July 14, 1997

The National Semiconductor Metrology Program at the Commerce Department's National Institute of Standards and Technology is a key enabler for the semiconductor development and manufacturing goals described in the Semiconductor Industry Association's National Technology Roadmap for Semiconductors. NSMP programs provide the semiconductor industry with the high-resolution, precise and increasingly sensitive measurement tools needed to create the tinier, more powerful computer chips of the future. NSMP's goal is to help the U.S. semiconductor industry keep its multibillion dollar competitive edge into the 21st century.

The following three examples from the Physics Laboratory were excerpted from Fact Sheet

Software Unifies ``Scattered'' Data for Chip Makers

The ability to accurately define optical scattering--how light is spread after hitting a surface--by a silicon wafer provides invaluable data for the successful manufacture of semiconductor chips. Unfortunately, optical scatter instruments lack standardization, making it difficult to compare values obtained by devices from different companies. A new NIST software package soon will offer chip makers a way to reconcile and compare data derived from different systems.

The software uses an algorithm that compensates for the unique characteristics of individual optical scatter instruments and yields readings free of any variance. The purified data can be obtained for silicon wafers exhibiting microroughness.

The American Society for Testing and Materials is currently incorporating this method into documents describing wafer inspection systems. The software is expected to become available by the end of August 1997.

Technical Contact:
Thomas A. Germer
(301) 975-2876

Polarized Signatures May Lead to Tinier Chips

Optical scattering is used by the semiconductor industry to measure the microroughness of a silicon wafer as well as detect particulate contaminants and subsurface defects. However, the ability of optical scattering instruments to do the job is limited by problems with sensitivity. Light scattering due to microroughness can obscure or overwhelm the scattering caused by particles.

NIST researchers have discovered a way to cleanly distinguish the scattering shown by microroughness from that of particulate contamination or subsurface defects. Using a novel technique called bidirectional ellipsometry, they have found that light scattered by microroughness has a characteristic, well-defined, polarization ``signature.'' They also learned that other scattering sources, such as particulate contaminants and subsurface defects, scatter light with unique polarizations different than those exhibited by microroughness.

With this knowledge, the researchers designed an instrument that is blind to the microroughness optical scattering pattern. Therefore, nanoscale particles on silicon wafers can be detected and measured without the former problem of interference. A provisional patent has been filed on the device.

What this means to the semiconductor industry is that particles with diameters less than 0.1 micrometer soon may be routinely discernible. The Semiconductor Industry Association roadmap declared that, in the future, such ability would be a breakthrough advance toward the manufacture of tinier integrated circuits.

In addition to its usefulness in process inspection of silicon wafers, bidirectional ellipsometry is expected to become a powerful technique for identifying and characterizing defects in optical components, disk storage materials and film coatings.

Technical Contact:
Thomas A. Germer
(301) 975-2876

Index of Refraction Advances for Photolithography

Measurement of the index of refraction--the property that determines how a lens of a particular material focuses light--is critical to efforts to develop photolithographic exposure tools for the manufacture of future-generation integrated circuits. NIST scientists have made measurements of the index of refraction of fused silica and calcium fluoride at wavelengths near 193 nanometers.

NIST, in collaboration with MIT Lincoln Laboratory and SEMATECH, seeks to develop the infrastructure required to utilize 193 nanometer laser emission for 0.18 micrometer integrated circuit feature sizes (for products such as gigabyte memory chips). These index-of-refraction results keep industry on track to meet the National Technology Roadmap for Semiconductors' target date of 2001 for commercial production of these chips.

The NIST researchers have made temperature- and wavelength-dependent index-of-refraction measurements on optical materials that are required for such photolithography. Engineers require such precise data to design accurately components for the photolithographic tools. To make the index of refraction measurement accurate to 10 parts in a million, the researchers upgraded a precision refractometer, which is capable of temperature control of 0.1 degrees Celsius. For the longer term and for shorter wavelengths, they are developing interferometric methods capable of higher accuracy.

Technical Contact:
Rajeev Gupta
(301) 975-2325

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