Abstract:
A reflective mask inspection system comprises a short wavelength radiation source for irradiating a reflective mask. A detector system detects the short wavelength radiation reflected from the reflective mask and a controller compares reflectance images of the reflective mask from the detector to characterize the mask. The system analyzes the spatially resolved reflectance characteristics of the substrate from different angles with respect to normal to the substrate and/or at different angles of rotation of the substrate. This information can be used to then analyze the mask for buried defects and then characterize those defects. This technique improves over current systems that rely on atomic force microscopes, which can only provide surface information.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 62/090,746, filed on Dec. 11, 2014, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The optics for lithography systems for the extended ultraviolet (EUV) into the x-ray regime (short wavelength radiation) are generally reflective. Even the lithography masks are reflective optics. 
         [0003]    The short wavelength lithography masks include multilayer (ML) Bragg mirror stacks of many bilayers of high and low refractive index material. EUV masks, for example, usually have 40 bilayers of molybdenum and silicon, which have been successively coated onto a substrate. In general, a Bragg reflection of around 70% at 13.5 nanometer (nm) wavelength is targeted. 
         [0004]    The short wavelength lithography masks have additional layers. On top of the ML mirror stack is a capping layer for environmental protection. In addition, this capping layer acts as an etch stop during mask fabrication. As a result, the capping layer should have a low EUV absorption. The stack is finalized by an optional buffer layer (e. g. SiO2). Finally, an anti-reflecting absorber layer (e. g. TaN) is patterned to define the dark and bright (reflective) features of the mask. 
         [0005]    Because of the complex multilayer structure, these ML lithography masks are subject to defects known as multilayer (ML) defects (also called buried defects). These ML defects can come from the low thermal expansion material (LTEM) substrate in the form of pits, bumps or scratches that are created on the substrate surface from the chemical mechanical polish (CMP) and cleaning processes used to prepare the substrate prior to the deposition of the mirror stack. In fact, small substrate defects, e.g., below ˜20 nm, are considered process inherent during CMP and cleaning ML defects can also arise during the ML deposition process. The complex multilayer, typically consisting of 80 or 100 alternating layers of Si and Mo, is deposited on this substrate followed by a Ru capping layer. Ion beam deposition (IBD) is normally used for the ML deposition steps and over half of all killer blank defects can be traced to this deposition step. 
         [0006]    These ML defects are not easily detectable or capable of characterization by current inspection tools. Yet the defects present on the LTEM substrate or arising in the ML layers propagate through the ML mirror stack and will nevertheless print during EUV exposure if they affect reflective features of the final patterned photomask. 
       SUMMARY OF THE INVENTION 
       [0007]    Currently ML defect characteristics are calculated by measuring the defects at the mask surface with an atomic force microscope (AFM) and then performing a simulation to calculate the shape of the defect and the required repair. This technique has significant limitations, however, due to the fact that the AFM can only provide surface information. No information regarding the bulk of the defect or its propagation through the ML stack can be discerned and is therefore not taken into account. 
         [0008]    The present invention in contrast analyzes the spatially resolved reflectance characteristics of the substrate from different angles with respect to normal to the substrate and/or at different angles of rotation of the substrate. This information can be used to then analyze the mask for buried defects and then characterize those defects. 
         [0009]    In general, according to one aspect, the invention features a reflective mask inspection system. It comprises a short wavelength radiation source for irradiating a reflective mask. A detector system detects the short wavelength radiation reflected from the reflective mask to generate reflectance images, and a controller compares the reflectance images of the reflective mask from the detector to characterize the mask. 
         [0010]    In one embodiment, a rotation stage is provided to rotate the reflective mask relative to radiation from the radiation source between reflectance images captured by the detector system. In this way reflectance images at different rotation angles can be captured, compared, and used to characterize the mask. 
         [0011]    In another embodiment, an angular positioning stage is provided for changing a tilt axis of the mask relative to radiation from the radiation source between reflectance images captured by the detector system. In this way reflectance images at different tilt angles can be captured and used to characterize the mask. 
         [0012]    Preferably, the short wavelength radiation has a wavelength of less than 5 nanometers. This is less than half the operational wavelength of λ=13.5 nm that is common for EUV lithography systems. Typically, x-ray radiation is used, however. 
         [0013]    Also, in the preferred configuration, the chief ray angle (CRA) between the radiation from the radiation source and the reflective mask is higher than an operational CRA of a multilayer mirror deposited on the reflective mask. In current EUV lithography systems the operational CRA is about 6°. In embodiments, the CRA is higher than 15 degrees. 
         [0014]    In general, according to another aspect, the invention features a reflective mask inspection method. This method comprises irradiating a reflective mask with short wavelength radiation, detecting the short wavelength radiation reflected from the reflective mask to generate reflectance images, and comparing the reflectance images of the reflective mask to characterize the mask. 
