3-AXES TACTILE SENSOR WITH TUNABLE SENSING RANGE

Chih-Chieh Wen , Chia-Min Lin2, and Weileun Fang1,2Power Mechanical Eng. Dept., 2MEMS Institute, National Tsing Hua Univ., Hsinchu, Taiwan

(Tel: +886-3-5742923; E-mail: fanggpme.nthu.edu.tw)ABSTRACT: This study has successfully demonstrated a novel 3-axes polymer tactile sensor with built-in piezoresistive sensors. The sensor consists of polymer membrane and four sensing cantilevers withpiezoresistors on both top and side walls to measure the in-plane and out-of-plane loads. The fabricationprocess allows the changing of polymer membrane material easily. As a result, the sensing range of thepresent 3D tactile sensor is easily tuned by varying the polymer material.

Keywords: Tactile sensor, Side-wall doping, Tunable membrane, Piezoresistor

1. INTRODUCTION

The MEMS technology has already foundvarious applications in the area of micro sensors,for instance, the inertia sensor, the pressure sensor,the chemical sensor, etc. The MEMS sensors havemany commercialized products in the areas ofautomobile and consumer electronics. Presently,

r Qt the tactile sensor is regarded as one of the mostpromising MEMS sensors. There are various nonsilicon-based tactile sensor have been reported[1,2]. According to the characteristics highersensitivity, small size, and easy integration withother sensors, the silicon-based MEMS tactilesensor finds extensive applications in robots'fingers sensing, human body weight measurement,etc.

The piezoresistive sensing and capacitiveP sensing have been frequently employed for

silicon-based MEMS tactile sensor [3,4]. It isdifficult to tune the measurement range of thetactile sensor for a particular design [5]. Besides,it is not straight forward to determine the shearforce using the conventional piezoresistive sensor[6]. These issues give limitations in the designand implementation ofMEMS tactile sensors.

This study reports a novel 3-axes MEMStactile sensor with the capability of tunablesensing range. The present sensor consists ofsilicon-based piezoresistive sensing beamscovered by polymer membrane. In addition to thewell-known out-of-plane piezoresistive sensor,the in-plane piezoresistive sensors are alsorealized by employing the side-wall dopingmethod [7,8]. By varying the material of polymer

1

membrane, it is easy to tune the sensing range ofthe tactile sensor without changing the design ofMEMS structures and photo masks.

2. DESIGN AND CONCEPT

Figure 1 schematically illustrates the designconcept of the present 3D tactile sensor. Asshown in Fig. la-b, the sensor consists of foursingle-crystal-silicon sensing beams covered bypolymer membrane with tactile-bump. Thesensing beam has boron-doped piezoresistors ontop surface and side walls, as shown in Fig. 1 c.The load applying on the tactile-bump will lead to

(a)Tunabl

(b)Tunable thin film

..-..

(c)Top/Side-wall doping

Figure 1. The schematic of 3-axes and thin-film-tunabletactile sensor (a)concept of presented sensor (b)piezoresistive tactile sensor with tunable thin film (c) topand side-wall doping structure.

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the deformation of the sensing beams. Thepiezoresistors on top surface and side walls areused to measure the normal force and shear force,respectively. It is easy to change the polymermaterial during the fabrication process. Thus, thesensing range of the present tactile sensor can beeasily tuned by varying the material properties ofpolymer membrane.

The finite element model has beenestablished to analyze the performances of thesensor. In this study, the PDMS (Dow CorningSYLGARD 184) was used as the polymermaterial, and the elastic modulus of polymermembrane was modulated by mixing with Conano-particles. The polymer membranes of threedifferent PDMS/Co ratios were investigated. Themeasured elastic modulus of PDMS/Co filmswere 1.32MPa (no Co), 52.13MPa (PDMS/Co20/1) and 479.25MPa (PDMS/Co = 10/1),

> t respectively. These material properties were usedin the FEM analysis (ANSYS). Figure 2 showsthe typical FEM simulation models and results to

r jt predict the resistance variations of three° 20 PDMS/Co films under different normal and shear

forces. According to the analysis, the sensingbeams covered by the stiffer membrane willexperience a smaller deflection under the same

o applied load. As a result, the polymer membranewith larger stiffness has lower sensitivity yet

y 9 larger measurable load range. In conclusion, the

(a) (b)

sensitivity and sensing range of the tactile sensorare easily tuned using the present approach.

3. FABRICATION PROCESS

The key processes in this study are thefabrication of side-wall piezoresistors and theflexible membrane. The boron diffusion [7] butnot ion implantation [8] has been employed torealize the process. In addition, the moldingprocess to integrate polymer membrane andsilicon piezoresistive sensor is also presented.

