# Patent

## Title
Method of selecting photomask blank substrates

## Abstract

A photomask blank substrate is selected for use in a process where at least a masking film or a phase shift film is deposited on a top surface of a photomask blank substrate to form a photomask blank, the deposited film is patterned to form a photomask, and the photomask is mounted in an exposure tool. The substrate is selected by simulating a change in shape in the top surface of the substrate, from prior to film deposition thereon to when the photomask is mounted in the exposure tool; determining the shape of the substrate top surface prior to the change that will impart to the top surface a flat shape when the photomask is mounted in the exposure tool; and selecting, as an acceptable substrate, a substrate having this top surface shape. The selected substrate has an optimized top surface shape that improves productivity in photomask fabrication.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2003-280464 filed in Japan on Jul. 25, 2003, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of selecting substrates for photomask blanks from which photomasks can be fabricated for use in photolithography associated with the manufacture of such products as semiconductor devices.
2. Prior Art
As semiconductor devices continue to become more highly integrated, there is a growing desire for smaller geometries in photolithography. Already, the design rule for devices has been lowered to 0.11 μm and patterns are required to be accurate to 10 nm or less. These trends have led to a number of technical challenges concerning the accuracy of photolithographic processes employed in semiconductor manufacturing.
One such challenge relates to the flatness of the photomask used in lithography. Photomask flatness is an important factor for achieving higher accuracy in the patterning step. As the minimum feature size becomes smaller, the focal depth allowed by the resist becomes smaller, making it impossible to disregard the flatness of the substrate as a cause of focal shift.
To reduce focal shift in lithography, the mask pattern serving as the original must be positioned with sufficient precision to enable the exposure pattern to be written at a given position and linewidth within the allowable error during wafer exposure. Hence, a deposited film, such as a patterned masking film or phase shift film, should ideally be formed on a substrate of exceptional flatness. However, when a photomask is mounted in a wafer exposure system, holding the photomask on the mask stage of the exposure system with a vacuum chuck in the manner disclosed in JP-A 2003-50458 can greatly deform the overall surface shape of the photomask, depending on the surface shape of the clamped portion of the photomask.
The photomask is generally fabricated by depositing on a transparent substrate a film such as a masking film, a semi-transparent film or a phase shift film, or a combination thereof, to produce a photomask blank. A resist is then applied onto the photomask blank to form a resist film, which is lithographically processed. Using the processed resist film as the etching mask, a mask pattern is formed. The resist film is finally stripped off.
A photomask is thus obtained after passing through a long series of operations. The transparent substrate used as the starting material in this process is prepared by cutting a slice from an ingot of synthetic quartz, and polishing the surfaces of the slice. Until now, this surface polishing operation has been carried out only with the intention of reducing surface defects or minimizing warpage of the substrate surface so as to provide a flat surface. Therefore, nothing has previously been known about how to modify the surface shape of the photomask substrate to achieve an even smaller minimum feature size in a pattern written on a wafer substrate; that is, specifically what shape the surface of the photomask should be given where it will be held with a vacuum chuck, based on the shape the photomask assumes when held by the vacuum chuck. Nor has anything been known about what sort of method may be used to form such a surface shape in order to enable the production of substrates having the desired surface shape in a good yield or about how to suppress the formation of defects on the substrate itself or of photomask defects that originate from the substrate.
SUMMARY OF THE INVENTION
It is therefore one object of the invention to provide a method of selecting photomask blank substrates capable of giving photomasks which can be used to expose patterns having a reduced minimum feature size to a high accuracy on a wafer substrate.
The inventor has discovered that, in a process where one or more layer of film, including at least a masking film or a phase shift film, is deposited on a top surface of a photomask blank substrate to form a photomask blank, the deposited film is patterned to form a photomask, and the photomask is mounted in an exposure tool, it is possible to optimize a substrate surface shape which takes into account the effects upon the substrate surface shape of the film pattern left on the substrate and of mounting at the time of photomask use and to minimize during substrate production the number of substrates which may result in a photomask having a defective patterned surface shape and thus prevent a given pattern exposure accuracy from being achieved by simulating the change in shape that takes place in the top surface of the substrate, from prior to film deposition thereon to when the photomask is mounted in the exposure tool; determining the shape of the substrate top surface prior to the change that will impart to the top surface a flat shape when the photomask is mounted in the exposure tool; and selecting, as an acceptable substrate, a substrate having this top surface shape. Furthermore, to achieve these desired results, the inventor has found it advantageous for the top surface shape that is imparted in a patterning region on the top surface of the substrate when the photomask is mounted in the exposure tool to be a flat shape if the substrate top surface shape prior to the change in shape has a difference between maximum and minimum values for height from a patterning-region least squares plane to the patterning region of at most 0.5 μm.
