The present invention relates to the field of determining at least one optical property of at least one deposition material used for a lithographic mask. In particular, the at least one deposition material can be used in a repair process of a lithographic mask having at least one clear defect.

2. Background of the invention

As a result of the constantly increasing integration density in the semiconductor indus try, lithographic or photolithographic masks have to project smaller and smaller struc tures onto a photosensitive layer, i.e., a photoresist dispensed on a wafer. In order to fulfil this demand, the exposure wavelength of lithographic masks has been shifted from the near ultraviolet across the mean ultraviolet and the deep ultraviolet (DUV) into the extreme ultraviolet (EUV) region of the electromagnetic spectrum. Presently, mainly a DUV wavelength of 193 nm is used for the exposure of the photoresist on wa fers. But the application of photolithographic exposure systems operating in the EUV wavelength range (approximately in a range of 10 nm to 15 nm) rapidly gains im portance. As a consequence, the manufacturing of photolithographic masks having ever smaller pattern elements is becoming more and more complex, and thus more and more expensive as well.

Typically, defect-free photolithographic masks cannot be fabricated with a reasonable yield due to their tiny pattern elements. The defects of photomasks must be corrected at the end of the manufacturing process whenever possible. In transmissive photomasks, the exposure radiation typically incidents on the mask symmetrically with respect to the optical axis of the mask. This means that a CRA (chief ray angle) is zero. The situa tion changes when using reflective lithographic masks. To separate the incident and the reflective radiation, radiation (e.g., EUV radiation) incidents on a reflective mask typi cally with a CRA in a range of 5 ⁰ to 8° with respect to the optical axis. As a consequence, three-dimensional (3D) effects play an important role when optimizing the operational behaviour of an EUV mask. Exemplary 3D effects are shadowing, dependence of the CD (critical dimension) on the feature orientation, best focus shift of different pattern ele ments, just to name a few. For example, shadowing effects of an absorbing layer can be minimized when reducing the layer thickness or layer height of the absorbing layer as much as possible. But, on the other hand, the function of the absorbing layer may not be compromised so that, for example, the optical contrast in the photoresist arranged on a wafer does not deteriorate.

For precisely determining an optimal layer thickness of an absorbing layer, it is neces- sary to know the optical properties of the material used for absorbing the EUV photons.

However, it is challenging to determine the optical properties of very thin layers, that is layers having a height or thickness as well as lateral dimensions in the two-digit na nometer range. In the article “Validation of optical constants in the EUV wavelength range”, Proc. SPIE 11147, Intern. Conf. on Extreme Ultraviolet Lithography, Sept. 26, 2019, https: / / doi.org/ 101117/ 12.2536644, the authors Q. Saadeh et al. describe using

EUV reflectometry for determining the optical constants for candidate materials for EUV photomask absorbers. Further, the authors N. Davidova et al. describe in the arti cle “Experimental approach to EUV imaging enhancement by mask absorber height op timization”, 29 ^th European Mask and Lithography Conference, edited by U.F.W. Beh- ringer and W. Maurer, Proc. of SPIE, 2013, Vol. 8886, p. 88860A1- 88860-A15, an EUV lithography performance improvement by optimizing and fine-tuning an EUV mask, in particular by optimizing a height of the absorbing layer. The authors of both publications describe using radiation of a synchrotron source for measuring the refrac tive index and the absorption constant of an absorbing layer. However, a synchrotron source is not a common metrology tool in the semiconductor industry. This means, the respective measurements must be executed externally, and they typically require large samples surfaces. The situation is getting worse if the absorbing material is not used for fabricating EUV masks but is used for correcting clear defects of a reflective photomask. In a photomask repair process the material composition of a deposition material cannot be controlled as precisely as in a photomask fabrication process. This may result in a variation of the composition of the deposited material. Consequently, the optical properties of the de posited material may also vary. Thus, it is even more important for repair processes of reflective lithographic masks to precisely know the optical properties of the deposition material in order to be able to reliably correct defects of reflective photomasks. It is therefore one objective of the present invention to provide a method and apparat uses for optimizing the determination of optical properties of deposition materials used for lithographic masks.

3. Summary of the invention

According to a first aspect of the invention, a method according to claim 1 and apparat uses according to claims 18 and 19 are provided for solving the above problem at least partly. In a first embodiment, a method for determining at least one optical property of at least one deposition material used for a for lithographic mask comprises the steps: (a) deter mining a height value of the at least one deposition material deposited on a substrate for each of at least three different deposition heights of the deposition material, wherein the at least three different deposition heights are in a nanoscale range; (b) de- termining a reflectivity value of the at least one deposition material for each of the at least three different deposition heights, wherein determining the reflectivity values comprises using photons generated by an optical inspection system; and (c) determin ing the at least one optical property of the at least one deposition material by adapting simulated reflectivity data to the measured reflectivity values for each of the at least three different deposition heights.

