GB2412233A

GB2412233A - Electrostatic micro-actuator

Info

Publication number: GB2412233A
Application number: GB0405757A
Authority: GB
Inventors: Salah A Al-Chalabi
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-03-15
Filing date: 2004-03-15
Publication date: 2005-09-21
Anticipated expiration: 2024-03-15
Also published as: GB2412233B; GB0405757D0

Abstract

An electrostatic micro-actuator has a generated force which is a linear function of the signal voltage applied to a common electrode. A common electrode 3a, 3b is suspended for mobility and two other electrodes 1, 2 are in fixed relationship on either side. The common electrode may have the form of a back-to-back comb shape, in interlocking relationship with each of the other two comb electrodes. A method of making the device includes etching a conductive silicon layer of a silicon-on-insulator structure, down to an etch stop layer (figs 8A-8D). The dimensions of the fixed electrodes are greater than those of the mobile electrode so that a limited etch of the etch stop layer releases the common electrode but leaves the outer electrodes anchored by remaining material of the etch stop layer.

Description

1 241 2233

ELECTROSTATIC MICRO-ACTUATOR

This invention relates to electrostatic micro actuators.

It is known that microactuators can be fabricated as Micro-ElectroMechanical System (MEMS) devices. The microactuator is used as a force generator in the system. The microactuator can convert energy stored in an electrostatic, piezoelectric, magnetic, pressure, thermal...etc form to mechanical force. One of the most widely used and easy to fabricate devices is the electrostatic microactuator which, ha general, can be in the form of parallel plates or interdigitated combs. These electrostatic actuators have small gaps, of the order of a few micrometers or even sub-micrometer, between the electrodes which scales the electrostatic force between electrical charges on conductors.

The force in known electrostatic microactuators is a nonlinear function of the voltage difference applied to the electrodes. Referring to Figure 1, a known electrostatic microactuator is the comb microactuator in which two electrodes, referenced here as Electrodes A and B. are typically interdigitated combs as shown. An attracting force is generated, in use, in the y-direction between the two combs. The amplitude of this force is a quadratic function of the voltage applied to the combs, and is given by the following equation: F òrNW V2 Y g where 0 is the dielectric constant of free-space = 8.85 x I of coul2/N.m2; or is the relative dielectric constant of the medium; W is the width (depth) of the tooth; N is the number of inter-digital comb teeth or fingers; g is the gap separating the teeth of one comb and the teeth of the other comb, and V is the voltage difference between the two electrodes.

The force has a positive sign indicating that it acts to increase the overlap area of the teeth of the microactuator's two comb electrodes.

The electrostatic microactuator has been successfully used in switches and transducers where linearity is not required. It is also known to use them as a linear device by biasing the device at a high bias voltage and then applying a small voltage signal whose amplitude is much smaller than the bias voltage. This restricts the range of the amplitude of the signal and therefore of the device because harmonic distortions occur as the amplitude of the signal is increased.

It is known to use three interdigitated comb electrodes in an electrostatic microactuator. For example, an arrangement of this type is disclosed in the "Handbook of Sensors and Microactuators" published in 2000 by Elsevier and edited by S. Middelhoek. A push-pull driving arrangement is described at pages 173-4 where a middle electrode comprising two back-to-back combs interacts with two outer comb electrodes in use. A voltage applied to the middle electrode is fixed and voltages applied to the outer electrodes are (VO + Vs) and (VO- Vs) where VO is a fixed voltage and Vs is an alternating signal voltage. The arrangement can produce a forced vibration whose amplitude is linearly dependent on the product of VO and the amplitude of the signal voltage.

According to a first aspect of the present invention, there is provided an electrostatic microactuator comprising: i) first, second and third electrodes, the first and second electrodes being arranged on opposing sides of the third electrode for electrostatic interaction therewith in use; and ii) an electrical connection arranged to provide a signal voltage to the third electrode.

Preferably, electrical connections are arranged to provide voltages to each of the electrodes.

More preferably, electrical connections are arranged to provide DC voltages which remain constant in use to the first and second electrodes, and to provide a variable voltage to the third electrode which comprises both a DC bias voltage component and a variable signal voltage component.