Accordingly, the invention provides a method of selecting a substrate for photomask blanks, which method is characterized by, in a process where one or more layer of film, including at least a masking film or a phase shift film, is deposited on a top surface of a photomask blank substrate to form a photomask blank, the deposited film is patterned to form a photomask, and the photomask is mounted in an exposure tool, the steps of simulating a change in shape by the top surface of the substrate from prior to film deposition thereon to when the photomask is mounted in the exposure tool; determining the shape of the substrate top surface prior to the change that will impart to the top surface a flat shape when the photomask is mounted in the exposure tool; and selecting, as an acceptable substrate, a substrate having this top surface shape prior to film deposition.
Preferably, the top surface shape that is imparted in a patterning region on the top surface of the substrate when the photomask is mounted in the exposure tool is a flat shape if the substrate top surface shape prior to the change in shape has a difference between maximum and minimum values for height from a patterning-region least squares plane to the patterning region of at most 0.5 μm.
The inventive method makes it possible to produce photomask blank substrates having an optimized surface shape which takes into account the effects upon the substrate top surface shape of the film pattern left on the substrate and of mounting at the time of photomask use. In addition, the inventive method can minimize during substrate production the number of substrates which may result in photomasks having a defective patterned surface shape and prevent the required pattern exposure accuracy from being achieved, and thus makes it possible to improve productivity in the manufacture of photomasks.



BRIEF DESCRIPTION OF THE DIAGRAMS
FIG. 1 shows strip-like regions, a quadrangular ring-shaped region and patterning regions on the top surfaces of substrates.
FIG. 2 shows sectional views illustrating the concepts of a least squares plane for a surface and of a height from such a least squares plane.
FIG. 3 shows sectional views of some examples of top surface shapes on substrates.
FIG. 4 is a schematic cross-sectional view of a single side polishing machine such as may be used to produce substrates of an acceptable surface shape that are selected by the method of the invention.
FIG. 5A shows a schematic perspective view of the top surface shape of a photomask blank substrate prior to vacuum chucking in the simulation conducted in Example 1. FIG. 5B shows a similar view of the simulated top surface shape of the same substrate when vacuum-chucked.
FIG. 6A shows a schematic perspective view of the top surface shape of a photomask blank substrate prior to vacuum chucking in the simulation conducted in Comparative Example 1. FIG. 6B shows a similar view of the simulated top surface shape of the same substrate when vacuum-chucked.



DETAILED DESCRIPTION OF THE INVENTION
The method of selecting photomask blank substrates according to the invention involves, in a process where one or more layer of film, including at least a masking film or a phase shift film, is deposited on a top surface of a photomask blank substrate to form a photomask blank, the deposited layer is patterned to form a photomask, and the photomask is mounted in an exposure tool, the steps of simulating a change in shape by the top surface of the substrate from prior to film deposition thereon to when the photomask is mounted in the exposure tool; determining the shape of the substrate top surface prior to the change that will impart to the top surface a flat shape when the photomask is mounted in the exposure tool; and selecting, as an acceptable substrate, a substrate having this top surface shape prior to film deposition.
When a film such as a masking film is formed on a substrate, stress by the film deforms the substrate. Because substrate deformation by such film stress primarily effects a change in the warp of the substrate, it alters the shape of the top surface of the substrate. Hence, when determining the shape of the substrate top surface, by predicting, at a substrate stage prior to deposition of the masking film, the stress of the film to be formed and the shape of the photomask when it is held by a chuck in a wafer exposure system, mask production can be carried out to an even better yield.
