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Strategies in deep wet etching of Pyrex glass
Sensors and Actuators A 133 (2007) 395–400
Strategies in deep wet etching of Pyrex glass Ciprian Iliescu a,∗ , Francis E.H. Tay a,b , Jianmin Miao c a
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos #04-01, Singapore 138669, Singapore Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore c School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b
Received 8 July 2005; received in revised form 10 January 2006; accepted 20 March 2006 Available online 18 July 2006
Abstract This paper establishes the strategies for deep wet etching of one of the most common glasses: Pyrex. There are two way for increasing the etch depth: increasing the etch rate or increasing the resistance of the mask in the etching solution. The paper analyzes the methods for increasing the glass etch rate in HF solutions: annealing, concentration, ultrasonic agitation and temperature. The generation of the defects is investigated. The main factors that affect the degradation of the mask are: type, value and gradient of the residual stress and the hydrophilicity of the surface. Cr/Au mask is used for illustration. A new method for deep wet etching of glass using Cr/Au mask and photoresist is established. The result of this method is the best thus far as reported in the literature: 85 min deep wet etching in HF 49% which is equivalent to etching of more than 500 m deep in the Pyrex glass material. © 2006 Elsevier B.V. All rights reserved. Keywords: Wet etching of glass; Residual stress; Masking layer
1. Introduction Glass is a widely used material in MEMS and bio-chip device fabrication. The use of glass is common in the packaging process, as die for thermal compensation, on two of the most commercialized MEMS devices: piezoresistive pressure sensors and accelerometers. In these applications, glass is used due to its good bondability to silicon and also to the similar coefficient of expansion with silicon. The good isolation property makes glass a very good material for RF-MEMS applications. Due to its optical properties, glass is also very well used in MOEMS devices. Bio-chips are fabricated on glass substrate due to its optical transparency, hydrophilicity, chemical stability and biocompatibility. A microflow cells for single molecule handling of DNA is presented in [1], Luginbuhl et al. reported a microinjector for DNA mass spectrometry in [2], a microPCR device for DNA amplification is described in [3] or a dielectrophoretic chip with 3D electrodes in a “sandwich” structure glass/silicon/glass is presented in [4]. Most of these applications require methods and technologies for glass micropatterning.
∗
Corresponding author. Tel.: +65 68247137; fax: +65 68749082. E-mail address: [email protected] (C. Iliescu).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.06.044
There are three major groups of technique used for glass etching: mechanical, dry and wet. Mechanical methods include traditional drilling, ultrasonic drilling, electrochemical discharge or powder blasting. However, smooth surfaces cannot be generated using such methods. Dry etching of glass had been reported in [5] using SF6 . However, the etching rate is relatively low. Wet etching remains the most common and low cost method. The etching solution is based on HF. The masking layer depends on the application and on the “thermal budget” of the fabrication process of the device. Photoresist is very often used as mask layer [6–8], but its area of application is limited. A very commonly used mask is Cr/Au [8,9], where Cr layer is used to improve the adhesion of gold to glass. Bu et al. [10] reported etching through 500 m thick glass wafer using a multilayer of metal, Cr/Au/Cr/Au, in combination with a thick SPR220-7 photoresist, by etching from both sides of the wafer. Another very commonly used mask material for glass etching is silicon, deposited by different methods: PECVD (amorphous silicon) [8,11], LPCVD (polysilicon) [8,12] or even bulk silicon [13]. The maximum reported depth was 320 m by Bien et al. [8] with a mask of polished polysilicon (1.5 m) and SU-8 (50 m) as etching mask. We establish in this paper the strategies for deep wet etching of glass. There are two main directions. First is to increase the
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etching rate. For this aim a maximum concentration of HF (in our case 49%) must be used. Second is the improvement of mask quality. This can be achieved by a low stress value in the masking layer, a multiple deposition (for Au layer) and by changing the hydrophilicity of the masking layer (keeping the photoresist mask). As a result a Pyrex glass wafer is etched through using a Cr/Au and photoresist mask. This is the best result reported in the literature. In addition, defects generation is discussed in this paper. 2. Selecting the glass By definition glass is a mixture of oxide. The composition of glass and the concentration of these oxides give the main properties of the glass material. As a result, there are a huge number of glasses available on the market, each with different properties, and due to the different structure the microfabrication techniques and especially the wet etching process can be different. We select for our study a well-known Pyrex glass: Corning7740. The main solution for wet etching of glass is based on HF. Some oxides such as CaO, MgO or Al2 O3 give insoluble products after reacting with HF. These insoluble products can act like a masking layer and reduce the etch rate or can increase the roughness of the generated surface. An example is shown in Fig. 1 where the variation depth etch during 1 h etch in HF 40% for three different glasses (Corning 7740, Hoya SD-2 and soda lime) is presented. As can be observed, only Corning 7740 has a constant etch rate, while the other two glasses have as upper limit of etch depth up to 250–300 m. The large amount of Al2 O3 (about 20%) in Hoya glasses and CaO (8.8%), MgO (4%) for soda lime glasses compared with only 2% Al2 O3 for Corning 7740 can explain the parabolic variation of depth with time for the first two glasses. We proposed in [14] a method to improve the wet etching process by adding HCl. The resultant insoluble products (CaF2 , MgF2 or AlF3 ), after the reaction with HCl, will generate soluble products. The amount of HCl must be optimized from case to case. For soda lime and Pyrex the optimal ratio is 10:1 HF (49%)/HCl (37%) [14].
