JP2020024224A - Photomask blank defect inspection method, selection method and manufacturing method - Google Patents

Abstract

SOLUTION: An inspection method, which uses an inspection optical system to detect a defect existing in a front surface part of a photomask blank having at least one layer formed on a substrate, is configured to set a distance between the defect and an objective lens of an inspection optical system to a defocus distance; irradiate the defect with inspection light via the objective lens; collect reflection light of an area irradiated with the inspection light as an enlargement image via the objective lens; specify a change part of light intensity of the enlargement image; and determine an irregularity shape of the defect from the change part of the light intensity of the enlargement image.EFFECT: An optical defect inspection method is used to discriminate irregularity shapes of a defect with high reliability, and can inspect the defect of a photomask blank. Further, the defect inspection method of the present invention is applied to enable the photomask blank having a concavity defect serving a fatal defect to be surely eliminated without making wrong determinations of convexity defects as the concavity defects, which in turn the photomask blank not including the fatal defect can be provided at a lower cost and a high yield.SELECTED DRAWING: Figure 11

Description

  The present invention relates to a defect inspection method for a mask blank used for manufacturing a photomask used in the manufacture of a semiconductor device (semiconductor device) and the like, and more particularly to a defect inspection method effective for determining the uneven shape of the surface of a fine defect. . The present invention also relates to a photomask blank sorting method and a manufacturing method to which a photomask blank defect inspection is applied.

  2. Description of the Related Art A semiconductor device (semiconductor device) irradiates a mask (transfer mask) such as a photomask on which a circuit pattern is drawn with exposure light, and converts a circuit pattern formed on the mask through a reduction optical system into a semiconductor substrate (transfer mask). It is manufactured by repeatedly using a photolithography technique for transferring onto a semiconductor wafer). The transfer mask is manufactured by forming a circuit pattern on a substrate (mask blank) on which an optical thin film is formed. Such an optical thin film is generally a thin film containing a transition metal compound as a main component or a thin film containing a transition metal-containing silicon compound as a main component, and depending on the purpose, a film functioning as a light-shielding film or a phase shift film. A film or the like that functions as a film is selected.

  A transfer mask such as a photomask is used as an original for manufacturing a semiconductor element having a fine pattern, so it is required that the mask be defect-free. Will be required. Also, when forming a circuit pattern, a resist film for processing is formed on a mask blank on which an optical thin film is formed, and the pattern is finally formed through a normal lithography process such as an electron beam drawing method. Form. Therefore, the resist film is required to be free from defects such as pinholes. Under such circumstances, many studies have been made on techniques for detecting defects in a transfer mask or mask blank.

  Japanese Patent Application Laid-Open No. 2001-174415 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2002-333313 (Patent Document 2) describe a method of irradiating a substrate with a laser beam and detecting a defect or a foreign substance from light that is irregularly reflected. In particular, a technique is described in which a detection signal is given asymmetry to determine whether it is a convex defect or a concave defect. Japanese Patent Application Laid-Open No. 2005-265736 (Patent Document 3) describes a technique in which DUV (Deep Ultra Violet) light used for performing a pattern inspection of a general optical mask is used as inspection light. . Furthermore, Japanese Patent Application Laid-Open No. 2013-19766 (Patent Document 4) describes a technique in which inspection light is divided into a plurality of spots for scanning, and reflected beams are respectively received by photodetectors. On the other hand, Japanese Patent Application Laid-Open No. 2007-219130 (Patent Document 5) discloses a technique for distinguishing irregularities of defects of an EUV mask blank using EUV (Extreme Ultra Violet) light having a wavelength of about 13.5 nm as inspection light. ing.

JP 2001-174415 A JP 2002-333313 A JP 2005-265736 A JP 2013-19766 A JP 2007-219130 A

  With the continued miniaturization of semiconductor devices, ArF lithography technology using argon fluoride (ArF) excimer laser light with a wavelength of 193 nm has been used, and a process called multi-patterning, which combines exposure and processing processes multiple times, has been developed. By adopting the technique, a technique for finally forming a pattern having a size sufficiently smaller than the exposure wavelength has been vigorously studied. As described above, since the transfer mask is used as an original of a fine pattern, all defects on the transfer mask that hinder the fidelity of pattern transfer must be eliminated. Therefore, it is necessary to detect all the defects that hinder the formation of the mask pattern even in the mask blank manufacturing stage.

  In a transfer mask, a concave defect, particularly a pinhole defect, is fatal in forming a mask pattern. On the other hand, a convex defect may not be fatal in forming a mask pattern depending on the height of the defect. Further, a convex defect caused by a foreign substance attached to the surface is not a fatal defect if it can be removed by cleaning. Therefore, if all of these convex defects are regarded as fatal defects and the mask blank is excluded as a defective product, the yield is reduced. Therefore, in the defect inspection, it is extremely important to distinguish the concave and convex shapes of the defect with high accuracy in terms of both reliable elimination of a mask blank having a fatal defect and securing a yield.

  The inspection devices described in Patent Documents 1 to 4 are all devices that employ an optical defect detection method. The optical defect detection method has an advantage that a wide area defect inspection can be performed in a relatively short time, and a fine defect can be precisely detected by shortening the wavelength of a light source. The present invention also provides a method that can determine the irregularities of a defect from the relationship between the positions of the bright and dark portions of an inspection signal obtained by an inspection optical system using an oblique illumination method or a spatial filter. Further, Patent Literature 5 describes a method for distinguishing unevenness of a phase defect, although an inspection target is limited to an EUV mask blank.

However, according to an actual inspection experiment based on the inspection apparatus described in the above Patent Documents 1 to 4, some of the defects determined as concave defects based on the arrangement of the light and dark of the inspection signal of the photomask blank include the defect. Was confirmed by observation of a real image with an atomic force microscope or an electron microscope or the like, and it was found that convex defects were sometimes included. That is, in the inspection apparatuses described in Patent Literatures 1 to 4, it is not always possible to distinguish the uneven shape of a defect with high accuracy. In addition, the method described in Patent Document 5 is applied to a phase defect unique to an EUV mask blank, and is difficult to apply to a current mainstream photomask blank using a KrF excimer laser, an ArF excimer laser, an F ₂ laser, or the like. Is the way. For this reason, it has been desired to establish a method that prevents a convex defect from being erroneously determined as a concave defect, which was difficult with the conventional methods.

  The present invention has been made in order to solve the above-mentioned problem, and it is possible to use an optical defect detection method to reliably determine the uneven shape of a defect in a photomask blank without erroneously determining a convex defect as a concave defect. An object of the present invention is to provide a defect inspection method that can be distinguished by characteristics, and a method for selecting and manufacturing a photomask blank to which the defect inspection is applied.

