Silicon nitride (SiN) deposited by plasma-enhanced chemical vapor deposition (PECVD) is the dielectric of choice for use in amorphous silicon thin-film transistors for active-matrix liquid crystal displays. Our nitride films deposited in a 40 MHz, coldwall PECVD system on chromium-coated glass substrates exhibit lower etch rates in buffered HF, as well as lower hydrogen contents, lower deposition rates, and higher indexes of refraction than nitride films deposited under supposedly identical conditions on bare glass. We use a simple heat-flow model to show that thermal radiation from the substrate surface in the heated susceptor/coldwall design of our deposition chamber leads to different substrate temperatures for different surface types, thus causing the substrate dependent deposition. We present SiN film data as a function of temperature and as a function of thickness to support this theory. Additionally, the length scale over which this thermal effect is relevant, and its effect on film and device uniformity, are discussed.