The fabrication process steps of the presenttactile sensor are shown in Fig. 3. The processbegan with a n-type SOI substrate of 20[pm thickdevice layer (p 1OQ-cm). The silicon nitride filmwas deposited and patterned to act as the diffusionmask for the following boron doping process.First, the silicon nitride film was deposited andpatterned to define the location of top layerpiezoresistors. After that, the photolithographyand DRIE were used to define the location ofside-wall piezoresistors. As illustrated in Fig. 3a,a boron doped process was employed afterremoving the photoresist, and the top surface andside-wall piezoresistors were realized. As shownin Fig. 3b, the conducting wires were depositedand patterned using lift-off process. The silicondevice layer was then patterned to form sensingbeams inside a cavity using DRIE, as shown in

(a)

(b)

(e)- _I ...... ...p

.......

- _I

(c)6 - -.E=1.32MPa5 - E=52.13MPa4- E=479.25MPa

-- .3.

00.00 0.05 0.10 0.15 0.20

Normal Force (N)

(d)6-_- E=1.32MPa5 - E=52.13MPa4-. E=479.25MPa3-,.

X2- ,- ,1 U

0.00 0.05 0.10 0.15 0.20

Shear force (N)

Figure 2. (a, b)typical normal and shearforce simulation(c,d) diferent materials' maximum stress under normal andshearforce.

Sensing beam

(c)

Cavity(d) *

_ .... _ ~~~~~ ~~~~~~~~~~~~~.........

(f).... _

-. ........ .......

.....

(g)

* C * * *Si SiO2 Si3N4 Boron Au PDMS Glass

doped

Figure 3. Fabrication process steps.

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Fig. 3c. The top and side-wall piezoresistors wereplaced on the sensing beams. As illustrated in Fig.3d, the sensing beams were fully suspended afterremoving silicon oxide layer with HF. In thepresent case, the sensing beams had a 24m gapwith the handling silicon substrate underneath.The polymer was then dripped into the cavitydefined in Fig. 3c using a pneumaticallycontrolled micro pipette. As shown in Fig. 3e, thecavity acted as a mold to define the planar shapeof the polymer membrane, and the sensing beamswith piezoresistors were fully covered by thispolymer membrane. After patterned withphotoresist, the handling silicon layer was thenbackside etched by DRIE to fully release thepolymer membrane from substrate, as shown inFig.3f. Finally, the tactile sensor was mounted topyrex 7740 glass, to protect the sensor frombroken, as depicted in Fig. 3g.

Figure 4 shows the SEM photos of sometypical suspended sensing beams after the process

~8 C of Fig. 3d. Figure 5a-b shows the SEM photos ofr Qt sensing beams covered by the PDMS membrane

after the process of Fig. 3e. In this case, the edgeof square membrane is 3mm in length, and the

w: sensing beam is 200[pm in length. Because ofsurface tension, the PDMS thin film was molded

o in the square cavity defined on silicon device? z layer. In addition, the PDMS automatically

formed a 200[pm-high tactile-bump, and themembrane thickness increased from 20ptm at itsedge to 200[pm at its center, as in Fig. 5b. Finally,Figs. 5c-d show the photos of sensor chip after theprocess. The four sensing beams can be clearlyobserved in the photo of Fig. 5c since the PDMSmembrane is transparent. In short, the in-planedimensions (length and width) of the sensingbeam and membrane can be easily tuned using the

(a) Cavity (b) Sensing beam

Figure 4. SEMphotos offabrication results, (a) the sensingchip without polymer, and (b)zoom-in ofa piezoresistor.

(a) (b)

I(c) Thin film (d)

Figure 5. The photos of (a-b) tactile sensor after polymerdispensed, (c-d) photos ofcompleted sensor chip.

photolithography. The thickness of the sensingbeam and membrane can be changed by varyingthe device layer thickness of SOI wafer. Moreover,the height and shape of tactile-bump are tuned bythe volume of polymer.

