Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 399 Mã bài: 98 Cảm biến gia tốc trục z vi cơ với mạch tích hợp đọc tín hiệu lối ra kiểu tụ điện chuyển mạch Micromachined Z-Axis Accelerometer with Switched-Capacitor Readout Integrated Circuit Chu Manh Hoang 1, , Nguyen Quang Long 1 , Chu Duc Trinh 2 and Vu Ngoc Hung 1 1 International Institute for Materials Science, Hanoi University of Science and Technology, Hanoi, Vietnam 2 MEMS Dept., Faculty of Electronics and Telecommunications, University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam e-Mail: hoangcm@itims.edu.vn Tóm tắt: Trong bài báo này, nghiên cứu thiết kế, chế tạo và đặc trưng của cảm biến gia tốc kiểu điện dung trục z sẽ được trình bày. Bản cực tụ điện nhạy gia tốc được cấu tạo từ bề mặt khối gia trọng được treo bởi các lò xo gồm các thanh bim gấp được tạo thành từ lớp silíc dày 30 micromét trên bề mặt phiến SOI. Lớp điện môi giữa khối gia trọng và bản tụ điện đối diện với nó là lớp không khí được tạo ra bằng cách ăn mòn lớp oxít đệm của phiến SOI. Cảm biến gia tốc được thiết kế trên cơ sở phân tích phần tử hữu hạn với phần mềm ANSYS. Đặc trưng hoạt động của cảm biến được kháo sát bằng mạch đọc tín hiệu đầu ra kiểu điện dung chuyển mạch. Mạch đo có thể đo lường chính xác sự thay đổi điện dung bằng cách giảm điện dung ký sinh với chức năng căn chỉnh độ nhạy và độ lệch không ban đầu. Hoạt động của cảm biến đã được khảo sát trong dải gia tốc từ 0 tới 4g. Độ nhạy của cảm biến đo được bằng thực nghiệm là 1,6 mV/g. Abstract: Design, fabrication and measured performance of a z-axis capacitance accelerometer are presented. The acceleration sensing capacitor plate/the proof mass and folded beam springs are constructed from 30 µm thick top silicon layer of SOI wafer. The dielectric layer between the proof mass and the counter capacitance plate is air gap which is defined by removing 2µm thick buried oxide layer of SOI wafer. The accelerometer was designed using finite element analysis with ANSYS software. The operation of the accelerometer is investigated by switched capacitor readout circuit. The measurement circuit enables to detect precisely the change of capacitance by reducing parasitic capacitance error with offset and sensitivity calibration functions. The operation of the accelerometer was investigated in the range of acceleration from 0 to 4g. The measured sensitivity of the accelerometer was 1.6 mV/g. 1. Introduction Compared to other types of acceleration sensors, capacitive type acceleration sensors have realized intensive research interest due to their advantages such as high sensitivity, high reliability, low temperature dependence, and low power consumption. The development of MEMS techniques such as bulk and surface micromachining allow to substantially improve the performance and decrease the cost of the MEMS inertial accelerometers. For bulk micromachining, the masses and spring suspensions are formed by etching through the complete wafer [1, 2]. Consequently, the masses available for the acceleration-to-force conversion are relatively large which enables the fabrication of sensitive devices. For surface micromachining, the masses and spring suspension are formed by selective etching of sacrificial layers, which have a thickness of typically several microns [3]. As a result, the masses are rather small. This technique requires more sophisticated processing techniques with on-chip electronics, because the sensor signals are rather small and parasitic effects are relatively large. The combination of bulk and surface micromachining was also used for fabricating the accelerometer [4]. Recently, MEMS accelerometer fabricated on SOI wafer has advantages of the superior material properties of a single crystal silicon material, low cost and high reliability mass production, and feasibility of IC integration. There are several reports on the integration of SOI accelerometer and sensing electronic circuit. Especially, the integration of the SOI accelerometers in hybrid systems is preferred due to its production yield. In [5], the accelerometer was integrated in a closed-loop system, in which bandwidth, linearity and dynamic range of the sensor was improved. In another 400 Chu Manh Hoang, Nguyen Quang Long, Chu Duc Trinh and Vu Ngoc Hung VCM2012 approach, the hybrid integration was reported using CMOS circuit [6]. In this paper, we develop a capacitive type z-axis accelerometer based on SOI micromachining. Analysis and design of the accelerometer was carried out using finite element analysis with ANSYS software. The design of the proof mass and spring is considered to keep the device small size and desired sensitivity. The interfaced electronic circuit for investigating the characteristic of the accelerometer is designed, which is based on switched capacitor readout circuit. The measurement circuit enables to detect precisely the change of capacitance by reducing parasitic capacitance error with offset and sensitivity calibration functions. 