Method and apparatus for providing image data for constructing an image of a region of a target object
Embodiments of the present invention provide a method ( 200 ) of providing image data for constructing an image of a region of a target object, comprising detecting, by at least one detector ( 40 ), at least a portion of radiation scattered by a target object ( 30 ) with the incident radiation ( 10 ) or an aperture at a predetermined probe position, determining an offset vector ( 203 ) for reducing an error associated with the probe position ( 201 ), estimating a wavefront ( 210 ) based on a probe function having the offset vector applied to the probe position, and providing image data responsive to the detected radiation.
1. A method of providing image data for constructing an image of a region of a target object, comprising:
detecting, by at least one detector of a diffractive imaging device, at least a portion of radiation scattered by the target object with the incident radiation or an aperture at a predetermined probe position;
determining, by a processing unit, an offset vector for reducing an error associated with the probe position;
estimating, by the processing unit, a wavefront based on a probe function having the offset vector applied to the probe position; and
providing via an iterative process image data responsive to the detected radiation.
2. The method of claim 1 , comprising:
updating a current best offset vector associated with the probe position when a current best error metric associated with the probe position is improved.
3. The method of claim 2 , wherein the updating of the current best offset vector comprises:
determining an error metric for the offset vector based on a difference between the estimated wavefront and a wavefront corresponding to the radiation detected by the at least one detector; and
comparing the error metric for the offset vector with a current best error metric associated with the current best offset vector for the probe position.
4. The method of claim 1 , comprising:
determining an offset vector for each of a plurality of probe positions.
5. The method of claim 1 , wherein the offset vector is determined in an at least partly random manner.
6. The method of claim 1 , comprising:
determining a plurality of offset vectors;
estimating a plurality of wavefronts based on a probe function each having a respective one of the offset vectors applied to the probe position; and
selecting one of the plurality of offset vectors having a lowest associated error.
7. The method of claim 6 , wherein the plurality of offset vectors are determined during each iteration of the method.
8. The method of claim 1 , comprising:
determining one or more global correction factors for correcting one or more errors associated with a plurality of probe positions.
9. The method of claim 8 , wherein the global correction factors correct for one or more of scaling, drift and rotation.
10. The method of claim 1 , wherein the, or each, offset vector is determined in a trial-and-error manner.
11. The method of claim 1 , wherein the method is performed iteratively and the offset vector is determined during a plurality of iterations of the method.
12. The method of claim 11 , wherein the offset vector is first applied to the probe function during a second or greater iteration of the method.
13. The method of claim 2 , wherein the current best offset vector is an offset vector having an associated current best error metric according to one or more predetermined criteria.
14. The method of claim 13 , wherein the one or more predetermined criteria includes a lowest current best error metric.
15. The method of claim 11 , wherein the offset vector is determined at least in part according to a scaling factor such that a magnitude of the offset vector is reduced with increasing iterations of the method.
16. The method of claim 2 , wherein the offset vector is determined based on the current best offset vector.
17. The method of claim 2 , wherein the offset vector V is determined according to:
V=CBC +(random x ,random y )
wherein CBC is the current best offset vector for the probe position and randomx and randomy are randomly determined coordinates.
18. The method of claim 3 , wherein the error metric E is determined according to the equation:
E
=
∑
u
(
T
ψ
j
′
-
I
j
)
2
∑
u
I
j
wherein ψ′ j is an exit wave determined for a jth probe position, T is a transform for transforming the exit wave to a plane of the at least one detector and I j is a measured intensity at the measurement plane of the at least one detector.
19. An apparatus for providing image data for constructing an image of a region of a target object, comprising:
a data store storing data indicating an intensity of detected radiation scattered by the target object and detected by a detector of a diffractive imaging device with the incident radiation or an aperture at a predetermined probe position;
a processing means arranged to determine an offset vector for reducing an error associated with the probe position, to estimate a wavefront based on a probe function having the offset vector applied to the probe position, and to provide via an iterative process image data responsive to the detected radiation.
20. The apparatus of claim 19 , wherein the processing means is arranged to update a current best offset vector associated with the probe position when a current best error metric associated with the probe position is improved.
21. The apparatus of claim 20 , wherein the processing means is arranged to:
determine an error metric for the offset vector based on a difference between the estimated wavefront and a wavefront corresponding to the data indicating an intensity of radiation held in the data store, and
to compare the error metric for the offset vector with a current best error metric associated with the current best offset vector for the probe position.
22. The apparatus of claim 19 , wherein the processing means is arranged to determine an offset vector for each of a plurality of probe positions.
23. The apparatus of claim 19 , wherein the processing means is arranged to perform operations comprising:
determining a plurality of offset vectors;
estimating a plurality of wavefronts based on a probe function each having a respective one of the offset vectors applied to the probe position; and
selecting one of the plurality of offset vectors having a lowest associated error.
24. The apparatus of claim 19 , wherein the processing means is arranged to perform operations comprising:
determining one or more global correction factors for correcting one or more errors associated with a plurality of probe positions.
25. The apparatus of claim 24 , wherein the global correction factors correct for one or more of scaling, drift and rotation.
26. The apparatus of claim 19 , wherein the offset vector is determined in an at least partly random manner.
27. The apparatus of claim 19 , wherein the processing means is arranged to determine, estimate, and provide iteratively and the offset vector is determined during each of a plurality of iterations.
28. The apparatus of claim 27 , wherein the offset vector is first applied to the probe function during a second or greater iteration.
29. The apparatus of claim 20 , wherein the current best offset vector is an offset vector having an associated current best error metric according to one or more predetermined criteria.
30. The apparatus of claim 29 , wherein the one or more predetermined criteria includes a lowest current best error metric.
31. The apparatus of claim 27 , wherein the offset vector is determined at least in part according to a scaling factor, such that a magnitude of the offset vector is reduced with increasing iterations.
32. A computer program tangibly stored on a non-transitory recording medium, which, when executed by a computer, is arranged to perform the method of claim 1 .