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Available online at www.sciencedirect.com

Sensors and Actuators A 142 (2008) 192–195

CMOS integrated tactile sensor array by porous Si bulk micromachining

Maria Adam a,∗, Tibor Mohacsy a, Peter Jonas b, Csaba Ducso a,Eva Vazsonyi a, Istvan Barsony a

a Research Institute for Technical Physics and Materials Science - MFA, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 49, Hungaryb Computer and Automatic Institute - SZTAKI, Hungarian Academy of Sciences, H-1502 Budapest, P.O. Box 63, Hungary

Received 29 September 2006; received in revised form 13 June 2007; accepted 1 August 2007Available online 6 August 2007

bstract

An integrated 8 × 8 element 3D force sensor array was developed for tactile sensing. The 300 × 300 �m2 sensors are formed from mono-rystalline silicon membranes with spacing of 700 �m. The signals of the individual transducers are read out line-by-line via an on-chip CMOSecoder. The porous silicon bulk micromachining was implemented as one of the final steps of the n-well CMOS compatible processing.

2007 Elsevier B.V. All rights reserved.

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eywords: Tactile sensor; CMOS compatible; Bulk micromachining

. Introduction

Artificial tactile sensors play similar role as mechanorecep-ors in human tactile sensing. Signals provided by a 3D forceensor capable to resolve the vectorial components of the attack-ng load in the range of �N . . . N, and operates up to a speed offew hundred Hz, can be considered to fulfill the analog func-

ions as Merkel-discs and Meissner-corpusculae in human skin.esides modeling of human perception, tactile sensor arraysith spatial resolution of ca. 1 mm and size of up to 1 cm2 cane efficiently applied in industrial and surgery robotic manipu-ations.

Arrays for haptic sensation reported so far were processedither by alkaline etching [1] or by surface micromachiningechniques [2,3]. Latter method provides higher flexibility inateral geometry design (size, density) and offers the possibilityf integration of CMOS driving and read-out circuitry for a largerray. However, it suffers from the limited mechanical stabilitynd reduced sensitivity of polycrystalline Si membranes.

Integrated tactile sensing solutions with capacitive or piezore-

istive read-out make use of surface or bulk micromachining,atter being less suited for CMOS compatibility. The need ofensitivity enhancement in tactile sensing led to the exploitation

∗ Corresponding author. Tel.: +36 1 3922222/3885; fax: +36 1 3922235.E-mail address: adam@mfa.kfki.hu (M. Adam).

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924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2007.08.003

f the more complex bulk micromachining for this purpose. Theerforated single crystalline membrane formed by the presentrocess offers the advantage of higher sensitivity piezoresistiveensing elements, compared to the similar size of a conven-ional surface micromachined reference [4,5]. The tactile sensorrray is manufactured by combining porous Si bulk microma-hining with a conventional CMOS process. To the best ofur knowledge, this is the first reported demonstration of themplementation of perforated single crystalline membranes in aMOS sequence.

. Sensor design

The single crystalline sensing elements (Fig. 1) consist of a1 0 0) central plate, suspended by four bridges over an etchedavity. When load is applied, deformation occurs in the bridgelements only, since the central plate of the membrane is rela-ively rigid compared to the narrow suspensions. The center holen the membrane is provided for the insertion of an optional load-ransmitting element in order to enhance the shear sensitivity ofhe device.

Each of the four bridges includes a [1 1 0]-oriented p+

iezoresistor, acting as an independent strain gauge, providing

he signals for resolving the vector components of the load.

The nominal value of piezoresistors is 2 k�. Each strain-ensing resistor is connected to an n-channel MOS transfer gate.ignals of the sensing elements of the 8 × 8 array can be read

M. Adam et al. / Sensors and Actua

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Fig. 1. Schematics of an individual sensing element.

ut row-by-row at a time. With this set-up, the simultaneousetection of 8 × 4 individual signals is achieved.

The addressed row is selected through the eight outputecoders, composed of CMOS inverters according to Fig. 2.

Each piezoresistor–transfer gate combination is driven by a p-hannel MOS transistor in current generator mode. In a selectedow all the transfer gate transistors are turned on, allowing theurrent to pass the strain-sensing resistors. Operation is enabled,ccording to the binary address code, by biasing one of the

ight outputs at high level, while the others are grounded. Theirperating points are set by a CMOS divider. Therefore, in theddressed row the voltage across the individual strain gaugesan be measured at the 32 outputs.

Fig. 2. Diagram of read-out and addressing electronics.

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. Device fabrication

The sensor array and the on-chip circuits were manufacturedy an n-well CMOS process. The reason for this selection iswofold:

in bulk micromachining the selectivity of porous etching canbe exploited for the formation of n-type perforated siliconmembrane andp-type piezoresistors can be designed with the well-knownoptimum performance for piezoresistive transduction.

