Lithography: Transition to Sensing

Unforeseen Lithography Challenge Forces Expansion of the Group's Capabilities (1993–2008)

Within a few months of installing the SVGL 193-nm step-and-scan system in the Microelectronics Laboratory in 1993, an unexpected problem arose: the image quality at the wafer plane had degraded considerably. Detailed analysis revealed that some of the optical surfaces were covered with a layer of contaminant, whose chemical composition or source was unknown. The Submicrometer Technology Group hypothesized that small amounts of volatile compounds present in the chamber were being photodissociated by the short-wavelength 193-nm radiation, leading to a soot-like deposit. Here was a new effect that could potentially be fatal to 193-nm lithography. It certainly had to be understood and fully resolved.

The team set up a separate test bed, specifically dedicated to studying and analyzing 193-nm photocontamination. The test bed included a state-of-the-art highly sensitive chemical analysis system, coupled to a photodissociation chamber into which trace amounts of various compounds could be introduced. It soon became apparent that controlling volatile impurities was not enough to avoid surface deposits. Perfectly nonvolatile materials in the chamber were being dissociated by minute amounts of scattered laser light, yielding volatile fragments that migrated into the laser beam and eventually decomposed and formed the observed deposits.

When the Submicrometer Technology Group first reported their findings, they realized that they had hit a raw nerve. Several semiconductor companies who had just started using 248-nm lithography in mass production were encountering serious optics contamination issues, but had kept the information as highly proprietary. It turned out that photoinduced outgassing and contamination were not unique to 193 nm after all. It took place also at 248 nm, and even at longer wavelengths, and it presented a major challenge in manufacturing.

Almost overnight, the test bed became a unique resource for lithographers everywhere. In response to requests from the semiconductor industry, the Submicrometer Technology Group analyzed the photoinduced outgassing at 248 and 193 nm of numerous types of photoresists, adhesives, ceramics, vacuum-sealing materials, and even the surface finish of metal tubing. The test bed provided invaluable information to the semiconductor industry and led to new industry-wide standards for lithography tool assembly and gas purging, and was the topic of the plenary presentation presented by the Submicrometer Technology Group at the prestigious SPIE Microlithography Conference in 2004. In the meantime, similar effects have plagued suppliers of ultraviolet lasers and sensing systems requiring their use, and again the group’s team was able to identify the cause of optics degradation and recommend alternative designs that improved operational performance.

Furthermore, the team used its accumulated knowledge, coupled with the test bed–related infrastructure, as a springboard to expand into new areas of research. During the period between 2003 and 2008, the group brought online a specialized chemical-vapor sensor test bed, conducted field measurements of trace explosive residues, and explored the use of ultraviolet lasers for remote detection of these compounds—all because of the degradation of the first 193-nm lithography system. It was shortly after this that the group's name was changed from the Submicrometer Technology Group to the Chemical, Microsystem, and Nanoscale Technologies Group to reflect the expanding breadth of its mission.


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