A line plasma source (20) comprises a plasma chamber (30) configured so that plasma (32) is situated remotely and on-edge with respect to a polycrystalline silicon surface (20S) to be treated, thereby preventing damage to the surface, facilitating treatment of large substrates, and permitting low temperature operation. Active species exit the plasma chamber through a long narrow ("line") outlet aperture (36) in the plasma chamber to a reaction zone (W) whereat the active species react with a reaction gas on the polycrystalline silicon surface (e.g., to form a deposited thin film). The polycrystalline silicon surface is heated to a low temperature below 6000.degree. C. Hydrogen is removed from the reactive surface in the low temperature line plasma source by a chemical displacement reaction facilitated by choice of dominant active species (singlet delta state of molecular oxygen). Reaction by-products including hydrogen are removed by an exhaust system (100) comprising long narrow exhaust inlet apertures (114L,114R) extending adjacent and parallel to the outlet aperture of the plasma chamber. An ionizing electric field is coupled to the plasma across a smallest dimension of the plasma, resulting in uniform production of active species and accordingly uniform quality of the thin film. The polycrystalline silicon surface to be treated is translated with respect to the plasma line source in a direction perpendicular to the outlet aperture of the plasma line source for integrating thin film quality in the direction of translation (22).