JP6927273B2 - Defect inspection method, sorting method and manufacturing method of photomask blank - Google Patents

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) or the like, particularly a defect inspection method effective for determining the uneven shape of the surface of fine defects. .. The present invention also relates to a method for selecting and manufacturing a photomask blank to which a defect inspection of the photomask blank is applied.

In a semiconductor device (semiconductor device), a mask (transfer mask) such as a photomask on which a circuit pattern is drawn is irradiated with exposure light, and the circuit pattern formed on the mask is transferred to a semiconductor substrate (transmission optical system) via a reduction optical system. It is manufactured by repeatedly using photolithography technology that transfers 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 a film or a phase shift film that functions as a light-shielding film depending on the purpose. A film or the like that functions as a function is selected.

Since a transfer mask such as a photomask is used as an original plate for manufacturing a semiconductor element having a fine pattern, it is required to be defect-free, and this naturally means that the mask blank is also defect-free. Will be required. Further, 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 a pattern is finally formed through a normal lithography process such as an electron beam drawing method. Form. Therefore, it is required that the resist film also has no defects such as pinholes. Under these circumstances, many studies have been made on defect detection techniques for transfer masks and mask blanks.

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 defects and foreign substances from diffusely reflected light. In particular, a technique for imparting asymmetry to a detection signal to determine whether it is a convex defect or a concave defect is described. Further, Japanese Patent Application Laid-Open No. 2005-265736 (Patent Document 3) describes a technique of using DUV (Deep Ultra Violet) light used for pattern inspection of a general optical mask as inspection light. .. Further, 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 and scanned, and the reflected beam is received by a photodetector. On the other hand, Japanese Patent Application Laid-Open No. 2007-219130 (Patent Document 5) discloses a technique for distinguishing the unevenness of defects of an EUV mask blank using EUV (Extreme Ultra Violet) light having a wavelength near 13.5 nm as inspection light. ing.

Japanese Unexamined Patent Publication No. 2001-174415 Japanese Unexamined Patent Publication No. 2002-333313 Japanese Unexamined Patent Publication No. 2005-265736 Japanese Unexamined Patent Publication No. 2013-19766 JP-A-2007-219130

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

In a transfer mask, concave defects, especially pinhole defects, are fatal in mask pattern formation. On the other hand, convex defects may not be fatal in mask pattern formation, depending on the height of the defects. Further, the convex defect caused by the foreign matter adhering 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 lowered. Therefore, in the defect inspection, it is extremely important to distinguish the uneven shape of the defect with high accuracy from both the viewpoints of surely eliminating the mask blank having a fatal defect and ensuring the yield.

The inspection devices described in Patent Documents 1 to 4 are devices that employ an optical defect detection method. The optical defect detection method has an advantage that wide area defect inspection can be performed in a relatively short time, and precise detection of fine defects can be performed by shortening the wavelength of the light source. Further, the present invention provides a method of determining the unevenness of a defect from the relationship between the arrangement position of the bright part and the dark part of the inspection signal obtained by the inspection optical system using the orthorhombic illumination method or the spatial filter. Further, Patent Document 5 describes a method for distinguishing the unevenness of the phase defect, although the inspection target is limited to the EUV mask blank.

However, according to an actual inspection experiment based on the inspection apparatus described in Patent Documents 1 to 4, some of the defects determined to be concave defects based on the arrangement of the light and dark of the inspection signal of the photomask blank are the defects. Was confirmed by observing a real image with an atomic force microscope or an electron microscope, and it was found that convex defects may also be included. That is, the inspection devices described in Patent Documents 1 to 4 cannot always distinguish the uneven shape of a defect with high accuracy. Further, the method described in Patent Document 5 is applied to a phase defect peculiar to an EUV mask blank, and is difficult to be applied to a currently mainstream photomask blank using _{a KrF excimer laser, ArF excimer laser, F 2 laser or the like.} The method. Therefore, it has been desired to establish a method in which a convex defect is not erroneously determined as a concave defect, which has been difficult with the conventional methods.

The present invention has been made to solve the above problems, and the uneven shape of a defect in a photomask blank is highly reliable by using an optical defect detection method without erroneously determining a convex defect as a concave defect. It is an object of the present invention to provide a defect inspection method that can be distinguished by sex, and a method for selecting and manufacturing a photomask blank to which this 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 regarded as the concave defect. Although it is judged, this is for convex defects that occur when foreign matter such as particles whose material is different from that of the thin film adheres to the surface of the thin film of the photomask blank or is partially buried in the thin film. Was sometimes erroneously determined as a concave defect.

Therefore, as a result of diligent studies to solve the above problems, the present inventors have focused on the inspection optical system from the focus state determined to be a concave defect with respect to the defect determined to be a concave defect by the conventional method. When the inspection image of the defect was collected in the so-called defocused state where the position was shifted and the light intensity distribution of the inspection image was evaluated, particularly the arrangement of light and darkness and the difference in light intensity between light and dark, it was determined to be a concave defect in the focused state. We have found that defects can be further distinguished into true concave defects and convex defects, and have come to the present invention.

