DE102021200109A1 - Method for the areal determination of a map of at least one structure parameter of a structured surface of a diffractive optical element - Google Patents

Abstract

The invention relates to a method for the areal, in particular simultaneous determination of a map of at least one structural parameter of a structured surface (1) of a diffractive optical element, in particular a computer-generated hologram, CGH (2), or a wafer, with one or more interferograms being measured by means of an interferometric measuring method (3) of the structured surface (1) are recorded, from which phase values of the measuring light beams (16a, 20) emanating from the CGH (2) or wafer are determined in the zeroth diffraction order, and from the at least one interferogram (3) of the structured surface (1) the map of the at least one structural parameter is determined.

Description

The invention relates to a method for the areal, in particular simultaneous determination of a map of at least one structural parameter of a structured surface of a diffractive optical element, in particular a computer-generated hologram, CGH, or a wafer.

The invention also relates to the use of a map of at least one structural parameter determined areally for a CGH that has actually been manufactured, for calculating a correction wavefront.

Scatterometric (diffraction) methods are often used to determine profile parameters or structural parameters of a CGH. With these methods, the intensity of the light of diffraction orders is measured in transmission or reflection. Furthermore, a model for the structured surface or the profile of the CGH is set up, which describes the profile via structure parameters. Typical structure parameters are an etching depth, a duty cycle or a flank angle. Light intensities are now calculated for this modeled profile, for example using RCWA (rigorous coupled wave analysis). The measured intensities are compared to the calculated intensities. The structural parameters with the best match represent the measurement result. It is therefore a model-based measurement. Various systems are known from practice that work according to this principle and are used to determine profile parameters for structured wafers. In this regard, see Morten H. Madsen and Poul. E Hansen; Surface Topography: Metrology and Properties 4 023003 2016. Intensities are measured for several wavelengths, polarizations and, in some cases, angles. The large number of different measured intensities enables a stable determination of the profile parameters. A disadvantage of many of these systems is that they only measure at one point or measurement point at a time. For the correction of the test wavefront of a CGH, however, the structural parameters are required over the entire surface of the CGH. For a two-dimensional, in particular full-surface, map, many individual points must be measured with these methods and combined to form a map, which is very time-consuming and can therefore typically only be carried out for a few points.

It is known to use a so-called diffraction measurement stand for the areal, in particular full-area, and in particular simultaneous determination of the profile parameters of a CGH. The transmission (more precisely the intensity of the first diffraction order of the radiation in transmission) of a CGH is determined in a spatially resolved manner at different wavelengths and polarizations of the input radiation. This allows the profile parameters to be determined over the entire CGH area. For this, on the U.S. 2006/0274325 A1 and the DE 10 2018 200 568 A1 referred.

The optical methods mentioned above use the light intensities of the diffraction orders as the measurement signal. The accuracy of the measurement is thus limited by the accuracy with which light intensities can be determined. The spatially resolved measurement on a diffraction measuring system uses CCD or CMOS sensors, the accuracy of which is limited to around 1%. With complex calibrations, about 0.1% can also be achieved there. More precise sensors such as photodiodes or photomultipliers, on the other hand, do not allow spatially resolved measurements.

Furthermore, reflections in the optics used lead directly to errors in the measurement signal and thus to incorrectly determined profile parameters or structural parameters. It is therefore disadvantageous in the prior art that only measuring means based on the principle of the diffraction measuring stand are available for a spatially resolved measurement and their measuring accuracy for structural parameters of CGHs or wafers is limited.

There are a number of alternative, non-optical methods for measuring profile parameters, such as Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), as well as X-ray diffraction. So far, however, none of these methods has made it possible to carry out an area-based, in particular full-area, measurement with many measurement points on the area to be measured in an acceptable measurement time.

The present invention is therefore based on the object of specifying a method and a use of the type mentioned at the outset which avoid the disadvantages of the prior art and in particular have greater accuracy.

With regard to the method, this object is achieved according to the invention by claim 1 .

With regard to the use, this object is achieved by claim 12.

According to the invention, a method for the areal, in particular simultaneous or simultaneous, determination of a map of at least one structure parameter of a structured surface of a diffractive optical element, in particular a computer-generated hologram, CGH, or of a wafer, wherein one or more interferograms of the structured surface are recorded by means of an interferometric measuring method, from which phase values of the measuring light beams emitted and/or manipulated by the diffractive optical element, in particular the CGH or wafer, are determined in the zeroth diffraction order, and wherein the map of the at least one structure parameter is determined from the at least one interferogram of the structured surface.

The basic principle of the invention consists in the interferometric measurement of the phase effect generated by the CGH in the zeroth order of diffraction, in particular at different wavelengths, polarizations and/or angles both in transmission and in reflection. In contrast to the known methods, it is not the intensity of the radiation in the zeroth diffraction order that is measured, but the phase position. The interferometric determination of profile parameters or structural parameters (for example an etching depth) of a diffractive optical element, in particular a CGH or wafer, enables the profile parameters to be determined particularly precisely. It is intended to measure the phase of the zeroth diffraction order of the radiation emanating from the CGH at different wavelengths, polarizations and/or angles in transmission and in reflection.

$${\mathrm{\psi }}_0=\text{a tan}\left(\frac{\text{A}\cdot \text{D}\cdot \text{sin}\left(\mathrm{\phi }\right)}{\text{A}\cdot \left(1-\text{D}\right)+\text{A}\cdot \text{D}\cdot \text{cos}\left(\mathrm{\phi }\right)}\right)\cdot \frac{\mathrm{\lambda }}{2\mathrm{\pi }}$$

The basic principle is explained below using the example of measuring a CGH under perpendicular incidence of light in a reflection configuration. For this purpose, the scalar approximation for the calculation of the intensities and phases is used in the following for simplification and clarification instead of, for example, the RCWA already mentioned at the beginning. This has the advantage that a closed formula for determining the intensity and the phase from the structure parameters can be given here. In the present case, a CGH, which is designed as a simple line grating, is assumed to be the diffractive optical element. The grating profile is modeled with only two parameters, the etch depth and the duty cycle. This results in the intensity and the phase of the zeroth diffraction order as follows: $${\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="DE102021200109A1\_0026.png,DE102021200109A1\_0028.png"\; state="\{\{state\}\}">n</figure-callout>}}_0={\text{A}}^2\cdot {\left(1-\text{D}\right)}^2+{\text{A}}^2{\text{<figure-callout id="2" label="D" filenames="DE102021200109A1\_0023.png,DE102021200109A1\_0024.png" state="{{state}}">D</figure-callout>}}^2+2{\text{A}}^2\text{D}\cdot \left(1-\text{D}\right)\text{cos}\left(\mathrm{\phi }\right)$$

$${\mathrm{\psi }}_0=\text{a tan}\left(\frac{\text{A}\cdot \text{D}\cdot \text{sin}\left(\mathrm{\phi }\right)}{\text{A}\cdot \left(1-\text{D}\right)+\text{A}\cdot \text{D}\cdot \text{cos}\left(\mathrm{\phi }\right)}\right)\cdot \frac{\mathrm{<figure-callout\; id="2"\; label="\lambda "\; filenames="DE102021200109A1\_0023.png,DE102021200109A1\_0024.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}{2\mathrm{\pi }}$$

• Ψ₀ die Phasenänderung der reflektierten Prüfwelle, also der nullten Beugungsordnung;
• η₀ die Effizienz (Beugungswirkungsgrad) der nullten Beugungsordnung;
• A der Reflexionskoeffizient der reflektierenden Oberfläche;
• D das Tastverhältnis der Gitterstruktur; und
• φ der Phasenhub des diffraktiven optischen Elements, insbesondere des CGHs, wobei für den Fall in Reflexionskonfiguration $\mathrm{\phi }=4\mathrm{\pi }\cdot \frac{\text{T}}{\mathrm{\lambda }}$
  
  gilt mit der Ätztiefe T und der verwendeten Prüfwellenlänge λ.

where:

• Ψ ₀ the phase change of the reflected test wave, i.e. the zeroth diffraction order;
• η ₀ the efficiency (diffraction efficiency) of the zero diffraction order;
• A is the reflection coefficient of the reflecting surface;
• D is the duty cycle of the grating structure; and
• φ is the phase deviation of the diffractive optical element, in particular of the CGH, where for the case in reflection configuration $\mathrm{\phi }=4\mathrm{\pi }\cdot \frac{\text{T}}{\mathrm{\lambda }}$
  
  applies to the etching depth T and the test wavelength λ used.

