Optimum spacing (d) between the RF powered electrode and the substrate in an RF PECVD reactor is usually between 1 and 5 cm for a-Si deposition. Smaller spacing is more desirable for a uniform deposition, while a larger spacing is easier to maintain a plasma. Some of the silicon atoms in the gases directed into the chamber are deposited onto the substrates or the chamber walls; the remainder is pumped to the exhaust.