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Sensors and Actuators A 220 (2014) 194–203

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

j ourna l ho me page: www.elsev ier .com/ locate /sna

imulation, fabrication and characterization of an all-metalontact-enhanced triaxial inertial microswitch with low axialisturbance

enguo Chena,∗, Yan Wanga, Guifu Dinga, Hong Wanga, Xiaolin Zhaoa,huoqing Yanga,b

National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Shanghai Jiao Tong University, Shanghai 200240,eople’s Republic of ChinaNational Key Laboratory of Fundamental Science of Micro/Nano-Device and System Technology, Chongqing University, Chongqing 400044,eople’s Republic of China

r t i c l e i n f o

rticle history:eceived 8 May 2014eceived in revised form 12 August 2014ccepted 31 August 2014vailable online 2 October 2014

eywords:ll-metalontact timexial disturbanceriaxial inertial switchicroelectromechanical system (MEMS)

a b s t r a c t

An all-metal inertial microswitch that is sensitive to three axial accelerations (+x, +y and +z) is fabricatedby low-temperature photoresist modeled metal-electroplating technology. The inertial switch consists offour main parts: a quartz wafer with anti-stiction strips as the substrate; a proof mass suspended by con-joined serpentine springs as the movable electrode; two L-shaped flexible cantilevers and a multi-holecrossbeam as horizontal and vertical fixed electrodes, respectively; two anchors located in the middleof proof mass as limit blocks. ANSYS software is used to simulate the dynamic contact process in themicroswitch, and the simulation results reveal that the flexible fixed electrode can prolong the con-tact time and eliminate the rebound during the contact process. The axial disturbance among differentsensitive directions has been discussed by dynamic simulation. The modal analysis, crosstalk betweenhorizontal and vertical directions, cross-axis sensitivity, and the disturbance under overload shock alongthe reverse sensitive direction are also simulated and discussed. The suspension and gap in the devicestructure can be precisely controlled utilizing the photoresist modeled metal-electroplating technologyto reduce the axial disturbance effectively. Finally, the prototype is fabricated successfully and tested

by dropping hammer system. It is shown that the test threshold acceleration is 255–260 g in horizontaldirections (+x and +y), ∼75 g in vertical direction. The contact time of the switch with elastic contactpoint is ∼60 �s in horizontal direction and ∼80 �s in vertical direction. The crosstalk between horizontaland vertical direction, cross-axis and overload disturbance have been also demonstrated by test results,which indicate the axial disturbance is low in the present inertial switch.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

An inertial switch is essentially both a threshold sensor and anctuator, which has functions of both threshold detection and clos-ng action. With the development of implantable devices, inertialwitch has the second largest sales volume after pressure sensorsue to the distinctive characteristics of small size, lightweightnd low power consumption and manufacturing cost. The inertial

witch fabricated by micromachining technology is frequentlysed in many industrial applications such as accessories, toys,ealth monitoring and automobiles [1–4]. Especially in recent

∗ Corresponding author. Tel.: +86 21 34206686.E-mail addresses: wyyw@sjtu.edu.cn (Y. Wang), gfding@sjtu.edu.cn (G. Ding).

ttp://dx.doi.org/10.1016/j.sna.2014.08.014924-4247/© 2014 Elsevier B.V. All rights reserved.

years internet of things (IOT) has grown rapidly, the inertial switchis used for detecting the vibration shock and then sending corre-sponding overload signal [5,6]. The reported inertial switch hadbeen fabricated on the silicon substrate with etching technologymainly [7–11]. Compared with all-metal device, the elastic-plasticproperty of silicon is poor. The switch fabricated using metal (i.e.,nickel) could withstand higher impact due to the lower brittleness.In addition, the contact resistance of all-metal device is lower andhence it can minimize the heat loss [12].