Therefore, the present invention provides the following photomask blank defect inspection method, sorting method, and manufacturing method.
Claim 1:
A method of inspecting defects existing on the surface of a photomask blank on which at least one thin film is formed on a substrate by 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 to the focus distance, and in the state where the focus distance is set, the inspection light is emitted and the optical axis thereof is the above. The step of irradiating the defect through the objective lens with oblique illumination that is inclined with respect to the surface of the photomask blank, and
(A2) A step of collecting the reflected light of the region irradiated with the inspection light as a first magnified image of the region via the objective lens, and
(A3) A first determination step of identifying a portion of the light intensity change of the first magnified image and determining the uneven shape of the defect from the change of the light intensity of the portion of the first magnified image where the light intensity changes. Including and
Only when the defect shape is determined to be concave in the first determination step, further
(B1) The distance between the defect and the objective lens of the inspection optical system is set to a positive or negative defocus distance deviating from the focus distance by a predetermined positive or negative defocus amount , and the positive and negative defocus distances are set. At each of the defocus distances, with the defocus distance set, the inspection light is irradiated to the defect through the objective lens by oblique illumination whose optical axis is inclined with respect to the surface of the photomask blank. And the process to do
(B2) A step of collecting the reflected light of the region irradiated with the inspection light as a second magnified image of the region via the objective lens, and
(B3) The change portion of the light intensity of the second magnified image is specified, and the true concave portion at the negative defocus distance is obtained from the change of the light intensity of the changed portion of the light intensity of the second magnified image. The reversal of the light-dark positional relationship of the cross-sectional profile of the defect light intensity distribution and the difference (absolute value) between the light intensity of the defect-free region and the light intensity of the dark region at the above positive defocus distance, of the defect-free region Based on the ratio of the difference (absolute value) between the light intensity and the light intensity of the bright part, the second determination step of re-determining the uneven shape of the defect is performed to re-determine the uneven shape of the defect. A featured photomask blank defect inspection method.
Claim 2:
In the step (B3), the arrangement of the bright part and the dark part in the defective part of the second enlarged image is changed.
Convex defects that do not invert at a negative defocus distance,
Convex defects, which are inverted at a negative defocus distance and whose brightness and darkness increase at a positive defocus distance,
Concave defects when inverted at a negative defocus distance and the brightness intensity did not increase at a positive defocus distance
The defect inspection method according to claim 1, wherein the uneven shape of the defect is redetermined.
Claim 3
In the (B3) step, advance to obtain the simulation of light intensity change in the variation of the light intensity of the true recessed defect, a light intensity change resulting change in the light intensity of the second enlarged image The defect inspection method according to claim 1 or 2, wherein the uneven shape of the defect to be inspected is redetermined by comparison with the change in light intensity of the portion.
Claim 4 :
The defect inspection method according to any one of claims 1 to 3, wherein the inspection light is light having a wavelength of 210 to 550 nm.
Claim 5:
Claim 1 is characterized in that, in both the steps (A2) and (B2), a spatial filter that shields 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. The defect inspection method according to any one of 4 to 4.
Claim 6:
In the step (A1), the photomask blank is placed on a stage that can move in the in-plane direction, and the stage is moved in the in-plane direction so that the defect and the objective lens of the inspection optical system are brought close to each other. The defect inspection method according to any one of claims 1 to 5, wherein the defect inspection method is performed.
Claim 7:
The defect inspection method according to any one of claims 1 to 6, wherein the defocus amount is more than 0 nm and 300 nm or less.
Claim 8:
From the photomask blank obtained by performing the above steps (B1) to (B3) based on the uneven shape of the defect re-determined in the second determination step of the defect inspection method according to any one of claims 1 to 7. A method for selecting a photomask blank, which comprises selecting a photomask blank that does not contain concave defects.
Claim 9:
The process of forming at least one thin film on the substrate and
A method for producing a photomask blank, which comprises a step of determining the uneven shape of a defect existing in the thin film by the defect inspection method according to any one of claims 1 to 7.

