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Focused ion beam lithography for fabrication of suspended nanostructures on highly corrugated surfaces
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 335302 (http://iopscience.iop.org/0957-4484/25/33/335302) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 15/05/2017 at 10:30 Please note that terms and conditions apply.
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Nanotechnology Nanotechnology 25 (2014) 335302 (7pp)
doi:10.1088/0957-4484/25/33/335302
Focused ion beam lithography for fabrication of suspended nanostructures on highly corrugated surfaces M Erdmanis1, P Sievilä1, A Shah1, N Chekurov1, V Ovchinnikov2 and I Tittonen1 1 2
Department of Micro- and Nanosciences, Aalto University, PO Box 13500, FI-00076 Aalto, Finland Department of Aalto Nanofab, Aalto University, PO Box 13500, FI-00076 Aalto, Finland
E-mail: mikhail.erdmanis@aalto.fi Received 17 April 2014, revised 16 June 2014 Accepted for publication 23 June 2014 Published 30 July 2014 Abstract
We propose a nanofabrication method that allows for patterning on extremely corrugated surfaces with micrometer-size features. The technique employs focused ion beam nanopatterning of ion-sensitive inorganic resists formed by atomic layer deposition at low temperature. The nanoscale resolution on corrugated surfaces is ensured by inherently large depth of focus of a focused ion beam system and very uniform resist coating. The utilized TiO2 and Al2O3 resists show high selectivity in deep reactive ion etching and enable the release of suspended nanostructures by dry etching. We demonstrate the great flexibility of the process by fabricating suspended nanostructures on flat surfaces, inclined walls, and on the bottom of deep grooves. Keywords: focused ion beam, nanofabrication, silicon (Some figures may appear in colour only in the online journal) on top of supporting elements. On a flat sample, such nanonetworks are extensively used for experiments devoted to thermal conductance [6, 7], thermoelectric properties [8], controlled resistivity [9], and radiative enhancement of heat transfer by optical near field [10]. Ultimately, the possibility of fabricating suspended nanonetworks on strongly corrugated surfaces and on the bottom of deep microchannels would significantly increase the number of their applications via the integration with microfluidics, plasmonics, photonics, etc.
1. Introduction Nanofabrication on multilevel surfaces, trenches, and micro grooves has attracted a lot of attention due to its great importance for three-dimensional (3D) integration and interdisciplinary research. In particular, the fabrication of nanopillars within micron-scale grooves and channels enables local control of wetting behavior through a combination of hydrophobic properties of nanopillar arrays [1] with hydrophilic behavior of microfluidic channels [2] and grooves [3]. Analogously, it also gives a new way to confine, isolate, and manipulate analytes within nanoscale volumes or to separate molecules via the infiltration through microchannels. For sensing and spectroscopy, gold-or silver-coated nanopillars [4] fabricated in microcavities can enhance the signal and allow for operation with a very small amount of analyte. In addition, nanoscale periodic patterns within micron-scale grooves can be utilized in multiple photonic and plasmonic resonant schemes for optical sensing and signal filtering [5]. The functionality of described nanostructures can be further extended by adding a network of suspended nanowires 0957-4484/14/335302+07$33.00
1.1. Challenges in nanofabrication on strongly corrugated samples
Nanofabrication on surfaces with a strong relief is a great challenge even with the most accurate state-of-the-art fabrication methods. For standard lithographies, samples with irregular surfaces disrupt both a uniform distribution of photoresist and a good focusing during the exposure. There are special techniques that improve the quality of photoresist coating on highly corrugated samples. For example, one can use commercially available spraycoating facilities that rely on 1
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raster scanning nozzles or utilize a multistep photoresist coverage where the bottom and the upper resists have different molecular weights [11]. However, the techniques are not universal, and they should be optimized for a particular sample profile. An alternative way of nanopatterning on irregular surfaces is to use the resistless e-beam lithographies that have emerged during the last two decades. Such lithographies rely on various physics, from synthesis of nanoparticles upon electron beam (e-beam) irradiation [12] and local formation of silicides due to the heating [13], to surface passivation opening the exposed areas for mask formation [14]. In general, these methods return patterns with a very high resolution; however, the robustness and selectivity of the formed mask cannot match the requirements for the fabrication of released nanostructures. Apart from the problem of uniform photoresist coating, both resist and resistless lithographies on highly irregular surfaces suffer from a degraded resolution and disturbed pattern features. For maskless lithographies, it originates from the poor depth of focus, which is usually below 10 μm for ebeam lithography. The mitigation of the problem requires the use of dynamic focusing and dose correction across the sample [15] that is difficult to implement for irregular samples with arbitrary surface features. Similarly, masked contact and proximity lithographies that work perfectly for patterning on flat samples cannot provide the highest resolution on nonplanar samples. More complex methods, such as nanoimprint lithography based on embossing with elastomeric pattern transfer [16], which allow for patterning even on spherical surfaces, require a complicated setup alignment and controllable low pressure under the mask during exposure. Consequently, there is no one universal method that can tolerate strong arbitrary variations of sample height and similarly provide a straightforward patterning process with fast exposure times.
In most cases, dry release is beneficial due to better resolution and repeatability. Consequently, the material that is used for released nanostructures should be of high quality and mechanical strength to withstand the impact of dry etching. Therefore, there is a need for a nanofabrication process that can provide patterning of robust mask material and release by dry etching with high selectivity that can also simultaneously tolerate strong and arbitrary corrugation of the sample profile.
