Method and system for discriminating defects present on a frontside from defects present on a backside of a transparent substrate

An inspection system for discriminating defects present on a frontside of a substrate from defects present on a backside of the substrate, the substrate being made of a material transparent at an inspection wavelength, the system comprising:

• at least one optical source coupled to an optical system for emitting a first light beam and a second light beam, the first and second light beams having the inspection wavelength and being contained and intersecting in a reference plane;
• a support to receive the substrate and to position the substrate relative to the optical system such that the first and second light beams intersect at a measurement spot on the frontside of the substrate and also illuminate the backside of the substrate at two respective illumination spots separated by a distance, the support and the optical system being movable relative to each other;
• a controller for controlling the relative movement of the support and the optical system to scan the measurement spot along a measurement path on the frontside of the substrate, the controller being configured to control the relative movement such that the reference plane is kept tangential to the measurement path;
• a collector for collecting at least a portion of the light scattered by defects present on the frontside and/or on the backside of the substrate;
• a detector associated to the collector for receiving the collected light and for providing a signal representing a variation in intensity of the collected light as the measurement spot is scanned along the measurement path;
• and
• a processing device connected to the detector, the processing device being configured to process the signal to identify a first pattern corresponding to the light scattered by a defect on the backside of the substrate, the first pattern presenting two peaks separated from each other by a determined separation interval corresponding to a time necessary for the defect to be moved over the distance separating the two illumination spots.