Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 459 Mã bài: 108 Nghiên cứu thiết kế và chế tạo con quay vi cơ fork tuning có độ nhạy cao Design and Fabrication of an Enhanced Sensitivity Tuning Fork Micro-Gyroscope Nguyen Quang Long 1 , Chu Manh Hoang 1,  , Trinh Quang Thong 2 , Chu Duc Trinh 3 and Vu Ngoc Hung 1 1 International Training Institute for Materials Science, Hanoi University of Science and Technology 2 The Institute of Engineering Physics, Hanoi University of Science and Technology 3 MEMS Dept., Faculty of Electronics and Telecommunications, University of Engineering and Technology, Vietnam National University e-Mail: hoangcm@itims.edu.vn Tóm tắt: Chúng tôi trình bày kết quả nghiên cứu thiết kế và chế tạo cảm biến con quay vi cơ fork tuning có độ nhạy cao. Độ nhạy vận tốc góc của cảm biến được khuếch đại bởi hệ số phẩm chất của mốt nhạy khi điều kiện cộng hưởng cơ trong mốt nhạy và mốt chấp hành được thỏa mãn. Cấu trúc điện cực răng lược trong mốt nhạy cũng được cải tiến để tăng cường độ nhạy của cảm biến. Các thông số thiết kế của con quay vi cơ thỏa mãn điều kiện cộng hưởng cơ của mốt nhạy và mốt chấp hành đã đạt được từ phân tích phần tử hữu hạn. Con quay vi cơ đã đươc chế tạo bằng công nghệ vi cơ khối tỷ lệ cạnh cao trên phiến SOI. Tần số cộng hưởng của mốt nhạy và mốt chấp hành của cảm biến chế tạo đo được là 11125 Hz và 11250 Hz. Kết quả này phù hợp với kết quả phân tich phần tử hữu hạn. Hệ số phẩm chất của mốt nhạy và mốt chấp hành được xác định bẳng thực nghiệm trong môi trường không khí là 44,5 và 140. Abstract: We report design and fabrication of an enhanced sensitivity tuning fork micro-gyroscope. The angular rate sensitivity of the gyroscope is amplified by the mechanical quality factor of the sense resonant mode when the resonant frequencies of the driving and sensing modes are closely matched. Added comb electrodes in the parallel-plate sensing comb structure were also designed for enhancing the sensitivity of the sensor. From finite element analysis, the design parameters for a gyroscope structure having closely matched resonant frequencies were obtained. The gyroscope was fabricated by SOI-based high-aspect ratio micromachining process. The drive and sense mode resonant frequencies of the fabricated device were experimentally measured to be 11250 Hz and 11125 Hz, respectively, which are in good agreement with the designed prediction by FEA. The measured quality factors of driving and sensing modes in air are 140 and 44.5, respectively. 1. Introduction Micromachined vibratory gyroscope is a key element for applications in space navigation, automobile and consumer electronics [1]. Recently, micromachined capacitive type tuning fork gyroscopes has attracted a great deal of attention due to their advantages such as low cost, small size, low power consumption using capacitance sensing mechanism. In addition, the feature structure design of tuning fork is considered to be insensitive to vibration and possible to reject common-mode acceleration inputs by a differential Coriolis measurement [2]. However, the detection of the sensor signal in micromachined gyroscopes is quite challenging due to small output capacitance signal. In order to improve the sensitivity of a vibratory gyroscope, the driving and sensing vibration modes of the gyroscope need designed and fabricated with the matched resonant frequencies [3]. When driven at the resonant condition, the amplitude of sensing mode vibration is amplified by its mechanical quality factor. In [4], the sensing-mode quality factor is increased by eliminating the energy dissipation through the substrate using the anti- phase operation of a dual mass tuning fork gyroscope architecture and the mode-matching operation in vacuum condition. In a latest report [5], the research showed that the tuning fork gyroscopes having decoupled sense and drive masses with an anchored drive mass is less 460 Nguyen Quang Long, Chu Manh Hoang, Trinh Quang Thong, Chu Duc Trinh, Vu Ngoc Hung VCM2012 sensitive to vibration than tuning fork gyroscope designs featured in literature. To increase signal to noise ratio, the mass and capacitance of a capacitive type gyroscope are also required to be as large as possible. In this paper, design and fabrication of a symmetric tuning fork gyroscope are reported. The driving and sensing vibration modes of the gyroscope are designed and fabricated with the closely matched resonant frequencies for enhancing the sensitivity. Added comb electrodes in the parallel-plate sensing comb structure were also designed for enhancing the sensitivity of the sensor. The design parameters for a gyroscope structure having closely matched resonant frequencies were obtained by finite element analysis. The gyroscope was fabricated by SOI- based high-aspect ratio micromachining process. The operation characteristics of the fabricated device were investigated by designed electronic interface circuit. 