Patent Publication Number: US-7220969-B2

Title: Mask blanks inspection tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/971,786 filed Oct. 21, 2004 now U.S. Pat. No. 7,005,649, which claims the benefit of the earlier filing date of co-pending Japanese Application No. 265399/2004, filed Sep. 13, 2004, by Intel Corporation, titled “Mask Blanks Inspection Method and Mask Blank Inspection Tool”. 

   BACKGROUND 
   1. Field 
   Circuit patterning and more particularly to masks used to pattern light sensitive material on substrates or wafers. 
   2. Background 
   Patterning is the series of operations that results in the removal of selected portions of surface layers added on a substrate, such as a wafer. Patterning creates the surface parts of devices that make-up a circuit. One goal of patterning is to create in or on the wafer surface, the parts of the device or circuit in the exact dimensions (feature size) required by the circuit design and to locate the parts in their proper location on the wafer surface. 
   Generally, patterning is accomplished through photolithography techniques. For example, photolithography may be a multi-operation pattern transfer process wherein a pattern contained on a reticle, photo mask, etch mask, or multi-layers mask is transferred onto the surface of a wafer or substrate through a lithographic imaging operation, and a light sensitive material (e.g., photoresist) is developed on the wafer. One goal of circuit designers is to reduce the feature size (the critical dimension) of devices of a circuit, i.e., reduce the smallest feature patternable. A reduction in wavelength of light used in patterning will reduce the critical dimension. Thus, the patterning wavelength can be reduced to under 200 nanometers, and can lie in the extreme ultraviolet (EUV) light region to reduce the critical dimension to 100 nanometers or less. 
   In the general course of patterning, the image of a reticle or photo mask is projected onto a wafer or substrate surface by an imaging system. EUV light radiation, however, does not pass through quartz or glass, and is therefore typically projected using reflective optics. For example, a reticle or photo mask for EUV light patterning of a light sensitive material may include a multi-layer mask that is created by forming light absorbing material on certain portions of a substrate covered with multiple layers of a reflective material (e.g., a patterning mask). The substrate having only multiple layers of reflective material may be referred to as a “mask blank” (e.g., such as a substrate having multiple layers of reflective material, prior to forming the light absorbing material). 
   It is important to be able to inspect an EUV light mask blank for defects that may cause errors in the imaging or patterning of the light sensitive material, such as by causing unwanted variations in the image of features (e.g., such as critical features) patterned on that material by the patterning mask formed from that mask blank. Specifically, because of the wavelength of EUV light used to expose the light sensitive material, a small bump with a height as low as two nanometers on the surface of a multi-layered mask blank may cause errors in the imaging or patterning of the light sensitive material, and thus be a defect in the mask blank. Therefore, EUV light patterning photo mask blanks may be inspected during manufacture, after manufacture, prior to shipment, or after shipment to detect “critical defects” (e.g., such as defects that may cause an error in patterning) while minimizing detection of “false defects” (e.g., such as defects that do not cause errors in patterning substantial enough to affect the critical dimension of features to be formed). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various features, aspects and advantages will become more thoroughly apparent from the following detailed description, the set of claims, and accompanying drawings in which: 
       FIG. 1  is one embodiment of a system for inspecting a multi-layered mask blank. 
       FIG. 2  is a cross-sectional view of a multi-layered mask blank. 
       FIG. 3  is a top perspective view of a multi-layered mask blank. 
       FIG. 4  is a top perspective view of a multi-layered mask blank. 
       FIG. 5  is a flow diagram of a process for locating defects in a multi-layered mask blank. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is one embodiment of a system for inspecting multi-layered mask blanks.  FIG. 1  shows system  100  having source  110  to illuminate substrate  105 . Source  110  may provide (e.g., reflect) incident light IL to substrate  105  using source mirror  112  (e.g., such as a spherical mirror or aspherical mirror). Light from source  110  may be focused by source mirror  112  onto surface  115  of substrate  105 , below surface  115  of substrate  105 , or above surface  115  of substrate  105 . 
   In addition, source  110  may be an illumination apparatus, such as a source of light, ultraviolet (UV) light, extreme ultraviolet (EUV) light, or other light appropriate for patterning a light sensitive material on a substrate or wafer, or for inspecting an etch or patterning mask blank. Accordingly, source  110  may provide light having a wavelength in the range of 10 to 400 nanometers (e.g., UV light), such as by providing light having a wavelength of 20 nanometers, 50 nanometers, 100 nanometers, 150 nanometers, or 175 nanometers. Specifically, source  110  may produce light such that incident light IL is EUV light having a wavelength of between 10 nanometers and 200 nanometers, such as by having a wavelength of 11 nanometers, 12 nanometers, 13 nanometers, 13.5 nanometers, 14 nanometers, 15 nanometers, or 16 nanometers. In addition, light source  110  may include a filter, such as a filter to ensure that the wavelength or incident light IL is in a desired range, such as within the EUV range of 10 nm to 200 nm, as specified above. 
