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Wet anisotropic etching characteristics of Si{111} in NaOH-based solution for silicon bulk micromachining

Wet anisotropic etching characteristics of Si{111} in NaOH-based solution for silicon bulk... Silicon bulk micromachining is extensively employed method in microelectromechanical systems (MEMS) for the for- mation of freestanding (e.g., cantilevers) and fixed (e.g., cavities) microstructures. Wet anisotropic etching is a popular technique to perform silicon micromachining as it is low-cost, scalable, and suitable for large scale batch processing, which are the major factors considered in the industry to reduce the cost of the product. In this work, we report the wet anisotropic etching characteristics of Si{111} in sodium hydroxide (NaOH) without and with addition of hydroxy- lamine (NH OH). 10M NaOH and 12% NH OH are used for this study. The effect of NH OH is investigated on the etch 2 2 2 rate, etched surface roughness and morphology, and the undercutting at mask edges aligned along < 112 > direction. These are the major etching characteristics, which should be studied in a wet anisotropic etchant. A 3D laser scan- ning microscope is utilized to measure the surface roughness, etch depth, and undercutting length, while the etched surface morphology is examined using a scanning electron microscope (SEM). The incorporation of NH OH in NaOH significantly enhances the etch rate and the undercutting at the mask edges that do not consist of {111} planes. To fabricate freestanding structure (e.g., microcantilever) on Si{111} wafer, high undercutting at < 112 > mask edges is desirable to reduce the release time. Moreover, the effect of etchant age on the abovementioned etching character - istics are investigated. The etch rate and undercutting reduce significantly with the age of the modified NaOH. The present paper reports very interesting results for the applications in wet bulk micromachining of Si{111}. Keywords: Silicon, Si{111}, Anisotropic etching, MEMS, NaOH, NH OH Introduction [7–12]. The Si{111} wafers can also be used to control Wet anisotropic etching is a fundamental process for the gap between the freestanding structure and the bot- the fabrication of variety of components in the field of tom surface [10, 11]. Silicon wet anisotropic etching microelectromechanical systems (MEMS) [1–5]. Many is usually carried out in alkaline solution, which can be kinds of MEMS components (e.g., cantilever, cavity, dia- either an organic (e.g., tetramethylammonium hydrox- phragm, etc.) are fabricated through wet anisotropic ide (TMAH)) or an inorganic compound (e.g., potas- etching-based silicon bulk micromachining of {100}, sium hydroxide (KOH), and sodium hydroxide (NaOH)) {110} and {111} oriented silicon wafers for different appli - [15–32]. TMAH and KOH are comprehensively studied cations [1–14]. In addition, wet anisotropic etching of alkaline solutions for silicon wet anisotropic etching. Si{111} is utilized to fabricate complex structures using However, limited number of studies have been reported deep reactive ion etching (DRIE) assisted wet etching for other kinds of alkaline solutions such as NaOH, eth- ylenediamine pyrocatechol water (EDP), cesium hydrox- ide (CsOH), ammonium hydroxide (NH OH), Hydrazine, *Correspondence: prem@iith.ac.in etc. [25–36]. Recently, we reported the etching character- Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana, istics of Si{100} and Si{110} in NaOH without and with India © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Purohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 2 of 10 addition NH OH. A significant improvement in etch - etching temperature, agitation during etching, and the ing characteristics is observed when NH OH is added presence of an additive in the etchant. Lately, the influ - into NaOH solution [31, 32]. Hence, it is very important ence of hydroxylamine (N H OH) on the etching proper- to investigate the etching characteristics of Si{111} in ties of KOH and TMAH is reported [37–43]. NaOH-based solution for application in MEMS. In this paper, we present the wet anisotropic etch- In all kinds of alkaline solutions, Si{111} planes are ing characteristics of Si{111} in NaOH-based solution. the slowest etch rate planes. Hence, the mask edges NaOH solution is modified by addition of NH OH to aligned along a crystallographic direction comprising alter the etching characteristics. The main objective of {111} planes exhibit least undercutting. When an arbi- the present work is to enhance the undercutting rate at trary mask opening is etched in alkaline