Abstract:
A MEMS apparatus is provided for scanning an optical beam. The MEMS apparatus is formed out of a pre-fabricated multi-layer device and comprises at least one tilting micro-mirror formed in a first active layer of that pre-fabricated multi-layer device and a support structure formed in a second layer of the pre-fabricated multi-layer device, and wherein the support structure is preferably formed by etching parts of the second layer to obtain a pre-determined shape of the supporting structure.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to micromachined structures and their fabrication and more particularly to rigid microelectromechanical systems (“MEMS”) structures having reduced deformations. 
     BACKGROUND OF THE INVENTION 
     Multimedia devices become increasingly more popular. As the technology advances, the devices shrink and consequently so do their respective displays. Nowadays people watch television broadcast in their cell phone having an average of less than 10 cm 2  screens. Thus, the need for a light, removable, reliable and high resolution projector has never been bigger. One type of the devices that is being developed in the recent year is the personal projectors in which fast micro mirrors are used in controlling the path of a laser beam thereby providing the required images. 
     One of the difficulties in the design and fabrication of the fast mirrors is the requirement to deliver both large tilting angles and larger resonance frequencies which may be improved by reducing the mass of the mirror. However, reducing the mirror&#39;s mass can only be done by thinning the mirror (assuming that both the material of which it is made and the beam diameter are kept constant), leads to a new problem—development of dynamic deformations. 
     When the mirror vibrates, it experiences deformations resulting from its undeformed flat surface due to its inertial force and torque from the torsion bars referred to hereinafter as dynamic deformation, which disrupt the optical beam wavefront and damage the achievable resolution. 
     Some solutions have been suggested in the past, among which is the common solution of adding support structures to the mirror. These support structures add mass to the mirror and by careful design, they increase the mirror&#39;s rigidness thereby reducing the dynamic deformation, without adding too much to its mass. 
     US 2003039021 discloses a MEMS mirror and method for fabricating the mirror. The mirror has a plurality of structures that operatively rotate around a support structure. The mirror is fabricated, such that the silicon components are separated from a glass structure having electrodes to prevent shorting of the electrodes to the mirror. 
     US 2004190110 describes a MEMS device having a deformable segmented mirror. The mirror includes a plurality of movable segments supported on a substrate using spring vertices, each vertex having a fixed plate and one or more springs. Due to the fixed plates, motion of each movable segment is substantially decoupled from that of the adjacent segments, which makes the shape of the segmented mirror relatively easy to control. 
     US 2004245888 discloses a MEMS device having a deformable mirror and includes a deformable membrane supporting a plurality of light-reflecting segments that form the deformable mirror. The actuator(s) which is configured to apply torque to the side of the membrane, is used to deform the membrane. Membrane deformation causes the segments to change orientation and thereby change the shape of the mirror, reduce it deformation and improve the resolution. 
     US 2007019280 describes MEMS devices that comprise a conductive movable layer spaced apart from a conductive fixed layer by a gap, and supported by rigid support structures, or rivets, overlying depressions in the conductive movable layer, or by posts underlying depressions in the conductive movable layer. It also describes cases where portions of the rivet structures extend through the movable layer and contact underlying layers. 
     In “Electrostatic 1D Micro Scanner with Vertical Combs for HD Resolution Display” by Jin-Woo Cho et al. Proc. Of SPIE vol. 6466, pp. 64660B-1 to 64660B-12, an electrostatic one dimensional scanning mirror is described in which a vertical comb drive was used to tilt the micro mirror, and a rib was used to minimize the moment of inertia and maximize the tilting angle of the mirror. The rib described was patterned on the upper silicon layer before making the SOI wafer. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a method of fabricating supporting elements to thin layers MEMS structures using SOI or DSoI based technology. 
     It is another object of the present invention to provide a low mass MEMS scanning micro-mirror device which has low deformations. 
     It is yet another object of the present invention to provide a method for using DSoI wafers as the basis for fabricating MEMS structures out of a very thin layer of silicon and ruggedizing the thin structure with support structures patterned from the second silicon layer, thereby providing structures that are lightweight and rigid, enabling superior dynamic performance in fast moving applications such as scanning mirrors. 