The n-type single crystalline membrane and the n-well cane prepared partially or even completely simultaneously, whilehe source/drain doping allows the formation of the p+ piezore-istors, too. Both the membrane thickness as well as the n-wellepth and surface concentration can be tailored in accordanceith the requirements by adjusting the respective implantationose and drive-in conditions.

In the fabrication of the force-sensing elements the possibil-ty of selective formation and dissolution of porous Si was madese of. Since porous silicon can not withstand high-temperaturerocesses, the electrochemical etching and the dissolution of theacrificial layer have to be among the last steps in the processingequence. Therefore, after completing all the high-temperaturereatments in the conventional CMOS process and opening the

asking SiNx layer around the membrane, the circuitry was pas-ivated (Fig. 3a) to withstand the electrochemical etching of Sin HF-electrolyte. The porous Si sacrificial layer is protected byl during the subsequent metallization and photolithographicrocesses in order to maintain integrity and planarity of theicromachined part of the chip. After removal of the passi-

ation, the formation of contact holes and Al wiring followsFig. 3b). The dissolution of porous silicon under photoresistrotection releases the suspended perforated membrane. As theafers are exposed to a severe mechanical stress during dicing,

he sacrificial porous silicon was only dissolved on the individ-al chips, in order to avoid a damage of the suspended fragiletructures (Fig. 3c).

. Results

The processed sensor chip is shown in Fig. 4.The size of a sensing element is 300 × 300 �m2 with 400 �m

pacing between individual, the membrane thickness is 8 �m.he active area of the tactile sensor is 7 × 6 mm2. In order tovoid mechanical damage of wiring, the bonding pads are placedt a distance of several mm, thereby the chip size is 7 × 14 mm2.

To mimic the structure of human skin, in real applications theensor array has to be covered with an elastic layer [4]. The elas-ic cover plays multiple roles in device operation: it protects theragile membrane against intensive mechanical shocks, transfershe attacking force to the sensing elements in a complex mode

nd expands the receptive field. These effects are described inore detail in [4–6]. Here we just emphasize the increased sen-

itivity for the shear components and demonstrate the observedodification in sensitivity and linearity.

194 M. Adam et al. / Sensors and Actuators A 142 (2008) 192–195

Fig. 3. Main steps in the processing sequence. (a) The CMOS circuit is coveredwith a protective multilayer in order not to be damaged during the HF-basedelectrochemical etching process, (b) Al protection of porous sacrificial layer forproviding appropriate surface for metallization and lithography, (c) final step:dissolution of the porous layer. 1: p-type Si substrate; 2A, 2B: n-well for CMOSand for the single crystalline membrane; 3, 4: SiO2; 5: poliSi gate; 6: n+ Si;7Sp

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Fig. 4. Scanning electron micrograph of the sensor array (top) and two sensingelements (bottom).

A, 7B: p+ Si; 8: CVD SiO2; 9: SiNx; 10: perforation; 11: poliSi; 12: porousi; 13: protecting Al layer; 14: photoresist; 15: metallization; 16: photoresistrotection.

The correct interpretation of the signals provided by theensor elements [7–9] and visualization of the force vector com-onents is beyond the scope of this work.

Fig. 5a represents the response of a single bare element tooads normal to the surface. The difference in the slopes is theonsequence of the indefinite positioning of the loading tip. Inig. 5b the typical characteristics of the same element modifiedy an elastic cover are shown.

The shape and elasticity of the cover are also essential inespect of signal transduction: for different applications plainlastomers or surfaces with various patterns might be preferred.herefore, the fabrication of the elastic cover is inherent part of

he sensor back-end processing.For embossing the elastomer Si molds of various shapes can

e fabricated by using porous Si micromachining. A high degreef freedom is offered in realization by the isotropic nature of theorous etching. The capability of this method is demonstratedn Fig. 6, where the multifunctional cover of periodic structure

Fig. 5. (a) Linear response of four piezoresistive elements of a single taxelwithout elastic cover. (b) Responses of the same structure modified by a 500 �mthick plain elastomer cover.

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ig. 6. Micrograph of a mounted 8 × 8 element tactile sensor chip covered witheriodic silicon rubber bumps protruding 300 �m from the surface.

f hemispheric bumps protects the 8 × 8 element sensor arrayhip.

. Conclusions

By integrating the decoder circuitry, current generators andransfer gates with an 8 × 8 element single crystalline force sen-or array, we proved for the first time, that front side porousilicon bulk micromachining can successfully be combinedith an n-well CMOS process. The same high-element den-

ity, which was achieved so far by CMOS compatible surfaceicromachining only, was realized by the elaborated methodith bulk-micromachined single crystalline sensing elementsf higher sensitivity and versatility.

cknowledgements

The assistance of A. L. Toth with SEM imaging is acknowl-dged. This work was partially supported by the Hungarianesearch Found (OTKA) under contract TO47002 and by theational Office for Research and Technology in the NKFProject AQUANAL.

eferences

1] Z. Chu, P.M. Sarro, S. Middelhoek, Silicon three-axial tactile sensor, Sens.Actuators A 54 (1996) 505–510.

