Ring test aberration determination and device lithography correlation

The exposure tool is a critical enabler to continue improving the packing density and transistor speed in the semiconductor industry. In addition to increasing resolution (improving packing density), a scanner is expected to provide tight linewidth control across the chip, ACLV (transistor speed). An important component of ACLV is lens aberrations. Recently techniques that allow the measurement in-situ of aberrations using Zernike coefficients have become available. We have measured the first 25 Zernike coefficients in two ASML PAS 500/700D DUV Step & Scan systems. The measured Zernikes are in agreement with PMI (Phase Measurement Interferometry) data collected at the lens manufacturer within 3.8 nm or less. We find good agreement between the variation of the Z 5 (first order astigmatism) coefficient and the optimum focus offset between horizontal and vertical lines measured using FOCAL. There is also good agreement between Z 5 and the linewidth difference between 160 nm horizontal and vertical lines with a 330 nm pitch. The lines were printed using an NA equals 0.68, (sigma) equals 0.70 on 3,800 angstrom of resist on top of an inorganic BARC. We find good correlation between the Z 7 coefficient (first order coma) and linewidth variation across the slit. We also found that the effect of the aberrations as measured by linewidth range is a function of pitch. Linewidth range decreases as the duty ratio increases, reaching a minimum at a duty ratio of 1:1.44, and then increases again as the lines become isolated. This is surprising because these intermediate pitches also have the smallest focus-exposure window. We conclude that knowing the Zernike coefficients provides us with a very powerful tool to characterize our exposure tools. However to fully realize the benefit of this new tool we must improve the accuracy of our simulation tools.