8:3570–3575. Jahng J, Lee M, Noh H, Seo Y, Jhe W. 2007. Active Q control in tuning-fork-based atomic force microscopy. Appl Phys Lett 91:023103–023103-3. Ju BF, Chen YL, Fu MM, Chen Y, Yang YH. 2009. Systematic study of electropolishing technique for improving the quality and production reproducibility of tungsten STM probe. Sensors Actuators A 155:136–144. Karrai K, Grober RD. 1995. Piezo-electric tuning fork tip-sample distance control for near field optical microscopes. Ultramicroscopy 61:197–205. Kim J, Won D, Sung B, An S, Jhe W. 2014. Effective stiffness of qPlus sensor and quartz tuning fork. Ultramicroscopy 141:56–62. King GM, Lamb JS, Nunes G. 2001. Quartz tuning forks as sensors for attractive-mode force microscopy under ambient conditions. Appl Phys Lett 79:1712–1714. Kulakov M, Luzinov I, Kornev KG. 2009. Capillary and surface effects in the formation of nanosharp tungsten tips by electropolishing. Langmuir 25:4462–4468. Kulawik M, Nowicki M, Thielsch G, et al. 2003. A double lamellae dropoff etching procedure for tungsten tips attached to tuning fork atomic force microscopy/scanning tunneling microscopy sensors. Rev Sci Instrum 74:1027–1030. Lee D, Lee H, Lee NS, Kim KB, Seo Y. 2012. High-speed atomic force microscopy with phase-detection. Curr Appl Phys 12:989–994. Liu AW, Hu XT, Liu WH, Ji GJ. 1997. An improved control technique for the electrochemical fabrication of scanning tunneling microscopy microtips. Rev Sci Instrum 68:3811– 3813. McKendry JE, Allen CS, Critchley K, et al. 2008. Magnetic field enhanced nano-tip fabrication for four-probe STM studies. Nanotechnology 19:085201–085201-5. Nam AJ, Teren A, Lusby TA, Melmed AJ. 1995. Benign making of sharp tips for Stm and Fim – Pt, Ir, Au, Pd, and Rh. J Vacuum Sci Technol B 13:1556–1559. Oiko VTA, Martins BVC, Silva PC, Rodrigues V, Ugarte D. 2014. Development of a quartz tuning-fork-based force sensor for measurements in the tens of nanoNewton force range during nanomanipulation experiments. Rev Sci Instrum 85:035003– 035003-7. Putman CAJ, Degrooth BG, Vanhulst NF, Greve J. 1992. A detailed analysis of the optical beam deflection technique for use in atomic force microscopy. J Appl Phys 72:6–12. Rychen J, Ihn T, Studerus P, Herrmann A, Ensslin K. 1999. A lowtemperature dynamic mode scanning force microscope operating in high magnetic fields. Rev Sci Instrum 70: 2765–2768. Seo Y, Cadden-Zimansky P, Chandrasekhar V. 2005. Lowtemperature high-resolution magnetic force microscopy using a quartz tuning fork. Appl Phys Lett 87:103103–103103-3. Seo Y, Choe H, Jhe W. 2003. Atomic-resolution noncontact atomic force microscopy in air. Appl Phys Lett 83:1860– 1862. Seo Y, Jhe W. 2008. Atomic force microscopy and spectroscopy. Rep Prog Phys 71:016101–016101-23. Seo Y, Jhe WH, Hwang CS. 2002. Electrostatic force microscopy using a quartz tuning fork. Appl Phys Lett 80:4324–4326. Seo Y, Lee J, Yu I. 2007. Amplitude change of a quartz crystal microbalance. J Korean Phys Soc 51:1948–1952.