2. Design The design of micromachined turning fork gyroscope is showed in Fig. 1 with the lateral dimension of 4500 μm x 4350 μm. The thickness of the structure is 30μm. The device would be fabricated by one-mask process using Silicon On Insulator (SOI) wafer. Fig. 1 Schematic of micromachined Tuning Fork gyroscope: (1) outer mass frame, (2) inner mass frame, (3) drive comb electrodes, (4) sense electrodes, (5) folded beam, (6) anchor, (7) lozenge coupling spring, and (8) self-rotation ring The main structure of the Turning Fork Gyroscope design comprises two proof masses, each of which includes the outer frame for driving and the inner one for sensing. The drive comb electrode set is attached to the outer frame and designed such that in driving mode the masses oscillate in opposite direction along x-axis due to electrostatics force. The rotation rate is measured by a capacitance change. A pair of sense electrode set using the balanced scheme is placed symmetrically within each inner mass frame. In order to increase the change of capacitance, we here design four comb banks each side of sense electrode set. The dimension of comb finger is 30 μm length and 3 μm width. The gap between two adjacent fingers is 2.5 μm. Upon the rotation the Coriolis force excites these mass frames in out of plane motion that the differential capacitance can be detected. This design also employs the folded beams for suspension. The suspensions of the proof mass are designed to allow the structure to oscillate in two orthogonal modes. After we have the designed model, the sensor was verified by finite element analysis (FEA) using ANSYS software. In this case, SOLID45 element was employed for modeling and simulation. The dimension parameters of the proof mass and the suspension beam were investigated to have the optimal the designed mechanical structure of the gyroscope because vibration modes are strongly dependent on these parameters. Figure 2 shows the FEA result of gyroscope for sensing and driving modes. The resonant frequencies of sensing and driving modes are determined as 10,038 kHz and 9,918 kHz, respectively. In fact, when the two modes are matched, the output signal is amplified by the quality factor of the sense mode, thereby increasing the sense displacements by orders of magnitude. Then, the amplitude along sensing direction achieves the maximum. However, it leads to a problem that the response time would be long. The response of the gyroscope to time varying rotation rate gives an indication of the bandwidth of the sensor. The larger the bandwidth, the quicker is the response of the sensor. That is why there must be the difference between these frequencies causing a width of the range of frequencies. For Turning Fork Gyroscope, the larger bandwidth is attained at the cost of sensitivity. In this case, the drive and sense mode frequencies have a mismatch of about 100 Hz corresponding to the sensor bandwidth. This result satisfies the requirement to optimize sensibility and bandwidth. Using SIMULINK model, the sensitivity of sensor was determined to be 11 fF/deg/s. To increase further the sensitivity, the natural frequencies of the sensor structure need decreased. Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 461 Mã bài: 108 Fig. 2 FEA result of gyros obtained by ANSYS, (a) sensing and (b) driving mode 3. Fabrication process The gyroscope has been fabricated by SOI-based MEMS technology as shown schematically in Fig. 3. The main steps in the fabrication process are described as follows. The 4-inches Silicon-On- Insulator (SOI) wafer was used for fabrication with thickness of a device layer is 30 µm, buried silicon dioxide layer is 4 µm and substrate is about 500 µm (Fig. 3 (a)). First, the SOI wafer has been cleaned by SC process. Next, the sensors patterns were transferred to the surface of SOI wafer after photolithography and developing processes (Fig. 3 (b)) with positive photoresist layer. This layer has a role as protecting mask, which used for DRIE process in next step. Fig. 3 Main steps of the fabrication process Then, DRIE process was performed to a depth of 30 µm to reach the buried dioxide layer of SOI wafer (Fig. 3 (c)). The SOI wafer was then diced to separate each sensor. Vapor HF etching process was done to etch the SiO 2 underneath the device layer and release the movable electrodes and beams (Fig. 3 (d)). 