WO2019233685A1

WO2019233685A1 - Computer-generated hologram (cgh), and method for characterising the surface form of an optical element

Info

Publication number: WO2019233685A1
Application number: PCT/EP2019/061533
Authority: WO
Inventors: Jochen Hetzler; Alexander Wolf
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2018-06-08
Filing date: 2019-05-06
Publication date: 2019-12-12
Also published as: DE102018209175B4; DE102018209175A1

Abstract

The invention relates to a computer-generated hologram (CGH) and a method for characterising the surface form of an optical element. In accordance with one aspect a CGH has a first region (121, 321, 421) having a useful structure (A) and a second region (122, 322, 422), which is spatially separate from said first region (121, 321, 421), the second region forming a monitoring region for detecting contamination forming on the CGH (120, 320, 420, 520).

Description

Computer generated hologram (CGH) as well

Method for characterizing the

Surface shape of an optical element

The present application claims the priority of German Patent Application DE 10 2018 209 175.7 filed on 8 June 2018. The content of this DE application is incorporated by reference into the present application text by "incorporation by reference".

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a computer-generated hologram (CGH) and to a method for characterizing the surface shape of an optical element.

State of the art

Microlithography is used to fabricate microstructured devices such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which has an illumination device and a projection objective. In this case, the image of a mask (= reticle) illuminated by means of the illumination device is projected onto a photosensitive layer (photoresist) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection objective (eg, a silicon wafer) to project the mask structure onto the photosensitive layer Transfer coating of the substrate. In projection lenses designed for the EUV sector, ie at wavelengths of, for example, about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process because of the lack of availability of suitable light-transmissive refractive materials. Typical projection lenses designed for EUV, for example as known from US 2016/0085061 A1, can have, for example, an image-side numerical aperture (NA) in the range of NA = 0.55 and form an object field (eg ring segment-shaped) in the image plane or wafer plane from.

Increasing the image-side numerical aperture (NA) typically involves an increase in the required mirror surfaces of the mirrors used in the projection exposure apparatus. This in turn means that in addition to the production and the examination of the surface shape of the mirror is a challenging challenge.

In particular, interferometric measuring methods using computer-generated holograms (CGH) are used for the high-precision testing of the mirrors. It is u.a. It is also known that in one and the same CGH, in addition to the functionality required for the actual test (ie the CGH structure designed in accordance with the mirror shape for shaping the wavefront mathematically corresponding to the test piece shape), at least one further "calibration functionality" for providing a To encode for calibration or error correction serving reference wavefront.

6 shows a schematic representation for explaining the possible structure of an interferometric test arrangement for testing a mirror.

According to FIG. 6, the illumination radiation produced by a light source (not shown) and emerging from the exit face of an optical waveguide 601 emerges as an input shaft 605 with a spherical wavefront, passes through a beam splitter 610 and subsequently strikes a complex coded one CGH 620. The CGH 620 generates in transmission in the example according to its complex coding from the input shaft 605 a total of four output waves, of which an output wave as a test wave on the surface of the test object in the form of a mirror 640 with one to the desired shape of the surface this mirror hits 640 matched wavefront. In addition, the CGH 620 generates three further output waves from the input shaft 605 in transmission, each of which encounters a respective reflective optical element 631, 632 and 633, respectively. With "635" is called a shutter. The CGH 620 also serves to superimpose the test wave reflected by the test object or mirror 640 as well as the reference waves reflected by the elements 631-633, which again strike the beam splitter 610 as convergent beams and from there in the direction of a CCD. Camera-oriented interferometer camera 660, wherein they pass through an eyepiece 650. The interferometer camera 660 detects an interferogram generated by the interfering waves, from which the actual shape of the optical surface of the test object 640 is determined via an evaluation device (not shown).

Knowledge of the grating profile of the CGH used in the interferometric measurement as well as the prediction of any changes in this grating profile is required in order to be able to clearly distinguish, if a phase deviation is detected in the interferometric measurement setup, if this phase deviation is due to existing errors on the CGH or on the CGH Area is due.

Here, in practice, the problem arises that changes in the grating profile may also be caused by contamination with e.g. Hydrocarbons can be caused, for example, in vacuum systems contaminants may be present for example by fats. For measurements in the atmosphere, the surface of the CGH may be e.g. form a water film of varying thickness.

FIG. 7 shows, in a merely schematic and greatly simplified representation, a CGH 720 with a contamination matrix located on a lattice structure 821. Layer 722. The homogeneous area contamination indicated in FIG. 7 also results in a change in the optically effective grating profile due to effective narrowing in the grating structure 721, even if the refractive indices of contamination layer 722 on the one hand and grating structure 721 or CGFI substrate match substantially located valleys.