2. Methods and materials 2.1. Focused ion beam lithography
Focused ion beam (FIB) has been extensively used in microfabrication and integrated circuit analysis from the development of bright liquid metal ion sources in the mid 1970s [21]. It also gave rise to a new concept of FIB lithography (FIBL), which utilizes the phenomena that bombarding ions get implanted into the target material and modify its surface layer [22]. With modern gallium ion sources that provide stable operation and a long lifetime, FIB becomes a more competitive tool in comparison to e-beam lithography (EBL). Although the resolution achievable by FIBL is lower than that of EBL, the remarkable advantages of FIBL are a significantly larger depth of focus and a very small scattering of ions in a resist layer. In FIBL, patterning can be performed either on the original material (resistless lithography) or on an extra resist layer. When a sample is subjected to, e.g., a gallium ion beam, the irradiated areas exhibit etch-retarding behavior due to several mechanisms. The first one is a physical change of lattice constant caused by impurity atoms, inducing the corresponding strain effects and slowing down the wet and dry etching rates [23, 24]. Another effect is a change in the nature of the surface chemical reaction across implanted areas. For example, in dry etching with fluorine chemistries, a nonvolatile GaFx masking compound is formed on irradiated areas [25, 26]. In a case when oxygen is present in the etching gas mixture, the etch retardation effect can also be provided by in situ formation of a thin gallium oxide layer [22]. Micro and nanofabrication with FIBL can be realized with various resist materials, from standard PMMA [27] to different compounds and multilayer coatings [28–30]. Resistless FIBL with direct ion implantation into, e.g., pure silicon, can also facilitate the fabrication of high aspect ratio structures via inductively coupled plasma reactive ion etching (ICP-RIE) at cryogenic temperatures [31]. However, the ultimate advantage of utilizing resists is that the sample surface is protected, and the surface layer does not become amorphous and contaminated by gallium dopants. In addition, high quality resist materials can provide fine control over the surface profile of nanostructures formed by dry etching, even when patterning is done with a commonly used FIB system [32].
1.2. Existing methods for fabrication of suspended nanostructures
The number of methods for fabrication of suspended nanostructures on very complex geometries has been continuously growing during the last decade. Recent advances include direct fabrication of suspended nanowires from a gas phase by e-beam deposition [17] or sequential drawing of fibers from a polymer droplet by microscale tip [18]. Such methods, however, are craftwork, and they cannot compete with lithographies when used for large area patterning. For this reason, the most widely used methods for fabrication of suspended nanostructures rely on standard process steps. Typically, they include patterning, mask development, structural etching that forms supporting geometry, and release stage returning suspended structures. In the case of an irregular sample, the accuracy of patterning will similarly degrade from nonuniform resist coating and limited depth of focus of the exposure system. Another factor defining the resolution is a type of process used for release of suspended structures, which can be either wet [8, 19] or dry etching [20]. 2
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2.2. Deposition methods and materials
When nanofabrication is to be performed on a strongly corrugated sample, the role of the mask deposition method becomes crucial, as it should ensure a uniform coating across the sample. From modern deposition techniques, a very attractive option providing nm thick uniform coating is known as atomic layer deposition (ALD) [33]. This method perfectly tolerates highly irregular surfaces, since it is based on self-terminating chemical reactions. One of the most established and canonic ALD processes is growing high quality Al2O3 films from trimethylaluminum (TMA) and water precursors. In silicon microfabrication, Al2O3 is widely used as a mask material for dry etching. It has been shown that the Al2O3 etch rate in fluorine-based cryogenic ICP-RIE can be close to zero and provide a unique selectivity of 1 : 70000 [34]. In practice, it means that only a 6–10 nm thick Al2O3 layer is sufficient for through-wafer etching of silicon. At the same time, Al2O3 films received great interest due to their impressive mechanical properties [35]. For this reason, Al2O3 appears to be a perfect material for the fabrication of nm thick released structures [36].
Figure 1. Schematic drawing of fabrication process flow. Sequence
(A) with single layer TiO2 resist, sequence (B) with bilayer TiO2/ Al2O3 resist.
throughout the paper for the processes employing single layer TiO2 and bilayer TiO2/Al2O3 resists, respectively. Both ALD films are deposited at 120 °C in a Beneq TFS 500 reactor with TiCl4 and H2O precursors for TiO2, and TMA with H2O precursors for Al2O3 deposition. The typical thicknesses of the films are 50 nm for TiO2 and 15 nm for Al2O3. The thickness of TiO2 was selected to be large enough to prevent Ga+ ions from achieving the underlying Al2O3 layer [39]. FIB patterning was performed with the acceleration voltage of 30 kV, an ion beam current of 9.7 pA, and an exposure dose around 1.5 × 1016 ions/cm2. Both processes employ two dry etching steps—first, the developing of negative patterns formed from TiO2 film; and second, the structural etching for the fabrication of silicon structures. The developing etching in both cases is done by RIE with a CF4 gas mixture. The differences between (A) and (B) are in the presence of the additional Al2O3 wet etching step in sequence (B) and in the strength of a final mask. The TiO2 mask in (A) is not very robust, which implies that the structure should be formed by dry etching with a very low bias and high selectivity such as, e.g., cryogenic ICP-RIE. In contrast, in (B) the underlying Al2O3 mask is more resistant against dry etching, and therefore structural etching can also be realized by room temperature RIE with lower selectivity. Both RIE and cryogenic ICP-RIE structural etching steps are performed with SF6/O2 gas mixtures, which can be used for etching of various polymers [40] in addition to silicon. The achievable resolution and influence of the main parameters on the final structures will be discussed throughout the manuscript within the context of fabricated structures.
