Method for calibrating optical inspection device, and method for manufacturing reflection-type mask blank

Abstract

This method for calibrating an optical inspection device comprises: preparing a calibration substrate having a plurality of defects with different dimensions; preparing measurement data of dimensions of standard defects measured by an inspection device different from the optical inspection device; measuring the dimensions of the standard defects using the optical inspection device; and creating a correlation expression between measurement data of the optical inspection device and measurement data of the other inspection device in relation to the dimensions of the standard defects. The calibration substrate includes a defect-forming film in which the standard defects are formed, and a base substrate on which the defect-forming film is formed. The uppermost layer of the defect-forming film contains at least one element selected from the group consisting of Cr, Ta, Ir, Pt, Pd, Au, and Ru.

Description

Method for calibrating optical inspection equipment and method for manufacturing reflective mask blanks

[0001] The present disclosure relates to a method for calibrating an optical inspection apparatus and a method for manufacturing a reflective mask blank.

[0002] In recent years, with the miniaturization of semiconductor devices, EUV lithography (EUVL), an exposure technology using extreme ultraviolet (EUV) light, has been developed. EUV includes soft X-rays and vacuum ultraviolet light, and specifically refers to light with a wavelength of approximately 0.2 nm to 100 nm. At present, EUV light with a wavelength of approximately 13.5 nm is mainly being considered.

[0003] In EUVL, a reflective mask is used. The reflective mask has, for example, a conductive film, a glass substrate, a multilayer reflective film, a protective film, and an absorbing film, in that order. The conductive film is used to attach the reflective mask to an electrostatic chuck of an exposure tool. The multilayer reflective film reflects EUV light. The protective film protects the multilayer reflective film when an opening pattern is formed in the absorbing film. The absorbing film absorbs EUV light. In EUVL, the opening pattern in the absorbing film is transferred to a target substrate such as a semiconductor substrate. Transferring includes reducing and transferring.

[0004] A method for manufacturing a reflective mask includes preparing a reflective mask blank and forming an aperture pattern in an absorbing film. The reflective mask blank has the same configuration as a reflective mask, except that the absorbing film does not have an aperture pattern. The reflective mask blank has, for example, a conductive film, a glass substrate, a multilayer reflective film, a protective film, and an absorbing film, in this order. Before manufacturing the reflective mask, a defect inspection of the reflective mask blank is performed. The target of the defect inspection is, for example, at least one of the conductive film and the absorbing film.

[0005] Patent Document 1 discloses a method for adjusting the optical axis of a defect inspection device used in the manufacturing process of a photomask blank. The method described in Patent Document 1 involves capturing an image of a square defect formed on the surface of a substrate, creating a cross-sectional profile of the defect intensity in the image, and adjusting the optical axis of the defect inspection device based on the symmetry of the cross-sectional profile.

[0006] Patent Document 2 discloses correcting the defect dimensions measured by a defect inspection device according to the defect dimensions obtained separately. The defect dimensions are corrected using, for example, a linear equation (y = ax + b). x is the dimension before correction, y is the dimension after correction, a is a correction coefficient, and b is an offset value. a and b are obtained from the relationship between the defect dimensions measured by the defect inspection device and the defect dimensions measured using an SEM. When obtaining a and b, a standard wafer with defects built in or a product wafer with standard particles sprayed on it is used.

[0007] Japanese Patent Publication No. 2020-16629 Japanese Patent Publication No. 2007-24737

[0008] Before manufacturing a reflective mask, a reflective mask blank is subjected to defect inspection. The reflective mask blank has, for example, a conductive film, a glass substrate, a multilayer reflective film, a protective film, and an absorbing film, in this order. The target of defect inspection is, for example, at least one of the conductive film and the absorbing film. The conductive film contains at least one element selected from Cr and Ta. The absorbing film contains at least one element selected from Ta, Ir, Cr, Pt, Pd, Au, and Ru.

[0009] One aspect of the present disclosure provides a technique for accurately measuring the dimensions of defects contained in the surface of an object when the surface of the object contains at least one element selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru.

[0010] A calibration method for an optical inspection device according to one aspect of the present disclosure is a method for calibrating an optical inspection device that measures dimensions of defects contained in the surface of an object by capturing an image of the surface of the object, the method comprising: preparing a calibration substrate having a plurality of standard defects of different dimensions; preparing measurement data of the dimensions of the standard defects measured by an inspection device other than the optical inspection device; measuring the dimensions of the standard defects with the optical inspection device; and creating a correlation equation regarding the dimensions of the standard defects between the measurement data of the optical inspection device and the measurement data of the other inspection device. The calibration substrate includes a defect-forming film on which the standard defects are formed and an underlying substrate on which the defect-forming film is formed. The uppermost layer of the defect-forming film contains at least one element selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru.

[0011] According to one aspect of the present disclosure, when the surface of an object contains at least one element selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru, the dimensions of defects contained in the surface of the object can be measured with high accuracy.

