A planar waveguide grating (PWG) sensor is described herein which exhibits a low signal drift and an enhanced sensitivity due to the use of a fully dense silicon-rich nitride surface layer. In the preferred embodiment, the silicon rich silicon nitride surface layer has a composition which includes Si and N, and optionally H, Ge and/or O, where a Si/N atomic ratio is greater than 0.75. In addition, the silicon rich nitride surface layer has a refractive index that is greater than 2.45 and less than 3.2 at a wavelength of operation. A method is also described herein for manufacturing the PWG sensor with acceptable costs and high yields by utilizing well known semiconductor processes and tools.