Dark field digital holographic microscope and associated metrology method

1 . A dark field digital holographic microscope configured to determine a characteristic of interest of a structure, comprising:

an illumination device configured to provide at least:

a first beam pair comprising a first illumination beam of radiation and a first reference beam of radiation and

a second beam pair comprising a second illumination beam of radiation and a second reference beam of radiation; and

an imaging branch configured to

capture a first scattered radiation scattered by the structure being illuminated by the first illumination beam of radiation; and

capture a second scattered radiation scattered by the structure being illuminated by the second illumination beam of radiation,

the imaging branch having a detection NA greater than 0.1, wherein the illumination device is configured such that:

the first illumination beam of radiation and the first reference beam of radiation are at least partially temporally and spatially coherent;

the second illumination beam of radiation and the second reference beam of radiation are at least partially temporally and spatially coherent; and

the illumination device is configured to impose temporal and/or spatial incoherence between the first beam pair and second beam pair.

2 . The dark field digital holographic microscope of claim 1 , wherein the illumination device is configured

to direct the first illumination beam of radiation so as to illuminate the structure from a first direction and

to direct the second illumination beam of radiation so as to illuminate the structure from a second direction, the second direction being different from the first direction.

3 . The dark field digital holographic microscope of claim 1 , wherein:

the imaging branch further comprises a sensor,

the dark field digital holographic microscope is configured to simultaneously capture on the sensor

a first interference pattern resulting from interference of the first scattered radiation and the first reference beam and

a second interference pattern resulting from interference of the second scattered radiation and the second reference beam, and

the dark field digital holographic microscope is configured such that the first interference pattern and second interference pattern at least partially, spatially overlap on the sensor.

4 . The dark field digital holographic microscope of claim 3 , wherein

the first reference beam of radiation and the second reference beam of radiation are configured to be each incident at respective different azimuthal angles with respect to an optical axis of the dark field digital holographic microscope, and

the azimuthal angle of the first reference beam of radiation and the azimuthal angle of the second reference beam of radiation is configured to comprise a sufficient large difference such that the first and second interference patterns are separable in the spatial frequency domain.

5 . The dark field digital holographic microscope of claim 3 , wherein the first reference beam of radiation and the second reference beam of radiation are configured to be each incident at respective different angles of incidence with respect to an optical axis of the dark field digital holographic microscope.

6 . The dark field digital holographic microscope of claim 1 , wherein:

the illumination device further comprises a coherence matching arrangement configured

to adjustably delay one of the first illumination beam and first reference beam with respect to the other of the first illumination beam and first reference beam so as to maintain the beams of the first beam pair to be at least partially coherent and

to adjustably delay one of the second illumination beam and second reference beam with respect to the other of the second illumination beam and second reference beam so as to maintain the beams of the second beam pair to be at least partially coherent.

7 . The dark field digital holographic microscope of claim 6 ,

wherein:

the illumination device comprises a time delay arrangement configured to impose incoherence between the first beam pair and the second beam pair, and

the time delay arrangement comprises an adjustable time delay arrangement configured to adjustably delay one of the first beam pair and the second beam pair with respect to the other of the first beam pair and the second beam pair to impose the incoherence.

8 . The dark field digital holographic microscope of claim 7 , wherein:

the illumination device comprises a first branch configured to provide the first pair of beams and a second branch configured to provide the second pair of beams;

the time delay arrangement comprises at least a delay line configured to impose a delay on one of the first branch or the second branch with respect to the other of the first branch or the second branch; and

the coherence matching arrangement comprises a first coherence matching arrangement in the first branch configured to adjustably delay at least one of the first reference beam and first illumination beam, and a second coherence matching arrangement in the second branch configured to adjustably delay at least one of the second reference beam and second illumination beam.

9 . The dark field digital holographic microscope of claim 7 , wherein: the illumination device comprises a first branch configured to provide the first illumination beam and the second illumination beam and a second branch operable to provide the first reference beam and the second reference beam; and

the coherence matching arrangement and the time delay arrangement are implemented via co-optimization of at least a first adjustable delay line configured to impose an adjustable delay on one of the first branch or the second branch with respect to the other of the first branch or the second branch, a second adjustable delay line in the first branch configured to impose an adjustable delay on at least one of the first illumination beam and second illumination beam, and a third adjustable delay line in the second branch configured to impose an adjustable delay on at least one of the first reference beam and the second reference beam.

10 . The dark field digital holographic microscope of claim 1 , wherein the first illumination beam of radiation is configured to illuminate the structure at a first angle of incidence and the second illumination beam of radiation is configured to illuminate the structure at a second angle of incidence, different to the first angle of incidence.

11 . The dark field digital holographic microscope of claim 1 , wherein the illumination device comprises a single radiation source from which the illumination device is configured to generate the first beam pair and the second beam pair.

12 . The dark field digital holographic microscope of claim 11 , wherein the single light source is configured to emit at least partially coherent radiation.

13 . The dark field digital holographic microscope of claim 1 , wherein the illumination device is configured such that the first reference beam and the second reference beam are each generated at a first power level, and the first illumination beam and second illumination beam are each generated at a second power level, the second power level being greater than the first power level.

14 . A metrology apparatus for determining a characteristic of interest of a structure on a substrate comprising a dark field digital holographic microscope of claim 1 .

15 . The dark field digital holographic microscope of claim 1 , further comprising:

a first delay line configured to impose the temporal and/or spatial incoherence between the first beam pair and second beam pair, and

at least one of a second delay line configured to impose a delay on the first beam pair and a third delay line configured to impose a delay on the second beam pair.

16 . The dark field digital holographic microscope of claim 1 , further comprising:

a first delay line configured to impose the temporal and/or spatial incoherence between the first beam pair and second beam pair, and

at least one of a second delay line configured to impose a delay on the first illumination beam and the second illumination beam and a third delay line configured to impose a delay on the first reference beam and the second reference beam.

17 . A method comprises:

illuminating a target formed by a lithographic process on a substrate with a first illumination beam of radiation;

capturing, using an imaging branch having a detection NA greater than 0.1, resultant first scattered radiation having been scattered from the target;

illuminating the target with a second illumination beam of radiation and capturing, using the imaging branch, resultant second scattered radiation having been scattered from the target;

imposing spatial and/or temporal incoherence between a first beam pair comprising the first illumination beam and a first reference beam and a second beam pair comprising the second illumination beam and a second reference beam, such that:

the beams of the first beam pair are at least partially spatially and temporally coherent,

the beams of the second beam pair are at least partially spatially and temporally coherent, and

any beam of the first beam pair is spatially and/or temporally incoherent to any beam of the second beam pair; and

simultaneously generating a first interference pattern resulting from interference of the first scattered radiation and a first reference beam of radiation; and a second interference pattern resulting from interference of the second scattered radiation and a second reference beam.

18 . The method of claim 17 , further comprising:

imposing the spatial and/or temporal incoherence between the first beam pair and the second beam pair using a first delay line; and

imposing a delay on the first beam pair or on the second beam pair using a second delay line.

19 . The method of claim 17 , further comprising:

imposing the spatial and/or temporal incoherence between the first beam pair and the second beam pair using a first delay line; and

imposing a delay on the first illumination beam and the second illumination beam or on the first reference beam and the second reference beam using a second delay line.