electrostatic actuation

Sensors and Acrualors A, 37-38 (1993) 684-692

684

A silicon microvalve with combined electromagnetic/electrostatic actuation D Bosch, B Heunhofer, G Muck, H Seldel, U Thumser and W Welser Deutsche Aerospace AG, P 0 Box 801109, D-8000 Munach 80 (Germany)

Abstract We report on the design, fabncatlon and performance of a s&on nucrovalve for small gas flows It consists of two mlcromachmed components which are bonded together One part contams the gas flow inlet, the other part a deflectable silicon membrane Tlus membrane may be activated by a combmation of electromagnetic and electrostatic forces This pnnnple 1sespecially useful for muumlzmg the power consumption The mam expernnental results are as follows The maxImum pressure agamst which the valve can fully operate 1s 160 mbar With tlus pressure, a flow of up to 3 ml/mm can be controlled Typically, current pulses of 200 mA and voltage amplitudes of 30 V are applied for electromagnetic and electrostatic actuation, respectively The sv&chmg time 1s below 0 4 ms

1. Intraduction Dunng the attempts to establish advanced rmcrosysterns contaming components fabricated by means of the s&on integrated-cucmt (IC) technology m combmation with stltcon rmcromechamcs, research on rmcroactuators has become more and more attractive An

unportant field m these actlvltles IS the development of mlcromuuature valves and pumps useful for many kinds of flow control systems Concerning nucrovalves, several force transducing prmaples have been mvestlgated and realized m the form of prototypes, e g , electrostatic [ 1,2], electromagnetic [ 3-61, piezoelectrlc [7,8], thermopneumatic [9, lo] and thermoelectnc [ 1l] principles For special apphcatlons, the advantages and drawbacks of these methods have to be analyzed The tirn of our work was the development of a gas valve for space apphcatlons controlling the flow of the workmg medunn m an ion thrust engme The requlrements gven were the followmg ones flow rate I - 10 ml/ mm, operating pressure 1-2 bar and surltchmg tune 2-5 ms The working medium preferred IS xenon gas Specml attention has to be piud to low power consumption Furthermore, the vibrational and accelerational speclficatlons for satellite components must be met As a result of our analysis consldermg the V~IIOUS actuation prmclples to meet the requirements mentioned above, we decided to use a combined electromagnetlc/electrostatlc force transducing mechanism integrated m the valve structure The relevant forces are (1) Lorentz force on a current carrymg conductor m a magnetic field, and (u) Coulomb force between two conductmg, electncally-charged plates

0924-4247/93/$600

To our knowledge an apphcatlon of this kmd of combined actuation to s&on nucrovalves has not been reported on so far

2. Principle of operation The basic concept of the valve IS illustrated m Fig 1 The mam components are two nucromachmed silicon parts and two permanent magnets Part 1 contains the amsotroplcally etched gas flow mlet, whereas part 2 contams the deflectable membrane which IS suspended on four cantilever beams Figure 2 shows a more detailed cross section m a perspective view On the bottom of part 1 a shallow cavity v&h a depth of about 10 pm 1s etched, whch contams an electrode and an insulating layer This cavity hmlts the path of deflection for the membrane on top of part 2 The membrane is coated urlth a metal conductor, which IS contacted at both ends At the site of the membrane, two permanent

Pm2 Fig 1 Schematic mew of the sd~con mlcrovdve with electromag-

netic/electrostak actuation

@ 1993- Elsevxx Sequoia All nghts reserved

685

other hand, gves the electrostatic holding force m the ‘closed’ state Both voltages have to be synchromzed as indicated on the right side of Fig 3

3. Deqn

membrane

Rg 2 Cross se&on of the nncrovalve showmg the generatlon of the Lorentz force FL actmg on the deflectable membrane (pennanent magnets arc not shown)

magnets (not shown m Rg

2) produce a strong and homogeneous magnetic field of up to 2 5 kGs m the dtrectton mdtcated by arrow ‘B’ An electnc current ‘I’ running across the membrane perpendicular to the field wdl cause an opening or closing deflection (depending on the current &rectlon) according to the Lorentz force ‘FL’

