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A novel inertial switch with an adjustable acceleration threshold using an MEMS digital-to-analog converter
Microelectronic Engineering 110 (2013) 374–380
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A novel inertial switch with an adjustable acceleration threshold using an MEMS digital-to-analog converter Cheng-Wen Ma a, Po-Cheng Huang a, Jui-Chang Kuo a, Wen-Cheng Kuo b, Yao-Joe Yang a,⇑ a
Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan Department of Mechanical and Automation Engineering and Graduate Institute of Industrial Design, National Kaohsiung First University of Science and Technology, Kaoshiung, Taiwan b
a r t i c l e
i n f o
Article history: Available online 4 March 2013 Keywords: Inertial switch MEMS digital-to-analog converter Parylene KOH etching
a b s t r a c t This study presents the development of an inertial switch that uses an MEMS digital-to-analog converter (M-DAC) to adjust acceleration thresholds. The proposed device consists of an M-DAC layer with a proofmass, latching layer, and PDMS cap. Various PDMS caps can be used to push the selected adjusting plates of the M-DAC layer, generating specific displacement states of the proof-mass, thereby enabling the adjustment of the acceleration thresholds. When an applied acceleration exceeds the specified acceleration threshold, the proof-mass moves up and latches onto the latching layer to record the inertia impact. In addition, the unlatching of the device can be easily achieved by rotating the proof-mass of the M-DAC layer using needles. The suspensions of the M-DAC layer are fabricated with parylene-C to achieve a low stiffness. The latching layer is fabricated using simple KOH etching with corner compensation. The PDMS cap is constructed using SU-8 molds. The acceleration thresholds can be varied from 40 to 75 g by using various PDMS caps. The measured results are in good agreement with the analytical results. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Impacts can cause damage to fragile devices. Monitoring acceleration can predict the likelihood of device damage from impacts. Dissimilar to typical accelerometers [1] that sense the level of acceleration, inertial switches chiefly detect acceleration thresholds and can be used for safety and protection in airbags, transportation systems, crash recorders, and arming and firing systems [2]. Because of the cost advantage of the batch process, MEMS-based accelerometers are inexpensive. However, the total cost of a typical microsensor system, which includes an MEMS sensor package, printed circuit boards, and external power sources [3], may not be trivial. Therefore, passive inertial switches have been proposed. Recently, numerous inertial switches have been developed for various applications [4,5]. Hensen et al. [6] proposed an inertial switch that uses a bi-stable mechanism that switches states when the external acceleration exceeds a threshold value. An acceleration latching switch with cylindrical contacts was presented in [7]. Special contacts can decrease the contact resistance and effectively latch the switch. In [8], an inertial switch with a v-shaped ⇑ Corresponding author. Address: Department of Mechanical Engineering, National Taiwan University, No. 1 Roosevelt Rd., Sec. 4, Taipei, Taiwan 10617, ROC. Fax: +886 2 23631755. E-mail address: [email protected] (Y.-J. Yang). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.02.069
beam was proposed. The contact stability and anti-disturbance capability were studied. An inertial switch using a mechanical memory was presented in [9], in which the proposed device relies on the pure plastic extension of the sensing section that is realized by the elastic buckling of a thin wire. An inertial micro-switch with a bridge-type elastic electrode was proposed in [10], and the bridge-type elastic beams can effectively enhance the contact of the micro-switch. In [11], a threshold accelerometer based on a fully compliant bistable mechanism was presented. Yoo et al. [12] proposed an inertial switch that uses liquid–metal as the activation mechanism. A liquid–metal droplet in a microstructured channel serves as the activation component of the inertial switch. Most of the mentioned inertial switches only allow a pre-defined threshold acceleration that is typically determined during the mask layout design. This study proposes a novel inertial switch that employs an M-DAC mechanism to adjust the acceleration thresholds. The proposed device consists of an M-DAC layer, latching layer, and PDMS cap. Various PDMS caps can be used to move the selected adjusting plates of the M-DAC layer, which generates specific displacement states of the proof-mass, thereby enabling the adjustment of the acceleration threshold. When an input acceleration exceeds a threshold, the proof-mass latches with the latching layer. The unlatching of the device can be easily achieved by rotating the proof-mass of the M-DAC layer using needles. The latching layer is fabricated using simple KOH etching with corner compensation. The PDMS cap is fabricated using SU-8 molds. The
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suspensions of the M-DAC layer are fabricated with parylene-C to achieve low stiffness. This paper is organized as follows: the design, principle, and fabrication of the proposed adjustable inertial switch are presented in Sections 2 and 3; the measurement results and discussions are provided in Section 4; and, finally, a conclusion is offered in Section 5.
Inclined surface
Latching block
Z
a
Z
Latching beam
Proof mass
a
(a)
(b)
2. Device design and operational principle Fig. 1(a) shows a schematic of the proposed inertial switch. Fig. 1(b) shows the exploded view of the proposed inertial switch. The device consists of three layers: the silicon M-DAC layer, silicon latching layer, and PDMS cap. The M-DAC on the M-DAC layer can generate eight discretized displacement states that can be used to adjust eight acceleration thresholds. There are eight designs of the PDMS caps, and each PDMS cap can induce a state of the M-DAC. The latching block on the latching layer and the latching beam on the proof-mass of M-DAC layer are critical components for threshold detection. Fig. 2 shows the detailed schematic of the latching mechanism of the inertial switch. The mechanism consists of a latching beam (on the proof-mass of the M-DAC layer) and a latching block (on the latching layer). As shown in Fig. 2(a), when an external acceleration exists along the negative-Z direction, the proof-mass moves in the positive-Z direction. Fig. 2(b) and (c) shows the latching beam of the proof-mass contacting the inclined surface of the latching block, and sliding onto the top surface of the latching block. Fig. 2(d) shows the latched state. Note that metal films are completely coated on
B’ PDMS cap A’
Z
Z
a
Latching block
a
(c)
(d)
Fig. 2. The detailed schematic of the latching mechanism of the inertial switch.
