Deformable mirrors

The invention resides in the innovative use of deposition techniques in deformable mirrors. The use of deposition techniques allows contact pads for electrodes to be located wherever convenient. When applied to bimorph mirrors the invention enables both electro-restrictive layers to become active, increasing the stroke of the deformable mirror whilst retaining temperature insensitivity. A controllably-deformable mirror according to the invention comprises a deposition layer; a first electro-restrictive plate; a mirror surface; and a plurality of electrodes comprising a set of electrodes defined as regions on a first surface of the first electro-restrictive plate, the set of electrodes comprising a first electrode and a second electrode; and a common electrode on a second surface of the first electro-restrictive plate.

This invention relates to an improved deformable-mirror, and more particularly, though not exclusively, to a deformable mirror comprising a passivation layer.

Deformable mirrors are often used in the field of adaptive optics. For example, phase distortions in a signal may be sensed by a wavefront sensor and these distortions may be corrected for by an adaptive mirror. Such adaptive mirrors may be employed in numerous fields, including:imaging, for example adaptive mirrors are used in astronomy to improve the resolution of earth-based telescopes that are otherwise affected by atmospheric distortions;laser sensing, where the amount of laser light that can be delivered onto a target is significantly increased by using an adaptive mirror to correct for atmospheric distortions—this enables either better information to be obtained or objects to be identified at a greater range; andlaser generation, where an adaptive mirror can be used intracavity within a high power laser to counter the thermal blooming that can be otherwise induced by the high concentration of laser light inside the cavity.

The two main operational parameters of a deformable mirror are its bandwidth and its stroke. Bandwidth determines how quickly the mirror can be deformed and hence, for example, how quickly the mirror can respond to the variations in atmospheric turbulence. Stroke corresponds to the maximum displacement of the mirror when deformed and this determines, for example, the level of turbulence that can be corrected. Ideally, both bandwidth and stroke would be maximised.

One known type of deformable mirror comprises a layer of an active electro-restrictive material glued to a passive mirror substrate layer. The electro-restrictive material can be a piezo-electric material such as PZT, a ceramic material comprised of lead, zinc, titanium and oxygen. A possible alternative is PMN, an electro-restrictive material comprised of lead, magnesium, niobium and oxygen. On application of an electric field, the electro-restrictive material deforms. This deformation can be used to deform the mirror in a controllable manner. By placing an array of electrodes on one surface of the active layer, and a continuous electrode on its other surface, the applied electric field can be varied such that a degree of control can be exerted over the mirror deformation.

Since the passive substrate and the electro-restrictive layer are made from different materials, their response to changes in the ambient temperature will differ, creating an additional bending movement that must be compensated for by the mirror. This reduces the effective stroke of the mirror.

A possible solution to this problem is to use a second piece of passive electro-restrictive material as the mirror substrate. This avoids the above problem since both the passive substrate and the active layer respond in the same way to a change in ambient temperature. However, standard, readily available forms of electro-restrictive materials such as PZT are ceramic, and cannot easily be polished to a mirror finish since they are too granular. Only some advanced and expensive forms of PZT ceramic can be polished directly. In WO 02/059674, M J Northcott and J E Graves disclose a method for forming a mirror surface on a passive PZT substrate by epoxy replication, wherein the mirror layer is first formed separately and then adhered to the surface of the passive PZT layer. To ensure that the two pieces are as near identical as possible, thus minimising the thermal sensitivity of the mirror, electrode layers are formed on both the passive and the active PZT layers. It is notable that there remains a passive PZT layer.

A number of problems exist with the prior art mirror described above. Firstly, since epoxy is used to attach the mirror to the deforming structure, the deformable mirror is not suitable for high-power applications: the local high temperatures created in the mirror when it is subjected to high power radiation would irreparably damage the epoxy. Even for low-power applications, the epoxy bond is weak and can result in a short working life for the deformable mirror. However, the epoxy cannot simply be removed since a mirror surface cannot be applied directly to the PZT plate. Secondly, the stroke is not at a maximum since one layer of PZT must remain passive. The method of manufacture does not allow for a plurality of electrodes to be placed on all surfaces of the PZT, and so an electric field cannot be applied independently across each PZT layer. Furthermore, the array of electrodes must be soldered to individually, with a wire at the back of the mirror for each electrode. This method of connection is inconvenient if the mirror is to be used in multiple driver systems or mounts, or if it is to be used in a variety of applications, since the soldering process can be a very delicate task when there are a large number of electrodes. The process is also time consuming: it may take several days for the soldering to be completed if there are a large number of electrodes.

Against this background, one object of the invention is to substantially reduce or overcome some of the above-mentioned drawbacks. It is a further object of the invention to provide a low-cost deformable mirror that can be easily adapted for use in any application or driver system. Another object of the present invention is to provide a temperature-insensitive deformable mirror with a greater stroke than prior-known mirrors.

