Scatterometry was applied in a 90 nm logic process to monitor etched polysilicon gate
profiles and establish correlations of inline dimensional measurements to end-of-line
electrical test data. Scatterometry data were acquired on test wafers patterned on full-loop
production routes, with etched polysilicon profiles intentionally skewed across wide profile
ranges, bracketing the nominal 75 nm linewidth target. Scatterometry profiles were
benchmarked to cross-section SEM images, and optimal correlations were established across
wide process skews to both average top-down SEM linewidths and to end-of-line electrical
test data for electrical-CDs and overlap capacitance. Scatterometry measurements were made
with commercial Rotating-Compensator Spectroscopic Ellipsometers, with model inversions
on four independent spectral components of 0-th order diffracted signals from grating test
structures. Profile regression and analysis were based on both real-time parallel computations,
and on pre-computed databases. Analyses of linewidth error propagation, correlations, and
sensitivities were made using computed databases and measured spectral covariance matrices
for the four signal components. Calculations of measurement uncertainties for polysilicon
linewidths closely matched cross-tool measurements of 0.1 nm 1-σ site-level precision. At
wafer-level, bottom CD mean matching of < 0.1 nm was demonstrated between two
production metrology tools in our fab in short-term precision measurements.
KEYWORDS: Semiconducting wafers, Scatterometry, Finite element methods, Databases, Scanning electron microscopy, Reflectivity, Lithography, Data modeling, Critical dimension metrology, Process control
Spectroscopic Scatterometry was employed for definition of control metrics for lithography process tool optimization. Normal incidence unpolarized reflectance data was acquired in active device regions on resist gratings that defined the first gate level for the core array of a flash memory circuit. Calculations of the interactions of incident boradband light with the scattering structures were perforemd utilizing both a database of pre-computed spectra as well as a real-time method. Results from both inversion techniques were highly correlated. Scatterometry-based CDs were also highly correlated with dimensions determiend by CD-SEM and the modeled profiles clsoely matched cross section SEM data. Test wafer sets were patterned both at uniform exposure and as focus-exposure matrices, and measured to determine resist critical dimensions and thicknesses. Process variations were tracked across fields, across wafers and over time. Analytical models were applied to the profile data to determine rpocess windows and to define optimal scanner settings.
Scatterometry is gaining acceptance as a technique for critical dimension (CD) metrology that complements the more established scanning electron microscopy (SEM) techniques. Scatterometry determines the dimensions of the submicron structures by inverse diffraction calculations. SEM and scatterometry are complementary in many respects. Therefore, they are likely to coexist in the foreseeable future. Scatterometry and CD-SEM instruments can be integrated to take advantage of the complementary nature of the two techniques. To explore the joint use of scatterometry and SEM measurements, we measured a set of photoresist grating samples with CDs ranging from 240 nm through 40 nm by scatterometer (Sensys CD-i) and cross-section SEM (Hitachi S-4700). Although a cross-section SEM was used as an absolute standard for comparison of profiles and CDs, our conclusions range to include CD-SEM techniques. It was found that for measurements of profiles that were patterned with high uniformity within the measurement area, scatterometry was very effective, and correlated best with SEM measurements. However, in cases of substantial line-to-line profile variations, or for isolated or non-periodic lines, SEM is the more appropriate measurement method.
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