Fabrication of Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 109

the use of clear ield photomask and positive photoresist can obtain
the same island patterns as the use of dark ield photomask and
negative photoresist. As for which selection is better for the inal
results, the accumulation of user experience could make a proper
judgment.

Photoresist polarity Photoresist polarity Photoresist polarity

Negative Positive Negative Positive Negative Positive

Clear
field

Clear
field

Clear
field

Hole Hole Hole

Island Island Island

Photomask
polarity

Photomask
polarity

Photomask
polarity

Dark
field

Dark
field

Dark
field

Island Island Island Hole Hole Hole

Figure 6.2 Photomask and photoresist polarity results.

An aligner system consists of two major parts: (1) movable
mechanism to correctly position the patterns of the photomask on
the wafer surface and (2) the exposure subsystem. Two major factors
determine the quality of the aligner: resolution and registration
capacities. The resolution is mainly a function of the wavelength
of the exposing light. The exposing light with smaller or narrower
wavelength possesses higher resolution ability due to the diffraction
effect. Also, the short wavelength light reduces the scattering effect
in the photoresist and from the wafer surface. The registration
capacity is determined by how well the images are correctly placed
to each other. This requires precise translation/rotation stages
and associated mechanisms as well as the operators' experience of
inding the best image matching position.
Three aligner types are commonly used: contact, proximity
(or soft-contact), and projection. When the contact type aligner is
operated, the photomask is positioned over the chunk, which the
processed wafer is vacuumed onto. During alignment, the chunk
is translated in XY plane, and rotated with an angle θ . Once the
wafer and photomask are aligned properly, the chunk is pushed to
contact the photomask and followed by exposure. Similar to contact
aligner, the major difference of proximity type aligner is that a tiny
gap existing between the wafer and photomask during exposure.
Therefore, the contact type can make smaller pattern transfer to
the wafer compared to the proximity type, but the wear degree of
the photomask for the contact type is more severe compared to