  As described above, when the defect inspection is performed by the conventional method of distinguishing the unevenness from the arrangement of the bright part and the dark part in the inspection image, the concave defect such as the pinhole formed in the thin film of the photomask blank is correctly determined as the concave defect. Although it is judged, foreign substances such as particles different in material from the thin film adhere to the surface of the thin film of the photomask blank, or a convex defect caused by being partially buried in the thin film. Was erroneously determined to be a concave defect.

  The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, regarding a defect determined as a concave defect by the conventional method, the focus state of the inspection optical system is changed from the focus state determined as the concave defect. When the inspection image of the defect is collected in a so-called defocus state where the position is shifted, and the light intensity distribution of the inspection image is evaluated, in particular, the arrangement of light and dark and the difference between the light intensity of light and dark are determined to be a concave defect in the focused state. The inventors have found that defects can be further distinguished into true concave defects and convex defects, and have accomplished the present invention.

Accordingly, the present invention provides the following photomask blank defect inspection method, selection method, and manufacturing method.
Claim 1:
A method for inspecting a defect present on a surface portion of a photomask blank having at least one thin film formed on a substrate using an inspection optical system,
(A1) The defect and the objective lens of the inspection optical system are brought close to each other, the distance between them is set as a focus distance, and inspection light is transmitted through the objective lens while the focus distance is set. Irradiating the defect with
(A2) collecting reflected light of the region irradiated with the inspection light as a first enlarged image of the region via an objective lens;
(A3) A first determination step of specifying a portion where the light intensity of the first magnified image changes, and judging the irregular shape of the defect from the light intensity change of the portion where the light intensity of the first magnified image changes And
In the first determining step, only when the defect shape is determined to be concave,
(B1) The distance between the defect and the objective lens of the inspection optical system is set to a defocus distance deviated from the focus distance by a predetermined defocus amount, and the inspection light is set in a state where the defocus distance is set. Irradiating the defect through the objective lens,
(B2) collecting reflected light of the region irradiated with the inspection light as a second enlarged image of the region via an objective lens;
(B3) A second determination in which the light intensity change portion of the second magnified image is specified, and the irregular shape of the defect is re-determined from the light intensity change of the light intensity change portion of the second magnified image. A defect inspection method for a photomask blank, wherein the method further comprises the steps of:
Claim 2:
In the step (B3), the light intensity change of the light intensity change portion of the true concave defect is obtained in advance by simulation, and the obtained light intensity change and the light intensity change portion of the second enlarged image are obtained. 2. The defect inspection method according to claim 1, wherein the irregular shape of the defect to be inspected is re-determined based on a comparison with the light intensity change.
Claim 3:
3. The defect inspection method according to claim 1, wherein the inspection light is light having a wavelength of 210 to 550 nm.
Claim 4:
4. The method according to claim 1, wherein in both the steps (A1) and (B1), the inspection light is irradiated by oblique illumination whose optical axis is inclined with respect to the surface of the photomask blank. The defect inspection method according to any one of claims 1 to 4.
Claim 5:
2. The method according to claim 1, wherein in both of the steps (A2) and (B2), a spatial filter that blocks a part of the reflected light is provided on an optical path of the reflected light, and the reflected light is collected through the spatial filter. The defect inspection method according to any one of claims 1 to 3.
Claim 6:
In the step (A1), the photomask blank is placed on a stage capable of moving in the in-plane direction, and the stage is moved in the in-plane direction to bring the defect and the objective lens of the inspection optical system into close proximity. The defect inspection method according to claim 1, wherein the defect inspection is performed.
Claim 7:
7. The defect inspection method according to claim 1, wherein the defocus amount is more than 0 nm and 300 nm or less.
Claim 8:
8. The method according to claim 1, further comprising: A method for selecting a photomask blank, comprising selecting a photomask blank that does not include a concave defect.
Claim 9:
Forming at least one thin film on the substrate;
8. A method for manufacturing a photomask blank, comprising the step of: determining a concave / convex shape of a defect present in the thin film by the defect inspection method according to any one of claims 1 to 7.

  ADVANTAGE OF THE INVENTION According to this invention, the defect of a photomask blank can be inspected by distinguishing the uneven | corrugated shape of a defect with high reliability using an optical defect inspection method. Further, by applying the defect inspection method of the present invention, a photomask blank having a concave defect, which is a fatal defect, can be reliably eliminated without erroneously determining a convex defect as a concave defect. It is possible to provide a photomask blank that does not include any serious defects at a lower cost and a higher yield.

It is explanatory drawing which shows the outline of an example of the process of manufacturing a photomask from a photomask blank, and is sectional drawing in each stage of a manufacturing process. It is sectional drawing which shows the example in which a concave defect exists in a photomask blank, (A), (B) is a photomask blank in which a concave defect exists, (C) is manufactured from a photomask blank in which a concave defect exists. FIG. 4 is a diagram illustrating a photomask that has been used. It is sectional drawing which shows the example in which a convex defect exists in a photomask blank, (A) is a photomask blank in which a convex defect exists, (B) is a photomask manufactured from the photomask blank in which a convex defect exists. FIG. FIG. 2 is a diagram illustrating an example of a configuration of an inspection optical system used for defect inspection of a photomask blank. (A) is a conceptual diagram showing a mode of reflected light with respect to inspection light illuminated by oblique illumination on a concave defect on a photomask blank, and (B) is a diagram showing a cross-sectional profile of a light intensity distribution of an inspection image. is there. (A) is a conceptual diagram showing a mode of reflected light with respect to inspection light radiated by oblique illumination to a convex defect on a photomask blank, and (B) is a diagram showing a cross-sectional profile of a light intensity distribution of an inspection image. is there. (A) is a plan view of a photomask blank having a convex defect in which part of a foreign substance made of a substance having a low refractive index protrudes from the optical thin film, (B) is a cross-sectional view thereof, and (C) is a sectional view thereof. (D) is a diagram showing a cross-sectional profile of the light intensity distribution of the inspection image. (A) is a cross-sectional view of a photomask blank having a convex defect in which a part of foreign matter protrudes from the optical thin film, (B) is a cross-sectional view of a photomask blank having a true concave defect, and (C) is (D) is a diagram showing a cross-sectional profile of a light intensity distribution of an inspection image of each defect in a positive defocus state, and (E) and (F) are inspections of each defect in a focus state FIGS. 3G and 3H are diagrams showing the cross-sectional profiles of the light intensity distribution of the image, and FIGS. 3G and 3H are diagrams showing the cross-sectional profiles of the light intensity distribution of the inspection image in the negative defocus state of each defect. (A) is a plan view of a photomask blank having a convex defect formed by an attachment made of a material substantially transparent to inspection light, (B) is a cross-sectional view thereof, and (C) is a cross-sectional view thereof. (D) is a diagram showing a cross-sectional profile of the light intensity distribution of the inspection image. (A) is a cross-sectional view of a photomask blank having a convex defect formed by an attachment made of a material substantially transparent to inspection light, and (B) to (D) respectively show positive photomask blanks. It is a figure showing a section profile of light intensity distribution of an inspection picture in a defocus state, a focus state, and a negative defocus state. 6 is a flowchart illustrating an example of steps of a defect inspection method. 11 is a flowchart illustrating another example of the steps of the defect inspection method. (A) is sectional drawing of the photomask blank which has a convex defect of Example 1, (B)-(D) is each in a positive defocus state, a focus state, and a negative defocus state. It is a figure showing a section profile of light intensity distribution of an inspection picture. (A) is a sectional view of a photomask blank having a true concave defect to be compared, and (B) to (D) show a positive defocus state, a focus state, and a negative defocus state, respectively. 5 is a diagram showing a cross-sectional profile of a light intensity distribution of the inspection image of FIG. (A) is sectional drawing of the photomask blank which has a convex defect of Example 2, (B)-(D) is each in a positive defocus state, a focus state, and a negative defocus state. It is a figure showing a section profile of light intensity distribution of an inspection picture.