In this regard, reference is made to Ping Zhou and James H. Burge; APPLIED OPTICS 2007 Vol. 46, No. 5 referenced in 2007.

If one now measures the phase of the wave reflected at the CGH at different wavelengths by switching the test wavelength of the interferometer, one obtains different phase values for the different wavelengths, in particular since the phase change Ψ ₀ through the CGH changes with the wavelength according to formula 2. If one now has at least two phase changes Ψ ₀ (λ ₁ ) and Ψ ₀ (λ ₂ ) through the CGH at one location of the CGH, the etching depth suitable for these phase changes and the duty cycle suitable for these phase changes can be determined using formula 2. If one uses more, ie at least two different, wavelengths λ1, λ2, then one has more phase changes. This ensures that as a result, for example, the etching depth and the pulse duty factor can be determined, for which the sum of the squared deviations of the calculated phase changes and the measured phase changes is minimal. This assignment of measured values to suitable structural parameters is referred to as inverse calculation, with methods such as RCWA being used to calculate the values for higher accuracy.

The structural parameters determined in this way do not appear to be unambiguous at first. The phase deviation Φ through the etching depth can only be determined modulo 2π. The etching depth typically moves in a sufficiently small band around a predefinable setpoint etching depth of the etching process. Only one value for the etching depth is therefore realistic in practice. If necessary, the etching depth can also be determined at some support points with another sensor (e.g. by means of atomic force microscopy or AFM).

In the case of an areal or areal, in particular spatially resolved, interferometric measurement, the relative phase of different locations and not the absolute lute phase measured. i.e. the absolute phases are initially not available as an input signal for the inverse calculation.

It can be advantageous if at least one reference area with structures with a known phase effect is applied to the surface of the diffractive optical element, in particular the CGH or the wafer, and by forming a difference between the phase values to be determined of the structured surface and the known phase values of the reference range an unknown global phase of the at least one interferogram is eliminated.

An area with a known phase can now advantageously be applied to the CGH or wafer. This reference area cannot be etched, for example (D=0, T=0 and thus Ψ ₀ =0). The difference between the measured, relative phases and the measured phase of the reference area can then be viewed as absolute phase values for Ψ ₀ at this point or area, which are included in the inverse calculation as input signals. Absolute phase values for Ψ ₀ can thus be formed from the measured relative phase values of the at least one interferogram of the structured surface by means of the reference area. As with the intensity-based methods, the profile parameters or structure parameters that best match the measured values, phase values now adjusted for the global phase determined, can now be determined via the inverse calculation.

Several interferograms of the structured surface can be created using different light wavelengths λ _i ,λ _j of the measuring light beams, where the following applies to the light wavelengths λ _i ,λ _j involved: $2\cdot \frac{{\mathrm{\lambda }}_{\text{i}}-{\mathrm{\lambda }}_{\text{j}}}{{\mathrm{\lambda }}_{\text{i}}+{\mathrm{\lambda }}_{\text{j}}}>\mathrm{0.001.}$

As a result, a number of independent measurement signals can be generated and the structure parameters can thus be determined more precisely and more robustly, that is to say more independently of other manufacturing errors. The interferometer to be used can be designed in such a way that a measurement with different wavelengths of the test wave or the measuring light beams is possible by switching. For example, a Fizeau interferometer, a Twyman-Green interferometer, or other types of interferometers can be used.

It is advantageous if several interferograms of the structured surface are created using different polarization directions of linearly polarized measuring light beams, where the following applies to the angles α _i ,α _j that define the respective polarization direction: $2\cdot \frac{{\mathrm{a}}_{\text{i}}-{\mathrm{a}}_{\text{j}}}{{\mathrm{a}}_{\text{i}}+{\mathrm{a}}_{\text{j}}}>\mathrm{0.001.}$

This also allows additional independent measurement signals to be added. A polarizer can be inserted into the measuring beam path between the interferometer and CGH or wafer, which can be rotated in discrete steps.

gilt: $$2\cdot \frac{\frac{E_{xi}}{\left|{\overrightarrow{E}}_i\right|}-\frac{E_{xj}}{\left|{\overrightarrow{E}}_j\right|}}{\frac{E_{xi}}{\left|{\overrightarrow{E}}_i\right|}+\frac{E_{xj}}{\left|{\overrightarrow{E}}_j\right|}}>\mathrm{0,001}$$

und/oder $$2\cdot \frac{\frac{E_{yi}}{\left|{\overrightarrow{E}}_i\right|}-\frac{E_{yj}}{\left|{\overrightarrow{E}}_j\right|}}{\frac{E_{yi}}{\left|{\overrightarrow{E}}_i\right|}+\frac{E_{yj}}{\left|{\overrightarrow{E}}_j\right|}}>\mathrm{0,001}$$

und/oder $$2\cdot \frac{{\phi }_i-{\phi }_j}{\left|{\phi }_i\right|+\left|{\phi }_j\right|}>\mathrm{0,001.}$$

Alternatively, it is provided that several interferograms of the structured surface are created using different polarization directions of measuring light beams polarized as desired, for example linearly, circularly or elliptically, with Jones vectors defining the respective polarization $${\overrightarrow{E}}_i=\left(\begin{array}{c}{E}_{xi}\\ E_{yi}\ast e^{i{\phi }_i}\end{array}\right)\text{and}{\overrightarrow{E}}_j=\left(\begin{array}{c}{E}_{xj}\\ E_{yj}\ast e^{i{\phi }_j}\end{array}\right)$$

is applicable: $$2\cdot \frac{\frac{E_{xi}}{\left|{\overrightarrow{E}}_i\right|}-\frac{E_{xj}}{\left|{\overrightarrow{E}}_j\right|}}{\frac{E_{xi}}{\left|{\overrightarrow{E}}_i\right|}+\frac{E_{xj}}{\left|{\overrightarrow{E}}_j\right|}}>0.001$$

and or $$2\cdot \frac{\frac{E_{yi}}{\left|{\overrightarrow{E}}_i\right|}-\frac{E_{yj}}{\left|{\overrightarrow{E}}_j\right|}}{\frac{E_{yi}}{\left|{\overrightarrow{E}}_i\right|}+\frac{E_{yj}}{\left|{\overrightarrow{E}}_j\right|}}>0.001$$

and or $$2\cdot \frac{{\phi }_i-{\phi }_j}{\left|{\phi }_i\right|+\left|{\phi }_j\right|}>\mathrm{0.001.}$$

It is advantageous if several interferograms of the structured surface are created at different angles of the mean surface normal of the structured surface to an incident measuring light beam, with the participating
angle $${\mathrm{\theta }}_{\text{i}},{\mathrm{\theta }}_{\text{j}}\text{is applicable}:2\cdot \frac{{\mathrm{\theta }}_{\text{i}}-{\mathrm{\theta }}_{\text{j}}}{{\mathrm{\theta }}_{\text{i}}+{\mathrm{\theta }}_{\text{j}}}>\mathrm{0.001.}$$

This also allows additional independent measurement signals to be added. For lines with low frequency, angle changes and waves behave in the scalar approximation length changes very similar in the scalar approximation. However, a change in angle in a measuring arrangement can sometimes be achieved more easily. A commercially available interferometer is therefore sufficient and no interferometer with several different adjustable wavelengths is required. Several different measurement channels can be created using an interferometer with only one wavelength by changing the angle and/or changing the directions of polarization.

