The National Bureau of Standards (NBS) has had a continuing program to develop optical linewidth standards for the integrated circuit industry for over 10 years. The past work has concentrated on the development and the certification of photomask linewidth and pitch standards. The recent work is directed at extending the feature sizes on these standards to cover the range from 0.5 to 30 μm, and at doubling the certification accuracy to 0.025 μm. Features with heights larger than approximately 1/4 wavelength of light cannot be modeled as zero-thickness layers as is done for photomasks. The development of models to handle this thick-layer case and to develop practical edge-detection criteria are currently under development at NBS. However, at the present time, it is generally not possible to interpret the image profiles of thick features and thereby measure an accurate linewidth. The basic obstacles that must be overcome to achieve accurate submicrometer feature size measurements for these features will be reviewed and the prospects for future NBS optical standards for features such as photoresist lines on silicon wafers will be assessed. Some suggestions about what to do until these standards become available will be given.
Robert D. Larrabee,
"Submicrometer Optical Linewidth Metrology", Proc. SPIE 0775, Integrated Circuit Metrology, Inspection, & Process Control, (17 April 1987); doi: 10.1117/12.940410; https://doi.org/10.1117/12.940410