Hereinafter, the present invention will be described in more detail.
First, a process of manufacturing a photomask from a photomask blank will be described. FIG. 1 is an explanatory diagram of an example of a process of manufacturing a photomask from a photomask blank, and is a cross-sectional view of a photomask blank, an intermediate, or a photomask at each stage of the manufacturing process. In the photomask blank, at least one thin film such as an optical thin film and a processing auxiliary thin film is formed on a transparent substrate.

  In the photomask blank 100 shown in FIG. 1A, an optical thin film 102 functioning as a light-shielding film, a phase shift film such as a halftone phase shift film or the like is formed on a transparent substrate 101, and the optical thin film 102 is formed on the optical thin film 102. , A hard mask film (processing auxiliary thin film) 103 is formed. When manufacturing a photomask from such a photomask blank, first, a resist film 104 for processing is formed on the hard mask film 103 (FIG. 1B). Next, a resist pattern 104a is formed from the resist film 104 through a lithography process such as an electron beam drawing method (FIG. 1C), and the lower hard mask film 103 is processed using the resist pattern 104a as an etching mask. A hard mask film pattern 103a is formed (FIG. 1D), and the resist pattern 104a is removed (FIG. 1E). Further, when the lower optical thin film 102 is processed using the hard mask film pattern 103a as an etching mask, an optical thin film pattern 102a is formed. After that, the hard mask film pattern 103a is removed to obtain a photomask 100a (FIG. 1 ( F)).

  If a concave defect such as a pinhole defect exists in the thin film of the photomask blank, it eventually causes a defect of a mask pattern on the photomask. FIG. 2 shows an example of a typical photomask blank concave defect. FIG. 2A shows an example of a photomask blank 100 in which a concave defect DEF1 exists in a hard mask film 103 formed thereon for performing high-precision processing of an optical thin film 102 formed on a transparent substrate 101. FIG. 2B is a cross-sectional view showing an example of a photomask blank 100 in which a concave defect DEF2 exists in the optical thin film 102 formed on the transparent substrate 101.

  In any of the photomask blanks, when a photomask is manufactured from such a photomask blank by a manufacturing process as shown in FIG. 1, the photomask blank is changed to a photomask 100a as shown in FIG. The resulting concave defect DEF3 becomes a photomask existing in the optical thin film pattern 102a. The concave defect DEF3 causes a pattern transfer error in exposure using a photomask. Therefore, it is necessary to detect the concave defect of the photomask blank at a stage before processing the photomask blank, to eliminate the photomask blank having the defect, or to correct the defect.

  On the other hand, FIG. 3 shows a case where a convex defect such as a particle defect exists on a photomask blank, and FIG. 3A shows a convex defect on an optical thin film 102 formed on a transparent substrate 101. It is sectional drawing which shows the example of the photomask blank 100 in which DEF4 exists. When a photomask is manufactured from such a photomask blank by a manufacturing process as shown in FIG. 1, a convex defect DEF4 is formed on the optical thin film pattern 102a as in a photomask 100a shown in FIG. The remaining photomask is obtained. However, a convex defect may not be fatal depending on the size of the defect, and a convex defect caused by a foreign substance attached to the surface is not a fatal defect if it can be removed by cleaning.

  As described above, whether a defect present in a photomask blank is a concave defect such as a pinhole that is a fatal defect or a convex defect that is not a fatal defect in many cases is determined by quality assurance of the photomask blank. Thus, the key to the yield in photomask blank production is to be held. Therefore, there is a demand for a method capable of distinguishing the irregular shape of a defect with high reliability by an optical method.

  Next, an inspection optical system suitably used for defect inspection of the photomask blank, specifically, an inspection optical system suitably used for determining the uneven shape of the defect on the surface portion of the photomask blank will be described. FIG. 4 is a conceptual diagram showing an example of the basic configuration of the inspection optical system, which includes a light source ILS, a beam splitter BSP, an objective lens OBL, a stage STG on which a photomask blank MB can be placed and moved, and an image detector SE. . The light source ILS is configured to emit light having a wavelength of about 210 nm to 550 nm, and the inspection light BM1 emitted from the light source ILS is bent by the beam splitter BSP and passes through the photomask through the objective lens OBL. A predetermined area of the blank MB is irradiated. The light BM2 reflected on the surface of the photomask blank MB is collected by the objective lens OBL, passes through the beam splitter BSP and the lens L1, and reaches the light receiving surface of the image detector SE. At this time, the position of the image detector SE is adjusted so that an enlarged inspection image of the surface of the mask blank MB is formed on the light receiving surface of the image detector SE. Then, the data of the enlarged inspection image collected by the image detector SE is subjected to an image processing operation, whereby the size calculation of the defect and the determination of the unevenness are performed, and the results are recorded as defect information. I have.

  In the enlarged inspection image, for example, the image detector SE is a detector in which a large number of photodetectors such as a CCD camera are arranged as pixels, and the light BM2 reflected on the surface of the photomask blank MB is transmitted through the objective lens OBL. The enlarged image to be formed can be collected by a direct method of collectively collecting as a two-dimensional image. Further, the inspection light BM1 is scanned over the surface of the photomask blank MB by the scanning means, and the light intensity of the reflected light BM2 is sequentially collected by the image detector SE, photoelectrically converted and recorded, and the entire two-dimensional image is recorded. A method of generating an image may be employed. Further, a spatial filter SPF that blocks a part of the reflected light BM2 may be provided on the pupil position of the inspection optical system, for example, on the optical path of the reflected light BM2, particularly, between the beam splitter BSP and the lens L1. In this case, if necessary, a part of the optical path of the reflected light BM2 is blocked, and the enlarged inspection image can be captured by the image detector SE. The incident angle of the inspection light BM1 can be set to a predetermined angle with respect to the photomask blank MB. The defect to be inspected may be positioned at a position where the defect to be inspected can be observed by the objective lens OBL. In this case, the photomask blank MB is placed on the mask stage STG, and the mask stage By moving the STG, it can be positioned at a position observable by the objective lens OBL.