Mechanically Stable Tuning Fork Sensor With High Quality Factor for the Atomic Force Microscope KWANGYOON KIM, JUN-YOUNG PARK, K.B. KIM, NAESUNG LEE,
AND
YONGHO SEO
Faculty of Nanotechnology and Advanced Material Engineering, HMC, and GRI, Sejong University, Seoul, Korea
Summary: A quartz tuning fork was used instead of cantilever as a force sensor for the atomic force microscope. A tungsten tip was made by electrochemical etching from a wire of 50 mm diameter. In order to have mechanical stability of the tuning fork, it was attached on an alumina plate. The tungsten tip was attached on the inside end of a prong of a tuning fork. The phase shift was used as a feedback signal to control the distance between the tip and sample, and the amplitude was kept constant using a lock-in amplifier and a homemade automatic gain controller. Due to the mechanical stability, the sensor shows a high quality factor (103), and the image quality obtained with this sensor was equivalent to that of the cantilever-based AFM. SCANNING 36:632–639, 2014. © 2014 Wiley Periodicals, Inc. Key words: tuning fork sensor, atomic force microscope (AFM), scanning probe microscopy (SPM), quartz tuning fork
Introduction Cantilevers have been mostly used as force sensors for atomic force microscopes (AFM) (Binnig et al., ’86). In order to detect the motion of the cantilever, the optical deflection technique (Putman et al., ’92) is adopted using a laser beam and a position sensitive photo-diode. The optical deflection technique is widely used not only for
Contract grant sponsor: Ministry of Science; Contract grant number: 2013R1A1A1A05005298; Contract grant sponsor: Ministry of Education; Contract grant number: 2010-0020207. Address for reprints: Yongho Seo, Faculty of Nanotechnology and Advanced Material Engineering, HMC, and GRI, Sejong University, Seoul 143-747, Korea E-mail: [email protected] Received 7 May 2014; revised 1 August 2014; Accepted with revision 26 August 2014 DOI: 10.1002/sca.21169 Published online 17 September 2014 in Wiley Online Library (wileyonlinelibrary.com).
topographic measurement, but also for almost all other scanning probe microscopies (SPM), including electrostatic force microscopy, magnetic force microscopy, and near-field scanning optical microscopy. However, there are disadvantages to using the optical deflection technique, particularly for measurements in dark environments and at low temperatures. To overcome these problems of the optical deflection technique, SPM adopting a quartz tuning fork (QTF) as a force sensor was developed, and has been applied to almost all kinds of SPM (Karrai and Grober, ’95; Edwards et al., ’97; Giessibl, ’98; Seo et al., 2002, 2003; An et al., 2014a,b). First of all, the QTF has a high Q-value (quality factor), which is an essential feature to achieve good force sensitivity and high resolution images (Jahng et al., 2007; Oiko et al., 2014). As a representative example of QTF, qPlus sensor (Giessibl, ’98, 2000; Kim et al., 2014), which was developed by Giessibl, has a Qvalue of a few thousands in ambient pressure. The qPlus sensor has a unique design; one prong of the tuning fork is attached to a hard material and the other prong has free motion to sense the force. An ultra-high resolution AFM image with a sub-atomic scale, even better than the scanning tunneling microscopic (STM) image, was achieved using the qPlus sensor (Giessibl et al., 2000). Moreover, QTF needs no laser source as it operates only electrically as a sensor, and a variety of experiments can be performed compared with the conventional SPM. For example, photocurrent imaging in nanometer scale or near field optics in dark conditions is possible. Also, SPM measurements in a high vacuum and at low temperatures are available using QTF (Seo et al., 2005). In the case of conventional SPM, many optical units are not compatible in a vacuum or at low temperatures. Not only the QTF is compatible in a vacuum, but its Q-value is enhanced up to 105. Also, the power dissipation is low enough to be used at ultra-low temperatures (Rychen et al., ’99). Because of these advantages, the QTF sensor has been used widely in SPM field and has been linked to commercialization (Akiyama et al., 2010). In this paper, we suggest a method of fabricating a QTF based AFM sensor including the tip etching and mounting, designing of the sensor holder, and electronic phase detection for
K. Kim et al.: Mechanically Stable Tuning Fork Sensor
feedback control, in order to facilitate the use of QTF based AFM for industrial purposes.