In an advantageous development of the method according to the invention, it can be provided that the at least one interferogram of the structured surface is created in transmission in double passage and/or in reflection and/or in double reflection via a mirror. As a result, further independent measurement signals can also be added or a simple measurement setup can be implemented or a setup with good sensitivity for a specific structural parameter.

According to one embodiment, it is provided that a model or grating profile of the structured surface is created or modeled for each partial area of the surface for which a phase value was measured in the at least one interferogram, using the at least one structure parameter, with a phase value of the zeroth diffraction order for this Partial areas are calculated using the model or the modeled grating profile by solving Maxwell's equations, after which that value of the structure parameter or structural parameters is determined for the respective partial area in which the calculated phase value or the calculated phase values match the measured phase value or the measured phase values as best as possible corresponds and in this way the map of the structured surface comprising the partial regions is determined with values of the at least one specific structure parameter.

The at least one structure parameter or profile parameter can include an etching depth, a pulse duty factor, a trenching radius and/or a flank angle of structures of the diffractive optical element, in particular of the CGH or of the wafer.

Such structural parameters or profile parameters are important for the characterization of a diffractive optical element, in particular a CGH, and their precise knowledge is of great advantage. The optical properties of the CGH such as intensity and phase of transmitted and reflected diffraction orders as well as the scattering properties are influenced by the structural parameters such as etching depth, trench radius, edge offset and flank angle. Knowledge of the local expression of said parameters. The use of these parameters as structure parameters enables the parameters that can be controlled by the surface structuring processes usually used, such as etching processes, to be recorded. As a result, quality control of precisely those parameters and thus of the manufacturing processes can be achieved.

It is advantageous if a phase effect of a substrate, in particular a pass of the diffractive optical element or wafer, is taken into account when determining the at least one structural parameter.

Since the measured phase distribution is also influenced by the phase effect or the fit or the fit error of the CGH substrate, this measure can prevent the reconstructed profile parameters from being significantly falsified. The shape deviation, in particular of an optical surface of an optical element, is referred to as a yoke or fit error.

It is advantageous if at least one reference interferogram of the unstructured substrate of the diffractive optical element, in particular of the CGH or wafer, is determined, which is subtracted from the at least one interferogram of the structured surface.

1. 1. Das unstrukturierte Substrat wird vermessen. Bei einer Messung in Reflexionskonfiguration kann bereits eine Messung ausreichen. Bei einer Messung in Transmissionskonfiguration sollten aufgrund der Brechungsindexinhomogenität alle Wellenlängen gemessen werden. Damit erhält man die beiden Wellenfronten W_λ1,unstrukt und W_λ2,unstrukt.
2. 2. Das strukturierte CGH wird bei zwei unterschiedlichen Wellenlängen vermessen. Dabei erhält man die zwei Wellenfronten W_λ1,strukt und W_λ2,strukt.
3. 3. Danach werden Differenzen Δ_λ1 = W_λ1,strukt - W_λ1,unstrukt und Δ_λ2 = W_λ2,strukt - W_λ2,unstrukt gebildet.

The yoke of the unstructured substrate is determined before structuring. The substrate pass is then subtracted from all measurements of the structured CGH. The remaining wavefront then comes from the profiles. In the simple case already described (with at least two different wavelengths λ1, λ2 for two different parameters, for example etching depth and duty cycle), the following procedure can be followed (the procedure described below for taking into account the phase effect of the substrate is referred to as method 1 in this case):

1. 1. The unstructured substrate is measured. When measuring in reflection configuration, one measurement can already be sufficient. When measuring in transmission configuration, all wavelengths should be measured due to the refractive index inhomogeneity. This gives the two wave fronts W _{λ1,unstructured} and W _{λ2,unstructured} .
2. 2. The structured CGH is measured at two different wavelengths. Included one obtains the two wavefronts W _λ1,struct and W _λ2,struct .
3. 3. Then differences Δ _λ1 = W _λ1,struct - W _λ1,unstruct and Δ _λ2 = W _λ2,struct - W _{λ2,unstruct are} formed.

This means that there are now two measured variables that are linked to the two unknown profile parameters. The unknown profile parameters can now be determined by the inverse calculation.

In the above method for considering the substrate pass, a difference between the substrate pass and the pass of the structured CGH substrate can lead to a measurement error of the profile parameters. This could occur, for example, due to differences in the recording of the substrate or a coating of the CGH after structuring of the CGH.

In an advantageous development of the invention, it can be provided that at least one reference interferogram of the structured surface is created at a reference wavelength, a reference angle and a reference polarization, with comparisons or differences between the phase values of the at least one interferogram of the structured surface at a wavelength different from the reference wavelength and/or an angle different from the reference angle and/or a polarization different from the reference polarization compared to the at least one reference interferogram of the structured surface can be taken into account when determining the at least one structure parameter.

This procedure allows the profile parameters to be determined without having previously measured the unstructured substrate pass.

1. 1. Messung des CGH bei drei verschiedenen Wellenlängen, wodurch man die drei Passen W_λ1,strukt, W_λ2,strukt und W_λref,strukt erhält.
2. 2. Bilden der Differenzen Δ_λ1 = W_λ1,strukt - W_λref,strukt und Δ_λ2 = W_λ2,strukt - W_λref,strukt.

For this purpose, measurements can first be carried out at a reference wavelength. Then the measurements of the other wavelengths are related to this reference measurement. Thus, only the differences between the measurement of the other wavelengths and the reference wavelength are considered. Only the respective differences in the phase of the reference wavelength to the measurement wavelength then also go into the inverse calculation. The pass of the substrate is included in the measurement for all wavelengths. Another advantage of this procedure is that all absolute errors in the test setup are eliminated. Only the reproducibility of the test setup is used. In a simple example with three different wavelengths for two different parameters, e.g. etch depth and duty cycle, one can proceed as follows (the procedure described below for taking into account the phase effect of the substrate is referred to here as method 2):

1. 1. Measurement of the CGH at three different wavelengths, whereby the three passes W _λ1,struct , W _λ2,struct and W _λref,struct are obtained.
2. 2. Forming the differences Δ _λ1 = W _λ1,struct - W _λref,struct and Δ _λ2 = W _λ2,struct - W _λref,struct .

This means that there are now again two measured variables which are linked to the two unknown profile parameters. The unknown profile parameters can now be determined by the inverse calculation.

The difference in sensitivity between the measurement wavelengths and the reference wavelength can be used with this method, but not the sensitivity itself.

The invention also relates to the use of a map of at least one structural parameter determined over an area for an actually manufactured CGH using the method according to the invention for calculating a correction wavefront, in particular a deviation of a wavefront that can actually be generated on the basis of an actual structure of the actually manufactured CGH compared to a predetermined wavefront of a CGH a target structure, to correct the measurement errors caused by manufacturing errors in the actually manufactured CGH during the interferometric testing of optical elements using the actually manufactured CGH.