In our previous work, a serious of inertial switches with uni-directional or omnidirectional sensitivities had been reported

[13–17]. However, some of them still used a rigid block withoutelastic contact structure as the fixed electrode has only ∼5 �s shortcontact time, which will be an obstacle for circuit analyzing in manyapplications. In the report [7], a latching acceleration switch had

W. Chen et al. / Sensors and Actuators A 220 (2014) 194–203 195

F a) 3Dv

beavefi6siwisat

faotaistsittsaiewt2ce3tsla

ig. 1. The schematic diagram and structure parameters of triaxial inertial switch. (iew of one half of triaxial inertial switch.

een proposed, where the movable electrode is latched by fixedlectrode when the applied acceleration reaches to its threshold,nd the contact time can be prolonged enough. But this system isery difficult to be restarted after pre-measurement. Matsunagat al. [18] have proposed an acceleration switch utilizing squeezelm effect to extend holding time and it can be prolong to about

ms. As a result, the response of this kind of switch will be verylow. On the contrary, in our design the squeeze film must be elim-nated as far as possible. This is because the squeeze film effect

ould change dynamic response when the acceleration is appliedn the horizontal direction, increasing the complexity in triaxialwitch design. In this paper, an L-shaped fixed contact cantilevernd multi-hole crossbeam will be proposed to effectively prolonghe contact time and reduce the damage during the contact process.

In addition, different application environments give differentunctional requirements for the inertial switch. When the shockcceleration from different directions needs to be detected, usingne triaxial switch instead of three unidirectional switches inhree individual directions is more reasonable. In the practicalpplication, the triaxial switch can also avoid the sensibility loss,nstallation errors and simultaneously keep costs down. At theame time, the inertial switch is usually also required to be resistanto reverse impact in some specific environments for applicationsuch as transportation of special goods. In the reference [8], a triax-al inertial switch is proposed to sense in ±x, ±y, ±z directions. Buthis design of symmetrical sensitive directions will possibly lead torigger in reflected directions due to the non-flexible contact in theensitive direction, and the axial disturbance is severe. In this paper,n all-metal triaxial inertial microswitch has been proposed, whichs used for fall protection system of special goods (e.g. chemicals,tc.). When the shock acceleration exceeds a certain threshold (i.e.hich is determined by the height and weight), the protective sys-

em must be activated. In our experiment, the threshold is about50–280 g. The microswitch is connected to signal processing cir-uit, which can output trigger signal with long duration to protectquipment as long as the contact time of microswitch is more than0 �s. So, the design objective of contact time is more than 30 �s in

he present work. Finally, a triaxial inertial microswitch with largeuspension and precise gap limit block will be proposed in order toower the axial disturbance. The prototype will be also fabricatednd evaluated in details.

sketch of the triaxial switch. (b) Top view of one half of the triaxial switch. (c) Side

2. Structure design

A novel inertial microswitch that can detect threshold accel-eration in three directions (+x, +y and +z) was designed based onlow-temperature photoresist modeled metal-electroplating tech-nology on quartz wafer. Compared with the switch sensitive tosymmetrical multiple directions (±x, ±y, ±z), the difficult pointin structure design is axial disturbance in negative directions (−x,−y and −z). A novel structure mainly includes four parts as shownin Fig. 1(a): the quartz substrate and the anti-stiction raised bar,supports; a hollow proof mass suspended by conjoined serpentinesprings as the movable electrode; two L-shaped elastic cantileverbeams suspended by fixed block in the middle of proof mass ashorizontal stationary electrodes, a multi-hole crossbeam abovethe proof mass as vertical fixed electrode; two anchors located inthe middle of proof mass as limit blocks. The movable electrodesuspended by conjoined serpentine spring was designed to be acentrosymmetric structure in order to reduce the cross-axis sen-sitivity, increasing the axis sensitivity. The horizontal electrodesfabricated with L-shaped cantilever beam can prolong the contacttime and release the inertial impaction during the contact process,which prevents the damage from overload acceleration, increasingthe safe reliability of system and prolonging service life simulta-neously.

In a real world application, the switch was shocked in all direc-tions, but many shock direction must be constrained. Two fixedblocks have been set in the inside of proof mass to restrain thecontact bounce from trigger position and restrain the overloadimpaction from the opposite direction. The spurious triggering inreflection sensitive direction (−x, −y) would be avoided due to thetiny gap between proof-mass and limit blocks, and the gap was setas 10 �m in this structure based on surface micromachining tech-nology. The details of elastic cantilever beams and fixed blocks areshown in Fig. 1(a), where the gap (x01) between blocks and proofmass is designed to be much narrower than the gap (x0) betweencontact point and proof mass in favor of reducing the bounce dis-placement. Fig. 1(b) and (c) show the top view and side view of

one half of triaxial switch, where symmetric structure of the wholedevice ensures threshold accelerations in horizontal direction isconsistent, and the proof mass thickness H is designed to be muchlarger than the spring thickness t in order to minimize the area