Materials and Methods Fabrication of Tungsten Tip
The electrochemical etching method was applied to make a tungsten (W) tip from W-wire of 50 mm diameter. As the tip should be mounted at the end of a prong of QTF, a low mass of tip is important not to reduce the Q-value of the tuning fork, and that is the reason why thin wire was chosen. Conventional methods fabricating the W-tip for STM are well known (Ibe et al., ’90; Ekvall et al., ’99). Electrochemical etching is composed of two electrodes and an electrolyte, where one of the electrodes (anode) is the tungsten wire. By applying an external voltage, a redox reaction occurs at the electrodes, and the tungsten wire narrows until it becomes a sharp tip. The chemical reaction can be described by equations (Ekvall et al., ’99; Kulakov et al., 2009) Cathode : 6H2 O þ 6e ! 3H2 ðgÞ þ 6OH Anode : WðsÞ þ 8OH ! WO2 4 þ 4H2 O þ 6e
Overall : WðsÞ þ 2OH þ 2H2 O ! WO4 2 þ 3H2 ðgÞ:
ð1Þ In etching tungsten wire, KOH solution (2 M) was used as an electrolyte and platinum wire was used as a cathode. In order not to disturb the liquid near the tungsten wire, which is excited by the H2 gas bubble generated at the cathode (Ekvall et al., ’99), the two electrodes were separated by a plastic plate. To make the tip sharp enough to image samples with high resolution, the optimum condition of etching was explored. The external voltages were applied in both AC (Nam et al., ’95) and DC (Liu et al., ’97) modes to find which was preferable. In each case, 30 tips were made
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and inspected by a scanning electron microscope (SEM). Representative SEM images of the tips made in AC and DC modes are shown in Figure 1(a, b), respectively. Not only is the tip in ac mode blunt, but it also looks as if it is contaminated by other materials. Meanwhile, the tip made in DC mode looks very sharp without residue. As the etching process is very sensitive to the other effects, the quality of the tips was statistically evaluated. The sharp tips having radius less than 50 nm were selected as desirable. The result was that 6 sharp tips were fabricated in AC mode while 24 sharp tips in DC mode, as shown in Figure 1(c). There is an issue whether the DC or AC voltage is effective in the electrolyte dissolving of tungsten (Nam et al., ’95). The etching processes are quite complicated, typically involving redox reaction and H2 gas bubble formation, as well as diffusion of anions and cations (Nam et al., ’95). From our result, it is supposed that the oxidized tungsten was accumulated at the end of the tip by an electrophoresis effect due to the AC field. The tungsten wire was cut into 2 mm long before the etching, and the half of it (1 mm) was submerged into the etchant. We attempted to shut off the etching at the exact moment when the wire was divided into two pieces. At that time, the tip was supposed to be its sharpest. By monitoring current while etching it electrochemically, the disconnecting moment when the current dropped abruptly was able to be observed. However, after the abrupt current dropping, lagging current was found and disappeared in less than a second. Due to the lagging current, the tip can be blunt, which is called over-etching (Ju et al., 2009). In order to prevent over-etching, a shutoff circuit was manufactured, as shown in Figure 2. Our circuit consisted of a high-speed comparator, a buffer, and a relay. This shut-off circuit stopped the etching with a 1 ms ‘shut-off’ time, once the current dropped below a set point. This fast termination of the etching current improved the tip sharpness (McKendry et al., 2008). The etching current was divided into two resistors (0.5 and
Fig 1. Tungsten tips were fabricated by electrochemical etching. (a) The tip made in ac shows a blunt apex with a 160 nm radius, and some residues were attached to the tip. (b) The other tip made in dc looks much sharper and no residues were found. (c) Numbers of successful tips (radius < 50 nm) among 30 samples made in ac and dc, respectively, are compared in a histogram.
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Fig 2. Diagram for shut-off circuit using a solid-state relay. The response time was improved by adapting an optoelectronic relay.