both sides of the M-DAC layer (including each latching beam). Also, a metal film is patterned on the top side of the latching layer (including the top side of each latching blocks). Therefore, as the latching beams are latched with the latching blocks, Electrode-A and Electrode-B (see Fig. 1) will be shorted via the proof-mass, which can be easily detected using a multimeter. Fig. 3(a) shows a detailed schematic of the 3-bit M-DAC. The MDAC consists of a proof-mass, six adjusting plates, two latching beams, and a set of parylene suspensions that connect the adjusting plate to the proof-mass. The spring constants of these meandering parylene suspensions are specially designed so that M-DAC can generate specific displacements based on the input bits. The adjusting plates are either at its original position, or pushed down by a bump on the PDMS cap. More details about the PDMS cap will be provided in next section. By pushing with the selected adjusting plates of the M-DAC mechanism, various
Adjusting plate A
M-DAC Layer
Latching Layer B
Latching beam
Parylene suspension
(a) PDMS cap
Proof mass
Unlatching hole
Latching block
Bit2
Bit1
Bit3
Latching Layer Electrode B Electrode A
(a) Latching beam
a
Proof mass
b
1
2
n
3
M-DAC Layer Adjusting plate
Parylene suspension
(b) Fig. 1. (a) The schematic of the proposed inertial switch. (b) The exploded view of the inertial switch.
(b)
w
Fig. 3. (a) The schematic of the 3bit M-DAC layer. (b) The detailed schematic of a meandering spring with design parameters.
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The schematic of a meandering parylene suspension is shown in Fig. 3(b). The analytical model [14] for estimating the spring constants of the meandering springs is given by:
Table 1 The parameters of the 3-bit M-DAC. Connection springs
Bit1
Bit2
Bit3
Primary meander length (lm): a Secondary meander length (lm): b Number of periods: n Spring constant (N/m): K
100 275 8 0.0787
100 321 6 0.1572
100 450 4 0.3147
K¼
Table 2 The parameters of the meandering suspensions. Parameters
Definitions/values
Structure thickness: t Beam width: w Young’s modulus: E Shear modulus: G x-Axis moment of inertia: Ix Torsion constant : J
20 (lm) 40 (lm) 4 (GPa) 1.67 (GPa) wt3/12 (lm4) 0.229 wt3 (lm4)
Dz ¼
kp þ ð2N 1Þk
!
1 2N 1
N X 2i1 bi
! ð1Þ
i¼1
where k, 2 k,. . ., 2N1 k are the spring constants of the meandering suspensions that connect the proof-mass and adjusting plates, kp is the spring constant of the proof-mass suspensions, D is the displacement of the adjusting plate, bi is the binary control bit, and i = 1,2,. . .N. The detailed design parameters of the 3-bit M-DAC are shown in Table 1.
A-A
na2 ½2na þ ð2nþ1Þb 2 EIx GJ 2ðEIax þ GJb Þ
nb 2
2
a b þ GJ EIx
#1 ð2Þ
The dimensions and design parameters for the meandering suspensions which were employed in the device are listed in Table 2. Fig. 4 shows how the M-DAC adjusts the acceleration threshold. The A–A0 cross-sectional view of the M-DAC device (Fig. 1) at the Initial State (State-A) is illustrated in Fig. 4(a). Also, the A–A0 cross-sectional views of the device at the Adjusted State (State-B) is illustrated in Fig. 4(b). Similarly, Fig. 4(c) and Fig. 4(d) are the B–B0 cross-sectional views of the M-DAC device at State-A and State-B. At State-A, the initial distance between the lower edge of the latching beam and the upper edge of the latching block is Z0. As the PDMS cap attaches on the top of the M-DAC device, the adjusting plates of the M-DAC are pushed downward by the bumps of the PDMS cap, which in turn generates an M-DAC displacement of Dz (i.e., State-B). The distance between the latching beam and latching block at State-B is farther than that at State-A. Therefore, a greater external acceleration for State-B is required to move the proofmass to latch with the latching block. The primary function of the M-DAC is to generate different Dz by using different types of PDMS caps, and thus to change the acceleration thresholds. Fig. 5 shows the schematic of eight designs of the PDMS caps that can be used to impose eight states of the M-DAC. The bumps on the PDMS cap are used to press the adjusting plates of the
displacement states of the proof-mass can be obtained. Therefore, the acceleration threshold can be altered. In addition, the states of the M-DAC can be changed using a PDMS cap. The displacement of the proof-mass for an N-bit device is given by [13]:
ð2N 1Þk D
" 3 8n3 a3 þ 2nb abn½3b þ ð2n þ 1Þð4n þ 1Þa þ 3GJ 3EIx
cross section State-A Latching block
Z0
Proof-mass Latching beam A-A
(a) PDMS cap
cross section State-B
Z0 + Δz
(b) B-B
cross section State-A
Adjusting plate B-B
(c)
cross section State-B
D
(d) Fig. 4. (a) The A–A0 cross section of inertial switch of State-A, and (b) State-B. (c) The B–B0 cross section of inertial switch of State-A and (d) State-B. Note that Dz is generated by the M-DAC after the PDMS cap is placed on the top of the M-DAC.
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Bump
Unlatching hole
000
001
010
011
100
101
110
111
Fig. 5. The schematic of eight different designs of the PDMS caps.
Needle Unlatching holes
PDMS cap
Latching beam
Proof mass Latching layer
(a)
Latching block
(b) Needle moves down
Unlatching
(c)
(d)
Fig. 6. The schematic of unlatching mechanism.
