JP2018120211A - Method of inspecting defect of photomask blank, method of screening, and method of manufacturing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical method of inspecting defects for inspecting a photomask blank at more low cost and with a high yield.SOLUTION: The concave convex shape of defects existing on a surface part of a photomask blank is determined on the basis of a feature amount of a light intensity distribution extracted from an enlarged image, and an acceptance criterion of defects corresponding to a structure of an optical film by irradiating the surface of the photomask blank formed with at least one layer of a thin film on a transparent substrate with an inspecting light, and collecting a reflective light from a region irradiated with the inspecting light through an inspection optical system to form the enlarged image.SELECTED DRAWING: Figure 8

Description

  The present invention relates to a defect inspection method for a photomask blank used for manufacturing a photomask (transfer mask) used in manufacturing a semiconductor device (semiconductor device) and the like, and in particular, a thickness formed on the photomask blank. The present invention relates to a defect inspection method for a photomask blank which is effective for determining a concave shape such as a pinhole existing in a thin film having a thickness of 10 nm or less. The present invention also relates to a photomask blank sorting method and manufacturing method to which a photomask blank defect inspection method is applied.

  A semiconductor device (semiconductor device) irradiates a pattern transfer mask such as a photomask on which a circuit pattern is drawn with exposure light, and the circuit pattern formed on the mask is transferred to a semiconductor substrate (semiconductor wafer) via a reduction optical system. ) It is manufactured by repeatedly using a photolithographic technique to be transferred onto. With continuous miniaturization of circuit patterns of semiconductor devices, the wavelength of exposure light is mainly 193 nm using argon fluoride (ArF) excimer laser light, which is a combination of multiple exposure processes and processing processes. By adopting a patterning process, it is possible to finally form a pattern having a sufficiently small size compared to the exposure wavelength.

  The pattern transfer mask is manufactured by forming a circuit pattern on a substrate (mask blank) on which an optical film is formed. Such an optical film (thin film) is generally a film containing a transition metal compound as a main component or a film containing a transition metal-containing silicon compound as a main component. A film functioning as a phase shift film is selected. Furthermore, a hard mask film which is a processing auxiliary film for the purpose of high-precision processing of the optical film is also included.

  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. This is naturally also defect-free for a photomask blank. Will be required. Also, when forming a circuit pattern, a resist film for processing is formed on the photomask blank on which the film is formed, and the pattern is finally formed through a normal lithography process such as an electron beam drawing method. Form. Therefore, the resist film is required to have no defects such as pinholes. Under such circumstances, many studies have been made on techniques for detecting defects in photomasks and photomask 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 matter from irregularly reflected light. In particular, a technique is described in which an asymmetry is given to a detection signal to determine whether the defect is a convex defect or a concave defect. Japanese Patent Laying-Open No. 2005-265736 (Patent Document 3) describes a technique of using DUV (Deep Ultra Violet) light, which is used for performing pattern inspection of a general optical mask, as inspection light. . Furthermore, Japanese Patent Laid-Open No. 2013-19766 (Patent Document 4) describes a technique in which inspection light is divided into a plurality of spots, a plurality of spots are scanned on a substrate, and a reflected beam is received by a light detection element. Has been. On the other hand, Japanese Unexamined Patent Application Publication No. 2007-219130 (Patent Document 5) discloses a technique for determining the uneven shape of defects in an EUV mask blank using EUV (Extreme Ultra Violet) light having a wavelength of around 13.5 nm as inspection light. Has been.

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

  All of the inspection apparatuses described in Patent Documents 1 to 4 adopt an optical defect method, and enable wide area defect inspection and defect unevenness determination in a relatively short time. Further, as far as EUV mask blanks are concerned, Patent Document 5 describes a method capable of determining the unevenness of phase defects.

  However, when the present inventors examined, according to an inspection experiment using an atomic force microscope or an electron microscope, the conventional method for examining the arrangement of the bright part and the dark part of the inspection signal of the photomask blank can determine the unevenness. It turns out that there are cases where it is not possible. That is, there are cases where the positional relationship between the bright part and the dark part for distinguishing irregularities in the inspection signal for pinhole defects is unclear. In particular, it has been found that in the defect inspection of the processing auxiliary layer formed for processing the tip mask, that is, the hard mask thin film having a thickness of 10 nm or less, the above-described problem that the unevenness determination is difficult to occur.

  From such a situation, according to the actual inspection experiment based on the inspection apparatus described in Patent Documents 1 to 4, it is not always possible to determine the uneven shape on the surface of the defective portion with high accuracy. Is. Further, the method described in Patent Document 5 is applied to phase defects inherent to EUV mask blanks, and is difficult to apply to photomask blanks currently used in mainstream ArF lithography. For this reason, it has been desired to establish a method for accurately determining the concave and convex shape of the defect existing in the hard mask thin film, which has been difficult with the conventional method.

  The present invention has been made in order to solve the above-mentioned problems, and it is possible to perform defect inspection of a photomask blank capable of highly reliably determining irregularities in the surface shape of a defective portion using an optical defect inspection method. Pinhole defect by applying the method, in particular, the method for determining the unevenness of the defect portion present in the hard mask thin film used as a processing auxiliary layer in mask pattern processing, and the method for determining the unevenness of the defect portion of the photomask blank It is an object of the present invention to provide a photomask blank selection method and manufacturing method that eliminates a substrate containing a substrate.

