Abstract:
A MEMS device having a proof mass resiliently mounted above a substrate has projections formed on adjacent surfaces of the mass and substrate. The device is formed by creating a plurality of holes in the upper layer. A substance suitable for removing the intermediate layer without substantially removing the upper layer and substrate is introduced through the holes. A substance removing the upper layer, the substrate, or both, is then introduced through the holes to remove a small amount of the substrate and upper layer. Portions of the intermediate layer between the projections are then removed. The dimple structure fabricated from this process will prevent MEMS device stiction both in its final release and device operation.

Description:
BACKGROUND OF THE INVENTION 
     Many Micro Electromechanical Systems (MEMS) include a structure, such as shown in  FIGS. 1A and 1B , in which a mass  10  is resiliently mounted above a substrate  12 . For example, a comb drive is formed on a proof mass  10  elastically mounted to a substrate  12 , having a plurality of teeth  14  extending therefrom. Corresponding teeth  16  are rigidly mounted to the substrate and electrically coupled to signal generating and sensing circuits. The capacitance between the teeth  14  of the proof mass  10  and the rigidly mounted teeth  16  is used to sense movement of the mass  10  and to actuate the mass  10 . 
     In such systems, the proof mass  10  is typically a very small distance from the substrate  12 . As a result, it is possible for the proof mass  10  to contact the substrate. Because the forces involved are typically extremely small, adhesion between the proof mass  10  and the substrate  12  due to intermolecular forces, such as van der Waals interactions, become significant. The contacting surfaces of the proof mass and substrate are therefore prone to adhere to one another when brought into contact with one another due to a phenomenon known as “stiction.” Stiction may cause the mass and substrate to fail to separate during the fabrication process. CO 2  drying or other drying methods may be used to release stiction during fabrication. However, stiction can still occur during device operation. Sometimes this stiction is permanent and the device then cannot be recovered. In some cases, stiction during device operation is temporary and can be overcome by inertial forces exerted on the mass  10 . However, even where stiction is overcome, measurements derived from the device and movements performed by the device will be subject to errors caused by the temporary adhesion of the proof mass to the substrate. 
     In view of the foregoing, it would be an advancement in the art to provide a MEMS device not subject to errors caused by stiction. 
     SUMMARY OF THE INVENTION 
     A Micro Electromechanical (MEM) device includes a substrate having an upper surface and a mass having a lower surface positioned over the upper surface vertically separated from the upper surface by a small gap. The mass is resiliently mounted to the substrate such that the mass may move relative to the substrate. A lower surface of the mass, the upper surface of the substrate, or both have a plurality of small local projections formed thereon. The height of the projection is typically substantially less than the width of the gap. 
     The device may be formed in a workpiece having an upper layer, an intermediate layer, and a substrate. The intermediate layer is formed of a material different then that forming the upper layer, the substrate, or both. In one embodiment, the upper layer and substrate are formed of silicon and the intermediate layer is formed of an oxide of silicon. 