[0012] FIG. 1 is a cross-sectional view showing a reflective mask blank according to an embodiment. FIG. 2 is a flowchart showing a method for manufacturing a reflective mask blank according to an embodiment. FIG. 3 is a cross-sectional view showing a reflective mask according to an embodiment. FIG. 4 is a flowchart showing a method for manufacturing a reflective mask according to an embodiment. FIG. 5(A) is a cross-sectional view showing an example of step S201, FIG. 5(B) is a cross-sectional view showing an example of step S202, and FIG. 5(C) is a cross-sectional view showing an example of step S203. FIG. 6 is a cross-sectional view showing an example of EUV light reflected by the reflective mask of FIG. 3. FIG. 7 is a diagram showing an optical inspection apparatus according to an embodiment. FIG. 8 is a flowchart showing a method for calibrating an optical inspection apparatus according to an embodiment. FIG. 9 is a cross-sectional view showing an example of a calibration substrate. FIG. 10 is a cross-sectional view showing another example of a calibration substrate. FIG. 11 is a plan view showing an example of a standard defect. FIG. 12 is a diagram showing measurement data from an optical inspection apparatus, measurement data from another inspection apparatus, and an example of a correlation equation therebetween.

[0013] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or corresponding components are denoted by the same reference numerals, and their description may be omitted. In the specification, the symbol "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits. The range of values ​​includes the range rounded up or down.

[0014] In each drawing, the X-axis direction, the Y-axis direction, and the Z-axis direction are perpendicular to one another. The Z-axis direction is perpendicular to the first main surface 10a of the substrate 10. The X-axis direction is perpendicular to the plane of incidence of the EUV light (the plane including the incident light beam and the reflected light beam). As shown in Figure 6, the incident light beam is tilted more in the positive Y-axis direction as it moves in the negative Z-axis direction, and the reflected light beam is tilted more in the positive Y-axis direction as it moves in the positive Z-axis direction.

[0015] A reflective mask blank 1 according to one embodiment will be described with reference to FIG. 1 . The reflective mask blank 1 includes, for example, a substrate 10, a multilayer reflective film 11, a protective film 12, an absorbing film 13, and a hard mask film 14, in this order. The multilayer reflective film 11, the protective film 12, the absorbing film 13, and the hard mask film 14 are formed on a first main surface 10a of the substrate 10 in this order. The multilayer reflective film 11 reflects EUV light. The protective film 12 protects the multilayer reflective film 11 from a first etching gas during processing of the absorbing film 13. The absorbing film 13 absorbs EUV light. The absorbing film 13 may not only absorb EUV light but also shift the phase of the EUV light. In other words, the absorbing film 13 may be a phase shift film. The hard mask film 14 protects a portion of the absorbing film 13 from the first etching gas during processing of the absorbing film 13.

[0016] The reflective mask blank 1 may have a conductive film 15 on the side opposite to the multilayer reflective film 11 with respect to the substrate 10. That is, the reflective mask blank 1 may have the conductive film 15, substrate 10, multilayer reflective film 11, protective film 12, absorbing film 13, and hard mask film 14, in this order. The conductive film 15 is formed on the second main surface 10b of the substrate 10. The second main surface 10b is the surface facing opposite to the first main surface 10a. The conductive film 15 is used, for example, to attract the reflective mask 2 to an electrostatic chuck of an exposure tool.

[0017] The reflective mask blank 1 may further have a functional film not shown in Fig. 1. For example, the reflective mask blank 1 may have a diffusion barrier film (not shown) between the multilayer reflective film 11 and the protective film 12. The diffusion barrier film suppresses diffusion of metal elements contained in the protective film 12 into the multilayer reflective film 11.

[0018] Although not shown, the reflective mask blank 1 may have a buffer film between the protective film 12 and the absorbing film 13. The buffer film protects the protective film 12 from the first etching gas that forms the opening pattern 13a in the absorbing film 13. The buffer film is etched more slowly than the absorbing film 13. Unlike the protective film 12, the buffer film will ultimately have the same opening pattern as the opening pattern 13a in the absorbing film 13.

[0019] Next, a method for manufacturing a reflective mask blank 1 according to one embodiment will be described with reference to FIG. 2. The method for manufacturing a reflective mask blank 1 includes, for example, steps S101 to S108 shown in FIG. 2. In step S101, a substrate 10 is prepared. In step S102, a conductive film 15 is formed on the second main surface 10b of the substrate 10. In step S103, the dimensions of any defects in the conductive film 15 are measured using an optical inspection device calibrated using a calibration method described below. The defects in the conductive film 15 may be concave or convex. The measurement results in step S103 are used, for example, to determine whether the reflective mask blank 1 is good or bad. In step S104, a multilayer reflective film 11 is formed on the first main surface 10a of the substrate 10. In step S105, a protective film 12 is formed on the multilayer reflective film 11. In step S106, an absorbing film 13 is formed on the protective film 12. In step S107, the dimensions of any defects in the absorbing film 13 are measured using an optical inspection device calibrated using a calibration method described below. In step S107, not only the size of the defect in the absorbing film 13 but also the position of the defect in the absorbing film 13 may be measured. The defect in the absorbing film 13 may be concave or convex. The measurement result in step S107 is used, for example, to determine the quality of the reflective mask blank 1 or to correct the formation position of the opening pattern 13a in the absorbing film 13. In step S108, a hard mask film 14 is formed on the absorbing film 13.