  Next, using the inspection optical system shown in FIG. 4, the distance between the defect and the objective lens of the inspection optical system is set to the focus distance (focus point), and the concave defect and the convex defect when the reflected light is collected. 5 and 6 will be described with reference to FIGS. FIG. 5A shows an example in which the inspection light BM1 is illuminated from the left oblique direction to the surface MBS of the photomask blank including the typical concave defect DEF5 from the inspection optical system shown in FIG. It is a conceptual diagram. Such oblique illumination, for example, changes the position of the inspection light BM1 emitted from the light source ILS shown in FIG. 4 to the photomask blank MB to the position of the aperture (located between the light source ILS and the beam splitter BSP). It can be realized by controlling. In this case, the reflected light BM2 reflected by the left side surface LSF of the concave defect DEF5 in the drawing is concentrated on the right side of the objective lens OBL due to regular reflection, and is not sufficiently taken into the objective lens OBL. On the other hand, the reflected light reflected by the right side surface RSF of the concave defect DEF5 in the drawing is sufficiently taken into the objective lens OBL by regular reflection. As a result, the light intensity distribution of the inspection image obtained by the image detector SE has a dark portion on the left side of the concave defect DEF5 and a bright portion on the right side, and has a cross-sectional profile PR1 as shown in FIG. 5B.

  On the other hand, FIG. 6A shows an example in which the inspection light BM1 is illuminated from the left oblique direction to the surface MBS of the photomask blank including the typical convex defect DEF6 from the inspection optical system shown in FIG. FIG. In this case, the reflected light BM2 reflected on the left side surface LSF of the convex defect DEF6 in the drawing is sufficiently taken into the objective lens OBL by regular reflection. On the other hand, the reflected light reflected by the right side surface RSF of the convex defect DEF6 in the drawing is concentrated on the right side of the objective lens OBL due to regular reflection, and is not sufficiently taken into the objective lens OBL. As a result, the light intensity distribution of the inspection image obtained by the image detector SE has a bright portion on the left side of the convex defect DEF6 and a dark portion on the right side, and has a cross-sectional profile PR2 as shown in FIG. 6B.

  As described above, by applying the oblique illumination, it is possible to determine the uneven shape of the defect from the positional relationship between light and dark in the obtained inspection image. 5 and 6 show an example of oblique illumination from the left side in the drawings, the illumination direction can be set arbitrarily. In the obtained inspection image, the inspection image is compared with the inspection light incident side. Similarly, it is possible to determine the concave-convex shape of the defect from the light-dark positional relationship or the light intensity difference.

  Also, as shown in FIG. 4, in the inspection optical system, when a spatial filter SPF that blocks a part of the reflected light is provided on the optical path of the reflected light, and the reflected light is collected through the spatial filter SPF, Even when the surface of the photomask blank is irradiated with the inspection light in the vertical direction, the inspection image can be made bright and dark as in the case of using the oblique illumination described above. In this case, for example, if half of the optical path of the reflected light is shielded, the uneven shape of the defect can be determined from the light-dark positional relationship or the difference in light intensity of the inspection image with reference to the inspection light incident side. .

  However, in the case where foreign matter such as particles is embedded in the optical thin film on the surface portion of the photomask blank and a part of the foreign matter protrudes from the optical thin film to form a convex defect, the inspection image described above is used. In some cases, it is not possible to correctly determine whether a defect is a concave defect or a convex defect only by the positional relationship between light and dark. FIGS. 7A and 7B are a plan view and a cross-sectional view, respectively, of the photomask blank 100 having such a convex defect (first mode). These are convex defects DEF7 formed by a foreign substance made of a substance having a lower refractive index than the optical thin film 102 on the surface of an optical thin film 102 made of a MoSi-based material formed on a quartz substrate 101 transparent to inspection light. Indicates a state where is present.

  With respect to the convex defect DEF7, the distance between the defect and the objective lens of the inspection optical system is set to a focus distance (focus point), and the concave defect shown in FIG. 5 or the convex defect shown in FIG. When the inspection optical system shown in FIG. 4 is used to irradiate the surface MBS of the photomask blank with inspection light from the left side in the figure by oblique illumination and collect reflected light, the light intensity shown in FIG. An inspection image of the distribution is obtained, and the light intensity distribution has a profile PR3 as shown in FIG. 7D in a cross section along the line AA ′ in FIG. In this case, when compared with the cases shown in FIGS. 5 and 6, it is determined that the defect is a concave defect, but it is actually a convex defect.

  However, as shown in FIGS. 7A and 7B, the distance between the defect and the objective lens of the inspection optical system for the convex defect that is determined to be a concave defect and the true concave defect is as follows. When the reflected light is collected by setting the defocus distance to a predetermined defocus distance from the focus distance, there is a difference between the inspection image and the light intensity distribution in a state where the defocus distance is set (defocus state). I understood.

  FIG. 8A is a cross-sectional view similar to FIG. 7B. On the other hand, FIG. 8B is a cross-sectional view of a photomask blank 100 having a true concave defect DEF8 on an optical thin film 102 made of a MoSi-based material formed on a quartz substrate 101 transparent to inspection light. .

  The convex defect DEF7 and the concave defect DEF8 are obliquely formed on the surface of the photomask blank by using the inspection optical system shown in FIG. 4 like the concave defect shown in FIG. 5 or the convex defect shown in FIG. When the inspection light is irradiated from the left side in the drawing by illumination and the reflected light is collected, the focus state where the distance between the defect and the objective lens of the inspection optical system is set to the focus distance (Δz = 0, Δz is the focus state) In the case of indicating the difference from the distance (hereinafter the same), the sectional profile PR6 of the light intensity distribution of the convex defect DEF7 (FIG. 8E) and the sectional profile PR7 of the light intensity distribution of the concave defect DEF8 (FIG. 8). (F)), there is no difference in the positional relationship between light and dark. Further, the distance between the defect and the objective lens of the inspection optical system is set to a positive defocus distance, that is, the mask stage STG on which the photomask blank MB is mounted is raised to be closer to a predetermined defocus amount than the focus distance. Also in the case of the positive defocus state (Δz> 0), the cross-sectional profile PR4 of the light intensity distribution of the convex defect DEF7 (FIG. 8C) and the cross-sectional profile PR5 of the light intensity distribution of the concave defect DEF8 (FIG. D)), there is no difference in the positional relationship between light and dark. On the other hand, the distance between the defect and the objective lens of the inspection optical system is set to a negative defocus distance, that is, the mask stage STG on which the photomask blank MB is placed is lowered to be a predetermined defocus amount longer than the focus distance. In the case of the negative defocus state (Δz <0), the cross-sectional profile PR8 of the light intensity distribution of the convex defect DEF7 (FIG. 8 (G)) and the cross-sectional profile PR9 of the light intensity distribution of the concave defect DEF8 (FIG. H)), the light and dark positional relationship is reversed.