According to the present invention, it is possible to inspect the defects of the photomask blank by distinguishing the uneven shape of the defects with high reliability by using the optical defect inspection method. Further, by applying the defect inspection method of the present invention, it is possible to reliably eliminate a photomask blank having a concave defect, which is a fatal defect, without erroneously determining a convex defect as a concave defect, which is fatal. It is possible to provide a photomask blank containing no 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 at each stage of a manufacturing process. It is sectional drawing which shows the example which the concave defect exists in a photomask blank, (A), (B) are manufactured from the photomask blank which has a concave defect, (C) is manufactured from the photomask blank which has a concave defect. It is a figure which shows the photomask. It is sectional drawing which shows an example in which a convex defect exists in a photomask blank, (A) is a photomask blank which has a convex defect, (B) is a photomask manufactured from a photomask blank which has a convex defect. It is a figure which shows. It is a figure which shows an example of the structure of the inspection optical system used for defect inspection of a photomask blank. (A) is a conceptual diagram showing the mode of reflected light with respect to the inspection light irradiated to the concave defect on the photomask blank by oblique illumination, and (B) is a diagram showing a cross-sectional profile of the light intensity distribution of the inspection image. be. (A) is a conceptual diagram showing the mode of reflected light with respect to the inspection light irradiated to the convex defect on the photomask blank by oblique illumination, and (B) is a diagram showing a cross-sectional profile of the light intensity distribution of the inspection image. be. (A) is a plan view of a photomask blank having a convex defect in a state where a part of a foreign substance made of a substance having a low refractive index protrudes from an optical thin film, (B) is a sectional view of the same, and (C) is a cross-sectional view. , The inspection image of the convex defect, (D) is a figure which shows the 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 a state where a part of foreign matter protrudes from an optical thin film, (B) is a cross-sectional view of a photomask blank having a true concave defect, and (C). (D) is a diagram showing a cross-sectional profile of the light intensity distribution of the inspection image of each defect in the positive defocus state, and (E) and (F) are inspections of each defect in the focus state. The figure which shows the cross-sectional profile of the light intensity distribution of an image, (G) and (H) is the figure which shows the cross-sectional profile 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 convex defects formed by deposits made of a material substantially transparent to inspection light, (B) is a cross-sectional view of the same, and (C) is a cross-sectional view. , The inspection image of the convex defect, (D) is a figure which shows the cross-sectional profile of the light intensity distribution of the inspection image. (A) is a cross-sectional view of a photomask blank having convex defects formed by deposits made of a material substantially transparent to inspection light, and (B) to (D) are positive, respectively. It is a figure which shows the cross-sectional profile of the light intensity distribution of the inspection image in the defocus state, the focus state, and the negative defocus state. It is a flowchart which shows an example of the process of a defect inspection method. It is a flowchart which shows the other example of the process of a defect inspection method. (A) is a cross-sectional view of a photomask blank having a convex defect of Example 1, and (B) to (D) are a positive defocus state, a focus state, and a negative defocus state, respectively. It is a figure which shows the cross-sectional profile of the light intensity distribution of an inspection image. (A) is a cross-sectional view of a photomask blank having a true concave defect to be compared, and (B) to (D) are a positive defocus state, a focus state, and a negative defocus state, respectively. It is a figure which shows the cross-sectional profile of the light intensity distribution of the inspection image of. (A) is a cross-sectional view of a photomask blank having a convex defect of Example 2, and (B) to (D) are a positive defocus state, a focus state, and a negative defocus state, respectively. It is a figure which shows the cross-sectional profile of the light intensity distribution of an inspection image.

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 view 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 that functions as a phase shift film such as a light shielding film and a halftone phase shift film is formed on the transparent substrate 101, and is formed on the optical thin film 102. , A hard mask film (processing auxiliary thin film) 103 is formed. When producing a photomask from such a photomask blank, first, a resist film 104 for processing is formed on the hard mask film 103 (FIG. 1 (B)). Next, a resist pattern 104a is formed from the resist film 104 through a lithography process such as an electron beam drawing method (FIG. 1 (C)), and the underlying hard mask film 103 is processed using the resist pattern 104a as an etching mask. The hard mask film pattern 103a is formed (FIG. 1 (D)), and the resist pattern 104a is removed (FIG. 1 (E)). Further, when the optical thin film 102 of the lower layer is processed using the hard mask film pattern 103a as an etching mask, the optical thin film pattern 102a is formed, and then when the hard mask film pattern 103a is removed, a photomask 100a is obtained (FIG. 1 (FIG. 1). F)).

The presence of concave defects, such as pinhole defects, in the thin film of the photomask blank ultimately causes defects in the mask pattern on the photomask. An example of a concave defect of a typical photomask blank is shown in FIG. FIG. 2A shows an example of a photomask blank 100 in which a concave defect DEF1 is present in the hard mask film 103 formed on the optical thin film 102 formed on the transparent substrate 101 in order to perform high-precision processing. 2 (B) is a cross-sectional view showing an example of a photomask blank 100 in which the optical thin film 102 itself formed on the transparent substrate 101 has a concave defect DEF2.

In any photomask blank, when a photomask is manufactured from such a photomask blank by a manufacturing process as shown in FIG. 1, the photomask blank is as shown in the photomask 100a shown in FIG. 2C. The derived concave defect DEF3 becomes a photomask existing in the optical thin film pattern 102a. Then, this 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 the stage before processing the photomask blank, eliminate the photomask blank having the defect, or correct the defect.

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

In this way, determining whether the defect existing in the photomask blank is a concave defect such as a pinhole, which is a fatal defect, or a convex defect, which is not a fatal defect in many cases, is a quality assurance of the photomask blank. , Will hold the key to yield in photomask blank manufacturing. Therefore, a method capable of distinguishing the uneven shape of a defect with high reliability by an optical method is desired.

Next, an inspection optical system preferably used for defect inspection of the photomask blank, specifically, an inspection optical system preferably 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, and 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 a beam splitter BSP and photomasked through an objective lens OBL. Irradiate a predetermined area of the blank MB. 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 a magnified 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 magnified inspection image collected by the image detector SE is subjected to image processing calculation to perform dimensional calculation of defects and determination of unevenness, and these results are recorded as defect information. There is.

