Abstract
The invention relates to a device for measuring a substrate for semiconductor lithography, comprising an illumination optical unit, an imaging optical unit and a recording device arranged in the image plane of the imaging optical unit, a diffractive element being arranged in the pupil of the imaging optical unit.
The invention also relates to a method for measuring a substrate for semiconductor lithography with a measuring device, the measuring device comprising an imaging optical unit with a pupil, with the following method steps: arranging a diffractive element in the pupil of the imaging optical unit for producing a multifocal imaging, capturing the imaging of a partial region of the substrate, and evaluating the imaging.
Claims
1. A device for measuring a substrate for semiconductor lithography, comprising an illumination optical unit, an imaging optical unit and a recording device arranged in the image plane of the imaging optical unit, wherein a diffractive element is arranged in the pupil of the imaging optical unit.
2. The device of claim 1, wherein the illumination optical unit comprises a stop arranged in the field plane.
3. The device of claim 1, wherein the diffractive element is formed as an amplitude grating.
4. The device of claim 1, wherein the diffractive element is formed as a phase grating.
5. The device of claim 1, wherein the diffractive element is formed as a combination of an amplitude grating and a phase grating.
6. The device of claim 3, wherein the grating comprises a locally varying period in one direction.
7. The device of claim 3, wherein the grating comprises a locally varying period in two directions perpendicular to one another.
8. The device of claim 3, wherein the grating comprises a distortion corresponding to a non-paraxial wavefront of the defocus.
9. The device of claim 1, wherein the diffractive element is formed such that the imaging comprises a number of partial imagings, which respectively correspond to an imaging with different distances between the substrate and the imaging optical unit, without the use of a diffractive element.
10. The device of claim 9, wherein the diffractive element is formed such that the radiation energy is allocated equally to the individual partial imagings with deviations of a few percent, in particular less than 2%.
11. The device of claim 1, wherein the diffractive element is formed such that it can bring about aberrations of a higher order in the imaging.
12. The device of claim 11, wherein the diffractive element is formed such that the aberrations of a higher order that are brought about by the diffractive element correct aberrations of a higher order of the imaging optical unit.
13. The device of claim 1, wherein the diffractive element is formed such that it is adjustable.
14. The device of claim 1, wherein the diffractive element can be pivoted into the pupil of the imaging optical unit.
15. The device of claim 1, wherein the diffractive element is formed such that it is exchangeable.
16. The device of claim 1, wherein the device comprises a magazine with a number of differently formed diffractive elements.
17. A method for measuring a substrate for semiconductor lithography with a measuring device, the measuring device comprising an imaging optical unit with a pupil, with the following method steps: arranging a diffractive element in the pupil of the imaging optical unit for producing a multifocal imaging, capturing the imaging at least of a partial region of the substrate, and evaluating the imaging.
18. The method of claim 17, wherein the imaging comprises a number of partial imagings of the partial region of the substrate.
19. The method of claim 18, wherein the imaging comprises nine partial imagings of the substrate, which correspond to different defocus positions.
20. The method of claim 18, wherein the radiation energy respectively allocated to the partial imagings is normalized.
21. The method of claim 17, wherein the evaluation of the imaging comprises the determination of the location of the best focus.
22. The method of claim 18, wherein a contrast value is determined for a number of partial imagings.
23. The method of claim 22, wherein the distance of the substrate from the best focus is determined on the basis of the contrast values of the partial imagings.
24. The method of claim 18, wherein a structure position (x,y).sub.n on a structure of the substrate that is visible in the image field is determined for each partial imaging.
25. The method of claim 18, wherein the line width of a part of a structure is determined for each partial imaging.
26. The method of claim 25, wherein the region in which the line width of the structure corresponds to a previously determined setpoint value is determined on the basis of the line widths of the partial imagings.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0043] Exemplary embodiments and variants of the invention are explained in more detail below with reference to the drawing. In the figures:
[0044] FIG. 1 shows a basic representation of a device according to the invention,
[0045] FIGS. 2A-2C show a view of a detail of a diffractive element,
[0046] FIGS. 3A and 3B show a view of a detail of a structure and of a further diffractive element,
[0047] FIGS. 4A-4C show imaging captured by the device, a normalized imaging and a diagram of the determined contrast values,
[0048] FIGS. 5A and 5B show a further, normalized imaging captured by the device and a diagram of the contrast values determined from it,
[0049] FIGS. 6A and 6B show a schematic representation of a focus stack from the prior art and an imaging captured by the device according to the invention with a number of partial imagings, and
[0050] FIG. 7 shows a flow diagram of a method according to the invention.
