DE102019206651A1 - Method for three-dimensional determination of an aerial image of a lithography mask - Google Patents

Abstract

For three-dimensional determination of an aerial image of a lithography mask (7) as the measurement intensity result of an image with anamorphic projection exposure imaging optics of a projection exposure system, the 3D aerial image to be determined having a wavefront that deviates in a predetermined manner from a defocus value, the procedure is as follows: In In an initial measuring step, a 3D aerial image is measured in a plurality of operating situations, each of which corresponds to a defocus value. This is done with the aid of a metrology system (14) with measuring optics with measuring imaging optics (15) with isomorphic numerical aperture and at least one displaceable and / or deformable measuring optics component (Mi). The measurement takes place using an aperture diaphragm (16a) with an aspect ratio different from 1 and under the influence of a targeted misalignment (Δα →) of the measurement imaging optics, in each case according to an operating situation. As part of the determination method, spectra (F1 ... N) are also reconstructed as Fourier transforms of a field of imaging light (1) in each case in a specific section of a pupil of an illumination setting of an illumination of the lithography mask (7). The measurement result obtained in the initial measurement step (imeasured) is corrected for each defocus value (zw) by means of correction terms taking into account the reconstructed spectra. The result is a 3D aerial image determination with high accuracy.

Description

The invention relates to a method for three-dimensional determination of an aerial image of a lithography mask as the result of an image using an anamorphic projection exposure imaging optics of a projection exposure system.

Such a method and a metrology system are known from US Pat US 2017/0131528 A1 (Parallel document WO 2016/012 425 A2 ) and from the US 2017/0132782 A1 .

It is an object of the present invention to improve the accuracy of a 3D aerial image determination of a lithography mask that is to be imaged by means of anamorphic projection exposure imaging optics when using a metrology system with measuring imaging optics with an isomorphic imaging scale.

According to the invention, this object is achieved by an aerial image determination method having the features specified in claim 1.

According to the invention, it was recognized that an approach to defocused aerial images of the projection exposure imaging optics, which are necessary to complete the three-dimensional aerial image, namely to capture the third aerial image dimension perpendicular to an image plane of the imaging optics, can be displaced and / or deformed by a targeted misalignment Measurement optics components of the measurement optics of the metrology system, can be improved through the use of correction terms. The respectively reconstructed spectra of an illumination setting are included in these correction terms. The correction terms take into account the lighting setting influence of the difference between the defocus dependency of the imaging optics of the projection exposure system on the one hand and the misalignment dependency of the measuring optics of the metrology system on the other. The two correction terms are used when correcting the measurement result obtained in the initial measurement step, preferably with different signs. Since the same reconstructed spectra are included in the two correction terms, errors that occur in the reconstruction of the spectra are then highlighted by using the two correction terms.

By using isomorphic measurement imaging optics of the metrology system, the 3D aerial image of the lithography mask, imaged with the anamorphic projection exposure imaging optics, can be determined very precisely with the determination method. This can be used to optimize the original structures on the lithography mask to improve their imaging performance in the production of semiconductor components, in particular memory chips. The use of anamorphic measurement imaging optics is not required. In addition, it is not necessary to shift a field perpendicular to the field plane when measuring with the measurement imaging optics.

Several displaceable and / or deformable measuring optics components according to claim 2 for generating the targeted misalignment of the measuring optics for specifying the different defocus values in each case increase the number of degrees of freedom available when minimizing the difference between the wavefront, generated by the imaging by the projection exposure Imaging optics on the one hand, and the wave front, generated by imaging the measurement imaging optics, which is intended to approximate this wave front, on the other hand. The effects of a displacement and / or deformation of the respective displaceable and / or deformable measuring optics component on the wavefront are preferably linearly independent of one another. The difference between the wavefronts of the projection exposure imaging optics on the one hand and the measurement imaging optics on the other hand, which is to be minimized in the initial measuring step, can thus advantageously be kept small. The different defocus values of the projection exposure imaging optics can thus be simulated well with the measuring optics. The measuring imaging optics can have exactly one displaceable and / or deformable measuring optics component, can have exactly two displaceable and / or deformable measuring optics components, can have more than two displaceable and / or deformable measuring optics components, for example three, four, five or more have more displaceable and / or deformable measuring optics components for targeted misalignment of the measuring imaging optics to simulate corresponding defocus values of the projection imaging optics.

A subdivision of an illumination setting pupil according to claim 3 improves the accuracy of the spectrum reconstruction. The subdivision takes into account the physical fact that for lithography masks used in practice, an approach also known as the Hopkins approximation, according to which a shift in a direction of illumination only causes a shift in a mask spectrum, only for small ones Changes in the direction of illumination is a good approximation. This is also referred to below as the “local Hopkins approximation”.

A spectrum reconstruction according to claim 4 improves the accuracy of the determined spectra.

An aerial image determination method according to claim 5 generates 3D aerial image data even with higher defocus values, which is advantageous for predicting a stability of the projection exposure operation. The range of defocus values that are covered in the aerial image determination process can deviate from an ideal focus position by more than 20 nm, by more than 30 nm, by more than 50 nm, or by more than 100 nm.

A diffraction spectrum measurement according to claim 6 enables, for example, a comparison with the reconstructed spectra. This can make the determination of the correction term or terms more accurate.

