Electrostatic forces and their effects on capacitive mechanical sensors

E LSEVI ER

Sensorsand ActuatorsA 56 (1996) 203-210

Electrostatic forces and their effects on capacitive mechanical sensors Robert Puers *, Daniel Lapadatu Katholieke UniversiteitLeuven. Dept. ElektrotechniekESAT-Micas. Kardinaal Mercierlaon 94, B-3001 Heverlee. Belgium

Received31 October 1995; revised 8 March ]996; accepted22 Match 1996

Abstract

This paper describes the electrostatic forces developed between the plates of capacitive mechanical sensors built in crystalline silicon and their effects on the measurement and the fabrication process. In single capacitive sensors the electrical forces can introduce offset errors in the measuring process, or can cause the collapse of the sensing structure or jeopardize the functionality of the final device due to the sticking or even bonding of the movable parts. Also it is investigated how the limits are affected when sealing down the dimensions of the sensors. Finally, some solutions to avoid the negative effects of the electrostatic forces are proposed. Keywordv: Capacitive mechanicalsensors;Electrostaticforces

1. Capacitive m e c h a n i c a l s e n s o r s An increasing number of applications demand implantable sensors for measuring pressure or movement inside the human body [ 1--4]. Hence, one of the designer's goals is to miniaturize mechanical sensors as much as possible without degrading their sensitivity. The capacitive transducing principle is being widely used for implantable sensors because of its inherent low power consumption. First a short description of the capacitive mechanical sensor and its features is given. A more detailed one can be found in Ref. [5]. A mechanical capacitive sensor looks more or less as in Fig. 1. A mechanical input signal, such as a pressure or an acceleration, deflects the thin membrane or the mass, in this way changing the capacitance between two conductive plates. By monitoring the change in capacitance, information about the mechanical input can be obtained. It is worth mentioning that for pressure sensors a full membrane, mostly not bossed, is used [ 1,2,6], while for acceleration sensors a proof mass is required. Also for accelerometers the suspension system is not necessarily a full membrane [ 3,7,8 ]. An important feature o f each sensor is its sensitivity. The sensitivity is a quantity that reflects the sensor capability in transducing the input signal into an electrical output signal, in our case a capacitance change. * Correspondingauthor. Phone: + 32 16 32 1082. Fax: + 32 16 32 1986. E-mail: [email protected]. 0924-4247/96/$15.00 © 1996ElsevierScience S.A. All rights reserved Pil $0924-4247 (96) 01310-6

mass Fig. I. A capacitivemechanicalsensor. For a mechanical capacitive sensor the sensitivity can be defined in two different ways, somehow equivalent: (a) the mechanical sensitivity, Sz, which relates the vertical displacement of the movable plate, z, to the input signal; (b) the electrical sensitivity, So, which relates directly the electrical change (a capacitance or a voltage) to the input signal, as follows: z=S~F,,

V=S~F~

(1)

where F~ is here considered to be the mechanical input (a pressure or an acceleration, both being directly proportional to the input force). It must be observed that the mechanical sensitivity is nothing other than the reverse of the elastic spring constant o f the suspension system, k~, as follows from Hooke's law:

~=k.z= S~= l/k~

(2)

Hence, the more flexible a suspension system is, the more sensitive the capacitive sensor becomes. The sensitivity, in its turn, depends on the geometrical and material features o f the suspension system. For this reason,

R. Puers. D. Lapadatu /Sensors and Actuators A 56 (1996) 203-210

204

scaling down a sensor will influence the sensitivity. In general, when scaling down the device dimensions, the sensitivity gets worse, since the suspension system becomes more rigid. Typical values range between 0.01 and 0.5 p m / t N - 1 [1-81. Another important feature of a capacitive sensor is its rest capacitance, Co, the capacitance when no input signal is applied. It is defined only by the gap at rest between the plates, do, and the plate area, b 2, assuming the plates to be square shaped: Eob2

co= W

(3)