Fig. 1. Variation of depth vs. time for three different glasses: Corning 7740, Hoya SD-2 and soda lime.
3. Experiments Our experiments were performed on Pyrex glass (Corning 7740), both on unannealed and annealed glass wafers (at 560 ◦ C in a N2 environment for 6 h). The wafers were first cleaned in piranha solution (H2 SO4 /H2 O2 2/1), then rinsed in DI water and spun-dried. Before mask deposition the wafers were baked in an oven at 120 ◦ C for 30 min. For patterning of Cr/Au mask, positive photoresist AZ7220 (from Clariant) and wet Cr and Au etchant were used. The thickness of photoresist layer was 2 m. A hard baking process on a hot plate at 120 ◦ C for 30 min was performed. The wet etching process of the glass wafers was performed in a sealed Teflon container with a slow magnetic stirring. A silicon wafer was bonded to the glass wafer using wax to protect the side of the glass wafer uncovered by masking layer. 4. Strategies for increasing the etch rate 4.1. Influence of HF concentration An important factor in deep wet etching of glass is the etch rate. In some cases of wet process the selectivity of the etching (defined as the ratio of the etch rate of masking layer to the etch rate of materials to be processed) is the preferred parameter of the process. In wet etching of glass materials used as masking layers, mainly silicon and gold, are inert in the HF-based etchant. In such cases the etching process is limited by the defects of the masking layer and the penetration of the etchant through these defects. For this reason a fast etch rate of glass will lead to a deeper etching while the defect generation will maintain at the same rate each time. The etch rate is a characteristic for each type of glass, especially due to different oxides and different composition used during fabrication. The etch rate is determined by the concentration of HF enchants. To achieve a high etch rate, a standard concentration of 49% should be used. Fig. 2 presents the influence of HF concentration on the etch rate for Corning 7740 Pyrex glass. It should be noted that by increasing the HF concentration from 40% to 49% a rapid increase of etch rate of 50–60% could be achieved (from 4.4 to 7.6 m/min for non-annealed glass). At the same time no major difference in the resistance of mask in the etchant was observed.
Fig. 2. Influence of HF concentration on glass etch rate.
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4.2. Influence of glass annealing The annealing process has a strong influence on the etch rate of glasses. Each type of glasses has its optimum annealing point. The annealing influence is also presented in Fig. 2. A similar variation was noted, but the increase in the etch rate was from 9.1 to 14.3 m/min when the HF concentration was increased from 40% to 49%. We can conclude that annealing is an important process not only for the reductions of the internal stress, but also for increasing the etch rate. The increase of etching rate can be attributed to a redistribution of oxides. It is noted that after etching the generated surface became rough. 4.3. Other methods Other methods tested, for increasing the etch rate, were: warming the solution at 50 ◦ C and using ultrasonic for agitation. The first method was tested in a sealed Teflon container warmed in hot water at the above-mentioned temperature. The increasing of etch rate was significant (about twice). However, this method is not recommended to be use for safety reasons. By applying ultrasonic agitation we also noticed increases of the etch rate, but the resistance of the masking layer in the etchant is drastically reduced. 5. Strategies for masking layers 5.1. Defects generation There are two defects type: pinholes and notching defects on the edges. These could be observed after certain etch time, and were the result of the interaction between the etchant and mask. Fig. 3 illustrates these defects. Fig. 3a shows the intersection of two channels etched in HF 49% using a Cr/Au mask (50 nm/400 nm) after removing the masking layer, while Fig. 3b presents the masking layer near the edge of a channel, with a breakage of the masking material, and pinholes. The main reasons of defect generation are: the residual stress in the masking layer, the stress type (tensile or compressive), the gradient of stress (for multilayer mask such as Cr/Au or Cr/Cu), and the hydrophilicity of the surface. The residual stress type, as we mentioned in [11], have a strong influence on the defect generation. For example the Cr/Au layer, for a deposition of 50 nm/400 nm on sputtering equipment the stress was in the range between 250 and 300 MPa tensile. The small defects on the surface, which were generated during the cooling process after the deposition, if the stress is tensile, can create small microchannels in the masking layer. If the surface of the layer is hydrophilic the etching solution is “suck” in this microchannels and in a short time pinholes will start to be generated. For this reason a masking layer with compressive stress (such as amorphous silicon or polysilicon) is, sometimes, preferred. From our experience, the notching defects on the edges are a characteristic of metal masks, which are at the same time multilayers (such as Cr/Au or Cr/Cu) and have tensile stress. These defects can be generated either by the stress gradient (the stress in
Fig. 3. (a) Optical image of 100 m deep channels etched in glass with Cr/Au mask with tensile stress of 250–300 MPa. (b) Typical defects after deep etching of glass with high tensile stress.