  That is, from the inspection image and the light intensity distribution obtained in the focus state or the positive defocus state, both are determined to have the same shape. However, from the inspection image and the light intensity distribution obtained in the negative defocus state, in the case of the concave defect DEF8, which is a true concave defect, the left side in the figure is a bright part, and the right side is a dark part, and the arrangement of light and dark is shown. However, in the convex defect DEF7 formed in a state where a part of the foreign matter protrudes from the optical thin film, the bright part and the dark part are not reversed. The light intensity distribution of the light and dark parts of the inspection image changes depending on the width, height, depth, defocus amount, etc. of the defect, but in any case, the positional relationship between the light and dark parts of both Causes a difference in a negative defocus state. By utilizing this difference, it is possible to correctly determine a defect which is actually a convex defect but which is determined as a concave defect in the focus state, as a convex defect.

  Next, a case will be described in which an attachment made of a material substantially transparent to the inspection light exists as a defect as shown in FIG. FIGS. 9A and 9B are a plan view and a cross-sectional view, respectively, of the photomask blank 100 having such a convex defect (second mode). These are convexes formed on the surface of an optical thin film 102 made of a MoSi-based material formed on a quartz substrate 101 transparent to the inspection light, by using an attachment made of a material substantially transparent to the inspection light. The state where the defect DEF9 exists is shown. In this case, the optical thin film 102 itself is flat.

  For this convex defect DEF9, the distance between the defect and the objective lens of the inspection optical system is set to a focus distance (focus point), and the concave defect shown in FIG. 5 or the convex defect shown in FIG. When the inspection optical system shown in FIG. 4 is used to irradiate the surface MBS of the photomask blank with inspection light from the left side in the figure by oblique illumination and collect reflected light, the light intensity shown in FIG. An inspection image of the distribution is obtained, and the light intensity distribution has a profile PR10 as shown in FIG. 9D in a cross section along the line AA ′ in FIG. In this case, when compared with the cases shown in FIGS. 5 and 6, it is determined that the defect is a concave defect, but it is actually a convex defect.

  However, the distance between the defect and the objective lens of the inspection optical system for the convex defect that is determined to be a concave defect and the true concave defect as shown in FIGS. When the reflected light is collected by setting the defocus distance to a defocus distance deviated from the focus distance by a predetermined amount, also in this case, the inspection image and the light intensity distribution remain in the state where the defocus distance is set (defocus state). I found a difference.

  FIG. 10A is a cross-sectional view similar to FIG. 9B. On the other hand, a cross-sectional view of the photomask blank 100 having the true concave defect DEF8 is shown in FIG.

  The convex defect DEF9 and the concave defect DEF8 are obliquely formed on the surface of the photomask blank by using the inspection optical system shown in FIG. 4 like the concave defect shown in FIG. 5 or the convex defect shown in FIG. When the inspection light is illuminated from the left side in the figure by illumination and the reflected light is collected, when the focus state (Δz = 0), the positive defocus state (Δz> 0), and the negative defocus state (Δz) In each case of <0), the cross-sectional profile PR12 of the light intensity distribution of the convex defect DEF9 (FIG. 10C) and the cross-sectional profile PR7 of the light intensity distribution of the concave defect DEF8 (FIG. 8F), respectively. ), And between the sectional profile PR11 of the light intensity distribution of the convex defect DEF9 (FIG. 10B) and the sectional profile PR5 of the light intensity distribution of the concave defect DEF8 (FIG. 8D). of The intensity distribution of the cross-sectional profile PR13 (Fig. 10 (D)), with the light intensity distribution in the cross section profile PR9 of concave defect DEF8 (FIG 8 (H)), no difference of the positional relationship between the light and dark. Therefore, in the same method as in the first aspect described above, although a defect is actually a convex defect, a defect determined as a concave defect in the focus state cannot be distinguished.

  However, in the positive defocus state, when comparing the light intensity distribution of the convex defect DEF9 formed by the deposit made of the material substantially transparent to the inspection light with the light intensity distribution of the true concave defect DEF8. The ratio of the difference (absolute value) between the light intensity of the defect-free region and the light portion of the bright portion to the difference (absolute value) between the light intensity of the defect-free region and the light intensity of the dark portion (hereinafter referred to as the light-to-dark ratio) However, the convex defect DEF9 is higher. As described above, in the case of the convex defect formed by the deposit made of the material substantially transparent to the inspection light, the bright portion tends to be emphasized. The difference between the light intensity of the defect-free region and the light intensity of the bright portion, and the difference between the light intensity of the defect-free region and the light intensity of the dark portion change depending on the size of the convex defect, but are sufficiently far from the convex defect. The reference intensity obtained in the defect-free region is specified by the structure of the optical thin film of the photomask blank, and is constant in the defect-free region. Further, with respect to a true concave defect, it is possible to grasp in advance the light intensity of the bright part and the dark part for various sizes and depths as a light-dark ratio by actual measurement or simulation. Therefore, the light intensity in the non-defect area sufficiently distant from the defect is set as the reference intensity, and by comparing the light intensity of the bright portion and the dark portion with respect to the reference intensity, for example, a predetermined threshold is determined for the light-dark ratio, If a defect that is less than or equal to 0.9 (e.g., 0.9 or less) is a true concave defect and a defect that exceeds the threshold is a convex defect, a defect that is actually a convex defect but is determined to be a concave defect in the focus state is a convex defect. Can be correctly determined.

  In the first and second embodiments, examples of defects existing in the optical thin film made of a MoSi-based material on a quartz substrate have been described. However, other optical thin films used for photomask blanks, Defects existing in a thin film such as a thin film, for example, a thin film made of a chromium-based material are also targeted by the defect inspection method of the present invention.

In the present invention, when a defect existing on a surface portion of a photomask blank having at least one layer of a thin film formed on a substrate is inspected using an inspection optical system, first, the following (A1) to (A3) steps: That is,
(A1) The defect and the objective lens of the inspection optical system are brought close to each other, the distance between them is set as a focus distance, and the inspection light is irradiated to the defect via the objective lens in a state where the focus distance is set. The process of
(A2) collecting reflected light of the region irradiated with the inspection light as a first enlarged image of the region via an objective lens;
(A3) A first determining step of specifying a portion where the light intensity of the first magnified image changes, and judging the uneven shape of the defect from the light intensity change of the portion where the light intensity of the first magnified image changes Then, the uneven shape of the defect is determined in the focus state, and then the following steps (B1) to (B3), ie,
(B1) The distance between the defect and the objective lens of the inspection optical system is set to a defocus distance deviated from the focus distance by a predetermined defocus amount, and the inspection light is transmitted to the objective lens with the defocus distance set. Irradiating the defect via
(B2) collecting reflected light of the area irradiated with the inspection light as a second enlarged image of the area via an objective lens;
(B3) a second determination step of specifying a portion where the light intensity of the second magnified image changes, and re-determining the concave / convex shape of the defect from the light intensity change of the portion where the light intensity of the second magnified image changes Thus, the irregular shape of the defect is determined again in the defocused state. By inspecting the defect by such a method, for example, the irregular shape of the defect can be more accurately determined without erroneously determining a defect that is originally a convex defect as a concave defect.