A phase retrieval algorithm according to claim 7 has proven itself in connection with the measurement of the diffraction spectra. The person skilled in the art can find information on such an algorithm in US 2017/0132782 A1 .

The advantages of a metrology system according to claim 8 correspond to those which have already been explained above with reference to the 3D aerial image determination method. The metrology system can be used to measure a lithography mask, which is provided for projection exposure to produce semiconductor components with the highest structural resolution, which is, for example, better than 30 nm and, in particular, can be better than 10 nm.

• 1 schematisch eine Projektionsbelichtungsanlage für die EUV-Lithographie, aufweisend eine anamorphotische Projektionsbelichtungs-Abbildungsoptik zur Abbildung einer Lithographiemaske;
• 2 schematisch ein Metrologiesystem zur Bestimmung eines Luftbildes der Lithographiemaske, aufweisend eine Mess-Abbildungsoptik mit isomorphem Abbildungsmaßstab, eine Aperturblende mit sich von 1 unterscheidendem Aspektverhältnis und mindestens eine verlagerbare Messoptik-Komponente;
• 3 beispielhaft eine Intensitätsverteilung von Abbildungslicht in einer Bildebene bei der Abbildung der Lithographiemaske mit der Projektionsbelichtungsanlage nach 1 bei einem spezifischen Defokuswert, also einer Abweichung einer Messebene von einer idealen Fokalposition der Bildebene;
• 4 eine Abbildungslicht-Intensität gemessen mit dem Metrologiesystem nach 2, wobei die verlagerbare Messoptik-Komponente so eingestellt ist, dass ein Defokuswert entsprechend dem Defokus nach 3 durch eine gezielte Dejustage der Mess-Abbildungsoptik angenähert ist;
• 5 eine Abfolge von Abbildungslichtintensitäts-Messergebnissen in der Bildebene des Metrologiesystems bei der Abbildung des Retikels mit jeweils unterschiedlichen Verlagerungspositionen der verlagerbaren Messoptik-Komponente, die verschiedenen Defokuswerten entsprechen;
• 6 schematisch ein Vorgehen bei der Bestimmung eines Luftbildes unter Verwendung von Spektren, die Fouriertransformierte eines Feldes des Abbildungslichts in jeweils einen bestimmten Abschnitts einer Pupille eines Beleuchtungssettings einer Beleuchtung der Lithographiemaske darstellen, wobei dieses Bestimmen der Spektren nach Art einer lokalen Hopkins-Approximation erfolgt; und
• 7 die einzelnen Beiträge bei der Luftbild-Bestimmung, nämlich oben rechts das gemessene Luftbild der Messoptik des Metrologiesystems, unten links als ein Korrekturterm das berechnete Luftbild, erhalten durch Simulation einer Abbildung mit der anamorphotischen Projektionsbelichtungs-Abbildungsoptik unter Einbeziehung rekonstruierter Spektren nach 6 und unten rechts ein weiterer Korrekturterm in Form eines berechneten Luftbildes, erzeugt durch Simulation einer Abbildung mit der Messoptik des Metrologiesystems unter Einbeziehung der Spektren, wobei die verschiedenen Luftbilder jeweils dem gleichen Defokuswert zugeordnet sind.

An exemplary embodiment of the invention is explained in more detail below with reference to the drawing. In this show:

• 1 schematically a projection exposure system for EUV lithography, having anamorphic projection exposure imaging optics for imaging a lithography mask;
• 2 schematically a metrology system for determining an aerial image of the lithography mask, having a measuring imaging optics with isomorphic imaging scale, an aperture stop with an aspect ratio different from 1 and at least one displaceable measuring optics component;
• 3 an example of an intensity distribution of imaging light in an image plane when imaging the lithography mask with the projection exposure system 1 with a specific defocus value, that is to say a deviation of a measuring plane from an ideal focal position of the image plane;
• 4th an imaging light intensity measured with the metrology system 2 , wherein the displaceable measuring optics component is set so that a defocus value according to the defocus 3 is approximated by a targeted misalignment of the measurement imaging optics;
• 5 a sequence of imaging light intensity measurement results in the image plane of the metrology system when imaging the reticle, each with different displacement positions of the displaceable measuring optics component, which correspond to different defocus values;
• 6th schematically a procedure for the determination of an aerial image using spectra that represent the Fourier transform of a field of the imaging light in each case a specific section of a pupil of an illumination setting of an illumination of the lithography mask, the spectra being determined in the manner of a local Hopkins approximation; and
• 7th the individual contributions to the aerial image determination, namely the measured aerial image of the measurement optics of the metrology system at the top right, the calculated aerial image as a correction term at the bottom left, obtained by simulating an image with the anamorphic projection exposure imaging optics, including reconstructed spectra 6th and at the bottom right another correction term in the form of a calculated aerial image, generated by simulating an image with the measurement optics of the metrology system taking into account the spectra, the different aerial images being assigned to the same defocus value.

1 shows, in a section corresponding to a meridional section, a beam path of EUV illumination light or imaging light 1 in a projection exposure system 2 with anamorphic projection exposure imaging optics 3 that are in the 1 is shown schematically by a box. The illuminating light 1 is generated in a lighting system 4th the Projection exposure system 2 , which is also shown schematically as a box. The lighting system 4th contains an EUV light source and lighting optics, each of which is not shown in detail. The light source can be a laser plasma source (LPP; laser produced plasma) or a discharge source (DPP; discharge produced plasma). In principle, a synchrotron-based light source can also be used, for example a free electron laser (EFL). A useful wavelength of the illuminating light 1 can be in the range between 5 nm and 30 nm. In principle, in a variant of the projection exposure system 2 a light source for other useful light wavelengths can also be used, for example for a useful wavelength of 193 nm.