In broad terms, the present invention resides in the innovative use of passivation techniques in deformable mirrors. The use of passivation allows contacts for the electrodes to be located wherever convenient. When applied to bimorph mirrors the invention enables both electro-restrictive layers to become active, increasing the stroke of the deformable mirror whilst retaining the advantageous temperature insensitivity of the prior-known mirror of WO 02/059674.

According to a first aspect of the invention, there is provided a controllably-deformable mirror comprising a first electro-restrictive plate; a mirror surface; a plurality of electrodes comprising a set of electrodes defined as regions on a first surface of the first electro-restrictive plate, the set of electrodes comprising a first electrode and a second electrode; a common electrode on a second surface of the first electro-restrictive plate; and a passivation layer adapted to mutually insulate the set of electrodes. The passivation layer allows a bias voltage to be applied to each electrode in the set of electrodes independently by insulating each electrode from the other electrodes, and from any conducting tracks that may run across it.

Advantageously, the deposition layer confers a large degree of flexibility on the mirror design. Conveniently, the controllably-deformable mirror further comprises a set of contacts removed from the electrodes and positioned such that an external mirror-driver operable to apply voltages to the electrodes can be interchangeably electrically connected to the contacts, the set of contacts including a first contact electrically connected to the first electrode and a second contact electrically connected to the second electrode. Contacts to the electrodes can be made wherever may be convenient for the particular application of the mirror, and can be adapted to suit any preferred method of support or mirror-holder. For example, if the mirror is to be supported at its periphery, contact to the electrodes can be made from below. Alternatively, if the mirror is to be supported from below, contact to the electrodes can be made at the periphery of the mirror. There may be a set of conducting tracks to electrically connect the set of electrodes to the set of contacts, the set of conducting tracks being formed such that the passivation layer is interposed between the set of conducting tracks and the set of electrodes. This leads to further flexibility in the mirror design, since this layer allows the conducting tracks to be run across the set of electrodes. Thus, in contrast to the prior art mirror of WO 02/059674, embodiments of the present invention provide a compact deformable mirror with no need for trailing wires leading to each electrode.

Conveniently, the passivation layer and the set of conducting tracks are provided by a flexi-circuit. The flexi-circuit may comprise a first portion shaped to correspond with the shape of the first electro-restrictive plate and a second portion extending from the mirror to provide the set of contacts, the first portion being adapted to provide the passivation layer. The flexi-circuit may comprise a set of conducting copper tracks embedded in a polyimide substrate. The polyimide substrate is non-conducting, and thus, in this embodiment, the passivation layer may comprise polyimide. The flexi-circuit may be glued, or otherwise bonded, to the first surface of the first electro-restrictive plate, resulting in a particularly quick and simple construction method, since there is no need to provide an additional interconnect in order to operate the deformable mirror with an external mirror driver, and the steps of forming the passivation layer and forming the conducting tracks are both accomplished by the single step of bonding the passivation layer to the electro-restrictive plate.

There may be a second electro-restrictive plate attached to the first electro-restrictive plate such that the passivation layer is interposed between the first and second electro-restrictive plates. Advantageously, electrical connections can then be made to electrodes on both electro-restrictive plates, so that both electro-restrictive plates can be independently and actively deformed.

The passivation layer may comprise a deposition layer. Whilst the formation of a deposition layer is not as quick or as simple a process as the bonding of a flexi-circuit to the first electro-restrictive plate, the use of deposition techniques enables materials that better match the thermal properties of the mirror to be used for the passivation layer, and therefore leads to a more temperature insensitive mirror. Conveniently, the contacts can then be arranged in a peripheral region of the mirror at or near the circumference of the first electro-restrictive plate.

Optionally, the deposition layer is on the first surface of the first electro-restrictive plate. Optionally, the mirror surface is on the first surface of the first electro-restrictive plate. Conveniently, the deposition layer is interposed between the mirror surface and the first surface of the electro-restrictive plate. Where the invention is embodied in a bimorph deformable mirror, this arrangement allows both layers of electro-restrictive material to be independently active, since in prior known mirrors, such as that disclosed in WO 02/059674, it was not possible to incorporate a set of electrodes between the mirror surface and the upper electro-restrictive plate. Advantageously, this enhances the stroke of the mirror over the prior-art mirror. Alternatively, the mirror may further comprise a second electro-restrictive plate, and the deposition layer may be interposed between the first and second electro-restrictive plates.

Preferably, the deformable mirror also comprises a planarization layer on which the mirror surface is formed. The planarization layer obviates the need for prior-known techniques, such as epoxy replication, to be used before applying the mirror surface. Preferably, the planarization layer is formed by deposition techniques. The planarization layer may comprise any one of silicon dioxide and silicon nitride. Such materials are better able to deal with the conditions generated by exposure to high power radiation than epoxy. Thus the mirror is more versatile than those in which epoxy replication is used. Advantageously, the planarization layer smoothes out deviations from optical flatness such that a mirror surface can be formed. The planarization layer may be deposited above the passivation layer, or may be deposited directly onto the electro-restrictive plate. In the case of a unimorph mirror, the planarization layer may be used to form a passive substrate for the mirror.

