Closed-loop control of X-ray knife edge

Apparatus for X-ray scatterometry, comprising:

• an X-ray source, which is configured to generate and direct an X-ray beam to be incident at a grazing angle on an area of a surface of a sample;
• an X-ray detector, which is configured to measure X-rays scattered from the area in response to the incident X-ray beam;
• a knife edge, which is arranged parallel to the surface of the sample in a location adjacent to the area on which the X-ray beam is incident so as to define a gap between the surface and the knife edge and to block a portion of the X-ray beam that does not pass through the gap;
• a motor, which is configured to move the knife edge in a direction perpendicular to the surface of the sample so as to control a size of the gap;
• an optical rangefinder, which is configured to receive optical radiation reflected from the surface of the sample and to output a signal indicative of a distance of the knife edge from the surface of the sample responsively to the received optical radiation;
• and
• control circuitry, which is configured to drive the motor responsively to the signal output by the optical rangefinder in order to regulate the size of the gap.