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We have previously demonstrated our method to obtain unbias dense line’s LER (Line Edge Roughness) processed by EUV (Extreme Ultra-Violet) lithography. No special edge-detection, simulation modeling, beam-scan, nor image processing is required, except optimization of beam-dose parameters. For instance, threshold method that is a conventional edge-detection is used on SEM (Scanning Electron Microscopy) image taken by a conventional beam-scanning with a conventionally used beam-conditions. However, especially for EUV-lithographed features we must determine carefully landing voltage and beam-dose that cause artificial reduction of LER and LWR (Line Width Roughness) due to shrink of photo resist materials.
Note that we made edge-detection interval at every 5 nm, that is as small as probe-beam diameter. For instance, we set the vertical magnification, the pixel-density, and the sum-line-per-point parameter at 52.7k, 1024, and 2, respectively. Although we can make the interval as small as sub-nanometer, it is not reliable to measure the sub-nanometer object that is only one-tenth of the probe-beam diameter.
Then, we verified accuracy of our unbiased LWR by using two independent experimental methods: TEM and FIB-SEM. We obtained spectra of PSD (Power Spectrum Density) from the TEM, the FIB-SEM and our unbiassing CDSEM-method. We found the three PSDs agreed very well to each others. This result strongly implies that three independent methods measured an identical PSD, of true LWR. By our unbiassing CDSEM-method we can measure the true LWR which is calculated from the true PSD.
In this work, by using the similar experimental-methods we verify accuracy of our wiggling measurement. Wiggling is a hot issue in production yield of post dry-etch process. Especially about 20 nm or less in the line-width, wiggling starts appearing due to decreased elastic-stiffness of the line feature.
A reason why we study the accuracy of LER, LWR, and wiggling is inline monitor of stochastic failures. As they increase significantly in EUV lithography process, production yield may not be promising. Bisshop et al. [1] investigated pitch and/or space-width correlate to incidence of stochastic-failures, such as breaking-line and bridging-line, down to ppm (parts per million). However, the incidence of the stochastic failures should be reduced less to ppb (parts per billion) or ppt (parts per trillion) which is as small as tolerable particle contamination on a wafer.
This implies significance of accurate LWR and/or wiggling monitor because in narrow line or space a-few-nanometer-LWR narrows locally them the more and causes the more stochastic failures.
In this work, in order to control ppb-stochastic failures we figure out a correlation between LWR and the failures’ incidence. We need to measure billions of features, that requires too long measurement time for practical process-monitor. To shorten the measurement time we test an effective method to estimate such very few number of failures.
Reference
[1] Stochastic effects in EUV lithography: random, local CD variability, and printing failures, Peter De Bisschop, J. of Micro/Nanolithography, MEMS, and MOEMS, 16(4), 041013 (2017).
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