Generation of multi-level microstructures using a wavelength-selective photoresist and mask-less grayscale lithography

Precise and fast structuring polymeric materials is important for almost all applications in microsystems engineering, microfluidics and microoptics. In addition to 3D printing and soft lithography, grayscale lithography is a promising technique for structuring of complex geometries. It allows for the direct fabrication of relief structures, such as microoptical lenses and microfluidic channel structures, by illuminating a photo-curable material with a light pattern of gradually changing light intensity. However, standard grayscale resists are limited in terms of penetration depth and contrast: a low contrast, i.e. a flat slope of the polymerization rate vs. exposure dose curve is desired for generating accurate grayscale patterns of high resolution. The achievable polymerization depth of these patterns is limited. Hence we have developed a new photoresist for grayscale lithography that makes various polymerization depths independently accessible. The acrylate-based photoresist contains a combination of different initiators and absorbers, which can be polymerized at different wavelengths from ultraviolet to visible light, producing individual polymerization depths, each with a low contrast. Due to the wavelength-dependent absorptions of the initiators and absorbers, the different polymerization depths are addressed independently. The intensity gradients were generated using a custom-built mask-less microlithography system based on a digital mirror device (DMD). In this work we will demonstrate that 3D structures with a high complexity can be fabricated fast and easily via grayscale lithography using the multi-level photoresist.