Dry etch damage is a potential worry when etching III-Vsemiconductors. Even though very low levels of damage are possible in moderndry etch processes, it is nevertheless of interest to be able to detect anyresidual damage and understand its origins. We have used measurements ofphotoluminescence intensity from GaAs and InGaAs quantum well heterostructuresto probe the damage in a non-invasive way. At low damage levels, two factorsinfluencing possible damage penetration have been found: the ratio of atomicto molecular ions in the discharge and the effect of ambient light. Sampleswere etched using SiCl4 reactive ion etching and also a more complexmulti-component discharge. Comparative studies were made on samples bombardedby the separate constituent ions of a SiCl4 discharge using amass-resolving ion implanter fitted with a deceleration lens. The experimentsshow that molecular ions contribute less to deep damage than do atomic ions.This is relevant, for example, if using high pump powers to sustain thedischarge, such as in ICP or ECR. Further studies of laser illumination on thesample during etching show that a form of radiation enhanced diffusion canmodify the damage distribution. This is relevant, for example, if laserinterferometry is used for end-point detection. Thus for low damage inlow damage etch processes, etch parameters that minimize atomic ion content inthe discharge should be used and ambient illumination should be avoided ifpossible.