For the magnified 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 magnified image to be formed can be collected as a two-dimensional image by a direct method. Further, the inspection light BM1 is scanned on the surface of the photomask blank MB by scanning means, the light intensity of the reflected light BM2 is sequentially collected by the image detector SE, photoelectrically converted and recorded, and the whole is two-dimensional. A method of generating an image may be adopted. Further, a spatial filter SPF that shields a part of the reflected light BM2 may be arranged at 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, a part of the optical path of the reflected light BM2 can be shielded as necessary, and the magnified 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 target defect can be observed by the objective lens OBL. In this case, the photo mask 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, when the reflected light is collected by setting the distance between the defect and the objective lens of the inspection optical system to the focus distance (focus) using the inspection optical system shown in FIG. 4, concave defects and convex defects The difference between the inspection images of the above will be described with reference to FIGS. 5 and 6. FIG. 5A shows an example in which the inspection light BM1 is illuminated from the left oblique direction with respect to the surface MBS of the photomask blank containing the typical concave defect DEF5 from the inspection optical system shown in FIG. It is a conceptual diagram. In such oblique illumination, for example, the position of the inspection light BM1 emitted from the light source ILS shown in FIG. 4 to the photomask blank MB is set 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 side surface LSF on the left side of the concave defect DEF5 in the drawing is concentrated on the right side of the objective lens OBL due to specular reflection, and is not sufficiently taken into the objective lens OBL. On the other hand, the reflected light reflected by the side surface RSF on the right side in the figure of the concave defect DEF5 is sufficiently taken into the objective lens OBL by specular 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 and a bright portion on the right side of the concave defect DEF5, and has a cross-sectional profile PR1 as shown in FIG. 5 (B).

On the other hand, FIG. 6A shows an example in which the inspection light BM1 is illuminated from the left oblique direction with respect to the surface MBS of the photomask blank containing the typical convex defect DEF6 from the inspection optical system shown in FIG. It is a conceptual diagram which shows. In this case, the reflected light BM2 reflected by the side surface LSF on the left side in the drawing of the convex defect DEF6 is sufficiently captured by the objective lens OBL by specular reflection. On the other hand, the reflected light reflected by the side surface RSF on the right side of the convex defect DEF6 in the drawing is concentrated on the right side of the objective lens OBL due to specular 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 and a dark portion on the right side of the convex defect DEF6, and has a cross-sectional profile PR2 as shown in FIG. 6B.

By applying the oblique illumination in this way, the uneven shape of the defect can be determined from the positional relationship between the light and dark of the obtained inspection image. In FIGS. 5 and 6, an example of oblique illumination from the left side in the figure is shown, but the illumination direction can be set arbitrarily, and in the obtained inspection image, the inspection image is based on the incident side of the inspection light. Similarly, it is possible to determine the uneven shape of a defect from the positional relationship between light and dark or the difference in light intensity.

Further, as shown in FIG. 4, when the inspection optical system is provided with a spatial filter SPF that shields a part of the reflected light on the optical path of the reflected light and is configured to collect the reflected light through the spatial filter SPF. Even if the surface of the photomask blank is irradiated with the inspection light from the vertical direction, it is possible to make the inspection image 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 positional relationship between the brightness and darkness of the inspection image or the difference in light intensity with reference to the incident side of the inspection light. ..

However, when foreign matter such as particles is embedded in the optical thin film on the surface of the photomask blank and a part of the foreign matter is projected from the optical thin film to form a convex defect, the above-mentioned inspection image is obtained. It may not be possible to correctly determine whether the defect is a concave defect or a convex defect only by the positional relationship between the light and dark. 7 (A) and 7 (B) are a plan view and a cross-sectional view of the photomask blank 100 having such a convex defect, respectively (first aspect). These are convex defects DEF7 formed 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 by a foreign substance made of a substance having a refractive index lower than that of the optical thin film 102. Indicates that is present.

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

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

FIG. 8 (A) is a cross-sectional view similar to that of FIG. 7 (B). 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. ..

These convex defects DEF7 and concave defects DEF8 are oblique to the surface of the photomask blank using the inspection optical system shown in FIG. 4, such as the concave defects shown in FIG. 5 or the convex defects shown in FIG. When the inspection light is irradiated from the left side of the figure by illumination and the reflected light is collected, the focus state (Δz = 0, where Δz is the focus) in which the distance between the defect and the objective lens of the inspection optical system is set as the focus distance. In the case of showing the difference from the distance (the same applies hereinafter), the cross-sectional profile PR6 (FIG. 8 (E)) of the light intensity distribution of the convex defect DEF7 and the cross-sectional profile PR7 (FIG. 8) of the light intensity distribution of the concave defect DEF8. There is no difference in the positional relationship between light and dark with (F)). 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 placed is raised to be closer to a predetermined defocus amount than the focus distance. Even in the positive defocus state (Δz> 0), the cross-sectional profile PR4 (FIG. 8 (C)) of the light intensity distribution of the convex defect DEF7 and the cross-sectional profile PR5 of the light intensity distribution of the concave defect DEF8 (FIG. 8 (FIG. 8)). There is no difference in the positional relationship between light and dark with D)). 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 set a predetermined defocus amount farther than the focus distance. In the case of a 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. 8 (FIG. 8)). The positional relationship between light and dark is reversed between H)).

That is, from the inspection image and the light intensity distribution obtained in the focused state or the positive defocused 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 the bright part and the right side is the dark part, and the arrangement of light and dark is arranged. Although it is inverted, in the convex defect DEF7 formed in a state where a part of the foreign matter protrudes from the optical thin film, the bright portion and the dark portion are not inverted. The light intensity distribution of the bright and dark areas of the inspection image changes depending on the width, height, depth, defocus amount, etc. of the defect, but in each case, the positional relationship between the bright and dark areas of both Makes a difference in the negative defocus state. By utilizing this difference, a defect that is actually a convex defect but is determined to be a concave defect in the focused state can be correctly determined as a convex defect.

