Abstract:
A micro-electro-mechanical system (MEMS) device includes a mirror having a top surface with trenches, a beam connected to the mirror, rotational comb teeth connected to the beam, and one or more springs connecting the beam to a bonding pad. The mirror can have a bottom surface for reflecting light. The mirror can include a top flange and a bottom flange joined by a web, wherein the top and the bottom flanges form the top and the bottom surfaces, respectively. The rotational comb teeth can have a tapered shape. Stationary comb teeth can be interdigitated with the rotational comb teeth either in-plane or out-of-plane. Steady or oscillating voltage difference between the rotational and the stationary comb teeth can be used to oscillate or tune the mirror.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of application Ser. No. 10/778,742, filed Feb. 13, 2004 now U.S. Pat. No. 7,046,421, which is incorporated herein by reference. 

   FIELD OF INVENTION 
   This invention relates to micro-electro-mechanical system (MEMS) devices, and more particularly to MEMS scanning mirrors. 
   DESCRIPTION OF RELATED ART 
   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. Typically a MEMS scanning mirror is driven at its main resonance to achieve a large scan angle. To ensure a stable operation, it is crucial to ensure the mirror and its associated movable structure will vibrate in the desired mode shape at the lowest and main resonant frequency. In many applications, the mirror size has to be large and the mirror surface has to be flat to ensure high optical resolution. The mirror vibration/scanning speed also has to be fast for many applications. It is known that when the mirror size and scanning speed increase, the mirror dynamic flatness deteriorates. Without a flat mirror surface, the scanning mirror is of little use to many applications. In addition, this main frequency has to be separated far from other structural vibration frequencies to avoid potential coupling between the desired and the undesired mode shapes. 
   The undesired structural vibrations will increase the mirror dynamic deformation and result in degraded optical resolution. Furthermore, some of the structural vibration modes may cause the rotationally movable and stationary comb teeth to come into contact and break the actuator all together. Two or more structural vibration modes with close resonant frequencies may be coupled to produce high vibration amplitude that leads to hinge failure. Thus, an apparatus and a method are needed in the design of MEMS scanning mirrors to effectively improve the vibration stability at resonance, and to ensure optical resolution of these devices. 
   SUMMARY 
   In one embodiment of the invention, a MEMS device includes a mirror having a top surface with trenches, a beam connected to the mirror, rotational comb teeth connected to the beam, and one or more springs connecting the beam to a bonding pad. The mirror can have a bottom surface for reflecting light. The mirror can include a top flange and a bottom flange joined by a web, wherein the top and the bottom flanges form the top and the bottom surfaces, respectively. The rotational comb teeth can have a tapered shape. The beam can be connected to the mirror at multiple locations. Stationary comb teeth can be interdigitated with the rotational comb teeth either in-plane or out-of-plane. A steady or oscillating voltage difference between the rotational and the stationary comb teeth can be used to oscillate or tune the mirror. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A ,  1 B, and  1 C illustrate top views of the layers in a MEMS device in one embodiment of the invention. 
       FIGS. 2A ,  2 B, and  2 C illustrate top views of the layers in a MEMS device in another embodiment of the invention. 
       FIG. 3  illustrates a deformation of a scanning mirror in one embodiment of the invention. 
       FIGS. 4A ,  4 B,  4 C,  4 D,  4 E,  4 F, and  4 G illustrate a MEMS device in another embodiment of the invention. 
       FIGS. 4H ,  4 I,  4 J, and  4 K illustrate the MEMS device of  FIG. 4A  with different power schemes in embodiments of the invention. 
       FIG. 5  illustrates process for manufacturing the device of  FIG. 4A  in one embodiment of the invention. 
       FIGS. 6A ,  6 B,  6 C,  6 D,  6 E, and  6 F illustrate a MEMS device in another embodiment of the invention. 
       FIGS. 6G ,  6 H,  6 I, and  6 J illustrate the MEMS device of  FIG. 6A  with different power schemes in embodiments of the invention. 
       FIG. 7  illustrates process for manufacturing the device of  FIG. 6A  in one embodiment of the invention. 
       FIG. 8  illustrates comb teeth in one embodiment of the invention. 
   

   Use of the same reference numbers in different figures indicates similar or identical elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A  illustrates a MEMS scanning mirror device  100  in one embodiment of the invention. Device  100  includes a top layer  100 A ( FIG. 1B ) and a bottom layer  100 B ( FIG. 1C ). 
   Referring to  FIG. 1B , top layer  100 A includes rotational comb teeth  108  that are connected on opposing sides of beam-like structures  103 A and  103 B. Proximate ends of beams  103 A and  103 B are connected by multiple support attachments  102  to opposing sides of a scanning mirror  101 . In other words, each beam is connected at multiple locations to scanning mirror  101 . The positions and the number of support attachments  102  can be refined through finite element analysis to improve the vibration stability and to minimize dynamic deformation of scanning mirror  101 . By improving the vibration stability and reducing dynamic deformation of scanning mirror  101  with support attachments  102 , the optical resolution of device  100  is improved. 
   Beams  103 A and  103 B are attached by eight serpentine springs/hinges  105 A to  105 H to bottom layer  100 B ( FIG. 1C ) in a distributed manner along the rotational axis (e.g., the x-axis) of scanning mirror  101 . Specifically, the distal end of beam  103 A is connected by spring/hinge  105 A to anchor  104 A, and the distal end of beam  103 B is connected by spring/hinge  105 H to anchor  104 H. Along their lengths, beam  103 A is connected by springs/hinges  105 B to  105 D to corresponding anchors  104 B to  104 D, and beam  103 B is connected by springs/hinges  105 E to  105 G to corresponding anchors  104 E to  104 G. In one embodiment, springs  105 B to  105 G are located within beams  103 A and  103 B. Anchors  104 A to  104 H are mounted to bottom layer  100 B ( FIG. 1C ). 
