Patent Publication Number: US-8986913-B2

Title: Method and apparatus for inspecting a mask substrate for defects, method of manufacturing a photomask, and method of manufacturing a semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-059421, filed Mar. 15, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a method of inspecting a mask substrate for defects, and the like. 
     BACKGROUND 
     If a mask blank has defects, the mask pattern formed on a photomask cannot be correctly transferred to a semiconductor substrate. It is therefore important to inspect the mask blank before the photomask is prepared. 
     A method of inspecting a mask blank for defects is known, in which a dark-field optical system acquires dark-field images. If the mask blank has defects, intense scattered light will emanate from the defects. In any dark-field image, the defects are observed as bright points. Hence, in order to detect a defect signal, a threshold level is set, and any signal having a level higher than the threshold level is regarded as a defect signal. 
     In the course of the inspection, however, noise signals may be detected, erroneously as defect signals. For example, if the threshold level is too low, weak defect signals can be detected, but a frequency of (the number of) erroneously detecting noise signals increases. On the other hand, if the threshold level is too high, fewer noise signals are detected, but defect signals having intensity lower than the threshold level cannot be detected. 
     Thus, it has hitherto been difficult to detect noise signals accurately in the process of inspecting the mask substrate (e.g., mask blank) for defects. It has inevitably hard to inspect, with high reliability, the mask substrate for defects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing the configuration of an apparatus according to an embodiment, which is configured to inspect a mask substrate for defects; 
         FIG. 2  is a flowchart showing a method according to the embodiment, which is designed to inspect a mask substrate for defects; 
         FIG. 3  is a diagram showing a partial region of a mask blank and an inspection region of the mask blank; 
         FIG. 4  is a diagram showing a set of signals stored in a storage unit; 
         FIG. 5  is a diagram showing distribution of the number of noise signals and signal intensities; 
         FIG. 6  is a function block diagram showing a configuration that performs various processes according in the embodiment; 
         FIG. 7  is a diagram showing a modification of the embodiment; and 
         FIG. 8  is a flowchart showing a method of manufacturing a photomask and a method of manufacturing a semiconductor device. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a method of inspecting a mask substrate for defects, the method comprising: acquiring a defocus image of a partial region of a mask substrate using a dark-field optical system; acquiring a just-focus image of the partial region of the mask substrate using the dark-field optical system; generating a set composed of first signals obtained from the defocus image and having signal intensities equal to or higher than a first threshold value; excluding, from the set, the first signals pertaining to parts in which signal intensities of signals obtained from the just-focus image are equal to or higher than a second threshold value; determining an inspection threshold value for signal intensities, on the basis of the first signals not excluded from, and remaining in, the set; acquiring an image of an inspection region of the mask substrate using the dark-field optical system; and determining, based on the image of the inspection region, that any part having a signal intensity equal to or higher than the inspection threshold value has a defect. 
     The embodiment will be described below, with reference to the accompanying drawings. 
       FIG. 1  is a diagram schematically showing the configuration of an apparatus according to an embodiment, which is configured to inspect a mask substrate for defects. 
     The extreme ultraviolet (EUV) light emanating from a light source  11  is applied via an elliptical mirror  12  and a planer mirror  13  to a mask substrate  14 . In this embodiment, the mask substrate is a mask blank. The mask blank  14  is a mask blank for a reflection-type photomask used for EUV exposure. More specifically, the mask blank  14  has a multilayer reflection film formed on a glass substrate. The mask blank  14  is placed on a stage  15  that can be moved in X direction, Y direction and Z direction. 
     The light applied to the mask blank  14  is scattered at the surface of the mask blank  14 . Any light beam with a radiation angle smaller than a prescribed angle is shielded by a shielding unit (i.e., convex mirror)  16 . Any light beam with a radiation angle larger than the prescribed angle is collected by a concave mirror  17  and applied to the shielding unit (i.e., convex mirror)  16 . Any light beam coming from shielding unit (i.e., convex mirror)  16  is focused at the imaging surface of a time-delay integration (TDI) camera  18 . The optical system descried above is a dark-field optical system. The TDI camera  18  therefore takes a dark-field image. 
     The signal based on the image obtained by the TDI camera  18  is supplied to a personal computer  19  that functions as an operation unit. The personal computer  19  performs an operation for determining defects. The personal computer  19  is connected to a storage unit  20 . The storage unit  20  stores the signal based on the image obtained by the TDI camera  18 . 
       FIG. 2  is a flowchart showing a method according to the embodiment, which is designed to inspect a mask substrate for defects. The method of inspecting the substrate for defects will be explained with reference to the flowchart of  FIG. 2 . 
     First, the mask blank  14  is placed on the stage  15  (S 11 ). The mask blank  14  placed on the stage  15  is moved to the defocus position. The defocus position is spaced from the just-focus position by such a distance that relatively small defects (non-coarse defects), if any on the surface of the mask blank  14 , are riot focused at the TDI camera  18 . More precisely, the distance between the just-focus position and the defocus position is preferably about 30 times λ/NA 2  (where λ is the wavelength of the exposure light and NA is the numerical aperture of the optical system). 
