SU-8 Processing on a Variety of Substrates
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ABSTRACT SU-8 has become a popular material for micromachining high aspect ratio structures. Typically, SU-8 is spun on a polished silicon wafer for processing. After patterning, the SU-8 is used for micromachined structures directly (such as fluidic channels) or as a mold for electroforming. Non-silicon substrates offer the possibility of cheaper processing, improved mold designs, and multi-material devices. Successful SU-8 processing depends strongly on surface properties of the substrate itself as well as environmental conditions during the processing. We explore the issues involved in transferring SU-8 technology to non-silicon substrates such as glass, plastics and metals. Issues such as wettability, adhesion, and surface tension are explored in this study. The findings indicate the merits of non-spinning approaches, such as dipping, spraying, and brushing and point to new SU-8 processes. INTRODUCTION SU-8 has become a popular material for micromachining in recent years [1-3]. The typical process is to spin SU-8 on a polished silicon or glass wafer, similar to the way that standard photoresist is done. However, due to the thickness requirements of the SU-8 (often up to several hundred microns), the spin rate must be slow, and a post-spin settling period is usually required to allow the SU-8 liquid to flatten out [4]. It is not obvious that spinning SU-8 on silicon is the most appropriate method for building SU-8 structures. The spinning process wastes most of the SU-8 (most of it spins off), and a resettling of the material is often required. Furthermore, for many applications, such as molding or electroforming, a silicon substrate is unnecessary. In this study, we explore the possibility of using non-traditional materials such as plastics for SU-8 substrates. Furthermore, we explore new methods of applying SU-8 to these surfaces, including brushing, spraying and dipping. Our hope is that these methods can result in cheaper, more reliable SU-8 processes which result in less material waste. EXPERIMENT Substrate preparation The materials studied were silicon (Si), glass, chrome (Cr), gold (Au), aluminum (Al), acrylic (PMMA), polypropylene (PP), polyvinylchloride (PVC), polycarbonate (PC), polyurethane (PU), and high density polyethylene (HDPE). Silicon wafers and glass slides (3" x 1") were cleaned by RCA clean (DI water/hydrogen peroxide/ammonium hydroxide acid, 3 /1/1) at 70 °C for 25 minutes, then dehydrated in the oven at 120 °C for 1 hour. Glass slides were coated with chromium, gold and aluminum, with chromium as an adhesion layer in the latter two cases. All plastic samples were cleaned by soapy water followed by a DI water rinse then dehydrated for 24 hours in 50 'C ambient air.
91 Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
SU-8-10 from Micro Chem Corporation was used in this paper. Because materials properties of substrates are different, the process of patterning SU-8 is slightly different for the plastics. Various test and resolution masks were used during the patterning.
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