196 W. Chen et al. / Sensors and Actuators A 220 (2014) 194–203

F withe

cashetstewhns

3

saaugiatBT

TD

ig. 2. The FE model of triaxil inertial switch. (a) The whole model of triaxial switchnlarged view of contact pairs.

overage while enabling the desired sensitivity. When the acceler-tion (i.e. meets or surpasses set threshold) was applied on inertialwitch, the proof mass moves towards the stationary electrodes inorizontal directions or vertical direction, and the L-shaped elasticlectrodes and multi-holes crossbeam can realize a flexible con-act to prolong the contact time, and maintain system stabilityimultaneously. Fig. 1(c) shows the height d2 of horizontal elas-ic contacts higher than bottom of proof mass avails to improve theffectiveness and reliability during contact process. The gap (d4)as designed higher enough to eliminate the crosstalk betweenorizontal and vertical stationary electrode. Three pads were con-ected to the external circuit, forming a closed circuit with thewitch when the system in the conduction state.

. The simulation

The commercial ANSYS finite-element software was used toimulate the dynamic contact process, which can provide us

convenient and precise method for evaluating the thresholdccelerations and contact times. Fig. 2 shows the finite element sim-lation model of triaxial switch and the contact pairs, whose theeometric parameters and specifications of the designed triaxialnertial switch is shown in Table 1. Fig. 2(a) is the FE model of equiv-

lent structure of triaxial inertial switch, which is used to simulatehe contact process in every sensitive direction. The grid type SOLIDrick 185 was chosen to mesh the model with SWEEP method. TheRAGE170 and CONTACT 174 were chosen to establish the contact

able 1esign geometric parameters and specifications of designed triaxial inertial switch.

Component Geometric parameter Value (�m)

Proof mass Length A 1400Length a 600Thickness H 40

Distance x0 20x01 10

Gap d1 45d2 15d3 35d4 40

Spring Thickness t 15Width w 12Inside radius r 25Length L 530Length l 438

Electrodes Thickness t01 10Width w01 12Thickness h 20

horizontal elastic electrodes, vertical stationary electrode and fixed blocks. (b) The

pairs. The end sections (A-P) of all suspended springs, fixed elec-trodes in horizontal, vertical directions and blocks are constrainedto be zero in all degree of freedom. Fig. 2(b) shows the close-upview of contact pairs between proof mass and fixed electrodes andblocks. Ni metal was chosen to as the structural material. From ourexperimental results [19], the material properties of Ni are as fol-lows: Young’s modulus is 170 Gp and density is 8.96 kg m−3, thePoisson’s ratio is 0.3. Considering that the response process is one-time active in this simulation, the dissipation of energy is so smallthat you can ignore the material damping. But, squeeze-film damp-ing is one important factor in the designed system. In our previouswork, the squeeze-film damping constant is about 0.34, which hadbeen demonstrated in the reference [20]. Therefore, the equivalentdamping constant is generally selected as 0.34 for FEM simulation.

3.1. Simulations of threshold acceleration and contact time

The simulation results of contact process are shown in Fig. 3.In practical application, the switch usually suffer from accelera-tion that changes with time, in our simulation experiments, theaccelerations applied on switch are half-sine waves with differ-ent peak values and the pulse width of which are one millisecond.Fig. 3(a) shows the dynamic response displacement–time curveswith different acceleration magnitudes in horizontal direction (+x),from which it is clear to see that the threshold acceleration is230 g under the given parameters. Fig. 3(b) shows the contact pro-cess of proof mass and elastic cantilever beam, which indicatesthat they maintain consistent movement after the switch trigger-ing, at point 1 to point 2 during the movement, approximately75 �s contact time was obtained due to the elastic deformation.The deformation increases with increasing acceleration due to thegreater impact from proof mass, obviously. About 100 �s contacttime was achieved under acceleration of 240 g. It is thus clear thatthe elastic cantilever is designed as fixed electrode, which doesnot just contribute to prolong the contact time, but also estab-lishes a stable inertial system. Fig. 3(c) and (d) show the dynamicresponse of inertial switch in +z direction, which demonstrate thatthe threshold acceleration is about 70 g and contact time is about55 �s, where the contact process similar to horizontal fixed elec-trode due to elastic deformation of multi-hole crossbeam, too.

