AUTOFOCUSING METHOD FOR AN IMAGING DEVICE

Abstract

The invention relates to an autofocusing method for an imaging device (for semiconductor lithography) comprising an imaging optical unit, an object to be measured and an autofocusing device having a reflective illumination, comprising the following method steps: a) defining at least three basis measurement points M(x.sub.j, y.sub.j) on a surface of the object, b) determining the deviation A.sub.z(M)j of a nominal position of the surface of the object from the focal plane of the autofocusing device at the defined basis measurement points M(x.sub.j, y.sub.j), c) storing the deviations A.sub.z(M)j from at least three basis measurement points M(x.sub.j, y.sub.j), d) using the stored deviation A.sub.z(M)j for determining a deviation A.sub.z(P)k at an arbitrary point P(x.sub.k, Y.sub.k) of the surface, and e) using the deviation A.sub.z(P)k for focusing onto the point P(x.sub.k, Y.sub.k).

Claims

1. An autofocusing method for an imaging device (for semiconductor lithography) comprising an imaging optical unit, an object to be measured and an autofocusing device having a reflective illumination, comprising the following method steps: a) defining at least three basis measurement points M(x.sub.j, y.sub.j) on a surface of the object, b) determining the deviation A.sub.z(M)j of a nominal position of the surface of the object from a focal plane of the autofocusing device at the defined basis measurement points M(x.sub.j, y.sub.j), c) storing the deviations A.sub.z(M)j from at least three basis measurement points M(x.sub.j, y.sub.j), d) using the stored deviation A.sub.z(M)j for determining a deviation A.sub.z(P)k at an arbitrary point P(x.sub.k, y.sub.k) of the surface, and e) using the deviation A.sub.z(P)k for focusing onto the point P(x.sub.k, y.sub.k).

2. The autofocusing method of claim 1, wherein only basis measurement points M(x.sub.j, y.sub.j) are used for which a deviation A.sub.z(M)j can be determined with a required accuracy.

3. The autofocusing method of claim 2, wherein a design description of the object is taken into account in the definition of the basis measurement points.

4. The autofocusing method of claim 1, wherein only points without structures are taken into account in the definition of the basis measurement points M(x.sub.j, y.sub.j).

5. The autofocusing method of claim 1, wherein during the imaging of a point P(x.sub.k, y.sub.k) of the surface by use of the imaging optical unit, the deviation A.sub.z(P)k is interpolated on the basis of the stored deviations A.sub.z(M)j and is taken into account in the focusing.

6. The autofocusing method of claim 5, wherein the interpolation for predicting the deviation A.sub.z(P) for arbitrary points P(x.sub.k, y.sub.k) on the surface of the object is based on a linear or polynomial interpolation model or an interpolation model based on a thin plate basis function, a Legendre polynomial or a Zernike polynomial.

7. The autofocusing method of claim 5, wherein the deviations A.sub.z(M)j are determined only for as many basis measurement points M(x.sub.j, y.sub.j) as are needed for the interpolation.

8. The autofocusing method of claim 1, wherein the method is carried out before the actual measurement of the object in the imaging device.

9. The autofocusing method of claim 8, wherein the method is carried out during the temperature regulation and/or stabilization of the imaging device.

10. The autofocusing method of claim 1, wherein the deviation A.sub.z(P)k for at least one measurement point P(x.sub.k, y.sub.k) is corrected by a correction value ΔA.sub.k during operation.

11. The autofocusing method of claim 10, wherein the correction value ΔA.sub.k is determined on the basis of changes of pressure, temperature, air humidity or mechanical drift.

12. The autofocusing method of claim 10, wherein the correction value ΔA.sub.k is determined on the basis of a focus measurement of the imaging device.

13. The autofocusing method of claim 12, wherein the correction value ΔA.sub.k for the interpolated deviations A.sub.z(P)k at a measurement point P(x.sub.k, Y.sub.k) is determined on the basis of a focus measurement at the previous measurement point P(x.sub.k—1, y.sub.k−1).

14. The autofocusing method of claim 13, wherein the correction value ΔA.sub.k is summed recursively.

15. The autofocusing method of claim 1, wherein the object is embodied as a photomask of a projection exposure apparatus for semiconductor lithography.

16. The autofocusing method of claim 1, wherein the object is embodied as a substrate for a photomask of a projection exposure apparatus for semiconductor lithography.

