Kentaro Kasa

Effective-exposure-dose monitoring technique in EUV lithography

EUV lithography is a promising candidate for 2x-nm-node device manufacturing. Management of effective dose is important to meet the stringent requirements for CD control. Test pattern for a lithography tool evaluation, the effective dose monitor (EDM), shows good performance in the dose monitoring for optical lithography, for example, KrF lithography. The EDM can measure an exposure dose with no influence on defocus, because the image of an EDM pattern is produced by the zero-th-order ray in diffraction only. When this technique is applied to EUV lithography, the mask shadowing effect should be taken into consideration. We calculated the shadowing effect as a function of field position and applied it to correction of the experimental dose variation. We estimated the dose variation in EUV exposure field to be 2.55 % when corrected by the shadowing effect. We showed that the EDM is useful for EUV lithography.

Improvement of CD-SEM mark position measurement accuracy

CD-SEM is now attracting attention as a tool that can accurately measure positional error of device patterns. However, the measurement accuracy can get worse due to pattern asymmetry as in the case of image based overlay (IBO) and diffraction based overlay (DBO). For IBO and DBO, a way of correcting the inaccuracy arising from measurement patterns was suggested. For CD-SEM, although a way of correcting CD bias was proposed, it has not been argued how to correct the inaccuracy arising from pattern asymmetry using CD-SEM. In this study we will propose how to quantify and correct the measurement inaccuracy affected by pattern asymmetry.

Effective exposure dose monitoring technique in extreme ultraviolet lithography

Extreme ultraviolet (EUV) lithography is a promising candidate for 2x-nm-node device manufacturing. Management of effective dose is important to meet the stringent requirements for critical dimension control. As a test pattern for a lithography tool evaluation, the effective dose monitor (EDM) demonstrates sound performance in dose monitoring for optical lithography, such as KrF lithography. The EDM can measure an exposure dose with no influence on defocus, because the image of an EDM pattern is produced by the zeroth-order ray in diffraction only. When this technique is applied to EUV lithography, the mask shadowing effect should be taken into consideration. We calculated the shadowing effect as a function of field position and applied it to correction of the experimental dose variation. We estimated the dose variation in EUV exposure field to be 2.55% when corrected by the shadowing effect. We showed that the EDM is useful for EUV lithography.

Overlay sampling optimization by operating characteristic curves empirically estimated

Operating Characteristic (OC) curves, which are probabilities of lot acceptance as a function of fraction defective p, are powerful tools for visualizing risks of lot acceptance errors. The authors have used OC curves for the overlay sampling optimization, and found that there are some differences in probability of acceptance between theoretical calculation and empirical estimation. In this paper, we derive a theoretical formulation of the probability of acceptance for several simple cases by decomposing overlay errors, and show that the origin of the differences is the use of stratified sampling in overlay inspection.

Skewness and Kurtosis Risks of Quality Control in Overlay Inspection

The population of overlay errors in layer-to-layer structures in semiconductor manufacturing often obeys non-normal distribution functions such as elliptic, rectangular, and skewed distributions. Thus, there can be many wrong diagnoses of quality control that can lead to consumers risk and producers risk. In a lot acceptance test, a product should be judged by variables rather than by attributes because of the small sample size. A new acceptance test by variables was introduced for a non-normal population. It included sample skewness and kurtosis as well as mean and standard deviations. Using operating characteristic (OC) curves, it was shown that sampling inspection by variables can be applied safely to a strong non-normal population with new variables.

Focus Monitor Using Asymmetric Diffraction Rays by Off-Axis Monopole Illumination

Some methods of focus monitoring for lithography tools use nontelecentric imaging delivered from phase shift masks and require double exposure. A newly proposed focus monitoring method requires neither a special mask nor double exposure. A mask pattern of 1:1 duty ratio is illuminated by an off-axis monopole source. The positive first-order diffraction ray from the pattern is designed to advance in the direction of the optical axis. Then, the second-order diffraction ray is not generated. Only zeroth-order and the first-order diffraction rays contribute to imaging. At defocus, the pattern shifts sideways. On the other hand, a double-pitch pattern does not shift at defocus because rays are generated at symmetrical positions against the optical axis. The focal value is obtained by measuring their relative position. For an ArF immersion exposure tool of 1.3 numerical aperture (NA), pattern sizes of 86 nm half pitch and 43 nm half pitch were used. The focus position was measured from the relative position between the two patterns.