The first and second electrodes can be arranged in opposing positions in relation to the third electrode such that forces generated in use of the actuator are aligned. An important aspect of the arrangement is that the first and second electrodes have the same voltage- force relationship with the third electrode.

In embodiments of the invention, the response of an electrostatic actuator can be rendered a linear function of the amplitude of an applied voltage signal without the use of a high bias voltage. This can be achieved as described below.

First and second microactuator electrodes, the first and second electrodes referred to above, are arranged in opposition on either side of the third electrode which provides a common middle electrode. Voltage Vat is applied to the common middle electrode, this comprising a bias voltage Vb and a signal voltage Vs. and DC voltages Vat and V2 are applied to the first and second microactuator electrodes respectively.

The electrostatic force "Fir" between the first electrode and the common middle electrode is: Fy, = c, (V, _ V3)2 where cat is a function of the microactuator's geometry and the dielectric constant of the material filling the gap between the first electrode and the common middle electrode.

(Normally the material might be air which has a relative dielectric constant of 1. The dielectric constant of free space is 8.85x1 on The electrostatic force between the second electrode and the common middle electrode is: Fy2 = C2(V2 _V3)2 where c2 is a function of the microactuator's geometry and the dielectric constant.

If Cal = c2 = c, then the resultant force acting on the middle common electrode has a static force in addition to a force linearly dependent on V3. If V3 is chosen to consist of a bias voltage Vb and a signal voltage Vs. i.e. V3 =vb+vs and if Vb is chosen as the arithmetic average voltage between Vat and V2, i.e. 3 0 V = y then the resultant force acting on the common middle electrode is F. - Fy2 = -2C(V, - V2)Vs which is a linear function of the product of the voltage Vs and the voltage difference (VV2) without any static force term. If V' and V2 are fixed, then (V-V2) and Vb are also fixed, and the force acting on the common middle electrode is a linear function of the voltage Vs This is different from the push-pull driving arrangement described in the "Handbook of Sensors and Microactuators" published in 2000 by Elsevier and edited by S. Middelhoek where the applied voltage to the middle electrode is fixed and the voltages applied to the outer electrodes are (VO + Vs) and (VO - Vs) where VO is a fixed voltage and Vs is the signal's I O voltage.

In a simple arrangement to achieve cat = c2, the microactuator is preferably symmetrical in its geometry and material. This applies for instance where the electrodes are comb electrodes, the common middle electrode having combs on either side, interdigitated with respective ones of the first and second electrode combs. The dimensions and materials of the fingers of the combs are preferably matched to provide symmetry about the centre line of the common middle electrode of the microactuator.

Further inventive aspects of embodiments of the invention are as set out in the claims hereto.

Three part electrostatic actuators according to embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawing in which: Figure I shows schematically, in three quarter view from above, the structure of a two part electrostatic comb actuator according to the prior art; Figure 2 shows schematically, in plan view, the electrodes of a three part electrostatic comb actuator in a back to back configuration showing the forces acting on a common middle electrode; Figure 3 shows schematically, in plan view, the comb actuator of Figure 2 with a supporting spring system for a middle electrode; Figure 4 shows schematically, in plan view, the comb actuator and spring system of Figure 3 together with electrical connections; Figure 5 shows a cross section of a three part parallel plates actuator in back to back configuration showing the forces acting on a common middle electrode; Figure 6 shows the cross section of Figure 5 but including electrical connections to the parallel plates of the actuator; Figure 7 shows part of the comb actuator of Figures 3 and 4 having two cross sections (A-A, B-B) indicated; and Figures 8A to 8F show a series of steps in fabrication of the comb actuator, using views along the two cross sections of Figure 7, in the directions indicated by the respective arrows. s

It should be noted that none of the drawings is intended to be drawn to scale.

Referring to Figure 1, as discussed above, electrostatic comb actuators are known in which there are two interdigitated combs A, B of the type shown. This arrangement is not further discussed herein.

