Page 22 - Principles and Applications of NanoMEMS Physics

8 Chapter 1

at these wavelengths, however, the method of image projection employed is
proximity printing through a mask containing x-ray absorbing patterns. The
mask is separated from the PR-wafer a distance of just about 25 µ m, but
since dust particles with low atomic number do not absorb x-rays, no
damage is caused to the pattern. Despite the potential for highest resolution
germane to x-ray lithography, two factors have been identified as potentially
limiting it. Both factors originate in geometrical aspects of the illumination.
In particular, there is the possibility that a significant penumbral blur
δ = φ g L be introduced on the position of the resist image by the extended
point source of diameter φ located a distance L above a mask separated
from the wafer a distance g. Also, a potential for lateral magnification error
is present, due to the divergence of the x-ray from the point source and the
finite mask to wafer separation. Accordingly, images of the projected mask
are shifted laterally by an amount d = r g L .
Even with perfect resolution, pattern formation quality depends on how
the PR responds to the impinging lightwave or electron beam. This is
addressed next.

1.2.1.2.2 Photoresist

The mechanism for image transfer to the PR involves altering its
chemical or physical structure so the exposed area may subsequently be
easily dissolved or not dissolved. According to the previous sections, pattern
formation is effected on optical resists, electron beam resists, or x-ray resists.
Optical lithography resists may be negative or positive. The fundamental
difference, in terms of how they affect the resolution of the image
transferred, is rooted in their chemical composition.
In the negative resist, which combines a cyclized polyisopropene polymer
material with a photosensitive compound, the latter becomes activated by the
absorption of energy with wavelengths in the 2000- to 4500-Å range. The
photosensor acts as an agent that causes cross linking of the polymer
molecules by transferring to them the received energy. As a result of the
cross linking, the molecules’ molecular weight increases and this elicits their
insolubility in the developing system. The highest resolution limit of a
negative PR derives from the fact that during development the exposed
(cross linked) areas swell, whereas the unexposed low molecular weight
areas are dissolved. The minimum resolvable feature when using a negative
resists is typically three times the film thickness [23].
In response to light the positive resist, which also contains a polymer and
a photosensitizer, the latter becomes insoluble in the developer and, thus,
prevents the dissolution of the polymer. Since the photosensitizer precludes