A process and apparatus is provided in which improved control of gas phase radicals is provided. In one embodiment, a system generating atomic oxygen is provided in which gases which generate the atomic oxygen are mixed prior to injection in a process space. The mixing may occur within a showerhead or prior to entrance into the showerhead. In another embodiment, a showerhead is provided which includes multiple zones. Some of the zones of the showerhead may inject the mixture of gases which generate the atomic oxygen into the process space, while other zones do not inject that mixture. In one embodiment, the mixture of gases which generates the atomic oxygen is injected into a main zone, while a subset of those gases is injected into inner and outer zones of the showerhead. The process and apparatus provides a uniform density of atomic oxygen across the substrate being processed.