Polyimide tape (Kapton™ or equivalent) has been found to be effective as a material for masking selected areas of a semi- conductor wafer that is about to be processed in a reactive-ion etching (RIE) apparatus. The conventional etch- resistant masking materials for protecting selected areas against etching are photoresists and oxides. These materials are patterned into masks by standard photolithographic techniques. The use of polyimide tape is an inexpensive option that provides additional flexibility for increasing the protection of some areas and for implementing lastminute design changes without having to incur the high cost of making optical masks for photolithography.
The tape is cut to fit the areas that are to be protected against etching, then secured to the wafer by use of its own adhesive backing (see figure). The tape is smoothed down to remove pockets of air that would otherwise become trapped between the wafer and the adhesive. The tape is thin enough (≈50 μm) that it can easily be made to conform to previously patterned changes in surface height. When the RIE process is finished, the tape is simply pulled off.
In experiments, polyimide tape proved effective in preventing RIE of masked areas. Specimens of tape that were exposed to RIE and vacuum for as long as 3 hours exhibited no measurable change in thickness, no loss of adhesion, no changes in color, and no melting or other surface changes.
This work was done by Stephen Vargo of Caltech for NASA’s Jet Propulsion Laboratory.