Algorithm Enables CD-SEM Shape Metrology
Alexander E. Braun, Senior Editor -- Semiconductor International, 7/1/2003
Edge position measurement accuracy
determines CD measurement uncertainty. Presently, the ITRS calls for precision close to 1 nm and <10% bias of the feature size. Optical scatterometry has proven effective for linewidth measurement because of its repeatability and capability of measuring sidewall shape. Now this capability is available for the CD-SEM performing in-circuit measurements.
Since incomplete use is made of SEM information, International SEMATECH (Austin, Texas) funded research work at NIST (Gaithersburg, Md.) to develop an analogous and complementary SEM technique that, like scatterometry, measures wall angles as well as feature widths. The algorithm finds a match between the measured signal from an unknown sample and a library of signals developed for known samples. This differs from scatterometry, in that the signal is an image instead of a scattering pattern, and the probe particles are electrons instead of photons.
Working with NIST, Schlumberger Verification Systems (Concord, Mass.) has developed what it terms Critical Shape Metrology, a software application for its Yosemite Ultra Low Voltage CD-SEM, which measures features based on a model library, using a proprietary code validated against NIST's Monte-Carlo simulation engine. The algorithm enables the measurement of features such as linewidth, line edge roughness, and sidewall angle — all of which have been characterized. Other capabilities will be developed for measuring resist height, top rounding, resist footing, and other factors. Because this is an algorithm, no hardware calibrations are required. Different setups of the various angles required for tilt measurements, etc., which add to the process engineer's work, become unnecessary.
Although there are several different ways to make 3-D profile measurements, the new software offers advantages: Capturing additional images is not required because it uses the standard top-down CD measurement image; the 3-D information is obtained from the same measurement used by the SEM to determine the CD. The algorithm quantifies the data like scatterometry would, because the underlying structure is understood by the modeling. There is no throughput penalty since no additional information is collected. The sample dose is the same as with any CD measurement.
CSM is aimed at <100 nm process nodes. It significantly improves standard CD measurement performance. This is because, instead of using a point-to-point algorithm, the complete feature is employed for the correlation, so there are considerably more data points and an improvement in signal-to-noise ratios. An entire edge is correlated as opposed to looking at just one point on an edge from which to measure. This provides as much as a threefold precision improvement. On a system measuring 1.0 nm (3σ), for example, this can go to 0.3 nm. Although the algorithm requires time to execute, throughput is unaffected because it is executed concurrently with the system measurements.
The algorithm matches the measurement feature's intensity profile with a library of profiles determined for the actual geometries to arrive at a CD result whose value corresponds directly to the physical structure. Thus, it is possible to measure a CD at a specific height along the feature, as required by SEMI P19-92. This method has provided an ancillary benefit of improved precision.
The official introduction will take place at SEMICON West this month.
For additional information on inspection, measurement and test, go to www.semiconductor.net/imt.