solution, severe the mask edges aligned along < 112 > direction for the fast undercutting takes place at the edges that do not contain release of the microstructures such as microcantilever. {111} planes. The undercutting continues till it encoun - Moreover, the effect of etchant age on the etching char - ters {111} planes. To fabricate an etched profile of con - acteristics is methodically investigated. trolled dimensions, the edges of the mask patterns must be aligned along the directions comprising {111} planes (e.g., < 110 > directions on Si{100} and Si{111} sur- Experimental details faces, < 112 > and < 110 > directions on Si{110} surface). As In this work, Cz-grown p-type doped Si{111} one side stated in previous paragraph, Si{111} is an important ori- polished four-inch wafers with a resistivity of 1–10 Ωcm entation to fabricate complicated microstructures using are used for the investigation of etching characteris- wet anisotropic etching or DRIE assisted wet anisotropic tics. An oxide layer of 1  µm thickness deposited using etching [10, 11]. Stereographic projection, which is sche- thermal oxidation method is employed as mask and/or matically presented in Fig.  1, provides very important structural layer. This oxide layer is patterned using pho - information about the crystallographic planes on wafer tolithography in which positive photoresist (AZ1512HS) surface that appear at specific crystallographic directions. is used as mask for selective etching of oxide layer in Moreover, it is very useful to know the angles between buffered hydrofluoric acid (BHF). A thorough rinse in different planes and directions. In Fig.  1, solid blue circle DI water is performed once the photoresist is removed represents the {111} planes projected from the top hemi- using acetone. Subsequently, the wafer is diced into small sphere, while open blue circle indicates the {111} planes samples. These samples are cleaned in a piranha bath projected from the bottom hemispheres. {111} planes (H SO :H O ::1:1) followed by a DI water rinse. A thin 2 4 2 2 denoted by solid blue circle make an angle of 109.5º with layer of oxide is chemically deposited in piranha bath wafer surface plane and 60º with each other, however during cleaning process that delays silicon etching pro- the {111} planes depicted by open blue circle form an cess. The cleaned samples are therefore dipped in 1% HF angle of 70.5º with the top surface of the wafer and 60º for 1 min to remove chemically grown oxide layer. There - with each other as illustrated in Fig.  1. The < 110 > cr ys - after, the samples are thoroughly rinsed in DI water. Now tallographic directions on Si{111} surface, as shown in the etching is performed in pure and 12% N H OH-added Fig. 1, comprise {111} planes that emerge during wet ani- 10M sodium hydroxide (NaOH) at 70 ± 1 °C temperature. sotropic etching process. Various types of shapes formed 10M of NaOH is selected because this concentration pro- by < 110 > cr ystallographic directions at which {111} vides the high etch rate [29]. The etching experiments planes appear during wet anisotropic etching process are are conducted in 1-L etchant solution. To prepare a 1-L described in Fig.  2. These shapes are schematically pre - solution of 10M NaOH, 400  g of pellets are dissolved sented on Si{111} wafer surface in Fig.  3a. Sidewalls of in 1000  ml deionized (DI) water. In the case of 1-L 12% the etched profiles are created by {111} planes when these NH OH-added 10M NaOH, 400-gm NaOH pellets, shapes are etched in an anisotropic etchant as shown in 240 ml of 50% N H OH and 760 ml DI water are used. To Fig.  3b. The cross-sectional views of the etched profiles keep the temperature constant during the etching experi- are exhibited in Fig. 3c. ment, a constant temperature water bath is used. An In the fabrication of microstructures, etch rate is one etching container made of Teflon is used for the process. of the most important characteristics as it influences The continuous heating of the etchant during etching the fabrication time, which eventually affects the over - process changes the concentration of the etchant due to all production cost. High etch rate is advantageous for the evaporation of water. Henceforth, a reflex condenser the higher industrial throughput. The salient factors made of thick glass equipped with a double-layered nar- that control the etching characteristics are etching time, row opening is used. A 3D measuring laser microscope P urohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 3 of 10 Fig. 1 Schematic representation of stereographic projection of {111} silicon: a (111) plane