Abstract:
A visible light method for detecting sub-100 nm size defects on mask blanks used for lithography. By using optical heterodyne techniques, detection of the scattered light can be significantly enhanced as compared to standard intensity detection methods. The invention is useful in the inspection of super-polished surfaces for isolated surface defects or particulate contamination and in the inspection of lithographic mask or reticle blanks for surface defects or bulk defects or for surface particulate contamination.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to surface particle detection, and more specifically, it relates to the detection of defects on lithographic mask blanks. 
     2. Description of Related Art 
     The proposed 1999 SIA Technology Roadmap for Semiconductors is accelerating the reduction in dense line critical dimensions (CDs) to 23 nm by the year 2011. This will put a tremendous burden on mask fabrication, particularly in the area of defect detection and reduction. Mask defects as small as one-eighth the equivalent CD are printable and may cause chip failure. Table 1 shows the maximum permissible defect size for each lithography generation out to the year 2011. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Year of first 
                 1999 
                 2002 
                 2005 
                 2008 
                 2011 
                 2014 
               
               
                 shipment 
               
               
                 Generation (nm) 
                  180 
                  130 
                  100 
                  70 
                  50 
                  35 
               
               
                 Maximum mask 
                  90 
                  65 
                  50 
                  35 
                  25 
                  18 
               
               
                 defect size (nm) 
               
               
                   
               
               
                 (assuming a lithography tool magnification of 0.25)  
               
             
          
         
       