The highly accurate determination of the grating profile of the CGFI presents an increasingly challenging challenge, since in present and future measurement arrangements a reproducibility on the order of 0.1 nm over a period of 1 year (corresponding to the maximum permissible change of the measurement result with identical test specimen) may be required. The problem of a high-precision characterization of the C G H gene rprof i Is is made difficult by the fact that a temporary removal of the CGFI from the interferometric test arrangement in view of the high reproducibility to be ensured and also for reasons of time is not desirable.

For the state of the art, reference is merely made by way of example to DE 10 2015 209 490 A1, DE 10 2012 217 800 A1 and US 2006/0274325.

SUMMARY OF THE INVENTION

Against the above background, it is an object of the present invention to provide a computer-generated hologram (CGH) and a method of characterizing the surface shape of an optical element which, in an interferometric test set, provides increased measurement accuracy while at least partially avoiding the above allow described problems.

This object is achieved by the computer-generated hologram (CGH) according to the features of independent patent claim 1 and the method according to the features of independent patent claim 11. According to one aspect of the invention, a computer-generated hologram (CGH), in particular for use in an interferometric test arrangement for testing an optical element of a microlithographic projection exposure apparatus, has:

- A first area having at least one Nutzstruktur; and

a second area spatially separated from this first area;

- This second area forms a control area for detecting a forming on the CGH contamination.

According to one embodiment, the second region has two spatially separated substructures, wherein these substructures differ from one another with regard to the contamination dependency of their phase effect on light incident on the CGH during operation.

According to one embodiment, these substructures differ with regard to the edge portion, which is defined for each of the substructures in each case as a quotient of the area structured with a flank angle of at least 80 ° and the total area occupied by the structure, by at least 30% relative to the larger one Value, from each other.

The invention further relates to a computer-generated hologram (CGH), in particular for use in an interferometric test arrangement for testing an optical element of a microlithographic projection exposure apparatus, with

- A first area having at least one Nutzstruktur; and

a second area spatially separated from this first area;

- wherein the second region has two spatially separated substructures, wherein these substructures with respect to the edge portion, which for each of the substructures in each case as a quotient of the surface with a flank angle of at least 80 ° structure occupied and total structural area is defined to differ from each other by at least 30%, based on the respective larger value.

In particular, the invention is based on the concept of providing a control region which is spatially separated from the actual useful structure for detection and monitoring of a contamination located on a CGH, which in turn allows the wavefront measurement carried out in the interferometric test arrangement to be traced back to the said contamination ,

In this case, the invention is based on the consideration that a sensitivity of the CGH or the measurement results provided by the CGH in the test arrangement can be varied with regard to contamination, based on the schematic representation of FIG. 7 discussed above the flank proportion or the flank area acting between mountains and valleys of the lattice structure and acting as a contact surface for the contamination is varied. By now providing two regions or substructures on a CGH according to the invention in said control region which differ from each other with regard to this flank surface, it is achieved that these regions react differently to a contamination forming on the CGH and change accordingly Phase effect enables the clear detection of the relevant contamination.

Specifically, the invention makes use of the circumstance that an area located in the control region in the form of a comparatively finer substructure with an overall greater total area of the flank areas located between mountains and valleys has a comparatively stronger due to the effectively larger area of attack for contamination Sensitivity of the phase effect to contamination has as a second region with a comparatively coarser substructure. It should be noted here that regardless of the different substructures present in the control area of the CGH according to the invention, there is a "matching basic functionality" for the control area or the different substructures, as it is ensured that the waves originating from the control area or said substructures remain unchanged be detected and measured in the interferometer. In embodiments of the invention, this "matching basic functionality" can be achieved in particular in that there is a match in the grating period or in one of the multiple-coded grating periods. In further embodiments, said "matching basic functionality" can also be achieved by designing the structure comprising the substructures in the control area such that at least a part of the incident light (in accordance with the Littrow condition) is bent back in the direction of incidence becomes.

In other words, the principal principal functionality of the CGH also remains for the differently contamination-sensitive substructures present in a separate control area, since these substructures merely have the purpose of reacting differently to contamination and a consideration in this respect in the interferometric test arrangement to obtain the results of the measurements.

Due to the existence of at least two different contamination-sensitive areas on the CGH according to the invention, it is also possible to detect changes in the measurement signal due to contamination caused by signal changes, e.g. may result from positional tolerances of the different optical elements to distinguish reliably.

