DE102019215800A1 - Method for determining an optical phase difference of measuring light of a measuring light wavelength over a surface of a structured object - Google Patents

Abstract

To determine an optical phase difference (φabs, - φML |) of measuring light (1i, 1j) of a measuring light wavelength over an area of a structured object (8), the following measures are used: First, a series of 2D images of the object (8) is recorded in measured at different focal planes to take a 3D aerial image of the object (8). Subsequently, an image-side field distribution including amplitude and phase of an electric field of the 3D aerial image is reconstructed from the 3D aerial image. The phase difference (| φabs- φML |) is then determined from the reconstructed field distribution with the aid of a phase calibration. The phase difference (| φabs- φML |) is the difference between an absorber structure phase φabsdes measuring light 1i, which is reflected by absorber structures (9) of the object (8), and a reflector structure phase φML of the measuring light (1j), which is reflected by reflector structures (10) of the Object (8) is reflected. The phase difference (| φabs- φML |) is determined as a parameter that applies overall over an object structure to be measured. A metrology system with an optical measuring system is used to carry out this determination method. The result is a phase difference determination method which leads to useful values when optimizing an image contrast when using the structured object as a reflection lithography mask.

Description

The invention relates to a method for determining an optical phase difference of measuring light of a measuring light wavelength over a surface of a structured object.

Phase measurement systems and measurement methods that can be carried out with them are known from the specialist articles "Phase-shift / Transmittance measurements in micro pattern using MPM193EX" by H. Nozawa et al., Photomask and Next-Generation Lithography Mask Technology XVI, proceedings of SPIE Vol. 7379, 737925 and "Phame ™: a novel phase metrology tool of Carl Zeiss for in-die phase measurements under scanner relevant optical settings" , by S. Perlitz et al., proceedings of SPIE, March 2007, Art. 65184 R .

It is an object of the present invention to create a phase difference determination method which leads to useful values when optimizing an image contrast when using the structured object as a reflection lithography mask.

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

According to the invention, it was recognized that a phase difference determination from a reconstructed field distribution in which both amplitude and phase of an electric field are included, in comparison to phase determination methods according to the prior art, which regularly only consider the phase values but not the associated amplitude values Leads to results, since phase values in particular can be weighted less with small amplitudes. The resulting phase difference is a parameter that can be used overall to qualify the image contrast of the measured structured object. This makes it possible to optimize the design of object structures on the basis of the phase differences measured in each case, so that the highest possible image contrast results. The determination method according to the invention is not dependent on a communication of a phase value over a possibly arbitrarily determined area of the object structure. This also increases the reliability and reproducibility of the determined optical phase difference.

Object structures for which the phase difference can be determined are line structures, contact hole or contact pin structures as well as general, in particular periodic, structural shapes extending in two dimensions.

A phase calibration according to claim 2 can be implemented in a computational algorithm so that the phase calibration can be carried out in an automated manner. In the adaptation method, output functions that are initially non-linear can be linearized.

Modeling according to claim 3 allows different modeling parameters to be used for the real part on the one hand and the imaginary part on the other hand of the image-side field distribution. This can increase the stability of the calculation method.

With the aid of an iterative adaptation method according to claim 4, it is possible to track non-linear functional dependencies by means of linear adaptations in each iteration step.

An independent adaptation of the real and imaginary part of the image-side field distribution according to claim 5 can increase the accuracy of the adaptation method.

A Fourier transform according to claim 6 can simplify the adaptation method.

A measurement according to claim 7 allows the phase difference to be determined precisely. A higher number than two diffraction orders can also be guided by the projection optics from the object to the image side, for example three, four or even more diffraction orders.

The advantages of a metrology system according to claim 8 or 9 correspond to those which have already been explained above with reference to the determination method according to the invention.

A metrology system according to claim 10 allows a measurement with a very high resolution. A measurement light wavelength that is provided by the EUV light source can be in a wavelength range between 5 nm and 30 nm. The measurement light wavelength is thus adapted to typical illumination wavelengths of projection exposure systems in which the structured objects to be measured can be used as lithography masks in semiconductor chip production.

