JP4842696B2 - Photomask blank manufacturing method and photomask blank - Google Patents

Description

  The present invention relates to a method for manufacturing a photomask blank used for manufacturing a semiconductor integrated circuit, a CCD (Charge Coupled Device), a color filter for LCD (Liquid Crystal Display Device), and a photomask used for fine processing such as a magnetic head.

  In recent years, advanced semiconductor microfabrication technology has become an extremely important elemental technology in order to meet the demand for circuit pattern miniaturization accompanying the high integration of large-scale integrated circuits. For example, high integration of a large scale integrated circuit requires a thinning technique for a wiring pattern constituting a circuit and a miniaturization technique for a contact hole pattern for wiring between layers constituting a cell. The reason why pattern miniaturization of large-scale integrated circuits is accelerated is because of its high-speed operation and low power consumption, and the most effective method is that of pattern miniaturization.

  Since most of such advanced microfabrication is performed by a photolithography technique using a photomask, the photomask is a basic technique that supports the miniaturization technique together with the exposure apparatus and the resist material. For this reason, technological development for forming a finer and more accurate pattern on the photomask blank is underway for the purpose of realizing a photomask having the above-described thinned wiring pattern and miniaturized contact hole pattern. Has been.

  In order to form a highly accurate photomask pattern on a photomask substrate, it is premised that a resist pattern formed on a photomask blank is patterned with high accuracy. Since photolithography when microfabricating a semiconductor substrate is performed by a reduction projection method, the size of the pattern formed on the photomask is about four times the size of the pattern formed on the semiconductor substrate. This does not mean that the accuracy of the pattern formed on the photomask is relaxed, but rather it is required to form the photomask pattern with higher accuracy than the pattern accuracy obtained on the semiconductor substrate after exposure.

  By the way, in order to form a photomask pattern, usually, a photoresist film is formed on a photomask blank provided with a light-shielding layer on a transparent substrate, and this photoresist film is irradiated with an electron beam to draw a pattern. And developing the photoresist film to obtain a resist pattern. Then, using the resist pattern as an etching mask for the light shielding layer, the light shielding layer is patterned to obtain a photomask pattern. In order to obtain a fine photomask pattern by such a method, it is important to reduce the thickness of the photoresist film and select the material of the light shielding layer.

  Many materials have already been proposed as light shielding film materials for patterning using a photoresist as an etching mask. Among these, the chromium compound film has a large amount of information for etching, and in practice, the chromium compound has always been used as a light shielding film material, and has been established as a practical standard processing step. For example, Patent Documents 1 to 3 disclose a configuration example of a photomask blank in which a light-shielding film having a light-shielding characteristic required for a photomask blank for ArF exposure is formed of a chromium compound.

  However, even if the material for the light-shielding film of the photomask blank is properly selected, if defects such as particles or pinholes are present in the actually formed film, this defect is a mask pattern defect (mask defect). The chip to which such a pattern is transferred becomes a defective product.

  The causes of such mask defects include defects originally present in the photomask blank in addition to those caused by the process during mask manufacturing. The mask defect caused by the defect of the photomask blank can be dealt with by performing a defect inspection in the process of manufacturing the photomask blank and selecting a blank having a defect level higher than a predetermined level.

  By the way, in recent years, a demand for reducing mask defects is severe. For example, a mask used for a 120 nm node (nodes) is required to have no defect with a diameter of 80 nm (or more). In defect inspection of photomask blanks to eliminate such minute mask defects, the intensity of the reflected light is monitored by irradiating light converged on the main surface (mask pattern forming surface) of the blank. The defect detection method is effective, and the position of the defect can be specified by scanning the irradiation light on the surface of the photomask blank and monitoring the reflected light intensity and the scanning coordinates. When monitoring the intensity change of the reflected light, it is possible to introduce a sufficient amount of light to the detector by using the bright field light. Here, the measurement based on the detection of scattered light is referred to as dark field measurement, and the measurement method in which a detector is placed on the incident optical axis to detect a change in reflectance is referred to as measurement using bright field light.

  In such a defect detection method, if the irradiation light is converged by a lens to reduce the irradiation diameter, and the optical system is set so that the focal point of the irradiation light is tied to the blank surface, defect detection can be performed. Although the resolution (sensitivity) is improved and finer defects can be detected, the amount of scanning on the photomask blank surface increases as the irradiation light is narrowed down, which is required for defect inspection. The time will be longer.