inside a unit cell, b unit cell at the center of a sphere which is used to project different crystallographic planes on 2D surface, and c stereographic projection exhibiting different planes. The {111} planes projected from the top and bottom hemispheres are shown by solid and open blue circles, respectively. Three {111} planes shown by blue color dots make an angle of 109.5º to the silicon wafer surface plane and 60º with each other, while another three {111} planes indicated by open blue circle form an angle of 70.5º to the surface plane and 60º to each other Purohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 4 of 10 Fig. 2 Different shapes formed by < 110 > crystallographic directions at which {111} planes appear during wet anisotropic etching process such as hexagonal, rhombus, triangle, isosceles trapezoid (OLYMPUS OLS4000) is used to measure the etch depth, samples. The etched surface morphology is characterized undercutting length and surface roughness. The meas - using scanning electron microscope (SEM). urements are carried out at different locations of the P urohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 5 of 10 Fig. 3 Schematic demonstration of the wet anisotropically etched profiles of various shapes mask geometries on Si{111} wafer: a mask patterns, b etched profiles after etching in wet anisotropic etchants, and c cross sectional view along different directions. Dashed lines in a show the directions where wet etching will terminate due to the appearance of {111} planes Results and discussion The etching characteristics (e.g., etch rate, surface rough - ness, and undercutting) of Si{111} in 10M NaOH without and with the addition of 12% NH OH are investigated. Etch rate and undercutting are determined by measuring the etch depth and lateral undercutting length, respec- tively, for different etching times. Surface roughness is measured at different locations of the same sample, and then its mean and standard deviation are calculated. The etched surface morphology is analyzed using a SEM. In the subsequent subsections, etching characteristics of Si{111} are systematically presented. To determine the impact of etchant aging on the etching characteristics, successive etching experiments are performed in the same solvent for next 15 days. Fig. 4 Etch rate of Si{111} in pure and NH OH-added NaOH at 70 °C Etch Rate The etch rate is one of the most important parameters to NH OH-added NaOH, etch depths are measured on be measured when the etching characteristics of an etch- Si{111} samples etched for different etching times. The ant are investigated. It is characterized as vertical etch results are presented in Fig. 4. The etch rate of Si{111} sig - depth per unit time as illustrated in Fig.  4. It is required nificantly increases when the NH OH is added in NaOH to estimate other parameters such as etching time for the solution, which is useful to reduce the etch time to form formation of different kinds of microstructures such as the microstructures, for instance, cavities. Several factors cavities/grooves. To calculate the etch rate in pure and Purohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 6 of 10 [1, 2]. When the NH OH is incorporated, the reactive − − species HO, OH , and NH O increases [44]. These 2 2 extra species might be produced from the decomposi- tion of NH OH as intermediate and final products in the presence of alkaline solutions [44–47]. Therefore, it is speculated that the etch rate in NH OH-added NaOH increases due to the presence of extra reactive species. To investigate the effect of etchant age on the etching characteristics, the same etchant solution is continu- ously used for the next 15 days. Silicon etching is carried out after every one day for the next five days. Thereafter, etching is performed after every five days. The impact of etchant age on the etch rate is presented in Fig.  5. It can easily be noticed that the etch rate is considerably reduced with etchant age during etching process. It is Fig. 5 Eec ff t of etchant age (12%NH OH + 10 M NaOH) on the etch speculated that the accessibility (or production) of extra rate reactive species may decline as the etchant age increases, may affect the etch rate in an etchant such as etchant that may lead to decrease in the etch rate [44]. concentration, etching temperature, loss of reactive spe- cies (HO, OH , etc.), the amount of substrate dissolved in the etchant, impurities/additives in the etchant, etc. Fig. 6 Average surface roughness and corresponding SEM micrographs of Si{111} etched in pure and NH OH-added NaOH 2 P urohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 