Referring to Figure 2, the three part electrostatic comb microactuator according to an embodiment of the present invention has first and second outer electrodes 1, 2 and a middle electrode 3. The first outer electrode 1, and the second outer electrode 2 are simple comb structures each forming a half of an electrostatic comb actuator. The third electrode 3 which is the common electrode has a simple structure of two back to back comb structures 3a, 3b forming the other halves of the two electrostatic comb actuators. Voltages V' and V2 are applied to the two outer electrodes 1 and 2 and voltage V3 iS applied to the common middle electrode 3. The pulling force on the common middle electrode 3 due to the voltage difference between the comb of the first electrode 1 and one of the combs 3a of the common electrode 3

IS

F. = <' r I (V -V)2 g] Similarly, the pulling force on the common middle electrode 3 due to the voltage difference between the comb of the second electrode 2 and the other of the combs 3b of the common electrode 3 is F 2 = 0 r 2 W (V -V)2 g2 These forces will be in opposite directions. Assuming N'=N2 and g=g2, the resultant force acting on the common middle electrode 3 with the two back to back comb actuators 3a, 3b is Fyl - Fy2 = 0 [(V, - V3) - (V2 - V3) ] If V3 is chosen to consist of a bias voltage Vb and a signal voltage Vs' V3 = Vb + Vs and if Vb is chosen as the average voltage between Vat and V2, i.e. V = V, + V2 b 2 Then Fy, - Fy2 = - ' [2(V, - V2)V5] The acting force on the common electrode 3 with double combs is a linear function of the voltage Vs which can be larger than (V2-V). The overall micro-actuator has a linear response until two electrodes touch, or other nonlinear effects start dominating the normal electrostatic forces. This occurs for example when comb teeth clip a facing electrode.

Referring to Figure 3, in practice the common middle electrode 3 can be connected to a beam 4 which acts as a spring and an electrical connection. This configuration provides a simple spring and electrostatic microactuator system. Although shown straight, in practice the beam 4 is likely to be constructed in a folded fashion, having for instance a quite deeply castellated form of outline in plan view, this providing its spring action.

Referring to Figure 4, the electrical connections of the spring and electrostatic microactuator device are as follows: the first electrode 1 is connected to an electrical connection pad 5, the second electrode 2 is connected to an electrical connection pad 6, and the common middle electrode 3 is connected via the spring beam 4 to two connection pads 7, one at each end of the beam 4.

The connection pads 5, 6, 7 are insulated from one another by a wide gap 8 or insulating layers.

The spring beam 4, which is attached in its middle portion to the moving electrode 3 and at each end to a stationary connection pad 7, is itself conductive and thus acts as a wire connection to the middle electrode 3. Both the spring beam 4, which as mentioned above can be folded to give the required spring constant/elasticity, and the electrodes 1,2,3 are made of a polysilicon/silicon material which is highly doped to make it conductive. This is a standard material used in the semiconductor industry. For example, silicon wafers of conductivity of 0.01 ohm-cm can be obtained by doping with impurity (e-type or p-type: the latter is more common) concentration of 10 exponential 18 per cm cube. This resistivity can be reduced by increasing the doping level. For comparison, aluminium and other very good metal conductors have resistivity in the range of 0.000002-0.000010 ohm-cm.

The device can be fabricated using standard Micro Electro-Mechanical System ("MEMS") technologies. These include LIGA fabrication techniques and silicon micromachining (or Deep Reactive Ion Etching (DRIE)). The most widely used material in MEMS devices is silicon, and silicon micromachining is also the most mature fabrication technology. The following describes a silicon micromachining technique for fabricating a comb microactuator as shown in Figures 3 and 4.

Referring to Figure 7, this shows a plan view of part of the comb actuator of Figures 3 and 4 with two cross sections "A-A" and "B-B" indicated. Figures 8A to 8C show steps in fabrication viewed along the cross section "A-A" and Figures 8D to 8F show steps in fabrication viewed along the cross section "B-B". Figures 8A to 8C thus show a cross section during steps in fabrication of the broad back portion of the first outer electrode 1, one finger of that electrode and the relatively narrow back portion of a comb 3a of the middle electrode 3. Figures 8D to 8F show a cross section during steps in fabrication of four interdigitated fingers, two fingers taken from the first outer electrode 1 and two fingers taken from of a comb 3a of the middle electrode 3.

