Abstract:
Optical MEMS scanning micro-mirror comprising: —a movable scanning micro-mirror ( 101 ), being pivotally connected to a MEMS body ( 102 ) substantially surrounding the lateral sides of the micro-mirror, —a transparent window ( 202 ) substantially covering the reflection side of the micro-mirror; —wherein a piezo-actuator assembly ( 500 ) and a layer of deformable transparent material ( 501 ) are provided on the outer portion of said window ( 202 ); —the piezo-actuator assembly ( 500 ) being arranged at the periphery of the layer of transparent material ( 501 ); —said piezo-actuator assembly ( 500 ) and transparent material ( 501 ) cooperating so that when actuated, the piezo-actuator assembly ( 500 ) causes micro-deformation of the transparent material ( 501 ), thereby providing an anti-speckle effect. The invention also provides the corresponding micro-projection system and method for reducing speckle.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an optical MEMS scanning micro-mirror comprising 
         [0002]    a movable scanning micro-mirror being pivotally connected to a MEMS body substantially surrounding the lateral sides of the micro-mirror, a substrate covering a back face of said mirror and a transparent window substantially covering the reflection side of the micro-mirror. The present invention also relates to a micro-projection system comprising such a micro-mirror, a corresponding method for reducing speckle and a method for manufacturing the optical MEMS scanning mirror. 
       BACKGROUND OF THE INVENTION 
       [0003]    Speckle is a phenomenon created with laser light sources, due to the fact that laser light is coherent. Parallels and synchronized wavefronts simultaneously hit the projection surface. When the light hits the surface, it creates constructive and destructive interference. The first category of interference induces an image deterioration that is often visible by human eye and/or by sensors. In addition to a loss of image quality, visual comfort of the viewer may also be affected. 
         [0004]    Several techniques are used in order to remove or reduce speckle. In many cases, light coherence reduction techniques are used. For instance, the light hitting the projection surface is provided from various projection angles. Polarized laser light hitting a depolarized film is also used. Otherwise, illumination using various laser wavelengths may also be used. 
         [0005]    Another approach consists in using vibration of the projection surface. The resulting systems are complex, expensive, and involve very specific hardware material. 
         [0006]    WO2009/077198 describes an optical system comprising a coherent light source and optical elements for directing light from the source to a target. The optical elements include at least one diffusing element arranged to reduce a coherence volume of light from the source and a variable optical property element. A control system controls the variable optical property element such that different speckle patterns are formed over time at the target with a temporal frequency greater than a temporal resolution of an illumination sensor or an eye of an observer so that speckle contrast ratio in the observed illumination is reduced. The variable optical property element may be a deformable mirror with a vibrating thin plate or film. This solution requires modifying the projection system in order to integrate additional components, such as diffusing elements. 
         [0007]    WO2007/112259 describes a system and method for reducing or eliminating speckle when using a coherent light source. A refracting device, comprising a birefringent material, is positioned such that the refracting device intercepts the coherent light. The refracting device rotates, thereby causing the ordinary and/or extraordinary beams to move. The human eye integrates the movement of the beams, reducing or eliminating laser speckle. The refracting device may include one or more optical devices formed of a birefringent material. Wave plates, such as a one-half wave plate, may be inserted between optical devices to cause specific patterns to be generated. Multiple optical devices having a different orientation of the horizontal component of the optical axis may also be used to generate other patterns. Furthermore, the refracting device may include an optical device having multiple sections of differing horizontal components of the optical axis. This solution involves a complex and expensive component, the rotating refracting device. Moreover, the integration of such device requires a specific global design. 
         [0008]    CN101477234 discloses a piezo-driven optical lens, which comprises a lens body, rails for providing an axial motion path, a piezoelectric element for providing a driving force and an elastic element for providing pre-stress for contacting the piezoelectric element with the rails. The lens body is at least provided with a hollow seat body, a lens barrel which is capable of moving axially and is positioned in the hollow seat, and a lens group fixed on the lens barrel. The piezoelectric element contacts the rails through the elastic element to drive the lens barrel to move linearly along the rails. This solution provides an efficient auto-focus system, but is not adapted to reduce speckle. 
