Selective formation of porous silicon

DETAILED DESCRIPTION OF THE INVENTION The invention is based on forming a latent pattern in the surface of a silicon substrate that on exposure to etchant forms a pattern of porous silicon.
The latent pattern is formed by selectively damaging the crystal on the surface in the form of the pattern to provide crystal defects that are preferentially etched to form porous silicon.
The crystal can be damaged by scratching the surface with a sharp instrument such as a diamond scribe.
A pattern could be formed by stamping or translation of a scribe in the x-y axis controlled by a servomechanism.
Higher resolution damage patterns can be formed by applying energetic beams to the surface.
The energy of the beams determine the depth of damage in the surface of the crystal.
Implantation of ion such as at energies above 10 keV such as silicon and boron produces deep patterns of damage greater than 5000 Angstroms deep.
Since most devices can be formed with films of porous silicon from 100 Angstroms to 5000 Angstroms deep, less energetic beams such as plasma and ion milling with energies with energies from about 100 volts to 1000 volts are preferred.
Suitable ion milling beams can comprise argon and plasma beams can contain CF.
sub.
4 or Ar ions.
The ion dose is typically above 10.
sup.
14 cm.
sup.
-2 suitably from 1 to 5 10.
sup.
15 ions cm.
sup.
-2 in order to form crystal defects that can be converted to porous silicon at an acceptable rate.
The ion beam tools can be translated across the surface of the chip to directly write a damage pattern.
For purpose of automation and cost efficiency, it is preferable to form the damage patterns by opening windows in a photoresist mask layer opaque to an ion beam by lithographic techniques.
The photoresist can be a positive or negative photoresist.
The uncured areas of the mask are removed with solvent to form windows exposing the silicon substrate.
An energetic ion beam is then applied to the surface of the silicon substrate exposed through the windows.
The mask is then removed and the surface exposed to etchant