BISTABLE MICROACTUATOR WITH COUPLED MEMBRANES

Abstract: The invention concerns a bistable electrostatic actuator with pneumatic or liquid coupling. The electrostatic membrane actuator has enclosed metallic electrodes (21, 22; 23). It can be used for a microvalve or a micropump. Said actuator having buckled membrane sections (20a, 20b, 20x) in pairs and curved substrate electrodes (21, 22, 23), locally associated with said membrane sections (20a, 20b).

Description: Bistable Microactuator With Coupled Membranes The invention concerns a bistable electrostatic actuator with pneumatic or liquid coupling. From power and safety considerations mechanically bistable actuators are of considerable importance. However, only a few bistable microactuators fabricated in silicon technology have been reported 1 - 2,3 . Due to the low power consumption electrostatic drives are often preferred to electromagnetic and thermomechanic actuators. A drawback of electrostatics is its low range. High voltages are needed to achieve large deflections. Different concepts have been reported to produce non-planar driving electrodes 4 - 5 ' 6 ' 7 . These technologies are however rather specific and not for general-purpose use. Further prior art is sensitive, since electrodes are free to the environment, e.g. particles. This can be improved, when enclosed electrodes, separated from the gaseous or liquid medium are formed 4 . B. Halg, On a micro-electro-mechanical nonvolatile memory cell, IEEE Trans. Electron. Dev. 37 (1990) 2230-2236 H. Matoba, T Ishikawa, C -J Kim, R S Muller, A bistable snapping microactuator, Proc MEMS 94 , Oiso, 45-50 M. A Huff, A.D. Nikolich, M.A Schmidt, A threshold pressure switch utilizing plastic deformation of silicon, Proc. Transducers 91 , San Francisco, 177-180 J. Branebjerg, P. Gravesen, A new electrostatic actuator providing improved stroke length and force, Proc. MEMS 92, 6-11 A Bertz, T. Werner, The formation of very flat silicon sidewalls by dry etching, J Micromech. Microeng. 4 (1994) 23-27 M. Shikida, K. Sato, T. Harada, Fabrication of an electrostatic microactuator with an S-shaped film, Proc. Transducers 95 & Eurosensors IX, Stockholm, 426-429 P Rangsten, L Smith, L Rosengren, B Hδk, Electro-statically excited diaphragm driven as loudspeaker, Proc Transducers 95 & Eurosensors IX, Stockholm, 430-433 see footnote 4. The invention is set out in the claims. Its underlying problem is the reduction of deflection voltage to make the actuator suitable for a general purpose use. According to the invention two buckled membranes span over connected cavities with enclosed driving electrodes (claims 1,13) . The membranes operate in counteraction (claim 11) , if one membrane section is pulled down electrostatically, the other membrane section is pushed up and vice versa. The coupling may be air (pneumatic) or liquid (claim 12) . The actuator module is designed to achieve a deflection of ±lOμm and can be integrated in a microvalve for controlling the flow of fluids. With completed actuator modules the electrostatic driving principle is used in examples for a valve application, e.g. an on/off valve or a multi-way-valve, a switch application, e.g. an on/off-switch or a changeover switch, a micropump application. Grey-tone lithography may be applied to fabricate the curved driving electrodes on the curved cavity bottom (claim 5) . Compared to flat electrodes the driving voltage can thus be reduced up to a factor of five. The curved cavity bottom ("dimple") also improves the pneumatic or liquid coupling since the enclosed air or liquid volume is minimized. The design and technology of the bistable electrostatic actuator with enclosed electrodes for a microvalve application uses curved shape of the electrodes by employing greytone lithography, a method to produce arbitrary relief-type surfaces. To produce a membrane deflection away from the substrate a pneumatic coupling of two cavities is incorporated slightly related to the suggestion 8 of MEMS 92, Travemunde. The two buckling sections of the membrane are oppositely stable (claim 15) and close to each other in the same wafer. K.J. Gabriel, O. Tabata, K Shimaoka, S. Sugiyama, H Fujita, Surface-normal electrostatic/pneumatic actuator, Proc. MEMS 92, Travemunde, 128-132 Examples are described with