High rate etching of through-substrate via holes are essential to many GaAs electronic and photonic device applications. The backside via holes are relevant to monolithic microwave integrated circuits for low inductance grounding and increased circuit complexity. Via holes have also become important to photonic devices such as transmission modulators and vertical cavity surface emitting lasers (VCSELs) fabricated on absorbing substrates. We have investigated and compared reactive ion etch (RIE) and electron cyclotron resonance (ECR) etch results for GaAs via holes patterned with either photodefinable polyimide masks or conventional thick photoresist masks. We report GaAs etch rates for 5 min plasma exposures of similar to 8000 nm/min in a RIE-generated Cl-2/BCl3/SiCl4 plasma and similar to 3200 nm/min in a Cl-2/BCl3 ECR-generated plasma.