The illuminating light 1 is used in the lighting optics of the lighting system 4th conditioned in such a way that a certain lighting setting of the lighting is provided, i.e. a specific lighting angle distribution. A specific intensity distribution of the illuminating light corresponds to this lighting setting 1 in an illumination pupil of the illumination optics of the illumination system 4th .

A Cartesian xyz coordinate system is used below to facilitate the representation of positional relationships. The x-axis runs in the 1 perpendicular to the drawing plane out of this. The y-axis runs in the 1 to the right. The z-axis runs in the 1 up.

The illuminating light 1 illuminates an object field 5 an object level 6th the projection exposure system 2 . In the object level 6th is a lithography mask 7th arranged, which is also referred to as a reticle. Above the object level 6th , which runs parallel to the xy plane, is in the 1 schematically a structural section of the lithography mask 7th shown. This structural section is shown in such a way that it is in the plane of the drawing 1 lies. The actual arrangement of the lithography mask 7th is perpendicular to the plane of the drawing 1 in the object level 6th .

The illuminating light 1 is from the lithography mask 7th , as shown schematically in the 1 displayed, reflected and enters an entrance pupil 8th the imaging optics 3 in an entrance pupil plane 9 one. The entrance pupil used 8th the imaging optics 3 is edged elliptically.

Inside the imaging optics 3 propagates the illumination or imaging light 1 between the entrance pupil plane 9 and an exit pupil plane 10 . In the exit pupil plane 10 is a circular exit pupil 11 the imaging optics 3 . The imaging optics 3 is anamorphic and generated from the elliptical entrance pupil 8th the circular exit pupil 11 .

The imaging optics 3 forms the object field 5 in an image field 12 in one image plane 13 the projection exposure system 2 from. 1 shows below the image plane 13 schematically an imaging light intensity distribution I _scanner , measured in a plane that is in the z-direction from the image plane 13 is spaced apart by a value z _w , that is to say an imaging light intensity at a defocus value z _w . Another example of such a measured imaging light intensity distribution Iscanner when imaging with the projection exposure imaging optics 3 shows 3 .

Between the object level 6th and the image plane 13 results in particular due to the components of the imaging optics 3 a wavefront error φ, which in the 1 is shown schematically as a defocus deviation of a wavefront actual value from a wavefront setpoint value (defocus = 0).

The imaging light intensities Iscanner (xy) at the various z values around the image plane 13 around are also called 3D aerial photos of the projection exposure system 2 designated. The projection exposure system 2 is designed as a scanner. The lithography mask 7th on the one hand and one in the picture plane 13 arranged wafers, on the other hand, are scanned synchronously with one another during the projection exposure. This creates the structure on the lithography mask 7th transferred to the wafer.

2 shows a metrology system 14th for measuring the lithography mask 7th . The metrology system 14th is used for the three-dimensional determination of an aerial image of the lithography mask 7th as an approximation to the actual Iscanner (xyz) aerial image of the projection exposure system 2 used.

Components and functions described above with reference to 1 have already been explained in the 2 the same reference numbers and will not be discussed again in detail.

In contrast to anamorphic imaging optics 3 the projection exposure system 2 is a measurement imaging optics 15th of the metrology system 14th designed as isomorphic optics, i.e. as optics with isomorphic image scale. A measurement entrance pupil 16 is, apart from a global image scale, true to shape in a measurement exit pupil 17th convicted. The metrology system 14th has in the entrance pupil plane 9 an elliptical aperture stop 16a . The design of such an elliptical aperture diaphragm 16a in a metrology system is known from the WO 2016/012 426 A1 . This elliptical aperture stop 16a generates the elliptical measurement entrance pupil 16 the measurement imaging optics 15th . The inner border of the aperture diaphragm 16a gives the outer contour of the measurement entrance pupil 16 in front. This elliptical measurement entrance pupil 16 is in the elliptical measurement exit pupil 17th convicted. An aspect ratio of the elliptical measurement entrance pupil 16 can be the same size as that of the elliptical entrance pupil 8th the imaging optics 3 the projection exposure system 2 . With regard to the metrology system, reference is also made to the WO 2016/012 425 A2 .

The measurement imaging optics 15th has at least one displaceable and / or deformable measuring optics component. Such a measuring optics component is in the 2 schematically at M _i shown as a mirror. The measurement imaging optics 15th may have a plurality of mirrors M1, M2 ... and may have a corresponding plurality M _i , M _{i + 1 of} such measuring optics components.

der verlagerbaren und/oder deformierbaren Messoptik-Komponente M_i ergibt sich ein Wellenfrontfehler φ(α), der ähnlich wie in der 1 auch in der 2 schematisch dargestellt ist.A displaceability or manipulation of the displaceable and / or deformable measuring optics component M _i is in the 2 schematically by a manipulator lever 18th indicated. A degree of freedom of manipulation is in the 2 indicated as a double arrow α. Depending on any misalignment set $\Delta \overrightarrow{\alpha }$

the displaceable and / or deformable measuring optics component M _i results in a wavefront error φ (α), which is similar to that in 1 also in the 2 is shown schematically.