An optical inspection system may be any metrology system which may be used for in specting, reviewing, and/or verifying a lithographic mask and/or a wafer. In particular, the optical inspection system may use an aerial image measuring principle. Typically, numerical values of optical properties of very thin layers having specific ma terial compositions and or densities are not known with sufficient accuracy in the DUV and EUV wavelength range, or are not known at all, in order to optimize a layer thick- ness of an absorbing layer of a reflective photomask. The inventive method determines reflectivity values for three or more different deposition heights by using photons, pref erably photons of the actinic wavelength of the lithographic mask. Hence, the deter mined at least one optical property of the at least one deposition material is obtained at the condition, the lithographic mask is later subjected to. Further, it avoids the execu- tion of measurements by using metrology tools which are uncommon in the semicon ductor industry and may exclusively be based on metrology tools which are available in the semiconductor industry. Moreover, since photons can usually be controlled well in lithographic metrology tools, as for example optical inspection systems, small (lateral) sample sizes may be sufficient according to the aspects outlined herein. Furthermore, optical inspection systems are well established in the semiconductor industry and are more compact as for example synchrotron sources.

Moreover, the inventive method varies a numerical value of the at least one optical property to adapt simulation data to experimental values in order to determine the at least one optical property with a highest possible accuracy. Hence, it uses a combina tion of experiments and simulations for simultaneously optimizing both the accuracy with which the at least one optical property is determined and the effort necessary for obtaining the result. For example, if accuracy is needed only to a more limited extent, one or more values maybe used instead of three or more. For example, a height value of the at least one deposition material may be determined for one or more deposition heights of the depo sition material, wherein the at least three different deposition heights are in a na noscale range. A reflectivity value may be determined for each of the one or more dif- ferent deposition heights, wherein determining the reflectivity values comprises using photons generated by an optical inspection system. The at least one optical property may be determined by adapting simulated reflectivity data to the measured reflectivity value(s) for each of the one or more different deposition heights. For example, one or more values maybe used and at the same time higher accuracy may be achieved, if one or more optical properties of the at least one deposition mate rial is already known. Then the known optical property may be used for partly deter mining the simulated reflectivity data that is then adapted to the measured reflectivity value(s) to determine at least one other optical property.

Determining the height values of the at least one deposition material may comprise measuring the height values of the at least one deposition material. Determining the height values may also comprise determining the height values based on calibration data. For example, a height value may be determined based on a number of deposition steps and/or a deposition time (and calibration data linking the steps and/or time to a deposition height value).

Additionally, or alternatively, determining the reflectivity values of the at least one dep- osition material may comprise measuring the reflectivity values of the at least one dep osition material using photons generated by an optical inspection system.

The method described above may obtain or receive deposition height values from a first metrology tool. Further, the method may also obtain measured reflectivity values from an external metrology tool. Thus, the method may perform simulations and may deter mine the at least one optical property by comparing simulated reflectivity data and measured reflectivity values. However, it is also possible that the method performs both types of experiments and the simulations. Moreover, it is also conceivable that the method executes a first portion of the experiments and obtains experimental data for a second portion of the experiments or vice versa.

Determining the at least one optical property may comprise determining at least one of the at least one deposition material: a refractive index and an absorption constant. Apart from the composition of a material, its density may also have an impact on the at least one optical property. Further, the substrate on which a material is deposited may influence the at least one optical property of a thin layer of the deposited material. Moreover, presently available materials used for fabricating lithographic masks for the EUV wavelength range show a refractive index which is < 1. Further, at the moment, there are no materials available which are essentially optically transparent in the EUV wavelength range. This means that the absorption constant of presently known materi als is larger than zero.

Determining the at least one optical property may comprise determining the at least one optical property at an actinic wavelength of the lithographic mask.

This feature may ensure that the at least one optical property is measured under essen tially identical conditions the lithographic mask is operated later in a semiconductor factory (wafer fab). It is an advantage of the method described in this application that the at least one optical property is measured essentially under the conditions the litho graphic mask is operated at a later time.

Within this application, the term “essentially” means measuring results obtained at dif ferent sites when using state of the art metrology tools.

The deposition material may comprise an absorbing material. The absorbing material may have a relatively large absorption constant in the extreme ultraviolet wavelength range. Such a large absorption constant (k) may have a numerical value in the extreme ultraviolet wavelength range which is k > 0.05.

The substrate may comprise a substrate of a lithographic mask for the extreme ultravio let wavelength range having a multilayer structure and may further comprise deposit ing the at least one deposition material on the multilayer structure. It is also possible to use any substrate for depositing the deposition material. Further, it is also conceivable to deposit a specific layer on any substrate which provides an optical interface which is essentially identical to the surface on which the at least one deposition material is de posited (e.g., on a lithographic mask).

A top surface of the deposition heights of at least one deposition material may comprise an area of equal to or less than: 64 pm ² , preferably 16 pm ² , more preferred 4 pm ² , even more preferred 1 pm ² , and most preferred 0.5 pm ² .

It is a beneficial effect of the described method that the various deposition heights of the at least one deposition material may have a small area on which the reflectivity data can be measured. Thus, the effort of depositing the various deposition heights is rather low. In contrast to this, measuring the reflectivity data by means of a synchrotron source comprises depositing areas of the at least one deposition material which are ap- proximately larger by a factor of too. Further, small volumes of deposition materials maybe deposited by using a particle beam induced deposition process. Therefore, dep osition materials may be generated having a material composition which is very close to the deposition material used for repairing clear defects of lithographic masks. The nanoscale range may comprise deposition heights of the at least one deposition material of: < 200 nm, preferred < 150 nm, more preferred < 100 nm, and most pre ferred < 80 nm.