3. Fabrication process flow In this work, we present a method for nanofabrication on strongly corrugated surfaces that combines the advances of FIB lithography and ALD. The minor distortions of patterned features are ensured by the large depth of focus of FIB [37] and the uniformity of ALD coating. We concentrate on negative tone ALD ion-sensitive resists (exposed areas correspond to the formed masked pattern), as this type of resist is highly desirable for many applications where patterned structures should be surrounded by large empty areas. Originally, irradiation of Al2O3 films by a Ga+ beam promotes the destruction of exposed areas in consequent dry etching (equal to the positive tone resist) [38]. However, fabrication of negative tone nanopatterns from Al2O3 is very attractive due to its superb mechanical and etch-stop properties. Therefore, we suggest using an extra layer to make the inversion of resist tone and to finally provide negative Al2O3 patterns. We have found that one suitable candidate for such an inversion layer is low-temperature amorphous TiO2, which acts as a high-contrast negative ion-sensitive resist in fluorinebased dry etching. Our observation complies with earlier reports on the general ion-sensitive behavior of other transition metal oxides [29]. Moreover, in certain conditions TiO2 can act as a mask material itself when a high enough etching selectivity is provided. Consequently, we propose two distinct masking processes—with a single layer TiO2 resist and with a bilayer resist (TiO2 on top of Al2O3). Due to the high quality of ALD films, both materials allow for the fabrication of suspended nanostructures via dry etching release. Figure 1 shows the two process sequences with different types of resists that are utilized for fabrication of pillars on flat and inclined surfaces. Abbreviations (A) and (B) are kept
4. Fabricated nanostructures 4.1. High aspect ratio nanostructures
Figure 2 shows the examples of high aspect ratio nanostructures fabricated with the process sequence (A). Figure 2(a) presents an array of pillars with a 110 nm diameter and a 380 nm height that are connected by thin walls. The walls have nearly perfect vertical profiles and only 25–30 nm width (shown by magnified side view in figure 2(b)). Figure 2(c) provides the results of the line width 3
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Figure 3. (a) Schematic drawing of groove cross section. (b) SEM
image of the cleaved groove facet taken with a 45 degree angle with respect to surface normal. Scale bar is 10 μm.
Figure 2. Nanostructures fabricated with TiO2 mask and cryogenic ICP-RIE. SEM images were taken with a 52 degree angle with respect to surface normal. Pillars connected by fine nanowalls: (a) front view and (b) magnified side view. (c) Resolution test. Scale bar is 500 nm.
tests where fine 30–35 nm-wide lines were etched for approximately 420 nm with a slightly positive sidewall angle. As seen, when the line-to-gap ratio starts to approach 1:1, the mask peels off due to the undercut (figure 2(c) right). Consequently, the resolution of deeply etched nanostructures that are located close to each other would be generally limited by the parameters of dry etching. The resolution of patterning of a single line, however, can be further improved by using a smaller FIB exposure current and a thinner TiO2 layer. Another important aspect is that the TiO2 mask shows a very high selectivity in silicon cryogenic dry etching. We experimentally verified that for our process, it is about 1 : 2000 with the following cryogenic ICP-RIE parameters: process temperature − 120 °C, SF6/O2 gas mixture, 3 W forward power, and 800 W ICP power. As a result, FIB patterning of the TiO2 mask would practically allow for combining of nano- and microfabrication with the etching depths achieving a few tens of micrometers.
Figure 4. Lines with a TiO2/Al2O3 mask patterned on the groove bottom and wall. The SEM image was taken with a 52 degree angle with respect to surface normal. Scale bar is 5 μm.
((CH3)4NOH) wet etching of [100] silicon wafers with a SiO2 mask, providing a target etching angle of 54.7 degrees. The appearance of irregular steps on the walls (seen on the left wall in figure 3(b)) is caused by misalignment between a mask and a silicon wafer flat.
4.3. Patterning on the groove wall with TiO2/Al2O3 resist
The presence of wet Al2O3 etching in process sequence (B) significantly complicates the application of the process onto the inclined groove walls. In particular, the difficulties originate from the unequal etching speeds across the pattern due to the different accessibility of the top and bottom regions for wet etchant. Here we show that without any optimization it is possible to obtain well-defined 170 nm-wide line patterns that are formed on the groove bottom, raised up along the wall, and are etched for 530 nm (figure 4). The key aspect here is that the process retains its universal nature, as we have not used any dose correction algorithms across the pattern to compensate for variations in the wet etching speed. Therefore, sequence (B) allows for fast prototyping on multilevel surfaces with practically unlimited structure height due to the high selectivity of the Al2O3 mask. Eventually, the resolution can be further improved if one implements a proper dose correction algorithm and reduces the thickness of the Al2O3 layer.
