Abstract:
Inspection system and method for high-throughput inspection, the system and method is capable to generate and sense transmitted and/or reflected short duration beams. According to one embodiment of the invention the transmitted and reflected short duration beams are generated and sensed simultaneously thus provide a reflected image and a transmitted image simultaneously. The reflected and transmitted short duration radiation beams are manipulated either in the frequency domain or are distinctly polarized such that they are directed to the appropriate area sensors. According to another aspect of the invention the system changes the manipulation of a short duration beam of radiation to selectively direct the short duration beam to distinct area sensors.

Description:
RELATED APPLICATIONS 
   The present application is related to, incorporates by reference and is a continuation of the following U.S. Patent Application, assigned to the assignee of the present application: U.S. patent application Ser. No. 10/215,972, filed Aug. 8, 2002, now U.S. Pat. No. 6,930,770, entitled “High Throughput Inspection System And Method For Generating Transmitted And/Or Reflected Images.” 

   FIELD OF THE INVENTION 
   The present invention relates to a system and method for high throughput inspection of an object using short duration reflective and transmitted radiation beams, such as but not limited to radiation beams. 
   BACKGROUND OF THE INVENTION 
   Systems and methods of inspecting an article to determine the condition of the article, such as a mask (also referred to as reticle or photomask) are known in the art. Optical inspection systems and methods involve directing a radiation beam onto an inspected object and detecting the radiation reflected from the system or the radiation transmitted through the object. 
   The size of transistors is constantly being reduced and there is a need to inspect masks (also known as reticles) with higher resolution. In spite of the required higher resolution there is a need to perform optical inspections in a time efficient manner. There is therefore a need to provide a system and method for inspection that is characterized by both high throughput and high resolution. 
   SUMMARY OF THE INVENTION 
   The invention provides a system and method for high throughput optical inspection, whereas the method includes the steps of: (i) Reflecting a first beam of radiation from one face of an area of the object to produce a short duration reflected beam and simultaneously transmitting a second beam of radiation through the area of the object including the first face and a second face to provide a short duration transmitted beam; (II) Sensing the short duration reflected beam and the short duration transmitted beam and in response generating output signals reflecting a condition of the area of the object; (III) Periodically repeating steps (I) and (II) until a predefined portion of the object is irradiated; and (IV) Processing the output signals to provide an indication of the condition of the predefined portion of the object. 
   The invention provides an optical inspection system that has reflected and transmitted radiation paths, that enable short duration reflected and transmitted radiation beams to be simultaneously generated and directed towards area sensors to simultaneously provide a transmitted and reflected images of the inspected objects. Accordingly, the system and method enable simple comparison between transmitted and reflected images of an area (and accordingly simplify the registration process and even eliminate the need for performing registration between transmitted and reflected images) as both a transmitted image and a reflected image of an area are taken simultaneously. 
   The invention provides an optical inspection system of high throughput by manipulating either reflected or transmitted beams so that images are formed at alternating area detectors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will be apparent from the description below. The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
       FIGS. 1   a – 1   c  are schematic diagrams illustrating optical inspection systems constructed in accordance with the present invention; 
       FIGS. 2   a – 2   c  illustrate transmitted and reflected images of an area, in accordance with an embodiment of the invention; 
       FIG. 3  illustrates a scanning scheme in accordance with an embodiment of the invention; 
       FIGS. 4–5  are schematic diagrams of optical inspection systems in accordance with other embodiments of the present invention; and 
       FIG. 6  is a flow chart illustrating a method for inspecting an object, according to embodiments of the invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   As indicated earlier, the method and apparatus of the present invention are particularly useful for optically inspecting photomasks in order to detect defects in reflecting and/or transmissive areas of the photomask. It is noted that some photomasks have clear areas and opaque areas alone, while other photomasks may include areas that are characterized by reflection and/or transmission levels between full reflection/ transmission and zero reflection/transmission. For example, a half tone area permits only about % of light to pass through it. For simplicity of explanation alone it is assumed that the photomask has clear and opaque areas. 