It is therefore provided that a map determined for a CGH using the method according to the invention of at least one structure parameter for calculating a correction wavefront, in particular a deviation of a real wavefront due to a actually manufactured structure of the CGH compared to an ideal wavefront of a perfectly manufactured CGH, for correcting by Manufacturing errors caused measurement errors by means of a test piece interferometer is used. Advantageously, the map of the at least one structural parameter can thus be used to improve the reliability of a wavefront emanating from a CGH.

Advantageous refinements and developments of the invention result from the dependent claims.

In the following, exemplary embodiments of the invention are described in principle with reference to the drawing.

Elements with the same function are provided with the same reference symbols in the figures.

• 1 eine EUV-Projektionsbelichtungsanlage;
• 2 eine DUV-Projektionsbelichtungsanlage;
• 3 ein Interferometersystem zur Prüfung von optischen Elementen mittels eines CGHs;
• 4 eine vereinfachte Schnittansicht eines Teilbereichs einer strukturierten Oberfläche eines CGHs;
• 5 eine Darstellung einer ersten Ausführungsform einer interferometrischen Beugungsmessung eines CGHs in der Reflexionskonfiguration bei senkrechtem Einfall eines Messlichtstrahls;
• 6 eine Darstellung einer zweiten Ausführungsform einer interferometrischen Beugungsmessung eines CGHs in Transmissionskonfiguration in doppeltem Durchtritt;
• 7 eine Darstellung einer dritten Ausführungsform einer interferometrischen Beugungsmessung eines CGHs in doppelter Reflexion;
• 8a und 8b Schaubilder der Effizienz der Strahlung in nullter Beugungsordnung in Transmission und Reflexion;
• 9a und 9b Schaubilder von Sensitivitäten von Strukturparametern für deren Messung mittels Intensität in Transmissionskonfiguration;
• 10a und 10b Schaubilder von Sensitivitäten von Strukturparametern für deren Messung mittels Intensität in Reflexionskonfiguration;
• 11a und 11b Schaubilder von Sensitivitäten von Strukturparametern für deren Messung mittels Phase in Transmissionskonfiguration; und
• 12a und 12b Schaubilder von Sensitivitäten von Strukturparametern für deren Messung mittels Phase in Reflexionskonfiguration.

They show schematically:

• 1 an EUV projection exposure system;
• 2 a DUV projection exposure system;
• 3 an interferometer system for testing optical elements using a CGH;
• 4 a simplified sectional view of a portion of a structured surface of a CGH;
• 5 a representation of a first embodiment of an interferometric diffraction measurement of a CGH in the reflection configuration with perpendicular incidence of a measuring light beam;
• 6 a representation of a second embodiment of an interferometric diffraction measurement of a CGH in transmission configuration in double pass;
• 7 an illustration of a third embodiment of an interferometric diffraction measurement of a CGH in double reflection;
• 8a and 8b Diagrams of the efficiency of radiation in the zeroth diffraction order in transmission and reflection;
• 9a and 9b Graphs of sensitivities of structural parameters for their measurement by means of intensity in transmission configuration;
• 10a and 10b Graphs of sensitivities of structural parameters for their measurement by means of intensity in reflection configuration;
• 11a and 11b Graphs of sensitivities of structural parameters for their measurement by phase in transmission configuration; and
• 12a and 12b Graphs of sensitivities of structural parameters for their measurement by phase in reflection configuration.

1 shows an example of the basic structure of an EUV projection exposure system 400 for semiconductor lithography, for which the invention can be used. In particular, the invention can be used by installing one or more optical elements, which are combined with a diffractive optical element, in particular a CGH 2 (see 3 ) were checked, which was measured using the method according to the invention. In addition to a radiation source 402 , an illumination system 401 of the projection exposure system 400 has optics 403 for illuminating an object field 404 in an object plane 405 . A reticle 406 which is arranged in the object field 404 and is held by a reticle holder 407 shown schematically is illuminated. Projection optics 408, shown only schematically, are used to image the object field 404 in an image field 409 in an image plane 410. A structure on the reticle 406 is imaged on a light-sensitive layer of a wafer 411 arranged in the area of the image field 409 in the image plane 410, which is wafer holder 412, also shown in part, is held.

The radiation source 402 can emit EUV radiation 413, in particular in the range between 5 nanometers and 30 nanometers, in particular 13.5 nm. To control the radiation path of the EUV radiation 413, optically differently designed and mechanically adjustable optical elements are used. The optical elements of the in 1 The EUV projection exposure system 400 shown is configured as an adjustable mirror in suitable embodiments that are mentioned below only by way of example.

The EUV radiation 413 generated with the radiation source 402 is aligned by means of a collector integrated in the radiation source 402 in such a way that the EUV radiation 413 passes through an intermediate focus in the region of an intermediate focal plane 414 before the EUV radiation 413 impinges on a field facet mirror 415. Downstream of the field facet mirror 415, the EUV radiation 413 is reflected by a pupil facet mirror 416. Field facets of the field facet mirror 415 are imaged in the object field 404 with the aid of the pupil facet mirror 416 and an optical assembly 417 with mirrors 418, 419, 420.

In 2 an exemplary DUV projection exposure system 100 is shown. The projection exposure system 100 has an illumination system 103, a device known as a reticle stage 104 for receiving and precisely positioning a reticle 105, by means of which the subsequent structures on a wafer 102 are determined, a wafer holder 106 for holding, moving and precisely positioning the wafer 102, and an imaging device , namely a projection lens 107, with a plurality of optical elements 108 which are held in a lens housing 140 of the projection lens 107 by means of sockets 109.

The optical elements 108 as individual refractive, diffractive and / or reflective optical elements 108 such. B. lenses, mirrors, prisms, end plates and the like, be formed.

The basic functional principle of the projection exposure system 100 provides that the structures introduced into the reticle 105 are imaged onto the wafer 102 .

The illumination system 103 provides a projection beam 111 in the form of electromagnetic radiation that is required for imaging the reticle 105 onto the wafer 102 . A laser, a plasma source or the like can be used as the source for this radiation. The radiation is shaped in the illumination system 103 via optical elements in such a way that the projection beam 111 has the desired properties in terms of diameter, polarization, shape of the wave front and the like when it strikes the reticle 105 .

An image of the reticle 105 is generated by means of the projection beam 111 and transmitted to the wafer 102 by the projection lens 107 in a correspondingly reduced size. The reticle 105 and the wafer 102 can be moved synchronously, so that areas of the reticle 105 are imaged onto corresponding areas of the wafer 102 practically continuously during a so-called scanning process.

An air gap between the last lens or the last optical element 108 and the wafer 102 can be replaced by a liquid medium which has a refractive index>1. The liquid medium can be, for example, ultrapure water. Such a structure is also referred to as immersion lithography and has an increased photolithographic resolution.

In 3 an interferometric measuring system for measuring a surface 18, in particular an asphere or free-form surface, of the optics 19 to be tested, for example a mirror, is shown by way of example using a computer-generated hologram or CGHs 2.

The interferometer 5 shown here is a Fizeau interferometer, and the method presented can also be carried out with other interferometer types such as a Twymann-Green interferometer or others.

The interferometer 5 consists of a light source 6, for example a laser light source. The emitted light 7 is focused by a focusing lens 8 onto a pinhole or an aperture 9 , so that a divergent beam 10 of coherent light emanates from the aperture 9 . The divergent light beam 10 is collimated by a lens group 11 to form a raw measurement light beam 12 which has a substantially planar wavefront.

