Understanding Bossung curve asymmetry and focus shift effect in EUV lithography

Pei-yang Yan

doi:10.1117/12.458302

11 March 2002 Understanding Bossung curve asymmetry and focus shift effect in EUV lithography

Pei-yang Yan

Proceedings Volume 4562, 21st Annual BACUS Symposium on Photomask Technology; (2002) https://doi.org/10.1117/12.458302
Event: Photomask 2001, 2001, Monterey, CA, United States

Abstract

As minimum semiconductor critical dimension feature sizes become smaller and smaller, many adverse effects in lithography printing due to mask topography start to stand out. These effects existed in the optical lithography in the past many generations. Usually, these effects contributed to a negligible depth of focus (DOF) consumption, and therefore, were neglected. In the case of Extreme Ultraviolet Lithography (EUVL), due to the drastic reduction in wavelength, some of these adverse effects that related to the wavelength tend to magnify in the case of EUVL as compared to the deep ultraviolet (DUV) lithography. For example, the asymmetry of Bossung curves and focus shift for semi-isolated to isolated lines is found to be much less in DUV optical lithography. However, in the case of EUVL, a maximum 40nm focus offset between the dense and the isolated lines exists. This focus shift effect in EUVL has been previously discussed by C. Krautschik, et al1 and explained via the complex phase behavior for the EUVL mask that is fundamentally different from the thin mask. In this paper, we will present a detailed and simpler explanation for the cause of the Bossung curve asymmetry and focus shift effect. Our simulation study indicated that the asymmetry and focus shift effect is simply due to a phase error at the mask pattern edge. This phase error is primarily caused by the interference of the reflected light from the reflective substrate with the diffracted light at the absorber edge or boundary and with the partially transmitted light from the absorber edge. This effect is not unique for the reflective masks; it exists in transmission masks as well. The analogy of this effect to the conventional thin transmission mask can be simply made and will also be discussed in detail.

Citation Download Citation

Pei-yang Yan "Understanding Bossung curve asymmetry and focus shift effect in EUV lithography", Proc. SPIE 4562, 21st Annual BACUS Symposium on Photomask Technology, (11 March 2002); https://doi.org/10.1117/12.458302

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