M-DAC. When the PDMS cap clips onto the top of the M-DAC layer, the bumps push the adjusting plates of the M-DAC, generating an M-DAC displacement. The PDMS caps can be easily fabricated using an SU-8 mold. The relationship of the acceleration thresholds and the M-DAC motion step can be described as:
F ¼ kðz0 þ N . . . DzÞ ¼ mða0 þ N DaÞ ¼ m ath ;
N ¼ 0; 1; 2 . . . 7 ð3Þ
where k is the total spring constant of the connection springs that connect the proof-mass and adjusting plates, z0 is the initial displacement, a0 is the initial acceleration, m is mass of the proof-mass, and F is the external force. Also, the available acceleration thresholds which can be adjusted by the M-DAC can be written as:
ath ¼ a0 þ N Da;
N ¼ 0; 1; 2 . . . 7
ð4Þ
Fig. 6 shows the schematic of the unlatching mechanism. By manually and gently inserting needles into the unlatching holes, the proof-mass is rotated clockwise, and thus the latching beam detaches from the latching block and the proof-mass is released. 3. Device fabrication The fabrication process of the M-DAC layer is described in Fig. 7(a)–(i). A silicon wafer (300 lm) was the starting material.
ICP silicon etching was used to define the structure, as shown in Fig. 7(b). Next, the parylene-C was deposited (Fig. 7(c)). Thereafter, a chromium and gold layer was deposited and patterned as the etching mask for the parylene springs, as shown in Fig. 7(d) and (e). Then, parylene-C spring structures were formed using RIE etching (Fig. 7(f)). The potential etching reactions that govern parylene removal have been previously noted [15]. The metal etching mask was removed using wet etchant (Fig. 7(g)), and the entire wafer was etched using an isotropic silicon etching technique (Fig. 7(h)). This solution should be mixed several hours in advance to yield a stable etch rate [16]. The M-DAC structure was protected by the parylene during the wet etching process. Finally, the proofmass, springs, and adjusting plates were formed. An additional chromium and gold metal layer was deposited to create the conductive layer, as shown in Fig. 7(i). The fabrication process of the latching layer is described in Fig. 7(j)–(r). A silicon wafer (300 lm) was used as the starting material. The first step was to pattern the silicon oxide layers, which were used as the etching masks for the structure, as shown in Fig. 7(k). The wafer was immersed in KOH etchant, as shown in Fig. 7(l). Then, the parylene-C was deposited, as shown in Fig. 7(m). Thereafter, the parylene layer and the oxide layer were patterned using lithography and RIE, and the etching mask was formed, as shown in Fig. 7(n) and (o). Next, the wafer was etched by KOH etching, as shown in Fig. 7(p). Finally, the parylene-C was removed using RIE etching, as shown in Fig. 7(q), and the chromium and
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Latching Layer
M-DAC Layer
300 µm (a) 300um silicon wafer
(j) 300 um silicon wafer
(b) ICP etching (150um)
(k) patterning etching mask
(c) Parylene deposition
(l) KOH etching
(d) Cr/Au deposition
(m) Parylene deposition
(e) Cr/Au patterning
(n) Parylene etching
(f) RIE etching of parylene
(o) Patterning etching mask
(g) Cr/Au layer removing
(p) Backside etching
(h) Isotropic silicon etching
(q) Parylene etching
(i) Electrode deposition
(r) Electrode patterning
(b)
100 µm
1 mm
(c)
(a)
300 µm
200 µm
(e)
(d)
Si
Cr/Au
Parylene
Oxide
Fig. 9. (a) Top view of latching block. (b) The closer top view of latching block. (c) The front view of latching block. (d) The M-DAC layer. (e) The adjust plate.
1 mm
2 mm (a)The M-DAC layer
(b)The latching layer
(s) Assembled
Fig. 7. The fabrication process of the inertial switch.
Etching Mask
Etched Geometry The top surface of the convex corner created by KOH etching
2H
1 mm
3 mm
(c)The PDMS cap
3.2H
(d)The assembled device
Fig. 10. The fabricated components and assembled device. (a) The M-DAC layer. (b) The latching layer. (c) The PDMS cap. (d) The assembled device.
H = etching depth Fig. 8. The etching mask design and the etched geometry of the corner compensation techniques.
PC with Labview and DAQ card
gold layer was deposited using a shadow mask as the electrodes, as shown in Fig. 7(r). The schematic view when the M-DAC layer and the latching layer were assembled together is shown in Fig. 7(s). The latching blocks on the latching layer were realized using KOH etching. Because two convex corners exist on each latching block, the corner compensation technique [17] was employed to ensure the corners of the latching blocks will not be over-etched during the KOH-etching process. Fig. 8 shows the etching mask design as well as the etching geometry of the corner compensation techniques.
Acceleration switch Accelerometer Power Amplifier a Shaker
Fig. 11. The experimental setup for measuring the acceleration thresholds.
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Then, the M-DAC layer and the latching layer were bonded to form the inertial switch chip. Polymeric adhesive (epoxy) was employed as the glue to bond these two layers. Before the layers were aligned, the adhesive was dispensed on the bonding surfaces close to the edges of the layers. The alignment was performed by using a microscope and a linear XY stage. During the alignment, the two pairs of the latching beams (on the M-DAC layer) and the latching blocks (on the latching layer), which are symmetric to the center of the device, were used as the alignment marks. The acceleration thresholds can be manually adjusted by placing different PDMS caps on the top of the switch chip. Currently, we use PMMA frames to fix the PDMS cap with the switch chip. In the future, the PDMS cap and PMMA frames can be replaced by a plastic cap which is mass-produced by injection-molding process. Fig. 9(a) shows the fabricated latching layer. Fig. 9(b) and 9(c) shows the top view of the latching block and the front view of latching block, respectively. Figs. 9(d) and 9(e) is the SEM pictures of M-DAC layer. The dimensions of the latching block are 250 100 25 lm. Pictures of the fabricated components and assembled device are shown in Fig. 10. Fig. 10(a) shows the MDAC layer that consists of a proof-mass, adjusting plate, unlatching hole, and parylene suspensions. The thickness of the layer is 150 lm. The latching layer, which consists of latching block, adjusting holes, and electrodes, is shown in Fig. 10(b). The size of unlatching holes is 400 lm. Fig. 10(c) shows the PDMS cap which consists of unlatching holes and adjusting bumps is 475 lm. Fig. 10(d) shows a picture of the assembled inertial switch that is fixed onto a PMMA frame.
