Design and fabrication of a mems filter bank for

Poster 110

DESIGN AND FABRICATION OF A MEMS FILTER BANK FOR HEARING AIDS APPLICATIONS Shih-Hsorng Shen' , Shuenn-Tsong Young' , Weileun Fang2 'Institute of Biomedical Engineering, National Yang Ming University, Taipei, Taiwan. Dept. of Power Mechanical Engineering, National Tsing Hua University, Hsin-Chu, Taiwan. Abstract - A novel MEMS filter bank was developed for hearing aids application. It directly processes the sound signals by separating the sound into specific frequency bands. Therefore, the filter bank can replace some function of the microphone and the processor within the modern hearing aids. Such a MEMS filter bank works as a passive component, and it has the advantages of lower energy consumption and without computation time. Combining with its excellent aging and thermal stability characteristics, the novel MEMS filter bank will be a potential device for effective sound signal processing in modern hearing aids. The paper shows the fabrication process of the novel MEMS filter bank. Our preliminary results show that the device dimensions of the filter bank are about millimeter level for the low operating frequency requirement, lOOHz to 10000Hz. The devices are then apparent bending by residual stress with low stress silicon nitride film. The phenomenon will be discussed and resolved in the future study.

Keywords - MEMS bandpass filter, filter bank, sound signal processing, hearing aids application, piezoresistive cantilevers

I. INTRODUCTION Many people have the hearing problem, for instance, as indicated in [11, more than 28 million Americans are deaf or hard of hearing. People with hearing loss are confused and hindered from the social communication, and they are usually suggested to wear hearing aids for enhancing the worse communication status of human life. With the speech characteristics, a crucial issue is to divide the input signal into multiple frequency bands for tuning individual frequency properties in modem hearing aids. The rapid development of semiconductor and digital techniques make digital signal processor (DSP) to become a substantial core for such techniques [2,3]. However, the contemporary DSPs are of deficiencies that need higher power consumption and computation time for some applications. The mechanical filter then provides as an alternative for solving this crucial issue if its size can reduce. The mechanical filters have been well known at least five decades ago [4]. They are usually used for extracting signals from the specific frequency band, and are designed as electrical filter functionality. Recently, by means of the microelectromechanical system (MEMS) techniques, the mechanical filters with micron dimensions have demonstrated more feasible for some extended applications, such as the utilization for signal processing from high

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frequency (HF) to radio frequency (RF) region [5-81. An electromechanical filter bank constructed by silicon beams was also investigated for HF communication system [9]. Within the cited filter bank, each filter was comprised of only one beam structure with its sharp frequency response. The sharp frequency response always results poor linearity for the passband signals in the filter. To develop a revolutionary signal processor in hearing aids, we proposed a novel MEMS filter bank in this study. Its characteristic and design parameters were investigated by simulation approach throughout [lo]. The MEMS filters were simulated by multiple mechanical structures, and the simulation was conducted on their critical parameters, such as the structural damping factors and the MEMS Q factor of the filters. The MEMS Q factor is a new factor, which is proposed and defined in [lo]. The simulation result shows that the MEMS filter is capable of achieving different filter performance in acoustic frequency region. Besides, the fabrication process of MEMS filter bank was proposed in this paper. The MEMS filter bank was also successful realized by multiple piezoresistive cantilevers. With the excellent temperature and aging characteristics in mechanical filters [ll], the novel MEMS filter bank is expected to be a revolutionary device for modem hearing aids in future. 11. THEORY

As shown in Fig. 1, the proposed MEMS filter bank for speech signal processing is consisted of multiple mechanical structures with microns in dimensions. The mechanical structure has specific natural frequencies and mode shapes as its dynamic characteristics [ 121. The structures respond to acoustic stimulations, and produce the associated electrical signals by their sensing circuits, e.g. piezoresistive circuits. By utilizing a differential amplifier to process the picked-up electrical signals, we can have the phase and amplitude response of individual mechanical structure on frequency domain. The result signal can then be as a signal filtered by a filter with special frequency response. Therefore, if the responses of multiple structures are coupled, the preset bandpass filter functionality can be implemented in the MEMS filter, as given in Fig. 2. For unobvious effects on higher resonating mode and convenient illustration, we considered the first resonating vibration mode of each structure. We also discussed the filter contain only two structures for simplicity in this paper.

