a chemical vapor deposition method includes a step of maintaining a hydrogen plasma at low pressure in a processing chamber . the processing chamber has a long , wide , thin geometry to favor deposition of thin - film silicon on sheet substrates over the chamber walls . the sheet substrates are moved through between ends . a pair of opposing radio frequency electrodes above and below the workpieces are electrically driven hard to generate a flat , pancaked plasma cloud in the middle spaces of the processing chamber . a collinear series of gas injector jets pointed slightly up on a silane - jet manifold introduce 100 % silane gas at high velocity from the side in order to roll the plasma cloud in a coaxial vortex . a second such silane - jet manifold is placed on the opposite side and pointed slightly down to further help roll the plasma and maintain a narrow band of silane concentration . a silane - concentration monitor observes the relative amplitudes of the spectral signatures of the silane and the hydrogen constituents in the roll - vortex plasma and outputs a process control feedback signal that is used to keep the silane in hydrogen concentration at about 6 - 7 %.