OVERLAY MEASUREMENT SYSTEM USING LOCK-IN AMPLIFIER TECHNIQUE

A detection system (200) includes an illumination system (210), a first optical system (232), a phase modulator (220), a lock-in detector (255), and a function generator (230). The illumination system is configured to transmit an illumination beam (218) along an illumination path. The first optical system is configured to transmit the illumination beam toward a diffraction target (204) on a substrate (202). The first optical system is further configured to transmit a signal beam including diffraction order sub-beams (222, 224, 226) that are diffracted by the diffraction target. The phase modulator is configured to modulate the illumination beam or the signal beam based on a reference signal. The lock-in detector is configured to collect the signal beam and to measure a characteristic of the diffraction target based on the signal beam and the reference signal. The function generator is configured to generate the reference signal for the phase modulator and the lock-in detector.