- Email: [email protected]
MEMS acceleration switch with bi-directionally tunable threshold
Accepted Manuscript Title: MEMS acceleration switch with bi-directionally tunable threshold Author: Hyunseok Kim Yun-Ho Jang Yong-Kweon Kim Jung-Mu Kim PII: DOI: Reference:
S0924-4247(14)00005-3 http://dx.doi.org/doi:10.1016/j.sna.2014.01.003 SNA 8623
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
14-9-2013 3-1-2014 3-1-2014
Please cite this article as: H. Kim, Y.-H. Jang, Y.-K. Kim, J.-M. Kim, MEMS acceleration switch with bi-directionally tunable threshold, Sensors and Actuators: A Physical (2014), http://dx.doi.org/10.1016/j.sna.2014.01.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Special virtual Transducers 2013 volume of Sensors and Actuators A: Physical Original Paper Number in Transducers 2013: T3P.004
MEMS acceleration switch with bi-directionally tunable threshold Hyunseok Kim1, Yun-Ho Jang1, Yong-Kweon Kim1, and Jung-Mu Kim2* 1
Department of Electrical and Computer Engineering, Seoul National University, Seoul, Republic of Korea Department of Electronic Engineering, Chonbuk National University, Jeonju, Republic of Korea
ip t
2
*corresponding author: Department of Electronic Engineering, Chonbuk National University, Deok-jin dong, Deok-jin gu, 561756, Jeonju, Republic of Korea (ZIP code : ASI|KR|KS004|JEONJU, phone: +82 063-270-3382, email: [email protected])
an
us
cr
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 x 3.1 x 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.
M
Keywords Acceleration switch, threshold acceleration, bi-directional tuning, MEMS inertial sensor
Ac ce p
te
d
1. 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 [3-5] 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 [6-9]. 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. 2. Design and simulation Figure 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, d2x dx +β + kx = ma (t ) (1) 2 dt dt , 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]. Equation (1) is rewritten as follows, m
Page 1 of 15
d 2 x ωn dx + + ωn2 x = a (t ) dt 2 Q dt
(2)
, where Q is the quality factor and ωn is the undamped resonant frequency of the proof mass.
ωn =
k m
(3)
d2x dx +β + kx = ma (t ) + ( Fcontact + Fpush − Fpull ) dt dt 2
Ac ce p
m
te
d
M
an
us
cr
ip t
In steady state, the displacement from the original position is, m xd = astatic (4) k , where astatic and xd are the acceleration and displacement in steady state. Therefore the mechanical contact between the proof mass and the electrode occurs if the input acceleration is higher than the threshold acceleration (ath) and the proof mass travels the initial gap. The threshold acceleration is simply expressed as follow. kd ath = (5) m However, the acceleration is not steady state but half-sinusoidal in the case of mechanical shock induced by crashing or collision. The maximum deflection is then determined by additional parameters such as amplitude and duration of waveform, resonant frequency of the proof mass and quality factor (Q factor). Figure 2 shows the normalized displacement of proof mass when half-sinusoidal acceleration is applied. Q factor is assumed to be 50, because laterally actuating MEMS devices typically have the Q factor of the order of ten [13, 14]. The resonant frequency is set to 1.1 kHz. As shown in Figure 2(a), normalized displacement of the proof mass is directly proportional to the applied acceleration. At the half-sinusoidal input of 10 ms duration, normalized value of maximum displacement is 1.04 as shown in Figure 2(b), which implies that 4 % larger deflection of proof mass occurs by the half-sinusoidal acceleration. If the duration of half-sinusoidal waveform becomes 2 ms which is similar to the resonant frequency of the proof mass, 26 % larger deflection occurs as shown in Figure 2(c). These results imply that the magnitude of threshold acceleration is different for two cases of steady state and impulse input. Moreover the difference becomes substantial for the certain duration of the impact. Since the impulse input causes the false initiation of the acceleration switch, operating environment and characteristics of input acceleration should be considered at the design stage of the acceleration switch. To minimize the threshold discrepancy between steady state and half-sinusoidal input, the resonant frequency of proof mass should be located higher than the duration of input waveform. If the resonant frequency of proof mass becomes closer to the duration of input waveform, the simulation and calculation is needed to avoid the aforementioned false initiation. Electrostatic forces are used to tune the threshold acceleration and the forces involved are generated by the electric potential between contact parts (Fcontact), pushing comb (Fpush), and pulling comb (Fpull) as shown in Figure 1, respectively. The motion of proof mass is then expressed as,
Fcontact =
Fpush = Fpull =
εA 2 Vcontact 2( d − x ) 2
ε N push h d comb
ε N pull h d comb
2 V push
2 V pull
(6)
(7) (8) (9)
, where ε is the permittivity, A is the overlapped area of contact parts, Npush and Npull is the number of comb fingers for pushing and pulling combs, h is the height of proof mass, dcomb is the gap between comb fingers, and Vcontact, Vpush, Vpull is electrical potential between contact parts, pushing combs, and pulling combs, respectively [15]. It is noted that equation(1) is a special form of equation(6) without electric potential except that Fcontact should be always considered because Vcontact is always present to detect switch contact. The electrostatic forces by two comb actuators, Fpush and Fpull, are key components for the bi-directional tuning of threshold acceleration. In steady state, the threshold acceleration is expressed as follows. Fpull Fpush kd F (10) ath = ( − contact ) + − m m m m As shown in equation(10), the threshold acceleration can be changed by Fpull and Fpush. The threshold acceleration decreases by applying Vpush, whereas it increases by applying Vpull. The magnitude of threshold acceleration by tuning can also be adjusted by controlling the magnitude of Vpush and Vpull. Table 1 shows the geometrical parameters of suggested acceleration switch. lspring and dspring represent the length and width of folded-beam spring. Based on the designed dimensions, the resonant frequency is calculated as 1097 Hz that matches well with the simulation (1102 Hz) in Figure 3. Figure 4 shows the schematic diagram of Simulink simulation based on equation(6) and given parameters in Table 1. Vcontact is
Page 2 of 15
ip t
fixed as 30 V, and the occurrence of bouncing or chattering at the contact parts is not considered in the simulation. Figure 5 shows the displacement of proof mass under steadily increasing input acceleration. Without tuning voltage (Vpush=Vpull=0 V), the switch is not turned on up to 13 g (Figure 5(a-1)) while the proof mass reaches the contact point abruptly and the switch is turned on around 14 g (Figure 5(a-2)). From these two graphs, it is interpreted that threshold acceleration is between 13 g and 14 g for no tuning potentials. At the presence of tuning voltage Vpush=20 V, threshold acceleration decreases to 10 g, while threshold increases to 19 g if Vpull=20 V as shown in Figure 5(b) and (c), respectively. It is noted that the calculated steady state threshold accelerations from equation(6) are 13.83 g, 9.38 g, and 18.27 g for no tuning voltage, Vpull=20 V, and Vpush=20 V, respectively. We examined analytical equations and solved them with a numerical method to expect the performance of the switch. The designed comb actuators generate electrical potentials enough to alter the threshold acceleration and therefore bi-directionally tunable switch can be implemented.