For example, MAGICS M2350 manufactured by Lasertec Co., Ltd. is known as a typical defect inspection apparatus using light irradiation. In the case of this apparatus, although it is supposed to have a capability of detecting defects having a diameter of 80 nm or more, a photomask blank In order to scan the irradiation light over the entire inspection area, a time of about 20 minutes is required.
JP 2003-195479 A JP 2003-195483 A Registered Utility Model No. 3093632

  FIG. 1 is a cross-sectional view conceptually showing a film configuration of a photomask blank for a halftone phase shift mask. The blank shown in this figure is halftone on one main surface of a quartz glass substrate 11. A phase shift film (HT film) 12 and a light shielding film (Cr layer) 13 are sequentially laminated.

  In general, the defect inspection of a photomask blank is performed at the final stage after forming the light shielding film (Cr layer) 13 for the purpose of shortening the total time required for the inspection. According to the above, even if the conventional defect inspection as described above is performed on the blank after the light shielding film (Cr layer) 13 is formed, the defect generated in the previous process of forming the light shielding film (Cr layer) 13 is detected. It turned out to be difficult.

  Therefore, in order to further reduce the defect of the photomask blank, the defect is generated both after the formation of the HT film 12 on the quartz glass substrate 11 and after the formation of the light shielding film 13 on the HT film 12. Ideally, the inspection should be performed to accurately grasp the occurrence of defects after each film formation process. However, when such an inspection process is adopted, the total time required for defect inspection is long. There is a problem that the number of steps required for manufacturing the photomask blank increases and the manufacturing cost also increases.

  The present invention has been made in view of such problems, and its object is to suppress the occurrence of defects in the optical film formed on the transparent substrate (the defective rate of the photomask blank) and to reduce the photomask. Photomask blank that enables defect detection with high accuracy without performing defect inspection after each film formation in a photomask blank manufacturing process that requires a plurality of film formation processes in order to increase the blank production yield It is in providing the manufacturing method of.

In order to solve such a problem, the present invention provides a photomask blank manufacturing method according to claim 1, wherein (a) an inspection light is applied to a surface of an optical film formed on a transparent substrate. Forming a transparent film having a high transmittance with respect to the refractive index greater than 1 and not sensitive to the inspection light ; and (b) forming the transparent film on the optical film. Irradiating the inspection light while scanning, detecting a change in the reflected light intensity of the inspection light, and (c) obtaining the number of defects in the optical film surface based on the change information of the reflected light intensity A defect inspection step having a step.

  The invention according to claim 2 is the method for producing a photomask blank according to claim 1, wherein the transparent film is at least one of a polyhydroxystyrene copolymer, a novolac resin, an acrylic resin, and a methacrylic resin. It is characterized by forming with the polymeric material containing this.

According to a third aspect of the present invention, in the photomask blank manufacturing method according to the first or second aspect , the transparent film can be used as a photoresist film at the time of mask pattern formation. .

  According to a fourth aspect of the present invention, in the method for manufacturing a photomask blank according to any one of the first to third aspects, the thickness of the transparent film is set in a range of 50 nm to 20000 nm (20 μm). It is characterized by.

According to a fifth aspect of the present invention, in the photomask blank manufacturing method according to any one of the first to fourth aspects, the wavelength of the inspection light is selected in a range of 480 to 500 nm. .

According to a sixth aspect of the present invention, in the method for manufacturing a photomask blank according to any one of the first to fifth aspects, the change in reflected light intensity of the inspection light is detected by a bright field optical system. It is characterized by.

According to the present invention, the “transparent film” having a high transmittance with respect to the inspection light and having a refractive index larger than 1 is provided on the outermost surface of the sample. The degree of optical unevenness corresponding to (d) (optical distance: d _opt ) is increased, and the defect detection sensitivity is increased. As a result, it is possible to improve the quality evaluation accuracy of the photomask blank and reduce the number of defect inspections, thereby contributing to low-cost production of a high-quality photomask blank.

  The best mode for carrying out the present invention will be described below with reference to the drawings. First, the present inventors have studied and obtained from “latent defects” generated in the photomask blank manufacturing process. The findings obtained will be explained.

  FIG. 2 explains the result of inspecting defects on the surface after each film-forming step of manufacturing the photomask blank having the film configuration shown in FIG. 1 using a defect inspection apparatus by light irradiation (MAGICS M2350 manufactured by Lasertec Corporation). FIG. 2A shows a test result of a sample in which a HT film made of MoSiON having a thickness of 70 nm is formed on a quartz glass substrate after fine foreign substances are previously attached to the quartz glass substrate. ) Shows the inspection results after forming a light-shielding film (Cr layer) with a film thickness of 53 nm on the HT film. The upper figure of each figure is a map of the defects detected on the blank, and the lower figure is The defect size distribution (pixel histogram) is shown.