2. Design The accelerometer consists of a sensing element/proof mass which is suspended by four folded flexible beams. The sensing element is a movable capacitor plate constructed from the top silicon layer. The Fig. 1 Top view schematic of z-axis accelerometer Fig. 2 Oscillation mode in z-axis simulated by FEM counter capacitor plate is formed by the substrate layer of SOI wafer. In order to have a reasonable capacitance change for detecting applied acceleration, the proof mass and counter capacitor plate needs separated by a narrow gap. In this study, the SOI wafer having 2µm thick buried oxide layer is used. Moreover, to release the proof mass after Dry Reactive Ion Etching (DRIE), perforated holes are opened for etching buried oxide layer as shown in Fig. 1. The design of perforated holes are also to decrease the squeezed air film damping between the proof mass and counter capacitor plate. By changing the dimensions of the spring beams, we model and simulate the performance of the accelerometer. Figure 2 shows mode analysis result of the accelerometer. The natural frequency of z-axis oscillation is 7.8 kHz. The outermost device area is designed to be 2mm x 2mm. The spring is composed of five single beams, each beam has dimensions of 6µm wide, 236 µm long and 30 µm thick. 3. Fabrication The fabrication process starts with SOI wafer having the top silicon top 30 µm thick, the buried oxide layer 2µm thick and the bottom silicon layer 400 µm thick. The wafer is thermally oxidized to have a protective oxide mask layer in the fabrication of device. The fabrication process is one-mask photolithography process. First, the structure of sensor including the proof mass frame, springs, hole contacts, and holes for post-releasing device are patterned by a positive photoresist polymer device (Fig. 3 (a)). The patterned surface then is etched by Deep Reactive Ion Etching (DRIE) device (Fig. 3(b)). To release the device for the operation, the buried oxide layer is removed by a HF solution for 30 min device (Fig. 3 (c)). Fig. 3 Fabrication process of accelerometer Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 401 Mã bài: 98 4. Measurement setup The block diagram of the measurement system is used for testing the capacitive sensor shown in Fig. 4. The measurement system is composed of an interface electronic circuit, analog to digital converter, a parallel port and a control computer. The interface electronic circuit is a switched- capacitor integrator type differential open-loop capacitive readout circuit. The circuit consists of a charge amplifier, low-pass filter, and a buffer for amplification. It outputs a voltage that is proportional to the change in Fig. 4 Block diagram of measurement system Fig. 5 Static characteristic of input switched-capacitor integrator for: (a) C F =1.197 pF and C S2 = 0.798 pF and (b) C F = 0.209 pF and C S2 =0.798 pF capacitance. C S1 and C S2 are the internal trimming capacitances that can be adjusted to balance the external capacitance C S . The acceleration sensor is connected as C S , and the internal capacitance C S1 is used to balance the circuit, that is to eliminate any offset in the baseline of the voltage output. When an external acceleration applies on the sensor, the capacitance of C S is changed; this is reflected as the output voltage, as the bridge is no longer balanced. Charge amplifiers are commonly used in read-out circuitry as they have the advantage of measuring very small charges thus enabling small capacitance measurement. A charge amplifier consists of an operational amplifier with a feedback capacitor (C F ). The charge amplifier is followed by a low pass filter (LPF) which filters out the high frequency components of the signal; noise tends to be high frequency. The maximum frequency response of the circuit is limited by the LPF, the break- frequency of which is adjustable from 500 Hz to 8 kHz. The final stage is the amplification of the signal using buffering components and amplifiers. The signal from the output buffer is converted to digital mode using an analog to digital (A/D) converter on a National Instruments data acquisition board controlled via LabView software. The circuit is placed in the socket of the evaluation board and it is interfaced to the computer via parallel port. The characteristics of interface electronic circuit were investigated. Figure 5 (a) and (b) shows static characteristics for two capacitors sets with feedback capacitance values of 1.197 pF and 0.209 pF respectively. Thus, by adjusting C F , the sensitivity and measured range of system can be controlled. 