   Electromagnetic radiation beams may be characterized by their polarization. The electric field of a linearly polarized optical wave lies only in a single plane. The electric fields of a circularly polarized optical wave lie in two orthogonal planes and are phased shifted by a quarter wavelength (or an odd amount of quarter wavelengths) of the optical wave. Polarizing beam splitters divide an optical wave that has electric field in two orthogonal planes into two orthogonally polarized optical waves. Phase retardation involves making an optical path length for one out of two orthogonal linear polarizations different than the other. Quarter wave retarders convert linearly polarized optical waves into circularly polarized optical waves and vice versa. Variable retarders are able to change their retardance and accordingly are able to change the relative phase shift between the electrical fields in two orthogonal planes, thus introducing a phase shift. Variable wave retarders may change their retardance between zero and a portion of a wavelength. Variable wave retarders are characterized by the maximal amount of phase shift they introduce. For example a half wavelength variable retarder is able to change its retardance between zero and a half wavelength. Phase retarders such as but not limited to quarter wavelength retarders and polarizing beam splitters are known in the art. 
     FIG. 1   a  illustrates an optical inspection system  10 , in accordance with an embodiment of the invention. System  10  includes a radiation source. Preferably the radiation has a wavelength of about 193 nm. It it further noted that the radiation source is located below a plane in which the inspected object is located, but this is not necessarily so. 
   System  10  includes a linearly polarized radiation source  12 , controller  14 , first quarter wave retarder  11 , beam splitter  16 , first reflector  22 , stage  60 , objective lens  36 , beam splitter  24 , relay lens  26 , second quarter wavelength retarder  28 , polarized beam splitter  30 , optics such as transmissive objective lens  59 , first area sensor  32  and second area sensor  32 . 
   It is noted that polarized radiation source  12 , first quarter wave retarder  11 , beam splitter  16 , first reflector  22 , objective lens  36 , transmissive objective lens  59 , beam splitter  24 , relay lens  26 , second quarter wavelength retarder  28  and polarized beam splitter  30  define a illumination system having a reflected and transmitted paths. 
   Polarized radiation source, such a laser  12  is operable to generate short duration radiation beams of a linear polarization, such as a horizontal polarization (e.g.—the electrical fields of the radiation beam lie in the XZ plane, while the short duration radiation beam propagates along the X axis.). Controller  14 , coupled to laser  12 , is operable to control the generation of the short duration radiation beams in accordance with an irradiation pattern (also termed illumination pattern). Conveniently, the irradiation pattern includes a series of time spaced pulses. The irradiation pattern is responsive to various parameters such as the radiation source parameters (usually maximal duty cycle), and required throughput. Those of skill in the art will appreciate that as the wavelength of radiation pulses continues to decrease the complexity and cost of high duty cycle lasers substantially increases. 
   Laser  12  is followed by a quarter wave retarder  11  that produces a circularly polarized (assuming Right Hand Circularly (i.e.—RHC) polarized) short duration radiation beam  13 . The RHC polarized short duration radiation beam  13  is split by beam splitter  16  to a first and second short duration radiation beams  15  and  17  respectively. The first short duration radiation beam  15  is directed towards first reflector  22  to be reflected towards the lower face of the inspected object  8 , and especially towards an lower face of an area AR  9  of the inspected object, whereas AR  9  is defined by the cross section of the first short duration radiation beam  15 . It is noted that the intensities of the first and second short duration radiation beams  15  and  17  may be equal but this is not necessarily so. 
   The first short duration radiation beam  15  is partially transmitted through clear portions of area AR  9  to produce short duration transmitted beam  21 . The second short duration radiation beam  17  is partially reflected from opaque portions of area AR  9  to produce a short duration reflected beam  23 . Short duration reflected beam is RHC polarized, while short duration transmitted beam  21  is LHC polarized, as the polarization of the former is reversed as result of the reflection. 
