Optical channel waveguiding has been observed in a structure consisting of a silicon nitride ridge on a multilayer GaAlAs heterostructure. The silicon nitride layer was deposited and then patterned to form 3.5 micron wide ridges. The optical field intensity radiating from the endface of the channel waveguide is found to have two peaks symmetric about the center of the silicon nitride ridge. Raman microprobe spectroscopy indicates a significant stress-induced shift in the GaAs longitudinal phonon frequency. Rapid thermal annealing reduces the stress and changes the optical field intensity distribution to a single symmetric peak.