PTB uses a mask metrology system LMS 2020 (manufacturer: Leica) for coordinate measurement on photomasks, which was modified according to special PTB requirements.
In this instrument the photomask is mounted on top of an x-y-stage, which moves on air bearings and is driven by friction rods. The position of the stage is measured and controlled by means of reference plane mirror interferometry. See drawing! The coordinate microscope allows simultaneous detection of line edge positions in x- and y-direction. The high-NA microscope objective has to be constantly kept in focus, which is realized by an integrated laser autofocus system measuring the z-profile of the substrate surface.
To obtain highest measurement precision, careful adjustment of the comparator as well as application of sophisticated correction procedures of systematic error contributions is necessary. The mask comparator is mounted on a vibration isolating support and housed within a separate clean room cabin which offers temperature stabilities of a few 0,01 K. The wavelength of the laser light used for diplacement interferometry in air is continously measured by means of a tracking refractometer. Wavelength comparisons in reference to an iodine stabilized HeNe-laser can be performed regularly without removing the laser head from the mask comparator.
By all these measures and accompanied by different comparison measurements, traceable calibrations of microstructure coordinates on standard 6 inch masks can be performed with uncertainties of u95 = 35 nm. More details are given in table Calibration services.
The increasing integration density in microchip manufacturing requires the development of measurement instruments with higher resolution and higher precision. A scanning electron microscope (SEM) developed on behalf of PTB combines the high resolution of the electron beam probe with a large lateral measuring range of 300 mm x 300 mm. The laser-interferometer used for 2D coordinate measurements excels in a small measurement uncertainty. The optical path of the interferometer completely runs in a vacuum.
The figure shows the SEM based Electron-Optical Metrology System (EOMS) installed in PTB's cleanroom centre. The electron column used in this instrument is a low-voltage scanning electron microscope (LV-SEM) with an annular secondary electron detector that allows a symmetrical electron detection. The measurements performed up to now with EOMS at photomasks showed a very good reproducibility of coordinate measurement results (ca. 5 nm). Comparison with measurements performed with the optical mask measuring instrument LMS2020 resulted in a good agreement of coordinate measurements at photomasks (better than 20 nm). Low voltage SEMs are used to an increasing degree in structure width measurements at wafers and photomasks. Thus comparisons of structure width measurements with other measuring techniques are of great interest. These investigations are supported by modeling of image formation at structural edges at microstructures. The interaction of electrons and specimen is characterized by means of Monte Carlo simulations.