where eo is the free-space permittivity. Typical values for the rest capacitance range from 0.3 to 12 pF [ 1-5,9]. In general, the parasitic capacitances are in the same range of magnitude. Without an implemented system for suppressing the parasitic capacitances [ 1 ], the rest capacitance must have much higher values. This is in strong conflict with the miniaturization requirements [ 5 ]. During its functioning, a mechanical capacitive sensor is subjected to four different forces: the mechanical input (a pressure acting on the membrane or an inertial force acting on the proof mass); the elastic force generated in the suspension system, in response to its deformation; the damping force generated by friction with the surrounding gas; and the electrostatic force generated between the capacitor plates, which always tends to pull them together. In conditions of equilibrium,the damping can be neglected. Hence, in Eq. (1) the force F z is in fact the sum of the mechanical input and the electrostatic force. At equilibrium they are counterbalanced by the elastic force built into the suspension system, deformed by the amount z. Therefore it is desired to minimize as much as possible the influence of the electric force in the measurement process. Another characteristic of a mechanical sensor is the operating range, which can be defined as the range of the mechanical input for which the sensor provides a reliable electrical output. It mainly depends on the device sensitivity. First the capacitance value will be computed and then the electric forces. The main section will deal with the effects of the electrostatic forces. The case of bossed-type structures is fully discussed below (the case for accelerometers and bossed-membrane pressure sensors). In the case of nonbossed-membrane pressure sensors an identical approach can be performed, yielding similar results.

2. The capacitance and the electrostatic forces between the capacitor plates When subject to a mechanical input, the movable plate of the capacitor (the lower plate for the case shown in Fig. 1) will deflect vertically, either remaining parallel or becoming tilted with respect to the fixed plate. The behaviour of the

Fig 2. The geometryof a capacitivemechanicalsensorwith the movable platehavinga verticaldeflectionZmin the middleand tilt anglesO~,Oy. capacitance in both cases will be considered in the following sections. The position of the movable capacitor plate, in the general case, is described by two tilt angles, Ox and O.~, and the vertical displacement at the middle of the plate, Zm (see Fig. 2). For simplicity, it will be considered that at least one tilt angle is zero. Therefore two distinct cases will be analysed: (a) the capacitor with non-parallel plates, described by Zmand ~ (b) the capacitor with parallel plates, described only by z,~ ( ~ = 0 ) . The capacitance can be easily computed, for small values of ~, according to the following formulae: e.ob ['2do + boP- 2Zm-[ or C[¢=01 =

(5)

d o - Zm

where Eq. (5) stands for the capacitor with parallel plates. Both q~ and Zm are directly proportional to the applied mechanical input. However, the relationship depends on the suspension system used. In either case the capacitance increases with b. Two different drive modes can be used to measure a capacitance: Q-drive (the capacitor is biased with a constant charge Qo) and V.drive (the capacitor is biased with a constant voltage Vo). As it will be seen, the behaviour of the electric force between the plates is different in these two drive modes. From here on, the voltage over the capacitor plates is denoted by V. The total electric force, F, is given by the following formula where both the tilt angle (b and the mean displacement Zm depend on F~: F=

26oV2b2 ( 2d o - b o P - 2Zm) (2do + b ~ -

2zm)

(6)

R. Puers, D. Lapadatu/Sensors and Actuators A 56 (1996) 203-210

.o

Fig. 3, A mass suspendedeither by one (non-parallelplate) or by two (parallelplme)beams. In the case of V-drive mode, the voltage V is the constant applied voltage Vo. In the case of Q-drive mode, the voltage Vappearing in Eq. (6) is linked to the constant stored charge Qo through the following formula:

v=

¢Qo r~+b~-2,°] b~olnL~ j

205

11..

