Wavelength shift, caused by temperature fluctuation, critically limits the application of photonic systems. Here, the waveguide geometry is optimized to minimize the wavelength shift due to temperature change and fabrication error. A temperature-insensitive Mach-Zehnder interferometer filter is proposed for a wavelength locker, based on a silicon nitride waveguide. The proposed device achieves a 0.6 pm/K spectral shift over the C-band, which meets the requirements of a wavelength locker for application in dense wavelength division multiplex systems.