         [0015]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
           [0017]      FIGS. 1A and 1B  are a SEM image and a cross-sectional SEM image, respectively, of a pit defect in the substrate illustrating the surface information and the propagation of the defect through the ML; 
           [0018]      FIGS. 2A and 2B  are a SEM image and a cross-sectional SEM image, respectively, of a bump defect in the substrate illustrating the surface information and the propagation of the defect through the ML; 
           [0019]      FIG. 3  is a cross-sectional SEM image of a bump defect in the substrate illustrating the surface information and the lateral translation of the defect through the successive layers of the ML; 
           [0020]      FIG. 4  is a schematic diagram of an EUV reflective mask blank inspection system according to an embodiment of the present invention; 
           [0021]      FIGS. 5A and 5B  are top plan views showing the rotation of the mask blank 90 degrees between the capture of two reflectance images of an ML defect; 
           [0022]      FIG. 6A  is schematic cross-sectional view of a bump defect illustrating the reflection of the incoming beam by the multilayer mirror in a region around the defect when the substrate is positioned at 0°; 
           [0023]      FIG. 6B  is a schematic plot showing the change in reflectance of the substrate across the bump defect when the substrate is positioned at 0°, e.g., maximum reflectivity of undistorted ML minus actual reflectivity in presence of buried defect; 
           [0024]      FIG. 7A  is schematic cross-sectional view of the bump defect illustrating the reflection of the incoming beam by the multilayer mirror around the defect when the substrate is positioned at 180°; 
           [0025]      FIG. 7B  is a schematic plot showing the change in reflectance of the substrate across the bump defect when the substrate is positioned at 180°; 
           [0026]      FIGS. 8A and 8B  is a schematic diagram of an EUV reflective mask blank inspection system according to another embodiment of the present invention in which the chief ray angle (CRA) of the mask blank is changed between successive reflectance images; 
           [0027]      FIGS. 9A and 9B  are schematic cross-sectional views in which the CRA angle of the mask blank is changed between successive reflectance images using the inspection system shown in  FIGS. 8A and 8B ; and 
           [0028]      FIGS. 10A, 10B, and 10C  are schematic cross-sectional views in which the CRA angle of the mask blank is changed between successive reflectance images using an aperture system. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]      FIG. 1A  shows a pit surface defect  112  at the surface of the lithography mask blank  110 . The surface defect  112  arose from a pit defect in the mask&#39;s substrate  116 . The defect  112  presents as only a shallow depression at the mask&#39;s surface. 
         [0030]      FIG. 1B  shows the pit defect in cross-section. It arose from a pit  114  in the substrate  116 . Then, as the successive layers of the ML  118  were deposited, the pattern of the substrate pit  114  propagated through the layers. In general, the cross-section of pit defects  112  measured at the surface are similar in size to the substrate pits  114  because the propagation through the ML remains relatively constant in width. 
         [0031]      FIG. 2A  shows a bump surface defect  120  at the surface of the lithography mask blank  110 . The surface defect  120  arose from a bump defect in the mask&#39;s substrate. The defect presents as only a small mesa. 
         [0032]      FIG. 2B  shows the bump defect in cross-section. It arose from a bump  122  in the substrate  116 . Then, as the successive layers of the ML  118  were deposited, the pattern of the substrate bump  122  propagated through the layers. In general, the cross-section of bump defects  120  measured at the surface are much larger than that at the substrate due to the nature of the propagation through the ML. 
         [0033]      FIG. 3  shows another bump defect  112  in cross-section illustrating lateral translation of the bump defect through the successive layers of the ML  118 . The result of this lateral translation is that the surface measurement techniques often lead to biased defect position due to the assumption that the defect propagates in an entirely vertical (normal to substrate surface) direction. 
         [0034]    Recent investigations of defect propagation through the ML  118  predict that the defects propagate at up to a 6° angle (Amano and Terasawa 2013, SPIE 86791P) with respect to the normal. As a result, there can be a significant shift of the surface defect  112 ,  120  with respect to the position of the substrate defect  114 ,  122 . Whether the ML defect propagates vertically or at an angle can have an impact on the shape required for the compensational repair. 
         [0035]    Additionally, a smoothing of substrate defects throughout the ML  118  is expected that might be a function of the multilayer deposition process parameters. But in general it will also be unknown to the user. 
         [0036]      FIG. 4  shows an EUV reflective mask blank inspection system  200  that has been constructed according to the principles of the present invention. 
         [0037]    The system  200  generally includes the x-ray source  210  for illuminating the lithography mask blank  110 . 
         [0038]    In one implementation, a synchrotron source is used. Such sources can produce highly collimated, narrow wavelength radiation. 
         [0039]    In another implementation, the source  210  is a “laboratory x-ray source”. Examples include an x-ray tube, in which electrons are accelerated in a vacuum by an electric field and shot into a target piece of metal, with x-rays being emitted as the electrons decelerate in the metal. Typically, such sources produce a continuous spectrum of background (bremsstrahlung) x-rays combined with sharp peaks in intensity at certain energies that derive from the characteristic lines of the selected target, i.e., depending on the type of metal target used. Furthermore, the x-ray beams are divergent and lack spatial and temporal coherence. Preferably a transmission configuration is used in which the electron beam strikes the thin target  212  from its backside. The x-rays emitted from the other side of the target are used as the beam  220 . 
