features with various thicknesses, which can be controlled by the spin speed during
coating. For exposure i-line (365 nm) is used and exposure time correlates to the
thickness of the layer. That information is provided in the datasheet from the
manufacturer. The SU-8 has excellent adhesion properties to silicone substrates;
demonstrates limited adherence to glass or quartz, the later needs to be treated with
HDMS prior to coating. It is important to remember that the illuminated areas are
rendered insoluble for negative photoresist. Overexposure is required for complete
polymerization, as the process starts from the top, where the light hits first, to the
bottom, where the resist adheres to the substrate. Major consequence is that for full
polymerization of very thick layers a long overexposure is needed. The scattered
light during the extra time diminishes the resolution to about 2–3μm([ 6], p. 7).
Multi-step processes are possible when thinner layers are applied in consecutive
coating and patterning cycles. However, such multi-layer depositions require align-
ment and also very thin films can result in non-homogeneous coating in the
substrate and pin whole formation. The resulting polymerized SU-8 has excellent
optical properties and is suitable for producing integrated optical guides on silicon.
As a mould the SU-8 can form undercuts, protruding edges at the top surface that
hinder demoulding and can lead to breakage of the silicone master. As shown in
Fig.3.4 the tapered angle can be reversed and such impractical demoulding can be
avoided during the fabrication step [3] by using transparent substrate (Pyrex)
instead of silicon and by exposing from the bottom.
Typical SU-8 25 series forviasand channels follows established protocol [7],
which can vary in terms of baking and exposure times related to the thickness
(15μm) of the resist layer. First SU-8 is poured to cover 60 % of the wafer surface.
Fig. 3.4 Top: standard front side exposure of negative resist tends to form undercut structures at
underexposed sidewalls.Bottom: backside exposure reversed the effect to form tapered sidewalls
(taper angleα¼ 6 ), which allowed facile demolding. Reprinted by permission from Institute of
Physics Publishing: [Journal of Micomechanics and Microengineering, 2007] [3]
90 N. Dimov