abhängig von der jeweiligen Dejustage $\Delta \overrightarrow{\alpha }$

der verlagerbaren und/oder deformierbaren Messoptik-Komponente M_i gezeigt. Ein weiteres Beispiel für eine derartige Intensitätsmessung Imeasured zeigt 4.In one measuring plane 19th of the metrology system 14th , which represents an image plane of the measurement imaging optics, is a spatially resolving detection device 20th arranged, which can be a CCD camera. Similar to the 1 is in the 2 below the measuring plane 19th a result of an intensity measurement ${\text{I.}}_{\text{measured}}\left(\text{x},\text{y},\Delta \overrightarrow{\alpha }\right)$

depending on the respective misalignment $\Delta \overrightarrow{\alpha }$

the displaceable and / or deformable measuring optics component M _i shown. Another example of such an intensity measurement is shown by Imeasured 4th .

From the measurement results of the metrology system 14th in the measuring plane 19th can take the aerial photo of the projection exposure system 2 can be determined as explained in detail below.

First, a wavefront error φ of the imaging optics 3 the projection exposure system 2 calculated at a defocus value z _r which has the amount of a Rayleigh unit λ / NA ² _wafer . λ is the wavelength of the illuminating light 1 and NA _wafer is the image-side numerical aperture of the imaging optics 3 the projection exposure system 2 . This wavefront error is determined for the wave vectors k.

dieser Zernike-Entwicklung des Scanner-Wellenfrontfehlers in der Bildebene 13. Es wird dann diejenige Manipulatorposition Δα beziehungsweise diejenige Kombination von Manipulatorpositionen Δα_i gesucht, die zu einem Wellenfrontfehler φ der Mess-Abbildungsoptik 15 führt, dessen Zernike-Entwicklung zu Zernike-Koeffizienten ${\overrightarrow{\text{Z}}}_{\text{actual}}$

führt, die den Koeffizienten ${\overrightarrow{\text{Z}}}_{\text{target}}$

am nächsten liegen. Bei dieser Manipulatorposition beziehungsweise diesem Satz von Manipulatorpositionen wird dann mit dem Metrologiesystem 14 ein Bild der Lithographiemaske 7 mit Hilfe der Detektionseinrichtung 20 aufgenommen.This wavefront error is then written as an expansion of Zernike functions and the target Zernike coefficients then result ${\overrightarrow{Z}}_{tarGet}$

this Zernike development of the scanner wavefront error in the image plane 13 . The manipulator position Δα or that combination of manipulator positions Δα _{i is then} sought which results in a wavefront error φ of the measurement imaging optics 15th whose Zernike expansion leads to Zernike coefficients ${\overrightarrow{\text{Z}}}_{\text{actual}}$

which leads to the coefficient ${\overrightarrow{\text{Z}}}_{\text{target}}$

be closest. With this manipulator position or this set of manipulator positions, the metrology system 14th an image of the lithography mask 7th with the help of the detection device 20th recorded.

This method is now repeated for different defocus values, with the wavefront error in this defocus of the imaging optics initially 3 the projection exposure system 2 is determined and then the set of manipulations Δα and the set of Zernike coefficients of the measurement imaging optics are determined, which best simulate this defocus wavefront error.

in der Messebene 19. Bei jedem dieser Defokuswerte erfolgt also eine Manipulatoreinstellung so, dass die Zernike-Koeffizienten ${\overrightarrow{\text{Z}}}_{\text{actual}}$

des zugehörigen Wellenfrontfehlers der Mess-Abbildungsoptik mit jeweils kleinstem Fehler an die Zernike-Koeffizienten ${\overrightarrow{\text{Z}}}_{\text{target}}$

des Wellenfrontfehlers der Abbildungsoptik der Projektionsbelichtungsanlage 2 angepasst sind.This can be done, for example, for the multiple n = -2, -1.5, -1, -0.5, 0, 0.5, 1, 1.5 and 2 of the Rayleigh unit. 5 shows the corresponding results for the intensity measurement ${\mathrm{I.}}_{measured}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{actual}\right)$

in the measuring plane 19th . For each of these defocus values, a manipulator setting is carried out so that the Zernike coefficients ${\overrightarrow{\text{Z}}}_{\text{actual}}$

of the associated wavefront error of the measurement imaging optics with the smallest error in each case to the Zernike coefficients ${\overrightarrow{\text{Z}}}_{\text{target}}$

the wavefront error of the imaging optics of the projection exposure system 2 are adapted.