         [0040]    In another example, the source  210  is a metal jet x-ray source such as are available from Excillum AB, Kista, Sweden. This type of source uses microfocus tubes in which the anode is a liquid-metal jet. Thus, the anode is continuously regenerated and already molten. Other related examples include a rotating anode x-ray source or a micro-focus x-ray source. 
         [0041]    The x-ray beam  220  generated by source  210  is preferably conditioned to suppress unwanted energies or wavelengths of radiation. For example, undesired wavelengths present in the beam are eliminated or attenuated, using, for instance, an energy filter  218  (designed to select a desired x-ray wavelength range (bandwidth)). Typically, the energy filter, if used, is configured to select the energy associated with a single characteristic line of the target metal and suppress other wavelengths, including the bremsstrahlung radiation, when a laboratory x-ray source is used. 
         [0042]    Since the laboratory sources produce a relatively weak and diverging radiation beam as compared to stronger sources, such as the synchrotrons, a condenser system  214  is preferably used to collimate the beam  220 . 
         [0043]    Different types of condensers can be used including fiber optic or zone plate devices. In one example, the condenser  214  is a capillary tube-based system. Specifically, the capillary tube condenser is preferably made out of a glass capillary tube that is circularly symmetric around the center optical axis A. This capillary tube has been formed, such as, by introducing a pressurized gas into the capillary tube, while heating it to soften the glass forming the tube. Preferably, the inner wall  216  is controlled to have an ellipsoidal or half-ellipsoidal curvature. 
         [0044]    In one embodiment, the inner wall  216  of the capillary tube condenser is coated with a material that is reflective to the x-ray radiation beam  220 . Typically, this is a high Z material, such as tungsten or gold. As a result, the radiation emitted by the source  210  is reflected due to the low angle of incidence on the inner surface  216  to enable the efficient relay of the radiation to the target mask blank  110 . 
         [0045]    The radiation is thus converted into a collimated beam of radiation  220 , directed at the mask blank  110 . 
         [0046]    The mask blank  110  is preferably held on a rotation stage or goniometer  222 , which allows for its controlled rotation (see arrow  224 ) about an axis  225  that is orthogonal to the surface of the mask blank  110 . In a preferred embodiment, a positioning stage  226  is provided to position the goniometer  222  and thus the mask blank  110  in the two dimensional plane of the mask blank surface to thereby enable the step-wise scanning of the entire surface of the mask blank  110  while maintaining a constant angle between the mask normal axis  225  and optical axis A of the incoming radiation beam  220 . 
         [0047]    In one example, the positioning stage  226  is controlled by a system controller  250  to locate a region of interest of the surface of the mask blank  110  in the beam  220 . Then, a first reflectance image of the mask blank  110  is acquired. Then the mask blank  110  is rotated by 180 degrees, for example, using rotation stage  222  and a second reflectance image is acquired of the same region of interest. In other examples, the mask blank is rotated to 90 and/or 270 degrees and additional reflectance image(s) are acquired. 
         [0048]    The reflectance images are acquired when the x-ray beam  220  is reflected by the mask blank  110  and then detected by a detector system  280 . In the illustrated example, the detector system  280  includes a scintillator  282  that converts the x-rays into photons of lower energy (typically within or near the visible range of the electromagnetic spectrum). This is required when the energy of the x-rays is such that they cannot be directly detected by a sensor chip. 
         [0049]    An optical magnification system of the detector system  280  images the light from the scintillator  282  onto a camera  292 , which typically includes a charge coupled device (CCD) or CMOS sensor chip. The spatially resolved image generated by the camera  292  is provided to the system controller  250 . 
         [0050]    In one example, the optical magnification system preferably includes a magnification lens system  284  held within a housing  290  of the detector system  280 . A couplet  285  can be used to condition the optical signal from the magnification lens  284 . A final lens couplet  288  forms the image on the detector or camera  292 . 
         [0051]    In general, suitable arrangements that can be used are described, for instance, in U.S. Pat. No. 7,130,375 B1, issued to Yun et al. on Oct. 13, 2006, the contents of which are incorporated herein by reference in their entirety. 
         [0052]    In some examples, a turning mirror  286  is included in the optical portion of the detector system  280 . It is located prior to the camera  292  to prevent damage from the x-rays. 
         [0053]    In still other examples, flat panel detectors (direct or indirect) are used. 
         [0054]    In operation, the controller  250  acquires reflectance images of the same region of the mask blank  110  but at different rotation angles of the mask blank  110  around axis  225 . 
         [0055]      FIG. 5A  shows the incoming radiation beam irradiating the ML defect  112 ,  120 . This enables the capture a first reflectance image. 
         [0056]    As shown in  FIG. 5B , the mask blank  110  is then rotated by −90 degrees, for example, and the incoming radiation beam again irradiates the ML defect  112 ,  120 . This enables the capture a second reflectance image. 
         [0057]      FIG. 6A  shows the incoming beam  220  reflecting off the multilayer  118  of the mask blank  110  around the bump  122  on the substrate  116  when the mask blank  110  is rotated to a first angle such as 0°. 
