More Photons Are Good

By Michael P.C. Watts

Scatterometry is the favored approach to monitor complex 3D nano-structures in production. At SPIE Advanced Lithography, KLA introduced a new generation of their scatterometry metrology system. The new system expands the number of different measurements; know internally as multi-multi-multi.

Scatterometry relies on measuring the diffraction patterns from a test diffraction grating that mimics the device structure. The measured diffraction is compared to a library of simulated diffraction patterns from gratings with different dimensions. The best match to the measured diffraction gives the dimensions of the test gratings. In order to probe different shapes, the KLA tool measures the diffraction at multiple wavelengths, multiple angles in 2 axes, and multiple polarizations. At my request they provided a schematic to illustrate the dimensions that can be measured on a typical device using these new degrees of freedom.

Schematic of measurement capability, provided by KLA

To enable all these measurements, quicker and using a smaller gratings area, KLA use a new lights source with 30x increased brightness. The classic broadband light source in these systems is a Xenon arc lamp. The source in this new system is a Xenon arc where the Xenon is heated by a laser. The net result is a much brighter light bulb. Given that measurement limits, are always determined by the signal to noise ratio, a brighter light source (More Photons) increases the signal and enables all sorts of improvements (Are Good).

In related news, KLA also announced an improved cluster defect inspection tool that can detect sub micron particles on the front and backside of wafers. In this case they use a scanned LED RGB light source for both light and dark field illumination. Backside particles are a real problem these days, where a backside particle can bow the surface of a wafer out of the depth of field of the lens. The imprint guys are also very sensitive to backside particles.

“More photons are good” is a not-particularly–profound insight that came to me when working on positive x-ray resists. The sources were so weak that the resists had to disintegrate if a photon wandered in its general direction….. and be resistant to plasma etching ! I concluded that a brighter light source would really help. Eventually Grant Willson’s team at IBM solved the problem by inventing chemical amplification to make resists two orders of magnitude more sensitive. However in getting there Hiroshi Ito evaluated polyphthalaldehyde, which disintegrated so well (unzipped) that he put in in a SEM and promptly killed the electron source of the SEM due to contamination….another high point in resist chemist/SEM engineer relations.

About the Author

Mike Watts has been patterning since 1 um was the critical barrier, in other words for a longtime. I am a tall limey who is failing to develop a Texas accent here in Austin. I have a consulting shingle at www.impattern.com.

My blog “ImPattering” will focus on the latest developments in the business and technology of patterning. I am particularly interested in trying to identify how the latest commercial applications evolve.