  In one or both of the step (A3) and the step (B3), the above-mentioned steps (B1) to (B3) are actually performed on a target concave defect or convex defect, particularly a true concave defect. By comparing the change in the light intensity with the change in the light intensity of the first enlarged image or the second enlarged image, the uneven shape of the defect to be inspected may be determined. Determining the irregular shape of a defect to be inspected by comparing a light intensity change obtained by simulation for a defect, particularly a true concave defect, with a light intensity change of the first enlarged image or the second enlarged image. Is also possible. In this case, a light intensity change of a true concave defect is obtained by simulation, and a defect corresponding to the light intensity change is determined as a concave defect, and a defect corresponding to the light intensity change is determined as a convex defect, thereby enabling efficient determination. .

  In addition, a photolithography method including a step of forming at least one thin film such as an optical thin film and a processing auxiliary thin film on a substrate, and a step of determining an uneven shape of a defect existing in the thin film by the defect inspection method of the present invention. If a mask blank is manufactured, a photomask blank having a fatal defect is eliminated, and a photomask blank having a non-fatal defect such as a removable defect or a repairable defect is selected and used as it is or It can be provided after playback.

  In particular, the present invention performs the above steps (A1) to (A3), and executes the above steps (B1) to (B3) when the defect shape is determined to be concave in the first determination step. Then, re-determining the concave and convex shape of the defect is particularly effective because a defect that is originally a convex defect can be determined to be an original convex shape without erroneously determining a concave defect. Then, by applying such a defect inspection method, a concave defect can be obtained from the photomask blank that has been subjected to the above steps (B1) to (B3) based on the concave and convex shape of the defect re-determined in the second determination step. Can be sorted out.

  In the defect inspection method of the present invention, the inspection light is preferably light having a wavelength of 210 to 550 nm. In one or both of the steps (A1) and (B1), the inspection light may be irradiated by oblique illumination whose optical axis is inclined with respect to the surface of the photomask blank; In one or both steps of B2), a spatial filter that blocks a part of the reflected light may be provided on the optical path of the reflected light, and the reflected light may be collected through the spatial filter. The defocus distance depends on the size and depth of the defect, but is preferably Δz = −300 nm to +300 nm (defocus amount is 300 nm or less), and more preferably Δz = −250 nm to +250 nm (defocus amount is 250 nm or less). Range. In any case, 0 nm is excluded, but it is particularly preferable to set the defocus distance in a range excluding Δz = −100 nm to less than +100 nm (defocus amount is less than 100 nm).

  Further, in the step (A1), the photomask blank is placed on a stage capable of moving in the in-plane direction, and the stage is moved in the in-plane direction so that the defect is brought close to the objective lens of the inspection optical system. If this is the case, the defect can be easily aligned, and a continuous defect inspection can be performed on a plurality of defects existing on the photomask blank, thereby contributing to efficiency.

Next, the defect inspection method of the present invention will be described more specifically with reference to the flowchart shown in FIG.
First, a photomask blank to be inspected having a defect (photomask blank to be inspected) is prepared (step S201). Next, position coordinate information of a defect existing on the photomask blank is captured (step S202). As the position coordinates of the defect, the position coordinates of the defect specified by a known defect inspection can be separately used.

  Next, as a step (A1), the position of the defect is adjusted to the inspection position of the inspection optical system. More specifically, the defect is brought close to the objective lens of the inspection optical system, and the defect is brought into contact with the objective lens of the inspection optical system. Is set as the focal distance (focus distance), and the inspection light is emitted from the oblique direction via the objective lens while maintaining the focus distance (step S203). The alignment is performed by placing the photomask blank to be inspected on a stage capable of moving in the in-plane direction thereof, and moving the stage in the in-plane direction based on the position coordinates of the defect of the photomask blank to be inspected, thereby obtaining the defect. And the objective lens of the inspection optical system may be brought into close proximity. Next, as a step (A2), the reflected light in the area irradiated with the inspection light is collected as a first enlarged image of the area including the defect via the objective lens of the inspection optical system (step S204). Next, as a step (A3), a portion where the light intensity of the inspection image changes at the defect portion is specified from the collected image data (inspection image) of the first enlarged image (step S205), and the incidence side of the inspection light is determined. As a reference, a first determination step of determining the uneven shape of the defective portion from the positional relationship between the bright and dark portions of the inspection image is performed (step S206).

  Here, if it is not determined in step S206 that the defect is a concave defect, the defect information is recorded as a convex defect (determination D201, step S212).

  On the other hand, if it is determined in step S206 that the defect is a concave defect, in step (B1), the distance between the defect and the objective lens of the inspection optical system is different from the in-focus distance and deviates from the focus distance by a predetermined defocus amount ( (Positive or negative defocus distance), and the inspection light is irradiated from an oblique direction through the objective lens while maintaining the defocus distance (step S207). Next, as a step (B2), the reflected light of the area irradiated with the inspection light is collected as a second enlarged image of the area including the defect via the objective lens of the inspection optical system (step S208). Next, as a step (B3), a portion where the light intensity of the inspection image changes at the defect portion is specified from the collected image data (inspection image) of the second enlarged image (step S209). In the case of the second embodiment, the light intensity in a defect-free region sufficiently distant from the defect is defined as a reference intensity based on the positional relationship between the bright part and the dark part of the inspection image with reference to the incident side of the inspection light. A second determination step of determining the uneven shape of the defective portion by comparing the light intensity of the bright portion and the light intensity of the dark portion with respect to the reference intensity is performed (step S210).

  Here, if it is determined in step S210 that the defect is a concave defect, the defect information is recorded as a concave defect (determination D202, step S211). Conversely, if it is not determined that the defect is a concave defect, the defect information is recorded as a convex defect (determination D202, step S212). Next, it is determined whether the inspection has been completed for all the defects specified in advance (determination D203). If the inspection has not been completed, the position of a new defect is specified (step S213), and the process returns to step S203. Steps (A1) to (A3), and further steps (B1) to (B3) are repeated. When it is determined that the inspection has been completed for all the defects specified in advance (determination D203), the defect inspection ends.

  Next, a convex defect (first mode) formed by a foreign substance made of a substance having a low refractive index and a convex defect (second mode) formed by an attachment made of a material substantially transparent to inspection light. In the following, an example in which inspection is performed in a series of steps will be described with reference to the flowchart shown in FIG. In this case, in the flowchart shown in FIG. 11, steps S207 to S210 corresponding to steps (B1) to (B3) and determination D202 and steps S211 and 212 subsequent thereto are performed as follows.