the Cr layer can be in the range between 800 MPa and 1 GPa) [11] or by the breakage of the edge of the mask. During isotropic etching process, due to underetching, the edge of the mask becomes a freestanding structure. Due to the stress and/or stress gradient the mask can break and leave some areas uncovered. The value of stress can be also an important factor. As presented in [11] for amorphous silicon mask, by annealing the amorphous silicon mask at 400 ◦ C the value of the residual stress can be modify. For low stress value of the masking layer the etching performance of the amorphous silicon layer was improved up to 200 m deep etch. A similar value can be achieved with polysilicon layer (deposited in a furnace at 530 ◦ C). The stress value of polysilicon layer was 50 MPa compressive. Hydrophilicity or hydrophobicity of the surface, as we previous mention has also an important role. A small defect (microcreep) on a hydrophobic surface will be very difficult to be filling with the etching solution. Silicon surface present a hydrophobic surface and for this reason, generally the amorphous silicon or polysilicon masks, give better results are preferred comparing with Cr/Au mask. 5.2. Improving of masking quality There are three major directions to improve the performance of the masking layer: a low value of the residual stress in the masking layer, a multideposition of the masking layer and generation of a hydrophobic surface. In this section we will discuss
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the Cr/Au masking layer (the optimization of the amorphous silicon layer was presented in [11]). A low stress value (about 120 MPa tensile) Cr/Au layer was achieved when the deposition was performed on an e-beam evaporator (CHA equipment). It is noted that for Cr/Au annealing, one cannot modify the stress value. In addition, by increasing the thickness of the gold layer (from 400 nm to 1.2 m) no significant modification of the stress value was observed. The main problems for Cr/Au masking layer, as we previously mentioned are given by the small microcreeps that are generated during the cooling process of the wafer after the deposition. A method for removing this effect is to make a multiple deposition of the Au layer (Fig. 4). After each deposition there is a “relaxation” time of 30 min for cooling. In this period the microcreeps are generated, the next deposition will cover these defects, and new defects are generated in different location, mainly in the second layer. The process can continue with three to four depositions. The results of etching with such a mask (Cr/Au 50 nm/1 m with the Au layer deposited in four successive layers of 250 m) are presented in Fig. 5. Fig. 5a shows an image with a microchannel obtained “through” the glass substrate after etching in HF 49% for 30 min (around 200 m deep). The edges are very well defined without notching defects. Fig. 5b present the image with the Cr/Au mask used to define the intersection of two channels of about 250 m deep. The bending of the mask indicates a tensile stress; the integrity of the mask shows the low stress value. The surface of the mask is without pinholes. After removing the mask (Fig. 5c) the image shows very good definitions of the edges with no pinholes on the protected surface. The maximum time with the above mentioned Cr/Au mask was approximated to be 40 min. The hydrophilicity of the surface can be changed while retaining the photoresist mask. In our case we use a positive photoresist AZ7220 that was hard baked. The other advantage of using hard baked photoresist was that during the baking process the photoresist filled the microcreeps of the mask. By hard baking the layer, the adhesion of the photoresist at the microcreeps walls increases, and even the photoresist will be peeled off from the plain surface of the mask; the removal of the photoresist from these microchannels is very difficult. This phenomenon was confirmed when the Cr/Au layer was removed. The photoresist mask was removed using a classical stripping process with acetone and IPA. After the removal of the Au and Cr layers in classical Au and Cr etchants, a lot of metallic “dots” are present on the surface. This proved that photoresist was not completely removed from the microchannels. Only after a strong cleaning process in a resist stripper (NMP) at 80 ◦ C for 15 min in an ultrasonic bath, and after a second immersion in Au and Cr etchants the wafer was perfectly cleaned. The results of the etching process
Fig. 4. Deposition of the Au layer for an improved mask quality.
Fig. 5. (a) Optical image of a 200 m deep channel etch with a Cr/Au mask, viewed through the glass wafer. (b) Image with intersection of two channels and holes (250 m depth) with the Cr/Au masking layer. (c) Same image after removing the masking layer.
are presented in Fig. 6a and b where a cross-section through a hole etched in a 500 m thick glass wafer and a image with the whole wafer etched through are presented. It can be observed that the wafer edges are a little affected (due to the manipulation of the wafer) while the main part of the wafer is clean without pinholes. The resistance of the Cr/Au + photoresist mask in the etchant (HF 49%) was approximately 85 min.