According to one embodiment, the substructures successively merge with respect to the edge portion. According to one embodiment, the computer-generated hologram (CGH) is designed to generate, in addition to a test wave, a reference wave for interferometric superposition with the test wave after reflection of the reference wave at a reference mirror.

According to one embodiment, the computer-generated hologram (CGH) for providing the reference wave has a complex coding.

According to one embodiment, the second region has a basic structure which corresponds in its lattice period to the useful structure.

According to one embodiment, the second region has a basic structure designed as a Littrow grating.

The invention further relates to an interferometric test arrangement for testing an optical element, in particular an optical element of a microlithographic projection exposure apparatus, wherein the test arrangement comprises a computer-generated hologram and wherein a test at least a partial surface of the optical element by interferometric superimposition of one of these Computer-generated hologram on the optical element steered test shaft and a reference wave is feasible, wherein the computer-generated hologram is designed according to the features described above.

The invention further relates to a method for characterizing the surface form of an optical element, using an interferometric test arrangement, this test arrangement having a computer-generated hologram with the features described above.

According to one embodiment, the method comprises the steps:

Determining a model parameter based on an interferometric measurement based on the control area of the CGH; and Correction of interferometric measurement on the basis of the useful structure having the first portion of the CGH obtained measurement results based on this model parameter.

According to one embodiment, the determination of the model parameter takes place on the basis of a comparison of a wavefront calculated model-based for the control region of the CGH with measurement results obtained in the case of interferometric measurement based on the control region of the CGH in the test arrangement.

Further embodiments of the invention are described in the description and the dependent claims.

The invention will be explained in more detail with reference to embodiments shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Show it:

Figure 1-5 are schematic representations to illustrate exemplary

Embodiments of the present invention;

Figure 6 is a schematic representation of a possible construction of an interferometric test arrangement;

FIG. 7 shows a schematic illustration for explaining a possible problem occurring when using a CGH in an interferometric test arrangement;

Figure 8 is a schematic representation of a designed for operation in the EUV projection exposure system. DETAILED DESCRIPTION PREFERRED

EMBODIMENTS

FIG. 8 initially shows a schematic representation of an exemplary projection exposure apparatus designed for operation in the EUV, which has mirrors that can be tested using a method according to the invention.

According to FIG. 8, a lighting device in a projection exposure apparatus 10 designed for EUV has a field facet mirror 3 and a pupil facet mirror 4. On the field facet mirror 3, the light of a light source unit, which comprises a plasma light source 1 and a collector mirror 2, directed. In the light path after the pupil facet mirror 4, a first telescope mirror 5 and a second telescope mirror 6 are arranged. A deflecting mirror 7, which directs the radiation impinging on it onto an object field in the object plane of a projection objective comprising six mirrors 21-26, is arranged downstream of the light path. At the location of the object field, a reflective structure-carrying mask 31 is arranged on a mask table 30 which is imaged with the aid of the projection lens into an image plane in which a photosensitive layer (photoresist) -coated substrate 41 is located on a wafer table 40 ,

For example, in the case of the optical element tested in an interferometric test set-up (e.g., the structure already described with reference to Figure 6) and the optical element tested using a CGH according to the invention, it may be e.g. to act on any mirror of the projection exposure system 10.

Hereinafter, embodiments of the invention will be described with reference to the schematic figures in Figs. 1-5.

1 shows a schematic representation for explaining the basic structure and the mode of operation of a CGH 120 according to the invention. The CGH 120 has, as shown in a highly simplified manner in FIG. 1, in addition to a first area 121 comprising the actual useful structure A for generating the test wave in the interferometric measurement, a second area 122 spatially separate therefrom, which serves as a control area for detecting a contamination forming on the CGH 120.

In order to be able to detect the waves emanating from the control area 122 in the interferometric test arrangement likewise in the interferometric measurement, the grating period of the grating structure (basic structure B) located in the control area 122 coincides with that of the user structure A. However, in order to provide a contamination-sensitive functionality within the control region 122 according to the invention, at least two mutually different substructures B1 and B2 are present in said control region 122 as described below.

The essential effect of these substructures B1 and B2 is that, as a result of the different sensitivities of the substructures B1 and B2 compared to a contamination forming on the CGH 120 or the control region 122, an unambiguous detection of the relevant (eg, positional tolerances of the optical elements) Contamination is made possible.

