Fabrication of Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 108

and the spinning speed strongly affect the layer formed. Hence, the
process optimization needs to focus on these two parameters. When
the processed wafer is not lat, spray method could be a good choice
to apply photoresist. The spray system uses a spray nozzle, which
generates a distribution of droplets of micrometer size. During spray
coating, rotating the wafer slowly could minimize the centrifugal
force and allow photoresist coverage in all the angle of the cavities.
To get the proper droplet size distribution of photoresist, a viscosity
of less than 20 cSt is necessary.
Soft bake step is necessary after the photoresist coating. The
purpose is to evaporate the solvent contained in the photoresist, which
allows for better adhesion and avoid interfering with photosensitive
components in the photoresist during exposure. Temperature and
time are two major parameters for soft baking. Usually photoresist
manufacturers will provide the required information and user could
follow and ine-tuned for different applications. Convection ovens
and hot plates are commonly used for this process.

6.1.3 Photomask Design/Fabrication and Alignment

Photomasks are high-precision plates containing microscopic
images of speciic patterns designed by users. It transfers the
deined tiny feature-sized patterns onto the wafer. Commercial
available substrate materials for making photomasks include quartz
and soda lime glass due to their good optical transparent property
and small thermal expansion coeficients. To block ultraviolet (UV)
light the following materials are commonly coated on one-side of
the mask plate: chrome, iron oxide, and emulsion. To fabricate a
high resolution photomask is costly. The e-beam directly write the
designed pattern on the speciic photoresist which is coated on the
block material, followed by development and etch process to remove
unwanted block material. If the resolution is larger than 10 microns
the transparency-type photomasks made by printing would be an
alternative, especially for some microluidics or biochip research
areas.
When readers design the photomasks, two polarities of masks
are needed to consider: light ield (most regions are clear) and dark
ield (most regions are dark). Through different combination of
light/dark ield photomask and negative/positive photoresist, the
intended patterns can be achieved in two ways. Figure 6.2 shows