Photoresist (such as PMMA) thin films are among the most important materials in semiconductor fabrication processes. Photolithography exclusively uses photoresist to construct the circuit pattern. The quality of a pattern relies on the uniformity of a photoresist layer. However, thermal expansion mismatch between the photoresist and silicon substrate during the subsequent thermal cycling process inevitably produces undesirable residual strains/stresses in the resist layer. Such residual fields can deteriorate the desired quality of the final pattern. A recently developed method is used here to record full-field strain information in the thin resist film.
Traditional methods such as wafer curvature measurement or x-ray diffraction (for crystalline films) have not been effective for measuring the full-field strains. However, using a deposited dot array in the interface between the film and substrate, one can measure the strains in the thin film, full-field, by analyzing the displacements of individual dots. A biquadratic surface model is employed to fit these dot pattern displacements. An IDL (Interactive Data Language, Research Systems Inc.) program is utilized repeatedly to accurately locate the positions of the individual dots and transform that information into displacement and strain data by differentiating the biquadratic surface. This study successfully demonstrates a straightforward approach to determine the residual strains, full-field, in photoresist thin films.