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

With respect to this convex defect DEF9, the distance between the defect and the objective lens of the inspection optical system is set to the focus distance (focusing), and the same as the concave defect shown in FIG. 5 or the convex defect shown in FIG. When the surface MBS of the photomask blank is irradiated with the inspection light from the left side in the drawing by oblique illumination using the inspection optical system shown in FIG. 4 and the reflected light is collected, the light intensity shown in FIG. 9C is obtained. An inspection image of the distribution is obtained, and the light intensity distribution is the profile PR10 as shown in FIG. 9 (D) in the cross section along the line AA'in FIG. 9 (C). In this case, it is determined to be a concave defect when compared with the cases shown in FIGS. 5 and 6, but it is actually a convex defect.

However, with respect to the convex defect determined to be a concave defect and the true concave defect as shown in FIGS. 9A and 9B, the distance between the defect and the objective lens of the inspection optical system is determined. When the reflected light is collected by setting the defocus distance deviating from the focus distance by a predetermined defocus amount, the inspection image and the light intensity distribution are also displayed in the state where the defocus distance is set (defocus state). It turns out that there is a difference.

FIG. 10 (A) is a cross-sectional view similar to that of FIG. 9 (B). On the other hand, a cross-sectional view of the photomask blank 100 having a true concave defect DEF 8 is shown in FIG. 8 (B).

These convex defects DEF9 and concave defects DEF8 are oblique to the surface of the photomask blank using the inspection optical system shown in FIG. 4, such as the concave defects shown in FIG. 5 or the convex defects shown in FIG. When the inspection light is irradiated from the left side of the figure by illumination and the reflected light is collected, 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 (FIG. 10 (C)) of the light intensity distribution of the convex defect DEF9 and the cross-sectional profile PR7 (FIG. 8 (F)) of the light intensity distribution of the concave defect DEF8, respectively. ), The cross-sectional profile PR11 (FIG. 10 (B)) of the light intensity distribution of the convex defect DEF9 and the cross-sectional profile PR5 (FIG. 8 (D)) of the light intensity distribution of the concave defect DEF8. There is no difference in the positional relationship between light and dark between the cross-sectional profile PR13 of the light intensity distribution (FIG. 10 (D)) and the cross-sectional profile PR9 (FIG. 8 (H)) of the light intensity distribution of the concave defect DEF8. Therefore, in the same method as the first aspect described above, it is not possible to distinguish a defect determined to be a concave defect in the focused state, although it is actually a convex defect.

However, in the positive defocused state, comparing the light intensity distribution of the convex defect DEF9 formed by deposits made of a 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 intensity of the bright part (absolute value) to the difference (absolute value) between the light intensity of the defect-free region and the light intensity of the dark part (hereinafter referred to as the light-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 a material substantially transparent to the inspection light, the bright part tends to be emphasized. The difference between the light intensity in the defect-free region and the light intensity in the bright part, and the difference between the light intensity in the defect-free region and the light intensity in the dark part vary depending on the size of the convex defect, but are sufficiently separated from the convex defect. The reference strength 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, regarding a true concave defect, it is possible to grasp in advance the light intensity of the bright part and the dark part as a light-dark ratio by actual measurement or simulation for various sizes and depths. Therefore, the light intensity in the defect-free region sufficiently distant from the defect is set as the reference intensity, and by comparing the light intensity of the bright part and the dark part with respect to the reference intensity, for example, a predetermined threshold value for the light-dark ratio is determined, and this threshold value is set. If the following (for example, 0.9 or less) is a true concave defect, and the one exceeding the threshold value is a convex defect, a defect that is actually a convex defect but is determined to be a concave defect in the focused state is a convex defect. Can be correctly determined.

Further, in the first and second aspects described above, examples of defects existing in the optical thin film made of MoSi-based material on the quartz substrate are shown, but other optical thin films used for the photomask blank and processing assistance are shown. Defects existing in thin films such as thin films, for example, thin films made of chromium-based materials, are also subject to the defect inspection method of the present invention.

In the present invention, when inspecting a defect existing on the surface of a photomask blank on which at least one thin film is formed on a substrate by using an inspection optical system, first, the following steps (A1) to (A3), 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 to the focus distance, and the inspection light is applied to the defect through the objective lens with the focus distance set. And the process to do
(A2) A step of collecting the reflected light of the region irradiated with the inspection light as a first magnified image of the region via the objective lens, and
(A3) By the first determination step of identifying the light intensity change portion of the first magnified image and determining the uneven shape of the defect from the light intensity change of the light intensity change portion of the first magnified image. , The uneven shape of the defect is determined in the focused state, and then the following steps (B1) to (B3), that is,
(B1) The distance between the defect and the objective lens of the inspection optical system is set to a defocus distance that deviates from the focus distance by a predetermined defocus amount, and in a state where the defocus distance is set, the inspection light is emitted to the objective lens. And the process of irradiating defects through
(B2) A step of collecting the reflected light of the region irradiated with the inspection light as a second magnified image of the region via the objective lens, and
(B3) A second determination step of identifying the changed portion of the light intensity of the second magnified image and re-determining the uneven shape of the defect from the change of the light intensity of the changed portion of the light intensity of the second magnified image. Redetermines the uneven shape of the defect in the defocused state. By inspecting the defect by such a method, for example, the uneven shape of the defect can be more accurately determined without erroneously determining the defect which is originally a convex defect as a concave defect.