   Substrate  105  may be a substrate or wafer including multiple layers of a reflective material, a multi-layered patterning mask, a patterning mask, an etch mask, a lithographic mask, a photolithography mask, a photo mask, or a mask blank thereof. Thus, it is contemplated that source  110  and source mirror  112  are sufficient to illuminate bumps or irregularities in the height of surface  115  or layers below surface  115  of substrate  105 . For instance, substrate  105  may be a plate or substrate having multiple layers of reflective material of molybdenum (Mo) and silicon (Si) formed thereon, and source  110  may provide incident light IL having an appropriate wavelength, intensity, and focus to illuminate or penetrate all of the layers of Mo and Si formed thereon. 
   Thus, substrate  105  may be a patterning or etch mask blank, such as a substrate or plate having multiple layers of molybdenum (Mo) or silicon (Si) formed thereon in an alternating layered order, where surface  115  is the surface of a top layer of molybdenum (Mo) or silicon (Si), prior to the mask blank having a buffer layer and absorber layer deposited on surface  115 . Specifically, surface  115  may have a buffer layer formed thereon; an absorber layer formed on the buffer layer; a pattern patterned, written, or etched in the absorber layer; and then have a pattern written or etched into the buffer layer (e.g., to form a patterning mask). It is also to be appreciated that this process may include repair of the absorber layer (such as during patterning, writing, or etching). Thus, substrate  105  may be a mask blank of a mask to be formed for patterning or etching a silicon wafer, such as a wafer on which electronic semiconductor devices are being formed, using EUV patterning of a light sensitive material formed or layered on the wafer. 
   It is also contemplated that substrate  105  may have between 10 and 60 layers (e.g., such as by having between 10 and 60 layers of Mo and Si material). For example, substrate  105  may include 20, 25, 30, 35, 40, 45, or 50 layers of Mo and Si material where each layer is one layer of either Mo or Si material of an alternating Mo Si layer structure. In addition, each of the Mo or Si layers may be a reflective layer, such as a layer capable of partially reflecting EUV light. For example, incident light IL may penetrate all of the multi-layers of substrate  105  (e.g., such as by penetrating 35, 40, or 45 layers of Mo and Si without being completely absorbed and/or reflected by those layers). 
   Because the EUV light can have light intensity spread over a light frequency bandwidth (e.g., such as by having light intensity across a bandwidth of frequencies as large as about two percent of the selected or desired frequency of the light), it is useful to describe the frequency of EUV light, such as incident light IL, in terms of “centroid wavelength”. A centroid wavelength may represent a wavelength at which is located the mean value across the bandwidth of the intensity of EUV light used or radiated. 
   According to embodiments, source  110  may generate incident light IL having an EUV light whose centroid wavelength is between 1.005 and 1.010 times the centroid wavelength of a light used to pattern or expose portions of a light sensitive material on a wafer during a photolithographic process where portions corresponding to a pattern formed on substrate  105  of reflective multi-layers (as compared to light absorbing material formed on the reflective multi-layers) are formed in the light sensitive material of the wafer. For example, incident light IL may have a centroid wavelength of 1.006, 1.007, 1.008, or 1.009 times a centroid wavelength of a patterning light used to expose portions of light sensitive material on a wafer being patterned with a patterning mask formed from the mask blank being inspected by incident light IL. 
   Moreover, it is considered that the light used to expose the light sensitive material on the wafer may be incident upon surface  115 , substrate  105 , and/or the light sensitive material of the wafer at an angle of between six degrees and eight degrees from perpendicular to surface  115  (e.g., such as by being at an angle of between 6 degrees and 8 degrees from axis AX of mirror  120 , as shown in  FIG. 1 ). For example, the patterning light used to expose the light sensitive material on the wafer may be incident at an angle of 6.5 degrees, 7 degrees, or 7.5 degrees from axis AX. 
     FIG. 1  also shows scattered light SL such as dark-field, imaging light scattered, diffused, or reflected from substrate  105 . Specifically, scattered light SL may include reflected light, scattered light, diffused light, caused by illumination of multiple Mo and Si layers of substrate  105  by incident light IL. However, according to embodiments, scattered light SL may exclude a portion of or all of specular reflection of incident light IL from substrate  105 . Thus, according to embodiments, incident light IL may be completely reflected by layers of reflective material that are part of substrate  105  (e.g., such as layers of Mo or Si material formed on a substrate or plate as described herein). Specifically, incident light IL may be reflected, scattered, or diffused by the layer that forms surface  115  of substrate  105  as well as one or more layers of multi-layer material of substrate  105  below surface  115  (e.g., such as multiple layers of Mo and Si material on which a surface layer of Mo or Si is formed). 
   Furthermore, according to embodiments, scattered light SL may include a portion or all of incident light IL reflected, scattered, or diffused by substrate  105 . Thus, when incident light IL encounters a bump or irregularity on or below surface  115 , the magnitude or brightness of scattered light SL may increase while the magnitude or brightness of specular reflection decreases. 
