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We have implemented traditional CD-SEM metrology complimented with the 3D imaging capability of the VERASEM 3D CD-SEM from Applied Materials. 3D imaging is performed by tilting the SEM beam to capture images at two unique angles. Reconstruction of these images allows for the determination of resist thickness and sidewall angle at the same point the critical dimension, CD, is measured. These three output parameters provide the user with automated multi-metric lithographic process control. We have used these techniques to characterize e-beam lithography of isolated lines in ~0.6μm of negative resist at CDs between ~50 and ~100 nm. The flexibility of our e-beam lithography system allows us to expose an array of identical features with 30 distinct dose values over a small area of a wafer. We have characterized the resist CD and thickness as a function of small incremental decreases in dose. As the dose decreases so does the CD of the isolated resist line at a rate of ~1 nm per 1μC/cm2 of area exposure. At a nominally high dose where the isolated line CD is ~100 nm the resist is measured by 3D imaging to be close to full thickness. The main observation is that the resist thickness erodes at a rate of ~5 nm in height per every 1nm decrease in CD down to the resolution limit of 50-60 nm. As the dose is further lowered the resist is then completely washed away. This subtle but significant loss in resist etch mask integrity could not have been observed by traditional top-down CD-SEM metrology alone. This also demonstrates the tilt capability of the VERASEM 3D to measure very thin resist films of ~100 nm. Additionally, we have successfully used this methodology to characterize this effect as a function of isolated line length from ~0.5-2.0μm, and resist thickness from ~0.25-0.6 μm. The CD is strongly correlated with the total isolated line length due to the e-beam proximity effect, while the resist erosion rate remains fairly constant. The resist erosion rate is also similar for the resist films regardless of initial thickness. However, we also confirm the trend that identical area doses produce larger CDs for thicker resist films with some subtle effects for the thinner films.
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