Abstract:
A micro-electro-mechanical system (MEMS) mirror device includes a mirror coupled to a rotating frame by a first torsional hinge along a rotational axis. The rotating frame has a body that defines a frame opening, and a group of rotational teeth extending from the body. A first bonding pad is located in the frame opening and coupled to the rotating frame by a second torsional hinge along the rotational axis. A second bonding pad is coupled to the rotating frame by a third torsional hinge along the rotational axis.

Description:
FIELD OF INVENTION  
       [0001]     This invention relates to micro-electro-mechanical system (MEMS) devices, and more particularly to MEMS scanning mirrors.  
       DESCRIPTION OF RELATED ART  
       [0002]     Various electrostatic comb actuator designs for MEMS scanning mirrors have been proposed. The extensive applications of these devices include barcode readers, laser printers, confocal microscopes, projection displays, rear projection TVs, and wearable displays (e.g., micro displays). For these applications, the MEMS scanning mirrors typically need to have a great range of rotation. Thus, what is needed is a MEMS scanning mirror that has a great range of rotation.  
       SUMMARY  
       [0003]     In one embodiment of the invention, a micro-electro-mechanical system (MEMS) mirror device includes a mirror coupled to a rotating frame by a first torsional hinge along a rotational axis. The rotating frame has a body that defines a frame opening, and a group of rotational teeth extending from the frame body. A first bonding pad is located in the frame opening and coupled to the rotating frame by a second torsional hinge along the rotational axis. A second bonding pad is coupled to the rotating frame by a third torsional hinge along the rotational axis. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1A  illustrates a perspective cut-away view of a MEMS mirror device in one embodiment of the invention.  
         [0005]      FIG. 1B  illustrates a perspective cut-away view of a lower layer in the MEMS mirror device of  FIG. 1A  in one embodiment of the invention.  
         [0006]      FIGS. 2 and 3  illustrate partial top views of the layers in the MEMS mirror device of  FIG. 1A  along a vertical line of symmetry in one embodiment of the invention.  
         [0007]      FIG. 4  illustrates a cross-sectional view of the MEMS mirror device of  FIG. 1A  in one embodiment of the invention.  
         [0008]      FIGS. 5 and 6  illustrate partial top views of the layers in a MEMS mirror device along a vertical line of symmetry in another embodiment of the invention. 
     
    
       [0009]     Use of the same reference numbers in different figures indicates similar or identical elements.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0010]      FIG. 1A  partially illustrates a MEMS mirror device  100  along a vertical line of symmetry  103  in one embodiment of the invention. Device  100  includes an upper layer  102  bonded to but electrically insulated from a lower layer  202  (shown more clearly in  FIG. 1B ) by an insulation layer  105 . Components on upper layer  102  and lower layer  202  can be formed from semiconductor wafers using semiconductor processing techniques.  
         [0011]     Referring to  FIG. 2 , upper layer  102  includes a mirror  104  connected by a torsional hinge  106  to a first end of a rotating frame  108  along a rotational axis  110 . A second end of rotating frame  108  is connected by a torsional hinge  112  to a bonding pad  114  along rotational axis  110 .  
         [0012]     In one embodiment, mirror  104  defines a slot  115  that divides itself into a tab portion  104 A that is connected to a reflector portion  104 B above and below slot  115 . In this embodiment, torsional hinge  106  is connected between tab portion  104 A and rotating frame  108 .  
         [0013]     Rotating frame  108  is a beam-like structure having a body that defines one or more frame openings  116  (only one is illustrated for clarity). Each frame opening  116  accommodates a bonding pad  117 . Opposing sides of each bonding pad  117  are connected by torsional hinges  118  and  119  to rotating frame  108  along rotational axis  110 .  
         [0014]     Rotating frame  108  has two opposing sides  108 A and  108 B relative to rotational axis  110 . Rotational comb teeth  120  (only one is labeled for clarity) extend from side  108 A while rotational comb teeth  122  (only one is labeled for clarity) extend from side  108 B.  