     It is another object of the present invention to provide a method for combining a simple fabrication process with robustness of SOI or DSoI based devices, whereby obtaining devices that can be efficiently used for static and resonant operation of titling micro devices including micro mirrors, optical switches and attenuators and micro manipulators. 
     Other objects of the invention will become apparent as the description of the invention proceeds. 
     According to a first embodiment of the present invention there is provided a MEMS apparatus for scanning an optical beam formed from a pre-fabricated multi-layer device (such silicon-on-insulator (“SoI”) substrate or a double active layer silicon-on-insulator (“DSoI”) substrate) and comprising at least one tilting micro-mirror formed in a first active layer of the pre-fabricated multi-layer device and a support structure formed in a second layer of the pre-fabricated multi-layer device. 
     In accordance with another preferred embodiment of the invention, the pre-fabricated multi-layer device is a DSoI device, and the MEMS apparatus comprises at least one tilting micro-mirror formed in a first active layer of the DSoI device and a support structure formed in a second active layer of the DSoI device, and wherein an insulator, such as an oxide layer, separates between both these active layers. 
     In the alternative, the pre-fabricated multi-layer device is a SoI device, and the MEMS apparatus comprises at least one tilting micro-mirror formed in the active layer of the SoI device being the first active layer of the pre-fabricated multi-layer device and a support structure formed out of the substrate layer of the SoI device, and wherein an insulator, such as an oxide layer, separates between both these layers. 
     The technology of silicon on insulator (SOI) refers to the use of a layered silicon-insulator-silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics. SoI-based devices differ from conventional silicon-built devices in that the silicon junction is above an electrical insulator, typically silicon dioxide. The terms “active layer”, “silicon layer” or “Si layer” as used herein and throughout the specification and claims, should be understood as interchangeable with each other. 
     In accordance with a preferred embodiment of the invention, the support structure is formed by etching the full thickness of the second layer at pre-defined locations of that layer. In other words, by etching the full thickness of the second layer at certain sections, a pre-defined shape of the supporting structure of the MEMS apparatus is formed. 
     Preferably, the support structure is formed in a shape of a cross surrounded by a ring. 
     In accordance with another embodiment of the invention there is provided a method for fabricating at least one supporting element in a MEMS apparatus for scanning an optical beam formed from a pre-fabricated multi-layer device, the method comprising: 
     a. providing a wafer having a front side and a back side which comprises a at least one active layer and a second layer; 
     b. etching parts of the second layer to obtain a pre-determined shape of a supporting structure. 
     Step b. of the method described above is preferably carried out by etching parts of the second layer surface, where the etching is carried out through the full thickness of that second layer, e.g. until the layer separating the two active layers is reached. 
     By still another preferred embodiment of the invention, the wafer comprises a first Si layer, a second Si layer and an oxide layer interposed therebetween, and wherein said etching step is carried out in said second Si layer. 
     By yet another preferred embodiment of the invention the method provided further comprising a step of forming a reflecting mirror at the front side of the first of said at least one active layer. 
     In accordance with still another embodiment of the invention, the supporting structure is formed in a shape of a cross surrounded by a substantially circular shape. 
     According to yet another preferred embodiment of the invention, the method provided further comprises a step of selecting the initial thickness of the oxide layer so as to ensure that the support structure is made in a stop etch process of the required parts of the of second Si layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  presents a schematic view of a pre-fabricated DSOI wafer; 
         FIGS. 2A-2O  illustrate a schematic process of preparing the MEMS device with the supporting structure of the present invention; 
         FIG. 3  presents a view of an exemplified supporting structure in accordance with an embodiment of the present invention; 
         FIG. 4  shows a schematic isometric view of a device comprising the supporting structure of  FIG. 3 ; and 
         FIGS. 5A and 5B  illustrate differences between the device of the present invention ( FIG. 5A ) and a device fabricated in accordance with prior art methods ( FIG. 5B ). 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be understood and appreciated more fully from the following detailed examples taken in conjunction with the drawings. 