  According to the inspection results shown in FIG. 2, the total number of defects detected after the HT film is formed is 175, whereas the total number of defects detected after the light shielding film (Cr layer) is formed is 18. It is evaluated that the number of defects is an order of magnitude less after the formation of the light shielding film (Cr layer). However, this blank is not subjected to defect removal such as cleaning after the HT film is formed, nor is the defect disappeared during the formation of the light shielding film (Cr layer).

  FIG. 3 is a diagram for explaining a phenomenon in which the number of detected defects obtained after each film formation changes greatly, that is, a result of an experiment conducted to verify a phenomenon in which the obtained defect information is different from a true value. . In this experiment, several levels (the number of foreign objects) of fine foreign substances are attached in advance on a quartz glass substrate, an HT film is formed on these quartz glass substrates under the same conditions as described above, and each sample is evaluated. The number of detected defects was determined. Then, a light shielding film (Cr layer) is formed on these HT layers under the above-described conditions, and the number of detected defects is obtained again. In this figure, the horizontal axis represents the number of detected defects in the HT film, and the vertical axis represents the number of detected defects in the light shielding film (Cr layer).

  According to the results shown in FIG. 3, the number of detected defects obtained from the sample after forming the light shielding film (Cr layer) is approximately 20 irrespective of the number of detected defects obtained from the sample after forming the HT layer. Front and rear (14 to 28). This fact means that the number of detected defects obtained from the sample after forming the light-shielding film (Cr layer) does not thrive on previous defect information.

  According to the examination by the present inventors, the reason why the number of defects is evaluated to be small after the light shielding film (Cr layer) is formed is that the HT film and the HT film are formed even after the light shielding film (Cr layer) is formed. Defects existing at the interface between the film and the light-shielding film (Cr layer) are merely in a state where they are not detected by defect inspection by light irradiation, and are not detected by such defect inspection. The knowledge that the defect becomes a “latent defect” and becomes apparent during the photomask manufacturing process and causes the generation of the mask defect is obtained.

  The present invention has been made based on this finding, and as a result of examining a method for accurately detecting defects generated in the previous process even after film formation of a light shielding film (Cr layer) or the like, It has been found that it is effective to further provide a “transparent film” on the surface of the outermost layer.

  FIG. 4 is a diagram for explaining the effect of the “transparent film” provided on the surface of the outermost layer of the photomask blank (here, the light shielding film (Cr layer)). FIG. 4A and FIG. (B) is the same figure as FIG. 2 (A) and FIG. 2 (B) already explained, and FIG. 4 (C) is the top of the light shielding film (Cr layer) subjected to the defect inspection of FIG. 4 (B). Further, as a “transparent film”, a positive resist (SEBP-8332 manufactured by Shin-Etsu Chemical Co., Ltd.) having a polyhydroxystyrene copolymer as a main component is formed with a thickness of 240 nm, and a defect inspection is performed. The device used for defect inspection is MAGICS M2350 manufactured by Lasertec.

  As already described with reference to FIG. 2, the total number of defects detected after the deposition of the HT film is 175, whereas the total number of defects detected after the deposition of the light shielding film (Cr layer) is The number of defects is evaluated to be one order of magnitude less after the formation of the light shielding film (Cr layer). However, when a defect inspection is performed by providing a “transparent film” on the light shielding film (Cr layer), FIG. As shown in FIG. 4C, the total number of detected defects is 252, and defects larger than the total number of defects 175 detected after the HT film is formed are detected. Then, it can be confirmed that the defect in-plane distribution (the upper diagram in FIG. 4C) is very similar to that in FIG.

  This result occurs in the previous process even when the outermost layer (here, the light-shielding film (Cr layer)) is formed, when a “transparent film” is provided on the surface of the outermost layer and defect inspection is performed. It is shown that the detected defect can be detected with high accuracy.