[0020] The order of steps S101 to S108 is not limited to the order shown in FIG. 2. For example, the order of steps S102 to S103 and steps S104 to S108 may be reversed. Furthermore, the method for manufacturing the reflective mask blank 1 does not have to include all of steps S101 to S108. For example, the method for manufacturing the reflective mask blank 1 may include only one of steps S103 and S107. Furthermore, the method for manufacturing the reflective mask blank 1 may further include a step of forming a functional film not shown in FIG. 2.

[0021] Next, a reflective mask 2 according to one embodiment will be described with reference to FIG. 3. The reflective mask 2 is fabricated using, for example, the reflective mask blank 1 shown in FIG. 1 and includes an opening pattern 13a in an absorbing film 13. In EUVL, the opening pattern 13a in the absorbing film 13 is transferred to a target substrate such as a semiconductor substrate. Transferring includes reducing and transferring. Note that the hard mask film 14 shown in FIG. 1 is not included in the reflective mask 2.

[0022] Next, a method for manufacturing a reflective mask 2 according to one embodiment will be described with reference to Figures 4 and 5. The method for manufacturing a reflective mask 2 includes steps S201 to S204 shown in Figure 4. In step S201, a reflective mask blank 1 is prepared, as shown in Figure 5(A). The reflective mask blank 1 includes a resist film 16, as shown in Figure 5(A). The resist film 16 is formed on a hard mask film 14. An opening pattern to be transferred to the absorption film 13 is formed in the resist film 16.

[0023] 5B, the hard mask film 14 is processed using the resist film 16 having an opening pattern. In the openings in the resist film 16, the hard mask film 14 is exposed to a second etching gas, and the second etching gas etches the hard mask film 14. At the end of step S202, the resist film 16 remains. As a result, the opening pattern of the resist film 16 is transferred to the hard mask film 14.

[0024] The second etching gas is selected depending on the combination of the material of the resist film 16 and the material of the hard mask film 14, and is not particularly limited, but may include, for example, a fluorine-based gas. The fluorine-based gas may be, for example, CF ４ Gas, CHF ３ Gas, C ２ F ６ Gas, C ３ F ６ Gas, C ４ F ６ Gas, C ４ F ８ Gas, CH ２ F ２ Gas, CH ３ F gas, C ３ F ８ Gas, F ２ Gas, SF ６ Gas and NF ３ The second etching gas may contain at least one selected from the group consisting of a fluorine-based gas, an active gas, and an inert gas. The active gas may be, for example, O ２ The inert gas includes, for example, N ２ The second etching gas preferably contains at least one selected from the group consisting of a nitrogen gas, a He gas, and an Ar gas. The second etching gas is preferably a plasma.

[0025] In step S203, as shown in FIG. 5C , the absorber film 13 is processed using the hard mask film 14 having an opening pattern. In the openings in the hard mask film 14, the absorber film 13 is exposed to a first etching gas, and the first etching gas etches the absorber film 13. The hard mask film 14 has higher resistance to the first etching gas than the absorber film 13. At the end of step S203, the hard mask film 14 remains. As a result, the opening pattern of the hard mask film 14 is transferred to the absorber film 13.

[0026] The first etching gas is selected depending on the combination of the material of the hard mask film 14 and the material of the absorption film 13, and is not particularly limited, but includes, for example, a chlorine-based gas and an oxygen-based gas. The chlorine-based gas is, for example, Cl ２ Gas, SiCl ４ Gas, CHCl ３ Gas, CCl ４ Gas and BCl３ The oxygen-based gas includes at least one selected from the group consisting of O ２ Gas and O ３ The first etching gas may contain at least one selected from the group consisting of a chlorine-based gas, an oxygen-based gas, and an inert gas. The inert gas may be, for example, N ２ The first etching gas preferably contains at least one selected from the group consisting of a nitrogen gas, a He gas, and an Ar gas. The first etching gas is preferably a plasma.

[0027] In step S204, although not shown, the hard mask film 14 is removed. A third etching gas, for example, is used to remove the hard mask film 14. The third etching gas, like the second etching gas, contains, for example, a fluorine-based gas. The third etching gas is preferably a plasma gas. A chemical solution may be used to remove the hard mask film 14.

[0028] Next, referring back to FIG. 1, the substrate 10, the multilayer reflective film 11, the protective film 12, the absorbing film 13, the hard mask film 14, and the conductive film 15 will be described in this order.

[0029] The substrate 10 is, for example, a glass substrate. The material of the substrate 10 is TiO ２ The quartz glass has a smaller coefficient of linear expansion and a smaller change in dimension due to temperature changes than common soda lime glass. ２ 80% by mass to 95% by mass of TiO ２ It may contain 4% to 17% by mass of TiO ２ When the content is 4% by mass to 17% by mass, the linear expansion coefficient is approximately zero around room temperature, and there is almost no dimensional change around room temperature. ２ and TiO ２ The substrate 10 may contain a third component or impurities other than the above. The material of the substrate 10 may be crystallized glass in which a β-quartz solid solution is precipitated, silicon, a metal, or the like.

