WO2006097522A1

WO2006097522A1 - An actuator

Info

Publication number: WO2006097522A1
Application number: PCT/EP2006/060824
Authority: WO
Inventors: Clyde Warsop; Martyn Hucker; Andrew Press; Roger WHATMORE; Renaud JOURDAIN; Stephen Wilson
Original assignee: Bae Systems Plc
Priority date: 2005-03-18
Filing date: 2006-03-17
Publication date: 2006-09-21

Abstract

The invention relates to a microvalve actuator (14) primarily for the purpose of controlling airflow to and from an aircraft wing to control aerodynamic flow thereon. The actuator (14) is manufactured using microfabrication methods and is formed from a laminate of a shim supporting layers of piezoelectric material to which voltage can be applied at rates of up to 10kHz or more to flex the laminate and open and/or close the valve orifice (20), a plurality of such actuators being operable to open and/or close orifices at a wing surface through which air jet pulses can be emitted at the wing surface.

Description

AN ACTUATOR

This invention is concerned with actuators and is especially concerned with micro-electro-mechanical systems (MEMS) actuators which may be used as valves to control the flow of gasses and liquids. An example of the application for such a device is the manipulation of airflow about an aircraft wing to promote or delay flow separation to control, for example, vortex bursting thereon. In our co-pending UK patent application no. GB 0420293.3 filed on 10 Sep 2004, there is disclosed a methodology for emitting controlled pulsed jets of air from outlets at or adjacent a leading edge of an aircraft wing. The present invention is concerned with an arrangement for effecting such control. The device described, however has a much broader range of application as a generic fluid control valve for a variety of applications.

Accordingly the present invention provides a method of making a bimorph actuator which comprises assembling and bonding flat outer layers of piezoelectric material to a flat intermediate reinforcement layer while the inner layer is in an expanded condition relative to the outer layers whereby, when all the layers are cooled to substantially the same temperature, a lateral compressive stress develops in the outer layers, the layers of the piezoelectric material and the metallic layer forming a laminate, forming electrically conductive coatings on external surfaces of the outer layers and etching the coatings to define aligned tongue portions at least in the plane of each of the coatings, bonding the laminate to a substrate to form a composite structure and etching or otherwise shaping the composite structure to remove material from the substrate and the laminate to create an actuator as a flexible tongue in the plane of the laminate.

Preferably the piezoelectric material is a piezoceramic material which may be lead zirconate titanate. The intermediate layer preferably comprises titanium, stainless steel or copper alloy. Each outer layer of piezoelectric material has a surface flatness of about +/- 1 μm and preferably a total thickness of the order of about 20 to 100 μm. In carrying out a method according to the present invention, each outer layer of piezoelectric material is provided with a coating of an electrically conductive material on that surface thereof facing the intermediate metallic layer; preferably the coating is formed of chromium. Preferably, both surfaces of each layer of piezoelectric material is coated with a planarising layer prior to application of the coating of electrically conductive material; the planarising layer is preferably provided by a coating of benzocyclobutene or by a coating of a sol-gel comprising a material which is compatible with the piezoelectric material of the outer layers. The sol-gel may comprise lead zirconate titanate material. The planarising layer is preferably curable at a temperature in the range of 100 degrees C to 300 degrees C. The bonding layer applied as a coating to the inner surfaces of the planarised piezoelectric layers may again be a material comprising benzocyclobutene, which may be spun onto the surfaces of the piezoelectric layers to a thickness of the order of 1.5 to 2.0 μm. Alternatively, other means of bonding may be applied (eg. Eutectic, adhesive, direct fusion, thermocompression).

In carrying out a method according to the present invention, the substrate preferably comprises a silicon substrate, which may have a silicon oxide buffer layer on each major surface thereof.

In order to provide alignment of the laminate with the substrate, alignment holes/marks can be provided in the substrate.

An electrically conductive layer is provided on that surface of the substrate to which the laminate is adhered, the layer being formed by metallisation of a material such as gold and/or chromium. The electrically conductive layer provided on the surface of the substrate is preferably etched to form electrical contacts that are aligned during assembly of the laminate with the substrate to make electrical contact with the electrically conductive coatings on the external surfaces of the outer layers of the laminate. The laminate is bonded to the substrate using a material comprising benzocyclobutene, which may be spun onto the surfaces of the piezoelectric layers to a thickness of the order of 1.5 to 2.0 μm. Alternatively, other means of bonding may be applied (eg. Eutectic, adhesive, direct fusion, thermocompression). The present invention also provides a method of making a flexible actuator which comprises bonding a flat piezoelectric layer to a flat substrate layer, grinding at least one of the two layers to a desired reduced thickness and patterning at least one layer of reduced thickness as desired.

