Method and apparatus for determining line centers in a microminiature element.

摘要

Method and an apparatus for determining line centers in a microminiature element such as on a semiconductor wafer or a mask having linewidths and other feature spacings below the one micron range down to 0.25 micron. The invention uses a laser beam illumination arrangement which generates a laser beam which is directed on the object. The light reflected by the object is measured. Two focussed laser scanner spots are directed on the object, both being illuminated successively with a distance below the classic resolving power. The reflected light and light which is outcoupled before directing the laser beam on the object are conducted to detectors which generate an analog measurement and reference signal. High frequency noise components produced by the laser and the scanning are eliminated in order to provide an appropriate measurement signal. This is accomplished by a tuned amplifier. Subsequently the maximum and minimum values of the filtered measurement signal and the reference signal are digitized. The measurement signal and the reference signal are synchronously processed to generate a correlated measurement signal with eliminated low frequency noise. With the digitized values of the correlated measurement signal two sine shaped curves can be generated. The sine shaped curves represent the envelope of the AC component signal and are both correlated to another. They are formed by maximum/minimum and minimum/maximum values, respectively, lying on the respective curve of an envelope. The following evaluation of the digital values of the sine shaped curves is accomplished by computing the intersection point of the two sine shaped curves. With the method according to this invention it is possible to determine the center of lines in the submicron range with a small error and in real time.