Abstract:
A micro-electro-mechanical component comprising a movable element with comb electrodes, and two stationary elements with comb electrodes aligned and stacked on each other but electrically insulated by a layer of insulation material. The movable element is supported by multiple torsional hinges and suspended over a cavity such that the element can oscillate about an axis defined by the hinges. The comb electrodes of the movable element are interdigitated with the comb electrodes of one stationary element in the same plane to form an in-plane comb actuator. The comb electrodes of the movable element are also interdigitated in an elevated plane with the comb electrodes of another stationary element to form a vertical comb actuator. As a result, the micro-electro-mechanical component is both an in-plane actuator and a vertical comb actuator, or a multiple-plane actuator. Methods of fabricating such actuator are also described.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a micro-electro-mechanical component, and more particularly to actuator. 
   Micro-electro-mechanical mirrors have great potential in wide variety of optical applications including optical communication, confocal microscope, laser radar, bar code scanning, laser printing and projection display. For some optical scanning applications such as laser printing and scanning projection display, the mirror needs to achieve large optical scanning angle at specific frequency. Large optical angle is also a key to optical resolution and smaller product footprint. For scanning mirror, this requirement poses a challenge in the design of actuator to generate large actuation force. A variety of micro-electro-mechanical actuator designs have been proposed to steer or scan light beam for various applications. In order to achieve deflection or movement of the micro-component out of the chip plane, it is known to design a movable element containing electrodes and a stationary element containing counter-electrodes such that the movable element can be driven by the electrical force. 
   In U.S. Pat. No. 6,595,055, Harald Schenk, et al described a micromechanical component with both the oscillating body and the frame or stationary layer located on the same chip plane. Capacitance is formed between the lateral surfaces of the oscillating body and the frame layer and will vary as the movable body oscillates about a pivot axis out of the chip plane. The structure is suspended and supported by an insulating layer and a substrate to allow out-of-plane motion of the oscillating body. They described in “Large Deflection Micromechanical Scanning Mirrors for Linear Scan and Pattern Generation” in Journal of Selected Topics in Quantum Electronics, Vol 6, No 5, 2000 that the scanning mirror can scan at large angle with low driving voltage at low frequency. However, movable comb electrodes located on the mirror perimeter will increase dynamic deformation of the mirror or movable body. Excessive dynamic deformation of scanning mirror will increase divergence of reflected light beam and significantly deteriorate optical resolution of the device for high speed scanning applications such as printing and scanned display. Additional electrode insulated from the structure may be required to perturb the symmetry of the setup in order to quickly initiate oscillation of the mirror. Furthermore, the setup only allows analog operation (scanning) but not digital operation (static angle positioning) of the movable body. 
   R. Conant describes in “Staggered Torsional Electrostatic Combdrive and Method of Forming SAME” (Patent Application US2003/0019832), a comb-drive actuator with a stationary comb teeth assembly and a moving comb teeth assembly with a mirror and a torsional hinge, and the method of fabricating such devices. The moving assembly is positioned entirely above the stationary assembly by a predetermined vertical displacement during resting state. The actuator is able to scan at relative high frequency with mirror dynamic deformation lower than the Rayleigh limit. However, the optical scan angle which dominates the optical resolution is notably smaller than what Schenk has reported despite a relative high voltage is applied. An alternate design was proposed with additional stationary comb teeth assemblies stacked on top of the stationary comb teeth assembly. This stacked comb teeth assemblies were claimed to be used for the purpose of capacitive sensing and frequency tuning of the movable assembly despite that the method of frequency tuning was not described. In the fabrication process steps, a process step is required to open alignment windows by etching through the top wafer to reach the insulating oxide layer then removing the oxide layer in order to use features located on the bottom wafer for alignment of subsequent steps. If the top wafer is thick for the purpose of minimizing dynamic deformation, this process could be time-consuming and hence, expensive. 