reference to the enclosed Figures. Figure 1 is a schematic cross-section of the a bistable microvalve with two buckling membrane sections 20a, 20b of a membrane 20. Figure la is a total view of the membrane 20 with the buckling sections 20a, 20b in a top view and considerably enlarged. Figure lb is a side view in section of only one smoothly curved flat dimple with an overlying buckling membrane, representing both membrane sections 20a, 20b and both dimples 11,12 of Figure 1. Figure lc shows a comparative example having a flat shaped electrode 21f below the membrane section 20a. Figure 2 shows an exploded view of the bistable microvalve of Figure 1. Visible is a coupling channel 30 between the cavities 11,12. The outlet 8 is just behind the channel 30. Figure 2a is the side view of the anodic bonded three part embodiment as seen in Figure 2, having an actuator chip 100 with bonded membrane 20 and on top the valve seat chip 200. Figure 3 is a finite-element calculation of the buckling of circular membranes, 2mm (A-points) and 3mm (O-points) in diameter. The membranes have a silicon thickness of 7μm and are covered single sided with silicon oxide. Figure 4 is an iterative electrostatic finite-element calculation of the snapping process of bistable membrane sections 20a, 20b in the membrane 20. The steps are marked (1) ... (5) . Figure 5 is a profile plot of a grey-tone lithograhic structured hollow (dimple) in silicon. This substantially cosinus-square contour is the optimum shape for the dimple bottom with associated driving electrodes. Figure 6a to Figure 6h are steps (1) to (22) in an exemplary manufacturing process to provide the actuator chip 100 with bonded membrane 20. Each step of manufacturing is represented in the figures; each figure 6a to 6h simultaniously shows a section in side view (righthand) and top view (lefthand) . Figure 7a to Figure 7c are an exemplary process with manufacturing steps (1) to (11) for providing the valve seat 200, to be anodic bonded to the actuator chip turned upside down and shown in Figure 2a as side sectional view. The buckling of the membrane 20 is for clarity reasons not shown in Figures 6, 7 and 2a, but in the schematic views of Figure 1 and lb, lc. Figure 8 is a three/two way valve with two outlets 8a and 8b and on inlet 7. Figure 9 is an electronic switch providing a contact above membrane section 20a. The contact is shown in closed condition wherein the metallic valve seat 9a cooperates with a metallic contact section 28a, applied to the top of the membrane section 20a. A second cooperating contact pair 9b, 28b is provided associated to membrane section 20b. Thus two contacts A, B are provided, which can be operated as changeover switch, depending on the metallic structures connecting the contact sections 28a, 28b. Figure 10 and Figure 11 are micropump applications. An electrostatic attraction force at membrane 20a is transferred via the channel 30 in a repulsing force for membrane section 20b. Thus a "repulsing electrostatic force" is generated, although physically only attracting electrostatic forces can be generated. The space below the membrane section 20a, 20b is a closed cavity 11, 12, 30, not changing its volume and providing steady stationary and dynamic coupling. Figure 1 shows a schematic cross-section of a first example of a valve with buckling membranes. Figure 2 shows an exploded view of the bistable microvalve. Figure 2a is its side view. The manufacturing of Figures 6 and 7 leads to the Figure 2a embodiment. The lower active chip 100 of the valve contains two silicon membranes 20b, 20b, which are buckling due to intrinsic compressive stress. The cavities 11,12 below the closely spaced membranes 20a, 20b are air-filled and connected by a channel 30. Thus, the membranes are pneumatically coupled in order to operate in counteraction. Each membrane section 20a, 20b can be switched down by electrostatic forces using an underlying driving electrode 21 and 22 as can also be seen from Figure 8, which displays the same principle of membrane buckling in a multi-way application (3/2 way valve) . By switching one membrane down the air is pressed through the channel 30 and pushes the second membrane up. The use of two coupled membranes has the advantage that the valve can be operated