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References
Fig. 6. (a) Cross-section view of through etched hole with a Cr/Au/photoresist mask for an annealed glass (Corning 7740). (b) Etch through glass wafer.
The results and the explanation of the phenomenon presented here confirm the previous improvement in deep wet etching of glass reported by Bu et al. [10]. In the above-mentioned report the authors used Cr/Au/Cr/Au + photoresist mask deposited on the both sides of the wafer for etching through a 500 m thick Pyrex glass wafer. 6. Conclusions This paper establishes the strategies for deep wet etching of glass. First, considerations regarding the selection of the glass material for the wet etching process are made. Second, the main factors that influence the etch rate such as: concentration of HF solution, annealing of the glass, temperature of the solution are analyzed. The generation of the defects is explained. The factors that limit deep wet etching processes are: the type of residual stress in the masking layer, the value of the residual stress, the stress gradient and the hydrophilicity of the surface. According to these considerations a new masking layer Cr/Au (50 nm/1 m) plus photoresist (AZ7220) was designed for a deep wet etching of glass. In order to avoid the penetration of the solution through the microcreeps generated after the Au deposition, the 1 m thick gold layer was deposited in four successive processes at 30 min time interval. The results of the etching, with a 500 m thick glass wafer being etched through, is the best result report in the literature and confirms that the strategies offered in this paper are good.
[1] C. Rusu, R. van Oever, M.J. de Boer, H.V. Jansen, J.W. Berenschot, M.L. Bennink, J.S. Kanger, B.G. de Grooth, M. Elwenspoek, J. Greve, J. Brugger, A. van den Berg, Direct integration of micromachined pipettes in a flow channel for single DNA molecule study by optical tweezers, J. Microelectromech. Syst. 10 (2001) 238–246. [2] Ph. Luginbuhl, P.F. Indermuhle, M.A. Gretillat, F. Willemin, N.F. de Rooij, D. Gerber, G. Gervasio, J.L. Vuilleumier, D. Twerenbold, M. Duggelin, R. Guggenheim, Micromachined injector for DNA mass spectrometry, Proc. Transducers 99 (1999) 1130–1133. [3] P.J. Obeid, T.K. Christopoulos, H.J. Crabtree, C.J. Backhouse, Microfabricated device for DNA and RNA amplification by continuous-flow polymerase chain reaction and reverse transcription-polymerase chain reaction with cycle number selection, Anal. Chem. 75 (2003) 288– 295. [4] C. Iliescu, G.L. Xu, V. Samper, F.E.H. Tay, Fabrication of a dielectrophoretic chip with 3D silicon electrodes, J. Micromech. Microeng. 15 (2005) 494–500. [5] X. Li, T. Abe, M. Esashi, Fabrication of high-density electrical feedthroughs by deep-reactive-ion etching of Pyrex glass, J. Microelectromech. Syst. 1–6 (2002) 625–630. [6] M. Stjernstr¨om, J. Roeraade, Method for fabrication of microfluidic systems in glass, J. Micromech. Microeng. 8 (1998) 33–38. [7] A. Grosse, M. Grewe, H. Fouckhardt, Deep wet etching of fused silica glass for hallows capillary optical leaky waveguides in microfluidic devices, J. Micromech. Microeng. 11 (2001) 257–262. [8] D.C.S. Bien, P.V. Rainey, S.J.M. Mitchel, H.S. Gamble, Characterization of masking materials for deep glass micromachining, J. Micromech. Microeng. 13 (2003) S34–S40. [9] S. Shoji, H. Kikuchi, H. Torigoe, Low-temperature anodic bonding using lithium aluminosilicate--quartz glass ceramic, Sens. Actuators A 64 (1997) 95–100. [10] M. Bu, T. Melvin, G.J. Ensell, J.S. Wilkinson, A.G.R. Evans, A new masking technology for deep glass etching and its microfluidic application, Sens. Actuators A 115 (2–3) (2004) 476–482. [11] C. Iliescu, J. Miao, F.E.H. Tay, Stress control in masking layers for deep wet micromachining of Pyrex glass, Sens. Actuators A 117 (2) (2005) 286– 292. [12] M.A. Grettilat, F. Paoletti, P. Thiebaud, S. Roth, M. Kondelka-Hep, N.F. de Rooij, A new fabrication method for borosilicate glass capillary tubes with lateral inlets and outlets, Sens. Actuators A 60 (1997) 219– 222. [13] T. Corman, P. Enokson, G. Stemme, Deep wet etching of borosilicate glass using anodically bonded silicon substrate as mask, J. Micromech. Microeng. 8 (1998) 84–87. [14] C. Iliescu, J. Ji, F.E.H. Tay, J. Miao, T.T. Sun, Characterization of masking layers for deep wet etching of glass in an improved HF/HCl solution, Surf. Coat. Technol. 198 (1–3) (2005) 314–318.