   S. Olav describes in “Self-Aligned Vertical Combdrive Actuator and Method of Fabrication” (US Patent Application US2003/0073261), a vertical comb-drive actuator with small gaps between comb teeth for increased torsional deflection, a double-sided vertical comb-drive actuator for dual-mode actuation, vertical piston and scan, and the method of making them. Despite the proposed fabrication process steps allow self-alignment of the embedded comb teeth, the process of vertical comb-drive actuator requires highly skilled techniques to etch the bottom comb teeth and twice deep silicon trench etching of the bottom substrate. For dual-mode vertical comb-drive actuator, the fabrication process steps start with deep silicon trench etching of the device layer of a Silicon-On-Insulator (SOI) wafer then fusion bonding to another silicon wafer that resulting in a complex five-layer structure, two insulation oxide layers and three silicon layers. To form the bottom comb teeth highly skilled self-alignment etching techniques and twice deep silicon trench etching are still required. 
   SUMMARY OF THE INVENTION 
   It is the objective of the present invention to provide a micro-electro-mechanical actuator with in-plane comb electrodes and a supporting substrate with a cavity of specific depth. 
   It is the objective of the present invention to provide a micro-electro-mechanical actuator with both in-plane and vertical comb electrodes that increase the actuation force on the movable element, and the methods of fabricating such device. 
   It is a further objective of this invention to provide a micro-electro-mechanical actuator with both in-plane and dual-side vertical comb electrodes that increase the actuation force on the movable element, and the methods of fabricating such devices. 
   It is another objective of this invention to provide a method to support and fan out the bottom electrodes of the vertical comb electrodes. 
   It is another objective of this invention to provide a torsional hinge design with built-in electrodes that can be used to increase the effective torsional stiffness of the hinges such that the resonance frequency of the movable element in an actuator can be adjusted. 
   It is another objective of this invention to provide a method to decrease the effective torsional stiffness of the torsional hinges such that the resonance frequency of the movable element in an actuator can be adjusted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A ,  1 B, and  1 C show the top views of the top, middle and bottom layers of one embodiment of the present invention. 
       FIGS. 1D ,  1 E and  1 F illustrate the fabrication process flow steps of the embodiment described in  FIGS. 1A ,  1 B and  1 C. 
       FIGS. 1G and 1H  illustrate another fabrication process flow steps of the embodiment described in  FIGS. 1A ,  1 B and  1 C. 
       FIGS. 2A˜2D  illustrate another side view of the embodiment described in  FIG. 1  and show the relationship of actuation force of in-plane and vertical comb electrodes when the mobile element of top layer is in oscillation motion. The vertical comb electrodes on the bottom layer are located only on one side of the torsional hinges. 
       FIG. 3  illustrates one example of the relationship between the phase of mirror deflection angle and the phase of applied voltage sources for MEMS actuator depicted in  FIG. 2 . 
       FIG. 4  illustrates the three dimensional view of the present invention where the mobile element is supported by a pair of torsional hinges and actuated by both in-plane and vertical comb structure. 
       FIGS. 5A ,  5 B, and  5 C show the top views of the top, middle and bottom layers of another embodiment of present invention where vertical comb electrodes on the bottom layer are electrically isolated into two halves of the different sides of the torsional hinges. Three voltage sources can be applied to achieve large actuation force on the mobile element. 
       FIG. 5D  illustrates another design of the bottom layer of the embodiment as depicted in  FIG. 5C . The two sets of electrically isolated vertical comb electrodes are reinforced through thin film deposition processes. 
       FIGS. 6A˜6D  illustrate one fabrication process flow steps of the embodiment as described in  FIGS. 5A ,  5 B and  5 C. 
       FIGS. 7A˜7F  illustrate the fabrication process flow steps of the embodiment as described in  FIGS. 5A ,  5 B and  5 D. 
       FIGS. 8A˜8D  illustrate the side view of the embodiment as described in  FIG. 5  and show the relationship of actuation force of in-plane and vertical comb electrodes when the mobile structure of top layer is in oscillation motion. The vertical comb electrodes on the bottom layer are electrically isolated on each side of the torsional hinges. 
       FIG. 9  illustrates one example of the relationship between the phase of mirror deflection angle and the phase of applied voltage sources for MEMS actuator depicted in  FIG. 8 . 
       FIGS. 10A ,  10 B, and  10 C illustrate the methods to connect the two set of electrically isolated vertical comb electrodes located on the bottom layer of the actuator as described in  FIG. 1  and  FIG. 5 . 
