Abstract:
The present disclosure provides a micro-machined switchable optical mirror device with a fast response speed. The mirror device includes a substrate defining a gap space, and a mirror assembly disposed on the substrate and deflectable in the gap space, the mirror assembly including a free end cantilever and a reflector on the cantilever, wherein the cantilever is anchored on the substrate adjacent a side of the gap space through an elastic member. In one aspect, the mirror device further includes a stop spring at an end of the cantilever opposing the elastic member.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with government support under Contract Number W909MY-12-C-0018 awarded by the US Army Contracting Command, Subcontract Number SA-04, awarded by Banc 3, Inc. under Prime Contract No. W909MY-12-D-0005-0002 awarded to Banc 3, Inc. by the US Army and Subcontract Number SA-05, awarded by Banc 3, Inc. under Prime Contract No. W909MY-12-D-0005-0002 awarded to Banc 3, Inc. by the US Army. The government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to a micro-machined optical mirror switch and a method for fabricating the same. More particularly, the present disclosure relates to a silicon-based, micro-machined optical mirror switch with a fast response speed and a method for fabricating the same. 
         [0003]    Micro-Electro-Mechanical Systems (MEMS) is a fast growing manufacturing technology that produces ultra-fine mechanical devices at a very low cost. MEMS benefits from the economics of scale by employing the batch fabrication established in the semiconductor industry. Moreover, MEMS can be constructed using single-crystal silicon, which is an ideal material for mechanical devices, partly because single-crystal silicon has virtually no hysteresis and hence almost no energy dissipation. Further, single-crystal silicon is less prone to fatigue damages, and thus allows for a prolonged service lifetime. For example, single-crystal silicon may sustain over trillions of mechanical flexing cycles without breaking. 
         [0004]    MEMS based moving mirrors have been widely used in communication components such as switches and attenuators and extensively use in digital light projectors (DLPs) and laser scanners. However, conventional MEMS mirror switches are limited in switching speed. There is an acute need to have fast MEMS optical mirrors for fast optical switches supporting the insatiable growth of internet bandwidth and other applications such as micro-scanners, laser Q-switching, optical shutters, etc. Fast optical MEMS mirror finds extensive applications in telecommunications, astrophysics, biology, medical imaging, etc. 
       SUMMARY 
       [0005]    In light of the above, the present disclosure provides a MEMS optical mirror switch that is optimized to achieve a significantly increased electrical response speed. For example, the fast electrical response speed may be achieved by operating the MEMS optical mirror switch in a near breakdown field region. 
         [0006]    In one aspect, the mirror switch of the present disclosure includes a substrate defining a gap space, and a mirror assembly disposed on the substrate and deflectable in the gap space, the mirror assembly including a free end cantilever and a reflector on the cantilever, wherein the cantilever is anchored on the substrate adjacent a side of the gap space through an elastic member. The mirror device may further include a stop spring at an end of the cantilever opposing the elastic member. The mirror switch may further include an insulating layer disposed between the substrate and the mirror assembly. The mirror switch may further include a first electrode electrically coupled to the substrate, and a second electrode electrically coupled to the mirror assembly. The mirror switch may further include a highly reflective coating layer on the reflector. The mirror switch may further include a stop spring at an end of the cantilever opposing the elastic member. The mirror assembly of the mirror switch may further include an obstacle disposed adjacent a side of the gap space proximate the stop spring. The substrate of the mirror switch may include a plurality of apertures, the apertures opening the gap space to an exterior of the switchable optical mirror device. 
         [0007]    The mirror assembly is switchable between a neutral state and a deflected state in response to an electrical voltage applied to the mirror assembly. In one embodiment, the cantilever may be substantially parallel to the substrate when the mirror assembly is at the neutral state. In an altemative embodiment, the cantilever may be substantially parallel to the substrate when the mirror assembly at the deflected state. 
         [0008]    A number of other embodiments and fabrication of the mirror switch of the present disclosure are also disclosed herein below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present disclosure is to be read in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  illustrates a sectional view of a mirror switch at an OFF state, in accordance with one embodiment of the present disclosure; 
           [0011]      FIG. 2  illustrates a sectional view of a mirror switch at an ON state, in accordance with one embodiment of the present disclosure; 
           [0012]      FIG. 3  illustrates a sectional view of a mirror switch in accordance with another embodiment of the present disclosure; 
           [0013]      FIGS. 4A through 4D  illustrate a process for fabricating an optical member of a mirror switch in accordance with one embodiment of the present disclosure; 
           [0014]      FIGS. 5A through 5D  illustrate a process for fabricating a support member of a mirror switch in accordance with one embodiment of the present disclosure; and 
           [0015]      FIG. 6  illustrates a process for fabricating a mirror switch including the optical member and the support member, in accordance with one embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following detailed description is of the best currently contemplated modes of carrying out the present disclosure. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the present disclosure, because the scope of the present disclosure is defined by the appended claims. 