4.2. Groove definition
As discussed above, due to the very large FIB depth of focus and uniform coverage of ALD resists, the proposed method should work even on extremely corrugated sample surfaces. In order to test such a nanofabrication, we prepared a set of samples with a strong surface relief. Since generally performing lithography in grooves and trenches is more challenging than, e.g., on top of volumetric structures, we fabricated a set of samples with long grooves. All the grooves have a similar depth of about 13 μm and sidewalls angles close to 55 degrees; the width of the grooves varies from 4 to 10 μm. The schematic drawing in figure 3(a) illustrates an example of the groove cross section dimensions, and the SEM image in figure 3(b) shows a cleaved facet of the fabricated groove. The grooves were fabricated by TMAH 4
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Figure 5. (a), (b), (c) Network of suspended nanowires on pillars and
(d) long released nanowires without intermediate supports. SEM images were taken with a 45 degree angle with respect to surface normal. Scale bar is 500 nm. 4.4. Suspended thin Al2O3 nanowires
In this section, we utilize excellent mechanical and etch-stop properties of Al2O3 to fabricate highly robust suspended nanowires. We apply process sequence (B) and modify the parameters of the last etching step. The dry release of suspended nanostructures is realized by room temperature RIE, which is tuned to a lower anisotropy. Figure 5(a) shows an example of a suspended Al2O3 nanowire network that is fabricated on top of a pillar array (seen as semitransparent). The release etching with low anisotropy removes the TiO2 mask from suspended nanowires, leaving predominantly Al2O3 as a nanowire material. The width of Al2O3 nanowires can be adjusted with great flexibility. For the given pillar dimensions (220 nm height and 120 nm width), it can be defined within the range from 20–100 nm. We show the two examples where Al2O3 bridges are approximately 30 and 60 nm wide and 15 nm thick (magnified views of figures 5(b) and (c), respectively). The fabricated nanobridges are very robust, and they do not break or collapse, even without intermediate supporting pillars, when their length approaches 4 μm (figure 5(d); the observable hanging down is caused by SEM imaging). The advantage of such fabrication of suspended nanowire networks is that the utilized RIE enables low etching rates, which greatly improve the accuracy and repeatability.
Figure 6. Nanopillar arrays on the groove bottom connected by (a)
thin solid walls and (b) released nanobridges. Closeup views are given for areas indicated by dashed lines. SEM images were taken with a 45 degree angle with respect to surface normal. Scale bar for closeup views is 500 nm.
is close to 480 nm, and the width of suspended TiO2 nanobridges is around 50 nm. The release of the structures shown in figure 6(b) is done by the corresponding tuning of ICP-RIE parameters. Fabrication of similar structures by standard lithography would be very difficult due to the problems with nonuniform resist spinning and development. In our case, the process also ensures a good repeatability, since both resist development and release etching steps are performed by dry etching. The nanowires are fabricated from TiO2 and are less robust in comparison to Al2O3 suspended structures; however, their mechanical properties are still sufficient for many applications. 4.6. Suspended structures on the groove walls
Some applications of suspended nanonetworks require them to follow the sample relief (e.g., the steep wall of a groove). Fabrication of released nanostructures on inclined surfaces implies patterning as well as developing and release etching processes to be greatly insensitive to height variation. We demonstrate that the process sequence (A) can fulfill these stringent demands and provide the fabrication of solid and released nanostructures across the inclined walls, as shown in figures 7(a) and (b) respectively. The suspended nanobridges extend for about 7 μm across the wall, which has a slope angle close to 55 degrees. The nanobridges are approximately 55-nm wide, and they retain their width from top to bottom. The key detail here is that the developing RIE should be optimized to provide a good uniformity of TiO2 resist removal speed across the slope. As also seen, patterning on inclined surfaces leads to a deformation of final structures with respect to written layout. This is a drawback of the proposed method since, e.g., originally circular pillars will get stretched and will change their shape to oval. However, such
4.5. Suspended nanostructures on the groove bottom
Formation of suspended structures on the bottoms of deep grooves and channels belongs to the most challenging types of micro- and nanofabrication. Here we show that it can be realized by the proposed process sequence (A), which does not contain wet etching steps and tolerates very strong irregularities of surface relief. Process scheme (A) with cryogenic ICP-RIE was applied to fabricate nanopillar arrays on the bottom of deep grooves. Even when a groove depth is about 13 μm and sidewall angles equal to 55 degrees, the method allows for fabrication of nanopillars that can be connected either by thin solid walls (figure 6(a)) or by released nanobridges (figure 6(b)). The diameter of the shown nanopillars is about 80 nm, the height 5
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very small sensitivity to height variation and retain the nanometer resolution. A bilayer TiO2/Al2O3 resist is more suitable for application on flat samples, where its ultimate robustness allows for fabrication of a few micrometers long and only a few tens of nm wide suspended nanowires. Finally, due to the absence of high-temperature steps, the method is also applicable to a broad variety of polymers and other temperature-sensitive materials.
Acknowledgments Mikhail Erdmanis acknowledges Tekniikan Edistämissäätiö for the personal research grant. The work was funded by the Energy Efficiency Programme (project 9158101), the Academy of Finland (project 13140009), and the EMRP REG (project 602167). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. We also acknowledge the provision of technical facilities of the Micronova Nanofabrication Centre of Aalto University.
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(a) solid walls and (b) suspended nanobridges. SEM images were taken with a 40 degree angle with respect to surface normal. Scale bar is 2 μm.
a distortion of patterns can be compensated for a particular angle of wall inclination mainly by implementing pattern and dose correction algorithms.
