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High aspect ratio anisotropic silicon etching for x-ray phase contrast imaging grating fabrication
Materials Science in Semiconductor Processing xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
High aspect ratio anisotropic silicon etching for x-ray phase contrast imaging grating fabrication ⁎
Patrick S. Finnegan , Andrew E. Hollowell, Christian L. Arrington, Amber L. Dagel Sandia National Laboratories, 1515 Eubank Blvd SE, Albuquerque, NM 87123, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: XPCI Anisotropic aqueous silicon etch Potassium Hydroxide Etching Deep Silicon Etching
Lab based x-ray phase contrast imaging (XPCI) systems have historically focused on medical applications, but there is growing interest in material science applications for non-destructive analysis of low density materials. Extending this imaging technique to higher density materials or larger samples requires higher aspect ratio gratings, to allow the use of a higher energy x-ray source. In this work, we demonstrate the use of anisotropic silicon (Si) etching in potassium hydroxide (KOH), to achieve extremely high aspect ratio gratings. This method has been shown to be effective in fabricating deep, uniform gratings by taking advantage of the etch selectivity of differing crystalline planes of silicon. Our work has demonstrated a method for determining Si crystalline plane directions, specific to (110) Si wafers, enabling high alignment accuracy of the etch mask to these crystalline planes.
1. Introduction Grating-based laboratory implementation of x-ray phase contrast imaging (XPCI) depends on micro-fabricated gratings: a source grating, a phase grating, and an analyzer grating. The source and analyzer gratings are transmission gratings; their depth determines the contrast that can be achieved. In the phase grating, the depth is directly correlated to the phase shift imposed on the x-ray wave front. Small feature sizes are necessary in the analyzer grating for higher sensitivity to measuring phase changes in a Talbot-Lau system. This combination of large grating depths and small feature sizes sets the requirement for extremely high aspect ratio features, pushing the limits on state-of-the-art grating fabrication. Lab-based XPCI systems currently operate in the 10's of keV because of the current limits in grating fabrication. To extend XPCI to source energies as high as hundreds of keV, while maintaining large fields-of-view, higher aspect ratio (AR) diffraction gratings are necessary. The higher the AR, the higher the source energies that can be used [1,2]. A variety of techniques have been used to fabricate XPCI gratings, including Lithographie, Galvanoformung, Abformung (LIGA), Metal assisted chemical etching (MACE), angled evaporation, Deep reactive ion etch (DRIE), and KOH etching [1–6]. LIGA fabricated gratings have limitations in realizing unsupported large fields-of-view. MACE has challenges with etch uniformity across large areas but has realized impressive aspect ratios. Angled evaporation and DRIE are ultimately limited in aspect ratio due to geometric constraints and RIE lag respectively. KOH etching has some promising attributes as it can achieve
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extremely high aspect ratios over a large area if accurate alignment of the lithographically defined mask to the crystalline plane can be achieved. In this work, we investigate anisotropic KOH etching using pre-etched features to facilitate accurate grating alignment. The preetch has the potential to drastically increase the attainable aspect ratios and involves a simple contact alignment method. The benefit of our approach is that no unwieldly optical set ups are required [1,7,8]. The end goal of producing high AR gratings in Si is to fabricate a rigid template for a metallization that provides a dense material to act as the x-ray absorption material. A precision electroformed gold can be used to achieve specific space-to-gap dimensions and provide the density needed for absorption. Different methods of conformal metal coatings over rigid Si templates have been explored, and we have demonstrated this capability on smaller AR gratings [3]. KOH etched gratings have been demonstrated with AR's of > 100:1 when etching small features over small areas [9]. Here we utilize KOH etching to demonstrate XPCI gratings with geometries suitable for high energy xray radiation, 8 µm pitch, 159 µm depth with an AR of 80:1. This is accomplished across the surface of an entire 150 mm (110) Si wafer with potential to yield over 600 mm2. 2. Experimental details 2.1. Determining crystallographic orientation of Silicon wafers Each crystalline plane of silicon etches at a different rate in KOH.
Corresponding author. E-mail address: psfi[email protected] (P.S. Finnegan).
https://doi.org/10.1016/j.mssp.2018.06.013 Received 31 January 2018; Received in revised form 8 June 2018; Accepted 14 June 2018 1369-8001/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Finnegan, P.S., Materials Science in Semiconductor Processing (2018), https://doi.org/10.1016/j.mssp.2018.06.013
Materials Science in Semiconductor Processing xxx (xxxx) xxx–xxx
P.S. Finnegan et al.
Fig. 1. Alignment of an etch mask to differing Si crystalline planes on a (110) Si wafer results in different and predictable profiles. A) Trench profiles which are parallel to a {111} plane yield sidewalls 90° to the wafer surface and a minimal hard mask undercut resulting from KOH anisotropic etching. B) Trench profiles parallel to a {111} results in a 35.3° angled sidewall profile, with minimal hard mask undercut. C) Trench profiles which are perpendicular alignment to < 100 > direction results in 54.7° sloped sidewall and undercut of the etch mask D) Trench profiles which are parallel to a {110} plane straight sidewalls with mask undercut.
Fig. 2. A) SEM image of a circular opening in Si3N4 hard mask on (110) Si that was KOH etched forming a self-stopping hexahedron cavity bound by 6 {111} crystal planes. B) SEM cross section of a (1̅11̅) crystal plane normal to the (110) wafer surface. C) SEM cross section shows the 35.3° intersection of the (111̅) and (1̅1̅1̅) crystal planes with the surface of the (110) wafer, perpendicular (11̅1̅) and (1̅11̅) and {100} plane. D) SEM cross section shows all relevant crystal planes forming the boundaries of the self-stopping KOH etch.
