Characteristic determination
Embodiments of the present invention provide a method of determining at least one characteristic of a target surface, comprising providing a phase map for a first region of a target surface via radiation having a first wavelength, providing n further phase maps for the first region via radiation having a further n wavelengths each different from the first wavelength, and determining at least one characteristic at the target surface responsive to the first and further phase maps.
1. A method of determining at least one characteristic of a target surface, comprising:
providing a phase map for a first region of a target surface via radiation at least partly reflected from the target surface having a first wavelength;
providing two further phase maps for the first region via radiation at least partly reflected from the target surface having a further two wavelengths each different from the first wavelength;
determining a map of height variations for the first region of the target surface based on the first and two further phase maps; and
determining a relative phase shift on reflection at the target surface responsive to the first and two further phase maps and the map of height variations, wherein the relative phase shift on reflection (PSoR) is indicative of a change in phase resulting from a material composition of the target surface.
2. The method of claim 1 , wherein the relative PSoR is determined by:
RelativePSoR
(
λ
n
)
=
Φ
n
-
4
π
d
λ
n
where λ n is one of the first and two further wavelengths and Φ is a phase change at one of the first and two further wavelengths.
3. The method of claim 1 , comprising determining:
ΔΦ
12
=
Φ
λ
1
-
Φ
λ2
=
(
a
1
-
4
π
d
λ
1
λ
2
)
(
λ
1
-
λ
2
)
Δ
Φ
23
=
Φ
λ
2
-
Φ
λ3
=
(
a
1
-
4
π
d
λ
2
λ
3
)
(
λ
2
-
λ
3
)
where a 1 is a constant, λ 1 , λ 2 , λ 3 are the first wavelength and two further wavelengths, respectively, and Φ λ1 , Φ λ2 , Φ λ3 are the phase change at the first and two further wavelengths.
4. The method according to claim 1 , comprising scaling each phase map prior to determining the PSoR.
5. The method according to claim 1 , further comprising:
for each m, determining
d
=
ΔΦ
m
-
ΔΦ
m
+
1
Δλ
.4
π
(
1
λ
m
λ
m
+
1
-
1
λ
m
+
1
λ
m
+
2
)
where ΔΦ m =Φ n+1 −Φ n and Φ is a phase change and n is one of the first and two further wavelengths.
6. The method according to claim 1 , further comprising:
determining a change in phase via an iterative process.
7. The method according to claim 1 , further comprising:
determining a change in optical or dielectric properties at the target surface.
8. The method according to claim 1 , further comprising:
providing a reference portion of known material having a known PSoR with the target sample.
9. The method of claim 8 , wherein the reference portion comprises a dielectric material having a constant PSoR.
10. The method of claim 1 , wherein the PSoR changes linearly with wavelength.
11. A processor configured with instructions which, when executed, implement the method of determining at least one characteristic of a target surface as claimed in claim 1 .
12. An apparatus for determining at least one characteristic of a target surface, comprising:
a receiver for receiving a phase map for a first region of a target surface generated using radiation having a first wavelength, and for receiving two further phase maps for the first region each generated using radiation having a respective one of two further wavelengths, each different than the first wavelength; and
a processor arranged to determine:
a map of height variations for the first region of the target surface based on the first phase map and the two further phase maps; and
a relative phase shift on reflection at the target surface responsive to the first phase map and two further phase maps and the map of height variations, wherein the relative phase shift on reflection (PSoR) is indicative of a change in phase resulting from a material composition of the target surface.
13. The apparatus of claim 12 , wherein the relative PSoR is determined by:
Relative
PSoR
(
λ
n
)
=
Φ
n
-
4
π
d
λ
n
where λ n is one of the first and two further wavelengths and Φ is a phase change at one of the first and two further wavelengths.
14. The apparatus of claim 12 , wherein the processor is arranged to determine
ΔΦ
12
=
Φ
λ
1
-
Φ
λ2
=
(
a
1
-
4
π
d
λ
1
λ
2
)
(
λ
1
-
λ
2
)
Δ
Φ
23
=
Φ
λ
2
-
Φ
λ3
=
(
a
1
-
4
π
d
λ
2
λ
3
)
(
λ
2
-
λ
3
)
where λ 1 , λ 2 , λ 3 are the first wavelength and two further wavelengths, respectively, and Φ λ1 , Φ λ2 , Φ λ3 are the phase change at the first and two further wavelengths.