5. Conclusion In summary, we proposed a new type of FIB lithography that can be used to fabricate suspended nanostructures on samples with micrometer-size surface corrugation. The process employs large FIB depth of focus and uniform coverage of ALD layers. The proposed single and bilayer inorganic FIB resists are deposited by ALD at a moderate temperature (120 °C). A single layer TiO2 resist provides a feature size of about 30 nm and a selectivity of up to 1 : 2000 for structures formed by cryogenic ICP-RIE. When a bilayer TiO2/Al2O3 resist is used, TiO2 acts as an imaging layer and provides negative tone patterning of underlying Al2O3. A minimum feature size in this case degrades to about 50 nm; however, the selectivity in cryogenic ICP-RIE achieves supreme values of 1 : 70000. Utilizing the proposed processes, we realized suspended nanostructures on planes inclined surfaces, and on the bottoms of 13 μm-deep grooves. A good repeatability of the fabrication process is ensured by the implementation of dry etching for both resist development and release etching steps. The structures formed with a single layer TiO2 resist have a 6
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Focused ion beam lithography for fabrication of suspended nanostructures on highly corrugated surfaces
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Nanotechnology Nanotechnology 25 (2014) 335302 (7pp)
doi:10.1088/0957-4484/25/33/335302
Focused ion beam lithography for fabrication of suspended nanostructures on highly corrugated surfaces M Erdmanis1, P Sievilä1, A Shah1, N Chekurov1, V Ovchinnikov2 and I Tittonen1 1 2
Department of Micro- and Nanosciences, Aalto University, PO Box 13500, FI-00076 Aalto, Finland Department of Aalto Nanofab, Aalto University, PO Box 13500, FI-00076 Aalto, Finland
E-mail: mikhail.erdmanis@aalto.fi Received 17 April 2014, revised 16 June 2014 Accepted for publication 23 June 2014 Published 30 July 2014 Abstract
We propose a nanofabrication method that allows for patterning on extremely corrugated surfaces with micrometer-size features. The technique employs focused ion beam nanopatterning of ion-sensitive inorganic resists formed by atomic layer deposition at low temperature. The nanoscale resolution on corrugated surfaces is ensured by inherently large depth of focus of a focused ion beam system and very uniform resist coating. The utilized TiO2 and Al2O3 resists show high selectivity in deep reactive ion etching and enable the release of suspended nanostructures by dry etching. We demonstrate the great flexibility of the process by fabricating suspended nanostructures on flat surfaces, inclined walls, and on the bottom of deep grooves. Keywords: focused ion beam, nanofabrication, silicon (Some figures may appear in colour only in the online journal) on top of supporting elements. On a flat sample, such nanonetworks are extensively used for experiments devoted to thermal conductance [6, 7], thermoelectric properties [8], controlled resistivity [9], and radiative enhancement of heat transfer by optical near field [10]. Ultimately, the possibility of fabricating suspended nanonetworks on strongly corrugated surfaces and on the bottom of deep microchannels would significantly increase the number of their applications via the integration with microfluidics, plasmonics, photonics, etc.
1. Introduction Nanofabrication on multilevel surfaces, trenches, and micro grooves has attracted a lot of attention due to its great importance for three-dimensional (3D) integration and interdisciplinary research. In particular, the fabrication of nanopillars within micron-scale grooves and channels enables local control of wetting behavior through a combination of hydrophobic properties of nanopillar arrays [1] with hydrophilic behavior of microfluidic channels [2] and grooves [3]. Analogously, it also gives a new way to confine, isolate, and manipulate analytes within nanoscale volumes or to separate molecules via the infiltration through microchannels. For sensing and spectroscopy, gold-or silver-coated nanopillars [4] fabricated in microcavities can enhance the signal and allow for operation with a very small amount of analyte. In addition, nanoscale periodic patterns within micron-scale grooves can be utilized in multiple photonic and plasmonic resonant schemes for optical sensing and signal filtering [5]. The functionality of described nanostructures can be further extended by adding a network of suspended nanowires 0957-4484/14/335302+07$33.00
1.1. Challenges in nanofabrication on strongly corrugated samples
Nanofabrication on surfaces with a strong relief is a great challenge even with the most accurate state-of-the-art fabrication methods. For standard lithographies, samples with irregular surfaces disrupt both a uniform distribution of photoresist and a good focusing during the exposure. There are special techniques that improve the quality of photoresist coating on highly corrugated samples. For example, one can use commercially available spraycoating facilities that rely on 1
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raster scanning nozzles or utilize a multistep photoresist coverage where the bottom and the upper resists have different molecular weights [11]. However, the techniques are not universal, and they should be optimized for a particular sample profile. An alternative way of nanopatterning on irregular surfaces is to use the resistless e-beam lithographies that have emerged during the last two decades. Such lithographies rely on various physics, from synthesis of nanoparticles upon electron beam (e-beam) irradiation [12] and local formation of silicides due to the heating [13], to surface passivation opening the exposed areas for mask formation [14]. In general, these methods return patterns with a very high resolution; however, the robustness and selectivity of the formed mask cannot match the requirements for the fabrication of released nanostructures. Apart from the problem of uniform photoresist coating, both resist and resistless lithographies on highly irregular surfaces suffer from a degraded resolution and disturbed pattern features. For maskless lithographies, it originates from the poor depth of focus, which is usually below 10 μm for ebeam lithography. The mitigation of the problem requires the use of dynamic focusing and dose correction across the sample [15] that is difficult to implement for irregular samples with arbitrary surface features. Similarly, masked contact and proximity lithographies that work perfectly for patterning on flat samples cannot provide the highest resolution on nonplanar samples. More complex methods, such as nanoimprint lithography based on embossing with elastomeric pattern transfer [16], which allow for patterning even on spherical surfaces, require a complicated setup alignment and controllable low pressure under the mask during exposure. Consequently, there is no one universal method that can tolerate strong arbitrary variations of sample height and similarly provide a straightforward patterning process with fast exposure times.
In most cases, dry release is beneficial due to better resolution and repeatability. Consequently, the material that is used for released nanostructures should be of high quality and mechanical strength to withstand the impact of dry etching. Therefore, there is a need for a nanofabrication process that can provide patterning of robust mask material and release by dry etching with high selectivity that can also simultaneously tolerate strong and arbitrary corrugation of the sample profile.