• 1 schematisch ein Metrologiesystem zum Ermitteln eines Luftbildes eines zu vermessenden Objektes in Form einer Lithographiemaske, aufweisend ein Beleuchtungssystem, eine abbildende Optik und eine ortsauflösende Detektionseinrichtung, wobei zusätzlich Aufsichten auf das zu vermessende Objekt einerseits und auf ein beispielhaft erzeugtes 2D-Bild des Objekts als Teil-Datensatz eines 3D-Luftbildes dargestellt sind;
• 2 im Vergleich zur 1 stark vergrößert einen Querschnitt eines Abschnittes des zu vermessenden Objekts einschließlich eines Absorber-Flächenabschnitts und eines Reflektor-Flächenabschnitts des Objekts, wobei beispielhaft zwei Beleuchtungs- beziehungsweise Messlichtstrahlen dargestellt sind, die einerseits von Schichten des Absorber-Flächenabschnitts und andererseits von Schichten des Reflektor-Flächenabschnitts reflektiert werden;
• 3 in einem Diagramm eine Abhängigkeit einer Phasendifferenz Δφ zwischen einer Absorberstrukturphase des Beleuchtungs- beziehungsweise Messlichts, welches von den Absorber-Flächenabschnitten des Objekts reflektiert wird, und einer Reflektorstrukturphase des Messlichts, welches von den Reflektorflächenabschnitten des Objekts reflektiert wird, von einer Dicke beziehungsweise Höhenerstreckung h_abs einer Absorberstruktur der Absorber-Flächenabschnitte, wobei diese Abhängigkeit dargestellt ist für zwei verschiedene Absorber-Materialzusammensetzungen AM1 und AM2;
• 4 ein Ablaufschema zur Bestimmung der optischen Phasendifferenz Δφ und zur Optimierung von Objektstrukturen des zu vermessenden, strukturierten Objektes derart, dass ein Bildkontrast beim Einsatz des strukturierten Objekts in der Projektionslithographie optimiert ist;
• 5 Phasenwerte der Absorberflächenphase und der Reflektorflächenphase als Ergebnis einer Rekonstruktion einer bildseitigen Feldverteilung im Rahmen des Verfahrens nach 4 am Beispiel eines Objektes nach 1 mit einer Linienstruktur;
• 6 eine anschauliche Darstellung einer Berechnung einer bildseitigen Feldverteilung mit Hilfe eines Modells, welches von einer bestimmten Objektverteilung ausgeht, die abhängt von einer Objektperiode, von einer kritischen Dimension des Objekts, von einer komplexen Reflektivität der Absorber-Flächenabschnitte und von einer komplexen Reflektivität der Reflektor-Flächenabschnitte.

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

• 1 schematically a metrology system for determining an aerial image of an object to be measured in the form of a lithography mask, having an illumination system, an imaging optics and a spatially resolving detection device, with additional views of the object to be measured on the one hand and of an exemplary 2D image of the object as part Data set of a 3D aerial image are shown;
• 2 in comparison to 1 greatly enlarged a cross-section of a section of the object to be measured, including an absorber surface section and a reflector surface section of the object, two illuminating or measuring light beams being shown as an example, which reflect on the one hand from layers of the absorber surface section and on the other hand from layers of the reflector surface section become;
• 3 In a diagram, a dependence of a phase difference Δφ between an absorber structure phase of the illumination or measurement light, which is reflected by the absorber surface sections of the object, and a reflector structure phase of the measurement light, which is reflected by the reflector surface sections of the object, on a thickness or height extension h _abs an absorber structure of the absorber surface sections, this dependency being shown for two different absorber material compositions AM1 and AM2;
• 4th a flow chart for determining the optical phase difference Δφ and for optimizing object structures of the structured object to be measured in such a way that an image contrast is optimized when the structured object is used in projection lithography;
• 5 Phase values of the absorber surface phase and the reflector surface phase as a result of a reconstruction of an image-side field distribution within the framework of the method 4th using the example of an object 1 with a line structure;
• 6th a clear representation of a calculation of a field distribution on the image side with the help of a model which is based on a certain object distribution that depends on an object period, a critical dimension of the object, a complex reflectivity of the absorber surface sections and a complex reflectivity of the reflector surface sections .

1 shows, in a section corresponding to a meridional section, a beam path of EUV illuminating light or imaging light 1 in a metrology system 2 . The illuminating light 1 is generated by an EUV light source 3 .

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 plane of the drawing out of this. The y-axis runs in the 1 To the right. The z-axis runs in the 1 up.

