Effects of graded superlattice on endpoint detection for low damage heterojunction bipolar transistor etching

Abstract

Endpoint detection for the emitter etch of an AlInAs/GaInAs heterojunction bipolar transistor with a graded nine period AlInAs/GaInAs superlattice was investigated. Ga optical emission at 417.2 nm was detected after ∼10 nm of the superlattice had been etched. Once this occurred, an additional overetch of 30 nm was required to completely remove the superlattice and the lightly doped p-type GaInAs spacer layer. The etch-induced damage was characterized by measuring the contact resistivity (ρc) of the base layer and comparing this to a wet etched sample. Two step etching was studied to minimize surface roughness and reduce damage. This requires etching at high rf power initially to maintain smooth surface morphology and then etching at reduced rf power to remove damage created in the first part of the etch. By etching the first 50% of n-type GaInAs at 160 W rf power and the rest at 30 W rf power, a nearly damage free smooth surface can be achieved. Etching with just 30 W rf power can completely eliminate the damage. Damage to both n-type and p+-GaInAs has been studied. For n-type material, damage decreases ρc and for p+-type material damage increases ρc when compared to the wet etched samples. The effect of annealing on the transmission line measurements was also studied. For the p+-GaInAs, an annealed contact on the sample etched with low rf power had a contact resistivity of 8.1×10−6 Ω cm2. This is lower than the wet etched sample, where ρc was 1.6×10−5 Ω cm2. This could be related to the smoother surface that was obtained from dry etching when compared to wet etching.