[0030] The substrate 10 has a first major surface 10a and a second major surface 10b facing opposite to the first major surface 10a. A multilayer reflective film 11 and the like are formed on the first major surface 10a. In plan view (viewed in the Z-axis direction), the substrate 10 measures, for example, 152 mm long and 152 mm wide. The vertical and horizontal dimensions may be 152 mm or greater. The first major surface 10a and the second major surface 10b each have, for example, a square quality assurance area at their centers. The quality assurance area measures, for example, 142 mm long and 142 mm wide. The vertical and horizontal dimensions may be 142 mm or greater. The quality assurance area on the first major surface 10a preferably has a root-mean-square roughness (Rq) of 0.15 nm or less and a flatness of 100 nm or less. Furthermore, the quality assurance area on the first major surface 10a preferably does not have defects that cause phase defects.

[0031] The multilayer reflective film 11 reflects EUV light. The multilayer reflective film 11 is formed by alternately stacking, for example, high-refractive-index layers and low-refractive-index layers. The high-refractive-index layers are made of, for example, silicon (Si), and the low-refractive-index layers are made of, for example, molybdenum (Mo), so that a Mo / Si multilayer reflective film is used. Note that other films that can be used as the multilayer reflective film 11 include a Ru / Si multilayer reflective film, a Mo / Be multilayer reflective film, a Mo compound / Si compound multilayer reflective film, a Si / Mo / Ru multilayer reflective film, a Si / Mo / Ru / Mo multilayer reflective film, a Si / Ru / Mo / Ru multilayer reflective film, and a Si / Ru / Mo multilayer reflective film.

[0032] The thickness of each layer constituting the multilayer reflective film 11 and the number of repeating units of the layers can be appropriately selected depending on the material of each layer and the reflectivity for EUV light. When the multilayer reflective film 11 is a Mo / Si multilayer reflective film, in order to achieve a reflectivity of 60% or more for EUV light at an incident angle θ (see FIG. 6 ) of 6°, Mo layers with a thickness of 2.3±0.1 nm and Si layers with a thickness of 4.5±0.1 nm can be stacked so that the number of repeating units is 30 to 60. The multilayer reflective film 11 preferably has a reflectivity of 60% or more for EUV light at an incident angle θ of 6°. The reflectivity is more preferably 65% ​​or more.

[0033] The protective film 12 is formed between the multilayer reflective film 11 and the absorbing film 13 to protect the multilayer reflective film 11. The protective film 12 protects the multilayer reflective film 11 from the first etching gas when processing the absorbing film 13, i.e., in step S203. The protective film 12 is not removed even when exposed to the first etching gas, but remains on the multilayer reflective film 11.

[0034] The protective film 12 contains at least one element selected from, for example, Ru, Rh, and Si. When the protective film 12 contains Rh, the protective film 12 may contain only Rh, or may contain an Rh compound. The Rh compound may contain, in addition to Rh, at least one element selected from the group consisting of Ru, Nb, Mo, Ta, Ir, Pd, Zr, Y, and Ti.

[0035] The protective film 12 may be a film consisting of a single layer, or may be a multilayer film having a lower layer and an upper layer. The lower layer of the protective film 12 is a layer formed in contact with the uppermost surface of the multilayer reflective film 11. The upper layer of the protective film 12 is in contact with the lowermost surface of the absorbing film 13. By making the protective film 12 have such a multilayer structure, materials with excellent predetermined functions can be used for each layer, thereby making the protective film 12 as a whole multifunctional.

[0036] The upper layer of the protective film 12 preferably contains at least one element selected from Ru and Rh, more preferably Rh, and even more preferably a Rh compound. The lower layer of the protective film 12 preferably contains at least one element selected from Ru, Rh, Nb, Mo, Zr, Y, and Si, more preferably Ru. Furthermore, in order to suppress the crystallinity of the protective film 12, the lower layer of the protective film 12 preferably contains, in addition to these elements, at least one element selected from C, N, and B. When the protective film 12 is a multilayer film, the thickness of the protective film 12 below refers to the total film thickness of the multilayer film. A mixing layer formed by mixing components contained in the multilayer reflective film 11 and components contained in the lower layer of the protective film 12 may be formed between the multilayer reflective film 11 and the lower layer of the protective film 12.

[0037] The thickness of the protective film 12 is preferably 1.0 nm to 4.0 nm, more preferably 2.0 nm to 3.5 nm, and even more preferably 2.5 nm to 3.0 nm. If the thickness of the protective film 12 is 1.0 nm or more, the etching resistance is good. Furthermore, if the thickness of the protective film 12 is 4.0 nm or less, the reflectance to EUV light is good.