   Mirror  120  may be used to gather all or a portion of scattered light SL. Mirror  120  may be a spherical mirror having perimeter P 1  (e.g., such as a diameter) and opening OP. In addition, mirror  120  may be designed to reflect scattered light SL to mirror  130 . Mirror  130  may also be a spherical mirror having perimeter P 2  (e.g., such as a diameter) smaller than perimeter P 1 . Mirror  130  may be designed to reflect a portion or all of the scattered light received from mirror  120  to a detector. For example, mirror  130  may reflect that light through opening OP to detector  140 . 
   According to embodiments, mirror  120  and mirror  130  may be mirrors having a spherical shape including less than 20% of a sphere by surface area. Specifically, mirror  120  may have a numerical aperture (NA) of 0.2 and have a concave spherical shape. Likewise, mirror  130  may have an NA of 0.1 and have a convex-shaped spherical reflective surface. 
   Furthermore, according to embodiments, opening OP may have a diameter less then or equal to perimeter P 2  of mirror  130 . More particularly, opening OP (e.g., such as a diameter of opening OP) may be larger than the trace or cross-sectional shape of a light ray which is reflected at the edges or perimeter P 2  of mirror  130 . Mirror  120 , mirror  130 , and opening OP may be part of a “Schwarzschild Optics” device. 
   Detector  140  may be a device for detecting light, such as UV or EUV light. For example, detector  140  may be a camera or electronic-type image sensing array (ISA), such as a charge-coupled device (CCD), or various other appropriate pixel imaging technology able to capture scattered light reflected to detector  140 . 
   As shown in  FIG. 1 , source mirror  112  may be aligned or disposed along axis AX of mirror  120 . Similarly, detector  140  may be oriented or disposed along axis AX. Moreover, axis AX may be oriented perpendicular to surface  115  of substrate  105 . Also, it is considered that there may be angle AN between the outermost ray of the illumination of incident light IL or the half cone angle of the illumination of incident light IL and surface  115 . In one case, angle AN may be between 85 and 90 degrees, such as by being 86 degrees, 87 degrees, 88 degrees, or 89 degrees. 
     FIG. 1  also shows moving apparatus  160  attached to substrate  105 . Moving apparatus  160  may be an apparatus sufficient to move surface  115  in three dimensions with respect to axis AX. For example,  FIG. 1  shows axes  170  having an “X” axis and a “Y” axis forming a two dimensional plane that may be parallel to surface  115 . Thus, moving apparatus  160  may move surface  115  along the “X” “Y” plane of axes  170 , in two dimensions with respect to axis AX. Specifically, moving apparatus  160  may move substrate  105  with respect to incident light IL so that incident light IL is incident upon all or a portion of surface  115 . Moreover, moving apparatus  160  may move substrate  105  so that source mirror  112 , mirror  120 , and detector  140  are oriented or disposed along axis AX at an angle with respect to surface  115  as described above (e.g., such as where axis AX is oriented perpendicular to surface  115 ). 
   In addition, to focus the Schwarzschild Optics with respect to substrate  105  (e.g., such that the focus of mirror  112  and/or mirror  120  are at surface  115 , or at a desired depth below surface  115 ), moving apparatus  160  may move substrate  105  along axis AX (e.g., the third dimension with respect to axis AX) so that source mirror  112 , mirror  120 , and detector  140  are oriented or disposed along axis AX (e.g., such as by those devices being located along axis AX at an appropriate distance from surface  115  to focus one or more of the mirrors as desired). 
   According to embodiments, moving apparatus  160  can be a servo stage that can have a mask blank set on it and that can be controlled by a computer. For instance, moving apparatus  160  may have a platform or surface on which substrate  105  is placed or attached (e.g., such as removably attached by physical restraints or adhesive). Moving apparatus may further include one or more servos, that are controlled by a computer (e.g., such as according to a machine accessible medium having instructions for execution by a machine, or a software routine), to move the substrate in three dimensions, as described above. 
   In addition, system  100  or components thereof may exist in a vacuum setting, for example, light provided by source  110 , incident light IL, substrate  105 , scattered light SL, mirror  120 , mirror  130 , and light reflected by mirror  130  to detector  140  may exist in a vacuum sufficient to allow for propagation of EUV light sufficient for systems and processes described herein. Thus, it can be appreciated that system  100 , substrate  105 , multiple layers of reflective material on substrate  105  (e.g., such as alternating layers of Mo and Si formed on and below surface  115 ) and a patterning mask formed from substrate  105  (e.g., such as where substrate  105  is a mask blank) may be designed, configured, and use optics and pressure appropriate for transmission of EUV light). For instance, those devices may be designed without lenses or glass through which the EUV light is to pass (e.g., since EUV light does not pass through glass) and may be designed to only have the EUV light travel in a vacuum (e.g., since EUV light does not travel far in an atmosphere such as air). 