3.2. Simulation of axial disturbance performance

The modal analysis results are shown in Fig. 4, which indicatethat the vibration state of fourth or fifth order mode (Fig. 4d ande) tends to sensitive directions. The first mode (Fig. 4a) reveals

that the proof mass moves to +z direction when the frequencyis 551 Hz, which demonstrates that the natural frequency in +zdirection is lower compared with horizontal direction, so the trig-ger will never happen in horizontal direction when the switch is

W. Chen et al. / Sensors and Actuators A 220 (2014) 194–203 197

Fig. 3. The ANSYS simulation results of the contact process. (a) Dynamic response displacement–time curves in horizontal directions (x or y) under different accelerations.(b) Magnified plot of the contact period in dashed rectangle of (a). (c) Dynamic response displacement–time curves in vertical direction under different accelerations. (d)Magnified plot of the contact period in dashed rectangle of (c).

F ode. (bm

s(abmdCfwe

ig. 4. The first five order mode of suspension movable electrode. (a) First order mode.

hocked in +z direction. The second order mode or third order modeFig. 4b and c) indicates that the corner of proof mass swings upnd down along z direction. On account of the low frequency spanetween first order mode and second order mode or third orderode, instead of swing, the proof mass tends to vibrate up and

own more when the inertial switch is shocked by acceleration.

ompared the natural frequency of first order mode (551 Hz) withourth or fifth order mode (1805 Hz), the vibration displacementill be small attributed to the far enough frequency span. But, to

liminate the crosstalk between horizontal and vertical direction,

) Second order mode. (c). Third order mode. (d) Fourth order mode. (e) Fifth order

the displacement of proof mass in +z direction was investigatedwhen the proof mass was subjected to acceleration in horizontalsensitive direction.

Fig. 5 shows the simulation results when the inertial switch isshocked by acceleration of 240 g (i.e., the threshold) in horizontaldirection (+x). Fig. 5(a) shows the von mises stress distribution

of suspension movable electrode shocked by 240 g horizontalsensitive acceleration. The maximal stress is about 214 MPa, whichis lower than the yield strength of Ni (i.e., 317 MPa). Fig. 5(b) is theresponse process of proof mass in +z direction when the inertial

198 W. Chen et al. / Sensors and Actuators A 220 (2014) 194–203

Fig. 5. The simulation results under the acceleration of 240 g in +x direction. (a) The diagram of von mises stress under 240 g horizontal sensitive acceleration. (b) Thedisplacement–time curve of proof mass in +z direction under 240 g horizontal sensitive acceleration.

F s strea

stctdedvembh

wd

Fe

ig. 6. The simulation results of non-axis performance. (a) The diagram of von misend horizontal fixed electrode under 500 and 600 g.

witch is shocked in horizontal direction, which indicates thathe bounce distance is not more than 8 �m. This is because theontact process is completed in a short span of time apart fromhe far enough frequency space between horizontal and verticalirections, there is not enough time for the proof mass to absorbnergy from shock wave, so that the vibration distance in +zirection is short. To avoid the crosstalk between horizontal andertical directions, the gap between proof mass and vertical fixedlectrode is designed to be 40 �m. Based on photoresist modeledetal-electroplating technology, the height of the gap is controlled

y the thickness of photoresist. So, the gap can be designed veryigh by laminated plating process.

In order to evaluate the cross-axis sensitivity, the accelerationas applied on proof mass in 45◦ with the horizontal sensitiveirection. The simulation results are shown in Fig. 6. The von mises

(a)

ig. 7. The ANSYS simulation results of contact process in negative direction (−x) under alectrode and fixed block while the distance is 15 �m and (b) The distance between mov

ss under 600 g acceleration. (b) The dynamic response curves of movable electrode

stress distribution of suspension movable electrode under 600 gacceleration is shown in Fig. 6(a), which indicates the maximalstress is about 280 MPa (i.e., Ni, 317 MPa) in the corner of spring.Fig. 6(b) shows the dynamic response curves of movable electrodeand horizontal fixed electrode under 500, 600 g, which indicatesthat the threshold acceleration is about 600 g at the angle of 45◦

with the horizontal sensitive direction. Compared with the thresh-old acceleration (i.e., 240 g) in horizontal direction, the cross-axissensitivity of this design is low due to the design of centrosymmetrystructure.