  In order to solve the above-mentioned problems, the present inventors have studied the light intensity distribution of the inspection signal in defects existing in various optical films from both the inspection experiment and the simulation. As a result, it has been found that depending on the value of the complex refractive index with respect to the inspection light of the above optical film and the optical film below it, the change in the brightness of the defect observation image and the positional relationship between the bright part and the dark part are different. As a result of further various studies, the present invention has been made.

Therefore, the present invention provides the following photomask blank defect inspection method and photomask blank selection method and manufacturing method to which the method is applied.
[1]
An inspection light is irradiated on the surface of the photomask blank having at least one thin film formed on the surface of an optically transparent substrate, and the reflected light from the region irradiated with the inspection light is captured to capture the photomask blank. A method for inspecting a defect existing on a surface portion,
(A1) preparing a photomask blank having at least one thin film;
(A2) The photomask blank is moved to move a defect existing on the surface portion of the photomask blank to the observation position of the inspection optical system, and the inspection light is irradiated to the region including the defect, and the inspection light is irradiated. Collecting reflected light from the region as an enlarged image of the region through the inspection optical system;
(A3) extracting the feature amount of the magnified image;
(A4) A method for inspecting a defect of a photomask blank, comprising a step of determining a shape of a defect based on a combination of the feature amount and a thin film mode of the photomask blank.
[2]
The magnified image in the step (A2) is generated by a diffraction component that passes through the inspection optical system in the reflected light, and is a positive and negative asymmetric high-order diffraction component with respect to the zero-order diffraction component (regular reflection component) of the reflected light. The defect inspection method for a photomask blank according to [1], wherein the defect inspection method is an enlarged image formed in step (1).
[3]
The step (A3) includes a processing step of comparing the change in the light intensity level of the defective portion in the magnified image with the light intensity level of the peripheral portion of the defect, and the intensity difference between the bright portion having the high light intensity and the dark portion having the low light intensity, and The defect inspection method for a photomask blank according to [1] or [2], wherein a feature amount of a defect inspection image that is an arrangement positional relationship between a bright part and a dark part is extracted.
[4]
The step (A4) is a table that can be selected as a pinhole defect or a convex defect, which is created in advance based on optical simulation or experimental data, based on the information on the feature quantity of the magnified image and the thin film mode of the photomask blank. The method for inspecting a defect of a photomask blank according to [1], [2], or [3], wherein the defect shape is a step of determining a defect shape.
[5]
In the step (A3), when the feature amount that the enlarged image of the defect is an image in which the bright part is dominant is extracted, and the outermost surface of the photomask blank to be inspected is a thin film transparent to the inspection light, [4] The defect inspection method for a photomask blank according to [4], wherein the detected defect is determined as a pinhole defect.
[6]
In the step (A3), when the feature amount that the dark part is the dominant image is extracted from the enlarged image of the defect, the inspection light reflectance of the thinnest film on the photomask blank to be inspected is the inspection light reflectance of the lower layer. The defect inspection method for a photomask blank according to [4], wherein if the film structure is higher, the detected defect is determined as a pinhole defect.
[7]
The defect inspection method for a photomask blank according to any one of [1] to [6], wherein the thickness of the thin film is 10 nm or less.
[8]
The defect inspection method for a photomask blank according to any one of [1] to [7], wherein the inspection light is light having a wavelength of 210 to 550 nm.
[9]
The surface of the photomask blank is obtained by irradiating inspection light onto the surface of the thin film of a photomask blank in which at least one thin film is formed on an optically transparent substrate and capturing reflected light from the region irradiated with the inspection light. An inspection device for inspecting defects existing in
A defect inspection system for a photomask blank including a computer having a program for executing the steps of the defect inspection method for a photomask blank shown in any one of [1] to [8].
[10]
A photomask characterized in that a photomask blank that does not contain pinhole defects is selected based on the concavo-convex shape of the defect determined by the defect inspection method for a photomask blank according to any one of [1] to [8] Blank sorting method.
[11]
Forming at least one thin film on an optically transparent substrate;
[10] A method for producing a photomask blank, comprising a step of sorting a photomask blank that does not contain pinhole defects in the thin film by the photomask blank sorting method according to [10].

  According to the present invention, concave / convex defects in a photomask blank can be distinguished with high reliability using an optical defect inspection method, and a concave defect or pinhole defect which is a particularly fatal defect can be specified. In addition, by applying the defect inspection method of the present invention, a photomask blank having a concave defect which is a fatal defect can be surely eliminated, and a photomask blank which does not contain a fatal defect can be reduced. Cost and high yield can be provided.

