Recently, transition metal (Hf, Zr) oxide and oxynitride thin films have attracted interest because of their unique properties, particularly their high dielectric constant, relatively large band gap, and stability. Given their high dielectric properties, these films have been investigated as a potential replacement of dielectric to reduce the tunneling current through the gate-oxide of a field-effect transistor (FET). In the lithographic process, they have also been considered as a substitute for the phase-shift mask, because of their optical properties. In deep ultraviolet (DUV) lithography, the phase-shift masks (PSM) of Cr compounds and MoSiON have a low refractive index of less than 2.5. As a result, the thickness of the PSM should be greater than 90 nm to satisfy the requirements of transmittance and phase shift (20 ± 5% transmittance and 1800 phase shift at the exposure wavelength (193 nm) and less than 40% transmittance at the inspection wavelength (248nm)). Because this amount of thickness causes an error of patterns in the lithography process, the materials with higher refractive index such as transition metal (Hf, Zr) oxide and oxynitride should be used to decrease the film thickness and the an error of patterns.
Transition metal (Hf, Zr) oxide thin films were investigated to study the applicability to high transmittance attenuated phase shifting mask (HT-Att-PSMs). The optical properties such as transmittance and phase of films are dependant on their optical constants (n: refractive index and k: extinction coefficient) and thickness. It is important to understand the electronic structures of transition metal (Hf, Zr) oxide thin films, because it is related to the dispersion of optical constants (n: refractive index and k: extinction coefficient). The electronic structures including the transition states were calculated using first-principles calculations and the cluster models of (Hf4O18)-20 and (Zr4O17)-18. From the electronic structures, it was confirmed that transition metal (Hf, Zr) oxide thin films have the different bonds characteristics along the bond orientation. And by comparing the transition states with the XANES spectra for O K-edge, the different absorption mechanism for energies up to 10 eV above O K-edge was analyzed. The dispersion of optical constants for transition metal (Hf, Zr) oxide thin films was analyzed using the Cody-Lorentz (CL) model for spectroscopy ellipsometry (SE) analysis. Based on the optical properties (transmittance and optical constants), it was confirmed that hafnium oxide thin films had the applicability to HT-Att-PSMs if the transmittance at the inspection wavelength was decreased.
To reduce the transmittance at the inspection wavelength, hafnium oxynitride (Hf-O-N) thin films were suggested as the material for the phase shift mask. Before we researched the electronic structures and optical properties of Hf-O-N thin films, the local structures of hafnium oxynitride thin films with the different composition were analyzed using the extended x-ray absorption spectroscopy (EXAFS) and the structure optimization. Based on the local structures obtained with the EXAFS and the structure optimization, we suggested several local structures such as HfO2 (CN:7.0), Hf4O5N2 (CN: 6.25), Hf4O2N4 (CN: 5.5) and Hf4O2N4 (CN: 5.0) for hafnium oxynitride thin films with the different composition. Using the suggested local structures, we analyzed the effects of composition on the electronic structures including the transition states, and on the optical absorption of Hf-O-N thin films. The dispersion of optical constants for Hf-O-N thin films was analyzed using a four-phase model of ambient/graded/film/ substrate for SE analysis. Based on the SE analysis, it was confirmed that Hf-O-N thin films showed the different optical properties according to their compositions, and that single Hf-O-N thin film could not be used as HT-Att-PSM for ArF laser lithography.
For the solution, the bi-layer structures of Hf-O-N thin films were fabricated. In the bi-layer structures, Hf-O-N thin films with the low N/Hf ratios (the upper layers) were deposited on these with the high N/Hf ratios (the lower layers). The upper layers were used as the phase shifter because of their high refractive, and the lower layers were used as the absorber at the inspection wavelength (193 nm). Using the bi-layer structures, the ranges of the deposition conditions satisfying the transmittance requirements and the phase shift were analyzed.
높은 밴드갭, 안정성, 높은 유전 상수를 갖는 transition metal (Hf, Zr) oxide and oxynitride 박막이 field-effect transistor (FET)의 게이트 산화물, deep ultraviolet (DUV) lithography의 위상 변위 마스크 등의 분야에 연구 및 적용되고 있다. 특히, deep ultraviolet (DUV) lithography에서 transition metal (Hf, Zr) oxide and oxynitride 박막의 높은 굴절률로 인하여 90 nm 이하의 두께를 갖는 위상 변위 마스크를 개발 할 수 있을 것으로 생각된다.
먼저 Transition metal (Hf, Zr) oxide 박막의 전자 상태와 광학 특성을 연구하였다. 전자 상태 계산에 있어서는 (Hf4O18)-20 과 (Zr4O17)-18 의 모델을 이용하여 기저 상태와 천이 상태를 분석 하였다. 그 결과, transition metal (Hf, Zr) oxide 박막은 결합의 방향에 따라 다른 결합 특성을 가짐을 확이하였다. 그리고 전자 상태를 바탕으로transition metal (Hf, Zr) oxide 박막의 광학 특성을 SE 분석의 CL model을 이용하여 분석하였고, 나아가 위상 변위 마스크의 적용가능성을 분석하였다.
다음으로 Hf-O-N 박막을 위상 변위 마스크 물질로 선정하고 전자 상태와 광학 특성을 연구 하였다. 먼저, EXAFS 분석과 구조 최적화 법을 이용하여 전자 상태 계산에 필요한 Hf-O-N 박막의 local structure (HfO2 (CN:7.0), Hf4O5N2 (CN: 6.25), Hf4O2N4 (CN: 5.5) and Hf4O2N4 (CN: 5.0) )를 제안 하였다. 그리고 전자 상태 계산에서는 조성 변화에 따른 기저 상태와 천이 상태를 분석하고 Hf-O-N 박막의 광흡수 기구를 분석하였다. 분석된 광흡수 기구를 이용하여 Hf-O-N 박막의 SE 분석에 필요한 four-phase model을 제안하였다. 그리고 광학 분석 결과를 이용하여 위상 변위 마스크의 이용가능성을 분석하였다.
마지막으로 Hf-O-N 박막의 bi-layer 구조를 제안하였다. N/Hf 비율이 낮은 Hf-O-N 박막을 upper layer로 이용하였고, 이때 upper layer가 phase shifter 역할을 함을 확인하였다. 그리고 N/Hf 비율이 높은 Hf-O-N 박막을 lower layer로 이용하였고, 이때 lower layer가 absorber 역할을 함을 확인하였다. 다양한 조함의 bi-layer 구조를 이용하여 위상 변위 마스크의 요구 조건을 만족시키는 증착 조건을 확립하였다.