   Short duration transmitted beam  21  and short duration reflected beam  23  are collected by objective lens  36  that is positioned above the upper face of the inspected object  8 , whereas AR  9  is located at a focal plane of objective lens  36 . Short duration transmitted beam  21  and short duration reflected beam  23  pass through beam splitter  24  to propagate through relay lens  26 . Relay lens  26  is operative to match the size of the image of AR  9  or, as illustrated by  FIG. 3 , the size of an image of a rectangular portion of area AR  9  to the sensing surfaces of area sensors  34  and  32 . It is noted that the sensing surface of area sensors  32  and  34  are rectangular, while the cross section of the short duration reflected and transmitted radiation beams is circular, but this is not necessarily so, as the short duration reflected and transmitted radiation beam may be shaped to fit the shape of the sensing area, and vice verse. 
   After propagating through relay lens  26  the short duration transmitted beam  21  and short duration reflected beam  23  pass though second quarter wavelength retarder  28  that converts the LHC polarized short duration transmitted beam  21  and the RHC polarized short duration short duration reflected beam  23  to a linearly polarized radiation beam in the X direction (also referred to as p-polarized radiation beam)  25  and to a linearly polarized radiation beam in the Z direction (also referred to as s-polarized beam)  27 . Both beams  25  and  27  are directed towards polarizing beam splitter  30  that directs the p-polarized radiation beam  25  towards a first area sensor  32  and directs the s-polarized radiation beam  27  towards a second area sensor  34 . Thus, the first area sensor  32  receives a transmitted image of AR  9  (or of a portion of AR  9 ) while the second area sensor  34  receives a reflected image of AR  9  (or of a portion of AR  9 ), as illustrated in  FIG. 2   a – 2   c.    
     FIG. 1   a  illustrates the propagation of both reflected and transmitted radiation beams that enable the generation of a reflected and a transmitted image of an area respectfully, whereas  FIG. 1   b  illustrates the propagation of the short duration transmitted radiation beam alone and  FIG. 1   c  illustrates the propagation of the short duration reflected radiation beam alone. 
   Preferably, first area sensor  32  and second area sensor  34  are back illumination CCD area sensors having an array of 1024×1024 sensing elements. The 1024×1024 array is partitioned in multiple segments, for enabling parallel reading of the multiple segments and enhancing the system throughput. CCD area sensors are available from several vendors, such as Dalsa, Sarnoff or Feirchild. Typical data readout rates of a single CCD area sensor range between tens mega pixels per second to several hundreds mega pixels per second. Alternative configurations of detection elements and segments may also be used, as will be apparent to those skilled in the art. 
   The first area sensor  32  and second area sensor  34  are operable to (a) sense the short duration transmitted beam and the short duration reflected beam, respectively, and, in response, to (b) generate output signals reflecting a condition of the irradiated area of the object. The output signals reflect the charge of each sensing element, whereas the charge is responsive to the intensity of radiation that is incident on the sensing element. In other words, the output signals of first area sensor  32  represent a transmitted image received by the first area sensor  32 , while the output signals of second area sensor  34  represent a received image received by the second area sensor  34 . 
   Those of skill in the art will appreciate that other polarization schemes, such ellipsoid polarization and linear polarization may be utilized for separating the short duration reflected beam and short duration transmitted beam. 
     FIGS. 2   a – 2   c  illustrate an exemplary area AR  9  and especially a rectangular portion  9 (1) of AR  9 . Portion  9 (1) has opaque portions  100 ,  102 ,  104  and  106 , clear portions  101 ,  103  and  105  and foreign particle  110  and  120 . The clear and opaque portions are in the form of bright and dark areas in the transmitted image  92  of  FIG. 2   b  while being in the form of dark and bright areas in the reflected image  94  of  FIG. 2   c . Foreign particle  110  that is located above clear portion  103  can be seen as a radiation falloff ( 112 ) in the transmitted image  92  and as a spot ( 114 ) that has a different brightness than its surroundings in the reflected image  94 . Foreign particle  120  that is located above opaque portion  104  can be seen as spot  122  in the reflected image  94 . I 
   Referring back to  FIGS. 1   a – 1   c , controller  16  is operable to initiate the reflection, transmission and sensing of short duration radiation beams until a predefined portion of the object is radiated and is further operable to process the output signals to provide an indication of the condition of the predefined portion of the object. Various signal processing schemes are known in the art, such as a comparison between the reflected image and the transmitted image. As both images are acquired simultaneously, there is no need to perform a registration between these images, thus simplifying the processing stage and improving the accuracy of the image processing. 