The raw measurement light beam 12 then passes through a beam splitter 13 and impinges on wedged optics 14 with a Fizeau surface 15 which is oriented perpendicular to the raw measurement light beam 12 . The Fizeau surface 15 reflects back part of the raw measurement light beam 12 and the part 16b that is reflected back and is not shown here serves as reference light for the interferometric measurement.

The transmitted beam portion 16 of the raw measuring light beam 12 is converted by correction optics 17 into a measuring light beam 16a or into a plurality of measuring light beams 16a, in particular converted or manipulated in such a way that the wavefront of the measuring light beam 16a matches the shape of the surface 18 of the optics 19 to be tested. In other words: the measuring light beam 16a is shaped in such a way that it strikes the surface 18 perpendicularly at every point on the surface 18 . The measuring light beam 16a or the measuring light beams 16a are reflected on the surface 18 and travels the path back to the beam splitter 13, which reflects part of the measuring light beam 16a and the reference light beam 16b in the direction of a detector 23. The beam 20, which is combined from the measuring light beam 16a and the reference light beam 16b and is reflected at the beam splitter 13, is directed by an objective lens system 21 onto a camera 22 with the position-sensitive detector 23, so that the surface 18 to be checked is imaged on the position-sensitive detector 23. The portion 16a of the transmitted measuring light beam 16 reflected by the optics 19 to be tested and the reference beam 16b reflected by the Fizeau surface 15 are thereby superimposed on the position-sensitive detector 23 and form the interferogram 3.

The correction optics 17 have a lens 24 and the CGH 2 , one side of this correction optics 17 being provided with the structured surface 1 .

So that the surface 18 on the optics 19 to be tested can be measured, the structured surface 1 is calculated using methods such as ray tracing such that the measuring light beams 16a transmitted by the CGH 2 in a non-zero diffraction order are perpendicular to the surface 18 of the optics to be tested 19 hit. The calculated structure is then transferred to the CGH 2, for example by lithography and/or plasma etching. The actual or real manufactured structure will deviate from the ideal, calculated structure due to manufacturing errors. In particular, the structures can have structure parameters that deviate from the design, such as etching depth, width or flank angle. In other words, the actual structures deviate from the target structures or design structures, particularly with regard to the structural parameters of the etching depth, width or flank angle. These deviations from the design lead to wavefront errors in the measuring light beams 16a transmitted and reshaped by the CGH 2 . These wavefront errors can lead to significant errors in the measurement of the surface 18 under test.

The method according to the invention makes it possible to measure the production-related deviations in the structural parameters with high accuracy over the entire CGH 2 . With the measured structure parameters, the deviation of the wavefront or actual wavefront of the measuring light beams 16a from the design or from a target wavefront can be taken into account and thus a higher accuracy of the measurement of the surface 18 can be achieved.

The method according to the invention is not limited to CGHe 2 for testing optical elements of projection exposure systems 100, 200, 400, in particular not to optical elements of projection exposure systems 100, 200, 400 with the structure described. However, the invention is particularly suitable for optical elements of projection exposure systems, in particular for CGHe 2 for testing mirrors of EUV projection exposure systems.

Although the invention is described in the exemplary embodiments using CGHs 2 for testing optical elements of projection exposure systems, the disclosure is to be understood in such a way that the method according to the invention can be used for any CGHs 2 and also other structured surfaces, in particular of wafers.

4 shows a schematic, two-dimensional representation of a small section of a structured surface 1 as it is known as CGH 2 in an interferometric measuring system, as in 3 shown as an example, is used. In the case of a simple CGH 2, these structures can be imagined as curved lines perpendicular to the plane of the drawing with the cross-section shown. The spacing and curvature of the lines typically vary across the CGH 2.

In the present exemplary embodiment, the CGH 2 is implemented as a binary phase mask. Before the structured surface is manufactured, a CGH substrate 25 has a front side 26 and a rear side 27, each with a fit error (not shown in more detail) determined by the substrate manufacture. In accordance with the design of the CGH 2, material is removed during manufacture, for example by plasma etching, in certain regions 28 and raised regions 29 remain. The etched structures can be described approximately with structure parameters, of which an etching depth T, a width w, a pitch p and a flank angle 29a are shown here, for example. A duty cycle D results from D=w/p.

It can thus be provided that the at least one structure parameter includes the etching depth T, the duty cycle D, a trenching radius and/or the flank angle 29a of structures of the CGH 2 or of the wafer.

In transmission, the transmitted wave front of the measuring beams is influenced not only by the structure parameters to be determined but also by the fit of the front side 26 , the fit of the back side 27 and the homogeneity of the CGH substrate 25 . In reflection, the reflected wave front is influenced only by the fit of the front side 26 in addition to the structural parameters to be determined. Both can be taken into account in the presented method in order to be able to determine the structure parameters with high accuracy.

Provision can therefore be made for a phase effect of a substrate or a CGH substrate 25, in particular a pass of the diffractive optical element or wafer, to be taken into account when determining the at least one structural parameter.

The figures mentioned below represent the invention only by way of example and in a highly schematic manner.

For example by means of in the 5 until 7 In the measurement setups shown, the method according to the invention for the areal, in particular simultaneous determination of a map of at least one structural parameter of a structured surface 1 of a diffractive optical element, in particular a computer-generated hologram, CGH 2, or a wafer is carried out, with one or more interferograms 3 of the structured surface 1 are recorded, from which phase values of the measuring light beams 16a emanating from the CGH 2 or wafer and/or changed in the zeroth diffraction order are determined, and the map of the at least one structure parameter is determined from the at least one interferogram 3 of the structured surface 1.

“Map of the at least one structural parameter” means that the structural parameter is displayed or can be displayed on a map or in particular a two-dimensional surface. In this case, the map or area corresponds to a predeterminable part of the surface, several parts of the surface or the entire surface of the diffractive optical element. The structure parameter, in particular the etching depth, the pulse duty factor and/or the flank angle, can thus be represented in particular for each or at each point or location on the map or area. The structure parameter itself is displayed on the map, without being limited to this, in particular by means of definable identification colors. If the structure parameter is the etching depth, for example, it is provided in particular that a first color or identification color is used for a predeterminable first etching depth and a second color or identification color is used for at least one predeterminable second etching depth. The same applies to the other structural parameters described.

5 shows an implementation of the interferometric measuring method presented for measuring a CGH 2 in reflection or in a reflection configuration with perpendicular incidence of the transmitted beam component or measuring light beam 16. The interferometer 5 is only partially shown. In contrast to 3 the reference beam reflected on the Fizeau surface 15 is now brought to interference with the measuring light beam 16a reflected in the zeroth diffraction order at the CGH 2 and the phase measurement of the zeroth reflected diffraction order is thus realized according to the method presented.

In order to obtain several different measurements of the CGH 2, a polarizer 31 can be rotated and the polarization can thus be changed. In order to take into account the change in the wavefront of the measuring light beam 16a through the polarizer 31, the wavefront can be measured in advance for all measuring positions of the polarizer 31 with a calibration mirror instead of the CGH 2. In addition, any existing interferometer error can also be measured in this way.

Further different measurements can be achieved in this setup by changing the wavelength of the light source 6 of the interferometer 5. For example, by automatically exchanging the light source 6 for a light source with a different wavelength, or by using a broadband light source and placing different filters in front of it. Again, the interferometer error and an error of the polarizer 31 can be determined by measurements with a calibration mirror vora+b.

The polarizer 31 is not absolutely necessary for the measurements with different wavelengths and can also be removed if necessary.

