Inspection apparatus for lithography

1. An inspection apparatus comprising:

an optical element configured to focus a radiation beam onto a substrate at a range of incident and azimuth angles such that the radiation beam reflects from the substrate;

a polarizing device configured to polarize the reflected radiation beam into two different polarization directions;

a variable retarder configured to retard at least one of the two different polarization directions by a certain amount so as to apply a variable phase shift on the reflected radiation beam, the variable phase shift being dependent on a wavelength of the radiation beam; and

a detector system configured to detect simultaneously an angle-resolved spectrum of the two different polarization directions of the radiation beam.

2. The inspection apparatus according to claim 1 , wherein, if the applied variable phase shift is δ, the detector system is configured to:

output relative intensities of the two different polarization directions of the radiation beam,

output a sum of the intensities of the two different polarization directions and a difference between the intensities of the two different polarization directions,

apply an algorithm for each phase shift to output a file of relative phase differences (Δ Fourier ), and

solve

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tan

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(

Δ

Fourier

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=

(

2

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C

3

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S

+

2

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CS

3

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×

1

-

x

2

(

C

4

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C

2

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S

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×

tan

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(

ψ

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+

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S

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S

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tan

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(

ψ

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+

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C

2

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S

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x

where x=cos(Δ), C n =cos n (A), S n =sin n (A), S=sin(A) and Ψ=the angle between the two polarization directions, to obtain the value of relative phase differences (Δ) between the two polarization directions for all azimuth angles (A).

3. The inspection apparatus according to claim 1 , wherein the range of incident angles is about 0 to 80 degrees and the range of azimuth angles is about 0 to 360 degrees.

4. The inspection apparatus according to claim 1 , wherein the variable phase shift is from about 0 to 360 degrees.

5. The inspection apparatus according to claim 1 , further comprising a polarizing beamsplitter positioned downstream of the variable retarder and configured to split the phase-shifted radiation beam into two differently-polarized radiation sub-beams.

6. The inspection apparatus according to claim 1 , wherein the polarizing device and variable retarder are configured to cause the radiation beam reflected from the substrate to be elliptically polarized.

7. The inspection apparatus according to claim 1 , further comprising a focusing system and an optical wedge placed in an image plane of the focusing system configured to redirect the beam in different directions in dependence on the polarization directions, such that separate polarized reflected radiation sub-beams are received at different positions on the detector system.

8. The inspection apparatus according to claim 1 , further comprising a processor configured to:

measure an azimuth angle of the radiation beam;

detect a summed intensity of polarized reflected radiation sub-beams;

derive a reflectance of the polarized reflected radiation sub-beams from the azimuth angle and presummed intensity;

evaluate a ratio between polarization direction amplitudes of the polarized reflected radiation sub-beams; and

determine a property of the substrate surface resulting from a variation from a predetermined model of the ratio between the polarization direction amplitudes of the polarized reflected radiation sub-beams.

9. The inspection apparatus according to claim 8 , wherein the reflectance of the polarized reflected radiation sub-beams is derived using the equation:

m= 0.5( Rp 2 +Rs 2 )+0.5 cos(2 A )( Rp 2 −Rs 2 ),

where the summed intensity (m) is known from the detector, wherein Rp and Rs are the reflectances from the polarized reflected radiation sub-beams and A is the azimuth angle of the radiation beam.

10. The inspection apparatus according to claim 8 , wherein the ratio (tan ψ) between the polarization direction amplitudes of the polarized reflected radiation sub-beams is evaluated using the equation:

tan Ψ= Rp/Rs,

wherein Rp and Rs are the reflectances from the polarized reflected radiation sub-beams.

11. The inspection apparatus according to claim 1 , wherein the polarizing device further comprises a retarder such that the radiation is circularly polarized.

12. The inspection apparatus according to claim 1 , further comprising a processor configured to:

measure an azimuth angle of the radiation beam;

determine a value of the phase shift between polarized reflected radiation sub-beams prior to reflection from the substrate surface;

calculate a difference between the intensities of the polarized reflected radiation sub-beams from the azimuth angle and the phase shift;

derive a phase difference between the polarization directions of the polarized reflected radiation sub-beams post-reflection from the difference between the intensities of the sub-beams; and

determine a property of the substrate surface resulting from a variation from a predetermined model of the phase difference between the polarization directions of the polarized reflected radiation sub-beams.