Optionally, there may be a deposition layer formed symmetrically on the outer surfaces of both the first and second electro-restrictive plates. Advantageously, this balances the stresses caused by the formation of the deposition layers, and moreover allows the structure to be parallel polished, further enhancing the flatness of the as-made mirror.

In accordance with an exemplary embodiment of the invention, the passivation layer comprises silicon dioxide, and is between 16 μm and 20 μm thick, such that it is able to hold off a potential difference across it in the range of 800V to 1000V. In such an exemplary embodiment, the maximum bias voltage that can be safely applied to any one electrode is in the range of 400V to 500V, since a neighbouring electrode, or conducting track running across the electrode, may be at an equal bias voltage applied with opposite polarity (leading to a potential difference across the passivation layer in the range 800V to 1000V). The thickness of the planarization layer may lie in the range 5 μm to 30 μm, so as to be able to smooth out deviations from flatness in the surface to which it is applied. When the planarization layer is to be used as a passive substrate for a unimorph mirror, its thickness will be dependent on its mechanical properties. In such cases the planarization layer will be thicker so as to match the properties of the active layer.

Optionally, the mirror further comprises a flexi-circuit to electrically connect the contacts to a mirror driver. In this case, the flexi-circuit does not provide the passivation layer, but is used simply to provide a convenient interconnect between the contacts and an external mirror driver. Other forms of interconnect, such as rigid printed circuit board (PCB) interconnects, and wire bonders, may also be used to connect the mirror to an external mirror driver.

Optionally, the mirror is mounted in a deformable-mirror holder. The holder may comprise a support structure adapted to support the mirror from below. Advantageously, contact pads in the peripheral regions of the mirror are then easily accessible, thus facilitating electrical connections to external controls.

One of the disadvantages that unimorph or bimorph deformable mirrors in particular suffer from is the hysteresis effect associated with piezo-ceramics such as PZT.

Hysteresis in PZT arises from crystalline polarization and molecular effects. The absolute expansion of a PZT layer depends not only upon the voltage applied across the layer, but also on remnant polarization and therefore the recent history of the polarisation state, e.g. whether the PZT was previously energised by a higher or a lower field strength (and some other factors). Hysteresis is typically of the order of 10% to 15% of the commanded deformation. With hysteresis at such levels, use of unimorph or bimorph deformable mirrors has been limited to closed loop adaptive systems in which, due to continuous updating of the effects of applied deformations, hysteresis compensation is not necessarily required.

In preferred embodiments of the present invention, a controllably-deformable mirror is provided, further comprising a plurality of (preferably resistive) strain gauges provided in association with a layer of the mirror to provide information relating to deformation of the mirror. In particular, the strain gauges may be provided in regions of the mirror associated with electrodes.

Preferably, the strain gauges may be arranged as substantially mutually orthogonal pairs, or strain gauges may be provided in the form of a double spiral. However, in each case, the strain gauges are preferably deposited on or within a passivation layer of the mirror, wherever provided.

With strain gauges being used to accurately measure the bending strain resulting from the energising of particular electrodes, unimorph and bimorph deformable mirrors according to preferred embodiments of the present invention may be used in open loop adaptive systems.

According to a second aspect of the invention, there is provided a method of manufacturing a controllably-deformable mirror having an electro-restrictive plate, comprising the steps of:

(a) defining a set of electrodes, comprising a first electrode and a second electrode on a first surface of the electro-restrictive plate;

(b) depositing a passivation layer; and

(c) applying a mirror surface.

Advantageously, the step of depositing a passivation layer uses deposition techniques, a technology well-established in other fields. This innovative use of known technology leads to a cheap and effective manufacturing process. Optionally, the method further comprises the step of depositing a planarization layer. The thickness of the as-deposited planarization layer may be in the range of 21 μm to 30 μm. Preferably, the step of applying a mirror surface to the layer comprises the steps of grinding and polishing the planarization layer to optical flatness.

In the following description, the same reference numerals as used in different Figures are used to designate same/like parts.

FIG. 1shows a prior art mirror14described in WO 02/059674. The mirror comprises two PZT discs30and32, continuous electrodes34,36and42, segmented electrodes40-N and mirror surface48. These component layers are bonded together using epoxy46and38. The PZT disc32closest to the mirror surface remains passive in this case because electrodes36and42are both continuous and an electric field cannot therefore be controllably applied to this disc. Instead, disc32is a passive substrate against which active disc30is able to create a deforming force. Disc32simply ensures thermal insensitivity of the mirror, since discs32and30expand in the same manner under a changing temperature. Similarly, electrode42only serves to ensure symmetry of the mirror, thereby further reducing the temperature sensitivity of the mirror14. A number of wires W-1to W-N extend from the back of the mirror to enable electrical connection to the mirror.