.111

0

Fig.4. Electricforce( V-drivemodca~inon-paranel plales) vs. t ~ c a ~ dimensions.

m

(7)

t !/1

For the capacitor with parallel plates (~=- 0) Eqs. (6) and (7) give for the electric force the following values (respectively, for Y- and Q-drive modes): foV 2b2

F[ (ib-- 0,Vo] 2(do_Zm) 2

(8)

o

and F[ O = 0,Qo] =

•

•

•

•

•

,

b [ ~ l

2

(9)

As an example, the simplest accelerometer, consisting of a mass suspended by a beam or a pair of beams, is now considered (see Fig. 3). Using the specific values for a fabricated device [ 3 ], the mass thickness h = 230 ttm, the length, width and thickness of the beam, respectively, L = 200 bun, w = 4 0 Izm, t = l O l~m, I/'o=3 V or Qo=12 pC, and the applied acceleration az = lg=9.81 m s -2, Eq. (6), Eq. (8) and Eq. (9) can be plotted as a function of the capacitor lateral dimension b and for different plate separations d. Figs. 4-7 show the electric force between the capacitor plates operated in It'- or Q-drive mode, respectively, for nonparallel (one beam) and parallel (a pair of beams) plates, based on the design presented in Fig. 3. They remain qualitatively unchanged when other suspension systems are taken into account. Of course, the numerical values may differ. From Figs. 4 and 5, it is clear that for the V-drive mode the electric force increases with the capacitor area and decreases with the gap between its plates. From Figs. 6 and 7 it can be seen that the electric force decreases with the capacitor area and is slightly (for non-parallel plates) or not at all (for parallel plates) dependent on the gap between its plates. It is of paramount importance to consider that, under a given mechanical load, the plate separation changes. In the case of V-drive mode this implies an additional change in the

Fig. 5. Electxicforce ( Y-drivemodeand parallelplates) vs. the capacitor dimensions. 70, 60' 60,

~.~.

10,

O, Fig. 6. ~ force (Q-drive mode and nOn-lmral~lplales) vs. the ~tor dimensions. electrostatic forces, whereas in the Q-drive mode they remain

constant. The former results in an input-signal-dependent offset, the latter in a constant offset on the measured signal. A detailed elaboration will now follow.

3. T h e effects of the electrostatic forces The z-oriented force, F~, appearing in Eq. (1) is the total force acting on the movable plate, thus, at equilibrium, the

206

R. Puers, D. Lapadatu~Sensors and ActuatorsA 56 (1996) 203-210 70

Output ~

60-

~=o,

l........

=!

i

10'

)

0' 300

)

400

SO0

600

700

800

900

1000

b[pm]

Fig. 7. Electricforce (Q-drivemodeand parallelplates) vs. the capacitor dimensions. sum of the mechanical force and the electric force. It is clear that the presence of the electric forces will introduce a fixed or variable offset error. Another drawback of the presence of the electrostatic forces is that they can cause the collapse of the structures when they exceed the counteracting elastic force. This specific case will be investigated below. A last drawback arises from fabrication reasons. In order to define the capacitor and to enable operation under hermetic sealing, an anodic bonding process between a glass and a silicon wafer is usually performed. The required voltage for this bonding may induce a local electric force that can cause unwanted sticking and even bonding of the movable parts onto the glass wafer. 3.1. Offset errors

The change in capacitance, A C = C - Co, is, for small vertical deflections, directly proportional to the total applied force, Fz:

At. = - - - ~ - r Z

(10)

By scaling down the entire device (b, do, L) the electrical sensitivity, Sc (the coefficient of F= in Eq. (10)), will be considerably reduced, mainly due to the decrease of the mechanical sensitivity, $=. As already mentioned, F z is the sum of the mechanical signal and the electric force, hence, the latter can introduce big offset errors in the measurement process. In order to avoid those problems one solution could be to minimize the electric forces with respect to the mechanical input. For the Q-drive mode the electric force interferes as a constant offset, being independent of the vertical displacement (see Eq. (9)). An important drawback arises from the fact that in scaling down the device the offset increases and the sensitivity deteriorates. The rest gap, do, does not appear at all in Eq. (9), which means that it can be chosen in order to increase the sensor' s electrical sensitivity S= (see Eqs. ( 1) and ( I 0 ) ) . Another important drawback for this drive mode

