KR20220083774A - Method for determining the optical phase difference of the measurement light wavelength with respect to the surface of the structured object - Google Patents

Abstract

In order to determine the optical phase difference (|φ _abs - φ _ML |) of the measurement light 1 _i , 1 _j of the measurement light wavelength with respect to the surface of the structured object 8 , the following is performed, first, the object 8 ), a series of 2D images of the object 8 are measured at each different focal plane to record a 3D aerial image. Next, the image-side field distribution including the amplitude and phase of the electric field of the 3D aerial image is reconstructed from the 3D aerial image. Then, the phase difference (|φ _abs - φ _ML |) is determined from the reconstructed field distribution with the aid of phase correction. In this case, the phase difference (|φ _abs - φ _ML |) is the absorber structure phase (φ _abs ) of the measurement light 1 _i reflected by the upper structure 9 of the object 8 and the lower reflector of the object 8 . is the difference between the lower reflector structure phase φ _ML of the measurement light 1 _j reflected by the structure 10 . The phase difference (|ϕ _abs - ϕ _ML |) is determined as an applicable characteristic over the entire measured object structure. A metrology system with an optical measuring system is used to perform this determination method. The result is a phase difference determination method that yields very useful values while optimizing image contrast when using a structured object as a reflective lithography mask.

Description

Method for determining the optical phase difference of the measurement light wavelength with respect to the surface of the structured object

This patent application claims priority to the German patent application DE 10 2019 215 800.5, the content of which is hereby incorporated by reference.

The present invention relates to a method for determining the optical phase difference of a measurement light of a measurement light wavelength across the surface of a structured object.

Phase measurement systems and measurement methods that can be performed through them are described in Photomasks and Next-Generation Lithography Mask Technology XVI, SPIE Vol. 7379, 737925, Nozawa et al. "Measurement of phase shift/transmittance of micropatterns using MPM193EX" and the procedure of SPIE, March 2007, Art. 65184 R. to S. Perlitz et al., in papers such as "PhameTM: A new phase metrology tool by Carl Zeiss for in-die phase measurement in scanner-related optical settings".

SPIE Vol. 11147 111471F-1 to 1114721F-11, the paper "Measuring Phase of EUV Photomasks" by Sherwin et al. is further referenced.

J. Micro/ Nanolith. In MEMS MOEMS 18(1), 011005 (2018), Erdmann et al., "Attenuated phase shift masks for extreme ultraviolet radiation: Can the 3-D mask effect be mitigated?" It is known in a paper called

Phase shift masks for EUV lithography are Proc. It is discussed in a paper by Constancias et al. in SPIE 6151, Emerging Lithographic Technologies X, 61511W (23 Mar 2006).

The optimal mask and source pattern for printing a given feature is described in Rosenbluth et al., Proc. SPIE 4346, Optical Microlithography XIV, (September 14, 2001).

EUV source-mask optimization for the 7nm node and beyond is described in Liu et al., Proc. It is discussed in SPIE 9048, Extreme Ultraviolet (EUV) Lithography V, 90480Q (17 Apr 2014).

It is an object of the present invention to provide a method for determining the phase difference which yields very useful values during the optimization of image contrast when using a structured object as a reflective lithography mask.

This object is achieved according to the invention by a determination method having the features specified in claim 1 .

According to the present invention, both the amplitude and the phase of the electric field are included because the phase value can be less weighted, especially for small amplitudes, compared to the prior art phasing method, which regularly considers only the phase value and not the relevant amplitude value. It was found that the phase difference determination from the reconstructed electric field distribution produced meaningful results. The resulting phase difference is a parameter that can be used overall for image contrast qualification of the measured structured object. Therefore, it is possible to optimize the design of the object structure based on each measured phase difference, so that image contrast as strong as possible can be obtained. The determination method according to the invention does not rely on the communication of phase values over an arbitrarily determined surface area of the object structure. This also increases the reliability and reproducibility of the determined optical retardation.

The phase correction can be converted to a computing algorithm, which means that the phase correction can be performed automatically.

The object structures in which the phase difference can be determined are a line structure, a contact hole or a contact pin structure, and a general structure form extending in two dimensions, particularly a periodic structure form. An object having such an object structure can be used as a lithography mask.

The superstructure of the object can be embodied as an absorber structure, for example of an absorber material coated on the lower reflector structure. Alternatively or additionally, the superstructure of the object may be embodied as an upper reflector structure. In that case, the upper reflector structures on the one hand and the lower reflector structures on the other hand can be produced by etching the respective reflector structure, in particular the multilayer reflector structure.