For example by means of an in 5 With the measurement setup shown, an embodiment of the method according to the invention can therefore be implemented, with several interferograms 3 of the structured surface 1 being created using different light wavelengths λ _i , λ _j of the measuring light beams 12, 16, 16a, 20, with the light wavelengths λ _i , λ _j holds: $2\cdot \frac{{\mathrm{\lambda }}_{\text{i}}-{\mathrm{\lambda }}_{\text{j}}}{{\mathrm{\lambda }}_{\text{i}}+{\mathrm{\lambda }}_{\text{j}}}>\mathrm{0.001.}$

und ${\overrightarrow{E}}_j=\left(\begin{array}{c}{E}_{xj}\\ E_{yj}\ast e^{i{\phi }_j}\end{array}\right)$

gilt: $$2\cdot \frac{\frac{E_{xi}}{\left|{\overrightarrow{E}}_i\right|}-\frac{E_{xj}}{\left|{\overrightarrow{E}}_j\right|}}{\frac{E_{xi}}{\left|{\overrightarrow{E}}_i\right|}+\frac{E_{xj}}{\left|{\overrightarrow{E}}_j\right|}}>\mathrm{0,001}$$

und/oder $$2\cdot \frac{\frac{E_{yi}}{\left|{\overrightarrow{E}}_i\right|}-\frac{E_{yj}}{\left|{\overrightarrow{E}}_j\right|}}{\frac{E_{yi}}{\left|{\overrightarrow{E}}_i\right|}+\frac{E_{yj}}{\left|{\overrightarrow{E}}_j\right|}}>\mathrm{0,001}$$

und/oder $$2\cdot \frac{{\phi }_i-{\phi }_j}{\left|{\phi }_i\right|+\left|{\phi }_j\right|}>\mathrm{0,001.}$$

Furthermore, for example by means of an in 5 According to the measurement setup shown, an embodiment of the method according to the invention can be implemented, with several interferograms 3 of the structured surface 1 being created using different polarizations of any, for example linear, circular or elliptical polarized measurement light beams 12, 16, 16a, 20, with the respective Jones vectors defining polarization ${\overrightarrow{E}}_i=\left(\begin{array}{c}{E}_{xi}\\ E_{yi}\ast e^{i{\phi }_i}\end{array}\right)$

and ${\overrightarrow{E}}_j=\left(\begin{array}{c}{E}_{xj}\\ E_{yj}\ast e^{i{\phi }_j}\end{array}\right)$

is applicable: $$2\cdot \frac{\frac{E_{xi}}{\left|{\overrightarrow{E}}_i\right|}-\frac{E_{xj}}{\left|{\overrightarrow{E}}_j\right|}}{\frac{E_{xi}}{\left|{\overrightarrow{E}}_i\right|}+\frac{E_{xj}}{\left|{\overrightarrow{E}}_j\right|}}>0.001$$

and or $$2\cdot \frac{\frac{E_{yi}}{\left|{\overrightarrow{E}}_i\right|}-\frac{E_{yj}}{\left|{\overrightarrow{E}}_j\right|}}{\frac{E_{yi}}{\left|{\overrightarrow{E}}_i\right|}+\frac{E_{yj}}{\left|{\overrightarrow{E}}_j\right|}}>0.001$$

and or $$2\cdot \frac{{\phi }_i-{\phi }_j}{\left|{\phi }_i\right|+\left|{\phi }_j\right|}>\mathrm{0.001.}$$

6 shows an implementation of the presented interferometric measurement method for the measurement of a CGH 2 in transmission in double passage.

For example using the in 6 With the structure shown, an embodiment of the method according to the invention can be implemented, according to which the at least one interferogram 3 of the structured surface 1 is created in transmission in double passage via a mirror 32 .

The interferometer 5 is in 6 only partially shown. The reference beam reflected on the Fizeau surface 15 is brought to interference with the measuring beam 16a, which is transmitted on the outward and return path through the CGH 2 in the zeroth diffraction order at the angle Θ and reflected on the mirror 32, and thus the phase measurement of the zeroth transmitted Diffraction order realized according to the method presented.

In order to obtain several different measurements of the CGH 2, the polarizer 31 can be rotated and the polarization can thus be changed. In order to take into account the change in the wavefront of the transmitted measuring light beam 16 through the polarizer 31, the wavefront can be measured beforehand for all measuring positions of the polarizer 31 without CGH 2 . In this way one obtains the measurement error of the measurement setup including interferometer error, polarizer error and mirror error.

Further different measurements can be achieved in this setup by changing the wavelength of the light source 6 of the interferometer 5. In addition, other different measurements can be achieved in this setup by changing the angle Θ of the CGH 2 to the transmitted measurement light beams 16 .

For example using the in 6 With the measurement setup shown, an embodiment of the method according to the invention can therefore be implemented in which several interferograms 3 of the structured surface 1 are created at different angles Θ _i , Θ _j of a mean surface normal 33 of the structured surface 1 to the incident measuring light beam 16, with the participating
angle $${\mathrm{\theta }}_{\text{i}},{\mathrm{\theta }}_{\text{j}}\text{is applicable}:2\cdot \frac{{\mathrm{\theta }}_{\text{i}}-{\mathrm{\theta }}_{\text{j}}}{{\mathrm{\theta }}_{\text{i}}+{\mathrm{\theta }}_{\text{j}}}>\mathrm{0.001.}$$

7 shows an implementation of the presented interferometric measurement method for the measurement of the CGH 2 in double reflection in a 2-Θ arrangement. The interferometer 5 is only partially shown. The transmitted beam portion 16 is directed onto the CGH 2 at an angle Θ. The radiation in the zeroth order of diffraction is reflected again at the angle Θ and then directed again via the mirror 32 onto the CGH 2, at which it is reflected a second time. The measuring light beam 16a reflected in this way on the CGH 2 is then brought to interference with the reference beam reflected on the Fizeau surface 15 and the phase measurement of the radiation in the zeroth reflected diffraction order is thus implemented according to the method according to the invention.

In order to obtain several different measurements of the CGH 2, the polarizer 31 can be rotated and the polarization can thus be changed. In order to take into account the change in the wavefront of the transmitted measuring light beam 16a through the polarizer 31, the wavefront can be measured in advance for all measuring positions of the polarizer 31 with a calibration mirror instead of the CGH 2. In this way one obtains the measurement error of the measurement setup including interferometer error, polarizer error and mirror error.

Further different measurements can be achieved in this setup by changing the wavelength of the light source 6 of the interferometer 5. Furthermore, other different measurements can be achieved in this setup by changing the angle θ of the CGH 2 to the measurement light beam 16, in which case the position of the mirror 32 must also be adjusted to obtain the 2θ configuration.

It can be provided in one embodiment of the method according to the invention that at least one reference area with structures of known phase effect is applied to the surface of the CGH 2 or of the wafer, and that by forming a difference between the phase values to be determined of the structured surface 1 and the known Phase values of the reference range, an unknown global phase of the at least one interferogram 3 is eliminated.

Furthermore, in one embodiment of the method according to the invention, it can be provided that a model or grating profile of the structured surface 1 is created or modeled for each partial area of the surface for which a phase value was measured in the at least one interferogram 3, using the at least one structure parameter, wherein a phase value of the zeroth diffraction order for these sub-areas is calculated using the model or the modeled grating profile by solving Maxwell's equations, after which that value of the structure parameter is determined for the respective sub-area at which the calculated phase value matches the measured phase value as best as possible and on this Way the map of the structured surface 1 comprising the partial regions is determined with values of the at least one specific structure parameter.