Biographies Ciprian Iliescu received his BS and PhD from Polytechnic University of Bucharest in 1989 and 1999, respectively. From to 2001, he was with the IC company S.A. Baneasa in Bucharest; he was involved in the design and the process development pressure sensors. Between 2001 and 2003, he worked as a post-doctoral fellow in the Micromachines (MEMS) Center at the Nanyang Technological University, Singapore. In May 2003, he joined the Institute of Bioengineering and Nanotechnology in Singapore as a research scientist. He had published numerous papers on MEMS and its related areas. Francis E.H. Tay is currently an associate professor with the Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore. Dr. Tay is the deputy director (industry), for the Centre of Intelligent Products and Manufacturing Systems, where he takes charge of research projects involving industry and the Centre. Concurrently, he is the medical device group leader with the Institute of Bioengineering and Nanotechnology (IBN). Dr.
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Tay was also the founding director of the Microsystems Technology Initiative (MSTI), and had established the Microsystems Technology Specialization. He had also served as the technical advisor, in the Micro and Nano Systems Laboratory, Institute of Materials Research Engineering (IMRE). His research areas are MEMS, biotechnology, nanotechnology, and wearable devices. Jianmin Miao received his Dipl.-Ing. and Dr.-Ing. from the Department of Electrical Engineering and Information Technology at the Darmstadt University of
Technology, Germany in 1991, 1996, respectively. He was the head of R&D with S.A. Lectret (Switzerland and Singapore) with prime responsibility for the joint project of integrated silicon microphones with the Institute of Microelectronics in Singapore. In September 1998, he joined the Nanyang Technological University, Singapore as a faculty. He is currently an associate professor and the director of the Micromachines (MEMS) Center. His research interests include MEMS design, technology development and characterization. He is the member of the Institute of Electrical and Electronics Engineers.
Strategies in deep wet etching of Pyrex glass Ciprian Iliescu a,∗ , Francis E.H. Tay a,b , Jianmin Miao c a
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos #04-01, Singapore 138669, Singapore Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore c School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b
Received 8 July 2005; received in revised form 10 January 2006; accepted 20 March 2006 Available online 18 July 2006
Abstract This paper establishes the strategies for deep wet etching of one of the most common glasses: Pyrex. There are two way for increasing the etch depth: increasing the etch rate or increasing the resistance of the mask in the etching solution. The paper analyzes the methods for increasing the glass etch rate in HF solutions: annealing, concentration, ultrasonic agitation and temperature. The generation of the defects is investigated. The main factors that affect the degradation of the mask are: type, value and gradient of the residual stress and the hydrophilicity of the surface. Cr/Au mask is used for illustration. A new method for deep wet etching of glass using Cr/Au mask and photoresist is established. The result of this method is the best thus far as reported in the literature: 85 min deep wet etching in HF 49% which is equivalent to etching of more than 500 m deep in the Pyrex glass material. © 2006 Elsevier B.V. All rights reserved. Keywords: Wet etching of glass; Residual stress; Masking layer
1. Introduction Glass is a widely used material in MEMS and bio-chip device fabrication. The use of glass is common in the packaging process, as die for thermal compensation, on two of the most commercialized MEMS devices: piezoresistive pressure sensors and accelerometers. In these applications, glass is used due to its good bondability to silicon and also to the similar coefficient of expansion with silicon. The good isolation property makes glass a very good material for RF-MEMS applications. Due to its optical properties, glass is also very well used in MOEMS devices. Bio-chips are fabricated on glass substrate due to its optical transparency, hydrophilicity, chemical stability and biocompatibility. A microflow cells for single molecule handling of DNA is presented in [1], Luginbuhl et al. reported a microinjector for DNA mass spectrometry in [2], a microPCR device for DNA amplification is described in [3] or a dielectrophoretic chip with 3D electrodes in a “sandwich” structure glass/silicon/glass is presented in [4]. Most of these applications require methods and technologies for glass micropatterning.