In contrast to the prior art mirror14,FIG. 2shows a cross-sectional view of a controllably-deformable mirror2according to a first embodiment of the present invention. The mirror2comprises two discs of electro-restrictive material21and22that are bonded together with epoxy23with continuous electrodes24and25running between the two discs21and22. The two discs21and22of electro-restrictive material can be, for example, PZT. It is to be noted that there are two sets of segmented electrodes26and27on the mirror, with one set26beneath the PZT discs, and one set27between the PZT discs and the mirror surface56. The individual electrodes in the sets are labelled26-1,26-2and26-3, and27-1,27-2and27-3. It is to be noted that only three electrodes are shown for clarity. In fact, there may be any number of electrodes in each set. External drivers (not shown) are used to apply a voltage bias to the electrodes so as to deform the PZT discs21and22. The configuration of the sets of electrodes26and27will change according to the application of the mirror as is well known in the art. Furthermore, both PZT discs21and22can be used to actively deform the mirror2, enhancing the stroke of the mirror2over the prior art mirror14, shown inFIG. 1, whilst retaining the temperature stability gained from the use of the two PZT discs. This is achieved through use of a deposition layer, indicated generally by20, as will be described in more detail hereinafter.

The PZT discs21and22are poled parallel such that an electric field applied in the same direction to each disc will cause the discs to deform in the same way. This is indicated schematically by arrows28and29. In the embodiment shown, with the common electrodes24and25between the two PZT discs21and22, the electric field is applied to the discs in opposite directions, since the two common electrodes will be held at the same potential by the mirror driver. Thus when one disc expands, the other contracts. By applying a field in a controlled manner to each of the segmented electrodes in sets26and27, the mirror2can be made to deform as desired, through techniques that are well known in the art.

Alternatively, the PZT discs can be poled in opposite directions (not shown). In this case, an electric field is applied to only one PZT disc at a time, and thus whilst one disc is actively deformed by the application of electric field, the other remains passive. In this way, a simpler drive scheme is possible, in which field is applied to one disc or the other, depending on the direction of movement required. Furthermore, the maximum safe field Vsthat can be applied to a PZT disc depends on the direction of the field relative to the poling direction of the disc. Since a field up to five times greater (5 Vs) can be safely applied along the poling direction than against it, the stroke of the mirror is not adversely affected by the use of such a simpler driving scheme. This will still result in a uniformly bipolar response.

The method of manufacture of the embodiment shown inFIG. 2will now be described such that the structure of mirror2may be better understood.FIG. 3shows a first stage in the manufacture of mirror2. Two identical discs of PZT21and22are bonded together with epoxy23such that they are poled parallel as indicated by arrows28and29. Both include a wrap-around electrode (not shown) to enable electrical contact to the middle electrodes24and25. Sets of segmented electrodes27and26on the top and bottom of the mirror are defined as regions on the surface of the PZT discs, such that local deformations can be effected.

FIG. 4shows a second stage in the manufacture of mirror2. A passivation layer51has been has been applied to the upper set of segmented electrodes27. The passivation layer51mutually insulates the electrodes27-1,27-2and27-3and enables a voltage bias to be applied to each electrode independently. The passivation layer51may be silicon dioxide, polyimide, or any suitable insulating material. It can be applied by a number of deposition techniques to be described hereinafter. Vias55have been etched in the passivation layer51to allow access to the electrodes in set27.

FIG. 5shows a third stage in the manufacture of the mirror2. Metallisation has been put down to run conducting tracks52and53to form contacts57and58at the rim of the PZT disc21. Contacts57and58take the form of contact pads. For simplicity, only two tracks are shown: in fact, each electrode is connected to a contact on the rim of the mirror, and, as will be understood by the skilled reader, there may be many electrodes. The conducting tracks57and58make contact with the electrodes27at the vias55. Note that track53runs above PZT electrode27-3, but is stopped from making contact by the passivation layer51. The thickness of the passivation layer51must therefore be sufficient to insulate, for example, an electrode27-3at +Vmfrom a track53at −Vm, where Vmis the maximum bias voltage expected to be applied. Typically a maximum voltage of 400-500V is applied to the electrodes. Given the breakdown voltage of silicon dioxide of approximately 50V/μm, the necessary thickness of the passivation layer is 16-20 μm. It is possible for higher voltages to be applied to the electrodes, in which case the passivation layer51would need to be thicker. However, 400-500V is the maximum voltage that can currently be effectively applied using cost efficient drivers. The thickness of the passivation layer51will also vary depending on the material used. The metallisation can be put down, for example, by evaporation through a mask.