2. Methods and materials 2.1. Focused ion beam lithography
Focused ion beam (FIB) has been extensively used in microfabrication and integrated circuit analysis from the development of bright liquid metal ion sources in the mid 1970s [21]. It also gave rise to a new concept of FIB lithography (FIBL), which utilizes the phenomena that bombarding ions get implanted into the target material and modify its surface layer [22]. With modern gallium ion sources that provide stable operation and a long lifetime, FIB becomes a more competitive tool in comparison to e-beam lithography (EBL). Although the resolution achievable by FIBL is lower than that of EBL, the remarkable advantages of FIBL are a significantly larger depth of focus and a very small scattering of ions in a resist layer. In FIBL, patterning can be performed either on the original material (resistless lithography) or on an extra resist layer. When a sample is subjected to, e.g., a gallium ion beam, the irradiated areas exhibit etch-retarding behavior due to several mechanisms. The first one is a physical change of lattice constant caused by impurity atoms, inducing the corresponding strain effects and slowing down the wet and dry etching rates [23, 24]. Another effect is a change in the nature of the surface chemical reaction across implanted areas. For example, in dry etching with fluorine chemistries, a nonvolatile GaFx masking compound is formed on irradiated areas [25, 26]. In a case when oxygen is present in the etching gas mixture, the etch retardation effect can also be provided by in situ formation of a thin gallium oxide layer [22]. Micro and nanofabrication with FIBL can be realized with various resist materials, from standard PMMA [27] to different compounds and multilayer coatings [28–30]. Resistless FIBL with direct ion implantation into, e.g., pure silicon, can also facilitate the fabrication of high aspect ratio structures via inductively coupled plasma reactive ion etching (ICP-RIE) at cryogenic temperatures [31]. However, the ultimate advantage of utilizing resists is that the sample surface is protected, and the surface layer does not become amorphous and contaminated by gallium dopants. In addition, high quality resist materials can provide fine control over the surface profile of nanostructures formed by dry etching, even when patterning is done with a commonly used FIB system [32].
1.2. Existing methods for fabrication of suspended nanostructures
The number of methods for fabrication of suspended nanostructures on very complex geometries has been continuously growing during the last decade. Recent advances include direct fabrication of suspended nanowires from a gas phase by e-beam deposition [17] or sequential drawing of fibers from a polymer droplet by microscale tip [18]. Such methods, however, are craftwork, and they cannot compete with lithographies when used for large area patterning. For this reason, the most widely used methods for fabrication of suspended nanostructures rely on standard process steps. Typically, they include patterning, mask development, structural etching that forms supporting geometry, and release stage returning suspended structures. In the case of an irregular sample, the accuracy of patterning will similarly degrade from nonuniform resist coating and limited depth of focus of the exposure system. Another factor defining the resolution is a type of process used for release of suspended structures, which can be either wet [8, 19] or dry etching [20]. 2
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2.2. Deposition methods and materials
When nanofabrication is to be performed on a strongly corrugated sample, the role of the mask deposition method becomes crucial, as it should ensure a uniform coating across the sample. From modern deposition techniques, a very attractive option providing nm thick uniform coating is known as atomic layer deposition (ALD) [33]. This method perfectly tolerates highly irregular surfaces, since it is based on self-terminating chemical reactions. One of the most established and canonic ALD processes is growing high quality Al2O3 films from trimethylaluminum (TMA) and water precursors. In silicon microfabrication, Al2O3 is widely used as a mask material for dry etching. It has been shown that the Al2O3 etch rate in fluorine-based cryogenic ICP-RIE can be close to zero and provide a unique selectivity of 1 : 70000 [34]. In practice, it means that only a 6–10 nm thick Al2O3 layer is sufficient for through-wafer etching of silicon. At the same time, Al2O3 films received great interest due to their impressive mechanical properties [35]. For this reason, Al2O3 appears to be a perfect material for the fabrication of nm thick released structures [36].
Figure 1. Schematic drawing of fabrication process flow. Sequence
(A) with single layer TiO2 resist, sequence (B) with bilayer TiO2/ Al2O3 resist.
throughout the paper for the processes employing single layer TiO2 and bilayer TiO2/Al2O3 resists, respectively. Both ALD films are deposited at 120 °C in a Beneq TFS 500 reactor with TiCl4 and H2O precursors for TiO2, and TMA with H2O precursors for Al2O3 deposition. The typical thicknesses of the films are 50 nm for TiO2 and 15 nm for Al2O3. The thickness of TiO2 was selected to be large enough to prevent Ga+ ions from achieving the underlying Al2O3 layer [39]. FIB patterning was performed with the acceleration voltage of 30 kV, an ion beam current of 9.7 pA, and an exposure dose around 1.5 × 1016 ions/cm2. Both processes employ two dry etching steps—first, the developing of negative patterns formed from TiO2 film; and second, the structural etching for the fabrication of silicon structures. The developing etching in both cases is done by RIE with a CF4 gas mixture. The differences between (A) and (B) are in the presence of the additional Al2O3 wet etching step in sequence (B) and in the strength of a final mask. The TiO2 mask in (A) is not very robust, which implies that the structure should be formed by dry etching with a very low bias and high selectivity such as, e.g., cryogenic ICP-RIE. In contrast, in (B) the underlying Al2O3 mask is more resistant against dry etching, and therefore structural etching can also be realized by room temperature RIE with lower selectivity. Both RIE and cryogenic ICP-RIE structural etching steps are performed with SF6/O2 gas mixtures, which can be used for etching of various polymers [40] in addition to silicon. The achievable resolution and influence of the main parameters on the final structures will be discussed throughout the manuscript within the context of fabricated structures.
