Plasma silicon oxide films on garnet substrates: measurement of their thickness and refractive index by the prism coupling technique

The prism coupling technique has been used to measure the refractive index and thickness of plasma silicon oxide films deposited on garnet substrates. The film thickness is also measured with a mechanical stylus (Talystep) for comparison. A linearly polarized He-Ne laser at 632.8-nm wavelength or a He-Cd laser at 441.6-nm wavelength is used as the light source in the prism coupler. The use of the He-Ne laser is demonstrated to result in a fluctuation in the detector output due to light interference in the substrate. The He-Cd laser is shown to be superior because the substrate is sufficiently absorbing that interference is eliminated. It also permits thickness measurements on thinner films. The agreement between thickness measurements by the prism coupler and the mechanical stylus is within +/-0.015 microm for films of 0.4 microm or thicker and +/-0.010 microm for thinner films. The error in thickness measurement caused by an error in refractive index assumed or in determining the coupling angles for films thinner than 0.30 microm is also estimated.