In this paper, we introduce a novel approach measuring the pattern depth, using non-destructive CD-SEM platform. We derived a dimensionless metrics called as “depth index,” that is designed to be proportional to the pattern depth. The depth index is calculated by using the SEM signal intensity and the pattern geometry accessible by normal CD-SEM. As a proof-of-concept, the depth index is obtained on the etched hole patterns fabricated in 300 mm wafer with different depth, and the depth correlation with reasonable measurement repeatability of 1% is confirmed. The depth index has been applied to the process variation monitor in NAND Flash memory, and the local depth variation of the holes of 4% is confirmed. The intra-wafer variation of 7-10%, and the wafer-to-wafer variation have been also detected.
KEYWORDS: Critical dimension metrology, Monte Carlo methods, Electron microscopes, Scanning electron microscopy, Cadmium, Edge detection, Silicon, Optical simulations, Metrology, Inspection
The influence of the e-beam aperture angle on the critical dimensions (CD)-scanning electron microscope measurements for a high aspect ratio (AR) structure is investigated. The Monte Carlo simulator JMONSEL is used for evaluating the measurement sensitivity to the variation in the bottom CD. The aperture angle of the primary electron greatly influences the measurement precision of the bottom CD in the high AR structure. Then, we applied an energy-angular selective detection technique to the Monte Carlo simulation results and found that the measurement sensitivity for the large aperture angle was improved. In addition, the experimental results are qualitatively consistent with the results of the Monte Carlo simulation. These results indicate that the energy-angular selective detection technique is effective for improving the measurement resolution of CD at trench bottom of a high AR structure and the technique is also useful for the overlay measurement during after-etch inspection.
A CD-SEM metrology for an EUV resist that was compatible with low shrinkage and high spatial resolution was investigated by using primary electrons (PEs) with high energy. The shrinkage and an image sharpness were evaluated for the EUV resist using PEs with energies of 200, 800, and 4000 eV. As a result, the smallest shrinkage and image sharpness were obtained under the condition of PEs with an energy of 4000 eV. We believe that a high-voltage CD-SEM is a potential candidate for CD metrology tools in the EUV lithography era.
The influence of e-beam aperture angle on CD-SEM measurements for a high aspect ratio (HAR) structure was investigated. The Monte-Carlo simulator JMONSEL was used to evaluate the measurement sensitivity to the variation in the bottom CD. The aperture angle of the primary electron greatly influences the measurement accuracy of the bottom CD in the HAR structure. Then, we utilized a technique for energy-angular selective detection to the Monte-Carlo simulation results and found that the measurement sensitivity for the large aperture angle was improved. In addition, the experimental results were qualitatively consistent with the results of the Monte-Carlo simulation. These results indicate that the detection is effective for the bottom CD measurement of a HAR structure.
Voltage contrast (VC) images obtained using an energy filter (EF) were used to measure the bottom surface of high-aspect-ratio structures. The VC images obtained using the conventional EF were sensitive to variations in wafer potential. Since CD-SEM metrology requires precise EF voltage control when using VC images, we developed an EF voltage correction method to be used at each measurement point. Consequently, bottom-edge measurement, independent of the wafer potential fluctuations, was achieved using the newly developed EF. Our developed technique is effective for CD-SEM metrology using VC images.
Voltage contrast (VC) images obtained using an energy filter (EF) were used to measure the bottom surface of high-aspect- ratio (HAR) structures. The VC images obtained using the conventional EF were sensitive to variations in wafer potential. Since CD-SEM metrology requires precise EF voltage control when using VC images, we developed an EF voltage correction method to be used at each measurement point. Consequently, bottom-edge measurement, independent of the wafer potential fluctuations, was achieved by using the newly developed EF. Our developed technique is effective for CD-SEM metrology using VC images.
Systematic understanding of the mechanism of secondary-electron (SE) emission is important to simulate an SEM image of a high-aspect-ratio (AR) structure. The simulation technique for a high-AR structure is useful for optimizing the observation conditions of SEM. Trench patterns with AR between 0.5 and 8 were fabricated on the same substrate, and dependence of SE yield on AR of the trench patterns was determined from SEM images for several landing energies of primary electrons. In addition, to understand the SE emission inside a trench, Monte-Carlo simulation of the signal intensity for Si was performed. The SEM observations and simulation results indicate that SEM image contrast at the bottom of a trench improves with decreasing landing energy (owing to a positive charging effect) and that reflection of SEs at the sidewall of a trench is essential for accurately estimating SE emission for the high-AR structure with AR over eight.
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