The structured object may be a lithographic mask particularly suitable for EUV lithography. In particular, the structured object may be implemented as a phase mask, in particular a phase shift mask (PSM), for example a hard PSM. Such a phase mask may require an optical retardation of 180° between the etched substructure and the unetched superstructure.

Such a phase mask may include an etch stop layer (ESL) between the etched substructure and the unetched superstructure. The phase difference between the etched and unetched regions of this phase mask depends on the etch depth and thickness of the ESL. The dependence of the ESL-induced optical phase shift on the ESL thickness is non-linear and must be carefully determined.

The optical phase difference determination method may be used as a preparation method when manufacturing a phase mask. During this method a raw phase mask with a correction structure is created. This correction structure comprises a superstructure above the ESL which has the required height, ie produces the desired optical retardation of, for example, 180°. Also, this raw phase mask contains an ESL with a given thickness.

After fabrication of this raw phase mask, optical phase difference determination is performed to determine the optical phase difference between the upper and lower structures of the phase mask. Thereafter, the thickness of the ESL and/or the etch depth of the top (correction) structure is changed so that there is a desired optical retardation between the superstructure phase on the one hand and the bottom reflector structure phase on the other hand.

When a structured object whose phase difference is to be determined is used as a lithographic mask, this structured object is imaged to produce a 3D image of the mask structure in or near the image plane of a projection exposure apparatus used in a lithographic manufacturing process. The 3D imaging effect in such a projection exposure apparatus can lead to a decrease in contrast, i.e. low image quality. Additional effects affecting the displacement of the focal plane, effects affecting imaging telecentricity, and lateral displacement of structures in the image plane may contribute to this poor image quality. All of these contributions can be viewed as 3D effects, ie they are created, at least in part, by the structure of the mask to be imaged during the lithographic production process. These 3D effects should be minimized to achieve the desired image quality.

From the reference mentioned above, Erdmann et al., it is known to determine the ideal absorber phase and absorber reflectance for a given absorber material that minimizes this 3D effect. Then, phase metrology using the optical retardation determination method according to the invention is used to determine the complex reflectivity of the absorber material in particular. The thickness of the absorber structure of the structured object is then changed to adjust the desired phase and reflectivity of the absorber material to obtain a minimized 3D effect.

A method of determining the optical retardation may be used as part of a method for repairing a defect. During this correction method, in particular the method for determining the optical retardation according to the invention comprising phase metrology is used to determine the complex reflectivity of the hydraulic absorber material according to the thickness and composition of the hydraulic absorber material. Using this optical retardation determination method, in particular including phase measurement, the complex reflectance of the absorber material of the absorber structure of the mask to be repaired is also determined. After determining the complex reflectivity of the absorber material of the mask to be repaired on the one hand and the complex reflectance of the absorber material of the mask to be repaired on the one hand, each absorber material composition and the absorber material thickness are determined such that the complex reflectance of the repair material is equal to the complex reflectance of the absorber material of the mask to be repaired are chosen for the repair phase to match.

Furthermore, the method for determining the optical phase difference according to the present invention ensures, on the one hand, the creation of the desired structure on the wafer with a given mask layout and on the other hand, a large process possible using a given illumination setup in particular a partial coherent illumination distribution. An optimization process that guarantees a window can be used as part of it. Regarding this source mask optimization (SMO), Rosenbluth et al. and Liu et al.

Source mask optimization requires as an input parameter the complex reflectance of the absorber material of the absorber structure in the mask. The result of SMO depends on this complex reflectivity. Using the method for determining the optical phase difference according to the present invention may be part of a calibration process for optimizing parameters of a lithography mask for source mask optimization. In this correction process, in particular a method of determining optical retardation, including phase metrology, is used to determine the complex reflectivity of a given absorber material used in the absorber structure of the lithographic mask to be corrected. The complex reflectance determined by this determination method is then used as an input parameter for source mask optimization to determine the mask layout and to determine lighting settings.

During the fitting method, an output function that was initially non-linear can be linearized.

Modeling according to claim 3 allows the use of different modeling parameters for the real and imaginary parts of the image-side field distribution. This may increase the stability of the computing method.

With the help of the iterative fitting method according to claim 4, it becomes possible to trace the non-linear functional dependency via linear fitting at each iteration step.

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

The Fourier transform according to claim 6 can simplify the fitting method.

The measurement according to claim 7 allows an accurate determination of the phase difference. Diffraction of a second or higher order, for example a third, fourth or higher diffraction order, can also be guided from the object to the image side by the projection optical unit.

