Overlay metrology using X-rays

1. A method for inspection, comprising:

directing a beam of X-rays to pass through a sample from a first side to a second, opposing side of the sample via an area of the sample containing first and second features formed respectively in first and second thin film layers, which are overlaid on a surface of the second side of the sample;

detecting a pattern of the X-rays that have been transmitted through the sample and have been diffracted from the first and second features;

analyzing the pattern in order to detect a deviation in an alignment of the first and second features; and

adjusting a process parameter used in producing the features responsively to the deviation.

2. The method according to claim 1 , wherein directing the beam of X-rays comprises collimating and monochromatizing the beam.

3. The method according to claim 1 , wherein detecting the pattern comprises using an array of X-ray detectors to detect the diffracted X-rays as a function of angle relative to the beam.

4. The method according to claim 1 , wherein the pattern comprises diffraction lobes having respective amplitudes, and wherein analyzing the pattern comprises determining that the first and second features are misaligned responsively to a variation in the respective amplitudes of the diffraction lobes in the detected pattern.

5. The method according to claim 1 , wherein detecting the pattern comprises detecting a two-dimensional diffraction pattern, and wherein analyzing the pattern comprises assessing the alignment with respect to two axes.

6. The method according to claim 1 , wherein the sample comprises a semiconductor wafer, and wherein the first and second features comprise alignment targets formed by a photolithographic process on the surface of the wafer.

7. Apparatus for inspection of a sample having first and second opposing sides and containing first and second features formed respectively in first and second thin film layers, which are overlaid on a surface of the second side of the sample, the apparatus comprising:

an X-ray source, which is configured to direct a beam of X-rays to pass through a sample from the first side to the second side of the sample via an area of the sample containing the first and second features;

a detector, which is arranged to detect a pattern of the X-rays that have been transmitted through the sample and have been diffracted from the first and second features; and

a signal processor, which is coupled to the detector so as to receive and analyze the pattern in order to assess an alignment of the first and second features.

8. The apparatus according to claim 7 , wherein the X-ray source comprises an X-ray tube and X-ray optics for collimating and monochromatizing the beam.

9. The apparatus according to claim 7 , wherein the detector comprises an array of X-ray detectors, which are arranged so as to detect the diffracted X-rays as a function of angle relative to the beam.

10. The apparatus according to claim 7 , wherein the pattern comprises diffraction lobes having respective amplitudes, and wherein the signal processor is adapted to determine that the first and second features are misaligned responsively to a variation in the respective amplitudes of the diffraction lobes in the detected pattern.

11. The apparatus according to claim 7 , wherein the pattern comprises a two-dimensional diffraction pattern, and wherein the signal processor is adapted to analyze the pattern so as to assess the alignment with respect to two axes.

12. The apparatus according to claim 7 , wherein the sample comprises a semiconductor wafer, and wherein the first and second features comprise alignment targets formed by a photolithographic process on the surface of the wafer.