In one embodiment of the method according to the invention, it can also be provided that at least one reference interferogram of an unstructured substrate 25 of the CGH 2 or wafer is determined, which is subtracted from the at least one interferogram 3 of the structured surface 1 (method 1).

In one embodiment of the method according to the invention, it can also be provided that at least one reference interferogram of the structured surface 1 is created at a reference wavelength, a reference angle and a reference polarization, with comparisons or differences between the phase values of the at least one interferogram 3 of the structured surface 1 at one of the reference wavelength different wavelength and / or a angles different from the reference angle and/or a polarization different from the reference polarization compared to the at least one reference interferogram of the structured surface 1 are taken into account when determining the at least one structure parameter (method 2).

the 8a shows a diagram of the efficiency of the zeroth diffraction order of a CGH 2 in transmission with a measurement setup according to FIG U.S. 2006/0274325 A1 with different wavelengths. The resulting efficiency of the radiation in the zeroth diffraction order is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

the 8b shows a diagram of the efficiency of the zeroth diffraction order of a CGH 2 in reflection in the measurement setup according to FIG U.S. 2006/0274325 A1 at different wavelengths. The resulting efficiency of the radiation in the zeroth diffraction order is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

the 9a until 12b show diagrams of sensitivities of the structure parameters for different measurement setups. A sensitivity for the measurement of the etching depth T and for the measurement of the duty cycle D is shown. The sensitivities in the 9a until 10b were calculated for an intensity-based measurement stand, as is the state of the art according to U.S. 2006/0274325 A1 can be seen. the 11a until 12b show, in contrast, the sensitivities for a phase-based measurement system using the method according to the invention.

the 9a shows a graph of the sensitivity for the measurement of an etch depth difference of 1 nm with the from the U.S. 2006/0274325 A1 known measurement setup for measuring the intensity or the efficiency of the radiation in the zeroth diffraction order of a CGH 2 in transmission at different wavelengths. The sensitivity was calculated by forming the difference in the intensity or the efficiency of the radiation in the zeroth diffraction order for a binary phase grating with lines of etch depth T of 520 nm and duty cycle D of 0.55 and such a grating with an etch depth of 521 nm became. The resulting sensitivity for measuring the etching depth T is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

the 9b shows a graph of the sensitivity for the measurement of a duty cycle difference of 0.01 with the measurement setup according to FIG U.S. 2006/0274325 A1 for measuring the intensity or the efficiency of the radiation in the zeroth diffraction order of a CGH 2 in transmission at different wavelengths. The sensitivity was calculated by calculating the difference in the intensity or the efficiency of the radiation in the zeroth diffraction order for a binary phase grating with lines with an etching depth T of 520 nm and a duty cycle D of 0.55 and a similar grating with a duty cycle D of 0. 56 was formed. The resulting sensitivity for measuring the duty cycle D is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

If one takes for an evaluation of a measuring stand according to the U.S. 2006/0274325 A1 Assuming an uncertainty of the measured intensities of 0.1%, this results in an uncertainty in the etching depth T of approximately 0.3 nm with a fixed duty cycle D. Furthermore, there is an uncertainty in the duty cycle D of 0.0025 with a fixed etching depth T.

the 10a shows a graph of the sensitivity for the measurement of an etch depth difference of 1 nm with the from the U.S. 2006/0274325 A1 known measurement setup for measuring the intensity or the efficiency of the radiation in the zeroth diffraction order of a CGH 2 in reflection at different wavelengths. The sensitivity was calculated by calculating the difference in the intensity or the efficiency of the radiation in the zeroth diffraction order for a binary phase grating with lines of etch depth T of 520 nm and duty cycle D of 0.55 and a similar grating with an etch depth T of 521 nm was formed. The resulting sensitivity for measuring the etching depth T is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

the 10b shows a graph of the sensitivity for the measurement of a duty cycle difference of 0.01 with the measurement setup according to FIG U.S. 2006/0274325 A1 for measuring the intensity or the efficiency of the radiation in the zeroth diffraction order of a CGH 2 in reflection at different wavelengths. The sensitivity was calculated by calculating the difference in the intensity or the efficiency of the radiation in the zeroth diffraction order for a binary phase grating with lines of etch depth T of 520 nm and duty cycle D of 0.55 and a similar grating with a duty cycle D of 0, 56 was formed. The resulting sensitivity for measuring the duty cycle D is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

the 11a shows a graph of the sensitivity for the measurement of an etch depth difference of 1 nm with the method according to the invention and a measurement setup 6 for measuring the phase of the radiation in the zeroth diffraction order of a CGH 2 in transmission at different wavelengths. Assuming the setup without polarizer 31 and with a solid angle close to zero. The sensitivity was calculated by forming the difference in the phase of the radiation in the zeroth diffraction order for a binary phase grating with lines with an etch depth T of 520 nm and a duty cycle D of 0.55 and a similar grating with an etch depth T of 521 nm. The resulting sensitivity for measuring the etching depth T is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

the 11b shows a diagram of the sensitivity for the measurement of a duty cycle difference of 0.01 with the method according to the invention and a measurement setup 6 for measuring the phase of the radiation in the zeroth diffraction order of a CGH 2 in transmission at different wavelengths. Assuming the setup without polarizer 31 and with a solid angle close to zero. The sensitivity was calculated by forming the difference in the intensity or efficiency of the radiation in the zeroth diffraction order for a binary phase grating with lines of etch depth T of 520 nm and duty cycle D of 0.55 and a similar grating with a duty cycle D of 0.56 became. The resulting sensitivity for the measurement of the duty cycle T is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

the 12a shows a diagram of the sensitivity for the measurement of an etching depth difference of 1 nm with the method according to the invention and a measurement setup 5 for measuring the phase of the radiation in the zeroth diffraction order of a CGH 2 in reflection at different wavelengths, a measurement setup without a polarizer 31 being assumed. The sensitivity was calculated by forming the difference in the phase of the radiation in the zeroth diffraction order for a binary phase grating with lines with an etch depth T of 520 nm and a duty cycle D of 0.55 and a similar grating with an etch depth T of 521 nm. The resulting sensitivity for measuring the etching depth T is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

the 12b shows a diagram of the sensitivity for the measurement of a duty cycle difference of 0.01 with the method presented here and a measurement setup according to Figure 5 for measuring the phase of the radiation in the zeroth diffraction order of a CGH 2 in reflection at different wavelengths. The structure without polarizer 31 is assumed. The sensitivity was calculated by calculating the difference in the intensity or efficiency of the radiation in the zeroth diffraction order for a binary phase grating with lines of etch depth T of 520 nm and duty cycle D of 0.55 and a similar grating with a duty cycle D of 0.56 was formed. The resulting sensitivity for measuring the duty cycle D is plotted on the vertical axis, the wavelength of the radiation used on the horizontal axis.

For the sensitivities of the phase measurement of the 11a until 12b the phase difference of the grating structure used in relation to an unetched reference surface with a known phase was calculated for each phase in accordance with the method according to the invention.

The differences or sensitivities of the phases show broad resonances at the points at which a phase offset is an integral multiple of the circuit number π. In the transmission configuration, such resonances occur, for example, at a wavelength of approximately 478 nm. These resonances have good sensitivities. Wavelength ranges in a spectral proximity to such resonances are therefore particularly suitable for an interferometric profile measurement using the method according to the invention. From the 8a and 8b and it can be seen that in wavelength regions in the spectral vicinity of the resonances little light is transmitted or reflected back. In order to obtain a good contrast in an interferometric measurement, the intensity of the reference wave should be matched to the expected intensity of the test object wave, for example via the reflectivity of the Fizeau surface 15.