     
     A new infrastructure for mask inspection will be required to keep pace with this aggressive roadmap. Depending on the specific lithography used for a particular generation, mask inspection specifics may change, but the methodology will essentially remain the same. Mask blanks will have to undergo 100% area inspection for defects larger than a maximum acceptable size. Since masks are becoming a significant cost factor in the cost of ownership of lithography tools, this is a critical step—patterning defective mask blanks would be an economic disaster. 
     Inspecting mask blanks can be approached differently than patterned masks. Inspection does not necessarily have to be done at-wavelength since defects at the mask blank level will interact with visible light. Techniques using visible light are appealing because they are familiar to the user, relatively straightforward to manufacture and, if designed properly, extendable over many generations. 
     Current wafer inspection tools could play this role if silicon wafers are used as the mask blanks, but this is unlikely due to unfavorable thermal properties. Even then, detection of defects smaller than 100 nm has not been demonstrated with these tools. Wafer inspection tools operate by measuring the intensity of the light scattered by a surface defect. FIG. 1 shows a typical optical system used for commercial wafer inspection. However, scatter decreases as the sixth power of the defect size, so as the critical defect size decreases, defects become extremely difficult to detect, i.e., as the defect size decreases, the scatter decreases by the fractional decrease in defect size to the 6 th  power. Additionally, this scattered intensity inspection technique cannot distinguish surface defects from internal defects in transparent substrates such as ULE, a prime candidate for future mask blanks. As shown in FIG. 1, incident light beam  10  having S and P polarizations, is directed onto a mask blank  12  at the site of a defect  14 . Scattered light  16 , scattered from defect  14  is collected with collector mirrors  18  and  19  and directed to detector  20 . This technique cannot distinguish whether the defect is located on the surface or within the transparent substrate. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide new techniques for detecting a surface particle on a lithographic mask blank. 
     Defect detection, based on optical scattering, is a viable approach when there is sufficient signal-to-noise. Given a fixed amount of energy for illuminating the surface of the mask blank, typical instruments direct all of this light onto a given location on the mask blank and collect as much scattered light as possible (FIG.  1 ). If the fractional amplitude of the scattered light in a particular direction is 1/s, the intensity is proportional to 1/s 2 . Since the detected intensity falls off as the sixth power of the defect size, defects much smaller than the wavelength scatter very little light. For example, a 10 nm defect illuminated with a 1 mw beam focussed to a 10 um spot will scatter only 10 −12  mw into a 0.1 numerical aperture. 
     The detected signal can be significantly amplified by making the following modifications: 
     (a) use part of the energy to illuminate the mask blank to scatter from a defect and use the remaining energy as a probe beam to coherently interfere with the scattered light (FIG.  5 ); 
     (b) frequency shift the probe beam (˜10-1000 MHz) so that heterodyne detection can be used (FIG.  6 ); and 
     (c) the incident angle and polarization of the illumination and probe beams are chosen to maximize the particle scatter and to minimize the noise from the background scatter in the direction of the specularly reflected probe beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an optical system used in commercial wafer inspection tools, based on measurement of scattered intensity. 
     FIG. 2 shows the log angular scatter distribution from a 100 nm molybdenum particle. 
     FIG. 3 shows the log angular scatter distribution from a 0.5 nm rms rough glass substrate 
     FIG. 4 shows the log signal-to-noise and the detector position. 
     FIG. 5 shows an embodiment of the present invention that uses a probe beam to enhance scattered light from defects. 
     FIG. 6 shows a block type diagram that uses heterodyne detection to detect the presence of scattered light. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, the detected scattered light signal from a particle on a lithographic mask blank can be significantly amplified by (i) using a portion of the laser energy to illuminate the mask blank to scatter from a defect and using another portion of the laser energy as a probe beam to coherently interfere with the scattered light, (ii) frequency shifting the probe beam (˜10-1000 MHz) so that heterodyne detection can be used; and (iii) choosing the incident angle and polarization of the illumination and probe beams to maximize the particle scatter and to minimize the noise from the background scatter in the direction of the specularly reflected probe beam. 
     Accordingly, the detected signal is proportional to the amplitude, 1/s, of the scattered light, a factor of s higher than conventional configurations. Using the same numerical values as above, the detected signal is now six orders of magnitude greater. 
     Equally important is the noise rejection of the proposed method. In conventional instruments, the signal-to-noise for small defects is limited by the constant background scatter from the inherent surface roughness of the mask blank and ambient light scattered from optics and air within the instrument. In the proposed method, heterodyne detection eliminates the contribution of all scattered light from outside the coherent probe volume, thereby minimizing background noise. This permits the surface inspection of transparent substrates that produce back surface and bulk scatter, such as ULE glass. 
     The exact geometry of the illumination and probe beams is chosen to maximize the signal-to-noise. The angle of incidence and polarization of the illumination beam and the angle and polarization of detection of the scattered light is chosen to maximize the detected scattered light from the particle and to minimize the detected scattered light from the substrate due to surface roughness. These angles are chosen based on electromagnetic simulations for particles of the size and refractive index that are expected on the surface of the substrate. The incident angle and polarization of the probe is then chosen so that the specularly reflected beam matches the angle and polarization of the detected scattered light. 
     In general the angular distribution of scattered light from a particle resting on a surface depends on the particle size, shape, and refractive index, and on the illumination beam incidence angle, wavelength and polarization. To model this angular distribution, particle size, shape and refractive index are defined by the specific application and the wavelength is chosen from a list of commercially available lasers. Angular distributions of the scattered light are then calculated for various angles of incidence and both S and P polarizations. These angular distributions show where the detector should be placed for maximum signal. 