   Top layer  100 A may include stationary comb teeth  109 . In one embodiment, stationary comb teeth  109  provide the electrostatic biasing force used to increase the driving efficiency of the movable structure by tuning its modal frequency. In another embodiment, stationary comb teeth  109  provide the electrostatic driving force to drive scanning mirror  101 . In yet another embodiment, stationary comb teeth  109  provide both the electrostatic biasing force and the electrostatic driving force. 
   Referring to  FIG. 1C , bottom layer  100 B includes surfaces  106 A to  106 H that serve as anchoring surfaces for the movable structure in top layer  100 A ( FIG. 1A ). Specifically, anchors  104 A to  104 H are bonded to corresponding surfaces  106 A to  106 H. Cavity  107  accommodates the rotation of scanning mirror  101  without touching bottom layer  100 B. In one embodiment, stationary comb teeth  110  provide the electrostatic driving force to drive scanning mirror  101 . In another embodiment, stationary comb teeth  110  provide the electrostatic biasing force used to increase the driving efficiency of the movable structure. In yet another embodiment, stationary comb teeth  110  provide both the electrostatic driving force and the electrostatic biasing force. Stationary comb teeth  108  and  110  are interdigitated with rotational comb teeth  108  when viewed from above. 
   As described above, springs  105 A to  105 H are distributed along beams  103 A and  103 B. By carefully adjusting the distribution of the torsional and translational stiffness of these springs, all modal frequencies of the movable structure can be effectively separated and the desired rotational mode can be designed at the lowest resonance frequency. Since the main resonant frequency is the lowest and far apart from other structural modal frequencies, the mirror rotation driven by a sinusoidal AC voltage will not excite any other undesired vibration mode. 
   Using multiple springs, the maximum stress and strain on each individual spring are noticeably lower than conventional scanning mirror designs supported by only a pair of torsional beams. Therefore, the distributed spring design significantly improves the device reliability and increases the rotational angle. In summary, the system reliability and the servo and the optical performance are all improved with embodiments of the invention. 
     FIG. 2A  illustrates a MEMS scanning mirror device  200  in one embodiment of the invention. Device  200  includes a top layer  200 A ( FIG. 2B ) and a bottom layer  200 B ( FIG. 2C ). 
   Referring to  FIG. 2B , top layer  200 A includes a mirror  201  connected by multiple support attachments  202  to beams  203 A and  203 B. Mirror  201  and support attachments  202  are similar to those shown in  FIG. 1B . Rotational comb teeth  208  are connected to one side of beams  203 A and  203 B. 
   Beams  203 A and  203 B are connected by springs/hinges  205 A to  205 H to stationary surface  204  of top surface  200 A in a distributed manner along the rotational axis of scanning mirror  201 . Specifically, the distal end of beam  203 A is connected by spring/hinge  205 A to surface  204 , and the distal end of beam  203 B is connected by spring/hinge  205 H to surface  204 . Along their lengths, beam  203 A is connected by springs/hinges  205 B to  205 D to surface  204 , and beam  203 B is connected by springs/hinges  205 E to  205 G to surface  204 . 
   Referring to  FIG. 2C , bottom layer  200 B includes a cavity  207  that accommodates the rotation of scanning mirror  201  without touching bottom layer  200 B. In one embodiment, stationary comb teeth  210  provide the electrostatic driving force to drive scanning mirror  201 . In another embodiment, stationary comb teeth  210  provide the electrostatic biasing force used to increase the driving efficiency of the moving structure. In yet another embodiment, stationary comb teeth  210  provide both the electrostatic driving force and the electrostatic biasing force. Stationary comb teeth  210  are interdigitated with rotational comb teeth  208  when viewed from above. 
     FIG. 3  shows a typical mirror dynamic deformation of a mirror  301 . Mirror  301  rotates along the x-axis, which points in or out of the page. The total mirror dynamic deformation  302  is shown. The x-axis and the y-axis form a plane where the original mirror surface resides. The z-axis is used to describe the mirror out-of-plane motion. The mirror dynamic deformation is a function of mirror thickness, scanning frequency, mirror size, and rotation angle. The peak-to-peak dynamic deformation has to be smaller than one fourth of the wavelength to prevent diffraction from limiting the optical performance of the scanning mirror. It is estimated that the proposed mirror attachment structures and methods shown in  FIGS. 1A and 2A  reduce the mirror dynamic deformation up to 50 percent. 
     FIG. 4A  illustrates a MEMS scanning mirror device  400  in one embodiment of the invention. Device  400  includes a top layer  402  bonded atop but electrically insulated from a bottom layer  404 . 
     FIGS. 4B and 4C  illustrate the details of top layer  402 . Top layer  402  includes a top mirror layer  406  having an oblong shape. Top mirror layer  406  includes trenches/grooves  408  on its top surface. Trenches  408  reduce the mass of top mirror layer  406 , which in turn minimizes the total dynamic deformation. By minimizing the total dynamic deformation, the optical resolution of device  400  is improved. Although shown to run along the entire top surface, trenches  408  may be most effective when placed along the outer perimeter of top mirror layer  406  away from a rotational axis  414 . As described later, trenches  408  can be etched at the same time as other components by controlling their width so they are not etched through top mirror layer  406 . Alternatively, a shadow mask can be used to protect top mirror layer  406  during etching to prevent trenches  408  from etching through. The positions and the number of trenches  408  can be refined through finite element analysis. Gaps  409 A and  409 B separate top mirror layer  406  from the surrounding components in top layer  402 . As described later, the width of gaps  409 A and  409 B is designed to be greater than the widths of the gaps around more fragile components so that any trapped gas can escape around top mirror layer  406  instead of the fragile components during the etching process. 