     After moving the mask blank  14  to the defocus position, the dark-field optical system is used, acquiring a defocus image of a partial region included in the mask blank  14  (i.e., region included in the pattern-formation surface of the mask blank  14 ) (S 12 ).  FIG. 3  is a diagram schematically showing the positional relation between the mask blank  14  and the partial region. In  FIG. 3 , number  101  designates the mask blank, number  102  designates the partial region, and number  103  designates an inspection region, which will be described later. The defocus image is acquired by the TDI camera  18 . The image acquired by the TDI camera  18  is obtained as sets composed of the intensities of signals acquired at the respective pixels of the TDI camera  18  and the coordinates. 
     Of the signals obtained from the defocus image, the signals having intensities equal to or higher than a first threshold value are extracted (S 13 ). A set of signals (first signals) having intensities equal to or larger than the first threshold value is thereby generated. The set of signals, so generated, is stored in the storage unit  20  as a set  201  (S 13 ). The first threshold value is sufficiently smaller than an inspection threshold value, which will be descried later. Ordinary defects (non-coarse defects) existing on the mask blank  14  are not extracted from the defocus image. Therefore, any signal detected in this step, having intensity higher than the first threshold value, has derived from a. noise or a coarse defect. 
     Next, the mask blank  14  placed on the stage  15  is moved to the just-focus position. Then, the dark-field optical system is used, acquiring a just-focus image of the partial region  102  on the mask blank  14  (S 14 ). In the just-focus image, the signals resulting from coarse defects have far higher intensities than the noise signals. 
     Then, it is determined (S 15 ) whether any signal having intensity higher than a second threshold value exists in a region near the coordinates of the signal (having intensity higher than the first threshold value) included in the set  201  stored in Step S 13 . Assume that the second threshold value is larger than the intensity of any noise signal and far smaller than the intensity of any signal resulting from a coarse defect. Also assume that the region near the coordinates has a size falling within coordinate errors. 
     If the signal has intensity equal to or higher than the second threshold value, it is determined to have derived from a coarse defect, and is stored in the storage unit  20 , as a signal belonging to a set  202  shown in  FIG. 4  (S 16 ). As a result, a part, for which the signal generated from the defocus image has intensity equal to or higher than the first threshold value and the signal generated from the just-defocus image has intensity equal to or higher than the second threshold value, is extracted from the partial region. Further, the signal pertaining to that part is stored in the storage unit  20 , as belonging to the set  202 . 
     If the signal does not have intensity equal to or higher than the second threshold value, it is determined to have derived from noise. In this case, the signal is stored in the storage unit  20 , as a signal belonging to a set  203  (S 17 ). 
     Further, Steps S 15  to S 17  are repeated for all signals included in the set  201  shown in  FIG. 4  (S 18 ). 
     As the steps described above are performed, any signals generated from the just-focus image and having intensities equal to or higher than the second threshold value are excluded from the set  201 , and a set  202  composed of the excluded signals is obtained. The signals remaining, not excluding from the set  201 , constitute a set  203 . 
     Next, with respect to the signals (noise signals) included in the set  203  shown in  FIG. 4 , the relationship between a certain signal intensity and the number of signals having intensities equal to or higher than the certain signal intensity are plotted. As a result, such a distribution of the number of noise signals and the signal intensities is obtained in the partial region, as indicated by plot P 1  in  FIG. 5  (S 19 ). Plot P 1  shown in  FIG. 5  indicates the number of signals having intensities equal to or higher than the certain signal intensity with respect to the certain signal intensity on the horizontal axis of  FIG. 5 . That is, plot P 1  shows the number of positions where the signal intensity is equal to or higher than the certain value. 
     Then, the distribution of  FIG. 5  (plot P 1 ) is used, determining a distribution of the number of noise signals and the signal intensities in a desired inspection region (i.e., region  103  shown in  FIG. 3 ) (S 20 ). To be more specific, the area of the image acquiring region is considered proportional to the number of noise signals of the image acquiring region. The area ratio of the inspection region  103  to the partial region  102  is therefore calculated, and plot P 1  shown in  FIG. 5  is multiplied the area ratio. The distribution of the number of noise signals and signal intensities in the inspection region  103  is thereby determined, as plot  92  shown in  FIG. 5 . 
     Further, fitting approximation is performed by using a given function, on the distribution of the number of noise signals and the signal intensities (i.e., plot P 2 ) in the inspection region. Such an approximation curve C 1  as shown in  FIG. 5  is thereby obtained (S 21 ). 
     Next, used on the approximation curve C 1  shown in  FIG. 5 , in the inspection region  103 , the signal intensity which is obtained when the number of signals (the number of parts) of the vertical axis of  FIG. 5  is equal to or lower than a target value (prescribed value) Nth is determined as inspection threshold value Ith (S 22 ). The target value Nth is an expected value of a tolerable detection number of noise signals. If one noise is allowed in ten inspections, the target value Nth is 0.1. 
     The inspection threshold value Ith of the signal intensity is thus determined on the basis of the signals constituting the set  203 , which remain in the set  201  or have not been excluded from the set  201 . 