   As noted above, incident light IL may gradually penetrate, be reflected by and/or be absorbed by the multiple layers of substrate  105 . For example,  FIG. 2  is a cross-sectional view of a multi-layered mask blank.  FIG. 2  shows substrate  105  having substrate  205  and first layer L 1 , second layer L 2 , third layer L 3 , fourth layer L 4 , and fifth layer L 5  formed on top of substrate  205 . As mentioned above, it is contemplated that substrate  105  may include between 10 and 60 layers, thus layers L 1  through L 5  may be representative of a portion of the total layers of a multi-layer mask blank. As shown in  FIG. 2 , surface  115  is the surface of fifth layer L 5 . Thus, incident light IL, incident upon substrate  105 , may be scattered or diffused by defect DEF 1  which has form bump B 1  above surface  115 , such as is shown by scattered light SL 1  in  FIG. 2 . Similarly, where incident light IL is EUV light, defect DEF 2  may scatter or diffuse EUV light EUV by causing deformations or bumps in layers L 1  through L 5 , such as is shown by SL 2  in  FIG. 2 . 
   Thus, system  100  of  FIG. 1  may be able to detect-abnormalities, bumps, or defects within the layers of substrate  105 , by using dark-field imaging optics to gather the scattered, reflected, or diffused light resulting from the abnormalities, bumps, or defects in the multiple layers resulting from the defect. It is contemplated that system  100  may be able to detect defects having a height between 2 nanometers (nm) and 8 nanometers in height (e.g., see height H of defect DEF 2  of  FIG. 2 ), and having a width of between 35 nm and 94 nm (such as width W of defect DEF 2  as shown in  FIG. 2 ). For instance, a defect having a height and width as described above may cause abnormalities, bumps, or defects in the planarity of the multiple layers of the mask blank, thus causing diffusion or scattering of EUV light incident upon those layers. The scattered or diffused light may be gathered, measured or detected by system  100  such as by mirrors  120  and  130 , which reflect scattered light SL to detector  140 , which measures the intensity or amount of the scattered light. It can be appreciated that such defects may be a bump or abnormality at or below surface  115  (e.g., such as a defect or bump of unwanted material or space within the layers of substrate  105 ). 
   For instance, system  100  of  FIG. 1  may detect a defect having a width of between 60 to 70 nm, and a height of 2 nm or greater as a critical defect. Thus, apparatus  100  may be able to detect a defect at an approximate lower-layer level, such as at layers  35 - 40  below surface  115  of 40 layers, having a 60 nm width and a height of 2 nm or greater, that forms a bump of between 0.4 and 2.3 nm in height at surface  115 . Likewise, apparatus  100  may detect a defect at an approximate mid-layer lever, such as at layer  15  below surface  115  of 40 layers, that creates a bump at surface  115  of 2.3 nm in height. 
   Specifically, as shown in  FIG. 2 , system  100  may be able to detect, identify, and locate defect DEF 2  having height H of 3 nm, 4 nm, 5 nm, 6 nm, or 7 nm and producing bump height B 2  of zero or more nanometers at surface  115  by measuring scattered light SL 2  reflected by layers L 1  through L 5  of substrate  105  when those layers are illuminated by incident light IL 2 . Similarly, system  100  may detect, identify, and locate defect DEF 1  having bump height B 1  of 2 nm, 2.5 nm, 3 nm, or 4 nm at surface  115  by measuring scattered light SL 1  resulting from illuminating substrate  105  and defect DEF 1  with incident light IL 1 . 
     FIG. 1  also shows logic  150  connected to detector  140 . For example, logic  150  includes logic circuitry, gates, computer logic hardware, memories, comparators, and/or registers coupled to detector  140  to determine whether reflective, scattered, or diffused light received and measured from substrate  105  satisfies a criteria. Thus, logic  150  may include logic to detect, identify, and locate defects in substrate  105  (e.g., such as defects within the multiple layers of a multi-layer mask blank as described above with respect to  FIG. 2 ). Specifically, “detecting” a defect as used herein may include measuring reflected light intensity values and normalizing those values for a number of pixels, as described herein. Also, “identifying” a defect as used herein may include determining whether the measured and normalized value of reflective light from the pixels satisfies criteria to indicate that a defect exists, such as described herein. Next, “locating” a defect as used herein may include determining a location of a defect, such as a location at one or more pixels where the defect exists, as described herein. 
   In one instance, logic  150  may determine whether a pixel threshold value is satisfied by a pixel reflective light intensity value received by detector  140  for a first pixel of substrate  105 , where substrate  105  is a multi-layered patterning mask blank. It is considered that the pixel reflective light intensity value received by detector  140  may be normalized as described below with respect to  FIG. 3 , prior to logic  150  determining whether the threshold value is satisfied. For example, logic  150  may determine whether the normalized scattered light intensity value at center pixel CP is greater than a pixel threshold value, thus identifying center pixel CP as a candidate for a location with a defect. 
   In addition, logic  150  may determine whether a pixel block threshold value is satisfied by a sum of pixel reflective light intensity values received from detector  140  for a number of pixels of a pixel block of a multi-layer mask, where the pixel block includes the pixel compared to the first threshold value. It is considered that the pixel reflective light intensity values received by detector  140  for the number of pixels of the pixel block may each be normalized as described below with respect to  FIG. 3 , prior to logic  150  determining whether the second threshold value is satisfied. For example, logic  150  may determine whether the scattered light reflected by multiple pixels around and including center pixel CP, when normalized and summed together, satisfy a pixel block threshold, to determine whether the pixel block scatters a critical amount of light, such as an amount of reflected light that would correspond to a “critical defect” (e.g., such as a selected amount of light appropriate for apparatus  100 , substrate  105 , and the multi-layers of substrate  105  to satisfy a defect tolerance or threshold for a selected critical defect size). 
