EUV Lithography용 Thin Half-Tone PSM의 특성 향상을 위한 연구

Abstract

Extreme ultraviolet (EUV) lithography using 13.5 nm wavelength is the most promising patterning technology to be adopted for 22 nm half pitch (hp) and below. Since EUV light is strongly absorbed by most materials, reflective optics should be applied. Mask shadowing effect is an unique phenomenon caused by using mirror-based mask with oblique incident angle of light. This results in a horizontal-vertical (H-V) bias, an ellipticity in the contact hole pattern and eventually critical dimension (CD) non-uniformity. Reducing the absorber thickness is the most effective method to minimize the mask shadowing effect, but can deteriorate mask image contrast. A phase shift concept has been suggested as a potential solution to improve the image contrast with a thinner absorber stack. In this paper, we present an optimized thin half-tone phase shift mask (HT-PSM) stack having an ability to control reflectivity while maintaining total absorber stack thickness as thin as 40.5 nm which satisfies the out-of-phase condition in the case of tantalum nitride (TaN) as an absorber. By applying molybdenum (Mo) which has a similar delta (δ) but an attenuated beta (β) value compared with TaN absorber, the reflectivity can be increased by adjusting Mo thickness. However, the optimum absorber stack structure is quite different depending on pattern widths, pitches, and illumination conditions. The proposed PSM for 22 nm hp line and space pattern, for example, consists of 16.5 nm TaN absorber layer and 24 nm thick Mo phase shifter on 2 nm ruthenum (Ru) capped 40 pairs of Mo/Si multilayer. This Mo phase shifter results in 3.65 nm and 1.84 nm reduction in H-V CD bias compared to conventional binary intensity mask (BIM) with 70 nm thick TaN under 0.25 NA and 0.32 NA illumination conditions, respectively. And 10% improved image contrast was obtained with this attenuated PSM (13% reflectivity on the absorber stack) under 0.25 NA illumination condition through aerial image simulation. Finally, we manufactured the PSM and its basic optical properties were investigated.