us
cr
3. 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 (Figure 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 effects during the final DRIE process [16]. On the glass wafer 10 nm-thick chrome and 200 nm-thick nickel layer is patterned by thermal evaporation and lift-off process as electric paths to silicon structures for the application and detection of electrical signals (Figure 6(c)). The prepared two wafers are aligned and anodically bonded (EVG 501, EV Group Inc., Austria) under 400 N pressure, 800 V electrical bias, and 380 °C temperature for 15 minutes (Figure 6(d)). Chemical-mechanical polishing is followed
M
an
to make the thickness of silicon wafer to 40~45 μm (Figure 6(e)). After 200 nm-thick aluminum etch-mask is patterned by thermal evaporation and lift-off process as shown in Figure 6(f), DRIE process is followed until the proof mass is released (Figure 6(g)). Finally, chrome layer is removed by using ICP etcher (RIE80plus, Oxford instruments, UK) as shown in Figure 6(h). The SEM images of fabricated acceleration switch are shown in Figure 7. The size of the switch is 1.6 x 3.1 x 0.55 mm3. Movable silicon structures such as springs, comb drive actuators, and proof mass are effectively released. The initial gap between contact parts is measured to be 4.6 μm. As depicted in Figure 8, fabricated switches are diced, mounted on the PCB, and wirebonded for measurements.
Ac ce p
te
d
4. 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. Figure 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 varied from 1018 Hz to 1060 Hz. The measurement is about 4~8 % lower than the designed frequency (1.1kHz) as spring width loss happens during DRIE process and it consequently decreases spring constant and resonant frequency [17]. It is expected from equation(3) and (5) that threshold acceleration will be decreased 8~15 % by the decrease of resonant frequency. Next, switching characteristics are measured by using a centrifugal chamber as shown in Figure 10. A fabricated acceleration switch is fixed on the rotating disk and the rotation rate is externally controlled. As the disk rotates at the angular rate of ω rad/s, acceleration rω2 along the radial direction is applied on the proof mass, where r is the distance of switch from the center of disk. The centripetal acceleration that can be generated by the chamber ranges from 0 g to 40 g. A commercial accelerometer (MMA3202KEG, Freescale Semiconductor) with ±100 g detection range is also fixed on the same disk along with the fabricated acceleration switch to measure the magnitude of applied acceleration. The electrode for the contact part in Figure 1 is grounded through 100 MΩ-resistor to minimize the influence of contact resistance on the output signal, and the potential of 30 V is applied to the proof mass. The output voltage Vout across the resistor is an indicator for switch operation. Vout is 0 V for off switch state, while Vout has non-zero value when contact between proof mass and stationary electrode occurs. Vout is expressed as follows, 100 M Ω (10) Vout = × 30 [V] 100 M Ω + Rcontact + Rbulk , where Rcontact and Rbulk is contact resistance and resistance of silicon medium, respectively. Because 100 MΩ is much greater than Rcontact and Rbulk, Vout becomes approximately 30 V when the switch is turned on. Figure 11 shows the measured output of acceleration switch and accelerometer. Both in the case of Figure 11(a) and (b), output of commercial accelerometer shows that acceleration is steadily increased from 0 g to 27 g for about 4 seconds as the rotation rate of disk increases. Fabricated acceleration switch is turned on at about 10 g without tuning voltage, where the same switch is turned on at 6 g by applying tuning voltage of Vpush=20 V. As expected, output voltage of the switch is approximately 30 V when the switch is turned on. The switching characteristics explicitly show that threshold acceleration is successfully changed by applying tuning voltage. Figure 12 shows rising time of the acceleration switch measured. It takes about 9.7 ms for the switch output to transit from off-state to on-state. The overall tuning characteristics of fabricated acceleration switches are shown in Figure 13.
Page 3 of 15
Negative and positive tuning voltage on the x- axis implies that Vpush or Vpull is applied respectively, while the other tuning voltage is set to 0 V. Average threshold acceleration is measured to be 10.25 g without tuning voltage, which decreased to 2.0 g by applying Vpush=30 V. On the other hand, threshold acceleration increased to 17.25 g by applying the tuning voltage of Vpull=30 V. Standard deviation of measured threshold ranged from 0 g to 1.26 g, from which we can induce that all switches have similar performances. As expected from the resonant frequency measurement results, measured threshold accelerations are lower than the theoretical level. Variation of initial gap between contact parts and comb fingers can be other causes for the error. Considering such errors occurred during the fabrication process, measured threshold tuning characteristics reasonably match with theory.
ip t
5. 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.
Ac ce p
te
d
M
an
us
cr
5. 1. Safe/armed position convertibility In military applications such as a safety and arming unit (SAU), safe/armed position convertibility is required for storing the ammunitions safely [18]. The acceleration switch in such devices is initially at “safe” position, preventing the switch from false initiation by any accidental acceleration force. The switch is set to “armed” position by applying electrical signal when switch on state is intended under external acceleration. The switch is restored back to “safe” position again without arming signal [19]. Figure 14(a) shows the SEM images of fabricated acceleration switch. Two hook-shaped springs, which is at the left end of the proof mass, act as mechanical stopper. Without arming, the translation of the proof mass is restricted, because the stopper part hinders the proof mass from moving to the stationary electrode. The gap between the stopper and the proof mass is designed to be narrower than that of the contact part, so mechanical contact does not arise at the contact part, which implies that the acceleration switch is at “safe” position. The acceleration switch can be set to “armed” position by opening the stopper. Since the sidewall of the stopper springs is parallel to the electrode for transition in Figure 14(a), the springs are pulled-in under sufficient electrical potential between the spring and the electrode. At this “armed” position, the acceleration switch can be turned on under sufficient external acceleration. Also, threshold acceleration can be increased by applying voltage to the pulling comb. For the ease of measurements, pushing comb is not realized in the switch. Switching characteristics are measured using the centrifugal chamber mentioned above. Voltage for transition, which is the potential difference between hook-shaped spring and electrode for transition, is set to 0 V at the “safe” position. The stopper effectively hindered the proof mass to move along the x-direction, so the acceleration switch is not turned on by increasing the acceleration as depicted in Figure 14(b). When transited to the “armed” position by applying the potential of 30 V as shown in Figure 14(c), on the contrary, the switch is turned on by continuous increment of acceleration from 0 g to 27 g. Figure 14(d) shows the tuning characteristics of the acceleration switch at the “armed” position. The threshold acceleration is initially 10 g without tuning voltage, and it is increased to 28 g by applying Vpull=30 V. 5. 2. Mechanical latching-on tunable acceleration switch Some applications of the acceleration switches require extremely high reliability. For example, the acceleration switches used in airbags and rockets should avoid any malfunctioning including repeated operation due to mechanical bounce [20, 21]. Mechanically latching-on, single use acceleration switch is a good solution to minimize the risks of malfunction induced by too short contact time, chattering, or bouncing of proof mass is eliminated [22]. Once the switch is turned on by latching, switch “on” signal is maintained regardless of the external acceleration afterwards, so the switch can reliably hold the “on” state. SEM images of mechanically latching acceleration switch capable of threshold tuning are shown in Figure 15(a). When acceleration gradually increases, contact occurs at the latching parts in Figure 15(a). At the presence of higher acceleration, the springs bend by the pushing force applied by proof mass, eventually resulting in the latching between the proof mass and springs. Latched parts prevent the proof mass from moving back to the initial position, so the switch is consistently turned on regardless of afterwards acceleration. Consequently, proposed switch acts as mechanically latching but single use acceleration switch. Also, pulling comb is allocated for the tuning of threshold acceleration to lower level. Figure 15(b) and (c) shows the measurement results of fabricated latching switch. As shown in Figure 15(b), unstable contact occurs when applied acceleration is not high enough to latch the proof mass. The switch is latched when the applied acceleration is sufficiently high, and the “on” state of switch lasts even after external acceleration is decreased to 0 g. So, we can conclude that switch “on” signal is reliably sustained after latching. The threshold acceleration is measured to be 35 g without tuning voltage, and threshold decreased to 26 g by applying Vpush=20 V. The threshold acceleration is higher than contact type switches because additional inertial force is required to open the springs for latching. 6. 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
Page 4 of 15
the higher bound of threshold acceleration is more than 8 times higher than lower bound. The magnitude of non-tuned threshold and tuning range can also be changed by designing the spring constant, mass, or initial gap of the acceleration switch. We also described safe/armed position convertibility and mechanical latching-on functionalities for broad applications of the proposed acceleration switches. Achknowledgements 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.