  FIG. 5 is a diagram for explaining the result of an experiment conducted for verifying the effect of improving the defect detection sensitivity by providing the “transparent film” and performing the defect inspection. Also in this experiment, several levels (the number of foreign substances) of fine foreign substances are attached in advance on a quartz glass substrate, an HT film is formed on these quartz glass substrates under the same conditions as described above, and each sample is evaluated. The number of detected defects was determined. Then, the number of detected defects is obtained again in a state where a light-shielding film (Cr layer) is formed on these HT layers under the above-described conditions and a “transparent film” is further provided. In this figure, the horizontal axis represents the number of defects detected in the HT film, and the vertical axis represents the number of defects detected in the light shielding film (Cr layer) provided with the “transparent film”.

  As shown in this figure, between the number of detected defects obtained from the sample after forming the HT layer and the number of detected defects obtained from the sample provided with the “transparent film” on the light shielding film (Cr layer). Is highly correlated. Further, the slope of the straight line indicating the correlation is 1.38, and it can be seen that about 40% higher sensitivity is achieved than the defect detection sensitivity when the defect is detected in the sample after the HT layer is formed.

  As a mechanism for improving the defect detection sensitivity by forming the “transparent film”, the present inventors interpret as follows. In other words, a defect inspection apparatus using light irradiation (for example, MAGICS M2350 manufactured by Lasertec Corporation) monitors the reflected light intensity of the light irradiated on the sample surface and performs defect detection based on the intensity (change in intensity) of the reflected light. One factor that determines the intensity of reflected light is the unevenness of the sample surface, and the presence of this unevenness is evaluated as a defect. The greater the degree of unevenness (direction component perpendicular to the sample surface), the more easily the reflected light intensity is affected.

  By the way, since this defect inspection is based on the reflection phenomenon of light, the degree of unevenness (direction component perpendicular to the sample surface) when performing defect evaluation is not merely a geometrical unevenness, but an optical meaning. The degree of unevenness in the case becomes a problem. That is, even if the geometrical unevenness is the same, if the refractive index of the medium around the unevenness is different, the degree of unevenness is optically different, and the defect detection sensitivity is different.

  FIG. 6 is a diagram for schematically explaining the mechanism of improving the defect detection sensitivity by providing a “transparent film”. FIG. 6A shows a sample in which no “transparent film” is provided on the outermost surface of the sample. FIG. 6B is a schematic cross-sectional view of a sample provided with a “transparent film” on the outermost surface of the sample. In these drawings, reference numeral 10 denotes a photomask blank, reference numeral 20 denotes a defect, and reference numeral 30 denotes a “transparent film”.

Since a sample (conventional sample to be inspected) in which no “transparent film” is provided on the outermost surface of the sample is measured in air, the refractive index n around the defect 20 is 1.0. On the other hand, in a sample provided with a “transparent film” on the outermost surface of the sample, the refractive index n around the defect 20 is the refractive index of the “transparent film”, for example 1.5. When a “transparent film” having a refractive index (n> 1.0) larger than the refractive index of air is provided in the uppermost layer, the degree of optical unevenness (optical distance: d _opt ) depends on the geometry of the defect 20. It becomes larger than the degree (d) of the geometrical unevenness, and the defect detection sensitivity is increased accordingly.

  Such a “transparent film” is a film having a high transmittance with respect to inspection light and a refractive index larger than 1, but a film made of an inorganic material such as a MoSiON film or polyhydroxystyrene A film made of a polymer material such as a copolymer, a novolac resin, an acrylic resin, or a methacrylic resin is exemplified, and a material including at least one of these can also be used. Since these polymer materials are photoresist compositions for manufacturing semiconductor devices, a technique for forming a stable “transparent film” with little contamination by foreign substances has been established. Further, the “transparent film” may not be simply used for defect inspection, but may be a photoresist film made of the above material.

  In addition, regarding the thickness of the `` transparent film '', when the thickness is about the same as the inspection wavelength, the effect of interference and multiple reflection can be superimposed, and an improvement in detection sensitivity can be expected, but if it is too thin ( If it is much thinner than the inspection wavelength, it becomes difficult to obtain the optical effect described above, and improvement in defect detection sensitivity cannot be expected. For these reasons, the film thickness is preferably 50 nm or more.

  On the other hand, if the “transparent film” is too thick, it becomes difficult to form an optically uniform “transparent film”. For example, when the “transparent film” is applied and formed by a spin coater, it is necessary to increase the viscosity of the resin to be applied in order to increase the film thickness of the “transparent film”. It is not easy to filter out foreign substances or remove bubbles. Therefore, the film thickness of the transparent film formed of the “transparent film” material as described above is preferably 20000 nm (20 μm) or less.