Referring to Figures 8A and 8D, the silicon fabrication process can start from a polished SOI (Silicon On Insulator) wafer with a top silicon layer 70 for example 10 microns thick, an embedded oxide etch stop layer 71 and a bulk silicon wafer 72. The bulk silicon wafer 72 might be for example 100 microns thick. The top silicon layer 70 is heavily doped to make it highly conductive.

Referring to Figures 8B and 8E, photolithography and reactive ion etching are used for defining the comb's teeth 1, 3a and the beam 4 that supports the middle electrode 3. The etching automatically stops at the embedded oxide etch stop layer 71.

Referring to Figures 8C and 8F, a limited etch of the embedded oxide etch stop layer 71 is next performed. Because of their dimensions, the narrow teeth of the interdigitated combs 1, 2, 3 will all at this stage be fully exposed as the oxide is etched away from the sides of the teeth, leaving perhaps just a pimple 73 (in cross section) of oxide material lying centrally under the teeth. However, as can be seen in Figure 8C, the broad back portion 74 of the first outer electrode 1 protects its underlying oxide layer and thus remains undercut but largely supported by remaining oxide material 71. Thus the first outer electrode 1 remains anchored.

Although not shown, this also applies to second outer electrode 2. The whole middle electrode 3 and the spring beam 4 conversely are dimensioned more narrowly. As a result, the limited etch fully exposes this whole structure, in the manner of the narrow teeth of the interdigitated combs 1, 3a shown suspended in Figure 8F. The middle electrode 3 and the spring beam 4 can now move freely.

The remaining embedded oxide layer 71 provides electrical insulation between the anchored electrodes 1, 2 and the movable middle electrode 3 so that different voltages can be applied to each electrode.

The reference to "LIGA" above is to the known technique invented in Germany which stands for "Lthographie", "Galvanoformung" and "Abformung". These respectively in English are lithography, electroforming and plastic moulding.

Two worked examples of comb microactuators of the type shown in Figures 2 to 4 are now given below.

Example 1: V,= (-V2)=Vup For A= 50 fingers, W= 20 Am, g=1 Am, and Vat= (V2)=VsUp= 5 volts, Vs=2 volts, So = 8.85x10-2 coul2/N.m2, or = 1, then the bias voltage applied to the middle common electrode 3 is 3 5 Vb = = 0 and the resultant force acting on the middle common electrode 3 is F.-F. = _ E rNW [4V V] = - 0.354' yl y2 sup s In a simple micro-actuator and spring system with a spring constant of 1 N/m, the resultant movement of the middle common electrode 3 is - 0.354 lam.

Example 2: V'= Vsup and V2=0 For N= 50 fingers, W= 20 m, g=1 Am, and Vat= Vs'p = 5 volts, V2=0 volts, Vs=2 volts, So = 8.85x10-2 coul2/N.m2, Or = 1, then the bias voltage applied to the middle common electrode 3 is V, + V2 = V9UP = 2.5Volts 2 2 and the resultant force acting on the middle common electrode 3 is Fy, - Fy2 = - [2VsUpVs] = -0.1 77pN In a simple micro-actuator and spring system with a spring constant of 1 N/m, the resultant movement of the middle common electrode 3 is - 0.177 lam.

In Figures 5 and 6, the same reference numerals are assigned to parts of the microactuator which correspond to parts of the microactuator shown in Figures 2 to 4.