7 of 10 Fig. 7 Eec ff t of etchant age (12%NH OH + 10M NaOH) on the etched surface roughness and morphology Etched surface morphology vary from one laboratory to another laboratory and Etched surface morphology is a major concern specifi - sample to sample in the same type of etchant under cally when the microstructures with uniform depth need similar etching parameters. To investigate the effect to be fabricated or the surface is used for optical applica- of etchant age, the same methodology, which is used tions. The etching parameters, which affect the etch rate, for etching rate study, is followed. Figure  7 exhibits also influence the etched surface roughness and mor - the consequence of etchant aging on the average sur- phology. It is basically a result of non-uniform removal face roughness. It can be seen from the results that the of silicon atoms from the surface during etching process. aging of the etchant deteriorates the etched surface It happens mainly due to the micro-masking effect [1, roughness. 2]. Surface contaminations and hydrogen bubbles pre- dominantly inhibit surface reactions and therefore act as micro-mask during etching process [1, 2, 48–51]. Undercutting at mask edges Figure  6 presents the average surface roughness (R ) Undercutting refers to the lateral etching that occurs and corresponding surface morphologies of Si{111} under the masking layer [52]. It has its own advantages etched in pure and N H OH-added NaOH. The meas - and disadvantages. It is a desirable feature for the fab- urement is obtained by scanning a 200 × 200  µm area rication of overhanging structures made up of materi- across different locations on the same sample using a als exhibiting high etch selectivity with silicon such 3D laser scanning microscope. It can be noticed from as SiO, Si N as schematically illustrated in Fig.  8. 2 3 4 the graph that the surface roughness is decreased when In this work, the undercutting rate is investigated at NH OH is added to NaOH. As the etched surface mor- the mask edges aligned < 112 > directions. The results phology depends on various factors, the results may are presented in Fig.  8. It can easily be noticed from Purohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 8 of 10 Fig. 8 Lateral undercutting rate at the mask edges aligned along < 112 > direction on Si{111} in pure and NH OH-added NaOH at 70 °C the experimental results that the undercutting rate demonstrate the application of modified etchant in sili - increases significantly with the addition of NH OH. con micromachining, freestanding cantilever beams are The primary reason behind the increase of undercut - fabricated, and the results are presented in Fig. 9. ting is the same as explained for etch rate in "Etch Rate" It is imperative to examine the effect of etchant aging section i.e., the reactive species increase on addition of on the undercutting process to understand the vari- NH OH that results in the increase of the undercutting ation in etching characteristics as the etchant ages. rate at mask edges aligned along < 112 > direction. To The same experimental procedure, which is employed Fig. 9 SEM images of the cantilever beams fabricated on Si{111} in NaOH + NH OH solution 2 P urohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 9 of 10 Acknowledgements We greatly acknowledge CSIR for financial support. Author contributions SP and VS did experiments and have made equal contributions. SP, VS and PP wrote the manuscript. AKP reviewed and edited the manuscript. All authors read and approved the final manuscript. Funding This work was supported by research grant from the Council of Scientific and Industrial Research (CSIR, Ref: 22(0824)/19/EMR-II,0527/NS). Availability of data and materials Not applicable. Declarations Ethics approval and consent to participate Not applicable. Fig. 10 Eec ff t of etchant age (12%NH OH + 10M NaOH) on the lateral undercutting rate Consent for publication All authors agreed to this publication. Competing interests The authors declare that they have no competing interests. for etch rate study in "Etch Rate" section, is followed to investigate the aging effect. The results are shown Received: 24 September 2022 Accepted: 14 November 2022 in Fig.  10. It can obviously be observed that the lat- eral undercutting is lessened with the age of the etch- ant. As discussed previously, the reactive species in References NH OH-added NaOH are reduced with etchant age, 1. Gad-el-Hak M (2002) The MEMS handbook. CRC Press LLC, Boca Raton which is the main reason behind the decrease in under- 2. Pal P, Sato K (2017) Silicon wet bulk micromachining for MEMS. Pan Stan- cutting rate with the age of the etchant. Although the ford Publishing, Singapore, p 412 3. Pal P, Sato K (2010) Fabrication methods based on wet etching process undercutting rate decreases with time, it is still higher for the realization of silicon MEMS structures with new shapes. Microsyst than that in pure NaOH. 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Wet anisotropic etching characteristics of Si{111} in NaOH-based solution for silicon bulk micromachining

