CMOS microelectromechanical bandpass filters

Sensors and Actuators A 90 (2001) 148±152

CMOS microelectromechanical bandpass ®lters Lung-Jieh Yanga, Tsung-Wei Huangb,*, Pei-Zen Changb a

Department of Mechanical Engineering, Tamkang University, Tamsui 25137, Taiwan, ROC b Institute of Applied Mechanics, National Taiwan University, Taipei 10617, Taiwan, ROC Accepted 23 December 2000

Abstract This work fabricates a laminated-suspension microelectromechanical ®lter, respectively, by a fully compatible CMOS 0.6 mm single poly triple metal (SPTM) process and CMOS 0.35 mm single poly quadri-metal (SPQM) process. Experimentally, due to the top metal layer being used as the etch-resistant mask during the subsequent dry etching. Therefore, this study performs maskless etching with plasma and obtains excellent results including high selectivity and full release of the structure. Additionally, the microelectromechanical ®lter can be driven by applying low-voltage of around 5 V and a measured center frequency of around 13.1 kHz and a quality factor of around 1871 were obtained for a single-comb resonator operated in air. The ®lter successful proposed herein has a monolithic integration capability with the relative electric circuits in the standard CMOS 0.35 mm process. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Microelectromechanical ®lter; CMOS; Maskless; Monolithic integration

1. Introduction Accelerating developments in semiconductor fabrication technology have led to a new generation of microelectromechanical systems (MEMS), designed using microfabrication equipment and processing methods. Various fabrication technologies, including LIGA, CMOS-MEMS, surface and bulk micromachining, have been developed to ful®ll various industrial requirements. Currently, many MEMS products, such as pressure sensors and accelerometers, have already been mass-produced. However, other products face lengthy delays before reaching the market place. What accounts for the difference pace of product development? We believe that monolithic integration, development time, and foundry services are the most important reasons for differing product development times. To ful®ll these requirements, the conventional CMOS process is the best choice. Applying the standard CMOS process to MEMS has several merits. First, the standard CMOS process can integrate micromachined devices and operated circuits in monolithic chips. This integration not only reduces noise between devices and

*

Corresponding author. Tel.: 886-2-23630979/ext. 433, 417; fax: 886-2-23639290. E-mail address: [email protected] (T.-W. Huang).

circuits, but also simpli®es the ®nal packaging of chips. Second, standard CMOS process can reduce the development time of products. Third, many professional IC foundry services using the standard CMOS process aid customers in manufacturing their products. Particularly in Taiwan, certain foundry services such as Taiwan Semiconductor Manufacture Company (TSMC), United Microelectronics Corporation (UMC) can provide fast, mass, economic, repeatable and reliable fabrication. Recently, MEMS has designed and fabricated resonant devices suitable for micromachined ®lters. In 1992, Tang et al. [1] developed an electrostatic comb drive levitation and control method. In their scheme, for compliant suspension, normal displacement of over 2 mm requires a comb bias of 30 V. Meanwhile, in 1997, Clark et al. [2] employed a polysilicon surface micromachining technology to build a parallel-resonator HF micromechanical bandpass ®lter. In their scheme, the center frequency is 14.5 MHz and ®lter Q ranges from 830 to 1600. Furthermore, in 1998, Lin et al. [3] used a polysilicon surface micromachining technology to construct a series-coupled resonator pair, design for operation at atmospheric pressure, with a measured center frequency of 18.7 kHz and a pass bandwidth of 1.2 kHz. Finally, in 1998, Wang et al. [4] used free±free beam, ¯exural-mode, micromechanical resonators to achieve measured Q's as high as 8400 at VHF frequencies from 30 to 90 MHz.

0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 4 5 1 - 4

L.-J. Yang et al. / Sensors and Actuators A 90 (2001) 148±152

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2. Operating principle 2.1. Comb drive force in x-direction The series ®lter concept has been implemented by using coupled electrostatically driven laminated-suspension resonant structures [5]. Fig. 1 shows the design layout of a single-comb resonator mechanical ®lter by the standard CMOS 0.6 mm (single poly triple metal) process. The input and output electrostatic combs function as linear electromechanical transducers provided they are biased at a dc voltage which is larger than the amplitude of the signal voltage (Drive Port). General analytic equations for the lateral comb drive force, Fx, as a function of comb ®nger width, wc , air gap between comb ®ngers, g, structure thickness, t, and sacri®cial spacer thickness, d, are derived by the expression [6] t Fx 1:12e0 N V 2 g where e0 is the permittivity of air, N the number of fingers in the movable comb drive, V the instantaneous voltage applied across the comb drive. To increase comb drive force in the x-direction, the thickness of this mechanical resonator was increased. The ®ve layers above the p-type silicon substrate are Poly, Metal 1, Metal 2, Metal 3 and Metal 4. Figs. 2 and 3 depict the design of Q-control scheme for a multi-port microresonator [7] and structural con®guration of CMOS 0.35 mm (single poly quadri-metal) process, respectively. Herein, CONT (connect poly and Metal 1), VIA 1 (connect Metals 1 and 2), VIA 2 (connect Metals 2 and 3), VIA 3 (connect Metals 3 and 4) are also used as sidewall electrodes for the ®xed electrodes. Metals 1, 2, 3, and 4, VIA 1, VIA 2, VIA 3 and CONT are alloys of aluminum, silicon and copper.