         [0015]     Rotating frame  108  also has beams  108 C and  108 D protruding from one end of rotating frame  108  to sandwich torsional spring  106 . Rotational comb teeth  120  extend from beam  108 C while rotational comb teeth  122  extend from beam  108 D. Similarly, rotating frame  108  has beams  108 E and  108 F protruding from the other end of rotating frame  108  to sandwich torsional spring  112 . Rotational comb teeth  120  extend from beam  108 E while rotational comb teeth  122  extend from beam  108 F. Note that sides  108 A and  108 B may extend further outward than beams  108 C,  108 D,  108 E, and  108 F to provide room to form bonding pads (e.g., bonding pad  117 ) within rotating frame  108  that would properly mount to anchoring pads (e.g., anchoring pad  218 ) below.  
         [0016]     Bonding pads  124  and  126  are formed on opposing sides of rotating frame  108 . Stationary comb teeth  128  (only one is labeled for clarity) extend from bonding pad  124  toward rotating frame  108  while stationary comb teeth  130  (only one is labeled for clarity) extend from bonding pad  126  toward rotating frame  108 . When rotation frame  108  is level (as shown), stationary comb teeth  128  and rotational comb teeth  120  are interdigitated while stationary comb teeth  130  and rotational comb teeth  122  are interdigitated. To match rotational comb teeth  120 , stationary comb teeth  128  are staggered with two side sections that extend closer to rotational axis  110  than a middle section. Stationary comb teeth  130  are also be staggered to match rotational comb teeth  122 .  
         [0017]     In one embodiment, the components of upper layer  102  are formed by etching a semiconductor wafer.  
         [0018]     Referring to  FIG. 3 , lower layer  202  includes an anchoring pad  218  onto which bonding pad  117  ( FIG. 2 ) is mounted. Lower layer  202  further includes an anchoring pad  206  having a horizontal sections  224  onto which bonding pad  124  ( FIG. 2 ) is mounted, a horizontal section  226  onto which bonding pad  126  ( FIG. 2 ) is mounted, and a vertical section  214  onto which bonding pad  114  is mounted.  
         [0019]     Stationary comb teeth  228  extend from horizontal section  224  toward rotational axis  110 . Stationary comb teeth  228  are staggered to match rotational comb teeth  120 . Similarly, stationary comb teeth  230  extend from horizontal section  226  toward rotational axis  110 . Stationary comb teeth  230  are also staggered to match rotational comb teeth  122 . Stationary comb teeth  228  and rotational comb teeth  120  are interdigitated at least when rotating frame  108  rotates in one direction (e.g., as shown in  FIG. 4 ). Conversely, stationary comb teeth  230  and rotational comb teeth  122  are interdigitated at least when rotating frame  108  rotates in the opposite direction.  
         [0020]     Lower layer  202  further includes an optional supporting rib structure  240  onto which mirror  104  ( FIG. 2 ) is mounted. Rib structure  240  includes vertical crossbeams  242  and horizontal crossbeams  244  (only one of each is labeled for clarity). When mounted to rib structure  240 , mirror  104  has less dynamic deformation and the optical resolution of device  100  is increased. Rib structure  240  is separated from the remainder of lower layer  202  by a gap  204 .  
         [0021]     In one embodiment, the components of lower layer  202  are formed by etching a semiconductor wafer so all the appropriate components are structurally supported by a floor  208 . The etching process also forms gap  204  around rib structure  240  to accommodate the rotation of mirror  104  ( FIG. 2 ).  
         [0022]      FIG. 4  is now used to explain the design benefits of device  100  in one embodiment. Typically stationary comb teeth  228 / 230 , anchoring pad  206  ( FIG. 3 ), anchoring pad  218  ( FIG. 3 ), and gap  204  ( FIG. 3 ) are formed by the same etching step. As the dimensions of stationary comb teeth  228 / 230  are much smaller than the dimensions of the other components, floor  208  around anchoring pad  218  is etched at a much faster rate than the spacing between stationary comb teeth  228 . Thus, the etching process is stopped before floor  208  is etched through and anchoring pad  218  becomes unsupported. However, when the etching process is stopped, the depth  302  of the spacing between stationary comb teeth  228 / 230  is much shallower than the depth  304  of floor  208 . This prevents rotational comb teeth  120 / 122  from reaching a rotation depth  306  required for some applications of mirror  104 . On the other hand, gap  204  is etched through to provide for the angle rotation of the mirror.  