5. Results and discussion In order to characterize the operation of the accelerometer, the device is bonded on a circuit board by expoxy for characteristic investigation. Figure 6 shows an array of six the accelerometers bonded on a circuit board. Wiring to the devices is carried out 402 Chu Manh Hoang, Nguyen Quang Long, Chu Duc Trinh and Vu Ngoc Hung VCM2012 Fig. 6 A six-accelerometer array bonded on a circuit board for characteristic investigation Fig. 7 Variation of the output voltage of the accelerometer measured as a function of z-axis applied acceleration using wedge bonding machine as seen in Fig. 6. To generate a z-axis acceleration, we fixed the device with measure circuit board on a vibrator. The acceleration that the vibrator can generate is ± 4g. The acceleration of vibrator is calibrated by a standard accelerometer which has the exactness of 0.1g. Figure 7 shows variation of the output voltage of the accelerometer measured as a function of z-axis applied acceleration. The operation characteristic of the accelerometer is linear in the range of investigated acceleration from 0 to 4 g. The sensitivity of the accelerometer was measured to be 1.6 mV/g. It is noticed that the exactness of output voltage measurement is 0.1 mV. Therefore, the sensitivity threshold of sensor is 0.07g. 6. Conclusion We presented the design, fabrication and characterization of a capacitive type z-axis accelerometer. Analysis and design of the accelerometer was carried out using finite element analysis with ANSYS software. The accelerometer was fabricated by SOI-based micromachining technology. The interfaced electronic circuit for investigating the characteristic of the accelerometer is designed using switched capacitor readout circuit. The measurement circuit enables to detect precisely the change of capacitance by reducing parasitic capacitance error with offset and sensitivity calibration functions. The sensitivity of the accelerometer was experimentally measured to be 1.6 mV/g. 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] Rudolf, F.; A micromechanical capacitive accelerometer with a two-point inertial-mass suspension, Sensors and actuators A 4,191- 198, 1983 [2] Hung, V. N.; Hang, N. T. M.; Tan, T. D.; Long, N. T., Hoang, C. M.; Thuy, N. P.; and Chien, N. D.; A highly sensitive micromachinined silicon based accelerometer, The 9th International Conference on Mechatronics Technology, December 5 – 8, Kuala Lumpur, Malaysia, 2005 [3] Howe, R. T.; Boser, B. E.; Pisano, A. P.; Polysilicon integrated microsystems technologies and applications, Sensors and Actuators A 56, 167-177, 1996 [4] Yazdi, N.; Nafafi, K.; An all-silicon single- wafer micro-g accelerometer with a combined surface and bulk micromachining process, Journal of Microelectromechanical systems, pp.1-8, 2000 [5] Kraft, M.; Lewis, C. P.; and Hesketh, T. G.; Closed-loop silicon accelerometers, IEEE Pro- Circuits Syst 146, 325, 1998 [6] Chen, W.; Chen, H.; Liu, X.; and Tan, X; A Hybrid micro-accelerometer system with CMOS readout circuit and self-test function Mechatronics, MEMS and Smart Materials SPIE 6040 (604004) 33-37, 2005 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 awarded the Fellowship of the Japanese Tuyển tập công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 403 Mã bài: 98 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. 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 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 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. . công trình Hội nghị Cơ điện tử toàn quốc lần thứ 6 399 Mã bài: 98 Cảm biến gia tốc trục z vi cơ với mạch tích hợp đọc tín hiệu lối ra kiểu tụ điện chuyển mạch Micromachined Z- Axis Accelerometer. cứu thiết kế, chế tạo và đặc trưng của cảm biến gia tốc kiểu điện dung trục z sẽ được trình bày. Bản cực tụ điện nhạy gia tốc được cấu tạo từ bề mặt khối gia trọng được treo bởi các lò xo gồm. khối gia trọng và bản tụ điện đối diện với nó là lớp không khí được tạo ra bằng cách ăn mòn lớp oxít đệm của phiến SOI. Cảm biến gia tốc được thiết kế trên cơ sở phân tích phần tử hữu hạn với