         [0009]    WO 9918456 describes a lens with variable focus comprising a chamber filled with a first liquid, a drop of a second liquid being provided on a first surface zone of the chamber wall, wherein the chamber wall is made of an insulating material. The first liquid is conductive, the second liquid insulating. The first and second liquid are immiscible, with different optical indices and substantially of the same density. Means are provided for positioning said drop in inoperative position on said zone, comprising electrical means for applying a voltage stress between the conductive liquid and an electrode arranged on said wall second surface, and centering means for maintaining the centering and controlling the shape of the drop edge while a voltage is being applied by electrowetting. This solution involves complex liquid/oil encapsulated system and electrostatic actuation. 
         [0010]    US2009040602 describes a stress-induced polarization converter in the form of a zero power optical window or, alternatively, a single element, positive or negative power optical lens, that is subject to a controlled amount of symmetric, peripheral stress. The stress may be provided by appropriate mechanical, thermal, hydraulic, electromagnetic/piezo, annealing/molding, or other known techniques. The applied symmetric stress will advantageously be trigonal or four-fold, but is not so limited. This solution involves symmetrical stress, which is not suited for speckle reduction. 
         [0011]    Thus, there is a need for a novel micro-projection system with reduced speckle having MEMS micro-mirrors and MEMS components in general, that does not present the above mentioned drawbacks, namely the complexity and costs problems caused by using specific configurations with additional components used only for speckle reduction. 
       SUMMARY OF THE INVENTION 
       [0012]    A general aim of the invention is therefore to provide an improved device and method for reducing or suppressing speckle in a laser micro-projection system. 
         [0013]    Still another aim of the invention is to provide such method and device for reducing or suppressing speckle, providing efficient performances at reasonable cost. 
         [0014]    Yet another aim of the invention is to provide such method and device for reducing or suppressing speckle, using components that can be fully integrated into a laser micro-projection device. 
         [0015]    These aims are achieved thanks to the optical MEMS scanning micro-mirror and to the micro-projection system defined in the claims. 
         [0016]    There is accordingly provided an optical MEMS scanning micro-mirror comprising: [0017] a movable scanning 1D or 2D micro-mirror, being pivotally connected to a MEMS body for deflecting a light beam; [0018] a layer of deformable transparent material arranged to be traversed by said light beam; [0019] a piezo-actuator assembly cooperating with said transparent material so that when actuated, the piezo-actuator assembly causes micro-deformation of the transparent material so as to reduce speckle. 
         [0017]    The anti-speckle effect is obtained by arranging and actuating the piezo-actuator so as to create deformations of the transparent material with a short spatial wavelength (preferably much shorter than the outer dimension of the transparent material, thus creating waves) and/or with a high temporal frequency, so as to change the deformation between each successive frame. 
         [0018]    In an advantageous embodiment, the MEMS scanning mirror further comprises a package with a transparent window substantially above a reflection side of the micro-mirror, said layer of deformable transparent being unitary with said window. 
         [0019]    In a further embodiment, the MEMS scanning mirror further comprises a substrate under a back face of said mirror; [0023] wherein said piezo-actuator assembly and said layer of deformable transparent material are provided on the outer portion of said window; [0024] wherein said piezo-actuator assembly is arranged at the periphery of the layer of transparent material. 
         [0020]    In an advantageous embodiment, the piezo-actuator assembly is substantially circumferential. 
         [0021]    In a variant, the piezo-actuator assembly is provided with a plurality of piezo-elements circumferentially arranged around the transparent material. 
         [0022]    The window is preferably provided with a substantially flat outer face on which the piezo-actuator assembly and the transparent material are attached. 
         [0023]    The piezo actuator assembly is advantageously driven on an irregular basis. 
         [0024]    In a preferred embodiment, the piezo-actuator assembly, the transparent layer and window are preferably fabricated at wafer-level. 
         [0025]    The invention also provides a micro-projection system for projecting light on a projection surface comprising: [0031] at least one coherent light source and preferably a plurality of light sources with a beam combiner; [0032] optical elements, in the optical path between said coherent light source and said projection surface, [0033] an optical MEMS scanning micro-mirror as previously described. 
         [0026]    Such a micro-projection system may comprise in addition to the micro-mirror and the light source(s), a quarter-wave plate, a beam splitter, beam combiner, etc. 