     The micro mirrors of the device constructed in accordance with the present invention, deflect the laser beam in both X and Y axis at a speed that is directly linked to the image resolution supported by the projector. The X axis deflector mirror normally operates in resonance mode at frequencies ranging from about 10 KHz to about 40 KHz, depending on the projector resolution. Typically, there is an inherent conflict in the design of micro mirrors, in order to support higher resolution one would need a combination of a larger mirror and a larger tilting angle while maintaining good surface quality of the micro-mirror. Any required resolution can be translated into a requirement of ΘD, i.e. the product of Θ (the micro-mirror tilt angle) and D (the micro-mirror&#39;s diameter). Higher resolutions imply higher ΘD number as well as higher scanning frequency in the X direction. 
     However, the tilting angle is inversely proportional to the mass and geometry of the mirror and inversely proportional to the square of the resonance frequency. This may be demonstrated by the following equation: 
               Θ   tilt     =     Q   ·     F         (     2   ⁢   π   ⁢           ⁢     f   n       )     2     ·   J               
Where: F is the force generated by the mirror actuator;
 
f n  is the resonance frequency;
 
Q is the system&#39;s quality factor; and
 
J is the mirror&#39;s moment of inertia (J is a function of the mass of the moving element and the geometry chosen).
 
     One of the difficulties in the design and fabrication of fast moving mirrors is the need to deliver both large tilting angles and larger resonance frequencies. This need leads to a requirement to reduce J in order to compensate for the increase in f n  which, in turn, leads to a requirement to reduce the mass of the mirror. However, reducing the mass of the mirror can only be done by reducing the mirror thickness since its area should remain fixed due to optical requirements. As previously explained, this reduction in mirror thickness might immediately lead to dynamic deformation. 
     This problem may be solved by adding support structure to the mirror which in turn adds mass to the mirror. Several methods were proposed in the past on how to add such a support. One such method is incorporating wafer bonding after separate patterning and partial etching of two wafers. However, the prior art methods result in fabrication inaccuracies which impose a limit upon the minimal mirror mass and other system performance parameters. 
     Let us now consider  FIG. 1  which presents a schematic example of a prefabricated commercially available double-active-layer SOI (Double Silicon on Insulator, “DSOI”) wafer  10 , prior to its processing. The DSOI wafer typically comprises either a substrate and two active layers or a substrate and one active layer by using the substrate itself as one of the two active layers, where the active layer(s) and the substrate are bonded together before the starting of the fabrication process. The DSOI presented in this Fig. has a first active layer  12 , an oxide layer  14 , a second active layer  16 , an oxide layer  18 , a silicon substrate  20 , and another oxide layer  22 . The three semiconductor layers, first active layer  12 , second active layer  16 , and substrate  20  are electrically isolated from one another by the oxide layers. Oxide layer  14  is located between the first and second active layers, and oxide layer  18  is located between the second active layer and the substrate. Each active layer may have a thickness of a few to a few tens of microns. 
     Details of a process used for fabricating a supported structure for bouncing mode electrostatic tilting micro-mirror are demonstrated in  FIGS. 2A-2O . All dimensions of the respective layers and of resulting product of the process should be understood as being illustrative for demonstration purposes only. First, a typical wafer  90  is shown in  FIG. 2A  which comprises 2 active layers  110  and  130 , a substrate layer (handle)  100  and two oxide layers  120  and  120 ′. Oxide layer  120  separates the first active layer  110  from the second active layer  130  and the second oxide layer  120 ′ separates the second active layer  130  from substrate  100 . In the following description of the process steps, only the two active layers  110  and  130 , and the oxide layer interposed between them  120  will be referred to out of the wafer original layers. 
