In this work we have investigated the photoresist thin film interference effect on different reflective substrates. Using computer simulated and experimental swing curves, the fitting parameters of an empirical formula based on the two mirror Fabry-Perot etalon are correlated to the reflectivity and resist characteristics. In particular, the phase of the swing curve which corresponds to the phase shift of the light wave at the resist/substrate interface was studied in detail. Therefore, the swing behavior on different reflective substrates (Si, Al, etc.) as well as on oxidized wafers was measured. It is postulated that this difference in phase shift is due to the different nature of the substrates, i.e. the complex refractive index in addition to the thickness. Simulations of the latent image of the photoresist predict that the axial shift of the latent image is also connected to the phase shift of the light wave at the resist/substrate interface and therefore related to the swing curve phase. To illustrate the axial shift of the latent image of the photoresist an improved cross section development technique based on a top coating before cleaving and development is proposed. Possible effects of this phase on the standing waves and on the profile at the resist/substrate interface are also examined with this improved method. Furthermore, Al substrates with a smaller grain size were used in order to explore this influence of diffuse reflectivity on the swing effect.
Ivan S. Daraktchiev,
"Influence of substrate reflectivity on linewidth control", Proc. SPIE 2440, Optical/Laser Microlithography VIII, (26 May 1995); doi: 10.1117/12.209292; https://doi.org/10.1117/12.209292