Calibration of linewidths on photomasks with light-optical procedures
PTB uses for optical linewidth measurements a modified Zeiss microscope, see Figure. The illumination wavelength is in the UV wavelength range at 365 nm. The device is installed at the PTB Clean Room Centre.
The measurement system enables the calibration of linewidths in the range of 0.3 µm to 200 µm. The reproducibility of the measurement values lies in the range of a few nanometers. On high quality test samples expanded uncertainties of U = 20 nm (k = 2) are achieved.
The microscope uses the object scanning procedure. Here, the light coming from the sample is detected behind a scanning slit, while the sample is moved perpendicular to the structure edges. The sample displacement is measured with a laser interferometer and an additional capacitive displacement sensor. In this way a direct link to the length unit “meter” is realized.
The structure linewidth is determined by a comparison of measured and modeled signals. The relevant parameters of the optical microscope as well as information about the object enter into these model calculations.