Cl2/Ar based inductively coupled plasma etching of GaN/AlGaN is investigated using photoresist mask in a restricted
domain of pressure < 10mTorr and RF power <100W, for selective mesa etching. The etch characteristics and rootmean-
square surface roughness are studied as a function of process parameters viz. process pressure, Cl2 percentage in
total flow rate ratio, and RF bias at a constant ICP power, to achieve moderately high GaN etch rate with anisotropic
profiles and smooth surface morphology. The etch rate and resultant rms roughness of etched surface increased with
pressure mainly due to dominant reactant limited etch regime. The etch rate also increased with increasing Cl2 % as a
result of increased chlorine radicals that enhances chemical etching. The etch rate and rms roughness showed strong
dependence on RF power with former increasing and later decreasing with applied RF power up to 80W. The
process etch yield variation with applied RF bias is also reported. Negligible etch selectivity was observed between GaN
and AlGaN up to 25% aluminum concentration with etch rate ~120nm/min. The studied etch parameters resulted in
highly anisotropic mesa structures with Ga rich etched surface.
The formation of multilevel interconnects in GaAs MMIC fabrication depends mainly on the adhesion of the interconnect metals with the intermediate dielectric and with the substrate. The metal lines in our experimental study were formed by depositing multilayer films by rf sputtering technique over spin-coated polyimide and Plasma Enhanced Chemical Vapor Deposited (PECVD) silicon nitride (Si3Nx). It is observed that samples with a poor adhesion between the second level metal and Si3Nx show bubbling of blistering in the metal layer when subjected to subsequent high temperature cycles in the fabrication process. It is also found that the choice of certain metal schemes develops a stress between the first level metal and GaAs at the bond pads which peel off at the time of bonding. The present experimental study was undertaken to avoid the loss of the circuit at such advanced stages of fabrication like bonding or second level interconnect formation. The study brings out the dependence of the bondability of the circuit bond pads and the formation of the interconnect lines on the rf sputtering parameters, the premetallization treatment, the choice of the multilayer metal schemes and the thickness of the metals in the multilayer metallization.