Closed-loop control of X-ray knife edge

1. 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.

2. The apparatus according to claim 1 , wherein the X-ray detector is configured to measure an angular spectrum of the X-rays scattered from the area, and comprising a processor, which is configured to analyze the angular spectrum so as to assess a property of the sample.

3. The apparatus according to claim 1 , wherein the knife edge comprises a single crystal of a semiconductor or metal material.

4. The apparatus according to claim 1 , wherein the motor comprises a piezoelectric motor, which is configured to set the size of the gap with a resolution finer than 1 μm.

5. The apparatus according to claim 1 , wherein the optical rangefinder is connected to move with the knife edge under control of the motor.

6. The apparatus according to claim 5 , wherein the optical rangefinder comprises a laser, which is configured to direct an optical beam to impinge on the surface of the sample in proximity to the knife edge, and a detector, which is configured to sense the optical beam that is reflected from the surface.

7. The apparatus according to claim 6 , wherein the detector comprises a position-sensitive detector, which is arranged so that a position of the reflected optical beam on the detector varies with movement of the knife edge.

8. The apparatus according to claim 1 , wherein the control circuitry is configured to drive the motor to set the size of the gap to a target height responsively to an intensity of the scattered X-rays measured by the X-ray detector, which varies with the size of the gap, and thereafter to maintain the size of the gap at the target height responsively to the signal output by the optical rangefinder.

9. A method for X-ray scatterometry, comprising:

directing an X-ray beam to be incident at a grazing angle on an area of a surface of a sample;

measuring X-rays scattered from the area in response to the incident X-ray beam;

positioning a knife edge 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;

receiving in an optical rangefinder optical radiation reflected from the surface of the sample and outputting a signal from the optical rangefinder that is indicative of a distance of the knife edge from the surface of the sample responsively to the received optical radiation; and

moving the knife edge in a direction perpendicular to the surface of the sample responsively to the signal output by the optical rangefinder so as to control a size of the gap.

10. The method according to claim 9 , wherein measuring the X-rays comprises acquiring an angular spectrum of the X-rays scattered from the area, and analyzing the angular spectrum so as to assess a property of the sample.

11. The method according to claim 9 , wherein the knife edge comprises a single crystal of a semiconductor or metal material.

12. The method according to claim 9 , wherein moving the knife edge comprises setting the size of the gap with a resolution finer than 1 μm.

13. The method according to claim 9 , wherein the optical rangefinder is connected to move together with the knife edge under control of a motor.

14. The method according to claim 13 , wherein the optical rangefinder comprises a laser, which is configured to direct an optical beam to impinge on the surface of the sample in proximity to the knife edge, and a detector, which is configured to sense the optical beam that is reflected from the surface.

15. The method according to claim 14 , wherein the detector comprises a position-sensitive detector, which is arranged so that a position of the reflected optical beam on the detector varies with movement of the knife edge.

16. The method according to claim 9 , wherein positioning the knife edge comprises setting the size of the gap to a target height responsively to a measurement of an intensity of the scattered X-rays, which varies with the size of the gap, and wherein moving the knife edge comprises maintaining the size of the gap at the target height responsively to the signal output by the optical rangefinder.