[0038] The absorbing film 13 absorbs EUV light. The absorbing film 13 is a film in which an opening pattern 13a is to be formed. The opening pattern 13a is not formed in the manufacturing process of the reflective mask blank 1, but is formed in the manufacturing process of the reflective mask 2. The absorbing film 13 may not only absorb EUV light, but also shift the phase of the EUV light. In other words, the absorbing film 13 may be a phase shift film. The phase shift film shifts the phase of the second EUV light L2 relative to the first EUV light L1 shown in FIG. 6 .

[0039] The first EUV light L1 is light that passes through the opening pattern 13a without passing through the absorbing film 13, is reflected by the multilayer reflective film 11, and passes through the opening pattern 13a without passing through the absorbing film 13 again. The second EUV light L2 is light that passes through the absorbing film 13 while being absorbed by the absorbing film 13, is reflected by the multilayer reflective film 11, and passes through the absorbing film 13 while being absorbed by the absorbing film 13 again.

[0040] The phase difference (≧0) between the first EUV light L1 and the second EUV light L2 is, for example, 170° to 250°. The phase of the first EUV light L1 may be ahead of or behind the phase of the second EUV light L2. The absorbing film 13 improves the contrast of the transferred image by utilizing interference between the first EUV light L1 and the second EUV light L2. The transferred image is an image obtained by transferring the opening pattern 13a of the absorbing film 13 onto the target substrate.

[0041] In EUVL, a so-called shadowing effect occurs. The shadowing effect refers to the occurrence of a region in the vicinity of the sidewall of the opening pattern 13 a where the sidewall blocks the EUV light due to the incident angle θ of the EUV light being not 0° (for example, 6°), resulting in a positional or dimensional deviation of the transferred image. In order to reduce the shadowing effect, it is effective to reduce the height of the sidewall of the opening pattern 13 a, and it is also effective to thin the absorbing film 13.

[0042] The thickness of the absorbing film 13 is, for example, 60 nm or less, and preferably 50 nm or less, in order to reduce the shadowing effect, and is preferably 20 nm or more, and more preferably 30 nm or more, in order to ensure a phase difference between the first EUV light L1 and the second EUV light L2.

[0043] In order to reduce the thickness of the absorbing film 13 so as to reduce the shadowing effect while ensuring the phase difference between the first EUV light L1 and the second EUV light L2, it is effective to reduce the refractive index n of the absorbing film 13. Furthermore, in order to reduce the reflectance for EUV light, it is effective to increase the extinction coefficient k of the absorbing film 13. Thus, the absorbing film 13 is required to have excellent optical properties.

[0044] The absorbing film 13 preferably contains at least one metal element selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru. These metal elements have a relatively small refractive index, so the film thickness of the absorbing film 13 can be reduced while maintaining a phase difference. The absorbing film 13 preferably contains a compound of a metal element. The compound of a metal element preferably contains at least one element selected from O, B, C, and N. By adding at least one element selected from O, B, C, and N, crystallization can be suppressed while suppressing deterioration of optical properties, and roughness of the side surfaces of the opening pattern 13a can be reduced.

[0045] In this embodiment, the absorbing film 13 is a film consisting of a single layer, but it may also be a multilayer film having a lower layer and an upper layer. The lower layer and upper layer constituting the absorbing film 13 are formed in this order on the protective film 12. The uppermost layer of the absorbing film 13 is the layer farthest from the protective film 12. The uppermost layer of the absorbing film 13 preferably contains at least one metal element selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru, and more preferably contains a compound of the metal element. When the absorbing film 13 is a multilayer film, the thickness of the absorbing film 13 means the total film thickness of the multilayer film.

[0046] The hard mask film 14 is formed on the opposite side of the protective film 12 with respect to the absorbing film 13, and is used to form an opening pattern 13a in the absorbing film 13. The hard mask film 14 enables the resist film 16 to be made thinner.

[0047] The hard mask film 14 preferably contains at least one metal element or semi-metal element selected from Al, Hf, Y, Cr, Nb, Ti, Mo, Ta, and Si. The hard mask film 14 preferably contains a compound of the above metal element or semi-metal element. The compound preferably contains at least one element selected from O, N, C, and B.

[0048] The thickness of the hard mask film 14 is preferably 2 nm or more and 30 nm or less, more preferably 2 nm or more and 25 nm or less, and further preferably 2 nm or more and 10 nm or less.

[0049] The conductive film 15 is formed on the opposite side of the substrate 10 from the multilayer reflective film 11, and is used to attract the reflective mask 2 to an electrostatic chuck of an exposure tool. The conductive film 15 preferably contains at least one metal element selected from Cr and Ta. The conductive film 15 preferably contains a compound of the above metal element. The compound preferably contains at least one element selected from O, N, C, and B.

[0050] In this embodiment, the conductive film 15 is a film consisting of a single layer, but it may also be a multilayer film having a lower layer and an upper layer. The lower layer and upper layer constituting the conductive film 15 are formed on the substrate 10 in this order. The uppermost layer of the conductive film 15 is the layer farthest from the substrate 10. The uppermost layer of the conductive film 15 preferably contains at least one metal element selected from Cr and Ta, and more preferably contains a compound of the above metal element. When the conductive film 15 is a multilayer film, the thickness of the conductive film 15 means the total film thickness of the multilayer film.