At the light source 3 it 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 (FEL). 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 in a lighting optics 4th a lighting system of the metrology system 2 conditioned in such a way that a certain lighting setting of the lighting is provided, i.e. a specific lighting angle distribution. A certain intensity distribution of the illuminating light corresponds to this lighting setting 1 in an illumination pupil of the illumination optics 4th . The lighting optics 4th have a setting diaphragm not shown in the drawing.

Together with imaging optics or projection optics 8th provides the lighting optics 4th an optical measuring system of the metrology system 2 represent.

The illuminating light 1 illuminates an object field with the lighting setting set in each case 6th in one object level 7th of the metrology system 2. In the object level 7th is a lithography mask as a reflective object 8th arranged, which is also referred to as a reticle. The object level 7th runs parallel to the xy plane.

The object 8th has a line structure with linear absorber structures 9 that run parallel to each other and parallel to the x-direction. Between two neighboring absorber structures 9 lies a reflector structure 10 .

In the 1 above the object level 7th is a plan view of a reticle to be measured for illustration purposes 8th shown, which in comparison to the reticle 8th is tilted in the measuring position about the y-axis by 90 ° into the top view position shown. The absorber structures 9 and the respective intermediate reflector structures 10 can be seen in this plan view as line structures running parallel to the z-direction.

The absorber structures 9 have compared to the reflector structures 10 a height extension habs in the z-direction (cf. 2 ). The absorber structures 9 can each have a multilayer structure with comparatively few individual layers 9 ₁ , 9 ₂ .

The reflector structures 10 are highly reflective multilayer structures with a large number of individual layers 10i executed. The absorber structures 9 in turn lie on this multilayer layer.

1 also shows a formulaic representation for the electric field of the illuminating light 1 in the object field 6th : $$\left|{\mathrm{E.}}_{ret}\right|e^{i{\phi }_{ret}\left(x,y\right)}$$

E _ret denotes the electrical field strength of the illuminating light and φret the phase of the electrical field of the illuminating light 1 .

$$r_{ML}=\left|r_{ML}\right|e^{i{\phi }_{ML}}$$

For two rays 1 _i 1 _j of illuminating light 1 is in the 2 the effect of the respective layer structure of the absorber structures 9 as well as the reflector structures 10 illustrated on a respectively reflected portion of the light beam 1 _i , 1 _j. The light beam 1 _i falls on the in the 2 shown absorber structure 9 and the light beam 1 _j falls on the reflector structure 10 . In terms of formula, the 2 the reflectivities r _abs for the absorber structure 9 and r _ML for the reflector structure 10 reproduced. The following applies: $$r_{abs}=\left|r_{abs}\right|e^{i{\phi }_{abs}}$$

$$r_{\mathrm{M.}\mathrm{L.}}=\left|r_{\mathrm{M.}\mathrm{L.}}\right|e^{i{\phi }_{\mathrm{M.}\mathrm{L.}}}$$

φ _abs and φ _ML denote the phases of the reflected light rays 1 _i , 1 _j . A phase difference | φ _abs - φ _ML | is also referred to below as Δφ.

3 shows a dependence of the phase difference Δφ on the height h _{abs of} the absorber structure 9 using the example of two absorber material variants, which are designated with AM1 and AM2. With increasing height h _{abs of} the absorber structure 9 the phase difference Δφ increases. This growth is not monotonous and is periodic. In addition, different absorber material variants lead to different gradients of this growth of the phase difference Δφ depending on the height h _{abs of} the absorber structure 9 .

This ensures a good image contrast when depicting the structures of the object 8th through the projection optics 5 results, the phase difference Δφ should be in the range of 180 °. Absorber structure heights h _abs for which such a phase difference Δφ in the range of 180 ° is given are in the 3 each marked with an asterisk. It turns out that these structure _heights h abs for the absorber structure 9 strongly depends on the respective absorber material variant. For this reason it is necessary to use the metrology system 2 precisely determine the phase difference Δφ, this phase difference value Δφ being independent of a structural expansion of the absorber structures 9 in the object level 7th should take place, in particular independently of a pitch, that is to say of a periodicity of the absorber structures 9 over the object field 6th . This determination method is explained in more detail below.