Alternatively, a voltage can be applied between the membrane and the counter electrode on the upper part, causing the electrostatic Coulomb force Thus, the valve can be opened and closed with a short current pulse and it can be kept closed electrostatically agamst a specified gas pressure mth very low power consumption The contact pads for both prmclples of actuation are located on top of part 2 The electnc setup and the tnne sequence of the operating voltages for the valve are shown schematically m Fig 3 Application of voltage U, across the membrane wdl cause a current flow which produces electromagnetic deflection Voltage U,, on the

considerations

Startmg pomt for all conslderatlons concerning the design of the microvalve were the requirements gven for the xenon gas valve as mentioned in section 1 Three basic design parameters have been calculated from that data (I) diameter of valve opening lo-30 pm, (u) range of membrane deflection lo- 15 p, and (III) closmg forces lo-100 pN These results indicate that no large deflections are needed and a moderate force level should be sufficient to operate the valve The magnetic field necessary for generatlon of the Lore& force IS to be supplied by permanent magnets (Instead of cods) for energy savmg reasons The magnets are attached to the outer nm of the valve and can be a part of the housing As an alternative the mtegratlon of a permanent magnet to the moving part was considered, but ths would severely hmrt the dynanucal behavlour and the shock resistance Besides, the charactenstlcs of the valve would become dependent on the orlentatlon mth respect to the gravitational field and the earth’s magnebc field Based on the afore-mentioned main design parameters and the actuation prmclple selected, the layout of the two valve components was performed For both slhcon parts, we made several design vanatlons m order to be able to investigate valves with different geometries from the same wafer As an example, for the membrane contammg part we designed three ddferent shapes for the beams suspending the deflectable membrane, as shown m Fig 4 Going from the straight beams (left) to the S-shaped ones (third from left), the membrane suspension becomes mcreasmgly soft, 1 e , the assoclated sprmg constant decreases On the nght hand side

“I 0

l

“2

0 1

I

0

,I II 1l-ts

k

-t

Fig 3 Prmuple of electrical valve operation

Fig 4 Design of the valve components three membrane parts mth different cantilever beam shapes and assocmted nozzle part

of Fig 4, a standard layout of the nozzle part IS shown The overall dnnenslons of the chips are 8 mm m length and 1 5, 2 and 3 mm m width As the permanent magnets are to be mounted symmetncally on both long sides of the chips, the chip width determines the field strength attainable with gven magnets On the other hand, this width hmlts the area of the membrane and the counter eiectrode, which 1s an essential parameter for the electrostatic force generation

4. Force calculations In an approach to optlmlze the valve function theoretically, we tried to calculate the relevant forces as a function of the membrane position The geometrrcal design parameters used for the calculation are based on the dunenslons of a membrane chip of medmm width w=2mm The different forces may be expressed by the following equations (1) Lorentz force FL = 1,IB

where 1, 1s the effective length of the conductor, I the current, and B the magnetic field strength (assuming I,, Z and B to be orthogonal) (u) Coulomb force F,=-

qAU2 2(a - x)

where E 1s the &electnc constant of the isolator, A the area of membrane/electrode, U the voltage, a the maxlmum distance between electrodes, and x the membrane position The mechamcal reactlon force FR due to membrane displacement consists of two components, namely a bending force FR( B) and a tensile force FR(T) The following assumptions are made concernmg the bendmg force For the membrane/beam system we adopt the model of a car&lever clamped on both ends In a first order approximation, we used the dflerent membrane and beam urldths and calculated a we@ted mean value (w ) for the width of the total deflectable structure (see eqn (4) below) Furthermore, we assume a umform matenal (silicon), neglectmg the presence of thm SO, and metal layers on top of the s&on structure (see subsequent section) Concernmg the calculation of the tensile force, we only took mto account the elongation of the four beams and assumed the membrane itself to he rlgld This 1s reasonable for a coarse evaluation of forces, smce it represents a worst case approach

