Low damage InGaAs/AlGaAs etching was realized by electron cyclotron resonance (ECR) plasma with a Cl2/He mixture for heterojunction bipolar transistors (HBTs) emitter mesa formation. By optimizing the etching pressure and the Cl2/He ratio, n-InGaAs cap layer and n-AlGaAs emitter layer are successively etched and the smooth surface morphology was obtained. An optical emission measurement reveals that the enhancement of the ionized Cl etching plays the essential role for etching of InGaAs. Raman scattering spectra and the base contact resistance measurements indicate that the etching induced damage is extremely low. Moreover, it was found that the etching rate of p-AlGaAs base layer decreased down to two-thirds of that for n-AlGaAs emitter layer under the optimized etching condition. These results demonstrate the potentiality of ECR plasma etching with Cl2/He discharge providing degradation-free dry etching for AlGaAs/GaAs HBTs.