[0051] The thickness of the conductive film 15 is preferably 5 nm or more and 500 nm or less, more preferably 10 nm or more and 450 nm or less, and even more preferably 20 nm or more and 400 nm or less.

[0052] Next, an optical inspection apparatus 100 according to one embodiment will be described with reference to Fig. 7. The optical inspection apparatus 100 measures the dimensions of a defect, for example, in steps S103 and S107 shown in Fig. 2. The optical inspection apparatus 100 includes, for example, a light source 110, a beam splitter 120, a first lens system 121, an objective lens 122, a second lens system 123, a stage 130, an image detector 140, and an image processing device 150. The optical inspection apparatus 100 may further include an aperture filter 161 and a spatial filter 162.

[0053] The light source 110 irradiates light onto a position where an image is to be captured. The wavelength of the light is preferably 200 nm to 800 nm. The light passes through a first lens system 121, an aperture filter 161, a beam splitter 120, and an objective lens 122 in this order, and reaches an object held on a stage 130. The object is, for example, a reflective mask blank 1 or calibration substrates 201 and 202 (described later). After being reflected by the object, the light passes through the objective lens 122, the beam splitter 120, a spatial filter 162, and a second lens system 123 in this order, and reaches an image detector 140. The image detector 140 captures an image. The image detector 140 may be either an area sensor camera or a line sensor camera. The image processing device 150 is composed of, for example, a computer. The image processing device 150 processes the image captured by the image detector 140 to measure the dimensions of the defect. The image processing device 150 may measure the position of the defect in addition to the dimensions of the defect.

[0054] It should be noted that the optical inspection device 100 is not limited to the configuration shown in Fig. 7. The optical inspection device 100 may be any device that can non-destructively inspect the surface of an object using light. In this embodiment, the light is irradiated perpendicularly onto the surface of the object, reflected perpendicularly by the surface of the object, and reaches the image detector 140. However, the light may be irradiated obliquely onto the surface of the object, reflected perpendicularly or obliquely by the surface of the object, and reach the image detector 140.

[0055] Next, a calibration method for the optical inspection device 100 according to one embodiment will be described with reference to Fig. 8 to Fig. 12. The calibration method includes, for example, steps S301 to S304 shown in Fig. 8. Note that the order of steps S301 to S304 is not limited to the order shown in Fig. 8. For example, the order of steps S302 and S303 may be reversed.

[0056] In step S301, for example, a calibration substrate 201 shown in Fig. 9 or a calibration substrate 202 shown in Fig. 10 is prepared. The calibration substrate 201 is used to create a correction formula for correcting the dimensions of a defect in the absorbing film 13. The calibration substrate 202 is used to create a correction formula for correcting the dimensions of a defect in the conductive film 15.

[0057] The calibration substrate 201 includes a substrate 210, a multilayer reflective film 211, a protective film 212, and an absorbing film 213, in this order. The substrate 210 has the same configuration as the substrate 10, the multilayer reflective film 211 has the same configuration as the multilayer reflective film 11, the protective film 212 has the same configuration as the protective film 12, and the absorbing film 213 has the same configuration as the absorbing film 13. The calibration substrate 201 does not include the conductive film 215 shown in FIG. 10 , but may include the conductive film 215. The conductive film 215 is formed on the opposite side of the substrate 210 from the multilayer reflective film 211.

[0058] The calibration substrate 201 has a plurality of standard defects 220 of different dimensions in an absorbing film 213. The substrate 210 is an example of a base substrate. The absorbing film 213 is an example of a defect-forming film. The top layer of the defect-forming film and the top layer of the absorbing film 13 preferably contain one or more of the same elements selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru. The calibration substrate 201 and the reflective mask blank 1 can have the same or similar background for defects in an image, thereby aligning the measurement conditions for the dimensions of the defects in the image. This allows the defect dimensions to be corrected with high precision, improving the measurement accuracy of the defect dimensions.

[0059] It is more preferable that the uppermost layer of the defect-forming film and the uppermost layer of the absorber film 13 have the same composition. Specifically, it is more preferable that the uppermost layer of the defect-forming film and the uppermost layer of the absorber film 13 have the same most abundant element and the difference (absolute value) in the content of the most abundant element is 0 at% to 3 at%. The difference is preferably 0 at% to 1 at%, and more preferably 0 at% to 0.5 at%. The most abundant element is selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru.

[0060] The calibration substrate 202 includes a substrate 210 and a conductive film 215. The substrate 210 has the same configuration as the substrate 10, and the conductive film 215 has the same configuration as the conductive film 15. The calibration substrate 202 does not include the multilayer reflective film 211, the protective film 212, and the absorbing film 213 shown in Fig. 9, but may include these films. The multilayer reflective film 211, the protective film 212, and the absorbing film 213 are formed in this order on the opposite side of the substrate 210 from the conductive film 215.

