The Applied Research On The Thickness And Optical Constants Of Absorbing Thin Films Deposited On Opaque Substrates By Spectroscopic Ellipsometry

The precise measurement of both the optical constants and thickness of the thin film is of great value in both academic and industrial areas. Spectroscopic Ellipsometry(SE) is a high-sensitive and non-destructive in situ characterization method. One of the remarkable features of SE is the high precision of the measurement, and very high thickness sensitivity(～0.1?) can be obtained even for conventional instruments. However, SE is not direct measurement of film thickness and optical constants. Instead, the sample structure must be inferred from polarization or intensity change versus wavelength and angle by computer.The determination of the optical constants of absorbing films, particularly on opaque substrates, is a difficult problem when approached solely with SE. First, unwanted backside reflections are incoherent with the desired reflection from the front side, if this is not taken into account, inaccurate optical constants will be fitted from the acquired data. Second, the optical constants of substrate must be carefully characterized in advance, as any small absorption in the substrate would be mixed into the film’s overall optical constants. Third, strong correlation between thickness and optical constants may prevent a unique solution for absorbing films. For the above reasons, quartz, glass slide, cover glass and float glass substrates were studied. Backside reflections of the substrates were suppressed by index matching technique. The results showed that the simple technique work well for substrate materials with refractive index in the range of 1.43 to 1.64, including materials such as fused silica, float glass, etc over the spectral range from 190 nm to 1700 nm. After that, the refractive index and extinction coefficient of the substrates were fitted by ellipsometric ψ data and the normal spectral transmittance T0. The results were consistent with the literature reported. Finally, a Combined ellipsometry and transmission approach(SE + T) was used to determine the thickness and optical constants of the diamond-like carbon(DLC) film coated on the quartz and amorphous silicon(a-Si) film coated on the glass slide and cover glass accurately.Thickness and optical constants could be obtained easily and rapidly using SE + T method. However, noise, minor contamination on the surface of the film or any small absorption in the substrate would be mixed into the film’s optical constants. In order to determine the optical constants of ultrathin DLC and a-Si film accurately, the optical constants were parameterized by combined SE + T and Optical constant parameterization approach. The results show that the adopted method is able to uniquely and accurately measure optical constants from a unique solution while maintaining smooth, continuous, and often Kramers-Kronig consistent optical properties for the ultrathin DLC film and a-Si film. This method is expected to be of value for optical properties measurement of other amorphous films with a thickness just a few tens of nanometers.