FIG. 6shows a fourth stage in the manufacture of the mirror2in which a planarization layer54has been added. Layer54is applied so that any deviations from planarity created by the passivation layer51and the conducting tracks52and53can be removed by grinding and polishing. The passivation layer51is uneven since vias55are etched through it to enable access to the electrodes27. The planarization layer54must therefore be at least as thick as the passivation layer51, and preferably slightly thicker, so that there is a small amount of planarization material remaining after grinding and polishing. For a passivation layer51of thickness 16 μm to 20 μm, therefore, the thickness of the as-deposited planarization layer54is in the range 21 μm to 30 μm. After grinding and polishing, therefore, in the regions above the passivation layer51, the remaining thickness of planarization material will vary from 5 μm to 10 μm. Above the vias55, there will be a greater thickness of planarization material remaining. Suitable materials must be hard and able to be polished to optical flatness. Examples include silicon dioxide, silicon nitride and chemical vapour deposition silicon carbide (CVD SiC). The planarization layer54is applied using the deposition techniques described below.

Once the planarization layer54has been ground and polished, the mirror surface56can be applied. The specific material used for the mirror surface56will depend on the wavelength of the radiation to be reflected. The result is the embodiment shown inFIG. 2.

As described above, the planarization procedure is only carried out on the mirrored piece of PZT21. This is adequate if it can be assumed that the deposition layer20will not affect bending or temperature sensitivity of the mirror2. If more robust temperature invariance is required, the procedure can be carried out on both PZT discs. Applying the procedure symmetrically has the additional benefits that any stress built up during the application of the deposition layers will be equalised, and that the structure can be parallel polished, further improving the flatness of the as-made mirror2.

The use of the deposition techniques enables contacts, such as pad57, to be formed on the mirror2wherever may be convenient. Location of the contacts can easily be altered: the use of the passivation layer51allows conducting tracks, such as tracks52and53, to be run across the electrodes27to any location. The contacts can therefore be placed according to the mounting of the mirror2, or the application for the mirror2. A particularly convenient position for the contacts is the peripheral region of the mirror. The contacts may be equally spaced around the circumference of the mirror, or could be grouped as may be convenient.

Contact can be achieved by using a wire bonder, as described in the Applicant's pending UK Patent Application No. 0412851.8. The interconnection PCB could either be a rigid PCB with solder pins or it could be a flexi-circuit. A possible shape for the flexi-circuit7is shown inFIG. 7. The flexi circuit is shaped as an annulus70from which two cables74and75extend. The flexi-circuit7has an array of bond pads71on the inside of the annulus70which map one-to-one with the array of contact pads (labelled57inFIG. 5) on the periphery of the bimorph mirror2. At the other end, the flexi-circuit7has contacts in the form of bond pad arrays72and73designed to plug into a connector or similar standard arrangement. The flexi-circuit7shown has two flexible circuits emanating from the annulus, but its shape could differ depending on the desired use of the mirror2.

For embodiments in which the bimorph mirror is to be mounted by clamping to the base structure, the clamp mechanism can also contain a pressure connector arrangement which clamps the flexi-circuit onto the array of contacts around the periphery of the mirror. Advantageously, this removes the need to make individual connections to each electrode.

Alternatively, a surface mount connector could be soldered directly to the contact pads at the periphery of the PZT disc. This could connect to a flex-cable or flexi-circuit to achieve the interconnect. A PCB edge connector is designed to inter-connect a mother and daughter board, typically at right angles. The connector clamps onto an array of pads lined up at the edge of the board. A similar style connector with a flexi-circuit and flex cable added could be used to clamp onto the pads at the periphery of the PZT disc. Some types of flexi-circuit and flex-cable are designed to be soldered directly down onto an array of bond pads. If this technique is used, no clamping arrangement is necessary, but the process is likely to require special ‘hot bar’ tooling. The use of flexi-circuits, rather than, for example, ribbon cable, minimises the loading added by the interconnect.

FIG. 8shows an alternative flexi-circuit8shaped to make contact with each electrode on a PZT disc that has not been planarized. Flexi-circuit8has finger-like portions81designed to make contact to the electrodes individually. This arrangement could be used, for example, for connecting to the set of electrodes26on the underside of the mirror2shown inFIG. 2.

FIG. 9shows a mirror9according to a second embodiment of the invention in which the mirror surface54is formed onto a continuous electrode91. In this case, it is only necessary to deposit a single deposition layer20, which can then be polished to create an optically flat surface. In this embodiment, the mirror9is a true bimorph, in which two PZT discs poled in opposite directions are bonded together and the field is applied across both such that one will expand and one will contract, as indicated by arrows28and29. Similarly, only a single deposition layer is needed when the mirror surface is to be formed onto a rough surface.