etched small circular openings in the (110) wafer surface [10]. Although anisotropic KOH etching is not new, in this work we clarify ambiguity, poorly communicated, and misinterpreted nomenclature used in silicon micromachining literature and common practice. Much literature exists on these topics; our approach is aimed at clarifying nomenclature used in silicon crystallography and cited in KOH etch literature with a simple pre-etch that enables unambiguous identification of specific crystalline planes. The ability to precisely align to specific crystalline planes is essential to achieve unprecedented aspect ratios with aqueous KOH etch. For instance, most literature quotes the use of a (110) wafer referring to a specific crystal plane when instead {110} should be used referring to an equivalent family of crystalline planes. We’ve applied a simple pre-etch to clearly identify crystalline orientation. We used this technique on two {110} Si wafers with major flats of different orientations to evaluate the crystallographic orientation of each. Low pressure chemical vapor deposition (LPCVD) Si3N4 films of 300 nm were grown on each wafer and patterned with small circular openings. AZ4330 photoresist was spin cast onto each wafer at
Aqueously etched trenches in silicon yield different etch profiles. The orientation of the etch mask on the surface of the wafer and the crystalline orientation of the wafer can produce predictable etch profiles, resulting in trenches with vertical or sloped sidewalls, illustrated in Fig. 1 [10,11]. XPCI gratings target a vertical sidewall, 90° to the wafer surface. It has been reported that etch rates of a (110) plane are twice that of a (100) and up to 143 times faster than the etch rate of the {111} plane [12]. A deep trench etch is most effectively achieved by etching the (110) plane with the direction perpendicular to the surface of the sidewalls aligned in the < 111 > direction. This results in a rapid, selective etch in the vertical (110) direction and a very slow etch in the lateral direction, bound by {111} planes. For this reason, we used (110) silicon wafers and align our grating features to the {111} planes in pursuit of high aspect ratio gratings. Achieving a small period, high aspect ratio analyzer grating with an anisotropic aqueous etch requires precise alignment to a specific plane of the Si wafer. To identify the crystalline orientation in our wafers, we used a method described by Bassous in the late 1970s, where KOH 2
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removed in an O2 plasma asher. The grating pattern was then etched in a 6 M KOH concentration at 75 °C for 1 h, which resulted in 76 µm deep trenches. However, aligning to the cleaved edge of the crystal plane resulted in poor alignment accuracy. This misalignment led to limited areal uniformity and lack of dimensional control, producing a grating with approximately 0.8 µm Si fins and approximately 2.4 µm gaps. The tall Si fins were stuck together in places, and the Si3N4 hard mask delaminated due to stresses caused by fin distortion. The fin distortion was likely due to the extremely thin and long Si fins and surface tension forces of the aqueous etch and rinse methods that were followed by an N2 blow dry, as seen in Fig. 4. This can be overcome by super critical drying of the wafer. The alignment process along a cleaved edge produced a promising result, but a more accurate alignment was needed for better results. It is likely the cleave did not create a break perfectly along a single {111} plane and produced an edge that was not straight. Furthermore, misalignment of the mask using this contact alignment method is inevitable because it is difficult to see a cleaved edge and align it to a grating pattern through the MA6 objectives. To achieve a more accurate and repeatable crystallographic alignment, a preliminary layer 0 etch was performed on the wafer prior to aligning and patterning the gratings. Building upon a method developed by Vangbo et al. [15], a series of stepped rotationally arrayed alignment forks at 0.1°, from + 3° to − 3° was produced. These forks allow us to accurately align to a crystalline plane by selecting the set of forks that has the most symmetrical etch pattern. In addition, a series of parallel lines arrayed at 0.1° in a fan pattern were included to demonstrate the ability to identify the (111) plane. A layer 0 mask was produced and was used to pattern only the forked comb arrays and the series of parallel lines in a fanned array. The second mask, layer 1, is aligned to the forked comb structures and the parallel lines. A (110) Si wafer was processed through LPCVD, lithography, and ICP etch as others before described above. Once the layer 0 was patterned, the Si3N4 was etched and photoresist removed, the wafer was etched for 25 min in a 6 M KOH concentration at 75 °C. This revealed an obvious pattern within the fork structures. The most symmetrically etched fork displays the most accurate theta adjustment to align to the (111) plane, and allowed very accurate alignment to a crystalline plane, shown in Fig. 5. Once a proper rotational correction was identified by the most symmetrically etched combed fork structure, a corresponding alignment mark on the subsequent layer allowed for accurate alignment. We observed cross-sections of the series of parallel lines with an 8 µm pitch with a 50% duty cycle aligned properly parallel to the {111} plane, with an un known error. The results showed good straight sidewalls and a depth of 60 µm. The Si gratings showed no distortion; however, they
3000 rpm, and exposed using an MA6 contact aligner with a 160 mJ/cm broadband dose and developed in AZ400K 1:4 developer for 90 s. The LPCVD Si3N4 hard mask was etched using a CF4/Ar based ICP etch. The resist was stripped in a Tepla barrel asher using an O2 plasma at 600 W, 600 sccm O2 at 1 mT for 15 min. The wafers were then etched for 5 min in a 6 M KOH solution at 75 °C. Fig. 2 shows how the KOH etch undercut the nitride hard mask and formed a self-stopping hexahedron cavity bound by six {111} crystal planes. A V-groove was formed in the bottom of the cavity identifying the (111̅) and (1̅1̅1̅) planes which intersect the (110) wafer surface at 35.3° and meet each other at 109.5°. The (11̅1̅) and (1̅11̅) planes are normal to the (110) wafer surface and meet each other at 109.5° [13]. To achieve straight sidewalls and deep etched trenches, it is imperative to accurately align the sidewalls of the diffraction gratings parallel to the (11̅1̅) or (1̅11̅) planes that are perpendicular to the (110) surface. By using the etched cavity method, the aspect of the {111} planes were identified for each wafer in relationship to the major flat. One wafer had a flat in the < 111 > direction and the other wafer's flat was in the < 110 > direction. Plane direction signified by carrots, < xyz > , is defined as the direction perpendicular to any crystalline plane signified by parenthesis, (xyz). As shown in Fig. 3, {111} plane abcd and defa, intersected the (110) surface of each wafer at a 35.4° angle, while the other four {111} planes ab, bc, de and ef were perpendicular to the wafers (110) surface [14]. 2.2. Accurate crystallographic alignment The major flat of most Si wafers is aligned to the crystalline plane with ± 2° accuracy. The etch mask must be aligned within 0.1° of a crystal plane to maintain critical dimension control of the final etched grating without undercutting the etch mask [2,8–10,15]. We used a 1X contact photolithography mask with 2 cm2 diffraction gratings with a 4 µm pitch and 50% duty cycle. A 300 nm LPCVD Si3N4 was grown on the wafers. Si3N4 etches approximately 100 times slower than Si in KOH. The wafer was coated with AZ5214 photoresist, at a thickness of 1.4 µm, using a ACS 200 plus track system. In first attempts of aligning to the crystal plane, the wafer was cleaved in the < 111 > direction. This created a straight edge to the wafer along a {111} crystalline plane. This straight edge of the wafer was then used to align our photolithography mask so that the pattern was parallel to this cleaved edge. An MA6 contact aligner was used for the alignment and exposure of the diffraction gratings mask with 46 mJ/cm2 broadband dose. The resist was then developed on the ACS 200 track using NMDW developer, creating the grating pattern. The Si3N4 was etched in a reactive ion etch (RIE) using CF4/Ar plasma chemistry. The photoresist was then
