3D Imaging of isolated lines of negative e-beam resist

摘要

We have implemented traditional CD-SEM metrology complemented with the 3D imaging capability of the VERASEM 3D from Applied Materials. 3D imaging is performed by tilting the SEM beam to capture images at both ～2 and ～7 degrees. Reconstruction of these images allows for the determination of resist thickness and side-wall angle at the same point the critical dimension 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 negative resist at critical dimensions below 100nm. 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 and therefore minimization of the CD. The CD decreases by 0.7 nm per 1 μC/cm~2 of exposure. At a nominally high dose, where the isolated line critical dimension is 140 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 3 nm in height per every 1 nm decrease in CD down to 60 nm, below which the resist has been completely removed. This subtle but severe 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 thin resist films of 100nm. Additionally, we have characterized line width, CD, as a function of isolated line length from 200nm to 2.0m and resist thickness, from 0.24 to 0.57 μm. The top-down critical dimension is strongly correlated with the total isolated line length due to the e-beam proximity effect. We also report the trend that identical area doses produce larger CD's for thicker resist films with some subtle effects for the thinner films.