   Stage  60  is operable to hold the inspected object and translate it such that a predefined portion of the inspected object is illuminated during a series of reflection, transmission and detections iterations. The illuminated areas and especially the portions that are later imaged on the area sensors overlap, thus reducing the sensitivity of system  10  to mechanical vibrations and for preventing gaps in the coverage of the inspected object. Usually, stage  60  translates the inspected object such that a predefined portion of the inspected object is irradiated. Preferably, the inspected object is raster scanned, but other scanning schemes may also be implemented. 
     FIG. 3  illustrates a scanning scheme in which the inspected object is translated along a scan (X) axis and a row of partially overlapping circular areas  90 (m, 1)– 90 (m,n) is illuminated during a series of time spaced short duration radiation pulses. It is noted that a rectangular portion (denoted  92 (m, 1)– 90 (m,n)) of each of said circular areas  90 (m, 1)– 90 (m,n) is imaged on the sensing surfaces of first area sensor  32  and second area sensor  34 , but this is not necessarily so. For example, the radiation beams may be shaped as to illuminate a rectangular area, or the first area sensor and second area sensor may have a circular shaped sensing surface. 
     FIG. 4  illustrates an optical inspection system  10 , in accordance with another embodiment of the invention. System  110  differs from system  10  in that the differentiation between the transmitted and reflected beams that generate the transmitted and reflected images is based upon wavelength but not upon polarization. In other words, the short duration reflected radiation beam differs from the short duration transmitted radiation beam by wavelength. It is noted that the generation of short duration radiation beams of distinct wavelength may be implemented by using distinct radiation sources, but when dealing with ultra short radiation pulses (such as picosecond to nanosecond radiation pulses) the synchronization between distinct radiation sources is very complex, thus using a single radiation source for generating the short duration radiation pulses is more feasible and much more accurate. Accordingly, a single radiation source generates a multi-wavelength short duration radiation pulses that are later filtered to split multiple short duration radiation beams of distinct wavelength. 
   As system  110  is based upon wavelength separation, the polarizing and polarization based elements of system  10  (such as first quarter wave retarder  18 , second quarter wavelength retarder  28 , polarized beam splitters  30 ) are replaced by dichronic beam splitter  116  and  130 . 
   System  110  includes polychromatic radiation source  112  that generates multi-wavelength short duration radiation beams  111 , that are directed towards dichroic beam splitter  116 , that splits said beam to provide a first wavelength short duration beam  115  that is directed towards first reflector  22 , and to provide a second wavelength short duration beam  117  that is directed towards second reflector  20 . 
   First wavelength short duration beam  115  is reflected from first reflector  22 , passes through optics, such as transmissive objective lens  159 , and passes through clear portions of illuminated area AR  9 , is collected by objective lens  36 , passes through relay lens  26  and is split by diachronic beam splitter  130  to two portions  125  and  135 . First portion  125  passes through first spectral filter  116  and arrives to first area sensor  32  and forms a transmitted image of AR  9 , while a second portion  135  is blocked by second spectral filter  114  thus does not arrive to second area sensor  34 . 
   Second wavelength short duration beam  117  is reflected from second reflector  20 , is reflected from opaque portions of illuminated area AR  9 , is collected by objective lens  36 , passes through relay lens  26  and is split by beam splitter  130  to two portions  127  and  137 . First portion  127  is blocked by first spectral filter  112  thus does not arrive to first area sensor  32 , while second portion  137  passes through second spectral filter  114  and arrives to second area sensor  34  and forms a reflected image of AR 9 . I 
   First and second array sensors  32  and  34  simultaneously send to controller  14  electrical signals representative of a transmitted and reflected images of area AR 9 , respectively. Controller  14  processes the images to determine the condition of area AR 9 . 
   It is noted that polychromatic radiation source  112 , diachronic beam splitters  116  and  130 , first reflector  22 , relay lens  26  and transmissive objective lens  159  define an illumination system that has a reflective and transmitted radiation paths. 
     FIG. 5  illustrates an optical inspection system  210 , in accordance to a further 
   embodiment of the invention. System  210  generates only transmitted images but is characterized by a very high throughput. 