         [0023]     To address this challenge, rotating frame  108  ( FIG. 2 ) is torsionally, instead of fixedly, connected to mirror  104  ( FIG. 2 ) by torsional hinge  106  ( FIG. 2 ). As rotating frame  108  rotates, its rotational motion is transferred to mirror  104  by torsional hinge  106 . Torsional hinge  106  in turn amplifies the rotational motion so that mirror  104  is rotated at a greater angle. The exact amplification of mirror  104  can be determined by studying the vibration mode shape through computer simulation of device  100 . For example, to amplify the rotational amplitude of mirror  104  relative to the rotational amplitude of rotating frame  108 , the stiffness of hinge  106  need to be reduced.  
         [0024]     Device  100  can be operated in a variety of fashion. In one embodiment, rotational comb teeth  120  and  122  are coupled via bonding pad  114  to receive a reference voltage (e.g., DC). Stationary comb teeth  128  and  130  are coupled via bonding pads  124  and  126 , respectively, to receive an oscillating voltage with a steady voltage bias (e.g., an AC+DC voltage). Stationary comb teeth  228  and  230  are coupled via bonding pad  206  to receive an oscillating voltage (e.g., an AC voltage source). The two oscillating voltages have a phase shift of 180 degrees. Thus, a steady (e.g., DC) voltage difference between rotational comb teeth  120 / 122  and stationary comb teeth  128 / 130  changes the natural frequency of device  400 , whereas oscillating (e.g., AC) voltage differences between rotational comb teeth  120 / 122  and stationary comb teeth  128 / 130 / 228 / 230  oscillate the mirror at the desired scanning frequency and at the desired scanning angle. The DC voltage difference between rotational comb teeth  120 / 122  and stationary comb teeth  128 / 130  is adjusted by adjusting the steady voltage bias of the oscillating voltage provided to stationary comb teeth  128  and  130 .  
         [0025]      FIGS. 5 and 6  partially illustrate an upper layer  402  and a lower layer  502  of another MEMS mirror device in one embodiment of the invention. This mirror device is similar to mirror device  100  ( FIG. 1A ) but for the following.  
         [0026]     Referring to  FIG. 5 , layer  402  is similar to layer  102  except that mirror  104  is replaced with a mirror  404 . Like mirror  104 , mirror  404  is connected by torsional hinge  106  to a first end of rotating frame  108  along rotational axis  110 . However, mirror  404  further defines an opening  406  to accommodate a bonding pad  410 . Bonding pad  410  is connected by a torsional hinge  412  to mirror  404  along rotational axis  110 .  
         [0027]     Referring to  FIG. 6 , layer  502  is similar to layer  202  except that additional anchoring pads  510 A and  510 B are formed in lower layer  502  to support bonding pad  410 .  
         [0028]     As described above, mirror  404  is connected by torsional hinge  106  to rotating frame  108  and by torsional hinge  412  to bonding pad  410 . The stiffnesses of hinges  106  and  412  are adjusted to control the rotational amplitude of mirror  404 . For example, to amplify the rotational amplitude of mirror  404  relative to the rotational amplitude of rotating frame  108 , the stiffness of hinge  412  should be reduced and the stiffness of hinge  106  should be made relatively larger. Conversely, to dampen the rotational amplitude of mirror  404  relative to the rotational amplitude of rotating frame  108 , the stiffness of hinge  412  should be increased and the stiffness of  106  should be made relatively smaller. The exact amplification and dampening rotational amplitude of mirror  404  is also related to the structure inertia distribution and the stiffnesses of the other hinges, which can be determined by studying the vibration mode shape through computer simulation of the device. The device can be operated in the same manner as device  100 .  
         [0029]     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.