electrostatically in both directions without applying a voltage across the fluid. The actuator is separated from the medium. Another advantage of the coupled membranes is its insensitivity against external pressure changes. The electrodes 21,22 are curved to decrease the gap towards the outside clamping areas 20c, 20d. Thus the electrostatic forces are increased and/or the switching voltage can be reduced. The electrodes 21,22 are implanted in the dimples to allow a Si-Si bonding process for the membrane formation without metallic feedthroughs. The pneumatic coupling is improved by the curved cavity bottom 11,12. The enclosed air volume is as small as possible. The microvalve is completed by etching an outlet opening 7 in the lower chip 100 and bonding of an upper chip 200 carrying the inlet 8 and valve seat 9a. This is shown in Figure 2a. Valve seat 9a may be metallic, to provide switch applications according to Figure 9. If similar reference numerals are used along the description, they are designated to show the same elements. In Figure 1 the buckled membranes 20a, 20b are provided with metallic layers, cooperating with the electrodes 21, 22 on the bottom of the cavities 11, 12. The electrodes 21, 22 and the metallic layers can be guided via contact paths 2b to contact pads 2a, applied near or in further distance of the valve, shown in Figure 8 or 1. The substrate 10 is identified as actuator chip 100 in Figure 2a and manufactured according to Figures 6a to 6h. The valve seat chip 9 is identified as upper chip 200 in Figure 2a and manufactured according to Figure 7a to 7c . An inlet 7 and outlet 8 is provided close to one of the membranes 20a, each passage 7, 8 is provided in one of the valve seat chip 200 and the actuator chip 100. The flow direction of inlet and outlet may be reversed. A valve seat 9a is provided opposite the buckling area 20a above cavity 12, in which the electrode 22 is provided. In Figur 1 the membrane section 20a directly cooperates with the valve seat, although according to Figure 9 a contact pad may be provided between them, however for a different use. The description focusses on the bistable actuator in valve application, however pump and switch application (1/2- or 3/2-valve) are possible, as outlined in Figures 8 to 11. The theory of buckling beams and square membranes has been described 1 ' 9 ' 10 byHaelg, Sδderkvist, Popescu. In the case of 9 J. Sόderkvist, U. Lindberg, Characteristics of quasi buckling, Sensors and Materials, 6 (1994) 293-309 10 D.S. Popescu, T.S. Lammerink, M. Elwenspoek, Buckled membranes for microstructures, Proc. MEMS 94, Oiso, 188-192 circular membranes the critical compressive stress is given by 14,68 E-t 2 cr 12 (1-n 2 where R is the radius, t the thickness and E and n are the elasticity constants of the membrane, see Pepescu 11 . For stress values σ above σ cr the membrane buckles out. In good approximation the membrane deflection has a cosinus-square shape π- r d(r) d n ■ cos 2 2-R The center deflection d Q grows proportional to sqrt ( σ/ σ cχ .- l The target specification of the valve of Figure 1 is to have a center membrane deflection of ±lOμm. The mechanical force to switch the buckled membrane should be about 1 mN. This leads to a design with a silicon membrane thickness of 7μm. Membranes with diameters of 2 mm and 3 mm were investigated. The membranes are fabricated as a double layer of stress-free single-crystal silicon covered with thin silicon oxide on one side. For thermally grown SiC an intrinsic compressive stress of about 3-10 8 N/m2 has been measured. The manufacturing of such membrane 20 is step by step shown in Figures 6d,6e steps (10) to (13) . The electromechanical behaviour of the bistable membrane 20 has been calculated with finite-element-analysis (FEA) using ANSYS software. In Figure 3,4 the FEA result of 7μm thick silicon membranes is given. Above threshold the deflection increases with the square-root of the Si0 2 thickness (single sided with silicon oxide) . The 2mm membrane uses 585 nm silicon oxide, the 3mm membrane 240 nm silicon oxide to buckle lOμm. In the further simulation the thickness of the Si0 2 layer was fixed in order to achieve a deflection of lOμm. The major results of the calculation are given in Table 1 below. 