Film stress varies depending on the film type and composition, as well as the method of manufacture. For example, a chromium-based film causes tensile stress, whereas a MoSi-based film generates compressive stress. Deformation of the substrate top surface due to film stress can be determined by depositing a given film on a substrate whose top surface shape and height have already been measured, then measuring and analyzing the shape and height of the substrate top surface following film deposition. Therefore, through measurement of the amount of deformation before and after film deposition at various positions on the top surface of a substrate, preliminary measurement of stress and degree of deformation and, based on these measurements, simulation from the shape of the substrate on which no film has been formed, it is possible to predict the shape of the substrate following film deposition and to determine, on the basis of the predicted shape, the shape the photomask will have when it is clamped in the exposure tool. An even more accurate simulation is possible by also taking into account the film (patterned area) coverage on the substrate top surface when a mask pattern has been formed as the photomask. Hence, in a process where one or more layer, including at least a masking layer or a phase shift layer, is deposited on a top surface of a photomask blank substrate to form a photomask blank, the deposited layer is patterned to form a photomask, and the photomask is mounted in an exposure tool, by simulating a change in the shape of the top surface of the substrate from prior to film deposition thereon to when the photomask is mounted in the exposure tool, determining the shape of the substrate top surface prior to the change that will impart to the top surface a flat shape when the photomask is mounted in the exposure tool, and selecting, as an acceptable substrate, a substrate having this top surface shape prior to deposition, the shape and height of the substrate top surface are varied in accordance with such considerations as the type of film to be deposited on the substrate and changes in the shape of the substrate top surface due to the film, making it possible to produce in a higher yield substrates which are capable of conferring a good flatness at the time of photomask use.
In the practice of the invention, it is especially preferable for the top surface shape that is imparted in a patterning region on the top surface of the substrate when the photomask is mounted in the exposure tool to be a flat shape if the substrate top surface shape prior to the change in shape has a difference between maximum and minimum values for height from a patterning-region least squares plane to the patterning region of at most 0.5 μm.
No particular limitation is imposed on the acceptable substrate selected as a result of simulation. However, an example of an acceptable substrate for photomask blanks that may be thus selected is one where, in a pair of strip-like regions which extend from 2 to 10 mm inside each of a pair of opposing sides along the outer periphery of the top surface of the substrate on which a mask pattern is to be formed but exclude at each end in the lengthwise direction thereof a 2 mm edge portion, the strip-like regions are each inclined downward from at least an intermediate position in the width direction thereof toward the outer periphery of the substrate, and the difference between maximum and minimum values for height from a least squares plane for the strip-like regions on the top surface to the strip-like regions themselves is at most 0.5 μm.
The term “strip-like region” as used herein is explained below while referring to FIG. 1A, which shows the substrate top surface 1 on which a suitable film such as a film having light-shielding properties, or a film pattern thereof, is formed when a substrate is rendered into a photomask blank or a photomask. In the diagram, a pair of strip-like regions 2 extend from 2 mm to 10 mm inside each of a pair of opposing sides along an outer periphery of the top surface 1, but exclude at each end in the lengthwise direction thereof a 2 mm edge portion. These strip-like regions have a shape and height characterized by the shape and values described above. The film pattern (mask pattern) is formed in a patterning region 4 which is situated between the strip-like regions 2 and begins 10 mm inside the respective sides making up the outer periphery of the top surface. When a photomask is fabricated using such a substrate and the photomask is held, near a pair of opposing sides, on the mask stage of a wafer exposure system with a vacuum chuck, the vacuum chucking positions are included within these strip-like regions. Hence, the shape of these strip-like regions governs the shape of the entire top surface of the substrate during wafer exposure; i.e., the shape of the entire top surface of the substrate on which the mask pattern is formed. That is, accurate exposure of the pattern with respect to position and linewidth requires that the substrate top surface, and in particular the patterning region, have a shape which can, at the time of pattern exposure, be disposed parallel to the exposure light without tilting the mask pattern. Accordingly, it is possible, using the inventive method of selection, to optimize as described above the shape and height of the strip-like regions which primarily govern the shape of the entire top surface of the substrate, and especially the shape of the patterning region.
In this embodiment, the strip-like regions extend from 2 to 10 mm inside each of a pair of opposing sides along the outer periphery of the top surface of the substrate on which a mask pattern is formed, but exclude at each end in the lengthwise direction thereof a 2 mm edge portion. If the strip-like regions satisfy the particular shape and value ranges mentioned above, they will satisfy the above-described shape and value ranges at the vacuum chucking positions (substrate holding positions) as well, enabling flatness to be achieved at the time of photomask use. Alternatively, it is acceptable for the above-described shape and values to be satisfied at least at the vacuum chucking positions or the substrate holding positions.
It is desirable for each strip-like region as a whole to have a shape which is inclined downward toward the periphery of the substrate. However, the patterning region situated between the strip-like regions is not subject to any particular limitation, and may have various shapes, such as a planar shape, convex shape or concave shape.