FIG. 10shows a mirror10according to a third embodiment of the invention in which a deposition layer20is used as a passive substrate for a unimorph mirror with only one PZT disc21. This is possible where the unimorph mirror10is small and the mirror thickness is less than 1 mm thick. For larger mirrors, the time taken to deposit layer20is impractical and it becomes progressively difficult to deposit layers sufficiently thick that are of sufficient quality.

FIG. 11shows a fourth embodiment of the invention in which a symmetric bimorph mirror11is created by bonding two thin passive discs111and112of a hard material such as glass, fused silicon, silicon or CVD silicon carbide to the top and bottom surfaces of a standard bimorph mirror in which two PZT discs are bonded directly together, poled in opposite directions (as indicated by arrows28and29), and the electric field applied across both discs. The mirror11has a set of electrodes26on the bottom surface of the structure, and a continuous electrode113on the top surface. Bonding is accomplished using, for example, epoxy114and115. The epoxy114,115can be applied by screen printing such that there is no epoxy directly beneath the vias55. The thickness of the layers111and112will vary in dependence on the diameter of the mirror but in most cases will be between 0.5 mm and 1 mm. The hard material must be able to take a mirror finish. The structure is then parallel polished, and a mirror surface56applied to the top coating. Vias55are then etched in the bottom surface, and metallisation is put down to run tracks52,53to contact pads around the outside of the mirror (not shown). The formation of vias55in the layer of hard material112can be accomplished, either before or after the layer is bonded to the standard bimorph, by etching or ion-beam milling. This embodiment is particularly insensitive to temperatures due to its symmetry. Furthermore, the use of parallel polishing enables the as-manufactured mirror to be substantially free from distortions.

In a further embodiment of the invention (not shown), the PZT plates are bonded together such that the common electrode for each plate is facing outwards, and the sets of electrodes are between the two plates. Electrical contact to the sets of electrodes can be established either by passivating and planarising the two sets of electrodes before the plates are bonded together, so that a set of peripheral contact pads can be used; or by using a flexi-circuit8(as shown inFIG. 9) sandwiched between the PZT plates. The flexi-circuit is three layered, with the outer layers arranged to make contact to the electrodes on the inner surfaces of the PZT plates, and the inner layer providing the tracking and interconnections to an external control driver. Two passive discs are then applied to the outer surfaces of the structure, as in the embodiment shown inFIG. 11, although without vias. The structure can then be parallel polished to create a symmetric bimorph deformable mirror. If a flexi-circuit8is used it will not have the same thermal expansion properties as the PZT plates. However, the symmetry of the overall structure, and the parallel polishing process, will reduce thermally induced distortions. There may be a reduction in sensitivity of the bimorph since the soft flexi-circuit does not efficiently couple the two PZT plates. However, the convenience, low cost and ease of manufacture of this embodiment may outweigh this potential problem.

FIG. 12shows a fifth embodiment of the invention in which a flexi-circuit201is used to provide the passivation layer. The flexi-circuit201comprises conducting copper tracks202set in a polyimide substrate203. As manufactured, flexi-circuit201is provided with conducting copper tracks202that terminate at vias204drilled through the plane of the flexi-circuit. To facilitate electrical connection, vias204are plated with a conducting material. The copper tracks202and vias204are formed in a predetermined pattern such that the flexi-circuit201can provide an electrical connection to each of the electrodes206on the underside of PZT plate61. In the embodiment shown inFIG. 12, a copper track is provided by the flexi-circuit201for each of the electrodes206on the underside of the PZT plate207, such that a copper track extends from each electrode to a contact that may conveniently be connected to an external mirror driver (not shown). The flexi-circuit201is bonded to the PZT plate61using a three-stage process. Firstly, it is held in place, at a fixed distance from the electrodes206, using spacers. Secondly, a glue-writer is used to inject a conductive epoxy into each via such that each electrode is connected to a desired conducting track in the flexi-circuit by conducting epoxy, as indicated at208. Thirdly, a low viscosity, non-conducting epoxy209is used to fill the remaining gaps between the flexi-circuit201and the PZT disc61, and to increase the mechanical strength of the bond between the PZT disc61and the flexi-circuit201.

As will be readily apparent to those skilled in the art, it is not necessary for deposition techniques to be used to form a separate passivation layer in the embodiment described above with reference toFIG. 12. Passivation is provided primarily by the polyimide component203of the flexi-circuit in which the conducting copper tracks202are embedded. Thus it will be understood that the layer of polyimide indicated generally at203inFIG. 12, between the conducting track202and electrode206provides the passivation layer in this embodiment. This results in a quicker and simpler manufacturing process, since the need for a deposition stage in the manufacture of the mirror is obviated. Furthermore, the flexi-circuit201provides a direct interconnect between the electrodes206on the PZT plate61and the external mirror driver (not shown), further simplifying the manufacture process.