Fig. 7 shows the contact process of movable electrode and limitblock, where the distance was designed to be 10 �m in our model

and 1200 g acceleration with pulse width is one microsecond wasapplied to the inertial switch in −x direction. Fig. 7(a) shows thedynamic curves of movable electrode and fixed block when the gap

(b)

cceleration of 1200 g. (a) Dynamic response displacement–time curves of movableable electrode and fixed block is 10 �m.

W. Chen et al. / Sensors and Actuators A 220 (2014) 194–203 199

F hotors

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towscH4pTttwtla

sFrd

ig. 8. (a) The drying program for multi-layer photoresist. (b) The fourth layer of pchematic of electroplating method.

x01) is 15 �m, which indicates that the maximal rebound distances not more than the gap (x01) between electrode and block. Theibration wave is a sine wave which is centered with the originalosition. In addition, the second peak of the vibration wave demon-trates that movement locus of movable electrode is an attenuationrocess. Fig. 7(b) shows the dynamic curves when the gap (x01) washanged to 10 �m, which indicates that the maximal rebound dis-ance is still not more than 10 �m. The simulation results in Fig. 7ndicate that the maximal rebound distance is less than or equal tohe gap between proof mass and limit block during the contact pro-ess when the inertial switch is shocked by overload acceleration inpposite direction. So, in this designed triaxial inertial switch, there-set gap x0 is designed far larger than x01, which can avoid thepurious triggering in sensitive direction when the inertial switchs shocked in opposite sensitivity direction.

. Microfabrication process

The triaxial inertial microswitch was fabricated with low-emperature photoresist modeled metal-electroplating technol-gy. The Ni metal was chosen as the structure material and Cr/Cuas used as seed material. The fabrication process mainly includes

puttering, electroplating and sacrificial layer technologies. Theompositions of Ni electrolyte are Ni [NH2SO3]2(600 g L−1),3BO3(25 g L−1), NiCl2·6H2O(10 g L−1), and the conditions are PH.0, temperature 50 ◦C and current density 1.25 A/dm2. Positivehotoresist (AZP 4903) was used as model and sacrificial layers.he stylus profiler (Dektak 6 M, Veeco, USA) was used to monitorhe entire manufacturing process, including the measurements ofhickness of model and height of electroplated Ni. The Ni metalas electroplated as high as the model when the designed struc-

ure was patterned. The fabrication mainly consists of sputtering,ithography and electroplating. The details of fabrication processre described as follow:

Sputtering: Before the first sputtering, the surface of quartz sub-

trate is treated to improve the binding force of sputtered Cr/Cu.irst, a polishing machine (Minitech 333, PRESI, France) is used toemove the inorganics, during the process, the nano Al2O3 pow-er is used as polishing suspension. Second, the substrate is boiled

esist and electroplated spring structure. (c) The sixth layer of photoresist. (d) The

with sulfuric acid (H2SO4, 98%) to remove organics. Third, an ultra-sonic instrument is used to remove the surface impurities. Whilethe Cr/Cu is sputtered on Ni metal structure, the surface of Niis activated by diluted hydrochloric acid (HCL) and treated byplasma.

Lithography: The lithography is key factor in fabrication process.First, the drying method would affect the roughness of photore-sist, in the present experiment, the number of photoresist layer is9 in the whole process. The drying program is shown in Fig. 8(a),and the dried model is shown in Fig. 8(b) and (c). Fig. 8(b) is thefourth layer, which indicates that the photoresist is smooth afterthe second Cr/Cu seed is sputtered, and the electroplated springstructure is intact. Fig. 8(c) indicates that the photoresist is stillintact although the photoresist is stacked to the sixth layer, whichdemonstrates that the photoresist strip and crack is little. Second,the photoresist is polished by nano-powder for realizing a smoothflat surface, which will provide the substantial benefit to the stackof multi-layer and make sure the vertical threshold is generallyuniform all around the substrate. Third, the layers of proof massare fabricated by laminated photoresist and each layer is smallerthan 15 �m to ensure the sidewall of photoresist molds straight-ness farthest. The alignment mark is fabricated on the back ofsubstrate, which can provide the substantial benefit to accuratealignment, and then, the gap between electrodes can be controlledaccurately.

