Abstract:
A method of manufacturing a vertical comb structure for a micro electromechanical (MEMS) device. Tooth structures are formed on a first wafer. A second wafer is then bonded to the tooth structures of the first wafer. The tooth structures are then released to form a comb structure. Forming the tooth structures on the first wafer includes using oxidation, photolithography, etching, epitaxy, and chemical and mechanical polishing to create the tooth structures on the first wafer.

Description:
BACKGROUND OF THE INVENTION 
   The manufacture of microelectromechanical system (MEMS) devices is generally well-known, and there are numerous devices used in different applications and numerous methods of making those devices. An example of a method of manufacturing a MEMS device is contained in U.S. Pat. No. 6,925,710 titled “METHOD FOR MANUFACTURING MICROELECTROMECHANICAL COMBDRIVE DEVICE” TO Scalf et al., herein incorporated by reference. An example of a MEMS device is contained in U.S. Pat. No. 6,705,166 titled “SMALL SIZE, HIGH CAPACITANCE READOUT SILICON BASED MEMS ACCELEROMETER” to Leonardson, herein incorporated by reference. 
   The Leonardson accelerometer provides an inexpensive force measurement device having high pick-off sensitivity in a high-G input range which can operate in a high-G shock environment by providing a capacitance pick-off force sensor having a proof mass with spaced-apart tooth-type electrodes (i.e., a comb structure) that is suspended by an annular suspension member. 
   One limitation of current methods of manufacturing the comb structure of the Leonardson accelerometer is the practice of manufacturing the proof mass and the cover plate separately, each including tooth-type electrodes, and then mechanically assembling the proof mass and the cover plate. This method requires mechanical alignment of the teeth of the proof mass with the recesses of the cover plate, and vice versa. Mechanical assembly in this way can achieve close tolerances between cover plate electrodes and adjacent proof mass electrodes, but not tolerance levels needed for comb drive devices. A need exists, therefore, for manufacturing methods that will allow smaller tolerances between adjacent electrodes than permitted by current manufacturing processes that use mechanical assembly. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a method of manufacturing a comb structure on a MEMS device which can achieve much smaller tolerances between opposing teeth of the comb structure than current manufacturing methods. 
   According to one aspect of the method of the present invention, tooth structures are formed on a first wafer. A second wafer is then bonded to the tooth structures of the first wafer. The tooth structures are then released to form a comb structure. 
   According to another aspect of the present invention, forming the tooth structures on the first wafer includes using oxidation, photolithography, etching, epitaxy, and chemical and mechanical polishing to create the tooth structures on the first wafer. 
   According to a further aspect of the present invention, releasing the tooth structures to form a comb structure includes forming holes through one of the first or second wafers and introducing etchant through the holes to release the tooth structures and form a comb structure. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  shows a cross-sectional view of a finished comb structure manufactured according to a method of the present invention; 
       FIGS. 2A-2H  show a comb structure after various steps of the method; and, 
       FIG. 3  shows a top view of the structure of  FIG. 2H  after through holes have been formed in the second substrate layer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a cross-section view of a finished comb structure  10  manufactured according to a method of the present invention. The structure  10  includes a first substrate layer  12  and a second substrate layer  14 . By way of example and without loss of generality, the substrate layers  12 ,  14  may be silicon wafers. The first substrate layer  12  includes discretely formed first teeth  16  attached to the first substrate layer  12  at a first proximal end  17 , and the second substrate layer  14  includes discretely formed second teeth  18  attached to the second substrate layer  14  at a second proximal end  19 . First and second teeth  16 ,  18  have associated first and second recesses  20 ,  22  between a distal end  24 ,  26  of each tooth  16 ,  18  and the opposite substrate layer  14 ,  12 , respectively. The structure additionally includes a separation distance  28  and spaces  29  between adjacent teeth  16 ,  18  and an overlap distance  30  between adjacent teeth  16 ,  18 . 
     FIGS. 2A-2H  illustrate the steps performed in the manufacture of the comb structure  10 .  FIG. 2A  shows the first substrate layer  12  including a first insulating layer  32 . In the example of the silicon first substrate layer  12 , the first insulating layer  32  may be an oxide, such as silicon oxide produced by oxidizing a portion of the exposed surface of the first substrate layer  12 , or the first insulating layer  32  may be a nitride or other sacrificial material. Portions of the first insulating layer  32  are selectively removed to produce the structure of  FIG. 2B . Selective removal may be effected by masking, photolithography, and etching. 
   As shown in  FIG. 2C , a first device layer  34  is deposited on the remaining portions of the first insulating layer  32  and on the exposed portions of the first substrate layer  12 . Epitaxial growth may be used to effect the deposition. The first device layer  34  may be of the same material as the first substrate layer  12 , or it may be a different material, including electrically conductive materials. The portions of the first device layer  34  deposited on the exposed portions of the first substrate layer  12  are the first teeth  16  of the finished comb structure  10 , and the portions of the first device layer  34  deposited on the first insulating layer  32  will form the second teeth  18  of the finished device  10 . 
   As shown in  FIG. 2D , after deposition, a second insulating layer  36  is created through, for example, oxidation of the upper surfaces of the first device layer  34 , and portions of the second insulating layer  36  are removed, using, for example, chemical and mechanical polishing, leaving the structure shown in  FIG. 2E . Those portions of the second insulating layer  36  left on the first device layer  34  after removal will eventually be removed to leave first recesses  20  of first teeth  16 . 
   After removal of portions of the second insulating layer  36 , portions of the first device layer  34  are removed, resulting in a structure including second teeth  18 , spaces  29  between adjacent teeth  16 ,  18 , and separation distance  28 , as shown in  FIG. 2F . Removal may be accomplished by masking, photolithography, and etching. 
   After removal of portions of the first device layer  34 , the second teeth  18  and the remaining portions of the second insulating layer  36  are bonded to a second substrate layer  14 , the second substrate layer  14  including a third insulating layer  38  and a handle substrate layer  40 . Bonding may be effected using any suitable method such as metal eutectic bonding or silicon fusion bonding. This step forms the structure shown in  FIG. 2G . After bonding, the handle substrate layer  40  and the third insulating layer  38  are removed by thinning using, for example, grinding or chemical and mechanical polishing, to form the structure shown in  FIG. 2H . 
   After thinning, through holes  42  are formed in the second substrate layer  14  ( FIG. 3 ) such that the through holes  42  terminate at either portions of the second insulating layer  36  or between adjacent teeth  16 ,  18 . The through holes  42  should not terminate at the second proximal ends  19  of second teeth  18 . The through holes  42  may be formed by photolithography and etching. 
   After the through holes  42  are formed in the second substrate layer  14 , an etchant (not shown) is introduced into the holes  42  where the etchant may remove the remaining portions of the insulating layers  32 ,  36  to release the comb structure  10  as shown in  FIG. 1 . The holes  42  may be sealed after releasing the comb structure  10 , but it is not necessary. 
   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.