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DOI
10.1186/s40486-022-00162-7
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Abstract

Silicon bulk micromachining is extensively employed method in microelectromechanical systems (MEMS) for the for- mation of freestanding (e.g., cantilevers) and fixed (e.g., cavities) microstructures. Wet anisotropic etching is a popular technique to perform silicon micromachining as it is low-cost, scalable, and suitable for large scale batch processing, which are the major factors considered in the industry to reduce the cost of the product. In this work, we report the wet anisotropic etching characteristics of Si{111} in sodium hydroxide (NaOH) without and with addition of hydroxy- lamine (NH OH). 10M NaOH and 12% NH OH are used for this study. The effect of NH OH is investigated on the etch 2 2 2 rate, etched surface roughness and morphology, and the undercutting at mask edges aligned along < 112 > direction. These are the major etching characteristics, which should be studied in a wet anisotropic etchant. A 3D laser scan- ning microscope is utilized to measure the surface roughness, etch depth, and undercutting length, while the etched surface morphology is examined using a scanning electron microscope (SEM). The incorporation of NH OH in NaOH significantly enhances the etch rate and the undercutting at the mask edges that do not consist of {111} planes. To fabricate freestanding structure (e.g., microcantilever) on Si{111} wafer, high undercutting at < 112 > mask edges is desirable to reduce the release time. Moreover, the effect of etchant age on the abovementioned etching character - istics are investigated. The etch rate and undercutting reduce significantly with the age of the modified NaOH. The present paper reports very interesting results for the applications in wet bulk micromachining of Si{111}. Keywords: Silicon, Si{111}, Anisotropic etching, MEMS, NaOH, NH OH Introduction [7–12]. The Si{111} wafers can also be used to control Wet anisotropic etching is a fundamental process for the gap between the freestanding structure and the bot- the fabrication of variety of components in the field of tom surface [10, 11]. Silicon wet anisotropic etching microelectromechanical systems (MEMS) [1–5]. Many is usually carried out in alkaline solution, which can be kinds of MEMS components (e.g., cantilever, cavity, dia- either an organic (e.g., tetramethylammonium hydrox- phragm, etc.) are fabricated through wet anisotropic ide (TMAH)) or an inorganic compound (e.g., potas- etching-based silicon bulk micromachining of {100}, sium hydroxide (KOH), and sodium hydroxide (NaOH)) {110} and {111} oriented silicon wafers for different appli - [15–32]. TMAH and KOH are comprehensively studied cations [1–14]. In addition, wet anisotropic etching of alkaline solutions for silicon wet anisotropic etching. Si{111} is utilized to fabricate complex structures using However, limited number of studies have been reported deep reactive ion etching (DRIE) assisted wet etching for other kinds of alkaline solutions such as NaOH, eth- ylenediamine pyrocatechol water (EDP), cesium hydrox- ide (CsOH), ammonium hydroxide (NH OH), Hydrazine, *Correspondence: prem@iith.ac.in etc. [25–36]. Recently, we reported the etching character- Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana, istics of Si{100} and Si{110} in NaOH without and with India © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Purohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 2 of 10 addition NH OH. A significant improvement in etch - etching temperature, agitation during etching, and the ing characteristics is observed when NH OH is added presence of an additive in the etchant. Lately, the influ - into NaOH solution [31, 32]. Hence, it is very important ence of hydroxylamine (N H OH) on the etching proper- to investigate the etching characteristics of Si{111} in ties of KOH and TMAH is reported [37–43]. NaOH-based solution for application in MEMS. In this paper, we present the wet anisotropic etch- In all kinds of alkaline solutions, Si{111} planes are ing characteristics of Si{111} in