4. Measurement and discussion Fig. 11 shows the experimental setup for measuring the acceleration thresholds. The proposed inertial switch was set up on a shaker table to measure the device performance and experimentally verify the analytical models. The acceleration applied to the inertial switch was monitored with a commercial reference accelerometer mounted on the shaker table. The electrodes of the latching layer were connected to a DAQ card to detect whether the electrodes were shorted as the device was latched. The DAQ card generates a square wave signal to a power amplifier to drive the shaker. A data acquisition program was developed using LabVIEW to simultaneously record the output of the reference accelerometer mounted on the shaker and to monitor the voltage output from the inertial sensor. Fig. 12 shows the measured results when a shock of 40 g was applied to the device with a PDMS cap of the ‘‘000’’ M-DAC state. This state also corresponds to the initial acceleration threshold a0. The variation of the input acceleration is about 10%, and was probably caused by the contact behaviors between latching block and latching beam. In Fig. 12, the measured voltage, which was filtered using a de-bouncing circuit, indicated that the latching beam contacted the latching block when the acceleration exceeded the threshold. As the PDMS cap was attached on the switch chip, each bump on the PDMS cap contacts and pushes the corresponding adjusting plate. The displacement of the adjusting plate (D) is about 175 lm. According to Eq. (1), the motion step of the M-DAC structure is 25 lm. Based on Eq. (3), the acceleration step (Da) for each motion
30
1.2
20
1
Acceleration (g)
0.8
0
0.6
-10 -20
0.4
Acceleration output -30
Measured output
normalized voltage
10
0.2
-40 -50
0 0
50
100
150
Time (ms) Fig. 12. The measured results of acceleration output.
85 Experiment Results Analytical Results
65
55
45
Input Binary Code Fig. 13. The measured results of acceleration threshold.
111
110
101
100
011
010
001
35 000
Acceleration (g)
75
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C.-W. Ma et al. / Microelectronic Engineering 110 (2013) 374–380
step is about 5 g. By using different PDMS caps (i.e., 001, 010, 011 and so on), different M-DAC displacement outputs can be calculated by using Eq. (1), and different acceleration thresholds can be estimated by using Eq. (4). Fig. 13 shows the measured results of the inertial switch with various M-DAC configurations. Each data point on the curve is the average result obtained by measuring the device five times. By using Eq. (4), the acceleration thresholds were estimated as 40 g (0 0 0), 45 g (0 0 1), 50 g (0 1 0), 55 g (0 1 1), 60 g (1 0 0), 65 g (1 0 1), 70 g (1 1 0) and 75 g (1 1 1). The error bars indicate the measured maximal and minimal values, and are about 8%– 12%. The measured results were in good agreement with the analytical results. 5. Conclusions This study developed an inertial switch with the capability of adjustable acceleration thresholds. The device comprises an MEMS digital-to-analog converter (M-DAC) layer, a latching layer, and PDMS caps. Eight different PDMS caps can be used to push the selected adjusting plates of the M-DAC layer, generating various displacement states of the proof-mass, thereby altering the acceleration threshold. The suspensions of the M-DAC layer were fabricated with parylene-C. The latching layer was fabricated using simple KOH etching with corner compensation. The PDMS cap was fabricated using SU-8 molds. In addition, the unlatching mechanism was actuated by rotating the proof-mass of the MDAC layer with needles. The acceleration thresholds can be adjusted from 40 to 75 g using different PDMS caps. The measured results were in good agreement with the analytical results.
