Method of forming a membrane with nanometer scale pores and application to biofiltration

Abstract

A method of forming a membrane having nanometer scale pores includes forming an etch stop layer on a substrate and forming a base layer on the etch stop layer. Advantageously, a silicon nitride etch stop layer is formed on a silicon substrate and the base layer is a thermally grown oxide layer. Micron scale holes are etched through the base layer and, advantageously, partially through the underlying etch stop layer. A sacrificial base layer of controlled thickness is formed on the base layer and lining the holes. A thermally grown oxide is advantageously used as the sacrificial base layer. A plug layer is then formed on the base layer, on the sacrificial base layer and filling the holes. Polysilicon is advantageously used as the plug layer. The plug layer is planarized followed by the creation of an aperture in the backside of the wafer. Release of the etch stop layer and the sacrificial base layer results in a membrane having pores therein with lateral dimensions determined by the thickness of the sacrificial base layer, typically less than about 50 nm. Such membranes are shown to be favorably used in biofiltration and bioseparation applications.

What is claimed is:  
     
         1 . A method of forming a porous membrane comprising: 
 a) forming an etch stop layer on a substrate; and,    b) forming a base layer on said etch stop layer; and,    c) patterning and etching holes through said base layer and not completely through said etch stop layer; and,    d) forming a sacrificial base layer on said base layer and lining said holes, wherein said sacrificial base layer has a nanometer scale thickness, substantially uniform across the wafer; and,    e) forming a plug layer on said sacrificial base layer and filling said holes, wherein said plug layer is selectively removable in comparison with said sacrificial base layer without doping; and,    f) planarizing said plug layer; and,    g) patterning and etching an aperture through the backside of said substrate and through said etch stop layer, exposing thereby said plug layer and said sacrificial base layer; and,    h) selectively removing said sacrificial base layer, forming thereby nanometer scale pores through said base layer.    
     
     
         2 . A method as in  claim 1  further comprising the use of protective layers: 
 immediately following step f; 
 f 1 ) forming protective layers on the frontside and backside of said wafer; and,  
 
 immediately following step g; 
 g 1 ) removing remaining portions of said backside protective layer and said frontside protective layer; and.  
 
 
     
     
         3 . A method as in  claim 1  further comprising the use of support ridges: 
 immediately following step a; 
 a 1 ) forming support ridges on said substrate.  
 
 
     
     
         4 . A method as in  claim 1  further comprising the use of anchor points: 
 immediately following step d;  
 d 1 ) forming anchor points in said sacrificial base layer; and.  
 
     
     
         5 . A method as in  claim 1  wherein said substrate is a silicon wafer.  
     
     
         6 . A method as in  claim 5  wherein said etch stop layer is silicon nitride.  
     
     
         7 . A method as in  claim 6  wherein said sacrificial base layer is thermally formed oxide.  
     
     
         8 . A method as in  claim 7  wherein said plug layer is polysilicon.  
     
     
         9 . A method as in  claim 8  wherein said selective removal of said sacrificial base layer is with HF or SF 6 /oxygen plasma.  
     
     
         10 . A method as in  claim 1  wherein said nanometer scale pores are less than about 50 nanometers in lateral extent.  
     
     
         11 . A biocompatible membrane produced according to the method of  claim 1  having nanometer scale pores.  
     
     
         12 . A membrane as in  claim 11  wherein said membrane is derived from silicon compounds and has sub-fifty nanometer scale pores therein.  
     
     
         13 . A membrane as in  claim 12  wherein said membrane has a glucose diffusion test of at least 1 mg/dl and an albumin diffusion test of at most 0.1 g/dl over approximately 330 minutes.  
     
     
         14 . A method of separating biological substances comprising filtering a mixture of said biological substances through a membrane as in  claim 11  having nanometer scale pores therein.