FIGS. 13 to 18show embodiments of the invention in which the mirror is supported from below by a compliant support structure. Similar compliant support structures are described in the Applicant's co-pending UK Patent Application No. 0412851.8. In each embodiment, there is an electro-restrictive PZT plate61on which electrodes are formed, and a compliant disc60that supports the mirror structure.

FIG. 13shows a sixth embodiment in which a set of electrodes62is beneath the PZT plate61. A passivation layer is applied using deposition techniques as described above such that the compliant support60can support the mirror12from below, whilst contact pads63are located around the lower rim of the PZT disc61. Connection to the mirror12can then be made at its periphery. The mirror surface64is formed above a passive substrate65(that may be, for example, glass). There is a common electrode66between the passive substrate65and the PZT disc61. The passive substrate65slightly overlaps the PZT disc61.

FIG. 14shows a seventh embodiment of the invention that is similar to the embodiment shown inFIG. 12. Like parts in these two Figures are referenced alike, and are not described further. The seventh embodiment, shown inFIG. 14, includes a compliant support60to support the mirror structure as described above with reference toFIG. 13.

FIG. 15shows a mirror13according to an eighth embodiment that is similar to the sixth embodiment shown inFIG. 13. However, in the embodiment ofFIG. 15the PZT disc61is larger than the passive substrate65so that connections to the periphery of the mirror are facilitated since the contact pads63can be moved outwards. Note that the common electrode66does not extend beyond the passive substrate65and does not overlap the contact pads63, so that there is no net electric field around the rim of the PZT disc.FIG. 16shows a ninth embodiment of the invention that is similar to the eighth. In the ninth embodiment, the contact pads63have been wrapped around the periphery of the disc such that connection to the mirror14can be made from the top.

FIG. 17shows a tenth embodiment in which the compliant support60is used as the passive substrate. A thin planarization layer67is applied to the common electrode66on the top surface of the PZT plate61. The lower surface is passivated to enable connections to external drivers to be made at the periphery. Alternatively, the common electrode66could be placed at the bottom surface of the PZT plate61(in which case there would be no need to passivate the lower surface) and the set of electrodes62at the top. The upper surface in this case would need to be both passivated and planarised.

FIG. 18shows an exaggeration of the deformation that would result if an equal field were to be applied to each electrode in the electrode array62of the mirror15. For clarity, the set of electrodes, the passivation layer and the planarization layer have been omitted from the figure.

A preferred embodiment of the present invention will now be described with reference toFIG. 19in which strain gauges are provided to give information about the deformation of the mirror. This information may be used by an associated mirror control system to take account of hysteresis effects that are known to occur in piezo-electric materials.

Referring toFIG. 19, a portion of a preferred mirror is shown in cross-section. In common with the embodiments shown inFIGS. 13 and 15above, for example, a common electrode66is provided on one side of a PZT layer61between the PZT layer61and a layer of passive substrate65. On the other side of the PZT layer61are provided electrodes27over which a passivation layer51has been deposited. Vias105have been created through the passivation layer51to enable electrical connections to be made to the underlying electrodes27. Resistive strain gauges101and102are shown, provided on the passivation layer51in the region overlying one of the electrodes27to enable strain and hence deformation of the mirror in that region to be detected and measured, for example as a result of energising the respective electrode27.

In an alternative arrangement, not shown inFIG. 19, a further passivation layer may be provided between the common electrode66and the layer of passive substrate65so that resistive strain gauges may be provided within that further passivation layer rather than, or in combination with, strain gauges101,102provided as shown inFIG. 19. Within the further passivation layer, being closer to the reflective surface64of the mirror, strain gauges so positioned are able to provide strain information more representative of the deformation occurring at the reflective surface64. A flexi-connector of a type similar to that shown inFIG. 8may be included on or within the further passivation layer to enable electrical connection to be made to each of the strain gauges deposited therein from outside the mirror.

Referring toFIG. 20, a preferred arrangement is shown in which a pair of orthogonally positioned resistive strain gauges101,102are deposited on or within a passivation layer overlying an electrode27. In the particular example shown inFIG. 20, the electrode27is located close to the outer edge of a mirror and this enables electrical connections to the strain gauges101,102to be extended to peripheral bond pads110. Electrical connection111to the electrode27itself may be made at a convenient point on the electrode27by means of a via105provided through the passivation layer. However, as described above, all electrical connections to electrodes and to strain gauges may preferably be extended to bond pads around the perimeter of the mirror using flexi-connectors.

It has been found that the surface profile of a deforming mirror may be measured more effectively using two orthogonally disposed strain gauges101,102, as shown inFIG. 20, in the region of each electrode27. This is because the local curvature of a mirror will be influenced by the states of neighbouring electrodes. In some circumstances, especially where the electrode shapes are square or hexagonal, it may be possible to only use one gauge, which could be in the form of a double spiral.