Acknowledgement This study was supported in part by the National Science Council, Taiwan, ROC (Contract No: NSC 100-2221-E-002-075 -MY3). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
H. Campanella et al., Microelectronic Engineering 86 (2009) 1254–1257. S. Michaelis et al., Sensors & Actuators A – Physical 85 (2000) 418–423. C.Z. Wei et al., Microelectronic Engineering 91 (2012) 167–173. C.Y. Chan et al., IEEE-ASME Transactions on Mechatronics 7 (2002) 220–234. T. Matsunaga et al., Sensors & Actuators A – Physical 100 (2002) 10–17. B.J. Hansen, Smart Materials and Structures 16 (2007) 1967–1972. Z.Y. Guo et al., Journal of Micromechanics and Microengineering 20 (2010) 5. J. Zhao et al., Mechatronic and Embedded Systems and Applications (2006) 1– 5. A. Mita et al., Smart Materials and Structures 12 (2003) 204–209. Y. Zhuoqing et al., IEEE Transactions on Electron Devices 55 (2008) 2492–2497. Zhao, et al., IEEE Sensors Journal 10 (2010) 1019–1024. Yoo Kwanghyun, et al., Sensors & Actuators – Part A – Physical 166 (2011) 234–240. L. Hsin-Hung et al., Journal of Micromechanics and Microengineering 20 (2010) 10. G.Y. Zhuo et al., Journal of Microelectromechanical Systems 13 (2004) 770– 778. E. Meng et al., Journal of Micromechanics and Microengineering 18 (2008) 045004. K.R. Williams et al., Journal of Microelectromechanical Systems 12 (2003) 761–778. H. Sandmair et al., in Proc. International Conference on Solid-State Sensors and Actuators (Transducers 1991) 456–459.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A novel inertial switch with an adjustable acceleration threshold using an MEMS digital-to-analog converter Cheng-Wen Ma a, Po-Cheng Huang a, Jui-Chang Kuo a, Wen-Cheng Kuo b, Yao-Joe Yang a,⇑ a
Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan Department of Mechanical and Automation Engineering and Graduate Institute of Industrial Design, National Kaohsiung First University of Science and Technology, Kaoshiung, Taiwan b
a r t i c l e
i n f o
Article history: Available online 4 March 2013 Keywords: Inertial switch MEMS digital-to-analog converter Parylene KOH etching
a b s t r a c t This study presents the development of an inertial switch that uses an MEMS digital-to-analog converter (M-DAC) to adjust acceleration thresholds. The proposed device consists of an M-DAC layer with a proofmass, latching layer, and PDMS cap. Various PDMS caps can be used to push the selected adjusting plates of the M-DAC layer, generating specific displacement states of the proof-mass, thereby enabling the adjustment of the acceleration thresholds. When an applied acceleration exceeds the specified acceleration threshold, the proof-mass moves up and latches onto the latching layer to record the inertia impact. In addition, the unlatching of the device can be easily achieved by rotating the proof-mass of the M-DAC layer using needles. The suspensions of the M-DAC layer are fabricated with parylene-C to achieve a low stiffness. The latching layer is fabricated using simple KOH etching with corner compensation. The PDMS cap is constructed using SU-8 molds. The acceleration thresholds can be varied from 40 to 75 g by using various PDMS caps. The measured results are in good agreement with the analytical results. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Impacts can cause damage to fragile devices. Monitoring acceleration can predict the likelihood of device damage from impacts. Dissimilar to typical accelerometers [1] that sense the level of acceleration, inertial switches chiefly detect acceleration thresholds and can be used for safety and protection in airbags, transportation systems, crash recorders, and arming and firing systems [2]. Because of the cost advantage of the batch process, MEMS-based accelerometers are inexpensive. However, the total cost of a typical microsensor system, which includes an MEMS sensor package, printed circuit boards, and external power sources [3], may not be trivial. Therefore, passive inertial switches have been proposed. Recently, numerous inertial switches have been developed for various applications [4,5]. Hensen et al. [6] proposed an inertial switch that uses a bi-stable mechanism that switches states when the external acceleration exceeds a threshold value. An acceleration latching switch with cylindrical contacts was presented in [7]. Special contacts can decrease the contact resistance and effectively latch the switch. In [8], an inertial switch with a v-shaped ⇑ Corresponding author. Address: Department of Mechanical Engineering, National Taiwan University, No. 1 Roosevelt Rd., Sec. 4, Taipei, Taiwan 10617, ROC. Fax: +886 2 23631755. E-mail address: [email protected] (Y.-J. Yang). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.02.069
beam was proposed. The contact stability and anti-disturbance capability were studied. An inertial switch using a mechanical memory was presented in [9], in which the proposed device relies on the pure plastic extension of the sensing section that is realized by the elastic buckling of a thin wire. An inertial micro-switch with a bridge-type elastic electrode was proposed in [10], and the bridge-type elastic beams can effectively enhance the contact of the micro-switch. In [11], a threshold accelerometer based on a fully compliant bistable mechanism was presented. Yoo et al. [12] proposed an inertial switch that uses liquid–metal as the activation mechanism. A liquid–metal droplet in a microstructured channel serves as the activation component of the inertial switch. Most of the mentioned inertial switches only allow a pre-defined threshold acceleration that is typically determined during the mask layout design. This study proposes a novel inertial switch that employs an M-DAC mechanism to adjust the acceleration thresholds. The proposed device consists of an M-DAC layer, latching layer, and PDMS cap. Various PDMS caps can be used to move the selected adjusting plates of the M-DAC layer, which generates specific displacement states of the proof-mass, thereby enabling the adjustment of the acceleration threshold. When an input acceleration exceeds a threshold, the proof-mass latches with the latching layer. The unlatching of the device can be easily achieved by rotating the proof-mass of the M-DAC layer using needles. The latching layer is fabricated using simple KOH etching with corner compensation. The PDMS cap is fabricated using SU-8 molds. The
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C.-W. Ma et al. / Microelectronic Engineering 110 (2013) 374–380
suspensions of the M-DAC layer are fabricated with parylene-C to achieve low stiffness. This paper is organized as follows: the design, principle, and fabrication of the proposed adjustable inertial switch are presented in Sections 2 and 3; the measurement results and discussions are provided in Section 4; and, finally, a conclusion is offered in Section 5.
Inclined surface
Latching block
Z
a
Z
Latching beam
Proof mass
a
(a)
(b)
2. Device design and operational principle Fig. 1(a) shows a schematic of the proposed inertial switch. Fig. 1(b) shows the exploded view of the proposed inertial switch. The device consists of three layers: the silicon M-DAC layer, silicon latching layer, and PDMS cap. The M-DAC on the M-DAC layer can generate eight discretized displacement states that can be used to adjust eight acceleration thresholds. There are eight designs of the PDMS caps, and each PDMS cap can induce a state of the M-DAC. The latching block on the latching layer and the latching beam on the proof-mass of M-DAC layer are critical components for threshold detection. Fig. 2 shows the detailed schematic of the latching mechanism of the inertial switch. The mechanism consists of a latching beam (on the proof-mass of the M-DAC layer) and a latching block (on the latching layer). As shown in Fig. 2(a), when an external acceleration exists along the negative-Z direction, the proof-mass moves in the positive-Z direction. Fig. 2(b) and (c) shows the latching beam of the proof-mass contacting the inclined surface of the latching block, and sliding onto the top surface of the latching block. Fig. 2(d) shows the latched state. Note that metal films are completely coated on
B’ PDMS cap A’
Z
Z
a
Latching block
a
(c)
(d)
Fig. 2. The detailed schematic of the latching mechanism of the inertial switch.
