Fabrication of Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 107

6.1.2 Photoresist Selection and Application

Photoresists are the heart of the masking process. Their preparation,
bake, exposure, and remove process needs to be well treated so that
the designated pattern can be formed on the substrate. Basically,
the photoresist is manufactured to respond to speciic wavelengths
of light and different exposing sources, as well as formulated with
speciic thermal low characteristics to adhere to speciic surfaces.
Four major components included in the photoresist: light (energy)
sensitive polymers, solvents, sensitizers, and additives. The light-
sensitive polymers are functioned to change the structure of the
photoresist from soluble to polymerized (or vice versa) after
exposure. The solvents maintain the photoresist in liquid form and
allow a thin photoresist layer to be applied to the wafer by spinning/
spraying. The sensitizers act as the control of chemical reaction
of photoresist while exposure. The additives are mixed with the
photoresist to obtain various purposes, such as the dyes to absorb
light rays in the photoresist.
Common photoresists are designed to respond to ultraviolet or
laser sources. Some special photoresists react to X-ray or electron
beams. Typically, two kinds of photoresists can be chosen for an
application: positive (e.g., AE5214) or negative (e.g., SU-8) type, which
is related to the contained light-sensitive polymers in photoresists.
The basic positive photoresist polymer is the phenol-formaldehyde
novolak resin. Before exposure, the polymer is insoluble. It converts
to a soluble state in a special solvent (called development) after
exposure. As for the negative photoresist, the exposure makes
the polymer switched from unpolymerized structure to a cross-
linked material which is etch-resistant (not soluble in developing
solution).
To apply the photoresist on the wafers, adhesion promotion can
be considered (e.g., HMDS) to get better adhesion, especially dealing
with features less than 1 micron. Two methods are commonly used:
spin coating and spray. Spin coating is a mature technique and the
standard coating method for lat wafers in IC technology. Regularly,
two spin speeds are used in one coating. The slow acceleration and
spin speed is applied irst, which allows the liquid photoresist to
low and spread the whole wafer. Then a fast spin speed causes the
coating as uniform as possible. The viscosity of the resist solution