4. Results and discussion The fabrication of gyroscope have been successfully performed by using only one photo mask and standard MEMS processes such as photolithography, DRIE processes and vapor HF etching. The 4-inches SOI wafer with device layer of 30 µm and buried SiO 2 layer of 4 µm has been used to fabricate hundreds of sensors. After fabrication, these sensors are ready for testing of performances. The fabricated gyroscope is characterized by SEM. Fig. 4 SEM pictures of fabricated sensor: top view of whole structure (a), holes for releasing structure (b) and zoom-in comb-fingers (c) (a) (b) 462 Nguyen Quang Long, Chu Manh Hoang, Trinh Quang Thong, Chu Duc Trinh, Vu Ngoc Hung VCM2012 Figure 4 shows SEM pictures of gyroscope after fabrication process. The pictures show that the good fabrication process has been achieved. All the moving parts seem to be released. The edges are very well etched and not broken or damaged in structure and sensing electrodes. Packaging is a final step to complete the fabricated sensors. Firstly, the sensor chip is glued on a device holder using two components epoxy, which may be either made of hard plastic or metal with pins for use. Next step is wire bonding process. In this work, the Westbond 7400C was used to package the device. Figure 5 shows a picture of packaged device. When the fabrication period is finished, basic tests are performed on capacitive micromachined sensors. Fig. 5 Sensor after packaging Fig. 6 The 1-port actuation and detection scheme for frequency response extraction These are the stiction test, short circuit test, and capacitance test. These basic tests were carried out using probe station equiped a microscope, a milimeter and an impedance analyzer. In order to improve the sensitivity of a vibratory gyroscope, the driving and sensing vibration modes of the gyroscope need fabricated with the closely matched resonant frequencies. To characterize frequency response of the driving and sensing modes, the sensor is driven with a varying frequency AC signal from one end of the drive or sense electrode and then the output is taken from the other end of the electrode. Figure 6 is schematic diagram of frequency response extraction circuit. The magnitude and the phase of the output shows the magnitude and phase response of the system. From this data, we can see that the peak value of the magnitude response, which corresponds to the resonance frequency of the sensor. A problem existing in this test is that the output signal is naturally small. So, an amplifier configuration is used in output end to amplify the signal. In this test, the OP37 IC chip was used. Beside the output signal is small, another problem is that it is very hard to control both DC and AC applied voltage, so the circuit to control magnitude, frequency of AC and DC voltage applied on sensor have also been built. Fig. 7 Frequency response of driving mode Fig. 8 Frequency response of sensing mode The measured quality factors of driving and sensing are 140 and 44.5, respectively. 5. Conclusion Design and fabrication of a symmetric tuning fork gyroscope with enhanced sensitivity were presented. The closely matched resonant Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 463 Mã bài: 108 frequencies were obtained for the driving and sensing vibration modes of the gyroscope. In order to enhance the sensitivity of the gyroscope, parallel-plate sensing comb structure with increasing number of comb electrodes was also designed. The gyroscope was fabricated by SOI- based high-aspect ratio micromachining process. Acknowledgement This work is supported by the Ministry of Science and Technology (MOST), Vietnam under the NAFOSTED project coded MS 103.02-2010.23. References [1] Yazdi, N.; Ayazi, F.; Najafi, K.; Micromechanical inertial sensors, in : Proceedings of the IEEE, pp. 1640-1659, 1998 [2] Weinberg, M.S.; Kourepenis, A.; Error sources in in-plane silicon tuning-fork MEMS gyroscopes, J. Microelectromech. Syst. 15 (3), pp. 479-491, 2006 [3] Maenaka, K., Fujita, T.; Konish, Y.; Maeda; M.; Analysis