There now follows a detailed description, which is to be read with reference to the accompanying drawings, of an actuator and of a method of making that actuator, in accordance with an illustrative embodiment of the present invention. It is to be clearly understood that this embodiment has been selected for description to illustrate the invention by way of example and not by way of limitation.

In the drawings:- Figure 1 is a diagrammatic cross section of an actuator according to one embodiment of the invention;

Figure 2 is a photographic illustration in plan view of an actuator such as is shown in Figure 1 ;

Figure 3 is a photographic illustration showing how a plurality of actuators can be formed on a single silicon substrate;

Figures 4 to 8 are diagrammatic cross sections illustrating sequential stages in the production of a laminate part of an actuator according to the present invention;

Figures 9 to 12 are diagrammatic cross sections illustrating sequential stages in the preparation of a substrate in the manufacture of an actuator according to the present invention; and

Figures 13 to 18 are diagrammatic cross sections illustrating the sequential stages in assembling the actuator.

It should be clearly understood that the actuator according to the present invention which is shown in the accompanying drawings was designed primarily for the purpose of controlling a compressed gas source and as part of a system for injecting a high velocity jet into an airflow traversing an aircraft wing, in order to control flow separation. It is however also to be clearly understood that an actuator such as is described can also serve to control airflow or other gas or liquid flow other than over the aerodynamic structure of an aircraft. - A -

In Figure 1 is shown an actuator according to the invention for controlling airflow on an aircraft wing. The wing surface structure is shown generally at 10 and set into the wing so as to be flush with its surface is a cell unit generally indicated at 12, the cell unit comprising an actuator 14 according to the present invention supported by a housing 16 of the cell unit 12. The cell unit 12 comprises a surface plate 18 flush with the wing surface and having an orifice or outlet port 20 through which air can be pulsed. The housing also has an inlet 22 through which air under pressure can be introduced to the interior of the cell unit. The actuator comprises a cantilever arm 24 which is held in position by side walls 26 of the unit, as hereinafter described. The cell unit itself has an overall length, in this embodiment, of approximately 3 mm when viewed from left to right in Figure 1.

A plan view of such an actuator is shown in the photograph in Figure 2 where the cantilever arm 24 is in the form of an elongate tongue extending into an aperture 27 surrounding the arm on three sides and formed as described below. The cantilever arm 24 can be operated by application of an appropriate applied voltage to move between a position in which it can alternately open and close the orifice or outlet port 20, as is also described below.

For the purpose of controlling airflow over a wing, typical optimum jet orifice 20 sizes are of the order 200μm diameter and modelling and experimentation has shown that to achieve the high velocities needed to influence airflow, a pressure differential of 30-8OkPa is required. In addition, modelling and experimentation have suggested that to reduce fluidic losses in the orifice and increase jet velocity the opening distance that needs to be travelled by the closing part of the cantilever arm 24 needs to be at least 50-

60μm for a 200μm diameter orifice.

As described with reference to Figures 4 to 8, the applicants created and designed a bimorph cantilever arm as this configuration offered a useful combination of force and deflection and the planar construction would be compatible with existing microfabrication processes. Using such processes, it is possible, as will become clear, to mass produce actuators for this, and other purpose as is shown in Figure 3 where, for proof of concept purposes, sixteen such actuators were experimentally produced on a single silicon wafer.

As described with reference to Figures 4 to 8, the bimorphs were built up from two layers or strata 30, 31 of lead zirconate titanate (PZT) piezoelectric ceramic on either side of a titanium centre layer or vane 32.

Referring briefly therefore to Figures 4 to 8, which shows the formation of a single actuator only, a central or intermediate layer in the form of a shim or vane 32 is sandwiched between two outer layers 30, 31 with the intermediate layer 32 held in tension. The two outer layers 30, 31 are heated so that they expand and are then bonded as described below to the intermediate layer 32 so that, as they cool a compressive force is generated within the outer layers 32 (Figure 5). Each of the two outer layers is formed of a piezoelectric material which may initially be a piezoelectric substrate in the form of a commercial wafer. When the laminate or sandwich 33 of the three layers is formed, one 30 of the two outer layers is reduced in thickness (by grinding for example) as shown in Figure 6.

On this reduced thickness layer is formed a planarising layer and a layer of electrically conductive material as shown at 34 in Figure 7, and, as shown in Figure 8, that layer 34 of electrically conductive material is then etched as indicated at 36 to form the outline as indicated in Figure 2.

The laminate is mounted on a supporting substrate 40, such as a silicon wafer, which has oxidised surface layers as indicated at 42 in Figure 9. To assist with alignment of the laminate on the substrate, alignment apertures or other marks 44 are formed in the substrate, as shown in Figure 10, these apertures being of the order of 2 mm in diameter so that visual judgement can be utilised to obtain the correct alignment.

