We present the details of our study on the internal stresses and adhesion strengths of SU-8 structures to different substrate seed layers. The effect of adhesion promoter-methacryloxy [propl] trimethoxysilane (MPTS), and OmniCoat-and different seed layer combinations (Ti/Cu/Ti, Ti/Cu, Cr/Au, and Cr/Au/Cr) was examined for internal stress and adhesion strength in 650-µm-thick SU-8 films. Increased stress and poor adhesion have led to the delamination of SU-8 in some cases. Adhesion and stress have proven to be the function of process parameters such as soft bake (time and temperature), exposure dose, post-exposure bake (time and temperature), and development time. We have found that a 100 silicon wafer containing a titanium-copper-titanium (Ti/Cu/Ti) seed layer with MPTS as the adhesion promoter yielded a thick SU-8 film with a lower value of stress and consequently better adhesion for processing in deep x-ray lithography (DXRL). A detailed correlation of the effects of adhesion and internal stress on the SU-8 film is discussed. An analysis of the possible chemical bonding interactions occurring between SU-8, and its adhesion promoter and/or substrate is also presented.
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 and MPTS) on the adhesion of SU-8 structures. In addition, parameters such as SU-8 thickness (450 μm, 650 μm, 900 μm) and substrate roughness values (silicon, Ra < 10 nm and Ra = 0.5 μm) 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 μm) 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 μm) 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 μm) showed poor adhesion. A detailed correlation of the effect of these parameters on SU-8 adhesion will be discussed in this paper.
This paper mainly reviews the current status of the LIGA fabrication technology for use in microfluidics applications. It also presents the work that the group is involved in the fabrication of 3-D high aspect ratio structures. This work explored the possibility of using LIGA and excimer laser micromachining as complimentary processes in two sequential steps for making some of the components required for fluidic applications. A microflask is designed and fabricated using this combination of processing technology. The details of the design and fabrication technology are presented in this paper.
Synthetic opal has been used as a template for making 3D inverse opals of silica, titania and silicone rubber. The materials are mesoporous with connected pores and channels and have better opalescence than the opal templates they replace. Thin films of synthetic opal have been grown onto glass substrates by spin coating and these have also been used as templates for making thin films of inverse opal and as masks for depositing metal nanodots. This method produced hexagonally patterned 50 nm gold dots on a flat graphite substrate.