50 V) connected to a real ground and a virtual ground (input of a current amplified), respectively. In order to be used in both AC and DC modes, a selection switch (1) was inserted to determine whether the signal passes through a RMS-to-DC converter, or not, depending on the modes. The current was compared with a set point, and the resultant triggering signal activated the relay. While a voltage of 4 VDC was applied between the anode and cathode using a power supply the current was
fed into a current-to-voltage converter. The converted signal was buffered and connected into the comparator. Simultaneously, the signal was monitored by an oscilloscope. As the abrupt current drops were mostly found between 4 and 0.5 mA, and the set points input into the comparator were selected as 3, 2, and 1 mA. In each case, the SEM image of a representative tip is shown in Figure 3. When the set point was 3 mA (Fig. 3 (a)), the etching time was insufficient and the apex was
Fig 3. SEM images of tips with different shut off voltages: (a) 3 mA, (b) 2 mA, and (c) 1 mA. While panel (a) shows under-etching and (c) over-etching, panel (b) shows desirable result with a suitable set point.
K. Kim et al.: Mechanically Stable Tuning Fork Sensor
too long. When it was 1 mA (Fig. 3(c)), over etching occurred, and the tip became blunt. Case 2 mA (Fig. 3 (b)) exhibited the best result in terms of shape and sharpness of the tip. The tip apex radius was estimated less than 20 nm. Based on these results, the tip for the QTF sensor was fabricated.
Fabrication of QTF Sensor
The etched tip was mounted on an inside surface of a prong of QTF using an instant glue (cyanoacrylate adhesive), as shown in Figure 4(a). The location of the tip was chosen between two prongs, that is, near the center part of the QTF to improve compatibility for conventional cantilever based AFM and tip-enhanced Raman spectroscopy. Gold electrodes were deposited on an alumina plate using an evaporator. As an adhesion layer, Ti was deposited first with 5 nm thickness, and Au 30 nm was done successively. Instant glue was used to attach the QTF to the alumina plate in a stable manner, and then the electrodes on the QTF and alumina plate were connected by a conductive epoxy (silver paste), as shown in Figure 4(b). As the electrodes in the holding part of the
Fig 4. (a) Etched tip was attached at the end of a prong of QTF. (b) A QTF was mounted on an alumina plate with Ti/Au electrodes. (c) Sensor was located above a sample mounted on a tube scanner. The tuning fork excitation direction was parallel to the sample surface, and the tip can be observed from the top of the microscope.
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2
QTF has very small areas (0.1 mm ), the interconnection of the electrodes was a very delicate process not to cause a short-circuit, which was performed under a microscope. Figure 4(c) shows how the QTF was located in the AFM, where the tuning fork was excited in horizontal direction, so that the tip had shear vibration. It was challenging to attach the tiny tip to the end of the prong of the QTF while keeping the high Q-value of the QTF. For that reason, other groups attached the wire before it was etched (Kulawik et al., 2003), or thick wires were used. Scotch tape was used to hold the tip, and its position was adjusted by an xyz translator while being monitored by an optical microscope.
Signal Detection and Feedback Control
The amplitude change and phase shift reflect the interaction between the tip and sample, which were measured by a lock in amplifier (LIA). At first, the
Fig 5. Schematic diagram for overall QTF based AFM system. The excitation signal from a function generator was fed into an AGC and connected into one electrode of the QTF. The induced signal from the other electrode was detected by a lock-in amplifier. From the phase shift, a proper z-position was controlled by a PI controller.
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Fig 6. Circuit diagram of AGC. AGC is composed of a PI controller and multiplier. The amplitude of the QTF is detected by a lock-in amplifier, and the set point was subtracted from it. The resultant signal was integrated and multiplied by the excitation signal. There are two switches to adjust the polarity.