The illuminating light 1 is from the lithography mask 8th , as shown in the 1 shown, reflected and enters an entrance pupil of the imaging optics 5 in an entrance pupil plane. The used entrance pupil of the imaging optics 5 can have a circular or elliptical border.

Inside the imaging optics 5 propagates the illumination or imaging light 1 between the entrance pupil plane and an exit pupil plane. A circular exit pupil of the imaging optics lies in the exit pupil plane 5 .

The imaging optics 5 forms the object field 6th in an image field 11 in one image plane 12th of the metrology system 2 from. The image plane 12th is also referred to as the measuring plane. An image scale for the image through the projection optics 5 is greater than 500. Depending on the design of the projection optics 5 the magnifying imaging scale can be greater than 100, can be greater than 200, can be greater than 250, can be greater than 300, can be greater than 400 and can also be significantly greater than 500. The imaging scale of the projection optics 8th is regularly less than 2000.

The projection optics 5 is used to map a section of the object 8th into the image plane 12th .

Similar to the representation of the top view of the object 8th in the 1 is there below the image plane perpendicular to the plane of the drawing 12th a top view of an aerial photograph 13th the image of the object 8th shown. This aerial view 13th is a partial data set of an entire 3D aerial image, the measurement of which will be explained below.

The electric field of the illuminating light 1 in the field of view 11 can be described as: $$\left|{\mathrm{E.}}_{cam}\right|e^{i{\phi }_{cam}\left(x,y\right)}$$

When transferring the electric field of the illuminating light 1 from the object field 6th in the image field 11 The shape of the electric field is affected by aberrations and a defocus of the projection optics 5 .

In the image plane 12th is a spatially resolving detection device 14th of the metrology system 2 arranged. This can be a CCD camera. With the detection device 14th an intensity I is measured for which the following applies: $$\mathrm{I.}=\left(x,y\right)={\left|{\mathrm{E.}}_{cam}\right|}^2$$

The detection device 14th is displaceable in the z-direction and is in the 1 in a lowered position away from the image plane 12th shown. The detection device is in measurement mode 14th in or near the image plane 12th arranged. One level of detection 15th the detection device 14th so can with the image plane 12th coincide or be at a defined distance from this.

The metrology system 2 is used to carry out a method for determining the optical phase difference Δφ as a parameter that applies overall over a measuring object structure. With the help of this parameter, the object 8th with regard to its contrast properties in an image of the object 8th be qualified by a projection exposure system.

The main steps of the determination process are additionally described using the 4th ff. explained.

In one measuring step 16 becomes a series of two-dimensional images I (x, y) of the object 8th each in different focal planes for taking a three-dimensional aerial image of the object 8th with the projection optics 5 measured. After each 2D image measurement in which 2D image intensity values I (x, y) are recorded, the detection device 14th shifted by a predetermined increment .DELTA.z with the aid of a detection displacement device, not shown. For example, five, seven, nine, eleven or thirteen such 2D images I (x, y) are recorded for the complete measurement of the 3D aerial image at different z values. With this measurement at least two diffraction orders of the object 8th diffracted illuminating light or measuring light 1 through the projection optics 5 towards the image field 11 , i.e. to the image side of the metrology system 2 guided.

In a subsequent reconstruction step 17th an image-side electric field distribution f _rec (x, y) including an amplitude and a phase of the electric field E of the 3D aerial image is reconstructed.

5 shows an example of a phase distribution over the image field 11 as a result of the reconstruction step 17th for the object 8th with the line structure. The absorber structures 9 have a different absolute phase φ in the range of 30 ° than the reflector structures 10 with an absolute phase in the range of 210 °.

The reconstruction step 17th can be processed with the aid of a method that is known from the WO 2017/207 297 A1 .

This is followed by a phase calibration step 18th the determination of the phase difference from the reconstructed field distribution f _rec . During the phase calibration, an image-side field distribution f _im is calculated by introducing a model which is based on an object field distribution that depends on an object period or a pitch p of the object 8th , from a critical dimension CD of the object 8th , of a complex reflectivity r _{abs of} the absorber structures 9 and from a complex reflectivity r _{ML of} the reflector structures 10 . The image-side field distribution f _im results from the convolution of the object field distribution with a coherent point distribution function PSF (Point Spread Function) of the projection optics 5 . The coherent point spread function is the Fourier transform of the coherent optical transfer function.

