MEMS sensor structure and microfabrication process therefor

Abstract

A micro-electro-mechanical structure including a semiconductor layer mounted to an annular support structure via an isolation layer wherein the semiconductor layer is micromachined to form a suspended body having a plurality of suspension projections extending from the body to the rim and groups of integral projections extending toward but spaced from the rim between said suspension projections. Each projection in said groups has a base attached to the body and a tip proximate the rim. The structure includes a plurality of inward projections extending from and supported on the rim and toward the body. Each such projection has a base attached to the rim and a tip proximate the body; wherein the grouped projections and the inward projections are arranged in an interdigitated fashion to define a plurality of proximate projection pairs independent of the suspension elements such that a primary capacitive gap is defined between the projections of each projection pair. Also, a process is disclosed for fabricating the micro-electro-mechanical structure including the steps of removing a highly doped etch termination layer and thereafter etching through a lightly doped epitaxial layer to thereby define and release the structure.

1 . A MEMS sensor structure of the type comprising the uniplanar combination of a body defining a proof mass supported for compliance in a predetermined mode by one or more integral suspension projections which extend from said body to a support member wherein the improvement comprises: 
 a first plurality of rigid sensory projections between each of said suspension projections extending from said body toward but spaced from said support member; and    a second plurality of complemental and electrically isolated sensory projections extending from said support member toward but spaced from said body; 
 said first and second pluralities of projections being arranged in complemental pairs and defining on one side of each of said projections a primary sensing gap of essentially uniform width over the entire longitudinal extent thereof.  
   
     
     
         2 . A sensory structure as defined in  claim 1  wherein the proof mass is essentially circular, the projections are radial and the compliance mode is rotary.  
     
     
         3 . A sensor as defined in  claim 1  wherein the first and second projections have rounded tips which form contiguous portions of the essentially uniform width primary sensing gap.  
     
     
         4 . A sensor structure as defined in  claim 2  wherein the second plurality of sensory projections are integrally grouped and joined with arcuate peripheral portions overlying said support plane, said suspension projection lying between said groups and spaced therefrom by a gap which is not less than said primary sensing gap.  
     
     
         5 . A sensor structure as defined in  claim 2  wherein the first plurality of sensory projections are integrally grouped and joined with arcuate portions of said proof mass overlying said support plane, said suspension projection lying between said groups and spaced therefrom by a gap which is not less than said primary sensing gap.  
     
     
         6 . A sensor structure for a micro-electro-mechanical rotary accelerometer of the type comprising the uniplanar combination of a generally circular body and, integral therewith, a plurality of radially outwardly extending projections arranged in complemental pairs with electrically isolated inwardly extending projections characterized by arranging some of said radial extensions to operate only as suspension elements which are coupled with the inwardly extending projections by capacitive couplings which are weak relative to the capacitive coupling of said complemental pairs.  
     
     
         7 . A sensor structure as defined in  claim 6  wherein the complemental pairs are coupled by uniform capacitive gaps.  
     
     
         8 . A micro-electro-mechanical structure comprising: 
 a semiconductor layer defining a body ( 10 ) and a rim structure ( 16 ) surrounding said body;    an underlying support means for said rim structure;    a first plurality of flexible projections ( 18 ) extending from said body to said support means;    a second plurality of projections ( 20 ) extending integrally from said body toward but spaced from said rim structure; each said second projection having a base integral with said body and a tip proximate but spaced from said rim; and    a third plurality of projections ( 30 ) extending from said rim structure substantially toward but spaced from said body, each such third projection having a base attached to said rim, and a tip proximate said body;    wherein said second projections ( 20 ) and said third projections ( 30 ) are arranged in a substantially interdigitated fashion such that each said second projection is positioned proximate to a different one of said third projections to thereby define a plurality of proximate projection pairs, such that a minimum capacitive gap is substantially defined between said second projection and said third projection of each said projection pair, with each said projection pair being separated from any adjacent projection pair by a gap greater than said primary capacitive gap, such that each said tip of each of said second projections is spaced from said rim by a distance substantially equal to said primary capacitive gap, and such that each said tip of each of said third projections is spaced from said body by a distance substantially equal to said primary capacitive gap.    
     
     
         9 . The micro-electro-mechanical structure according to  claim 8 , wherein said rim is shaped such that the circumference of said tip of each said second projection is substantially uniformly spaced from said rim by a distance substantially equal to said primary capacitive gap.  
     