NaOH-based solution. the slowest etch rate planes. Hence, the mask edges NaOH solution is modified by addition of NH OH to aligned along a crystallographic direction comprising alter the etching characteristics. The main objective of {111} planes exhibit least undercutting. When an arbi- the present work is to enhance the undercutting rate at trary mask opening is etched in alkaline solution, severe the mask edges aligned along < 112 > direction for the fast undercutting takes place at the edges that do not contain release of the microstructures such as microcantilever. {111} planes. The undercutting continues till it encoun - Moreover, the effect of etchant age on the etching char - ters {111} planes. To fabricate an etched profile of con - acteristics is methodically investigated. trolled dimensions, the edges of the mask patterns must be aligned along the directions comprising {111} planes (e.g., < 110 > directions on Si{100} and Si{111} sur- Experimental details faces, < 112 > and < 110 > directions on Si{110} surface). As In this work, Cz-grown p-type doped Si{111} one side stated in previous paragraph, Si{111} is an important ori- polished four-inch wafers with a resistivity of 1–10 Ωcm entation to fabricate complicated microstructures using are used for the investigation of etching characteris- wet anisotropic etching or DRIE assisted wet anisotropic tics. An oxide layer of 1  µm thickness deposited using etching [10, 11]. Stereographic projection, which is sche- thermal oxidation method is employed as mask and/or matically presented in Fig.  1, provides very important structural layer. This oxide layer is patterned using pho - information about the crystallographic planes on wafer tolithography in which positive photoresist (AZ1512HS) surface that appear at specific crystallographic directions. is used as mask for selective etching of oxide layer in Moreover, it is very useful to know the angles between buffered hydrofluoric acid (BHF). A thorough rinse in different planes and directions. In Fig.  1, solid blue circle DI water is performed once the photoresist is removed represents the {111} planes projected from the top hemi- using acetone. Subsequently, the wafer is diced into small sphere, while open blue circle indicates the {111} planes samples. These samples are cleaned in a piranha bath projected from the bottom hemispheres. {111} planes (H SO :H O ::1:1) followed by a DI water rinse. A thin 2 4 2 2 denoted by solid blue circle make an angle of 109.5º with layer of oxide is chemically deposited in piranha bath wafer surface plane and 60º with each other, however during cleaning process that delays silicon etching pro- the {111} planes depicted by open blue circle form an cess. The cleaned samples are therefore dipped in 1% HF angle of 70.5º with the top surface of the wafer and 60º for 1 min to remove chemically grown oxide layer. There - with each other as illustrated in Fig.  1. The < 110 > cr ys - after, the samples are thoroughly rinsed in DI water. Now tallographic directions on Si{111} surface, as shown in the etching is performed in pure and 12% N H OH-added Fig. 1, comprise {111} planes that emerge during wet ani- 10M sodium hydroxide (NaOH) at 70 ± 1 °C temperature. sotropic etching process. Various types of shapes formed 10M of NaOH is selected because this concentration pro- by < 110 > cr ystallographic directions at which {111} vides the high etch rate [29]. The etching experiments planes appear during wet anisotropic etching process are are conducted in 1-L etchant solution. To prepare a 1-L described in Fig.  2. These shapes are schematically pre - solution of 10M NaOH, 400  g of pellets are dissolved sented on Si{111} wafer surface in Fig.  3a. Sidewalls of in 1000  ml deionized (DI) water. In the case of 1-L 12% the etched profiles are created by {111} planes when these NH OH-added 10M NaOH, 400-gm NaOH pellets, shapes are etched in an anisotropic etchant as shown in 240 ml of 50% N H OH and 760 ml DI water are used. To Fig.  3b. The cross-sectional views of the etched profiles keep the temperature constant during the etching experi- are exhibited in Fig. 3c. ment, a constant temperature water bath is used. An In the fabrication of microstructures, etch rate is one etching container made of Teflon is used for the process. of the most important characteristics as it influences The continuous heating of the etchant during etching the fabrication time, which eventually affects the over - process changes the concentration of the etchant due to all production cost. High etch rate is advantageous for the evaporation of water. Henceforth, a reflex condenser