Antireflective (AR) coating on waveguides with high refractive index is imperative to minimize insertion losses. In fabricating silicon, rectangular, suspended waveguides on silicon-on-insulator (SOI) wafers, a single Si3N4 layer is deposited on the waveguide walls. For the purpose of applying an optimum layer, we develop an integrated micromechanical gauge to determine the coating width by measuring the induced stress in the silicon. Gauges at different sites on a wafer produce results with a standard deviation of about 0.5%. The insertion loss due to the waveguides is measured directly by coupling a laser beam at 1550 nm from a single-mode fiber to the waveguide, then to another fiber and a detector. Tests are run on six wafers and two types of devices: a waveguide with two facets and a waveguide with a gap, presenting four facets. The optimal silicon-nitride thickness is found at 200 nm, featuring a fiber-waveguide-fiber insertion loss of about 1 dB for a two-facet device and 1.7 dB for a four-facet device.