Logo PTB
Production sequence of Si-spheres and interferometrical determination of the sphere volume

Displacement Interferometry

Working Group 5.22

Electron Optical Metrology System (EOMS)



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.