Referring to Figures 5 and 6, a second embodiment of the invention is a quasi-linear electrostatic actuator where the three electrodes 1, 2, 3 are parallel plates rather than of a comb structure. The resulting device is approximately a linear function of the applied signal voltage. The force acting perpendicular to each of the pairs of parallel plates 1, 3a and 2, 3b, across the gap between the capacitor's plates in the g- direction, is directly proportional to the square of the applied voltage and inversely proportional to the square of the distance separating the two plates and is given by the equation: EOrLpWp v2 g 2g2 where Lp is the overlapping length of the parallel plates 1, 3a and 2, 3b, Wp is the overlapping width of the plates 1, 3a and 2, 3b, and g is the gap between the overlapping plates. (In Figure 5, the W direction is perpendicular to the plane of the drawing and the L direction is parallel to the plane of the drawing.) The minus sign indicates that the force acts such as to reduce the gap between the electrodes. The resultant force acting on the common middle electrode 3 is given by: F F L W [ (V, _V3)2 (V2 _V3)2] gI g2 0 r p P 2(g -fog) 2(go g) where gO is the initial equilibrium separation between the plates of the common middle electrode 3 and the other two electrodes 1, 2, and Ag is the change in the separation from its equilibrium position go. If Ag is much less than gO and if V3 iS chosen to consist of a bias voltage Vb and a signal voltage Vs' i.e. V3 = Vb + Vs and if Vb is chosen as the average voltage between V' and V2, i.e. 2 5 V = y V: Then F. - Fg2 (V, - V2)Vs o which is also a linear function of the signal's applied voltage Vs when Vat and V2 are fixed voltages.

The device can be fabricated using the same type of processes described above in relation to the comb actuator of Figures 2 to 4.

Claims

1. An electrostatic microactuator comprising: i) first, second and third electrodes, the first and second electrodes being arranged on opposing sides of the third electrode for electrostatic interaction therewith in use; and ii) an electrical connection arranged to provide a signal voltage to the third electrode.

2. A microactuator according to Claim I wherein the first and second electrodes are in fixed relation to one another and the third electrode is mounted to be mobile.

3. A microactuator according to Claim 2 wherein the third electrode is mounted by means of a conductive spring beam.

4. A microactuator according to any one of the preceding claims wherein electrical connections are arranged to provide voltages to each of the electrodes.

5. A microactuator according to Claim 4 wherein electrical connections are arranged to provide first and second DC voltages which remain constant in use to the first and second electrodes, and to provide a variable voltage to the third electrode which variable voltage comprises both a third DC voltage component and a variable signal voltage component.

6. A microactuator according to Claim 5 wherein the third DC voltage component is at least substantially equal to the arithmetic average of the first and second DC voltages.

7. A microactuator according to any one of the preceding claims wherein the first and second electrodes are arranged in opposing positions in relation to the third electrode such that forces generated in use of the actuator are aligned.

8. A microactuator according to any one of the preceding claims wherein the microactuator is symmetrical in its geometry and material about a centre line of the common middle electrode, the centre line being normal to the direction or directions in which force is generated in use of the microactuator.

9. A microactuator according to any one of the preceding claims comprising comb electrodes.

10. A microactuator according to any one of Claims 1 to 8 comprising parallel plate electrodes.

11. A microactuator according to any one of the preceding claims implemented as a Micro-machined Electro-Mechanical System (MEMS) device.

12. A microactuator according to any one of Claims 3 to 11, comprising comb electrodes, the first and second comb electrodes comprising a back portion and fingers attached to the back portion, wherein the back portion of the first and second electrodes is wider in cross section than at least one dimension of any part of the third electrode or the conductive spring beam.

13. A method of making a microactuator according to Claim 12, the method comprising the steps of: i) in a silicon wafer carrying an etch stop layer and a layer of conductive silicon, etching the conductive layer of silicon as far as the etch stop layer to create the conductive structures of the first to third electrodes and the conductive spring beam; and ii) carrying out a limited etch of the etch stop layer so as to release the third electrode and the conductive spring beam while retaining the first and second electrodes anchored by their back portions to remaining material of the etch stop layer.

14. A method according to Claim 13 wherein the conductive spring beam is provided at either end with anchoring pads, the anchoring pads having dimensions greater in cross section than at least one dimension of any part of the third electrode or the conductive spring beam such that the limited etch of the etch stop layer retains the conductive spring beam anchored to the anchoring pads.

15. A method according to either one of Claims 13 or 14 wherein the etch stop layer comprises an insulating material.