A coating 46 of electrically conductive material is applied to one 48 of the oxidised surfaces of the substrate, as shown in Figure 11 and the coating is then etched, as shown at 50 in Figure 12 to create a pattern that will correspond to the configuration of the etched electrically conductive coating of the laminate

33. The laminate 33 is then placed on the substrate 40 in alignment with its etched configuration aligned with that of the laminate as determined via the apertures or other marks 44, and is then bonded thereto by a bonding layer 46, as shown in Figure 13, and described in further detail below. Once bonded to the substrate, the layer 31 can then be reduced in thickness in the same manner as the layer 30, as shown in Figure 14.

A coating 48 of a planarising layer and an electrically conductive material is then applied to the exposed surface of the laminate 33, as shown in Figure 15, and then the conductive coating is etched to shape the electrode to form an electrode pattern 52, as shown in Figure 16.

Having shaped the electrode, a mask 54 corresponding in shape to the desired shape of the actuator is superimposed on the upper surface of the laminate, as shown in Figure 17, and the laminate is then etched to remove material from the laminate as shown in Figure 18, to define a cantilever shaped actuator such as is shown in Figure 2. Alternatively, the actuator can be formed by powder blasting or laser cutting of the laminate as discussed below.

The actuator 14 is designed to flex under the influence of applied voltage potential across the electrodes. The bending or flexing action is obtained by driving the PZT layers 30,31 in opposite directions. The centre vane 32, preferably of titanium, provides physical support, electrical contact to the centre electrodes and gives a degree of toughness to the otherwise brittle ceramic layers 30, 31 by the introduction of a residual compressive stress

For piezoelectric materials to display electromechanical behaviour the majority of the polarisable domains within their molecular structure need to be aligned. This process is commonly referred to as poling and is usually achieved by heating the material to around 50% of its Curie temperature and applying an electric field for a given time period. The material retains a remnant polarisation on cooling and removal of the electric field. The polarisation axis is normally referred to by the subscript 3, and the in-plane axes are referred to by the subscripts 1 and 2 (d₃i for example). The direction of the polarisation depends on the polarity of the applied field. When the material is driven by an electric field that has the same polarity as the poling field this is referred to as forward bias, the material elongates in the direction of the applied field and contracts in the plane normal to the field. If the electric field polarity opposes the polarisation field then the material is being driven in reverse bias, in this case the material contracts in the direction of the applied field and expands in the plane normal to it. Thus, a bimorph cantilever actuator 14 can be constructed by coupling two pieces of piezoelectric material 30, 31 on either side of a centre support 32, with each side initially polarised in the same direction. If the sides are driven in opposite directions one expands in plane and the other contracts resulting in a bending motion.

In the illustrated embodiment, titanium shim provides the intermediate layer 32. Typically the intermediate layer has a thickness of the order of 12.5μm. Once the three layers 30, 31 and 32 have been bonded together, the thickness of the layer 31 is reduced by grinding the outer surface until the layer has a thickness in the range of 20 to 100 μm. Thereafter, the thickness of the layer 31 is similarly reduced until the layer has a thickness in the range 20- 100μm.

As previously referred to, two methods were investigated to cut out the cantilever actuators 14 from the PZT laminate. The first used a combination of powder blasting and electrochemical etches to remove the PZT and shim material. The second method used an in-house laser dicing process to define the beams. The laser based system is computer controlled and can be driven directly from the same CAD data used to generate photolithography masks. As this process is controlled directly it does not require any masks or subsequent microfabrication processes thus simplifying processing and allowing rapid design changes to be implemented. Cut depth is adjusted by changing the scanning rate and output power of the laser.

When mounted in an aircraft wing, a plurality of such actuators can be installed in a wing of an aircraft and operated to permit airjet pulses to be emitted from the orifices such as that shown at 20 in Figure 1 to control airflow across the surface of the wing. Recent tests on a bimorph actuator design embodying the invention have further demonstrated tip deflections of the order 60-70μm on a 3mm long bimorph for a drive voltage of +/-30V (i.e. an applied field of 1V/μm). On these devices both ceramic elements were 30μm thick as opposed to 60-70μm on the previous prototype (before, the titanium centre vane was 12.5μm thick). Selection of the thinner ceramic elements improved the performance by reducing the mechanical stiffness of the cantilever and in addition reduced the voltage required to achieve the electric fields required for driving and poling the devices (1V/μm and 4-6V/μm respectively). Further enhancements have also been achieved by the inventors by use of a PZT ceramic with a higher piezoelectric activity than previously used (TRS610 d₃i=-340pC/N used instead of Ferroperm PZ27d₃i=-170pC/N). Previous problems with electrical breakdown were found to be overcome as the TRS material had a finer grain structure and a much lower void population. Advantageously, breakdown was further reduced by means of applying a polyimide coating to the ground surface of the PZT and etching this back with oxygen plasma to expose the uppermost surface of the ceramic. This had the effect of filling any remaining pores whilst allowing intimate contact to be made between the electrode metal and the PZT. With these modifications in place the inventors found that it was possible to achieve poling fields of at least 4V/μm without breakdown occurring.