both sides of the M-DAC layer (including each latching beam). Also, a metal film is patterned on the top side of the latching layer (including the top side of each latching blocks). Therefore, as the latching beams are latched with the latching blocks, Electrode-A and Electrode-B (see Fig. 1) will be shorted via the proof-mass, which can be easily detected using a multimeter. Fig. 3(a) shows a detailed schematic of the 3-bit M-DAC. The MDAC consists of a proof-mass, six adjusting plates, two latching beams, and a set of parylene suspensions that connect the adjusting plate to the proof-mass. The spring constants of these meandering parylene suspensions are specially designed so that M-DAC can generate specific displacements based on the input bits. The adjusting plates are either at its original position, or pushed down by a bump on the PDMS cap. More details about the PDMS cap will be provided in next section. By pushing with the selected adjusting plates of the M-DAC mechanism, various
Adjusting plate A
M-DAC Layer
Latching Layer B
Latching beam
Parylene suspension
(a) PDMS cap
Proof mass
Unlatching hole
Latching block
Bit2
Bit1
Bit3
Latching Layer Electrode B Electrode A
(a) Latching beam
a
Proof mass
b
1
2
n
3
M-DAC Layer Adjusting plate
Parylene suspension
(b) Fig. 1. (a) The schematic of the proposed inertial switch. (b) The exploded view of the inertial switch.
(b)
w
Fig. 3. (a) The schematic of the 3bit M-DAC layer. (b) The detailed schematic of a meandering spring with design parameters.
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The schematic of a meandering parylene suspension is shown in Fig. 3(b). The analytical model [14] for estimating the spring constants of the meandering springs is given by:
Table 1 The parameters of the 3-bit M-DAC. Connection springs
Bit1
Bit2
Bit3
Primary meander length (lm): a Secondary meander length (lm): b Number of periods: n Spring constant (N/m): K
100 275 8 0.0787
100 321 6 0.1572
100 450 4 0.3147
K¼
Table 2 The parameters of the meandering suspensions. Parameters
Definitions/values
Structure thickness: t Beam width: w Young’s modulus: E Shear modulus: G x-Axis moment of inertia: Ix Torsion constant : J
20 (lm) 40 (lm) 4 (GPa) 1.67 (GPa) wt3/12 (lm4) 0.229 wt3 (lm4)
Dz ¼
kp þ ð2N 1Þk
!
1 2N 1
N X 2i1 bi
! ð1Þ
i¼1
where k, 2 k,. . ., 2N1 k are the spring constants of the meandering suspensions that connect the proof-mass and adjusting plates, kp is the spring constant of the proof-mass suspensions, D is the displacement of the adjusting plate, bi is the binary control bit, and i = 1,2,. . .N. The detailed design parameters of the 3-bit M-DAC are shown in Table 1.
A-A
na2 ½2na þ ð2nþ1Þb 2 EIx GJ 2ðEIax þ GJb Þ
nb 2
2
a b þ GJ EIx
#1 ð2Þ
The dimensions and design parameters for the meandering suspensions which were employed in the device are listed in Table 2. Fig. 4 shows how the M-DAC adjusts the acceleration threshold. The A–A0 cross-sectional view of the M-DAC device (Fig. 1) at the Initial State (State-A) is illustrated in Fig. 4(a). Also, the A–A0 cross-sectional views of the device at the Adjusted State (State-B) is illustrated in Fig. 4(b). Similarly, Fig. 4(c) and Fig. 4(d) are the B–B0 cross-sectional views of the M-DAC device at State-A and State-B. At State-A, the initial distance between the lower edge of the latching beam and the upper edge of the latching block is Z0. As the PDMS cap attaches on the top of the M-DAC device, the adjusting plates of the M-DAC are pushed downward by the bumps of the PDMS cap, which in turn generates an M-DAC displacement of Dz (i.e., State-B). The distance between the latching beam and latching block at State-B is farther than that at State-A. Therefore, a greater external acceleration for State-B is required to move the proofmass to latch with the latching block. The primary function of the M-DAC is to generate different Dz by using different types of PDMS caps, and thus to change the acceleration thresholds. Fig. 5 shows the schematic of eight designs of the PDMS caps that can be used to impose eight states of the M-DAC. The bumps on the PDMS cap are used to press the adjusting plates of the
displacement states of the proof-mass can be obtained. Therefore, the acceleration threshold can be altered. In addition, the states of the M-DAC can be changed using a PDMS cap. The displacement of the proof-mass for an N-bit device is given by [13]:
ð2N 1Þk D
" 3 8n3 a3 þ 2nb abn½3b þ ð2n þ 1Þð4n þ 1Þa þ 3GJ 3EIx
cross section State-A Latching block
Z0
Proof-mass Latching beam A-A
(a) PDMS cap
cross section State-B
Z0 + Δz
(b) B-B
cross section State-A
Adjusting plate B-B
(c)
cross section State-B
D
(d) Fig. 4. (a) The A–A0 cross section of inertial switch of State-A, and (b) State-B. (c) The B–B0 cross section of inertial switch of State-A and (d) State-B. Note that Dz is generated by the M-DAC after the PDMS cap is placed on the top of the M-DAC.
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Bump
Unlatching hole
000
001
010
011
100
101
110
111
Fig. 5. The schematic of eight different designs of the PDMS caps.
Needle Unlatching holes
PDMS cap
Latching beam
Proof mass Latching layer
(a)
Latching block
(b) Needle moves down
Unlatching
(c)
(d)
Fig. 6. The schematic of unlatching mechanism.