Under these conditions, we used the followmg equations (1) Bending force [ 121

F,(B) =

32E(w)t3x l3

with, (w>=

41, Wb+ 1, w, l

where E IS Young’s modulus, lb the mdlvldual beam length, wb the beam width, 1, the length of membrane, w, the uldth of membrane, I the total length of movmg structure, and t the membrane/beam thickness (u) Tensile force

&(T) =

4X[J(Zb2 +x2) - I,]w,tE

(5) lbJ@S? Blmetal forces resultmg from current heating on the slhcon/metal membrane were not included m this model In section 6, a formula is p;lven to deduce numerical values of the bnnetal force from expenmental results In Fig 5, a plot of the three forces FL, F, and FR as well as the total force F IS gven as a function of membrane deflection x Numerical values used m the calculations are B = 1 4 kGs, Z = 0 2 A, 1, = 3 mm, A=08mm2, U=30V,a=11~,f,=15mm,w,= 60~, l,,,=2mm, w,,,=O4mm, I=5mm, and t=lOlm The Lorentz force FL IS constant m this very small x-range due to the homogeneity of the magnetic field

0

2

4

6

8 x/pm 10

Fig 5 Plot of calculated forces as a fun&Ion of membrane position, FL Lorentz force, Fc Coulomb force, FR mechamcal reaction force, F,, force due to gas pressure m the closed state

687

At x = 10 pm (valve closed) we have a total force of about 3 mN, which 1s slgruficantly larger than the maximum counter force due to the gas pressure (mdlcated by F,,,) of 0 1 mN (see section 2) Thus, the proper valve function should be ensured The swltchmg tune can be estimated by using the mean dnvmg force m the Interval 0 < x < 10 pm and the mass of the structure to be moved The result 1s about 0 1 ms

5. Fabrication The fabrication process comprises standard ICtechnolo@es and additional &con nucromachmmg techniques As a starting matenal for both valve components 4 inch double polished (100) s&on wafers with a thckness of 525 pm were used The technologcal sequences to fabncate part 1 and part 2 wafers are shown schematically m Figs 6 and 7, mcludmg a short descnptlon of the mdlvldual steps Figure 6 shows the sequence for part 1 which contains the valve nozzle Special features of this part are a sputtered Pyrex glass layer to achieve anodlc bonding to the membrane part and a gold contact area which estabhshes the electrical contact from the electrostatic electrode to the surface of part two, where the wire bond areas are located Access to these areas 1s possible via openmgs etched through the wafer as shown at the left side of the last picture of Fig 6 In Fig 7 we see the processmg sequence of the membrane part An electrochemical etch stop for form-

cmd0tlci-l LFcvoaimde

Rg 6 Fabncatlon sequence for the nozzle component of the valve

LtmO CcJnt~ (FS) wdo etching

h4etaszanQn llN/lllAu

LHho metd (Fs) Gold &ctrodc~M~ RN and Au etching

Electrochemkd Oxide etching

etching

Fig 7 Fabneatlon sequence for the membrane component of the valve

mg the membrane/beam structure dunng KOH etching 1s provided by the unplantatlon of an n-well mto the p-type substrate A special feature of tlus structure 1s the metalllzatlon of the membrane surface with a sufficiently thck gold layer (between 0 5 and 2 pm) for canymg the operating current of the valve, avoiding exccsslve productlon of local heat As mentioned before, the fully processed wafers are attached to one another by means of anodlc bondmg wth the aid of the mtermedlate layers conslstmg of slhcon dloxlde and sputtered Pyrex glass [ 131 Figure 8 shows a scanning electron rmcrograph of two fully processed membrane cbps mth dlfferent shapes of the cantilever beams The hghter surface areas mdlcate the metalhzed parts of the structure On the nght hand side of the chips, three contact pads for the wire bond connections can be seen