11 S.P. Timoshenko, J.M. Gere, Theory of elastic stability, McGraw-Hill 1961 , pp. 389 Table 1 shows the results of the FEA for 7μm thick silicon membranes covered on one side with silicon oxide. The Si0 2 thickness is determined in order to obtain buckling with a lOμm center deflection. membrane diameter [mm] 2.0 3.0 Si0 2 thickness [nm] 585 240 mech. snapping force [mN] 1.35 0.6 U flat electrode [V] 245 105 ^curved electrode r , g min = l.Oμm [V] 88 38 TTcurved electrode r , 9min = °- 5 ^ m [v] 47 22 The mechanical forces to snap the membranes are 1.35 mN and 0.60mN, respectively. As mentioned above, the electrostatic switching voltages strongly depend on the distance and shape of the lower driving electrodes. Two examples are shown in Figure lb and lc. In Figure lc a flat electrode 21f on the bottom of a lOμm deep cavity is assumed. In Figure lb curved electrodes 21,22 with the cos 2 -shape, substantially corresponding to each down-buckled membrane 20a, 20b are calculated. The maximum depth is lOμm again. Of interest is the minimum air gap at the clamping. Two values, lμm and 0.3μm have been chosen for this parameter. The calculation of driving voltages was performed in an iterative electrostatic-mechanical manner. Figure 4 shows the membrane deflection in five steps of the calculation as being moved from upwards stable condition (1) to downwards stable position (5) . When snapping over, the membrane forms an intermediate state which is sine-shape. This instable shape could not be calculated. The switching voltages are also listed in Table 1. Compared to the flat electrodes 21f the cosinus-square electrode 21,22 with lμm minimum gap g m - j _ n reduce the voltage to 36%. With a gap of g mj _ n 0.3μm a voltage of about 20% of the flat-electrode value is sufficient. The process sequence of Figures 6a to 6h, Figures 7a to 7c and Figure 2a starts on double-sided polished 525μm thick n-type wafers 10 (5-15Ωcm) with the etching of the coupled cavities 11,12. In a first complete fabrication run the process to realize cavities 11,12 with curved bottom electrodes (see Figure lb) was not yet included, it was performed later (after priority, before actual filing date) . Cavities with a constant depth of 4.5μm were etched using an isotropic etching solution (HN0 3 :HF:CH 3 C00H = 8:1:3) and a silicon oxide mask. After removal of the Si0 2 mask the wafers are oxidized again (50 nm) . The next step is an implantation defining the area of the two isolated substrate electrodes 21,22 (boron, 70 keV, 1-10 16 cm -2 ) seen in Figure 6b, step (4) . A photolitography with a 6μm thick resist is used to achieve a good step coverage. This prevents undesired doping of the coupling channel between the two cavities. The boron ions are activated by an 1150°C drive-in process for 7 hours in an N 2 /0 2 gas mixture to achieve a homogeneous dopant distribution in the vertical side walls of the cavities. The following second implantation (phosphorus, 70 keV, l-lO 1 ^ cm -2 ) defines the n + -contact area to the n-type substrate. The implantation is activated by a 1000°C annealing in nitrogen for 1 hour, see steps (7) and (8) in Figure 6c. The membranes of the actuator module are fabricated by silicon bonding of an SOI-wafer (silicon on insulator) , as seen in steps (10) to (13) in Figures 6d, 6e. Commercial SOI wafers with a 7μm thick silicon layer 63 on a lμm intermediate silicon oxide 62 were used. Before bonding, the SOI wafer is oxidized to grow a silicon oxide 60 of 380nm. This layer 60 is responsible for the membrane buckling. The thickness is a medium value for the 2mm and 3mm membranes. To achieve a high bonding strength the bonded wafer pairs are annealed at 1000°C in nitrogen and then oxidized at 1000°C to a thickness of about 500 nm. In the next step (11) the substrate 63 of the SOI-wafer is etched back completely in 25% aqueous TMAH at 90°C. The lμm thick intermediate Si0 2 62 acts as an etch stop layer. This stopping oxide 62 is thinned down to about 200nm by dry etching, see step (13) . The following lithography defines the access areas to the implanted substrate electrodes beneath the membrane areas 20a, 20b. After opening of the oxide by RIE the silicon membrane is locally removed in 25% aqueous TMAH at 90°C. After defining the contact holes to the implanted electrodes, to