Ac ce p
te
d
M
an
us
cr
ip t
References [1] N. Yazdi, F. Ayazi, and K. Najafi, "Micromachined inertial sensors," Proceedings of the IEEE, vol. 86, pp. 1640-1659, 1998. [2] W. D. Frobenius, S. A. Zeitman, M. H. White, D. D. O'Sullivan, and R. G. Hamel, "Microminiature ganged threshold accelerometers compatible with integrated circuit technology," Electron Devices, IEEE Transactions on, vol. 19, pp. 37-40, 1972. [3] D. R. Ciarlo, "A latching accelerometer fabricated by the anisotropic etching of (110) oriented silicon wafers," Journal of Micromechanics and Microengineering, vol. 2, p. 10, 1992. [4] J. Zhao, Y. Yang, K. Fan, P. Hu, and H. Wang, "A Bistable Threshold Accelerometer With Fully Compliant ClampedClamped Mechanism," Sensors Journal, IEEE, vol. 10, pp. 1019-1024, 2010. [5] K. Yoo, U. Park, and J. Kim, "Development and characterization of a novel configurable MEMS inertial switch using a microscale liquid-metal droplet in a microstructured channel," Sensors and Actuators A: physical, vol. 166, pp. 234-240, 2011. [6] H. Cai, G. Ding, Z. Yang, Z. Su, J. Zhou, and H. Wang, "Design, simulation and fabrication of a novel contact-enhanced MEMS inertial switch with a movable contact point," Journal of Micromechanics and Microengineering, vol. 18, p. 115033, 2008. [7] Z. Yang, G. Ding, H. Cai, X. Xu, H. Wang, and X. Zhao, "Analysis and elimination of the'skip contact'phenomenon in an inertial micro-switch for prolonging its contact time," Journal of Micromechanics and Microengineering, vol. 19, p. 045017, 2009. [8] A. Selvakumar, N. Yazdi, and K. Najafi, "A wide-range micromachined threshold accelerometer array and interface circuit," Journal of Micromechanics and Microengineering, vol. 11, p. 118, 2001. [9] Z. Yang, B. Zhu, W. Chen, G. Ding, H. Wang, and X. Zhao, "Fabrication and characterization of a multidirectionalsensitive contact-enhanced inertial microswitch with a electrophoretic flexible composite fixed electrode," Journal of Micromechanics and Microengineering, vol. 22, p. 045006, 2012. [10] M. Jia, X. Li, Z. Song, M. Bao, Y. Wang, and H. Yang, "Micro-cantilever shocking-acceleration switches with threshold adjusting and'on'-state latching functions," Journal of Micromechanics and Microengineering, vol. 17, p. 567, 2007. [11] H.-S. Kim, Y.-H. Jang, Y.-K. Kim, and J.-M. Kim, "MEMS acceleration switch capable of increasing threshold acceleration," Electronics Letters, vol. 48, pp. 1614-1616, 2012. [12] W. Ma, Y. Zohar, and M. Wong, "Design and characterization of inertia-activated electrical micro-switches fabricated and packaged using low-temperature photoresist molded metal-electroplating technology," Journal of Micromechanics and Microengineering, vol. 13, p. 892, 2003. [13] H. Xie and G. K. Fedder, "Fabrication, characterization, and analysis of a DRIE CMOS-MEMS gyroscope," Sensors Journal, IEEE, vol. 3, pp. 622-631, 2003. [14] P. Yang, C. Peng, H. Zhang, S. Liu, D. Fang, and S. Xia, "A high sensitivity SOI electric-field sensor with novel combshaped microelectrodes," Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS), 2011 16th International Conference on, pp. 1034-1037, 2011. [15] W. C. Tang, T.-C. H. Nguyen, and R. T. Howe, "Laterally driven polysilicon resonant microstructures," Sensors and actuators, vol. 20, pp. 25-32, 1989. [16] Y.-S. Lee, Y.-H. Jang, Y.-K. Kim, and J.-M. Kim, "Thermal de-isolation of silicon microstructures in a plasma etching environment," Journal of Micromechanics and Microengineering, vol. 23, p. 025026, 2013. [17] Y.-H. Jang, J.-W. Kim, J.-M. Kim, and Y.-K. Kim, "Design of etch holes to compensate spring width loss for reliable resonant frequencies," Journal of Micromechanics and Microengineering, vol. 22, p. 057002, 2012. [18] P. J. Smith, "MEMS Based Fuzing/Safety and Arming Systems," RTO AVT Lecture Series on MEMS Aerospace Applications(RTO-EN-AVT-105), Montreal, Canada, p. 3, 2002. [19] H. R. Last, M. Deeds, D. Garvick, R. Kavetsky, P. A. Sandborn, E. B. Magrab, and S. K. Gupta, "Nano-to-millimeter scale integrated systems," Components and Packaging Technologies, IEEE Transactions on, vol. 22, pp. 338-343, 1999. [20] J. Zhao, J. Jia, H. Wang, and W. Li, "A novel threshold accelerometer with postbuckling structures for airbag restraint systems," Sensors Journal, IEEE, vol. 7, pp. 1102-1109, 2007. [21] H. Pezous, C. Rossi, M. Sanchez, F. Mathieu, X. Dollat, S. Charlot, and V. Conédéra, "Fabrication, assembly and tests of a MEMS-based safe, arm and fire device," Journal of Physics and Chemistry of Solids, vol. 71, pp. 75-79, 2010. [22] Z. Guo, Z. Yang, L. Lin, Q. Zhao, H. Ding, X. Liu, X. Chi, J. Cui, and G. Yan, "Design, fabrication and characterization of a latching acceleration switch with multi-contacts independent to the proof-mass," Sensors and Actuators A: physical, vol. 166, pp. 187-192, 2011.