als Messintensität in Abhängigkeit von einem Defokuswert z_w, also über eine Mehrzahl von Defokus-Messebenen, die jeweils einen Defokuswert (z_w) entsprechen, mit Hilfe des Metrologiesystems 14 mit der Messoptik 15 mit isomorpher numerischer Apertur und der mindestens einen verlagerbaren Messoptik-Komponente M_i gemessen. Diese Messung geschieht unter Verwendung der elliptischen Aperturblende 16a für die Eintrittspupille 16 mit sich um mehr als 10% von 1 unterscheidendem Aspektverhältnis in der Mess-Abbildungsoptik 15. Diese Messung geschieht weiterhin unter Einfluss der jeweils dem Defokuswert zugeordneten gezielten Dejustage der Mess-Abbildungsoptik 15. Durch diese gezielte Dejustage erfolgt, wie vorstehend erläutert, eine Minimierung eines Unterschiedes zwischen einer Wellenfront $\mathrm{\phi }\left({\overrightarrow{\text{Z}}}_{\text{target}}\right),$

die durch die Abbildung der Lithographiemaske mittels der Abbildungsoptik 3 der Projektionsbelichtungsanlage 2 entsteht, und einer Wellenfront $\mathrm{\phi }\left({\overrightarrow{\text{Z}}}_{\text{actual}}\right),$

die durch die Abbildung der Lithographiemaske 7 mittels der Mess-Abbildungsoptik 15 mit gezielt verlagerter Messoptik-Komponente M_i erfolgt.In an initial measuring step of the three-dimensional determination method for the aerial image Iscanner of the lithography mask 7th is the 3D aerial photo ${\mathrm{I.}}_{measured}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{actual}\right)$

as a measurement intensity as a function of a defocus value z _w , that is to say over a plurality of defocus measurement planes, each corresponding to a defocus value (z _w ), with the aid of the metrology system 14th with the measuring optics 15th with isomorphic numerical aperture and the at least one displaceable measuring optics component M _i measured. This measurement is done using the elliptical aperture diaphragm 16a for the entrance pupil 16 with an aspect ratio that differs by more than 10% from 1 in the measurement imaging optics 15th . This measurement continues to take place under the influence of the targeted misalignment of the measurement imaging optics assigned to the respective defocus value 15th . As explained above, this targeted misalignment minimizes the difference between a wave front $\mathrm{\phi }\left({\overrightarrow{\text{Z}}}_{\text{target}}\right),$

by the imaging of the lithography mask by means of the imaging optics 3 the projection exposure system 2 arises, and a wave front $\mathrm{\phi }\left({\overrightarrow{\text{Z}}}_{\text{actual}}\right),$

by the mapping of the lithography mask 7th by means of the measurement imaging optics 15th takes place with specifically relocated measuring optics component M _i .

i bei den Manipulatorpositionen, die den verschiedenen Vielfachen (n = -2, ... n = 2) der Rayleigh-Einheit entsprechen, sowie die korrespondierenden Zernike-Koeffizienten $n{\overrightarrow{Z}}_{actual},$

die bei einer fehlerminimierten Anpassung der zugehörigen Wellenfrontfehler resultieren, und das bei der Messung verwendete Beleuchtungssetting, welches dem Beleuchtungssetting entspricht, das bei der Projektionsbelichtung zum Einsatz kommt, werden nun zur Rekonstruktion von Masken-Spektren verwendet.The series with the metrology system 14th measured aerial photographs ${\text{I.}}_{\text{measured}}\left(\overrightarrow{\text{r}},{\tilde{\text{z}}}_{\text{w}}{\overrightarrow{\text{Z}}}_{\text{actual}}\right)$

i for the manipulator positions, which correspond to the various multiples (n = -2, ... n = 2) of the Rayleigh unit, as well as the corresponding Zernike coefficients $n{\overrightarrow{Z}}_{actual},$

which result in an error-minimized adaptation of the associated wavefront errors, and the lighting setting used in the measurement, which corresponds to the lighting setting used in the projection exposure, are now used to reconstruct mask spectra.

An approximation is used here which is known in the literature as the Hopkins approximation. This approximation is based on the assumption that the respective mask spectrum for two different directions of illumination is identical except for one shift. The Hopkins approximation is only used locally, i.e. for lighting directions that are close to one another. This takes into account the fact that for lighting directions that are further apart, shading due to a three-dimensional structure of the lithography mask leads to different lighting spectra. Details on the Hopkins approximation are explained in chapter, for example 15th of the textbook "Advances in FDTD Computational Electrodynamics", A. Taflove (ed.), Artech House, 2013.

zugeordnet werden. Eine Änderung des Beleuchtungswinkels innerhalb des jeweiligen Pols σ_i führt entsprechend der lokalen Hopkins-Näherung zu einer Frequenzverschiebung des jeweiligen Beugungsspektrums Fi der Lithographiemaske 7. 6th shows on the left an exemplary lighting setting, shown as an intensity distribution in an illumination pupil plane 21st (see. 1 and 2 ) of the lighting system 4th . The lighting setting is designed as a quadrupole lighting setting, with the individual lighting poles σ in the 6th are shown on the left with σ ₁ to σ ₄ depending on pupil coordinates q _x , q _y . Each of these poles σ ₁ to σ ₄ represents a section of the pupil of the illumination setting. According to the local Hopkins approximation, each of these sections σ ₁ to σ _{4 can have} a Fourier-transformed F ₁ to F ₄ as a function of a wave vector $\overrightarrow{\text{k}}$

be assigned. A change in the illumination angle within the respective pole σ _i leads to a frequency shift of the respective diffraction spectrum Fi of the lithography mask in accordance with the local Hopkins approximation 7th .