3. Fabrication process flow In this work, we present a method for nanofabrication on strongly corrugated surfaces that combines the advances of FIB lithography and ALD. The minor distortions of patterned features are ensured by the large depth of focus of FIB [37] and the uniformity of ALD coating. We concentrate on negative tone ALD ion-sensitive resists (exposed areas correspond to the formed masked pattern), as this type of resist is highly desirable for many applications where patterned structures should be surrounded by large empty areas. Originally, irradiation of Al2O3 films by a Ga+ beam promotes the destruction of exposed areas in consequent dry etching (equal to the positive tone resist) [38]. However, fabrication of negative tone nanopatterns from Al2O3 is very attractive due to its superb mechanical and etch-stop properties. Therefore, we suggest using an extra layer to make the inversion of resist tone and to finally provide negative Al2O3 patterns. We have found that one suitable candidate for such an inversion layer is low-temperature amorphous TiO2, which acts as a high-contrast negative ion-sensitive resist in fluorinebased dry etching. Our observation complies with earlier reports on the general ion-sensitive behavior of other transition metal oxides [29]. Moreover, in certain conditions TiO2 can act as a mask material itself when a high enough etching selectivity is provided. Consequently, we propose two distinct masking processes—with a single layer TiO2 resist and with a bilayer resist (TiO2 on top of Al2O3). Due to the high quality of ALD films, both materials allow for the fabrication of suspended nanostructures via dry etching release. Figure 1 shows the two process sequences with different types of resists that are utilized for fabrication of pillars on flat and inclined surfaces. Abbreviations (A) and (B) are kept
4. Fabricated nanostructures 4.1. High aspect ratio nanostructures
Figure 2 shows the examples of high aspect ratio nanostructures fabricated with the process sequence (A). Figure 2(a) presents an array of pillars with a 110 nm diameter and a 380 nm height that are connected by thin walls. The walls have nearly perfect vertical profiles and only 25–30 nm width (shown by magnified side view in figure 2(b)). Figure 2(c) provides the results of the line width 3
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Figure 3. (a) Schematic drawing of groove cross section. (b) SEM
image of the cleaved groove facet taken with a 45 degree angle with respect to surface normal. Scale bar is 10 μm.
Figure 2. Nanostructures fabricated with TiO2 mask and cryogenic ICP-RIE. SEM images were taken with a 52 degree angle with respect to surface normal. Pillars connected by fine nanowalls: (a) front view and (b) magnified side view. (c) Resolution test. Scale bar is 500 nm.
tests where fine 30–35 nm-wide lines were etched for approximately 420 nm with a slightly positive sidewall angle. As seen, when the line-to-gap ratio starts to approach 1:1, the mask peels off due to the undercut (figure 2(c) right). Consequently, the resolution of deeply etched nanostructures that are located close to each other would be generally limited by the parameters of dry etching. The resolution of patterning of a single line, however, can be further improved by using a smaller FIB exposure current and a thinner TiO2 layer. Another important aspect is that the TiO2 mask shows a very high selectivity in silicon cryogenic dry etching. We experimentally verified that for our process, it is about 1 : 2000 with the following cryogenic ICP-RIE parameters: process temperature − 120 °C, SF6/O2 gas mixture, 3 W forward power, and 800 W ICP power. As a result, FIB patterning of the TiO2 mask would practically allow for combining of nano- and microfabrication with the etching depths achieving a few tens of micrometers.
Figure 4. Lines with a TiO2/Al2O3 mask patterned on the groove bottom and wall. The SEM image was taken with a 52 degree angle with respect to surface normal. Scale bar is 5 μm.
((CH3)4NOH) wet etching of [100] silicon wafers with a SiO2 mask, providing a target etching angle of 54.7 degrees. The appearance of irregular steps on the walls (seen on the left wall in figure 3(b)) is caused by misalignment between a mask and a silicon wafer flat.
4.3. Patterning on the groove wall with TiO2/Al2O3 resist
The presence of wet Al2O3 etching in process sequence (B) significantly complicates the application of the process onto the inclined groove walls. In particular, the difficulties originate from the unequal etching speeds across the pattern due to the different accessibility of the top and bottom regions for wet etchant. Here we show that without any optimization it is possible to obtain well-defined 170 nm-wide line patterns that are formed on the groove bottom, raised up along the wall, and are etched for 530 nm (figure 4). The key aspect here is that the process retains its universal nature, as we have not used any dose correction algorithms across the pattern to compensate for variations in the wet etching speed. Therefore, sequence (B) allows for fast prototyping on multilevel surfaces with practically unlimited structure height due to the high selectivity of the Al2O3 mask. Eventually, the resolution can be further improved if one implements a proper dose correction algorithm and reduces the thickness of the Al2O3 layer.
