Fabrication of macroporous silicon

Abstract

A process for the fabrication of macroporous silicon from silicon wafers. Preferred embodiments include a two-step-etch process that results in a single, macroporous layer of silicon with pore depths of several microns and relatively uniform equivalent diameters with an operator chosen mean equivalent diameter. The mean diameter determined by the operator is a mean diameter within the range of about 40 nm to about 250 nm and is determined, at least in part, by selection of an applied current density. Uniformity of equivalent pore diameter is greatly improved as compared to prior art porous silicon fabrication techniques. In a first electrochemical anodisation step, a macroporous layer is created that is covered by a shallower combination microporous-mesoporous layer. The wafer is removed and the top surface of the wafer is dissolved under alkaline conditions, producing “pits”. These “pits” serve as defect sites during a second electrochemical anodisation step, resulting in a single, uniform, macroporous layer in the silicon wafer. Preferably, the wafer with its porous silicon surface is quickly rinsed in acetone and then pentane to preserve the structure of the pores and prevent collapsing of pore walls. The process of the present invention eliminates the upper, nanoporous region produced by prior art processes. In a preferred embodiment, utilizing a current density of 181.8 mA/cm 2 in the two-etch-step process, the resulting mean equivalent pore diameter is about 100 nm with more than half of the equivalent pores diameters within about ±50 nm of the mean equivalent pore diameter. By increasing or decreasing the anodisation current, porous layers with mean diameters within the range of about 40 nm to about 250 nm can be created.

1 . A process for the fabrication of porous silicon from silicon wafers with pore depths of several microns and average equivalent pore diameters of about 40 nm to about 250 nm comprising the steps of: 
 A) in a double-tank etch chamber, comprising a positive electrode and negative electrode and an etch solution, immobilizing a silicon wafer between said positive and negative electrodes,    B) initiating a first electrochemical anodisation step to etch on a surface of said silicon structure a macroporous layer that is covered by a shallower combination microporous-mesoporous layer,    C) dissolving the combination microporous-mesoporous layer and most of the mesoporous layer leaving pits in the surface of said silicon structure, and    D) initiating a second electrochemical anodisation step to etch on the surface of said silicon structure a mesoporous layer with said pits serving as defect sites to define locations of pores having average diameters of about 40 nm to about 250 nm.    
     
     
         2 . The process as in  claim 1  wherein said average diameter of between 40 nm and 250 nm is determined by choice of current density applied in said first and second anodisation steps.  
     
     
         3 . The process of  claim 1  wherein said dissolving step is accomplished under alkaline conditions.  
     
     
         4 . The process as in  claim 1  and further comprising the step of rinsing said silicon structure in acetone and then pentane prior to initiating said second anodisation step.  
     
     
         5 . The process as in  claim 1  and further comprising a step of modifying the porous surface by molecular vapor deposition of silane compounds.  
     
     
         6 . The process as in  claim 1  wherein said etch solution is an ethanolic HF solution.  
     
     
         7 . The process as in  claim 5  wherein said ethanolic HF solution comprises about 25 percent hydrogen fluoride.  
     
     
         8 . The process as in  claim 1  wherein a current density in the range of about 162 mA/cm 2  to 404 mA/cm 2  is applied through said silicon structure during said second electrochemical anodisation step.  
     
     
         9 . The process of  claim 1  wherein said silicon structure has resistivity values in the range of about 0.001 to 0.0035 ohms-cm.  
     
     
         10 . The process of  claim 8  wherein an electric current density in the range of about 162 mA/cm 2  to 404 mA/cm 2  is applied through said silicon structure during said first electrochemical anodisation step.  
     
     
         11 . The process as in  claim 1  wherein said double-tank etch chamber defines a cathode region and an anode region isolated electrically from each other by said wafer and a seal.  
     
     
         12 . The process as in  claim 1  wherein said seal is a fluoroelastomer gasket.