The disclosure relates to microlens actuators, and in particular piezoelectric microlens actuators along with methods and an apparatus for actuating and/or controlling movement of a lens in an out-of-plane direction.

Background Art

Micro-optics applications and miniaturised confocal microscopy require precise movement of a microlens in an out-of-plane direction. Out-of-plane movement may be used to allow for auto-focus, optical zooming, changing the focal plane of a laser beam along the optical axis or other optical or camera applications. Multiple aligned micro-lenses with each microlens being capable of out-of-plane movement in the optical axis is required to realize miniaturized cameras. Movements in an out-of-plane direction or in the z-axis that extends through the lens have been known to be provided through thermal or electrostatic actuation. Although thermal actuation operates at low voltage, it suffers from slow response time and large power consumption. On the other hand, even though electrostatic actuation has a fast response time and low power consumption, it requires large actuation voltage and pliable mechanical platform to yield large displacement due to its low energy density. It would be valuable to find a way of providing out-of-plane movement with a fast or faster response time and a lower power consumption while.

The above references to the background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art.

The above references are also not intended to limit the application of the actuator, method of manufacturing an actuator and composition as disclosed herein.

Summary

According to a first aspect of the disclosure, a piezoelectric microlens actuator is disclosed.

The microlens actuator provides out-of-plane deflection. In some forms the microlens actuator minimises tilt while maximising out-of-plane deflection. In other forms the microlens actuator may control tilt.

In some forms, disclosed is an array of piezoelectric actuation members fabricated such that a voltage applied to any one piezoelectric actuation member results in a change of dimension of that piezoelectric actuation member, the piezoelectric actuation members engaged with a lens, the actuation members and the lens being configured such that the change in dimension of at least one piezoelectric actuation member results in movement of the lens out of a plane in which the array extends.

In some forms of the disclosed embodiments, the microlens actuator comprises a lens support and an array of piezoelectric actuation members. The piezoelectric actuation members are fabricated such that a voltage applied to any one of the piezoelectric actuation members results in a change of dimension of the piezoelectric layer of that piezoelectric actuation member resulting in a bending moment in an out-of-plane direction. The piezoelectric actuation members extend from the lens support and engage with the lens support by a plurality of engagement elements. The engagement elements and the lens support are configured such that the bending moment of any one of the piezoelectric actuation members results in a movement of the lens support in an out-of-plane direction.

In some forms the movement of the actuation members is a change in the length of the piezoelectric layer of the actuation member. That is, application of the voltage results in an expansion or contraction of the length of the piezoelectric layer which results in a bending moment and then in a movement of the lens support in the direction of the z-axis.

In some forms the movement of the lens support is out of plane in the facing direction of a lens supported by the lens support.

In some forms the engagement members comprise biasing members, which can be in the form of springs.

In some forms the piezoelectric actuation members comprise a laminar structure including a piezoelectric layer and electrodes positioned such that application of voltage to the electrodes results in the change in dimension of the piezoelectric actuation members.

In some forms the piezoelectric layer and the electrodes are positioned such that a portion of the piezoelectric layer is intermediate the electrodes.

The actuator may have the benefits of providing substantial out-of-plane movement of the lens with minimal tilt at a relatively fast speed with lower energy requirements than some forms found in the prior art. The actuator may have the benefit of controlling tilt at relatively fast speed with lower energy requirements than alternative actuators. The actuator may simply provide an alternative actuation method than those known. The actuator may find particular application in micro-optics applications specifically auto-focus applications and zooming applications in miniature cameras or confocal microscopy.

According to a second aspect, disclosed is a method to enhance out of plane displacement ranges by manipulation of residual stress in a support layer. It should also be appreciated that the lens actuator disclosed herein may find application in technologies not previously envisaged, due to its ability to maximise out-of- plane movement in response to the voltage applied to a piezoelectric layer.

