Abstract:
A method of fabricating a self-aligned vertical combdrive is described. The method includes the steps of etching in a semiconductor wafer a first comb with a coarse set of teeth. A second semiconductor wafer is bonded to the first set of teeth. A set of accurately positioned teeth is etched in the second wafer with teeth overlapping the teeth in the first comb. The lower teeth are etched using the overlapping teeth as a mask to assure proper alignment. One variation in this fabrication method whereby the first coarse comb teeth are etched on semiconductor-on-insulator instead, allows creation of double-sided comb actuators with increased torsional deflection range. Another variation to this fabrication method that keeps the electrically isolated upper masking teeth allows creation of dual-mode vertical comb actuators after an initial assembly step.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority to provisional application Serial No. 60/315,552 filed Aug. 28, 2001. 
     
    
     GOVERNMENT SUPPORT  
       [0002] This invention was made with Government support awarded by the Defense Advanced Research Projects Agency. The Government has certain rights in this invention. 
     
    
     
       BRIEF DESCRIPTION OF THE INVENTION  
         [0003]    This invention relates generally to vertical combdrive actuators and to a method of fabrication, and more particularly to a method of fabrication, which provides narrow gap high force combdrive actuators. This invention also describes a double-sided torsional vertical combdrive actuator for increased torsional deflection and a related fabrication method. Finally, this invention presents a fabrication method variation for design of dual-mode vertical combdrive actuators.  
         BACKGROUND OF THE INVENTION  
         [0004]    There is great demand for high-speed, high-resolution micromirrors in a variety of optical applications including optical scanning, optical switching, free-space optical communications, optical phased arrays, optical filters, external cavity lasers, adaptive optics and other applications. For many of these applications, electrostatic combdrives are the preferred actuation mechanism. Combdrives provide high speed and relatively high force. Furthermore, they can be fabricated using standard materials.  
           [0005]    Combdrives produce large deflections at relatively low voltages with continuous stable control over the full range of motion. In vertical combdrives, two sets of comb teeth are staggered in the vertical direction. A voltage applied between the movable top comb array and the static bottom comb array produces a vertical electrostatic force that can be applied to create torsional or piston-like motion. In standard horizontal combdrives the two sets of comb teeth are in-plane (in the horizontal direction) and the piston-like motion is also in-plane in the horizontal direction.  
           [0006]    A critical aspect of combdrive design is the spacing or gap between adjacent comb teeth, because the generated force is inversely proportional to this gap. Combdrives with small spacing are, however, more susceptible to misalignment between the interleaved comb teeth. For proper actuator operation, the misalignment tolerance level between the moveable and stationary teeth should be an order of magnitude smaller than the gap width. Micromachined vertical combdrives have been demonstrated in the past. However, the fabrication processes for these devices are either quite complex or create comb teeth that are not accurately aligned.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0007]    It is a general object of the present invention to provide a method of fabrication for vertical combdrives with small gaps or spacings between adjacent comb teeth, thereby providing high speed and high force actuation.  
           [0008]    It is a further object of the present invention to provide a fabrication process that provides self-aligned vertical combdrives.  
           [0009]    It is another object of this invention to provide a double-sided torsional vertical combdrive actuator that increases the range of torsional motion, and a method of fabrication for this device.  
           [0010]    Another object of this invention is to present a modified fabrication process and a simple assembly technique to create vertical combdrives for independently controlled torsional and piston motion (dual-mode actuation).  
           [0011]    The foregoing and other objects of the invention are achieved by a method of fabrication in which both the top and bottom comb teeth of the vertical combdrive actuator are defined by a single fabrication mask with accurately spaced gaps. This pattern is first transferred to the top comb layer to form the top comb teeth and a mask for the bottom comb teeth. This mask for the bottom comb teeth is then used to fabricate bottom comb teeth that are aligned automatically to the top comb teeth. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention will be more clearly understood from the following description when read in connection with the accompanying drawings in which:  
         [0013]    FIGS.  1 A- 1 E show process flow steps in the fabrication of a self-aligned vertical combdrive actuator.  
         [0014]    [0014]FIG. 2 illustrates a self-aligned combdrive actuator driving a hinged mirror.  
         [0015]    [0015]FIG. 3 illustrates another self-aligned combdrive actuator driving a hinged mirror.  
         [0016]    FIGS.  4 A- 4 E show the process flow steps of another method of fabrication of a self-aligned combdrive actuator for double-sided operation.  
