Abstract:
A method and apparatus for inspecting patterned substrates, such as photomasks, for unwanted particles and features occurring on the transmissive as well as pattern defects. A transmissive substrate is illuminated by a laser through an optical system comprised of a laser scanning system, individual transmitted and reflected light collection optics and detectors collect and generate signals representative of the light transmitted and reflected by the substrate. The defect identification of the substrate is performed using transmitted and reflected light signals from a baseline comparison between two specimens, or one specimen and a database representation, to form a calibration pixelated training set including a non-defective region. This calibration pixilated training set is compared to a transmitted-reflected plot map of the subject specimen to assess surface quality.

Description:
This application is a continuation of U.S. patent application Ser. No. 11/046,493, filed Jan. 28, 2005, now U.S. Pat. No. 7,046,355, which is a continuation of U.S. patent application Ser. No. 10/191,765, filed Jul. 9, 2002, now U.S. Pat. No. 6,850,321, entitled, “Dual Stage Defect Region Identification and Defect Detection Method and Apparatus,” both of which are hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to methods and apparatus for inspecting the surface of a substrate such as reticles, photomasks, wafers and the like (hereafter referred to generally as photomasks). More specifically, the present invention relates to an optical inspection system and method for scanning specimens at a high speed and with a high degree of sensitivity. 
   2. Description of the Related Art 
   Integrated circuits are produced using photolithographic processes, which employ photomasks or reticles and a light source to project a circuit image onto a silicon wafer. Photomask surface defects are highly undesirable and adversely affect the resulting circuits. Defects can result from, but not limited to, a portion of the pattern being absent from an area where it is intended to be present, a portion of the pattern being present in an area where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or pattern layer. Since it is inevitable that defects will occur, these defects are preferably located and repaired prior to use. Blank substrates can also be inspected for defects prior to patterning. 
   Methods and apparatus for detecting defects have been available. For example, inspection systems and methods utilizing laser light are available to scan the surface of substrates such as photomasks, reticles and wafers. These laser inspection systems and methods generally include a laser source for emitting a laser beam, optics for focusing the laser beam to a scanning spot on the surface of the substrate, a stage for providing translational travel, collection optics for collecting either transmitted and/or reflected light, detectors for detecting either the transmitted and/or reflected light, sampling the signals at precise intervals and using this information to construct a virtual image of the substrate being inspected. By way of example, representative laser inspection systems are described in U.S. Pat. No. 5,563,702 to Emery et al., U.S. Pat. No. 5,737,072 to Emery et al., U.S. Pat. No. 5,572,598 to Wihl et al., and U.S. Pat. No. 6,052,478 to Wihl et al., each of which are incorporated herein by reference. 
   Although such systems work well, ongoing work in the area seeks to improve existing designs to enable higher degrees of sensitivity, increase the ability to classify and quantify defects, and to allow faster scanning speeds and higher throughput. As the complexity of integrated circuits has increased, the demands on the inspection of the integrated circuits have also increased. Both the need for resolving smaller defects and for inspecting larger areas have resulted in greater magnification requirements and greater speed requirements. 
   Various methods exist to perform detailed inspections of patterned masks or reticles. One inspection method is a die-to-die comparison which uses transmitted light to compare either two adjacent dies or a die to the CAD database of that die. These comparison-type inspection systems are quite expensive because they rely on pixel-by-pixel comparison of all the dies and, by necessity, rely on highly accurate methods of alignment between the two dies used at any one time for the comparison. Apart from their high costs, this method of inspection is also unable to detect particles on opaque parts of the reticle which have the tendency to subsequently migrate to parts that are transparent and then cause a defect on the wafer. One such die-to-die comparison method of inspection is described in U.S. Pat Nos. 4,247,203 and 4,579,455, both by Levy et al. 
   The second method for inspecting patterned masks is restricted to locating particulate matter on the mask. This method makes use of the fact that light scatters when it strikes a particle. Unfortunately, the edges of the pattern also cause scattering and for that reason these systems are unreliable for the detection of particles smaller than one micrometer. Such systems are described in a paper entitled “Automatic Inspection of Contaminates on Reticles” by Masataka Shiba et al.,  SPIE Vol.  470  Optical Microlithography III , pages 233-240 (1984). 
   A third example of a system for performing photomask inspection is disclosed in U.S. Pat. No. 5,563,702 to David G. Emery, issued Oct. 8, 1996. The system disclosed therein acquires reflected images, in addition to transmitted images, to locate defects associated with contaminants, particles, films, or other unwanted materials. Since this system locates defects without reference or comparison to a description or image of the desired photomask pattern, it may in certain circumstances not locate defects outside of known boundaries, such as where the transmitted and reflected images differ from an expected amount by a threshold amount. 
