Biomedical Engineering Reference
In-Depth Information
Fig. 10
Plot of gel fraction (
F
g
)inexposed
hb
-P
32
(4) film versus exposure time (
t
)
Because of its well-known high photoreactivity, benzophenone has been
introduced into natural (e.g., protein) and synthetic polymers (e.g., poly-
imide) to serve as a photo-cross-linker, and its photoreactions in various
polymer matrixes such as PS, PMMA, PC and poly(vinyl alcohol) have
been well documented [133-136]. As a matter of fact, benzophenone-
containing polyimides such as Ciba-Geigy 412 are commercially available
high-performance photoresist materials. The
hb
-PAAs contain numerous
aroylbenzene units and, as expected, exhibited very high photo-cross-linking
efficiencies. For example, a thin film of
hb
-P
76
on a glass plate could be read-
ily cross-linked by the irradiation with a hand-held UV lamp at room tem-
perature. The cross-linking may have proceeded through the well-established
radical mechanism [133-136]: a carbonyl group abstracts a hydrogen atom
from a benzyl unit, creating a stable benzyl radical. Coupling or combination
of two radicals leads to cross-linking and hence gel formation [105].
Figure 11 depicts the dose effect on the gel formation of
hb
-PAA films
(
l
F
= 1-2
m) after they have been exposed to a weak UV light with a power
of about 1 mW
µ
cm
2
. Although the photo-cross-linking conditions had not
been optimized, all the four
hb
-PAAs already exhibited much higher sensitivi-
ties (
D
0.5
= 43-180 mJ
/
cm
2
) than those of commercial poly(amic ester)-based
photoresists (
D
0.5
= 650-700 mJ
/
cm
2
) [137].
Well-resolved patterns with line widths of
/
m were readily formed
when a film of
hb
-P
76
was exposed to a UV dose of 1 J
∼
1.0
µ
cm
2
(Fig. 12A).
Patterns with submicron resolutions (line width down to 500 nm) were
also achievable, as demonstrated by the examples given in panels B and C
of Fig. 12. Clearly,
hb
-P
76
is an excellent photoresist material. Similar to
hb
-PAAs,
hb
-PYs were also photosensitive. Well-resolved, defect-free pho-
/