This is illustrated in the 6th . On the left, this shows the object field distribution f _obj , which depends on the spatial coordinate x, on the reflectivity r _{ML of} the reflector structures 10 , on the reflectivity r _{abs of} the absorber structures 9 , of a duty cycle (sampling rate) d and of a period (pitch) p. The following applies to duty cycle d: d = CD / p.

Starting from this object structure f _obj , which is divided into a real part (index r) and an imaginary part (index i), the influence of the projection optics is now taken into account 5 when generating the image of the object field distribution by folding with the point spread function PSF _{coh of} the projection optics 5 . This is in the 6th Shown in the middle, the real part and the imaginary part being specified for the various parameters r, d. As a result of the convolution of the object field distribution with the point spread function PSF, a real part f _im and an imaginary part f _{im of} the image-side field distribution f _im , which are shown in the 6th is shown schematically on the right. This image-side field distribution f _{im in} turn depends on the spatial coordinate x, on the reflectivities r _ML , r _{abs of} the reflector structures 10 and the absorber structures 9 , from the imaginary part di and from the real part d _{r of} the duty cycle and from the pitch p.

_{The image-side field distribution f im} calculated in this way as a model is then compared with the reconstructed image-side field distribution f _rec. For this purpose, a difference between these field distributions is minimized with the aid of an adaptation method or fitting method by varying at least one of the model parameters object period (pitch p), critical dimension CD or complex reflectivities r _ML , r _abs . This is done with the help of a merit function that is viewed and minimized in an integrated manner over the xy field extension. This merit function M can be written as follows: $$\begin{array}{l}\mathrm{M.}={\displaystyle \int {\left|{}_{rec}\left(x,y\right)-{}_{im}\left(x,r_{\mathrm{M.}\mathrm{L.}},r_{abs},d_r,d_i,p\right)\right|}^{2}dxdy=}\\ {\displaystyle \int {\left|{}_{rec}^r\left(x,y\right)-{}_{im}^r\left(x,r_{\mathrm{M.}\mathrm{L.}}^r,r_{abs}^r,d_r\right)\right|}^{2}dxdy+}{\displaystyle \int {\left|{}_{rec}^i\left(x,y\right)-{}_{im}^i\left(x,r_{\mathrm{M.}\mathrm{L.}}^i,r_{abs}^i,d_r,p\right)\right|}^{2}dxdy}\end{array}$$

To minimize this merit function, which occurs independently for the real part on the one hand and the imaginary part on the other hand, the reconstructed image-side field distribution f _rec and the calculated image-side field distribution f _{im are} Fourier transformed. These Fourier transforms F _rec , F _im then depend on the spatial frequencies v _x , v _y .

The merit function M can be written as follows with the help of these Fourier transforms F _rec , F _im , which in turn are split into real and imaginary parts: $$\begin{array}{l}\mathrm{M.}={\displaystyle \int {\left|{\mathrm{F.}}_{rec}^r\left(v_x,v_y\right)-{\mathrm{F.}}_{im}^r\left(v_x,v_y,r_{\mathrm{M.}\mathrm{L.}}^r,r_{abs}^r,d_r,p\right)\right|}^{2}d{v}_{x}d{v}_y}\\ {\displaystyle \int {\left|{\mathrm{F.}}_{rec}^i\left(v_x,v_y\right)-{\mathrm{F.}}_{im}^i\left(v_x,v_y,r_{\mathrm{M.}\mathrm{L.}}^i,r_{abs}^i,d_i,p\right)\right|}^{2}d{v}_{x}d{v}_y}\end{array}$$

The integration in the frequency space can be replaced for periodic structures by a summation over those frequencies in which the respective arguments deviate from 0, whereby it is the observed line structure of the object 8th it is about the frequencies v _x = j / p for j = -j _max ... j _max .

j _max is the maximum of the projection optics 5 transmitted diffraction order.

So that the projection optics 5 the condition | j _max | ≥ 2 fulfilled, must for the pitch p of the object 8th apply: p ≥ 2λ / NA.

λ is the wavelength of the illuminating light 1 and NA is the object-side numerical aperture of the projection optics 5 .