In one or both of the steps (A3) and (B3), the above steps (B1) to (B3) are actually carried out for the target concave or convex defects, particularly the true concave defects. The uneven shape of the defect to be inspected may be determined by comparing the change in light intensity with the change in light intensity of the first magnified image or the second magnified image. Determining the uneven shape of a defect to be inspected by comparing the change in light intensity obtained by simulation for a defect, particularly a true concave defect, with the change in light intensity of the first magnified image or the second magnified image. Is also possible. In this case, it is possible to make an efficient determination by obtaining a change in the light intensity of a true concave defect by simulation and determining a defect corresponding to the change in light intensity as a concave defect and a defect that does not correspond to the change in light intensity as a convex defect. ..

Further, by a 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 the uneven shape of a defect existing in the thin film by the defect inspection method of the present invention, a photophotomask is used. Manufacture of mask blanks eliminates photomask blanks with fatal defects and also selects photomask blanks with non-fatal defects such as removable and repairable defects and either as is or It will be possible to provide it after playing it.

In particular, in the present invention, the above steps (A1) to (A3) are carried out, and when the defect shape is determined to be concave in the first determination step, the above steps (B1) to (B3) are carried out. Then, when the uneven shape of the defect is re-determined, the defect which is originally a convex defect can be determined to be the original convex shape without being erroneously determined as a concave defect, which is particularly effective. Then, by applying such a defect inspection method, a concave defect is obtained from the photomask blank obtained by performing the above steps (B1) to (B3) based on the uneven shape of the defect redetermined in the second determination step. It is possible to select photomask blanks that do not contain.

In the defect inspection method of the present invention, the inspection light is preferably light having a wavelength of 210 to 550 nm. Further, in one or both steps of (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, and (A2) and (A2) and ( In one or both steps of B2), a spatial filter that shields 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). Is the range of. In any range, 0 nm is excluded, but it is particularly preferable to set the defocus distance in a range other than Δz = −100 nm and 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 thereof, and the stage is moved in the in-plane direction so that the defect and the objective lens of the inspection optical system are brought close to each other. For example, defects can be easily aligned, and continuous defect inspection can be performed on a plurality of defects existing on the photomask blank, which contributes 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 (photomask blank to be inspected) having a defect is prepared (step S201). Next, the position coordinate information of the defect existing on the photomask blank is taken in (step S202). As the position coordinates of the defect, the position coordinates of the defect identified by a known defect inspection can be used separately.

Next, as the step (A1), the position of the defect is aligned with the inspection position of the inspection optical system. Specifically, the defect and the objective lens of the inspection optical system are brought close to each other, and the defect and the objective lens of the inspection optical system are combined. Is set to the focal length (focus distance), the focus distance is maintained, and the inspection light is irradiated from an oblique direction through the objective lens (step S203). For alignment, the photomask blank to be inspected is placed on a stage that can be moved in the in-plane direction, and the stage is moved in the in-plane direction based on the position coordinates of the defect of the photomask blank to be inspected to obtain the defect. And the objective lens of the inspection optical system may be brought close to each other. Next, as the step (A2), the reflected light of the region irradiated with the inspection light is collected as a first magnified image of the region including the defect through the objective lens of the inspection optical system (step S204). Next, as the step (A3), the changed portion of the light intensity of the inspection image in the defective portion is identified from the collected image data (inspection image) of the first enlarged image (step S205), and the incident side of the inspection light is determined. Using the reference, the first determination step of determining the uneven shape of the defective portion from the positional relationship between the bright portion and the dark portion of the inspection image is carried out (step S206).

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

On the other hand, when it is determined to be a concave defect in the step S206, as the step (B1), the distance between the defect and the objective lens of the inspection optical system is different from the focal length, and the distance deviates from the focus distance by a predetermined defocus amount ( The defocus distance is set to 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 the step (B2), the reflected light of the region irradiated with the inspection light is collected as a second magnified image of the region including the defect through the objective lens of the inspection optical system (step S208). Next, as the step (B3), the portion where the light intensity of the inspection image changes in the defective portion is specified from the collected image data (inspection image) of the second enlarged image (step S209), and in the case of the first aspect. In the second aspect, the light intensity in the defect-free region sufficiently distant from the defect is set as the 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 is carried out by comparing the light intensities of the bright portion and the dark portion with respect to the reference intensity (step S210).

Here, if it is determined to be a concave defect in step S210, the defect information is recorded as a concave defect (determination D202, step S211). On the contrary, when it is not determined to be 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), and if not completed, the position of a new defect is specified (step S213), and the process returns to step S203. The steps (A1) to (A3) and further the steps (B1) to (B3) are repeated. Then, when it is determined that the inspection has been completed for all the defects specified in advance (determination D203), the defect inspection is completed.

Next, a convex defect formed by a foreign substance having a low refractive index (first aspect) and a convex defect formed by an deposit made of a material substantially transparent to the inspection light (second aspect). An example of the case where the aspect) is inspected 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, instead of steps S207 to S210 corresponding to the steps (B1) to (B3) and subsequent determinations D202, steps S211 and 212, the following steps are carried out.