         [0027]    The invention further provides a method for reducing speckle in a micro-projection system adapted for projecting light on a projection surface, comprising: [0036] providing a light with at least one coherent light source; [0037] directing light from the light source to the projection surface; [0038] providing a scanning micro-mirror for deviating light from said light source so as to scan a projected image onto said projecting surface; [0039] providing a layer of deformable transparent material arranged to be traversed by said light beam; [0040] providing a piezo-actuator assembly cooperating with said transparent material so that when actuated, the piezo-actuator assembly causes micro-deformation of the transparent material so as to reduce speckle. 
         [0028]    The piezo-actuator assembly and deformable material configuration enables to deform the transparent material to get an irregular surface varying according to the crossing position of the beams. Thus, several parallel light beams crossing the deformed surface are impacted differently (spatial deformation) while the deformation of each point also varies with time. Therefore, the deformed surface is specifically adapted to slightly modify the angle between adjacent light beams, resulting in a reduced coherence, and reduced speckle effect. 
         [0029]    In an advantageous embodiment, the piezo-actuator assembly is actuated so as to create deformations of the transparent material with a spatial wavelength shorter than the average outer dimension of the transparent material, thus creating waves in said transparent material. 
         [0030]    In a variant, the piezo-actuator assembly is actuated with a temporal frequency adapted to change the deformation between each successive pixel. 
         [0031]    In a further variant, the above two deformation modes are combined, in order to enhance the performance. 
         [0032]    The invention further provides a method for manufacturing an optical MEMS scanning micro-mirror, comprising: [0046] providing a MEMS micro-mirror for deflecting a light beam; [0047] providing a transparent substrate arranged to be traversed by said light beam; [0048] deposing a piezo-actuator assembly on one side of the substrate, said assembly forming a substantially central cavity; [0049] providing said cavity with a layer of deformable transparent material. 
         [0033]    In a preferred embodiment, the substrate is advantageously a glass window. 
         [0034]    In the above method, the piezo-actuator assembly is preferably obtained by: 
         [0035]    deposing a metallic layer on the surface of the substrate, for forming an electrode; 
         [0036]    removing the excess of material to leave a substantially circumferential portion for receiving the piezo material; [0054] deposing piezo material on said electrode portion; 
         [0037]    removing the excess of piezo material to leave a substantially circumferential piezo-actuator assembly; [0056] deposing a metallic layer on said piezo-actuator assembly; [0057] providing at least one layer of deformable transparent material within the piezo-actuator assembly. 
         [0038]    The method advantageously further comprises the step of providing a spacer wafer (for instance with Silicon or glass) on said substrate, on the side opposite to said piezo-actuator assembly. 
         [0039]    In a preferred embodiment, the method still further comprises the step of attaching the substrate (for instance by anodic bonding, gluing, eutectic bonding, glass frit bonding, etc) to the reflection side of the micro-mirror. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
           [0041]      FIG. 1  describes a movable micro-mirror; 
           [0042]      FIG. 2A  and  FIG. 2B  describe respectively a protected micro-mirror and its cross section; 
           [0043]      FIG. 3  is a schematic representation of a projection or scanning system comprising a light source and a micro-mirror; 
           [0044]      FIG. 4  shows an optical MEMS scanning micro-mirror provided with a piezo-actuator assembly in accordance with the invention; 
           [0045]      FIG. 5  shows the optical MEMS scanning micro-mirror of  FIG. 4  with a schematic representation of the deformable transparent material when the piezo-actuator assembly is actuated. 
           [0046]      FIG. 6  shows the optical MEMS scanning micro-mirror of  FIG. 4  with a schematic representation of the deformable transparent material when the piezo-actuator assembly is actuated, and the resulting light reflections and transmission principles; 
           [0047]      FIGS. 7   a  to  7   f ,  8   a  to  8   f  and  9   a  to  9   c  are schematic illustrations of an improved manufacturing process for an anti-speckle MEMS scanning mirror in accordance with the invention; 
           [0048]      FIG. 10  shows a process flow chart with certain process steps carried out in a fabrication process for fabricating the MEMS components of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0049]    For clarity, as is generally the case in representation of microsystems, the various figures are not drawn to scale. 