   Opposing sides of top mirror layer  406  are connected by multiple support attachments  410  to the proximate ends of beam-like structures  412 A and  412 B. By connecting top mirror layer  406  at multiple locations to beams  412 A and  412 B, the dynamic deformation of top mirror layer  406  is minimized. The positions and the number of support attachments  410  can be refined through finite element analysis. 
   Opposing sides of beams  412 A and  412 B about a rotational axis  414  are connected to rotational comb teeth  416 . Rotational comb teeth  416  each has a tapered body that consists of an end rectangular section that has a smaller cross-section than a base rectangular section. By reducing the size and thus the weight of rotational comb teeth  416  at its ends, the inertia of the entire structure is reduced. By reducing the structural inertia, the scanning speed can be increased or/and the driving voltage can be reduced. In one embodiment, rotational comb teeth  416  provide the electrostatic biasing force used to increase the driving efficiency of the movable structure by tuning its modal frequency. In another embodiment, rotational comb teeth  416  provide the electrostatic driving force to driver the mirror. In yet another embodiment, rotational comb teeth  416  provide both the electrostatic biasing force and the electrostatic driving force. 
   Beams  412 A and  412 B are connected by serpentine springs to bonding pads mounted atop bottom layer  404 . Specifically, beam  412 A has a distal end connected by a serpentine spring  422 - 1  to a bonding pad  424 , and a midsection connected by serpentine springs  422 - 2  and  422 - 3  to a bonding pad  426  formed within beam  412 A. Similarly, beam  412 B has a distal end connected by a serpentine spring  428 - 1  to a bonding pad  430 , and a midsection connected by serpentine springs  428 - 2  and  428 - 3  to a bonding pad  432  formed within beam  412 B. Thus, beams  412 A and  412 B are connected by springs in a distributed manner along rotational axis  414  of top mirror layer  406 . Beams  412 A and  412 B may include holes  433  to reduce their mass. 
   By carefully adjusting the distribution of the stiffness and location of the springs, all modal frequencies of the movable structure can be effectively separated and the desired rotational mode can be designed at the lowest resonance frequency. Since the main resonant frequency is the lowest and far apart from other structural modal frequencies, the mirror rotation will not excite any other undesired vibration mode. By using multiple springs, the maximum stress and strain on each spring are lower than conventional scanning mirror designs supported by only a pair of torsional beams. Since the stress and strain on each spring are reduced, the reliability of each spring is improved and the rotational angle is increased. 
   Top layer  402  may include stationary comb teeth  434  that are interdigitated in-plane with rotational comb teeth  416 . Stationary comb teeth  434  may have a tapered body like rotational comb teeth  416 . In one embodiment, stationary comb teeth  434  provide the electrostatic biasing force used to increase the driving efficiency of the movable structure by tuning its modal frequency. In another embodiment, stationary comb teeth  434  provide the electrostatic driving force to drive top mirror layer  406 . In yet another embodiment, stationary comb teeth  434  provide both the electrostatic biasing force and the electrostatic driving force. Stationary comb teeth  434  are connected to bond pad  436  mounted atop bottom layer  404 . 
     FIGS. 4D ,  4 E,  4 F, and  4 G illustrate the details of bottom layer  404 . Bottom layer  404  includes a bottom mirror layer  460  having a protrusion  462  from an oblong plate  464 . A gap  465  separates bottom mirror layer  460  from the surrounding components in bottom layer  404 . As shown in  FIG. 4F , the bottom surface of plate  464  serves as the reflecting surface and other structures can be aligned with the mirror using assembly alignment marks  466  on the bottom surface of bottom layer  404 . The top surface  467  of bottom mirror layer  460  is bonded with the bottom surface of top mirror layer  406  to form the final mirror. As shown in  FIG. 4G , the final mirror has an I-beam like structure where top mirror layer  406  forms the top flange, protrusion  462  forms the web, and plate  464  forms the bottom flange. The I-beam like structure removes most of the mirror mass and stiffens the mirror structure. Therefore, it minimizes the dynamic deformation of the bottom mirror surface. By minimizing the total dynamic deformation of bottom mirror surface, the optical resolution of device  400  is improved. The shape of the I-beam like structure can be refined through finite element analysis. 
   Bottom layer  404  includes surfaces for anchoring the bonding pads of the movable structure in top layer  402 . Specifically, anchoring pads  468  and  470  provide surfaces for mounting corresponding bonding pads  426  and  432 , and anchoring pad  472  provides a surface for mounting bonding pads  424 ,  430 , and  436 . 
   Bottom layer  404  includes stationary comb teeth  474  that are out-of-plane interdigitated with rotational comb teeth  416 . In other words, they are interdigitated when viewed from above or when the final mirror is rotated. Stationary comb teeth  474  may have a tapered body like comb teeth  416  and  434 . Referring to  FIG. 4E , a gap  482  is provided between stationary comb teeth  474  and anchoring pad  472 . Gap  482  has a width greater than gaps  484  between adjacent stationary comb teeth  474  so that gap  482  is etched deeper into bottom layer  404  than gaps  484 . A deeper gap  482  allows rotational comb teeth  416  to rotate at a greater angle without contacting bottom layer  404 . In one embodiment, stationary comb teeth  474  provide the electrostatic driving force to drive the final mirror. In another embodiment, stationary comb teeth  474  provide the electrostatic biasing force used to increase the driving efficiency of the movable structure. In another embodiment, stationary comb teeth  474  provide both the electrostatic driving force and the electrostatic biasing force. In yet another embodiment, the capacitance between rotational comb teeth  416  and stationary comb teeth  474  is sensed to determine the rotational position of the mirror. 
     FIG. 5  illustrates a method  500  for making device  400  in one embodiment of the invention. The process starts at a step  0  with a silicon wafer  502  having a silicon dioxide layer  504  formed on the top surface and a silicon dioxide layer  506  formed on the bottom surface. Wafer  502  is used to form bottom layer  404  ( FIG. 4E ) of device  400 . 
