This paper presents results of our study of the chemical-mechanical polishing (CMP) of plasma-enhanced chemical vapor deposited (PECVD) silicon nitride with both alumina-powder-based and colloidal-silica-based slurries. The polish rate, polish rate uniformity and nitride surface condition were used as primary metrics, with nitride surface chemistry from ex-situ X-ray photoelectron spectroscopy and surface roughness from atomic force microscopy for more detailed evaluation. Excellent CMP results (polish rate of 70 nm min(-1), 4% uniformity across 125 mm diameter wafers, little change in nitride surface chemistry, as low as 0.2 nm average surface roughness) were obtained with a colloidal silica-based slurry and commercial pads. PECVD nitride stacks such as SiNx/SiO2/Si and SiNx/Ta/SiO2/Si demonstrate a three-fold increase in polish rate with thick metal underlayers. Colloidal silica-based slurries used for polishing oxides are a viable starting slurry chemistry for nitride polishing, with additional oxidizers improving both uniformity for thin remaining nitride (<150 nm) and surface roughness.