DETAILED DESCRIPTION
[0051] FIG. 1 shows a basic representation of a device 1 according to the invention with a light source 3, an illumination optical unit 4, an imaging optical unit 10 and a recording device formed as a camera 16. The camera 16 captures the imaging, projected onto the image plane 15 by the imaging optical unit 10, of the substrate formed in the example shown as a photomask 8, which is arranged in the object plane 9 of the imaging optical unit 10. The photomask 8 can be moved in the z direction, which corresponds to the direction of the optical axis 2 of the device 1, and can thereby be positioned in the best focus of the imaging optical unit 10. The illumination optical unit 4 comprises optical elements 5 which project the light of the light source 3 as an image onto the object plane 9 of the imaging optical unit 10. Apart from the optical elements 11 which project an image of the structure of the photomask 8 onto the image plane 15, the imaging optical unit 10 comprises a diffractive element 13, which is arranged in the pupil 12 of the imaging optical unit 10. The diffractive element 13 comprises a multifocal grating 14, which brings about the effect that the imaging of the illuminated partial region of the photomask 8 is formed multiply on the image plane 15. The multifocal grating 14 is formed as a two-dimensional grating with a locally varying period, whereby the partial imagings are formed as defocused in relation to one another. In the case of high-aperture objectives, as are usually used in mask inspection microscopes for semiconductor lithography, the grating also comprises a distortion corresponding to a non-paraxial wavefront of the defocus. The partial imagings are formed on the CCD chip 17 of the camera 16, which in the example shown has a size of 25 μm×25 μm. In order to prevent crosstalk of the various partial imagings, the illumination optical unit 4 comprises a stop 6, which is arranged in a field plane 7 of the illumination optical unit 4 and determines the region on the photomask 8 that is illuminated by the light. The region is in this case chosen such that the extent of the partial imagings in the imaging does not exceed a size of 5 μm×5 μm. The rays of light represented in the figure after the diffractive element by variously broken lines show the path of rays of the partial imagings brought about by the diffractive elements 13. These respectively correspond to an imaging of the photomask 8 that was defocused from the best focus in the z direction, which is illustrated by corresponding photomasks 8.1, 8.2 represented by broken lines.
[0052] FIG. 2A shows a detail of a diffractive element 13, in which a line grating 18.1 with two different partial regions 19.x, 20.x is represented. The partial regions 19.x, 20.x together form the grating 18.1 with a grating constant d, which is defined as the distance from one first partial region 19.1 to the next first partial region 19.2. If the grating 18.1 is formed as an amplitude grating, the first partial regions 19.x are formed as absorbing regions and the second partial regions 20.x are formed as transmitting regions, or vice versa. The same applies to reflective gratings. In the case of a phase grating, the first partial regions 19.x and the second partial regions 20.x are transparent or reflect the radiation, the second partial regions 20.x bringing about a phase shift of the radiation passing through the grating 18.1 in comparison with the first partial regions 19.x.
[0053] FIG. 2B shows a view of a detail of a diffractive element 13, in which a line grating 18.1 with a grating displaced locally by the value Δx is represented. In this case, the local phase shift m in relation to the zero order is given by
Φ.sub.m(x,y)=2π*mΔx(x,y)/d
and the location dependency of Δx is given by
Δx=W*d(x.sup.2+y.sup.2)/(λR.sup.2),
where
[0054] λ=the wavelength of the light,
[0055] R=the radius of the grating aperture,
[0056] W=a constant that determines the distance of the partial imagings in relation to one another
[0057] d=the fundamental period, and
[0058] (x.sup.2+y.sup.2)/(R.sup.2)=a location-dependent distortion of the basic grating.
[0059] The effect of the first 19.x and second 20.x partial regions is as described under FIG. 2A.
[0060] FIG. 2C shows a diffractive element 13 with a line grating 18.1 given by way of example, with W=3*λ.
[0061] FIG. 3A shows a view of a detail of a photomask 8 in which a structure formed as a cross 21 is represented. The cross 21 has a dimension of 4 μm x 4 μm with a width of the bars of the cross of 0.5 μm, whereby, with nine partial imagings and a size of the image of 25 μm x 25 μm, crosstalk of the partial imagings is avoided.
[0062] FIG. 3B shows a view of a detail of a schematically represented two-dimensional line grating 18.2, which is formed in accordance with the following formula:
Φ.sub.m,n(x,y)=2π*[mΔx(x,y)+nΔy(x,y)]/d
where
Δx=W.sub.x*d(x.sup.2+y.sup.2)/(λR.sup.2)
and
Δy=W.sub.y*d(x.sup.2+y.sup.2)/(λR.sup.2)
[0063] FIG. 4A shows a view of a detail of an imaging 23 captured with the device on which nine partial imagings 24.x of the cross 21 described in FIG. 3A are represented. The defocusing between the partial imagings 24.x is in each case 0.5 k. The size of the imaging 23, which is predetermined by the CCD chip not shown, is 25 μm, so that there is no crosstalk. The intensities of the individual partial imagings 24.x are very different, which is brought about by the order of diffraction of the individual partial imagings 24.x. The higher the order of diffraction of the partial imaging 24.x, the less radiation energy is allocated to each partial imaging 24.x. This corresponds to the phenomenon of decreasing intensity of the diffraction maximums known from a single gap or double gap.