         [0017]    As used herein, the singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise. 
         [0018]    Except where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and the claims are to be understood as being modified in all instances by the term “about.” Further, any quantity modified by the term “about” or the like should be understood as encompassing a range of ±10% of that quantity. 
         [0019]    For the purposes of describing and defining the present disclosure, it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0020]      FIG. 1  illustrates a sectional view of an optical mirror switch  100  at an OFF state, in accordance with one embodiment of the present disclosure. Switch  100  may be manufactured from silicon wafers using the MEMS technology. In one embodiment, switch  100  includes a suspended optical member  102  and a support member  104  on which optical member  102  is securely disposed. In one embodiment, optical mirror switch  100  is at an OFF state because no electrical voltage is applied thereon. 
         [0021]    Referring to  FIG. 1 , a suspended optical member  102  comprises support frame  14 , a mirror electrode  10 , a silicon cantilever spring  12 , silicon spring stopper  20 , and an upper stopper  18 . In one embodiment, a suspended optical member  102  may be formed from a single crystal silicon wafer by etching silicon. It is appreciated that a silicon cantilever spring  12  is mechanically coupled and suspend a mirror electrode  10  over bottom electrode  104 . In one embodiment, a spring stopper reduces the mechanical impact when the side of a suspended optical member  102  away from a cantilever spring  12  touches either bottom electrode or upper stopper during switching operation. In addition, suspended optical member  102  may optionally comprises a highly reflective layer  16  coated on mirror electrode element  10 . In one embodiment, suspended optical member  102  is a free end cantilever, that is, one end of suspended optical member  102  is supported by silicon cantilever spring  12  which is anchored to support frame  14 , while the other end of suspended optical member  102  is connected to spring stopper  20  which is free to fluctuate. 
         [0022]    Referring again to  FIG. 1 , support member  104  comprises a bottom electrode body  24 , a dielectric layer  30  on bottom electrode body  24 , and a spacer step  26 . Support member  104  may comprise a gap  22  formed by etching into bottom electrode body  24  to have a size commensurate with the size of a mirror electrode  10 , so as to provide sufficient space for a mirror electrode  10  to move or fluctuate therein. Further, support member  104  may comprise a plurality of apertures  28  formed by etching through bottom electrode body  24  so as to allow air to escape from gap  22  while a mirror electrode  10  moves in gap  22 . As shown in  FIG. 1 , optical member  102  and support member  104  securely are engaged with each other by aligning mirror electrode  10  and silicon spring stopper  20  of optical member  102  with gap  22  of support member  104 . In various embodiments, bottom electrode body  24  is electrically grounded. 
         [0023]      FIG. 2  illustrates a sectional view of a mirror switch  100  at an ON state, in accordance with one embodiment of the present disclosure. Mirror switch  100  in  FIG. 2  is substantially the same as mirror switch  100  in  FIG. 1 , except that a voltage Vd is applied to mirror electrode  10  and bottom electrode  24  of mirror switch  100  in  FIG. 2 , while no voltage is applied to mirror switch  100  in  FIG. 1 . Voltage Vd applied to mirror switch  110  may induce electrostatic force that attracts mirror electrode  10  to move toward bottom electrode  24 . As a result, MEMS mirror electrode  10  may be actuated and switched to an ON state in response to voltage Vd, as shown in  FIG. 2 . When mirror switch  100  is at an ON state, mirror electrode  10  is electrically insulated with bottom electrode  24  due to dielectric layer  30 . 
         [0024]    When voltage Vd is removed, mirror electrode  10  of switch  100  reverts back to the OFF state due to the restoration force of silicon spring  12 , as shown in  FIG. 1 . When mirror electrode  10  reverts back to the OFF state, upper stopper  18  of optical member  102  may physically contact stop spring portion  20  of mirror electrode  10  so as to prevent mirror electrode  10  from over overshooting. Further, stop spring  20  may absorb the kinetic energy of the movable portion of optical member  102  and avoid hard contact when snapping down or restoring back to neutral position. As a result, both switching ring effect and mechanical damage are illuminated to enhance the stability and reliability of switch  100 . 
         [0025]    The mirror switch  100  of the present disclosure may be operated in an acceleration mode to achieve faster speed than conventional devices. While not desiring to be bound by theory, one physical explanation is that when the actuation force is much larger than the intrinsic MEMS mechanical spring force, the MEMS structure is at a non-steady state. The rotation rate is very fast, and the higher the applied voltage Vd, the faster the rotation rate. The recovery time and maximum actuation frequency is affected by the primary mechanical resonant frequency of mirror electrode suspension including silicon suspension spring  12 , mirror electrode  10 , and spring stopper  20 . The mechanical resonant frequency depends mainly on the length and stiffness silicon spring  12 . The mechanical resonant frequency f of suspended mirror structure (including reference numerals  10 ,  12 ,  16 , and  20 ) may be related to the rotational spring constant K and rational inertia I of suspended mirror structure (including reference numerals  10 ,  12 ,  16 , and  20  with the following formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     = 
                     