For thick absorbing layers (layer thickness > ioo·l) the reflectivity of an absorbing layer is exclusively determined by the absorption constant of the deposition material.

However, for smaller thickness values, particularly in the nanoscale range or nanoscale wavelength range, the reflectivity behavior of an absorbing layer also depends on the thickness or height of the absorbing layer. A portion of the incident radiation is re flected at the front surface and another portion is reflected at the rear surface of the ab- sorbing layer. This may result in an interference effect of the electromagnetic radiation reflected from the absorbing layer. As a consequence, a swing curve is superimposed over the reflectivity curve that generally decreases as a function of the height of the ab sorbing layer in the nanoscale range. The periodicity of the swing curve for a CRA in a range of 5 ⁰ to 8° is approximately given by: h « wherein h is the height or thickness of the absorbing layer, l stands for the wavelength (e.g., the actinic wavelength of the lithographic mask), and n denotes the refractive index of the absorbing layer. Please note that the above approximation does not include the effect of the off-axis incidence of the actinic radiation. In order to find a minimum height of an absorbing layer which fulfills a pre determined amount of absorption, the interference between the front and the rear sur face must be considered.

There is a second interference effect which impacts on the performance of a reflective photomask. The actinic radiation reflected from the multilayer structure (BF, bright field) interferes with the radiation reflected from the absorbing layer (DF, dark field). The image contrast generated by the pattern elements of a reflective mask is maximized if the two reflected contributions have a phase difference of i8o°. This requirement may be fulfilled if: h « ₄ ^A _ .

Both interference effects dependent on the refractive index n of the absorbing layer. In particular, the BD/DF contribution strongly depends on the refractive index. Hence, the refractive index of the material of the absorbing layer must be known with high pre- cision to calculate a suitable optical height of the absorbing layer. The aspects described herein harness these effects to precisely determine the optical properties of the deposi tion material.

The at least three different height values of the at least one deposition material may comprise at least 10, preferably at least 20, more preferred at least 30, and most pre ferred at least 40 different height values of the at least one deposition material.

The height values of the at least one deposition material may comprise a range of 1 nm to 150 nm, preferred 2 nm to 100 nm, more preferred 5 nm to 80 nm, and most pre- ferred 10 to 60 nm.

An overall height difference of the at least three different deposition heights maybe larger than a wavelength of the photons used for determining the reflectivity values. To determine the periodicity of the swing curve superposed on the reflectivity curve, it may be helpful that the height range spanned by the at least three deposition heights is larger than a wavelength of the photons used for measuring the reflectivity values. For example, the overall height difference may be understood as the difference between the largest height value and the smallest height value of the at least three different deposi- tion heights.

A height difference between the at least three different deposition heights may not have a periodicity of a half wavelength, or integer multiples thereof, of the photons used for determining the reflectivity values. If the depositions heights have such a periodicity, the period of the swing curve superposed on the reflectivity curve may not be detected reliably. In other examples, a height difference between two, in particular between two adjacent, deposition heights may not have a value of half the wavelength or integer multiples thereof.

The photons may comprise photons of the extreme ultraviolet wavelength range.

As already indicated above, by using photons having essentially the wavelength range of the photons which illuminate the lithographic mask in its operation mode, the at least one optical property can be measured with high precision.

The lithographic mask may comprise at least one clear defect. The deposition material may be used to repair the at least one clear defect. The lithographic mask may comprise a lithographic mask for the EUV wavelength range. In some examples, the method de- scribed herein may be implemented with the substrate of the mask whose defect is to be repaired. However, it is also possible to use it with a substrate, and then deposit the deposition material on the mask to be repaired with an (optimum) height calculated based on the determined one or more optical properties. The photons may comprise photons of an actinic wavelength of the lithographic mask.

The optical inspection system may comprise at least one of: an inspection system for the lithographic mask, an aerial image metrology system, an optical scanning micro scope, and a microscope. Each of these may use an actinic wavelength of the litho- graphic mask.

An inspection system for inspecting masks in the deep ultraviolet (DUV) wavelength range may use a laser source as a light source for inspecting lithographic masks. An in spection system for inspecting masks in the EUV wavelength range may use a plasma source as a light source for inspecting lithographic masks. The plasma maybe gener ated by using pulses of a laser system as an energy source of high density. An aerial image metrology system may use a scanner of a lithographic exposure system but replace the projection lens with a magnifying objective which images a small sec tion of the intensity distribution of the mask on a camera with high resolution. Determining the reflectivity values may comprise using an optical inspection system for the extreme ultraviolet (EUV) wavelength range. The optical inspection system for the EUV wavelength range maybe an aerial image metrology system for the EUV wave length range (EUV aerial image metrology system). The method may further comprise the step of determining a deposition height function by interpolating between the at least three measured deposition heights.

The method may further comprise the step of plotting the measured reflectivity values as a function of the height values of the at least one deposition material. The method may further comprise the step of plotting the measured reflectivity values as a function of the deposition height.