   Specific types of photomasks called APSMs, or Alternating Phase Shift Masks, are typically designed with thickness variations in the glass or quartz which induce phase shift transitions between adjacent regions during photolithography. Phase defects can exist which are unwanted thickness variations created by phase etch process errors, and have similar optical image signatures during inspection. Hence APSM phase defects are difficult to distinguish from design phase features using the system shown in U.S. Pat. No. 5,563,702. Phase defects cannot be detected by this system without producing false defect readings on phase shift design features where no defects actually exist. However, the transmitted and reflected imaging capabilities and defect detection operators of this system can be useful to determine the presence of phase defects if all detected phase features are properly compared and contrasted to reference photomask image data, as in a die-to-die or die-to-database system. 
   The use of reflected light in combination with transmitted light may improve detection of phase defects. The difficulty with using reflected light is managing image artifacts, such as the bright chrome halos resulting from the removal of the antireflective chrome layer during quartz etching of phase shifters. Bright chrome halos may have variable widths resulting from second write level registration tolerances with intra-plate variations. These variations are not observable when solely using transmitted light inspection techniques. 
   Thus, in general, phase feature signals captured with brightfield transmitted light may vary widely depending on mask and defect characteristics, and phase feature signals captured in reflected light may be stronger. On the other hand, use of reflected light can be problematic in the presence of image artifacts such as bright chrome halos. Therefore, die-to-die or die-to-database photomask inspection with transmitted and reflected light may benefit from signal-to-noise enhancements as well as an enhanced ability to discern phase shift features and phase defects. 
   During inspection, different light sources can drastically affect the quality of the information received in the presence of certain types of defects. The phase defect signal in brightfield transmitted light can be substantially less than that of a similarly sized chrome defect, thereby complicating the ability to inspect the mask. The phase defect&#39;s signal depends on a variety of factors, including the height or phase angle of the phase defect, the depth of the phase shifters, and inspection system optical parameters. 
   An alternate method employed for defect detection is an extension of the die-to-database comparison method, wherein the system compares two die with an identical design. The images employed may be either reflected or transmitted light energy images. One of the two die is called the reference die, and the other is called the test die. In this method, if the difference between the pixels of the reference die and the test die exceed a predetermined value, the pixel is marked defective. The transmitted threshold may be called δT, while the reflected threshold is δR. Performance of a defect detection algorithm may be viewed as a ΔT−ΔR plane as illustrated in  FIG. 3 . From  FIG. 3 , ΔT represents the transmitted image difference between the test and reference dies, while ΔR is the reflected image difference between the test and reference dies. Previous methods have performed transmitted image detection independent from reflected image detection, where a defective zone is separated from a non-defective zone using constant thresholds as shown in  FIG. 3 . These previous methods employed constant thresholds for determining and quantifying defects. Non-defective zones used in the presence of constant thresholds have been found to be excessively large, resulting in declaring defects when no true defects exist, requiring additional post processing. A smaller non-defective zone is preferable, as it translates to higher defect detectability. In other words, if the non-defective zone of  FIG. 3  is small, a higher probability that a defect will be located exists. However, decreasing the non-defective zone excessively may result in too many anomalies being classified as defects. In actuality, many errors between dies exist, including errors between test and reference dies. If the threshold is too small, a large number of false or nuisance defects will develop, taking a great deal of time to inspect, decreasing overall throughput. 
   One type of defect identification system and method is presented in pending U.S. patent application Ser. No. 09/991,327, entitled “Advanced Phase Shift Inspection Method” to David G. Emery, filed on Nov. 9, 2001 and assigned to the assignee of the present invention. While the system disclosed therein provides for inspection of APSMs using transmitted and reflected light, certain defects may exist that may not be picked up by the Emery design. Further, the Emery design may require inspection of areas that are flagged as defective or potentially defective, potentially decreasing overall speed and throughput. 
   In view of the foregoing, there is a need for improved inspection techniques that provide for improved detectability while at the same time avoiding excess false and nuisance detections when scanning specific types of semiconductor specimens, such as APSM photomasks. 
   SUMMARY OF THE INVENTION 
   The present invention provides improved apparatus and methods for performing an inspection of a specimen, such as an APSM photomask. 
   In one aspect of the present invention, there is provided a method for inspecting a specimen, comprising performing a transmitted energy calibration inspection and a reflected energy calibration inspection of a plurality of calibration specimens, computing a training set based on the transmitted energy calibration inspection and reflected energy calibration inspection of the plurality of calibration specimens, performing a transmitted energy defect detection inspection and a reflected energy defect detection inspection of a plurality of specimens, computing a plot map based on the transmitted energy defect detection inspection and the reflected energy defect detection inspection of the plurality of specimens, and comparing the plot map to the training set to determine differences therebetween. 
