Dynamical formation of microlenses by the reflow method: numerical simulation and experimental study of the process fabrication

Abstract

In order to study the dynamics of microlens formation by the reflow method, we develop a model based on the Navier–Stokes equation that describes the formation of a microlens from its initial state (photolithographic dot) up to its final shape (rounded microlens). In CMOS imagers, photoresists used to form microlenses contain crosslinkers in order to obtain chemically and thermally stable microlenses at the end of the process. This model takes into account the effect of surface tension as well as the viscosity evolution with bake time, due to the crosslinking reaction. The results of the dynamic modelling show that the microlens passes through different intermediate steps before achieving its expected final spherical shape. It also shows that fast crosslinking kinetics leads to non-spherical microlenses. Indeed the fast increase of viscosity has the effect of blocking the formation of the microlens and freeze the microlens profile in an intermediate shape. Thus, with such type of photoresist materials, contrary to what is reported in the literature, the final shape of the microlens does not depend only on the initial volume or the aspect ratio of the resist pattern. This is the competition, during the melting bake, between the microlens formation under the effect of surface tension and the crosslinking reaction that determines, over all, the final shape that the microlens will adopt.