  If it is determined in step S206 that the defect is a concave defect, first, in step (B1), the distance between the defect and the objective lens of the inspection optical system is different from the focal length, and the negative distance deviates from the focus distance by a predetermined defocus amount. By setting the defocus distance and maintaining the negative defocus distance, the inspection light is irradiated from an oblique direction through the objective lens (step S221). Next, as a step (B2), the reflected light of the area irradiated with the inspection light is collected as a second enlarged image of the area including the defect via the objective lens of the inspection optical system (step S222). Next, as a step (B3), a portion where the light intensity of the inspection image changes at the defect portion is specified from the collected image data (inspection image) of the second enlarged image (step S223), and the incidence side of the inspection light is determined. Based on the reference, the second determination step of determining the uneven shape of the defective portion from the positional relationship between the bright and dark portions of the inspection image is performed (step S224).

  Next, the distance between the defect and the objective lens of the inspection optical system is set to a positive defocus distance that is different from the focal distance and deviates from the focus distance by a predetermined defocus amount to maintain the positive defocus distance. Then, the inspection light is irradiated from an oblique direction through the objective lens (step S225). Next, as a step (B2), the reflected light of the area irradiated with the inspection light is collected as a second enlarged image of the area including the defect via the objective lens of the inspection optical system (step S226). Next, as a step (B3), a change in light intensity of the inspection image at the defect portion is specified from the collected image data (inspection image) of the second enlarged image (step S227). The light intensity in the defect area is set as a reference intensity, and a second judgment step of judging the uneven shape of the defect portion is performed by comparing the light intensity of the bright part or the dark part with respect to the reference intensity (step S228).

  Here, if it is not determined in step S224 that the defect is a concave defect, the defect information is recorded as a convex defect of the first mode (determination D221, step S230). Conversely, if it is determined that the defect is a concave defect, the process proceeds to the determination result in step S228. Then, in step S228, when it is determined that the defect is a concave defect, the defect information is recorded as a true concave defect (decision D222, step S229). It is recorded as the convex defect of the mode (decision D222, step S230). Note that the series of steps may be moved back and forth within a range where the flow is matched. For example, after performing steps S225 to S228 performed at a positive defocus distance, step S221 performed at a negative defocus distance Steps S224 to S224 may be performed, or a step of performing the processing at a negative defocus distance and a step of performing the processing at a positive defocus distance may be performed alternately.

  By applying the defect inspection method of the present invention, which can distinguish the concave and convex shape of a defect with high reliability without erroneously determining it as a concave defect, to a photomask blank manufacturing process, a photo having a concave defect, particularly a pinhole defect, is obtained. The mask blank can be extracted with high reliability, and a photomask blank containing no pinhole defect can be selected. In addition, the information on the uneven shape of the defect obtained by the defect evaluation method of the present invention can be given to a photomask blank by a method such as attaching an inspection slip. Furthermore, a photomask blank that does not include a concave defect such as a pinhole can be selected based on the information given to the photomask blank. Conventionally, a convex defect caused by an adhering substance may be determined as a concave defect by optical inspection, and a photomask blank having a defect that is not necessarily a fatal defect may be excluded as a defective product. Because it was high, it was a factor of lowering the yield, but by the inspection method of the present invention, it is possible to selectively remove a photomask blank having a concave defect that is a fatal defect existing in the photomask blank, Photomask blanks that match product specifications can be provided with good yield.

  Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples.

[Example 1]
The defect inspection of the photomask blank including the convex defect of the first embodiment was performed. As the inspection optical system, the inspection optical system shown in FIG. 4 was used, the numerical aperture NA was 0.75, the inspection wavelength was 248 nm, and the inspection light was averaged from the upper left in the figure for defects on the photomask blank. Oblique illumination was performed at an incident angle of 38 degrees. As shown in FIG. 13A, a surface portion of an optical thin film 102 made of a MoSi-based material formed on a quartz substrate 101 transparent to inspection light is made of a substance having a lower refractive index than the optical thin film 102. Using a convex defect DEF7 formed by a foreign substance as an inspection target, an inspection image representing a light intensity distribution and a light intensity profile of a cross section thereof were obtained. Further, as shown in FIG. 14A, a true concave defect DEF8 existing on the surface of an optical thin film 102 made of a MoSi-based material formed on a quartz substrate 101 transparent to inspection light is removed. As an inspection target for comparison, an inspection image representing a light intensity distribution and a cross-sectional profile of the light intensity were obtained.

  For the convex defect DEF7, the height H1 of the protruding portion of the defect was 10 nm, the width W1 of the defect was 100 nm, and the depth D1 of embedding the defect was two types, 10 nm and 20 nm. FIG. 13B is a cross-sectional profile of light intensity when the defocus distance is set to a positive +200 nm (the defocus amount is 200 nm), and FIG. 13C is a focus distance, that is, a light intensity profile when focused. FIG. 13D shows a cross-sectional profile of light intensity when the defocus distance is set to a negative value of −200 nm (a defocus amount is 200 nm).

  On the other hand, the true concave defect DEF8 has a defect width W0 of 100 nm and the amount of change in the light intensity of the inspection image changes depending on the defect depth D0, so that the depth D0 is 20 nm, 40 nm, and 75 nm. There were three types. FIG. 14B is a cross-sectional profile of light intensity when the defocus distance is set to a positive +200 nm (defocus amount is 200 nm), and FIG. 14C is a focus distance, that is, a light intensity profile when focused. FIG. 14D shows a cross-sectional profile of light intensity when the defocus distance is set to −200 nm (the defocus amount is 200 nm).

  In the case of the focus distance (Δz = 0 nm), that is, in the case of the focused point, the distribution of the inspection image (cross-sectional profile of the light intensity) of the true concave defect DEF8 is dark on the left side, bright on the right side, and convex defect DEF7. However, similarly, since the left part is a dark part and the right part is a bright part, it is not possible to distinguish between the two. Also, in the case of a positive defocus distance (Δz = + 200 nm, defocus amount is 200 nm), in both cases, the left part is a dark part and the right part is a bright part. On the other hand, in the case of a negative defocus distance (Δz = −200 nm, the defocus amount is 200 nm), the convex defect DEF7 has a dark portion on the left side and a bright portion on the right side, but has a true portion. In the concave defect DEF8, the left side is a bright part and the right side is a dark part, and the positional relationship between light and dark is reversed.

  From this result, by comparing the light and dark positional relationship with the inspection image in the negative defocus state, in the conventional method, that is, in the inspection at the focused point, the embedded convex defect determined to be a concave defect is determined. It can be seen that a convex defect can be correctly determined.

[Example 2]
The defect inspection of the photomask blank including the convex defect of the second embodiment was performed. As the inspection optical system, the inspection optical system shown in FIG. 4 was used, the numerical aperture NA was 0.75, the inspection wavelength was 248 nm, and the inspection light was averaged from the upper left in the figure for defects on the photomask blank. Oblique illumination was performed at an incident angle of 38 degrees. As shown in FIG. 15A, a material substantially transparent to the inspection light is provided on the surface of an optical thin film 102 made of a MoSi-based material formed on a quartz substrate 101 transparent to the inspection light. An inspection image representing a light intensity distribution and a light intensity profile of a cross section thereof were obtained by using the convex defect DEF9 formed by the attached substance composed of the inspection object as an inspection target.