4.2. Groove definition
As discussed above, due to the very large FIB depth of focus and uniform coverage of ALD resists, the proposed method should work even on extremely corrugated sample surfaces. In order to test such a nanofabrication, we prepared a set of samples with a strong surface relief. Since generally performing lithography in grooves and trenches is more challenging than, e.g., on top of volumetric structures, we fabricated a set of samples with long grooves. All the grooves have a similar depth of about 13 μm and sidewalls angles close to 55 degrees; the width of the grooves varies from 4 to 10 μm. The schematic drawing in figure 3(a) illustrates an example of the groove cross section dimensions, and the SEM image in figure 3(b) shows a cleaved facet of the fabricated groove. The grooves were fabricated by TMAH 4
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Figure 5. (a), (b), (c) Network of suspended nanowires on pillars and
(d) long released nanowires without intermediate supports. SEM images were taken with a 45 degree angle with respect to surface normal. Scale bar is 500 nm. 4.4. Suspended thin Al2O3 nanowires
In this section, we utilize excellent mechanical and etch-stop properties of Al2O3 to fabricate highly robust suspended nanowires. We apply process sequence (B) and modify the parameters of the last etching step. The dry release of suspended nanostructures is realized by room temperature RIE, which is tuned to a lower anisotropy. Figure 5(a) shows an example of a suspended Al2O3 nanowire network that is fabricated on top of a pillar array (seen as semitransparent). The release etching with low anisotropy removes the TiO2 mask from suspended nanowires, leaving predominantly Al2O3 as a nanowire material. The width of Al2O3 nanowires can be adjusted with great flexibility. For the given pillar dimensions (220 nm height and 120 nm width), it can be defined within the range from 20–100 nm. We show the two examples where Al2O3 bridges are approximately 30 and 60 nm wide and 15 nm thick (magnified views of figures 5(b) and (c), respectively). The fabricated nanobridges are very robust, and they do not break or collapse, even without intermediate supporting pillars, when their length approaches 4 μm (figure 5(d); the observable hanging down is caused by SEM imaging). The advantage of such fabrication of suspended nanowire networks is that the utilized RIE enables low etching rates, which greatly improve the accuracy and repeatability.
Figure 6. Nanopillar arrays on the groove bottom connected by (a)
thin solid walls and (b) released nanobridges. Closeup views are given for areas indicated by dashed lines. SEM images were taken with a 45 degree angle with respect to surface normal. Scale bar for closeup views is 500 nm.
is close to 480 nm, and the width of suspended TiO2 nanobridges is around 50 nm. The release of the structures shown in figure 6(b) is done by the corresponding tuning of ICP-RIE parameters. Fabrication of similar structures by standard lithography would be very difficult due to the problems with nonuniform resist spinning and development. In our case, the process also ensures a good repeatability, since both resist development and release etching steps are performed by dry etching. The nanowires are fabricated from TiO2 and are less robust in comparison to Al2O3 suspended structures; however, their mechanical properties are still sufficient for many applications. 4.6. Suspended structures on the groove walls
Some applications of suspended nanonetworks require them to follow the sample relief (e.g., the steep wall of a groove). Fabrication of released nanostructures on inclined surfaces implies patterning as well as developing and release etching processes to be greatly insensitive to height variation. We demonstrate that the process sequence (A) can fulfill these stringent demands and provide the fabrication of solid and released nanostructures across the inclined walls, as shown in figures 7(a) and (b) respectively. The suspended nanobridges extend for about 7 μm across the wall, which has a slope angle close to 55 degrees. The nanobridges are approximately 55-nm wide, and they retain their width from top to bottom. The key detail here is that the developing RIE should be optimized to provide a good uniformity of TiO2 resist removal speed across the slope. As also seen, patterning on inclined surfaces leads to a deformation of final structures with respect to written layout. This is a drawback of the proposed method since, e.g., originally circular pillars will get stretched and will change their shape to oval. However, such
4.5. Suspended nanostructures on the groove bottom
Formation of suspended structures on the bottoms of deep grooves and channels belongs to the most challenging types of micro- and nanofabrication. Here we show that it can be realized by the proposed process sequence (A), which does not contain wet etching steps and tolerates very strong irregularities of surface relief. Process scheme (A) with cryogenic ICP-RIE was applied to fabricate nanopillar arrays on the bottom of deep grooves. Even when a groove depth is about 13 μm and sidewall angles equal to 55 degrees, the method allows for fabrication of nanopillars that can be connected either by thin solid walls (figure 6(a)) or by released nanobridges (figure 6(b)). The diameter of the shown nanopillars is about 80 nm, the height 5
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very small sensitivity to height variation and retain the nanometer resolution. A bilayer TiO2/Al2O3 resist is more suitable for application on flat samples, where its ultimate robustness allows for fabrication of a few micrometers long and only a few tens of nm wide suspended nanowires. Finally, due to the absence of high-temperature steps, the method is also applicable to a broad variety of polymers and other temperature-sensitive materials.
Acknowledgments Mikhail Erdmanis acknowledges Tekniikan Edistämissäätiö for the personal research grant. The work was funded by the Energy Efficiency Programme (project 9158101), the Academy of Finland (project 13140009), and the EMRP REG (project 602167). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. We also acknowledge the provision of technical facilities of the Micronova Nanofabrication Centre of Aalto University.
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5. Conclusion In summary, we proposed a new type of FIB lithography that can be used to fabricate suspended nanostructures on samples with micrometer-size surface corrugation. The process employs large FIB depth of focus and uniform coverage of ALD layers. The proposed single and bilayer inorganic FIB resists are deposited by ALD at a moderate temperature (120 °C). A single layer TiO2 resist provides a feature size of about 30 nm and a selectivity of up to 1 : 2000 for structures formed by cryogenic ICP-RIE. When a bilayer TiO2/Al2O3 resist is used, TiO2 acts as an imaging layer and provides negative tone patterning of underlying Al2O3. A minimum feature size in this case degrades to about 50 nm; however, the selectivity in cryogenic ICP-RIE achieves supreme values of 1 : 70000. Utilizing the proposed processes, we realized suspended nanostructures on planes inclined surfaces, and on the bottoms of 13 μm-deep grooves. A good repeatability of the fabrication process is ensured by the implementation of dry etching for both resist development and release etching steps. The structures formed with a single layer TiO2 resist have a 6
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