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Poster 110

Amp.

Fig. 1. The illustration of the proposed MEMS filter bank

After deriving the equations of motion of the MEMS filter, its dynamic behavior will be simulated by LabVIEW, a commercial program. As results, the optimized design parameters for the MEMS filter bank can be obtained. We found that the MEMS Q factors and damping factors affect the efficiency of the MEMS filters. The simulation results were summarized as Fig. 3. The detail explanations of the figure were discussed in [lo]. 111. DESIGN AND FABRICATION

Fig. 3. The ripple responsewith the coupling effect of the MEMS Q factor and the damping factor [lo].

where p and E are the density and the Young's modulus of the structure. To apply the MEMS filter bank in hearing aids, the natural frequency of each cantilever must be ranging from 1OOHz to 10000Hz. Moreover, the available materials and thickness of the micromachined cantilevers are limited to the micromachining processes. In this study, a 0.5 pm thick LPCVD Si,N, was selected as the material of the cantilevers for the prototype design. The length L of the cantilevers will then be determined by (1). Accordingly, the lengths of cantilevers are ranging from 200pm to 1200pm.

A. Structural and sensing circuits design

B. Fabrication process

In this paper, the micromachined cantilevers were exploited as sensing structures to demonstrate the feasibility of the novel MEMS filter. The natural frequency f of a cantilever with length L and thickness t can be expressed as r121.

The fabrication processes of the MEMS filters are given in Fig. 4, and the general steps are: 1. Deposit 0.5pm low stress Si,N, as the structure material and 0.2pm polysilicon using a low-pressure chemical vapor deposition (LPCVD), in sequence, onto a (1 1])-orientation silicon substrate; 2. Dope polysilicon film with phosphorus as the piezoresistive sensing circuits. Deposit phosphosilicate glass (PSG) onto polycrystalline silicon film at 950°C (P0Cl3+O2) and followed by drive-in annealing at 950 "C in the hmace. Furthermore, remove the PSG layer in buffered oxide etchant (BOE); 3. After photolithography, pattem the doped polysilicon film using reactive ion etching (RIE); 4. Plasma-enhanced chemical vapor deposition (PECVD) deposit a 0.5pm thick Si,N, as a protective layer during the structure-releasing etching step; 5. Pattern both of the LPCVD and PECVD Si,N, films by thick photoresist lithography and N E ; 6. Etch a 30pm deep cavity using inductively coupled plasma etching (ICP) and remove the PR; 7. Immerse wafers in 15% KOH solution at 80°C for 1 hour to release the structure; 8. Immerse wafers in BOE to remove PECVD Si,N, film; and

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Poster 1,lO

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Dry the devices and using isopropanol (IPA) to prevent the stiction problem.

Iv. RESULTS AND DISCUSSIONS The SEM photo in Fig. 5 shows the cantilevers fabricated through the above fabrication process. The cantilevers have various lengths to provide different first

0.5pm LPCVD Si,N, and 0.2pm LPCVD polysilicon are deposited on ( I 11) silicon wafer.

The polysilicon film is doped with phosphorus atoms by diffusion

V. CONCLUSIONS AND FUTURE WORKS

Pattem the doped polysilicon film for realizing the piezoresistive circuits

The design and fabrication of the proposed novel MEMS filter bank was implemented in the paper. Obviously, the residual stress remarkably affects the cantilever configuration with longer length. Therefore, the most important improvements should be hied in the future is the structure material and geometry should be modified for reducing the mechanical size and residual stress. Besides, the gauge factor of the piezoresistive material should be also investigated and improved.

