Photon Shot Noise Effect in Extreme Ultraviolet Lithography

Abstract

Extreme ultraviolet lithography (EUVL) which uses 13.5 nm wavelength has been widely accepted to be the predominant patterning process for 22 nm half pitch (hp) and below. Unlike conventional optics, extreme ultraviolet radiation around 13.5 nm is absorbed by most materials, and EUV lithography system requires the use of reflective optics owing to unavailability of transparent materials for EUV. In EUVL, photon shot noise effect is one of the leading cause of inferior imaging characteristics including deterioration of critical dimension uniformity (CDU) and more severe line width roughness (LWR). Photon shot noise effect can be described as quantum noise caused by the statistical fluctuations of photons as a consequence of discretization. Due to EUV's higher individual photon energy (93 eV) compared to that of ArF's (6 eV), fewer photons are required to react with photoresist for the same exposure energy. Two attenuated PSMs with 12% and 6% reflectivity that is known to increase image log slope (ILS) as well as maximizing the efficiency of the first-order diffracted photons has been used in order to ease the photon shot noise effect in EUVL. 12% attenuated phase shift mask (PSM) and 6% PSM consist of absorber layer and phase shift layer. For 12% PSM, 16.5 nm of tantalum nitride (TaN), which is known for high absorption at EUV wavelength, has been used as an absorber material. Below this TaN layer, 24 nm of molybdenum, which has relatively low extinction coefficient (k) but similar refractive index (n) compared to TaN, was deposited as a phase shift layer. Two layers were located on top of 2.5 nm-thick ruthenium capped Mo/Si multilayer. 6% PSM has 26.5 nm of TaN as an absorber and 14 nm of molybdenum as a phase shifter. The improved stochastic patterning properties of both PSMs were compared with those of conventional binary intensity mask with a 70 nm-thick TaN absorber for 14 nm and 18 nm line and space 1:1 dense pattern with 0.33 NA illumination conditions with a EUV generic resist model. The results showed that “informative” photons from the 1st diffraction orders are essential for mitigating the photon shot noise effect.| 13.5 nm 의 파장을 사용하는 극자외선 (Extreme Ultraviolet: EUV) 노광 기술은 22 nm 이하의 선폭의 반도체 양산 공정에 적용 될 가능성이 가장 높은 차세대 반도체 생산기술로 평가되고 있다. 이러한 극자외선 노광기술은 공기를 포함하는 모든 물질에서 흡수율이 높은 13.5 nm 파장의 빛을 사용하기 때문에 기존의 투과형 광학 시스템 대신 반사형 광학 시스템이 요구된다. 극자외선 리소그래피에서의 포톤 샷 노이즈 효과는, critical dimension uniformity (CDU)와 line width roughness (LWR) 등의 이미지 특성을 저하시키는 주요 원인 중 하나이다. 기존 ArF에 비해 극자외선은 14.3 배나 되는 에너지를 지니기 때문에, 더 적은 양의 광자가 포토 레지스트와의 반응을 요구하게 된다. 이러한 원인으로 CDU 저하와 line width roughness (LWR) 심화시킨다. 이러한 효과를 완화시키기 위하여 image log slope (ILS) 값을 증가시키며 1차광에 의한 광자 수를 늘리는데 효과적인 반사율을 12%와 6%를 갖는 위상 변위 마스크를 사용하여 실험을 진행하였다. 12%의 반사율을 갖는 위상 변위 마스크는 24 nm의 Mo 위상 변이 층 박막과 16.5 nm의 TaN 흡수체 박막으로 구성되어 있으며 이는 2 nm 두께의 Ru capping 박막이 증착된 40층의 Mo/Si 다층박막 거울 위에 증착된 구조이다. 6%의 반사율을 갖는 위상 변위 마스크는 14 nm의 두께의 Mo 층과 26.5 nm 두께의 TaN 흡수체 박막으로 구성되어 있다. 두 가지의 위상 변위 마스크를 사용하여, 70 nm 두께의 TaN 흡수체를 갖는 일반적인 마스크와 시뮬레이션을 통하여 이미지 특성에 대한 비교를 하였다. 14~18 nm 선폭의 1:1 line and space 패턴에 대하여 평가를 진행하였으며, ASML사의 High Volume Manufacturing (HVM) tool (NXE: 3300B)에 맞추어진 조명 조건을 각각 적용하여 평가하였다. 시뮬레이션 확인 결과 1차광의 의한 광자의 양이 증가하여 포톤샷 노이즈 효과를 완화시키는 효과를 확인할 수 있었다.; Extreme ultraviolet lithography (EUVL) which uses 13.5 nm wavelength has been widely accepted to be the predominant patterning process for 22 nm half pitch (hp) and below. Unlike conventional optics, extreme ultraviolet radiation around 13.5 nm is absorbed by most materials, and EUV lithography system requires the use of reflective optics owing to unavailability of transparent materials for EUV. In EUVL, photon shot noise effect is one of the leading cause of inferior imaging characteristics including deterioration of critical dimension uniformity (CDU) and more severe line width roughness (LWR). Photon shot noise effect can be described as quantum noise caused by the statistical fluctuations of photons as a consequence of discretization. Due to EUV's higher individual photon energy (93 eV) compared to that of ArF's (6 eV), fewer photons are required to react with photoresist for the same exposure energy. Two attenuated PSMs with 12% and 6% reflectivity that is known to increase image log slope (ILS) as well as maximizing the efficiency of the first-order diffracted photons has been used in order to ease the photon shot noise effect in EUVL. 12% attenuated phase shift mask (PSM) and 6% PSM consist of absorber layer and phase shift layer. For 12% PSM, 16.5 nm of tantalum nitride (TaN), which is known for high absorption at EUV wavelength, has been used as an absorber material. Below this TaN layer, 24 nm of molybdenum, which has relatively low extinction coefficient (k) but similar refractive index (n) compared to TaN, was deposited as a phase shift layer. Two layers were located on top of 2.5 nm-thick ruthenium capped Mo/Si multilayer. 6% PSM has 26.5 nm of TaN as an absorber and 14 nm of molybdenum as a phase shifter. The improved stochastic patterning properties of both PSMs were compared with those of conventional binary intensity mask with a 70 nm-thick TaN absorber for 14 nm and 18 nm line and space 1:1 dense pattern with 0.33 NA illumination conditions with a EUV generic resist model. The results showed that “informative” photons from the 1st diffraction orders are essential for mitigating the photon shot noise effect.