Fig. 3. KOH etched circular windows in (110) silicon wafer reveal a self-stopping cavity bound by six {111} crystal planes. This enables identification of the specific crystal orientation of the wafer. (111) planes abcd and adef, created a v-grooved etch with sidewalls at 35.3° with respect to the (110) surface. Other {111} planes intersect the surface at 90° at locations ab, bc, de and ef. High aspect ratio diffraction grating anisotropic etch in Si require precise alignment of feature walls parallel to either {111} planes ab, ed or fa, dc. A) (110) Si wafer with (111) planes ab and de 35.3° relative to the major flat. (111) planes fa and dc the major flat and the intersection of (111) planes abcd and defa are 57.4° to the major flat. B) (110) wafer with (111) planes ab and de are 70.6° relative to the major flat. (111) planes fa and dc are 144.7° to the major flat and the intersection of (111) planes abcd and defa are 90° to the major flat. C) Graphic depicting the crystalline orientation in relationship to the wafer flat. 3
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Fig. 4. A) Diagram of alignment method cleaving the wafer at 35° relative to the major flat and alignment of the diffraction grating parallel to that edge. B) ~ 0.8 µm Si gratings with ~ 2.4 µm gaps, 76 µ C) Close up of gratings with dimensions D) The fins are not structurally strong enough to support themselves; surface tension has distorted them from vertical and caused stiction. misalignment resulted in limited area/ lack of dimensional control. Si3N4 hard mask delaminating due to fin distortion.
Fig. 5. A) Optical image of comb structures arrayed at 0.1° increments from + 3° to − 3° with respect to the major flat. The individual fork that shows the best symmetry in the etched Si, relative to the pattern, identifies proper rotational correction for alignment B) SEM images of forked comb structures C) Comb structure with symmetric etch profile indicates alignment to crystalline plane.
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Fig. 6. A) Series of small gratings with 8 µm pitch and 50% duty cycle, arrayed at 0.1° from + 1° to − 1° relative to major flat B) Close-up of the arrayed gratings. C) Magnified SEM image after 90 mins etch time shows fin width is approximately 1.5 µm. D) Magnified SEM image of Si gratings after 60 mins KOH etch showing undercut of the Si3N4, with 2.27 µm wide Si gratings E) Cross-sectional SEM image after 90 min of etch time reveals an etch depth of 90 µm.
by the ability to achieve near perfect alignment and the lateral etch rates, which will cause narrower Si structures than designed. We observed approximately 80:1 vertical to lateral etch and no surface tension effects causing fin distortion. We can compensate for the lateral etch by biasing the pattern to achieve the desired pitch and feature dimensions. Based on the vertical to lateral etch rate, biasing the mask is limited by pitch and duty cycle. The pitch remains uniform and the duty cycle changes based on the 80:1 etch ratio. For example, if a desired grating is to be a 12 µm pitch and 25% duty cycle, a grating feature patterned at 10 µm wide, with a gap width of 2 µm, could theoretically be etched to a depth of 280 µm. This would result in a Si grating line width of 3 µm with a gap width of 9 µm, if the hypothesis holds true. There was some non-uniformity of the etch depth, shown in Fig. 7. This may be caused by hydrogen bubbles forming in the bottoms of the trenches, inhibiting the chemistry to effectively reach the bottom [15–17]. This can be overcome by adjusting process temperatures, adding surfactants to the chemistry and/or constant agitation or sonication during the etch. One concern is that agitation or sonication introduces a potential force that could damage the fragile silicon structures. The use of the combed forks for alignment enabled more accurate alignment and produced good results. Repeatability on subsequent
did have a small undercut of the Si3N4 etch mask. The same wafer was etched 30 min longer with a constant etch rate of 1 µm/minute. The depth after a total of 90 min was 90 µm with about a 1.5 µm undercut to both sides of the Si3N4 mask, Fig. 6. 3. Results and discussion Another wafer was prepared with the Layer 0 etch mask and etched in KOH. The most symmetrically formed comb structure was identified, indicating correct rotational adjustment needed for alignment of the second mask. The second mask containing an array of 2 cm2 gratings was aligned and patterned into the Si3N4, and etched in KOH for a total of one hour. The etch rate was faster than before which can be explained by the growth of native oxides in between each subsequent etch of the first wafer. Each time the wafer was taken out of the KOH chemistry it grew a native oxide. Oxide has a lower etch rate than Si in KOH, slowing the etch rate down each time the Si oxidize. The wafer that was etched for an uninterrupted hour was approximately 159 µm deep, which is nearly 1.8 µm/minute, most importantly resulting in an impressive 80:1 aspect ratio feature. The resulting Si gratings were about 2 µm wide, showing an undercut of the Si3N4 etch mask around 1 µm on each side of the mask. Ultimately, high AR gratings are limited
Fig. 7. A) Cross-sectional SEM image of KOH etched diffraction grating 159 µm deep in 90 min, resulting in an impressive 80:1 aspect ratio feature. B) SEM image showing straight side wall profiles, uniform 8 µm pitch, Si fin width near 2 µm and Si3N4 hard mask undercut approximately 1 µm. 5
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subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
wafers was difficult and did not perform as well as the etch described above. The comb structures work well for quantifying the misalignment of the crystalline plane relative to the flat, but they are not ideal for achieving accurate alignment of subsequent layers. Future development will include an alignment method coined as a Hexahedron alignment, by James et al. [16]. The self-stopping nature of these cavities clearly defines the directions of the {111} planes, as demonstrated above. For alignment, the pre-etched cavity {111} side walls are aligned by overlaying corresponding hexahedron pattern on a lithography mask. This alignment method simplifies alignment by eliminating the subjective nature of selecting a combed fork that looks most symmetrical. In combination with combed fork structures we will be able to identify a rotational offset and align directly to {111} planes defined in the preetch.