   In step  1 , a photoresist  508  is deposited on oxide layer  506 , exposed, and developed in a lithographic process to define one or more lithographic alignment marks  511  (shown in step  3 ). 
   In step  2 , the bottom surface of wafer  502  is etched to remove portions of oxide layer  506  left unprotected by photoresist  508 . In one embodiment, oxide layer  506  is dry etched. The top surface of wafer  502  is deposited with a photoresist  510  to protect it from the etching of the bottom surface. 
   In step  3 , the bottom wafer surface of wafer  502  is etched to remove portions of wafer  502  left unprotected by oxide layer  506  to form lithography alignment marks  511 . After the silicon dry etch, the remaining photoresists  508  and  510  are stripped. 
   In step  4 , photoresist  510  is reapplied and is exposed and developed in a lithographic process to define bottom mirror layer  460  ( FIG. 4E ), surfaces  468 ,  470 , and  472  ( FIG. 4E ), and stationary comb teeth  474  ( FIG. 4E ) on the top surface of wafer  502 . The mask used is aligned with the lithographic alignment marks  511  on the bottom wafer surface. 
   In step  5 , the top surface of wafer  502  is etched to remove portions of oxide layer  504  left unprotected by photoresist  510 . In one embodiment, oxide layer  504  is dry etched. 
   In step  6 , the top surface of wafer  502  is etched to remove portions of wafer  502  left unprotected by oxide layer  504  to form bottom mirror layer  460  ( FIG. 4E ), surfaces  468 ,  470 , and  472 , and stationary comb teeth  474  ( FIG. 4E ). Afterwards, the remaining photoresist  510  is stripped and oxide layers  504  and  506  are removed by a wet or dry etch. 
   In step  7 , a silicon wafer  512  is bonded to the top surface of wafer  502 . Wafer  512  has a silicon dioxide layer  514  formed on the top wafer surface and a silicon dioxide layer  516  formed on the bottom wafer surface. Wafer  512  is used to form top layer  402  ( FIG. 4C ) of device  400 . In one embodiment, wafers  512  and  502  are bonded by silicon fusion. 
   In step  8 , a photoresist  518  is deposited on oxide layer  514 , exposed, and developed in a lithographic process to define the components of top layer  402  ( FIG. 4C ). The mask used is aligned with lithographic alignment marks  511  on the bottom wafer surface. Also defined in step  8  are one or more lithographic alignment marks  521  (shown in step  10 ) and a separation trench  519  (shown in step  10 ). In order to etch trenches  408  ( FIG. 4C ), which are etched into wafer  512  at a particular depth, along with the gaps that surround the other components, which are etched through wafer  512 , the dimensions of trenches  408  and the gaps of the other components are differentiated. 
   In step  9 , the top surface of wafer  512  is etched to remove portions of oxide layer  514  left unprotected by photoresist  518 . In one embodiment, oxide layer  514  is dry etched. Afterwards, the remaining photoresist  518  is stripped. 
   In step  10 , the top surface of wafer  512  is etched to remove portions of wafer  512  left unprotected by oxide layer  514  to form the components of top layer  402  ( FIG. 4C ). In one embodiment, wafer  512  is etched using a DRIE process down to the etch stop formed by oxide layer  516 . When the top of device  400  is etched through, gas trapped between the bonded wafers  502  and  512  may escape and damage fragile components such as the comb teeth. To prevent such damage, gaps  409 A and  409 B ( FIG. 4C ) around top mirror layer  406  ( FIG. 4C ) are designed to be larger than the gaps around the other components so that oxide layer  516  beneath gaps  409 A and  409 B is etched through before the other gaps. This allows the air to escape around top mirror layer  406 , which is a structurally strong component. 
   In step  11 , the top surface of the mirror is protected by a shadow mask surface  522  to prevent the top mirror layer  406  from being etched through. This step is optional if inertia-reducing trenches  408  have a width that is smaller than other gaps so they are not etched through. However, the shadow mask may be preferred to create inertia-reducing trenches  408  having greater width, thereby removing more mass and further reducing the inertia of the rotating structure. 
   In step  12 , a photoresist  520  is deposited on the bottom surface of wafer  502 , exposed, and developed on the bottom surface of wafer  502  to define assembly alignment marks  466  ( FIG. 4F ), separation trench  509  (shown in step  13 ), and gap  465  ( FIG. 4E ) for separating bottom mirror layer  460  ( FIG. 4E ) from bottom layer  404  ( FIG. 4E ). The mask used is aligned with lithographic alignment marks  521  on the top wafer surface. 
   In step  13 , the bottom surface of wafer  502  is etched to remove portions of wafer  502  left unprotected by photoresist  520  to form assembly alignment marks  466  ( FIG. 4F ) and separation trench  509 , and to separate bottom mirror layer  460  ( FIG. 4E ) from layer  404  ( FIG. 4E ). In one embodiment, wafer  502  is etched using a DRIE process. 
   In step  14 , portions of oxide layer  516  are removed from the structure to release the various components of device  400  while maintaining the bonds between the corresponding bonding and anchoring pads. In one embodiment, portions of oxide layer  516  are removed using a hydrofluoric acid wet etch. 
   In step  15 , the bottom surface of bottom mirror layer  460  ( FIG. 4F ) is deposited with a reflective material (e.g., aluminum) to create a mirror surface. In one embodiment, a shadow mask is used to define areas to be deposited with the reflective material. 
   In step  16 , devices  400  made from wafers  502  and  512  are singulated. In one embodiment, wafers  502  and  512  are singulated by dicing through separation trenches  509  and  519  (shown in step  15 ). 