         [0050]      FIG. 1  presents a typical rectangular MEMS moving micro-mirror  101 , anchored to a fix body  102  by two beams  103 , and deflected along its central axis. 
         [0051]    An example of known type packaged MEMS mirror is presented in  FIG. 2A  and  FIG. 2B , where the MEMS mirror  101  is protected by a package comprising in this example transparent or semi-transparent surfaces  201  and  202  as the incoming light can either come from one side or from two sides of the mirror surfaces. 
         [0052]    The package of the encapsulated MEMS micro-mirror comprises a cap part with an optical window  202  that allows the light to penetrate and reflects on the micro-mirror surface. The cap optical window is typically made of glass such as borosilicate glass (for instance borofloat) or other type of glass, and has usually a flat surface. Micro-mirror surface can also be coated with reflective material such as gold, aluminum or silver, deposited in thin film, to obtain strong light reflection in the visible and Infra-Red wavelength. Optionally, the MEMS micro-mirror chip can also be packaged by a transparent or opaque substrate  201  from the other side of the MEMS micro-mirror chip. Ideally, each of the protection substrates made of transparent material should be coated on both sides with anti-reflective coating to avoid any parasitic light reflection. 
         [0053]      FIG. 3  presents a micro-projection system  404  where the light beam, coming from the light source  400 , is reflected into the optical projection system chip  403 , resulting in a projection image  402 . 
         [0054]    An aspect of the invention consists in reducing or suppressing speckle and therefore improves image quality and stability.  FIG. 4  illustrates a MEMS scanning micro-mirror provided with a piezo-actuator  500 , attached to the transparent window  202  of the package for the scanning mirror. The piezo-actuator is preferably arranged with a substantially circular configuration in order to form a cavity in which a layer of deformable transparent material  501  is placed. The latter two elements are in close cooperation so that piezo actuation creates a random or irregular deformation  502  of the transparent material  501 , as shown in  FIG. 5 . It is to be noted that the deformation shown in  FIG. 5  is voluntarily oversized, for illustrative purpose only. Actuation of the piezo-actuator causes alternate contractions and extensions of the transparent material  501 , thereby deforming it, as illustrated. The deformation reduces the laser coherence of light crossing the transparent material, which reduces the perceived speckle effect. 
         [0055]    In other words, when light beams providing one pixel or multiple adjacent pixels cross the transparent material in portions having different deformations, speckle may be reduced or suppressed. To provide such effect, in an embodiment, a high spatial frequency is used to generate waves  502 , as shown in  FIGS. 5 and 6 . It is even possible in one embodiment to vibrate the piezoelectric element fast enough to change the deformation of the deformable material during the projection time of one single pixel, thus reducing the speckle of each projected pixel. 
         [0056]    In a second embodiment, the various successive deformations are rapidly generated, so that deformations successively supported by the same pixels in different frames are different. This requires a substantially high temporal frequency, for instance at least equal to, and preferably greater than the frame frequency. 
         [0057]    In a variant, both previous techniques are combined to enhance the results. 
         [0058]      FIG. 6  illustrates exaggerated examples of input light and reflection when the piezo-actuator  500  is actuated. Input light  300  is reflected by the scanning mirror  101  and guided to the projection target in order to form a projected image. The reflected light  301  from the scanning mirror passes through the transparent window  202  and the deformed transparent material  501 . The deformation of the latter slightly modifies the direction of the reflected light, as shown with arrows  302 , illustrating the deviated light. The deviation is minimal and temporary, in order not to affect image quality, but sufficient to reduce the speckle effect. Also this pixel deviation can be compensated by software during the image projection, so as to change the phase of the laser pulse and send it slightly in advance or slightly delayed, thus changing its position. 
         [0059]    In other words, the different light beams cross the deformable material at different positions involving different orientations of the material, due to the vibrations of the material. The beams are slightly deviated, as shown in  FIG. 6 . This reduces the constructive and destructive interferences when the beams reach the screen. 