13. The inspection apparatus according to claim 12 , wherein the phase difference (Δ) between the polarization direction of the polarized reflected radiation sub-beams post-reflection from a difference (d) between the intensities of the sub-beams is derived using the equation:

d=I y −I x ={Rp 2 (cos 4 ( A )−cos 2 ( A )sin 2 ( A ))+ Rs 2 (sin( A )−cos 2 ( A )sin 2 ( A ))} cos(δ)+ . . . RpRs {cos(Δ)cos(δ)4 cos 2 ( A )sin 2 ( A )+sin(Δ)sin(δ)2(cos 3 ( A )sin( A )+cos( A )sin 3 ( A ))}

wherein Rp and Rs are the reflectances from the polarized reflected radiation sub-beams, A is the azimuth angle of the radiation Learn, δ is the phase shift, and Ix and Iy are the intensities of the sub-beams.

14. A method comprising:

providing a radiation beam with elliptical polarization;

reflecting the radiation beam off the surface of a substrate;

splitting the reflected radiation beam into two polarized sub-beams;

shifting a phase of a first sub-beam of the two sub-beams by a variable amount with respect to the second sub-beam of the two sub-beams, the variable amount being dependent on the wavelength of the radiation beam; and

simultaneously detecting the sub-beams.

15. The method according to claim 14 , further comprising:

measuring an azimuth angle of the radiation beam;

detecting a summed intensity of the sub-beams;

deriving a reflectance of the sub-beams from the azimuth angle and the summed intensity;

evaluating a ratio between polarization direction amplitudes of the sub-beams; and

determining a property of the substrate surface resulting from a variation from a predetermined model of the ratio between the polarization direction amplitudes of the sub-beams.

16. The method, according to claim 15 , wherein:

the reflectance of the sub-beams is derived by inserting the azimuth angle and the summed intensity into the equation:

m= 0.5( Rp 2 +Rs 2 )+0.5 cos(2 A )( Rp 2 −Rs 2 ),

wherein Rp and Rs are the reflectances of the sub-beams, A is the azimuth angle of the radiation beam and m is the summed intensity of the sub-beams; and

wherein the ratio (tan Ψ) between the polarization direction amplitudes of the sub-beams is evaluated using the equation:

tan Ψ= Rp/Rs.

17. The method according to claim 14 , further comprising:

measuring an azimuth angle of the radiation beam;

determining a value of the phase shift between the sub-beams prior to reflection from the substrate surface;

calculating a difference between intensities of the sub-beams;

deriving a phase difference between polarization directions of the sub-beams post-reflection; and

determining a property of the substrate surface resulting from a variation from a predetermined model of the phase difference between the polarization directions of the sub-beams.

18. The method according to claim 17 , wherein:

a difference (d) between the intensities of the sub-beams is calculated using the equation:

d=I y −I x ,

wherein Ix and Iy are the intensities of the sub-beams; and

wherein the phase difference (Δ) between the polarization directions of the sub-beams post-reflection is derived using the equation:

d=I y −I x ={Rp 2 (cos 4 ( A )−cos 2 ( A )sin 2 ( A ))+ Rs 2 (sin( A )−cos 2 ( A )sin 2 ( A ))} cos(δ)+ . . . RpRs {cos(Δ)cos(δ)4 cos 2 ( A )sin 2 ( A )+sin(Δ)sin(δ)2(cos 3 ( A )sin( A )+cos( A )sin 3 ( A ))}

wherein Rp and Rs are the reflectances of the sub-beams, A is the azimuth angle of the radiation beam and S is the phase shift.

19. A lithographic apparatus comprising:

an optical element configured to focus a radiation beam onto a substrate at a range of incident and azimuth angles such that the radiation beam reflects from the substrate;

a polarizing device configured to polarize the reflected radiation beam into two different polarization orientations;

a variable retarder with a variable phase shift configured to shift a phase of the reflected radiation beam, the variable phase shift being dependent on the wavelength of the radiation beam; and

a detector system configured to detect simultaneously an angle-resolved spectrum of the two polarization orientations of the radiation beam.

20. A lithographic cell comprising:

a radiation source configured to supply a radiation beam;

an optical element configured to focus a radiation beam onto a substrate at a range of incident and azimuth angles such that the radiation beam reflects from the substrate;

a polarizing device configured to polarize the reflected radiation beam into two different polarization orientations;

a variable retarder with a variable phase shift configured to shift a phase of the reflected radiation beam, the variable phase shift being dependent on the wavelength of the radiation beam; and

a detector system configured to detect simultaneously an angle-resolved spectrum of the two polarization orientations of the radiation beam.