It is sectional drawing which shows the example in which a defect exists in a photomask blank, (A), (B) is a photomask blank in which the pinhole defect which is a concave defect exists, (C) is a photomask in which a convex defect exists. It is a figure which shows a blank. It is a figure which shows an example of a structure of the inspection apparatus used for the defect inspection of a photomask blank. It is a figure which shows an example of the convex defect which exists in the surface of a photomask blank, and its inspection image, (A) is a photomask blank top view of a defective part, (B) is a photomask blank sectional view of a defective part, (C ) Is an inspection image of the convex defect, and (D) is a cross-sectional view of the light intensity distribution of the inspection image. It is a figure which shows the example of the concave defect which exists in the surface of a photomask blank, and its observation image, (A) is photomask blank sectional drawing of a defective part, (B) shows sectional drawing of the light intensity distribution of an inspection image. FIG. It is a figure which shows the cross-sectional profile of the structure and inspection image in a 1st film | membrane aspect, (A) is photomask blank sectional drawing in which the pinhole defect of a concave defect exists in an uppermost layer film | membrane, (B) is an inspection image of a defect. FIG. It is a figure which shows the structure in a 1st film | membrane form, and the cross-sectional profile of a test | inspection image, (A) is a photomask blank sectional drawing in which a concave defect exists in the 2nd layer from the top layer, (B) is an attached foreign material defect. Cross-sectional view of existing photomask blank, (C) is a cross-sectional view of photomask blank where convex defects of the same material as the top layer are present, (D), (E), (F) are (A), (B), It is a figure which shows the inspection image of the defect shown to (C). It is a figure which shows the structure and cross-sectional profile of a test | inspection image in a 2nd film | membrane aspect, (A) is photomask blank sectional drawing in which a pinhole defect exists in an uppermost layer film | membrane, (B) is the light of the test | inspection image of the defect It is a figure which shows an intensity | strength cross-sectional profile. It is a flowchart which shows an example of the process of the defect inspection method of a photomask blank. It is the table which matched the feature-value and film | membrane aspect of a defect inspection image, and defect shape. It is a flowchart which shows an example of the process of determining the quality of a photomask blank. It is a figure which shows the condition which scans the illumination spot for a test | inspection. (A) is sectional drawing of the photomask blank which has a concave defect of Example 1, (B) is a figure which shows the cross-sectional profile of the light intensity distribution of a test | inspection image. (A) is sectional drawing of the photomask blank which has a convex defect of Example 1, (B) is a figure which shows the cross-sectional profile of the light intensity distribution of an inspection image, (C) is a defect inspection image of a different size. It is a figure which shows the cross-sectional profile of light intensity distribution. (A) is sectional drawing of the photomask blank which has a concave defect of Example 2, (B) is a figure which shows the cross-sectional profile of the light intensity distribution of a test | inspection image. (A) is sectional drawing of the photomask blank which has a convex defect of Example 2, (B) is a figure which shows the cross-sectional profile of the light intensity distribution of a test | inspection image.

Hereinafter, the present invention will be described in more detail.
If defects such as pinholes are present in the thin film of the photomask blank, it will cause defects in the mask pattern on the photomask manufactured using this. An example of typical photomask blank defects is shown in FIG. FIG. 1A shows a photomask blank 100 in which an optical thin film 102 that functions as a light shielding film or a phase shift film for a halftone phase shift mask is formed on a transparent substrate 101. Here, the optical thin film 102 has a pinhole defect DEF1. FIG. 1B shows an optical thin film 102 that functions as a light-shielding film or a phase shift film for a halftone phase shift mask on a transparent substrate 101, and a processing auxiliary thin film for performing high-precision processing of the optical thin film 102. It is a figure which shows the photomask blank 100 in which 103 is formed. Here, a pinhole defect DEF2 exists in the processing auxiliary thin film 103. When a photomask is manufactured from such a photomask blank by a normal manufacturing process, a photomask in which defects derived from the photomask blank are present. This defect causes a pattern transfer error in exposure using a photomask. Therefore, it is necessary to detect a defect in the photomask blank at a stage before processing the photomask blank, to eliminate the photomask blank having the defect, or to correct the defect.

  On the other hand, FIG. 1C is a diagram illustrating an example of the convex defect of the photomask blank, and is a diagram illustrating an example of the photomask blank 100 in which the convex defect DEF3 exists on the optical thin film 102. The defect DEF3 may be a convex defect integrated with the optical thin film 102, or a foreign particle convex defect such as a particle. Even if a photomask is manufactured from such a photomask blank by a normal manufacturing process, a fatal pinhole defect is not necessarily formed. Moreover, if the foreign substance defect adhering to the surface can be removed by cleaning, it does not become a fatal defect.

  As described above, whether a defect existing in a photomask blank is a concave defect such as a pinhole which is a fatal defect or a convex defect which is not necessarily a fatal defect is determined by quality assurance of the photomask blank and This is the key to yield in mask blank manufacturing. Therefore, a method that can distinguish the concave and convex shape of the defect with high reliability in a short time by an optical inspection method is desired. Furthermore, considering that the wavelength of exposure light, which is currently the mainstream, is 193 nm using an argon fluoride (ArF) excimer laser beam, the unevenness of defects having a size of 200 nm or less, preferably 100 nm or less on the mask blank. A method that can distinguish shapes is desired.

  First, an inspection apparatus that is preferably used for defect inspection of a photomask blank, specifically, an inspection apparatus that is preferably used for determining the concavo-convex shape of defects on the surface portion of the photomask blank will be described. FIG. 2 is a conceptual diagram showing an example of a basic configuration of the defect inspection apparatus 150. The inspection optical system 151, the control apparatus 152, the recording apparatus 153, and the display apparatus 154 are main components. The inspection optical system 151 includes a light source ILS that emits inspection light, 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 be able to emit light having a wavelength of about 210 nm to 550 nm, and the inspection light BM1 emitted from the light source ILS is bent by the beam splitter BSP and passed through the objective lens OBL. Irradiate a predetermined area of the blank MB. The light BM2 reflected from the surface of the photomask blank MB is collected by the objective lens OBL and passes through the beam splitter BSP and the lens L1 and reaches the light receiving surface of the image detector SE. At this time, the position of the image detector SE is adjusted so that an enlarged inspection image of the surface of the mask blank MB is formed on the light receiving surface of the image detector SE. The data of the enlarged inspection image collected by the image detector SE is subjected to an image processing calculation, whereby a defect size calculation and a concavo-convex shape are determined, and the results are recorded as defect information. ing. The inspection device 150 operates under the control of the control device 152. The control device 152 has a control program and various image calculation programs. Further, it controls the operation of the recording device 153 for storing the inspection data and the display device 154 for performing various displays.

