WO2025078525A1

WO2025078525A1 - Actuating device structure

Info

Publication number: WO2025078525A1
Application number: PCT/EP2024/078560
Authority: WO
Inventors: Zeljko SKOKIC
Original assignee: SINTEF TTO AS
Current assignee: SINTEF TTO AS
Priority date: 2023-10-10
Filing date: 2024-10-10
Publication date: 2025-04-17
Anticipated expiration: 2026-04-10

Abstract

An actuating device (1) includes a first actuator element (7a) and a second actuator element (7b), each including a piezoelectric layer on a first side (2a) of the actuating device. Each of the first and second actuator elements has a respective width at least five times a respective thickness thereof. A moveable element (4) is connected to at least the first actuator element, such that actuation of the first actuator element causes movement of the moveable element. A reinforcing ring (8a) is on a second side (2b) of the actuating device opposite to the first side, opposite a location on the first side between the first actuator element and the second actuator element.

Description

Actuating device structure

Technical field

This invention relates to piezoelectric actuating devices for use in a range of applications, particularly, but not exclusively for use in moveable mirror devices.

Background

It is known to apply voltages to actuate mechanical structures for enabling rotation and displacement of small-scale devices such as micro-mirrors.

Existing microelectromechanical systems (MEMS) scanning micro-mirrors operate by actuating actuators surrounding a central mirror. Typically, the actuators receive an electric current and oscillate either on one or two axes. In micro-mirror applications, the actuators may be driven by electromagnetic, electrostatic, thermoelectric or piezoelectric effects. Magnetically actuated micro-mirrors, which use the Lorentz force, are the most commonly used in industry due to their suitability for static and dynamic operation.

The Applicant has proposed in their previous publication - WO 2022/172012 - actuating devices which achieve tilting motion and translational ‘piston’ motion. These piezoelectric actuating devices are operated by applying voltages to piezoelectric layers which are deposited on wide and thin actuator arms, i.e. which have a width over ten times their thickness.

One challenge recognised in the development of these actuating devices is that, owing to their particular structure, the actuating devices are prone to buckling which can impose limitations on their deflection range. This problem becomes more apparent for larger deflection angles as the actuator arms experience greater bending and torsion.

The present invention aims to address this problem. of Invention

From a first aspect, the invention provides an actuating device comprising: a first actuator element and a second actuator element each comprising a piezoelectric layer on a first side of the actuating device, wherein each of the first and second actuator elements has a respective width at least five times a respective thickness thereof; a moveable element, connected to at least the first actuator element, such that actuation of the first actuator element causes movement of the moveable element; and a reinforcing ring, on a second side of the actuating device opposite to the first side, opposite a location on the first side between the first actuator element and the second actuator element.

From a second aspect, the invention provides a method of manufacturing an actuating device comprising: providing a device layer and a bulk layer; depositing piezoelectric material on the device layer to form a first actuator element and a second actuator element on a first side of the actuating device; wherein the bulk layer is on a second side of the actuating device opposite to the first side; characterised by: etching away a portion of the bulk layer to form a reinforcing ring, on the second side of the actuating device, opposite a location on the first side between the first actuator element and the second actuator element.

Thus, it will be seen that, in accordance with the invention, a reinforcing ring is provided which may alleviate mechanical stress on the actuating device so that the risk of buckling is reduced. This beneficially helps to achieve an increased range of motion, particularly vertical deflection, for the actuating device compared to a similar device without a reinforcing ring. As will be appreciated by the skilled person, buckling is a sudden change of shape of a component under load and becomes a problem when actuating applications require precise positioning of components. The Applicant has found that actuating devices embodying the invention help to provide an increased maximum range of motion of the actuating device before buckling occurs.

With the strain alleviating structure described above, embodiments of the present invention may provide even greater deflection in devices of the type described in the Applicant’s earlier publication - WO 2011/033028 A1.

A vertical deflection of up to ± 35 pm (a 70 pm total stroke) at 20V of applied voltage has been achieved using embodiments of the invention. For comparison, some practical embodiments of the invention disclosed in WO 2011/033028 A1 had a 9 pm stroke. The long-stroke piston motion made possible by the present invention could find use in optical devices, e.g. allowing for three-dimensional image scans or use in MEMS Fourier transform infrared (MEMS FTIR) spectrometers. Applications outside the field of optics may also benefit from the non-buckling long-stroke motion achievable in accordance with the present invention, such as micropumps, microfluidic devices, and micro speakers.

