Inspection of a substrate using multiple cameras

An apparatus for inspection, comprising:

• an imaging assembly, comprising a plurality of cameras, which are mounted in different, respective locations in the imaging assembly and are configured to capture respective images of a sample;
• a motion assembly, which is configured to move at least one of the imaging assembly and the sample so as to cause the imaging assembly to scan the sample with a scan accuracy that is limited by a predetermined mechanical position tolerance;
• and
• an image processor, which is coupled to receive and process the respective images captured by the plurality of cameras, and which locates a defect in the sample, relative to a point of reference on the sample, with a position accuracy that is finer than the mechanical position tolerance,
• wherein each of the respective images contains a respective area of the sample, having a region of overlap with one or more neighboring images captured by the plurality of cameras in the imaging assembly, and
• wherein the image processor is configured:
• to recover a virtual position clock responsively to a periodic pattern on the sample that appears in the respective images captured by at least one of the plurality of cameras and to relative movement between the sample and the imaging assembly that is applied by the motion assembly, and
• to use the virtual position clock and one or more features appearing in the region of overlap in registering the respective images with one another.

A method for inspection, comprising:

• scanning a sample using an imaging assembly, comprising a plurality of cameras, which are mounted in different, respective locations in the imaging assembly, with a scan accuracy that is limited by a predetermined mechanical position tolerance;
• capturing respective images of the sample using the plurality of cameras while scanning the sample;
• and
• processing the respective images captured by the plurality of cameras, the processing comprising recovering a virtual position clock responsively to a periodic pattern on the sample that appears in the respective images captured by at least one of the plurality of cameras and to relative movement between the sample and the imaging assembly while scanning the sample, using the virtual position clock in registering the respective images with one another;
• and
• locating a position of a defect in the sample, relative to a point of reference on the sample, with respect to one or more features on the sample, thereby locating the defect with a position accuracy that is finer than the position tolerance.

An apparatus for inspection, comprising:

• an imaging assembly, comprising a plurality of cameras, wherein each of the plurality of cameras is fixedly mounted in a different, respective location in the imaging assembly and is configured to capture a respective image of a sample during a scan;
• a motion assembly, which is configured to move at least one of the imaging assembly and the sample so as to cause the imaging assembly to scan the sample with a scan accuracy that is limited by a predetermined mechanical position tolerance;
• and
• an image processor, which is coupled to receive and process the respective images captured by the plurality of cameras during the scan, which estimates relative positions among the respective images by computing an estimated shift and an angular skew with respect to each of the plurality of cameras by recovering a virtual position clock responsively to a periodic pattern on the sample that appears in the respective images and to relative movement between the sample and the imaging assembly that is applied by the motion assembly;
• which uses the virtual position clock in registering the respective images with one another;
• which sums the relative positions to obtain a global position estimate;
• and which locates a defect in the sample, relative to a point of reference on the sample, with a position accuracy that is finer than the mechanical position tolerance.