         [0060]      FIG. 6  also presents examples of input light  300 , either from the top of the scanning mirror or from the bottom direction. In the latter case, an angled reflector  310  is required in order to deviate the light to the scanning mirror  101 . Such reflector may be provided with silicon or glass coated with a metallic reflective layer, in this example directly under window  202 . 
         [0061]    The circular shape of the piezo-actuator  500  may be continuous along the periphery of the centrally placed deformable material  501 , or discontinuous, with regular or irregular interruptions along the circular profile. The transparent material may be a polymer, silicones such as Polydimethylsiloxane (PDMS) or a sufficiently viscous plastic, or other material that is capable of a light deformation when actuated by a piezo-actuator. 
         [0062]    The piezo element  500  and the deformable material  501  are preferably manufactured at wafer level with the transparent window  202  on which they are attached, as explained hereafter. 
         [0063]      FIGS. 7   a  to  7   f ,  8   a  to  8   f  and  9   a  to  9   c  are schematic illustrations of an improved manufacturing process for the anti-speckle MEMS scanning mirror of the invention. These Figures are completed with  FIG. 10  showing a process flow chart with certain process steps carried out in a fabrication process for fabricating the MEMS components. First, the core MEMS processing is shown (steps  2  to  14  of  FIG. 10 ). A known type SOI wafer  600  ( FIG. 7   a ) comprising two silicon layers  601  and  603  placed on each side of a silicon oxide layer  602  is used as a base material for manufacturing the scanning mirror (step  2 ).  FIG. 7   b  depicts the SOI wafer provided with metal deposition  604  such as aluminum or copper (step  4 ).  FIG. 7   c  shows the metal layer  605  after etching (step  6 ).  FIG. 7   d  presents the silicon  603  etching, for instance dry or wet etching, followed by oxide  602  etch (dry or wet) (steps  8  and  10 ).  FIG. 7   e  shows the silicon  601  dry etch to form the micro-mirror  101  portion (step  12 ). In  FIG. 7   f , a transparent glass window  201  made for instance with borosilicate glass (such as borofloat) or other type of glass, is bonded to the wafer (step  14 ). The glass window may be attached to the micro-mirror chip using any techniques, including but not limited to gluing, glass frit bonding, anodic bonding, eutectic bonding, molecular bonding, fusion bonding, low temperature direct bonding, soft soldering, metal thermo compression bonding, bonding with reactive multilayer, laser bonding, polymer attach, etc. 
         [0064]      FIGS. 8   a  to  8   f  show the different steps of the cap processing.  FIG. 8   a  shows the unprocessed substrate, in this case a transparent glass window (step  16 ). A metal deposition step is performed as shown in  FIG. 8   b , to add a thin layer  700  of metal alloy such as aluminum, to form an electrode  701  (step  18 ). Excess of metal is removed by etching (wet or dry) as shown in  FIG. 8   c  (step  20 ).  FIG. 8   d  shows the deposition of the piezo material (step  22 ). A material such as lead zirconate titanate (commonly designated “PZT”) or other natural or man-made piezo material such as Aluminum Nitride (AlN) may be used. Etching enables the removal of not-required piezo material, as shown in  FIG. 8   e , leaving the piezo-actuator  500  (step  24 ). The piezo-actuator section is preferably larger than the underlying electrode  701 . The second electrode  702  is provided on top of the piezo-actuator  500  by metal deposition, as shown in  FIG. 8   f  (step  26 ). 
         [0065]    A spacer wafer  203  such as Si or glass material is afterwards attached to the glass window, as shown in  FIG. 9   a  (step  28 ). The spacer may be attached to the micro-mirror chip using any techniques, including but not limited to gluing, glass frit bonding, anodic bonding, eutectic bonding, molecular bonding, fusion bonding, low temperature direct bonding, soft soldering, metal thermo compression bonding, bonding with reactive multilayer, laser bonding, polymer attach, etc. 
         [0066]    A similar type of attachment is used to connect the cap wafer stack to the MEMS mirror stack assembly, as illustrated in  FIG. 9   b  (step  30 ).  FIG. 9   c  show the addition of the deformable transparent material  501  within the cavity defined by the piezo-actuator  500  (step  32 ). The layer is preferably substantially flat for proper operation. The addition of the deformable material  501  can be done using different techniques, including plastic injection, PDMS or PMMA molding.