of hightly sensitive silicon gyroscope with cantilever beam as vibratin,g mass, Sen. Actuators A, 54, pp. 568-573, 1996 [4] Trusov, A.A.; Schofied A.R.; Shkel, A.M.; Study of substrate energy dissipation mechanism in in-phase and anti-phase micromachined vibratory gyroscopes, in: Proc. IEEE sensors, pp. 168-171, 2008 [5] Yoon, S.W.; Lee, S.; Najafi, K.; Vibration- induced errors in MEMS tuning fork gyroscopes, , Sen. Actuators A, 180, pp. 32-44, 2012 Long Quang Nguyen received the Diploma Engineer degree in material engineering from Hanoi University of Science and Technology (HUST) in 2010. Since 2009, he has been working as a research assistant at International Training Institute for Materials Science (ITIMS) in the field of micromechanical systems. His current interests are design and development of MEMS mechanical sensor. Chu Manh Hoang received the M.Sc. degree in materials science from the International Training Institute for Materials Science, Hanoi University of Science and Technology, Hanoi, Vietnam, in 2007 and the Dr. Eng. degree from the Graduate School of Mechanical Engineering, Tohoku University, Japan, in 2011. He was a Fellow of the Japanese Society for the Promotion of Science from April 2010 to March 2012. Since September 2012, Dr. Chu is a lecturer at Hanoi University of Science and Technology. His current research interests are MEMS inertial sensors, micro-mirrors, and nanophotonic. He is a reviewer for several international journals. Thong Quang Trinh received his PhD degree in electrical engineering from Dresden University of Technology, Germany, in 2006. From 1986 to 1999 he worked in the field of applied physics at the Institute for Applied Physics at Vietnamese Academy of Science and Technology (VAST). Since 2000 he has been Senior Scientist at Hanoi University of Science and Technology (HUST). His current research interests are focused on the design of sensors and sensor systems including the simulation of their components as well as development of MEMS mechanical sensors for different applications. Chu Duc Trinh received the B.S. degree in physics from Hanoi University of Science, Hanoi, Vietnam, in 1998, the M.Sc. degree in electrical engineering from Vietnam National University, Hanoi, in 2002, and the Ph.D. degree from Delft University of Technology, Delft, The Netherlands, in 2007. His doctoral research concerned piezoresistive sensors, polymeric actuators, sensing microgrippers for microparticle handling, and microsystems technology. He is currently an Associate Professor with the Faculty of Electronics and Telecommunications, University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam. Since 2008, he has been the Vice-Dean of the Faculty of Electronics and Telecommunications. He has been chair of Microelectromechanical Systems and Microsystems Department, since 2011. He has authored or coauthored more than 50 journal and conference papers. He was the recipient of the Vietnam National University, Hanoi, Vietnam Young Scientific 464 Nguyen Quang Long, Chu Manh Hoang, Trinh Quang Thong, Chu Duc Trinh, Vu Ngoc Hung VCM2012 Award in 2010, the 20th anniversary of DIMES, Delft University of Technology, The Netherlands Best Poster Award in 2007 and the 17th European Workshop on Micromechanics Best Poster Award in 2006. He is guest editor of the Special Issue of “Microelectromechanical systems” Vietnam journal of Mechanics, in 2012. Hung Ngoc Vu received the B.S. degree in physics from Kishinev University (USSR), in 1979 and the Ph.D. degree from Hanoi University of Technology (Vietnam), in 1991. His doctoral thesis dealt with the xeroradiography. At present, he is an Associate Professor with the International Training Institute for Materials Science (ITIMS), Hanoi University of Technology. His current research interests are in the area of MEMS inertial sensors and PiezoMEMS. . nghị Cơ điện tử toàn quốc lần thứ 6 459 Mã bài: 108 Nghiên cứu thiết kế và chế tạo con quay vi cơ fork tuning có độ nhạy cao Design and Fabrication of an Enhanced Sensitivity Tuning Fork. and Technology, Vietnam National University e-Mail: hoangcm@itims.edu.vn Tóm tắt: Chúng tôi trình bày kết quả nghiên cứu thiết kế và chế tạo cảm biến con quay vi cơ fork tuning có độ nhạy. cơ thỏa mãn điều kiện cộng hưởng cơ của mốt nhạy và mốt chấp hành đã đạt được từ phân tích phần tử hữu hạn. Con quay vi cơ đã đươc chế tạo bằng công nghệ vi cơ khối tỷ lệ cạnh cao trên phiến