Fig. 8. Outputsignal vs. the inputsignal for the idealcase (i.e., no electric forces), Q-driveand V-drive. is the difficulty of maintaining a constant charge in the capacitor, due to leak resistors. For the V-drive mode it can be seen that the electric force depends directly on the vertical displacement (see Eq. (8)), which in turn depends on the applied forces, including the electrical force. This complicates the problem. The higher the deflections, the bigger the electric force. Thus, in this drive mode, the electric force introduces input-signal-dependent offset errors. Their value depends on the actual vertical position of the movable plate. One possibility to reduce the errors is to reduce the capacitor area and to increase the rest gap between the plates. But, as can be seen from Eq. (10), this approach drastically reduces the electrical sensitivity of the sensor. From this point of view the Q-drive mode is reconunended, since it is easier to cope with constant offsets. In Fig. 8 the output signal is plotted versus the mechanical input for the ideal case, for Q-drive and V-drive modes, respectively. The maximum output signal is either defined by the electronic interface or by physical constraints ~such as the maximum possible displacement of the movab|e plate). Notice the difference in the offsets and in the input operating range. 3.2. Collapse o f the structures

If the elastic force of the suspension system is not high enough to counteract the electrical attraction of the plates, the structure will collapse. In the next sections, Figs. 9 and 10 are used to determine the deflection of the movable plate by looking for the equilibrium between the active components, which consist of the electrostatic force and the input mechanical force with the reactive component, which is the elastic force developed in the suspension. Below we consider separately the situation of Q-drive and V-drive modes. 3.2.1. Q-drive mode

For the Q-drive mode (Fig. 9), the lines passing through the origin represent the elastic force, F~, for different spring constants (Eq. (2)). OA represents the curve for a weak

K Puers.D. Lapadam/SensorsandActuatorsA 56 ~1996)203-210 s~
2a,2do Qo~

207 (ll)

which defines the elastic force OB in Fig. 9. On the other hand, for a given sensitivity S~, such as line OC, the limit of the al!owed charge that can be used without inducing collapse (line EC shows this situation ) is

Qo
Fig.9. Theelastic(OA,OB,OC) and the sumof theelectricand mechanical (DB, EC) forcesas a functionof the verticaldisplacement,for the Q-drive mode.

Fig. 10. The elastic (OA,OB, OC) and the sum of the electricand mechanical (DH, E,:;~ forces as a functionof the verticaldisplacement,for the bdrive mode. suspension, whereas OC is an example of a rigid one. OB is an intermediate situation and it will be the basis of the discussion below. The horizontal line DB represents the constant electric force (Eq. (9)), F¢s. The mechanical input, Fro, is added to this electric force generating other horizontal lines (dotted lines up to EC). The equilibrium is established when the sum of the electric force and the constant mechanical "nput equals the elastic force. The dark field in the right-hand side of the Figure represents a forbidden zone, where the fixed capacitor plate is touched (z equals do, the initial gap). It can be seen that for a highly sensitive device the elastic force, line OA in Fig. 9, is not able to balance the electric force F~t for any displacement. This means that, with Fcs present, the structure will collapse even in the absence of a mechanical input. To avoid the collapse in this case, the sensitivity must be low enough to allow for equilibrium points such as M, which correspond to an eq-ilibrium displacement, or offset, de. Two design approaches are possible: 1. The electrostatic force is specified by the electronic interface used, or 2. The mechanical sensitivity of the device is specified. If an electrostatic force is given (defined by Qo), such as line DB, the limit of the allowed mechanical sensitivity, S~.~,, is

(12)

For example, a capacitor with b = 5 2 0 pan, doffiO.5 fan, and a sensitivity Sz = O. 1/~n p,N- ~, as in Ref. [ 3 ], the maximum charge one can use in an overdamped sensor is, from Eq. ( 11 ), Q,~ = 4.89 pC. Using a bias charge that exceeds the critical value will result in the collapse of the stnmture even in the absence of a mechanical input. In the case of an undamped accelemmeter, it c,'m be easily shown that touching will occur for half the value predicted by Eq. (12). In real situations the critical charge lies between 50% (no damping) and 100% (overdamping) of the value predicted by Eq. (12). Looking again at Fig. 9, and considering that the mechanical sensitivity is defined by line OC and the electrostatic force by line DB, it can be. observed that starting the operation, with no mechanical input applied, the device goes directly to its equilibrium point M. Hence, an offset appears. Fortunately, it is constant and can easily be taken into account: A C o ~ = Co. d-'-~-~. = ~ S~Q°2 a o - a ~ "°2,4,2do- S~Qo~