         [0017]    [0017]FIG. 5 shows a mirror driven by a double-sided vertical combdrive actuator fabricated as shown in FIGS.  4 A- 4 E.  
         [0018]    FIGS.  6 A- 6 D show the modified process flow for fabricating self-aligned dual-mode vertical combdrive actuators.  
         [0019]    FIGS.  7 A- 7 B show a combdrive cross-section of the self-aligned dual-mode actuator fabricated as shown in FIGS.  6 A- 6 D.  
         [0020]    [0020]FIG. 8 illustrates a dual-mode self-aligned vertical combdrive actuated mirror.  
         [0021]    FIGS.  9 A- 9 B illustrate linear or piston-like operation of the self-aligned dual-mode combdrive of FIG. 8.  
         [0022]    FIGS.  10 A- 10 B illustrate torsional operation of the self-aligned dual-mode combdrive of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]    The steps in the fabrication of a combdrive actuator in accordance with one embodiment of the present invention are described with reference to FIGS.  1 A- 1 E. The self-aligned comb actuators are fabricated in semiconductor crystalline material preferably single crystal silicon using deep reactive ion etching (DRIE). The first step, FIG. 1A, is to create a coarse set of bottom comb teeth  11  in silicon wafer  12 . The teeth are fabricated by masking and then deep reactive ion etching for a predetermined time. Both silicon wafers  11  and  13  are oxidized to form oxide layer  14 . The wafer  13  is then bonded, for example by fusion bonding, to the top of the bottom comb teeth  11 . The silicon wafer  13  is then ground and polished to define the thickness of the top comb layer, FIG. 1B. The next step is to lithographically define both the upper and lower combs on the silicon wafer  13 . DRIE etching is used to etch through the top silicon layer  13  to pattern the comb teeth for both the upper and lower comb which, in accordance with the present embodiment, will create the movable top comb teeth  16  and a silicon mask for the bottom comb teeth  16   a,  FIG. 1C. The only alignment requirement is that, when formed, the upper or movable teeth overlap the coarse bottom comb teeth  11 . The alignment tolerances are in the order of the tooth spacing. The oxide layer  13  is then anisotropically or isotropically etched to expose the silicon of the coarse bottom teeth. This is followed by a DRIE etch of the exposed portion of the bottom teeth to define the final bottom comb teeth  11   a,  which are automatically aligned with the upper comb teeth  16   a,  FIG. 1D. In one method of fabrication, the masking silicon teeth of the top layer are removed, FIG. 1E, to provide an upper comb whose teeth are interleaved and accurately aligned with the teeth of the bottom comb and moveable with respect thereto. In a variation of this process, the masking teeth  16   a  are not removed, but are disconnected from the movable teeth  16  so that the movable teeth are free to move with respect to the static or fixed bottom and upper teeth  11   a  and  16   a.    
         [0024]    [0024]FIG. 2 shows a pair of actuators  21  and  22  having fixed or static teeth  23  and interleaved movable teeth  24 , staggered in the vertical direction, driving the torsional hinges  26  of mirror  27 . FIG. 3 shows a hinged mirror  28  driven by a combdrive actuator  29  having fixed combs  31  and movable combs  32  staggered in the vertical direction and connected to drive the mirror  28 . Actuation of the actuators of FIGS. 2 and 3 is accomplished by applying a voltage between the fixed and movable teeth. This voltage generates a torsional force about the torsional hinges  24  resulting in the rotation of the actuator and mirror in the corresponding direction. Although self-aligned vertical combdrives have been used in connection with mirrors in optical switching and scanning applications, it is to be understood that the combdrives can be used as actuators in many applications, such as micromirrors for optical switching, optical scanning for displays, barcode scanners, optical communications, optical coherence tomography, optical filters, external laser cavity lasers, adaptive optics, phase arrays, maskless lithography, confocal microscopy, displays, printers, spectroscopes, gyroscopes, resonators, micro-relay, sensors, accelerometers and for other micromanipulation or microtranslation.  