     
         10 . The micro-electro-mechanical structure according to  claim 8 , wherein said body is shaped such that the circumference of said tip of each said third projection is substantially uniformly spaced from said body by a distance substantially equal to said primary capacitive gap.  
     
     
         11 . The micro-electro-mechanical structure according to  claim 8 , wherein each said first projection, said semiconductor layer, said body, said second projections, and said third projections are comprised of an electrically conductive, doped semiconductor material such that capacitance is capable of being electrically measured between said second projections and said third projections.  
     
     
         12 . The micro-electro-mechanical structure according to  claim 11 , wherein said electrically conductive, doped semiconductor material is p-doped, epitaxial silicon.  
     
     
         13 . The micro-electro-mechanical structure according to  claim 8 , wherein each second projection is substantially straight and is substantially uniform in width along the length of said first projection.  
     
     
         14 . The micro-electro-mechanical structure according to  claim 13 , wherein one side of said second projection and one side of said third projection of each said projection pair is substantially uniformly spaced apart from each other, along said length of said second projection, by a distance substantially equal to said primary capacitive gap.  
     
     
         15 . The micro-electro-mechanical structure according to  claim 8 , said structure further comprising: 
 a substrate having a surface and a cavity defined within said surface; and    an isolation layer on top of said surface of said substrate and the lining of said cavity, wherein said semiconductor layer is on top of said isolation layer such that said body, said second projections, and said third projections are suspended over said cavity.    
     
     
         16 . The micro-electro-mechanical structure according to  claim 15 , wherein said isolation layer comprises silicon dioxide.  
     
     
         17 . A process for fabricating a micro-electro-mechanical structure having interdigitated projections, said process comprising the steps of: 
 providing a first substrate;    etching a cavity within said first substrate;    forming an isolation layer on said first substrate;    providing a second substrate;    doping the top portion of said second substrate to thereby form an etch termination layer;    forming a doped epitaxial layer on the etch termination layer portion of said second substrate such that said etch termination layer portion of said second substrate has a higher doping concentration than said epitaxial layer;    bonding said second substrate to said first substrate such that said epitaxial layer covers said cavity and is bonded to said isolation layer at the periphery of said cavity;    removing the non-termination layer portion of said second substrate from said etch termination layer portion of said second substrate;    removing said etch termination layer portion of said second substrate from said epitaxial layer;    applying photoresist on said epitaxial layer;    patterning said photoresist according to a predetermined shape of said micro-electro-mechanical structure;    anisotropically etching through sections of said epitaxial layer disposed over said cavity and revealed through said patterned photoresist to thereby define and release said micro-electro-mechanical structure above said cavity; and    removing said patterned photoresist.    
     
     
         18 . The process according to  claim 17 , wherein said top portion of said second substrate is doped with a p-type dopant comprising boron and germanium.  
     
     
         19 . The process according to  claim 17 , wherein said epitaxial layer is doped with a p-type dopant.  
     
     
         20 . The process according to  claim 17 , wherein the photoresist is a positive photoresist.  
     
     
         21 . The process according to  claim 17 , wherein the step of anisotropically etching through said epitaxial layer is accomplished by contacting said epitaxial layer with a plasma comprising sulfur hexafluoride and oxygen.  
     
     
         22 . The process according to  claim 21 , wherein the step of anisotropically etching through said epitaxial layer includes cooling the epitaxial layer to a cryogenic temperature of less than about 173 EK.  
     
     
         23 . The process according to  claim 17 , wherein the step of anisotropically etching through said epitaxial layer is accomplished by a pulsed halogen and carbon compound-forming gas process.  
     
     
         24 . The process according to  claim 23 , wherein the step of anisotropically etching through said epitaxial layer is performed at room temperature.  
     
     
         25 . The process according to  claim 17 , wherein said first substrate and said second substrate comprise silicon, and wherein said isolation layer comprises silicon dioxide.  
     
     
         26 . The process according to  claim 17 , wherein the step of patterning said photoresist according to a predetermined shape of said micro-electromechanical structure includes the steps of: 
 determining a minimum capacitive gap between said interdigitated projections of said micro-electro-mechanical structure which are nearest to each other, wherein each projection of said interdigitated projections has a base and a tip at opposite ends;    defining said predetermined shape such that each base of each said projection of said interdigitated projections is proximate to at least one tip of another said projection of said interdigitated projections by a distance substantially equal to said minimum capacitive gap; and    selectively removing said photoresist to reveal bare sections of said epitaxial layer according to said predetermined shape.