Actuation of the first actuator element and optionally the second actuator element may cause a dimensional and/or deformational change of that actuator element to result in the movement of the moveable element. A voltage may be applied to the piezoelectric layer of the actuator element to move the moveable element in a desired direction. Making the first actuator element in the form of a thin membrane, i.e. having a width at least five times its thickness, may allow for a large movement, e.g. deflection, of the moveable element. In a set of embodiments, the first actuator element has a width at least ten times its thickness. Similarly, in a set of embodiments, the second actuator element has a width at least ten times its thickness.

In a set of embodiments actuation of one or both of the actuator elements causes the moveable element to be translated vertically (i.e. in a direction normal to the plane of the piezoelectric layer). This vertical translation motion may be thought of as a ‘piston’ motion, in contrast to the more typical tilt motion. For example the moveable element may act like a diaphragm to generate acoustic waves. In such embodiments the moveable element is typically not optically reflective (although of course such a possibility is not excluded). In a set of embodiments, the actuator elements each comprise a ring shape. The first and second actuator elements may be concentric. Preferably, the ring shape is circular, i.e. the actuator elements are annular, however, they could have other shapes - e.g. oval . The first actuator element and second actuator element may have equal width (e.g. the width of an annulus being the difference between the radii of its outer and inner circles). In a set of embodiments, the first actuator element has a maximum dimension (e.g. outer width or outer diameter) smaller than a corresponding maximum dimension of the second actuator element. Thus the second actuator element may surround the perimeter of the first actuator element. For example, an outer edge of the first actuator element may be connected to an inner edge of the second actuator element.

There may be any number of further actuator elements (e.g. third, fourth, fifth, etc.). Similarly, as the number of further actuator elements scales, there may be further corresponding reinforcing rings. In a set of embodiments, the actuating device has a plurality of reinforcing rings, each reinforcing ring being opposite a location on the first side between adjacent actuator elements. In such a set of embodiments, there would be a corresponding plurality of actuator elements - e.g. an actuating device according to an embodiment of the invention may have four actuator elements and three reinforcing rings. The Applicant has determined that it is possible to increase the maximal possible deflection - e.g. vertical deflection, in particular, piston motion - of the actuating device, before buckling occurs, by increasing the number of actuator elements and reinforcing rings. In fact, the Applicant has found the maximal deflection to scale proportionally with the total number of reinforcing rings and actuator elements. Therefore, in a set of preferred embodiments, the actuating device comprises a plurality of reinforcing rings. For example, the actuating device may comprise at least three reinforcing rings.

In a set of embodiments, a plurality of piezoelectric actuator elements is independently addressable with respective voltages. The actuating device may be actuated by selectively applying voltages to one or more independently addressable piezoelectric elements. In a set of embodiments, each actuator element comprises an inner actuator portion and an outer actuator portion or set of portions. Thus, each actuator element may comprise a plurality of actuator portions. Both the inner actuator portion(s) and outer actuator portion(s) may comprise respective distinct piezoelectric areas. Both the inner actuator portion and outer actuator portion may comprise a continuous annulus having a width approximately half of the width of each actuator element. Alternatively, in a set of embodiments each actuator element is divided azimuthally into a plurality of portions, which may be independently addressable so that a voltage can be applied to each one selectively, e.g. each actuator element may comprise M independently addressable piezoelectric portions.

In a set of embodiments, actuator portions may be grouped, such that there may be a group of actuator portions which are independently addressable from those not in the group. In these embodiments, a voltage may be applied to each actuator portion within the group of actuator portions selectively, such that only the group of actuator portions actuate. In these embodiments, those actuator portions included in the group may not be able to be addressed independently of each other.

Each of the inner and outer actuator portions may comprise at least one distinct piezoelectric area, e.g. of Lead Zirconate Titanate (PZT). In a set of embodiments, the inner actuator portions and the outer actuator portions are independently addressable with a respective voltage. The inner actuator portion(s) and the outer actuator portion(s) may thus be actuated separately. Actuating the inner and outer actuator portions separately helps to allow both upwards and downwards deflection - e.g. upwards and downwards piston motion or tilting motion depending on the number of piezoelectric portions.

In a set of embodiments, the actuating device comprises N reinforcing rings and N+1 actuator elements. In embodiments where each actuator element comprises a single inner actuator portion and a single outer actuator portion the actuating device may comprise N reinforcing rings and 2(N+1) actuator portions.

In a set of embodiments, the actuating device comprises a substrate, e.g. a frame.

