Photomask Dimensional Metrology

Measurement of linewidth or critical dimension (CD) continues to be one of the most fundamental dimensional metrology needs in the semiconductor and nanomanufacturing industries.  The ideal technique for photomask linewidth measurements is the transmission optical UV microscope because optical transmission imaging produces relatively simple high contrast images with a well-defined baseline. Also, transmission optical microscopy emulates the way the photomask is used during the wafer exposure process, thus enhancing the effectiveness of the standard.

To provide traceability, the photomask metrology project will develop custom reference measurement instruments which have been highly engineered to provide traceability through the incorporation of the most accurate laser interferometry. Standard Reference Material (SRM) standards will be certified with these reference measurement instruments to accurately calibrate production instrumentation

The NIST scanning UV microscope uses on-axis image sampling to reduce aberration and distortion effects. The position of the scanning stage is measured by a laser interferometer. NIST will develop accurate physics-based modeling to deduce the object dimensions from the microscope image. Image modeling also extends the limits of optical metrology to feature sizes well below the wavelength of light used, as demonstrated by Dianna Nyyssonen at NIST in the 1980s when critical dimensions were micrometer in size. The challenge is to measure state of the art chromium photomasks and phase shifting masks with sub-0.25 micrometer structures.

NIST will also work on a next generation photomask standard SRM. This standard will likely contain, in addition to isolated lines, spaces (from ~100nm wide, or smallest manufacturable mask features), and pitch patterns, a variable duty cycle set of line/space arrays from near-isolated to very dense, as well as large scale 2-dimensional features in response to requests from the machine vision industry. This project will also develop an essential suite of techniques to calibrate and optimize the optical system errors. This new area of research is critical to improving model to experiment agreement in model-based linewidth measurements.

• Presented nanoscale metrology course at BACUS past 2 years.
• Installed filtered downdraft air cleaning system for the UV Microscope to reduce photomask contamination problems.
• Developed image library approach to evaluating effects of parametric uncertainties on optical mask measurements
• Collaborated directly with representatives of the photomask industry and SEMATECH Mask Advisory Steering Council to determine their requirements in the next generation NIST photomask linewidth standard.
• Completed a bilateral intercomparison on photomask linewidth standards between NIST and Physicalishe Technicshe Bundesanstalt (PTB) to ensure agreement between the leading two international suppliers of photomask calibration standards.