Various methods of producing the piezoelectric actuation members are envisaged including the deposition of low-cost, readily available technologies. Brief Description

Notwithstanding any other forms that may fall within the scope of the actuator, the method of manufacturing an actuator and composition as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 shows a perspective view of one embodiment of a microlens actuator of the present disclosure;

Fig. 2 shows a plan view of the microlens actuator of Figure 1 ;

Fig. 3 shows an enlarged view of a section of the microlens actuator of Figure 1 ;

Fig. 4 shows a cross-sectional view of a piezoelectric actuation member of one embodiment of the present disclosure;

Fig. 5 shows a cross-sectional view of a piezoelectric actuation member of one embodiment of the present disclosure;

Fig. 6 shows deflection of the microlens holding platform in respect to the device surface plane (zero on the y-axis) at various electric field (E); Fig. 7 shows deflection of the microlens holding platform in respect to the device surface plane (zero on the y-axis) at various electric field (E);

Fig. 8 shows deflection of a 600pm long 100p wide micro-cantilever actuator at various electric fields compared to FEM results. Detailed Description

Disclosed in some forms is a piezoelectric microlens actuator comprising: a lens support; an array of actuation members including a piezoelectric layer, the actuation members fabricated such that a voltage applied to an actuation member results in an out-of- plane bending moment on that actuation member, the piezoelectric actuation members extending from the lens support and engaged with the lens support by a plurality of engagement elements, the engagement elements and the lens support being configured such that the change in bending moment of the actuation member results in movement of the lens support in the desired direction.

In some forms the array of actuation members comprise a plurality of actuation members spaced apart around the lens support. In some forms the actuation members comprise a plurality of actuation members extending outwardly from proximal to a central point to form a wagon wheel shape. In some forms the lens is supported at the central point.

In some forms the array of actuation members is located on a substrate. In some forms each actuation member comprises an elongate piezoelectric beams extending outwardly from the lens support to engage the substrate.

In some forms the array of actuation members comprises between 4 and 12 actuation members. In some forms the array of actuation members comprises 6 actuation members.

In some forms the actuation members extend from and are spaced apart around the lens support such that there is an angle of between 30 and 90 degrees between longitudinal axes of the actuation members. In some forms the actuation members extend from and are spaced apart around the lens support such that there is an angle of 60 degrees between longitudinal axes of the actuation members.

In some forms, the change in dimension of the piezoelectric layer of the actuation members is a change in the length of the piezoelectric actuation member.

In some forms, the change in dimension of the piezoelectric layer results in a bending moment on the actuation members and this bending moment results in a movement of the lens support in an out-of-plane direction.

In some forms, the lens support is adapted to support a lens having two opposing faces and the movement of the lens support out-of-plane results in movement of the lens in the facing direction of one of the opposing faces. In some forms the engagement members comprise biasing members. In some forms the biasing members comprise springs. In some forms the springs are shaped such that expansion or contraction of the piezoelectric actuation members results in movement of the lens support out-of-plane. In some forms the springs comprise serpentine or torsional springs. In some forms the bending moment on the actuation members effect movement of the engagement members to move the lens.

In some forms the piezoelectric actuation members comprise a laminar structure including a piezoelectric layer and electrodes positioned with respect to the piezoelectric layer such that application of voltage to the electrodes results in a change in the bending moment of the piezoelectric actuation members.

In some forms the piezoelectric layer and the electrodes are positioned such that a portion of the piezoelectric layer is intermediate the electrodes.

In some forms the piezoelectric layer is sandwiched between electrodes and the bending moment direction is in the positive z-axis.

In some forms the electrodes are arranged in an interdigitated form, the bending moment direction is in the negative z-axis.

In some forms the laminar structure further comprises a support layer and an insulator layer.

In some forms the laminar structure comprises of ultra-high e-beam evaporated polysilicon as the support structural layer.

In some forms disclosed is a method to exploit or alter the intrinsic stress of the support structure layer of the piezoelectric microlens actuator to enhance out-of-plane displacement range without compromising the operating frequency.