                       
                         1 
                         
                           2 
                            
                           π 
                         
                       
                        
                       
                         
                           K 
                           I 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    The recovering time constant is 
         [0000]    
       
         
           
             
               
                 
                   τ 
                   = 
                   
                     
                       1 
                       
                         2 
                          
                         π 
                          
                         
                             
                         
                          
                         f 
                       
                     
                     = 
                     
                       
                         I 
                         K 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
         [0026]    By MEMS technology, one may design rotational spring constant K and rational inertia I, such that the rotational frequency f can be as high as 16 kHz, which may correspond to a recovering time constant τ of about 10 μSec. In one instance, the mechanical resonant frequency ranges from about 1 kHz to about 100 kHz. By confining displacement of the mirror electrode  10  to a space between the upper stopper  18  and a surface of the  22 , the optical mirror switch  100  is configured to withstand mechanical vibration from 10 to 2000 Hz and impact of to 2000 G. 
         [0027]    In sum, when mirror switch  100  is at the OFF state, mirror  10  is in a neutral position suspended by spring portion  12 , as illustrate in  FIG. 1 . When driving voltage Vd is applied, mirror switch  100  changes to the ON state by rotating mirror electrode suspension  10  to snap down to ground plate  24 . The rotation is then stopped by stop spring portion  20 , which lands on dielectric layer  30  on ground plate  24 , as illustrate in  FIG. 2 . When a light beam impinges on mirror switch  100  at the OFF state, the light beam is reflected to a first destination. When mirror switch  100  is changed to the ON state, the light beam is then reflected to a second destination different from the first destination. Accordingly, mirror switch  100  of the present disclosure can be used to quickly control the optical path of a light beam. 
         [0028]    As shown in  FIGS. 1 and 2 , mirror element  10  is suspended through silicon spring  12  over electrode plate  24  that is grounded. Gap  22  is formed between suspended mirror  10  and grounded electrode plate  24 . Mirror  10  may be made of either N or P type heavily doped single crystal silicon with low resistivity for applying a driving voltage to achieve mirror switch by electrostatic force. By using silicon micro-machining technology, gap  22  may be precisely defined by spacer  26  and can be as small as in the micron range, in which a large electrostatic driving force may be produced to achieve fast switching, even with a low driving voltage. In one embodiment, gap  22  may has a thickness of less than 100 micro meters. In addition, the reflective loss is minimized due to the high quality mirror surface  10  as well as the high reflective coating  16  on mirror surface  10 . 
         [0029]    On electrode plate  24 , arrays of through holes  28  are created to reduce air thin film squeeze damping and to increase switch speed. Stop spring portion  20  is created along the outer mirror edge away from suspended silicon spring  12  to avoid hard contact when mirror  10  switch down to ground plate  24 . When mirror  10  restores back to the neutral position, stop spring portion  24  absorbs the kinetic energy by contacting upper stopper  18 , thereby minimizing the ringing effect of switching. 
         [0030]    Although mirror  10  and ground plate  24  can be in parallel in order to reduce the complexity of assembly, as shown in  FIGS. 1 and 2 , mirror  10  and ground plate  24  can be arranged in a wedge form to further reduce driving voltage of the switch.  FIG. 3  illustrates a sectional view of a mirror switch  100  in accordance with another embodiment of the present disclosure. Mirror switch  100  in  FIG. 3  is substantially the same as mirror switch  100  in  FIGS. 1 and 2 , except that optical member  102  and support member  104  of mirror switch  100  in  FIG. 3  are not parallel with each other. Rather, optical member  102  is deposed on support member  104  with a wedged angle. In one embodiment, the wedge form of mirror switch  100  may be achieved by selectively over-etching spacer  26  of support member  104  prior to engaging optical member  102  with support member  104 . 