Fig 8 Scanmng electron rmcroscopy view of two membrane parts of the microvalve (dlmenslons 8 mm x 2 mm)

mated for the specified flow and pressure range 1s confirmed expernnentally The operational properties of the valve are essentially Influenced by the static and dynamic behavlour of the membrane The mvestlgations concerning this point have been done by means of a laser hsplacement meter (Keyence type LD 2500/2510) For all types of chips the membrane displacement as a function of operating current and structural parameters was measured The dommatmg features are the membrane and beam thlcknesses, the thickness of the current carrymg conductor and the shape of the beams We denote the three &fferent chip widths by 1, 2 and 3 and the different beam shapes by A, B and C, which means straight, shghtly curved and S-shaped beams, respectively Accordmgly, we have different sprmg constants of the structure to be moved (see Table 1) In general, the membrane deflection z as a function of current can be written

6. Experimental results z(I) = +, + z,(I) 6 1 Valve components

1 of the valve (nozzle part), a number of flow charactenstlcs were measured for several nozzle opemngs of square cross sectlon utlth side length rangmg from 4 to 33 Frn and for different gases As an example, Fig 9 shows the results for two gases of interest, namely air and xenon Due to the h@ ~scoslty of xenon gas (about 30% higher at amblent temperature as compared to an-), its flow 1s lower for a given pressure than for air The measurements show that the nozzle diameter range of lo-30 pm theoretically estlConcerntng

part

,f

31 x 31 (al0

26 x 26 (air)

"0

.wo

low

1500

TABLE 1 Properties of various membrane/beam structures from two wafers v&h different membrane thickness 1, sensitlvlty of membrane deflection S(O), and sprmg constant k

;Pm)

21 x 21 (air) 26 x 26 (Xe)

4

21 x 21 (Xe) 11 x 11 (air) 11 x 11 (Xe)

20

2000

pressure (mbar)

Rg 9 Measurements of gas flow at four different nozzle dlmenSlons as a fun&on of pressure for au and xenon gas

(6)

when z. IS the offset, z, the deflection by magnetic force, and zb the deflection by bnnetal force The offset displacement z, originates from net tenslle or compressive stresses m the membrane due to Its multiple layer structure The z, component m eqn (6) represents the Lorentz force deflection and IS a linear function of the current, whereas the blmetal component zb shows a quadratic dependence This 1s demonstrated m Fig 10 where the square root of zb 1s plotted versus current I1 The pure blmetal effect has been measured by operatmg the membrane without apphcatlon of a magnetic field In order to investigate the effect quantitatively, we made measurements usmg chips with different ratios of conductor thickness to substrate thickness From the

Wafer number

31 x 31 (Xe)

+ zb (1’)

k (N/m)

f “* (Hz)

0 05 0 07 0 22

83 64 20

3600 3380 2300

1B 1c

0 01 0 05

89 2 144

3800 3200

2A 2B 2c

0 01 0 02 0 06

50 I 244 73

4800 3420 2200

3B 3c

002 006

87 31

1750 1240

&P type

S(0) @+A)

10

2A 2B 2c

16

689

A

combined forces are plotted It clearly demonstrates the mfluence of the bnnetal force Its numerical value m special cases has been deduced empirically accordmg to the followmg considerations From the expenments made with an apphed magnetic field, we get the sprmg constant k The expetvnents made without field lead to the temperature trse for a gven current AT(Z) and the thermally induced deflection constant k,,, which 1s gven by the slope dz,/dT of the z,, versus temperature function The blmetal force, then, IS given by

1 SpmAuIlt3)pmSc

2

F,,(Z) = kk,, AT(Z)

“0

20

60

40

80

11 ImA

Rg 10 Blmetal effect dependence of the membrane deflectIon on the operatmg current for various Au/Slayer ratios

three examples shown m Fig 10, the followmg conclusions can be drawn for a grven substrate tluckness, the amount of zb 1s inversely proportional to the conductor thickness An increasing conductor thickness results m a decreasing electrrcal resistance and therefore in a reduced production of local heat for a given current Applymg 100 mA, the temperature nse at samples number 4 and number 7 was 25 and 6 “C, respectively On the other hand, dfferent substrate thicknesses lead to a change m mechanical sttiness and heat conduction properties, so that for a gven conductor thickness the membrane displacement ~11 be lower for the sample with the greater substrate thickness (compare samples number 7 and number 20 m Fig 10) In Fig 11, the characteristics of the two components of membrane displacement as well as the curve for the 18 I 5 4