An outermost actuator element may be connected to the substrate. Typically the actuating device comprises control electronics configured to control the actuation of the actuator elements - e.g. by selectively applying a voltage to one or more actuator elements or portions thereof. In a first mode of operation, a voltage may be applied to the outer portion(s) of each of the actuator elements. This may result in vertical deflection of the actuator elements and piston motion of the moveable element in a first direction. In a second mode of operation, a voltage may be applied to the inner portion(s) of each of the actuator elements. This may result in vertical deflection of the actuator elements and piston motion of the moveable element in a second direction, opposite to the first direction.

In a set of embodiments, the moveable element is connected to an innermost actuator element (e.g. the first actuator element). The moveable element and first actuator element may comprise a common edge. This common edge may be the outer perimeter of the moveable element and the inner edge of the first actuator element. In a set of embodiments, the substrate is connected to an outermost actuator element. The substrate and outermost actuator element may comprise a common edge. This common edge may be the outer perimeter of the outermost actuator element and the inner edge of the substrate. The substrate and the moveable element may each provide anchoring regions for the actuating device to move relative to.

In a set of embodiments the actuating device provides the moveable element with three degrees of freedom - e.g. tilt in both directions about two orthogonal axes and translation along a third mutually orthogonal direction. This may be achieved by each actuator element having a plurality of azimuthally adjacent piezoelectric portions. Such piezoelectric portions may have an annular sector shape. Each piezoelectric portion may subtend an angle between 45° and 180°. The angle subtended is preferably common for all the piezoelectric portions. For example, each piezoelectric portion may subtend an angle of 45°, 90° or 120°. When the actuator elements each comprise multiple azimuthally adjacent piezoelectric portions, a tip-tilt motion can be achieved by selective actuation of the piezoelectric portions. In a set of embodiments, each actuator element comprises an inner portion and an outer portion, each inner and outer portion comprising a respective plurality of azimuthally adjacent piezoelectric portions. The piezoelectric portions are preferably independently addressable. The reinforcing ring, first actuator element and second actuator element preferably comprise a common material, e.g. silicon. The actuating device may be fabricated by lithographic etching, e.g. of the device layer and the bulk layer, to form the reinforcing ring and the actuator elements. The actuator elements and any portions thereof may be formed by lithographic etching of the device layer and the reinforcing rings may be formed by etching of the bulk layer.

The piezoelectric material is preferably not directly deposited on the device layer. In a set of embodiments, the piezoelectric material is deposited between a first electrode and a second electrode on the device layer. In a set of embodiments, each piezoelectric layer is a piezoelectric stack, wherein the piezoelectric stack may comprise a layer of platinum, a layer of PZT and a layer of gold, the PZT being sandwiched between the gold and platinum layers. The device layer may comprise silicon. The device layer may have a thickness between 2-10 pm.

In a set of embodiments, the actuating device comprises, e.g. as a starting material, a silicon on insulator (SOI) wafer comprising the device layer, a buried oxide layer and the bulk layer. The device layer and the bulk layer may be made of silicon. The device layer may have a thickness from 2 to 10 pm. The buried oxide layer may have a thickness of approximately 500 nm. The buried oxide layer may have a thickness from 300 nm to 700 nm.

Each of the first actuator element and second actuator element may comprise a plurality of layers. In a set of embodiments, the first actuator element and second actuator element comprise three layers: a piezoelectric layer; a device layer and a bulk layer, e.g. arranged in that order. The reinforcing ring may be made of the bulk layer. While the device layer may have a uniform thickness across the actuating device, the bulk layer preferably has a non-uniform thickness across the actuating device - e.g. the bulk layer is thicker at the reinforcing ring and thinner at the actuator elements, or the bulk layer is completely removed below the actuator elements.

In a set of embodiments, the reinforcing ring protrudes away from the second side of the actuating device and has a thickness equal to the extent of the protrusion of between 100 m and 1 mm -e.g. between 300 pm and 500 pm. The reinforcing ring may have a width, e.g. the distance between a radially inner surface of the reinforcing ring and a radially outer surface of the reinforcing ring, of between 30 pm and 100 pm, e.g. between 30 pm and 90 pm. For example, in preferred embodiments, the reinforcing ring has a width of approximately 40 pm. The Applicant has found that these particular dimensions are optimal for reducing the risk of buckling whilst allowing for a sufficient range of motion. Furthermore, widths of around 40 pm to 50 pm are a good compromise between thinness, which preserves a large range of motion and reduces the area and mass taken up by the rings, with ease of manufacturing. This means that reinforcing rings, at these widths, may be produced in a controllable and repeatable manner. As will be appreciated by the skilled person, narrower reinforcing rings take up less space on the actuating device which, therefore, would allow more area to be used for the actuator elements.