Page 5 of 15
Biographies 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.
cr
ip t
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.
an
us
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.
Ac ce p
te
d
M
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.
Page 6 of 15
Special virtual Transducers 2013 volume of Sensors and Actuators A: Physical Original Paper Number in Transducers 2013: T3P.004
MEMS acceleration switch with bi-directionally tunable threshold Figure captions (15 figures) Figure 1. Schematic view of the proposed tunable acceleration switch
ip t
Figure 2. Normalized displacement of the proof mass by (a) steadily increasing acceleration, (b) half-sinusoidal acceleration of 10 ms duration, and (c) half-sinusoidal acceleration of 2 ms duration
cr
Figure 3. Resonant frequency of the acceleration switch Figure 4. Schematic diagram of the simulation model
us
Figure 5. Displacement of the proof mass under steadily increasing acceleration when (a) Vpull=0 V and Vpush=0 V, (b) Vpull=0 V and Vpush=20 V, (c) Vpull=20 V and Vpush=0 V
an
Figure 6. Fabrication process Figure 7. SEM images of fabricated acceleration switch
M
Figure 8. Optical photograph of wire-bonded acceleration switch on PCB
Figure 9. Resonant frequency, Q factor, and frequency response of the acceleration switch Figure 10. Measurement setup
d
Figure 11. Measured output of acceleration switch and accelerometer at (a) no tuning voltage, and (b) Vpush=20 V
te
Figure 12. Measured switching-on characteristics of the acceleration switch
Ac ce p
Figure 13. Measured and theoretical threshold acceleration according to the tuning voltage Figure 14. (a) SEM images of fabricated acceleration switch, (b) switch output at safe position, (c) switch output at armed position, (d) tuning characteristics at armed position Figure 15. (a) SEM images of fabricated acceleration switch, (b) switch output, and (c) tuning characteristics of the switch
Page 7 of 15
Special virtual Transducers 2013 volume of Sensors and Actuators A: Physical Original Paper Number in Transducers 2013: T3P.004
MEMS acceleration switch with bi-directionally tunable threshold Tables (1 table) Table 1. Geometrical parameters of the suggested acceleration switch 40 μm
A 2
1200 μm
d
dcomb
lspring
dspring
Npush
Npull
5 μm
4.5 μm
800 μm
5 μm
96
96
Ac ce p
te
d
M
an
us
cr
6.94e-08 kg
h
ip t
m
Page 8 of 15
Figures (15 figures) (a) comb actuator for threshold decreasing (pushing comb)
comb actuator for threshold increasing (pulling comb) metal interconnection
anchor electrode for the rotor
ip t
electrode for the contact part
lateral actuation
contact part
z
(b) folded beam spring
x
cr
y
us
Figure 1. Schematic view of the proposed tunable acceleration switch
10 ms
M
an
1.04
Ac
ce pt
ed
(c)
2 ms
1.26
Figure 2. Normalized displacement of the proof mass by (a) steadily increasing acceleration, (b) half-sinusoidal acceleration of 10 ms duration, and (c) half-sinusoidal acceleration of 2 ms duration
Page 9 of 15
ip t
f=1102 Hz
us
cr
Figure 3. Resonant frequency of the acceleration switch
an
spring, damper
M
Fpush and Fpull
ed
input acceleration a(t)
ce pt
Fcontact
contact detection
Ac
Figure 4. Schematic diagram of the simulation model
Page 10 of 15
(a-1) contact point
(a-2)
(b-1)
us
cr
contact point
ip t
contact point
(b-2)
M
an
contact point
ce pt
contact point
Ac
(c-2)
contact point
ed
(c-1)
Figure 5. Displacement of the proof mass under steadily increasing acceleration when (a) Vpull=0 V and Vpush=0 V, (b) Vpull=0 V and Vpush=20 V, (c) Vpull=20 V and Vpush=0 V
Page 11 of 15
(a)
A' A
A'
A' A
A'
20 mm
A
(b) A
(c)
5 mm
A' A
A'
A
A'
20 mm 5 mm
A'
Figure 8. Optical photograph of wire-bonded acceleration switch on PCB
A
(e)
A
cr
(d)
ip t
A
A'
us
A' A
(f)
A
A
A'
A
A'
A
(g)
M
A' A
A'
ed
A
(h)
an
1.036 kHz, 30.4 dB
A'
ce pt
A
: Glass
: Si
: Cr/Ni
: Photoresist
: Cr
: Al
Ac
Figure 6. Fabrication process
Figure 9. Resonant frequency, Q factor, and frequency response of the acceleration switch
centrifugal chamber control PC
contact part rotating disk
contact part
4.6 m
spring
acceleration switch & commercial accelerometer
comb actuator
oscilloscope & voltage source
Figure 10. Measurement setup
Figure 7. SEM images of fabricated acceleration switch
Page 12 of 15
(a) pushing comb voltage increment
pulling comb voltage increment
switch on at 10 g
ip t
bi-directional threshold tuning
(b)
cr
Figure 13. Measured and theoretical threshold acceleration according to the tuning voltage
ce pt
ed
M
Figure 11. Measured output of acceleration switch and accelerometer at (a) no tuning voltage, and (b) Vpush=20 V
an
us
switch on at 6 g
9.7 ms
on-state
Ac
threshold acceleration
off-state
Figure 12. Measured switching-on characteristics of the acceleration switch
Page 13 of 15
(a)
(a)
stopper
pulling comb
springs for latching
contact part
latch
electrode for transition moving direction pushing comb
stopper
latching parts
hook-shaped spring
latch (b)
ip t
(b)
cr
latched
switch off at “safe” position
an
us
unstable contact
(c)
M
(c)
Figure 15. (a) SEM images of fabricated acceleration switch, (b) switch output, and (c) tuning characteristics of the switch
Ac
(d)
ce pt
ed
switch on at “armed” position
Figure 14. (a) SEM images of fabricated acceleration switch, (b) switch output at safe position, (c) switch output at armed position, (d) tuning characteristics at armed position
Page 14 of 15
*Highlights (for review)
Special virtual Transducers 2013 volume of Sensors and Actuators A: Physical Original Paper Number in Transducers 2013: T3P.004
MEMS acceleration switch with bi-directionally tunable threshold
We firstly propose a bi-directionally tunable MEMS acceleration switch. Threshold decreased from 10.25 g to 2.0 g by applying tuning voltage. Threshold increased from 10.25 g to 17.25 g by applying tuning voltage. Additional functionality of safe/armed position convertibility is realized.
ce pt
ed
M
an
us
cr
Additional functionality of mechanical latching is realized.