     The device is formed by creating a plurality of holes in the upper layer. A substance suitable for removing the intermediate layer without substantially removing the upper layer and substrate is introduced through the holes to remove a substantial portion, but not the entire intermediate layer, to form isolated areas of material in the intermediate layer. A substance removing the upper layer, the substrate, or both, is then introduced through the holes to remove a small amount of the substrate and upper layer not shielded by the isolated areas of the intermediate layer to create small projections adjacent the isolated areas. Portions of the intermediate layer between the projections are then removed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1A  is a perspective view of comb drive device according to the prior art; 
         FIG. 1B  is a side cross-sectional of a comb drive device according to the prior art; 
         FIGS. 2A and 2B  are side cross-sectional views of a comb drive having a plurality of projections formed thereon, in accordance with an embodiment of the present invention; 
         FIG. 3  is a side cross-sectional view of an upper layer, intermediate layer, and substrate suitable for forming a MEMS device having projections formed thereon, in accordance with an embodiment of the present invention; 
         FIGS. 4A  is a top view of an upper layer of the layers of  FIG. 3  having a plurality of holes formed therein, in accordance with an embodiment of the present invention; 
         FIG. 4B  is a side cross-sectional view of the device of  FIG. 4A ; 
         FIG. 4C  is a side cross-sectional view of an upper layer formed into a comb drive having a plurality of holes formed therein, in accordance with an embodiment of the present invention; 
         FIG. 5  is a side cross-sectional view of the layers of  FIGS. 4A-4C  having a portion of the intermediate layer removed; 
         FIGS. 6A and 6B  are side cross-sectional views illustrating the formation of projections, in accordance with an embodiment of the present invention; and 
         FIG. 7  is a side cross-sectional view of the layers of  FIG. 3  having portions of the intermediate layer located adjacent the projections removed. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 2A and 2B , in an embodiment of the invention the proof mass  10  is provided with projections  18  on an undersurface  20  thereof. In some embodiments, projections  22  are formed on an upper surface  24  of the substrate  12 . The projections  22  may be aligned with the projections  18  or offset therefrom. The projections  18 ,  22  may be distributed in a regular or random pattern. The projections typically only occupy an insubstantial amount of the local region in which they are formed. For example, the projections may be less than 5% of the area of the local region in one embodiment. In another embodiment, the projections  18 ,  22  occupy less than 2% of the area of the local region. In yet another embodiment, the projections  18 ,  22  occupy less than 1% of the local region. The projections  18  may have a height  26   a  equal to a height  26   b  of the projections  22 . Alternatively the heights  26   a ,  26   b  may be unequal. The heights  26   a ,  26   b  of the projections  18 ,  22  are typically much less than the height  28  of the gap between the mass  10  and substrate  12 . In one embodiment, the heights  26   a ,  26   b  are 5% of the height  28 . In another embodiment, the heights  26   a ,  26   b  are less than 2% of the height  28 . In another embodiment, the heights  26   a ,  26   b  are less than 1% of the height  28 . The heights  26   a ,  26   b  are typically chosen to substantially reduce stiction by reducing the area of the mass  10  and substrate  12  that actually contact each other. The area occupied by the projections  18 ,  22  is also chosen to reduce stiction. 
     In the preferred embodiment, the projections  18 ,  22  occupy 5-10% of the area of the mass, the portion of the substrate  12  located beneath the mass  10 , or both, whereas in other areas of the MEMS device, the projections typically occupy less than 2% of the area. In the preferred embodiment, the heights  26   a ,  26   b  of the projections  18 ,  22  are between about 0.2 and 1 μm. 
     Referring to  FIG. 3 , the projections  18 ,  22  in accordance to the invention may be formed in an upper layer  30  positioned over the substrate  12 . An intermediate layer  32  is positioned between the upper layer  30  and the substrate  12 . The intermediate layer  32  is typically formed of a different material than the upper layer  30  and substrate  12 , such as an oxide of the material forming the substrate  12  or upper layer  30 . 
     In the illustrated embodiment, the upper layer  30  and substrate  12  are formed of silicon and the intermediate layer  32  is silicon oxide. In one embodiment, a silicon-on-insulator (SOI) wafer is used. The intermediate layer  32  is typically much thinner than the upper layer. The thickness of the intermediate layer  32  is typically chosen to be about the same size as, or just slightly smaller than, a gap between the projections  18 ,  22  of the proof mass  10  and the substrate  12  in the completed device. 
     Referring to  FIGS. 4A-4C , a plurality of holes  34  are then formed in the upper layer  30 . The distribution of the holes  34  may be random, quasi random, or according to a regular pattern. Within the local region in which holes  34  are formed, the holes  34  occupy an insubstantial amount of the area. For example, the holes  34  may be formed in the region where the proof mass  10  and teeth  14  are to be formed. In one embodiment, the holes  34  occupy less than 5% of the area of the region of the upper layer  30  in which they are formed. In another embodiment, the holes  34  occupy less than 2% of the region of the portion of the upper layer  30  in which they are formed. In another embodiment, the holes  34  occupy less than 1% of the area of the region of the upper layer  30  in which they are formed. 