In a set of embodiments, the moveable element has a mass per unit area greater than that of the first actuator element and the second actuator element. For example the moveable element could simply be thicker than the actuator elements or a supplementary mass could be attached to the moveable element, typically on the side opposite the outwardly facing surface of the moveable element in use. The greater mass per unit area of the moveable element may prevent the moveable element from being deformed upon actuation of the first actuator, for example, by increasing the stiffness of the moveable element. The aforementioned reinforcing ring may be of comparable thickness to the moveable element.

Where provided, the supplementary mass may be any suitable size or shape, however in a set of embodiments the supplementary mass comprises a cylindrical shape having a maximum width (e.g. diameter) equal to or greater than its thickness - e.g. at least twice its thickness - e.g. at least five times its thickness. The width of the mass may be the same as that of the moveable element.

In a set of embodiments, the moveable element comprises a plurality of individually addressable piezoelectric sections. Therefore, the moveable element may be a deformable moveable element which can change shape on actuation (e.g. the surface of the deformable moveable element may change curvature). Upon actuation there may be minimal (e.g. zero) lift around the perimeter of the deformable moveable element and maximal lift (e.g. of several hundred micrometres) at the centre of the deformable moveable element, thus giving a curved profile. The extent of this maximal lift may depend on the diameter of the moveable element - e.g. if a greater lift is desired, then a moveable element having a greater diameter can be selected. The deformable moveable element may be thicker or thinner than the actuator elements. For example, a deformable moveable element which is thicker than the actuator arms will provide a smaller maximal lift, however, a reduced flexibility may provide better optical reflecting properties. A more flexible deformable moveable element is likely to be prone to dynamic deformations during fast movement of the moveable element. Therefore, slightly reducing the flexibility of the deformable moveable element by controlling its thickness may help to prevent such unpredictable or undesired deformations.

In a set of embodiments, the deformable moveable element has a thickness equal to the actuator elements or within 25%, e.g. within 10% of the thickness of the actuator elements.

In a set of embodiments, the moveable element has an optically reflective surface (e.g. a gold coating or mirrored coating). This allows the moveable element to act as a moveable mirror.

When the actuating device is in an equilibrium state, i.e. when the actuator elements are not being actuated, at least one surface of the moveable element may be coplanar with the actuator elements.

The actuating device may have an overall width of less than 1 cm. The moveable element may have a width of between 0.3 mm and 25 mm and may have a thickness of approximately between 100pm and 400pm. Each actuator element may have a width of approximately between 600 pm and 700 pm. Each portion of the actuator elements may have a width of approximately between 300 pm and 350 pm.

Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap.

Brief Description Of The Drawings

Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of an actuating device embodying the invention;

Figure 2 is a second view of the actuating device of Figure 1 ;

Figure 3 is a cross-section of the actuating device of Figure 1 ;

Figure 4 is a perspective view of the cross-section shown in Figure 3;

Figure 5 illustrates some dimensions of a cross section of the actuating device;

Figure 6 shows a close-up view of a CAD drawing of a reinforcing ring of an actuating device embodying the invention;

Figure 7A shows a first actuation mode of the actuating device;

Figure 7B shows a second actuation mode of the actuating device;

Figures 8A and 8B show how the first and second actuation modes achieve piston motion in both directions;

Figures 9A and 9B show how the von Mises stress varies across the device in the first and second actuation modes;

Figure 10 is a photograph showing a second embodiment of the actuating device;

Figure 11 is a schematic diagram which shows how the actuating device according to the second embodiment can be actuated;

Figure 12 is a photograph showing a third embodiment;

Figure 13 is a schematic diagram which shows how the actuator elements of the actuating device according to the third embodiment can be actuated;

Figure 14A-B are schematic diagrams showing how a deformable moveable element can be actuated;

Figure 15 is a CAD drawing of a third embodiment of the actuating device;

Figure 16 is a photograph showing a fourth embodiment;

Figure 17 is a schematic diagram which shows how the actuating device according to the fourth embodiment can be actuated; and Figure 18 illustrates, by means of a schematic flow diagram, manufacturing steps A to F for fabricating an actuating device according to embodiments of the invention.

Detailed Description

Figure 1 shows an actuating device 1 according to a first embodiment of the invention. Figure 2 shows another perspective view of the actuating device 1 shown in Figure 1. The actuating device 1 has a first side 2a and a second, opposite, side 2b. While Figure 1 shows a perspective view from the second side 2b, i.e. the underside, Figure 2 shows a perspective view of the actuating device 1 from the first side 2a of the actuating device 1.