Measuring the deposition heights of the at least one deposition material may comprise using at least one of: a scanning probe microscope and a profilometer. The scanning probe microscope may be of any type of a scanning probe microscope.

The method may further comprise the step of depositing the at least one deposition ma terial for the at least three deposition heights on the substrate. The substrate may be prepared by depositing a layer providing an optical interface of the surface on which the at least three different height values of the at least one deposi tion material are deposited. The optical interface may be adapted essentially as an opti cal interface of the lithographic mask on which the deposition material may be depos ited for repairing a clear defect, for example.

Depositing the at least one deposition material may comprise performing a particle beam induced deposition process using at least one precursor gas. The at least one dep osition gas may comprise at least one element from the group of: a metal alkyl, a transi tion element alkyl, a main group alkyl, a metal carbonyl, a transition element carbonyl, a main group carbonyl, a metal alkoxide, a transition element alkoxide, a main group alkoxide, a metal complex, a transition element complex, a main group complex and an organic compound.

The metal alkyl, the transition element alkyl and the main group alkyl may comprise at least one element from the group of: cyclopentadienyl (Cp) trimethyl platinum (CpPtMe ₃ ), methylcyclopentadienyl (MeCp) trimethyl platinum (MeCpPtMe ₃ ), tetrame- thyltin (SnMe ₄ ), trimethylgallium (GaMe ₃ ), ferrocene (Cp ₂ Fe) and bisaryl chromium (Ar ₂ Cr).

The metal carbonyl, the transition element carbonyl and the main group carbonyl may comprise at least one element from the group of: chromium hexacarbonyl (Cr(CO) ₆ ), molybdenum hexacarbonyl (Mo(OO) _ό ), tungsten hexacarbonyl (W(CO) ₆ ), dicobalt octa- carbonyl (Co ₂ (CO)s), triruthenium docadecarbonyl (Ru ₃ (CO) ₁₂ ) and iron pentacarbonyl (Fe(CO) ₅ ).

The metal alkoxide, the transition element alkoxide and the main group alkoxide may comprise at least one element from the group of: tetraethyl orthosilicate (TEOS, Si(OC ₂ H ₅ ) ₄ ) and tetraisopropoxytitanium (Ti(OC ₃ H ₇ ) ₄ ). The metal halide, the transition element halide and the main group halide may comprise at least one element from the group of: tungsten hexafluoride (WF ₆ ), tungsten hexachloride (WCU), titanium hexa- chloride (TiCk), boron trichloride (BC1 ₃ ) and silicon tetrachloride (SiCl ₄ ).

The metal complex, the transition element complex and the main group complex may comprise at least one element from the group of: copper bis(hexafluoroacetylacetonate) (CU(C ₅ F ₆ H0 ₂ ) ₂ ) and dimethylgold trifluoroacetylacetonate (Me ₂ Au(C ₅ F ₃ H ₄ 0 ₂ )).

The organic compound may comprise at least one element from the group of: carbon monoxide (CO), carbon dioxide (C0 ₂ ), an aliphatic hydrocarbon, an aromatic hydrocar bon, a constituent of vacuum pump oils and a volatile organic compound.

Further, the particle beam induced deposition process may comprise at least one addi tive gas. The at least one additive gas may comprise at least one element from the group of: an oxidation agent, a halide, and a reducing agent. The oxidation agent may comprise at least one element from the group of: oxygen (0 ₂ ), ozone (0 ₃ ), water vapour (H ₂ 0), hydrogen peroxide (H ₂ 0 ₂ ), dinitrogen oxide (N ₂ 0), ni trogen oxide (NO), nitrogen dioxide (N0 ₂ ) and nitric acid (HN0 ₃ ). The halide may comprise at least one element from the group of: chlorine (Cl ₂ ), hydrochloric acid

(HC1), xenon difluoride (XeF ₂ ), hydrogen fluoride (HF), iodine (I ₂ ), hydrogen iodide (HI), bromine (Br ₂ ), hydrogen bromide (HBr), nitrosyl chloride (NOC1), phosphorus trichloride (PC1 ₃ ), phosphorus pentachloride (PC1 ₅ ) and phosphorus trifluoride (PF ₃ ). The reducing agent may comprise at least one element from the group of: hydrogen (H ₂ ), ammonia (NH ₃ ) and methane (CH ₄ ).

The particle beam may be an electron beam. The additional gas may support the depo sition process. In particular, the additional gas may help so that the at least one deposi tion material has a predetermined material composition.

The at least one precursor gas may comprise chromium hexacarbonyl (Cr(CC> ₆ )) and the additional gas may comprise nitride dioxide (N0 ₂ ).

The at least one deposition material may comprise chromium oxide (Cr _x O _y ), wherein x and y may vary within ranges of: o < x < 1.5 and o < y < 3.

The at least one deposition material may further comprise a carbon portion of < 30 atom-%, preferred < 20 atom-%, more preferred < 10 atom-%, and most preferred < 5 atom-%.

Determining the at least one optical property may comprise determining a deposition height function from the at least three different deposition heights and may further comprise simulating reflectivity data for the at least one deposition material as a depo sition height function of the at least one deposition material. The deposition height function may for example indicate a deposition height as a function of deposition steps.

