Correlation Between CD Bias and Fwhm Bandwidth on Immersion Exposure Systems
CD bias issue was occurred in 2x nm node production. This phenomenon was expected an aberration problem, presented equation like below. For improving CD bias, chromatic distortion (aberration) had to be minimized by reducing (λ-λo). (λ-λo) is a kind of bandwidth. Also it is called a FWHM.
Correlation between aberration and laser wavelength
δx(x) = (λ-λo) x dSx/dλ + (λ-λo) x3 dE/dλ
where, [δx(x) : chromatic distortion]
[(λ-λo) : laser wavelength deviation]
[dSx/dλ, dE/dλ : distortion coefficient]
[x : slit position]
The test of narrowing FWHM is done successfully from 0.3pm to 0.25 pm [Fig. 2]. As a result it is obtained better aberration values (coma Z7_0 0.7 nm à 0.4 nm). and CD bias issue was under controlled by E95 (FWHM). Although It is impossible for revealing the exact information about CD in this paper, this problem still occurred similar phenomena. CD bias issue was not easy to solve, However equation and simulations [Fig. 3, Fig. 4] could be explained clearly. Wavelength deviation caused best focus shift, which led to CD bias issue.
 Armen Kroyan et al., “Effects of 95% Integral vs. FWHM Bandwidth Specifications on Lithographic Imaging” Proc. SPIE Vol. 4346(2001)
 Kevin Huggins et al., “Effects of laser bandwidth on OPE in a modern lithography tool.” Proc. of SPIE Vol. 6154(2006)