4

2 I

6 p

;’

27

4

2

-5 0

B 8 i

4

-10 -12 -14 -LB -IS -20 -22

(7)

Consldermg the mdivldual membrane structures analysed, the ratios of bnnetal force to Lorentz force vary m the range of 0 6-2 5 at the maximum current apphed Depending on the stgn of blmetal force relative to the magnetic force, whch 1s fixed for a given design of the valve, the bnnetal force can be taken mto account either to enhance the closing force or the opening force In Fig 12, some examples of the dynamical behavlour of the membrane are given The lower curves of the osclllograms show the exetatlon function, the upper curves the response function of the membrane (analog output of the laser displacement meter) The current amplitude of the square wave excitation 1s 60 mA In addition, measurements with Sine wave excitation were done as well m order to determme the resonances of the membrane/beam structures Depending on the chip type mvestlgated, a vanety of resonance phenomena can be observed, which are mostly due to symmetrical and antlsymmetncal osclllatton modes In Fig 13, the mam resonances of three different chip types m the frequency range up to 10 kHz are shown As expected, the highest resonance frequency (4800 Hz) 1s correlated to chip 2A, which has the structure wth the hghest mechanical stiffness On the other hand, the highest osclllatlon amplitude of 90 pm was measured at chip 2C at a frequency of 2200 Hz With this chip, we did a long duration test for about 32 h, correspondmg to 2 5 x lo8 oscllations No change neither m the dynamical behavlour nor m sensitivity and offset was observed In Table 1, a summary of expenmental results concernmg some essential properties of various membrane structures 1s given Here, S(0) means the ‘zero-point sensitivity’ of the membrane, 1 e , the amount of deflectlon per current umt at the orlgm Knowing the equlvalent force, the associated spring constant can be derived

-24 -128

-,22

-26

-24

-22 membrane

2

a2

64

2a

122

162

currenthA

Rg II Charactenstlcs of the membrane deflection, curve 1 deflectIon due to bnnetal force, curve 2 deflectIon due to magnetlc force, and cume 3 deflectIon wth combined forces

6 2 Mmovalve Havmg investigated the properties of the mdlvldual slhcon components, the next step 1s to bond together these components (see Section 5) and to integrate them into some kmd of housing m order to set up a testable

690 Frqq Frqq 1 *

525f4lz 5254ti

49 2 nV.‘dlv

Vux#.

465rV

10 6 I*Cdlv

BBBV _

_

_

‘So F

Bwe. _

600

_

>

450

I

e

k- 30a 150

0

4

Rg 13 Mam re.sonanceS of three different membrane/beam structures, curves 1,2 and 3 represent chip types 2A, 2B and 2C, respectively

-_-______ _--

Louring Fig 12 Dynamic membrane deflectIon measurements at square wave frequenaes of 50, 210 and 1lOOHz (chip type 2B, Erregung = excitation, Antwort = response)

Fig 14 Cross section of the functional model showmg the mta grahon of mlcrovalve chops mto a conventlonal ceramic dual m-hne (DIL) package

valve A conventtonal ceramtc dual m-hne (DIL) package was used for this purpose It was mo&fied by a sprmg mount to fix the valve chips and by two gas connections, as illustrated m Fig 14 The associated mechanical adapters were JOlned to the rmcrovalve wth adhesives This setup 1s denoted as the ‘functional model The test eqmpment used contams the gas supply (pure argon), an automatic pressure control umt, some flowmeters (Honeywell type AWM 3100) and power supplies, mcludmg a function generator Figure 15 shows some results of the valve tests vvlth purely electromagnetic actuation for dnve frequencies up to 500 Hz The upper curves of the three patterns show the valve excltatlon function (current amphtude 100 mA),