Fig. 1. Layout of a single-comb resonator mechanical filter.

Fig. 2. Q-control schematic for a multi-port microresonator.

Compared to the ®xed electrodes, the movable ones have a thin oxide layer in which p plus ion is implanted instead of the polysilicon layer. Therefore, Metal 1 and substrate are short circuited via the layout of the CONT layer. 2.2. Theoretical resonance frequency The fundamental resonance frequency of this mechanical resonator is, again, determined largely by material properties and by geometry, and is given by the expression [8] s 1 2EhW=L3 f0 2p Mp Mt =4 12Mb =35 where Mp is the mass of shuttle, Mt the mass of the folding trusses, Mb the total mass of the suspending beams, W and h are the cross-sectional width and thickness, and E the Young's modulus. For this resonator design, a laminated-suspension structure with a thickness of approximately 5.005 mm. The folded suspensions are 1.5 mm wide beam lengths are 150 mm, ®nger widths are 2.5 mm, number of ®ngers is 26 and Young's modulus is 72:944 109 N/m2. According to geometric symmetry, we can easily obtain the shuttle mass is 307:820 10ÿ12 kg, the mass of the folding trusses is 7:930 10ÿ12 kg, the total mass of the suspending beams is 56:896 10ÿ12 kg. Therefore, at last, a typical theoretical center frequency for a 11.303 kHz folded-beam comb-driven micromechanical resonator was attained.

Fig. 3. Profile of high-aspect-ratio laminated microstructures.

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3. Fabrication The micromechanical ®lters are constructed using the CMOS 0.6 mm single poly triple metal (SPTM) and CMOS 0.35 mm single poly quadri-metal (SPQM) process available from TSMC. The process ¯ow in Fig. 4 illustrates development of laminated-suspension microstructures in cross section. Fig. 4(a) presents the schematic cross-section view after TSMC processing. All the passivation nitride is already removed and the top metal layer is exposed. Therefore, the top metal layer is used as the etch-resistant mask during the subsequent dry etching steps that create the laminated microstructure. Without wet chemical etching, sticking occurs rarely and laminated-suspension microstructures are achieved. Fig. 4(b) illustrates the anisotropic oxide etching through to the silicon substrate by CF4/O2 reactive ion etching (RIE). Meanwhile, Fig. 4(c) displays the silicon substrate etching that releases the whole structure. By employing the large undercut of SF6 RIE, 2.5 mm width of the beam could be released within 15 min via plasma etching. The ratio of CF4 and O2 is the decisional recipe when CF4 and O2 RIE are applied to etch oxide or silicon substrate anisotropically. According to Jansen's Black Silicon method [9], using only SF6 RIE to etch silicon substrate isotropically is the optimum choice. Fig. 5 shows a scanning

Fig. 4. Post-processing of microresonator: (a) after TSMC processing; (b) anisotropic oxide etching; (c) isotropic silicon etching.

Fig. 5. SEM of the laminated-suspension microstructure.

electron micrograph (SEM) of the laminated-suspension microstructures. 4. Results and discussion Fig. 6 displays a SEM of the released microstructures. After the structure is fully released, the resonator could be driven obviously around 5 V. Fig. 7 shows the SEM of circuits and microresonator monolithic integrate into a chip. Fig. 7(a) depicts using photoresist to protect the circuits during the plasma etching. On the other hand, the circuits have been made under Metal 3 layer to avoid destruction by dry etching (RIE) as shown in Fig. 7(b). The sense current is passed through a transresistance ampli®er and the output spectrum is observed on a spectrum analyzer. For a single-comb resonator operated in air, the measured center frequency is 13.1 kHz, which approaches the theoretical center frequency, indicating a quality factor of 1871 and an effective 3 dB-down bandwidth of 7 Hz when Ramp was 1 MO. Under varying values Ramp of transresistance ampli®er, different results can also be obtained on the side, as shown in Fig. 8. The aluminum layers are too thin for this process. To increase the thickness of the ®ngers, VIA1, VIA2 and CONT

Fig. 6. SEM of the released microstructures.


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Fig. 9. The free end of comb fingers would bend out-of-plane after being released.