When a concave defect is determined in step S206, as step (B1), first, the distance between the defect and the objective lens of the inspection optical system is different from the focal length, and a negative defocus amount deviates from the focus distance. The defocus distance is set, the negative defocus distance is maintained, and the inspection light is irradiated from the oblique direction through the objective lens (step S221). Next, as the step (B2), the reflected light of the region irradiated with the inspection light is collected as a second magnified image of the region including the defect through the objective lens of the inspection optical system (step S222). Next, as the step (B3), the portion where the light intensity of the inspection image changes in the defective portion is identified from the collected image data (inspection image) of the second enlarged image (step S223), and the incident side of the inspection light is determined. As a reference, a second determination step of determining the uneven shape of the defective portion from the positional relationship between the bright portion and the dark portion of the inspection image is carried out (step S224).

Next, set the distance between the defect and the objective lens of the inspection optical system to a positive defocus distance that is different from the focal length and deviates from the focus distance by a predetermined defocus amount, and maintain the positive defocus distance. , The inspection light is irradiated from an oblique direction through the objective lens (step S225). Next, as the step (B2), the reflected light of the region irradiated with the inspection light is collected as a second magnified image of the region including the defect through the objective lens of the inspection optical system (step S226). Next, as the step (B3), the changed portion of the light intensity of the inspection image in the defect portion is identified from the collected image data (inspection image) of the second enlarged image (step S227), and the defect is sufficiently separated from the defect. A second determination step of determining the uneven shape of the defective portion is carried out by using the light intensity in the defect region as a reference intensity and comparing the light intensity of the bright portion or the dark portion with respect to the reference intensity (step S228).

Here, if it is not determined to be a concave defect in step S224, the defect information is recorded as a convex defect of the first aspect (determination D221, step S230). On the contrary, when it is determined that the defect is concave, the process proceeds to the determination result in step S228. Then, in step S228, if it is determined to be a concave defect, the defect information is recorded as a true concave defect (determination D222, step S229), and if it is not determined to be a concave defect, the defect information is recorded as a second defect. It is recorded as a convex defect of the aspect (determination D222, step S230). The series of steps may be moved back and forth within a range in which the flows are consistent. For example, steps S225 to S228 carried out at a positive defocus distance and then steps S221 carried out at a negative defocus distance. ~ S224 may be carried out, or the steps carried out at a negative defocus distance and the steps carried out at a positive defocus distance may be alternately carried out.

By applying the defect inspection method of the present invention, which can distinguish the uneven shape of a defect with high reliability without erroneously determining it as a concave defect, to the manufacturing process of a photomask blank, a photo having a concave defect, particularly a pinhole defect. The mask blank can be extracted with high reliability to select a photomask blank that does not contain pinhole defects. Further, the information on the uneven shape of the defect obtained by the defect evaluation method of the present invention can be added to the photomask blank by a method such as attaching an inspection sheet. Further, it is also possible to select a photomask blank that does not contain concave defects such as pinholes based on the information given to the photomask blank. Conventionally, a convex defect caused by an deposit may be determined as a concave defect by an optical inspection, and there is a possibility that a photomask blank having a defect that is not necessarily a fatal defect is excluded as a defective product. Since it was high, it was a factor of lowering the yield. However, the inspection method of the present invention can selectively eliminate the photomask blank having a concave defect which is a fatal defect existing in the photomask blank. It is possible to provide a photomask blank that meets the product specifications with a high yield.

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

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

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

On the other hand, in the true concave defect DEF8, the width W0 of the defect is 100 nm, and the amount of change in the light intensity of the inspection image changes depending on the depth D0 of the defect. There were 3 types. FIG. 14 (B) shows a cross-sectional profile of the light intensity when the defocus distance is positive + 200 nm (the amount of defocus is 200 nm), and FIG. 14 (C) shows the focus distance, that is, the light intensity in the case of in-focus. The cross-sectional profile, FIG. 14 (D), shows a cross-sectional profile of light intensity when the defocus distance is negative −200 nm (defocus amount is 200 nm).

In the case of focus distance (Δz = 0 nm), that is, in the case of in-focus, in the case of true concave defect DEF8, the distribution of the inspection image (cross-sectional profile of light intensity) is dark on the left side, bright on the right side, and convex defect DEF7. However, similarly, since the left side is the dark part and the right side is the bright part, it is not possible to distinguish between the two. Further, even in the case of a positive defocus distance (Δz = + 200 nm, the defocus amount is 200 nm), the left side is the dark part and the right side is the bright part in both cases. On the other hand, in the case of a negative defocus distance (Δz = -200 nm, defocus amount is 200 nm), in the convex defect DEF7, the left side is the dark part and the right side is the bright part, whereas the true part is true. 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 positional relationship between light and dark with the inspection image in the negative defocus state, the embedded type convex defect determined to be a concave defect in the conventional method, that is, the inspection in the in-focus state, is obtained. , It can be seen that it can be correctly determined as a convex defect.

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

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

In this case, in the case of focus distance (Δz = 0 nm), that is, in the case of in-focus and in the case of positive defocus distance (Δz = + 200 nm, defocus amount is 200 nm), the left side is the dark part and the right side is the bright part. In the case of a negative defocus distance (Δz = -200 nm, defocus amount is 200 nm), the left side is the bright part and the right side is the dark part. Although it is not possible to distinguish between the two, in the positive defocus state, the light intensity level of the bright part of the inspection image of the convex defect DEF 9 is clearly higher than that of the true concave defect DEF 8.

