A new fabrication process for ultra-thick microfluidic microstructures utilizing SU-8 photoresist

Abstract

In this paper we describe a new process for fabricating ultra-thick microfluidic devices utilizing SU-8 50 negative photoresist (PR) by standard UV lithography. Instead of using a conventional spin coater, a simple 'constant-volume-injection' method is used to create a thick SU-8 PR film up to 1.5 mm with a single coating. The SU-8 PR is self-planarized during the modified soft-baking process and forms a highly-uniform surface without any edge bead effect, which commonly occurs while using a spin coater. Photomasks can be in close contact with the PR and a better lithographic image can be generated. Experimental data show that the average thickness is 494.32 ± 17.13 µm for a 500 µm thick film (n = 7) and the uniformity is less than 3.1% over a 10 × 10 cm 2 area. In this study, the temperatures for the soft-baking process and post-exposure baking are 120 • C and 60 • C, respectively. These proved to be capable of reducing the processing time and of obtaining a better pattern definition of the SU-8 structures. We also report on an innovative photomask design for fabricating ultra-deep trenches, which prevents the structures from cracking and distorting during developing and hard-baking processes. In this paper, two microfluidic structures have been demonstrated using the developed novel methods, including a micronozzle for thruster applications and a microfluidic device with micropost arrays for bioanalytical applications.

SU-8 Ka-band filter and its microfabrication

“…The thickness of 1000 µm is sufficient for the filter applications. The sidewall angle in these experiments is controlled within 90 ± 0.1 • , which is a significant improvement compared with those reported so far [8,17]. This high quality fabrication result is an important characteristic for a quality coaxial transmission line.…”

“…Figure 6 shows an SU-8 layer with a smooth and vertical finish on the edge, which is important to the quality of filtering. The highest aspect ratio presented in [8] is 15:1. The aspect ratio achieved by using the optimized process, however, has been much increased.…”

“…The optimized process for producing a 700 µm thick SU-8 layer consists of the following steps. In comparison with the ultrathick SU-8 process proposed in [8], the optimized process differs in several aspects. First, the soft bake temperature is kept at a maximum 95 • C rather than 120-130 • C. The relatively low soft bake time helps reduce the internal stress of the SU-8 structures, and reduce and eliminate cracks of the structures.…”

“…Harris et al pointed out in [7] that as an alternative to silicon filters, the same structure can be made of SU-8 with a simpler fabrication process. SU-8 structures in a single layer can be made as thick as 1.5 mm using a UV lithography process [8], as opposed to the 0.5 mm thickness in the Si fabrication using the DRIE process. This gives more options in the filter design.…”

Fabrication of multi-layer polymeric micro-sieve having narrow slot pores with conventional ultraviolet-lithography and micro-fabrication techniques

“…Baking steps are really crucial in order to reduce the internal stress in the final film. 19 After soft baking the SU-8 on the hot plate, a chrome coated glass mask with rectangular features (12 Â 9 lm) was used to transfer the patterns into the SU-8 photoresist. The pitch size (i.e., distance between mask features in x and y-directions) was 4 lm and 2 lm in x and y-directions, respectively (see Fig.…”