   Referring to  FIG. 4A , the operation of device  400  in one embodiment is explained hereafter. Rotational comb teeth  416  are coupled via bonding pad  424  to receive a reference voltage from a voltage source  476  (e.g., ground). Stationary comb teeth  434  are coupled via bonding pad  436  to receive a steady voltage from a voltage source  478  (e.g., a DC voltage source). Stationary comb teeth  474  ( FIGS. 4D and 4E ) are coupled via bonding pad  472  to receive an oscillating voltage from a voltage source  480  (e.g., an AC voltage source). Thus, a steady voltage difference between rotational comb teeth  416  and stationary comb teeth  434  changes the natural frequency of device  400 , whereas an AC voltage difference between rotational comb teeth  416  and stationary comb teeth  474  ( FIGS. 4D and 4E ) oscillates the mirror at the desired scanning frequency and at the desired scanning angle. 
   Referring to  FIG. 4H , the operation of device  400  in another embodiment is explained hereafter. Rotational comb teeth  416  are coupled via bonding pad  424  to receive a steady voltage from voltage source  476  (e.g., a DC voltage source). Stationary comb teeth  434  are coupled via bonding pad  436  to receive an oscillating voltage from AC voltage source  480 . Stationary comb teeth  474  ( FIGS. 4D and 4E ) are coupled via bonding pad  472  to receive a steady voltage from DC voltage source  478 . Between rotational comb teeth  416  and stationary comb teeth  434 , a steady voltage difference changes the natural frequency and the rotation amplitude of device  400  while an AC voltage difference oscillates the mirror at the desired scanning frequency and at the desired scanning angle. Furthermore, a steady voltage difference between rotational comb teeth  416  and stationary comb teeth  474  ( FIGS. 4D and 4E ) can also be used to change the amplitude of the rotational angle of device  400 . The capacitance between rotational comb teeth  416  and stationary comb teeth  474  can also be sensed through respective bonding pads  436  and  472  to determine the rotational angle of device  400 . 
   Referring to  FIG. 4I , the operation of device  400  in another embodiment is explained hereafter. Rotational comb teeth  416  are coupled via bonding pad  424  to receive an oscillating voltage from AC voltage source  480 . Stationary comb teeth  434  are coupled via bonding pad  436  to receive a steady voltage from DC voltage source  476 . Stationary comb teeth  474  ( FIGS. 4D and 4E ) are coupled via bonding pad  472  to receive a steady voltage from DC voltage source  478 . Between rotational comb teeth  416  and stationary comb teeth  434 , a steady voltage difference changes the natural frequency and the rotation amplitude of device  400  while an AC voltage difference between rotational comb teeth  416  and stationary comb teeth  434  oscillates the mirror at the desired scanning frequency and at the desired scanning angle. A steady voltage difference between rotational comb teeth  416  and stationary comb teeth  474  ( FIGS. 4D and 4E ) can also be used to change the amplitude of the rotational angle of device  400 . The capacitance between rotational comb teeth  416  and stationary comb teeth  474  can also be sensed through respective bonding pads  436  and  472  to determine the rotational angle of device  400 . 
   Referring to  FIG. 4K , the operation of device  400  in another embodiment is explained hereafter. Rotational comb teeth  416  are coupled via bonding pad  424  to receive an oscillating voltage from AC voltage source  480 A. Stationary comb teeth  434  are coupled via bonding pad  436  to receive a steady voltage from DC voltage source  476 . Stationary comb teeth  474  ( FIGS. 4D and 4E ) are coupled via bonding pad  472  to receive an oscillating voltage from AC voltage source  480 B. Between rotational comb teeth  416  and stationary comb teeth  434 , a DC voltage difference changes the natural frequency and the rotation amplitude of device  400  while an AC voltage difference oscillates the mirror at the desired scanning frequency and at the desired scanning angle. Between rotational comb teeth  416  and stationary comb teeth  474  ( FIGS. 4D and 4E ), a DC voltage difference can also be used to change the amplitude of the rotational angle of device  400  while an oscillating voltage difference can also be used to oscillate the mirror at the desired scanning frequency and at the desired scanning angle. The capacitance between rotational comb teeth  416  and stationary comb teeth  474  can also be sensed through respective bonding pads  436  and  472  to determine the rotational angle of device  400 . 
   Referring to  FIG. 4I , the operation of device  400  in another embodiment is explained hereafter. Rotational comb teeth  416  are coupled via bonding pad  424  to receive a steady voltage from DC voltage source  476 . Stationary comb teeth  434  are coupled via bonding pad  436  to receive an oscillating voltage from an AC voltage source  480 A. Stationary comb teeth  474  ( FIGS. 4D and 4E ) are coupled via bonding pad  472  to receive an oscillating voltage from an AC voltage source  480 B. The oscillating voltage provided by AC voltage source  480 B is out of phase (e.g., 180 degrees out of phase) with the oscillating voltage provided by voltage source  480 A. Between rotational comb teeth  416  and stationary comb teeth  434 , a steady voltage difference changes the natural frequency and the rotation amplitude of device  400  while an AC voltage difference oscillates the mirror at the desired scanning frequency and at the desired scanning angle. An AC voltage difference between rotational comb teeth  416  and stationary comb teeth  474  ( FIGS. 4D and 4E ) can also be used to oscillate the mirror at the desired scanning frequency and at the desired scanning angle. The capacitance between rotational comb teeth  416  and stationary comb teeth  474  can also be sensed through respective bonding pads  436  and  472  to determine the rotational angle of device  400 . 
     FIG. 6A  illustrates a MEMS scanning mirror device  600  in one embodiment of the invention. Device  600  includes a top layer  602  bonded atop but electrically insulated from a bottom layer  604 . 