the substrate and to the membrane in step (15) the wafer is metallized with aluminum in step (16) . The metal is structured by wet etching. Finally, the Si0 2 layer on top of the membrane is completely removed by dry etching. In a second exemplary manufacturing process the same steps as given above are employed, however, deviating at steps as given in Figure 6a to 6h with the full step sequence (1) to (22) . The process to obtain the cosine-shaped curvature of the cavity bottom has been developed separately in parallel. Basis is grey- tone lithography which uses a raster-screened photomask to produce arbitrarily curved resist profiles 12 . With a 5:1 projection lithography and mask pixels of sub-resolution size a smooth curved resist shape is obtained. In the lithography a 13μm thick photoresist is used. Subsequently the resist contour is transferred to the silicon substrate by dry etching. The RIE process is optimized in order to obtain nearly equal etching rates for silicon and the resist. The pneumatic coupling of two membrane sections 20a, 20b has been confirmed first with a passive test module. The chip containes two buckled Si/Si0 2 membranes with a diameter of 2mm. They cover two air-filled cavities connected by channel 30. In Figure la a completed bistable actuator module combining a 2mm membrane with a 3mm membrane is shown. The smaller membrane buckles up with a maximum deflection of 6.0μm. This value is in 12 B. Wagner, H.J. Quenzer, W. Henke, W. Hoppe, W. Pilz, Microfabrication of complex surface topographies using grey-tone lithography, Sensors and Actuators A 46-47 (1995) 89-94 accordance with the Si0 2 thickness of 380 nm. The larger membrane buckles down and is pressed on the bottom of the 4.5μm deep cavity. A breakdown voltage for the pn-isolation of 220V has been measured. With a voltage of 50V the 2mm membrane could be pulled down. This yields a buckling up of the 3mm membrane which was not quite complete. The membrane got into an asymmetric shape. The reason is probably that the pressure inside the cavity is lower than athmosperic pressure. This comes from the fact that the bonding has been performed in air. In the subsequent high temperature treatment the oxygen is lost which reduces the pressure inside. The 3mm membrane is switched down with 15V and pushes the 2mm membrane up completely. In Figure 5 a profile measurement plot of a dimple in the cosine-square-shape as the driving electrode 21,22 is shown. Figure 8 provides two passage ways 8a, 8b for alternative flow depending on the membrane position. In the displayed state passage way 8a is closed by buckled up membrane 20a and passage way 8b is clear for flow communication to passage way 7 in the actuator chip 10, or for vice versa flow from passage way 7 to passage way 8b. Applying voltage to electrode 22 will attract membrane section 20a, couple the fluid flow of dimple 12 via the passage way 30 between the membrane section 20a and 20b and push membrane section 20b upwards to close valve seat 9b and thereby close passage way 8b and open passage way 8a for flow communication to passage way 7. The opposite switch proceeding may be easily recognised. Figur 9 was described earlier and has no passage or inlet or outlet in the valve seat chip 9, but electric contact pads 9a, 9b for electric contact communication with contact pads 28a, 28b on the membrane section 20a, 20b. Applying voltage to electrode 22 will open electric contact 28a, 9a and close electric contact 28b, 9b. The reverse switch operation is easily recognised. It is self evident, to provide electric contact paths similar to contact path 2b also on valve seat chip 9 in cooperation with electric valve seats 9a, 9b. Figure 10 and Figure 11 are micropumps. Figure 1 employs check valves 80, 81 for charging and discharging the pump, which operates with cooperating membrane sections 20a, 20b in two parallel channels. Each pair of membranes 20a, 20b is coupled by an individual channel 30 and a multiplicity of rowed up membrane pairs is sequentially controlled to provide a pumping operation. The center C of each buckled membrane is defining - if connected by virtual line - the pumping direction. Figure 11 shows the curved electrode 23 below the membrane 20a and the pumping room 41 above the membrane 20a.

