Influence of residual layer on cross-sectional shape of thermal-reflowed photoresist structures

When photoresist structures are formed by employing various lithography technologies and followed by thermal reflow treatments, the mechanism that transforms the cross-section of a photoresist structure from a rectangular-shape into a circular-shape is seen as an integral constituent of the manufacturing method of microlens arrays. However, in the case, where a residual layer is absent, a photoresist film is completely exposed to the oncoming radiation down to the interface between the photoresist film and substrate. Even in the presence of a residual layer, it has been uncertain to the author if a photoresist structure with a circular cross-sectional shape could be obtained, and be made applicable for the fabrication of a microlens array. The author then executed a set of thermal reflow treatments under various conditions using a positive-tone photoresist AZP4903 known for its capability of forming relatively thick films. As a result, it became clear that the existence of a photoresist’s residual layer has large influence on the transformation of the cross-sectional shapes of photoresist structures. These observations can be attributed to whether the bottom surface of a photoresist structure is firmly fixed on a hard substrate, or if it happens to be in contact with a soft photoresist layer which can flow comparatively freely.