Harmonic suppression and defect enhancement using Schlieren processing

The Schlieren technique is a well-known coherent processing method that is usually applied to the visualization of phase objects. In this paper, we demonstrate that, when the Schlieren processing is applied to a light wave modulated in amplitude and possessing some periodicity, the harmonic contents of the resultant image decreases (i.e., the higher harmonics are suppressed). Also, we show that, when the amplitude-modulated (periodic) light wave possesses faults, the Schlieren processing produces an enhancement of the faults relative to the periodic carrier. This technique can be applied to defect detection in periodic structures such as photomasks used for LCD panels, integrated-circuit masks, or semiconductor wafers.