Photoelectrochemical fabrication of spectroscopic diffraction gratings

Photoelectrochemical etching was demonstrated as a means of fabricating a variety of periodic structures in semiconductors. The semiconductor is used as an electrode in an electrochemical cell, and is in contact with a liquid electrolyte. When the crystal is held at a positive voltage and illuminated, etching occurs in only the illuminated regions to a depth proportional to the illumination intensity and exposure time. In Phase 1, it was determined that diffraction gratings could be produced in gallium arsenide crystals by this method, using either a scanned focused laser beam or by uniform illumination of a ruling mask defined in metal or photoresist on the crystal surface. The latter approach was determined to produce V-grooves if the mask is oriented along certain crystallographic directions. These V-grooves were produced with an exceedingly smooth crystal morphology due to the highly controllable nature of the process and the mild electrolytes involved. The results form the basis for photoelectrochemical fabrication of deep, low pitch Eschelle gratings for use in high orders in NASA spectrographic instrumentation such as the Space Telescope Imaging Spectrograph.