(13)

The operating range, as previously defined, is the range of the mechanical input for which the sensor structure does not collapse, nor touch the fixed plate. As an example, the maximum mechanical force that can be measured is the force that deflects the lower plate exactly with do (see Fig. 9):

do

0° 2

F , , ~ = ~'~z- 2"-~

(14)

The range depends, of course, on the initial gap, sensitivity and capacitor dimensions. The more sensitive a sensor is, the smaller the range. See also Ftg. 8 for the offset and input range.

3.2.2. V-drive mode Plotting again the elastic force (Eq. (2)) and the electric force (Eq. (8)), Fig. 10 is obtained. As in Fig. 9, the lines passing through the origin represent the elastic force for different spring constants. The hyperbola DH represents the electric force Fcv The constant mechanical input, Fro, is added to this electric force generating other curves (dotted hyperbolae up to EG). The electric force goes toward infinity when z approaches

do. As in the Q-drive mode, for highly sensitive devices the elastic force (line OA) is too weak to balance the electric

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R. Puers, D. Lapadatu/Sensors and ActuatorsA 56 (1996)203-210

force, therefore the structure will collapse even in the absence of a mechanical input. By increasing the elastic spring constant of the suspension, i.e., making the suspension more rigid, the two curves defining the forces can intersect each other in points as M' and M" (line OC for the elastic force in Fig. 10). The same effect can be obtained by keeping the sensitivity constant while reducing the applied voltage, Vo (corresponding to lowering the hyperbolae). It is worth mentioning that M' is a stable equilibrium point, which corresponds to the vertical deflection de,, and M" is an unstable equilibrium point, which corresponds to the vertical deflection @. The critical voltage that produces collapse occurs when the equilibrium points M' and M" merge. Hence, after equalizing the slopes and values of the electric and elastic forces, for a given sensitivity S~ (such as line OB) and no mechanical input, the critical voltage that can cause the collapse of the structure can be obtained:

do

,/ gdo'

d~=-~- and V~= V 27eob~Sz

(15)

where dc is the critical equilibrium point (unstable). Note that when any mechanical input is present, the situation becomes worse. In order to avoid collapse, the applied voltage must not exceed the critical value. For the same reason explained before, in reality the critical voltage is even smaller. For a given electrostatic force (defined by Vo), such as hyperbola DH, the maximum allowed mechanical sensitivity, S~.~, is 8d°3 Sz<--Sz.cr 27 ~<~2Vo2

(16)

which defines the elastic force OB in Fig. 10. From Fig. lO and with a lower sensitivity such as line 0(2, it can be observed that starting the operation, with a voltage less than the critical one and no mechanical input applied (parabola DH), the device goes directly to its stable equilibrium point, M'. Hence, an offset appears. It has a higher value than its equivalent of the Q-drive mode (M' has a higher ordinate than D). The operating range of the sensor is directly defined by the distance between de, and de.. The bigger this distance, the larger the input mechanical signal can be before the structure will collapse (the hyperbola EG, which exceeds the elastic force over the entire z-range of values). From this point of view the V-drive mode is worse than the Q-drive mode, the operating range being smaller in the same conditions (see Fig. 8).