Fig. 7. SEM of circuits and microresonator monolithic integrate into a chip: (a) using photoresist to protect the circuits; (b) the circuits have been made under Metal 3 layer.

layers were used to connect Poly, Metal 1, 2 and 3. Using the VIA to connect each metal layer not only expands the electrode, but also protects the comb structure from the plasma etching. However, with the rugged surface caused by the VIA. The free end of comb ®ngers would bend out-ofplane following release as shown in Fig. 9. But this case occurs rarely here because design of I-shape structure and single material (alloys of aluminum, silicon and copper). To increase the stiffness of the comb structure, we try to turn it into I-shaped structure as shown in Fig. 4(a). The VIA layer must be smaller than the metal layer in order to meet the design rule of TSMC. Then we repeat this process to form a laminated comb structure. 5. Conclusion

Fig. 8. Experimental measurement of a single-comb resonator operated in air.

This investigation fabricates laminated-suspension micromachined ®lters by a fully compatible CMOS process. Experimentally, due to the top metal layer being used as the etch-resistant mask during the subsequent dry etching. Therefore, this study performs maskless etching with plasma and obtains excellent results, including high selectivity and full release of the structure. Finally, the micromechanical ®lter could be driven by applying a low-voltage of around 5 V, and has a measured center frequency of 13.1 kHz and a quality factor of around 1871. Due to the standard CMOS process, the micromechanical bandpass ®lter can easily be integrated with the relative electric circuits. In particular,

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micromechanical ®lters needed to extend their frequencies into the high VHF and UHF ranges are anticipated for all RF MEMS components. Acknowledgements The authors would like to thank JingHung Chiou, Kaihsiang Yen, Chiyuan Lee, Fuyuan Xiao, Hunglin Chen, Huiwen Huang of the Institute of Applied Mechanics, National Taiwan University, for their valuable assistance in the experiment, and the National Chip Implementation Center is also appreciated for their foundry support. In addition, we are grateful for discussions with Sujen Ji of the Advanced Instrumentation Center, National Taiwan University. References [1] W.C. Tang, M.G. Lim, R.T. Howe, Electrostatic comb drive levitation and control method, J. Microelectromech. Syst. 1 (4) (1992) 170± 178. [2] J.R. Clark, F.D. Bannon III, A.-C. Wong, C.T.-C. Nguyen, Parallelresonator HF micromechanical bandpass filters, Transducer 97 (1997) 1161±1164. [3] L. Lin, R.T. Howe, A.P. Pisano, Microelectromechanical filters for signal processing, J. Microelectromech. Syst. 7 (3) (1998) 28± 294. [4] K. Wang, Y. Yu, A.-C. Wong, C.T.-C. Nguyen, VHF Free±Free Beam High-Q Micromechanical Resonators, J. Microelectromech. Syst. 9 (3) (2000) 347±360.

[5] L. Lin, C.T.-C. Nguyen, R.T. Howe, A. P. Pisano, Microelectromechanical Signal Processor, US Patent 5455547 (1995). [6] W.A. Johnson, L.K. Warne, Electrophysics of micromrechanical comb actuators, J. Microelectromech. Syst. 4 (1) (1995) 49±59. [7] C.T.-C. Nguyen, R.T. Howe, Quality factor control for micromechanical resonators, IEDM (1992) 505±508. [8] W.C. Tang, T.-C. Nguyen, R.T. Howe, Lateral driven polysilicon resonator microstructures, Sens. Actuators 20 (1989) 25±32. [9] H. Jasen, H. Gardeniers, M. de Boor, M. Elwenspoek, J. Fluitman, A survey on the reactive ion etching of silicon in microtechnology, J. Macroelectromech. Syst. (1996) 14±28.

Biographies Lung-Jieh Yang received his MS degree from Tamkang University, Taiwan in 1991 and PhD degree from the Institute of Applied Mechanics of National Taiwan University, Taiwan in 1997. He is currently an Assistant Professor at the Department of Mechanical Engineering of Tamkang University with the research interests on micro-scaled fluid dynamics and microsensors technologies. Tsung-Wei Huang was born in 1973 in Taipei, Taiwan. He received his BS degree in the department of agricultural machinery engineering from National Chung-Hsing University, Taiwan, in 1998. He is working toward his MS in the Institute of Applied Mechanics, National Taiwan University. Pei-Zen Chang was born in Chia-Yi, Taiwan, ROC, in 1962. He received the BS degree in civil engineering from National Taiwan University, Taipei, Taiwan, in 1984, and the PhD degree in theoretical and applied mechanics from Cornell University, Ithaca, NY, in 1991. His PhD dissertation was about the mechanics of superconducting magnetic bearings. He joined the faculty of Institute of Applied Mechanics, National Taiwan University in 1991 and became a professor in 1999. His current research interests are in the area of micromachined sensors and actuators.