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Polymers, Photoresponsive 731

terns. Depending on the precise process conditions that are
employed, the final flood exposure step may be unneces-
sary. Also, it is not always necessary to add base to the
resistpriortoexposure.Alternateimagereversalprocesses
have been developed involving treatment of exposed pho-
toresist with a gaseous amine in a vacuum environment. 70

IV. CHEMICALLY AMPLIFIED RESISTS

As device feature sizes approached 0.25 µm and the indus-
try moved towards using 248 as the exposing wavelength
for advanced lithographic applications, the materials com-
munity saw the first truly revolutionary change in resist
FIGURE 10 The chemistry and imaging characteristics of a
novolac–poly(methyl-2-pentene sulfone) electron-beam resist; materials chemistry to be adopted (Table I). Conventional
the SEM image depicts nominal 0.25-µm line/space patterns de- resists are fundamentally too absorbant to allow uniform
veloped in aqueous tetramethyl ammonium hydroxide. imaging through the thickness of the film. Additionally,
the available light at the exposure plane of commercial,
248-nm exposure tools is insufficient to provide for man-
E. Image Reversal Chemistry ufacturable processes when the quantum efficiency of a re-
71
sist is less than 1. This knowledge laid the foundation for
Knowledge that carboxylic acids undergo base-catalyzed
thebreakthroughthatultimatelyledtotheadoptionof248-
decarboxylation led a number of groups to explore the
nm lithography as the technology of choice for advanced
possibility of creating negative tone images in conven- device fabrication: the announcement of what has been
tional positive photoresist. 69 For instance, addition of 72,73
termed the “chemically amplified” resist mechanism.
small amounts of base additives such as monazoline,
imidazole, or triethanolamine to diazoquinone novolac
resists, followed by exposure, post-exposure baking, and A. Deprotection Chemistry
finally development in aqueous base generates high-
The pioneering work relating to the development of
quality negative-tone images. The chemistry and pro-
chemically amplified resists based on deprotection mech-
cesses associated with this system are shown in Fig. 11. 72
anisms was carried out by Ito et al. These initial
Effectively, thermally induced, base-catalyzed decarboxy-
studies dealt with the catalytic deprotection of poly(4-
lation of the indene carboxylic acid destroys the aqueous-
tert-butoxycarbonyloxystyrene)(PTBS)inwhichthether-
base solubility of the exposed resist. Subsequent flood
mally stable, acid-labile tert-butoxycarbonyl group is used
exposure renders the previously masked regions soluble
to mask the hydroxyl functionality of poly(vinylphenol).
in aqueous base, allowing generation of negative tone pat-
As shown in Fig. 12, irradiation of PTBS films containing
small amounts of an onium salt such as diphenyliodo-
nium hexafluoroantimonate with UV light liberates an
acid species that, upon subsequent baking, catalyzes
cleavage of the protecting group to generate poly(p-
hydroxystyrene). Loss of the tert-butoxycarbonyl group
results in a large polarity change in the exposed areas
of the film. While the substituted phenol polymer is a
nonpolar material soluble in nonpolar lipophilic solvents,
poly(vinylphenol) is soluble in polar organic solvents and
aqueous base. These resists have been used successfully
in the manufacture of integrated circuit devices. 74
Alternative resins have been investigated for chemically
amplified resist applications. The parent polymer is typ-
ically an aqueous-base-soluble, high T g resin. Examples
include poly(hydroxystryene), 74 poly(vinyl benzoate), 75
FIGURE 11 The chemistry and process sequence for a conven- 76
tional diazonaphthoquinone–novolac photoresist used in an “im- poly(methacrylic acid), N-blocked maleimide-styrene
age reversal” mode. resins, 77 poly(hydroxyphenyl methacrylate), 78 and