[0061] The calibration substrate 202 has a plurality of standard defects 220 of different dimensions in a conductive film 215. The substrate 210 is an example of a base substrate. The conductive film 215 is an example of a defect-forming film. The top layer of the defect-forming film and the top layer of the conductive film 15 preferably contain one or more of the same elements selected from Cr and Ta. The calibration substrate 202 and the reflective mask blank 1 can have the same or similar background for defects appearing in an image, and measurement conditions for the dimensions of the defects appearing in the image can be made uniform. Therefore, the defect dimensions can be corrected with high precision, and the measurement accuracy of the defect dimensions can be improved.

[0062] It is more preferable that the uppermost layer of the defect-forming film and the uppermost layer of the conductive film 15 have the same composition. Specifically, it is more preferable that the uppermost layer of the defect-forming film and the uppermost layer of the conductive film 15 have the same most abundant element and that the difference (absolute value) in the content of the most abundant element is 0 at% to 3 at%. The difference is preferably 0 at% to 1 at%, and more preferably 0 at% to 0.5 at%. The most abundant element is selected from Cr and Ta.

[0063] Although not shown, a substrate may be prepared that serves as both the calibration substrate 201 and the calibration substrate 202. In this case, the calibration substrate is prepared to include a conductive film 215, a substrate 210, a multilayer reflective film 211, a protective film 212, and an absorbing film 213 in this order, and to have the standard defect 220 in both the absorbing film 213 and the conductive film 215.

[0064] As shown in FIG. 11 , the standard defect 220 is preferably rectangular in plan view, and more preferably square. Dimensions can be measured using a vertex or a side as a reference. In this embodiment, the dimension to be measured is the length L of one side of the square, but it may also be the length of the diagonal of the square. Furthermore, the dimension to be measured may be the length of the short side of the rectangle, the length of the long side of the rectangle, or the length of the diagonal of the rectangle. Furthermore, the rectangle may have slight chamfering of the corners or slight irregularities on the sides, as long as they do not affect the measurement of the dimension of the rectangle.

[0065] In plan view, the length L of one side of the standard defect 220 is preferably 0.03 μm to 50 μm, more preferably 0.2 μm to 40 μm, and even more preferably 0.3 μm to 35 μm. By making the size of the standard defect 220 and the size of the defect in the absorbing film 13 or the conductive film 15 approximately the same, it is possible to make uniform the measurement conditions for the dimensions of the defect that appear in the image. Therefore, the dimensions of the defect can be corrected with high precision, and the measurement accuracy of the dimensions of the defect can be improved.

[0066] The calibration method may include a step of forming the standard defect 220 on the calibration substrate 201 or 202 by dry etching or wet etching. Dry etching can improve the dimensional accuracy of the standard defect 220 compared to wet etching. The standard defect 220 is concave in this embodiment, but may be convex. For example, the calibration method may include a step of forming the standard defect 220 on the calibration substrate 201 or 202 by local film deposition, for example, by using a focused ion beam (FIB).

[0067] In step S302, measurement data of the dimensions of the standard defect 220 measured by an inspection device other than the optical inspection device 100 is prepared. The other inspection device is, for example, at least one selected from a critical dimension-scanning electron microscope (CD-SEM), an atomic force microscope (AFM), a microscope, and a laser microscope. Note that the other inspection device is not particularly limited as long as it is an inspection device that has been calibrated using a standard device that is more accurate than the standard defect 220.

[0068] In step S303, the dimensions of the standard defect 220 are measured by the optical inspection device 100. The optical inspection device 100 and the other inspection device measure the dimensions of the same standard defect 220. There are multiple standard defects 220 to be measured, and they have different dimensions. The optical inspection device 100 and the other inspection device measure the dimensions of each standard defect 220 multiple times. It is preferable to adopt the average value as the measurement data. This can reduce errors due to disturbances.

[0069] In step S304, a correlation equation is created between the measurement data of the optical inspection device 100 and the measurement data of another inspection device regarding the dimensions of the standard defect 220. The correlation equation is, for example, a linear equation (y = ax + b). Here, x is the measurement data of the optical inspection device 100, and y is the measurement data of the other inspection device. a is a correction coefficient, and b is an offset value. A correlation equation may be created for each numerical range of the dimensions of the standard defect 220, or multiple correlation equations may be created.

[0070] To create a linear equation, two standard defects 220 with different dimensions are sufficient, but three or more are preferable, and five or more are even more preferable, in order to improve accuracy. The more standard defects 220 with different dimensions, the better, but from a cost perspective, the number may be 20 or less. The correlation equation may also be an algebraic equation of degree two or higher. The correlation equation is determined, for example, by the least squares method. If an algebraic equation of degree two or higher results in a higher correlation coefficient than a linear equation, it is preferable to use an algebraic equation of degree two or higher.

[0071] FIG. 12 shows an example of measurement data from the optical inspection device 100, measurement data from another inspection device, and their correlation equations. In FIG. 12, the other inspection device is a CD-SEM, and R is the correlation coefficient. In FIG. 12, the standard defects 220 have a square shape in plan view, with the length of one side ranging from 0.03 μm to 32 μm. There are 12 standard defects 220, and there are two correlation equations. Separate correlation equations were created for dimensions of 6 μm or less and dimensions of more than 6 μm. From FIG. 12, it can be seen that the correlation can be expressed by a linear equation.