The advantages of the metrology system according to claim 8 or 9 correspond to those already described above with reference to the determination method according to the invention.

The metrology system according to claim 10 allows measurements with very high resolution. The measurement light wavelength provided by the EUV light source may be in a wavelength range between 5 nm and 30 nm. The measuring light wavelength is thus adapted to the general illumination wavelength of the projection exposure apparatus, wherein the structured object to be measured in the form of a lithographic mask can be used in semiconductor chip production.

Exemplary embodiments of the present invention are described in more detail below with reference to the drawings. In that drawing:
1 schematically shows a metrology system for confirming an aerial image of an object to be measured in the form of a lithographic mask having an illumination system, an imaging optical unit and a spatial resolution detection device, wherein further examples of and also examples of the object to be measured A top view of a 2D image, eg created as an example, of an object as a partial data set of a 3D aerial image is shown;
FIG. 2 is a greatly enlarged view compared to FIG. 1 and shows a cross-section of a portion of an object to be measured comprising an absorber surface superstructure part and a reflector surface substructure part of the object, wherein one of the two illuminating or measuring rays being reflected is exemplarily shown by a layer of an absorber surface part, the other by a layer of a reflector surface part;
3 shows the reflector structure of the absorber structure phase of illumination or measurement light reflected by the absorber surface portion of the object and the reflector structure of measurement light reflected by the reflector surface portion of the object on the thickness or height degree H _abs of the absorber structure of the absorber surface portion; A diagram of the dependence of the phase difference Δφ between phases is shown, wherein this dependence is plotted for two different absorber material compositions AM1 and AM2;
4 shows a flowchart for determining the optical phase difference Δφ and optimizing the object structure of the structured object to be measured such that the image contrast is optimized when the structured object is used in projection lithography;
5 shows the phase values of the absorber surface phase and the reflector surface phase as a result of reconstructing the image-side field distribution as part of the method according to FIG. 4 using the example of an object having a line structure according to FIG. 1 ;
6 shows an example of the calculation of the image-side field distribution with the aid of a model based on a determined object distribution which depends on the object period, the critical dimensions of the object, the complex reflectance of the absorber surface part and the complex reflectance of the reflector surface part;
Fig. 7 is a cross-sectional view similar to Fig. 2, a further embodiment of an object to be measured, the object being implemented as an EUV phase shift mask comprising a reflector surface superstructure part and a reflector surface substructure part;
FIG. 8 is a view according to FIG. 7 , in another embodiment of a structured object implemented as a phase shift mask with an etch stop layer positioned between the reflector surface superstructure parts and the reflector surface substructure parts, wherein the etch stop the layer covers the reflector surface substructure;
Fig. 9 is a view similar to Fig. 8, wherein one embodiment of a structured object re-implemented as a phase shift mask, wherein the etch stop layer is removed from the reflector surface substructure portion; In addition
FIG. 10 is a view similar to that of FIGS. 2 and 7 to 9 , a further embodiment of an object to be measured comprising an absorber surface superstructure portion and a reflector surface substructure portion, wherein one of the absorber surface superstructure portions is defective; It is a repair structure.

1 shows the beam path of EUV illumination light or EUV imaging light 1 in metrology system 2 in a cross-sectional view corresponding to a meridian section. Illumination light 1 is generated by EUV light source 3 .

To facilitate the expression of positional relationships, the Cartesian xyz coordinate system is used below. The x-axis of Figure 1 extends perpendicular to the plane of the figure and out of the plane of the figure. The y-axis of FIG. 1 extends to the right. The z-axis of FIG. 1 extends upward.

The light source 3 may be a laser plasma source (LPP; laser generated plasma) or a discharge source (DPP; discharge generated plasma). In principle, synchrotron-based light sources, such as free electron lasers (FELs), for example, can also be used. The used wavelength of the illumination light 1 may be in the range between 5 nm and 30 nm. In principle, in the case of a modification of the projection exposure apparatus 2, it is also possible to use a light source for a different wavelength of use light, for example, a wavelength of 193 nm.

The illumination light 1 is conditioned in the illumination optical unit 4 of the illumination system of the metrology system 2 such that a specific illumination setting of the illumination, ie a specific illumination angle distribution is provided. A specific intensity distribution of the illumination light 1 in the illumination pupil of the illumination optical unit 4 corresponds to said illumination setting. For specifying the lighting settings, the lighting optical unit 4 may have a setting stop not shown in the figure.