         [0025]    The steps in fabricating a vertical combdrive actuator in accordance with another embodiment of the invention are set forth in FIGS.  4 A- 4 E. These actuators are ideal for isolating comb electrodes in the bottom layer. The first step in forming the bottom stationary comb for the combdrive actuator is to select a silicon-on-insulator (SOI) wafer  36  having a lower silicon substrate layer  37 , an oxide layer  38  and a device silicon layer  39 , FIG. 4A. The wafer is then masked and DRIE-etched to form the coarse bottom comb teeth  41 . A silicon wafer  42  is then oxidized with an oxide layer  43  and fusion bonded to the bottom comb teeth. Alternatively, both the silicon wafer  42  and the bottom SOI wafer  36  can be oxidized prior to fusion bonding. The wafer is then polished to define the thickness of the top layer, FIG. 4B. The next step is to lithographically defined both the upper and lower combs on the wafer  42 . DRIE etching is used to etch through the top silicon layer  42  to pattern the comb teeth for both the upper and lower comb which, in accordance with the present embodiment, will create the movable top comb teeth  44  and a silicon mask  44 a for the bottom comb teeth  41 , FIG. 4C. The only alignment requirement is that, when formed, the upper or movable teeth overlap the coarse bottom teeth  41 . The alignment tolerances are in the order of the tooth spacing. The oxide layer  38  is then anisotropically or isotropically etched to expose the silicon of the coarse bottom teeth. This is followed by a DRIE etch of the exposed portion of the bottom teeth to define the final bottom comb teeth  41   a,  which are automatically aligned with the upper comb teeth  44 , FIG. 4D. In one method of fabrication, the masking silicon teeth of the top layer are removed, FIG. 4E, to provide an upper comb whose teeth are interleaved and accurately aligned with the teeth of the bottom comb and moveable with respect thereto.  
         [0026]    The process illustrated in FIGS.  4 A- 4 E facilitates isolation of bottom electrodes or teeth allowing fabrication of double-sided vertical combdriven actuators as shown in FIG. 5. The actuator of FIG. 5 includes upper or moveable comb teeth  44  extending outwardly from a drive member  51 . The drive member is shown connected to a mirror  52  with its ends suitably supported (not shown) so that it can be rotated. Since the stationary or bottom comb teeth are supported on oxide layer  38 , two isolated sets  53  and  54  can be supported on the substrate  37  with opposite voltages applied to move the comb teeth  44  in opposite directions. This doubles the range of torsional deflection of the actuator, since the actuator can be rotated clockwise with one bottom electrode and counterclockwise with the second bottom electrode. Such devices can also be operated as linear actuators.  
         [0027]    A variation to this process is shown in FIGS.  6 A- 6 D. This process is identical to the one illustrated in FIGS.  4 A- 4 E and includes the same reference numerals for like parts. However, it does not include the last step illustrated in FIG. 4E. Here, the masking silicon  44   a  in FIG. 4E is not removed, but is electrically isolated from the other electrodes. This configuration creates a set of static top comb teeth  44   a  in addition to the movable top comb teeth  44  and the static bottom comb teeth  41 . FIG. 8 shows a dual-mode actuator which can be fabricated using appropriate masking to define the teeth in the upper wafer  42  which serve as a mask for the fixed teeth. The actuator includes a set of movable teeth  44  connected to member  56  shown connected to drive mirror  57 . It is apparent that the member could be used to drive other appliances. Stationary comb drive teeth  41   a,    44   a  are supported on the oxide layer  38  on substrate  37 . FIG. 7A shows a crosssection taken along line  7 - 7  of FIG. 8 showing the static and movable comb teeth  41   a,    44   a  and teeth  44 . As shown in FIG. 7B, applying a bias voltage either between the movable top comb and the silicon substrate  37  or between the movable comb and the static bottom comb teeth  41   a  pulls the movable comb downward (FIG. 8). Positioning the movable comb between the top static and bottom static comb electrodes enables individual control of the torsional and piston-like motion of the device creating a dual-mode actuator.  
         [0028]    Operation of this dual-mode vertical comb actuator having the structure of FIG. 6D is illustrated in FIGS.  9 - 10 . FIGS. 9A, 9B illustrate operation of the dual-mode actuator as a piston. Voltages are applied between moveable teeth or electrodes  44  and both lower fixed teeth or electrodes  41   a  for downward motion. Similarly, voltages are applied between the moveable teeth  44  and both sets of upper fixed teeth  44   a  for upward motion.  
         [0029]    FIGS.  10 A- 10 B show operation of actuator for rotational or torsional motion. Voltages are applied between electrode or teeth  44  and diagonal electrodes  41   a  and  44   a  for clockwise rotation and counter-clockwise rotation. The mirror  57 , FIG. 8, can be rotated applying appropriate voltages between electrodes or teeth  44  and between diagonal electrodes or teeth  41   a  and  44   a.  Thus, by the application of appropriate voltages to the movable and fixed electrodes or teeth, the mirror can be moved up and down or rotated.  
         [0030]    The foregoing descriptions of specific embodiments of the present invention are presented for the purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.