   It is known in the art that area detectors that include multiple sensing elements, such as area CCD cameras, are limited by their data readout rate. It is known that although an image is formed in parallel at the sensing elements of a CCD camera, the sensing elements are read in a serial manner. In some CCD cameras the multiple sensing elements are partitioned to segments, whereas each segment includes sensing elements that are coupled to each other in a serial manner, whereas each segment may be read in parallel to the other segment, thus increasing the overall readout rate of the CCD camera, but this may not provide the required readout rate. Another method for multiplying the data readout rate involves buffering the sensing element readout within the CCD camera, but this solution is very costly. 
   System  210  enables an increase in the throughput of an inspection system by utilizing two CCD cameras while alternating the polarization of the radiation beam and accordingly alternating the area sensing element that generates the image. 
   System  210  may have a transmitted radiation path alone that includes first quarter wavelength retarder  16 , a fast variable half wavelength retarder  50 , first reflector  22 , stage  60 , objective lens  36 , relay lens  26 , second quarter wavelength retarder  28 , polarized beam splitter  30 , first area sensor  32  and second area sensor  34 . First quarter wavelength retarder  16 , fast variable half wavelength retarder  50 , first reflector  22 , stage  60 , objective lens  36 , relay lens  26 , second quarter wavelength retarder  28  and polarized beam splitter  30  define a illumination system that has a transmitted radiation path. 
   Fast variable half wavelength retarder  50  is able to change the polarization of the transmitted radiation beam from RHC polarization and LHC polarization, in response to control signals from controller  14 . The change rates may be adjusted/selected to fit the readout period out of each area sensor. It usually ranges between several hundred changes per second, but this is not necessarily so. 
   When the variable half wavelength retarder  50  does not change the polarization of the radiation beam the transmitted radiation beam arrives to the first area sensor  32 , while when the variable half wavelength retarder  50  introduces a phase shift of half a wavelength, the transmitted radiation beam arrives to the second area sensor  34 . 
   The timing of beam transmission and electrical transmission to processor  14  is illustrated by “N&#39;th cycle”, “(N−1)&#39;th cycle” and “(N+1)&#39;th cycle” reflecting that an image is directed towards second area sensor  34  during a (N−1)&#39;th cycle, that an image is directed towards first area sensor  32  and that output signals (that reflect the image that is generated during the (N−1)&#39;th cycle) are provided from second area sensor  34  to controller  14  during a N&#39;th cycle and that during the (N+1)&#39;th cycle output signals (that reflect the image that is generated during the N&#39;th cycle) are provided from first area sensor  32  to controller  14 . 
   Those of skill in the art will appreciate that system  210  may include a 
   reflective path alone, whereas the reflected path includes a half wavelength retarder, beam directing elements such as reflectors and beam splitters. 
   If is further noted that the elements of systems  10  and  210  may be combined, 
   to allow the generation of reflected and transmitted images, or to allow the generation of transmitted images alone or reflected images alone (when the half wavelength retarder is located at a reflected radiation path). 
   Referring to  FIG. 6  illustrating a method  400  for inspecting an object. 
   Method  400  starts at step  410  of reflecting a first beam of radiation from one face of an area of the object to produce a short duration reflected beam and simultaneously transmitting a second beam of radiation through the area of the object including the first face and a second face to provide a short duration transmitted beam. 
   Step  410  is followed by step  420  of sensing the short duration reflected beam and the short duration transmitted beam and in response generating output signals reflecting a condition of the irradiated area. 
   Step  420  is followed by step  430  of processing the electrical signals to provide an indication of the condition of an illuminated area of the object. Step  430  is followed by step  440  of determining whether another illumination is required (e.g.—if the predefined portion was already illuminated) and if so—step  440  is followed by step  410  such that steps  410 – 440  are periodically repeated until the predefined portion of the object is radiated. Else, step  440  is followed by “END” step  450 . 
   It is noted that  FIG. 6  illustrates a method in which the processing is done during the illumination and determination steps, but this is not necessarily so as the electrical signals may be stored and later on processed. 
   It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.