2] B.L. Gray, R.S. Fearing, A surface micromachined microtactile sensor array,in: Proceedings of the IEEE Robotics and Automation Conference, Min-neapolis, USA, April 22–28, 1996, pp. 1–6.

3] B.J. Kane, M.R. Cutkosky, G.T.A. Kovacs, A traction stress sensor array foruse in high-resolution robotic tactile imaging, J. Microelectromech. Syst. 9(4) (2000) 425–434.

4] G. Vasarhelyi, M. Adam, E. Vazsonyi, I. Barsony, C. Ducso, Effects of theelastic cover on tactile-sensor arrays, in: Proceedings of the XIX EUROSEN-

SORS Conference, Barcelona, Sept. 12-14, 2005, CD ROM. Sens. ActuatorsA 132 (2006) 245–251.

5] G. Vasarhelyi, M. Adam, E. Vazsonyi, Zs. Vızvary, A. Kis, I. Barsony, Cs.Ducso, Characterization of an integrable single-crystalline 3-d tactile sensor,IEEE Sens. J. 6 (4) (2006) 928–934.

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6] G. Vasarhelyi, B. Fodor, Enhancing tactile capabilities with elastic hemi-spheres, in: Proceedings of EuroHaptics International Conference EH 06,Paris, France, July 3–6, 2006.

7] A. Kis, F. Kovacs, P. Szolgay, 3D tactile sensor array processed by CNN-UM: a fast method for detecting and identifying slippage and twistingmotion, Int. J. Circuit Theory Appl. (CTA) (special issue on CNN) 34 (2006)517–531.

8] A. Kis, F. Kovacs, P. Szolgay, Hardware and software enviroment for a tactilesensor array, in: Proceedings of the Eurosensors XIX, Barcelona, Spain, Sept.12–14, 2005, pp. 324–328.

9] A. Kis, F. Kovacs, P. Szolgay, Grasp planning based on fingertip contactforces and torques, in: Proceedings of the Eurohaptics International Confer-ence EH 06, Paris, France, July 3–6, 2006, pp. 455–459.

iographies

aria Adam was born in Hungary, in 1948. She received the MSc degreen electrical engineering from the Technical University of Budapest, Hungary,n 1973. She was R&D Engineer at TUNGSRAM Ltd., at the Department ofemiconductors, from 1973 to 1982. From 1982 to 1985 she spent 3 yearsith the Microelectronics Co. Currently, she is research engineer at the MEMSaboratory of the Research Institute for Technical Physics and Materials Science

MFA). Her research interests include design, development and processing ofilicon-based mechanical and gas-microsensor structures. She is co-author ofbout 25 scientific papers and one patent.

ibor Mohacsy was born in Hungary, in 1934. He graduated as a chemicalngineer and electronic engineer in 1957 and 1967, respectively. Starting hisob in TUNGSRAM Ltd., took part in the development of several bipolar and

OS integrated circuits. In 1972 he joined the Central Research Institute forhysics where he continues the designing, measuring and developing of CMOSnd sensor devices. He authored one semiconductor brochure and co-authoredbout 15 scientific papers and 10 patents.

eter Jonas was born in Dunaujvaros, Hungary, in 1975. He received his MScegree in electrical engineering from Technical University of Budapest, Hun-ary, in 2003. He is currently a PhD student at Technical University of Budapest,lectrical Engineering Sciences Doctoral School, Budapest, Hungary. Main

esearch fields are analog and digital low power circuits.

saba Ducso was born in Hungary, in 1959. He graduated as a chemist in 1983nd received the PhD degree in chemistry from the Lorand Eotvos University,udapest, Hungary. Currently, he is head of the MEMS laboratory of MFA,udapest. His research interests include Si-based MEMS, with special emphasisn integrated gas and mechanical sensors and the development of the relatedrocessing technology. He is co-author of over 50 papers published in periodicalsnd conference proceedings.

´ va Vazsonyi was born in Hungary, in 1941. She received her MSc degreen chemical engineering from the Technical University of Budapest, Hun-ary, in 1965. She worked for the TUNGSRAM Ltd. at the Departmentf Semiconductors and later joined the Technical University as a researchellow. In 1972 she joined the Central Research Institute for Physics and cur-ently she works for one of its successor, the MEMS Laboratory MFA. Heresearch interests include fine line lithography and silicon micromachining.he has four patents and is the author and co-author of about 60 scientificapers.

stvan Barsony was born in Hungary, in 1949. He graduated in electrical engi-

irector of MFA, Budapest and professor of nanotechnology at the Universityf Veszprem, Hungary. His research interests are Si microtechnology, MEMSnd nanotechnology. He authored and co-authored over 100 scientific papersnd holds 13 patents.