Correlation Between CD Bias and Fwhm Bandwidth on Immersion Exposure Systems

Immersion lithography has been developed with great speed. This technology moved to volume production of memory devices about 7 years ago. As immersion lithography developed, various and unexpected problems  occurred under 30nm devices. In the act of investigating these problems, it was discovered correlation between Critical Dimension (CD) bias and full-width-at-half-maximum (FWHM). The relationship between wavelength and focus are well known issue [Fig. 1]. Also deviation of wavelength was become a crucial factor on CD bias. These similar phenomena will be frequently happened despite meeting the specification. Therefore this correlation will be able to be a solution for the similar cases.

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.

References

[1] Armen Kroyan et al., “Effects of 95% Integral vs. FWHM Bandwidth Specifications on Lithographic Imaging” Proc. SPIE Vol. 4346(2001)

[2] Kevin Huggins et al., “Effects of laser bandwidth on OPE in a modern lithography tool.” Proc. of SPIE Vol. 6154(2006)

[3] R. C. Peng et al., “Effects of laser bandwidth on Iso-Dense Bias and Line End Shortening at sub-micron process nodes” Proc. of SPIE Vol. 6520, 65203S, (2007)