Referring now to the Figures, Figs 1 and 2 show an actuator arrangement for a first embodiment of a microlens actuator of the present disclosure. The actuator arrangement 1 comprises a lens support 2 in the form of a holding platform. In the illustrated form the holding platform is an annular platform. The lens support 2 is tangentially engaged with a plurality of engagement members 3. In the illustrated form the engagement members are biasing members in the form of serpentine springs. Alternative engagement members and biasing members that provide a similar function are available for example the engagement members may be in the form of torsional springs or any spring that provides the necessary movement of the holding platform. The engagement members 3 engage the holding platform at one end and at the other engage one of an array of actuation members 4. In the illustrated form the actuation members are elongate laminar beams positioned to extend outwardly from the lens support 2 and are engaged with the lens support 2 by means of the engagement members 3 at their proximal end 5. The actuation members 4 extend from the engagement members 3 and across a substrate 6. The actuation members 4 are engaged with the substrate at a distal end 7. An electrical connection 8 is positioned at the perimeter of the substrate 6 in the illustrated form.

Referring now to Fig. 3, a serpentine spring 10 forms the engagement member 3 engaging the actuation member 4 with the holding platform 2. The engagement member in the illustrated form extends substantially coplanar with and in line with the actuation members 4 and the substrate 6.

Referring to Figs. 4 and 5, in this illustrated form, the actuation members 4 are in the form of laminar beams 12 having a plurality of layers. The laminar beams comprise a support layer 13 with an insulator layer 14 deposited thereon. Suitable insulating materials may include S1O2 or S13N4 and are not limited to it.

A piezoelectric layer 15 is deposited on the beam. In some forms, the piezoelectric material may be a lead-free piezoelectric material. In some forms, the piezoelectric material may alternatively or additionally be an inorganic piezoelectric material. .

The piezoelectric material may be one or more of: (Bi,Na)TiO3; (Ba,Ca)(Ti,Zr)O3; (K,Na)Nb0 ₃ , Pb(Zr,Ti)0 ₃ or PVDF.

As shown in Figs 4 and 5, different arrangements are possible to provide the necessary bending moment.

In the form shown in Fig. 4, the piezoelectric layer 15 is sandwiched between a positive electrode 17 and a negative electrode 18. When a voltage is applied between the electrodes an electric field is generated between the electrodes. The piezoelectric layer 15 responds by expanding in the same direction as the electric field. In this form, the expansion in the z-axis and contraction in the y-axis of the piezoelectric layer 15 produces a bending moment in the actuation member in the positive z-axis direction. The deflection is related to the piezoelectric coefficient of the film and to the magnitude of the applied electric field.

In this arrangement, polarisation leads to upward movement if the actuation member 4 is oriented as shown in the Figure. In the form shown in Fig. 5, the piezoelectric layer 15 is aligned with a series of electrode fingers in the form of positive electrode 17 and a negative electrode 18 that are interdigitated with one another. In some forms, such as that shown in Fig. 5, a buffer layer may be required as a seed layer depending on the type of piezoelectric material used. When a voltage is applied to the electrodes 18 the electric field is generated between the fingers along the length of the piezoelectric layer. In turn, the piezoelectric layer expands in the direction of polarisation resulting in a bending moment of the actuator in the negative z- axis.

When voltage is applied to a piezoelectric layer in the direction of polarisation, a change in the dimensions of the piezoelectric layer produces a moment which deflects the engagement members in the out-of-plane direction to displace the lens support. For example in the form shown in Fig. 4, if the voltage is applied in the direction of polarization, that is in the positive z-axis, then the contraction of the piezoelectric film in the x-y plane and expansion in the z-axis translates to an upward movement of the spring and upward displacement of the lens support.

The amount of displacement range of the lens support is influenced by the initial residual moment of the structure. Typically, this residual moment causes an initial deflection or deformation in the structure which restricts the displacement range of the actuator. In order to enhance the displacement range, the residual stress of the support layer can be manipulated to alter the initial deflection or deformation of the structure.