The actuating device 1 has four annular, concentric, actuator elements 7a-7d. In the centre of the actuating device 1 is a moveable element 4, connected to the innermost actuator element 7a. The actuator elements 7a-d and moveable element 4 are arranged such that actuation of the actuator elements 7a-d causes movement of the moveable element 4. The moveable element 4 has a mass-per-unit-area greater than the actuator elements 7a-d, thus allowing it to remain stiff and flat when the actuating device 1 is actuated.

On the underside 2b of the actuating device 1 are three concentric reinforcing rings 8a-c. The reinforcing rings 8a-c are positioned on the underside 2b at locations between neighbouring actuator elements 7a-d. The reinforcing rings 8a-c extend from the underside of the actuating device 1 in a direction normal to the x-y plane (i.e. the plane of the piezoelectric layer) in the form of concentric walls. The dimensions of these reinforcing rings 8a-c are described below with reference to Figures 3-5.

Each actuator element 7a-d is annulus-shaped having a width (the difference between the inner circle and outer circle radii) at least five times its thickness (the dimension normal to the width). In the embodiment shown in Figures 1 and 2, each actuator element 7a-d has two portions, an inner portion 5a-d and an outer portion 6a-d. The inner and outer portions 5a-d, 6a-d are annulus-shaped, like the actuator elements 7a-d themselves. These portions 5a-d, 6a-d each comprise a piezoelectric layer, e.g. made from Lead Zirconate Titanate (PZT). Each piezoelectric portion 5a-d, 6a-d can be independently addressed to apply a voltage thereto by a voltage supply module (not shown) to contract or expand the piezoelectric material.

Turning to Figure 3, a cross-sectional view of the actuating device 1 can be seen. In this view, the structure of both sides 2a, 2b of the actuating device 1 can be seen. Figure 4 provides another perspective view of the cross-section shown in Figure 3. Figures 3 and 4 show that each reinforcing ring 8a-c is positioned between neighbouring actuator elements 7a-d.

Toward the centre of the actuating device 1, the innermost reinforcing ring 8a is on the underside 2b at a location opposite the region between the innermost actuator element 7a and its neighbouring actuator element 7b. The inner portion 5a of the innermost actuator element 7a is next to the moveable element 4 and the outer portion 6a of the innermost actuator element 7a is next to the innermost reinforcing ring 8a.

At the periphery of the actuating device 1 , the outermost actuator element 7d is anchored to a substrate (not shown) at its edge. The inner portion 5d of the outermost actuator element 7d is next to the outermost reinforcing ring 8c and the outer portion 6d of the outermost actuator element 7d is next to the substrate.

The actuator elements 7b, 7c between the innermost and outermost actuator elements 7a, 7d are bounded by corresponding reinforcing rings 8a-c.

As can be seen in Figures 3-4, the actuating device 1 has an overall width of less than 1 cm. The moveable element 4 has a width of between 2-4 mm and a thickness of approximately 400 pm. Each reinforcing ring 8a-c has a width of approximately 40 pm and a thickness of approximately 400 pm. Each actuator element 7a-d, has a width of approximately 600 pm, each portion of the actuator elements 5a-d, 6a-d having a width of approximately 300 pm. These dimensions are illustrated more clearly in Figure 5. The above-described structure; in particular, the arrangement of the inner and outer portions 5a-d, 6a-d relative to the reinforcing rings 8a-c; helps to allow long-stroke piston motion of the actuating device 1 without buckling. With this structure, the Applicant has measured total stroke lengths of up to 70 pm.

The way in which this particular arrangement helps to achieve this long-stroke piston motion of the moveable element 4 will be described below.

To achieve the desired movement of the moveable element 4, specific piezoelectric portions must receive a voltage to actuate them. These specific portions and their resulting deflection will be described below in Figures 6 to 9B. In the drawings, the portions having a voltage applied thereto are indicated by the addition of '+’ signs on those portions. For the purposes of the embodiment presented in Figures 1-9B, the polarity of the applied voltage is such that the piezoelectric layer contracts radially. However, as the skilled person would appreciate, a voltage of the opposite polarity may have the inverse effect, i.e. the layer would expand.

A close-up view of one of the reinforcing rings 8a is shown in Figure 6. As can be seen in Figure 6, the reinforcing ring 8a is located on the second side 2b of the actuating device opposite to the first side 2a, opposite a location on the first side 2a between the first actuator element 7a and the second actuator element 7b. The first actuator element 7a is shown to have two distinct piezoelectric areas providing the inner and outer actuator portions 5a, 6a having a small gap therebetween. The second actuator element 7b similarly has two distinct piezoelectric areas providing the inner and outer actuator portions 5b, 6b having a small gap therebetween.