Acknowledgments Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. References [1] C. David, J. Bruder, T. Rohbeck, C. Gru¨nzweig, C. Kottler, A. Diaz, O. Bunk, F. Pfeiffer, Fabrication of diffraction gratings for hard X-ray phase contrast imaging, Microelectron. Eng. 84 (2007) 1172–1177. [2] T. Donath, F. Pfeiffer, O. Bunk, W. Groot, M. Bednarzik, C. Grünzweig, E. Hempel, S. Popescu, M. Hoheisel, C. David, Phase-contrast imaging and tomography at 60 keV using a conventional x-ray tube Source, Rev. Sci. Instrum. 80 (2009) 053701. [3] A.E. Hollowell, C.L. Arrington, J.J. Coleman, P.S. Finnegan, A.M. Rowen, A.L. Dagel, Extensively long high aspect ratio gold analyzer gratings, X-ray Neutron Phase Imaging Gratings (2015). [4] J. Mohr, T. Grund, D. Kunka, J. Kenntner, J. Leuthold, and J. Meiser, High Aspect Ratio Gratings for X-Ray Phase Contrast Imaging. in: Proceedings of the International Workshop on X-Ray and Neutron Phase Imaging with Gratings 1466 (2012) pp. 41–50. [5] J.H. Han, S. Radhakrishnan, C. Lee, A novel batch-processing method for accurate crystallographic axis alignment, J. Micromech. Microeng. 23 (2013) 055017 (7pp). [6] L. Romano, J. Vila-Comamala, M. Kagias, K. Vogelsang, H. Schift, M. Stampanoni, K. Jefimovs, High aspect ratio metal micrcasting by hot emboxxing for X-ray optics fabrication, Microelectron. Eng. 176 (2017) 6–10. [7] J. Vila-Comamala, L. Romano, V. Guzenko, M. Kagiasa, M. Stampanoni, K. Jefimovs, Towards sub-micrometer high aspect ratio X-ray gratings by atomic layer deposition of iridium, Microelectron. Eng. 192 (2018) 19–24. [8] Y. Wang, Z. Liu, Y. Zheng, K. Qiu, Y. Hong, High-accuracy alignment of the grating pattern along silicon 〈1 1 2〉 directions using a short rectangular array, J. Micromech. Microeng. 27 (2017) 065008 (8pp). [9] M. Ahn, R.K. Heilmann, M.L. Schattenburg, Fabrication of 200 nm period blazed transmission gratings on silicon-on-insulator wafers, JVSTB 26 (2008) 2179–2182. [10] E. Bassous, Fabrication of novel three-Dimentional microstructures by the anisotropic etching of (100) and (110) silicon, E, IEEE Trasactions Electron Devices 25 (10) (1978) 1178–1185. [11] K. Sato, Basic 2 Anisotropic Wet-etching of Silicon: Characterization and Modeling of Changeable Anisotropy, Dept. of Micro/Nano Systems Engineering, Nagoya University. 〈http://gcoe.mech.nagoya-u.ac.jp/basic/pdf/basic-02.pdf〉. [12] Mitsuhiro Shikida, Kazuo Sato, Kenji Tokoro, Daisuke Uchikawa, Differences in anisotropic etching properties of KOH and TMAH solutions, Department of Micro System Engineering, Nagoya UniÍersity, Chikusa, Nagoya 464-8603, Japan, 1999 (19 July). [13] D.L. Kendall, Vertical etching of silicon at very high aspect ratios, Annu. Rev. Mater. Sci. 9 (1979) 373–403. [14] D.R. Ciarlo, A latching accelerometer fabricated by the anisotropic etching of (110) oriented silicon wafers, J. Micromech. Microeng. 2 (1992) 10–13. [15] M. Vangbo, Y. Backlund, Precise mask alignment to the crystallographic orientation of silicon wafers using wet anisotropic etching, J. Micromech. Microeng. 6 (1996) 279–284. [16] (a) T.D. James, G. Parish, K.J. Winchester, C.A. Musca, A crystallographic alignment method in silicon for deep, long microchannel Fabrication, J. Micromech. Microeng. 16 (2006) 2177–2182; (b) Y. Zheng, K. Qiu, H. Chen, Y. Chen, Z. Liu, Y. Liu, X. Xu, Y. Hong, Alignment method combining interference lithography with anisotropic wet etch technique for fabrication of high aspect ratio silicon gratings, Opt. Express 22 (19) (2014) 23592–23604 (15). [17] J.H. Han, S. Radhakrishnan, C. Lee, A novel batch-processing method for accurate crystallographic axis alignment, J. Micromech. Microeng. 23 (2013) 055017 (7pp).
4. Conclusions XPCI needs high aspect ratio gratings to allow higher energy x-ray sources for nondestructive imaging of denser materials or larger samples. To fabricate high aspect ratio gratings for XPCI applications using KOH etched (110) Si wafers, it is necessary to understand the crystalline orientation of the wafer to be etched, and align trenches parallel to {111} crystal planes, which are normal to the (110) wafer surface. The preferential etch of the (110) plane to the slow etch rate of {111} planes have been used to demonstrate a method to achieve deep anisotropic KOH Si etched trenches. The small period, high aspect ratio gratings fabricated using anisotropic KOH etches can yield the large area required in XPCI systems. Crystalline orientation of (110) wafers was determined by performing a KOH etch though circular holes, which formed self-terminating hexahedron cavities bound by six {111} planes. Alignment methods were explored using the straight cleaved edge of a (110) wafer broken along an appropriate {111} plane and aligning gratings parallel to the cleaved edge. A more precise alignment method whereby trenches were aligned to specific crystalline planes by using a combed fork structure etched into the wafer was demonstrated. This method was effective at identifying the correct theta adjustment for accurate alignment to the {111} plane, but was difficult to execute a repeatable result given the inherent alignment error of contact lithography. The potential of moving toward stepper lithography would reduce this error and provide better alignment accuracy. The manipulation of preferential etch rates of Si crystalline planes was used to achieve deep silicon etches and produced gratings 159 µm deep, resulting in an impressive 80:1 aspect ratio feature. We continue to improve upon alignment methods with better repeatability, including implementation of a hexahedron alignment. Various etch chemistries, process temperatures, and the use of surfactants may improve grating quality to achieve improved aspect ratios, sidewall surface roughness and uniformity. Finally, mask feature biasing, in combination with the known vertical to lateral etch rates, can be manipulated to ultimately reach critical dimensions in a final etched grating. 5. Disclaimer This paper describes objective technical results and analysis. Any
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
High aspect ratio anisotropic silicon etching for x-ray phase contrast imaging grating fabrication ⁎
Patrick S. Finnegan , Andrew E. Hollowell, Christian L. Arrington, Amber L. Dagel Sandia National Laboratories, 1515 Eubank Blvd SE, Albuquerque, NM 87123, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: XPCI Anisotropic aqueous silicon etch Potassium Hydroxide Etching Deep Silicon Etching
Lab based x-ray phase contrast imaging (XPCI) systems have historically focused on medical applications, but there is growing interest in material science applications for non-destructive analysis of low density materials. Extending this imaging technique to higher density materials or larger samples requires higher aspect ratio gratings, to allow the use of a higher energy x-ray source. In this work, we demonstrate the use of anisotropic silicon (Si) etching in potassium hydroxide (KOH), to achieve extremely high aspect ratio gratings. This method has been shown to be effective in fabricating deep, uniform gratings by taking advantage of the etch selectivity of differing crystalline planes of silicon. Our work has demonstrated a method for determining Si crystalline plane directions, specific to (110) Si wafers, enabling high alignment accuracy of the etch mask to these crystalline planes.