Claims: Claims : 1. Electrostatic membrane actuator with enclosed metallic electrodes (21, 22;23) for a microvalve or a micropump, said actuator having buckled membrane sections (20a, 20b, 20x) in pairs and curved substrate electrodes (21,22,23) locally associated with said membrane sections (20a, 20b) . 2. Actuator according to claim 1, wherein the actuator has two membrane sections (20a, 20b) buckling in opposite directions and having intrinsic compressive stress as effect of at least one thin layer on the membrane sections, to buckle out and back and therewith have two stable operating points. 3. Actuator according to one of above claims, wherein the two membrane sections (20a, 20b) are coupled (30) to move the second membrane section of the actuator, which is the one not electrostatically attracted, in a direction opposite of the electrostatically attracted first membrane section. 4. Actuator according to one of above claims, wherein the membrane section both have metallic layers and are operatable in counteraction in a flip-flop manner. 5. Actuator chip (100) for actuator according to one of above claims, wherein the substrate (10) on which a flat area (20c,20d) outside the buckling sections (20a, 20b) of the membrane (20) is attached carries at least two dimples or shallow cavities (11,12) adapted closely to the shape of the buckled sections (20a, 20b) of the membrane (20) . 6. Actuator according to one of above claims, in which the shallow dimples or cavities (11,12) associated with the substrate (10) are connected via a linking channel (30) , allowing fluid to move from one (11) of said dimples or cavities (11,12) - when one membrane section (20b) is attracted by electrostatic force - to the other (12) of said dimples or cavities (11,12) and urge the other membrane section (20a) off (outwards) the other dimple. 7. Actuator according to one of above claims, wherein the channel (30) is provided in the substrate (10) and covered by the area of the membrane (20) between the buckling membrane sections (20a, 20b) . 8. Actuator according to one of above claims, having separate electrodes for each membrane section, incorporated in the dimples or cavities and closely adapted to their shape. 9. Actuator according to one of above claims, in which numerous pairs of membrane sections are lined up to form a pump line and always two neighboured dimples for each pair of membranes are coupled pneumatically or by fluid through a dimple connecting channel. 10. Actuator according to one of above claims, having an electrically conducting contact section (28a,28b) preferably in the centre of one or both buckling membrane sections (20a, 20b), adapted to operate with an electrically conducting seat (9a, 9b) arranged oppositely of the dimples or cavities (11,12) . 11. Method for operating two electrostatically deflectable buckled membranes (20a, 20b) in counteraction by only driving one (20a) of the membranes by electrically influenced forces and moving the other (20b) membrane by a flow of fluid through a channel (30) from below the first membrane (20a) to below the second membrane (20b) . 12. Method according to claim 10, in which the fluid is air, gas or liquid. 13. Method for manufacturing a membrane (20) for the above actuator or method, having at least two oppositely buckling membrane areas (20a, 20b) in which a stress-free flat carrier having a first substrate (63) is provided with a thin layer of isolator (60) on one side, providing intrinsic stress to the carrier; the flat carrier is bonded with the insulating layer (60) downwards to a second substrate (10) ; at least the first substrate (63) of the flat carrier is removed until the intrinsic stress providing layer (60) buckles out one or both of the membrane areas (20a, 20b) . 14. Method according to claim 13, in which the removal of the first substrate of the top-down-bonded wafer is stopped by a further insulating layer (62) , forming an intermediate layer between two semiconductor layers (61,63) , which stop layer (62) is partly removed, before metallizing the layer and structuring it to have a second electrode in or on the membrane (20) opposite to the first curved electrode (21,22) in the dimple (11,12) of the first substrate (10) . 15. Thin membrane (20,-20a, 20b) having two oppositely buckled sections (20a, 20b) for an actuator according to claim 1 or as product of a method for manufacturing according to claim 13, wherein said two oppositely buckling membrane sections (20a, 20b) are oppositely stable and are spaced from each other, but closely neighboured.

Inventors: QUENZER HANS JOACHIM, WAGNER BERND, QUENZER, HANS, JOACHIM, WAGNER, BERND
Applicants: FRAUNHOFER GES FORSCHUNG, QUENZER HANS JOACHIM, WAGNER BERND, FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWAND, QUENZER, HANS, JOACHIM, WAGNER, BERND
Application year: 1997
Main classification: H02N1
IPC codes: B81B3, B81C1, F04B43, F15C3, F15C5, F16K99, H01H59, H01L41, H02N1