Fig. I I. Photographof a partiallybondedsuspensionspring. The deviceis the capacitiveaccelerometerwith a spiral suspension beam described in Ref. [31. far the crttical value and high temperatures (usually around 400°C) are applied [ 10]. The presence of very strong electrostatic forces will bring the movable parts in contact with the fixed element, usually the glass wafer. Depending on the kind of materials used the results are either temporary sticking or, worse, permanent bonding. In Fig. 11 a photograph of a movable element partially bonded is shown. The indicated fringes show where the unbonded areas are. The fixed capacitor plate is made out of aluminium. The movable element is silicon covered with 120 nm oxide. The picture is taken through the top glass wafer. 3.4. Solutions to avoid the effects of the electric force Due to the fact that the electric force is always attractive, it can be cancelled out only by applying a similar force in the opposite direction, which suggested the idea of using double capacitor structures, such as the one shown in Fig. 12 [ 8,11 ]. A symmetrical structure keeps its plates parallel under any mechanical input. For the Q-orive mode the total electric force is zero, being independent of the vertical displacement. For V-drive mode the former statement is not true. This means that the electrical force for symmetrical structures is cancelled out when Q-drive mode is used, even for big deflections. In this mode, this structure is completely free of electric forces to a first order of approximation (during fabrication the full symmetry of the structure cannot be practically achieved). Hence, there is no danger of structure collapse during operation.

3.3. Effects during anodic bonding In order to perform anodic bonding between a glass and a silicon wafer either for defining the capacitor or for packaging reasons, high voltages (usually around 800 V) exceeding by

Fig. 12. A symmetricalaccelerometer.

R. Puers, D. Lapadam /Sensors and Actuators A 56 (1996) 203-210

Of course, there is a maximum mechanical input which brings the plates of one of the capacitors in contact, but this time the electric force plays no role in defining the input range of the sensor:

Fro. max =

d o / S~

(17)

Another possible approach, valid only for the Q-drive mode, is to use paired devices in a differential set-up to suppress the constant offset induced by the electric forces. In order to avoid the sticking and bonding of the movable parts during the anodic-bonding fabrication step, the following rules should be observed: (a) Avoid the use of aluminium. Aluminium bonds very easily on glass, silicon and other aluminium layers at g00 V and 350°C. As an alternative platinum is recommended. (b) The bare silicon or the silicon parts covered with a thin oxide should be protected against bonding by another layer. A silicon nitride thin film is recommended. Nitride does not bond at all on glass, metals or other nitride layers, at least for temperatures below 400°C. (c) Very rough surfaces prevent the bonding. Hence, aluminium or bare silicon can be used, provided, e.g., that the glass wafer is reactive ion etched locally just to increase its surface roughness.

4. Conclusions The effects of the electrical forces in capacitive mechanical sensors were investigated. It turns out that, when a single capacitor is used, the electrical forces can introduce offset errors in the measuring process of a mechanical input quantity, can cause the collapse of the sensing structure if critical voltages/charges are exceeded or jeopardize the functionality of the final device due to the sticking or bonding of the movable parts occurringd during the fabrication process. The two different drive modes, Q and V, act differently, the first one being finally more suitable for capacitive devices. The electric force increases with the capacitor surface in Vdrive mode and decreases with it in Q-drive mode. Thus, while for big sensors the electric force can be completely ignored in Q-~!rive mode, for miniaturized sensors the electric force is a real problem. In order to cancel out all the effects of the electrical force during the operation of the sensor, the use of either a fully symmetrical structure or paired devices in a differential setup is proposed. A good choice in materials and design can prevent all the negative aspects of the electrostatic forces during the fabrication process.

Acknowledgements This work has been carried out under the Belgian National Program IUAP-50. Mr Lapadatu's work has been sponsored

209

by a bursary from the research fund of the K.U. Leuven, DB/95/19.

