Materials Science and Engineering Laboratory FY 2004 ... - NIST

Technical Highlights
Profiling the Photoresist-Liquid Interface: Fundamentals
for Immersion Lithography and Polymer Dissolution
Recent advancements in the semiconductor
industry have resulted in new problems involving
the photoresist-liquid interface. For immersion
lithography, the water profile within a resist film
impacts pattern quality from changes in photoacid
generator diffusion or optical transparency.
For the development step, where a latent image
is realized into the final structure, an improved
understanding of photoresist swelling and
dissolution mechanisms is needed to address
stringent line-edge roughness requirements.
Data from neutron reflectivity measurements
provide critical insight needed to understand
and optimize next-generation photoresists and
process strategies.
Bryan D. Vogt and Vivek M. Prabhu
Introduction
Polymer thin film photoresists comprise the materials
foundation for the production of semiconductor
devices with nanoscale dimensions. The extension of
optical methods has been problematic due to challenges
arising from the implementation of shorter exposure
wavelengths. The past focus of the semiconductor
industry has been the development of sufficiently
transparent photoresist materials for future exposure
sources. However, future progress requires depth
profile information at the photoresist–liquid interface,
due to the emergence of immersion lithography and
the increased influence of the photoresist development
process on lithographic performance.
Using neutron reflectivity (NR), NIST successfully
quantified the profile of water and aqueous base
counterions in model photoresist films. NR provides
structural information regarding the composition
profile normal to the thin-film surface with isotopic
selectivity between protons and deuterium. Selective
deuteration of components in the system allows for
the quantification of water or counterion distribution
within photoresist films despite the negligible
differences in physical density. This data provide
critical insight needed to refine models for immersion
lithography and polymer dissolution.
Immersion Lithography: Water Profile
Recently, immersion lithography has emerged
as the key strategy to extend existing optical tools.
A liquid, such as water, is placed between the lens
and photoresist thin film to enhance resolution.
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The industry anticipates using immersion lithography
for production in 2007 at the 65 nm node. The role of
liquids in contact with photoresist films is important;
not only for component leaching and contamination,
but also due to the detrimental influence of trace levels
of water on the reaction and diffusion of photoacid
generators. Additionally, a non-uniform water profile
within thin films leads to incorrect assumptions
regarding the transmission and reflection at the
photoresist–anti-reflective coating interface.
Figure 1: Volume fraction of water distributed near the
PBOCSt/HMDS treated substrate during immersion, as
determined by neutron reflectivity. The excess of water at the
substrate reaches a maximum concentration of 17 % by volume.
Inset: Dependence of initial film thickness on the total film
swelling for two model photoresists in thin and ultrathin films.
Figure 1 shows the measured volume fraction
profile of water versus the distance from a
trimethylsilane-primed silicon oxide interface.
Far from the substrate, the 248 nm photoresist,
poly(4-tert-butoxycarbonlyoxystyrene) (PBOCSt),
shows bulk water absorption near 2.5 %. However,
significant deviations occur near the interface.
An excess of water, up to 17 %, extends 40 Å from
the substrate. This enriched interfacial water was
previously unknown and unaccounted for.
In contrast, a depletion of water was observed for
poly(4-hydroxystyrene) (PHOSt), the developer soluble
resist, even though the bulk of the film absorbs 25 %.
This depletion indicates the interface cannot accommodate
excess water. It appears that the relative hydrophobicity
between polymer and substrate controls the amount of
interfacial water.