resonance frequency and Q-value of the QTF sensor were measured by sweeping the frequency in the range of 31–33 kHz of the excitation signal. Once the resonance frequency was found, the excitation signal was fixed at that frequency, and the distance between the tip and sample was swept to investigate the force-vs.distance behavior. From the force–distance curve, the sensitivity of the QTF sensor was estimated. A schematic diagram for the overall feedback circuit is shown in Figure 5. A homemade automatic gain controller (AGC) (Couturier et al., 2003) was developed, basically composed of a proportional-integration (PI) controller and a multiplier. The circuit diagram of AGC is as shown in Figure 6. In the PI controller part, a set point was subtracted from the amplitude detected by the LIA, and then the output signal was integrated with a time constant (0.1–10 ms). The final output was multiplied by a sinusoidal signal from a function generator, and then it was fed into an electrode of QTF to be excited. The resultant excitation voltage was in the range of 50–100 mV. An analog device (AD633) was used as a multiplier, and two switches were added to adjust the polarity of the signal and to obtain the negative feedback control.
In the imaging mode, the amplitude was controlled by the AGC, which keeps the vibration amplitude constant, and the phase signal was used for z-directional feedback control to keep a constant force between the tip and sample. With the feedback control, the sample was raster-scanned, and a topography image was obtained. For the z-directional control, another PI controller was used. The circuit diagram of the analog PI controller was reported in our previous publication (Lee et al., 2012).
Results and Discussion Characteristics of QTF Sensor
Frequency spectrum of a representative QTF senor shows a high Q-value (103) as shown in Figure 7, where (a) shows the amplitude and (b) the phase. The original resonance frequency was 32,768 Hz, but it was shifted to 32,280 Hz, and the peak shape was deformed. This frequency shift and deformation are primarily related with mass the loading effect of the tip on the tuning fork. The mass loading effect can be approximated by (Seo et al., 2007)
K. Kim et al.: Mechanically Stable Tuning Fork Sensor
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Fig 7. (a) Amplitude and (b) phase versus frequency spectra are shown. The full width of half maximum was about 20 Hz, and the Q-value was about 1,600. (c) Phase versus distance curve was measured by z-scan. Sharp signal change after contact was found and there was no noticeable hysteresis between approaching (black) and retracting (red) data.
Df Dm ffi ; f m
ð2Þ
where Df is the frequency shift, Dm is loaded mass of the tungsten tip, and m is effective mass of two prongs. The tip mass, Dm, is estimated as 130 mg (50 times smaller than m) considering its dimension, and the result of Equation (2) is approximately fit to the measured frequency shift. The deformation of the resonance line shape is due to asymmetric geometry as the mass of the tip was loaded on only one of two prongs. In order to obtain higher sensitivity of the QTF sensor, a lower mass of the tip is required. The phase spectrum (Fig. 7(b)) shows a single-peak instead of a typical resonance curve. This problem is attributed to distortion of the simple harmonic oscillator behavior due to additional stray capacitance (An et al., 2013), as well as the unbalanced mass. Most sensors fabricated have Q-values in the range of 300–5,000. In spite of the spectrum distortion problem,
sustained high Q-values guaranteed a high sensitivity of the sensors. In order to evaluate the sensors, the force– distance curves were measured as shown in Figure 7(c). There is no conspicuous hysteresis between approach (black) and retract (red) processes, due to mechanical stability of the sensor. A sharp increment after the contact point means high sensitivity, which results from the high Q-value of the sensor. Slight dips near the contact point were found in these curves, which represent the attractive forces (King et al., 2001). The attractive force could be either van der Waals interaction or meniscus force due to adsorbed water layer.