[0072] The correlation equation obtained in step S304 is used as a correction equation in step S103 or S107 shown in FIG. 2. The correction equation is used to correct the dimensions of a defect in the absorbing film 13 or the conductive film 15. In the correction equation, x is the dimension before correction, and y is the dimension after correction. By performing correction using the correction equation, the dimensions of the defect can be measured with high accuracy using the optical inspection device 100.

[0073] In an image captured by the optical inspection device 100, the ratio (I1 / I2) of the luminance (I1) of the standard defect 220 to the luminance (I2) of the background of the standard defect 220 is preferably 0.1 to 0.9 or 1.1 to 10. The background of the standard defect 220 is an area that does not include the standard defect 220. The background of the standard defect 220 is an area outside the standard defect 220. If the ratio (I1 / I2) is 0.1 to 0.9 or 1.1 to 10, the contour of the standard defect 220 can be detected with high accuracy, and the dimensions of the standard defect 220 can be measured with high accuracy.

[0074] The defect size calibration method for an optical inspection device and a reflective mask blank manufacturing method according to the present disclosure have been described above, but the present disclosure is not limited to the above-described embodiments. Various changes, modifications, substitutions, additions, deletions, and combinations are possible within the scope of the claims. These naturally fall within the technical scope of the present disclosure.

[0075] This application claims priority based on Japanese Patent Application No. 2023-130450 filed with the Japan Patent Office on August 9, 2023, the entire contents of which are incorporated herein by reference.

[0076] 100 Optical inspection device 201 Calibration substrate 210 Substrate (base substrate) 213 Absorbing film (defect-forming film) 215 Conductive film (defect-forming film) 220 Standard defect

Claims

1. A method for calibrating an optical inspection device that measures dimensions of defects contained on a surface of an object by capturing an image of the surface of the object, comprising: preparing a calibration substrate having a plurality of standard defects of different dimensions; preparing measurement data for the dimensions of the standard defects measured by an inspection device separate from the optical inspection device; measuring the dimensions of the standard defects by the optical inspection device; and creating a correlation equation regarding the dimensions of the standard defects between the measurement data of the optical inspection device and the measurement data of the separate inspection device, wherein the calibration substrate has a defect-forming film in which the standard defects are formed and an underlying substrate on which the defect-forming film is formed, and wherein the uppermost layer of the defect-forming film contains at least one element selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru.

2. The method for calibrating an optical inspection device according to claim 1, wherein the standard defect is rectangular in plan view.

3. A method for calibrating an optical inspection device as described in claim 2, wherein the length of one side of the standard defect in a planar view is 0.03 μm to 50 μm.

4. The method for calibrating an optical inspection device according to claim 1 or 2, wherein the calibration substrate has three or more standard defects of different sizes.

5. A method for calibrating an optical inspection device according to claim 1 or 2, further comprising forming the standard defect on the calibration substrate by dry etching.

6. The method for calibrating an optical inspection apparatus according to claim 5, further comprising forming the standard defect on the calibration substrate by using a focused ion beam (FIB).

7. The method for calibrating an optical inspection device according to claim 1 or 2, wherein the other inspection device is at least one selected from a critical dimension-scanning electron microscope (CD-SEM), an atomic force microscope (AFM), a microscope, and a laser microscope.

8. The method for calibrating an optical inspection device according to claim 1 or 2, wherein the correlation equation is a linear equation.

9. A method for calibrating an optical inspection device according to claim 1 or 2, wherein the optical inspection device has a light source that irradiates light at a position where the image is captured, and the wavelength of the light is 200 nm to 800 nm.

10. A method for calibrating an optical inspection device as described in claim 1 or 2, wherein in the image, the ratio (I1 / I2) of the luminance (I1) of the standard defect to the luminance (I2) of the background of the standard defect is 0.1 to 0.9 or 1.1 to 10.

11. A method for manufacturing a reflective mask blank having a substrate, a multilayer reflective film, a protective film, and an absorbing film in this order, comprising: calibrating the optical inspection device with the calibration method described in claim 1 or 2; forming the multilayer reflective film, the protective film, and the absorbing film; and measuring the dimensions of defects in the absorbing film with the calibrated optical inspection device, wherein an uppermost layer of the absorbing film and an uppermost layer of the defect-forming film contain one or more of the same elements selected from Cr, Ta, Ir, Pt, Pd, Au, and Ru.

12. A method for manufacturing a reflective mask blank having a conductive film, a substrate, a multilayer reflective film, a protective film, and an absorbing film in this order, comprising: calibrating the optical inspection device with the calibration method described in claim 1 or 2; forming the conductive film; and measuring the dimensions of defects in the conductive film with the calibrated optical inspection device, wherein an uppermost layer of the conductive film and an uppermost layer of the defect-forming film contain one or more of the same elements selected from Cr and Ta.

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