  If the "transparent film" is formed of a photoresist film and light having a wavelength that does not expose the photoresist film is used as inspection light, the "transparent film" is a film for defect inspection. In addition, there is an advantage that it can be used as a photoresist film when forming a mask pattern. In this case, it is unnecessary to remove the “transparent film” before entering the mask manufacturing process, so that it is possible to suppress an increase in mask manufacturing cost and to prevent a decrease in yield.

  The shorter the wavelength of the inspection light, the better the inspection detection sensitivity. However, if the "transparent film" can be used as a photoresist film, the inspection light should not damage the photoresist film. Is done. The inspection light wavelength that does not damage the photoresist film (particularly, the electron beam photoresist film) and provides high defect detection sensitivity is preferably 480 to 500 nm.

  Thus, in the photomask blank manufacturing method of the present invention, various optical films (phase shift film, halftone film, light shielding film, antireflection film, etc.) required for the main surface of the photomask blank are laminated. In this state, the above-mentioned “transparent film” is formed on the outermost surface, the defect inspection is performed by light irradiation, and the non-defective / defective product is selected according to a predetermined selection standard. That is, (a) a transparent film having a high transmittance with respect to inspection light and having a refractive index larger than 1 is formed on the surface of the optical film formed on the transparent substrate, and (b) the optical film Irradiation is performed while scanning the inspection light through the transparent film to detect a change in the reflected light intensity of the inspection light, and (c) the number of defects in the optical film surface is obtained based on the obtained change information of the reflected light intensity. Will be.

  In addition, the inspection light irradiated on the surface of the sample to be inspected is focused by the lens so as to be focused by the lens in order to increase the defect detection sensitivity, and the reflected light intensity is monitored with a sufficient amount of light for the detector. In order to guide reflected light, it is preferable to use bright field light (a bright field optical system). In order to obtain high defect detection sensitivity, it is desirable to use a differential interference optical system.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

  Experiment 1 (Preparation of defect evaluation sample 1): A quartz glass substrate with a 152 mm square and a thickness of 6.35 mm is placed in a vacuum chamber equipped with a small orifice in the suction portion, and then sucked through the orifice, and the quartz substrate is placed on the quartz substrate. 20 samples with particles having a diameter of 60 nm or less attached thereto were prepared, and these were used as defect evaluation samples 1.

  (Comparative example 1): When the defect evaluation sample 1 (20 samples) was subjected to defect inspection (inspection wavelength 488 nm) with MAGICS M2350 manufactured by Lasertec, the number of detected defects was 2 to 5 per sample. .

  (Example 1): Five specimens of the above-described defect evaluation sample 1 were taken, and a MoSiON film having a thickness of 70 nm was formed as a “transparent film” by sputtering to prepare a defect evaluation sample 2. The transmittance of the MoSiON film was 60% at an inspection wavelength of 488 nm. When this defect evaluation sample 2 was inspected for defects, the number of detected defects was 180 to 230 per specimen.

  (Example 2): Five specimens of the above-described defect evaluation sample 1 were taken, and a defect evaluation sample 3 was prepared by sputtering a MoSiON film as a “transparent film” with a thickness of 50 nm. The transmittance of this MoSiON film was 70% at an inspection wavelength of 488 nm. When this defect evaluation sample 3 was inspected for defects, the number of detected defects was 150 to 200 per specimen.

  (Comparative Example 2): Five samples of the defect evaluation sample 1 are taken, and a Cr film is formed as a “opaque film” with a thickness of 33 nm, and a CrON film is formed as a “transparent film”. Defect evaluation sample 4 was prepared by forming a film. The transmittance of the “Cr layer” (total thickness 53 nm), which is a laminate of the “opaque film” and the “transparent film”, was 1.8% at an inspection wavelength of 488 nm. The transmittance of the “transparent film” CrON film (20 nm) alone was 60%. When this defect evaluation sample 4 was inspected for defects, the number of detected defects was 10 to 30 per specimen.

  (Comparative Example 3): Five specimens of the above-described defect evaluation sample 1 were taken, and a MoSiON film was sputtered to a thickness of 70 nm as a “transparent film”. Further, the “Cr layer” of Comparative Example 2 ( A defect evaluation sample 5 was fabricated by sputtering a 33 nm Cr film as an “opaque film” and a 20 nm CrON film as a “transparent film”. The transmittance of the MoSiON film is 60% at an inspection wavelength of 488 nm, and is adjusted to a transmittance of 6% and a phase difference of 177 ° at a wavelength of 193 nm. The transmittance of the “Cr layer” is 1.8% at the inspection wavelength of 488 nm, and the transmittance of the CrON film alone, which is the “transparent film”, is 60%. When this defect evaluation sample 5 was inspected for defects, the number of detected defects was 10 to 30 per specimen.