17. The autofocusing method of claim 2, wherein only points without structures are taken into account in the definition of the basis measurement points M(x.sub.j, y.sub.j).

18. The autofocusing method of claim 2, wherein during the imaging of a point P(x.sub.k, y.sub.k) of the surface by use of the imaging optical unit, the deviation A.sub.z(P)k is interpolated on the basis of the stored deviations A.sub.z(M)j and is taken into account in the focusing.

19. The autofocusing method of claim 2, wherein the deviation A.sub.z(P)k for at least one measurement point P(x.sub.k, y.sub.k) is corrected by a correction value ΔA.sub.k during operation.

20. The autofocusing method of claim 2, wherein the object is embodied as a photomask of a projection exposure apparatus for semiconductor lithography.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0032] Exemplary embodiments and variants of the invention are explained in more detail below with reference to the drawing. In the figures:

[0033] FIG. 1 shows a schematic illustration of an imaging device according to the prior art,

[0034] FIG. 2 shows a further schematic illustration of an imaging device according to the prior art,

[0035] FIG. 3 shows a schematic illustration of a photomask in a plan view, and

[0036] FIG. 4 shows a flow diagram concerning a method according to the invention.

DETAILED DESCRIPTION

[0037] FIG. 1 shows an embodiment of an autofocusing device 1 as known from the prior art, said autofocusing device being arranged in an imaging device for examining an object embodied as a lithography mask 3, said imaging device being embodied as a microscope 2. The microscope 2 comprises an illumination source 4, which emits incoherent, coherent or partially coherent illumination radiation having a wavelength of 193 nm. The illumination radiation is guided via a first deflection mirror 5 and a second deflection mirror 6 to the imaging objective 7 and is directed by use of the latter onto the lithography mask 3 for illumination purposes.

[0038] The object 3 is imaged by way of the imaging objective 7, the partly transparent deflection mirror 6 and also a tube optical unit 8, which together form an imaging optical unit 9, onto a CCD camera 10 in order to generate an image of a part of the object 3. By way of example, the lateral position of alignment marks of the lithography mask 3 can be determined highly accurately by use of the microscope 2. A CMOS camera or some other image sensor can also be used instead of a CCD camera.

[0039] The microscope 2 furthermore has an object stage 11, by use of which the object 3 can be positioned both laterally and in the observation direction, that is to say in the z-direction. As a result, the object 3 can be positioned such that it is situated at the focus of the imaging device 2, that is to say the focal plane 22 of the imaging device 2, said focal plane being indicated in a dashed manner.

[0040] The autofocusing device 1 uses the illumination source 4 and also the imaging objective 7 of the microscope 2 for illuminating the object 3 with a focusing image and uses the imaging objective 7, the tube optical unit 8 and the CCD camera 9 for recording the focusing image.

[0041] To that end, firstly the first deflection mirror 5 and secondly the deflection mirror 16 are embodied as displaceable, which is indicated by the double-headed arrows P1 and P2 in FIG. 1.

[0042] In contrast to the example shown, the imaging device 2 can also be operated in transmission as long as only the autofocusing device 1 is used in reflection.

[0043] FIG. 2 shows the same imaging device 2 during autofocusing operation. In this case, the deflection mirror 5 is moved out of the beam path of the illumination radiation coming from the illumination source 4, such that the illumination radiation is incident on a third deflection mirror 12, which directs the illumination radiation through a grating 13 tilted by 45° relative to the direction of propagation of the illumination radiation. However, the tilt angle can also be any other angle from the range of 1-89°. The deflection mirror 16 is displaced such that the grating structure is imaged by way of an autofocus optical unit 14, a further deflection mirror 15 and the deflection mirror 16 onto the second deflection mirror 6 and also through the imaging objective 7 onto the object 3.

[0044] The grating 13 can be embodied for example as a line grating having alternately transparent strips and nontransparent strips. The grating extends periodically in the x-direction.

[0045] For the purpose of focusing, that is to say positioning the object 3 into the focal plane 22 of the imaging device 2, the object is positioned in its nominal position at the focus of the imaging device 2. The surface deviations of the object 3 are in a range of a few μm, wherein the accuracy of the focus measurement is between 1 and 50 nm. This has the effect that the surface of the object 3 is usually not situated at the focus in the case of a nominal positioning. In order to determine the deviation of the surface from the nominal surface, the aerial image of the grating structure that is imaged on the CCD detector of the CCD camera 10 is fed to a control unit 17 of the autofocusing device 1. The control unit 17 determines the deviation of the surface of the object 3 from the nominal position thereof on the basis of an intensity distribution. This is used to drive the object stage 11 such that that region of the object 3 which is intended to be measured is positioned at the focus of the imaging device 2.