This summation for the real and the imaginary part of the merit function depends linearly on the real and imaginary parts of the reflectivities r _ML , r _abs . $${\mathrm{M.}}_{r,i}={\left|\overrightarrow{{\mathrm{F.}}_{rec}^{r,l}}-\widehat{\mathrm{S.}}\left(d_{r,i}\right)\left(\begin{array}{c}{r}_{\mathrm{M.}\mathrm{L.}}^{r,i}\\ r_{abs}^{r,i}\end{array}\right)\right|}^2$$

The coefficients of this linear notation also depend non-linearly on the real or imaginary parts d _r , di of the duty cycle d.

The optimization of the real part and the imaginary part d _r, d _{i of} the duty cycle and the reflectivities r _ML and r _{abs in order} to minimize the merit function can then take place via an iterative adaptation process, with starting values d ⁰ _{r, i} for the real and imaginary part of the first Duty cycle d is assumed and then corresponding starting values r ^{r, i} _{ML, 0} and r ^{r, i} _abs , 0 are calculated for the reflectivities.

For this purpose, the merit function can be written in the output iteration step as: $${\mathrm{M.}}_{r,i}={\left|\overrightarrow{{\mathrm{F.}}_{rec}^{r,l}}-\widehat{\mathrm{S.}}\left(d_{r,i}^0\right)\left(\begin{array}{c}{r}_{\mathrm{M.}\mathrm{L.}\mathrm{,\; 0}}^{r,i}\\ r_{abs\mathrm{,\; 0}}^{r,i}\end{array}\right)\right|}^2$$

Here, S denotes the coefficients for the reflectivities r, which in turn depend on the real and imaginary part of the duty cycle d. S is a matrix with j _max rows and, because of the effect of the matrix on the two reflectivities r _Ml , rabs, two columns. Starting from a start value of the duty cycles d ⁰ _{r, i} , the start values of the reflectivities r ^{r, i} _{ML, 0} and r ^{r, i} _{abs, 0 are} determined by linear optimization.

The individual matrix elements of S can now be expanded around the starting value of the duty cycles in a Taylor series. The second term of this Taylor series is linear in a value Δd ⁰ _{r, i} , that is to say a correction value for the real and imaginary parts for the starting value of the duty cycle d. This correction value is the one by which the starting value for the next step of the iteration process must be changed.

Inserting the Taylor expansion up to the order Δd in equation (8) results in a linear dependence on the reflectivities and on the correction value for the duty cycles. $${\mathrm{M.}}_{r,i}\approx \left|\overrightarrow{{\mathrm{F.}}_{rec}^{r,i}}-{\widehat{\mathrm{S.}}}_2\left(r_{\mathrm{M.}\mathrm{L.}\mathrm{,\; 0}}^{r,i},r_{abs\mathrm{,\; 0}}^{r,i},d_{r,i}^0\right)\left(\begin{array}{c}{r}_{\mathrm{M.}\mathrm{L.}\mathrm{,1}}^{r,i}\\ r_{abs\mathrm{,1}}^{r,i}\\ \Delta d_{r,i}^0\end{array}\right)\right|$$

The modified matrix Ŝ ₂ depends only on the known starting values for the duty cycles and the already calculated starting values for the reflectivities.

Δd ⁰ _{r, i} can now be determined again by linear optimization, so that, based on the starting value, the next value d ¹ _{r, i} for the iteration can be determined using the following formula: $$d_{r,\mathrm{<figure-callout\; id="1"\; label="i"\; filenames="DE102019215800A1\_0013.png,DE102019215800A1\_0016.png"\; state="\{\{state\}\}">i</figure-callout>}}^1=d_{r,\mathrm{<figure-callout\; id="0"\; label="i"\; filenames="DE102019215800A1\_0016.png"\; state="\{\{state\}\}">i</figure-callout>}}^0+\Delta d_{r,\mathrm{<figure-callout\; id="0"\; label="i"\; filenames="DE102019215800A1\_0016.png"\; state="\{\{state\}\}">i</figure-callout>}}^0$$

The matrix S in the above formula (9) is now updated with the new values for the duty cycle and a new linear optimization of the reflectivities is carried out.

This iteration process is repeated until, in an iteration step ^m , _{the new correction Δd m r , i is} below a predetermined threshold.

After this iterative adaptation method has converged, those reflectivity values for r _ML , r _abs for which the merit function M is minimized are known.