The difference between the maximum and minimum values for height from the least squares plane for the strip-like regions on the substrate top surface to the strip-like regions themselves is preferably not more than 0.5 μm, and more preferably not more than 0.3 μm.
As illustrated in FIGS. 2A and 2B, the least squares plane 21 refers herein to an imaginary plane obtained by using the method of least squares to arithmetically approximate as a flat plane a non-horizontal surface 22 such as a concave surface, convex surface or a surface having convex and concave areas. In the present invention, the height of each region is given with respect to the least squares plane for that respective region on the top surface of the substrate. However, for the sake of convenience, the least squares plane for the entire subsequently described principal surface region may be used as the reference plane for each respective region. The height from this least squares plane 21 to the non-horizontal surface 22 is the height taken perpendicular to this least squares plane 21. In FIG. 2, h1 represents the maximum value for height and is a positive number lying above the least squares plane 21, h2 is the minimum value for height and is a negative number lying below the least squares plane 21. The difference between the maximum and minimum values h1 and h2 is thus the same as the sum of the respective absolute values for h1 and h2.
No particular limitation is imposed on the patterning region, although the difference between the maximum and minimum values for height from the patterning-region least squares plane to the patterning region is preferably not more than 0.5 μm, more preferably not more than 0.3 μm, and most preferably not more than 0.2 μm.
An illustrative example of the acceptable substrate for photomask blanks that is selected is one where, in a quadrangular ring-shaped region which extends from 2 mm to 10 mm inside the respective sides making up the outer periphery on the top surface of the substrate on which a mask pattern is to be formed, the quadrangular ring-shaped region is inclined downward at least from an intermediate position in the width direction thereof toward the outer periphery, and the difference between the maximum and minimum values for height from the least squares plane for the quadrangular ring-shaped region on the substrate top surface to the quadrangular ring-shaped region itself is not more than 0.5 μm.
The term “quadrangular ring-shaped region” as used herein is explained below while referring to FIG. 1B, which shows the substrate top surface 1 on which a suitable film such as a film having light-shielding properties, or a film pattern thereof, is formed when a substrate is rendered into a photomask blank or a photomask. In the diagram, a quadrangular ring-shaped region 3 extends from 2 mm to 10 mm inside the respective sides making up the outer periphery of the top surface. The quadrangular ring-shaped region 3 has a shape and height characterized by the shape and values described above. The film pattern (mask pattern) is formed in a patterning region 4 which is situated within the inner periphery of the quadrangular ring-shaped region 3 and begins 10 mm inside the respective sides making up the outer periphery of the top surface. When this substrate is used to fabricate a photomask and the resulting photomask is held on the mask stage of a wafer exposure system with a vacuum chuck or the like near the outer periphery of the substrate along its entire perimeter, or near merely a pair of opposing sides, with the horizontal and vertical sides of the substrate being interchangeable, the vacuum chucking positions are included within this quadrangular ring-shaped region. Hence, the shape of the quadrangular ring-shaped region governs the shape of the entire top surface of the substrate during wafer exposure; i.e., the shape of the entire top surface of the substrate on which the mask pattern is formed. That is, accurate exposure of the pattern with respect to position and linewidth requires that the top surface of the substrate, and in particular the patterning region, have a shape which can, at the time of pattern exposure, be disposed parallel to the exposure light without tilting the mask pattern. Accordingly, it is possible, using the inventive method of selection, to optimize as described above the shape and height of the quadrangular ring-shaped region which primarily governs the shape of the entire top surface of the substrate, and especially the shape of the patterning region.
The quadrangular ring-shaped region in this case extends from 2 mm to 10 mm inside the respective sides making up the outer periphery of the top surface. If the quadrangular ring-shaped region satisfies the particular shape and value ranges mentioned above, it will satisfy these shape and value ranges at the vacuum chucking positions as well, enabling flatness to be achieved at the time of photomask use. Alternatively, it is acceptable for the above-described shape and values to be satisfied at least at the vacuum chucking positions or the substrate holding positions.
It is desirable for this quadrangular ring-shaped region as a whole to have a shape which is inclined downward toward the periphery of the substrate. However, the patterning region situated inside the quadrangular ring-shaped region is not subject to any particular limitation, and may have various shapes, such as a planar shape, convex shape or concave shape.
The difference between the maximum and minimum values for height from the least squares plane for the quadrangular ring-shaped region to the quadrangular ring-shaped region is preferably not more than 0.5 μm, and more preferably not more than 0.3 μm.