  (Example 3): A positive resist (SEBP-manufactured by Shin-Etsu Chemical Co., Ltd.) containing a polyhydroxystyrene copolymer as a main component as a “transparent film” for each of the defect evaluation samples 5 (5 samples of Comparative Example 3). 8332) was formed with a thickness of 240 nm to prepare a defect evaluation sample 6. The transmittance of this “transparent film” alone was 90% at an inspection wavelength of 488 nm. When this defect evaluation sample 6 was inspected for defects, the number of detected defects was 250 to 300 per specimen.

  Experiment 2 (Preparation of defect evaluation sample 7): After three samples of a quartz substrate on which particles are not adhered, a MoSiON film of 70 nm and a Cr film of 33 nm and then a CrON film of 20 nm are formed by the method described in Comparative Example 3 above. A positive resist film 240 nm was formed by the method described in Example 3 to obtain a defect evaluation sample 7 for background confirmation. When this defect evaluation sample 7 was inspected for defects, the number of detected defects was 10 to 20 per specimen.

  Table 1 summarizes the configurations of the defect evaluation samples 1 to 7 and the number of detected defects. As is apparent from the results shown in this table, the defect detection sensitivity is dramatically improved by forming a “transparent film” on the outermost surface of the sample. Further, based on the experimental fact that the number of detected defects is 10 to 30 in the defect evaluation sample 2 (Comparative Example 2), the thickness of the “transparent film” is insufficient at 20 nm, and the desirable “transparent film” thickness. Can be confirmed to be 50 nm or more.

  As mentioned above, although the Example demonstrated the manufacturing method of the photomask blank of this invention, the said Example is only an example for implementing this invention, and this invention is not limited to these. It is apparent from the above description that various modifications of these embodiments are within the scope of the present invention, and that various other embodiments are possible within the scope of the present invention.

  The present invention provides an effective manufacturing method for suppressing the occurrence of defects (defect ratio of a photomask blank) in an optical film formed on a transparent substrate and increasing the manufacturing yield of the photomask blank.

It is sectional drawing for notionally showing the film | membrane structure of the photomask blank for halftone phase shift masks. It is a figure for demonstrating the result of having test | inspected the defect on the surface after each film-forming process of photomask blank manufacture of the film | membrane structure shown in FIG. 1 using the defect inspection apparatus by light irradiation. It is a figure for demonstrating the experimental result performed in order to verify the phenomenon in which the defect detection number obtained after each film-forming changes large. It is a figure for demonstrating the effect of the "transparent film" provided in the surface of the outermost layer of a photomask blank. It is a figure for demonstrating the experimental result performed in order to verify the improvement effect of the defect detection sensitivity by providing a "transparent film" and performing a defect inspection. It is a figure for demonstrating notionally the mechanism in which defect detection sensitivity improves by providing a "transparent film".

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Photomask blank 11 Quartz glass substrate 12 Halftone phase shift film 13 Light-shielding film 20 Defect 30 Transparent film

Claims (6)

A photomask blank manufacturing method comprising a defect inspection step having the following steps.
(A) A photomask having a high transmittance with respect to the inspection light on the surface of the optical film formed on the transparent substrate and having a refractive index larger than 1 , which is not sensitive to the inspection light. Forming a resist film ; (b) irradiating the optical film with scanning inspection light through the transparent film; and detecting a change in reflected light intensity of the inspection light. (C) changing the reflected light intensity. Obtaining the number of defects in the optical film surface based on the information The photomask blank according to claim 1, wherein the transparent film is formed of a polymer material containing at least one of a polyhydroxystyrene copolymer, a novolac resin, an acrylic resin, and a methacrylic resin. Manufacturing method. The method of manufacturing a photomask blank according to claim 1 , wherein the transparent film is usable as a photoresist film when forming a mask pattern . 4. The method of manufacturing a photomask blank according to claim 1, wherein a film thickness of the transparent film is set in a range of 50 nm to 20000 nm (20 μm). 5. The method for manufacturing a photomask blank according to any one of claims 1 to 4, wherein a wavelength of the inspection light is selected in a range of 480 to 500 nm. 6. The method for manufacturing a photomask blank according to claim 1, wherein the detection of the reflected light intensity change of the inspection light is executed by a bright field optical system.