PROCEEDINGS ARTICLE | March 27, 2014
Proc. SPIE. 9051, Advances in Patterning Materials and Processes XXXI
KEYWORDS: Optical lithography, Deep ultraviolet, Polymers, Glasses, Molecules, Diffusion, Control systems, Polymerization, Extreme ultraviolet, Line edge roughness
Negative tone resists based on cross-linking via epoxide/cationic polymerization have a variety of potential advantages
over more traditional positive tone resists based on photoacid catalyzed deprotection including low outgassing, intrinsic
diffusion control, and improved pattern collapse performance through the higher modulus provided by a cross-linked
network. Based on the promising baseline performance achieved previously in simple negative tone systems composed
only of an epoxide functionalized molecular glass and a photoacid generator, a series of different methods and additives
that can be used to control the extent and rate of cross-linking in such systems have been developed and are reported here
which allow for even further improvement in resist performance. Simple addition of base quencher, as is used in
conventional chemically amplified resists, is ineffective in these systems because the patterning reaction mechanism is
different. Any control method must work by modifying the extent and rate of cationic polymerization of epoxides. By
adding molecules containing phenolic OH groups to such an epoxide resist, one can slow the extent of cross-linking due
to introduction of an additional reaction pathway and often a concomitant increase in the resist resin glass transition
temperature. Generalized additives similar to base quencher were also developed based on the addition of strong
nucleophiles such as triphenylphosphine which act essentially as chain termination agents. This approach allows for
improved resolution and LER in negative tone epoxide resist systems. A more superior additive was developed that can
be described as a photodecomposable nucleophile (PDN). The unexposed PDN acts similarly to the strong nucleophile
additives in that it terminates chain propagation. Upon exposure, the PDN can act like a chain transfer agent or an
additional initiator, but no longer has the effect of completely terminating chain propagation. This approach allows for
high levels of control in the nominally unexposed regions of the resist, but maintains high efficiency of cross-linking in
the most highly exposed regions. One particular implementation of a PDN used in this study is the blending of a PAG
(i.e. triphenylsulfonium triflate, TPS-Tf) with a more nucleophilic anion that plays the role of a PDN, with the common
and highly effective, non-nucleophilic PAG that is conventionally used in epoxide photopolymerizations (i.e.
triphenylsulfonium hexafluoroantimonate, TPS-SbF6). Addition of only a few percent of TPS-Tf to a baseline epoxide
resist formulation shows a 5-10 nm improvement in ultimate resolution and a reduction in LER to around 65% as
compared to the baseline resist without the PDN additive while only incurring a moderate increase in imaging dose. By
modulating the amount of the different polymerization control additives, the performance of a particular epoxide resist
was improved from a resolution of greater than 30 nm half pitch and an LER of around 9 nm to a resolution of ~20 nm
half pitch, with an LER of around 4 nm, and a sensitivity of 18 mJ/cm2. By increasing the additive loading even further,
the resolution was improved to ~18 nm half pitch, although with an increase in imaging dose to 39 mJ/cm2.