  In the enlarged inspection image, for example, the image detector SE is a detector in which a large number of light detection elements such as a CCD camera are arranged as pixels, and the light BM2 reflected by the surface of the photomask blank MB is passed through the objective lens OBL. The enlarged image to be formed can be collected by a direct method of collecting collectively as a two-dimensional image. Further, the inspection light BM1 is converged on the surface of the photomask blank MB to generate an illumination spot, and the light source ILS that emits the inspection light is provided with a scanning function to scan the illumination spot, and the light intensity of the reflected light BM2 is sequentially increased. Alternatively, a method may be adopted in which the image is collected by the image detector SE, photoelectrically converted and recorded, and an entire two-dimensional image is generated.

  Further, when the reflected light BM2 is collected in order to determine the unevenness of the defect, positive and negative high-order diffraction components are collected asymmetrically with respect to the zero-order diffraction component (regular reflection component) (centered on the regular reflection component). Also good. Specifically, a method of making the principal ray of the inspection light BM1 illuminating the surface of the photomask blank MB obliquely incident, or a spatial filter SPF that shields a part of the optical path of the reflected light BM2 although the principal ray is vertical illumination. A method of providing an enlarged inspection image with the image detector SE can be employed. By adopting these methods, it is generally possible to determine the uneven shape of the defect from the light / dark positional relationship of the inspection image light intensity distribution or the difference in light intensity.

  Next, the inspection light BM1 is focused and scanned on the surface of the photomask blank MB with the principal ray as vertical illumination, and in the inspection image obtained by sequentially collecting the light intensity of the reflected light BM2, convex defects and concave defects are detected. A difference between inspection images will be described. When collecting the light intensity of the reflected light BM2, it is assumed that the right half of the reflected light BM2 toward the image detector SE is shielded by the action of the spatial filter SPF.

  3A and 3B are a plan view and a cross-sectional view of a photomask blank 100 having a convex defect DEF4, respectively. In these, an optical thin film 102 made of MoSi-based material is formed on a transparent substrate 101 such as a quartz substrate that is transparent to inspection light, and a convex defect DEF4 made of MoSi-based material or other material is formed on the surface MBS thereof. Indicates the presence of.

  When the surface MBS of the photomask blank having the convex defect DEF4 is converged with the inspection light BM1 and illuminated and scanned, and the reflected light is collected through the spatial filter SPF, the light intensity distribution shown in FIG. An inspection image is obtained. The light intensity distribution in the cross section along the A-A ′ line in FIG. 3C is a cross-sectional profile PR1 as shown in FIG. The cross-sectional profile PR1 has a shape peculiar to a convex defect in which the left side of the convex defect DEF4 is a bright part and the right side is a dark part.

Similarly, FIG. 4A is a cross-sectional view of the photomask blank 100 having the concave defect DEF5, and FIG. 4B is a view showing a cross-sectional profile PR2 of the light intensity distribution of the inspection image obtained in this case. It is. The cross-sectional profile PR2 has a shape unique to the concave defect in which the left side of the concave defect DEF5 is a dark part and the right side is a bright part.
However, depending on the mode of the film of the photomask blank, it may not be possible to correctly determine whether the defect is a concave defect or a convex defect only with the above-described light / dark positional relationship of the inspection image. An example of such a case will be described below.

[First membrane embodiment]
FIG. 5A is a cross-sectional view of a photomask blank 100 having a concave defect. This is because the optical thin film 112 made of a MoSi-based material, the optical thin film 113 made of a Cr-based material, and the inspection light having a thickness of about 5 to 10 nm on a transparent substrate 101 such as a quartz substrate that is transparent to the inspection light. A hard mask thin film 114 made of a substantially transparent material, for example, silicon oxide, is formed, and the hard mask thin film 114 has a concave defect DEF6 such as a pinhole defect. When the inspection optical system shown in FIG. 2 is used to scan this concave defect DEF6 by converging and irradiating the surface of the photomask blank with inspection light from above and collecting the reflected light via the spatial filter SPF The cross-sectional profile of the light intensity distribution of the inspection image is a profile PR3 shown in FIG. In this case, the light intensity distribution of the inspection image is a portion of the concave defect DEF6, which is substantially only a bright part, and is clear as the light intensity distribution of the typical concave defect inspection image shown in FIG. The relationship between light and dark does not appear.

