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Practical cage‐effect model for crosslinking in a negative chemically amplified resist and its use in comparing e‐beam and optical exposure

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3 Author(s)
Tam, N.N. ; Electronics Research Laboratory, Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, California 94720 ; Ferguson, R.A. ; Neureuther, A.R.

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A closed‐form, accurate, and easy to fit model for crosslinking of chemically amplified acid‐hardened resist (Shipley SNR‐248) is presented and used to compare effects of e‐beam and optical exposure. In this model, the saturation of the acid‐catalyzed crosslinking reaction during the postexposure bake is assumed to be caused by the ‘‘cage‐effect’’ mechanism–restriction of segmental diffusion of the polymer chains as the reaction progress. [D. Seligson, S. Das, H. Gaw, and P. Pianetta, J. Vac. Sci. Technol. B 6, 2303 (1988)]. To model this cage effect, the rate coefficient in the rate equation is assumed to be a linearly decreasing function of the extent of the crosslinking with the asymptotic saturation level of the crosslinking as a parameter. According to this cage‐effect model, the order of the acid catalyst in the crosslinking reaction of e‐beam exposed SNR‐248 resists m is 1.37, which is similar to that of the deep‐ultraviolet (DUV) exposed resists (1.42). However, in e‐beam exposed resists, some crosslinking is induced by the electrons during exposure. As a result, this initial crosslinking might contribute to a slightly higher activation energy (0.866 versus 0.694 eV). The equilibrium conversions of the melamine crosslinking sites are also different between the two exposure types. The power of the acid concentration n in the expression for the equilibrium conversion for e‐beam exposed resists is ∼0.49 whereas in DUV exposed resists, n is 1.1. There is also a difference in the activation energies of the equilibrium constants which suggested the cage effect might have a stronger influence on the backward reaction in e‐beam exposed resists.  

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Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures  (Volume:10 ,  Issue: 6 )