Simulating reflectivity data may comprise taking a numerical value of the at least one optical property of the at least one deposition material (or of a similar material) of the literature as a starting value. Simulating reflectivity data may comprise calculating at least one reflectivity curve as a function of the deposition height (and, e.g., the refrac tive index and absorption constant), by using a simulation tool which numerically solves Maxwell’s equations. Dr.LITHO is an exemplary simulation tool which can be used for simulating reflectivity curves as a function of the height or thickness of an ab- sorbing layer. Examples of further simulation tools which can also be used are: PRO-

LITH, ProLE and HiperLith.

Adapting the simulated reflectivity data to the measured reflectivity values may com prise varying the at least one optical property of the at least one deposition material and may further comprise simulating the reflectivity data as a function of the deposi tion height. Varying the at least one optical property may comprise varying at least one numerical value of the at least one optical property.

Adapting the simulated reflectivity data to the measured reflectivity values may com- prise comparing simulated reflectivity data of various simulation runs having different numerical values of the at least one optical property with the measured reflectivity val ues.

The method defined above relies on experimental data and adapts simulated data or simulated curves to the experimental data by systematically varying at least one numer ical value of the at least one optical property.

Determining the at least one optical property may comprise extracting the at least one optical property from simulated reflectivity data having a best fit to the measured re- flectivity values.

The method defined above may further comprise the step of calculating an (optimal) deposition height of the at least one deposition material based on the determined at least one optical property in order to correct at least one clear defect of the lithographic mask. By precisely measuring the at least one optical property of the at least one deposition material, an absorbing layer may be fabricated which fulfils the predetermined absorp tion properties of the lithographic mask and at the same time minimizes 3D effects of the lithographic mask.

In some examples, the method may further comprise the step of depositing the at least one deposition material on a lithographic mask with the calculated (optimal) deposition height, to correct the at least one clear defect.

A computer program may have instructions to perform any of the method steps of the above discussed aspects when the computer program is executed on a computer sys tem.

Another aspect relates to a lithographic mask whose at least one defect is repaired ac cording to any of the method steps of the above-described aspects.

In a further embodiment, a computing apparatus maybe provided for determining at least one optical property of at least one deposition material used for a lithographic mask. The computing apparatus maybe operable to: (a) determine a height value of at least one deposition material for each of at least three different deposition heights, wherein the at least three different deposition heights are in a nanoscale range; (b) de termine a reflectivity value of the at least one deposition material for each of the at least three deposition heights, wherein the reflectivity values are measured by using photons generated by an optical inspection system; and (c) determine the at least one optical property of the at least one deposition material by adapting simulated reflectivity data to the obtained reflectivity values for the at least one three different deposition heights. In some examples, the computing apparatus may be operable to obtain a starting value for the at least one optical property of the at least one deposition material, before per forming step (c).

The computing apparatus may be further operable to at least two times simulate reflec tivity data for different numerical values of the at least one optical property as a func tion of a deposition height for photons having essentially the same wavelength distribu tion as the photons used for measuring the reflectivity values. The computing apparatus maybe further operable to compare the at least two simulated reflectivity data sets with the measured reflectivity values. Further, the computing apparatus may be operable to extract the at least one optical property from a simulated reflectivity data set having a best fit to the measured reflectivity values.

In another embodiment, an apparatus for determining at least one optical property of at least one deposition material for a lithographic mask, comprises: (a) means for de termining a height value of the at least one deposition material deposited on a substrate for each of at least three different deposition heights, wherein the at least three differ- ent deposition heights are in a nanoscale range; (b) means for measuring a reflectivity value of the at least one deposition material for each of the at least three different depo sition heights, wherein measuring the reflectivity values comprises using photons gen erated by an optical inspection system; and (c) means for determining the at least one optical property of the at least one deposition material by adapting simulated reflectiv- ity data to the measured reflectivity values for each of the at least three different depo sition heights.

The means for determining a height value may comprise means for measuring a height value, e.g., at least one of a scanning probe microscope, and a profilometer. The scan- ning probe microscope may comprise an AFM (atomic force microscope). The means for determining a height value may also comprise determining the height value based on calibration data. For example, a height value maybe determined based on a number of deposition steps and/or a deposition time (and calibration data linking the steps and/or time to a deposition height value).

An optical inspection system may comprise a light source. The light source may be a la ser light source, for example emitting in the DUV wavelength range, and/or a plasma light source, for example generating photons in the EUV wavelength range. The plasma light source may comprise a laser source for heating metal droplets. The metal droplets may comprise tin droplets and heating metal droplets may comprise vaporizing metal droplets.