  FIG. 6 shows an example in which various inspection images can be obtained depending on the type of defect even if the film structure is the same as that shown in FIG. In FIG. 6A, a concave defect already exists in the optical thin film 113 made of a Cr-based material, and a hard mask thin film 114 having a uniform film thickness without a defect is formed thereon, resulting in a concave shape on the surface. The state where the defect DEF7 exists is shown. Further, FIG. 6B shows a state in which a foreign substance mainly composed of silicon adheres to the surface as a convex defect DEF8 although there is no defect until the hard mask thin film 114 is formed. Further, FIG. 6C shows a state in which a part of the surface of the hard mask thin film 114 exists as a convex defect DEF9. The cross-sectional profiles of the inspection images of these defects DEF7, DEF8, and DEF9 are the profile PR4 shown in FIG. 6D, the profile PR5 shown in FIG. 6E, and the profile PR6 shown in FIG. 6F, respectively. The profile PR4 is a typical concave defect inspection image, but the outermost hard mask thin film 114 has no defect, the profile PR5 is a typical convex defect inspection image, and the profile PR6 is a glance. Although it appears as an inspection image of a concave defect, when this profile PR6 is obtained in the case of the first film mode, it is a convex defect of the hard mask thin film 114 that is transparent to the inspection light.

  From the above, in the inspection image of the defect in the first film mode, it can be determined that a pinhole defect which is a fatal defect exists when an inspection image in which the bright part is dominant is obtained. The criterion for the unevenness of the defect in the first film mode is a standard different from the case of the typical convex defect and the concave defect shown in FIGS. 3 and 4, and is a specific criterion for the case of the first film mode. It is. Furthermore, it is suitable when the film made of a material that is substantially transparent to the inspection light is thin, for example, when the film thickness is 10 nm or less, particularly 5 to 10 nm.

[Second membrane embodiment]
FIG. 7A is a cross-sectional view of a photomask blank 100 having a concave defect. In this, an optical thin film 122 made of a MoSi-based material and a hard mask thin film 123 made of a Cr-based material with a thickness of about 10 nm are formed on a transparent substrate 101 such as a quartz substrate that is transparent to inspection light. The figure shows a state in which a concave defect DEF10 such as a pinhole defect exists in the hard mask thin film 123. The feature of the second film mode is that the inspection light reflectance of the hard mask thin film 123 is higher than the inspection light reflectance of the optical thin film 122. When the concave defect DEF10 is scanned by converging and irradiating inspection light on the surface of the photomask blank from above and the reflected light is collected via the spatial filter SPF, the cross-sectional profile of the light intensity distribution of the inspection image is shown in FIG. The profile PR7 shown in FIG. In this case, the light intensity distribution of the inspection image is a portion of the concave defect DEF10 and is substantially only a dark portion, and a clear light and dark like the light intensity distribution of the typical concave defect inspection image shown in FIG. The positional relationship does not appear. The reason why the concave defect is observed only as a dark part in this case is that the depth of the concave defect DEF10 is shallow, so that the amount of reflected light from the side surface of the defect is small, and the influence of the reflectance of the inspection light on the change in light intensity. This is probably because it is larger.

  When a convex defect exists in the hard mask thin film 123, the inspection image is an inspection image in which a bright part and a dark part equivalent to the profile PR1 shown in FIG.

  From the above, in the defect inspection image in the second film mode, when an inspection image in which the dark part is dominant is obtained, it can be determined that a pinhole defect which is a fatal defect exists. The determination criteria for the unevenness of the defect in the second film mode are different from the typical convex and concave defects shown in FIGS. 3 and 4, and the determination criteria specific to the case of the second film mode. It is.

  Next, the defect inspection method of the present invention will be described more specifically with reference to the flowchart shown in FIG. First, as a step (A1), a photomask blank to be inspected having a defect (photomask blank to be inspected) is prepared (step S201). Next, the position coordinate information of the defect existing on the photomask blank is captured (step S202). As the defect position coordinates, the defect position coordinates specified by a known defect inspection method can be used.

  Next, as step (A2), the position of the defect is aligned with the inspection position of the inspection optical system, and inspection light is irradiated from above the photomask blank through the objective lens (step S203), and the inspection light is irradiated. The reflected light of the area is collected as an enlarged image of the area including the defect through the objective lens of the inspection optical system (step S204). For alignment, the photomask blank to be inspected is placed on a stage that can move in the in-plane direction, and the stage is moved in the in-plane direction based on the position coordinates of the defect in the photomask blank to be inspected. May be carried out by a method of maintaining the focal point of the objective lens of the inspection optical system.

  Next, from the collected light intensity distribution (image data (inspection image), cross-sectional profile, etc.) of the magnified image, the feature of the changed portion of the light intensity of the inspection image in the defect portion, that is, the feature amount of the magnified image is extracted (process) S205).

  Thereafter, as step (A4), the uneven shape of the defect is determined based on the feature amount of the enlarged image extracted in step S205 and the film structure (film mode) of the photomask blank (step S206). A specific example of the uneven shape determination step will be described later. The defect size can also be predicted by performing known image processing on the inspection image. Predicted values of the concavo-convex shape and defect size of these defects are recorded as defect information together with the defect position coordinates (step S207).

  Next, it is determined whether or not the inspection has been completed for all the defects based on the previously acquired defect position coordinate information (decision D201). If not completed, a new defect position is designated (step S208). Returning to step S203, the collection of inspection image data and the determination of the unevenness of the defect are repeated. When it is determined that the inspection has been completed for all the defects that have been captured in advance (determination D201), the defect inspection is completed.