When these light intensity levels were evaluated in detail, the light intensity at a position where there was no defect 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 part is 0.076 in the focused area, whereas it is 0.095 in the positive defocused state. Has increased to. On the other hand, the amount of change from the reference intensity in the dark part was 0.089 in the in-focus state, whereas it was 0.084 in the positive defocus state, which was a slight decrease. That is, the light-dark ratio in the in-focus state is 0.85, and the light-dark ratio in the positive defocus state is 1.13. In the positive defocus state, the average light intensity level of the inspection image increased, and the amount of change in the bright part with respect to the reference intensity exceeded the amount of change in the dark part. Further, even when the height H2 of the convex defect DEF9 was 60 nm, the amount of change with respect to the reference intensity of the bright part and the dark part was about the same (that is, the light-dark ratio was about 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 part is large in the positive defocus state. The amount of change from the reference intensity in the dark part was 0.024, and the light-dark ratio was 0.77, which was lower than the case of the convex defect DEF9. Further, in the true concave defect DEF8 having a depth D0 of 40 nm or 75 nm, the average light intensity level of the inspection image was lower than this, and the light-dark ratio was further lowered.

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

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 splitter DEF1, DEF2, DEF3, DEF5, DEF8 Concave Defect DEF4, DEF6, DEF7, DEF9 Convex Defect ILS Light Source L1 Lens LSF Defect Left Side MB Photomask Blank MBS Photomask Blank Surface OBL Objective Lens RSF Defect Right Side SE Image Detector STG Stage

Claims (9)

A method of inspecting defects existing on the surface of a photomask blank on which at least one thin film is formed on a substrate by 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 to the focus distance, and in the state where the focus distance is set, the inspection light is emitted and the optical axis thereof is the above. The step of irradiating the defect through the objective lens with oblique illumination that is inclined with respect to the surface of the photomask blank, and
(A2) A step of collecting the reflected light of the region irradiated with the inspection light as a first magnified image of the region via the objective lens, and
(A3) A first determination step of identifying a portion of the light intensity change of the first magnified image and determining the uneven shape of the defect from the change of the light intensity of the portion of the first magnified image where the light intensity changes. Including and
Only when the defect shape is determined to be concave in the first determination step, further
(B1) The distance between the defect and the objective lens of the inspection optical system is set to a positive or negative defocus distance deviating from the focus distance by a predetermined positive or negative defocus amount , and the positive and negative defocus distances are set. At each of the defocus distances, with the defocus distance set, the inspection light is irradiated to the defect through the objective lens by oblique illumination whose optical axis is inclined with respect to the surface of the photomask blank. And the process to do
(B2) A step of collecting the reflected light of the region irradiated with the inspection light as a second magnified image of the region via the objective lens, and
(B3) The change portion of the light intensity of the second magnified image is specified, and the true concave portion at the negative defocus distance is obtained from the change of the light intensity of the changed portion of the light intensity of the second magnified image. The reversal of the light-dark positional relationship of the cross-sectional profile of the defect light intensity distribution and the difference (absolute value) between the light intensity of the defect-free region and the light intensity of the dark region at the above positive defocus distance, of the defect-free region Based on the ratio of the difference (absolute value) between the light intensity and the light intensity of the bright part, the second determination step of re-determining the uneven shape of the defect is performed to re-determine the uneven shape of the defect. A featured photomask blank defect inspection method. In the step (B3), the arrangement of the bright part and the dark part in the defective part of the second enlarged image is changed.
Convex defects that do not invert at a negative defocus distance,
Convex defects, which are inverted at a negative defocus distance and whose brightness and darkness increase at a positive defocus distance,
Concave defects when inverted at a negative defocus distance and the brightness intensity did not increase at a positive defocus distance
The defect inspection method according to claim 1, wherein the uneven shape of the defect is redetermined. In the (B3) step, advance to obtain the simulation of light intensity change in the variation of the light intensity of the true recessed defect, a light intensity change resulting change in the light intensity of the second enlarged image The defect inspection method according to claim 1 or 2, wherein the uneven shape of the defect to be inspected is redetermined by comparison with the change in light intensity of the portion. The defect inspection method according to any one of claims 1 to 3, wherein the inspection light is light having a wavelength of 210 to 550 nm. Claim 1 is characterized in that, in both the steps (A2) and (B2), a spatial filter that shields 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. The defect inspection method according to any one of 4 to 4. In the step (A1), the photomask blank is placed on a stage that can move in the in-plane direction, and the stage is moved in the in-plane direction so that the defect and the objective lens of the inspection optical system are brought close to each other. The defect inspection method according to any one of claims 1 to 5, wherein the defect inspection method is performed. The defect inspection method according to any one of claims 1 to 6, wherein the defocus amount is more than 0 nm and 300 nm or less. From the photomask blank obtained by performing the above steps (B1) to (B3) based on the uneven shape of the defect re-determined in the second determination step of the defect inspection method according to any one of claims 1 to 7. A method for selecting a photomask blank, which comprises selecting a photomask blank that does not contain concave defects. The process of forming at least one thin film on the substrate and
A method for producing a photomask blank, which comprises a step of determining the uneven shape of a defect existing in the thin film by the defect inspection method according to any one of claims 1 to 7.

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