         [0058]      FIG. 6B  illustrates the reflectance of the multilayer  118  as a function of position. As illustrated, the reflectance changes across the bump  122 . The spatial reflectance changes as a function of a number of factors including the vertical distortion of the individual layers of the multilayer  118 , any compression or expansion in the thickness of those layers, and the baseline reflectance of the layers surrounding the bump. The reflectance, e.g., maximum reflectivity of undistorted ML minus actual reflectivity in presence of buried defect, as a function of position along the mask blank  110  around the underlying bump peaks on one side of the bump and then drops off. 
         [0059]      FIG. 7A  shows the reflectance of the multilayer when the angle of illumination has been changed such as by the rotation of the mask blank  110  by 180°. 
         [0060]    As illustrated in  FIG. 7B , the rotation of the mask blank  110  relative to the beam  220  changes the spatial location of the reflectance peak that arises due to the distortion precipitated by the bump defect  122 . 
         [0061]    As a result, the controller  250  by comparing the reflectance images taken from the different rotation angles of the mask blank can analyze the bump and/or pit defects, even when the bump/pit defects do not result in any surface changes, or only small surface changes, in the multilayer  118 . 
         [0062]      FIGS. 8A and 8B  are schematic diagrams of an EUV reflective mask blank inspection system  200  according to another embodiment of the present invention. In this embodiment the chief ray angle (CRA) angle of the mask blank  110  is changed between successive reflectance images. 
         [0063]    As in the previous embodiment, the system  200  generally includes the x-ray source  210  for illuminating the lithography mask blank  110 , along with possibly an energy filter  218  and a condenser system  214 , if required. 
         [0064]    In this embodiment, the mask blank  110  is held an angular positioning stage  226 A which is able to adjust the CRA angle between the mask normal axis  225  and optical axis A of the incoming radiation beam  220 . This functionality is illustrated between  FIGS. 8A and 8B  in which after a first reflectance image is captured with at a first CRA angle, the angular positioning stage  226 A changes the tilt axis of the mask blank  110  relative to the beam  220  to increase the CRA angle, in the illustrated example. 
         [0065]    In the example, the angular positioning stage  226 A is controlled by the system controller  250  to locate a region of interest of the surface of the mask blank  110  in the beam  220  by moving the mask blank in the plane of its surface. Then, a first reflectance image of the mask blank  110  is acquired with the orientation shown in  FIG. 9A . Then the mask blank  110  is tilted to a new angle and a second reflectance image is acquired of the same region of interest as shown in  FIG. 9B . 
         [0066]    Returning to  FIGS. 8A and 8B , the reflectance images are acquired when the x-ray beam  220  is reflected by the mask blank  110  and then detected by the detector system  280 . In the illustrated example, the detector system  280  position will also be changed between the capture of the two reflectance images due to the beam displacement arising from the change in the CRA angle. 
         [0067]    In operation, the controller  250  acquires reflectance images of the same region of the mask blank  110  but at different CRA angles of the mask blank  110  to the normal axis  225 . 
         [0068]      FIGS. 10A, 10B, and 10C  illustrate the operation of an alternative embodiment in which the CRA angle is also changed between successive reflectance images. 
         [0069]    Here, the CRA angle is changed by configuring the beam from the source  210  to be converging but then illuminating the mask blank  110  through an adjustable aperture  312 . 
         [0070]    In more detail, as illustrated in  FIG. 10  A, a first reflectance image is captured with the aperture  312  of the aperture plate  310  in a leftmost position. As result, the converging beam that passes through the aperture  312  forms a first, high CRA angle with respect to the substrate  110 . Then, the aperture plate  310  is adjusted to move the aperture  312  to an intermediate position, as shown in  FIG. 10B . As result, the beam  220  that strikes the substrate  110  now has a reduced CRA angle. Finally, as illustrated in  FIG. 10C , by further adjusting the aperture plate  310 , the aperture  312  is moved to still a third position, with a further reduced CRA angle. In this way, three different reflectance images of the substrate  110  and any ML defect  112 ,  120  can be acquired at different CRA angles. 
         [0071]    A number of different approaches can be used to process the reflectance images of the ML defects  112 ,  120  at different rotation angles of the mask blank  110  around axis  225  or different CRA angles. In one example, stereo-photogrammetry algorithms (triangulation) are utilized by the controller  250  using the parallax displacement which occurs by viewing a defect along two different lines of sight. Other examples provide analysis along more than two projection directions and potentially also the through-focus behaviour of the buried defect. Additionally, reconstruction algorithms for computed tomography, such as Algebraic Reconstruction Technique ART and/or Filtered Back Projection are used location and characterize the ML defects. 
         [0072]    As discussed above, in one example, ML defects are analyzed based on spatial changes in the reflectance of the ML  118  around the defects. Depending on the parameters of the multilayer, angle of illumination or chief ray angle (CRA), wavelength of the beam  220 , and the distortion of the multilayer ML by the defects, the defects will present as localized increase and/or decreases in reflectance. As a result, the selected wavelength of the beam  220  and the angle (CRA) of the beam  220  to the mask blank  110  must be selected to achieve constructive Bragg reflection: 
         [0000]        nλ= 2 d  sin θ,
 
