Measurement apparatus and method of measuring a target
The disclosure relates to measuring a target. In one arrangement, a measurement apparatus is provided that has an optical system configured to illuminate a target with radiation and direct reflected radiation from the target to a sensor. A programmable spatial light modulator in a pupil plane of the optical system is programmed to redirect light in each of a plurality of pupil plane zones in such a way as to form a corresponding plurality of images at different locations on the sensor. Each image is formed by radiation passing through a different respective one of the pupil plane zones.
1. A measurement apparatus comprising:
an optical system configured to illuminate a target with radiation and direct reflected radiation from the target to a sensor; and
a programmable spatial light modulator in a pupil plane of the optical system,
wherein the spatial light modulator is configured to:
redirect light in each of a plurality of pupil plane zones in such a way as to form a corresponding plurality of images at different locations on the sensor, each image being formed by radiation passing through a different respective one of the pupil plane zones;
impart a phase gradient in the pupil plane in each of two or more of the pupil plane zones; and
apply an optical correction to radiation passing through the pupil plane, wherein the optical correction comprises a focus adjustment or a correction for an optical aberration.
2. The measurement apparatus of claim 1 , wherein the phase gradient is constant along all radial directions in the pupil plane in each of two or more of the pupil plane zones.
3. The measurement apparatus of claim 1 , wherein the phase gradient in each pupil plane zone is such as to imitate an optical effect of a notional wedge-shaped optical element having a lower planar surface and an upper planar surface aligned obliquely with respect to the lower planar surface.
4. The measurement apparatus of claim 3 , wherein the notional wedge-shaped optical element corresponding to at least one of the pupil plane zones has a different orientation, a different angle between the lower planar surface and the upper planer surface, or both, relative to a notional wedge-shaped optical element corresponding to at least one other of the pupil plane zones.
5. The measurement apparatus of claim 1 , wherein each of the plurality of pupil plane zones consists of a zone within a different range of azimuthal angles relative to an optical axis of the optical system passing through the pupil plane.
6. The measurement apparatus of claim 1 , wherein:
the spatial light modulator is configured to form a plurality of groups of images on the sensor;
each group of images consists of images formed by redirecting light in each of the plurality of pupil plane zones to different locations on the sensor;
the spatial light modulator is configured to apply a different optical correction to each group of images; and
the spatial light modulator is configured to form each group of images in a different region on the sensor.
7. The measurement apparatus of claim 1 , wherein:
to impart the phase gradient, the spatial light modulator is configured to apply a modulation comprising a component of the form:
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k x and k y are wave vectors representing orthogonal Fourier components of radiation in the pupil plane; and
s ix and s iy define the phase gradient imparted by the spatial light modulator in each of four different pupil plane zones consisting of different quadrants in the pupil plane.
8. The measurement apparatus of claim 1 , wherein:
to apply the optical correction, the spatial light modulator is configured to apply a modulation comprising a component of the form:
H ( k x ,k y )=exp( j Δ( k x 2 +k y 2 ));
Δ represents an amount of the optical correction applied by the spatial light modulator; and
k x and k y are wave vectors representing orthogonal Fourier components of radiation in the pupil plane.
9. A method of measuring a target, comprising:
illuminating the target with radiation and detecting reflected radiation from the target on a sensor;
redirecting, using a programmable spatial light modulator, light in each of a plurality of pupil plane zones in a pupil plane to form a corresponding plurality of images at different locations on the sensor, each image being formed by radiation passing through a different respective one of the pupil plane zones;
imparting, using the programmable spatial light modulator, a phase gradient in the pupil plane in each of two or more of the pupil plane zones; and
applying, using the programmable spatial light modulator, an optical correction to radiation passing through the pupil plane, wherein the optical correction comprises a focus adjustment or a correction for an optical aberration.
10. The method of claim 9 , further comprising:
forming, using the programmable spatial light modulator, a plurality of groups of images on the sensor;
wherein:
each group of images consists of images formed by redirecting light, using the programmable spatial light modulator, in each of the plurality of pupil plane zones to different locations on the sensor;
a different optical correction is applied, using the programmable spatial light modulator, to each group of images; and
each group of images is formed, using the programmable spatial light modulator, in a different region on the sensor.
11. The method of claim 10 , further comprising identifying an optimal optical correction by comparing the different groups of images.
12. The method of claim 9 , wherein:
imparting the phase gradient comprises applying, using the programmable spatial light modulator, a modulation comprising a component of the form:
W
(
k
x
,
k
y
)
=
{
exp
(
j
(
s
1
x
k
x
+
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1
y
k
y
)
)
if
k
x
>
0
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k
y
<
0
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(
j
(
s
2
x
k
x
+
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2
y
k
y
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x
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0
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k
y
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(
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+
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x
<
0
,
k
y
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exp
(
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4
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k
x
+
s
4
y
k
y
)
)
if
k
x
>
0
,
k
y
>
0
;
k x and k y are wave vectors representing orthogonal Fourier components of radiation in the pupil plane; and
s ix and s iy define the phase gradient imparted by the spatial light modulator in each of four different pupil plane zones consisting of different quadrants in the pupil plane.
13. The method of claim 9 , wherein:
applying the optical correction comprises applying, using the programmable spatial light modulator, a modulation comprising a component of the form:
H ( k x ,k y )=exp( j Δ( k x 2 +k y 2 ));
Δ represents an amount of the optical correction applied by the spatial light modulator; and
k x and k y are wave vectors representing orthogonal Fourier components of radiation in the pupil plane.