The properties of polymethylmethacrylate (PMMA) as a pattern transfer mask in commonly used fluorine- and chlorine-based reactive-ion etching processes have been studied. A reduction in the PMMA etch rate in CHF3 and SiCl4 has been observed for a machine with a lower oxygen background level and an optimum selectivity of silicon nitride to PMMA in CHF3 reactive-ion etching (RIE) was obtained also. The machine with the lower base pressure does not exhibit high RIE lag and this allows better transfer of submicron patterns from PMMA to dielectric layers. This improved selectivity has led to a better intermediate mask for the fabrication of photonic band gap structures in AlGaAs.