Pre-photolithographic GaAs Surface Treatment for Improved Photoresist Adhesion
During Wet Chemical Etching and Improved Wet Etch Profiles
Keywords: Photoresist adhesion, Gallium arsenide, Wet etch, Surface treatment, Profile
Results of several experiments aimed at remedying photoresist adhesion failure during spray wet chemical etching of InGaP/GaAs NPN HBTs are reported. Several factors were identified that could influence adhesion and a Design of Experiment (DOE) approach was used to study the effects and interactions of selected factors. The most significant adhesion improvement identified is the incorporation of a native oxide etch immediately prior to the photoresist coat. In addition to improving adhesion, this pre-coat treatment also alters the wet etch profile of (100) GaAs so that the reaction limited etch is more isotropic compared to wafers without surface treatment; the profiles have a positive taper in both the [011'] and
 directions, but the taper angles are not identical. The altered profiles have allowed us to predictably yield fully probe-able HBTs with 5×5 μm emitters using 5200 Å evaporated metal without planarization.