   According to a second aspect of the present invention, there is provided a method for inspecting a specimen, comprising performing a transmitted energy calibration inspection and a reflected energy calibration inspection of a calibration specimen, computing a training set based on the transmitted energy inspection and reflected energy inspection of the calibration specimen and transmitted energy and reflected energy from a baseline specimen, performing a transmitted energy defect detection inspection and a reflected energy defect detection inspection of the specimen, computing a plot map based on the transmitted energy defect detection inspection and the reflected energy defect detection inspection of the specimen and transmitted energy and reflected energy from an alternate baseline specimen, and comparing the plot map to the training set to determine differences therebetween. 
   According to a third aspect of the present invention, there is provided a system for inspecting a specimen, comprising a training set table generator that generates a combined calibration transmitted energy and calibration reflected energy difference representation, and a defect detector that receives the combined calibration transmitted energy and calibration reflected energy difference representation and a transmitted and reflected energy representation of the specimen and determines the differences therebetween. 
   According to a fourth aspect of the present invention, there is provided a method for inspecting a specimen, comprising computing a training set based on a transmitted energy calibration inspection and reflected energy calibration inspection of at least one calibration specimen, computing a plot map based on a transmitted energy defect detection inspection and a reflected energy defect detection inspection of the specimen, and comparing the plot map to the training set to determine differences therebetween. 
   These and other objects and advantages of all of the aspects of the present invention will become apparent to those skilled in the art after having read the following detailed disclosure of the preferred embodiments illustrated in the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which: 
       FIG. 1  is a simplified block diagram of an optical inspection system in accordance with one aspect of the present invention; 
       FIG. 2  is a detailed block diagram of an optical inspection system for inspecting the surface of a substrate, in accordance with one aspect of the present invention; 
       FIG. 3  illustrates a transmitted-reflected (ΔT−ΔR) plane including a constant threshold non-defective zone; 
       FIG. 4  represents modifications of the non-defective threshold; 
       FIG. 5  is an expected ΔT−ΔR scan representation of a pair of images; 
       FIG. 6  represents the calibration performed by the present detection system using both transmitted and reflected images to produce a detection training set; 
       FIG. 7  illustrates a representative ΔT−ΔR plot as may be generated by the plot map generator; 
       FIG. 8  shows a core ΔT−ΔR training set as may be generated by the core training set generator; 
       FIG. 9  presents an enhanced TR training set as may be generated by the detection training set generator; and 
       FIG. 10  illustrates the defect detection processor of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will now be described in detail with reference to a few specific embodiments thereof and as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order not to unnecessarily obscure the present invention. 
     FIG. 1  is a simplified block diagram of an optical inspection system  10 , in accordance with one aspect of the present invention. The optical inspection system  10  is arranged for inspecting a surface  11  of a substrate  12 . The dimensions of various components are exaggerated to better illustrate the optical components of this embodiment. As shown, the optical inspection system  10  includes an optical assembly  14 , a stage  16 , and a control system  17 . The optical assembly  14  generally includes at least a first optical arrangement  22  and a second optical arrangement  24 . In general terms, the first optical arrangement  22  generates two or more beams incident on the substrate, and the second optical arrangement  24  detects two or more beams emanating from the sample as a result of the two or more incident beams. The first and second optical arrangement may be arranged in suitable manner in relation to each other. For example, the second optical arrangement  24  and the first optical arrangement  22  may both be arranged over the substrate surface  11  so that reflected beams resulting from incident beams generated by the first optical arrangement  22  may be detected by the second optical arrangement  24 . 
   In the illustrated system, the first optical arrangement  22  is arranged for generating a plurality of scanning spots (not shown) along an optical axis  20 . As should be appreciated, the scanning spots are used to scan the surface  11  of the substrate  12 . On the other hand, the second optical arrangement  24  is arranged for collecting transmitted and/or reflected light that is produced by moving the scanning spots across the surface  11  of the substrate  12 . 