Assuming that an interferogram 3 can be evaluated with good reproducibility in an intensity range of the measuring beam 16a from a quarter to four times the intensity of the reference wave and using a reflectivity of the Fizeau surface 15 of one percent in the case of a measurement in Reflection configuration, such as in 5 shown, results from 8b a usable wavelength range from 428 nm to 661 nm.

Assuming an uncertainty of measured phases of 0.1 nm, this results in an uncertainty in the etch depth T of 0.04 nm for a fixed duty cycle D and an uncertainty in the duty cycle D of 0.00014 for a fixed etch depth T.

If one compares the sensitivities and the measurement uncertainties determined therefrom for an intensity measurement and a phase measurement, one recognizes the higher accuracy to be expected of the phase-based measurement method according to the method according to the invention.

A map of at least one structure parameter determined over an area using the explained embodiments of the method according to the invention can be used to calculate a correction wavefront, in particular a deviation of a wavefront that can actually be generated based on an actual structure of the actually manufactured CGH 2 compared to a predetermined wavefront of a (perfectly manufactured, so to speak) CGH 2 a target structure, to correct the measurement errors caused by any manufacturing errors in the actually manufactured CGH 2 during the interferometric testing of optical elements or optics using the actually manufactured CGH 2 .

Reference List

1

textured surface

2

CGH

3

interferogram

5

interferometer

6

light source

7

emitted light

8th

focusing lens

9

cover

10

divergent ray

11

lens group

12

raw measurement light beam

13

beam splitter

14

wedged optics

15

Fizeau surface

16

transmitted part of the beam

16a

measuring light beam(s)

17

correction optics

18

Surface of the optics to be tested

19

optics to be tested

20

reflected measuring light beam

21

objective lens system

22

camera

23

position sensitive detector

24

lens

25

CGH substrate

26

substrate front

27

substrate backside

28

regions with erosion

29

lofty region

29a

flank angle

31

polarizer

32

mirror

33

mean surface normal

100

projection exposure system

102

wafers

103

lighting system

104

reticle stage

105

reticle

106

wafer holder

107

projection lens

108

optical element

109

version

111

projection beam

140

lens body

400

projection exposure system

401

lighting system

402

radiation source

403

optics

404

object field

405

object level

406

reticle

407

reticle holder

408

projection optics

409

image field

410

picture plane

411

wafers

412

wafer holder

413

EUV radiation

414

intermediate focal plane

415

field facet mirror

416

pupil facet mirror

417

optical assembly

418

mirror

419

mirror

420

mirror

T

etching depth

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US 2006/0274325 A1 [0004, 0095, 0096, 0097, 0098, 0099, 0100, 0101, 0102]
• DE 102018200568 A1 [0004]

Claims (12)

Method for the areal determination of a map of at least one structural parameter of a structured surface (1) of a diffractive optical element, in particular a computer-generated hologram, CGH (2), or a wafer, one or more interferograms (3) of the structured surface ( 1) are recorded, from which phase values of the measuring light beams (16a, 20) emanating from the diffractive optical element, in particular the CGH (2) or wafer, are determined in the zeroth diffraction order, and from the at least one interferogram (3) of the structured surface (1) the map of the at least one structural parameter is determined. procedure after claim 1 , where several interferograms (3) of the structured surface (1) are created using different light wavelengths λ _i ,λ _j of the measuring light beams (12,16,16,20), where the following applies to the light wavelengths λ _i ,λ _j involved: $2\cdot \frac{{\mathrm{\lambda }}_{\text{i}}-{\mathrm{\lambda }}_{\text{j}}}{{\mathrm{\lambda }}_{\text{i}}+{\mathrm{\lambda }}_{\text{j}}}>\mathrm{0.001.}$

procedure after claim 1 or 2 , wherein several interferograms (3) of the structured surface (1) are created using different polarization directions of linearly polarized measuring light beams (12,16,16a,20), wherein the angles α _i ,α _j that define the respective polarization direction apply: $$2\cdot \frac{{\mathrm{a}}_{\text{i}}-{\mathrm{a}}_{\text{j}}}{{\mathrm{a}}_{\text{i}}+{\mathrm{a}}_{\text{j}}}>\mathrm{0.001.}$$

procedure after claim 1 , 2 or 3 , where several interferograms (3) of the structured surface (1) are created at different angles Θ _i , Θ _j of the mean surface normal (33) of the structured surface (1) to an incident measuring light beam (16,16a), with the angles involved Θ _i ,Θ _j applies: $$2\cdot \frac{{\mathrm{\theta }}_{\text{i}}-{\mathrm{\theta }}_{\text{j}}}{{\mathrm{\theta }}_{\text{i}}+{\mathrm{\theta }}_{\text{j}}}>\mathrm{0.001.}$$

Procedure according to one of Claims 1 until 4 , wherein the at least one interferogram (3) of the structured surface (1) is created in transmission in double passage and/or in reflection and/or in double reflection via a mirror (32). Procedure according to one of Claims 1 until 5 , wherein at least one reference area with structures of known phase effect is applied to the surface (1) of the diffractive optical element, in particular of the CGH (2) or the wafer, and wherein by forming a difference between the phase values to be determined of the structured surface (1 ) and the known phase values of the reference range, an unknown global phase of the at least one interferogram (3) is eliminated. Procedure according to one of Claims 1 until 6 , wherein a model or grating profile of the structured surface (1) for each sub-area of the surface (1) for which at least one phase value was measured in the at least one interferogram (3) is created or modeled by the at least one structure parameter, with at least one Phase value of the zeroth diffraction order for each of these sub-areas is calculated using the model or the modeled grating profile by solving Maxwell's equations, after which that value of the at least one structure parameter is determined for the respective sub-area in which the at least one calculated phase value corresponds to the at least one measured phase value matches as best as possible and in this way the map of the structured surface (1) comprising the partial regions is determined with values of the at least one specific structure parameter. Procedure according to one of Claims 1 until 8th , wherein the at least one structural parameter comprises an etching depth (T), a duty cycle, a trenching radius and/or a flank angle of structures of the diffractive optical element, in particular of the CGH (2) or of the wafer. Procedure according to one of Claims 1 until 8th wherein a phase effect of a substrate (25), in particular a pass of the diffractive optical element, is taken into account when determining the at least one structural parameter. Procedure according to one of Claims 1 until 9 , wherein at least one reference interferogram of an unstructured substrate of the diffractive optical element, in particular of the CGH (2) or wafer, is determined, which is deducted from the at least one interferogram (3) of the structured surface (1). Procedure according to one of Claims 1 until 10 , wherein at least one reference interferogram of the structured surface (1) is created at a reference wavelength, a reference angle and a reference polarization, comparisons or differences between the phase values of the at least one interferogram (3) of the structured surface (1) at one of the references z wavelength and/or an angle different from the reference angle and/or a polarization different from the reference polarization compared to the at least one reference interferogram of the structured surface (1) are taken into account when determining the at least one structure parameter. Using a for a real manufactured CGH (2) by means of a method according to one of Claims 1 until 11 areally determined map of at least one structure parameter for calculating a correction wavefront, in particular a deviation of a wavefront that can actually be generated based on an actual structure of the actually manufactured CGH (2) compared to a predetermined wavefront of a CGH (2) with a target structure, for correcting the errors caused by manufacturing errors of the actually manufactured CGH (2) caused measurement errors in the interferometric testing of optical elements using the actually manufactured CGH (2).

Images