     FIGS. 6B and 6C  illustrate the details of top layer  602 . Top layer  602  includes a mirror  606  having an oblong shape. The bottom surface of mirror  606  serves as the reflecting surface. The top surface of mirror  606  includes trenches/grooves  608 A,  608 B,  608 C, and  608 D. Trenches  608 A are formed along the top outer perimeter of mirror  606  while trenches  608 B are formed along the bottom outer perimeter of mirror  606 . Trenches  608 C and  608  are formed on the midsection of mirror  606 . Trenches  608 A,  608 B,  608 C, and  608 D reduce the mass of mirror  606 , which in turn minimizes the dynamic deformation of mirror  606 . By minimizing dynamic deformation of mirror  606 , the optical resolution of device  600  is improved. As described later, trenches  608  can be etched at the same time as other components by controlling their width so they are not etched through mirror  606 . Alternatively, a shadow mask can be used to protect mirror  606  during etching to prevent trenches  608  from etching through. The mirror mass and inertia can be further reduced after the fabrication process by laser trimming. This method can adjust the mirror natural frequency. The effective place to remove the mirror mass is the area around the top and bottom outer perimeters of mirror  606 . Therefore, areas on mirror  606  can be reserved for the laser trimming process. 
   As described later, trenches  608  can be etched at the same times as other components by controlling their width so they are not etched through mirror  606 . The trenches were designed to remove the mirror mass around the mirror tips and outer diameter. This will effectively reduce the mirror inertia and reduce the mirror dynamic deformation. The positions and the number of trenches  608  can be refined through finite element analysis. Gaps  609 A and  609 B separate mirror  606  from the surrounding components. As described later, the width of gaps  609 A and  609 B is designed to be greater than the widths of gaps around more fragile components so that any trapped gas can escape around mirror  606  instead of the fragile components during the etching process. 
   Opposing sides of mirror  606  are connected by multiple support attachments  610  to the proximate ends of beam-like structures  612 A and  612 B. By connecting mirror  606  at multiple locations to beams  612 A and  612 B, the dynamic deformation of mirror  606  is minimized. The positions and the number of support attachments  610  can be refined through finite element analysis. 
   Opposing sides of beams  612 A and  612 B about a rotational axis  614  are connected to rotational comb teeth  616 . Rotational comb teeth  616  each has a tapered body having an end rectangular section that has a smaller cross-section than a base rectangular section. By reducing the size and thus the weight of rotational comb teeth  616  at its end, the inertia of the entire structure is reduced. By reducing the structural inertia, the scanning speed can be increased or/and the driving voltage can be reduced. In one embodiment, rotational comb teeth  616  provide the electrostatic biasing force used to increase the driving efficiency of the movable structure by tuning its modal frequency. In another embodiment, rotational comb teeth  616  provide the electrostatic driving force to driver mirror  606 . In yet another embodiment, rotational comb teeth  616  provide both the electrostatic biasing force and the electrostatic driving force. 
   Beams  612 A and  612 B are connected by serpentine springs to bonding pads mounted atop bottom layer  604 . Specifically, beam  612 A has a distal end connected by a serpentine spring  622 - 1  to a bonding pad  624 , and a midsection connected by serpentine springs  622 - 2  and  622 - 3  to a bonding pad  626  formed within beam  612 A. Similarly, beam  612 B has a distal end connected by a serpentine spring  628 - 1  to a bonding pad  630 , and a midsection connected by serpentine springs  628 - 2  and  628 - 3  to a bonding pad  632  formed within beam  612 B. Thus, beams  612 A and  612 B are connected by springs in a distributed manner along rotational axis  614  of top mirror  606 . Beams  612 A and  612 B may include holes  633  to reduce their mass. 
   By carefully adjusting the distribution of the location and stiffness of the springs, all modal frequencies of the movable structure can be effectively separated and the desired rotational mode can be designed at the lowest resonance frequency. Since the main resonant frequency is the lowest and far apart from other structural modal frequencies, the mirror rotation will not excite any other undesired vibration mode. By using multiple springs, the maximum stress and strain on each spring are lower than conventional scanning mirror designs supported by only a pair of torsional beams. Since the stress and strain on each spring are reduced, the reliability of each spring is improved and the rotational angle is increased. 
   Top layer  602  may include stationary comb teeth  634  that are interdigitated in-plane with rotational comb teeth  616 . Stationary comb teeth  634  may have a tapered body like rotational comb teeth  616 . In one embodiment, stationary comb teeth  634  provide the electrostatic biasing force used to increase the driving efficiency of the movable structure by tuning its modal frequency. In another embodiment, stationary comb teeth  634  provide the electrostatic driving force to drive mirror  606 . In yet another embodiment, stationary comb teeth  634  provide both the electrostatic biasing force and the electrostatic driving force. Stationary comb teeth  634  are connected to bonding pad  636  mounted atop bottom layer  404 . 
     FIGS. 6D ,  6 E, and  6 F illustrate the details of bottom layer  604 . Bottom layer  604  includes an opening  665  that accommodates the rotation of mirror  606  without touching bottom layer  604 . As shown in  FIG. 6F , the bottom surface of bottom layer  604  includes assembly alignment marks  666  for aligning other structures with mirror  606 . 
   Bottom layer  604  includes surfaces for anchoring the bonding pads of the movable structure in top layer  602 . Specifically, anchoring pads  668  and  670  provide a surface for mounting bonding pads  626  and  632 , and anchoring pad  672  provides a surface for mounting bonding pads  624 ,  630 ,  636 . 
   Bottom layer  604  includes stationary comb teeth  674  that are interdigitated out-of-plane with rotational comb teeth  616 . In other words, they are interdigitated when viewed from above. Stationary comb teeth  674  may have a tapered body like comb teeth  616  and  634 . Referring to  FIG. 6E , a gap  682  is provided between stationary comb teeth  674  and anchoring pad  672 . Gap  682  has a greater width than gaps  684  between adjacent stationary comb teeth  674  so that gap  682  is etched deeper into bottom layer  604  than gaps  684 . A deeper gap  682  allows rotational comb teeth  616  to rotate at a greater angle without contacting bottom layer  604 . In one embodiment, stationary comb teeth  674  provide the electrostatic driving force to drive the final mirror. In another embodiment, stationary comb teeth  674  provide the electrostatic biasing force used to increase the driving efficiency of the movable structure. In another embodiment, stationary comb teeth  674  provide both the electrostatic driving force and the electrostatic biasing force. In yet another embodiment, the capacitance between rotational comb teeth  616  and stationary comb teeth  674  is sensed to determine the rotational position of the mirror. 