       FIG. 11  illustrates another embodiment of the invention that additional in-plane comb electrodes are added to the torsional hinges and to the stationary structure on the top layer of the actuator. A voltage difference between the additional comb electrodes sets may be applied to increase the effective stiffness of the hinges. 
       FIG. 12  illustrates the torsional hinge with protrusion areas that may be removed by laser or other means to reduce the torsional stiffness of the hinge. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A ,  1 B and  1 C show the exploded top views of the three layers of a MEMS actuator in accordance with one embodiment of the present invention. Top layer  10 ,  FIG. 1A , is consisted of a stationary and a movable elements  11 ,  12  both made of electrically conductive material, typically doped single crystal silicon. Movable element  12  including comb electrodes  13  is supported by multiple torsional hinges  14  and is electrically isolated from stationary structure  11 . The stationary element  11  has comb electrodes  15  that are interdigitated in the same horizontal plane with the comb electrodes  13  of the movable element  12  such that the top layer  10  is an in-plane comb-drive actuator. Middle layer  20 ,  FIG. 1B , is made of electrically non-conductive material, typically silicon dioxide. Bottom layer  30 ,  FIG. 1C , consisting of a cavity  31  and stationary comb electrodes  32  located on one side of the torsional hinge  14 , is made of electrically conductive material, typically doped single crystal silicon. Stationary comb electrodes  32  on bottom layer  30  are interdigitated with comb electrodes  13  of the movable element  12  on top layer  10  such that the movable element  12  and the bottom layer  30  form a vertical comb-drive actuator. Middle and bottom layers  20 ,  30  support the top layer  10  while middle layer  20  electrically isolates top and bottom layers  10 ,  30 . As a result, the MEMS actuator  1  is consisted of both in-plane and vertical comb-drive actuators. 
   The movable element  12  is typically connected to electrical ground while the stationary element  11  on the top layer  10  is connected to a voltage source AC 1  and the bottom layer  30  is connected to another voltage source AC 2 .  FIG. 3  illustrates the phase and amplitude relationships between deflection angle of movable element  12  and applied voltage sources AC 1 , AC 2 . The waveform of the voltage source can be square, triangular, sinusoidal, half-sinusoidal or other shapes to meet specific angular velocity needs. 
     FIGS. 1D-1F  illustrate one method of fabricating the comb-drive actuator in accordance with one embodiment of the present invention as described in  FIGS. 1A-1C . The first step,  FIG. 1D , starts by etching the backside  41 ( 44 ) of a semiconductor wafer  40 , preferably single crystal silicon then etches the front-side  42 ( 43 ) using deep reactive ion etching (DRIE) with the etched features  44  on backside  41  for alignment  43 ,  44 . The next step is to fusion bond the double-side  42 ,  41  etched wafer  40  to another wafer  50  coated with silicon dioxide  60  then annealed to increase bonding strength. The bonded wafer  70  becomes a three layer  50 ,  60 ,  40  structure and the top layer  50  may be ground and polished to desired thickness and to the required surface quality,  FIG. 1E . The top layer  50  is then DRIE etched down to the middle layer  60  using the backside features ( 44 ) for alignment and the movable element  52  of the three-layer structure is released by removing the silicon dioxide ( 60 ) connecting to the stationary elements  45 ,  FIG. 1F . 
     FIGS. 1G and 1H  illustrate another fabrication method of the comb-drive actuator. The process starts with back-side  81  DRIE etching to the middle oxide layer  82  of a silicon-on-insulator (SOI) wafer  80 ,  FIG. 1G . The wafer  80  is then etched from the front-side  83  of the wafer  80  to the middle oxide layer  82 ,  FIG. 1H . The movable element  85  of the three-layer structure is then released by removing the silicon dioxide connecting to the stationary elements  84 . 
