MEMS acceleration switch with bi-directionally tunable threshold

doi:10.1016/j.sna.2014.01.003

Sensors and Actuators A: Physical

Volume 208, 1 February 2014, Pages 120-129

https://doi.org/10.1016/j.sna.2014.01.003 Get rights and content

Highlights

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We firstly propose a bi-directionally tunable MEMS acceleration switch.
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Threshold decreased from 10.25 g to 2.0 g by applying tuning voltage.
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Threshold increased from 10.25 g to 17.25 g by applying tuning voltage.
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Additional functionality of safe/armed position convertibility is realized.
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Additional functionality of mechanical latching is realized.

Abstract

A MEMS acceleration switch capable of tuning threshold acceleration to either higher or lower levels is designed and implemented with comb drive actuators as a mechanism of threshold tuning. A small sized switch (1.6 × 3.1 × 0.55 mm³) is successfully realized by patterning silicon structures on a glass wafer. The resonant frequency of fabricated switches agrees well with a designed frequency of 1.1 kHz. The threshold acceleration at no tuning voltage is 10.25 g and it is subsequently tuned to 2.0 g and 17.25 g by applying 30 V to pushing comb and pulling comb, respectively. The rising time is measured to be 9.8 ms. Additional functionalities such as safe/armed position convertibility and single use latching switch are also described for diverse applications of the tunable acceleration switches.

Introduction

There has been much recent interest on MEMS inertial sensors to take advantages of MEMS technology in the field of inertial sensors for small size, low fabrication cost, and high sensitivity. MEMS accelerometers and gyroscopes are good examples that have been successfully developed and widely used in many areas [1]. Another interesting device of the MEMS inertial sensor is an acceleration switch that is turned on at predetermined threshold acceleration. Because the MEMS technology brings additional advantages such as low power consumption, high reliability, and resistivity to electromagnetic noise, the MEMS-based acceleration switches are of great interest in many areas such as automotive, military, health-care, and other shock monitoring applications.

Since W. D. Frobenius et al. proposed the concept of MEMS acceleration switch [2], many studies have been done with different mechanisms; latching switch, bi-stable switch, magnetic switch or fluidic switch [3], [4], [5] in order to improve performance and build more versatile devices by realizing higher reliability, longer contact time, low or no power consumption, or multi-axis detection [6], [7], [8], [9]. Despite of these intensive research efforts, only few papers were trying to realize the tunable acceleration switch for the adjustment of the threshold acceleration [10]. If the threshold acceleration is tunable, the switch can be used in different applications and environments requiring different level of threshold acceleration. Furthermore, process deviations can be compensated after device fabrication. However, previously reported switches used an electrostatic force between contact parts; therefore, it is difficult to tune threshold acceleration to higher levels without electrical pull-in problems.

In our previous paper, we proposed an acceleration switch capable of increasing the threshold acceleration [11]. While the threshold increment is a useful feature, the decreasing of threshold acceleration was not realized in the previous design and the tunability was limited. In this paper, we propose a bi-directionally tunable acceleration switch for increasing or decreasing threshold acceleration in the same device. Such bi-directional tunability enables switches to be used in broad environments and applications where accurate threshold is required. We also describe additional functionalities such as safe/armed position convertibility for military application and mechanically latching tunable switch for more increased reliability in wide applications. All different versions of acceleration switches are fabricated with the same manufacturing process, making it possible to integrate multiple versions of switches in a single chip.

Section snippets

Design and simulation

Fig. 1 shows the schematic view of the suggested acceleration switch. Assuming that all motion and force are constrained in x-direction, the proof mass dynamics is expressed as, $m \frac{d^{2} x}{d t^{2}} + β \frac{d x}{d t} + k x = m a (t)$ where m is the mass of proof mass, β is the damping coefficient, k is the spring constant along the x-direction, and a(t) is the acceleration applied along the x-direction [12]. Eq. (1) is rewritten as follows, $\frac{d^{2} x}{d t^{2}} + \frac{ω_{n}}{Q} \frac{d x}{d t} + ω_{n}^{2} x = a (t),$ where Q is the quality factor and ω_n is the undamped resonant

Fabrication

A 525 μm-thick low-resistivity (100) silicon wafer and a 500 μm-thick Pyrex 7740 glass wafer are two base substrates for the fabrication of the acceleration switch. First, 5 μm-deep trenches are patterned on the silicon wafer by DRIE process (SLR-770, Plasma Therm Inc., US), where AZ4330 photoresist is used as etch-mask (Fig. 6(a)). Then 100 nm-thick chrome layer is deposited on trench bottom by thermal evaporation and lift-off process. The chrome layer prevents the thermal isolation or footing

Measurements

We measured the resonant frequency, switching characteristics, and tuning characteristics with four different switches.

The resonant frequency and Q factor of the switches are measured using a laser Doppler vibrometer and a network analyzer at atmospheric pressure. Fig. 9 shows the resonant frequencies and Q factors of fabricated switches, and frequency response of one of the switches. Q factor was in the range from 20 to 30, which is typical for MEMS resonators at 1 atm. Resonant frequencies

Applications

In this section, we describe two additional functionalities to broaden the application areas of the developed tunable acceleration switch. The first functionality is safe/armed position convertibility for military application and the other functionality is a single use mechanical latching-on switch for the application requiring high reliability.