   To elaborate further, the first optical arrangement  22  includes at least a light source  26  for emitting a light beam  34  and a first set of optical elements  28 . The first set of optical elements  28  may be arranged to provide one or more optical capabilities including, but not limited to, separating the light beam  34  into a plurality of incident light beams  36 , directing the plurality of incident light beams  36  to intersect with the surface  11  of the substrate  12 , and focusing the plurality of incident light beams  36  to a plurality of scanning spots (not shown in  FIG. 1 ) on the surface  11  of the substrate  12 . The amount of first beams produced generally corresponds to the desired inspection speed. In one embodiment, the optical elements are arranged to separate the beam  34  into three incident light beams  36 . By triplicating the beam, a wider scan is produced and therefore the resulting inspection speed is about three times faster than the speed produced for a non-triplicated single beam. Although only three light beams are shown, it should be understood that the number of separated light beams may vary according to the specific needs of each optical inspection system. For example, two beams may be used or four or more beams may be used. It should be noted, however, that the complexity of the optic elements is directly proportional to the number of beams produced. 
   Furthermore, the second optical arrangement  24  includes at least a second set of optical elements  30  and a light detecting arrangement  32 . The second set of optical elements  30  are in the path of a plurality of collected light beams  40 , which are formed after the plurality of incident light beams  36  intersect with the surface  11  of the substrate  12 . The plurality of collected light beams  40  may result from transmitted light that passes through the substrate  12  and/or reflected light that is reflected off the surface  11  of the substrate  12 . The second set of optical elements  30  are adapted for collecting the plurality of collected light beams  40  and for focusing the collected light beams  40  on the light detecting arrangement  32 . The light detecting arrangement  32  is arranged for detecting the light intensity of the collected light beams  40 , and more particularly for detecting changes in the intensity of light caused by the intersection of the plurality of incident light beams with the substrate. The light detecting arrangement  32  generally includes individual light detectors  42  that correspond to each of the second light beams  40 . Furthermore, each of the detectors  42  is arranged for detecting the light intensity and for generating signals based on the detected light. 
   With regards to the stage  16 , the stage  16  is arranged for moving the substrate  12  within a single plane (e.g., x &amp; y directions) and relative to the optical axis  20 , so that all or any selected part of the substrate surface  11  may be inspected by the scanning spots. In most embodiments, the stage  16  is arranged to move in a serpentine fashion. With regards to the control system  17 , the control system  17  generally includes a control computer  18  and an electronic subsystem  19 . Although not shown, the control system  17  may also include a keyboard for accepting operator inputs, a monitor for providing visual displays of the inspected substrate (e.g., defects), a database for storing reference information, and a recorder for recording the location of defects. As shown, the control computer  18  is coupled to the electronic subsystem  19  and the electronic subsystem  19  is coupled to various components of the optical inspection system  10 , and more particularly to the stage  16  and the optical assembly  14  including the first optical arrangement  22  and the second optical arrangement  24 . The control computer  18  may be arranged to act as an operator console and master controller of the system. That is, all system interfaces with an operator and the user&#39;s facilities may be made through the control computer  18 . Commands may be issued to and status may be monitored from all other subsystems so as to facilitate completion of operator assigned tasks. 
   On the other hand, the electronics subsystem  19  may also be configured to interpret and execute the commands issued by control computer  18 . The configuration may include capabilities for, but not limited to, digitizing the input from detectors, compensating these readings for variations in the incident light intensity, constructing a virtual image of the substrate surface based on the detected signals, detecting defects in the image and transferring the defect data to the control computer  18 , accumulating the output of the interferometers used to track the stage  16 , providing the drive for linear motors that move the stage  16  or components of the optical assembly  14 , and monitoring sensors which indicate status. Control systems and stages are well know in the art and for the sake of brevity will not be discussed in greater detail. By way of example, a representative stage, as well as a representative controller may be found in U.S. Pat. No. 5,563,702, which is herein incorporated by reference. It should be understood, however, that this is not a limitation and that other suitable stages and control systems may be used. 
   As should be appreciated, the optical inspection system  10  can be arranged to perform several types of inspection, for example, transmitted light inspection, reflected light inspection and simultaneous reflected and transmitted inspection. In transmitted light inspection, light is incident on the substrate, a photomask for example, and the amount of light transmitted through the mask is detected. In reflected light inspection, the light reflecting from a surface of the substrate under test is measured. In addition to these defect detection operations, the system is also capable of performing line width measurement. 
   In most of the defect detection operations a comparison is made between two images. By way of example, the comparison may be implemented by the electronic subsystem  19  of  FIG. 1 . Broadly speaking, the detectors  42  generate scan signals, which are based on the measured light intensity, and send the scan signals to the electronic subsystem  19 . The electronic subsystem  19 , after receiving the scan signals, correspondingly compares the scan signals with reference signals, which are either stored in a database or determined in a current or previous scan. 
