High-density plasma (HDP) etch systems are advantageous for small-dimension contact and via etch applications because they provide high etch rates and high selectivity between different dielectric films. However, controlling cross-wafer uniformity of the etch rate and selectivity can be difficult in large, single-wafer etch systems. In this article, we examine the effect of gas feed configuration and dilutent gas flow for a high-selectivity silicon oxide-to-silicon nitride etch process in a 200 mm wafer HDP etch system. The oxide etch rate and oxide-to-nitride selectivity were measured for both helium and argon dilution from the top (chamber roof) or bottom (around the base of the wafer chuck) configurations in the applied materials HDP dielectric etch system. These process parameters are compared with the emission spectra and rf parameters obtained during the etch process. The results indicate that the choice of helium or argon has little effect on the process results and increasing dilutent flows tend to make the cross-wafer etch rate and selectivity more nonuniform. However, with no dilutent, cross-wafer uniformity was the poorest and the rf match network could not completely compensate for the plasma impedance. Under all conditions for this process, center gas feed provided more uniform results.