   Hence, system  100  may detect abnormalities, bumps, or imperfections in the planarity or flatness of layers L 1  through L 5  of substrate  105  as shown in  FIG. 2  at a microscopic level (e.g., such as for pixels having a size of less than 1.0 μm) to identify and locate defects, such as errant materials or bumps of materials in or on the surface, or layers (e.g., such as layers L 1  through L 5 ) of substrate  105  by having logic  150  consider bright spots in the dark-field image of scattered or diffused light reflected from the surface and layers of substrate  120  when substrate  120  is illuminated with incident light IL. It is also contemplated that logic  150  may be implemented by hardware and/or software, such as digital code or instructions, or a machine-accessible medium containing instructions that cause a machine to perform the functionality described herein with respect to logic  150 . 
   Surface  115  of substrate  105  may include or define a grid of pixels (e.g., such as grid  300  of  FIG. 3  and grid  400  of  FIG. 4  as described below), that system  100  inspects, as described herein. Thus, reflected scattered light intensities may be measured for each pixel of the grid, then a normalized scattered light value may be calculated for each pixel of the grid, then logic  150  may determine whether criteria are met for various groups of the pixels of the grid to identify defects within the grid. 
   For example,  FIG. 3  is a top perspective view of a multi-layer mask blank.  FIG. 3  shows an array of pixels, such as grid of pixels  300 , center pixel CP, and next pixel NP in direction DIR from center pixel CP. Surrounding center pixel CP is inner perimeter of pixels T 1 , T 2 , T 3 , T 4 , T 5 , T 6 , T 7 , T 8 , T 9 , T 10 , T 11 , T 12 , T 13 , T 14 , T 15 , T 16 , T 17 , T 18 , T 19 , T 20 , T 21 , T 22 , T 23 , and T 24 . Surrounding inner perimeter of pixels T 1 -T 24  is outer perimeter of pixels S 1 , S 2 , S 3 , S 4 , S 5 , S 6 , S 7 , S 8 , S 9 , S 10 , S 11 , S 12 , S 13 , S 14 , S 15 , S 16 , S 17 , S 18 , S 19 , S 20 , S 21 , S 22 , S 24 , S 25 , S 26 , S 27 , S 28 , S 29 , S 30 , S 31 , and S 32 . 
   According to embodiments, grid of pixels  300  may be pixels on or associated with substrate  105 . For example, grid of pixels  300  may be pixels identified or mapped out with respect to surface  115 . Specifically, the pixels of grid of pixels  300  (e.g., such as center pixel CP, next pixel NP, each of inner perimeter of pixels T 1 -T 24 , and each of outer perimeter of pixels S 1 -S 32 ) may be pixels having a pixel size as described above with respect to  FIGS. 1 and 2 , and may include layers of a multi-layer patterning mask blank as described herein. Thus, pixels of grid of pixels  300  may correspond to pixels from which scattered, reflected, or diffused light intensity values or measurements are made as described herein. For example, system  100  may illuminate pixels of grid of pixels  300  with incident light IL and measure scattered light SL reflected by each pixel, as a result. 
   More particularly, according to embodiments, system  100 , mirrors  120  and  130 , source  110  and/or detector  140  may include Schwarzschild Optics or dark-field optics that have a magnification of 20×, use an incident light wavelength of 13.5 nm, and/or use a detector CCD having a pixel size of 13.5 μm to measure scattered light from pixels of grid of pixels  300 . For example, system  100  may detect scattered or diffused light, such as scattered light SL, for a pixel having a width and length of between 0.3 μm and 0.8 μm. Specifically, detector  140  may have a pixel size of 13.5 μm (e.g., pixel size of detection at detector  140 ) and the “Schwarzschild Optics” or dark-field imaging optics of system  100  may have a magnification of 20×, thus resulting in system  100  being able to sufficiently illuminate a pixel with incident light IL and measure the resulting scattered light SL for a resolution or pixel size as small as 13.5 μm divided by 20, or a pixel size equal to 0.675 μm (e.g., a pixel size of a pixel, such as center pixel CP of  FIG. 3 ). System  100  may also detect light from pixels having a size of 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.8 μm. 
   Referring to  FIG. 3 , a normalized light intensity value may be determined for center pixel CP by measuring a reflective light intensity value (e.g., such as a scattered or diffused reflection light value measured by dark-field imaging optics and/or system  10 ) for center pixel CP (e.g., such as a reflective light intensity value that includes reflection of incident light IL from the surface as well as layers below the surface of substrate  105  at pixel CP) and for at least one surrounding ring of other pixels. In one instance, reflective light intensity values for inner perimeter of pixels T 1  through T 24  are also measured. Likewise, reflective light intensity values for outer perimeter of pixels S 1  through S 32  are measured. Then, the normalized light intensity value for center pixel CP is calculated as the reflective light intensity value measured for center pixel CP divided by the average of the light intensity values for both the inner perimeter of pixels T 1  through T 24  and the outer perimeter of pixels S 1  through S 32 . In other words, (normalized light intensity value for center pixel CP)=(light intensity value for center pixel CP)/((some of light intensity values for pixels T 1  through T 24 + some of light intensity values for pixels S 1  through S 32 )/56). 