   More specifically, in die-to-die inspection mode, two areas of the substrate having identical features (dice) are compared with respect to each other and any substantial discrepancy is flagged as a defect. In the die-to-database inspection mode, a defect is detected by comparing the die under test with corresponding graphics information obtained from a computer aided database system from which the die was derived. In other defect detection operations, a comparison is made between two different modes of inspection. For example, in simultaneous reflected and transmitted inspection, a comparison is made between the light that is reflected off the surface of the substrate and light that is transmitted through the substrate. In this type of inspection the optical inspection system performs all of the inspection tasks using only the substrate to be inspected. That is, no comparisons are made between an adjacent die or a database. 
     FIG. 2  is a detailed block diagram of an optical assembly  50  for inspecting the surface  11  of the substrate  12 , in accordance with one embodiment of the present invention. By way of example, the optical assembly  50  may be the optical assembly  14  as described in  FIG. 1 . The optical assembly  50  generally includes a first optical arrangement  51  and a second optical arrangement  57 , both of which may respectively correspond to the first optical arrangement  22  and the second optical arrangement  24  of  FIG. 1 . As shown, the first optical arrangement  51  includes at least a light source  52 , inspection optics  54 , and reference optics  56 , while the second optical arrangement  57  includes at least transmitted light optics  58 , transmitted light detectors  60 , reflected light optics  62 , and reflected light detectors  64 . 
   The light source  52  is arranged for emitting a light beam  66  along a first path  68 . The light beam  66  emitted by the light source  52 , first passes through an acousto optic device  70 , which is arranged for deflecting and focusing the light beam. Although not shown, the acousto optic device  70  may include a pair of acousto-optic elements, which may be an acousto-optic prescanner and an acousto-optic scanner. These two elements deflect the light beam in the Y-direction and focus it in the Z-direction. By way of example, most acousto-optic devices operate by sending an RF signal to quartz or a crystal such as TeO 2 . The signal causes a sound wave to travel through the crystal. Because of the traveling sound wave, the crystal becomes asymmetric, which causes the index of refraction to change throughout the crystal. This change causes incident beams to form a focused traveling spot which is deflected in an oscillatory fashion. 
   When the light beam  66  emerges from the acousto-optic device  70 , it then passes through a pair of quarter wave plates  72  and a relay lens  74 . The relay lens  74  is arranged to collimate the light beam  66 . The collimated light beam  66  then continues on its path until it reaches a diffraction grating  76 . The diffraction grating  76  is arranged for flaring out the light beam  66 , and more particularly for separating the light beam  66  into three distinct beams, which are designated  78 A,  78 B and  78 C. In other words, each of the beams are spatially distinguishable from one another (i.e., spatially distinct). In most cases, the spatially distinct beams  78 A,  78 B and  78 C are also arranged to be equally spaced apart and have substantially equal light intensities. 
   Upon leaving the diffraction grating  76 , the three beams  78 A,  78 B and  78 C pass through an aperture  80  and then continue along path  68  until they reach a beam splitter cube  82 . The beam splitter cube  82  (working with the quarter wave plates  72 ) is arranged to divide the beams into paths  84  and  86 . Path  84  is used to distribute a first light portion of the beams to the substrate  12  and path  86  is used to distribute a second light portion of the beams to the reference optics  56 . In most embodiments, most of the light is distributed to the substrate  12  along path  84  and a small percentage of the light is distributed to the reference optics  56  along path  86 . It should be understood, however, that the percentage ratios may vary according to the specific design of each optical inspection system. In brief, the reference optics  56  include a reference collection lens  114  and a reference detector  116 . The reference collection lens  114  is arranged to collect and direct the second portion of the beams, now designated  115 A-C, on the reference detector  116 . As should be appreciated, the reference detector  116  is arranged to measure the intensity of the light. Although not shown in  FIG. 2 , the reference detector  116  is generally coupled to an electronic subsystem such as the electronic subsystem  19  of  FIG. 1  such that the data collected by the detector can be transferred to the control system for analysis. Reference optics are generally well known in the art. 
   The three beams  78 A,  78 B and  78 C continuing along path  84  are received by a telescope  88 . Although not shown, inside the telescope  88  there are a several lens elements that redirect and expand the light. In one embodiment, the telescope is part of a telescope system that includes a plurality of telescopes rotating on a turret. For example, three telescopes may be used. The purpose of these telescopes is to vary the size of the scanning spot on the substrate and thereby allow selection of the minimum detectable defect size. More particularly, each of the telescopes generally represents a different pixel size. As such, one telescope may generate a larger spot size making the inspection faster and less sensitive (e.g., low resolution), while another telescope may generate a smaller spot size making inspection slower and more sensitive (e.g., high resolution). 