Figure 7A shows the actuating device T in a first mode of operation, the first mode being denoted by a single prime (‘), where a voltage is applied to the outer portion 6a’-d’ of each of the actuator elements 7a’-7d’. T urning to Fig. 8A the resulting vertical deflection of the actuator elements 7a’-d’ and piston motion of the moveable element 4’ can be seen.

As will be appreciated by the skilled person, the voltage applied to the outer portions 6a’-6d’ causes the piezoelectric material of the outer portions 6a’-6d’ to contract in the radial direction. The width to thickness ratio of the actuator elements 7a-d means that they have some flexibility which allows strain on the piezoelectric layer to be transferred along the width of each actuator element. Thus, the contraction of the piezoelectric layer of the outer portions 6a’-6d’ causes the actuator element to curve so that the inner edge of each of the inner portions 5a’- 5d’ of each actuator element 7a’-d’ lifts vertically, i.e. translating each reinforcing ring 8a-c in the positive z-direction. The substrate (not shown) at the periphery of the actuating device 1 and the reinforcing rings 8a-c provide a stiff anchor for the outermost portions 6a’-6d’ to deform relative to.

Figure 8A shows, with an upwards arrow, that the moveable element 4’ is displaced normal to the x-y plane in the positive z-direction (i.e. the x-y plane being the plane of the moveable element 4’ at rest).

Figure 7B shows the actuating device 1” in a second mode of operation, the second mode being denoted by a double prime (“), where a voltage is applied to the inner portion 5a”-5d” of each of the actuator elements 7a”-7d”. Again, alternate piezoelectric portions have a voltage applied thereto. Figure 8B shows, with a downwards arrow, the resulting vertical deflection of the actuator elements 7a”-d” and ‘piston’ motion of the moveable element 4” in the second mode.

The voltage applied to the inner portions 5a”-5d” causes the piezoelectric material of these portions to contract in the radial direction. The flexibility of the actuator elements 7a”-d” allows the strain to be transferred across the width, so that the contraction of the piezoelectric layer of the outer portions 6a’-6d’ causes the actuator element to curve. The contraction causes the inner edges of each of the inner portions 5a’-5d’ of each actuator element 7a’-d’ to lower vertically, i.e. translating each reinforcing ring 8a-c in the negative z-direction. The moveable element 4” and the reinforcing rings 8a-c provide stiff anchors for the innermost portions 5a”-5d” to deform relative to. The moveable element 4” is, thus, displaced normal to the x-y plane in the negative z-direction (i.e. the x-y plane being the plane of the moveable element 4” at rest). The second mode thus results in the moveable element 4” moving in the opposite direction to the movement associated with the first mode. Figures 9A and 9B shows how the von Mises stress (N/m²) varies across the actuating device in the first mode and the second mode.

Figure 9A shows the von Mises stress on the actuating device T operating in the first mode, where the moveable element 4’ is deflected in the positive z-direction. Figure 9B shows the von Mises stress on the actuating device 1” operating in the second mode, where the moveable element 4’ is deflected in the negative z- direction. For both modes, the von Mises stress is between zero and 0.5 x 10⁸ N/m² at the inactive portions where a voltage is not being applied and approximately 1.5 x 10⁸ N/m² at the active portions where the voltage is applied. Thus, the von Mises stress is greatest where the piezoelectric layer is receiving a voltage. The presence of the reinforcing rings between each of the actuator elements limits the stress from becoming too high and thus reduces the risk of buckling.

Figure 10 shows an actuating device 10 according to a second embodiment of the invention. The structure of this device 10 is similar to the arrangement described above as there are four actuator elements 20a-d and three reinforcing rings (not shown) and a moveable element 40 in the centre of the actuating device 10. The actuating device 10 also has a substrate 21 to which the outer actuator element 20d is attached.

In this embodiment, each actuator element 20a-d has an inner ring of actuator portions and an outer ring of actuator portions, each segmented into four individual portions. Therefore, there are eight separate actuator portions, meaning that each portion has a separate piezoelectric area. For example, the innermost actuator element 20a has four inner actuator portions 12a, 13a, 14a, 15a and four outer actuator portions 16a, 17a, 18a, 19a. It can be seen from Figure 10 that the actuator portions are arranged in quadrants. Each of the piezoelectric portions 12a- 19a has an annular sector shape subtending an angle of 90°. These portions 12a- 19a are independently addressable which means that a voltage can be selectively applied to any one of their piezoelectric layers. By actuating a subset of the actuator portions a ‘tip-tilt’ motion can be achieved. The ‘piston’ motion described above can be achieved by applying a voltage to all inner portions, e.g. 12a-15a, of each actuator element 20a-d or all four outer portions, e.g. 16a-19a, of each actuator element 20a-d.