   It is also contemplated that the denominator of the calculation for determining the normalized light intensity value above may instead be the average of: (1) only perimeter of pixels T 1  through T 24 , (2) only outer perimeter of pixels S 1  through S 32 , or (3) one or more shapes or perimeters of pixels other than those shown in  FIG. 3 . For example, the denominator of the equation above for determining normalized light intensity values may be one or more perimeters in the shape of a trapezoid, a rectangle, a square, and a ring of pixels around center pixel CP. Furthermore, it may be appreciated that the calculations described above for determining a normalized light intensity value may be performed on each pixel in grid of pixels  300 . For example, the processes and calculations described above may be repeated for next pixel NP, by moving perimeter of pixels T 1  through T 24  and outer perimeter of pixels S 1  through S 32  one pixel in the direction of direction DIR, and recalculating as described above to determine the normalized light intensity value for next pixel NP. 
   Furthermore, according to embodiments, the light intensity values of EUV light reflected at pixels of grid of pixels  300  may be considered to determine criteria, such as to determine whether a defect exists in a multi-layered mask blank. For example,  FIG. 4  is a top perspective view of a multi-layered mask blank.  FIG. 4  is a grid of pixels having a center pixel and blocks of pixels including the center pixel.  FIG. 4  shows grid of pixels  400  having center pixel CP; a first block of four pixels including center pixel CP; a second block of nine pixels including the first block of pixels; and a third block of twenty-five pixels including the second block of pixels. For example, reflected light intensity values for the pixels of grid of pixels  400  may be considered, measured, or determined according to reflected EUV light intensity values of scattered light, diffused light, or reflected light resulting from incident light IL, as described above with respect to system  100 . Moreover, grid of pixels  400  may be a grid of pixels as described above with respect to grid of pixels  300 . Consequently, the pixels of grid of pixels  400  may be pixels on substrate  105  or surface  115 . 
   According to embodiments, whether a defect exists may be determined by considering a reflective EUV light intensity value for center pixel CP and by considering a light intensity value for a block of pixels including center pixel CP and (e.g., such as a block of more than one pixel). It is contemplated that considering the reflected EUV intensity light value of a pixel includes considering the normalized light intensity value as described above with respect to  FIG. 3 . Thus, identifying a defect at center pixel CP of grid of pixels  400  may be performed by considering whether the normalized light intensity value of center pixel CP qualifies center pixel CP as a candidate of a location with a defect, and by considering whether the normalized light intensity value of a block of pixels (e.g., such as a block having at least two pixels and including center pixel CP), verifies the criticality of the intensity of the normalized light values of the block of pixels as scattering or diffusing enough light to indicate a critical defect. 
   According to embodiments, the block of pixels to verify the criticality of the intensity may be a block of two pixels, a block of three pixels, a block of four pixels, a block of nine pixels, a block of ten pixels, a block of twenty-five pixels, or a block of thirty-six pixels. Specifically, as shown in  FIG. 4 , for instance, the block of pixels may include center pixel CP and pixel PB 1 . Also, the pixel block may include center pixel CP, pixel PB 1 , pixel PB 2 , and pixel PB 3 . Moreover, the pixel block may include pixels PB 1  through PB 3 , pixel PC 1 , pixel PC 2 , pixel PC 3 , pixel PC 4 , and pixel PC 5  (e.g., such as with or without center pixel CP). Additionally, the pixel block may include pixel PD 1 , pixel PD 2 , pixel PD 3 , pixel PD 4 , pixel PD 5 , pixel PD 6 , pixel PD 7 , pixel PD 8 , pixel PD 9 , pixel PD 10 , pixel PD 11 , pixel PD 12 , pixel PD 13 , pixel PD 14 , pixel PD 15 , and pixel PD 16 ; with or without pixels PB 1  through PB 3 , pixels PC 1  through PC 5 , and/or center pixel CP. 
   Thus, in one embodiment, it may be determined whether the normalized scattered or diffused light reflection at center pixel CP satisfies a first threshold and whether the normalized scattered or diffused light reflection for the pixel block satisfies a second threshold in order to identify if a defect is present at center pixel CP, or at the location of the pixel block. It is to be appreciated that determining whether a threshold value is satisfied may include determining whether a light intensity value is greater than a threshold value, or greater than or equal to a threshold value. 
     FIG. 5  is a flow diagram of a process for locating defects in a multi-layered mask blank. At block  510  a substrate is radiated with EUV light. For example, block  510  may correspond to descriptions above with respect to radiating or illuminating substrate  105  with incident light IL. In addition, block  510  may correspond to descriptions of radiating or illuminating grid of pixels  200  and/or grid of pixels  300  of substrate  105 . Moreover, block  510  may correspond to illuminating the surface and layers below the surface of a multi-layered patterning or etching mask blank as described herein. 
   At block  520 , the light intensity for a first pixel block is measured. Block  520  may correspond to measuring the scattered or diffused light for center pixel CP, or a pixel block as described above with respect to  FIG. 4 . For example, block  520  may include measuring reflected light, detecting reflected light, dark-field image detection, scattered light reflection measurement, or diffuse light reflection measurement of incident IL as reflected by the first pixel block as described above with respect to  FIG. 4 . 
