In the Nantero NRAM process, a carbon nanotube film is patterned using conventional photolithography and etch
techniques. CD SEM metrology of the printed resist image is straightforward. However, challenges arise when SEM
inspecting an etched nanotube pattern. Under conventional SEM inspection, a nanotube pattern is nearly invisible. In
order to facilitate nanotube pattern characterization, metrology structures have been developed which use passive
voltage contrast to cause electron emission from the nanotube pattern and associated conducting structures. These
enable manual inspection of the nanotubes, along with automated pattern recognition and automated CD measurement.
The voltage contrast is achieved by connecting the nanotubes to a remote "charge-sink" outside the image field
consisting of a large rectangle of metal. The voltage contrast occurs with no extra electrical connection to the wafer,
and without special SEM components or beam adjustment. The metrology structures are used in two general ways: 1.
Nanotubes are clearly imaged, enabling inspection, CD measurement, coat-quality characterization, etc. 2. Indicator
structures in associated process layers light up when contacted by nanotubes, enabling measurements of line-end
shortening; etch bias; overlay; etc. Various structures have been developed: 1. CD cells for manual and automated CD
measurement. 2. Vernier structures for characterization of overlay, line-end shortening and etch-bias. 3. Serpentine
structures for characterization of nanotube coat quality using conduction length.
ArF resist is critical in the production of today's state-of-the-art lithography. It is well documented that process control measurements via CD-SEM at landing energies greater than 200 eV significantly slims the ArF resist, leading to inaccurate measurements and changes in the final geometries of the feature measured in-circuit. Resist slimming is most frequently quantified as the difference between consecutive measurements of the same feature. This study uses an alternative method to measure the slimming caused by a single measurement on a resist feature. Measurements were taken of etched features that had been exposed on a CD-SEM to various beam conditions prior to etch. The slimming was calculated by measuring the delta between the exposed portion of the line and the adjacent un-exposed portion of the same line. Previous work and the results of this current work show that the slimming of the ArF resist carries over through the etch process and measurably altered the final CD. In this work a systematic study of various image acquisition conditions shows that the choice of landing energy dominates all other factors affecting the amount of slimming, with near zero slimming measured for the 100 eV landing energy.
Critical Shape Metrology (CSM), a Monte-Carlo simulation-based technique that extracts feature shape information from top-down CD-SEM images, is applied to study advanced process steps of etched polysilicon layers. True bottom CDs and sidewall angles are among the parameters obtained during real-time wafer inspection. Comparison is made to FIB cross-sections obtained independently from select test sites.
In this study, surface conditioning solutions were used during post-develop process to enhance the 193 nm lithography performance. These solutions were applied to the wafer surface in a surface treatment step between the DI water rinse and drying steps. Compared to the standard develop process, the formulated surface conditioning solution enabled a 24% reduction in line width roughness, particularly in the high frequency roughness components. The solution also improved the pattern collapse performance by enlarging the non-collapse window and extending the minimum CD feature size by 10 nm. Additional benefits provided by the formulated surface conditioner solution were minimal changes to CD and resist profile.
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