It will be clearly understood that although a specific embodiment of the invention has been described, the invention is not limited to the specific details disclosed and that variations thereon are possible within the scope of the accompanying claims.

Claims

1. A method of making a bimorph actuator which comprises assembling and bonding flat outer layers of piezoelectric material to a flat intermediate reinforcement layer while the inner layer is in an expanded condition relative to the outer layers whereby, when all the layers are cooled to substantially the same temperature, a lateral compressive stress develops in the outer layers, the layers of the piezoelectric material and the metallic layer forming a laminate, forming electrically conductive coatings on external surfaces of the outer layers and etching the coatings to define aligned tongue portions at least in the plane of each of the coatings, bonding the laminate to a substrate to form a composite structure and etching or otherwise shaping the composite structure to remove material from the substrate and the laminate to create an actuator as a flexible tongue in the plane of the laminate.

2. A method according to claim 1 wherein the piezoelectric material is a piezoceramic material.

3. A method according to claim 2 wherein the piezoceramic material is lead zirconate titanate.

4. A method according to claim 1 to 3 wherein the intermediate layer comprises a metallic material.

5. A method according to any one of claims 1 to 4 wherein the intermediate layer comprises titanium.

6. A method according to any one of claims 1 to 5 wherein each outer layer of piezoelectric material has a surface flatness of about +/- 1 μm or less.

7. A method according to any one of claims 1 to 6 wherein each outer layer of piezoelectric material has a total thickness of the order of about 20 to 100 μm.

8. A method according to any one of claims 1 to 7 wherein each outer layer of piezoelectric material is provided with a coating of an electrically conductive material on that surface thereof facing the intermediate metallic layer.

9. A method according to claim 8 wherein the coating is formed of chromium/chrome.

10. A method according to either one of claims 8 and 9 wherein said surface of each layer of piezoelectric material is coated with a planarising layer prior to application of the coating of electrically conductive material.

11. A method according to claim 10 wherein the planarising layer is provided by a coating of a material comprising benzocyclobutene.

12. A method according to claim 10 wherein the planarising layer is provided by a coating of a sol-gel comprising a material which is compatible with the piezoelectric material of the outer layers.

13. A method according to claim 12 wherein the sol-gel forms lead zirconate titanate material via subsequent processing.

14. A method according to any one of claims 11 to 13 wherein the planarising layer is curable at a temperature in the range of 100 degrees C to 300 degrees C.

15. A method according to any one of claims 11 to 14 wherein the coating is spun onto the surfaces of the intermediate layer.

16. A method according to claim 15 wherein the coating on each surface has a thickness of less than 3.0 μm.

17. A method according to any one of claims 1 to 16 wherein the substrate comprises a silicon substrate.

18. A method according to claim 17 wherein the silicon substrate has a silicon oxide buffer layer on each major surface thereof.

19. A method according to any one of claims 1 to 18 wherein alignment holes or marks are provided in the substrate for aligning the laminate on the substrate.

20. A method according to anyone of claims 1 to 19 wherein an electrically conductive layer is provided on that surface of the substrate to which the laminate is adhered.

21. A method according to claim 20 wherein the electrically conductive layer is formed by metallisation of a material selected from gold and chrome.

22. A method according to claim 21 wherein the electrically conductive layer provided on the surface of the substrate is etched to form electrical contacts that are aligned during assembly of the laminate with the substrate to make electrical contact with the electrically conductive coatings on the external surfaces of the outer layers of the laminate.

23. A method according to claim 22 wherein the electrically conductive coating on that surface of the laminate that is bonded to the surface of the laminate is pre-coated with a planarising coating.

24. A method according to claim 23 wherein the planarising pre-coating applied to the laminate is selected from the group consisting of materials comprising benzocyclobutene and a sol-gel.

25. A method of making a bimorph actuator substantially as hereinbefore described with reference to the accompanying drawings.

26. A bimorph actuator when produced by a method according to any one of claims 1 to 25.

27. A method of making a flexible actuator which comprises bonding a flat piezoelectric layer to a flat substrate layer, grinding at least one of the two layers to a desired reduced thickness and patterning at least one layer of reduced thickness as desired.