Together with the imaging optical unit or the projection optical unit 8 , the illumination optical unit 4 constitutes an optical measurement system of the metrology system 2 . A lithographic mask 8 , also called a reticle, is arranged as a reflective object in the object plane 7 . The reticle 8 is an example of a structured object in which the optical retardation is determined by a method to be described later. The object plane 7 extends parallel to the xy plane.

The object 8 has a line structure with a line-shaped absorber superstructure 9 extending parallel to each other and parallel to the x direction. The reflector substructure 10 is located in each case between two adjacent absorber structures 9 .

For the sake of illustration, FIG. 1 shows a top view of the reticle 8 to be measured, tilted by 90° with respect to the y-axis into the top view position shown in comparison to the reticle 8 in the measuring position, on the object plane 7 . . The absorber structure 9 and in each case the reflector structure 10 lying between the absorber structures can be seen in this plan view as a line structure extending parallel to the z-direction.

Compared to the reflector structure 10 , the absorber structure 9 has a height degree habs in the z-direction (see FIG. 2 ). The absorber structure 9 may have a multilayer configuration, each with a relatively small number of individual layers 9 ₁ , 9 ₂ .

The reflector structure 10 is designed to be a highly reflective multilayer structure having a plurality of individual layers 10i. The absorber structure 9 for that part is placed on said multilayer structure.

1 further shows an example according to the formula for the electric field of the illuminating light 1 in the object field 6 :

(1)

(One)

E _ret represents the electric field intensity of the illumination light, and phi _ret represents the electric field phase of the illumination light 1 .

For two rays 1 _i , 1 _j of illuminating light 1 , FIG. 2 shows each of the absorber structure 9 and the reflector structure 10 for the respectively reflected ratios of the rays 1 _i , 1 _j . The effect of layer composition is illustrated. In this case ray 1 _i is As shown in FIG. 2 , the ray 1 j is incident on the absorber structure 9 , and the ray 1 _j is incident on the reflector structure 10 . According to the formula, FIG. 2 shows the reflectance r _abs for the absorber structure 9 and the reflectance r _ML for the reflector structure 10 . The following is true:

(2)

(2)

(3)

(3)

는 Δφ라고도 한다.φ _abs and φ _ML represent the phases of the reflected rays 1 _i , 1 _j . In the following, the phase difference

is also called Δφ.

3 shows the function of the phase difference (Δφ) with respect to the height (h _abs ) of the absorber structure 9 using two examples of absorber material variants denoted AM1 and AM2. As the height h _abs of the absorber structure 9 increases, the phase difference Δφ also increases. This growth is not monotonous, but periodic. In addition, different absorber material deformations lead to different slopes of the growth of the phase difference Δφ as a function of the height h _abs of the absorber structure 9 .

The phase difference Δφ should be in the region of 180° so that excellent image contrast is obtained while imaging the structure of the object 8 using the projection optical unit 5 . The absorber structure height h _abs with such a phase difference Δφ in the region of 180° is marked with an asterisk, respectively, in FIG. 3 . It can be seen that this structure height h _abs for the absorber structure 9 is highly dependent on the respective absorber material deformation. For this reason, it is necessary to accurately determine the phase difference Δφ using the metrology system 2 , wherein the phase difference value Δφ is independent of the structural extension of the absorber structure 9 in the object plane 7 , i.e. in particular the pitch (pitch), ie the periodicity of the absorber structure 9 with respect to the object field 6 is independent. This determination method is described in more detail below.

The illumination light 1 is reflected by the lithography mask 8 as schematically shown in FIG. 1 and is incident on the entrance pupil of the imaging optical unit 5 in the plane of the incidence pupil. The used entrance pupil of the imaging optical unit 5 may have a circular or elliptical boundary.

Within the imaging optical unit 5 , illumination or imaging light 1 propagates between the incident pupil plane and the exit pupil plane. The circular exit pupil of the imaging optical unit 5 lies in the exit pupil plane.

The imaging optical unit 5 images the object field 6 into the image field 11 of the image plane 12 of the metrology system 2 . The image plane 12 is also referred to as the measurement plane. The imaging scale during imaging using the projection optical unit 5 is greater than 500. Depending on the embodiment of the projection optical unit 5 , the magnification imaging scale may be greater than 100, greater than 200, greater than 250, greater than 300, greater than 400, and even greater than 500. . The imaging scale of the projection optical unit 8 is regularly smaller than 2000.

The projection optical unit 5 serves to image a part of the object 8 into the image plane 12 .

Similar to the example of the top view of the object 8 in FIG. 1 , a top view of the aerial image 13 of the imaging of the object 8 is illustrated below the image plane 12 perpendicular to the plane of the drawing. This aerial image 13 is a partial data set of the entire 3D aerial image, and its measurement will be described later.

