A novel combined radical beam and ion beam etching (RBIBE) technique that uses a microwave‐excited radical beam combined with an Ar+ ion beam has been developed for smooth, low damage, and rapid etching of GaAs structures. Separate control of both argon ion (Ar+) energy and/or current and chlorine radical (Cl*) beam flux density has enabled us to realize highly chemically enhanced GaAs etching. This RBIBE technique differs qualitatively from and offers greater flexibility than reactive ion etching (RIE), ion beam assisted etching (IBAE), where the reactive etch gas is not plasma excited, or reactive ion beam etching (RIBE), where independent control of radical and ion beam flux is difficult. Our work examines etch rate as a function of flow rate, temperature, microwave power, and ion beam current. The etch rate and reactive sputter yield are typically eight times greater with the microwave plasma on (RBIBE) than with the plasma off (IBAE). With RBIBE, high etch rates (2.5 μm/min) have been realized at roo...