         [0073]    where 
         [0074]    d bilayer thickness (EUV mask: d≈4.1(Si)+2.8(Mo)=6.9 nm) 
         [0075]    θ angle between the incident ray and the object. 
         [0076]    The ML stacks of the masks are designed for EUV imaging (i.e. AIMS EUV). In a typically configuration, the operational chief ray angle (CRA)˜6°. CRA=90°−θ Further, the operational wavelength ML is λ=13.5 nm. 
         [0077]    In embodiments of the present invention, these same parameters could be used to analyze buried defects. A drawback, however is that the λ=13.5 nm wavelength requires a vacuum. 
         [0078]    In the preferred embodiments, a higher CRA angle or range of CRA angles is used, i.e., CRA&gt;6°. In one implementation, CRA is higher than 15 degrees and is preferably between 30-45°. Further, short wavelength radiation is used for the illumination beam  220 . This short wavelength radiation has a wavelength of about 5 nm or shorter and is preferably shorter than 1 nm (soft x-rays). 
         [0079]    For the given EUV mask blank geometry λ is a function of CRA is: 
         [0000]    
       
         
           
             λ 
             = 
             
               2 
                
               
                 d 
                 n 
               
                
               cos 
                
               
                   
               
                
               CRA 
             
           
         
       
     
         [0080]    Applying standard multilayer geometries used in EUV lithography, a number of possible wavelengths could be used as the illumination beam  220 . In one example, target  212  of the x-ray source  210  is aluminum and the Kα characteristic line of the emission is selected. In another example, the target  212  is chromium (Cr) and its Kα is again used. Finally, still another example is a copper target  212 . The different target options, the corresponding wavelengths, and the CRA angle for reflectivity are presented below: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 λ 
                 keV 
                 n 
                 CRA 
               
               
                   
                   
               
             
             
               
                   
                 0.84 nm 
                 1.49 (Al Kα) 
                 12 
                 43.4° 
               
               
                   
                 0.23 nm 
                 5.41 (Cr Kα) 
                 43 
                 44.4° 
               
               
                   
                 0.15 nm 
                 8.05 (Cu Kα) 
                 77 
                 30.7° 
               
               
                   
                   
               
             
          
         
       
     
         [0081]    Each of these wavelengths and CRA angles will result is reflection of the incoming beam  220  by the multilayer  118 . 
         [0082]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.