The electric field of the illumination light 1 in the image field 11 can be described as follows.

(4)

(4)

While the electric field of the illumination light 1 is transmitted from the object field 6 to the image field 11 , the aberration and defocus of the projection optical unit 5 influence the shape of the electric field.

The spatial resolution detection device 14 of the metrology system 2 is arranged in the image plane 12 . This detection device may be a CCD camera. A detection device 14 is used to measure the intensity I, for:

(5)

(5)

The detection device 14 is displaceable in the z-direction and is shown in FIG. 1 in a concave position at a distance from the image plane 12 . During the measurement operation, the detection device 14 is arranged in or near the image plane 12 . Accordingly, the detection plane 15 of the detection device 14 may coincide with or have a defined distance from the image plane 12 .

The metrology system 2 is used to carry out a method for determining the optical phase difference Δφ as a property applicable over the entire object structure to be measured. This property can be used to define the object 8 with respect to the contrast property of the object 8 while imaging the object 8 through the projection exposure apparatus.

The main steps of the determination method will be described further with reference to FIG. 4 .

In a measuring step 16 , a series of two-dimensional images I(x,y) of the object 8 are taken at different focal planes for recording a three-dimensional aerial image of the object 8 using the projection optical unit 5 . measured in each case. After each 2D image measurement during which the 2D image intensity value I(x,y) is recorded, the detection device 14 is displaced by the specified increment Δz with the aid of a detection displacement device (not shown). These 2D images I(x,y) of eg 5, 7, 9, 11 or 13 are recorded at different z-values for a complete measurement of the 3D aerial image. During this measurement, at least the second diffraction of the illumination light or measurement light 1 diffracted by the object 8 is guided by the projection optical unit 5 to the image field 11 , ie to the image side of the metrology system 2 . do.

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

5 shows as an example the phase distribution over the image field 11 as a result of the reconstruction step 17 for an object 8 having a line structure. The absorber structure 9 has an absolute phase ? in the region of 30° different from the reflector structure 10, which has an absolute phase in the region of 210°.

The reconstitution step 17 can work with the aid of the method known from WO 2017/207 297 A1.

Next, the phase difference is determined in the field distribution f _rec reconstructed by the phase correction step 18 . During phase correction, the image-side field distribution (f _im ) is the object period or pitch (p) of the object (8), the critical dimension (CD) of the object (8), the complex reflectance (r _abs ) of the absorber structure (9) and It is calculated by introducing a model based on the object field distribution that depends on the complex reflectance r _ML of the reflector structure 10 . The image-side field distribution f _im is the result of the convolution of the object field distribution with the coherent point spread function PSF of the projection optical unit 5 . The coherent point spread function is the Fourier transform of the coherent optical transfer function.

This is illustrated in FIG. 6 . 6 shows the spatial coordinate (x) on the left, the reflectance (r _ML ) of the reflector structure (10), the reflectance (r _abs ) of the absorber structure (9), the duty cycle (d) and the period (pitch) (p) dependent shows the object field distribution f( _obj ). For duty cycle (d): d = CD/p.

Based on this object structure f _obj divided into a real part (index r) and an imaginary part (index i), the influence of the projection optical unit 5 on the generation of imaging of the object field distribution is now ) is considered by convolution using the point spread function (PSF _coh ). This is shown in the center of figure 6, where the real and imaginary parts are defined for different parameters r, d in each case. The result of the convolution of the object field distribution using the point spread function (PSF) is the real part (f ^r _im ) and the imaginary part ( ^fi _im ) of the image-side field distribution ( f _im ), which are shown on the right side of FIG. 6 . schematically shown. This image-side field distribution (f _im ) is, in turn, the spatial coordinate (x), the reflectance (r _ML , r _abs ) of the reflector structure (10) and the absorber structure (9), the imaginary part (di) and the real part (di) of the duty cycle ( dr), and the pitch p.

Then, the image-side field distribution (fim) calculated in the model way is compared with the reconstructed image-side field distribution (f _rec ). To this end, the difference between the field distributions is minimized with the help of a fitting method by changing at least one of the model parameters object period (pitch p), critical dimension (CD) and complex reflectance (r _ML , r _abs ). This is done with the help of a merit function which is considered to be integrated and minimized over the xy field range. This merit function (M) can be written as

(6)

(6)

In order to minimize this merit function implemented according to Equation 6 above independently for the real and imaginary parts, the reconstructed image-side field distribution (f _rec ) and the computed image-side field distribution (f _im ) is Fourier transformed. These Fourier transforms F _rec, F _im depend on the spatial frequency (ν _x , ν _y ).