The patterning region is not subject to any particular limitation, although it is desirable for the difference between the maximum and minimum values for the height from the patterning-region least squares plane to the patterning region to be not more than 0.5 μm, preferably not more than 0.3 μm, and more preferably not more than 0.2 μm.
In the photomask blank substrate selected by the method of the invention, it is moreover desirable for the difference between the maximum and minimum values for height from the least squares plane for a principal surface region, which consists of the quadrangular ring-shaped region and the patterning region situated inside the inner periphery of this quadrangular ring-shaped region, to the principal surface region itself to be not more than 0.5 μm, and especially not more than 0.3 μm.
The shapes these strip-like regions, quadrangular ring-shaped regions and patterning regions on the top surface of the substrate, and their heights relative to a least squares plane, can be measured and analyzed using a device such as a flatness tester.
Thorough shape and height measurements of the strip-like regions or the quadrangular ring-shaped region are required to obtain the desired shape. Up until now, when fabricating a mask, measurements concerned particularly with substrate warp have been carried out on the area where the pattern is to be formed. However, because adequate measurements have not been carried out on the strip-like regions or quadrangular ring-shaped region which include the chucking area, it has been impossible to predict the change in shape of the substrate top surface after chucking. Therefore, when measuring the strip-like regions or quadrangular ring-shaped region on the top surface of the substrate in order to predict the change in shape after chucking, it is advantageous for these measurements to be carried out under the following conditions.

    (1) Areas where substrate shape is measured: The above-described strip-like regions or quadrangular ring-shaped region are measured together with positional information. Inadequate positional information may make it impossible to accurately predict the shape.
    (2) Measurement interval: 0.05 to 0.35 mm. A measurement interval that is too broad may make it impossible to achieve a sufficiently good prediction of the shape. On the other hand, too narrow a measurement interval may render measurement excessively cumbersome.
    (3) Measurement accuracy (error): 0.01 to 0.1 μm. A measurement error that is too large may make it impossible to achieve a sufficiently good prediction of the shape. On the other hand, a measurement error that is too small may render measurement excessively cumbersome and result in a poor efficiency.


Measurement involves the use of light interference. Because the shape and height of the surface are determined based on a reference plane for this surface, this reference plane must have a sufficient degree of accuracy (e.g., an error of not more than 0.02 μm, preferably not more than 0.01 μm).
The photomask blank substrate selected by the method of the invention may be made of a suitable material such as synthetic quartz. It is typically a quadrangular, and preferably square, substrate having a length on one side which is at least 6 inches (at least 152 mm), and is preferably 6 inches (152 mm). The substrate has a maximum size which, although not subject to any particular limitation, is preferably not more than 12 inches for reasons having to do with the weight of the substrate during wafer exposure and its handleability. The substrate has a thickness which, while not subject to any particular limitation, is preferably from 3 to 10 mm.
Marks for verifying the orientation of the substrate are sometimes formed on the substrate. Marks of this type are generally placed on the back (bottom surface) of the substrate. However, in cases where such marks are placed on the top surface, the area of the marks shall be excluded from substrate shape considerations according to the invention.
By using photomask blank substrates with strip-like regions or a quadrangular ring-shaped region of the above shape to produce photomask blanks, and using the resulting photomask blanks to fabricate photomasks, it is possible to manufacture in a good yield masks which undergo little patterning region deformation when chucked in a wafer exposure system.
Such photomask blank substrates may be produced by a method such as the following.
First, a starting substrate is subjected to an initial polishing step so as to give it a specific intermediate shape, after which a final polishing step is carried out. If the substrates do not pass through a stage in which they have a desirable intermediate shape, it will be difficult to obtain substrates of a desirable final shape in a good yield.
Here, depending on the combination of polishing techniques employed, two different types of desirable intermediates may be obtained, and so two different processes may be used. In one process, referred to below as Production Method 1, the substrate is polished under conditions that rapidly remove material from the center of the substrate, then is polished under conditions that rapidly remove material from the periphery of the substrate. In the other process, referred to below as Production Method 2, the substrate is polished under conditions that rapidly remove material from the periphery of the substrate, then is polished under conditions that rapidly remove material from the center of the substrate.
In this substrate production process, initial polishing is a double side polishing operation. The polishing machine used in this operation may be a double side polishing machine of a type that already exists. Such a machine uses a lower plate and an upper plate to polish both sides of the starting substrate at the same time. In the prior art, polishing has not been carried out to control the shape of the substrate, particularly the shape and height of the above-described strip-like regions or quadrangular ring-shaped region, and so this stage of polishing has not been subjected to any particular control. As a result, there has been substantial variation in the surface shape of the strip-like regions or the quadrangular ring-shaped region on the top surface of the resulting substrate.