1. Introduction Grating-based laboratory implementation of x-ray phase contrast imaging (XPCI) depends on micro-fabricated gratings: a source grating, a phase grating, and an analyzer grating. The source and analyzer gratings are transmission gratings; their depth determines the contrast that can be achieved. In the phase grating, the depth is directly correlated to the phase shift imposed on the x-ray wave front. Small feature sizes are necessary in the analyzer grating for higher sensitivity to measuring phase changes in a Talbot-Lau system. This combination of large grating depths and small feature sizes sets the requirement for extremely high aspect ratio features, pushing the limits on state-of-the-art grating fabrication. Lab-based XPCI systems currently operate in the 10's of keV because of the current limits in grating fabrication. To extend XPCI to source energies as high as hundreds of keV, while maintaining large fields-of-view, higher aspect ratio (AR) diffraction gratings are necessary. The higher the AR, the higher the source energies that can be used [1,2]. A variety of techniques have been used to fabricate XPCI gratings, including Lithographie, Galvanoformung, Abformung (LIGA), Metal assisted chemical etching (MACE), angled evaporation, Deep reactive ion etch (DRIE), and KOH etching [1–6]. LIGA fabricated gratings have limitations in realizing unsupported large fields-of-view. MACE has challenges with etch uniformity across large areas but has realized impressive aspect ratios. Angled evaporation and DRIE are ultimately limited in aspect ratio due to geometric constraints and RIE lag respectively. KOH etching has some promising attributes as it can achieve
⁎
extremely high aspect ratios over a large area if accurate alignment of the lithographically defined mask to the crystalline plane can be achieved. In this work, we investigate anisotropic KOH etching using pre-etched features to facilitate accurate grating alignment. The preetch has the potential to drastically increase the attainable aspect ratios and involves a simple contact alignment method. The benefit of our approach is that no unwieldly optical set ups are required [1,7,8]. The end goal of producing high AR gratings in Si is to fabricate a rigid template for a metallization that provides a dense material to act as the x-ray absorption material. A precision electroformed gold can be used to achieve specific space-to-gap dimensions and provide the density needed for absorption. Different methods of conformal metal coatings over rigid Si templates have been explored, and we have demonstrated this capability on smaller AR gratings [3]. KOH etched gratings have been demonstrated with AR's of > 100:1 when etching small features over small areas [9]. Here we utilize KOH etching to demonstrate XPCI gratings with geometries suitable for high energy xray radiation, 8 µm pitch, 159 µm depth with an AR of 80:1. This is accomplished across the surface of an entire 150 mm (110) Si wafer with potential to yield over 600 mm2. 2. Experimental details 2.1. Determining crystallographic orientation of Silicon wafers Each crystalline plane of silicon etches at a different rate in KOH.
Corresponding author. E-mail address: psfi[email protected] (P.S. Finnegan).
https://doi.org/10.1016/j.mssp.2018.06.013 Received 31 January 2018; Received in revised form 8 June 2018; Accepted 14 June 2018 1369-8001/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Finnegan, P.S., Materials Science in Semiconductor Processing (2018), https://doi.org/10.1016/j.mssp.2018.06.013
Materials Science in Semiconductor Processing xxx (xxxx) xxx–xxx
P.S. Finnegan et al.
Fig. 1. Alignment of an etch mask to differing Si crystalline planes on a (110) Si wafer results in different and predictable profiles. A) Trench profiles which are parallel to a {111} plane yield sidewalls 90° to the wafer surface and a minimal hard mask undercut resulting from KOH anisotropic etching. B) Trench profiles parallel to a {111} results in a 35.3° angled sidewall profile, with minimal hard mask undercut. C) Trench profiles which are perpendicular alignment to < 100 > direction results in 54.7° sloped sidewall and undercut of the etch mask D) Trench profiles which are parallel to a {110} plane straight sidewalls with mask undercut.
Fig. 2. A) SEM image of a circular opening in Si3N4 hard mask on (110) Si that was KOH etched forming a self-stopping hexahedron cavity bound by 6 {111} crystal planes. B) SEM cross section of a (1̅11̅) crystal plane normal to the (110) wafer surface. C) SEM cross section shows the 35.3° intersection of the (111̅) and (1̅1̅1̅) crystal planes with the surface of the (110) wafer, perpendicular (11̅1̅) and (1̅11̅) and {100} plane. D) SEM cross section shows all relevant crystal planes forming the boundaries of the self-stopping KOH etch.