In other words, the regions B1 and B2 react differently to contamination, which manifests itself in the schematically illustrated exemplary embodiment of FIG. 1 in that the comparatively finer substructure B2 forms an effectively larger surface compared to the substructure B1 and thus a correspondingly larger attack surface for contamination represents with the result that the dependence of the phase effect of the contamination in the actual interferometric measurement (as in the diagram shown in the lower section of FIG. 1) is different. As a quantitative criterion for describing the aforementioned surface variation between the substructures B1 and B2, it is possible in particular to use the effective flank surface formed for the relevant regions, whereby those wall regions with a slope greater than 80 ° can again be regarded as the "flank" , In this case, the flank surface thus defined between the substructures B1 and B2 preferably changes by at least 30% (based on the respective larger value).

2 shows a flow chart for explaining a possible procedure for correcting the measurement results obtained in the interferometric measurement on the basis of the useful structure A located in the first region 121 of the CGFI 120 with the aid of the control region 122 or the substructures B1 located there, B2 was done contamination detection.

According to FIG. 2, an interferometric measurement based on the actual useful structure A (step S210) is used to correct the obtained measurement results in step S270 on the basis of a model parameter P determined in step S260, which is, in particular, the layer thickness h of the contamination can act. The determination of the model parameter P again takes place on the basis of the interferometric measurement carried out on the basis of the control area or the substructures B1, B2 therewith (step S220), the measurement results obtained therefrom in step S230 with a wave front for area calculated model-based in step S240 B are compared. The model parameter P (S260) and the model-based calculated wavefront for the region A (S250) provide the correction of the measurement results in step S270.

3 and 4 show exemplary possible configurations of an interferometric measuring arrangement using a CGFI according to the invention (shown schematically in FIGS. 3b and 4b in each case and labeled "320" and "420", respectively). Fig. 3a shows the basic structure of a Fizeau interferometer. In this case, the illumination radiation generated by a light source (not shown) and emerging from the exit surface of an optical waveguide 301 passes through a beam splitter 310 as an input shaft 305 and passes through a collimator 335 and a Fizeau plate 325, whereupon the illumination radiation is directed to a CGH according to the invention 320 hits. The CGH 320 generates, via its payload structure A located in a first region, a test wave with wavefront matched to the sol I form of the surface of the test piece 340, whereas a second (control) region 322 of the CGH 320 has the substructures B1, B2 in Littrow Reflection is used. The incident on this Kontrol Ibereich 322 or Littrow grating light is thus reflected back into itself and interferes with the light reflected at the Fizeau plate 325 with the result that after reflection at the beam splitter 310 on the basis of an eyepiece 350 to the interferometer 360 radiation can be determined both for the substructures located in the control area of the CGH and for the actual useful structure of the CGH 320.

On the basis of the substructures B1, B2, in turn, as described above with reference to FIGS. 1 and 2, an unambiguous detection of the contamination is undertaken and used to correct the measurement results obtained on the basis of the user structure A. According to FIG. 3, the useful structure A generates only one wave in transmission and the region comprising the substructures B1, B2 only one wave in reflection. However, it remains ensured that the waves emanating from the control area or the substructures B1, B2 are also detected and measured in the interferometer setup

FIG. 4a shows a further possible configuration of an interferometric measurement setup, in which analogous or essentially functionally identical components with reference numbers increased by "100" are designated.

In contrast to FIG. 3 a, in the measuring setup of FIG. 4 a, the control area 422 on the CGH 420 according to the invention is not limited to (Littrow) Reflection but also in transmission or for diffraction in the direction of a reference mirror 431 used. The light reflected at the substructures B1, B2 of the control area of the CGH 420 thus interferes with light which has been diffracted in two transmissions by these substructures B1, B2 and reflected at the reference mirror 431. In order to generate the reference wave, the grating structure located on the CGH 420 (to the extent analogous to the conventional configuration described above with reference to FIG. 6) is encoded in a complex manner, so that in contrast to the embodiment of FIG. 3 a, the substructures B1 and B2 according to the invention formed in the example of Fig. 4a-4b doubly complex coded carrier grid.

5a-5b show schematic representations to illustrate a concrete embodiment of a CGH 520 according to the invention. In the upper part, FIG. 5a corresponds to an enlarged detail from FIG. 4a and shows two different, mutually remote regions or substructures of one in the middle part Control area 522 on the CGH 520. The complex coding comprises two basic gratings with grating periods of 478 lines / mm and 1373 lines / mm, respectively, taking advantage of this complex coding a Littrow reflection at 478 lines / mm and a double transmission at 1373 lines / mm takes place. For contamination detection, superimposition of the substructures according to the invention takes place on this basic grid, which is complexly coded over the entire control range of the CGH 520, as additional functionality, which in the present case is realized by superposition of a third grid having a period of 547 lines / mm or 774 lines / mm. As indicated in the middle part of FIG. 5a, the substructure shown on the right (corresponding to the period of 774 lines / mm) is comparatively finer in comparison with the substructure shown on the left (corresponding to the period of 547 lines / mm) and represents an effectively larger one Surface ready as an attack surface for contamination.