With the help of Fourier transforms F _{rec and} F _im , it is sequentially partitioned into a real part and an imaginary part, so that the merit function (M) can be written as

(7)

(7)

The integral in the frequency domain can be replaced by the summation over these frequencies where each factor for the periodic structure deviates from zero, where the considered line structure of the object 8 is for j = - j _max ... j _max . The frequency ν _x = j/p.

j _max is the maximum diffraction order transmitted by the projection optical unit 5 in this case.

The projection optical unit 5 sets the condition |j _max | In order to satisfy ≥ 2, the following must hold for the pitch p of the object 8: p ≥ 2λ/NA.

where λ is the wavelength of the illumination light 1 and NA is the object-side numerical aperture of the projection optical unit 5 .

This sum of the real and imaginary parts of the merit function is linearly dependent on the real and imaginary parts of the reflectances (r _ML , r _abs ).

(8)

(8)

The coefficients of this linear notation are further non-linearly dependent on the real and imaginary parts d _r, d _i of the duty cycle d. This non-linear duty cycle output function can be linearized as described below.

Optimization of the real and imaginary parts (d _r , d _i ) of the duty cycle and reflectance (r _ML , r _abs ) to minimize the merit function can be influenced by an iterative fitting method, where the duty cycle (d) Initial starting values for the real and imaginary parts of d ⁰ _r,i are taken and then the corresponding starting values for reflectivity r ^r,i _ML,0 and r ^r,i _abs,0 are calculated.

To do this, we can write the merit function in the initial iteration step like this:

(9)

(9)

는 반사율 r에 대한 계수를 나타내며, 이는 차례로 듀티 사이클 d의 실수부와 허수부에 종속된다.

는 이 경우 j_max 행 및 두 반사율(r_ML, r_abs)에 대한 행렬의 영향으로 인한 2개의 행을 갖는 행렬이다. 반사율(r_ML, r_abs)의 시작 값 r^r,i _ML,0 및 r^r,i _abs,0은 선형 최적화에 의해 듀티 사이클 d⁰ _r,i 의 시작 값을 기반으로 결정된다.here,

denotes the coefficient for the reflectance r, which in turn depends on the real and imaginary parts of the duty cycle d.

is in this case a matrix with j _max rows and 2 rows due to the effect of the matrix on both reflectances (r _ML , r _abs ). The starting values r ^r,i _ML,0 and r ^r,i _abs,0 of the reflectance r _ML , r _abs , are determined based on the starting value of the duty cycle d ⁰ _r,i by linear optimization.

의 개별 행렬 요소는 이제 Taylor 급수에서 듀티 사이클의 시작 값 주위로 확장될 수 있다. 이 Taylor 급수의 두 번째 항은 Δd⁰ _r,i 값에서 선형이다 즉, 듀티 사이클(d)의 시작 값에 대한 실수부와 허수부의 수정 값이다. 상기 수정 값은 반복 방법의 다음 단계를 위해 시작 값을 변경해야 하는 값이다.

The individual matrix elements of can now be extended around the starting value of the duty cycle in the Taylor series. The second term of this Taylor series is linear at the value of Δd ⁰ _r,i , i.e. it is the correction of the real and imaginary parts of the starting value of the duty cycle (d). The correction value is a value at which the starting value should be changed for the next step of the iterative method.

Inserting the Taylor extension up to the Δd order into Equation (8) gives rise to a linear dependence on the correction values for reflectance and duty cycle.

(10)

(10)

₂ 는 듀티 사이클에 대해 알려진 시작 값과 반사율의 이미 계산된 시작 값에만 종속된다.modified matrix

₂ depends only on the known starting value for the duty cycle and the already calculated starting value of the reflectance.

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 according to the formula:

(11)

(11)

은 이제 듀티 사이클에 대한 새로운 값으로 업데이트되고 반사율의 새로운 선형 최적화가 수행된다.In the above equation (9), the matrix

is now updated with the new value for the duty cycle and a new linear optimization of the reflectance is performed.

This iterative method is repeated until, in iteration step m, the repeated correction Δd ^m _r,i falls below the specified threshold.

After convergence of this iterative fitting method, the reflectance values r _ML, r _abs at which the merit function M is minimized can be found.

A desired value for the phase difference (Δφ) is obtained from the reflectance values (r _ML, r _abs ) obtained according to the following equation.