     Likewise, the similar calculations are performed under the same conditions for light reflected from the non-ideal substrate that supports the particle. Microscopic surface imperfections (commonly called roughness), present on all surfaces, produce an angular distribution of scattered light. These angular distributions show where the detector should be placed for minimum noise. 
     In general the two detector positions do not coincide. However, the signal-to-noise can be optimized by taking the ratio of the two angular distributions to determine the optimal angular position to place the detector. 
     This method can be extended from defect detection to defect identification by the use of multiple probe beams. Since the angular distribution of scattered light is a signature of the particle, being a function of particle size, shape and refractive index, measurement of the angular distributions can be used to determine characteristics of the particle. The angular distribution is measured at different angular locations by using multiple probe beams, with slightly different frequencies to prevent electronic crosstalk. 
     The angular distributions showing the detector placements for maximum signal and minimum noise are calculated. Electromagnetic simulation codes are used to perform these calculations. The exact geometry of the illumination and probe beams will be chosen after careful modeling of the scatter from particles of different size and refractive index and from inherent surface roughness. These electromagnetic simulation codes are known in the art. One such code is known as TSAR (developed at Lawrence Livermore National Laboratory). A description of TSAR is found in “Using the TSAR electromagnetic modeling system” UCRL-ID 115227, September 1993, incorporated herein by reference and published by the University of California at Lawrence Livermore National Laboratory. Other commercially available codes include EMFLEX (Weidlinger Associates), Maxwell EMINENCE (Ansoft Corp.), MAFIA (CST) and DDSURF (ASU). A description of DDSURF is shown in a document titled “DDSURF/RDSURF User&#39;s Manual” ©1998 Arizona State University, incorporated herein by reference. The referenced document was authored by Brent M. Nedeker and E. Dan Hirleman, both of the Laser Diagnostics Laboratory, Arizona State University. 
     The computer codes, such as TSAR, that calculate the electromagnetic field scattered by a particle resting on a surface (e.g., a flat surface) and illuminated by an incident electromagnetic field are based on the coupled dipole approximation method. The dipoles are placed in a lattice configuration that models the physical geometry and material properties of the scattering particle. The response of each dipole is related to the incident electromagnetic field so that the dipole moment distribution can be calculated within the scattering particle. Knowing this distribution, the external scattered field can then be calculated in any direction from the scattering particle. To perform this calculation, a particle size, shape and refractive index are specified; the incident electromagnetic wavelength, polarization and incident angle are specified; and the refractive index of the flat substrate is specified. 
     FIG. 2 shows the log angular scatter distribution from a 100 nm molybdenum particle. FIG. 3 shows the log angular scatter distribution from a 0.5 nm rms rough glass substrate. FIG. 4 shows the log signal-to-noise and the detector position. 
     FIG. 5 shows an embodiment of the present invention. A laser produces a beam which is split into an incident beam and a probe beam. The incident beam and the probe beam are each passed through a separate Bragg cell which is configured to produce a frequency difference between the incident and probe beam. As shown in the figure, incident beam  30 , comprising S and/or P polarized light, is focused by lens  32  onto mask blank  36  (comprising, e.g., ULE). Specularly reflected light  38  propagates out of the system. Defect  34 , if present on mask blank  36 , produces scattered light which is maximized at certain location(s). Mask blank  36  produces scattered light (background noise) which is minimized at certain locations. An electromagnetic simulation code is used to determine the position of maximum defect scatter and minimum background noise. Detector  42  is placed at the location of optimum signal-to-noise. Probe beam  40 , comprising S and/or P polarized light, is focused by lens  39  onto mask blank  36  at the same location as beam  30 . The angle of incidence is chosen so that the specularly reflected beam  41  is incident on detector  42 , interfering with scattered light  43  from defect  34 . The signal from detector  42  is sent through a synchronous detection module  44  and is then sent to a data acquisition system  46 . 
     Another embodiment of the invention is shown in FIG.  6 . In this embodiment, light beam  62  from a single laser  60  (e.g., an argon ion laser) is split into two beams by beamsplitter  64 . The portion of light beam  62  that reflects from beamsplitter  64  is beam  66 . The portion of light beam  62  that is transmitted through beamsplitter  64  is beam  68 . Beam  66  passes through acousto-optic (AO) frequency shifter  70 , which shifts the frequency of beam  66 . In the embodiment shown, AO frequency shifter  70  shifts the frequency of beam  66  by 61 MHz. Beam  68  reflects from mirror  72  and passes through acousto-optic (AO) frequency shifter  74 , which shifts the frequency of beam  68 . In the embodiment shown, AO frequency shifter  74  shifts the frequency of beam  68  by 81 MHz. Thus, a 20 MHz frequency difference is generated between beam  66  and beam  68  to enable heterodyne detection. One of these two frequency shifted beams is used as the incident beam and the other beam is used as the probe beam. 
     A reference signal from each AO frequency shifter is sent to a mixer  76  the output of which is sent to a mixer/integrator  78 . In a manner similar to the embodiment shown in FIG. 5, one of the output beams from the AO frequency shifter is used as an incident beam onto a defect or particle on mask blank  80  and the other output beam from the other AO frequency shifter is used as a probe beam onto mask blank  80 . Detector  82  is placed at the location of maximum signal-to-noise and the output from detector  82  is integrated in mixer/integrator  78 . The mixer/integrator output  78  is sent to a computer  84  for data acquisition. The embodiment may include input  86  from a computer (e.g., computer  78 ) to scanning stages connected to mask blank  80 , as are known in the art. 
     To enhance detection performance, the incident angle and polarization are chosen to maximize the particle scatter and to minimize the noise from the background scatter. 
     To further enhance detection performance, the invention exploits the angular and polarization dependence of the scattered fields. Scattering from a defect will generally have a unique polarization and angular distribution signature depending on the defect geometry and refractive index. To take advantage of this fact, the surface is illuminated with both s and p polarizations and the corresponding scattered components are measured with multiple probe beams. These measurements allow us to better identify the specific defect. 
     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.