     FIGS. 2A-2D  and  FIG. 3  show the operation of the MEMS actuator as described in  FIG. 1 . The movable element  12  is connected to electrical ground GND, the top stationary comb electrodes  15  and the bottom stationary comb electrodes  32  are connected to the first and the second AC voltage sources AC 1 , AC 2 , respectively as shown in  FIG. 2A . Top stationary and movable elements  11 ,  12  form an in-plane comb actuator whereas bottom stationary element  32  and top movable element  12  form a vertical comb actuator. The movable element  12  starts oscillation with respect to the torsional hinges  14  through either the unbalance of electrostatic force in the in-plane comb actuator or the electrostatic attraction from the vertical actuator,  FIG. 2A . The unbalance force in the in-plane comb may be introduced from manufacturing tolerances or intentional design features. Electrostatic attraction force from the vertical comb actuator will rotate the movable element  12  with respect to the torsional hinges  14  to the maximum deflection angle,  FIGS. 2A˜2B . After the movable element  12  reaches the largest deflection angle, electrostatic attraction force from the in-plane comb actuator will be applied to the movable element  12  until horizontal position is restored,  FIGS. 2B˜2C . The movable element  12  continues to rotate without actuation force to another maximum deflection angle,  FIG. 2C˜2D . After the movable element  12  reaches another maximum deflection angle, electrostatic attraction force from the in-plane comb actuator will again be applied to the movable element  12  until horizontal position is restored to complete one oscillation cycle,  FIG. 2D˜2A . 
     FIG. 3  illustrates the relationship of the applied voltage sources and the operation of the MEMS actuator corresponding to  FIG. 2 . The movable element  12  is typically designed to oscillate at or near its resonance frequency of primary oscillation mode. The movable element  12  including top movable comb electrodes  13  is connected electrical ground GND. The first voltage source AC 1  is applied to the top stationary structure ( 10 ) with in-plane comb electrodes  15 . The second voltage source AC 2  is applied to the bottom stationary comb electrodes  32  ( 30 ). The frequency of voltage source AC 1  is typically twice the oscillation frequency of the movable element  12 . The frequency of voltage source AC 2  is the same as the oscillation frequency of the movable element  12 . The waveform of AC 1  and AC 2  can be various shapes to achieve desired angular velocity of the movable element. Typically, waveform of square shape gives the highest efficiency in driving the movable element  12  to the largest rotation angle under given amplitude of AC 1  and AC 2 .  FIG. 4  shows a three-dimensional view of the MEMS actuator  1  with movable element  12  rotating to its largest angle. 
   The present invention combines both in-plane and vertical comb actuators to drive the movable element  12  to oscillate at large angle and at high frequency. Furthermore, the cavity  31  depth in the bottom layer  30  of the actuator, described in fabrication flow of  FIGS. 1D ,  1 E and  1 F, can be designed to be a mechanical stop to prevent excess deflection of the movable structure that could induce mechanical failure of the actuator. 
     FIGS. 5A ,  5 B and  5 C show the exploded top views of the three layers of a MEMS actuator  2  in accordance with another embodiment of the present invention. Top layer  90 ,  FIG. 5A , is consisted of a stationary and a movable elements  91 ,  92 , both made of electrically conductive material, typically doped single crystal silicon. Movable element  92  including comb electrodes  93  is supported by multiple torsional hinges  94  and is electrically isolated from stationary structure ( 91 ). The stationary element  91  has comb electrodes  95  that are interdigitated in the same horizontal plane with the comb electrodes  93  of the movable element  92  such that the top layer  90  is an in-plane comb-drive actuator. Middle layer  100 ,  FIG. 5B , is made of electrically non-conductive material, typically silicon dioxide. Bottom layer  110 ,  FIG. 5C , consisting of a cavity  111  and stationary comb electrodes  112 , is made of electrically conductive material, typically doped single crystal silicon. Comb electrodes  112  on the bottom layer  110  are electrically isolated into two halves  112 ′  112 ″ located on different sides of the torsional hinges  94 . Stationary comb electrodes  112  on bottom layer  110  are interdigitated with comb electrodes  93  of the movable element  92  on top layer  90  such that the movable element  92  and the bottom layer  110  form a vertical comb-drive actuator with dual-side driving capability. Middle and bottom layers  100 ,  110  support the top layer  90  while middle layer  100  electrically isolates top and bottom layers  90 ,  110 . As a result, the MEMS actuator  2  is consisted of both in-plane and vertical comb-drive actuators. 
