극 자외선 노광 공정 적용을 위한 Coherent Scattering Microscopy/In-situ Contamination System 구축 및 위상 반전 마스크 제작 연구

Abstract

Extreme ultraviolet lithography (EUVL) using 13.5 nm wavelength is one of the leading candidates for next-generation lithography technology for the 22 nm half-pitch node and beyond. EUVL mask yield and the development of defect inspection/review infra structure were selected as the main critical issues to be solved for transferring EUVL to the semiconductor manufacturing. The strong absorption of EUV light by all materials requires major modification of lithography system into reflective optics including the mask structure. Mask shadowing is a unique problem caused by use of mirror-based mask structure as well as oblique incident angle of light. In this study, we investigated the influence of the mask shadowing effect and suggested one of the best ways to minimize mask shadowing effect by applying the thin absorber stack using phase shift concept. Furthermore, optical design and system performance of the coherent scattering microscopy / in-situ accelerated contamination system (CSM/ICS) designed to analyze the actinic mask critical dimension (CD), characteristics of phase shift mask (PSM) and the impact of carbon contamination on the imaging performance were introduced. In chapter 2, the influence of mask shadowing effect on mask error enhancement factor (MEEF) was investigated. Imaging properties including horizontal-vertical (H-V) CD bias and MEEF change through the pitch according to absorber thickness and process condition were studied using aerial image simulation. The mask structure consisted of TaN absorber and 2 nm Ru capping layer on the 40 pairs of Mo/Si multilayer. As the absorber thickness increased and the pattern pitch decreased, both H-V CD bias and MEEF increased. At the illumination condition of 0.32 NA, the H-V CD bias variation through the pitch was negligible and slightly increased at 1:1 pitch, while it steeply increased at 1:1.2 and 1:1 pitch for NA of 0.25. The MEEF value was below 1.5 for all calculated absorber thicknesses when the pitch was from 1:1.2 to 1:5, whereas it was 3 with 64 nm thick TaN for 1:1 pitch at vertical pattern. With the increment of absorber thickness, the MEEF difference between the horizontal and vertical pattern increased. The H-V overlapping process window (PW) according to TaN thickness using 22 nm line and space (L/S) pattern was also calculated. As absorber thickness decreased, the overlapping zone in the exposure latitude (EL) of the focus-exposure plots between the horizontal and vertical features increased. In chapter 3, the optimization of PSM structure for EUVL was conducted to minimize the mask shadowing effect. The PSM structure consisted of a phase shift layer and a capping layer on the 40 pairs of Mo/Si multilayer. A phase shift layer was added below TaN absorber pattern. We analyzed the effect of the capping materials like Ru, Si and Mo with thickness variation. TaN absorber thickness for out-of phase condition shifted with the capping layer thickness change. The addition of phase shift layer showed a significant effect on the phase difference. Ru phase shift layer with a higher δ value than TaN absorber, shifted phase difference to the positive direction, whereas Si phase shift layer with a lower δ value than TaN absorber, shifted phase difference to the negative direction. However, phase difference was not shifted by the addition of Mo phase shift layer which has a similar δ value with TaN absorber. We also calculated the H-V overlapping PW and MEEF according to Mo phase shift layer thickness. As Mo phase shift layer thickness increased, the overlapping zone in the EL of the focus-exposure plots between the horizontal and vertical features increased and MEEF decreased. In addition, we manufactured proposed PSM structure, and evaluated the characteristics. In chapter 4, optical design and system performance observed during the first phase of the CSM/ICS integration were discussed. It was installed at 11B EUVL beam-line of the Pohang Accelerator Laboratory (PAL). The CSM/ICS is composed of the CSM for measuring imaging properties and the ICS for accelerating the carbon contamination. To improve the CSM measurement accuracy, optical and electrical noises of main chamber were minimized. The background noise level measured by CCD camera was ~ 8.5 counts (3 σ) when the EUV beam was off. The CSM, showing NA of 0.13, is expected to measure the actinic CD to the pattern size of the 12.5 nm L/S pattern and actinic CD measurement repeatability was < 1 Å (3 σ) at 17.5 nm L/S pattern. The CD and the reflectivity of the mask were compared before and after carbon contamination through acceleration exposure. The reflectivity degradation was 1.3 %, 2.1 % and 2.5 % after 1-, 2-, and 3-hour exposures, respectively, due to carbon contamination of 5, 10, and 20 nm as measured by using a Zygo interferometer. The change in the CD of the mask for a 22 nm L/S pattern was analyzed using the CSM and the CD SEM, and the result showed a similar trend, but a different absolute value. This difference confirmed the importance of the actinic inspection technique, which employs exactly the same imaging condition as the exposure tool. In conclusion, we suggested novel attenuated PSM structure for minimizing mask shadowing effect and successfully developed the CSM/ICS designed to analyze the actinic mask CD, characteristics of PSM and the impact of carbon contamination on the imaging performance.