The analysis and reduction of auto focus failure of advanced darkfiled inspection system

This work aims at the auto focus failure of dark field inspection tools used in 55/40 nm technologies. The diversification of progresses and products come along with the progress of semiconductor advanced; however, the auto focus (AF) system of traditional darkfield inspection riggings is insufficient to cover all process conditions, which could cause the interruption of the inspection process and additional dummy time. The AF system works with the inspection process in parallel, but the scan will be stopped once the wafer is too dark to reflect enough light to the AF system. The CIS (CMOS Image Sensor) product feature is unique, and which would lead to the low light signal and high failure rate. A series of experiments were carried out, and the high value of the incident light intensity can obviously reduce the fail rate (by wafer) from 40% to 10%. Furthermore, to switch the light sources into 780 nm wavelength, the problem can be fixed thoroughly, and the failure rate tended down to zero accordingly.