Using this non-local Hopkins approximation, the entire aerial image can be written as a superposition of the four spectra for the four poles of illumination $$\mathrm{I.}\left(\overrightarrow{r},\overrightarrow{Z},{\mathrm{F.}}_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)\right)={{\displaystyle \sum _{n,\overrightarrow{q}}{\sigma }_n\left(\overrightarrow{q}\right)\left|{\mathcal{F.}}^{-1}\left({\mathrm{F.}}_n\left(\overrightarrow{k}+\overrightarrow{q}\right)\mathrm{A.}\left(\overrightarrow{k}\right)e^{ip\left(\overrightarrow{k},\overrightarrow{Z}\right)}\right)\right|}}^2$$

• $\mathrm{\sigma }\left(\overrightarrow{\text{q}}\right)={\displaystyle {\sum }_{\text{n}}{\mathrm{\sigma }}_{\text{n}}\left(\overrightarrow{\text{q}}\right)}$
  
  das in N-Abschnitte, im vorliegenden Fall also in vier Abschnitte, unterteilte Beleuchtungssetting,
• $\text{A}\left(\overrightarrow{\text{k}}\right)$
  
  ist eine Amplituden-Apodisierungsfunktion der Projektionsoptik (1 innerhalb der zur Verfügung stehenden numerischen Apertur, 0 außerhalb);
• $\phi \left(\overrightarrow{k},\overrightarrow{Z}\right)$
  
  ist der Wellenfrontfehler der Abbildungsoptik, beschrieben als Entwicklung von Zernike-Funktionen mit Zernike-Koeffizienten $\overrightarrow{Z};$
  
• ${\mathrm{<figure-callout\; id="1"\; label="F"\; filenames="DE102019206651A1\_0051.png,DE102019206651A1\_0053.png"\; state="\{\{state\}\}">F</figure-callout>}}_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)$
  
  sind die vorstehend erläuterten Maskenspektren, zugeordnet zu jedem Pupillenabschnitt σi(i= 1....N).

Here are:

• $\mathrm{\sigma }\left(\overrightarrow{\text{q}}\right)={\displaystyle {\sum }_{\text{n}}{\mathrm{\sigma }}_{\text{n}}\left(\overrightarrow{\text{q}}\right)}$
  
  the lighting setting, which is divided into N sections, in this case into four sections,
• $\text{A.}\left(\overrightarrow{\text{k}}\right)$
  
  is an amplitude apodization function of the projection optics (1 inside the available numerical aperture, 0 outside);
• $\phi \left(\overrightarrow{k},\overrightarrow{Z}\right)$
  
  is the wavefront error of the imaging optics, described as the development of Zernike functions with Zernike coefficients $\overrightarrow{Z};$
  
• ${\mathrm{F.}}_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)$
  
  are the mask spectra explained above, assigned to each pupil segment σi (i = 1 .... N).

wird nun folgendermaßen vorgegangen:In the reconstruction of the mask spectra F1 ... N with the aid of the sequence of aerial image measurements according to FIG 5 and the associated Zernike coefficients ${\text{n}\overrightarrow{\text{Z}}}_{\text{actual}}$

proceed as follows:

A spectrum Fi is reconstructed for each section σ1 of the lighting setting. For this purpose, an output spectrum or raw spectrum Fi is initially used as a preliminary candidate value, which is generated raw, for example, by Fourier transformation of the respective aerial image measurement. Aerial images are then calculated from these raw spectra Fi, using the Zernike coefficients that were determined for the respective aerial image measurement in the initial measurement step. A difference Δ between the actual aerial image measurement and the simulation value is then determined for all pupil sections, for example for the four lighting poles: $$\Delta ={\displaystyle \sum _n\left|{\mathrm{I.}}_{measured}\left(\overrightarrow{r},n{\overrightarrow{Z}}_{actual}\right)-{\mathrm{I.}}_{sim}\left(\overrightarrow{r},n{\overrightarrow{Z}}_{actual},{\mathrm{F.}}_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)\right)\right|}$$

The raw spectra Fi are then iteratively adapted to minimize the difference Δ and the difference calculation is iterated multiple times if necessary.

und einer Simulation einer Abbildungslicht-Intensität $I_{sim}\left(n{\overrightarrow{Z}}_{actual},F_{\mathrm{1,..}N}\right)$

unter Einbeziehung eines jeweils vorläufigen Kandidatenwertes für das jeweilige Spektrum.Overall, the spectra Fi are thus Fourier transforms of a field of the imaging light 1 in each case a specific section σi of the pupil of the illumination setting of the illumination of the lithography mask 7th reconstructed. The difference Δ between one with the measuring optics is included in this reconstruction 15th using the targeted misalignment of the displaceable measuring optics component M _i measured imaging light intensity ${\text{I.}}_{\text{measured}}\left({\text{n}\overrightarrow{\text{Z}}}_{\text{actual}}\right)$

and a simulation of an imaging light intensity ${\mathrm{I.}}_{sim}\left(n{\overrightarrow{Z}}_{actual},{\mathrm{F.}}_{\mathrm{1,..}N}\right)$

taking into account a preliminary candidate value for the respective spectrum.