     The mask blank  14  is moved to the just-focus position. The dark-field optical system is used, acquiring a just-focus image of the inspection region  103  at the surface of the mask blank  14  (S 23 ). 
     Based on the image of the inspection region  103 , it is determined that any part, which has signal intensity equal to or higher than the inspection threshold value Ith, has a defect (S 24 ). 
     Next, the coordinates of the part which has been determined to have a defect, are output (S 25 ). If no parts are determined to have a defect, in Step S 24 , data indicating that no defects have been detected is output 
       FIG. 6  is a function block diagram showing the configuration that performs various processes described above. The processes are performed by mainly the personal computer  19 . 
     The configuration of  FIG. 6  comprises a defocus-image acquiring unit  31 , a just-focus image acquiring unit  32 , a set generating unit  33 , a signal excluding unit  34 , an inspection threshold-value determining unit  35 , an inspection-region image acquiring unit  36 , and a defect determining unit  37 . 
     The defocus-image acquiring unit  31  is configured to use the dark-field optical system, acquiring a defocus image of a partial region of the mask substrate. The just-focus image acquiring unit  32  uses the dark-field optical system, acquiring a just-focus image of the partial region of the mask substrate. The set generating unit  33  is configured to generate a set of first signals, each being acquired from the defocus image and each having signal intensity equal to or higher than the first threshold value. The signal excluding unit  34  is configured to exclude, from the set, any first signal at a part in which a signal intensity of a signal acquired from the just-focus image is equal to or higher than the second threshold value. The inspection threshold-value determining unit  35  is configured to determine the inspection threshold value for signal intensity, based on the first signal remaining in the set, or not excluded from the set. The inspection-region image acquiring unit  36  is configured to use the dark-field optical system, acquiring the image of the inspection region of the mask substrate. The defect determining unit  37  is configured to determine that any part which has signal intensity equal to or higher than the inspection threshold value, has a defect. These components perform the functions already specified above. 
     In this embodiment described above, a set of first signals generated from a defocus image and having signal intensities equal to or higher than the first threshold value is generated. First signal at a part in which signal intensity of a signal acquired from the just-focus image is equal to or higher than the second threshold value, is excluded from the set. Then, the inspection threshold value of signal intensities is determined based on the first signals not excluded from the set, or remaining in the set. It is therefore possible to detect only the noise signals from which the signals resulting from coarse defects have been excluded. Then, the inspection threshold value of the signal intensities is determined based on the noise signals. The threshold value for the noise signals generated during the defect inspection can therefore be appropriately determined, and the noise signals can be reliably detected. As a result, the mask substrate, such as mask blank, can be inspected for defects, with high reliability. 
     In the embodiment described above, the intensity distribution of illumination light may not be negligible, and the level change of the defect signal, resulting from the intensity difference between the illumination light components, may not be negligible. In this case, the region surrounding a region of interest may be taken into consideration. More precisely, the average signal intensity measured in the region outside the region of interest is subtracted from the signal intensity measured at the region of interest, and the resulting signal intensity may be used as signal intensity for the signal derived from the defocus image or just-focus image. The use of the above described method can reduce the influence of the intensity distribution of the illumination light. 
       FIG. 7  is a diagram showing the example described above. As shown in  FIG. 7 , the pixel AO of interest is the center pixel of a 9×9 pixel region. In this pixel region, all pixels, but the inner 5×5 pixel, define a region A 2 . The average signal intensity of the region A 2  is used as background level. Further, the background level of the region A 2  is subtracted from the signal intensity of each pixel of a 3×3 pixel region A 1 , in which the pixel A 0  of interest is the center pixel. The sum of the differences, each between the signal intensity of each pixel and the background level, of the region A 2 , is defined as peak intensity. The region A 1  (i.e., 3×3 pixel region in the embodiment) preferably has a size that is equivalent to the resolution of the focusing optical system, or equivalent to the signal expansion caused by electron diffusion in the TDI camera, Preferably, the region A 2  is sufficiently larger than the above-mentioned signal expansion. 
     In the embodiment described above, the mask substrate is a mask blank. Nonetheless, the method described above can be also applied to any mask substrate other than a mask blank. The method can be applied to, for example, a photomask having a cyclic pattern such as line-and-space pattern. Fine line-and-space pattern of a specific cycle has many lines and many spaces in each pixel of the TDI camera. Hence, the signal intensity for each pixel is considered constant if the mask substrate does not have a defect. The method described above can therefore detect defects of the photomask. 
     The method according to the embodiment can be applied to a method of manufacturing a photomask and also to a method of manufacturing a semiconductor device. 
       FIG. 8  is a flowchart showing a method of manufacturing a photomask and a method of manufacturing a semiconductor device. First, a mask blank is inspected by the method explained above (S 41 ). Next, using the mask blank inspected, a photomask is manufactured (S 42 ). Further, using the photomask so manufactured, a semiconductor device is manufactured (S 43 ). More specifically, the mask pattern (i.e., circuit pattern) formed on the photomask is transferred to the photoresist provided on a semiconductor substrate. Then, the photoresist is developed, forming a photoresist pattern. Further, using the photoresist pattern as mask, conductive films, insulating films or semiconductor films are etched. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.