the lower curves the correspondmg flow meter output The gas pressure m all cases was 100 mbar The measurements at 10 and 100 Hz indicate that the valve 1s partially opened and closed durmg one penod At 500 Hz, the flow slgnal amplitude 1s considerably reduced due to the llrmted operating frequency of the flowmeter The concept of operatmg the rmcrovalve wth combined electromagnetic and electrostatic actuation has been described m section 2 In order to venfy the tune sequence of the operatmg voltages as shown in Fig 3, a special electromcs part was developed wth the following features (1) generatlon of bipolar current pulses of vanable length up to amplitudes of 200 mA and fre-

691 Frrq Frmq I -

Graph w

18 04 Hz 18 82 Hz

Et 6 nV/dfv

v ..X v n," I : v mvc -

1 99 V

24 9 mwdiv

Found 2 I -- Notl999k

hamy 194 mv e999q

Gyh

m

II_

Frqq ' Frmq I . Graph m

v ma v ml" v IX I

1885Hr 115Hr

, ZBBmV/dxv

188

V

5 88 r/dlv

98BV

29 E r/dlv

9999l

____-____.

194v

R 888 s ,

‘m!r

Rg 16 Valve operation wltb combmed ekctromagnetic/electrostatic actuation (EM/ES), top current pulses 2OOmA/O4ms (EM), rmddle square wave voltage 25 V/l0 Hz (Es), and bottom flow-meter response at valve outlet

2F

operation,

Frmq - 582 : Hz Fraq I - Sc111ng7 Gaph

29 9 V/dlv

VU * 299mv v .I" l -194v VU I * 299v

m

, 18BmV/dlv

1 B

v mix - 181V v ml" -1BBv v 6l.XI - 194mV V

188

mvdlv

6 888 s ,

Rg 15 Valve operation wltb purely electromagneUc actuation for sme wave dnve frequencies of 10, 100 and 500 Hz (Strom = current, Fluss = flow)

quencles of 1 kHi, and (u) generation of monopolar, square wave drwe voltage of up to 30 V, wbch rs synchromzed to the above current pulses The results of the apphcatlon of this electronic setup m conjunction Hnth the functional model of the nucrovalve 1s demonstrated m Fig 16 The meanmg of the three signals shown 1sdescr&A m the legend The valve m this case operates at 10 Hi The short current pulses are sufiicient to swtch the valve, whereas a voltage of 25 V 1s able to generate the necessary holding force m the closed state The gas pressure used IS 100 mbar, causmg a flow m the open state of 1 5 seem From the mmnnum current pulse wdth necessary for a proper

a maxnnum

swttchmg

tune

of 0 4 ms IS

deduced In order to charactenze the static operation behavlour of the valve, measurements showmg the analogue flow control capability were made Figure 17 dlustrates the dependence of gas flow on the operatmg current m the pressure range up to 200mbar For example, at a pressure of lOOmbar a flow between 0 and 2 ml/mm can be contmuously controlled by a current m the range f200 mA Finally, the operatmg range for the present valve 1s shown m the flow versus pressure cllagram of Fig 18 By applymg the combmed forces, the mamum pres-

FI ml/

-200

-100

0

100 l,ImA200

Fig 17 Analogue gas-flow control by the operatmg current of the valve

692

4 flol, mllmlr 3

balance a gas pressure of 0 3 bar m the closed state This 1sessentially due to the fact that only a fraction of 10 to 20% of the membrane area gets mto contact with the counter electrode because of the membrane curvature m the deflected state As the voltage applied should not exceed 30 V, the present pressure hrmt can only be overcome by redesign of the valve structure