  Next, a specific example of the uneven shape determination step will be described. The recording device 153 connected to the control device of the defect inspection apparatus shown in FIG. 2 includes the defect information, the characteristics of the inspection signal when the defect is detected, and various optical mask blank optical films as shown in FIG. A table representing the relationship with the (thin film) structure is stored. The characteristics of the inspection signal are an image in which the bright part is dominant in the defective part, an image in which the dark part is dominant, the left side is the bright part and the right side is the dark part image, the left side is the dark part and the right side is the bright part image, etc. . Further, as the structure of the optical film (thin film), for example, the film structure A is the film mode 1 described above, and is a structure in which a hard mask thin film having a thickness of 10 nm or less and transparent to the inspection light is formed on the outermost surface. is there. Further, the film structure B is the film mode 2 described above, which is a case where the inspection light reflectance of the hard mask thin film having a thickness of 10 nm or less formed on the outermost surface is higher than the inspection light reflectance of the underlying optical thin film. The film structure C is a structure in which an optical thin film made of MoSi-based material is formed on the outermost surface, and the film structure D is an optical thin film made of Cr-based material having a thickness of 20 nm or more on the outermost surface. It is a structure that has been.

  Referring to the table shown in FIG. 9, when features of inspection images obtained by defect inspection are extracted in various film structures, it is identified whether the defect is a fatal pinhole defect or a convex defect. be able to. That is, since the feature amount of the magnified image is extracted in the above step S205, in step S206 for determining the uneven shape of the defect, the type of defect is specified from the film structure of the substrate to be inspected and the feature of the magnified image. With reference to the table shown in FIG. 9, the uneven shape of the defect can be identified. In particular, it is possible to determine whether or not the pinhole defect is fatal.

Note that the criteria for determining the concave defect in the table shown in FIG. 9 may change depending on the light shielding situation of the spatial filter SPF. For example, in the film structure C and the film structure D, if the light shielding portion of the spatial filter is set upside down, the determination of the concave defect and the convex defect corresponding to the arrangement position of the bright part and the dark part, which is a feature of the enlarged image, is also possible. Vice versa.
The table is not limited to the cross-sectional profile of the inspection image, and may be an image of a two-dimensional light intensity distribution. Furthermore, it is possible to sequentially add according to past defect inspection results and introduction of new film modes.

  Next, a photomask selection method employing the defect inspection method of the present invention will be described with reference to the flowchart shown in FIG. First, a photomask blank to be inspected is prepared (step S211). Subsequently, the above-described defect inspection of the photomask blank is performed, and defect information including concavo-convex shapes and sizes of all detected defects is recorded ( Step S212). Thereafter, it is examined whether or not the recorded defect information includes a concave defect such as a pinhole defect (decision D211). If a concave defect is included, the photomask blank is selected as a defective product (step S213). In the case where the concave defect is not included, if it is further determined that the predicted size of the defect is equal to or smaller than a predetermined allowable value (determination D212), the photomask blank is selected as a non-defective product (step S214). Conversely, if it is determined that the predicted size of the defect is equal to or greater than the predetermined allowable value (determination D212), the photomask blank is selected as a defective product (step S213).

  According to the defect inspection method of the present invention, when a thin film having a thickness of 10 nm or less, such as a hard mask thin film, is formed on the outermost surface portion of the photomask blank, the feature amount of the defect inspection image (enlarged image) , And by referring to a table that defines the defect concavo-convex shape specific to the film mode, the concavo-convex shape of the defect can be distinguished with high reliability.

  By applying the defect inspection method of the present invention, which can distinguish the uneven shape of the defect with high reliability, to the photomask blank manufacturing process, a photomask blank having a concave defect, particularly a pinhole defect, can be highly reliable. By extracting, a photomask blank that does not include a concave defect such as a pinhole defect can be selected. Moreover, the information of the uneven | corrugated shape of the defect obtained with the defect inspection method of this invention can be provided to a photomask blank by methods, such as attaching an inspection form.

  Conventionally, it was insufficient to understand that the observation images of pinhole defects differ depending on the film structure, so a photomask that misses a fatal pinhole defect or has a defect that is not necessarily a fatal defect There was a possibility of eliminating the blank as a defective product. Although this has been a factor in yield reduction, the defect inspection method of the present invention can selectively eliminate photomask blanks having concave defects that are fatal defects present in the photomask blank. A photomask blank that meets the product specifications can be provided with a high yield.

  EXAMPLES Hereinafter, although an Example is shown and this invention is demonstrated concretely, this invention is not limited to a following example.

[Example 1]
The defect inspection of the photomask blank including the concave defect and the convex defect of the first film mode was performed. As the inspection apparatus, an apparatus including the inspection optical system 151 shown in FIG. 2 was used. The wavelength of the inspection light emitted from the light source ILS is 532 nm, and the numerical aperture NA of the objective lens OBL is 0.95. The inspection light BM1 converges and illuminates the photomask blank MB from above via the objective lens OBL. As shown in FIG. 11, the focused illumination spot scans the surface MBS of the photomask blank 100 including the defect DEF6 in one direction by a scanning means (not shown). On the other hand, the stage STG on which the photomask blank MB is mounted moves intermittently or continuously in a direction perpendicular to the scanning direction. By combining the scanning of these illumination spots and the movement of the stage, the illumination spot was scanned two-dimensionally within a predetermined area including a defective portion. Then, the reflected light from the photomask blank obtained at each illumination spot is converged by passing through the objective lens OBL and the spatial filter SPF that shields the right half of the reflected light and the lens L1, and the light intensity is detected. Collected in vessel SE. A defect inspection image (enlarged image) was generated by two-dimensionally arranging the collected light intensity in accordance with the position of the illumination spot. Here, the size of the illumination spot on the surface of the mask blank was about 400 nm, and the two-dimensional investigation region including the defect portion was a rectangular region of about 30 μm × 30 μm.