By contrast, when a substrate is produced using the present method, polishing is carried out under conditions that rapidly remove material from the center of the substrate in the case of Production Method 1, and under conditions that rapidly remove material from the periphery of the substrate in the case of Production Method 2. Specifically, it is preferable to carry out polishing at a speed of about 1 to 200 rpm. In the resulting polished intermediate product, the shape and height of the quadrangular ring-shaped region or the patterning region on the polished intermediate product can be measured and analyzed by a method (i.e., apparatus and measurement conditions) like that described above.
Production Methods 1 and 2 are each described below.
[Production Method 1]
First, a polished intermediate product in which the substrate top surface (polished side) satisfies the conditions for intermediate shapes 1 and 2 below is prepared by initial polishing. The polished intermediate product is then final polished under different polishing conditions, thereby giving a substrate having a specific shape.
(Intermediate Shape 1)
This is a shape in which the difference between the maximum and minimum values for height from the principal surface-region least squares plane to the principal surface region is not more than 1.5 μm, and preferably not more than 1.0 μm, and in which the quadrangular ring-shaped region is inclined upward toward the periphery of the substrate.
(Intermediate Shape 2)
This is a shape in which the difference between the minimum and maximum values for height from the principal surface-region least squares plane to the principal surface region is not more than 1.5 μm, and preferably not more than 1.0 μm; the difference between the maximum and minimum values for height from the principal surface-region least squares plane to the four corners (vertices) of the principal surface region (which difference is sometimes abbreviated herein as “CIR”) is not more than 0.5 μm, and preferably not more than 0.3 μm; and the quadrangular ring-shaped region is inclined upward toward the periphery of the substrate.
Examples of substrate top surface shapes in which the quadrangular ring-shaped region is inclined upward toward the substrate periphery include shapes where, as shown in FIG. 3A, the patterning region at the center of the substrate top surface is planar and the quadrangular ring-shaped region 3 that is continuous from the outer periphery of the patterning region 4 is inclined so as to curve upward; and shapes which, as shown in FIG. 3B, have a concave surface that extends from the patterning region 4 to the quadrangular ring-shaped region 3 (i.e., shapes in which the principal surface region forms a concave surface). In FIG. 3, the numeral 5 designates the principal surface region.
The polished intermediate product having such an intermediate shape is then additionally polished, which operation can be carried out using a single side polishing machine like that shown in FIG. 4. FIG. 4 shows a substrate 11, a lower plate 12 and a top ring 13. In Production Method 1, the specific shape of the invention can be more effectively formed by holding the polished intermediate product in the single side polishing machine and polishing the side of the product on which a mask pattern is to be formed, with a reduced pressure acting on the side of the substrate opposite from the polished side. Moreover, in the polishing machine, a flat plate may be used as the bottom plate, although polishing with a convex plate enables a substrate of a better shape to be obtained. The outer periphery of the quadrangular ring-shaped region on the top surface of the substrate is generally chamfered on the outside.
[Production Method 2]
First, a polished intermediate product in which the substrate top surface (polished side) satisfies the conditions for intermediate shapes 3 and 4 below is prepared by initial polishing. The polished intermediate product is then final polished under different polishing conditions, thereby giving a substrate having a specific shape.
(Intermediate Shape 3)
This is a shape in which the difference between the maximum and minimum values for height from the principal surface-region least squares plane to the principal surface region is not more than 1.5 μm, and preferably not more than 1.0 μm, and in which the quadrangular ring-shaped region is inclined downward toward the periphery of the substrate.
(Intermediate Shape 4)
This is a shape in which the difference between the minimum and maximum values for the height from the principal surface-region least squares plane to the principal surface region is not more than 1.5 μm, and preferably not more than 1.0 μm; the difference between the maximum and minimum values for height from the principal surface-region least squares plane to the four corners (vertices) of the principal surface region (CIR) is not more than 0.5 μm, and preferably not more than 0.3 μm; and the quadrangular ring-shaped region is inclined downward toward the periphery of the substrate.