Topography Measured by QTF-AFM
As a reference sample, the surface of the data recording layer of a compact disk (CD) was scanned by conventional cantilever based AFM (a) and developed tuning fork based AFM (b) as shown in Figure 8. The digital versatile disk (DVD) surface also was scanned by
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Fig 8. Topographic images of data bits in a CD were measured by (a) a cantilever based AFM and (b) QTF based AFM. High quality AFM images of (c) DVD surface and (d) a standard grating sample were obtained by the QTF sensor
the QTF based AFM (Fig. 8(c)). For a standard CD (DVD), each pit is approximately 200 (160) nm deep by 500 (320) nm wide, and the pitch is about 1.6 (0.74) mm. As the line profiles show parabolic curves in the pits, the surface is supposedly covered by a polymer film as a protecting layer. The images (b, c) obtained by the QTF based AFM shows more detailed features, while cantilever based AFM image (a) shows a smooth surface without substructures. This can be explained by the fact that the QTF sensor has a high Q-value and the excitation amplitude was controlled at a low level (20 nm). The background noise in Figure 8(b)
indicates the mechanical vibration because the antivibration apparatus in the homemade AFM was not installed well, but it is irrelevant to the performance of the QTF sensor. A topography of a standard test sample with a step height 1 mm and pitch 3 mm (TGX 01, MIKROMASCH USA, Lady’s Island, SC, USA) was measured using QTF based AFM, as shown in Figure 8 (d). The line profile shows clear edges of the steps confirming that the tip was sharp and the vibration amplitude was low enough to scan steep slopes. On the other hand, critical disadvantage of QTF sensor is the slow response time, and the scanning time (30 min) in
K. Kim et al.: Mechanically Stable Tuning Fork Sensor
these measurements was much longer than that of the cantilever based AFM. The response time t of the amplitude change for a resonator is known as t ¼ 2Q=v0 , while t of the frequency change is as short as a single oscillation period (Seo and Jhe, 2008). However, it is not yet clear to us what determines the time constant t of the phase shift. We expected a fast response of the QTF as the phase shift was used as a feedback signal, but the result exhibited that the fast feedback loop caused instability of the control. Therefore, the scanning time was not significantly reduced.
Conclusion There are important advantages in the QTF based AFM, compared with cantilever based AFM. Particularly, the QTF based AFM has the potential to achieve high-resolution images by reducing the excitation amplitude, as it has a high Q-value. We fabricated a QTF based sensor with a design compatible with the conventional cantilever based AFM. In particular, a tiny tungsten tip was etched and mounted on the QTF without the loss of high Q-value. Topographic images obtained by using the QTF sensor showed greater detailed structures than that using the conventional AFM, due to low excitation amplitude.
References Akiyama T, de Rooij NF, Staufer U, et al. 2010. Implementation and characterization of a quartz tuning fork based probe consisted of discrete resonators for dynamic mode atomic force microscopy. Rev Sci Instrum 81:063706–063706-8. An S, Lee K, Kim B, et al. 2013. Compensation of stray capacitance of the quartz tuning fork for a quantitative force spectroscopy. Curr Appl Phys 13:1899–1905. An S, Lee K, Kim B, et al. 2014a. Nanopipette combined with quartz tuning fork-atomic force microscope for force spectroscopy/microscopy and liquid delivery-based nanofabrication. Rev Sci Instrum 85:033702–033702-7. An S, Sung B, Noh H, et al. 2014b. Position-resolved surface characterization and nanofabrication using an optical microscope combined with a nanopipette/quartz tuning fork atomic force microscope. Nano-Micro Lett 6:70–79. Binnig G, Quate CF, Gerber C. 1986. Atomic force microscope. Phys Rev Lett 56:930–933. Couturier G, Boisgard R, Nony L, Aime JP. 2003. Noncontact atomic force microscopy: stability criterion and dynamical responses of the shift of frequency and damping signal. Rev Sci Instrum 74:2726–2734. Edwards H, Taylor L, Duncan W, Melmed AJ. 1997. Fast, highresolution atomic force microscopy using a quartz tuning fork as actuator and sensor. J Appl Phys 82:980–984. Ekvall I, Wahlstrom E, Claesson D, Olin H, Olsson E. 1999. Preparation and characterization of electrochemically etched W tips for STM. Meas Sci Technol 10:11–18. Giessibl FJ. 1998. High-speed force sensor for force microscopy and profilometry utilizing a quartz tuning fork. Appl Phys Lett 73:3956–3958.