[0046] FIG. 3 shows an object 3 embodied as a lithography mask 3 with an x-axis and a y-axis, six basis measurement points 20 being arranged on said object. For said basis measurement points 20, the deviation A.sub.z(M)j of the surface in the z-direction is determined in each case. This can be determined according to a method known from the prior art as described with reference to FIG. 1 and FIG. 2, but any other highly accurate determination of the deviation A.sub.z(M)j of the surface from its nominal form, such as by use of an interferometer, for example, can also be used for this purpose. The deviations A.sub.z(M)j thus determined are stored for each basis measurement point M(x.sub.j, y.sub.j), thus giving rise to a table having deviations A.sub.z(M)j at the basis measurement points M(x.sub.j, y.sub.j). FIG. 3 furthermore also shows two measurement points 21 P(x.sub.k, y.sub.k) illustrated as circles left blank, said measurement points representing by way of example measurement points 21 at which a measurement is intended to be carried out by the imaging device 2. For the purpose of focusing the object 3 in the imaging device 2, which is not illustrated in FIG. 3, the deviation A.sub.z(P)k of the surface at the measurement points P(x.sub.k, y.sub.k) must be known. In contrast to the prior art, the deviation A.sub.z(P)k is not determined by use of a focusing method known from the prior art directly before the measurement, but rather is interpolated from the deviations A.sub.z(M)j of the basis measurement points 20 M(x.sub.j, y.sub.j). In the case of long-wave defects of the surface of the object 3, for example a fifth-order interpolation polynomial can already determine the deviation A.sub.z(P)k sufficiently accurately. Depending on the surface shape, the prediction of the deviation A.sub.z(P)k can also be based on a linear or some other polynomial interpolation model, such as, for example, a third- or seventh-order one or an interpolation model based on a thin plate basis function, a Legendre polynomial or a Zernike polynomial. In this case, the number of basis measurement points 20 required for a prediction depends on the interpolation model used. This method has the advantage that the time for focusing can be reduced to a minimum.

[0047] FIG. 4 describes a possible autofocusing method for an imaging device.

[0048] The first method step 30 involves defining at least three basis measurement points 20 M(x.sub.j, y.sub.j) on a surface of the object.

[0049] A second method step 31 involves determining the deviation A.sub.z(M)j of a nominal position of the surface of the object 3 from the focal plane of the autofocusing device at the defined basis measurement points 20 M(x.sub.j, y.sub.j).

[0050] A third method step 32 involves storing the deviations A.sub.z(M)j from at least three basis measurement points 20 M(x.sub.j, y.sub.j).

[0051] A fourth method step 33 involves using the stored deviations A.sub.z(M)j for interpolating a deviation A.sub.z(P)k at an arbitrary point 21 P(x.sub.k, y.sub.k) of the surface.

[0052] A fifth method step 34 involves focusing onto the point 21 P(x.sub.k, y.sub.k).

[0053] This method reduces the time for focusing to a minimum and avoids an excitation of the imaging device, such as, for example, as a result of a pivoting-in of deflection mirrors 5, 16 during the measurement of the partial regions of the objects.

LIST OF REFERENCE SIGNS

[0054] 1 Autofocusing device [0055] 2 Microscope [0056] 3 Lithography mask [0057] 4 Illumination source [0058] 5 First deflection mirror [0059] 6 Second deflection mirror [0060] 7 Imaging objective [0061] 8 Tube optical unit [0062] 9 Imaging optical unit [0063] 10 CCD camera [0064] 11 Object stage [0065] 12 Third deflection mirror [0066] 13 Grating [0067] 14 Autofocus optical unit [0068] 15 Further deflection mirror [0069] 16 Further deflection mirror [0070] 17 Control unit [0071] 20 Basis measurement points M(x.sub.j, y.sub.j) [0072] 21 Measurement points P(x.sub.k, y.sub.k) [0073] 22 Focal plane [0074] 30 Method step 1 [0075] 31 Method step 2 [0076] 32 Method step 3 [0077] 33 Method step 4 [0078] 34 Method step 5