etched small circular openings in the (110) wafer surface [10]. Although anisotropic KOH etching is not new, in this work we clarify ambiguity, poorly communicated, and misinterpreted nomenclature used in silicon micromachining literature and common practice. Much literature exists on these topics; our approach is aimed at clarifying nomenclature used in silicon crystallography and cited in KOH etch literature with a simple pre-etch that enables unambiguous identification of specific crystalline planes. The ability to precisely align to specific crystalline planes is essential to achieve unprecedented aspect ratios with aqueous KOH etch. For instance, most literature quotes the use of a (110) wafer referring to a specific crystal plane when instead {110} should be used referring to an equivalent family of crystalline planes. We’ve applied a simple pre-etch to clearly identify crystalline orientation. We used this technique on two {110} Si wafers with major flats of different orientations to evaluate the crystallographic orientation of each. Low pressure chemical vapor deposition (LPCVD) Si3N4 films of 300 nm were grown on each wafer and patterned with small circular openings. AZ4330 photoresist was spin cast onto each wafer at
Aqueously etched trenches in silicon yield different etch profiles. The orientation of the etch mask on the surface of the wafer and the crystalline orientation of the wafer can produce predictable etch profiles, resulting in trenches with vertical or sloped sidewalls, illustrated in Fig. 1 [10,11]. XPCI gratings target a vertical sidewall, 90° to the wafer surface. It has been reported that etch rates of a (110) plane are twice that of a (100) and up to 143 times faster than the etch rate of the {111} plane [12]. A deep trench etch is most effectively achieved by etching the (110) plane with the direction perpendicular to the surface of the sidewalls aligned in the < 111 > direction. This results in a rapid, selective etch in the vertical (110) direction and a very slow etch in the lateral direction, bound by {111} planes. For this reason, we used (110) silicon wafers and align our grating features to the {111} planes in pursuit of high aspect ratio gratings. Achieving a small period, high aspect ratio analyzer grating with an anisotropic aqueous etch requires precise alignment to a specific plane of the Si wafer. To identify the crystalline orientation in our wafers, we used a method described by Bassous in the late 1970s, where KOH 2
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removed in an O2 plasma asher. The grating pattern was then etched in a 6 M KOH concentration at 75 °C for 1 h, which resulted in 76 µm deep trenches. However, aligning to the cleaved edge of the crystal plane resulted in poor alignment accuracy. This misalignment led to limited areal uniformity and lack of dimensional control, producing a grating with approximately 0.8 µm Si fins and approximately 2.4 µm gaps. The tall Si fins were stuck together in places, and the Si3N4 hard mask delaminated due to stresses caused by fin distortion. The fin distortion was likely due to the extremely thin and long Si fins and surface tension forces of the aqueous etch and rinse methods that were followed by an N2 blow dry, as seen in Fig. 4. This can be overcome by super critical drying of the wafer. The alignment process along a cleaved edge produced a promising result, but a more accurate alignment was needed for better results. It is likely the cleave did not create a break perfectly along a single {111} plane and produced an edge that was not straight. Furthermore, misalignment of the mask using this contact alignment method is inevitable because it is difficult to see a cleaved edge and align it to a grating pattern through the MA6 objectives. To achieve a more accurate and repeatable crystallographic alignment, a preliminary layer 0 etch was performed on the wafer prior to aligning and patterning the gratings. Building upon a method developed by Vangbo et al. [15], a series of stepped rotationally arrayed alignment forks at 0.1°, from + 3° to − 3° was produced. These forks allow us to accurately align to a crystalline plane by selecting the set of forks that has the most symmetrical etch pattern. In addition, a series of parallel lines arrayed at 0.1° in a fan pattern were included to demonstrate the ability to identify the (111) plane. A layer 0 mask was produced and was used to pattern only the forked comb arrays and the series of parallel lines in a fanned array. The second mask, layer 1, is aligned to the forked comb structures and the parallel lines. A (110) Si wafer was processed through LPCVD, lithography, and ICP etch as others before described above. Once the layer 0 was patterned, the Si3N4 was etched and photoresist removed, the wafer was etched for 25 min in a 6 M KOH concentration at 75 °C. This revealed an obvious pattern within the fork structures. The most symmetrically etched fork displays the most accurate theta adjustment to align to the (111) plane, and allowed very accurate alignment to a crystalline plane, shown in Fig. 5. Once a proper rotational correction was identified by the most symmetrically etched combed fork structure, a corresponding alignment mark on the subsequent layer allowed for accurate alignment. We observed cross-sections of the series of parallel lines with an 8 µm pitch with a 50% duty cycle aligned properly parallel to the {111} plane, with an un known error. The results showed good straight sidewalls and a depth of 60 µm. The Si gratings showed no distortion; however, they
3000 rpm, and exposed using an MA6 contact aligner with a 160 mJ/cm broadband dose and developed in AZ400K 1:4 developer for 90 s. The LPCVD Si3N4 hard mask was etched using a CF4/Ar based ICP etch. The resist was stripped in a Tepla barrel asher using an O2 plasma at 600 W, 600 sccm O2 at 1 mT for 15 min. The wafers were then etched for 5 min in a 6 M KOH solution at 75 °C. Fig. 2 shows how the KOH etch undercut the nitride hard mask and formed a self-stopping hexahedron cavity bound by six {111} crystal planes. A V-groove was formed in the bottom of the cavity identifying the (111̅) and (1̅1̅1̅) planes which intersect the (110) wafer surface at 35.3° and meet each other at 109.5°. The (11̅1̅) and (1̅11̅) planes are normal to the (110) wafer surface and meet each other at 109.5° [13]. To achieve straight sidewalls and deep etched trenches, it is imperative to accurately align the sidewalls of the diffraction gratings parallel to the (11̅1̅) or (1̅11̅) planes that are perpendicular to the (110) surface. By using the etched cavity method, the aspect of the {111} planes were identified for each wafer in relationship to the major flat. One wafer had a flat in the < 111 > direction and the other wafer's flat was in the < 110 > direction. Plane direction signified by carrots, < xyz > , is defined as the direction perpendicular to any crystalline plane signified by parenthesis, (xyz). As shown in Fig. 3, {111} plane abcd and defa, intersected the (110) surface of each wafer at a 35.4° angle, while the other four {111} planes ab, bc, de and ef were perpendicular to the wafers (110) surface [14]. 2.2. Accurate crystallographic alignment The major flat of most Si wafers is aligned to the crystalline plane with ± 2° accuracy. The etch mask must be aligned within 0.1° of a crystal plane to maintain critical dimension control of the final etched grating without undercutting the etch mask [2,8–10,15]. We used a 1X contact photolithography mask with 2 cm2 diffraction gratings with a 4 µm pitch and 50% duty cycle. A 300 nm LPCVD Si3N4 was grown on the wafers. Si3N4 etches approximately 100 times slower than Si in KOH. The wafer was coated with AZ5214 photoresist, at a thickness of 1.4 µm, using a ACS 200 plus track system. In first attempts of aligning to the crystal plane, the wafer was cleaved in the < 111 > direction. This created a straight edge to the wafer along a {111} crystalline plane. This straight edge of the wafer was then used to align our photolithography mask so that the pattern was parallel to this cleaved edge. An MA6 contact aligner was used for the alignment and exposure of the diffraction gratings mask with 46 mJ/cm2 broadband dose. The resist was then developed on the ACS 200 track using NMDW developer, creating the grating pattern. The Si3N4 was etched in a reactive ion etch (RIE) using CF4/Ar plasma chemistry. The photoresist was then
Fig. 3. KOH etched circular windows in (110) silicon wafer reveal a self-stopping cavity bound by six {111} crystal planes. This enables identification of the specific crystal orientation of the wafer. (111) planes abcd and adef, created a v-grooved etch with sidewalls at 35.3° with respect to the (110) surface. Other {111} planes intersect the surface at 90° at locations ab, bc, de and ef. High aspect ratio diffraction grating anisotropic etch in Si require precise alignment of feature walls parallel to either {111} planes ab, ed or fa, dc. A) (110) Si wafer with (111) planes ab and de 35.3° relative to the major flat. (111) planes fa and dc the major flat and the intersection of (111) planes abcd and defa are 57.4° to the major flat. B) (110) wafer with (111) planes ab and de are 70.6° relative to the major flat. (111) planes fa and dc are 144.7° to the major flat and the intersection of (111) planes abcd and defa are 90° to the major flat. C) Graphic depicting the crystalline orientation in relationship to the wafer flat. 3
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Fig. 4. A) Diagram of alignment method cleaving the wafer at 35° relative to the major flat and alignment of the diffraction grating parallel to that edge. B) ~ 0.8 µm Si gratings with ~ 2.4 µm gaps, 76 µ C) Close up of gratings with dimensions D) The fins are not structurally strong enough to support themselves; surface tension has distorted them from vertical and caused stiction. misalignment resulted in limited area/ lack of dimensional control. Si3N4 hard mask delaminating due to fin distortion.
