Method for selective under-etching of porous silicon

Abstract

A method for making a solar cell is disclosed. In accordance with the method of the present invention a composite wafer is formed. The composite layer includes a single crystal silicon wafer, a silicon-based device layer and sacrificial porous silicon sandwiched therebetween. The composite wafer is treated to an aqueous etchant maintained below ambient temperatures to selectively etch the sacrificial porous silicon and release or undercut the silicon-based layer from the single crystal silicon wafer. The released silicon device layer is attached to a substrate to make a solar cell and the released single crystal silicon wafer is reused to make additional silicon device layer.

What is claimed is: 
     
         1 . A method of selectively etching a silicon wafer, the method comprising:
 a) providing a sacrificial porous silicon layer on a single crystal silicon wafer; and   b) selectively etching the sacrificial porous silicon layer with an aqueous etchant;   wherein the aqueous etchant is maintained at a temperature in a range of 0° C. to 10° C. during step b) and the aqueous etchant comprises one or more of potassium hydroxide, sodium hydroxide and hydrogen fluoride.   
     
     
         2 . The method of  claim 1 , wherein the aqueous etchant includes one or more of the potassium hydroxide, sodium hydroxide and hydrogen fluoride in a concentration of 5% or less by weight. 
     
     
         3 . The method of  claim 1 , wherein the aqueous etchant further comprises a surfactant. 
     
     
         4 . The method of  claim 3 , wherein the surfactant is selected from the group consisting of anionic surfactants, fatty acid surfactants, cationic surfactants, zwitterionic surfactants, and non-ionic surfactants. 
     
     
         5 . The method of  claim 1 , wherein the aqueous etchant comprises 1% or less by weight of the surfactant. 
     
     
         6 . The method of  claim 1 , wherein the aqueous etchant further comprises an alcohol. 
     
     
         7 . The method of  claim 6 , wherein the alcohol is isopropyl alcohol. 
     
     
         8 . The method of  claim 1  further comprising subsequent to step a) and prior to step b), forming a crystalline silicon device layer on the sacrificial porous silicon layer. 
     
     
         9 . The method of  claim 9 , wherein the crystalline silicon device layer is patterned with access grooves or holes sacrificial porous silicon layer. 
     
     
         10 . The method of  claim 10 , wherein the aqueous etchant contacts the sacrificial porous silicon layer via the access grooves or holes in the crystalline silicon device layer. 
     
     
         11 . The method of  claim 1 , wherein the sacrificial porous silicon layer comprises a material selected from the group consisting of a carbon doped oxide, a spin-on-glass (SOG) and fluoridated silicon glass (FSG). 
     
     
         12 . The method of  claim 8 , wherein the crystalline silicon layer is a p-doped crystalline silicon layer. 
     
     
         13 . A method of selectively etching a silicon wafer, the method comprising:
 a) providing a sacrificial porous silicon layer on a single crystal silicon wafer;   b) forming a p-doped crystalline silicon device layer on the sacrificial porous silicon layer, thereby forming a composite wafer; and   c) selectively etching the sacrificial porous silicon layer with an aqueous etchant;   wherein the aqueous etchant is maintained at a temperature in a range of 0° C. to 10° C. during step c) and the aqueous etchant comprises a surfactant.   
     
     
         15 . The method of  claim 13 , wherein the aqueous etchant comprises the surfactant in a concentration of 1% by weight or less. 
     
     
         15 . The method of  claim 13 , wherein the surfactant is an anionic surfactant selected from the group consisting of Perfluorooctanoate (PFOA or PFO), Perfluorooctanesulfonate (PFOS), Sodium dodecyl sulfate (SDS), ammonium lauryl sulfate, Sodium laureth sulfate, and Alkyl benzene sulfonate. 
     
     
         16 . The method of  claim 13 , wherein the surfactant is a cationic surfactant selected from the group consisting of Cetyltrimethylammonium bromide (CTAB), Cetylpyridinium chloride (CPC), Polyethoxylated tallow amine (POEA), Benzalkonium chloride (BAC) and Benzethonium chloride (BZT). 
     
     
         17 . The method of  claim 13 , wherein the surfactant is a zwitterionic surfactant selected from the group consisting of Dodecyl betaine, Cocamidopropyl betaine, and Coco ampho glycinate. 
     
     
         18 . The method of  claim 13 , wherein the surfactant is a non-ionic surfactant selected from the group consisting of Alkyl poly(ethylene oxide), Alkylphenol poly(ethylene oxide) Copolymers of poly(ethylene oxide) and poly(propylene oxide), Octyl glucoside, Decyl maltoside, Fatty alcohols, Cetyl alcohol, Oleyl alcohol, Cocamide MEA, cocamide DEA, Polysorbates and Dodecyl dimethylamine oxide. 
     
     
         19 . The method of  claim 13 , wherein the surfactant is a soap or a fatty acid salt. 
     
     
         20 . The method of  claim 13  further comprising during step c) treating the composite wafer and aqueous etchant to ultrasonic energy.