Tilting motion can be achieved by applying a voltage to one or more inner portions, e.g. 12a in one quadrant or half of the actuating device, and applying a voltage to the corresponding outer portions, e.g. 19a, in a quadrant or half of the actuating device on the side of the moveable element 40 diametrically opposite to the other quadrant or side.

Figure 11 shows a schematic diagram of the different piezoelectric portions that can be independently addressed by control electronics to actuate the actuating device of Figure 10. In the actuating device depicted in Figures 10 and 11 there are eight actuator portions per actuator element, e.g. 12a-19a, and there are four actuator elements in total. Thus for the actuating device 10 of Figures 10 and 11 , each actuator element has a North West Inner (NW-I), North West Outer (NW-0), North East Inner (NE-I), North East Outer (NE-O), South East Inner (SE-I), South West Inner (SW-I) and South West Outer (SW-0) actuator portion. For the four actuator elements, this amounts to a total of 32 piezoelectric actuator portions that can be independently actuated which allows both piston and tilting motion to be achieved. The different portions are indicated by different shading.

When applying a voltage to actuate the device, the control electronics may provide a voltage to all the piezoelectric portions having the same label - e.g. one or more of NW-I, NW-0, NE-I, NE-O, SE-I, SW-I and SW-0 - thus actuating a specific subset of piezoelectric portions. This may help to achieve large deflection angles and deflection heights.

Although Figures 10 and 11 show eight piezoelectric portions per actuator element, this is just one of a number of possibilities.

Figure 12 shows a photograph and Figure 15 shows a corresponding CAD drawing of an actuating device 100 according to a third embodiment of the invention . The actuating device 100 according to the third embodiment has six piezoelectric portions per actuator element 107a-d, each subtending an angle of approximately 120° surrounding a deformable moveable element 400. For example, the innermost actuator element 107a has three inner piezoelectric portions 101a, 102a, 103a and three outer piezoelectric portions 104a, 105a, 106a. The actuating device 100 also has a substrate 121 to which the outer actuator element 107d is attached. The deformable moveable element 400 has an annular independently addressable piezoelectric portion 141 which is annular in shape.

Similarly to Figure 11 , Figure 13 shows a schematic diagram of the different piezoelectric portions that can be independently addressed by control electronics to actuate the actuating device of Figure 12, i.e. according to the third embodiment. In the actuating device depicted in Figures 12 and 13 there are six piezoelectric portions per actuator element, e.g. 101a-106a, and there are four actuator elements in total. Thus for the actuating device 100 of Figures 12 and 13, each actuator element, has a 30 ° Inner (30 ° I), 30 ° Outer (30 ° O), 150 ° Inner (150 ° I), 150 ° Outer (150 ° O), 2700 ° Inner (270 ° I) and 270 ° outer (270 ° O) actuator portion. For the four actuator elements, this amounts to a total of 24 piezoelectric actuator portions that can be independently actuated which allows both piston and tilting motion to be achieved. The different portions are indicated by different shading.

Although the deformable mirror is only depicted in combination with the third embodiment, it should be appreciated that it could be combined with any of the other embodiments of the invention. Figures 14A and 14B show how the deformable mirror 400 may be implemented with one or two piezoelectric actuator portions. Figure 14A shows a deformable element 400 which has only one annular piezoelectric element 141 around the edge of the moveable element 400, providing defocusing functionality only. Figure 14B shows a deformable element 400 having a circular inner piezoelectric portion 140 and an annular outer piezoelectric portion 141. Having both independently addressable portions 140,141 may allow the deformable element 400 to deform in both directions, thus providing both focusing and defocusing functionality.

Figure 16 shows an actuating device 110 according to a fourth embodiment of the invention. The structure of this device 110 is similar to the first embodiment as there are four actuator elements 20a-d, each having an inner and outer piezoelectric actuator portion, three reinforcing rings (not visible) and a moveable element 440 in the centre of the actuating device 110. The actuating device 110 also has a substrate 221 to which the outer actuator element 207d is attached. Also shown in Figure 16 are control electronics, i.e. control lines 250, for applying a voltage to each of the piezoelectric portions from a power supply (not shown). These piezoelectric portions - 11, 01 , I2, 02, I3, 03, 14, 04 - are shown in Figure 17.