The optical inspection system may further comprise at least one optical element opera ble to guide and focus light generated by the light source onto a lithographic mask and/or a wafer. The at least one optical element may have a numerical aperture which is large enough so that the at least one optical element can image the deposition mate rial having a predetermined lateral size. The at least one optical element may generally image the deposition material of predetermined lateral size if it fulfils Raleigh’s resolu- tion criterion. This means, if d is the smallest lateral dimension of the deposition mate rial and l is the wavelength of the optical inspection system, the minimum numerical aperture (NA) is given by: NA = 1.22 . This means, the lateral size d of the deposition material determines the minimum NA required by the optical inspection system to re solve the deposition material. To resolve the deposition material, it is necessary for the numerical aperture of the at least one optical element to be: NA > 1.22 - , preferred:

NA > 2 1.22 , and most preferred: NA > 4 1.22 ^ . For example, the smallest lat eral dimension may have values of 0.5 pm, 1 pm, 2 pm, 4 pm, or 8 pm (for example us ing quadratic, rectangular (circular), etc. deposition geometries with corresponding edge lengths (diameters), respectively). For l, a typical (for EUV) value may be an ac- tinic wavelength of 13.5 nm. Hence, the numerical aperture NA may be directly linked to a minimum lateral dimension of the deposition geometry for each wavelength, for example l = 13.5 nm. For example, for a lateral dimension of 1 pm (and l = 13.5 nm), the minimum numerical aperture NA may be 0,016, preferred 0,033, most preferred 0,066. For example, for a lateral dimension of 0,5 pm (and l = 13.5 nm), the minimum numerical aperture NA may be 0,033, preferred 0,066, most preferred 0,132. For other lateral dimensions as outlined herein (cf. above), minimum numerical aperture values can be obtained in the same manner (and are understood as part of the present disclo sure). The lithographic mask may be arranged on a mask stage. Instead of a mask, a substrate may be arranged on the mask stage which comprises at least one deposition material having at least three different deposition heights.

Moreover, the optical inspection system may, but needs not, comprise a further optical element, e.g., a projection lens, operable to focus light reflected from the lithographic mask into a detector. The detector may be a CCD (charge-coupled device) camera. The projection lens may be a magnifying projection lens. The magnification of the projec tion lens may be > 50, preferred > 100, more preferred > 200, and most preferred > 400. The optical inspection system may comprise an optical inspection system for the EUV wavelength range. In particular, the aerial image metrology system may comprise an aerial image metrology system for the extreme ultraviolet wavelength range (EUV aerial image metrology system).

The apparatus may be operable to perform any of the method steps of the aspects de scribed above. 4. Description of the drawings

In order to better understand the present invention and to appreciate its practical ap plications, the following figures are provided and referenced hereafter. It should be noted that the figures are given as examples only and in no way limit the scope of the invention.

Fig. 1 schematically shows an overview of the method applied to determine an opti cal property of an absorbing layer of a reflective lithographic mask; Fig. 2 schematically represents a top view of deposition material deposited on a substrate;

Fig. 3 schematically depicts measured height values as a function of the number of deposition steps used for depositing the deposition material;

Fig. 4 schematically illustrates the principle of an aerial image metrology system on the basis of a lithographic exposure system as an example of an optical in spection system; Fig. 5 schematically presents measured reflectivity values and simulated reflectivity data as a function of the deposition height of an absorbing layer;

Fig-6 depicts a flow diagram of a method for determining at least one optical prop erty of at least one deposition material used for a lithographic mask; and Fig. 7 schematically illustrates an apparatus which can be used for executing the method presented in Fig. l. 5. Detailed description of preferred embodiments

In the following, the present invention will be more fully described hereinafter with ref erence to the accompanying figures, in which exemplary embodiments of the invention are illustrated. However, the present invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and will convey the scope of the invention to persons skilled in the art.

In the following, the present invention is described by taking absorbing reflective litho- graphic masks as examples. However, the present invention can also be applied to phase-shifting reflective masks. Further, the method described in the present applica tion may also be used for determining optical properties of deposition materials of transmissive photomasks. The optical properties of deposition materials of transmis sive photomasks maybe determined by performing transmission and/or reflection measurements. The following detailed description is restricted to measuring reflectivity data.

Apart from photomasks, the presented method may be used for determining optical properties of very thin layers deposited on various kinds of optical elements as for ex- ample mirrors and/ or lenses. Generally, the present invention may be applied for de termining optical properties of materials used for forming very thin layers in which in terference effects must be considered.

Fig. 1 schematically provides an overview of the method presented in the present appli- cation. Fig. 1 contains two parts. The upper part 105 presents the experimental portion of the method too, and the lower part 195 illustrates the simulation portion of the de termination of optical properties of deposition materials. In the following, the various parts of the method too are described in detail. In a first step of the experimental part, deposition material is deposited on a substrate having various height values. Diagram 200 of Fig. 2 depicts a top view on a substrate 210 on which deposition material 250 is deposited having various height values. In the exam- 5 pie presented in Fig. 2, the deposited material 250 is deposited in form of a square hav ing dimensions of 1 pm x 1 pm. Fig. 2 presents an image recorded with a scanning elec tron microscope (SEM). The line 230 separates the substrate 210 and the deposited material 250. However, the deposition material 250 maybe deposited in various geo metric forms of its top surface. For example, the deposition material 250 may be depos it) ited in form of a rectangle, circle, or any geometric shape (not shown in Fig. 2). The top surface 270 of the deposition material 250 may have an area of 1 pm ² or even less.