Ac
ip t
Highlights
Page 15 of 15
S0924-4247(14)00005-3 http://dx.doi.org/doi:10.1016/j.sna.2014.01.003 SNA 8623
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
14-9-2013 3-1-2014 3-1-2014
Please cite this article as: H. Kim, Y.-H. Jang, Y.-K. Kim, J.-M. Kim, MEMS acceleration switch with bi-directionally tunable threshold, Sensors and Actuators: A Physical (2014), http://dx.doi.org/10.1016/j.sna.2014.01.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Special virtual Transducers 2013 volume of Sensors and Actuators A: Physical Original Paper Number in Transducers 2013: T3P.004
MEMS acceleration switch with bi-directionally tunable threshold Hyunseok Kim1, Yun-Ho Jang1, Yong-Kweon Kim1, and Jung-Mu Kim2* 1
Department of Electrical and Computer Engineering, Seoul National University, Seoul, Republic of Korea Department of Electronic Engineering, Chonbuk National University, Jeonju, Republic of Korea
ip t
2
*corresponding author: Department of Electronic Engineering, Chonbuk National University, Deok-jin dong, Deok-jin gu, 561756, Jeonju, Republic of Korea (ZIP code : ASI|KR|KS004|JEONJU, phone: +82 063-270-3382, email: [email protected])
an
us
cr
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 x 3.1 x 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.
M
Keywords Acceleration switch, threshold acceleration, bi-directional tuning, MEMS inertial sensor
Ac ce p
te
d
1. 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 [3-5] 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 [6-9]. 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. 2. Design and simulation Figure 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, d2x dx +β + kx = ma (t ) (1) 2 dt dt , 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]. Equation (1) is rewritten as follows, m
Page 1 of 15
d 2 x ωn dx + + ωn2 x = a (t ) dt 2 Q dt
(2)
, where Q is the quality factor and ωn is the undamped resonant frequency of the proof mass.
ωn =
k m
(3)
d2x dx +β + kx = ma (t ) + ( Fcontact + Fpush − Fpull ) dt dt 2
Ac ce p
m
te
d
M
an
us
cr
ip t
In steady state, the displacement from the original position is, m xd = astatic (4) k , where astatic and xd are the acceleration and displacement in steady state. Therefore the mechanical contact between the proof mass and the electrode occurs if the input acceleration is higher than the threshold acceleration (ath) and the proof mass travels the initial gap. The threshold acceleration is simply expressed as follow. kd ath = (5) m However, the acceleration is not steady state but half-sinusoidal in the case of mechanical shock induced by crashing or collision. The maximum deflection is then determined by additional parameters such as amplitude and duration of waveform, resonant frequency of the proof mass and quality factor (Q factor). Figure 2 shows the normalized displacement of proof mass when half-sinusoidal acceleration is applied. Q factor is assumed to be 50, because laterally actuating MEMS devices typically have the Q factor of the order of ten [13, 14]. The resonant frequency is set to 1.1 kHz. As shown in Figure 2(a), normalized displacement of the proof mass is directly proportional to the applied acceleration. At the half-sinusoidal input of 10 ms duration, normalized value of maximum displacement is 1.04 as shown in Figure 2(b), which implies that 4 % larger deflection of proof mass occurs by the half-sinusoidal acceleration. If the duration of half-sinusoidal waveform becomes 2 ms which is similar to the resonant frequency of the proof mass, 26 % larger deflection occurs as shown in Figure 2(c). These results imply that the magnitude of threshold acceleration is different for two cases of steady state and impulse input. Moreover the difference becomes substantial for the certain duration of the impact. Since the impulse input causes the false initiation of the acceleration switch, operating environment and characteristics of input acceleration should be considered at the design stage of the acceleration switch. To minimize the threshold discrepancy between steady state and half-sinusoidal input, the resonant frequency of proof mass should be located higher than the duration of input waveform. If the resonant frequency of proof mass becomes closer to the duration of input waveform, the simulation and calculation is needed to avoid the aforementioned false initiation. Electrostatic forces are used to tune the threshold acceleration and the forces involved are generated by the electric potential between contact parts (Fcontact), pushing comb (Fpush), and pulling comb (Fpull) as shown in Figure 1, respectively. The motion of proof mass is then expressed as,
Fcontact =
Fpush = Fpull =
εA 2 Vcontact 2( d − x ) 2
ε N push h d comb
ε N pull h d comb
2 V push
2 V pull
(6)
(7) (8) (9)
, where ε is the permittivity, A is the overlapped area of contact parts, Npush and Npull is the number of comb fingers for pushing and pulling combs, h is the height of proof mass, dcomb is the gap between comb fingers, and Vcontact, Vpush, Vpull is electrical potential between contact parts, pushing combs, and pulling combs, respectively [15]. It is noted that equation(1) is a special form of equation(6) without electric potential except that Fcontact should be always considered because Vcontact is always present to detect switch contact. The electrostatic forces by two comb actuators, Fpush and Fpull, are key components for the bi-directional tuning of threshold acceleration. In steady state, the threshold acceleration is expressed as follows. Fpull Fpush kd F (10) ath = ( − contact ) + − m m m m As shown in equation(10), the threshold acceleration can be changed by Fpull and Fpush. The threshold acceleration decreases by applying Vpush, whereas it increases by applying Vpull. The magnitude of threshold acceleration by tuning can also be adjusted by controlling the magnitude of Vpush and Vpull. Table 1 shows the geometrical parameters of suggested acceleration switch. lspring and dspring represent the length and width of folded-beam spring. Based on the designed dimensions, the resonant frequency is calculated as 1097 Hz that matches well with the simulation (1102 Hz) in Figure 3. Figure 4 shows the schematic diagram of Simulink simulation based on equation(6) and given parameters in Table 1. Vcontact is
Page 2 of 15
ip t
fixed as 30 V, and the occurrence of bouncing or chattering at the contact parts is not considered in the simulation. Figure 5 shows the displacement of proof mass under steadily increasing input acceleration. Without tuning voltage (Vpush=Vpull=0 V), the switch is not turned on up to 13 g (Figure 5(a-1)) while the proof mass reaches the contact point abruptly and the switch is turned on around 14 g (Figure 5(a-2)). From these two graphs, it is interpreted that threshold acceleration is between 13 g and 14 g for no tuning potentials. At the presence of tuning voltage Vpush=20 V, threshold acceleration decreases to 10 g, while threshold increases to 19 g if Vpull=20 V as shown in Figure 5(b) and (c), respectively. It is noted that the calculated steady state threshold accelerations from equation(6) are 13.83 g, 9.38 g, and 18.27 g for no tuning voltage, Vpull=20 V, and Vpush=20 V, respectively. We examined analytical equations and solved them with a numerical method to expect the performance of the switch. The designed comb actuators generate electrical potentials enough to alter the threshold acceleration and therefore bi-directionally tunable switch can be implemented.