Electroplating: The schematic of electroplating method is shownin Fig. 8(d). At the edge of substrate, a conductive coil is designedto disperse the current. The current distribution will change whenthe contact is changed. Therefore, eight contacts are designed asconnect point. The conducting times of every contact are equal toeach other during the electroplating process of each layer. So, thedeposition rate is basically uniform, and the threshold from deviceto device is roughly equal.

The main process procedure is sketched in Fig. 9 and describedas follows:

(1) Chromium/copper seed layer was sputtered on the insulatingquartz substrate by magnetron sputtering machine.

200 W. Chen et al. / Sensors and Actuators A 220 (2014) 194–203

cess o

(

(

Fig. 9. The main fabrication pro

(2) After spin-on 10 �m thickness of photoresist and pattering,the electrode-lead and raised strips were electroplated.

(3) The raised strips were electroplated higher than the electrode-lead.

(4) The supports for springs and elastic cantilever were electro-plated, forming a gap between bottom electrode and proofmass.

(5) After the suspension gap was formed by the supports, the sec-ond chromium/copper seed layer was sputtered on supports,the serpentine springs and the bottom of the proof mass wereelectroplated.

(6) The proof mass was electroplated higher than spring to avoidthat the third seed layer adheres to springs.

(7) The third chromium/copper seed layer was sputtered on proofmass, the L-shaped cantilever beam as horizontal fixed elec-trode was electroplated, including middle layer proof mass.A cantilever beam fixed on limit block was electroplated toreduce the gap between proof mass and limit block. This noveldesign can reduce the error during laminated electroplate pro-cess, which enhanced the machining precision as much aspossible. The small the gap, the better performance to resistthe shock in reverse sensitive direction.

(8) The proof mass was fabricated by multi-layer electroplatinguntil it reaches the required thickness.

(9) After the gap between the proof mass and vertical electrodewas electroplated to designed height by laminated technol-ogy. The fourth chromium/copper seed layer was sputtered;the vertical fixed electrode was electroplated.

10) Finally, acetone was used to remove the PR, and an ammo-nia/peroxide solution was used to remove the seed layer. Thecompleted released inertial switch would be obtained after thephotoresist and chromium/copper seed layer were absolutelysacrificed.

11) The photo of released triaxial microswitch. The size of struc-

ture is 2.76 mm × 2.76 mm × 0.15 mm. A fabricated metalcap is used to package the released microswitch by epoxyadhesive bonding. The size of packaged device is about3.8 mm × 3.8 mm × 0.35 mm, as shown in Fig. 9.

f triaxial inertial microswitch.

Fig. 10 shows the scanning electron microscopy (SEM) photosof fabricated triaxial inertial switch. Fig. 10(a) shows that thetraixial inertial switch with complete structure was prepared bylow-temperature photoresist modeled metal-electroplating tech-nology, which demonstrated the designed model combined withmanufacturing engineering perfectly. The enlargements of sus-pended gaps were shown in Fig. 10(b), where it is clear to see thegap between movable electrode and substrate and vertical fixedelectrode. Fig. 10(c) shows the elastic contact points and Fig. 10(d)is limit block. The novel design of raised beam is used to simplifythe technology process, and which can raise the precision of man-ufacture.

5. Testing and characterization

Fig. 11(a) shows the drop hammer system for the testingof triaxial inertial switch. A standard commercial accelerometerADXL-193 (9.8 mv g−1) from the Analog Device Inc. was used asa calibration to the tested device and a multi-channel oscilloscope(MSO6034A) was utilized to detect the instantaneous output sig-nals from accelerometer and microswitch. A power source was usedto provide constant voltage (3 V) for testing and a resistance of700 � was introduced into the testing circuit to generate a highlevel signal when the switch was conducted. The tested inertialswitch was fastened to a hexahedral fixture which can be rotatedmulti-directionally. The sensitive directions of +x, +y and +z weretested by changing permanent position of fixture to ensure the sen-sitive direction perpendicular to the ground. The pole was droppedfreely from a scheduled height onto the base plate, generating half-sine wave acceleration with certain amplitude. Fig. 11(b) showsthe test directions of triaxial inertial switch. Fig. 11(c) shows theschematic diagram of test circuit, where the contact signals fromthree directions are inputted to the oscilloscope. When the accel-

eration is applied to triaxial inertial switch, the all contact signalsfrom horizontal direction and vertical direction can be captured.The performance of axial disturbance can be evaluated by this mea-sure.