A quantitative consideration, starting from the schematic illustration in FIG. 5b, shows that comparatively low contaminations or contamination layer thicknesses can also be determined on the basis of the inventive concept are detectable. While the filling factor change is calculated as (b ₀ -bi) / p, the following applies to the layer thickness d = (b ₀ -bi) / 2, so that the following applies to the relationship between layer thickness and fill factor change: Layer thickness = fill factor change * p / 2

A contamination-related filling factor change by a factor of 0.005 thus corresponds to a contamination layer thickness of 3 nm assuming an average grating period of p = 1.2 pm, which corresponds to a signal in the size of 6 nm in the interferometric test arrangement.

While the invention has been described in terms of specific embodiments, numerous variations and alternative embodiments, e.g. by combination and / or exchange of features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are intended to be embraced by the present invention and that the scope of the invention is limited only in terms of the appended claims and their equivalents.

Claims

claims

Computer-generated hologram (CGH), in particular for use in an interferometric test arrangement for testing an optical element of a microlithographic projection exposure apparatus, having

A first region (121, 321, 421) having at least one useful structure (A); and

A second area (122, 322, 422) spatially separated from this first area (121, 321, 421), this second area having a control area for detecting a CGH (120, 320, 420, 520) forming on the CGH Contamination forms.

2. Computer-generated hologram (CGH) according to claim 1, characterized in that the second region (122, 322, 422) has two spatially separated substructures (B1, B2), wherein these substructures (B1, B2) with respect to Distinguish the contamination dependence of their phase effect on in operation on the CGH (120, 320, 420, 520) incident light from each other.

3. Computer generated hologram (CGH) according to claim 2, characterized in that these substructures (B1, B2) with respect to the edge portion, which for each of the substructures (B1, B2) in each case as a quotient of the with a flank angle of at least 80th ° structurally defined area and the total structural area is defined by at least 30%, based on the respective larger value, differ from each other.

A first region (121, 321, 421) having at least one useful structure (A); and A second area (122, 322, 422) spatially separated from said first area (121, 321, 421);

• wherein the second region (122, 322, 422) has two spatially separated substructures (B1, B2), wherein these substructures (B1, B2) with respect to the edge component, which for each of the substructures (B1, B2) respectively is defined as a quotient of the area occupied by a flank angle of at least 80 ° and the total area occupied by the structure, differing by at least 30% relative to the respective larger value.

5. Computer-generated Flologram (CGFI) according to claim 3 or 4, characterized in that the substructures (B1, B2) with respect to the edge portion successively merge into each other.

6. Computer-generated Flologram (CGFI) according to any one of the preceding claims, characterized in that this is adapted to generate in addition to a test wave a reference wave for interferometric superposition with the test wave after reflection of the reference wave at a reference mirror (431, 531) ,

7. Computer-generated Flologram (CGFI) according to claim 6, characterized in that it has a complex coding for providing the reference wave.

8. Computer-generated Flologram (CGFI) according to any one of claims 1 to 7, characterized in that the second region (122, 422) in its lattice period with the Nutzstruktur (A) matching basic structure (B).

9. Computer-generated Flologram (CGFI) according to one of claims 1 to 7, characterized in that the second region (322) has a configured as Littrow lattice basic structure.

10. Interferometric test arrangement for testing an optical element, in particular an optical element of a microlithographic projection exposure apparatus, wherein the test arrangement comprises a computer-generated hologram (CGH) and wherein an examination of at least one partial surface of the optical element by interferometric superimposition of one of this computer generated hologram on the optical element steered test shaft and a reference wave is feasible, the computer-generated hologram (CGH) is designed according to one of claims 1 to 9.

A method of characterizing the surface shape of an optical element using an interferometric tester according to claim 10.

12. The method according to claim 11, characterized in that it comprises the steps:

Determining a model parameter on the basis of an interferometric measurement based on the control range of the CGH (120, 320, 420, 520); and

Correction of interferometric measurement based on the first region (121, 321, 421) of the CGH (120, 320, 420, 520) having the useful structure (A) on the basis of this model parameter.

13. Method according to claim 12, characterized in that the determination of the model parameter is based on a comparison of a wavefront calculated model-based for the control area of the CGH (120, 320, 420, 520) with interferometric measurement on the basis of the control area of the CGH (120, 320, 420, 520) measured results obtained in the test arrangement.