In some forms use of a semi-crystalline ultra-high vacuum e-beam evaporated polysilicon film with a percentage of crystallization above 95% allows residual stress manipulation. This is possible by depositing between 450-500°C at a deposition rate of 50 - 100 nm/min and subsequently annealing the film at elevated temperatures to achieve a fully crystallized tensile film.

Experimental Data

Fig 6 shows the actuation characteristic of the microlens actuator for three different residual moments, which determine the initial deflection of the actuator. In the figure, the gradient labelled“u” displayes the upward residual moment. The gradient labelled“z” displays the zero residual moment. The gradient labelled“d” displays the downward residual moment. For the case where residual moment is zero in the multilayer structure, the actuator displacement behaviour can be illustrated by curve‘z’. Since the polarization of PZT dictates upward deflection, the displacement range that can be achieved is dz. When the residual stress in the passive silicon layer is minimal, a positive residual moment results, and the displacement behaviour follows curve‘u’. In this case, the actuator will have an initial upward deflection which reduces the displacement range to δu. By manipulating the residual moment to cause an initial downward deflection, the displacement range (δd) will be enhanced significantly. To achieve an initial downward deflection in the microlens actuator, a possible solution is by manipulating the residual stress of the SiO ₂ and polysilicon films. A compressive stress in the S1O2 layer and tensile stress in the polysilicon structure is required in order to counteract the upward moment caused by the Ti/Pt and PZT films.

Referring now to table 1 below, four different types of thin-films that constitute to the formation of the piezoelectric actuators have been deposited on dummy silicon wafers to predetermine each residual stress in the film. The film residual stress have been measured as‘as-deposited’ and after annealing at various elevated temperatures above 650 _° C to determine manipulable stress range. In addition to residual stress, the thickness of the constituting films also plays an important role in determining the residual moment and are taken into account at the design stage. Another crucial condition that must be satisfied for annealing individual films is given by Ti≥ Ti+1. This condition requires that the annealing temperature of the subsequent deposited film (Ti+1) must be equal or less than the previous film annealing temperatures (Ti) in order to avoid further residual stress change in the films deposited prior, or possible delamination. For the design purposes, we set the design requirements for the microlens actuator as: (i) displacement (d) 100 pm at low driving electric field of 100 kV/cm, with d _3i coefficient of -88 pC/N determined from previous work2;(ii) resonance frequency≥ 2 kHz for the unloaded structure; (iii) maximizing the performance factor (v) which can be expressed as v = d · f, where δ and f are the displacement and free vibration frequency respectively;(iv) structure design remain the same without changing the footprint size of 35 mm ² .

Gradient-based optimization in COMSOL Multi-physics is used to find the optimal design parameters with the boundary conditions and design requirements. The optimal thickness of each thin-film to obtain a negative initial deflection under consideration of maximizing the performance factor is given in Table I, along with the mechanical properties used for simulation.

A thick UHVEEPoly film is deposited at 500 _° C substrate temperature on a silicon wafer with 0.5 pm S1O2 grown in wet oxidation. The measured thickness was 6.1 pm by a stylus profiler. The deposition rate of UHVEEPoly is 200 nm/min, with a base pressure of 1 x10 ^-9 Torr and deposition pressure in the order of 10 ^-8 Torr. The as-deposited UHVEEPoly film stress is determined to be around -50 MPa (compressive).

The film is annealed at 700 _° C in N2 ambient to obtain a tensile stress of 140 MPa.