∗
Corresponding author. Tel.: +65 68247137; fax: +65 68749082. E-mail address: [email protected] (C. Iliescu).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.06.044
There are three major groups of technique used for glass etching: mechanical, dry and wet. Mechanical methods include traditional drilling, ultrasonic drilling, electrochemical discharge or powder blasting. However, smooth surfaces cannot be generated using such methods. Dry etching of glass had been reported in [5] using SF6 . However, the etching rate is relatively low. Wet etching remains the most common and low cost method. The etching solution is based on HF. The masking layer depends on the application and on the “thermal budget” of the fabrication process of the device. Photoresist is very often used as mask layer [6–8], but its area of application is limited. A very commonly used mask is Cr/Au [8,9], where Cr layer is used to improve the adhesion of gold to glass. Bu et al. [10] reported etching through 500 m thick glass wafer using a multilayer of metal, Cr/Au/Cr/Au, in combination with a thick SPR220-7 photoresist, by etching from both sides of the wafer. Another very commonly used mask material for glass etching is silicon, deposited by different methods: PECVD (amorphous silicon) [8,11], LPCVD (polysilicon) [8,12] or even bulk silicon [13]. The maximum reported depth was 320 m by Bien et al. [8] with a mask of polished polysilicon (1.5 m) and SU-8 (50 m) as etching mask. We establish in this paper the strategies for deep wet etching of glass. There are two main directions. First is to increase the
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etching rate. For this aim a maximum concentration of HF (in our case 49%) must be used. Second is the improvement of mask quality. This can be achieved by a low stress value in the masking layer, a multiple deposition (for Au layer) and by changing the hydrophilicity of the masking layer (keeping the photoresist mask). As a result a Pyrex glass wafer is etched through using a Cr/Au and photoresist mask. This is the best result reported in the literature. In addition, defects generation is discussed in this paper. 2. Selecting the glass By definition glass is a mixture of oxide. The composition of glass and the concentration of these oxides give the main properties of the glass material. As a result, there are a huge number of glasses available on the market, each with different properties, and due to the different structure the microfabrication techniques and especially the wet etching process can be different. We select for our study a well-known Pyrex glass: Corning7740. The main solution for wet etching of glass is based on HF. Some oxides such as CaO, MgO or Al2 O3 give insoluble products after reacting with HF. These insoluble products can act like a masking layer and reduce the etch rate or can increase the roughness of the generated surface. An example is shown in Fig. 1 where the variation depth etch during 1 h etch in HF 40% for three different glasses (Corning 7740, Hoya SD-2 and soda lime) is presented. As can be observed, only Corning 7740 has a constant etch rate, while the other two glasses have as upper limit of etch depth up to 250–300 m. The large amount of Al2 O3 (about 20%) in Hoya glasses and CaO (8.8%), MgO (4%) for soda lime glasses compared with only 2% Al2 O3 for Corning 7740 can explain the parabolic variation of depth with time for the first two glasses. We proposed in [14] a method to improve the wet etching process by adding HCl. The resultant insoluble products (CaF2 , MgF2 or AlF3 ), after the reaction with HCl, will generate soluble products. The amount of HCl must be optimized from case to case. For soda lime and Pyrex the optimal ratio is 10:1 HF (49%)/HCl (37%) [14].
Fig. 1. Variation of depth vs. time for three different glasses: Corning 7740, Hoya SD-2 and soda lime.
3. Experiments Our experiments were performed on Pyrex glass (Corning 7740), both on unannealed and annealed glass wafers (at 560 ◦ C in a N2 environment for 6 h). The wafers were first cleaned in piranha solution (H2 SO4 /H2 O2 2/1), then rinsed in DI water and spun-dried. Before mask deposition the wafers were baked in an oven at 120 ◦ C for 30 min. For patterning of Cr/Au mask, positive photoresist AZ7220 (from Clariant) and wet Cr and Au etchant were used. The thickness of photoresist layer was 2 m. A hard baking process on a hot plate at 120 ◦ C for 30 min was performed. The wet etching process of the glass wafers was performed in a sealed Teflon container with a slow magnetic stirring. A silicon wafer was bonded to the glass wafer using wax to protect the side of the glass wafer uncovered by masking layer. 4. Strategies for increasing the etch rate 4.1. Influence of HF concentration An important factor in deep wet etching of glass is the etch rate. In some cases of wet process the selectivity of the etching (defined as the ratio of the etch rate of masking layer to the etch rate of materials to be processed) is the preferred parameter of the process. In wet etching of glass materials used as masking layers, mainly silicon and gold, are inert in the HF-based etchant. In such cases the etching process is limited by the defects of the masking layer and the penetration of the etchant through these defects. For this reason a fast etch rate of glass will lead to a deeper etching while the defect generation will maintain at the same rate each time. The etch rate is a characteristic for each type of glass, especially due to different oxides and different composition used during fabrication. The etch rate is determined by the concentration of HF enchants. To achieve a high etch rate, a standard concentration of 49% should be used. Fig. 2 presents the influence of HF concentration on the etch rate for Corning 7740 Pyrex glass. It should be noted that by increasing the HF concentration from 40% to 49% a rapid increase of etch rate of 50–60% could be achieved (from 4.4 to 7.6 m/min for non-annealed glass). At the same time no major difference in the resistance of mask in the etchant was observed.
Fig. 2. Influence of HF concentration on glass etch rate.
