Ultra-deep x-ray lithography of densely packed SU-8 features: II. Process performance as a function of dose, feature height and post exposure bake temperature

Ultra-tall (2–4 mm), densely packed arrays of high aspect ratio micro structures (HARMs) are required for a variety of heat transfer and mass transfer devices currently under development. Great success has been reported lithographically defining relatively tall features using SU-8 as an x-ray resist, but excellent results are scarce with respect to patterning ultra-tall features with tight packing and small gaps between features. Diffusion of a cross-linking species from exposed regions to unexposed regions is believed to be the primary mechanism that prevents such features from being defined. In a companion paper, solvent content is shown to be an extremely important parameter that controls the quality of ultra-tall, tightly packed micro structure arrays. In this paper, a simple quasi-two-dimensional model is developed to qualitatively show that the adverse effects of diffusion of a cross-linking species can be significant. Also, experiments were performed to isolate the effects of parameters other than solvent content on performance (post exposure bake temperature, dose and top-to-bottom dose ratio, and feature height). These effects are placed in the context of the general hypothesis that preventing diffusion of cross-linking species is a key to successfully defining ultra-tall, tightly packed SU-8 HARMs. Experimental results are provided that demonstrate the ability to define SU-8 features with heights of 3 mm, and gaps between adjacent features of 125 µm (the gap therefore having an aspect ratio of 24).