the higher industrial throughput. The salient factors made of thick glass equipped with a double-layered nar- that control the etching characteristics are etching time, row opening is used. A 3D measuring laser microscope P urohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 3 of 10 Fig. 1 Schematic representation of stereographic projection of {111} silicon: a (111) plane inside a unit cell, b unit cell at the center of a sphere which is used to project different crystallographic planes on 2D surface, and c stereographic projection exhibiting different planes. The {111} planes projected from the top and bottom hemispheres are shown by solid and open blue circles, respectively. Three {111} planes shown by blue color dots make an angle of 109.5º to the silicon wafer surface plane and 60º with each other, while another three {111} planes indicated by open blue circle form an angle of 70.5º to the surface plane and 60º to each other Purohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 4 of 10 Fig. 2 Different shapes formed by < 110 > crystallographic directions at which {111} planes appear during wet anisotropic etching process such as hexagonal, rhombus, triangle, isosceles trapezoid (OLYMPUS OLS4000) is used to measure the etch depth, samples. The etched surface morphology is characterized undercutting length and surface roughness. The meas - using scanning electron microscope (SEM). urements are carried out at different locations of the P urohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 5 of 10 Fig. 3 Schematic demonstration of the wet anisotropically etched profiles of various shapes mask geometries on Si{111} wafer: a mask patterns, b etched profiles after etching in wet anisotropic etchants, and c cross sectional view along different directions. Dashed lines in a show the directions where wet etching will terminate due to the appearance of {111} planes Results and discussion The etching characteristics (e.g., etch rate, surface rough - ness, and undercutting) of Si{111} in 10M NaOH without and with the addition of 12% NH OH are investigated. Etch rate and undercutting are determined by measuring the etch depth and lateral undercutting length, respec- tively, for different etching times. Surface roughness is measured at different locations of the same sample, and then its mean and standard deviation are calculated. The etched surface morphology is analyzed using a SEM. In the subsequent subsections, etching characteristics of Si{111} are systematically presented. To determine the impact of etchant aging on the etching characteristics, successive etching experiments are performed in the same solvent for next 15 days. Fig. 4 Etch rate of Si{111} in pure and NH OH-added NaOH at 70 °C Etch Rate The etch rate is one of the most important parameters to NH OH-added NaOH, etch depths are measured on be measured when the etching characteristics of an etch- Si{111} samples etched for different etching times. The ant are investigated. It is characterized as vertical etch results are presented in Fig. 4. The etch rate of Si{111} sig - depth per unit time as illustrated in Fig.  4. It is required nificantly increases when the NH OH is added in NaOH to estimate other parameters such as etching time for the solution, which is useful to reduce the etch time to form formation of different kinds of microstructures such as the microstructures, for instance, cavities. Several factors cavities/grooves. To calculate the etch rate in pure and Purohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 6 of 10 [1, 2]. When the NH OH is incorporated, the reactive − − species HO, OH , and NH O increases [44]. These 2 2 extra species might be produced from the decomposi- tion of NH OH as intermediate and final products in the presence of alkaline solutions [44–47]. Therefore, it is speculated that the etch rate in NH OH-added NaOH increases due to the presence of extra reactive species. To investigate the effect of etchant age on the etching characteristics, the same etchant solution is continu- ously used for the next 15 days. Silicon etching is carried out after every one day for the next five days. Thereafter, etching is performed after every five days. The impact of etchant age on the etch rate is presented in Fig.  5. It can easily be noticed that the etch rate is considerably reduced with etchant age during etching process. It is Fig. 5 Eec ff t of etchant age (12%NH OH + 10 M NaOH) on the etch speculated that the accessibility (or production) of extra rate reactive species may decline as the etchant age increases, may affect the etch rate in an etchant such as etchant