     FIG. 7  illustrates a method  700  for making device  600  in one embodiment of the invention. The process starts at a step A with a silicon wafer  702  having a silicon dioxide layer  704  formed on the top wafer surface and a silicon dioxide layer  706  formed on the bottom wafer surface. Wafer  702  is used to form bottom layer  604  of device  600 . 
   In step B, a photoresist  708  is deposited on oxide layer  706 , exposed, and developed in a lithographic process to define one or more lithographic alignment marks  711  (shown in step D). 
   In step C, the bottom surface of wafer  702  is etched to remove portions of oxide layer  706  left unprotected by photoresist  708 . In one embodiment, oxide layer  706  is dry etched. The top surface of wafer  702  is deposited with a photoresist  710  to protect it from the etching of the bottom surface. 
   In step D, the bottom surface of wafer  702  is etched to remove portions of wafer  702  left unprotected by oxide layer  706  to form lithographic alignment marks  711 . 
   In step E, photoresist  710  is exposed and developed in a lithographic process to define surfaces  668 ,  670 , and  672  ( FIG. 6E ), and stationary combs  674  ( FIG. 6E ) on the top surface of wafer  702 . The mask used is aligned and exposed with the lithographic alignment marks  711  on the bottom wafer surface. 
   In step F, the top surface of wafer  702  is etched to remove portions of oxide layer  704  left unprotected by photoresist  710 . In one embodiment, oxide layer  704  is dry etched. 
   In step G, the top surface of wafer  702  is etched to remove portions of wafer  702  left unprotected by oxide layer  704  to form anchoring pads  668 ,  670 , and  672  ( FIG. 6E ), and stationary combs  674  ( FIG. 6E ). In one embodiment, wafer  702  is etched using a deep reactive ion etching (DRIE) process. Afterwards, the remaining oxide layers  704  and  706  are removed. 
   In step H, a silicon wafer  712  is bonded to the top surface of wafer  702 . Wafer  712  has a silicon oxide layer  714  formed on the top wafer surface and a silicon dioxide layer  716  formed on the bottom wafer surface. Wafer  712  is used to form top layer  602  ( FIG. 6C ) of device  600 . In one embodiment, wafers  712  and  702  are bonded by silicon fusion. 
   In step I, a photoresist  718  is deposited on oxide layer  714 , exposed, and developed in a lithographic process to define the components of top layer  602  ( FIG. 6C ). Also defined in step I are one or more lithographic alignment marks  721  (shown in step K) and a separation trench  719  (shown in step K) In order to etch inertia-reducing trenches  608  ( FIG. 6C ), which are etched into wafer  712  at a particular depth, along with the gaps that surround the other components, which are etched through wafer  712 , the dimensions of trenches  608  and the gaps are differentiated. The mask used is aligned and exposed with lithographic alignment marks  711  on the bottom wafer surface. 
   In step J, the top surface of wafer  712  is etched to remove portions of oxide layer  714  left unprotected by photoresist  718 . In one embodiment, thermal oxide layer  714  is dry etched. 
   In step K, the top surface of wafer  712  is etched to remove portions of wafer  712  left unprotected by oxide layer  714  to form the components of top layer  602  ( FIG. 6C ), alignment marks  721  and separation trench  719 . In one embodiment, wafer  712  is etched using a DRIE process down to the etch stop formed by oxide layer  716 . When the top of device  600  is etched through, gas trapped between the bonded wafers  702  and  712  may escape and damage fragile components such as the comb teeth. To prevent such damage, gaps  609 A and  609 B ( FIG. 6C ) around mirror  606  ( FIG. 6C ) are designed to be larger than the gaps around the other components so that oxide layer  716  beneath gaps  609 A and  609 B is etched through before the other gaps. This allows the air to escape around mirror  606 , which is a structurally strong component. 
   In step L, the top surface of the mirror is protected by a shadow mask surface  722  to prevent the top mirror layer from being etched through. This step is optional if inertia-reducing trenches  608  have a width that is smaller than other gaps so they are not etched through. However, the shadow mask may be preferred to create inertia-reducing trenches  608  having greater width, thereby removing more mass and further reducing the inertia of the rotating structure. 
   In step M, a photoresist  720  is deposited on the bottom surface of wafer  702 , exposed, and developed on the bottom wafer surface of wafer  702  to define opening  665  ( FIG. 6E ) for mirror  606  ( FIG. 6C ). The mask used is aligned and exposed with lithographic alignment marks  721  on the top surface. 
   In step N, the bottom surface of wafer  702  is etched to remove portions of wafer  702  left unprotected by photoresist  720  to form opening  665  ( FIG. 6E ). In one embodiment, silicon wafer  702  is etched using a DRIE process. Afterwards, the remaining photoresist  720  is stripped. 
   In step O, portions of oxide layer  716  are removed from the structure to release the various components of device  600  while maintaining the bonds between the corresponding bonding and anchoring pads. In one embodiment, portions of oxide layer  716  are removed using a hydrofluoric acid wet etch. 
   In step P, the bottom surface of mirror  606  ( FIG. 6C ) is deposited with a reflective material (e.g., aluminum) to create a mirror surface. In one embodiment, a shadow mask is used to define areas to be deposited with the reflective material. 
   In step Q, devices  600  made from wafers  702  and  712  are singulated. In one embodiment, wafers  702  and  712  are singulated by dicing through separation trenches  709  and  719 . 