As soon as the iterating approximation of the spectra Fi to be reconstructed no longer results in an improvement for the value Δ, the reconstructed spectra F _{i are} available and two correction terms can now be calculated on the basis of these reconstructed spectra F _i .

ist hierbei ein berechnetes 3D-Luftbild beim zugehörigen Defokuswert z_w, welches durch Simulation einer Abbildung mit der anamorphotischen Projektionsbelichtungs-Abbildungsoptik 3 der Projektionsbelichtungsanlage 2 unter Einbeziehung der rekonstruierten Spektren F_i erzeugt wird.A first correction term ${\mathrm{I.}}_{sim}\left({\tilde{z}}_w{\overrightarrow{Z}}_{tarGet},{\mathrm{S.}}_{\mathrm{1,..}N}\right)$

is a calculated 3D aerial image at the associated defocus value z _w , which is obtained by simulating an image with the anamorphic projection exposure imaging optics 3 the projection exposure system 2 is generated taking into account the reconstructed spectra F _i .

beim zugehörigen Defokuswert z_w, welches durch Simulation einer Abbildung mit der Mess-Abbildungsoptik 15 unter Einbeziehung der rekonstruierten Spektren F_i erzeugt wird.A second correction term is a calculated 3D aerial image ${\mathrm{I.}}_{sim}\left({\tilde{z}}_w{\overrightarrow{Z}}_{actual},{\text{F.}}_{\mathrm{1,..}\text{N}}\right)$

at the associated defocus value z _w , which is determined by simulating an image with the measurement imaging optics 15th is generated taking into account the reconstructed spectra F _i .

und den beiden Korrekturtermen kann nun das Luftbild der anamorphotischen Projektionsbelichtungs-Abbildungsoptik 3, I_scanner, gemäß dem folgenden Ausdruck bestimmt werden: $$\begin{array}{l}{I}_{scanner}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{target}\right)\\ \approx I_{measured}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{actual}\right)+I_{sim}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{target},F_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)\right)\\ -I_{sim}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{actual},F_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)\right)\end{array}$$

From the result of the initial measuring step, ${\text{I.}}_{\text{measured}}\left({\tilde{\text{z}}}_{\text{w}}{\overrightarrow{\text{Z}}}_{\text{actual}}\right),$

and the two correction terms can now be the aerial image of the anamorphic projection exposure imaging optics 3 , I _scanner , can be determined according to the following expression: $$\begin{array}{l}{\mathrm{I.}}_{scanner}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{tarGet}\right)\\ \approx {\mathrm{I.}}_{measured}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{actual}\right)+{\mathrm{I.}}_{sim}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{tarGet},{\mathrm{F.}}_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)\right)\\ -{\mathrm{I.}}_{sim}\left(\overrightarrow{r},{\tilde{z}}_w{\overrightarrow{Z}}_{actual},{\mathrm{F.}}_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)\right)\end{array}$$

It turns out that simulation or reconstruction errors, since they appear in both correction terms with different signs, mutually emphasize one another.

dargestellt. Links unten ist der erste Korrekturterm durch Simulation anhand der Projektions-Abbildungsoptik 3, ${\text{I}}_{\text{sim}}\left(\overrightarrow{\text{r}},{\overrightarrow{\text{Z}}}_{\text{target}},{\text{F}}_{\mathrm{1,..}\text{N}}\left(\overrightarrow{\text{k}}\right)\right),$

und rechts unten der zweite Korrekturterm, also das berechnete Luftbild auf Basis einer Simulation der Messoptik $I_{sim}\left(\overrightarrow{r},{\overrightarrow{Z}}_{actual},F_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)\right),$

dargestellt. 7th clearly shows the various terms that go into the calculation of the 3D aerial image I _scanner according to the above formula. At the top left, before the calculation, with a question mark is the aerial image you are looking for with the actual wavefront errors caused by the anamorphic projection imaging optics 3 are caused. At the top right is an aerial photo corresponding to the initial measurement step, ${\text{I.}}_{\text{measured}}\left(\overrightarrow{\text{r}},{\text{n}\overrightarrow{\text{Z}}}_{\text{actual}}\right),$

shown. At the bottom left is the first correction term through simulation using the projection imaging optics 3 , ${\text{I.}}_{\text{sim}}\left(\overrightarrow{\text{r}},{\overrightarrow{\text{Z}}}_{\text{target}},{\text{F.}}_{\mathrm{1,..}\text{N}}\left(\overrightarrow{\text{k}}\right)\right),$

and the second correction term at the bottom right, i.e. the calculated aerial image based on a simulation of the measurement optics ${\mathrm{I.}}_{sim}\left(\overrightarrow{r},{\overrightarrow{Z}}_{actual},{\mathrm{F.}}_{\mathrm{1,..}N}\left(\overrightarrow{k}\right)\right),$

shown.

To determine at least one of the correction terms, a diffraction spectrum can also be used which is characterized by an in the US 2017/0132782 A1 described method is measured.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed 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 2017/0131528 A1 [0002]
• WO 2016/012425 A2 [0002, 0026]
• US 2017/0132782 A1 [0002, 0012, 0051]
• WO 2016/012426 A1 [0026]

Claims (8)