   At block  530 , the light intensity value for a second pixel block is measured. For example, the second pixel block may correspond to a pixel block as described above with respect to  FIG. 4  and including the first pixel block, such as where the second pixel block is larger than the first pixel block. Block  530  may correspond to measuring light intensity values as described above with respect to block  520 , but for a second pixel block as described above with respect to  FIG. 4 . 
   At block  540 , light intensity values for a third pixel block are measured. Block  540  may correspond to measuring light intensity values for a pixel block as described above with respect to  FIG. 4 , where the third pixel block includes and is larger than the second pixel block. Measuring light intensity values at block  540  may correspond to measuring light intensity values as described above with respect to block  520 , but for a third pixel block as described above with respect to  FIG. 4 . 
   At block  550 , the light intensity values measured at block  520 ,  530 , and  540  are normalized. For example, the light intensity measured for each pixel of the first, second, and third pixel block may be divided by an average of a number of UV light reflection measurements for a number of pixels surrounding each of the pixels of the first, second, and third pixel blocks. In addition, the normalizing of block  550  may correspond to descriptions above for normalizing light intensity values as described with respect to  FIG. 3 . 
   At decision block  560 , it is determined whether the normalized first pixel block value or values satisfy or are greater than first threshold Th 1 . For example, at block  560 , it may be determined whether the normalized reflected light intensity value of a first pixel block as described for  FIG. 4  (e.g., such as of center pixel CP) is greater than first threshold Th 1 . If at block  560  the normalized first pixel block value or values are not greater than first threshold Th 1 , then the process continues to block  570 . 
   At block  570 , it is determined whether the normalized pixel block values of the second pixel block are greater than second threshold Th 2 . For example, at block  570 , it may be determined whether the normalized reflected light intensity values for a second pixel block as described for  FIG. 4  (e.g., such as for center pixel CP plus pixels PB 1  through PB 3 ) are greater than second threshold Th 2 . If at block  570  the pixel block values are not greater than the second threshold, the process continues to block  590 . 
   If at block  560 , the pixel block values or value is greater than the first threshold, or if at block  570  the sum of the second pixel block values is greater than the second threshold, then the process continues to decision block  580 . At decision block  580 , it is determined whether the normalized third pixel block values are greater than third threshold Th 3 . For example, at block  580 , it may be determined whether the sum of the reflected light intensity values for a third pixel block as described for  FIG. 4  (e.g., such as for center pixel CP, plus pixels PB 1  through PB 3 , plus pixels PC 1  through PC 5 ) are greater than third threshold Th 3 . It is also considered that at block  580  it may be determined whether the reflected light intensity values for pixels PD 1  through PD 16 , pixels PB 1  through PB 3 , pixels PC 1  through PC 5 , and center pixel CP are greater than a third threshold. 
   If at block  580  the sum of the normalized third pixel block values is not greater than third threshold Th 3 , then the process continues to block  590 . 
   At block  590 , it is determined that the criteria is not satisfied. For example, at block  590 , it may be determined that a defect does not exist at center pixel CP, at the second pixel block, or at the third pixel block. Thus, according to  FIG. 5 , the criteria may not be satisfied when either the second or third pixel block fails to meet the second or third threshold, respectively. In other words, decision blocks  560  and  570  may be used to identify candidates of locations with defects, while block  580  may be used to verify whether there is a criticality of scattered light intensity by checking the summed intensity within nine to twenty-five pixels around center pixel CP because EUV light scattered by a defect may be imaged in a wide area due to the defect causing a large field with a curved focal plane within the multi-layers of the mask blank. 
   If at decision block  580  the sum of the normalized third pixel block values is greater than the third threshold, then the process continues to block  585 . At block  585 , it is determined that the criteria is satisfied. For example, at block  585 , it may be determined that a defect exists at center pixel CP, at the first pixel block, at the second pixel block, or at the third pixel block. Furthermore, at block  585 , the location of the defect, as described above, may be stored (e.g., such as storing in a memory or register the location or identification of the pixel or pixel block corresponding to the defect). 
   After block  585  or block  590 , it is contemplated that process  500  may move the location for center pixel CP to a next pixel of a grid of pixels and inspect the grid of pixels to determine whether a defect exists at or about the location of the next pixel. For example, after block  585  or block  590 , process  500  may continue and inspect next pixel NP as described above with respect to  FIG. 3  (e.g., such as by moving all pixel locations considered one pixel in direction DIR) and then considering new first, second, and third pixel blocks, such as is described above with respect to  FIGS. 4 and 5 . 
   In addition, it is contemplated that block  560  may correspond to the following equation:
 
