Method for determining a focus position of a lithography mask and metrology system for carrying out such a method

1. A method for determining a focus position of a lithography mask, the method comprising the following steps:

1.1. providing an optical system having an imaging optical unit for imaging lithography masks,

1.2. providing a lithography mask having at least one measurement region which is free of structures to be imaged,

1.3. recording a focus stack of the at least one measurement region of the lithography mask,

1.4. evaluating 2D intensity distributions of the recorded focus stack in a spatially resolved manner,

1.5. wherein evaluating the 2D intensity distributions comprises ascertaining the speckle contrast, and

1.6. wherein evaluating the 2D intensity distributions comprises ascertaining a focus position (z) for which the speckle contrast has a minimum;

wherein ascertaining the focus position for which the speckle contrast has a minimum comprises an interpolation method.

2. The method according to claim 1 , wherein evaluating the 2D intensity distributions comprises the following steps:

3.1. determining a spectrum S({right arrow over (v)}, z) of the 2D intensity distributions by Fourier transformation of the 2D intensity distributions,

3.2. determining a focus dependence of a real part RS(z) and an imaginary part IS(z) of a plurality of spectral components S(v xi , v yi ) of the spectrum ({right arrow over (v)}, z) in the frequency domain,

3.3. separating an imaging aberration contribution (Θ opt ) made to the spectrum S({right arrow over (v)}, z) by the imaging optical unit, and

3.4. representing the imaging aberration contribution (Θ opt ) as a linear combination of Zernike polynomials Z n ({right arrow over (v)}).

3. The method according to claim 2 , wherein representing the imaging aberration contribution (Θ opt ) as a linear combination of Zernike polynomials comprises ascertaining Zernike coefficients Z n .

4. The method according to claim 3 , wherein a linear regression method (least square fit) is used for ascertaining the symmetrical Zernike coefficients Z n .

5. The method according to claim 2 , wherein the focus position is ascertained directly from the fourth Zernike coefficient Z 4 .

6. The method according to claim 1 , wherein the evaluating comprises exclusively Fourier transformations and linear algebra.

7. The method according to claim 1 , wherein a mirror-symmetrical illumination setting is used for illuminating the measurement region.

8. The method according to claim 1 , wherein at least partially coherent illumination radiation is used for illuminating the measurement region.

9. The method according to claim 1 , wherein coherent illumination radiation is used for illuminating the measurement region.

10. A metrology system for carrying out a method according to claim 1 , the system comprising an illumination optical unit for illuminating the measurement region with illumination radiation and comprising an imaging optical unit for imaging the measurement region onto a spatially resolving detection device.

11. The metrology system according to claim 10 , wherein for evaluating the 2D intensity distributions of the recorded focus stack, a computing device is connected to the detection device in a data-transferring manner.

12. The metrology system of claim 11 in which the computing device is configured to evaluate the 2D intensity distributions of the recorded focus stack in a spatially resolved manner and ascertain the focus position (z) for which the speckle contrast has a minimum.

13. The metrology system of claim 11 in which the computing device is configured to use the interpolation method to ascertain the focus position for which the speckle contrast has a minimum.

14. The metrology system of claim 13 in which the computing device is configured to evaluate the 2D intensity distributions by performing the following steps:

determining a spectrum S({right arrow over (v)}, z) of the 2D intensity distributions by Fourier transformation of the 2D intensity distributions,

determining a focus dependence of a real part RS(z) and an imaginary part IS(z) of a plurality of spectral components S(v xi , v yi ) of the spectrum ({right arrow over (v)}, z) in the frequency domain,

separating an imaging aberration contribution (Θ opt ) made to the spectrum S({right arrow over (v)}, z) by the imaging optical unit, and

representing the imaging aberration contribution (Θ opt ) as a linear combination of Zernike polynomials Z n ({right arrow over (v)}).

15. The metrology system of claim 14 in which the computing device is configured to represent the imaging aberration contribution (Θ opt ) as a linear combination of Zernike polynomials by ascertaining Zernike coefficients Z n .

16. The metrology system of claim 15 in which the computing device is configured to use a linear regression method (least square fit) for ascertaining the symmetrical Zernike coefficients Z n .

17. The metrology system of claim 14 in which the computing device is configured to ascertain the focus position directly from the fourth Zernike coefficient Z 4 .

18. The metrology system of claim 11 in which the computing device is configured to evaluate the 2D intensity distributions using Fourier transformations and linear algebra.

19. A method for imaging a lithography mask onto a wafer, in which the lithography mask comprises a structure to be imaged and at least one measurement region that is free of structures to be imaged, the method comprising:

determining a focus position of the lithography mask, comprising the following steps:

providing an optical system having an imaging optical unit for imaging lithography masks,

recording a focus stack of the at least one measurement region of the lithography mask,

evaluating 2D intensity distributions of the recorded focus stack in a spatially resolved manner,

wherein evaluating the 2D intensity distributions comprises ascertaining the speckle contrast, and

wherein evaluating the 2D intensity distributions comprises ascertaining a focus position (z) for which the speckle contrast has a minimum,

aligning a position of the lithography mask in a projection exposure system according to the determined focus position of the lithography mask, and

exposing the lithography mask to transfer the structure to be imaged onto the wafer,

wherein ascertaining the focus position for which the speckle contrast has a minimum comprises an interpolation method.