A method for three-dimensional determination of an aerial image (I _scanner ) of a lithography mask (7) as a measurement intensity result of an image with anamorphic projection exposure imaging optics (3) of a projection exposure system (2), the 3D aerial image to be determined (I _scanner ) having a wavefront $\left(\mathrm{\phi }\left({\tilde{\text{z}}}_{\text{w}}{\overrightarrow{\text{Z}}}_{\text{target}}\right)\right)$

which deviates in a predetermined manner from a defocus value (z _w ), with the following steps: - In an initial measuring step, measuring a 3D aerial image (I _measured ) as a measurement intensity result in a plurality of operating situations $\left({\tilde{\text{z}}}_{\text{w}}{\overrightarrow{\text{Z}}}_{\text{actual}}\right),$

each corresponding to a defocus value (z _w ), with the aid of a metrology system (14) with measuring optics with measuring imaging optics (15) with isomorphic numerical aperture and at least one displaceable and / or deformable measuring optics component (M _i ), this measurement using an aperture diaphragm (16a) with an aspect ratio differing by more than 10% from 1 in the measurement imaging optics (15) and under the influence of a targeted misalignment $\left(\Delta \overrightarrow{\alpha }\right)$

the measurement imaging optics (15), each corresponding to an operating situation $\left({\tilde{\text{z}}}_{\text{w}}{\overrightarrow{\text{Z}}}_{\text{actual}}\right),$

takes place, - whereby through the targeted misalignment $\left(\Delta \overrightarrow{\alpha }\right)$

a minimization of a difference takes place between: --- a wavefront $\left(\mathrm{\phi }\left({\overrightarrow{\text{Z}}}_{\text{target}}\right)\right),$

)), which results from the imaging of the lithography mask (7) by means of the projection exposure imaging optics (3) at the respective defocus value (z _w ), and --- a wave front $\left(\mathrm{\phi }\left({\overrightarrow{\text{Z}}}_{\text{actual}}\right)\right),$

by the imaging of the lithography mask (7) by means of the measuring imaging optics (15) with targeted misalignment $\left(\Delta \overrightarrow{\alpha }\right),$

thus deliberately displaced and / or deformed measuring optics component (M _i ) arises, - reconstruction of spectra (F _{1 ... N} ) as Fourier transform of a field of imaging light (1) in a specific section (σ _i ) of a pupil of an illumination setting an illumination of the lithography mask (7), a minimization of a difference (Δ) between - one with the measuring optics (15) using the targeted misalignment being included in the reconstruction $\left(\Delta \overrightarrow{\alpha }\right)$

measured imaging light intensity $\left({\text{I.}}_{\text{measured}}\left({\text{n}\overrightarrow{\text{Z}}}_{\text{actual}}\right)\right)$

and a simulation of an imaging light intensity $\left({\mathrm{I.}}_{sim}\left(n{\overrightarrow{Z}}_{actual},{\mathrm{F.}}_{\mathrm{1,..}N}\right)\right)$

with the respective targeted misalignment $\left(\Delta \overrightarrow{\alpha }\right)$

taking into account a preliminary candidate value for the respective spectrum (F _{1 ... N} ), - correcting the measurement result (I _measured ) obtained in the initial measurement step for each defocus value (zw) using the following correction terms: - a calculated 3D aerial image $\left({\text{I.}}_{\text{sim}}\left({\tilde{\text{z}}}_{\text{w}}{\overrightarrow{\text{Z}}}_{\text{target}},{\text{F.}}_{\mathrm{1,..}\text{N}}\right)\right)$

at the associated defocus value (z _w ), which is generated by simulating an image with the anamorphic projection exposure imaging optics (3) of the projection exposure system (2) taking into account the reconstructed spectra (F _{1 ... N} ), - a calculated 3D aerial image $\left({\text{I.}}_{\text{sim}}\left({\tilde{\text{z}}}_{\text{w}}{\overrightarrow{\text{Z}}}_{\text{actual}},{\text{F.}}_{\mathrm{1,..}\text{N}}\right)\right)$

at the associated defocus value (z _w ), which is generated by simulating an image with the measurement imaging optics (15) taking into account the reconstructed spectra (F _{1 ... N} ). Procedure according to Claim 1 , characterized in that during the targeted misalignment, a shift and / or deformation of several shiftable measuring optics components (M _i , M _{i + 1} ) takes place. Procedure according to Claim 1 or 2 , characterized in that to determine the reconstructed spectra (F _{1 ... N} ) the pupil of the illumination setting is divided into more than two sections (σ _i ). Method according to one of the Claims 1 to 3 , characterized in that to determine the reconstructed spectra (F _{1 ... N} ) a measurement of the imaging light intensity $\left({\text{I.}}_{\text{measured}}\left({\text{n}\overrightarrow{\text{Z}}}_{\text{actual}}\right)\right)$

in a measuring plane (19) in several displacement positions $\left({\text{n}\overrightarrow{\text{Z}}}_{\text{actual}}\right)$

the at least one movable and / or deformable measuring optics component (n _i ) takes place, each of which corresponds to a defocus value (z _w ) of the projection exposure imaging optics (3). Method according to one of the Claims 1 to 4th , characterized in that the aerial image (I _scanner ) of the lithography mask (7) is determined three-dimensionally with absolute defocus values (z _w ) which deviate from an ideal focus position, ie an image plane (13) by more than 20 nm. Method according to one of the Claims 1 to 5 , characterized in that in order to determine at least one of the correction terms, a diffraction spectrum of the lithography mask (7) is measured under lighting conditions which correspond to those in the projection exposure. Procedure according to Claim 6 , characterized in that a phase retrieval algorithm is used to measure the diffraction spectrum. Metrology system (14) for performing the determination method according to one of the Claims 1 to 7th - with an illumination system (4) with illumination optics for illuminating the lithography mask (7) to be examined, - with imaging optics (15) for imaging a section of the lithography mask (7) in a measuring plane (19), and - with a spatially resolving one Detection device (20) arranged in the measuring plane (19).

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