  For the convex defect DEF9, the defect height H2 was set to two types of 60 nm and 80 nm, and the defect width W2 was set to 100 nm. FIG. 15B is a cross-sectional profile of light intensity when the defocus distance is set to a positive +200 nm (the defocus amount is 200 nm), and FIG. 15C is a focus distance, that is, a light intensity profile when focused. FIG. 15D shows a cross-sectional profile of light intensity when the defocus distance is set to a negative value of −200 nm (a defocus amount is 200 nm). On the other hand, the cross-sectional profiles of the light intensity in the case of the true concave defect DEF8 as shown in FIG. 14A, which is the inspection target for comparison, are as shown in FIGS. 14B to 14D.

  In this case, in the case of the focus distance (Δz = 0 nm), that is, in the case of the focused point and in the case of the positive defocus distance (Δz = + 200 nm, the defocus amount is 200 nm), the left part is a dark part and the right part is a bright part. In the case of a negative defocus distance (Δz = −200 nm, defocus amount is 200 nm), the left portion is a bright portion and the right portion is a dark portion. Although the two cannot be distinguished from each other, in the positive defocus state, the light intensity level of the bright portion of the inspection image of the convex defect DEF9 is clearly higher than that of the true concave defect DEF8.

  When these light intensity levels were evaluated in detail, the light intensity at a defect-free position sufficiently distant from the reference intensity defect was 0.166. On the other hand, when the height H2 of the convex defect DEF9 is 80 nm, the amount of change from the reference intensity in the bright portion is 0.076 at the in-focus point, and 0.095 in the positive defocus state. Increased to On the other hand, the amount of change from the reference intensity in the dark portion was 0.089 at the focused point, and slightly decreased to 0.084 in the positive defocus state. That is, the light-dark ratio at the focused point is 0.85, and the light-dark ratio in the positive defocus state is 1.13. In the case of the positive defocus state, the average light intensity level of the inspection image was increased, and the result was that the change amount of the bright portion with respect to the reference intensity exceeded the change amount of the dark portion. Further, even when the height H2 of the convex defect DEF9 was 60 nm, the amount of change in the reference intensity between the bright portion and the dark portion was substantially the same (that is, the contrast ratio was approximately 1).

  On the other hand, in the true concave defect DEF8, even when the average light intensity level of the inspection image is the highest and the depth D0 is 20 nm, the amount of change from the reference intensity in the bright portion in the positive defocus state is small. 0.024, the amount of change from the reference intensity in the dark part was 0.031, and the contrast ratio was 0.77, which was lower than the case of the convex defect DEF9. Further, in the case of the true concave defect DEF8 having a deeper depth D0 of 40 nm or 75 nm, the average light intensity level of the inspection image was lower than this, and the contrast ratio was further lower.

  From this result, the ratio of the difference (absolute value) between the light intensity of the defect-free region and the light portion of the bright portion to the difference (absolute value) between the light intensity of the defect-free region and the light intensity of the dark portion, that is, the contrast ratio In the positive defocus state, by comparing with the light-dark ratio of the true concave defect DEF8, the adhesion type convex defect determined as the concave defect in the conventional method, that is, the inspection at the focal point, is correctly convex. It can be seen that a defect can be determined.

REFERENCE SIGNS LIST 100 photomask blank 100a photomask 101 transparent substrate 102 optical thin film 102a optical thin film pattern 103 hard mask film 103a hard mask film pattern 104 resist film 104a resist pattern BM1 inspection light BM2 reflected light BSP beam splitters DEF1, DEF2, DEF3, DEF5, DEF8 Concave defects DEF4, DEF6, DEF7, DEF9 Convex defects ILS Light source L1 Lens LSF Left side MB of photomask blank MBS Front of photomask blank OBL Objective lens RSF Right side of defect SE Image detector STG stage

Claims (9)

A method for inspecting a defect present on a surface portion of a photomask blank having at least one thin film formed on a substrate using an inspection optical system,
(A1) The defect and the objective lens of the inspection optical system are brought close to each other, the distance therebetween is set as a focus distance, and inspection light is transmitted through the objective lens in a state where the focus distance is set. Irradiating the defect with
(A2) collecting reflected light of the region irradiated with the inspection light as a first enlarged image of the region via an objective lens;
(A3) A first determination step of specifying a portion where the light intensity of the first magnified image changes, and judging the irregular shape of the defect from the light intensity change of the portion where the light intensity of the first magnified image changes And
In the first determination step, only when the defect shape is determined to be concave,
(B1) The distance between the defect and the objective lens of the inspection optical system is set to a defocus distance deviated from the focus distance by a predetermined defocus amount, and the inspection light is set in a state where the defocus distance is set. Irradiating the defect through the objective lens,
(B2) collecting reflected light of the region irradiated with the inspection light as a second enlarged image of the region via an objective lens;
(B3) A second determination in which a portion where the light intensity of the second magnified image changes is specified and the uneven shape of the defect is re-determined from the light intensity change of the portion where the light intensity of the second magnified image changes. And a step of re-determining the concave and convex shape of the defect.   In the step (B3), the light intensity change of the light intensity change portion of the true concave defect is obtained in advance by simulation, and the obtained light intensity change and the light intensity change portion of the second enlarged image are obtained. 2. The defect inspection method according to claim 1, wherein the uneven shape of the defect to be inspected is re-determined based on a comparison with the light intensity change.   The defect inspection method according to claim 1, wherein the inspection light is light having a wavelength of 210 to 550 nm.   4. The method according to claim 1, wherein in both of the steps (A1) and (B1), the inspection light is irradiated by oblique illumination whose optical axis is inclined with respect to the surface of the photomask blank. The defect inspection method according to any one of claims 1 to 4.   2. The method according to claim 1, wherein in both of the steps (A2) and (B2), a spatial filter that blocks a part of the reflected light is provided on an optical path of the reflected light, and the reflected light is collected through the spatial filter. The defect inspection method according to any one of claims 1 to 3.   In the step (A1), the photomask blank is placed on a stage capable of moving in the in-plane direction, and the stage is moved in the in-plane direction to bring the defect and the objective lens of the inspection optical system into close proximity. The defect inspection method according to claim 1, wherein the defect inspection is performed.   7. The defect inspection method according to claim 1, wherein the defocus amount is more than 0 nm and 300 nm or less.   8. The method according to claim 1, wherein the step (B1) to the step (B3) are performed on the basis of the concave and convex shape of the defect re-determined in the second determining step. A method for sorting photomask blanks, comprising sorting photomask blanks that do not include concave defects. Forming at least one thin film on the substrate;
8. A method for manufacturing a photomask blank, comprising the step of: determining a concave / convex shape of a defect present in the thin film by the defect inspection method according to any one of claims 1 to 7.

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