An electron optical metrology system for pattern placement measurements

The demands on the uncertainties of pattern placement and overlay measurements in lithography are constantly growing due to the continuing reduction of feature size which allows higher density integration. Although nowadays optical projection lithography plays the dominant role in lithography and will continue to do so in the near future other technologies will be developed further in order to replace optical lithography soon after the beginning of the next century. A pattern placement metrology instrument should be able to perform measurements on the masks used today as well as on future types of lithography masks. The PTB has developed a new pattern placement metrology instrument using a high-resolution electron beam probe. The electron optical metrology system consists of a low-voltage scanning electron microscope with a large vacuum chamber and a 300 mm x-y-positioning stage, controlled by vacuum laser interferometry. The design principles of the instrument will be described and a comparison of measurements with those of an optical mask measuring system will be presented.