A m,n &gt;Th1   A
 
   wherein A m,n  is the normalized reflective light intensity value detected at a pixel located at position (m,n) of a grid of pixels (e.g., such as where m corresponds to a position in the X direction of axes  170 , and n corresponds to a position with respect to the Y direction of axes  170 , as shown in  FIG. 1 ); and Th 1  represents the first threshold. 
   Correspondingly, block  570  may correspond to the following equation: 
   
     
       
         
           
             
               
                 
                   
                     ∑ 
                     
                       i 
                       , 
                       
                         j 
                         = 
                         0 
                       
                     
                     1 
                   
                   ⁢ 
                   
                     A 
                     
                       
                         m 
                         + 
                         i 
                       
                       , 
                       
                         n 
                         + 
                         j 
                       
                     
                   
                 
                 &gt; 
                 Th2 
               
             
             
               B 
             
           
         
       
     
   
   wherein A m+i,n+j  represent the normalized reflected light intensity value at pixels located adjacent to and including pixel A m,n , as described above for equation A; and Th 2  represents a second threshold. Thus, equation B may represent the sum of normalized reflected light intensity values for a pixel block. 
   Next, block  580  may correspond to the following equation: 
   
     
       
         
           
             
               
                 
                   
                     ∑ 
                     
                       i 
                       , 
                       
                         j 
                         = 
                         
                           - 
                           1 
                         
                       
                     
                     1 
                   
                   ⁢ 
                   
                     A 
                     
                       
                         m 
                         + 
                         i 
                       
                       , 
                       
                         n 
                         + 
                         j 
                       
                     
                   
                 
                 &gt; 
                 Th 
               
             
             
               C 
             
           
         
       
     
   
   wherein A m+i,n+j  equals the normalized reflected light intensity values for pixels adjacent to or forming a perimeter around pixel A m,n , such as is described above with respect to equation A; and Th 3  is the third threshold. Thus, equation C may represent the sum of normalized pixel reflective light intensity values for a block of pixels larger than the block of pixels summed in equation B, including the pixel block summed in equation B, and/or including pixel A m,n . 
   In the foregoing specification, specific embodiments are described. However, various modifications and changes may be made thereto without departing from the broader spirit and scope of embodiments as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.