     The first step in the process is shown in  FIG. 2B . In this step a Plasma Enhanced Chemical Vapor Deposition (“PECVD”) layer  140  is prepared. In the next step, shown in  FIG. 2C  a photoresist layer  150  is deposited over the PECVD layer  140 , and patterned ( FIG. 2D ), thereby providing a support mask. The PECVD layer  140  is then etched ( FIG. 2E ), forming the same pattern of the mask in the PECVD layer. Next, the photoresist is stripped off ( FIG. 2F ), followed by etching the required support structure of the present invention in active layer  110  ( FIG. 2G ). The wafer is then flipped over ( FIG. 2H ) so that the second active layer,  130 , becomes now the upper layer. A new PECVD layer  160  is deposited on top of active layer  130  ( FIG. 2I ) and a new photoresist layer  170  is then deposited over PECVD layer  160  ( FIG. 2J ). The photoresist layer is patterned ( FIG. 2K ), thereby providing a mask suitable for the preparation of the required mirror and actuator. Next, The PECVD layer  160  is etched ( FIG. 2L ) in accordance with the pattern formed in photoresist layer  170  so that it may later serve as hard mask for etching active layer  130 . The next step is shown in  FIG. 2M , where the photoresist is stripped off, and then active layer  130  is etched in accordance with the etched PECVD layer  160  to obtain the required mirror and actuator for the device exemplified herein. This fabrication process ends in the step shown in  FIG. 2N , where the two PECVD layers  140  and  160  are stripped off, ending with a device that comprises the mirror and the actuator in one active layer ( 130 ) and with the appropriate support structure in active layer  110 . As may be seen from the above described process, the etching through both active layers in order to form both the support structure in layer  110  and the mirror and actuator in layer  130  is carried all the way through the respective layers, until reaching oxide layer  120  which is interposed therebetween. Thus, a very accurate etching is achieved as the solution according to the present invention does not rely on a stop etch method. 
       FIG. 3  exemplifies an embodiment of a supporting element of the present invention, prepared of a DSOI prefabricated wafer, e.g. during a process as demonstrated in the example of  FIGS. 2A-2O . The supporting element  300  has been etched from the back of the wafer using relatively large tolerances masks. The supporting element, in this example, designed as a cross ( 330 ) surrounded by a substantially circular shape such as a circular ring or an ellipsoid ring ( 320 ), made of the second active layer as described in the process exemplified in  FIGS. 2A-2O . Cavities  310 A,  310 B,  310 C and  310 D are etched in a DRIE process with due care to ensure fabricating the cross pillars thick enough to prevent deformation of the mirror during deflection, and wide enough not to be etched away during the release process. 
       FIG. 4  illustrates an isometric view of a device comprising supported micro-mirrors. The device shown in this FIG. is illustrated in an upside down position. In order to fabricate the device, a DSoI wafer was used. Wafer layer ( 410 ) is used for the mirror surface, while the support structure discussed hereinbefore, is made of the second wafer layer ( 430 ). The layers are separated from each other by an oxide layer ( 420 ).  FIGS. 5A-5B  further illustrate another aspect of the difference obtained between a device comprising a support structure that has been etched of the support structure in accordance with the present invention ( FIG. 5A ), and that of prior art ( FIG. 5B ). Device  500  comprises in its upper part a first active layer  510 , being the mirror layer, a second active layer  530  which is used for the support structure  535  and an oxide layer  520  located between the two active layers. Consequently, the support structure of the device of the present invention is not time based fabricated but is “stop etched” by the oxide itself. As opposed to device of the present invention, it may clearly be seen that layer  510 ′, i.e. the mirror layer of device  500 ′ which is fabricated by methods such as time stop methods referred to in the prior art, the considerably less accurate than the respective one of device  500 . 
     In addition, the method provided allows achieving not only very accurate support structures but also allows decoupling the mirror surface thickness selection from the design of the support structure. 
     While the present invention has been particularly described, persons skilled in the art will appreciate that many variations and modifications can be made. Therefore, the invention is not to be construed as restricted to the embodiments described, e.g. to any particular shape of the support structure or the steps of the fabrication process, rather the scope, spirit and concept of the invention will be more readily understood by reference to the claims which follow. For example, it should be clear to any person skilled in the art that the fabrication process&#39; steps may be carried out in a different order, e.g. preparing the two PECVD layers on both sides of the wafer prior to starting the etching step, preparing and/or stripping both photoresist layers at the same time, etc. It should be understood that any such shifting of the order in which the various steps are carried out is a matter of simple selection and can be done without departing from the scope of the invention. 
     The present invention has been described using non-limiting preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. Variations of embodiments described will occur to persons of the art. Furthermore, the terms “comprise”, “include”, “have” and their conjugates, shall mean, when used in the claims, “including but not necessarily limited to.” The scope of the invention is limited only by the following claims: