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728 Polymers, Photoresponsive

37
2 38,39
10 kV and 1 J/cm , respectively, have been required
for patterning purposes.
Efforts to improve the sensitivity of PMMA have
spawned many research efforts, including:

Introduction of electronegative substituents
40
Incorporation of bulky substituents
41
Incorporation of fluorine into the ester group
42
Generation of inter- and intramolecular anhydride

FIGURE 5 SEM micrographs depicting the effect of developer 43
linkages
selection on a solvent-developed, negative-acting resist.
Copolymerizing methyl methacrylate (MMA) with

other methacrylate-based monomers to effect improved
Fortheresistsdiscussedinthissection,lithographicper- 44 45
absorption characteristics or etching resistance
formance is hindered by the extent to which the materials
swell in organic developing solvents. This phenomenon is
Materials based upon poly(methyl isopropenyl ketone)
showninFig.5,whichdepictshowdeveloperselectioncan
(PMIPK) are also known to undergo radiation-induced
affect image quality in a styrene-based negative resist. The 46
chain scission.
type of pattern distortion shown in this figure ultimately
Copolymers of an alkene or vinylaryl compound with
limits the resolution capability of these resists. Novem- sulfur dioxide are another major class of chain scission re-
bre and co-workers 31 found that the extent of swelling
sists and are widely used for the fabrication of chromium
could be minimized by proper choice of developer us- 47
photomasks. Interest in the poly(olefin sulfone)s as re-
ing a method based upon the Hansen three-dimensional 48
sists arose from results of Bowmer and O’Donnell, who
solubility parameter model. They determined that devel-
reported a G(s) value of ∼10 for poly(1-butene sulfone).
opers found to be thermodynamically poor, but kinetically
Upon irradiation, the relatively weak main-chain carbon-
good,solventsaffordresistmaterialssuchaspoly(glycidyl 49
sulfur bond cleaves, inducing depolymerization to yield
methacrylate-co-3-chlorostyrene) (GMC) with enhanced
the starting monomers as the major products. The copoly-
resolution capability (Fig. 5a). This methodology facili-
mer containing 1-butene (PBS) (Fig. 6) is a highly sen-
tates selection of an optimal developer without the tedious 2 50
sitive (<1 µC/cm at 10 kV) electron-beam resist. An
trial-and-error approach commonly used.
O 2 -reactive,ion-etching-resistantmaterialcanbeobtained
using silyated monomers such as p-trimethylsilylstyrene
C. Main-Chain Scission Positive and p-pentamethyldisilylstyrene. 51
Resist Chemisty Linear silicon backbone polymers (i.e., polysilanes)
have also been reported to function as positive-acting
Positive resists exhibit enhanced solubility after exposure 52
deep-UV resists. Exposure results in cleavage of the
to radiation. The principles leading to this increased sol-
main-chain Si Si bond, resulting in a decrease in molec-
ubility are chain scission and/or polarity change. Positive
ular weight. These materials exhibit high quantum yields
photoresists that operate on the polarity change principle
forscission,nonlinearphotobleaching,andsubmicronres-
have been widely used for the fabrication of VLSI devices
olution. One interesting variant of this approach involves
because of their high resolution and excellent dry-etching
plasma-induced deposition of networked polysilane-like
resistance. The chain scission mechanism typically oper-
films from methylsilane. Irradiation of this material in the
ates at photon energies below 300 nm where the energy is
presence of oxygen affords a silicon-oxide-like material
sufficient to break main-chain bonds and will be reviewed
(Fig. 7). Either the irradiated, oxidized regions or the par-
first.
ent silane can then be removed through plasma processing
Substituted methacrylates are probably the most ex- 53
withtherequisitereagent. Thereaderisreferredtorecent
tensively investigated polymers that fall into this cat-
reviews of lithographic applications of silicon-containing
egory. Poly(methyl methacrylate) (PMMA) undergoes polymers for additional information. 54
36
chain scission when exposed to radiation. Coupled with
this scission process is dissolution behavior that leads to
minimal swelling resulting in extremely high-resolution
imaging capability. The ability to delineate submicron im-
ages in PMMA is, however, offset by its low sensitivity
and relatively poor dry-etching resistance. Electron-beam
2
and deep-UV exposure doses in excess of 50 µC/cm at FIGURE 6 Structural representation of poly(1-butene-sulfone).