Time dependent exposure effects in thin film semiconductor technology

Abstract

Photosensitive polymer films are used in semiconductor manufacturing as dielectric and passivation layers. Their chemical, mechanical, and interfacial properties depend not only on the total exposure dose applied, but also on the temporal characteristics of the exposure, which influence the photochemical reaction kinetics and the formation of the polymer network. Industrial exposure tools deliver the same dose with different intensity–time profiles. It remains an open question whether the commonly held assumption of exposure reciprocity applies to chemically complex systems such as high-performance polyimides. This study examines how exposure intensity and time influence the material behavior of a negative-type, photosensitive polyimide thin film. At the same wavelength and nominal dose, high-intensity Stepper exposure is compared with low-intensity Mask Aligner exposure, so that the two differ primarily in their intensity–time profiles. Three complementary methods are used on four sample conditions (two exposure doses, two curing temperatures), all processed identically and differing only in the exposure: Fourier-transform infrared spectroscopy (FTIR) for imidization, the shear test for adhesion and retention measurements as an indirect indicator of crosslinking density. The results of this study show: The reciprocity assumption does not hold for this material system. At the same dose, films exposed on a Stepper retain less layer thickness after development than those exposed on a Mask Aligner (9.1 and 4.2 percentage points difference at 75 and 150 mJ/cm^2, respectively), which indicates a lower crosslinking density entering the curing step. The degree of imidization at the surface, on the other hand, is determined exclusively by the curing temperature, not by the exposure tool or dose. For adhesion, both influences overlap, with a counterintuitive result: The samples cured at 300 °C, which are less fully imidized, adhere better than those cured at 400 °C (up to 9.4 MPa difference). Adhesion is thus inversely related to the degree of imidization for the investigated range. Based on current information, this is the first time this intensity-dependent reciprocity failure has been demonstrated for an ester-based photosensitive polyimide under conditions that isolate the intensity–time profile from other tool differences. Together, these findings form a chain of effects: The exposure tool does not influence adhesion through imidization, but most likely through the crosslinking density. The difference between tools in retention and shear strength narrows as the dose increases, whereas the surface degree of imidization shows no such dose dependence. Compensating for the cross-linking deficit caused by reciprocity and maximizing adhesion therefore require two independent, opposing adjustments: A higher dose directly reduces the difference in crosslinking density between the tools, while a lower curing temperature improves adhesion.