Conclusion

In this paper, we proposed a bi-directionally tunable MEMS acceleration switch for wide range of tunability. The performance of acceleration switch was analyzed and predicted by calculation and simulation. Proposed switch has its threshold acceleration at 10.25 g without tuning and higher and lower bounds at 17.25 g and 2.0 g, respectively, at 30 V tuning voltage, which implies that the higher bound of threshold acceleration is more than eight times higher than the lower bound. The magnitude of

Acknowledgements

This paper was supported by research funds of Chonbuk National University in 2013. We would like to thank Young-Suk Hwang of Microinfinity Co., Ltd., for helping with acceleration measurement.

Hyunseok Kim was born in Seoul, Korea, in 1987. He received the B.S. and M.S. degrees from the Department of Electrical Engineering, Seoul National University in 2011 and 2013, respectively. His master's thesis was about design, fabrication, and measurement of MEMS tunable acceleration switch.

From 2013, he is employed as a researcher in KITECH (Korea Institute of Industrial Technology). His current research interests are printed electronics, nanomaterials, and micro/nanofabrication technology.

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This paper proposed a novel microelectromechanical system (MEMS) inertial switch, which can not only capture an acceleration threshold, but also detect the impact direction in a hemisphere. The device includes a single sensitive mass-spring system as movable electrode, and 4 radial and 1 axial flexible stationary electrodes, it is essentially made of five individual electrical switches. The impact loads in different directions can make contact (switch-on) between the mass and one or several stationary electrodes, the acceleration direction intervals can be determined by identifying which electrode contacts with the mass. By dynamic finite element analysis and tests, the mass contacted to the electrodes under high g impact loads were determined to the corresponding acceleration direction intervals. The results of the dynamic tests were in good agreement with the numerical predictions, thus the proposed MEMS inertial switch can be used not only to sense the acceleration threshold, but also to detect the acceleration direction in a 3D space.
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Yun-Ho Jang received his B.S., M.S. and Ph.D. degrees from the Department of Electrical Engineering, Seoul National University in 1999, 2001, and 2005, respectively. During his academic stay, he studied and improved reliability of micromirror devices for optical and biological applications. In 2005, he joined the image development team at Samsung Electronics and worked there until he moved back to Seoul National Univeristy as a research professor in 2008. After completing a post-doctoral training at Harvard Medical School, he's currently working for FemtoFab to develop a high throughput 3D microfabrication system. His research interests include microfabrication techniques, microactuators and sensors.

Yong-Kweon Kim received the B.S. and M.S. degrees in electrical engineering from Seoul National University, Seoul, Korea, in 1983 and 1985, respectively, and the Dr. Eng. Degree from the University of Tokyo, Tokyo, Japan, in 1990.

In 1990, he joined the Central Research Laboratory, Hitachi Ltd., Tokyo, Japan, where he was a researcher involved with actuators of hard disk drives. In 1992, he joined Seoul National University, where he is currently a Professor with the School of Electrical Engineering and Computer Science. His current research interests are MEMS and their applications, especially inertial measurement units (IMUs), RF, optics, and biotechnology.

Jung-Mu Kim was born in Jeonju, Korea, in 1977. He received the B.S. degree in electrical engineering from Ajou University, Suwon, Korea, in 2000, the M.S. and Ph.D. degrees in electrical engineering and computer science from Seoul National University, Seoul, Korea, in 2002 and 2007, respectively.

From 2007 to 2008, he was a Postdoctoral Fellow at University of California, San Diego. In 2008, he joined the faculty of the Department of Electronic Engineering, Chonbuk National University, Jeonju, where he is currently an Associate Professor. His research interests include the IMU, Optical MEMS and RF MEMS.

^☆: Selected Paper based on the paper presented at The 17th International Conference on Solid-State Sensors, Actuators and Microsystems, June 16-20, 2013, Barcelona. Spain

View full text

MEMS acceleration switch with bi-directionally tunable threshold☆

Highlights

Abstract

Introduction

Section snippets

Design and simulation

Fabrication

Measurements

Applications

Conclusion

Acknowledgements

Sensor Actuat. A: Phys.

Sensor Actuat.

J. Phys. Chem. Solids

Sensor Actuat. A: phys.

Micromachined inertial sensors

Proc. IEEE

Microminiature ganged threshold accelerometers compatible with integrated circuit technology

IEEE Trans. Electron Dev.

A latching accelerometer fabricated by the anisotropic etching of (110) oriented silicon wafers

J. Micromech. Microeng.

A bistable threshold accelerometer with fully compliant clamped-clamped mechanism

IEEE Sens. J.

Design, simulation and fabrication of a novel contact-enhanced MEMS inertial switch with a movable contact point

J. Micromech. Microeng.

Analysis and elimination of the ‘skip contact’ phenomenon in an inertial micro-switch for prolonging its contact time

J. Micromech. Microeng.

A wide-range micromachined threshold accelerometer array and interface circuit

J. Micromech. Microeng.