Critical dimension measurements on phase-shift masks using an optical pattern placement metrology tool

A useful extension of the optical mask pattern placement metrology is the measurement of critical dimensions (CD), exploiting the outstanding mechanical resolution and stability of a corresponding mask metrology machine. In particular the CID measurement on phase-shift masks (PSMs) poses a challenge on the optical measurement method. The paper presents measurements and the corresponding computational modeling of the setup with respect to illumination beam path (reflection, transmission), PSM properties and measurement optics for a dedicated edge detection method. Variables have been the focus variation of the edge position and the critical dimension of the pattern. Based on the modeling outcome the alignment and the illumination have been improved and verification measurements have been performed on various machines of the type Vistec LMS IPRO3. The paper presents the measurements, the modeling and the comparison to the practical measurement results for original and improved setup, showing the achievement of the envisaged 2-nm repeatability.