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 Ma
Trang 1Cảm 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 Hoang1,, Nguyen Quang Long1, Chu Duc Trinh2 and Vu Ngoc Hung1
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
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
Trang 2approach, 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
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
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
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
Trang 34 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
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 CS1 and CS2 are the internal trimming capacitances that can be adjusted to balance the external capacitance CS The acceleration sensor is connected as CS, and the internal capacitance CS1
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 CS 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 (CF) 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
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 CF, 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
Trang 4Fig 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
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
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
Trang 5Society 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
material engineering from Hanoi
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
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
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
(Vietnam), in 1991 His doctoral
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