The advance in microlithography has greatly helped the development of micro optical elements. Large array of microlenses can now be fabricated in the same fashion as manufacturing of integrated circuit at low cost and high yield [1-2].
Because microlens array requires well-defined and continuous surface relief profile, special methods are needed to supplement the normal microlithography to produce those spherical structures . Various techniques have been developed, and the most widely used is multi-step photolithography with thermal resist reflow. However, the alternative grayscale photolithography technique appears to be the one as the most flexible and versatile method .
Indeed, this approach is a one-level lithography process enabling the development of 3D profiles in a photoresist masking layer. In addition, with the need to maintain or improve image quality at an ever-smaller pixel size, grayscale technic can offer one way to compensate the loss of the photosensitive area by achieving zero-gap microlens. One other advantage of grayscale is the possibility to have, from a single lithography, objects of different shapes, but also at the same time of different sizes (especially heights); which is possible with classical lithography only by doing multi-patterning.
There are several options for performing grayscale lithography, for example the HEBS mask (high energy beam sensitive) which darkens under exposure to electrons. The option that has been chosen is to use a grayscale reticle, with varying chromium features densities that locally modulate the intensity of transmitted UV light. Being non-uniformly exposed, this allows the creation of a relief structure in the resist layer after development. The resist height after development depends on the intensity of the incident light, the exposure time and the contrast of the resist. So contrary to conventional lithography where the goal is to achieve straight resist pattern profiles, grayscale lithography enables the realization of progressive profiles, which requires smooth resist contrast curve. The other specificity of these resists is that they must crosslink without flowing.
In this paper, we evaluate resists from different suppliers to generate microlenses smaller than 5μm via a grayscale mask. The study consists in establishing the contrast curves of these resists according to different process parameters, giving the designer great control of grayscale levels that can be achieved for a given resist. Then, pattern various microlenses shapes in these resists to evaluate the residual resist thickness according to the gray levels. With the final objective of establishing a relationship between these contrast curves and the profile variations at the microlens level to compute a suitable and accurate grayscale mask .
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.