(12)

(12)

Therefore, it is possible to calculate the phase difference value for a specific mask structure based on the object structure modeling, so that it is possible to check whether a specific structure design provides an optimal phase difference for image contrast.

7 shows another example of an object 20 for which an optical phase difference is determined using the method embodiment described above. Such an object 20 may be used in place of the object 8 discussed above in particular in relation to FIG. 2 .

The object 20 is a hard phase shift mask (PSM) having an etched substructure 22 and a superstructure 21 supported by a common base layer 23 . The superstructure 21 is embodied as a reflective surface superstructure part having a reflectance R1 . Substructure 22 is embodied as a reflector surface substructure etched from multiple layers establishing both the top and bottom structures. Substructure 22 has a reflectance R2. For the reflectance (R1, R2), the following substitution relationship can be established: R1 > R2, R1 = R2.

The upper/lower structures 21 , 22 create an optical retardation Δφ in the measurement light 1 reflected by the superstructure 21 on the one hand and the lower structure 22 on the other hand. The etch depth of the substructure 22 relative to the superstructure 21 is shown as h in FIG. 7 .

The measurement light ( The phase difference Δφ of 1) may be determined according to the method embodiment discussed above.

Unlike the object 8 , the superstructure 21 in the object 20 is also made of a multi-layered material. In particular, the object 20 is manufactured by etching the multilayer substrate.

8 shows another embodiment of an object 25 that may be used in place of the objects 8 or 20 described above. Components and functions discussed above, particularly with reference to FIGS. 2 and 7 , are given the same terms and reference numbers and are not described in detail again.

The object 25 has, between the superstructure 21 and the substructure 22 , an etch stop layer 26 of a material different from the layer material of the multilayer composition of the top and bottom structures 21 , 22 . This etch stop layer is used to differentiate in the etch production process of the object 25 between the superstructure part and the substructure part when etching is performed until reaching the etch stop layer 26 .

Of course, the reflectivity, material and thickness of the etch stop layer 26 of the object 25 affects the optical retardation of the measurement light 1 between the top structure phase on the one hand and the bottom reflector structure phase on the other hand. This can be used in the optimization process to create a hard phase shift mask. During this correction process, a corrected raw phase shift mask according to the object 25 is created. Then, the optical phase difference (Δφ) between the upper and lower reflector structure phases of such a primitive correction mask is determined. Thereafter, the thickness and/or the etching depth h of the etch stop layer 26 is changed to achieve a desired retardation Δφ, for example, a retardation of 180° (= π).

Figure 9 shows another embodiment of an object 27 that can also be used as a phase shift mask and can replace the objects 8, 20, 25 described above. Components and functions discussed above particularly with respect to FIGS. 2 and 8 are given the same terms and reference numerals and are not described in detail again.

Object 27 also includes an etch stop layer 26 . Unlike the layout of the object 25 , the etch stop layer 26 of the object 27 is removed from the substructure portion of the substructure 22 .

FIG. 10 shows another embodiment of an object 30 that may be used in place of the objects 8, 20, 25, 27 described above. Components and functions discussed above, particularly with respect to FIGS. 2 and 9, are given the same terms and reference numerals and are not described in detail again.

The object 30 comprises in particular an upper absorber structure 9 and a lower reflector structure 10 according to what has been described above with respect to FIG. 2 .

By way of example, one of such absorber structures 9, denoted _9D in FIG. 10, contains a defect. This defect is present by a reduced extension in the x-direction of the absorber structure 9 _D .

As a repair of this defect, the object 30 retains a structural supplement 31 made of repair absorber material. This repair absorber material is generally different from the absorber material of the original absorber structure 9 of the object 30 . Compared to the height h of the nominal absorber structure 9 , the hydraulic structure supplement 31 has a height h _D which is generally another h.

During the repair process, at least one embodiment of the phase metrology of the determination method discussed above is used to determine the complex reflectance according to the repair absorber material used for structural replenishment. Also, as described above, the complex reflectivity of the absorber material of the object 30 to be repaired is determined. After determining the complex reflectance of the repair absorber material on the one hand and the absorber material of the object to be repaired on the other hand, the repair absorber material and the height h _D are determined such that the complex reflectance of the repair absorber material is the absorber material of the mask to be repaired. is chosen to match the complex reflectance of

Optical phase difference determination methods and in particular phase metrology can further be used to optimize the phase difference Δφ to minimize the object 3D effect on imaging in a production projection exposure apparatus. Furthermore, this determination method, in particular phase metrology, can be used to correct object parameters and in particular lithographic mask parameters for simultaneous optimization of the illumination settings of the mask layout on the one hand and of the production projection exposure apparatus on the other hand.