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4.2. Influence of glass annealing The annealing process has a strong influence on the etch rate of glasses. Each type of glasses has its optimum annealing point. The annealing influence is also presented in Fig. 2. A similar variation was noted, but the increase in the etch rate was from 9.1 to 14.3 m/min when the HF concentration was increased from 40% to 49%. We can conclude that annealing is an important process not only for the reductions of the internal stress, but also for increasing the etch rate. The increase of etching rate can be attributed to a redistribution of oxides. It is noted that after etching the generated surface became rough. 4.3. Other methods Other methods tested, for increasing the etch rate, were: warming the solution at 50 ◦ C and using ultrasonic for agitation. The first method was tested in a sealed Teflon container warmed in hot water at the above-mentioned temperature. The increasing of etch rate was significant (about twice). However, this method is not recommended to be use for safety reasons. By applying ultrasonic agitation we also noticed increases of the etch rate, but the resistance of the masking layer in the etchant is drastically reduced. 5. Strategies for masking layers 5.1. Defects generation There are two defects type: pinholes and notching defects on the edges. These could be observed after certain etch time, and were the result of the interaction between the etchant and mask. Fig. 3 illustrates these defects. Fig. 3a shows the intersection of two channels etched in HF 49% using a Cr/Au mask (50 nm/400 nm) after removing the masking layer, while Fig. 3b presents the masking layer near the edge of a channel, with a breakage of the masking material, and pinholes. The main reasons of defect generation are: the residual stress in the masking layer, the stress type (tensile or compressive), the gradient of stress (for multilayer mask such as Cr/Au or Cr/Cu), and the hydrophilicity of the surface. The residual stress type, as we mentioned in [11], have a strong influence on the defect generation. For example the Cr/Au layer, for a deposition of 50 nm/400 nm on sputtering equipment the stress was in the range between 250 and 300 MPa tensile. The small defects on the surface, which were generated during the cooling process after the deposition, if the stress is tensile, can create small microchannels in the masking layer. If the surface of the layer is hydrophilic the etching solution is “suck” in this microchannels and in a short time pinholes will start to be generated. For this reason a masking layer with compressive stress (such as amorphous silicon or polysilicon) is, sometimes, preferred. From our experience, the notching defects on the edges are a characteristic of metal masks, which are at the same time multilayers (such as Cr/Au or Cr/Cu) and have tensile stress. These defects can be generated either by the stress gradient (the stress in
Fig. 3. (a) Optical image of 100 m deep channels etched in glass with Cr/Au mask with tensile stress of 250–300 MPa. (b) Typical defects after deep etching of glass with high tensile stress.
the Cr layer can be in the range between 800 MPa and 1 GPa) [11] or by the breakage of the edge of the mask. During isotropic etching process, due to underetching, the edge of the mask becomes a freestanding structure. Due to the stress and/or stress gradient the mask can break and leave some areas uncovered. The value of stress can be also an important factor. As presented in [11] for amorphous silicon mask, by annealing the amorphous silicon mask at 400 ◦ C the value of the residual stress can be modify. For low stress value of the masking layer the etching performance of the amorphous silicon layer was improved up to 200 m deep etch. A similar value can be achieved with polysilicon layer (deposited in a furnace at 530 ◦ C). The stress value of polysilicon layer was 50 MPa compressive. Hydrophilicity or hydrophobicity of the surface, as we previous mention has also an important role. A small defect (microcreep) on a hydrophobic surface will be very difficult to be filling with the etching solution. Silicon surface present a hydrophobic surface and for this reason, generally the amorphous silicon or polysilicon masks, give better results are preferred comparing with Cr/Au mask. 5.2. Improving of masking quality There are three major directions to improve the performance of the masking layer: a low value of the residual stress in the masking layer, a multideposition of the masking layer and generation of a hydrophobic surface. In this section we will discuss
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the Cr/Au masking layer (the optimization of the amorphous silicon layer was presented in [11]). A low stress value (about 120 MPa tensile) Cr/Au layer was achieved when the deposition was performed on an e-beam evaporator (CHA equipment). It is noted that for Cr/Au annealing, one cannot modify the stress value. In addition, by increasing the thickness of the gold layer (from 400 nm to 1.2 m) no significant modification of the stress value was observed. The main problems for Cr/Au masking layer, as we previously mentioned are given by the small microcreeps that are generated during the cooling process of the wafer after the deposition. A method for removing this effect is to make a multiple deposition of the Au layer (Fig. 4). After each deposition there is a “relaxation” time of 30 min for cooling. In this period the microcreeps are generated, the next deposition will cover these defects, and new defects are generated in different location, mainly in the second layer. The process can continue with three to four depositions. The results of etching with such a mask (Cr/Au 50 nm/1 m with the Au layer deposited in four successive layers of 250 m) are presented in Fig. 5. Fig. 5a shows an image with a microchannel obtained “through” the glass substrate after etching in HF 49% for 30 min (around 200 m deep). The edges are very well defined without notching defects. Fig. 5b present the image with the Cr/Au mask used to define the intersection of two channels of about 250 m deep. The bending of the mask indicates a tensile stress; the integrity of the mask shows the low stress value. The surface of the mask is without pinholes. After removing the mask (Fig. 5c) the image shows very good definitions of the edges with no pinholes on the protected surface. The maximum time with the above mentioned Cr/Au mask was approximated to be 40 min. The hydrophilicity of the surface can be changed while retaining the photoresist mask. In our case we use a positive photoresist AZ7220 that was hard baked. The other advantage of using hard baked photoresist was that during the baking process the photoresist filled the microcreeps of the mask. By hard baking the layer, the adhesion of the photoresist at the microcreeps walls increases, and even the photoresist will be peeled off from the plain surface of the mask; the removal of the photoresist from these microchannels is very difficult. This phenomenon was confirmed when the Cr/Au layer was removed. The photoresist mask was removed using a classical stripping process with acetone and IPA. After the removal of the Au and Cr layers in classical Au and Cr etchants, a lot of metallic “dots” are present on the surface. This proved that photoresist was not completely removed from the microchannels. Only after a strong cleaning process in a resist stripper (NMP) at 80 ◦ C for 15 min in an ultrasonic bath, and after a second immersion in Au and Cr etchants the wafer was perfectly cleaned. The results of the etching process
Fig. 4. Deposition of the Au layer for an improved mask quality.
