A SOI Sandwich Differential Capacitance Accelerometer with Low-stress Package

Yangxi Zhang12, Chengchen Gao12, Fanrui Meng1, and Yilong Hao1 1National Key Laboratory of Science and Technology on Micro/Nano Fabrication

2State Key Laboratories of Transducer Technology Beijing, China

gaocc@pku.edu.cn

Abstract-Thermal stress and device bending have significant effect on the performance of MEMS sensors. In this paper, a MEMS sandwich differential capacitance accelerometer with low-stress package is presented. The accelerometer is based on a thin silicon middle layer which has been bonded with two glass electrode plates. A metal hermetic package case is used for reliability and noise immunity. The purpose is to simplify manufacturing process and reduce thermal influence. The methods of reducing thermal stress and deflection were evaluated. The thickness of glass electrode plates was optimized for low-stress. A silicon multi-point supporting frame which could reduce thermal stress between glass electrode plate and metal case was designed and simulated. The stress model in this study provides useful information for sandwich structures. Test results in actual device show it has the sensitivity of 0.1124V/g and 0.435% nonlinearity error in test range of 0~50g, 0.02%/ zero temperature drift without temperature compensation.

Key words: Differential, Capacitance, Accelerometer, Low Stress Package

I. INTRODUCTION

With the speedy development of MEMS sensors and actuators, MEMS accelerometers have been proved to have variety applications in all fields from consumer electronic, personal navigation, to industrial inspection [1]. A full manufacturing process of a MEMS accelerometer contains two major parts, sensor fabrication and packaging. In sensor fabrication, the main issue is to combine low-cost with high performance. Sandwich capacitive accelerometer can be an effective solution to meet the demands of restraining cross-axis coupling effect, reducing environment influence, and improving thermal stability. A sandwich capacitive accelerometer which is made by bulk silicon process often needs 10~15 lithography steps and brings alignment error from complicated double-side processes[2][3], so a simplified process can be profitable. In MEMS packaging technology, the main challenges is to provide a firmly mounting base which allows the sensor to detect physical quantities outside, and avoiding unnecessary stress or disturbance to be introduced from package[4]. In sandwich structure manufacture process there are two bonding steps, which may cause stress and result in bending of the devices. As most unnecessary stress come from connecting of sensor die and package case, a four-dot die attach process can be used to avoid sensing part directly connecting to package case, but the precise control of the die-coat gel underflow may be difficult[5].

And different adhesives in packaging influence device performance [6].

In the present work, a simplified process of manufacturing sandwich accelerometers and a low-stress packaging method are reported (see Fig. 1). An easy processing sandwich differential capacitance structure with thick glass electrode and thin silicon middle layer was designed to achieve low thermal stress. The methods of reducing thermal stress and deflection were evaluated in simulation. The influence of glass electrode thickness to sandwich capacitance accelerometer was studied. An accurately made silicon four-point supporting frame which was optimized for sandwich capacitance accelerometer was designed to avoid direct mechanical contact between sensor chip and metal package case. The silicon supporting frame could reduce thermal mismatch stress in packaging significantly, and avoid shape control problem in existing four-dot die attach process. Based on results above, an actual 50g sandwich accelerometer with optimized glass electrode plate, silicon supporting frame and low thermal expansion coefficient epoxy was fabricated and tested.

Fig. 1. The 3D model of the accelerometer die (a) and the metal package design (b).

II. DESIGN AND SIMULATION

A. Manufacture process

Based on silicon device layer transfer technology from comb-finger capacitive and oscillating devices [7], a sandwich accelerometer fabrication process with only five lithography masks and six lithography steps is designed [8]. A brief process flow of the process is showed in Fig. 2. The thin device layer is moved from a SOI wafer to a glass electrode plate by anodic

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bonding and KOH etching. Most of structural parameters are precisely defined by a single DRIE dry etch-release step, avoiding alignment error between lithography steps. An improved TMAH Si-etching solution is used to reduce mismatch between two capacitive gaps. This process allows silicon middle layer thickness down to ten micro-meters that tolerates thicker glass electrode in production.

Fig. 2. A brief process flow of the accelerometer fabrication.

B. Geometry model

The sandwich structure is anodic bonded in 350~400�and used near room temperature. The stress comes from thermal expansion coefficient mismatch between glass and silicon. The sandwich sensor is a totally symmetry differential capacitive structure which can reduce thermal effect, but when it is glued to the case in packaging, the whole structure becomes asymmetric, which causes unnecessary thermal stress. The mechanical contact allows more stress to be introduced from package to sensor. To realize those stresses from bonding and packaging in sandwich differential capacitance accelerometer, some 3D models were developed (see Fig. 3) for simulation.