In particular, it has been recognized that the arrangement of the superstructure of each object affects the phase difference (Δφ) and thus may affect the imaging results of the production projection exposure apparatus.

These superstructure effects can be measured using the above-described embodiment of a method for determining optical retardation (Δφ) and stored for different superstructure arrays in a retardation library for later use in connection with optimization of lithographic masks. can

Regarding the optical retardation Δφ, the thickness of each layer of the multilayer structure and the complex refractive index, ie, the absorption effect of the refractive index in particular, are influencing parameters.

Claims (10)

A method for determining the optical phase difference (Δφ) of a measurement light (1) of a measurement light wavelength (λ) with respect to a surface of a structured object (8; 20; 25; 27; 30), the method comprising:
- the superstructure phase φ _abs of the measurement light 1 reflected by the superstructure 9; 21 of the object 8; 20; 25; 27; 30;
- the lower reflector structure phase (φ _ML ) of the measurement light 1 reflected by the lower reflector structure 10; 22 of the object 8; 20; 25; 27; 30
The phase difference (Δφ) between
The following steps:
- measure a series of 2D images of the object 8 ; 20 ; 25 ; 27 ; 30 in each case in a different focal plane in order to record a 3D aerial image of the object 8 using the projection optical unit 5 . step (16),
- reconstructing (17) the image-side field distribution (f _rec ) comprising the amplitude and phase of the electric field of the 3D aerial image from the 3D aerial image;
- determining the phase difference (Δφ) from the reconstructed field distribution (f _rec ) with the aid of a phase correction ( 18 ),
Here, the following steps in phase correction 18
-- the object period (p) of the object (8; 20; 25; 27; 30) and/or
-- the critical dimension (CD) of the object (8; 20; 25; 27; 30) and/or
-- the complex reflectance r ^r,i _abs of the superstructure 9; 21 and/or
-- complex reflectance (r ^r,i _ML ) of the reflector structure (10, 22)
- calculating the image-side field distribution (f _im ) by introducing a model based on the object field distribution (f _obj ) _dependent on is the result of the convolution of the coherent point spread function PSF of the projection optical unit 5 and the object field distribution f _obj ;
- comparing the calculated image-side field distribution (f _im ) with the reconstructed image-side field distribution (f _rec );
- the image-side field distribution (f _im ) calculated by the fitting method by changing the model parameters object period (p) and/or critical dimension (CD) and/or complex reflectance (r ^r,i _abs,ML ) and minimizing the difference between the reconstructed image-side field distributions (f _rec );
- calculating the phase difference Δφ from the model parameters resulting in a minimization for the complex reflectance r ^r,i _abs and r ^r,i _ML .
how it is done. The method according to claim 1, characterized in that in the fitting method, linearization of the initial nonlinear output function is achieved by minimizing the difference between the calculated image-side field distribution (f _im ) and the reconstructed image-side field distribution (f _rec ). The method according to claim 2, wherein in the calculation of the image-side field distribution (f _im ), the real part (f ^r _im ) and the imaginary part (f ⁱ _im ) are modeled with parameters independent of each other. 4. The method according to claim 2 or 3, characterized by an iterative fitting method. Method according to any one of claims 2 to 4, characterized in that the real part (f ^r _im ) and the imaginary part (f ⁱ _im ) of the image-side field distribution (f _im ) are fitted independently of each other. Method according to any one of claims 2 to 5, characterized in that at least one Fourier transform as a constituent part of the fitting method. 7. The method according to any one of the preceding claims, wherein during the measuring step (16), at least a second diffraction (j) of the measuring light (1) diffracted by the object (8; 20; 25; 27; 30) is Method, characterized in that it is guided towards the image side by the projection optical unit (5). A metrology system (2) having an optical measuring system for carrying out the method according to any one of claims 1 to 7, comprising:
- having an illumination optical unit 4 for illuminating the object to be inspected (8; 20; 25; 27; 30) with a specific illumination setting;
- an imaging optical unit 5 for imaging a part of the object 8; 20; 25; 27; 30 into the measuring plane 12;
- metrology system with spatial resolution detection device (14) arranged in measuring plane (12). 9. The imaging optical unit (5) according to claim 8, wherein the design of the imaging optical unit (5) is such that at least second-order diffraction (j) of the measurement light (1) diffracted by the object (8) during the measuring step (16) ) to be guided to the image side of the imaging optical unit (5). 10. Metrology system according to claim 8 or 9, characterized by an EUV light source (3).

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