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Understanding Acid Diffusion in Chemically Amplified Photoresists Michael D. Stewart,
Gerard M. Schmid, Sean D. Burns, & Hoang Vi Tran Problem and
Motivation The current mainstay of the semiconductor industry is deepultraviolet (DUV) wavelength lithography using photoresists designed on theprinciple of chemical amplification. Chemicallyamplified (CA) photoresists have played a major role in semiconductor microlithographyover the last decade. All foreseeablefuture lithography technologies (e.g. EUV, SCAPLE) will likely also require CAresists. It is important to have aclear understanding of how these CA resists function so that both resist designand processing can be improved to meet the challenges of sub-100 nmlithography. The Willson Group has individual projects studying almostall aspects of CA resist processing from exposure to development, but the workdiscussed in this section is on the very important post exposure bake (PEB)step. In a chemically amplified photoresist, thesolubility-switching chemistry necessary for imaging is not caused directly bythe exposure; rather exposure generates a stable catalytic species thatpromotes solubility-switching chemical reactions during a subsequent bakingstep. The term “chemical amplification”arises from the fact that each photochemically generated catalyst molecule canpromote many solubility-switching reaction events. The apparent quantumefficiency of the switching reaction is the quantum efficiency of catalystgeneration multiplied by the average catalytic chain length. The original exposure dose is “amplified” bythe subsequent chain of chemical reaction events. The catalytic chain length for a catalyst can be very long (up toseveral hundred reaction events) giving dramatic exposure amplification. Chemicalamplification
is advantageous in that it can greatly improve resistsensitivity, but it
is not without potential drawbacks.
For instance as a catalyst molecule movesaround to the several
hundred reactions sites, nothing necessarily limits it tothe region that
was exposed to the imaging radiation.
There is a potential trade-off between resist sensitivity
andimaging fidelity. Figure
1 belowdemonstrates the problem. The
catalystmolecules are depicted as acids, which is the typical catalyst
type. Figure 1: The Acid Diffusion Problem
Formuch
of the history of CA resists this trade-off was of little concern as
thecatalyst diffusion distances were insignificant relative to the
printed featuresize, but as feature sizes have decreased, the diffusion
distances haveremained roughly the same and catalyst diffusion has
emerged as a significantconcern. Ourwork
is geared towards developing a fundamental understanding of acid
diffusionso that: 1)
Process conditions can be found tominimize diffusion 2)
New resist materials/formulations can bedeveloped to limit
intrinsic diffusion 3)
Lithography simulators can modelcatalyst migration as
realistically as possible Experimental Ourgroup
uses several methods to study diffusion in photoresists.
Equipment used includes infrared spectrometers,UV spectrometers,
scanning electron microscopes, and spectroscopicellipsometers amongst
other things. Wealso have access to the microlithography facilities of
SEMATECH located here inAustin.
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