It is an advantage of method 100 that the surface 270 of the deposition material 250 may have a size which is close to the dimensions of clear defects which may be repaired 15 by using the deposition material 250. This allows depositing deposition materials 250 on the substrate 210 by using a particle beam induced deposition process. Thus, the material compositions of the deposition material 250 used for determining the optical properties of the deposition material 250 and of the deposition material 250 used for repairing clear defects may be very similar. For example, the deposition material 250 0 may be deposited by using an EBID (electron beam induced deposition) process.

In the example presented in Fig. 2, the deposition material 250 is chromium oxide (Cr _x O _y ) with o < x < 1.5 and o < y < 3. But the deposition material 250 is not restricted to chromium oxide. Rather, various metal oxides may be deposited on the substrate 25 210. Further, apart from metal oxides, for example metal nitrides maybe also used as materials for forming thin absorbing layers. Generally, the described method by be ap plied for the determination of optical properties of coating materials or any materials having a thickness in the nanoscale range. 0 The substrate 210 may be the upper surface of a multilayer structure of a reflective mask. Alternatively, the substrate 210 may be any substrate, as for example a wafer. If necessary, a layer may be deposited on a substrate 210 which provides an optical inter face which is essentially identical to the surface on which the deposition material 250 is deposited for correcting a clear defect of a lithographic mask. The diagram 300 of Fig. 3 shows a number of height values 330, 350, 370 of the deposi tion material 250 presented as a function of the number of deposition steps applied for depositing the deposition material 250. The deposition heights 330, 350, 370 of the deposition material 250 linearly increase with the number of deposition steps. The dot ted curve 310 provides a normalization curve for comparing measured reflectivity val ues and reflectivity data simulated as a function of the deposition height (see Fig. 5 be low). Therefore, as symbolized in Fig. 1, curve 310 may be an input parameter to the simulation tool 600.

Further, as illustrated by reference sign 400 in Fig. 1, substrate 210 having deposition material 250 with various height values 330, 350, 370 is measured by an optical inspec tion system. In the example presented in Fig. 1, the optical inspection system is an aer ial image metrology system. Diagram 400 of Fig. 4 schematically illustrates the meas- uring principle of an aerial image metrology system presented in the right partial image 455 in comparison with a lithographic exposure system illustrated on the left partial image 405. An aerial image metrology system is a presently preferred example of an op tical inspection system. In a lithographic exposure system, electromagnetic radiation of the actinic wavelength is focused onto a lithographic mask. A projection optical unit or a projection lens images the radiation passing through the photomask with reduction (typically 1:4 or 1:5) on a wafer or on a photoresist distributed on the wafer with a large numerical aperture (NA _W ).

The right partial image 455 in Fig. 4 shows some components of an optical mask in- spection system 450 which uses the aerial image metrology system principle. The expo sure system of the scanner and of an aerial image metrology system 450 are substan tially identical. This means that the image generation, for example of pattern elements of a lithographic mask, is substantially the same for both systems. Thus, the aerial im age metrology system 450 images a segment of the optical intensity distribution of a mask such as is incident to a photoresist arranged on the wafer. Unlike in the case of a scanner, however, in the case of the aerial image metrology system 450 a lens images a small segment of the optical intensity distribution of a photomask with great magnifica tion on a CCD (charge-coupled device) camera. Diagram 455 of Fig. 4 shows an aerial image metrology system 450 for a transmissive photomask. An EUV aerial image metrology system adapts the aerial image measure ment principle to reflective lithographic masks (not shown in Fig. 4). By using an EUV aerial image metrology system as an example of an EUV optical inspection system, it becomes possible to measure the reflectivity values of the deposition material 250 hav ing deposition heights 330, 350, 370 in the two-digit nanometer range with very high resolution.

Diagram 500 of Fig. 5 presents the reflectivity values 530, 550, 570 measured by the EUV aerial image metrology system discussed in the context of Fig. 4. In Fig. 5, the re flectivity values 530, 550, 570 are depicted as bullets. The measured reflectivity values 530, 550, 570 cover height values 330, 350, 370 of several wavelengths of the EUV aer ial image metrology system or more general of the EUV optical inspection system. Fur ther, the height difference between different height values 330, 350, 370 is not equidis- tant but is randomly selected to avoid that the height difference accidentally coincides with the periodicity of the wavelength of the EUV photons of the optical inspection sys tem, for example the aerial image metrology system of Fig. 1. It can be clearly recog nized from Fig. 5 that the measured reflectivity values 530, 550, 570 do not strictly monotonically decrease or fall as a function of the deposition height 330, 350, 370 of the absorbing deposition material 250. This means the EUV aerial image metrology system can clearly detect the swing curve superposed on the reflectivity curve falling as a function of the deposition height 330, 350, 370 of the deposition material 250.

Again, with respect to Fig. 1, the lower partial image 195 of diagram too symbolizes the simulation portion of the method described in the present application. The dotted curve 310 of Fig. 3 maybe provided as an input parameter to the simulation tool 600. The curve 310 forms the x-axis for simulating the reflectivity of the deposition material 250 as a function of the deposition height 330, 350, 370. Further, input parameters to the simulation tool 600 maybe 3D information of the deposition material 250 to be simulated.