The desired value for the phase difference Δφ is then obtained according to the following formula from the reflectivity values r _ML , r _abs : $$\Delta \phi ={\phi }_{\mathrm{M.}\mathrm{L.}}-{\phi }_{abs}=\text{bad}\left(r_{\mathrm{M.}\mathrm{L.}}\right)-\text{bad}\left(r_{abs}\right)$$

A phase difference value can thus be calculated for a specific mask structure on the basis of a modeling of the object structure, so that it can be checked whether a specific structure design results in an optimal phase difference with regard to the image contrast.

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

• WO 2017/207297 A1 [0046]

Non-patent literature cited

• H. Nozawa et al., Photomask and Next-Generation Lithography Mask Technology XVI, proceedings of SPIE Vol. 7379, 737925 and "Phame ™: a novel phase metrology tool of Carl Zeiss for in-die phase measurements under scanner relevant optical settings" , by S. Perlitz et al., proceedings of SPIE, March 2007, Art. 65184 R [0002]

Claims (10)

Method for determining an optical phase difference (Δφ) of measurement light (1) of a measurement light wavelength (λ) over a surface of a structured object (8), the phase difference (Δφ) between - an absorber structure phase (φ _abs ) of the measurement light (1), which is reflected by absorber structures (9) of the object (8), and - a reflector structure phase (φ _ML ) of the measuring light (1), which is reflected by reflector structures (10) of the object (8), as an overall parameter applicable over an object structure to be measured is determined, with the following steps: - measuring (16) a series of 2D images of the object (8), each in different focal planes, for recording a 3D aerial image of the object (8) with a projection optics (5), - reconstructing ( 17) an image-side field distribution (f _rec ) from the 3D aerial image including amplitude and phase of an electric field of the 3D aerial image, - determination of the phase difference (Δφ) from the reconstructed field distribution (f _{re c} ) with the help of a phase calibration (18). Procedure according to Claim 1 , characterized in that the following steps are carried out during the phase calibration (18): - Calculation of an image-side field distribution (f _im ) by introducing a model which is based on an object field distribution (f _obj ) which depends: on an object period (p) of the object (8) and / or - from a critical dimension (CD) of the object (8) and / or - from a complex reflectivity (r ^{r, i} _abs ) of the absorber structures (9) and / or - - of a complex reflectivity (r ^{r, i} _ML ) of the reflector structures (10), - the image-side field distribution (f _im ) by folding the object field distribution (f _obj ) with a coherent point spread function (PSF) of the Projection optics (5) result, - comparison of the calculated image-side field distribution (fi _m ) with the reconstructed image-side field distribution (f _rec ), - minimization of the difference between the calculated image-side field distribution (f _im ) with the reconstructed image itigen field distribution (f _rec ) using an adaptation method by varying the model parameters object period (p) and / or critical dimension (CD) and / or complex reflectivities (r ^{r, i} _{abs, ML} ), - Calculating the phase difference (Δφ) from the the resulting model parameters for the complex reflectivities (r ^{r, i} _abs and r ^{r, i} _ML ) during the minimization. Procedure according to Claim 2 , characterized in that when calculating the field distribution (f _im ) on the image side, a real part (f _im ) and an imaginary part (fi _m ) are modeled with parameters that are independent of one another. Procedure according to Claim 2 or 3 , characterized by an iterative adaptation process. Method according to one of the Claims 2 to 4th , characterized in that a real part (f _im ) and an imaginary part (fi _m ) of the field distribution (f _im ) on the image side are adapted independently of one another. Method according to one of the Claims 2 to 5 , characterized by at least one Fourier transform as part of the adaptation process. Method according to one of the Claims 1 to 6th , characterized in that when measuring (16) at least two diffraction orders (j) of measuring light (1) diffracted by the object (8) are guided through the projection optics (5) to one side of the image. Metrology system (2) with an optical measuring system for performing the method according to one of the Claims 1 to 7th - with lighting optics (4) for illuminating the object to be examined (8) with a predetermined lighting setting, - with imaging optics (5) for imaging a section of the object (8) in a measuring plane (12), and - with a spatially resolving detection device (14), arranged in the measuring plane (12). Metrology system according to Claim 8 , characterized by an embodiment of the imaging optics (5) such that when measuring (16) at least two diffraction orders (j) of measuring light (1) diffracted by the object (8) through the imaging optics (5) towards an image side of the imaging optics (5). Metrology system according to Claim 8 or 9 , characterized by an EUV light source (3).

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