     The holes  34  are typically formed by deep reactive ion etching (DIRE) or like process. The holes  34  and the projections  18 ,  22  may be formed prior to forming of the outline of the proof mass  10  and the teeth  14 ,  16 , as shown in  FIG. 4B . Alternatively, the process used to form the holes  34  may simultaneously form the proof mass  10  and the teeth  14 ,  16 , as shown in  FIG. 4C . The holes  34  may also be formed after formation of one or more of the proof mass  10  and teeth  14 ,  16 . 
     Referring to  FIG. 5 , a substance  36  suitable for removing the intermediate layer  32  without substantially removing the upper layer  30 , the substrate  12 , or both, is then introduced through the holes  34 . The substance  36  may be an oxide etchant such as a buffered oxide etchant (BOE) or a hydrogen fluoride (HF). HF is typically used at a concentration of about 49%. In an alternative embodiment, the substance  36  introduced is suitable for the intermediate layer and one or both of the substrate  12  and the upper layer  30 , however, the substance  36  removes the substrate  12 , upper layer  30 , or both at a much slower rate than the substance  36  removes the intermediate layer  32 . The substance  36  is maintained in contact with the intermediate layer  32  for sufficient time to remove all but small portions of the intermediate layer  30  located between the holes  34 . The small portion of the intermediate layer  32  corresponds to the size and location of the projections  18 ,  22  to be formed in the finished device. 
     Referring to  FIGS. 6A and 6B , a substance  38  is introduced that is effective to remove the upper layer  30 , the substrate  12 , or both is then introduced through the holes  34 . In some embodiments, the substance  38  is not effective, or not substantially effective, to remove the intermediate layer  32 . In the illustrated embodiment, a substance  38  effective to remove silicon is used such as potassium hydroxide (KOH) at high concentrations. At low temperatures, KOH will etch silicon very slowly while not substantially etching silicon oxide. The substance  38  is typically maintained in contact with the substrate  12  and upper layer  30  for sufficient time to remove a very small amount of material to a depth  40 . The depth  40  of material removed is typically chosen to be approximately equal to the height  26   a ,  26   b  of the projections  18 ,  22  of the final device. 
     Inasmuch as the substance  38  removes material from all exposed areas of the substrate  12  and upper layer  30 , the substrate  12  and upper layer  30  are sized such that the final size after the application of the substance  38  is suitable for an intended application. In instances where the mass  10  and teeth  14  are formed prior to the formation of the projections  18 ,  22 , the teeth  14  and mass  10  will be formed somewhat larger to compensate for material removed by the substance  38 . In the preferred embodiment, the depth  40  of material removed is from about 0.2 to 1 μm. 
     Referring to  FIG. 7 , all, or substantially all, of the intermediate layer  32  remaining between the projections  18 ,  22  is then removed to form a gap between the upper layer  30  and the substrate  12 . This is typically accomplished by introducing a substance  42 , similar to substance  36 , suitable for removing the intermediate layer  32 . The substance  42  may remain in contact with the intermediate layer  32  for no longer, or not substantially longer, than sufficient to remove the portion of the intermediate layer  32  between the projections  18 ,  22  such that substantial portions of the intermediate layer  32  remain to support structures formed in the upper layer  30  that are rigidly secured to the substrate  12 . The substance  42  is typically ineffective to remove the upper layer  30 , the substrate  12 , or both. Alternatively, the substance  42  removes the upper layer  30  and/or the substrate  12  at a substantially slower rate than the intermediate layer  32 . In the illustrated embodiment, the substance  42  is a substance suitable for removing silicon oxide, such as HF (e.g. at a 49% concentration), a BOE or the like. 
     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.