Acknowledgements The authors would like to thank the involved staff of the departments ZTA 12 and ZTA 13 for their valuable cooperation We further msh to thank W Kauschke and S Kahmng from Drager Werk AG for the performance of various flow measurements Especially we are grateful to the German Mm&y of Research and Technology (BMFT) for its financial support Rg 18 Operatmg range (typical) of the nucrovalve at shown electrlcal operating parameters

sure against which the valve can fully operate 1s 160 mbar Once closed, It can be kept m that state due to the electrostattc force up to 300mbar The overall range of flow which can be controlled under these conditions 1s about 3 ml/mm These data are typical for valves composed of membrane types 2B and 2C and for nozzle opemngs of about 20 p The leakage, measured at 100 mbar, 1sm the order of 0 5% This can certamly be Improved, because up to now, no efforts have been made to optlmlze the valve seat by design or by proper choice of a material Maximum power consumption of the valve m the analogue operation mode 1s typically 50 mW (internal resistances being m the order of 1 Q) If operated m the digital mode, as illustrated m Fig 16, the power consumptlon 1s frequency dependent, e g 0 5 mW at 10 Hz and 50mW at the maxnnum operation frequency of 1 kHz

7 Conclusions In the attempt to develop a mlcrovalve for gas flow control m space apphcatlons, the actuation prmclple with combined electromagnetic and electrostatic forces has proven to be successful m a lmuted range of ope.ratmg parameters The requirements given at the begmnmg of the development (see section 1) have mostly been fulfilled The only exception 1sthe specified operatmg pressure range of at least 1 bar as compared with 0 16 bar reached m the expenments Contrary to the result of the force calculations m section 4, the electrostatic force m practice was only able to counter-

References I M A Huff, M S Mettner, T A Lober and M A Schnndt, A pressure-balanced electrostatically-actuated nucrovalve, Tech Drgest, Proc IEEE Sohd-State Sensor and Actuaror Workshop, H&on Head, SC, USA, June 1990, p 123 2 T Ohnstem, T Fukmra, J Rldley and U Bonne, Mlcromachmed sillcon rmcrovalve, Proc IEEE Mcro Electra Mechamcal Systems, Napa Valley, CA, USA, Feb 1990, p 95 3 B Wagner and W Benecke, Mlcrofabncated actuator with moving permanent magnet, Proc IEEE Mrcro Electra Mechanrcal Systems, Nara, Japan, Jan /Feb 1991, p 27 4 R L Snuth, R W Bower and S D Colhns, The design and fabrication of a magnetically actuated rmcromachmed flow valve, Sensors and Actuators A, 24 (1990) 47-53 5 P Novak, Mikroventd, Ger Patent No 3621332 C2 (1990) 6 S Albarda, S Kahmng, W Thoren and P Vehrens, Vent& anordmmg aus nukrostruktunerten Komponenten, Eur Patent No 0339 528 BI (1992) 7 M Esaslu, Integrated rnlcro flow control systems, Sensors and Actuafors A, 21-23 (1990) 161-167 8 H T G van Lintel. F C M van de Pol and S Bouwstra. A plezoelectnc mlcro&mp based on nucromachmmg of s&on, sensors and Actuaiors, -15 (1988) 153-167 9 F C M van de Pol. D G J Wonmnk. M Elwensooek and .I H J Fhutman, A ihermo-pneumatic a&atlon pr&ple for a nncronnniature pump and other nucromechamcal devices, Sensors and Actuators, I7 (1989) 139- 143 10 W Benecke, Mkropumpe zur Forderung klemster Gasmengen, Ger Patent No 3802 545 C2 (1990) II C Donng, T Grauer, J Marek, M S Mettner, H P Trah and M Wllmann, Proc IEEE Micro Elecfro Mechanical Systems, Travemunde, Germany, Feb 4-7, 1992, pp 12-18 12 R A Buser and N F de Ro~J, Tuning forks m slbcon, Proc IEEE Mxro Electra Mechamcal Systems, Salt Luke CIfy, UT, USA, Feb 20-22, 1989, pp 94-95 13 A Hanneborg and P A Ohlckers, Ano& bondmg of tuhcon chtps using sputter-deposIted Pyrex 7740 thm films, Proc 12th Nordx SemuzonductorMeet, Jevnaker, Norway, June 8-11, 1986, pp 290-293