us
cr
3. 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 (Figure 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 effects during the final DRIE process [16]. On the glass wafer 10 nm-thick chrome and 200 nm-thick nickel layer is patterned by thermal evaporation and lift-off process as electric paths to silicon structures for the application and detection of electrical signals (Figure 6(c)). The prepared two wafers are aligned and anodically bonded (EVG 501, EV Group Inc., Austria) under 400 N pressure, 800 V electrical bias, and 380 °C temperature for 15 minutes (Figure 6(d)). Chemical-mechanical polishing is followed
M
an
to make the thickness of silicon wafer to 40~45 μm (Figure 6(e)). After 200 nm-thick aluminum etch-mask is patterned by thermal evaporation and lift-off process as shown in Figure 6(f), DRIE process is followed until the proof mass is released (Figure 6(g)). Finally, chrome layer is removed by using ICP etcher (RIE80plus, Oxford instruments, UK) as shown in Figure 6(h). The SEM images of fabricated acceleration switch are shown in Figure 7. The size of the switch is 1.6 x 3.1 x 0.55 mm3. Movable silicon structures such as springs, comb drive actuators, and proof mass are effectively released. The initial gap between contact parts is measured to be 4.6 μm. As depicted in Figure 8, fabricated switches are diced, mounted on the PCB, and wirebonded for measurements.
Ac ce p
te
d
4. 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. Figure 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 varied from 1018 Hz to 1060 Hz. The measurement is about 4~8 % lower than the designed frequency (1.1kHz) as spring width loss happens during DRIE process and it consequently decreases spring constant and resonant frequency [17]. It is expected from equation(3) and (5) that threshold acceleration will be decreased 8~15 % by the decrease of resonant frequency. Next, switching characteristics are measured by using a centrifugal chamber as shown in Figure 10. A fabricated acceleration switch is fixed on the rotating disk and the rotation rate is externally controlled. As the disk rotates at the angular rate of ω rad/s, acceleration rω2 along the radial direction is applied on the proof mass, where r is the distance of switch from the center of disk. The centripetal acceleration that can be generated by the chamber ranges from 0 g to 40 g. A commercial accelerometer (MMA3202KEG, Freescale Semiconductor) with ±100 g detection range is also fixed on the same disk along with the fabricated acceleration switch to measure the magnitude of applied acceleration. The electrode for the contact part in Figure 1 is grounded through 100 MΩ-resistor to minimize the influence of contact resistance on the output signal, and the potential of 30 V is applied to the proof mass. The output voltage Vout across the resistor is an indicator for switch operation. Vout is 0 V for off switch state, while Vout has non-zero value when contact between proof mass and stationary electrode occurs. Vout is expressed as follows, 100 M Ω (10) Vout = × 30 [V] 100 M Ω + Rcontact + Rbulk , where Rcontact and Rbulk is contact resistance and resistance of silicon medium, respectively. Because 100 MΩ is much greater than Rcontact and Rbulk, Vout becomes approximately 30 V when the switch is turned on. Figure 11 shows the measured output of acceleration switch and accelerometer. Both in the case of Figure 11(a) and (b), output of commercial accelerometer shows that acceleration is steadily increased from 0 g to 27 g for about 4 seconds as the rotation rate of disk increases. Fabricated acceleration switch is turned on at about 10 g without tuning voltage, where the same switch is turned on at 6 g by applying tuning voltage of Vpush=20 V. As expected, output voltage of the switch is approximately 30 V when the switch is turned on. The switching characteristics explicitly show that threshold acceleration is successfully changed by applying tuning voltage. Figure 12 shows rising time of the acceleration switch measured. It takes about 9.7 ms for the switch output to transit from off-state to on-state. The overall tuning characteristics of fabricated acceleration switches are shown in Figure 13.
Page 3 of 15
Negative and positive tuning voltage on the x- axis implies that Vpush or Vpull is applied respectively, while the other tuning voltage is set to 0 V. Average threshold acceleration is measured to be 10.25 g without tuning voltage, which decreased to 2.0 g by applying Vpush=30 V. On the other hand, threshold acceleration increased to 17.25 g by applying the tuning voltage of Vpull=30 V. Standard deviation of measured threshold ranged from 0 g to 1.26 g, from which we can induce that all switches have similar performances. As expected from the resonant frequency measurement results, measured threshold accelerations are lower than the theoretical level. Variation of initial gap between contact parts and comb fingers can be other causes for the error. Considering such errors occurred during the fabrication process, measured threshold tuning characteristics reasonably match with theory.
ip t
5. 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.
Ac ce p
te
d
M
an
us
cr
5. 1. Safe/armed position convertibility In military applications such as a safety and arming unit (SAU), safe/armed position convertibility is required for storing the ammunitions safely [18]. The acceleration switch in such devices is initially at “safe” position, preventing the switch from false initiation by any accidental acceleration force. The switch is set to “armed” position by applying electrical signal when switch on state is intended under external acceleration. The switch is restored back to “safe” position again without arming signal [19]. Figure 14(a) shows the SEM images of fabricated acceleration switch. Two hook-shaped springs, which is at the left end of the proof mass, act as mechanical stopper. Without arming, the translation of the proof mass is restricted, because the stopper part hinders the proof mass from moving to the stationary electrode. The gap between the stopper and the proof mass is designed to be narrower than that of the contact part, so mechanical contact does not arise at the contact part, which implies that the acceleration switch is at “safe” position. The acceleration switch can be set to “armed” position by opening the stopper. Since the sidewall of the stopper springs is parallel to the electrode for transition in Figure 14(a), the springs are pulled-in under sufficient electrical potential between the spring and the electrode. At this “armed” position, the acceleration switch can be turned on under sufficient external acceleration. Also, threshold acceleration can be increased by applying voltage to the pulling comb. For the ease of measurements, pushing comb is not realized in the switch. Switching characteristics are measured using the centrifugal chamber mentioned above. Voltage for transition, which is the potential difference between hook-shaped spring and electrode for transition, is set to 0 V at the “safe” position. The stopper effectively hindered the proof mass to move along the x-direction, so the acceleration switch is not turned on by increasing the acceleration as depicted in Figure 14(b). When transited to the “armed” position by applying the potential of 30 V as shown in Figure 14(c), on the contrary, the switch is turned on by continuous increment of acceleration from 0 g to 27 g. Figure 14(d) shows the tuning characteristics of the acceleration switch at the “armed” position. The threshold acceleration is initially 10 g without tuning voltage, and it is increased to 28 g by applying Vpull=30 V. 5. 2. Mechanical latching-on tunable acceleration switch Some applications of the acceleration switches require extremely high reliability. For example, the acceleration switches used in airbags and rockets should avoid any malfunctioning including repeated operation due to mechanical bounce [20, 21]. Mechanically latching-on, single use acceleration switch is a good solution to minimize the risks of malfunction induced by too short contact time, chattering, or bouncing of proof mass is eliminated [22]. Once the switch is turned on by latching, switch “on” signal is maintained regardless of the external acceleration afterwards, so the switch can reliably hold the “on” state. SEM images of mechanically latching acceleration switch capable of threshold tuning are shown in Figure 15(a). When acceleration gradually increases, contact occurs at the latching parts in Figure 15(a). At the presence of higher acceleration, the springs bend by the pushing force applied by proof mass, eventually resulting in the latching between the proof mass and springs. Latched parts prevent the proof mass from moving back to the initial position, so the switch is consistently turned on regardless of afterwards acceleration. Consequently, proposed switch acts as mechanically latching but single use acceleration switch. Also, pulling comb is allocated for the tuning of threshold acceleration to lower level. Figure 15(b) and (c) shows the measurement results of fabricated latching switch. As shown in Figure 15(b), unstable contact occurs when applied acceleration is not high enough to latch the proof mass. The switch is latched when the applied acceleration is sufficiently high, and the “on” state of switch lasts even after external acceleration is decreased to 0 g. So, we can conclude that switch “on” signal is reliably sustained after latching. The threshold acceleration is measured to be 35 g without tuning voltage, and threshold decreased to 26 g by applying Vpush=20 V. The threshold acceleration is higher than contact type switches because additional inertial force is required to open the springs for latching. 6. 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
Page 4 of 15
the higher bound of threshold acceleration is more than 8 times higher than lower bound. The magnitude of non-tuned threshold and tuning range can also be changed by designing the spring constant, mass, or initial gap of the acceleration switch. We also described safe/armed position convertibility and mechanical latching-on functionalities for broad applications of the proposed acceleration switches. Achknowledgements 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.