[0064] FIG. 4B shows a view of a detail of a normalized imaging 25 captured with the device, in which nine partial imagings 24.x of the cross 21 described in FIG. 3A are represented. The intensities of the partial imagings 24.x are normalized to the partial imaging 24.5 with the highest intensity. On the basis of the thus normalized intensities or energies, the contrast of the individual partial imagings 24.x is determined and the contrast value is plotted against the order of diffraction of the partial imaging 24.x.
[0065] FIG. 4C shows a diagram in which the contrast values 27.x determined on the basis of the partial imagings 24.x represented in FIG. 4B are represented in a contrast evaluation 26. In this case, the contrast values 27.x are plotted against the order of the diffraction of the respective partial imaging 24.x. A continuous contrast profile is determined from the individual contrast values with a quadratic fit 28. The partial imaging 24.5 in FIG. 4B is that with the highest contrast value 27.5 in FIG. 4C. In this case, the partial imaging 24.5 is identical to the partial imaging 24.x with the smallest order of diffraction, from which it can be deduced that the partial region of the photomask that is being considered is positioned in the best focus.
[0066] FIG. 5A shows a view of a detail of a normalized imaging 25 captured with the device, on which likewise nine partial imagings 24.x of the cross 21 described in FIG. 3A are represented.
[0067] FIG. 5B shows the diagram corresponding to the partial imagings 24.x, in which the contrast values 27.x plotted against the order of diffraction are represented in a contrast evaluation 26. As in FIG. 4C, a quadratic fit 28 has been placed over the individual contrast values 27.x. The maximum contrast value 27.3 is determined for the partial imaging 24.7. In this case, the partial region of the photomask that is being considered has been displaced from the best focus by plus 1λ. Thus, the evaluation of the contrast values 27.x allows the distance of the photomask from the best focus to be determined with just one captured imaging.
[0068] FIG. 6A shows a so-called focus stack 29 known from the prior art, that is to say nine imagings that have been recorded in each case with a different defocus in relation to one another, the displacement of the substrate expediently being carried out in the region of the expected best focus.
[0069] FIG. 6B shows an imaging 30 produced with a device represented in FIG. 1, which comprises nine partial imagings 31.x. These are divided into nine individual partial imagings 31.x during a post-processing of the imaging 30, as indicated in the figure by the arrow with the scissors symbol. In this way, a focus stack as defined by the prior art can be produced from just one captured imaging 30. This advantageously speeds up all processes that are based on such a focus stack, such as for example autofocusing of the photomask or the fixing of a process window for the imaging of a structure within predetermined tolerances.
[0070] FIG. 7 shows a flow diagram for a method according to the invention for measuring a substrate for semiconductor lithography with a measuring device, the measuring device comprising an imaging optical unit with a pupil.
[0071] In a first method step 41, a diffractive element 13 is arranged in the pupil 12 of the imaging optical unit 10.
[0072] In a second method step 42, the imaging of a partial region of the substrate 8 is captured.
[0073] In a third method step 43, the imaging is evaluated.
LIST OF REFERENCE SIGNS
[0074] 1 Apparatus [0075] 2 Optical axis [0076] 3 Light source [0077] 4 Illumination optical unit [0078] 5 Optical element (illumination) [0079] 6 Stop [0080] 7 Field plane (illumination optical unit) [0081] 8 Photomask [0082] 9 Object plane [0083] 10 Imaging optical unit [0084] 11 Optical element (imaging) [0085] 12 Pupil (imaging system) [0086] 13 Diffractive elements [0087] 14 Multifocal grating [0088] 15 Image plane [0089] 16 Camera [0090] 17 CCD chip [0091] 18.1,18.2 Line grating [0092] 19.1-19.x First partial region [0093] 20.1-20.x Second partial region [0094] 21 Cross (object structure) [0095] 23 Imaging [0096] 24.1-24.x Partial imaging [0097] 25 Normalized imaging [0098] 26 Contrast evaluation [0099] 27.1-27.x Contrast values [0100] 28 Fit of the curve [0101] 29 Focus stack (prior art) [0102] 30 Imaging [0103] 31.1-31.x Partial imaging [0104] 41 Method step 1 [0105] 42 Method step 2 [0106] 43 Method step 3