Fig. 3. The quarter model of the accelerometer frame (a) and package with silicon multi-point supporting frame (b).

The model in Fig. 3 (a) focuses on the residual stress that is caused by bonding in accelerometer frame. It consists of two symmetry glass electrode plates and a silicon hollow rectangular middle frame. In simulation, the symmetry model is reduced to a quarter for lower complexity. This model can expansive without extra constraint, which means no stress that comes from fixed point. The model is bonded in a preset anodic bonding temperature, and then cooled to operating temperature. It allows one to vary the size, shape and thickness of the sensor

frame, and study the thermal stress in operating temperature range.

The model in Fig. 3 (b) focuses on the residual stress that is caused by packaging process on metal case. The model contains a glass-silicon accelerometer frame model in Fig. 3 (a), a KOVER alloy substrate, some epoxy connection layers, and a silicon supporting frame which is glued between KOVER substrate and accelerometer frame. The bottom surface of package substrate is fixed in Z-Axis, following an application condition. The whole model is supposed to be assembled in epoxy curing temperature, and operated near room temperature. In this simulation part, the stress in bonding surface which have been simulated in model Fig. 3 (a) is neglected, so other effects in package can be evaluated more exactly. The model is also parameterized, so the compare between different shapes of the silicon supporting frame is possible. The effort of glass electrode thickness in package can also be simulated in this model.

C. Finite Element Analysis

The thermal stress and deflection of models above were calculated by Finite element analysis software ANSYS. Simulation results are discussed below.

1) The effects of glass electrode thickness

The effects of glass electrode plate thickness on thermal stress were studied in finite element simulation based on model in Fig.3 (a). In a capacitance sensor, the deflection of glass electrode surface (Du in up electrode, and Dd in down electrode, see Fig. 4(a)) are directly relate to output capacitance change. As a sandwich structure, the deflection difference of two electrodes (Du+Dd) is also meaningful, because effect of this deflection would not be removed by difference interface circuit. The stress intensity in the center of electrode plate surface (Su in up electrode, and Sd in down electrode) serves as a quantization index of stress. The model scale was similar to actual accelerometer design. The wide and length of glass electorate plate were 7000 m, with a hollow rectangular frame made by 60 m thick silicon. The thickness of the glass electrode was varied to study its effect to stress and deflection.

Fig. 4. Thermal deflection of accelerometer frame(a), and the relation between the thickness of glass electrode plates and the deflection of glass electrode plate in 298K (b).

In a symmetric sandwich accelerometer frame, stress only came from bonding surface, and the deflections of two electrode plates were equal in absolute value (see Fig. 4(b)). When the glass electrode plate thickness increased from 400 m to 800 m in 298K, the deflections decreased to 38.7%, but when glass became thicker then 800um, the deflections

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decreased slowly. This is because the electrode plates’ stiffness increasing with the thickness increasing.

Fig. 5. The deflection of glass electrode plate surface(a) and the stress intensity in the center of electrode plate surface change with glass electrode plate thickness in 298K.

In simulation base on model in Fig. 3 (b), the accelerometer frame was connected to package by a silicon four-point supporting frame (described in II.C.2). The results also show that using thick glass plate could significantly reduce deflections and stress in two electrode plates. Fig. 5 (a) shows the relation between glass electrode plate’s thickness and deflections of two electrode plates in 298K. The package process had broken the symmetric condition, so the deflections of two electrode plates were not symmetric in simulation. When the thickness of electrode plates increasing, Dd decreased slower then Du, which was affected less by stress from silicon supporting frame and package case. The result shows when the thickness of glass electrode plate increase to more then 2000 m, the deflections improves little, but the stresses have a minimum value near 1600 m (see Fig. 5 (b)). The results show that in the thickness range of 400~2000 m, a thicker glass electrode plates has advantages in thermal stability.

2) The effects of supporting frame

Fig.6. The sectional view (up) and stress intensity simulation results(down). (a)The direct bonding design, (b) the silicon shim design, (c) the four-point supporting frame design.

The packaging stress simulation was based on model in Fig. 3 (b). In this simulation, the thermal expansion effects on metal case and epoxy resin were included. Another focus was the silicon supporting frame design. A silicon multi-point supporting frame with high installation precision was designed

to release stress. Different shapes of the silicon supporting frame were compared. The simulation contained a four-point supporting frame design with four square supporting points in the corner, a silicon shim design, and a direct bonding design as control group (see Fig.6). The 400 m thick four-point supporting frame had four 1500 m wide square supporting points which were manufactured by wet etching. The silicon shim was a square piece of single silicon, with the same size of the four-point supporting frame. The frame and the shim were glued to a KOVER plate by 100 m thick low thermal expansion coefficient epoxy layers which were cured in 150 .

