High Temperature Oxygen Process

The most simply process uses pure $\rm O_2$ in a downstream plasma reactor. In this case the wafer temperature is of siginficant importance, as the process must be additionally thermally activated. To get some reasonable ashing rates ($>2\mu m/min$) for single wafer treatment temperatures of at least 130- $\rm 170^\circ C$ are required. The same process is carried out also in barrel reactors using lower temperatures. Throughput increases linear with temperature, but above appr. 250-$300^\circ C$ the resist starts to burn (reticulate), practically limiting temperatue. A huge benefit of the pure $\rm O_2$ process is its nearly infinite selectivity to Si, $\rm SiO_2$, Al and other important materials used in semiconductor manufacturing. But there is one important exception, low-k materials. Here problems with k value changes or side wall bowing might happen. For the above mentioned process some nitrogen addition can increase the ashing rate. This is sometimes attributed to the fact that the lifetime of the oxygen radicals is increased when nitrogen is present in the discharge.