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734 Polymers, Photoresponsive

melamine and benzyl alcohol derivatives as crosslink- Recent approaches for addressing this fundamental de-
ing agents, formulated with onium salt photoacid gener- sign challenge include: (1) careful tailoring of polymer
ators in novolac or poly(hydroxystyrene) binders, have properties to maximize lithographic performance with
shown 0.2-µm line space resolution. 96 Other additives minimalsacrificeinetchingperformance,and(2)develop-
that undergo acid-induced condensation include com- ment of three-component systems in which high-carbon-
pounds such as diphenylsilanediol, 99 polysiloxanes, 100 content alicyclic additives not only serve as dissolution
and diphenylcarbinol. 101 inhibitors but also enhance the resistance of the matrix as
a whole to plasma environments.
3. Electrophilic Aromatic Substitution
Mechanisms
A. Alicyclic Polymers for 193-nm Imaging
Photo-induced crosslinking can be achieved in styrene
As noted above, the first materials platforms that were ex-
polymers that are susceptible to electrophilic aromatic
amined for the purpose of providing transparent, 193-nm,
substitution by addition of latent electrophile (i.e., a car-
bocation precursor) and a photoacid generator. 102 The imagable matrices were based on methacrylate resin
chemistry (Fig. 16). While methacrylate-based resist plat-
photogenerated acid reacts with the latent electrophile
forms are attractive from an economic perspective, they
during a post-exposure bake step to generate a reactive
suffer from the fundamental drawback of possessing a lin-
carbocation that then reacts with an aromatic moiety in
ear, oxygen-rich scaffold whose poor plasma-etching sta-
the matrix affording a crosslinked network. The latent
bility can be offset only partially by functionalization with
electrophile may be either an additive or a monomer that
more stable pendant groups. In a more ideal resist plat-
is copolymerized into the polymer binder. Examples of la-
form, greater intrinsic plasma stability might be imparted
tent electrophiles include dibenzylacetate and copolymers
of acetyloxymethylstyrene 102 and 1,3-dioxlane-blocked through incorporation of alicyclic, etching-resistant moi-
benzaldehyde. 103 eties directly into the polymer backbone. In addition, min-
imizing oxygen content by designing oxygenated func-
tionalities to play only necessary imaging, adhesion, and
solubilizing roles would be beneficial. Alternative routes
V. POLYMERS FOR SUB-150-NM
IMAGING APPLICATIONS to achieve the goal of an “all alicyclic backbone” are
shown in Fig. 17 for the alicyclic alkene, norbornene.
These routes are ring-opening metathesis polymerization
The initial focus for designing 193-nm resists centered 109 110
(ROMP), metal-catalyzed vinyl polymerization, and
on derivatized acrylate and methacrylate copolymers. By 111
radical-promoted vinyl polymerization. Over the years,
and large, these polymers are effectively transparent at
several groups have taken such all-alicyclic approaches
193 nm and exhibit excellent resolution, but lack plasma- 112
towards 193-nm resins.
etching resistance and other requisite materials properties
for lithographic performance. For instance, the first exam-
ple of a high-speed, 193-nm acrylate resist was demon-
strated in a collaboration between MIT and IBM. This
material consisted of a methacrylate terpolymer contain-
ing acidolytically labile t-butylmethacrylate repeat units
formulated with a diphenyliodonium triflate onium salt
PAG. 104 Thus, the fundamental design challenge that has
emerged appears to be the necessary trade-off between
plasma-etching resistance and requisite materials prop-
erties for lithographic performance. On the whole, high-
carbon-content copolymers functionalized with pendant
alicyclic moieties possess adequate etching resistance but
tend to be brittle, display poor adhesion, and have sub-
optimal imaging characteristics due to poor aqueous base
solubility. Decreased alicyclic carbon content results in
improved lithographic performance at the cost of etch-
ing resistance. Examples of substituents that have been FIGURE 16 Examples of alicyclic esters of acrylate resins and
employed include menthyl, 105 adamantyl, 106 isobornyl, 107 selected dissolution inhibitors that have been examined in 193-nm
and tricyclodecyl. 108 lithographic applications.