   Referring back to  FIG. 6A , the operation of device  600  in one embodiment is explained hereafter. Rotational comb teeth  616  are coupled via bonding pad  624  to receive a reference voltage from a voltage source  676  (e.g., ground). Stationary comb teeth  634  are coupled via bonding pad  636  to receive a steady voltage from a voltage source  678  (e.g., a DC voltage source). Stationary comb teeth  674  ( FIGS. 6D and 6E ) are coupled via bonding pad  672  to receive an oscillating voltage from a voltage source  680  (e.g., an AC voltage source). Thus, a steady voltage difference between rotational comb teeth  616  and stationary comb teeth  634  changes the natural frequency and the rotation amplitude of device  600 , whereas an AC voltage difference between rotational comb teeth  616  and stationary comb teeth  674  ( FIGS. 6D and 6E ) oscillates the mirror at the desired scanning frequency and at the desired scanning angle. 
   Referring to  FIG. 6G , the operation of device  600  in another embodiment is explained hereafter. Rotational comb teeth  616  are coupled via bonding pad  624  to receive a steady voltage from a DC voltage source ground  676 . Stationary comb teeth  634  are coupled via bonding pad  636  to receive an oscillating voltage from AC voltage source  680 . Stationary comb teeth  674  ( FIGS. 6D and 6E ) are coupled via bonding pad  672  to receive a steady voltage from DC voltage source  678 . Between rotational comb teeth  616  and stationary comb teeth  634 , a steady voltage difference changes the natural frequency and the rotation amplitude of device  600  while an AC voltage difference oscillates the mirror at the desired scanning frequency and at the desired scanning angle. A steady voltage difference between rotational comb teeth  616  and stationary comb teeth  674  ( FIGS. 6D and 6E ) can be used to change the amplitude of the rotational angle of device  600 . The capacitance between rotational comb teeth  616  and stationary comb teeth  674  can also be sensed through respective bonding pads  636  and  672  to determine the rotational angle of device  600 . 
   Referring to  FIG. 6H , the operation of device  600  in another embodiment is explained hereafter. Rotational comb teeth  616  are coupled via bonding pad  624  to receive an oscillating voltage from AC voltage source  680 . Stationary comb teeth  634  are coupled via bonding pad  636  to receive a steady voltage from DC voltage source  676 . Stationary comb teeth  674  ( FIGS. 6D and 6E ) are coupled via bonding pad  672  to receive a steady voltage from DC voltage source  678 . Between rotational comb teeth  616  and stationary comb teeth  634 , a steady voltage difference changes the natural frequency and the rotation amplitude of device  600  while an AC voltage difference oscillates the mirror at the desired scanning frequency and at the desired scanning angle. A steady voltage difference between rotational comb teeth  616  and stationary comb teeth  674  ( FIGS. 6D and 6E ) can be used to change the amplitude of the rotational angle of device  600 . The capacitance between rotational comb teeth  616  and stationary comb teeth  674  can also be sensed through respective bonding pads  636  and  672  to determine the rotational angle of device  600 . 
   Referring to  FIG. 6J , the operation of device  600  in another embodiment is explained hereafter. Rotational comb teeth  616  are coupled via bonding pad  624  to receive an oscillating voltage from AC voltage source  680 A. Stationary comb teeth  634  are coupled via bonding pad  636  to receive a steady voltage from DC voltage source  676 . Stationary comb teeth  674  ( FIGS. 6D and 6E ) are coupled via bonding pad  672  to receive an oscillating voltage from AC voltage source  680 B. Between rotational comb teeth  616  and stationary comb teeth  634 , a DC voltage difference changes the natural frequency and the rotation amplitude of device  600  while an AC voltage difference oscillates the mirror at the desired scanning frequency and at the desired scanning angle. Between rotational comb teeth  616  and stationary comb teeth  674  ( FIGS. 6D and 6E ), a DC voltage difference can also be used to change the amplitude of the rotational angle of device  600  while an oscillating voltage difference can also be used to oscillate the mirror at the desired scanning frequency and at the desired scanning angle. The capacitance between rotational comb teeth  616  and stationary comb teeth  674  can also be sensed through respective bonding pads  636  and  672  to determine the rotational angle of device  600 . 
   Referring to  FIG. 6I , the operation of device  600  in another embodiment is explained hereafter. Rotational comb teeth  616  are coupled via bonding pad  624  to receive a steady voltage from DC voltage source  676 . Stationary comb teeth  634  are coupled via bonding pad  636  to receive an oscillating voltage from an AC voltage source  680 A. Stationary comb teeth  674  ( FIGS. 6D and 6E ) are coupled via bonding pad  672  to receive an oscillating voltage from an AC voltage source  680 B. The oscillating voltage provided by AC voltage source  680 B is out of phase (e.g., 180 degrees out of phase) with the oscillating voltage provided by voltage source  680 A. Between rotational comb teeth  616  and stationary comb teeth  634 , a steady voltage difference changes the natural frequency and the rotation amplitude of device  600  while an AC voltage difference oscillates the mirror at the desired scanning frequency and at the desired scanning angle. An AC voltage difference between rotational comb teeth  616  and stationary comb teeth  674  ( FIGS. 6D and 6E ) can also be used to oscillate the mirror at the desired scanning frequency and at the desired scanning angle. The capacitance between rotational comb teeth  616  and stationary comb teeth  674  can also be sensed through respective bonding pads  636  and  672  to determine the rotational angle of device  600 . 
     FIG. 8  illustrates comb teeth having another shape in one embodiment of the invention. Rotational comb teeth  816  each has a triangular body that tapers from the base to the end. By reducing the size and thus the weight of rotational comb teeth  816  at its ends, the inertia of the entire structure is reduced. By reducing the structural inertia, the scanning speed can be increased or/and the driving voltage can be reduced. Stationary comb teeth  834  and the stationary comb teeth in the lower layer can have the same triangular shape. 
   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.