Detailed mechanisms of sputter etch of silicon dioxide in argon plasma are studied using a novel test structure. We have found that a significant amount of the sputtered material (up to 50% of the sputter flux) returns, as an isotropic backscattered flux to the wafer. This backscattered flux results in significant deposition that cannot be accounted for by redeposition, i.e., line‐of‐sight deposition of the sputtered material, alone. A profile simulator is used to demonstrate a new physical model for the Ar sputter‐etch process, based on the interaction of three simultaneous processes: (1) sputtering, (2) direct (i.e., line‐of‐sight) redeposition of sputtered material, and (3) isotropic deposition of sputtered material backscattered from the gas phase. Simulated profiles show good agreement with experimental results on the test structure and a common device structure.