Fig. 5. A) Optical image of comb structures arrayed at 0.1° increments from + 3° to − 3° with respect to the major flat. The individual fork that shows the best symmetry in the etched Si, relative to the pattern, identifies proper rotational correction for alignment B) SEM images of forked comb structures C) Comb structure with symmetric etch profile indicates alignment to crystalline plane.
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Fig. 6. A) Series of small gratings with 8 µm pitch and 50% duty cycle, arrayed at 0.1° from + 1° to − 1° relative to major flat B) Close-up of the arrayed gratings. C) Magnified SEM image after 90 mins etch time shows fin width is approximately 1.5 µm. D) Magnified SEM image of Si gratings after 60 mins KOH etch showing undercut of the Si3N4, with 2.27 µm wide Si gratings E) Cross-sectional SEM image after 90 min of etch time reveals an etch depth of 90 µm.
by the ability to achieve near perfect alignment and the lateral etch rates, which will cause narrower Si structures than designed. We observed approximately 80:1 vertical to lateral etch and no surface tension effects causing fin distortion. We can compensate for the lateral etch by biasing the pattern to achieve the desired pitch and feature dimensions. Based on the vertical to lateral etch rate, biasing the mask is limited by pitch and duty cycle. The pitch remains uniform and the duty cycle changes based on the 80:1 etch ratio. For example, if a desired grating is to be a 12 µm pitch and 25% duty cycle, a grating feature patterned at 10 µm wide, with a gap width of 2 µm, could theoretically be etched to a depth of 280 µm. This would result in a Si grating line width of 3 µm with a gap width of 9 µm, if the hypothesis holds true. There was some non-uniformity of the etch depth, shown in Fig. 7. This may be caused by hydrogen bubbles forming in the bottoms of the trenches, inhibiting the chemistry to effectively reach the bottom [15–17]. This can be overcome by adjusting process temperatures, adding surfactants to the chemistry and/or constant agitation or sonication during the etch. One concern is that agitation or sonication introduces a potential force that could damage the fragile silicon structures. The use of the combed forks for alignment enabled more accurate alignment and produced good results. Repeatability on subsequent
did have a small undercut of the Si3N4 etch mask. The same wafer was etched 30 min longer with a constant etch rate of 1 µm/minute. The depth after a total of 90 min was 90 µm with about a 1.5 µm undercut to both sides of the Si3N4 mask, Fig. 6. 3. Results and discussion Another wafer was prepared with the Layer 0 etch mask and etched in KOH. The most symmetrically formed comb structure was identified, indicating correct rotational adjustment needed for alignment of the second mask. The second mask containing an array of 2 cm2 gratings was aligned and patterned into the Si3N4, and etched in KOH for a total of one hour. The etch rate was faster than before which can be explained by the growth of native oxides in between each subsequent etch of the first wafer. Each time the wafer was taken out of the KOH chemistry it grew a native oxide. Oxide has a lower etch rate than Si in KOH, slowing the etch rate down each time the Si oxidize. The wafer that was etched for an uninterrupted hour was approximately 159 µm deep, which is nearly 1.8 µm/minute, most importantly resulting in an impressive 80:1 aspect ratio feature. The resulting Si gratings were about 2 µm wide, showing an undercut of the Si3N4 etch mask around 1 µm on each side of the mask. Ultimately, high AR gratings are limited
Fig. 7. A) Cross-sectional SEM image of KOH etched diffraction grating 159 µm deep in 90 min, resulting in an impressive 80:1 aspect ratio feature. B) SEM image showing straight side wall profiles, uniform 8 µm pitch, Si fin width near 2 µm and Si3N4 hard mask undercut approximately 1 µm. 5
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subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
wafers was difficult and did not perform as well as the etch described above. The comb structures work well for quantifying the misalignment of the crystalline plane relative to the flat, but they are not ideal for achieving accurate alignment of subsequent layers. Future development will include an alignment method coined as a Hexahedron alignment, by James et al. [16]. The self-stopping nature of these cavities clearly defines the directions of the {111} planes, as demonstrated above. For alignment, the pre-etched cavity {111} side walls are aligned by overlaying corresponding hexahedron pattern on a lithography mask. This alignment method simplifies alignment by eliminating the subjective nature of selecting a combed fork that looks most symmetrical. In combination with combed fork structures we will be able to identify a rotational offset and align directly to {111} planes defined in the preetch.
