An investigation of SU-8 resist adhesion in deep X-ray lithography of high-aspect-ratio structures

The SU-8 negative photo resist has been recognised as an unique resist, equally useful for UV lithography and deep x-ray lithography (DXRL) applications; but it is in DXRL where SU-8 has shown a significant advantage over other resists. When compared with the common DXRL resist poly-methyl methacrylate (PMMA), SU-8 has been found to significantly reduce x-ray exposure time, processing time and cost, thus making SU-8 a strong candidate for commercial DXRL applications. Despite these advantages, several factors associated with SU-8 processing are not well understood. Resist-substrate adhesion, which is the key for successful lithography, is one such example. This paper examines the effect of substrate (silicon and graphite), seed layer (Ti/Cu/Ti, Ti/Cu, Cr/Au and Cr/Au/Cr), and the use of adhesion promoters (OmniCoat(TM) and MPTS) on the adhesion of SU-8 structures. In addition, parameters such as SU-8 thickness (450 mum, 650 mum, 900 gm) and substrate roughness values (silicon, Ra < 10 nm and Ra = 0.5 mum) have also been investigated. The results of our work highlight the importance of material selection for a given process and the relationship between the different parameters investigated. Increased stress for thicker films (>850 mum) has lead to the delamination of SU-8 on some substrates. The adhesion has also proven to be a function of process parameters such as pre-bake (time and temperature), exposure dose, development time and post exposure bake (time and temperature). We have found that a <100> silicon wafer (Ra = 0.5 mum) containing a titanium-copper-titanium (Ti/Cu/Ti) seed layer, provided an adequately adhered resist for DXRL, while a chromium-gold (Cr/Au) seed layer on silicon (Ra = 0.5 gm) showed poor adhesion. A detailed correlation of the effect of these parameters on SU-8 adhesion will be discussed in this paper.