  FIG. 12A is a cross-sectional view of a photomask blank 100 including a pinhole defect of the first film mode, on a quartz substrate 101 that is transparent to inspection light, and having a thickness of 75 nm made of a MoSi-based material. An optical thin film 112, a 44 nm thick optical thin film 113 made of a Cr-based material, and a 10 nm thick hard mask thin film 114 made of silicon oxide are formed, and a pinhole defect DEF6 having a diameter W1 exists in the hard mask thin film 114. It shows the state. Assuming a defect size (= diameter) W1 = 80 nm and 300 nm, the region including these defects is scanned with the illumination spot, and the defect of the inspection magnified image obtained as an array of reflected light intensity corresponding to the scanning position is detected. A cross-sectional profile of the included region is shown in FIG. The feature of this magnified image is that, in any defect size W1, the dark part hardly appears on the basis of the reflected light intensity of the area having no defect, and the bright part has a dominant profile. Since the hard mask thin film 114 is substantially transparent to the inspection light, it acts as an antireflection film. Therefore, the reflectance of the surface of the hard mask thin film 114 is lowered, and the reflectance of the pinhole defect portion where the lower layer surface is exposed becomes higher than the reflectance of the peripheral portion. As a result, the pinhole portion of the inspection image became a bright portion. At the stage of inspecting the photomask blank, the true concave-convex shape of the defect is unknown, but the photomask blank to be inspected has the optical thin film structure of the first film mode, and the feature of the defect observation image (enlarged image) Referring to the table shown in FIG. 9 based on the fact that the bright part is dominant, the defect was determined to be a pinhole defect.

  On the other hand, FIG. 13A is a cross-sectional view of a photomask blank 100 that has the same film structure as the film structure shown in FIG. 12A and includes a convex defect DEF8. As the composition of the convex defect portion, the same composition as the hard mask thin film 114 made of silicon oxide and two types of amorphous silicon (Si) were specified. FIG. 13B shows a cross-sectional profile of the region including the defect of the inspection enlarged image when the width W1 of the convex defect DEF8 is 80 nm and the height H1 is 10 nm and 30 nm. In the enlarged inspection image of the convex defect of W1 = 80 nm, the intensity level changes depending on the height and composition, but the defect portion becomes an inspection image in which the dark portion is dominant, and the inspection of the pinhole defect shown in FIG. The profile is different from the image. Therefore, it could be distinguished from pinhole defects.

  Further, FIG. 13C is a diagram showing a cross-sectional profile of a region including a defect in the inspection enlarged image when the convex defect size is W1 = 400 nm. Here, the defect height is H1 = 30 nm when the composition is silicon oxide, and H1 = 10 nm when the composition is amorphous silicon. In this case, when the composition was silicon oxide, an inspection image in which the defective portion was a dark portion was obtained, and when the composition was amorphous silicon, an inspection image in which the bright portion and the dark portion were aligned was obtained. Since both are different from the inspection image of pinhole defects, they can be distinguished.

  Here, the defect size can be predicted from the inspection image by arithmetic processing using the profile of the inspection image and the contrast of the image. In the defect inspection process in mask blank manufacturing, when the inspection image of FIG. 13C is obtained, the defect size can be estimated to be a value exceeding 300 nm. Here, for example, if the allowable defect size is 100 nm, it is determined that there is a convex defect that is not a fatal pinhole defect but exceeds the allowable value, and the photomask blank can be selected as a defective product.

  From the above, in a photomask blank in which a thin hard mask thin film acting as an antireflection film is formed on an optical thin film, if the light intensity distribution of the defect inspection image is dominant in the bright part, a fatal pin A hole defect is a convex defect if the dark part is dominant or the left part is bright and the right part is dark. These pieces of information were previously stored in the table shown in FIG. 9 as the characteristics of the inspection enlarged image in the film structure A. In the defect inspection in the first film mode, correct unevenness determination can be performed with reference to the film structure A on the table, and a fatal pinhole defect can be identified.

[Example 2]
The defect inspection of the photomask blank including the concave defect and the convex defect of the second film mode was performed. As the inspection apparatus, an apparatus including the inspection optical system 151 shown in FIG. 2 was used. However, the inspection wavelength was 355 nm, the numerical aperture NA of the objective lens OBL was 0.85, and the resolution was higher than that of the inspection optical system used in Example 1, so the illumination spot size was about 380 nm. The two-dimensional scanning area is the same as that in the first embodiment. A photomask blank 100 shown in FIG. 14A is made of a 75 nm-thick optical thin film 122 made of MoSi-based material and a 10-nm thick Cr-based material on a quartz substrate 101 that is transparent to inspection light. A hard mask thin film 123 is formed, and a concave defect DEF10 such as a pinhole defect is present on the hard mask thin film 123.

  Inspection when the width of the concave defect DEF10 is 80 nm, and the depth D2 is 5 nm (a concave defect that does not penetrate the hard mask thin film 123) and 10 nm (a concave defect that penetrates the hard mask thin film 123). FIG. 14B shows a cross-sectional profile of a region including a defect in the light intensity of the image. At any depth, the defect inspection image has a profile in which the dark portion is dominant and the bright portion does not appear.

