Abstract: An apparatus for producing improved large area photovoltaic devices by substantially reducing the warpage of relatively large area, relatively thin webs of magnetic substrate material which travel through a plurality of high temperature, low pressure glow discharge deposition chambers. As the web of the substrate material moves through the deposition chambers, it assumes a normal, elongated path of travel. Due to the elevated deposition temperature, the elongated path of travel, the force of gravity, etc., the web has a tendency to warp. Warpage of the web is undesirable as it promotes the deposition of non-uniform semiconductor alloy layers. The improvement of the present invention contemplates the establishment of at least one magnetic field within each deposition chamber which is adapted to urge the web of substrate material out of its normal path of travel into a flat, substantially planar path of travel.