4. EXPERIMENT SETUP AND RESULTS

The test setup in Fig.6 is established tocharacterize the present tactile sensor. AWheatstone bridge and amplifier circuit was usedto measure the resistance change during test. Theresistance change versus applied force wascharacterized by a micro-force gauge with aresolution of 0.001N. The tactile sensors withmembranes of different PDMS/Co ratio weretested; their Young's modules are 1 .32MPa,52.13MPa, and 479.25MPa, respectively. Figure7a shows the variation of resistance change (AR/R)after applying a normal force of 0.2N in Fig. 6a.The sensitivities of the tactile sensors withdifferent membrane stiffness are 3.36%/N,1.31%/N, and 0.52%/N, respectively; and thelinearity are approximately R2= 0.99. Figures 7b-c show the variation of AR/R after applying shearforces of 0.2N in x-axis and y-axis (in Fig. 6b),respectively. The results in Fig. 7b indicate thesensitivities of these three sensors are 2.28%/N,1.35%/N, and 0.95%/N with a linearity of R20.977, 0.996, and 0.986, respectively. In addition,

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(a) (b)Force gaugeF~~ oc ag

~ISensorIZ\ 14Sensing ~ Jj x Sensincircuit Stagecrcu

Figure 6. (a) The experimental setups for normal force, and(b) shearforces in two axes.

(a)

0.8. Pure PDMS0.7. PDMS:Co=20:10.6- PDMS:Co=10:1

0.3 .< 0.2-

0.10.00.00 0.05 0.10 0.15 0.20

Normal Force (N)

(b)0.80.70.6-

-0.5-0.4-

PI. 0.3-< 0.2-

0.1 -

0.0-0.00

Pure PDMSPDMS:Co=20PDMS:Co=1C

0.05 0.10

Shear Forc

(c)0.8 Pure PDMS

:):1 0.7- PDMS:Co=20:1:1 0.6 05 -PDMS:Co=10:1

.--0.4-~0.3< 0.2-

0.1 *

0.15 0.20 0.00.00 0.05 0.10 0.15 0.20ce (N) Shear Force (N)

Figure 7. The variation rate of resistance with applyingg z load of(a) normalforce (b, c) shearforces in two axes.

the results in Fig. 7c indicate the sensitivities ofthese three sensors are 2.23%/N, 1.46%/N, and

2: 0.96%/N with a linearity of R2 0.986, 0.994, and0.992, respectively. In summary, the tactile sensorwith a stiffer polymer membrane has a smallersensitivity. It is also expected form the linearrelation of measurement results that the stifferpolymer membrane has a larger sensing range ofthe applied load. The trend in Fig. 7 agreesqualitatively with the results predicted in Fig. 2.

5. CONCLUSIONS

This study has successfully demonstrated anovel 3-axes polymer tactile sensor with built-inpiezoresistive sensors. The sensor consists ofpolymer membrane and four sensing cantileverswith piezoresistors on both top surface and sidewalls to measure the in-plane and out-of-planeloads. The piezoresistors on both top and sidewalls were fabricated by boron doping technique.

Due to the surface tension of polymer, a tactile-bump is naturally formed on the membranewithout any additional process. Moreover, thefabrication process allows the changing ofpolymer membrane material easily. Thus, thesensing range of the present 3D tactile sensor canbe easily tuned by varying the polymer material.In applications, the sensing beams withpiezoresistive sensors cover by three differentPDMS/Co polymer composites have beenimplemented and tested.

6. ACKNOWLEDGEMENTS

This research was sponsored in part by theNSC of Taiwan under grant of NSC-94-2212-E-007-026. The authors wish to appreciate theNational Tsing Hua Univ. and National Chiao-Tung Univ. for providing the fabrication facilities.

7. REFERENCES

[1] E.-S. Hwang, J.-H. Seo, and Y.-J. Kim, IEEEMEMS'06, Istanbul, Turkey, Jan. 2006, pp.714-717.[2] J. Engle, J. Chen, Z. Fan, and C. Liu, Sensorsand Actuators, pp. 50-61, 2005.[3] Y Hasegawa, M Shikida, H Sasaki, KItoigawa, and K Sato, J. of Micromech. andMicroeng., pp. 1625-1632, 2006.[4] C.-T. Ko, J.-P. Wu, W.-C. Wang, C.-H. Huang,S.-H. Tseng, Y.-L. Chen, and M. S.-C. Lu, IEEEMEMS'06, Istanbul, Turkey, Jan. 2006, pp.642-645.[5] J. Engel, J. Chen, and C. Liu, J ofMicromech.and Microeng., pp.359-366, 2003.[6] K. Kim, K.R. Lee, D.S. Lee, N.-K. Cho, W.H.Kim, K.-B. Park, H.-D. Park, Y.K. Kim, Y.-K.Park, and J.-H. Kim, J of Phys., pp.399-403,2006.[7] B.K. Nguyen, K. Hoshino, K. Matsumoto, andI. Shimoyama, IEEE MEMS'06, Istanbul, Turkey,Jan. 2006, pp.662-665.[8] B.W. Chui, T.W. Kenny, H.J. Mamin, B.D.Terris, and D. Rugau, Appl. Phys. Lett., pp.1388-1390, 1998.

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