that may lead to decrease in the etch rate [44]. concentration, etching temperature, loss of reactive spe- cies (HO, OH , etc.), the amount of substrate dissolved in the etchant, impurities/additives in the etchant, etc. Fig. 6 Average surface roughness and corresponding SEM micrographs of Si{111} etched in pure and NH OH-added NaOH 2 P urohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 7 of 10 Fig. 7 Eec ff t of etchant age (12%NH OH + 10M NaOH) on the etched surface roughness and morphology Etched surface morphology vary from one laboratory to another laboratory and Etched surface morphology is a major concern specifi - sample to sample in the same type of etchant under cally when the microstructures with uniform depth need similar etching parameters. To investigate the effect to be fabricated or the surface is used for optical applica- of etchant age, the same methodology, which is used tions. The etching parameters, which affect the etch rate, for etching rate study, is followed. Figure  7 exhibits also influence the etched surface roughness and mor - the consequence of etchant aging on the average sur- phology. It is basically a result of non-uniform removal face roughness. It can be seen from the results that the of silicon atoms from the surface during etching process. aging of the etchant deteriorates the etched surface It happens mainly due to the micro-masking effect [1, roughness. 2]. Surface contaminations and hydrogen bubbles pre- dominantly inhibit surface reactions and therefore act as micro-mask during etching process [1, 2, 48–51]. Undercutting at mask edges Figure  6 presents the average surface roughness (R ) Undercutting refers to the lateral etching that occurs and corresponding surface morphologies of Si{111} under the masking layer [52]. It has its own advantages etched in pure and N H OH-added NaOH. The meas - and disadvantages. It is a desirable feature for the fab- urement is obtained by scanning a 200 × 200  µm area rication of overhanging structures made up of materi- across different locations on the same sample using a als exhibiting high etch selectivity with silicon such 3D laser scanning microscope. It can be noticed from as SiO, Si N as schematically illustrated in Fig.  8. 2 3 4 the graph that the surface roughness is decreased when In this work, the undercutting rate is investigated at NH OH is added to NaOH. As the etched surface mor- the mask edges aligned < 112 > directions. The results phology depends on various factors, the results may are presented in Fig.  8. It can easily be noticed from Purohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 8 of 10 Fig. 8 Lateral undercutting rate at the mask edges aligned along < 112 > direction on Si{111} in pure and NH OH-added NaOH at 70 °C the experimental results that the undercutting rate demonstrate the application of modified etchant in sili - increases significantly with the addition of NH OH. con micromachining, freestanding cantilever beams are The primary reason behind the increase of undercut - fabricated, and the results are presented in Fig. 9. ting is the same as explained for etch rate in "Etch Rate" It is imperative to examine the effect of etchant aging section i.e., the reactive species increase on addition of on the undercutting process to understand the vari- NH OH that results in the increase of the undercutting ation in etching characteristics as the etchant ages. rate at mask edges aligned along < 112 > direction. To The same experimental procedure, which is employed Fig. 9 SEM images of the cantilever beams fabricated on Si{111} in NaOH + NH OH solution 2 P urohit et al. Micro and Nano Systems Letters (2022) 10:21 Page 9 of 10 Acknowledgements We greatly acknowledge CSIR for financial support. Author contributions SP and VS did experiments and have made equal contributions. SP, VS and PP wrote the manuscript. AKP reviewed and edited the manuscript. All authors read and approved the final manuscript. Funding This work was supported by research grant from the Council of Scientific and Industrial Research (CSIR, Ref: 22(0824)/19/EMR-II,0527/NS). Availability of data and materials Not applicable. Declarations Ethics approval and consent to participate Not applicable. Fig. 10 Eec ff t of etchant age (12%NH OH + 10M NaOH) on the lateral undercutting rate Consent for publication All authors agreed to this publication. 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Journal

Micro and Nano Systems LettersSpringer Journals

Published: Dec 1, 2022

Keywords: Silicon; Si{111}; Anisotropic etching; MEMS; NaOH; NH2OH

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