극자외선 마스크 흡수체 측면 각도 측정과 그림자 효과
- 극자외선 마스크 흡수체 측면 각도 측정과 그림자 효과
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- Extreme Ultraviolet Lithography (EUVL) uses light of 13.5 nm wavelength, and it has been considered as a leading candidate for 22 nm node and beyond. Its implementation for researches of High-Volume Manufacturing was begun from Alpha Demo Tool to NXE:3100 at many companies. Wavelength of EUV 13.5 nm is easily absorbed to most materials. These are reasons that EUV light has been considered as Next Generation Lithography source and EUVL optical system changes basic platform into reflective components. EUVL must adopt its mask and optics system as not a projection type in conventional ArF lithography, but a reflective mirror type that shows relatively high reflectance response to EUV light of certain wavelength.
The one of the critical challenges in EUVL is the mask shadow effect caused by EUV light illumination with oblique incidence angle of 6° due to an asymmetric structure of EUVL scanner. The shadow effect, it can affect decisive effect on a resist critical dimension (CD) variation and shift on wafer. This shadow effect significantly depends on sidewall angle of absorber. A normalized image log slope (NILS) value of EUV mask is defined to evaluate EUV mask pattern contrast. NILS gets larger with higher image contrast of EUV mask pattern information. This value is decreased with sidewall angle variation due to the shadow effect of absorber.
In EUVL, the measurement of absorber sidewall angles has become more important than a conventional lithography method due to the shadow effect. Therefore, ITRS has required sidewall angle measurement tolerance <0.5°, and sidewall angle measurement must have a non-destructive method, high throughput, and an applicable method in mass production.
In this thesis, I used Field-Emission CD-SEM to measure sidewall angle. SEM has advantages of a non-destructive method, widely used equipment, and no degradation for long time measurement. Furthermore, SEM can measure various pattern shapes at any position of sample. Next, I calculated edge width by Matlab image processing to obtain sidewall angle. Very fast calculation is possible in various sidewall angles of EUV mask absorber. Then, experiment results were compared to TEM.
Also, I had simulation of the shadow effect estimation with various sidewall angles. I developed compensation methods to obtain acceptable NILS values with various sidewall angle using flexible illumination system in Sentaurus Lithography (Synopsys Inc.) simulator. In order to enhance NILS values, we had investigated the parameter changes of the conventional, annular, dipole, and quadrupole illumination depend on absorber sidewall angle. The NXE:3300B is expected to adopt Off-Axis illumination (OAI), so the study of the optimal flexible illumination shape and size are necessary to obtain high NILS values for enhanced resolution.
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