We have previously demonstrated selective area deposition of n+ microcrystalline silicon at 250°C using time modulated silane flow into a hydrogen plasma, and applied the technique to form high performance top-gate amorphous silicon TFT's with two mask sets. In this paper, we discuss issues related to process scale-up, including the effect of deposition rate on selectivity loss and non-uniformity. Uniformity can be achieved with higher growth rates by expanding the window for selectivity, and using conditions well within the process limits. We show that lower pressure and higher rf power can enlarge the window by enhancing the hydrogen-mediated silicon etching.