W. Chen et al. / Sensors and Actuators A 220 (2014) 194–203 201

Fig. 10. The scanning electron microscopy (SEM) photos of fabricated triaxial inertial switch. (a) The full view of fabricated triaxial inertial switch. (b) The side view ofsuspended structure. (c) The horizontal fixed electrode. (d) The limit block.

st dire

sse(dsleiFi

Fig. 11. (a) The drop hammer system. (b) The te

Fig. 12 shows the drop hammer experiment results of inertialwitch in three sensitive directions. The acceleration and switchignals captured by oscilloscope indicate that the threshold accel-rations in horizontal directions are about 255 g (Fig. 12a), 260 gFig. 12b) and the contact time is about 55–60 �s. There is minorifference of threshold acceleration between x and y directionsince the fabrication process are repeating steps, and the cumu-ative errors change the gap between movable electrode and fixed

lectrode. Obviously, the contact time is prolonged to 60 �s thats attributed to elastic deformation of L-shaped fixed electrodes.ig. 12(c) shows the threshold acceleration of triaxial inertial switchn +z direction is about 75 g, and contact time is about 80 �s. The

ctions. (c) The schematic diagram of test circuit.

tested results shown in Fig. 12 demonstrate that the crosstalkbetween horizontal and vertical directions is restrained. The con-tact in +z direction is avoided when the prototype is shocked byacceleration in +x and +y direction, as shown in Fig. 12(a) and (b).Likewise, the trigger does not appear in horizontal direction whenthe acceleration is applied in +z direction, as shown in Fig. 12(c).After the fabricated inertial switch is shock by the overload accel-eration (∼5 times of its threshold), the little damage of device

was observed in the microscope. And then, the device is shockedunder reversed sensitive direction, the test results are shown inFig. 13, which demonstrates the fabricated microswitch can with-stand high-g applied acceleration shock.

202 W. Chen et al. / Sensors and Actuators A 220 (2014) 194–203

Fig. 12. The tested results of threshold acceleration and contact time in x (a), y (b) and z (c) direction.

F hen ti

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6

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ig. 13. (a) The threshold acceleration and contact time of triaxial inertial switch wn non-sensitive direction (−x).

Fig. 13(a) shows the threshold acceleration of triaxial inertialwitch in the angle of 45◦ with +x direction, which indicates thathe threshold acceleration is about 632 g and the contact time isbout 50 �s. Compared with the threshold acceleration in +x, +yirections (255 and 260 g), this tested results demonstrate that theross-axis sensitivity is low. Fig. 13(b) is the test results in oppo-ite sensitive direction (−x). The triggers in +x and +z directionsave not appeared when 1200 g acceleration is applied in −x direc-ion. The overload resistance performance in reverse direction wasemonstrated.

. Conclusion

A prototype of all-metal triaxial inertial switch has beenabricated with low-temperature photoresist modeled metal-lectroplating technology. The dynamic response is simulated byNSYS software. The simulation results demonstrate that the flex-

ble contact structure can prolong the contact time. The simulatedhreshold acceleration is ∼240 g with the contact time of ∼75 �snd ∼70 g with the contact time of ∼55 �s in horizontal andertical directions, respectively. The axial disturbance is evalu-ted with modal analysis and threshold analysis, which indicateshat the crosstalk between horizontal and vertical axial direc-ions has been restrained effectively in the present microswitch.he overload resistance performance in the opposite direction islso characterized by the dynamic simulation under the reversehock acceleration of 1200 g. The simulation result reveals thathe rebound distance is much smaller than the setting gap. Inhe fabrication, the functional gap between electrodes and theimit block are realized precisely using the low-temperature pho-oresist modeled metal-electroplating. The fabricated prototype isested by dropping hammer system. The test threshold accelera-ion is 255–260 g with the contact time of ∼60 �s in the horizontalirection, and 75 g with the contact time of ∼80 �s in the verti-al direction, respectively. The cross-axis sensitivity and overload

esistance performance are also demonstrated by shock testing,hich verifies the simulated contact process in the triaxial inertial

nd indicates that the contact time can be prolonged effectively andhe axial disturbance is low.