   From the telescope  88 , the beams  78 A,  78 B and  78 C pass through an objective lens  90 , which is arranged for focusing the beams  78 A,  78 B and  78 C onto the surface  11  of the substrate  12 . As the beams  78 A-C intersect the surface  11  of the substrate  12  both reflected light beams  92 A,  92 B, and  92 C and transmitted light beams  94 A,  94 B, and  94 C may be generated. The transmitted light beams  94 A,  94 B, and  94 C pass through the substrate  12 , while the reflected light beams  92 A,  92 B, and  92 C reflect off the surface  11  of the substrate  12 . By way of example, the reflected light beams  92 A,  92 B, and  92 C may reflect off of an opaque surfaces of the substrate, and the transmitted light beams  94 A,  94 B, and  94 C may transmit through transparent areas of the substrate. The transmitted light beams  94  are collected by the transmitted light optics  58  and the reflected light beams  92  are collected by the reflected light optics  62 . 
   With regards to the transmitted light optics  58 , the transmitted light beams  94 A,  94 B,  94 C, after passing through the substrate  12 , are collected by a first transmitted lens  96  and focused with the aid of a spherical aberration corrector lens  98  onto a transmitted prism  100 . As shown, the prism  100  has a facet for each of the transmitted light beams  94 A,  94 B,  94 C that are arranged for repositioning and bending the transmitted light beams  94 A,  94 B,  94 C. In most cases, the prism  100  is used to separate the beams so that they each fall on a single detector in the transmitted light detector arrangement  60 . As shown, the transmitted light detector arrangement  60  includes three distinct detectors  61 A-C, and more particularly a first transmission detector  61 A, a second transmission detector  61 B, and a third transmission detector  61 C. Accordingly, when the beams  94 A-C leave the prism  100  they pass through a second transmitted lens  102 , which individually focuses each of the separated beams  94 A,  94 B,  94 C onto one of these detectors  61 A-C. For example, beam  94 A is focused onto transmission detector  61 A; beam  94 B is focused onto transmission detector  61 B; and beam  94 C is focused onto transmission detector  61 C. As should be appreciated, each of the transmission detectors  61 A,  61 B, or  61 C is arranged for measuring the intensity of the transmitted light. 
   With regards to the reflected light optics  62 , the reflected light beams  92 A,  92 B, and  92 C after reflecting off of the substrate  12  are collected by the objective lens  90 , which then directs the beams  92 A-C towards the telescope  88 . Before reaching the telescope  88 , the beams  92 A-C also pass through a quarter wave plate  104 . In general terms, the objective lens  90  and the telescope  88  manipulate the collected beams in a manner that is optically reverse in relation to how the incident beams are manipulated. That is, the objective lens  90  re-collimates the beams  92 A,  92 B, and  92 C, and the telescope  88  reduces their size. When the beams  92 A,  92 B, and  92 C leave the telescope  88 , they continue along path  84  (backwards) until they reach the beam splitter cube  82 . The beam splitter  82  is arranged to work with the quarter wave-plate  104  to direct the beams  92 A,  92 B, and  92 C onto a path  106 . 
   The beams  92 A,  92 B, and  92 C continuing on path  106  are then collected by a first reflected lens  108 , which focuses each of the beams  92 A,  92 B, and  92 C onto a reflected prism  110 , which includes a facet for each of the reflected light beams  92 A-C. The reflected prism  110  is arranged for repositioning and bending the reflected light beams  92 A,  92 B,  92 C. Similar to the transmitted prism  100 , the reflected prism  110  is used to separate the beams so that they each fall on a single detector in the reflected light detector arrangement  64 . As shown, the reflected light detector arrangement  64  includes three individually distinct detectors  65 A-C, and more particularly a first reflected detector  65 A, a second reflected detector  65 B, and a third reflected detector  65 C. Of course, each detector may be packaged separately or together. When the beams  92 A-C leave the prism  110 , they pass through a second reflected lens  112 , which individually focuses each of the separated beams  92 A,  92 B,  92 C onto one of these detectors  65 A-C. For example, beam  92 A is focused onto reflected detector  65 A; beam  92 B is focused onto reflected detector  65 B; and beam  92 C is focused onto reflected detector  65 C. As should be appreciated, each of the reflected detectors  65 A,  65 B, or  65 C is arranged for measuring the intensity of the reflected light. 