     FIGS. 6A-6D  illustrate one method of fabricating the comb-drive actuator in accordance with the embodiment as described in  FIGS. 5A-5C . The first step,  FIG. 6A , starts by etching the backside  121  of a semiconductor wafer  120 , preferably single crystal silicon then etches the front-side  122  using deep reactive ion etching (DRIE) with the etched features ( 124 ) on backside  121  for alignment ( 123 ,  124 ). Cavity  125  size and depth, and the stationary vertical comb electrodes  126  are defined. The next step is to fusion bond the double-side  121 ,  122  etched wafer  120  to another wafer  130  coated with silicon dioxide  140  then annealed to increase bonding strength,  FIG. 6B . The bonded wafer  150  becomes a three layer structure and the top layer  130  may be ground and polished to desired thickness and to the required surface quality. Backside ( 121 ) of the bonded wafer  150  is separated into two halves using 150′, 150″ DRIE,  FIG. 6C . Since the bottom layer  120  is bonded to the top layer  130  so the three layer structure remains intact. The top layer  130  is then DRIE etched down to the middle layer  140  using the backside ( 121 ) features ( 124 ) for alignment ( 131 ,  124 ) and the movable element  132  of the three-layer structure is released by removing the silicon dioxide ( 140 ) connecting to the stationary elements  126 ,  FIG. 6D . 
   The comb-drive actuator  2 , described in  FIGS. 5A ,  5 B and  5 C, can also be fabricated using process flow steps of  FIGS. 1G and 1H . The process starts with back-side DRIE etching of the bottom layer  161  to the middle oxide layer  162  of a SOI wafer  160  and also separates the bottom layer into two halves,  FIG. 1G . Since the bottom layer is bonded to the top layer so the three layer structure remains intact. The wafer  160  is then etched from the front-side  163  of the wafer  160  to the middle oxide layer  162 ,  FIG. 1H . The movable element of the three-layer structure is then released by removing the silicon dioxide connecting to the stationary elements  164 . 
     FIG. 5D  shows a variation of the bottom layer  110  as described in  FIG. 5C . The bottom layer  170  are electrically isolated into two halves  170 ′,  170 ″ and reinforced with thin film deposited materials  171 . The reinforcement materials ( 171 ) must have electrically non-conductive materials such as silicon dioxide. The comb-drive actuator, defined by  FIGS. 5A ,  5 B and  5 D, can be fabricated with process steps of  FIGS. 7A˜7F . Process steps of  FIGS. 7A˜7C  is the same as process steps of  FIGS. 6A˜6C . After the backside  181  of wafer  180  is etched and separated into two halves  182 ,  182 ′,  FIG. 7C , electrically isolated material such as silicon dioxide is deposited on the backside  181  and the opened channels  183  using thin film processes,  FIG. 7D . Another layer of material  184 , such as polysilicon, is further deposited on the backside  181  and the opened channels  183  to complete the reinforcement,  FIG. 7E . The thin film materials on the backside  181  may be removed by grinding and polishing. Top layer  185  is then DRIE etched down to the middle layer  186  using the backside  181  features ( 188 ) for alignment ( 187 ,  188 ) and the movable element  189  of the three-layer structure is released by removing the silicon dioxide connecting to the stationary elements,  FIG. 7F . 
     FIG. 8  and  FIG. 9  illustrate the operation of the MEMS actuator as described in  FIG. 5 . Movable element  92  on top layer  90  is connected to electrical ground GND while stationary comb electrodes  95  is connected the first AC voltage source AC 1 . The two sets of bottom ( 110 ) stationary comb electrodes  112  are connected to the second (AC 2 ) and the third (AC 3 ) AC voltage sources AC 2 , AC 3 , respectively as shown in  FIG. 8A . Movable element  92  starts oscillation with respect to the torsional hinges  94  through either the unbalance of electrostatic force in the in-plane comb electrodes  93 ,  95  or the electrostatic attraction from the vertical comb electrodes  112 ,  FIG. 8A . The unbalance force in the in-plane comb may be introduced from manufacturing tolerances or intentional design features. Electrostatic attraction force from one side of the vertical comb actuator will rotate the movable element  92  with respect to the torsional hinges  94  to the maximum deflection angle,  FIGS. 8A˜8B . After the movable element  92  reaches the largest deflection angle, electrostatic attraction force from the in-plane comb actuator will be applied to the movable element  92  until horizontal position is restored,  FIGS. 8B˜8C . Electrostatic attraction force from another side of the vertical comb electrodes will rotate the movable element  92  to another maximum deflection angle,  FIGS. 8C˜8D . After the movable element reaches another maximum deflection angle, electrostatic attraction force from the in-plane comb actuator will again be applied to the movable element until horizontal position is restored to complete one oscillation cycle,  FIGS. 8D˜8A . 