The actuating devices 1, 10, 100, 110 described herein may be manufactured by lithography or other etching methods. For example, the actuating device 1, 10,100, 110 may have a device layer 3 bonded to a bulk layer. Piezoelectric (e.g. PZT) material may be deposited on the device layer 3 to provide a distinct piezoelectric layer for each piezoelectric portion. Metal electrodes - e.g. Ag and Pt electrodes - may be provided above and below the piezoelectric layers for applying a voltage thereto.

During manufacture, portions of the bulk layer may be etched away to form the reinforcing rings 8a-c, on the second side 2b of the actuating device 1, 10, 100, 110 opposite a location on the first side 2a between neighbouring actuator elements.

Figure 18 illustrates steps A to F of a process of fabricating the actuating device 1 , 10, 100, 110. The diagrams shown in Figure 18 depict a cross section of one half of the actuating device 10, 100, 110.

The process starts with step A, where a 400 pm Silicon on Insulator (SOI) wafer is provided as a starting material having a 500 nm BOX (buried oxide) layer 201 sandwiched between an 8 pm device layer 202 and a 400 pm bulk (e.g. Si) layer 200.

Step B is an oxidation step where 1.6 pm SiO2 layers 203, 204 are formed both below the bulk layer 200 and above the device layer 202.

This is followed by the stack deposition step, i.e. step C. A PZT stack is deposited on the SiO2 layer 203 that is adjacent to the device layer 202. The PZT stack is made of a 2 pm PZT layer 206 sandwiched between a top electrode 207 and a bottom electrode 205. In this case, the top electrode 207 is a 250 nm layer of gold and the bottom electrode 205 is a 100 nm layer of platinum. Step D follows, where the structuring and etching of the PZT 206 and electrodes 205, 207 forms the individual piezoelectric portions. Here, cavities 208a-h are formed in the PZT stack forming distinct piezoelectric areas. Then, in step E, a moveable surface, e.g. a mirror, is defined by etching away the top layer of SiC>2 and the device layer 202 to form a central cavity where the moveable surface is located.

Lastly, step F involves etching away the bulk layer 200 and bottom SiC>2 layer 204 to form a plurality of reinforcing rings 8a,b,c between cavities 210a-d formed by etching.

It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.

Claims

1. An actuating device comprising: a first actuator element and a second actuator element each comprising a piezoelectric layer on a first side of the actuating device, wherein each of the first and second actuator elements has a respective width at least five times a respective thickness thereof; a moveable element, connected to at least the first actuator element, such that actuation of the first actuator element causes movement of the moveable element; and a reinforcing ring, on a second side of the actuating device opposite to the first side, opposite a location on the first side between the first actuator element and the second actuator element.

2. The actuating device of claim 1 , wherein the actuator elements are arranged such that actuation of one or both of the actuator elements causes the moveable element to be translated vertically

3. The actuating device of claims 1 or 2, wherein the actuator elements each comprise a ring shape.

4. The actuating device of any preceding claim, wherein each actuator element comprises a plurality of actuator portions.

5. The actuating device of claim 4, wherein at least some of the actuator portions are independently addressable with respective voltages.

6. The actuating device of claim 5, wherein each actuator portion is independently addressable with a respective voltage.

7. The actuating device of any preceding claim, wherein the actuating device comprises N reinforcing rings and N+1 actuator elements.

8. The actuating device of any preceding claim, wherein the actuating device is arranged to move the moveable element with three degrees of freedom.

9. The actuating device of any preceding claim, wherein the reinforcing ring protrudes away from the second side of the actuating device and has a thickness in the direction of the protrusion of between 100 pm and 1 mm.

10. The actuating device of any preceding claim, wherein the moveable element has a mass per unit area greater than that of the first actuator element and the second actuator element.

11. The actuating device of any preceding claim, wherein the moveable element comprises a plurality of individually addressable piezoelectric sections.

12. The actuating device of any preceding claim, wherein the moveable element has an optically reflective surface.

13. The actuating device of any preceding claim, wherein the actuating device comprises a silicon on insulator (SOI) wafer.

14. A method of manufacturing an actuating device comprising: providing a device layer and a bulk layer; depositing piezoelectric material on the device layer to form a first actuator element and a second actuator element on a first side of the actuating device; wherein the bulk layer is on a second side of the actuating device opposite to the first side; characterised by: etching away a portion of the bulk layer to form a reinforcing ring, on the second side of the actuating device, opposite a location on the first side between the first actuator element and the second actuator element.