Deposit PECVD Si,N, film for realizing the piezoresistive circuits

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bending frequency. Because of the gradient residual stress of the thin film materials, the cantilevers were bent upward significantly, especially for the longer one. In this study, the (111) Si substrate is employed to reduce the undercut etching time for longer beams, so as to prevent the variation of the beam thickness along the beam length [ 131. The octagon cavities appearing in the photo were formed by (1 11) crystal planes on the (1 11) substrate after KOH etching. In addition, four piezoresistive circuits associating with each cantilever were realized. Only the piezoresistive circuit padded on the root of the cantilever is a variable resistor, which is varied with the strain of cantilever, and the Wheatston bridge were designed for circuit powering and signal acquisition by acoustic stimulation.

Pattem PECVD and LPCVD Si,N,. film and etch by RIE

ACKNOWLEDGMENTS The authors would like to thank all the encouragements from the colleagues in the Micro-Device Lab in National Tsing Hua University and the Bioelectronics Lab in National Yang Ming University. Also, the authors would like to appreciate NSC Central Regional MEMS Research Center (Taiwan), Semiconductor Center of National Chiao Tung University (Taiwan), and National Nan0 Device Laboratories (Taiwan) in providing the fabrication facilities.

Etch the silicon substrate about 30pm by ICP

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Etch the silicon substrate by. KOH and immerse IPA solution for releasing the structure

LPCVD Si,N, 50008, Polysilicon 2000A Phosphorous doped polysilicon 20008, PECVD SLN, 5000A Fig. 4. Fabrication process of MEMS filter

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Figure 5 SEM photography of MEMS filter bank

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May 24,2002, Madison, Wisconsin USA - 0-7803-7480-0/02/$17.00 0 2 0 0 2 IEEE.

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REFERENCES [I] htto://www.nidcd.nih.eov/health/hearing [2] H. McDermott, “A programmable sound processor for advanced hearing aid research,” IEEE Transactions on Rehabilitation Engineering, vol. 6( I), March 1998, pp. 53 - 59 [3] J. Mitchell, W. Pruehsner, J.D. Enderle, “Digital hearing aid,” Proceedings of the IEEE 25th Annual Northeasf Bioengineering Conference, 1999, pp. 133 - 134 [4] R. Adler, “Compact electromechanical filter,” Electronics, April 1947, pp. 100 - 105 [5] L. Lin, R.T. Howe, A.P. Pisano, “Microelectromechanical filters for signal processing,” Journal of Microelectromechanical Systems, vol. 7(3), September 1998, pp. 286 - 294 [6] C.T.-C. Nguyen, “Frequency-selective MEMS for miniaturized low-power communication devices,” IEEE Transactions on Microwave Theory and Techniques, vol. 47(8), August 1999, pp. 1486 - 1503 [7] L.-J. Yang, T.-W. Huang, P.-Z. Chang, “CMOS Microelectromechanical

bandpass filters,” Sensors andActuaforsA, vol. 90,2001, pp. 148 - I52 [SI K. Wang, C.T.-C. Nguyen, “High-order micromechanical electronic filters,” Proceedings of the IEEE Tenth Annual Infernafional Workshop on Micro Electro Mechanical Sysfems, 1991, pp. 25 - 30 [9] M.F. HribSek, “Electromechanical silicon beam filter bank,” Microelectronics Journal, vol. 27,1996, pp. 525 - 530 [IO] S.-H. Shen, S. T. Young, W. Fang, “High-Performance MEMS Bandpass Filters for Acoustic Signal Processing Applications,” Proceedings of SPIE, Elecfronics and structures for MEMS II, vol. 4591,2001, pp.292-301 [ I I ] R.A. Johnson, Mechanical filters in electronic, John Wiley & Sons, New York, 1983, pp. 7 - 8 [I21 M.L. James, G.M. Smith, J.C. Wolford, P.W. Whaley, Mbration of mechanical and structural systems, HarperCollins, New York, 1994, pp. 136- 143 [I31 H.-H. Hu, H.-Y. Lin, W. Fang, B.C.-S. Chou, 2001, “Characteristics of the Micromachined Beams on the (1 1 I ) Substrate,” Sensors and Actuators A, VOI. 93, pp. 258 - 265

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