3D resist reflow compact model for imagers microlens shape optimization

There has been a significant increase of optical applications in the last decade, either embedded into complex multifunction devices such as smartphones, or for imaging purpose as cameras. Core of such optical systems are microlens arrays, used for light gathering or light emitting. The most commonly used manufacturing method by the industry is the thermal reflow of photoresist polymer. The method consists in melting previously patterned photoresist dots in order to form the lenses. But the resist shaping into a microlens is not as straight forward, since the final microlens needs to match shaping criteria to maximize the device optical efficiency. The optimization of the microlens 3D shape is thus an empiric and iterative work, where several lithography and reflow process variations are explored. Photomask reorder might also be needed in order to finally reach the final targeted microlens. All of this results in a costly and time consuming process tuning work. A low cost alternative option to overcome this practical issue and make the overall microlens optimization process easier would be to have at disposal a resist reflow simulation tool, which could predict the photoresist shaping evolution through melt and cure steps. This would help designers and lithographer to evaluate beforehand the final shape of a certain design at the end of the process flow. It would then offer the possibility to identify from the start the correct design to embed onto the photomask guaranteeing the fabrication of the desired microlens. A 3D compatible and computation efficient reflow simulation software is proposed in this paper, in line with a Design Process Technology Co-optimization (DTCO) approach. It allows the fast 3D reflow simulations of hundreds of different resist patterns, taking as input a CAD design and returning the corresponding 3D microlens that will be formed. The purpose of this paper is to present the developed reflow modeling software solution and its calibration methodology. The use of the proposed alternative simulation flow for microlens optimization in a Resolution Enhancement Technics (RET) environment will also be described.