Examples of substrate top surface shapes in which the quadrangular ring-shaped region is inclined downward toward the substrate periphery include shapes where, as shown in FIG. 3C, the patterning region at the center of the substrate top surface is planar and the quadrangular ring-shaped region that is continuous from the outer periphery of the patterning region is inclined so as to curve downward; and shapes which, as shown in FIG. 3D, have a convex surface that extends from the patterning region to the quadrangular ring-shaped region (i.e., shapes in which the principal surface region forms a convex surface).
In Production Method 2, the specific shape of the invention can be more effectively formed by holding the polished intermediate product in a single side polishing machine like that shown in FIG. 4 and polishing the side of the product on which a mask pattern is to be formed, with a pressure applied to the side of the substrate opposite from the polished side. Moreover, in the polishing machine, a flat plate may be used as the bottom plate, although polishing with a convex plate enables a substrate of a better shape to be obtained. The outer periphery of the quadrangular ring-shaped region on the top surface of the substrate is generally chamfered on the outside.
By polishing in stages that pass through operations like those described above, substrates having a specific shape at given places on the top surface thereof can be produced in a good yield.
In the patterning region of the polished intermediate product, it is desirable for the difference between the maximum and minimum values for height from the patterning-region least squares plane to the patterning region to be not more than 0.5 μm, preferably not more than 0.3 μm, and most preferably not more than 0.2 μm. Moreover, the patterning region may have any suitable shape, including a planar shape, convex shape or concave shape.
A film having light shielding properties, such as a masking film or a phase shift film, is suitably selected according to the intended use of the photomask, and is formed on the resulting substrate to give a photomask blank. Masking films and phase shift films such as halftone phase shift films are generally formed by a sputtering process. In this invention, “a film having light shielding properties” includes films which substantially block the passage of exposure light, such as halftone phase shift film.
In the production of a photomask blank for a binary mask, a layer of a suitable material is deposited as the masking film on the substrate selected by the method of the invention. Examples of materials suitable for this purpose include metallic chromium; chromium compounds such as chromium oxide, chromium nitride, chromium oxide nitride and chromium oxide nitride carbide; and metal silicide compounds such as molybdenum silicide, titanium silicide and zirconium silicide, as well as oxides, nitrides, oxide nitrides and oxide nitride carbides thereof. This masking film is generally formed by using two or more layers of materials having different refractive indices, or by using a material within which the components have been provided with a composition gradient, so as to confer an anti-reflection capability. It is preferable for the stress in these films to be low.
In the production of a phase shift mask blank for a halftone phase shift mask, a substrate selected by the inventive method has formed thereon a halftone phase shift film which attenuates exposure light to a degree where the light substantially does not sensitize the photoresist, and which also has the ability to shift the phase of the light 180°. Examples of such materials that may be used include chromium compounds such as chromium oxide, chromium nitride, chromium oxide nitride and chromium oxide nitride carbide; metal silicide compounds such as molybdenum silicide, titanium silicide and zirconium silicide, as well as oxides, nitrides, oxide nitrides and oxide nitride carbides thereof; and also silicon nitride and silicon oxide.
The halftone phase shift film may be a single-layer or multilayer film. However, to provide both resistance to post-patterning operations such as chemical washing and also good processability, it is preferable to use a multilayer film in which the materials or compositions on the surface side and on the substrate side differ, or a film having a composition gradient. As in the case of the masking film, it is preferable for the combination of materials making up the halftone phase shift film to be selected such as to minimize film stress.
In addition, the above-described masking film is generally formed on top of the halftone phase shift film. If exposure is repeated at different positions on a photoresist-coated wafer, the peripheral portion of the halftone phase shift mask where transmittance has been lowered to prevent sensitization may be subjected to overlapping exposure. As a result, the areas of overlap are exposed a plurality of times and may become sensitized, something which must be prevented from happening. To this end, it is generally desirable to deposit a masking film on the peripheral portion of the halftone phase shift mask where the mask pattern is not written. It is thus preferable, in terms of the overall order of the operations, for the masking film to be already formed at the stage of halftone phase shift mask blank production. Accordingly, to obtain this type of mask, the masking film is deposited together with the halftone phase shift film during production of the halftone phase shift mask blank. Moreover, a phase shift mask can be formed by depositing on the substrate a high-transmittance phase shift film, as is done in the case of Levenson type phase shift masks. If necessary, an electrically conductive film and an etching stopper film can be deposited over any of the above films.
The film deposited on the substrate selected by the method of the invention, while not subject to any particular limitations, has a film stress which, when expressed as the amount of film warp by the substrate, is preferably not more than 0.5 μm, more preferably not more than 0.3 μm, even more preferably not more than 0.2 μm, and most preferably not more than 0.1 μm. At a large film stress, substrate deformation may worsen depending on the percent coverage by the film (patterning area) over the photomask, possibly preventing the desired flatness from being achieved.