   There are multiple inspection modes that can be facilitated by the aforementioned optical assembly  50 . By way of example, the optical assembly  50  can facilitate a transmitted light inspection mode, a reflected light inspection mode, and a simultaneous inspection mode. With regards to transmitted light inspection mode, transmission mode detection is typically used for defect detection on substrates such as conventional optical masks having transparent areas and opaque areas. As the light beams  94 A-C scan the mask (or substrate  12 ), the light penetrates the mask at transparent points and is detected by the transmitted light detectors  61 A-C, which are located behind the mask and which measure the light of each of the light beams  94 A-C collected by the transmitted light optics  58  including the first transmitted lens  96 , the second transmitted lens  102 , the spherical aberration lens  98 , and the prism  100 . 
   With regards to reflected light inspection mode, reflected light inspection can be performed on transparent or opaque substrates that contain image information in the form of chromium, developed photoresist or other features. Light reflected by the substrate  12  passes backwards along the same optical path as the inspection optics  54  but is then diverted by a polarizing beam splitter  82  into detectors  65 A-C. More particularly, the first reflected lens  108 , the prism  110  and the second reflected lens  112  project the light from the diverted light beams  92 A-C onto the detectors  65 A-C. Reflected light inspection may also be used to detect contamination on top of opaque substrate surfaces. 
   With regards to simultaneous inspection mode, both transmitted light and reflected light are utilized to determine the existence and/or type of a defect. The two measured values of the system are the intensity of the light beams  94 A-C transmitted through the substrate  12  as sensed by transmitted light detectors  61 A-C and the intensity of the reflected light beams  92 A-C as detected by reflected light detectors  65 A-C. Those two measured values can then be processed to determine the type of defect, if any, at a corresponding point on the substrate  12 . 
   More particularly, simultaneous, transmitted and reflected detection can disclose the existence of an opaque defect sensed by the transmitted detectors while the output of the reflected detectors can be used to disclose the type of defect. As an example, either a chrome dot or a particle on a substrate may both result in a low transmitted light indication from the transmission detectors, but a reflective chrome defect may result in a high reflected light indication and a particle may result in a lower reflected light indication from the same reflected light detectors. Accordingly, by using both reflected and transmitted detection one may locate a particle on top of chrome geometry which could not be done if only the reflected or transmitted characteristics of the defect was examined. In addition, one may determine signatures for certain types of defects, such as the ratio of their reflected and transmitted light intensities. This information can then be used to automatically classify defects. 
   By way of example, representative inspection modes including, reflected, transmitted and simultaneous reflected and transmitted modes, may be found in U.S. Pat. No. 5,563,702, which is herein incorporated by reference. It should be understood, however, that these modes are not a limitation and that other suitable modes may be used. 
   Further details of the system of  FIGS. 1 and 2  wherein the present invention may be employed may be found in U.S. patent application Ser. No. 09/636,124, filed Aug. 10, 2000, and U.S. patent application Ser. No. 09/636,129, filed Aug. 10, 2000, both assigned to the assignee of the present invention, the entirety of which are incorporated herein by reference. As may be appreciated to those skilled in the art, other systems and designs may be used to practice the present invention. 
   Transmitted and Reflected Processing 
   Transmitted light and reflected light exhibit reverse behavioral characteristics such that when transmitted light intensity decreases, reflected light intensity usually increases. Considering this phenomenon in the ΔT−ΔR plane, ΔT and ΔR will follow the relationship that ΔT * ΔR is less than zero. Phase errors, in contrast to chrome defects, typically exist when the transmitted and reflected light exhibit similar properties, such as when both transmitted and reflected differences are greater than or are both less than zero. The non-defective threshold is modified using these relationships as shown in  FIG. 4 , where modified non-defective zone  401  is bordered by removed region  402 , representing the sensitivity increase over the ΔT and ΔR constant thresholds. In practice, after scanning test and reference images and subtracting transmitted and reflected pixel quantities, the result is as shown in  FIG. 5 .  FIG. 5  is a ΔT−ΔR scan representation of a pair of images, test and reference, where chrome defects generally exist when ΔT and ΔR have opposite signs, one positive and one negative. Phase defects exist where ΔT and ΔR have identical signs, i.e. where both ΔT is positive and ΔR is positive or where both ΔT is negative and ΔR is negative. From the roughly negative 45 degree sloping graphical representation, the area of likely defect locations can be more precisely assumed, drawn, and considered to more accurately assess the presence of defects. 
   Defect detection according to the present invention employs a two step process, a calibration step followed by a defect detection step. These processes or procedures may be embodied as part(s) of the control system  17 , control computer  18 , and/or electronics subsystem  19  of  FIG. 1 , or may be included in a separate processing subsystem not illustrated but interacting with the inspection optics design such that transmitted and reflected image representations may be received and processed according to the following description. Dual stage defect detection according to the present invention may be performed by hardware, such as an ASIC, software, firmware, or in a manner known to those skilled in the art. 