FIG. 21shows an optional arrangement for the two orthogonal gauges101,102ofFIG. 20, where they are required to fit into a square area. A potential option of sharing a common contact is also shown. Use of this type of strain gauge pair would advantageously reduce the number of electrical connections required.

The deposition layer, including the passivation layer51and the planarization layer54are applied using thin film deposition techniques. The main thin film deposition techniques fall into three categories as will be described hereinafter: spin coating, Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD).

Spin coating is a widely used process for applying a thin film to a flat substrate. A small quantity of a polymer solution is dispensed onto the centre of a clean substrate. The substrate is then rotationally accelerated to a high speed, typically 2000-5000 rpm. The spinning causes most of the solution to be ejected from the substrate, leaving a thin film which flows outwards from the centre of the substrate under centrifugal forces. After spinning the thin film must be thermally cured into a smooth structural layer. Since the polymer is applied in liquid form, these materials tend to fill narrow gaps in the sub-dielectric surface and produce a good degree of planarization.

Polyimides can be deposited in this way. Polyimides produce films which have excellent thermal stability, toughness and chemical resistance. Polyimides can be built up into thick films and are suitable for passivation or planarization layers. However, a low temperature curing polymer is advantageous so as to ensure that the PZT does not need to be re-poled after the application of the dielectric.

All PVD techniques require the deposition to be carried out in a vacuum chamber. A good vacuum is required to increase the mean free path for collisions of atoms and high-energy ions. A source material is heated in a high vacuum such that its vapour pressure exceeds that of its environment and it is quickly vaporised. The substrate to be coated is placed in close vicinity of the source material. Upon contacting the cooler surfaces of the substrate the vapour condenses thereby creating a thin film of source material on the substrate.

One possible method for heating the source material is to place it in a boat wrapped with high resistance wire and apply a current through the wire. This is known as thermal, or resistive, evaporation. Flash evaporation can be used to deposit alloys. Small portions of an alloy powder are placed on a vibrating incline such that they fall into a boat that is kept at sufficiently high temperature to vaporise all the constituents as soon as they contact the boat. Electron beam evaporation can be used to deposit refractory metals and dielectrics. In this case an e-beam is focussed on the source material causing it to heat and vaporise.

The above PVD techniques deposit films in a ‘line of sight’ manner. For large substrates the uniformity of the film thickness will be poor. The deposition of thick films is possible, although a large amount of source material may be required.

Sputtering is a special case of PVD in which a target of the source material is bombarded with highly energetic positive ions. These ions cause ejection of particles from the target. The ‘sputtered’ particles deposit as thin films on substrates that have been placed on an anodic or grounded holder.

For DC sputtering, a diode or parallel plate system is used. The material to be sputtered is attached to the cathode plate while the substrate is placed on a facing plate which is either positively charged or grounded. An argon plasma is generated between the plates. Argon ions in the plasma are attracted to the cathode where they strike the target with sufficient energy to sputter particles of the target material. During this process highly energetic secondary electrons are emitted which create more positively charged Ar ions so that the plasma is self-sustaining.

The DC sputtering process is limited to electrically conductive targets. RF sputtering, where the target is subjected to alternating positive ion and electron bombardment, is used for non-conducting targets. RF sputtering can be used to deposit metals, alloys and almost any dielectric materials at low temperature and pressure, and a film of the thickness required for the passivation or planarization layers of a mirror according to an embodiment of the invention would be easily achievable.

Chemical vapour deposition occurs under a wide range of conditions. Deposition temperatures vary from 100° C. to 1000° C. and pressures from atmospheric to 10−2Torr. The energy for the reaction can be supplied thermally, by photons or by a glow discharge.

For atmospheric CVD, the substrate lies on a heater and reactant gasses flow over the surface at high velocity. The film is formed by chemical reactions at the substrate surface. Recently this process has been improved with low pressure hot wall reactors in which a more uniform film can be achieved at lower temperatures, typically 100-500° C.

A variant involves striking a plasma to enhance the chemical reaction rates of the precursor gasses. Plasma enhanced CVD (PECVD) allows deposition at lower temperatures with excellent control over the film properties. However the reducing atmosphere of the deposition chamber may deleteriously affect the composition of the PZT.

The two most suitable deposition techniques for forming the deposition layer are spin coating polyimide and RF sputtering. Both can achieve a uniform film with relatively low stress in the thickness range required. In addition, the stress of the sputtered film can be reduced by depositing alternate films with compressive and then tensile stress to build up a stress-free final film. PZT has a maximum working temperature of just over 100° C. At higher temperatures it will de-pole. Re-poling is possible, but the overall process will be simpler and quicker if high temperatures can be avoided. The RF sputtering process is performed at sufficiently low temperatures to avoid re-poling, but the polyamide would have to be carefully chosen to be curable at low temperatures.