         [0031]    Hereafter, a process for fabricating mirror switch  100  in accordance with on embodiment of the present disclosure is described. 
         [0032]      FIGS. 4A through 4D  illustrate a process for fabricating optical member  102  of mirror switch  100  in accordance with one embodiment of the present disclosure. Referring to  FIG. 4A , the fabrication process begins from silicon wafer In one embodiment, the top surface of silicon wafer may be the prime polished surface that is ideal for an optical mirror surface. 
         [0033]    Referring to  FIG. 4B , silicon is etched from one side to define the bottom boundaries of mirror electrode suspension including silicon suspension spring  12 , mirror electrode  10 , and spring stopper  20  It is noted that mirror electrode  10 , and suspending spring  12  are mechanically and electrically coupled with each other. 
         [0034]    Referring to  FIG. 4C , silicon is etched from the other side, to define mirror element  10  and thickness of suspension spring  12  and spring stopper  20 . 
         [0035]    Referring to  FIG. 4D , a high reflective (HR) layer  16 , in which reflectivity is more than 99.5%, is coated on an upper surface of mirror element  10 . In one embodiment, HR layer  16  may be formed by coating a HR material to an entire upper surface of optical element  102  and then etching the HR material such that only the portion on mirror element  10  remains. It is appreciated that, in other embodiments, HR layer  16  may be formed after the bonding of optical member  102  and support member  104 . This completes the fabrication of optical element  102  of switch  100 , as shown in  FIGS. 1 and 2 . 
         [0036]      FIGS. 5A through 5C  illustrate a process for fabricating support member  104  of mirror switch  100  in accordance with one embodiment of the present disclosure. Referring to  FIG. 5A , the fabrication process begins from providing silicon wafer. 
         [0037]    Referring to  FIG. 5B , silicon is etched to form spacer  26  on electrode plate  24 , thereby defining gap  22 . In one embodiment, gap  22  may have a dimension commensurate to that of mirror electrode suspension including silicon suspension spring  12 , mirror electrode  10 , and spring stopper  20 . 
         [0038]    Referring to  FIG. 5C , silicon is etched through to form an array of apertures  28 . In one embodiment, apertures  28  are through holes that permit air to communicate between two sides of the silicon electrode  24 . 
         [0039]    Referring to  FIG. 5C , a dielectric layer  30  is deposited on the top of bottom electrical plate  24 . This completes the fabrication of support element  104  of switch  100 , as shown in  FIGS. 1 and 2 . 
         [0040]      FIG. 6  illustrates a process for fabricating mirror switch  100  including optical member  102  and support member  104 , in accordance with one embodiment of the present disclosure. Referring to  FIG. 6 , optical member  102  as shown in  FIG. 4D  and support member  104  as shown in  FIG. 5D  are bonded together by facing and aligning mirror electrode suspension including silicon suspension spring  12 , mirror electrode  10 , and spring stopper  20  with gap  22  of support member  104 . Upper stopper is aligned between the edge of bottom electrode  24  and spring stopper  20 , and bonded to the bottom electrode  24 . 
         [0041]    Referring to  FIG. 6 , once optical member  102  and support member  104  are bonded, mirror switches  100  fabricated on a large wafer may be separated into individual chip units. Each individual mirror switch  100  may then be wire bonded with electrical terminals such that a driving voltage may be applied to mirror electrode  10  and that bottom electrode  24  may be grounded. It is appreciated that, in alternative embodiments, a driving voltage may be applied to bottom electrode  24 , while mirror electrode  10  may be grounded. 
         [0042]    In view of the foregoing, it can be seen that the present disclosure provides a MEMS optical mirror switch that is optimized to achieve a significantly increased electrical response speed. It is to be understood that the method and the apparatus of the present disclosure are described for exemplary and illustrative purposes only. Various modifications and changes may be made to the disclosed embodiments by persons skilled in the art without departing from the scope of the present disclosure as defined in the appended claims.