Tunable Vertical Comb for Driving Micromirror Realized by Bending
IEICE TRANS. ELECTRON., VOL.E90–C, NO.1 JANUARY 2007
147
LETTER
Special Section on Microoptomechatronics
Tunable Vertical Comb for Driving Micromirror Realized by Bending Device Wafer Minoru SASAKI†a) , Member, Masahiro ISHIMORI† , JongHyeong SONG† , and Kazuhiro HANE† , Nonmembers
SUMMARY An electrostatically driven micromirror is described. The vertical comb of a three-dimensional microstructure is realized by bending the device wafer having microstructures. By resetting the bending angle, the tuning of the vertical gap between moving and stationary combs is possible. The characteristics of the vertical comb drive actuator can be tuned, confirming the performance. key words: micromirror, tuning, vertical comb, wafer bending
1.
Introduction
Fabricating micromirrors from a silicon-on-insulator (SOI) wafer is now a well-recognized method of obtaining a micro-electro-mechanical system (MEMS) devices. A micromirror is usually required for rotation in the out of plane direction, which cannot be obtained from the lateral comb drive actuators. A vertical comb drive actuator is developed by preparing a bonded wafer with microstructures inside [1]. The vertical comb requires paired combs at different heights. For realizing this three-dimensional structure, many methods have been tried. The direct method is dry etching from the front and back sides of the wafer [2]. The delayed mask is a useful method [3]. Another approach is postprocessing. A paired comb can be accurately patterned using a single mask. After preparing planer structures, some parts are moved from their original positions. The mechanisms reported are local plastic deformation [4], the surface tension of the melt resist material [5], and film stress in a layer of the unimorph structure [6]. In this study, the wafer-bending method [7], [8] is applied for realizing a vertical comb. This is a combination of micromachining and conventional machining. Tuning the bending angle or vertical gap between combs is possible. Although micromirrors are usually fixed in their characteristics after fabrication, the tuning of micromirror characteristics is demonstrated. 2.
Micromirror Design
Figure 1 shows a schematic of the wafer-bending process for preparing the vertical comb. Microstructures are prepared using MEMS processes (dry and wet etchings). The mirror, and stationary and moving combs are in the device Manuscript received July 19, 2006. Manuscript revised September 15, 2006. † The authors are with Tohoku University, Sendai-shi, 9808579 Japan. a) E-mail: [email protected] DOI: 10.1093/ietele/e90–c.1.147
Fig. 1 Schematic drawing for explaining wafer-bending for generating vertical comb. (a) and (b) are before and after wafer-bending process, respectively.
layer. V-grooves and the cavity beneath the mirror in the back-side handle layer are prepared using wet anisotropic Si etching. The wafer is segmented into three parts. The mirror, having moving combs, is at the center (part 2). Stationary combs are at the end of the cantilevers supported by the left and right parts (parts 1 and 3). The V-grooves are filled with photoresist connecting the three parts. Sacrificial buried oxide was previously etched out. By heating the wafer to greater than the glass transition temperature of the resist, the wafer can be bent. This motion lifts up the stationary combs, making the vertical comb. The bending angle is controlled by the jig with the appropriate slopes for parts 1 and 3. The jig is prepared using normal machining. After cooling, the wafer is maintained and fixed on the jig. The angular accuracy is ∼0.2◦ [7]. An arbitrary angle can be set using different jigs. By resetting the bending angle, tuning is possible. Figure 2 shows an image of the fabricated micromirror. The bending angle is 1.8◦ , generating a 27 µm height difference between the combs. The mirror size is 1.0 × 0.7 mm2 . The surface roughness of the mirror is 2–4 nm Rrms . On a 2 × 2 cm2 wafer, seven arrayed mirrors are fabricated. The thickness of the device Si layer is 10 µm. The comb finger width is 6 µm. The lateral gap between combs is 3 µm. The error in the lateral displacement generated by the wafer bending is ∼0.3 µm. When the sample is baked, the comb fingers are confirmed to align toward the gap center owing
c 2007 The Institute of Electronics, Information and Communication Engineers Copyright
IEICE TRANS. ELECTRON., VOL.E90–C, NO.1 JANUARY 2007
148
Fig. 2
Fabricated micromirror.
to the shape memory effect of the resist material. 3.
Results and Conclusion
Figure 3 shows the mirror rotation angle as a function of driving voltage. The driving voltage is applied to the stationary combs. The mirror, handle layers, and jig are connected to ground (0 V), as shown in Fig. 1. The bending angles are 1.8, 1.5 and 0.94◦ . When the angle is 1.8◦ , the mirror rotation is subtle at a low driving voltage. The pullin phenomenon, the instability due to an abrupt increase in the electrostatic force, is observed at 330 V. The rotation angle jumps to 9◦ , showing hysteresis. This instability can be attributed to the large initial comb-to-comb gap of 17 µm [5]. The net controllable rotation angle before the pull-in is 2◦ . When the bending angle is reduced to 1.5 or 0.94◦ , this instability is removed. The mirror rotation angle shows an S-shaped curve. At 0.94◦ , the mirror rotation angle reaches 3.5◦ at 180 V. Note that the S-shaped curve of the bending angle of 0.94◦ has a larger rotation angle at a lower driving voltage (0–75 V). The overlap between comb fingers, which increases the change in capacitance against the mirror movement, generating a larger electrostatic force, is obtained from the initial state for the bending angle of 0.94◦ [6]. Although the maximum rotation angle is 3.9◦ with the bending angle of 1.5◦ , the S-shaped curve is steeper. To control the rotation angle, a gentle slope is preferred. The microscopic characteristics are confirmed to be tuned with the bending angle. SOI wafer usually has a thickness variation of about +/−1 µm in the device Si layer owing to polishing accuracy. This will affect the characteristics of the mi-
Fig. 3 Mirror rotation angle as function of driving voltage. The parameter is bending angle.
cromirror. The tuning ability of the micromirror realized by the wafer-bending method will be useful. The resonant frequency is 1.36 kHz and does not change with bending angle because this is determined by the inertia of the mirror and the spring constant of the torsion bar. References [1] R.A. Conant, J.T. Nee, K.Y. Lau, and R.S. Muller, “A flat highfrequency scanning micromirror,” Proc. Solid-State Sensor and Actuator Workshop, Hilton Head Island, pp.6–9, June 2000. [2] X. Mi, H. Soneda, H. Okuda, O. Tsuboi, N. Kouma, Y. Mizuno, S. Ueda, and I. Sawaki, “A multi-chip directly mounted 512-MEMSmirror array module with a hermetically sealed package for large optical cross-connects,” J. Opt. A, Pure Appl. Opt., vol.8, pp.S341–S346, July 2006. [3] V. Milanovic, “Multilevel beam SOI-MEMS fabrication and applications,” J. Microelectromech. Syst., vol.13, pp.19–30, Feb. 2004. [4] J. Kim and L. Lin, “Electrostatic scanning micromirrors using localized plastic deformation of silicon,” J. Micromech. Microeng., vol.15, pp.1777–1785, Sept. 2005. [5] D. Hah, P.R. Patterson, H.D. Nguyen, H. Toshiyoshi, and M.C. Wu, “Theory and experiments of angular vertical comb-drive actuators for scanning micromirrors,” IEEE J. Sel. Top. Quantum Electron., vol.10, no.3, pp.505–513, May/June 2004. [6] M. Sasaki, D. Briand, W. Noell, N. de Rooij, and K. Hane, “Threedimensional SOI-MEMS constructed by buckled bridges and vertical comb drive actuator,” IEEE J. Sel. Top. Quantum Electron., vol.10, no.3, pp.455–461, May/June 2004. [7] M. Ishimori, J.H. Song, M. Sasaki, and K. Hane, “Si-wafer bending technique for a three-dimensional microoptical bench,” Jpn. J. Appl. Phys., Part 1, vol.42, no.6B, pp.4063–4066, June 2003. [8] S. Endou, M. Ishimori, M. Sasaki, and K. Hane, “Compact triangulation sensor array constructed by wafer bending,” Jpn. J. Appl. Phys. 1, vol.44, no.4B, pp.2874–2878, April 2005.
147
LETTER
Special Section on Microoptomechatronics
Tunable Vertical Comb for Driving Micromirror Realized by Bending Device Wafer Minoru SASAKI†a) , Member, Masahiro ISHIMORI† , JongHyeong SONG† , and Kazuhiro HANE† , Nonmembers
SUMMARY An electrostatically driven micromirror is described. The vertical comb of a three-dimensional microstructure is realized by bending the device wafer having microstructures. By resetting the bending angle, the tuning of the vertical gap between moving and stationary combs is possible. The characteristics of the vertical comb drive actuator can be tuned, confirming the performance. key words: micromirror, tuning, vertical comb, wafer bending
1.
Introduction
Fabricating micromirrors from a silicon-on-insulator (SOI) wafer is now a well-recognized method of obtaining a micro-electro-mechanical system (MEMS) devices. A micromirror is usually required for rotation in the out of plane direction, which cannot be obtained from the lateral comb drive actuators. A vertical comb drive actuator is developed by preparing a bonded wafer with microstructures inside [1]. The vertical comb requires paired combs at different heights. For realizing this three-dimensional structure, many methods have been tried. The direct method is dry etching from the front and back sides of the wafer [2]. The delayed mask is a useful method [3]. Another approach is postprocessing. A paired comb can be accurately patterned using a single mask. After preparing planer structures, some parts are moved from their original positions. The mechanisms reported are local plastic deformation [4], the surface tension of the melt resist material [5], and film stress in a layer of the unimorph structure [6]. In this study, the wafer-bending method [7], [8] is applied for realizing a vertical comb. This is a combination of micromachining and conventional machining. Tuning the bending angle or vertical gap between combs is possible. Although micromirrors are usually fixed in their characteristics after fabrication, the tuning of micromirror characteristics is demonstrated. 2.
Micromirror Design
Figure 1 shows a schematic of the wafer-bending process for preparing the vertical comb. Microstructures are prepared using MEMS processes (dry and wet etchings). The mirror, and stationary and moving combs are in the device Manuscript received July 19, 2006. Manuscript revised September 15, 2006. † The authors are with Tohoku University, Sendai-shi, 9808579 Japan. a) E-mail: [email protected] DOI: 10.1093/ietele/e90–c.1.147
Fig. 1 Schematic drawing for explaining wafer-bending for generating vertical comb. (a) and (b) are before and after wafer-bending process, respectively.
layer. V-grooves and the cavity beneath the mirror in the back-side handle layer are prepared using wet anisotropic Si etching. The wafer is segmented into three parts. The mirror, having moving combs, is at the center (part 2). Stationary combs are at the end of the cantilevers supported by the left and right parts (parts 1 and 3). The V-grooves are filled with photoresist connecting the three parts. Sacrificial buried oxide was previously etched out. By heating the wafer to greater than the glass transition temperature of the resist, the wafer can be bent. This motion lifts up the stationary combs, making the vertical comb. The bending angle is controlled by the jig with the appropriate slopes for parts 1 and 3. The jig is prepared using normal machining. After cooling, the wafer is maintained and fixed on the jig. The angular accuracy is ∼0.2◦ [7]. An arbitrary angle can be set using different jigs. By resetting the bending angle, tuning is possible. Figure 2 shows an image of the fabricated micromirror. The bending angle is 1.8◦ , generating a 27 µm height difference between the combs. The mirror size is 1.0 × 0.7 mm2 . The surface roughness of the mirror is 2–4 nm Rrms . On a 2 × 2 cm2 wafer, seven arrayed mirrors are fabricated. The thickness of the device Si layer is 10 µm. The comb finger width is 6 µm. The lateral gap between combs is 3 µm. The error in the lateral displacement generated by the wafer bending is ∼0.3 µm. When the sample is baked, the comb fingers are confirmed to align toward the gap center owing
c 2007 The Institute of Electronics, Information and Communication Engineers Copyright
IEICE TRANS. ELECTRON., VOL.E90–C, NO.1 JANUARY 2007
148
Fig. 2
Fabricated micromirror.
to the shape memory effect of the resist material. 3.
Results and Conclusion
Figure 3 shows the mirror rotation angle as a function of driving voltage. The driving voltage is applied to the stationary combs. The mirror, handle layers, and jig are connected to ground (0 V), as shown in Fig. 1. The bending angles are 1.8, 1.5 and 0.94◦ . When the angle is 1.8◦ , the mirror rotation is subtle at a low driving voltage. The pullin phenomenon, the instability due to an abrupt increase in the electrostatic force, is observed at 330 V. The rotation angle jumps to 9◦ , showing hysteresis. This instability can be attributed to the large initial comb-to-comb gap of 17 µm [5]. The net controllable rotation angle before the pull-in is 2◦ . When the bending angle is reduced to 1.5 or 0.94◦ , this instability is removed. The mirror rotation angle shows an S-shaped curve. At 0.94◦ , the mirror rotation angle reaches 3.5◦ at 180 V. Note that the S-shaped curve of the bending angle of 0.94◦ has a larger rotation angle at a lower driving voltage (0–75 V). The overlap between comb fingers, which increases the change in capacitance against the mirror movement, generating a larger electrostatic force, is obtained from the initial state for the bending angle of 0.94◦ [6]. Although the maximum rotation angle is 3.9◦ with the bending angle of 1.5◦ , the S-shaped curve is steeper. To control the rotation angle, a gentle slope is preferred. The microscopic characteristics are confirmed to be tuned with the bending angle. SOI wafer usually has a thickness variation of about +/−1 µm in the device Si layer owing to polishing accuracy. This will affect the characteristics of the mi-
Fig. 3 Mirror rotation angle as function of driving voltage. The parameter is bending angle.
cromirror. The tuning ability of the micromirror realized by the wafer-bending method will be useful. The resonant frequency is 1.36 kHz and does not change with bending angle because this is determined by the inertia of the mirror and the spring constant of the torsion bar. References [1] R.A. Conant, J.T. Nee, K.Y. Lau, and R.S. Muller, “A flat highfrequency scanning micromirror,” Proc. Solid-State Sensor and Actuator Workshop, Hilton Head Island, pp.6–9, June 2000. [2] X. Mi, H. Soneda, H. Okuda, O. Tsuboi, N. Kouma, Y. Mizuno, S. Ueda, and I. Sawaki, “A multi-chip directly mounted 512-MEMSmirror array module with a hermetically sealed package for large optical cross-connects,” J. Opt. A, Pure Appl. Opt., vol.8, pp.S341–S346, July 2006. [3] V. Milanovic, “Multilevel beam SOI-MEMS fabrication and applications,” J. Microelectromech. Syst., vol.13, pp.19–30, Feb. 2004. [4] J. Kim and L. Lin, “Electrostatic scanning micromirrors using localized plastic deformation of silicon,” J. Micromech. Microeng., vol.15, pp.1777–1785, Sept. 2005. [5] D. Hah, P.R. Patterson, H.D. Nguyen, H. Toshiyoshi, and M.C. Wu, “Theory and experiments of angular vertical comb-drive actuators for scanning micromirrors,” IEEE J. Sel. Top. Quantum Electron., vol.10, no.3, pp.505–513, May/June 2004. [6] M. Sasaki, D. Briand, W. Noell, N. de Rooij, and K. Hane, “Threedimensional SOI-MEMS constructed by buckled bridges and vertical comb drive actuator,” IEEE J. Sel. Top. Quantum Electron., vol.10, no.3, pp.455–461, May/June 2004. [7] M. Ishimori, J.H. Song, M. Sasaki, and K. Hane, “Si-wafer bending technique for a three-dimensional microoptical bench,” Jpn. J. Appl. Phys., Part 1, vol.42, no.6B, pp.4063–4066, June 2003. [8] S. Endou, M. Ishimori, M. Sasaki, and K. Hane, “Compact triangulation sensor array constructed by wafer bending,” Jpn. J. Appl. Phys. 1, vol.44, no.4B, pp.2874–2878, April 2005.