The simulation tool 695 numerically solves Maxwell’s equations for discrete deposition heights 330, 350, 370 or thicknesses of the deposition material 250. Typically, the re- fractive index n and the absorption constant k specify the optical properties of a deposi tion material 250. For performing a useful simulation of the reflectivity behavior of the deposition material 250, numerical values of n and k are required as starting or initial values. Numerical values of n and k of the literature are used for the deposition mate- rial 250. If no data are available for a specific deposition material, numerical values of n and k are used for a deposition material having a material composition which is close to deposition material 250 to be investigated.

In the example presented in Fig. 1, the simulation tool Dr.LITHO 695 is used for simu- lating the reflectivity behavior of the deposition material 250 as a function of the depo sition height 330, 350, 370. However, the simulation part 195 can be performed by any conventional simulation tool which numerically solves Maxwell’s equations. For exam ple, the simulation tool PROLITH is an alternative to the software package Dr.LITHO. To determine the optical properties n and k of the deposition material 250, the reflec tivity as a function of the deposition height 350 is repeatedly simulated, wherein the starting values of n and k are systematically varied as indicated by the reference sign 795 in Fig. 1. The simulated reflectivity curves for various n and k combinations are presented in Fig. 5 as reflectivity data 730, 750 and 770. As expected, the simulated re- flectivity data 730, 750, 770 predict a sharp drop of the reflectivity with increasing height of the deposition material 250. Further, the simulated reflectivity data 730, 750, 770 reveal a swing curve superposed on the reflectivity drop.

As indicated by the reference sign 800 in diagram too of Fig. 1, the various simulated reflectivity data 730, 750, 770 or reflectivity data sets 730, 750, 770 are compared with the measure reflectivity values 530, 550, 570. In the example presented in Fig. 5, the re flectivity data 750 best fit to the reflectivity values 530, 550, 570. The reflectivity data 750 are simulated with a refractive index of n=m and an absorption constant of k=k . Thus, as symbolized by the reference sign 900 in Fig. 1, the requested optical properties of the deposition material 250 are n=m and k=k . It is also possible to determine either the refractive index or the absorption constant if the other quantity of the deposition material 250 is already known with high accuracy. Fig. 6 presents a flow diagram 6oo of the method for determining at least one optical property of at least one deposition material 250 used for a lithographic mask. The method begins at 610. At step 620, a height value of the at least one deposition material 250 deposited on a substrate 210 is determined for each of at least three deposition heights 330, 350, 370 of the deposition material 250, wherein the at least three deposi tion heights 330, 350, 370 are in a nanoscale range. For example, the deposition height 330, 350, 370 may be measured by an AFM or it may be determined based on a number of deposition steps, e.g., as outlined with reference to Fig. 3. At step 630, a reflectivity value 530, 550, 570 of the at least one deposition material 250 is determined for each of the at least three different deposition heights 330, 350, 370, wherein the determination of the reflectivity values 530, 550, 570 comprises the usage of photons generated by an optical inspection system, in particular of photons of the EUV wavelength range. The reflectivity values 530, 550, 570 may be measured by using an AIMS™ EUV of the applicant.

At step 640, at least one optical property of the at least one deposition material 250 is determined by adapting simulated reflectivity data 730, 750, 770 to measured reflectiv ity values 530, 550, 570 for each of the at least three different deposition heights 330, 350, 370. This method step may be performed by a computing apparatus. Then the method ends at 650.

Finally, Fig. 7 schematically depicts an apparatus 1000 which can be used for perform ing the method schematically presented in Fig. 1. The apparatus 1000 may combine an AFM 1010, as an example of a scanning probe microscope, a computing apparatus 1030 and / or an EUV aerial image metrology system 1070, as an example of an optical in spection system. The computing apparatus 1030 may be connected to the AFM 1010 via the connection 1015. The computing apparatus 1030 may control the AFM 1010 via the connection 1015 and may obtain measuring data, in particular deposition heights 330, 350, 370 from the AFM 1010.

The computing apparatus 1030 may comprise a non-volatile memory 1040 for storing a simulation tool 1050. The simulation tool 1050 maybe the simulation tool 695 of Fig. 1. Further, the computing apparatus 1030 may comprise a processor 1060 which operable to execute instructions of the simulation tool 1050. The hardware implementation of the processor 1060 may be adapted to the requirements of the simulation tool 1050.

The computing apparatus 1030 may be connected to the EUV aerial image metrology system 1070 by means of the connection 1075. The computing apparatus 1030 may control the EUV aerial image metrology system 1070 via the connection 1075. Further, the computing apparatus 1030 may obtain measuring data from the EUV aerial image metrology system 1070 via the connection 1075. In particular, the computing apparatus 1030 may receive reflectivity values 530, 550, 570 from the EUV aerial image metrology system 1070.

The apparatus 1000 may further have an interface 1090. The computing apparatus 1030 of the apparatus 1000 may receive experimental data via the connection 1095 from the interface 1090. The experimental data received by the computing apparatus 1030 via the interface 1090 may comprise depositions heights 330, 350, 370 of the dep osition material 250 and/or reflectivity values 530, 550, 570 of the deposition material