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Giessibl FJ. 2000. Atomic resolution on Si(111)–(77) by noncontact atomic force microscopy with a force sensor based on a quartz tuning fork. Appl Phys Lett 76:1470–1472. Giessibl FJ, Hembacher S, Bielefeldt H, Mannhart J. 2000. Subatomic features on the silicon (111)–(77) surface observed by atomic force microscopy. Science 289:422–425. Ibe JP, Bey J, Brandow SL, et al. 1990. On the electrochemical etching of tips for scanning tunneling microscopy. J Vacuum Sci Technol A 8:3570–3575. Jahng J, Lee M, Noh H, Seo Y, Jhe W. 2007. Active Q control in tuning-fork-based atomic force microscopy. Appl Phys Lett 91:023103–023103-3. Ju BF, Chen YL, Fu MM, Chen Y, Yang YH. 2009. Systematic study of electropolishing technique for improving the quality and production reproducibility of tungsten STM probe. Sensors Actuators A 155:136–144. Karrai K, Grober RD. 1995. Piezo-electric tuning fork tip-sample distance control for near field optical microscopes. Ultramicroscopy 61:197–205. Kim J, Won D, Sung B, An S, Jhe W. 2014. Effective stiffness of qPlus sensor and quartz tuning fork. Ultramicroscopy 141:56–62. King GM, Lamb JS, Nunes G. 2001. Quartz tuning forks as sensors for attractive-mode force microscopy under ambient conditions. Appl Phys Lett 79:1712–1714. Kulakov M, Luzinov I, Kornev KG. 2009. Capillary and surface effects in the formation of nanosharp tungsten tips by electropolishing. Langmuir 25:4462–4468. Kulawik M, Nowicki M, Thielsch G, et al. 2003. A double lamellae dropoff etching procedure for tungsten tips attached to tuning fork atomic force microscopy/scanning tunneling microscopy sensors. Rev Sci Instrum 74:1027–1030. Lee D, Lee H, Lee NS, Kim KB, Seo Y. 2012. High-speed atomic force microscopy with phase-detection. Curr Appl Phys 12:989–994. Liu AW, Hu XT, Liu WH, Ji GJ. 1997. An improved control technique for the electrochemical fabrication of scanning tunneling microscopy microtips. Rev Sci Instrum 68:3811– 3813. McKendry JE, Allen CS, Critchley K, et al. 2008. Magnetic field enhanced nano-tip fabrication for four-probe STM studies. Nanotechnology 19:085201–085201-5. Nam AJ, Teren A, Lusby TA, Melmed AJ. 1995. Benign making of sharp tips for Stm and Fim – Pt, Ir, Au, Pd, and Rh. J Vacuum Sci Technol B 13:1556–1559. Oiko VTA, Martins BVC, Silva PC, Rodrigues V, Ugarte D. 2014. Development of a quartz tuning-fork-based force sensor for measurements in the tens of nanoNewton force range during nanomanipulation experiments. Rev Sci Instrum 85:035003– 035003-7. Putman CAJ, Degrooth BG, Vanhulst NF, Greve J. 1992. A detailed analysis of the optical beam deflection technique for use in atomic force microscopy. J Appl Phys 72:6–12. Rychen J, Ihn T, Studerus P, Herrmann A, Ensslin K. 1999. A lowtemperature dynamic mode scanning force microscope operating in high magnetic fields. Rev Sci Instrum 70: 2765–2768. Seo Y, Cadden-Zimansky P, Chandrasekhar V. 2005. Lowtemperature high-resolution magnetic force microscopy using a quartz tuning fork. Appl Phys Lett 87:103103–103103-3. Seo Y, Choe H, Jhe W. 2003. Atomic-resolution noncontact atomic force microscopy in air. Appl Phys Lett 83:1860– 1862. Seo Y, Jhe W. 2008. Atomic force microscopy and spectroscopy. Rep Prog Phys 71:016101–016101-23. Seo Y, Jhe WH, Hwang CS. 2002. Electrostatic force microscopy using a quartz tuning fork. Appl Phys Lett 80:4324–4326. Seo Y, Lee J, Yu I. 2007. Amplitude change of a quartz crystal microbalance. J Korean Phys Soc 51:1948–1952.