As described above, when a photomask fabricated using the substrate selected by the inventive method is chucked in a wafer exposure system, the substrate provides an excellent flatness. It is therefore possible to accurately align the mask pattern at a predetermined position, enabling an exposure pattern of small minimum feature size to be written on the wafer to high position and linewidth accuracies. Such a substrate fully meets the requirements for smaller pattern geometries in photolithography, and is especially preferred as a photomask blank substrate for the fabrication of photomasks used in the exposure of very fine patterns having a linewidth of 0.12 μm or less, and especially 0.10 μm or less.
EXAMPLES
The following examples are given by way of illustration and not by way of limitation.
Example 1
A simulation was carried out wherein a 152 mm (6 inch) square photomask blank substrate having a principal surface region of a flatness like that shown in FIG. 5A and having a horizontal top surface had formed on the top surface a mask pattern that imparts thereto a concave shape having a difference between the maximum and minimum values for height from the top-surface least squares plane to the top surface of 0.25 μm. The substrate flatness when the resulting photomask is vacuum chucked onto the mask stage of a wafer exposure system was simulated, giving a substrate top surface shape like that shown in FIG. 5B. This top surface shape had a difference between the maximum and minimum values for height from the top-surface least squares plane to the top surface of less than 0.5 μm, and so substrates with this top surface shape were selected as acceptable substrates for imparting a good flatness during fabrication into photomasks.
Substrates having a top surface shape adjusted to substantially the selected shape described above were fabricated, and a mask pattern like that in the simulation was formed on the substrates. The resulting photomasks were vacuum chucked to the mask stage of the wafer exposure system, then subjected to an exposure test in the exposure tool, yielding good results.
Comparative Example 1
A simulation was carried out wherein a 152 mm (6 inch) square photomask blank substrate having a principal surface region of a flatness like that shown in FIG. 6A and having a horizontal top surface had formed on the top surface a mask pattern that imparts thereto a concave shape having a difference between the maximum and minimum values for height from the top-surface least squares plane to the top surface of 0.25 μm. The substrate flatness when the resulting photomask is vacuum chucked onto the mask stage of a wafer exposure system was simulated, giving a substrate top surface shape like that shown in FIG. 6B. Because this top surface shape had a difference between the maximum and minimum values for height from the top-surface least squares plane to the top surface of more than 0.5 μm, substrates having this top surface shape were regarded as incapable of achieving sufficient flatness during fabrication into photomasks.
Substrates having a top surface shape adjusted to substantially the shape described above were fabricated, and a mask pattern like that in the simulation was formed on the substrate. The resulting photomasks were vacuum chucked to the mask stage of the wafer exposure system, then subjected to an exposure test in the exposure tool. In this case, the photomasks had large focal shifts, making them unfit for use.
Japanese Patent Application No. 2003-280464 is incorporated herein by reference.
Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

## Claims

1. A method of selecting photomask blank substrates for use in a process where one or more layer of film, including at least a masking film or a phase shift film, is deposited on a top surface of a photomask blank substrate to form a photomask blank, the deposited film is patterned to form a photomask, and the photomask is mounted in an exposure tool, the method comprising the steps of:
simulating a change caused by a film stress of the masking film or the phase shift film to be formed on the substrate in shape by the top surface of the substrate from prior to film deposition thereon to when the photomask is mounted in the exposure tool;
determining the shape of the substrate top surface prior to said change that will impart to the top surface a flat shape when the photomask is mounted in the exposure tool; and
selecting, as an acceptable substrate, a substrate having this top surface shape prior to film deposition.

2. The method of claim 1 wherein the top surface shape that is imparted in a patterning region on the top surface of the substrate when the photomask is mounted in the exposure tool is a flat shape if the substrate top surface shape prior to the change in shape has a difference between maximum and minimum values for height from a patterning-region least squares plane to the patterning region of at most 0.5 μm.

3. The method of claim 1 wherein the film stress, when expressed as the amount of film warp by the substrate, is not more than 0.5 μm.

4. The method of claim 1 wherein the photomask blank substrate is a quartz substrate.

5. The method of claim 1 wherein said stress is tensile stress caused by film deposition.

6. The method of claim 1 wherein said stress is compressive stress caused by film deposition.