(a)

(b)

Fig.7. The comparison of deflection(a) and stress(b) in different design of supporting frame.

The simulation results are showed in Fig. 7. In the direct bonding design, the down electrode plate was almost totally fixed, but the stress which was transferred to up electrode plate caused big deflection. A transition layer with similar thermal expansion coefficient with glass-Si frame could significantly reduce stress in electrode plate. Compared to the direct bonding design, the silicon shim design’s Du+Dd was lower than 70%. In multi-point supporting frame design, the down electrode plate was not contact to package case directly. The frame could be an effective stress release. In 298K, the 1500 m wide four-point supporting frame design could reduce deflection to lower than

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50% and stress in the center of electrodes to lower than 15% when compared to the direct bonding design.

Fig.8. The effects of supporting point side length (Ls) in four-point supporting frame design. Ld stands for the side length of accelerometer device in 298K.

The effects of supporting point size were also simulated. simulation results are showed in Fig. 8. In the four-point supporting frame design, when the ratio of square supporting point side length (Ls) to device side length (Ld) increased to more than 0.25, the deflection in down electrode plate decreased because more suspension area was fixed, which cause undesirable stress. When the ratio decreased from 0.25 to 0.15, the deflection and stress both decreased. The result shows small supporting points could have better performance. But bonding strength requiring a certain amount of contact area that means supporting point size is limited.

III. RESULTS

Based on the results above, a 50g single-axis accelerometer with low-stress package has been designed and fabricated (see Fig. 9). As a compromise between thermal stress and production condition, 800 m glass sheets were used in design as electrode plate, twice than usually thickness (400 m).

Fig. 9. (a). Photograph of the accelerometer die and metal package (1)Accelerometer in Package, (2)Metal hermetic package, (3)Silicon frames, (4)A one-dime coin, (5) Accelerometer dies. (b). The photograph of accelerometer unit with a capacitive interface circuit.

The accelerometer has two 800 m glass electrode plates, a 60 m thick silicon middle layer, and a four-point supporting frame in package. Each square point was 1500 m wide in trade off bonding strength and thermal deflection. Glass electrode plates and silicon middle layer were anodic bonded in 400 , while the epoxy glue was cured in 150 . The accelerometer was packaged in a KOVER hermetic package case to improve reliability and noise immunity.

The accelerometer which is fabricated by the design shows 0.1124V/g sensitivity and 0.435% nonlinearity error in 0~50g

centrifuge test (see Fig. 10). The final sensor unit shows 0.02%/ zero temperature drift without temperature compensation

(see Fig. 11)

Fig. 10. The result of 0~50g centrifuge test shows 0.1124V/g sensitivity and 0.435% nonlinearity error.

Fig.11. Test result of zero temperature drift without temperature compensation .

IV. CONCLUSION In this paper, a convenient process of manufacturing

sandwich accelerometers is reported. Based on the process, the influences of different methods to reduce thermal stress were investigated and discussed. The study gives some ideas of improving thermal stability for sandwich accelerometer. A thick glass electrode thickness can not only reduce deflection, but also reduce stress on electrode plates in sandwich differential capacitance accelerometer. To releasing stress from packaging, a silicon four-point supporting frame was designed and simulated to avoid direct contact between sensor and metal package case. The frame could significantly reduce thermal stress in packaging and avoid underflow in existing four-dot die attach process.

According to the above results, a sandwich differential capacitance structure accelerometer was designed to achieve low thermal stress. The test results in actual device proved that

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the accelerometer has the potential to be applied as a serviceable scheme in low-cost micro accelerometer fabrication.

ACKNOWLEDGEMENTS

This work is supported by National High Technology Research and Development Program of China (2012AA041201). Special thanks go to the partners in the National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Peking University for supporting this paper. The work on capacitive interface circuit by Harbin Institute of Technology is also gratefully acknowledged.

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[4] W. Jian , V. Sarihan, and B. Myers, "A Multidisciplinary Approach for Effective Packaging of MEMS Accelerometer" , ECTC 2010, pp.1173-1177, June 2010.

[5] G. Li, AA. Tseng, "Low Stress Packaging of a Micromachined Accelerometer", Electronics Packaging Manufacturing, Vol. 24, pp.18 - 25, Jan 2001.

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