Ac ce p
te
d
M
an
us
cr
ip t
References [1] N. Yazdi, F. Ayazi, and K. Najafi, "Micromachined inertial sensors," Proceedings of the IEEE, vol. 86, pp. 1640-1659, 1998. [2] W. D. Frobenius, S. A. Zeitman, M. H. White, D. D. O'Sullivan, and R. G. Hamel, "Microminiature ganged threshold accelerometers compatible with integrated circuit technology," Electron Devices, IEEE Transactions on, vol. 19, pp. 37-40, 1972. [3] D. R. Ciarlo, "A latching accelerometer fabricated by the anisotropic etching of (110) oriented silicon wafers," Journal of Micromechanics and Microengineering, vol. 2, p. 10, 1992. [4] J. Zhao, Y. Yang, K. Fan, P. Hu, and H. Wang, "A Bistable Threshold Accelerometer With Fully Compliant ClampedClamped Mechanism," Sensors Journal, IEEE, vol. 10, pp. 1019-1024, 2010. [5] K. Yoo, U. Park, and J. Kim, "Development and characterization of a novel configurable MEMS inertial switch using a microscale liquid-metal droplet in a microstructured channel," Sensors and Actuators A: physical, vol. 166, pp. 234-240, 2011. [6] H. Cai, G. Ding, Z. Yang, Z. Su, J. Zhou, and H. Wang, "Design, simulation and fabrication of a novel contact-enhanced MEMS inertial switch with a movable contact point," Journal of Micromechanics and Microengineering, vol. 18, p. 115033, 2008. [7] Z. Yang, G. Ding, H. Cai, X. Xu, H. Wang, and X. Zhao, "Analysis and elimination of the'skip contact'phenomenon in an inertial micro-switch for prolonging its contact time," Journal of Micromechanics and Microengineering, vol. 19, p. 045017, 2009. [8] A. Selvakumar, N. Yazdi, and K. Najafi, "A wide-range micromachined threshold accelerometer array and interface circuit," Journal of Micromechanics and Microengineering, vol. 11, p. 118, 2001. [9] Z. Yang, B. Zhu, W. Chen, G. Ding, H. Wang, and X. Zhao, "Fabrication and characterization of a multidirectionalsensitive contact-enhanced inertial microswitch with a electrophoretic flexible composite fixed electrode," Journal of Micromechanics and Microengineering, vol. 22, p. 045006, 2012. [10] M. Jia, X. Li, Z. Song, M. Bao, Y. Wang, and H. Yang, "Micro-cantilever shocking-acceleration switches with threshold adjusting and'on'-state latching functions," Journal of Micromechanics and Microengineering, vol. 17, p. 567, 2007. [11] H.-S. Kim, Y.-H. Jang, Y.-K. Kim, and J.-M. Kim, "MEMS acceleration switch capable of increasing threshold acceleration," Electronics Letters, vol. 48, pp. 1614-1616, 2012. [12] W. Ma, Y. Zohar, and M. Wong, "Design and characterization of inertia-activated electrical micro-switches fabricated and packaged using low-temperature photoresist molded metal-electroplating technology," Journal of Micromechanics and Microengineering, vol. 13, p. 892, 2003. [13] H. Xie and G. K. Fedder, "Fabrication, characterization, and analysis of a DRIE CMOS-MEMS gyroscope," Sensors Journal, IEEE, vol. 3, pp. 622-631, 2003. [14] P. Yang, C. Peng, H. Zhang, S. Liu, D. Fang, and S. Xia, "A high sensitivity SOI electric-field sensor with novel combshaped microelectrodes," Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS), 2011 16th International Conference on, pp. 1034-1037, 2011. [15] W. C. Tang, T.-C. H. Nguyen, and R. T. Howe, "Laterally driven polysilicon resonant microstructures," Sensors and actuators, vol. 20, pp. 25-32, 1989. [16] Y.-S. Lee, Y.-H. Jang, Y.-K. Kim, and J.-M. Kim, "Thermal de-isolation of silicon microstructures in a plasma etching environment," Journal of Micromechanics and Microengineering, vol. 23, p. 025026, 2013. [17] Y.-H. Jang, J.-W. Kim, J.-M. Kim, and Y.-K. Kim, "Design of etch holes to compensate spring width loss for reliable resonant frequencies," Journal of Micromechanics and Microengineering, vol. 22, p. 057002, 2012. [18] P. J. Smith, "MEMS Based Fuzing/Safety and Arming Systems," RTO AVT Lecture Series on MEMS Aerospace Applications(RTO-EN-AVT-105), Montreal, Canada, p. 3, 2002. [19] H. R. Last, M. Deeds, D. Garvick, R. Kavetsky, P. A. Sandborn, E. B. Magrab, and S. K. Gupta, "Nano-to-millimeter scale integrated systems," Components and Packaging Technologies, IEEE Transactions on, vol. 22, pp. 338-343, 1999. [20] J. Zhao, J. Jia, H. Wang, and W. Li, "A novel threshold accelerometer with postbuckling structures for airbag restraint systems," Sensors Journal, IEEE, vol. 7, pp. 1102-1109, 2007. [21] H. Pezous, C. Rossi, M. Sanchez, F. Mathieu, X. Dollat, S. Charlot, and V. Conédéra, "Fabrication, assembly and tests of a MEMS-based safe, arm and fire device," Journal of Physics and Chemistry of Solids, vol. 71, pp. 75-79, 2010. [22] Z. Guo, Z. Yang, L. Lin, Q. Zhao, H. Ding, X. Liu, X. Chi, J. Cui, and G. Yan, "Design, fabrication and characterization of a latching acceleration switch with multi-contacts independent to the proof-mass," Sensors and Actuators A: physical, vol. 166, pp. 187-192, 2011.