Fig. 5. (a) Optical image of a 200 m deep channel etch with a Cr/Au mask, viewed through the glass wafer. (b) Image with intersection of two channels and holes (250 m depth) with the Cr/Au masking layer. (c) Same image after removing the masking layer.
are presented in Fig. 6a and b where a cross-section through a hole etched in a 500 m thick glass wafer and a image with the whole wafer etched through are presented. It can be observed that the wafer edges are a little affected (due to the manipulation of the wafer) while the main part of the wafer is clean without pinholes. The resistance of the Cr/Au + photoresist mask in the etchant (HF 49%) was approximately 85 min.
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References
Fig. 6. (a) Cross-section view of through etched hole with a Cr/Au/photoresist mask for an annealed glass (Corning 7740). (b) Etch through glass wafer.
The results and the explanation of the phenomenon presented here confirm the previous improvement in deep wet etching of glass reported by Bu et al. [10]. In the above-mentioned report the authors used Cr/Au/Cr/Au + photoresist mask deposited on the both sides of the wafer for etching through a 500 m thick Pyrex glass wafer. 6. Conclusions This paper establishes the strategies for deep wet etching of glass. First, considerations regarding the selection of the glass material for the wet etching process are made. Second, the main factors that influence the etch rate such as: concentration of HF solution, annealing of the glass, temperature of the solution are analyzed. The generation of the defects is explained. The factors that limit deep wet etching processes are: the type of residual stress in the masking layer, the value of the residual stress, the stress gradient and the hydrophilicity of the surface. According to these considerations a new masking layer Cr/Au (50 nm/1 m) plus photoresist (AZ7220) was designed for a deep wet etching of glass. In order to avoid the penetration of the solution through the microcreeps generated after the Au deposition, the 1 m thick gold layer was deposited in four successive processes at 30 min time interval. The results of the etching, with a 500 m thick glass wafer being etched through, is the best result report in the literature and confirms that the strategies offered in this paper are good.
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Biographies Ciprian Iliescu received his BS and PhD from Polytechnic University of Bucharest in 1989 and 1999, respectively. From to 2001, he was with the IC company S.A. Baneasa in Bucharest; he was involved in the design and the process development pressure sensors. Between 2001 and 2003, he worked as a post-doctoral fellow in the Micromachines (MEMS) Center at the Nanyang Technological University, Singapore. In May 2003, he joined the Institute of Bioengineering and Nanotechnology in Singapore as a research scientist. He had published numerous papers on MEMS and its related areas. Francis E.H. Tay is currently an associate professor with the Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore. Dr. Tay is the deputy director (industry), for the Centre of Intelligent Products and Manufacturing Systems, where he takes charge of research projects involving industry and the Centre. Concurrently, he is the medical device group leader with the Institute of Bioengineering and Nanotechnology (IBN). Dr.
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Tay was also the founding director of the Microsystems Technology Initiative (MSTI), and had established the Microsystems Technology Specialization. He had also served as the technical advisor, in the Micro and Nano Systems Laboratory, Institute of Materials Research Engineering (IMRE). His research areas are MEMS, biotechnology, nanotechnology, and wearable devices. Jianmin Miao received his Dipl.-Ing. and Dr.-Ing. from the Department of Electrical Engineering and Information Technology at the Darmstadt University of
Technology, Germany in 1991, 1996, respectively. He was the head of R&D with S.A. Lectret (Switzerland and Singapore) with prime responsibility for the joint project of integrated silicon microphones with the Institute of Microelectronics in Singapore. In September 1998, he joined the Nanyang Technological University, Singapore as a faculty. He is currently an associate professor and the director of the Micromachines (MEMS) Center. His research interests include MEMS design, technology development and characterization. He is the member of the Institute of Electrical and Electronics Engineers.