References [ 1] B. Pner~,E. Peeters,A. Van Den Bosscheand W. Sansen,A capacitive pressuresensorwithlow impedanceontpnta~! activesuppressionof parasiticeffect,Sensors and Actuatars, A21-A?~ (1990) 108-114. [2l S. Shoji, T. Nisese, M. Esashi and T. Matsuo. Fabricationof an implan~blecapacitivetypepressoresensor,Proc. 4th Int. Co~, SolidState Sensors and Actuators (Transducers "87). Tokyo, Japan, 2-5 June, 1987, pp. 305-308. [3] B. Puers and D. Lapadatu. Exu-emely miniaturized capacitive movement se~ors using new suspension systems, Sensors and Actuators A, 41-42 (1994) 129-135. [4] L. Rosengren,P. Rangaten,Y. B~klond, B. Hok, B. Svedherghand G. Selen,A systemfor passiveimplaatablepressuresensors,Proc. 8th l.~t. Conf. Solid-State Sensors and Actuators (Transducers "93), Yokohama. Japan. 7-10June. 1993, pp. 588-591. [5 ! R. Puers,Capacitivesensov~:whenand how to use them.Sensorsand Acmatars A, 37-38 (1993) 93-105. [6] J.T. Sumito,G.-J. Yeh, T.M. Speaxand W.H. Ko, Silicondiap.hr',',',',',',',~m capacitive sensor for Inussme, flow, acceleration and attitude raeasorements, Proc. 4th Int. Conf. Solid-State Sensors and Acmators (Trar~ducers "87). Tokyo, Japan. 2-5 June. 1987. pp 36-39. 17] Y, Matsun~toandM. Esushi,Integratedcapacitiveaccelemmeterwith noveleicctr~tatic force balancing,Tech. Digest, 1 lth Sensor Syrup.. Japan, 1992,pp. 47-50. 18] H. Seidel,H. Riedel,R. Kolbuck,G. Muck,W. Kupkeand M. Konigor, Capacitive silicon accelerometerwith highly s y ~ design, Sensors and Acmatars. A21-A23 (1990) 312-315. 19] J. Ji, S. Cho, Y. Zhang, K. Najuli and K. Wise, An ultraminiature CMOSp ~ m e sensorfor a multiplexedcardiovascularcafi~'ter,Proc. 7th Int. Conf. Solid-State Sensors and Actuators (Trcmsducers "91), S~m Francisco. CA. USA. 2,1L28June, 1991, pp. 43---~. [ 10] A. Cozrnaand R. I'ners,Characterizationof the eicctrustaticbonding of silicon and Pyrex glass, J. Micromech. Microeng., 5 (1995) 98102. [ 11] D. Lapedatu,M, De Coomanand R. Pners,A doublesidedcapacitive accelerometerbasedon phntovoltaicetch stop technique,Sensorsand ActuatorsA, 52-54 (1996) 261-266.

Biographies R o b e r t P u e r s was born in Antwerp, Belgium, in 1953. He received his B.S. degree in electrical engineering in Ghent in 1974, and his M.S. degree from the Katholieke Universiteit te Leuven (K.U. Leuven) in 1977, where he obtained his Ph.D. in 1986. He is professor at the K.U. Leuven and technical coordinator of the ESAT laboratories. He is director of the cleanroom facilities for silicon wafer processing and for hybrid circuit technology. His main interests are in biotelemetry systems and transducers, as a designer of low-power mixed digit~ and analog integrated circuits and pressure-sensitive devices. Since 1987 he has been establishing a group working on mechanical sensors and silicon micromachining, and on

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R. Puers, D. Lapadatu/Sensors and ActuatorsA 56 (1996) 203-210

packaging technologies, for biomedical implant systems as well as for high performant industrial devices. Professor Puers is teaching courses in 'Production techniques for electronic circuit manufacturing' and 'Biomedical instrumentation and stimulation'. He is a reviewer for several international scientific journals and is a member of many conference steering committees in the field. He is the author of more than 120 papers on biotelemetry, sensors or packaging in journals or international conferences. He is a member of IEEE, the International Society on Biotelemetry and the International Society for Hybrid Microelectronics.

Daniel Lapadatu was born in Turnu Magurele, Romania, in 1967. He received an engineering degree in electronics from the Polytechnic Institute of Bucharest, Romania, in 1991 and a master's degree in engineering from the K.U. Leuven in 1992. He is presently working towards his Ph.D. degree within the ESAT-Micas division of the electrotechnic department of the K.U. Leuven. He was involved in the development and characterization of the photovoltaic etch stop technique and in several designs of capacitive accelerometers. He has been a member of the IEEE since 1991.