Biomedical Engineering Reference
In-Depth Information
the soft-contact type. As for the projection type aligner, the major
discrepancy from the above two types is the mask image projected
onto photoresist by lenses and the projected image on wafer at 1:1
or 5:1 or 10:1 reduction. Hence, good lenses and optical system are
must. Besides, due to its small projected region, step-by-step repeat
projection onto a large wafer is needed. There is a trade-off between
the speed and resolution.
6.1.4
Photoresist Exposure and Development
The photoresist sensitivity relates to the energy of speciic
wavelength provided by exposing source. Common positive and
negative photoresists respond to UV light. Some are particularly
designed to reactive to G, H, I lines, whose wavelengths are 365 nm,
405 nm, and 432 nm, respectively, because of current lithography
systems use the high pressure Hg lamp which has high intensity in
these lines. To enhance resolution, there are speciic photoresists
sensitive to deep ultraviolet (DUV) light spectrum which excited by
excimer lasers.
The photoresist sensitivity is a parameter to measure the
amount of energy required to initiate the basic chemical reaction.
Users could usually ind the information of speciic photoresist from
manufacturer's data sheet. The unit is often expressed as millijoules
per square centimeter (mJ/cm
2
) for a given thickness of coated
photoresist. In practical operation, the intensity of light source
will be tested irst, and users can calculate the necessary time for
exposure. Overexposure and underexposure could affect the idelity
of the designed patterns, especially when dealing with the micron-
range features. Some process recipes will suggest the postexposure
bake for a suitable temperature and bake time to reduce the standing
wave effect.
After the wafer inishes the alignment and exposure steps,
the designed pattern is transferred to the photoresist as exposed
and unexposed regions. Development technique is to use the
speciic chemicals to dissolute the unpolymerized region (negative
photoresist) or photosolublized part (positive photoresist) that an
exact duplicate of patterns is formed on the photoresist.
The chemicals for developing photoresist associated with speciic
photoresists can be found from the data sheet provided by the
photoresist manufacturers. Generally speaking, the factors affecting
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