M-DAC. When the PDMS cap clips onto the top of the M-DAC layer, the bumps push the adjusting plates of the M-DAC, generating an M-DAC displacement. The PDMS caps can be easily fabricated using an SU-8 mold. The relationship of the acceleration thresholds and the M-DAC motion step can be described as:
F ¼ kðz0 þ N . . . DzÞ ¼ mða0 þ N DaÞ ¼ m ath ;
N ¼ 0; 1; 2 . . . 7 ð3Þ
where k is the total spring constant of the connection springs that connect the proof-mass and adjusting plates, z0 is the initial displacement, a0 is the initial acceleration, m is mass of the proof-mass, and F is the external force. Also, the available acceleration thresholds which can be adjusted by the M-DAC can be written as:
ath ¼ a0 þ N Da;
N ¼ 0; 1; 2 . . . 7
ð4Þ
Fig. 6 shows the schematic of the unlatching mechanism. By manually and gently inserting needles into the unlatching holes, the proof-mass is rotated clockwise, and thus the latching beam detaches from the latching block and the proof-mass is released. 3. Device fabrication The fabrication process of the M-DAC layer is described in Fig. 7(a)–(i). A silicon wafer (300 lm) was the starting material.
ICP silicon etching was used to define the structure, as shown in Fig. 7(b). Next, the parylene-C was deposited (Fig. 7(c)). Thereafter, a chromium and gold layer was deposited and patterned as the etching mask for the parylene springs, as shown in Fig. 7(d) and (e). Then, parylene-C spring structures were formed using RIE etching (Fig. 7(f)). The potential etching reactions that govern parylene removal have been previously noted [15]. The metal etching mask was removed using wet etchant (Fig. 7(g)), and the entire wafer was etched using an isotropic silicon etching technique (Fig. 7(h)). This solution should be mixed several hours in advance to yield a stable etch rate [16]. The M-DAC structure was protected by the parylene during the wet etching process. Finally, the proofmass, springs, and adjusting plates were formed. An additional chromium and gold metal layer was deposited to create the conductive layer, as shown in Fig. 7(i). The fabrication process of the latching layer is described in Fig. 7(j)–(r). A silicon wafer (300 lm) was used as the starting material. The first step was to pattern the silicon oxide layers, which were used as the etching masks for the structure, as shown in Fig. 7(k). The wafer was immersed in KOH etchant, as shown in Fig. 7(l). Then, the parylene-C was deposited, as shown in Fig. 7(m). Thereafter, the parylene layer and the oxide layer were patterned using lithography and RIE, and the etching mask was formed, as shown in Fig. 7(n) and (o). Next, the wafer was etched by KOH etching, as shown in Fig. 7(p). Finally, the parylene-C was removed using RIE etching, as shown in Fig. 7(q), and the chromium and
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Latching Layer
M-DAC Layer
300 µm (a) 300um silicon wafer
(j) 300 um silicon wafer
(b) ICP etching (150um)
(k) patterning etching mask
(c) Parylene deposition
(l) KOH etching
(d) Cr/Au deposition
(m) Parylene deposition
(e) Cr/Au patterning
(n) Parylene etching
(f) RIE etching of parylene
(o) Patterning etching mask
(g) Cr/Au layer removing
(p) Backside etching
(h) Isotropic silicon etching
(q) Parylene etching
(i) Electrode deposition
(r) Electrode patterning
(b)
100 µm
1 mm
(c)
(a)
300 µm
200 µm
(e)
(d)
Si
Cr/Au
Parylene
Oxide
Fig. 9. (a) Top view of latching block. (b) The closer top view of latching block. (c) The front view of latching block. (d) The M-DAC layer. (e) The adjust plate.
1 mm
2 mm (a)The M-DAC layer
(b)The latching layer
(s) Assembled
Fig. 7. The fabrication process of the inertial switch.
Etching Mask
Etched Geometry The top surface of the convex corner created by KOH etching
2H
1 mm
3 mm
(c)The PDMS cap
3.2H
(d)The assembled device
Fig. 10. The fabricated components and assembled device. (a) The M-DAC layer. (b) The latching layer. (c) The PDMS cap. (d) The assembled device.
H = etching depth Fig. 8. The etching mask design and the etched geometry of the corner compensation techniques.
PC with Labview and DAQ card
gold layer was deposited using a shadow mask as the electrodes, as shown in Fig. 7(r). The schematic view when the M-DAC layer and the latching layer were assembled together is shown in Fig. 7(s). The latching blocks on the latching layer were realized using KOH etching. Because two convex corners exist on each latching block, the corner compensation technique [17] was employed to ensure the corners of the latching blocks will not be over-etched during the KOH-etching process. Fig. 8 shows the etching mask design as well as the etching geometry of the corner compensation techniques.
Acceleration switch Accelerometer Power Amplifier a Shaker
Fig. 11. The experimental setup for measuring the acceleration thresholds.
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Then, the M-DAC layer and the latching layer were bonded to form the inertial switch chip. Polymeric adhesive (epoxy) was employed as the glue to bond these two layers. Before the layers were aligned, the adhesive was dispensed on the bonding surfaces close to the edges of the layers. The alignment was performed by using a microscope and a linear XY stage. During the alignment, the two pairs of the latching beams (on the M-DAC layer) and the latching blocks (on the latching layer), which are symmetric to the center of the device, were used as the alignment marks. The acceleration thresholds can be manually adjusted by placing different PDMS caps on the top of the switch chip. Currently, we use PMMA frames to fix the PDMS cap with the switch chip. In the future, the PDMS cap and PMMA frames can be replaced by a plastic cap which is mass-produced by injection-molding process. Fig. 9(a) shows the fabricated latching layer. Fig. 9(b) and 9(c) shows the top view of the latching block and the front view of latching block, respectively. Figs. 9(d) and 9(e) is the SEM pictures of M-DAC layer. The dimensions of the latching block are 250 100 25 lm. Pictures of the fabricated components and assembled device are shown in Fig. 10. Fig. 10(a) shows the MDAC layer that consists of a proof-mass, adjusting plate, unlatching hole, and parylene suspensions. The thickness of the layer is 150 lm. The latching layer, which consists of latching block, adjusting holes, and electrodes, is shown in Fig. 10(b). The size of unlatching holes is 400 lm. Fig. 10(c) shows the PDMS cap which consists of unlatching holes and adjusting bumps is 475 lm. Fig. 10(d) shows a picture of the assembled inertial switch that is fixed onto a PMMA frame.