  On the other hand, FIG. 15A is a cross-sectional view of the photomask blank 100 including the convex defect DEF11 in the second film mode that is the same as the film structure shown in FIG. The region including the defect of the inspection enlarged image when the composition of the convex defect DEF11 is two types of the hard mask thin film 123 itself made of Cr-based material and the silicon foreign substance (particle), the width W2 is 80 nm, and the height H2 is 10 nm. FIG. 15B shows a cross-sectional profile. The inspection magnified image of the convex defect had a bright part on the left side of the defective part and a dark part on the right side of any defect in any composition. Although the intensity level varies depending on the composition, the light / dark positional relationship is similar to the light intensity distribution (cross-sectional profile PR1) of a typical convex defect shown in FIG.

  From the above, in a photomask blank in which a thin film such as a hard mask thin film made of a high-reflectance material is formed on an optical thin film, if the light intensity distribution of the defect inspection image is dominant in the dark area, a fatal pin A hole defect is a convex defect if the left side is bright and the right side is dark.

  These pieces of information when the inspection wavelength is 355 nm are stored in advance in the table shown in FIG. 9 as the characteristics of the inspection enlarged image in the film structure B. In the defect inspection in the second film mode, correct unevenness determination can be performed with reference to the film structure B on the table, and a fatal pinhole defect can be specified.

100 Photomask blank 101 Transparent substrate 102, 103, 112, 113, 122 Optical thin film 103, 114, 123 Processing auxiliary thin film or hard mask thin film 150 Defect inspection device 151 Inspection optical system 152 Control device 153 Recording device 154 Display device BM1 Inspection light BM2 Reflected light BSP Beam splitter DEF1, DEF2, DEF5, DEF6, DEF7, DEF10 Concave defect or pinhole defect DEF3, DEF4, DEF8, DEF9, DEF11 Convex defect ILS Light source L1 Lens MB Photomask blank OBL Objective lens SE Photodetector STG Stage SPF spatial filter

Claims (11)

An inspection light is irradiated on the surface of the photomask blank having at least one thin film formed on the surface of an optically transparent substrate, and the reflected light from the region irradiated with the inspection light is captured to capture the photomask blank. A method for inspecting a defect existing on a surface portion,
(A1) preparing a photomask blank having at least one thin film;
(A2) The photomask blank is moved to move a defect existing on the surface portion of the photomask blank to the observation position of the inspection optical system, and the inspection light is irradiated to the region including the defect, and the inspection light is irradiated. Collecting reflected light from the region as an enlarged image of the region through the inspection optical system;
(A3) extracting the feature amount of the magnified image;
(A4) A method for inspecting a defect of a photomask blank, comprising a step of determining a shape of a defect based on a combination of the feature amount and a thin film mode of the photomask blank.   The magnified image in the step (A2) is generated by a diffraction component that passes through the inspection optical system in the reflected light, and is a positive and negative asymmetric high-order diffraction component with respect to the zero-order diffraction component (regular reflection component) of the reflected light. 2. The defect inspection method for a photomask blank according to claim 1, wherein the defect inspection method is a magnified image formed by the step (1).   The step (A3) includes a processing step of comparing the change in the light intensity level of the defective portion in the magnified image with the light intensity level of the peripheral portion of the defect, and the intensity difference between the bright portion having the high light intensity and the dark portion having the low light intensity, and 3. The defect inspection method for a photomask blank according to claim 1, wherein a feature amount of a defect inspection image which is a positional relationship between a bright part and a dark part is extracted.   The step (A4) is a table that can be selected as a pinhole defect or a convex defect, which is created in advance based on optical simulation or experimental data, based on the information on the feature quantity of the magnified image and the thin film mode of the photomask blank. The method for inspecting a defect of a photomask blank according to claim 1, wherein the defect shape is determined by referring to FIG.   In the step (A3), when the feature amount that the enlarged image of the defect is an image in which the bright part is dominant is extracted, and the outermost surface of the photomask blank to be inspected is a thin film transparent to the inspection light, 5. The defect inspection method for a photomask blank according to claim 4, wherein the detected defect is determined as a pinhole defect.   In the step (A3), when the feature amount that the dark part is the dominant image is extracted from the enlarged image of the defect, the inspection light reflectance of the thinnest film on the photomask blank to be inspected is the inspection light reflectance of the lower layer. 5. The photomask blank defect inspection method according to claim 4, wherein if the film structure is higher, the detected defect is determined as a pinhole defect.   The photomask blank defect inspection method according to claim 1, wherein the thin film has a thickness of 10 nm or less.   The defect inspection method for a photomask blank according to claim 1, wherein the inspection light is light having a wavelength of 210 to 550 nm. The surface of the photomask blank is obtained by irradiating inspection light onto the surface of the thin film of a photomask blank in which at least one thin film is formed on an optically transparent substrate and capturing reflected light from the region irradiated with the inspection light. An inspection device for inspecting defects existing in
A defect inspection system for a photomask blank including a computer having a program for executing the steps of the defect inspection method for a photomask blank according to any one of claims 1 to 8.   9. A photomask blank, wherein a photomask blank that does not contain pinhole defects is selected based on the concavo-convex shape of the defect determined by the defect inspection method for a photomask blank according to any one of claims 1 to 8. Sorting method. Forming at least one thin film on an optically transparent substrate;
A method for producing a photomask blank, comprising the step of sorting a photomask blank that does not contain pinhole defects in the thin film by the photomask blank sorting method according to claim 10.