     FIG. 9  illustrates the relationship of the applied voltage sources and the operation of the MEMS actuator  2  corresponding to  FIG. 5 . The movable element  92  is typically designed to oscillate at or near its resonance frequency of primary oscillation mode. The movable element  92  including top movable comb electrodes  93  is connected electrical ground GND. First voltage source AC 1  is applied to the top stationary structure ( 90 ) with in-plane comb electrodes  95 . Second voltage source AC 2  is applied to one set of the bottom stationary comb electrodes ( 112 ′). Third voltage source AC 3  is applied to another set of the bottom stationary comb electrodes  112 ( 112 ″). The frequency of voltage source AC 1  is typically twice the oscillation frequency of the movable element  92 . The frequency of voltage sources AC 2  and AC 3  are the same as the oscillation frequency of the movable element  92  but at different phases. The waveform of AC 1 , AC 2  and AC 3  can be various shapes to achieve desired angular velocity of the movable element. Typically, waveform of square shape gives the highest efficiency in driving the movable element to the largest rotation angle under given amplitude of AC 1 , AC 2  and AC 3 . 
     FIG. 10A  illustrates a method to form electrical connections to the bottom layer  110  of the actuator  2  with SOI structure. Additional openings  190  on the top layer  90  are etched in DRIE etching process step as described in  FIGS. 1F ,  1 H,  6 D or  7 F to expose access to the middle layer  100 . Electrical insulation material of the middle layer  100  in the exposed area is then removed during structure release process. Connections can be made to the bottom layer  110  through conventional methods such as wire-bonding after deposition of metallic contact pad. 
     FIGS. 10B and 10C  illustrate another method to form electrical connections to the bottom layer  110  of the actuator  2  with SOI structure. The SOI structure is connected to a substrate  201  through a layer of electrically conductive material  200  which is separated into two halves  200 ′,  200 ″ to avoiding electrical bridging. The conductive material  200  may be conductive paste, conductive film, solder paste, etc. The substrate  201  is configured for fan-out of the bottom comb electrodes. Dielectric material  202  is disposed on the substrate  201  which insulates the metal conductor pads  203  on the substrate  201 . Fan-out can be done from the top side conductor pads  203  of the substrate  201 ,  FIG. 10B  or from bottom side conductor pads  204  connecting to top side conductor pads  203  through via holes  205 ,  FIG. 10C . 
     FIG. 11  illustrates one invention embodiment to adjust the structural resonance frequency of the movable element by increasing the effective torsional stiffness of the torsional hinges. Torsional hinges  211  are designed with comb electrodes  212  and are interdigitated with a set of comb electrodes  213  on the stationary structure of the top layer  210 . This set of comb electrodes  213  on the top stationary structure are connected to a DC voltage source and are electrically isolated from the rest of the comb electrodes  214  on the top layer  210 . During oscillation motion of the movable element  215 , the voltage difference between the DC voltage and the ground GND will generate electrostatic attraction force between the additional comb electrodes  212 ,  213  which will suppress the torsional rotation of the portion of hinge  211  with additional electrodes  212 . By adjusting the voltage difference between DC and ground, the effective torsional stiffness of the hinges  211  can be increased such that resonance frequency of the movable element can be tuned. 
     FIG. 12  illustrates another invention embodiment to adjust the structural resonance frequency of the movable element by thinning portions or trimming portions of protrusions  221  on the torsional hinges  220 . The protrusions  221  may be removed selectively utilizing techniques such as laser trimming, E-beam lithography, etc without damaging structural integrity. The effective torsional stiffness of the torsional hinges are reduced such that the resonance frequency of the movable element can be tuned.