   In the present invention, an initial calibration step determines the non-defective region on the ΔT−ΔR plane for a specific pair of dies, namely a test and reference die. This calibration generates a training set to represent the non-defective region on the ΔT−ΔR plane. Multiple image pairs may be used to generate this calibration training set. In practice, one may select the pattern on the mask having the greatest density or most complicated pattern when generating the training set. Other parameters may be used to select calibration patterns used for developing the training set. Selection of the high density or most complicated pattern can, in certain circumstances, avoid certain false defects. 
     FIG. 6  represents the calibration functions performed by the detection system using both transmitted and reflected image data. From  FIG. 6 , the transmitted image (for both the test and reference specimens) is pre-processed at block  601  into a transmitted reference image and a transmitted test image. The reflected image (for both the test and reference specimens is pre-processed at block  602  into a reflected test image and a reflected reference image. Preprocessing here includes obtaining fixed pixel subimages of the test and reference specimens, such as a  70  by  70  pixel,  120  by  120  pixel,  180  by  180  pixel, or other sized subimage of the specimens. 
   Fine sub-pixel alignment and subtraction blocks  603  and  604  align the test and reference images for both transmitted and reflected representations, resulting in a ΔT(i,j) for the transmitted images and a ΔR(i,j) for the reflected images. Alignment may be performed before subtraction in blocks  603  and  604 . These aligned and subtracted values are passed through two 2 by 2 FIR (Finite Impulse Response) low pass filters  605  and  606 , resulting in reduced noise ΔT(i,j) and ΔR(i,j) values. Use of either or both FIR lowpass filters is optional. Generally, if no significant false defect readings are expected or actually occur for a mask, the filter can optionally be overlooked. 
   The system generates a ΔT−ΔR plot map using the ΔT and ΔR values at plot map generator  607 . The system then generates a core ΔT−ΔR training set at core training set generator  608 . A threshold is applied along with the core training set from core training set generator  608  at detection training set table generator  609 . Application of the threshold provides a buffer zone around the core ΔT−ΔR training set, and the threshold may vary depending on circumstances. For example, a zero to five pixel border, or other sized border, may be applied around all pixels from the core ΔT−ΔR training set to include close cases. 
   A representative ΔT−ΔR plot as may be generated by plot map generator  608  is illustrated in  FIG. 7 . A core ΔT−ΔR training set as may be generated by core training set generator  608  is presented in  FIG. 8 .  FIG. 9  presents an enhanced or dilated TR training set as may be generated by detection training set generator  609 . As compared with the core training set of  FIG. 8 , certain stray artifacts are buffered using detection training set generator  609 , and the pixels of  FIG. 9  represent the non-defective region used for subsequent processing. The non-defective region resulting from the detection training set generator  609  may be converted to a look up table for reference purposes. 
   Defect detection processes four images: T T (i,j), the transmitted image from the test die; T R (i,j), the reflected image from the test die; R T (i,j), the transmitted image from the reference die; and R R (i,j), the reflected image from the reference die. i and j vary from 1 to the number of pixels in each direction within the image.  FIG. 10  illustrates the defect detection processor. From  FIG. 10 , test die image preprocessor  1001  processes the test die into test reflected and test transmitted representations. Reference die image preprocessor  1002  processes the reference die into reference reflected and reference transmitted representations. These four components are then aligned and subtracted in alignment and subtraction blocks  1003  and  1004  to produce ΔT and ΔR values, which are passed to two 2 by 2 FIR filters  1005  and  1006  for noise reduction. Again, use of either or both of these lowpass FIR filters is optional. Generally, use of each lowpass FIR filter at this stage depends on whether the filter was applied on the calibration stage, i.e. was used in the design of  FIG. 6 . The ΔT and ΔR components are used to generate a ΔT−ΔR plot map at plot map generator  1007 . The generated ΔT−ΔR plot map is compared against the ΔT−ΔR training set table generated at detection training set table generator  609  in defect detection and classification block  1008 . If the ΔT−ΔR plot from plot map generator  1007  includes pixels outside of the pixels in the training set representation provided by detection training set generator  609 , or outside the non-defective region, those pixels are flagged as defective. After the system processes all pixels for all images, all defects are identified for potential post processing. Post processing may entail an operator reviewing the regions to determine the acceptability of the region and/or presence of a defect, or automated review of the particular area, or any method of defect review known in the art. Post processing may alternately entail computing the number of defects or potential defects and considering the sample bad if the number of defects exceeds a predetermined threshold. Post processing is an optional part of the present procedure. 
   While the invention has been described in connection with specific embodiments thereof, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within known and customary practice within the art to which the invention pertains.