As part of the elastic structural layer, a thick compressive SiO ₂ film is needed to achieve a downward residual moment. The ways to deposit SiO ₂ films are either by RF magnetron sputtering or PECVD techniques. For sputtered S1O2 films, the residual stress becomes more compressive after annealing at elevated temperatures. Whereas for PECVD oxide films, the stress becomes more tensile after a long period of annealing. Due to a lengthy process of the PZT deposition at a high temperature in the subsequent steps, it is crucial for the SiO ₂ film to maintain a fairly compressive stress to enable the downward residual moment once the structure is released. Therefore, the sputtered oxide is chosen to be part of the elastic structural layer in this work. An optimized process for the sputtering of SiO ₂ film is used to achieve the desired stress level and thickness. The film is sputtered using a 4” diameter 99.995% fused quartz target. In this form, the deposition distance is 6 cm, with an RF power of 150 W and Ar pressure of 15 mTorr. The uniformity and refractive index is confirmed by optical waveguiding technique (Metricon 2010/M Prism Coupler). The uniformity is less than 5% between the center and the edge of a 2” wafer. The average thickness and refractive index are 2.25 miti and 1 .4473 respectively. The SiO ₂ film is annealed at 700°C to obtain a compressive stress of 1 10 MPa. A bi-layer metal film consisting of Ti(15 nm)/Pt(100 nm) is then deposited by e-beam evaporation on the sputtered S1O2 layer as the bottom electrode. The film deposition is followed by annealing at 650°C to control the film stress and to ensure a crack-free PZT film deposition after sintering. A sol-gel PZT solution with Zr/Ti ratio of 52/48 and 10% excess Pb is spun at 4000 rpm, dried at 300°C for 5 min on a hot-plate, and crystallized at 650°C for 15 min in O2 ambient furnace to achieve approximately 1 15 nm thin- film. This process is consecutively repeated to build up approximately 2.2 pm thick PZT film.

The top electrode, Ti(15 nm)/Pt(100 nm), is also deposited by the similar e-beam evaporation process as that of the bottom electrode. The top electrode remained as- deposited without subsequent annealing step. However, the top Ti/Pt electrode will be subjected to 300°C for approximately 30 mins for 1 pm PECVD Si02 deposition which conformally covers the step height of the top electrode for Al pad patterning. This is to avoid short-circuiting between the top and bottom electrodes. The PECVD oxide on top of the actuators and microlens holding platform is etched away by RIE. Three micro-cantilever actuators of different lengths have also been fabricated on the same chip to characterize and verify the residual moment generated from the thin-films.

The theoretical prediction using the measured residual stresses listed in Table I state that all the actuators will deflect in a downward position under those stresses. The deflection of the microlens holding platform is -47 pm from the contact pad as the point of reference, as compared to -54.5 pm predicted by simulation.

For the 600 pm test cantilever, the initial deflection measured at the tip was -5.04 pm compare to -5.16 pm from the simulation. Simulation modelling of the microlens actuator also show that the out-of-plane displacement with stress manipulation will generate a larger displacement, as compared to that without.

With the assumption of an average piezoelectric coefficient of d _3i = -108 pC/N determined from the test devices, the stress manipulation can enhance the displacement by 36%. The actuator is driven by a 1 Hz square-wave from 2 - 22 V (9.09 - 100 kV/cm) in which the displacement is measured by the vibrometer at a fixed point on the microlens holding platform. At 100 kV/cm, the actuator has a measured displacement of 145 pm at the lens platform, which is about 9.8% higher than the simulated result considering constant d _3i coefficient. The unloaded microlens actuator has a measured resonance frequency of 1 .961 kHz which is close to the simulated result of 1.934 kHz. The methodology to enhance out-of-plane displacement range by residual stress manipulation can be applied across different MEMS actuator structures like micro-bridges or diaphragms. The initial deflection of a PZT based micro-actuator can be modified due to stress manipulation of the polysilicon layer to enhance displacement without significantly influencing the resonance frequency.

Referring to Fig. 7 deflection of the microlens holding platform in respect to the device surface plane (zero on the y-axis) at various electric fields (E) is shown. Fig. 8 shows deflection of a 600μm long 100p wide micro-cantilever actuator at various electric fields compared to FEM results. It will be understood to persons skilled in the art that many other modifications may be made without departing from the spirit and scope of the actuator, method of

manufacturing an actuator and composition as disclosed herein.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations thereof such as“comprises” or“comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the actuator, method of manufacturing an actuator and composition as disclosed herein.