Acknowledgments Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. References [1] C. David, J. Bruder, T. Rohbeck, C. Gru¨nzweig, C. Kottler, A. Diaz, O. Bunk, F. Pfeiffer, Fabrication of diffraction gratings for hard X-ray phase contrast imaging, Microelectron. Eng. 84 (2007) 1172–1177. [2] T. Donath, F. Pfeiffer, O. Bunk, W. Groot, M. Bednarzik, C. Grünzweig, E. Hempel, S. Popescu, M. Hoheisel, C. David, Phase-contrast imaging and tomography at 60 keV using a conventional x-ray tube Source, Rev. Sci. Instrum. 80 (2009) 053701. [3] A.E. Hollowell, C.L. Arrington, J.J. Coleman, P.S. Finnegan, A.M. Rowen, A.L. Dagel, Extensively long high aspect ratio gold analyzer gratings, X-ray Neutron Phase Imaging Gratings (2015). [4] J. Mohr, T. Grund, D. Kunka, J. Kenntner, J. Leuthold, and J. Meiser, High Aspect Ratio Gratings for X-Ray Phase Contrast Imaging. in: Proceedings of the International Workshop on X-Ray and Neutron Phase Imaging with Gratings 1466 (2012) pp. 41–50. [5] J.H. Han, S. Radhakrishnan, C. Lee, A novel batch-processing method for accurate crystallographic axis alignment, J. Micromech. Microeng. 23 (2013) 055017 (7pp). [6] L. Romano, J. Vila-Comamala, M. Kagias, K. Vogelsang, H. Schift, M. Stampanoni, K. Jefimovs, High aspect ratio metal micrcasting by hot emboxxing for X-ray optics fabrication, Microelectron. Eng. 176 (2017) 6–10. [7] J. Vila-Comamala, L. Romano, V. Guzenko, M. Kagiasa, M. Stampanoni, K. Jefimovs, Towards sub-micrometer high aspect ratio X-ray gratings by atomic layer deposition of iridium, Microelectron. Eng. 192 (2018) 19–24. [8] Y. Wang, Z. Liu, Y. Zheng, K. Qiu, Y. Hong, High-accuracy alignment of the grating pattern along silicon 〈1 1 2〉 directions using a short rectangular array, J. Micromech. Microeng. 27 (2017) 065008 (8pp). [9] M. Ahn, R.K. Heilmann, M.L. Schattenburg, Fabrication of 200 nm period blazed transmission gratings on silicon-on-insulator wafers, JVSTB 26 (2008) 2179–2182. [10] E. Bassous, Fabrication of novel three-Dimentional microstructures by the anisotropic etching of (100) and (110) silicon, E, IEEE Trasactions Electron Devices 25 (10) (1978) 1178–1185. [11] K. Sato, Basic 2 Anisotropic Wet-etching of Silicon: Characterization and Modeling of Changeable Anisotropy, Dept. of Micro/Nano Systems Engineering, Nagoya University. 〈http://gcoe.mech.nagoya-u.ac.jp/basic/pdf/basic-02.pdf〉. [12] Mitsuhiro Shikida, Kazuo Sato, Kenji Tokoro, Daisuke Uchikawa, Differences in anisotropic etching properties of KOH and TMAH solutions, Department of Micro System Engineering, Nagoya UniÍersity, Chikusa, Nagoya 464-8603, Japan, 1999 (19 July). [13] D.L. Kendall, Vertical etching of silicon at very high aspect ratios, Annu. Rev. Mater. Sci. 9 (1979) 373–403. [14] D.R. Ciarlo, A latching accelerometer fabricated by the anisotropic etching of (110) oriented silicon wafers, J. Micromech. Microeng. 2 (1992) 10–13. [15] M. Vangbo, Y. Backlund, Precise mask alignment to the crystallographic orientation of silicon wafers using wet anisotropic etching, J. Micromech. Microeng. 6 (1996) 279–284. [16] (a) T.D. James, G. Parish, K.J. Winchester, C.A. Musca, A crystallographic alignment method in silicon for deep, long microchannel Fabrication, J. Micromech. Microeng. 16 (2006) 2177–2182; (b) Y. Zheng, K. Qiu, H. Chen, Y. Chen, Z. Liu, Y. Liu, X. Xu, Y. Hong, Alignment method combining interference lithography with anisotropic wet etch technique for fabrication of high aspect ratio silicon gratings, Opt. Express 22 (19) (2014) 23592–23604 (15). [17] J.H. Han, S. Radhakrishnan, C. Lee, A novel batch-processing method for accurate crystallographic axis alignment, J. Micromech. Microeng. 23 (2013) 055017 (7pp).
4. Conclusions XPCI needs high aspect ratio gratings to allow higher energy x-ray sources for nondestructive imaging of denser materials or larger samples. To fabricate high aspect ratio gratings for XPCI applications using KOH etched (110) Si wafers, it is necessary to understand the crystalline orientation of the wafer to be etched, and align trenches parallel to {111} crystal planes, which are normal to the (110) wafer surface. The preferential etch of the (110) plane to the slow etch rate of {111} planes have been used to demonstrate a method to achieve deep anisotropic KOH Si etched trenches. The small period, high aspect ratio gratings fabricated using anisotropic KOH etches can yield the large area required in XPCI systems. Crystalline orientation of (110) wafers was determined by performing a KOH etch though circular holes, which formed self-terminating hexahedron cavities bound by six {111} planes. Alignment methods were explored using the straight cleaved edge of a (110) wafer broken along an appropriate {111} plane and aligning gratings parallel to the cleaved edge. A more precise alignment method whereby trenches were aligned to specific crystalline planes by using a combed fork structure etched into the wafer was demonstrated. This method was effective at identifying the correct theta adjustment for accurate alignment to the {111} plane, but was difficult to execute a repeatable result given the inherent alignment error of contact lithography. The potential of moving toward stepper lithography would reduce this error and provide better alignment accuracy. The manipulation of preferential etch rates of Si crystalline planes was used to achieve deep silicon etches and produced gratings 159 µm deep, resulting in an impressive 80:1 aspect ratio feature. We continue to improve upon alignment methods with better repeatability, including implementation of a hexahedron alignment. Various etch chemistries, process temperatures, and the use of surfactants may improve grating quality to achieve improved aspect ratios, sidewall surface roughness and uniformity. Finally, mask feature biasing, in combination with the known vertical to lateral etch rates, can be manipulated to ultimately reach critical dimensions in a final etched grating. 5. Disclaimer This paper describes objective technical results and analysis. Any
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