[

he acceleration is applied in the angle of 45◦ with +x direction. (b) The test results

Acknowledgements

This work is supported by the National Natural Science Foun-dation of China (No. 51205254), Visiting Scholar Foundation ofChongqing University (No. 2014MS02), the Important Foundationof Ministry of Education of China (No. 625010105), and the Pre-research Project (No. 51308050304).

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iographies

Wenguo Chen received the B.Sc. and the M.Sc. degreesfrom Kunming University of Science and Technology,Kunming, China, in 2008 and 2011, respectively. He iscurrently working toward the Ph.D. degree in microelec-tronics and solid state electronics at the National keyLaboratory of Science and Technology on Micro/Nano Fab-rication, Shanghai Jiao Tong University, Shanghai, China.His research interests include the design, simulation andfabrication of MEMS/NEMS devices.

Yan Wang received the Ph.D. degree in microelec-tronics from Shanghai Jiao Tong University, Shanghai,China, in 2009. She is now a lecturer at Instituteof Micro/Nano Science and Technology, Shanghai JiaoTong University. Her fields of expertise are carbon

nanotube enhanced electrodes, micromachining process,and microelectromechanical system devices. Her currentresearch is supported by National Nature Science Founda-tion of China and the Science and Technology Commissionof Shanghai Municipality.

tors A 220 (2014) 194–203 203

Guifu Ding received his B.Sc. and the M.Sc. degree fromFudan University, Shanghai, China, in 1984 and 1987,respectively. He is now a professor and the vice-director ofthe National Key Laboratory of Science and Technology onMicro/Nano Fabrication, Research Institute of Micro/NanoScience and Technology, Shanghai Jiao Tong University.He is also currently with the National Key Laboratoryof Science and Technology on Micro/Nano Fabrication,Shanghai Jiao Tong University, Shanghai, China. His mainresearch interests include the nano materials and thedesign, simulation and fabrications of the MEMS/NEMSdevices.

Hong Wang was born in 1959. She received the B.Sc.degree from the Department of Chemistry, NortheasternUniversity, Liaoning, China, in 1982, and the M.Sc. degreefrom the Department of Applied Chemistry, Kyushu Insti-tute of Technology, Kitakyushu, Japan, in 1998. Sheworked as a Researcher at Hamada Heavy Industries Ltd.,Kitakyushu, and Fuji Finetec Company Ltd., Tokyo, Japan,from 1998 to 2001. She is currently a professor of theResearch Institute of Micro/Nano Science and Technol-ogy, Shanghai Jiao Tong University, Shanghai, China. Hercurrent research interests include electrochemical micro-machining and figuration technology.

Xiaolin Zhao received his B.Sc. degree from ShanghaiTechUniversity, Shanghai, China, in 1975. He is currently aDirector of the National Key Laboratory of Nano/MicroFabrication Technology, Research Institute of Micro/NanoScience and Technology, Shanghai Jiao Tong University,Shanghai, China. He is also currently a professor withthe National Key Laboratory of Nano/Micro FabricationTechnology, Shanghai Jiao Tong University. His currentresearch interests include the design, simulation and fab-rications of microelectromechanical system devices, andmicro-fabrication technologies for non-silicon devices.

Zhuoqing Yang became a Member of IEEE in 2012. Hereceived the B.Sc. and the M.Sc. degree in electrome-chanical engineering from Harbin Engineering University,Harbin, China, in 2003 and 2005, respectively. He receivedthe Ph.D. degree in microelectronics and solid state elec-tronics from Shanghai Jiao Tong University, Shanghai,China, in 2010. Now he is an associate professor of theResearch Institute of Micro/Nano Science and Technol-ogy, Shanghai Jiao Tong University. He worked as a JSPSpostdoctoral fellow in National Institute of Advanced

Industrial Science and Technology (AIST) from 2011 to2013 in Japan. He is also currently with the National KeyLaboratory of Science and Technology on Micro/Nano Fab-

rication, Shanghai Jiao Tong University. His research interests include the design,simulation and fabrications of the MEMS/NEMS devices.