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Polymers, Photoresponsive 739

TABLE III Resist Thickness Required to Achieve carbon platforms do not provide sufficient transparency,
an Optical Density of 0.4 for Three Classes of Resist fluorinated analogs do show potential, as do siloxane-
Matrix Polymers 23
based materials. Etching resistance and adhesion of the
Thickness to achieve fluorinated materials to silicon substrates are concerns that
Materials platform OD = 0.40 will need to be addressed. Interestingly, the Kunz study
demonstrated that the absorbance of many of the standard
Acrylic 46–87 nm
photoacid generator materials that have been used for both
Phenolic 48 nm
248- and 193-nm chemically amplified resists have either
Cycloolefin 77 nm
similar or even lower absorbance at 157 nm than at the
longer wavelengths, 130 thus alleviating concerns regard-
ing the PAG component. An aspect of materials design
imaging of 200 to ∼400 nm, respectively. 131 Thus, av-
that will be increasingly important at 157 nm is that of
enues leading to decreased absorbance need to be identi-
resist outgassing, or rather, the level of volatile species
fied and explored for this application. This said, commer-
evolving from the resist film during exposure. These may
cially available phenolic-, acrylate-, and cycloolefin-based
include residual solvent, volatile resist components, or
resins have been evaluated as “tool testing” resists where
by-products generated upon irradiation of the resist film.
“thinness” is not an obstacle. However, even in this appli-
Clearly, the evolution of volatile species must be kept to a
cation there is a need to suppress unwanted photochemical
minimum at 157 nm so that these products do not deposit
processes such as crosslinking or outgassing. The former
onto critical lens surfaces, deleteriously affecting the lens
leads to undesirable negative-tone behavior in a positive
transmission characteristics.
resist, while the latter can lead to outgassing which can
Several groups are beginning to explore materials al-
deposit on the objective element of the exposure tool and 133
ternatives for 157 nm applications. Willson has uti-
lead to tool downtime. Such design considerations are cru-
lized a “modular approach” in which chemical approaches
cial for all 157-nm systems. Of particular concern is out-
to instilling necessary functionality into the 157-nm ma-
gassing of silicon-containing volatiles which can lead to
terial is first tested in a model system. In this manner,
irreversible damage and cannot be cleaned by irradiation
he and his colleagues have identified the hexafluoroiso-
in the presence of oxygen as is the case for carbon-based propyl group as an effective aqueous-base-solubilizing
lens deposits. 132
moiety and have shown that it can be protected with
A list of representative polymeric alternatives along
acid-labile alkyl acetal protecting groups while main-
with their 157-nm absorbance characteristics was reported taining 157-nm transparency. Building from the 193-nm
byKunzetal. 130 andispresentedinTableIV.Whilehydro-
materials research that demonstrated the effectiveness of
alicyclic backbone polymers for providing etching resis-
TABLE IV Survey of 157-nm Absorbance Characteristics of tance, Willson 133 also showed that substitution of nor-
Selected Polymeric Platforms
bornene with an electron-withdrawing group such as flu-
Film thickness orine or even a carbonyl group may afford resins with
Absorbance (in nm for sufficient 157-nm transparency. For instance, the par-
Polymer (µm −1 ) an OD = 0.4)
tially fluorinated poly(norbornene) shown in Fig. 25a
Si-O Backbone has an absorbance of only 1.7 AU/micron. Although
Poly(hydrosilsequioxane) 0.06 6667 this absorbance is still too high for practical appli-
Poly(dimethylsiloxane) 1.61 248 cations, it demonstrates substantial improvement over
Poly(phenylsiloxane) 2.68 149 nonfluorinated analogs which can have absorbances as
Carbon Backbone high as 7 AU/micron, and it represents a promising
Fluorocarbon, 0.70 571 starting point for the design of new materials. Ober
100% fluorinated et al. 134 have reported two design approaches to achiev-
Hydrofluorocarbon, 1.34 298 ing 157-nm transparency. One system is based upon a
30% fluorinated poly(trifluoromethylvinyl alcohol-co-vinyl alcohol) resin
Partially esterified 2.60 154 protected with acid-labile THP protecting groups, while
hydrofluorocarbon,
in another approach they investigated the introduction
28% fluorinated
of hexafluoropropyl groups onto cyclized polyisoprene
Poly(vinyl alcohol) 4.16 96
(Fig. 25b). The material described to date does not have
Ethyl cellulose 5.03 80
good transparency at 157 nm, but it does exhibit a high T g
Poly(methyl methacrylate) 5.69 70
◦
(120–170 C) and good etching resistance. It is anticipated
Poly(norbornene) 6.10 66
that hydrogenation of the olefinic moiety will address the