4. Measurement and discussion Fig. 11 shows the experimental setup for measuring the acceleration thresholds. The proposed inertial switch was set up on a shaker table to measure the device performance and experimentally verify the analytical models. The acceleration applied to the inertial switch was monitored with a commercial reference accelerometer mounted on the shaker table. The electrodes of the latching layer were connected to a DAQ card to detect whether the electrodes were shorted as the device was latched. The DAQ card generates a square wave signal to a power amplifier to drive the shaker. A data acquisition program was developed using LabVIEW to simultaneously record the output of the reference accelerometer mounted on the shaker and to monitor the voltage output from the inertial sensor. Fig. 12 shows the measured results when a shock of 40 g was applied to the device with a PDMS cap of the ‘‘000’’ M-DAC state. This state also corresponds to the initial acceleration threshold a0. The variation of the input acceleration is about 10%, and was probably caused by the contact behaviors between latching block and latching beam. In Fig. 12, the measured voltage, which was filtered using a de-bouncing circuit, indicated that the latching beam contacted the latching block when the acceleration exceeded the threshold. As the PDMS cap was attached on the switch chip, each bump on the PDMS cap contacts and pushes the corresponding adjusting plate. The displacement of the adjusting plate (D) is about 175 lm. According to Eq. (1), the motion step of the M-DAC structure is 25 lm. Based on Eq. (3), the acceleration step (Da) for each motion
30
1.2
20
1
Acceleration (g)
0.8
0
0.6
-10 -20
0.4
Acceleration output -30
Measured output
normalized voltage
10
0.2
-40 -50
0 0
50
100
150
Time (ms) Fig. 12. The measured results of acceleration output.
85 Experiment Results Analytical Results
65
55
45
Input Binary Code Fig. 13. The measured results of acceleration threshold.
111
110
101
100
011
010
001
35 000
Acceleration (g)
75
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C.-W. Ma et al. / Microelectronic Engineering 110 (2013) 374–380
step is about 5 g. By using different PDMS caps (i.e., 001, 010, 011 and so on), different M-DAC displacement outputs can be calculated by using Eq. (1), and different acceleration thresholds can be estimated by using Eq. (4). Fig. 13 shows the measured results of the inertial switch with various M-DAC configurations. Each data point on the curve is the average result obtained by measuring the device five times. By using Eq. (4), the acceleration thresholds were estimated as 40 g (0 0 0), 45 g (0 0 1), 50 g (0 1 0), 55 g (0 1 1), 60 g (1 0 0), 65 g (1 0 1), 70 g (1 1 0) and 75 g (1 1 1). The error bars indicate the measured maximal and minimal values, and are about 8%– 12%. The measured results were in good agreement with the analytical results. 5. Conclusions This study developed an inertial switch with the capability of adjustable acceleration thresholds. The device comprises an MEMS digital-to-analog converter (M-DAC) layer, a latching layer, and PDMS caps. Eight different PDMS caps can be used to push the selected adjusting plates of the M-DAC layer, generating various displacement states of the proof-mass, thereby altering the acceleration threshold. The suspensions of the M-DAC layer were fabricated with parylene-C. The latching layer was fabricated using simple KOH etching with corner compensation. The PDMS cap was fabricated using SU-8 molds. In addition, the unlatching mechanism was actuated by rotating the proof-mass of the MDAC layer with needles. The acceleration thresholds can be adjusted from 40 to 75 g using different PDMS caps. The measured results were in good agreement with the analytical results.
Acknowledgement This study was supported in part by the National Science Council, Taiwan, ROC (Contract No: NSC 100-2221-E-002-075 -MY3). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
H. Campanella et al., Microelectronic Engineering 86 (2009) 1254–1257. S. Michaelis et al., Sensors & Actuators A – Physical 85 (2000) 418–423. C.Z. Wei et al., Microelectronic Engineering 91 (2012) 167–173. C.Y. Chan et al., IEEE-ASME Transactions on Mechatronics 7 (2002) 220–234. T. Matsunaga et al., Sensors & Actuators A – Physical 100 (2002) 10–17. B.J. Hansen, Smart Materials and Structures 16 (2007) 1967–1972. Z.Y. Guo et al., Journal of Micromechanics and Microengineering 20 (2010) 5. J. Zhao et al., Mechatronic and Embedded Systems and Applications (2006) 1– 5. A. Mita et al., Smart Materials and Structures 12 (2003) 204–209. Y. Zhuoqing et al., IEEE Transactions on Electron Devices 55 (2008) 2492–2497. Zhao, et al., IEEE Sensors Journal 10 (2010) 1019–1024. Yoo Kwanghyun, et al., Sensors & Actuators – Part A – Physical 166 (2011) 234–240. L. Hsin-Hung et al., Journal of Micromechanics and Microengineering 20 (2010) 10. G.Y. Zhuo et al., Journal of Microelectromechanical Systems 13 (2004) 770– 778. E. Meng et al., Journal of Micromechanics and Microengineering 18 (2008) 045004. K.R. Williams et al., Journal of Microelectromechanical Systems 12 (2003) 761–778. H. Sandmair et al., in Proc. International Conference on Solid-State Sensors and Actuators (Transducers 1991) 456–459.