Abstract

For real-time nondestructive evaluation of periodic structures (e.g., masks), it is preferable to suppress the intended periodic features to enhance defect visualization (e.g., scratches and digs). Previous demonstrations¹ employed a phase-conjugate mirror (PCM) placed at the spatial Fourier transform plane of an optical processor. The spatially transformed defect features, which are broadband and typically of low intensity, can be enhanced in the presence of the transformed intense periodic structure(s) by exploiting pump depletion of the PCM reflectivity for the latter features. The dynamic range and response time of the PCM are typically constrained by the intensity ratios of the defect and periodic features. Our approach relaxes these constraints by using a spatial light modulator (SLM) at the transform plane, followed by a PCM.¹ The SLM performs the function of intensity discrimination by suppressing the intense periodic features of the mask, while allowing the low-intensity defect features to be passed onto the PCM. The PCM ensures system auto-alignment and compensation for propagation errors, resulting in reverse transit of the desired information back through the transform lens for subsequent mask inspection. Our system is capable of large dynamic range and can operate at video frame rates.

© 1991 Optical Society of America