Page 5 of 15
Biographies 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.
cr
ip t
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.
an
us
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.
Ac ce p
te
d
M
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.
Page 6 of 15
Special virtual Transducers 2013 volume of Sensors and Actuators A: Physical Original Paper Number in Transducers 2013: T3P.004
MEMS acceleration switch with bi-directionally tunable threshold Figure captions (15 figures) Figure 1. Schematic view of the proposed tunable acceleration switch
ip t
Figure 2. Normalized displacement of the proof mass by (a) steadily increasing acceleration, (b) half-sinusoidal acceleration of 10 ms duration, and (c) half-sinusoidal acceleration of 2 ms duration
cr
Figure 3. Resonant frequency of the acceleration switch Figure 4. Schematic diagram of the simulation model
us
Figure 5. Displacement of the proof mass under steadily increasing acceleration when (a) Vpull=0 V and Vpush=0 V, (b) Vpull=0 V and Vpush=20 V, (c) Vpull=20 V and Vpush=0 V
an
Figure 6. Fabrication process Figure 7. SEM images of fabricated acceleration switch
M
Figure 8. Optical photograph of wire-bonded acceleration switch on PCB
Figure 9. Resonant frequency, Q factor, and frequency response of the acceleration switch Figure 10. Measurement setup
d
Figure 11. Measured output of acceleration switch and accelerometer at (a) no tuning voltage, and (b) Vpush=20 V
te
Figure 12. Measured switching-on characteristics of the acceleration switch
Ac ce p
Figure 13. Measured and theoretical threshold acceleration according to the tuning voltage Figure 14. (a) SEM images of fabricated acceleration switch, (b) switch output at safe position, (c) switch output at armed position, (d) tuning characteristics at armed position Figure 15. (a) SEM images of fabricated acceleration switch, (b) switch output, and (c) tuning characteristics of the switch
Page 7 of 15
Special virtual Transducers 2013 volume of Sensors and Actuators A: Physical Original Paper Number in Transducers 2013: T3P.004
MEMS acceleration switch with bi-directionally tunable threshold Tables (1 table) Table 1. Geometrical parameters of the suggested acceleration switch 40 μm
A 2
1200 μm
d
dcomb
lspring
dspring
Npush
Npull
5 μm
4.5 μm
800 μm
5 μm
96
96
Ac ce p
te
d
M
an
us
cr
6.94e-08 kg
h
ip t
m
Page 8 of 15
Figures (15 figures) (a) comb actuator for threshold decreasing (pushing comb)
comb actuator for threshold increasing (pulling comb) metal interconnection
anchor electrode for the rotor
ip t
electrode for the contact part
lateral actuation
contact part
z
(b) folded beam spring
x
cr
y
us
Figure 1. Schematic view of the proposed tunable acceleration switch
10 ms
M
an
1.04
Ac
ce pt
ed
(c)
2 ms
1.26
Figure 2. Normalized displacement of the proof mass by (a) steadily increasing acceleration, (b) half-sinusoidal acceleration of 10 ms duration, and (c) half-sinusoidal acceleration of 2 ms duration
Page 9 of 15
ip t
f=1102 Hz
us
cr
Figure 3. Resonant frequency of the acceleration switch
an
spring, damper
M
Fpush and Fpull
ed
input acceleration a(t)
ce pt
Fcontact
contact detection
Ac
Figure 4. Schematic diagram of the simulation model
Page 10 of 15
(a-1) contact point
(a-2)
(b-1)
us
cr
contact point
ip t
contact point
(b-2)
M
an
contact point
ce pt
contact point
Ac
(c-2)
contact point
ed
(c-1)
Figure 5. Displacement of the proof mass under steadily increasing acceleration when (a) Vpull=0 V and Vpush=0 V, (b) Vpull=0 V and Vpush=20 V, (c) Vpull=20 V and Vpush=0 V
Page 11 of 15
(a)
A' A
A'
A' A
A'
20 mm
A
(b) A
(c)
5 mm
A' A
A'
A
A'
20 mm 5 mm
A'
Figure 8. Optical photograph of wire-bonded acceleration switch on PCB
A
(e)
A
cr
(d)
ip t
A
A'
us
A' A
(f)
A
A
A'
A
A'
A
(g)
M
A' A
A'
ed
A
(h)
an
1.036 kHz, 30.4 dB
A'
ce pt
A
: Glass
: Si
: Cr/Ni
: Photoresist
: Cr
: Al
Ac
Figure 6. Fabrication process
Figure 9. Resonant frequency, Q factor, and frequency response of the acceleration switch
centrifugal chamber control PC
contact part rotating disk
contact part
4.6 m
spring
acceleration switch & commercial accelerometer
comb actuator
oscilloscope & voltage source
Figure 10. Measurement setup
Figure 7. SEM images of fabricated acceleration switch
Page 12 of 15
(a) pushing comb voltage increment
pulling comb voltage increment
switch on at 10 g
ip t
bi-directional threshold tuning
(b)
cr
Figure 13. Measured and theoretical threshold acceleration according to the tuning voltage
ce pt
ed
M
Figure 11. Measured output of acceleration switch and accelerometer at (a) no tuning voltage, and (b) Vpush=20 V
an
us
switch on at 6 g
9.7 ms
on-state
Ac
threshold acceleration
off-state
Figure 12. Measured switching-on characteristics of the acceleration switch
Page 13 of 15
(a)
(a)
stopper
pulling comb
springs for latching
contact part
latch
electrode for transition moving direction pushing comb
stopper
latching parts
hook-shaped spring
latch (b)
ip t
(b)
cr
latched
switch off at “safe” position
an
us
unstable contact
(c)
M
(c)
Figure 15. (a) SEM images of fabricated acceleration switch, (b) switch output, and (c) tuning characteristics of the switch
Ac
(d)
ce pt
ed
switch on at “armed” position
Figure 14. (a) SEM images of fabricated acceleration switch, (b) switch output at safe position, (c) switch output at armed position, (d) tuning characteristics at armed position
Page 14 of 15
*Highlights (for review)
Special virtual Transducers 2013 volume of Sensors and Actuators A: Physical Original Paper Number in Transducers 2013: T3P.004
MEMS acceleration switch with bi-directionally tunable threshold
We firstly propose a bi-directionally tunable MEMS acceleration switch. Threshold decreased from 10.25 g to 2.0 g by applying tuning voltage. Threshold increased from 10.25 g to 17.25 g by applying tuning voltage. Additional functionality of safe/armed position convertibility is realized.
ce pt
ed
M
an
us
cr
Additional functionality of mechanical latching is realized.
Ac
ip t
Highlights
Page 15 of 15