Abstract:
Apparatus for imaging a surface, including scanning optics, which are configured to scan and focus one or more traveling beams onto the surface so as to form one or more traveling spots thereon. The apparatus also includes collection optics, which are arranged to collect radiation scattered from the one or more traveling spots and to focus the radiation to form one or more image spots traveling along a line. The apparatus also has a detecting assembly, which consists of a detector which is configured to generate a signal in response to the one or more traveling image spots, and a detector entry port interposed between the collection optics and the detector, which is coincident with the line. The apparatus also includes phase and/or polarization altering elements for the traveling beams.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application 60/736,983, filed Nov. 15, 2005, U.S. Provisional Patent Application 60/736,645, filed Nov. 15, 2005 and U.S. Provisional Patent Application 60/736,646, filed Nov. 15, 2005, which is are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to imaging an object, and specifically to imaging the object by scanning it with one or more sources of radiation, typically for the purpose of wafer and/or mask inspection. 
   BACKGROUND OF THE INVENTION 
   In production of a semiconductor die, imaging of the wafer upon which the dice are formed is an integral part of the fabrication process, and images are typically generated for many stages of the fabrication process. Typically, the wafer images are used for the purpose of inspection and/or quality control of the specific stages. One of the methods used to produce the images is to scan the wafer, and form the scanned images, and/or determine characteristics of the section of the wafer being scanned, from radiation returning from the section. The scanning process is relatively time-consuming, and methods for reducing the scanning time, while maintaining the quality of the received signals, are well known in the semiconductor art. One such method is to use apparatus that performs multiple scans simultaneously. A number of other methods and techniques have also been used for enhancing the production of the image of a scanned object. The references below describe some of these methods and techniques. 
   U.S. Pat. No. 5,355,252 to Haraguchi, whose disclosure is incorporated herein by reference, describes a scanning laser microscope using a single beam. 
   U.S. Pat. No. 6,674,522 to Krantz et al., whose disclosure is incorporated herein by reference, describes an optical technique for inspecting photomasks. The techniques are based on multiple modified radiation collection techniques. 
   U.S. Pat. No. 6,853,475, to Feldman et al., whose disclosure is incorporated herein by reference, describes a method for producing multiple optical beams which are scanned across the surface of a wafer. The method uses an acousto-optical device wherein multiple traveling lenses are generated, each lens forming a respective traveling beam which is focused onto, and which is scanned over, the wafer surface. 
   An article entitled “Bright field-bright future: Material defect detection with a laser scanning system,” by Larson et al., published in Sep. 1997 in  Solid State Technology , is incorporated herein by reference. The article describes splitting a single laser beam into two correlated beams. The two beams irradiate a surface, and the returning radiation is combined into one beam. The recombined beam provides information on the surface. The system uses a differential interference contrast (DIC) technique, first described by Nomarski et al. in  Rev. Metallurgie  L11, 121, 1955. In DIC the prism used to split the beam is a Wollaston prism, or a Nomarski prism. 
   Basic confocal microscopy principles were described in U.S. Pat. No. 3,013,467, to Minsky, whose disclosure is incorporated herein by reference. In a confocal microscope, a light beam is focused to a spot in the object plane, and this spot is imaged onto a small circular aperture (often a pinhole) placed in front of a detector. Confocal microscopy improves the discrimination of objects in the focal plane compared with those not in the plane. 
   In articles titled “Scanning mirror microscope with optical sectioning characteristics: applications in ophthalmology” by C. J. Koester, published in  Appl. Opt.,  19, pgs 1749-1757, 1980, and “Confocal Microscopes with slit apertures” by C. J. R. Sheppard et al., published in  J. Mod. Opt.  35, pgs 1169-1185 (1988), the use of a slit (“one-dimensional confocal”) instead of a small circular aperture is described. Both articles are incorporated herein by reference. The use of a slit source with a slit aperture allows a larger signal to be detected, compared with a circular aperture with a diameter of the size of the slit width. 
   U.S. patent application Ser. No. 2005/0225849 to Gouch, whose disclosure is incorporated herein by reference, describes a confocal microscope having a slit source. The slit source is focused onto an object, and radiation from the object is focused onto a linear array of detectors. 
   U.S. Pat. No. 5,241,364 to Kimura, whose disclosure is incorporated herein by reference, describes a confocal phase contrast scanning microscope. The optics of the microscope includes an annular phase plate, via which a collimated beam is passed, before being focused to a point on an object. Light from the object is focused onto the entrance of a fiber optic, which transfers the received light to a detector. Scanning is performed by mechanically moving the optics and the object independently. 
   In an article titled “Tandem-scanning reflected-light microscope” by Petran et al., published in  J. Opt. Soc. Am.,  58, pgs 661-664 (1968), whose disclosure is incorporated herein by reference, methods are proposed to allow simultaneous detection of signals from a large number of apertures in a scanning confocal microscope. The methods include using aperture arrays and Nipkow discs. 
   Other configurations of scanning confocal microscopes for inspection applications have been proposed. Examples are provided in U.S. Pat. No. 6,248,988 to Krantz, U.S. Pat. No. 6,429,897 to Derndinger et al., and U.S. patent application Ser. No. 2003/0156280 to Reinhorn, all of whose disclosures are incorporated herein by reference. 
   SUMMARY OF THE INVENTION 
   In embodiments of the present invention, a scanning microscope comprises an element, typically an acousto-optic (AO) element, which generates a multiplicity of traveling beams. The traveling beams are focused to spots at a surface being inspected, and returning radiation from the surface is imaged and detected at an array of detectors. The characteristics of the image may be improved by introducing one or more image enhancing elements into the optics irradiation and collection path. 
   In one embodiment, the image enhancing element comprises a detector assembly having a one-dimensional entry port. The entry port may comprise an element separate from the detectors, and is typically a slit positioned before the detectors, which are implemented as a one-dimensional linear array. Alternatively, the entry port may comprise the front surface of the detectors, in which case the detectors are also implemented as a one-dimensional array. Further alternatively, the detector assembly may comprise detectors coupled to first ends of fiber optics, the second ends of the fiber optics being configured as the one-dimensional entry port. Hereinbelow, unless otherwise stated, the entry port is assumed to be a slit. 
   The slit is aligned parallel with the traveling direction of the spots, and with the detectors, and dimensions of the slit and its position are set so that in a direction orthogonal to the slit, confocal effects are introduced, whereas in the slit direction, normal imaging takes place. Introduction of a one-dimensional slit reduces depth discrimination while relaxing the auto-focus requirements of the system, compared to a confocal two-dimensional circular aperture. 
   In an alternative embodiment, the image enhancing element comprises a Wollaston, Nomarski, or equivalent prism followed by an analyzer. The prism divides each of the traveling beams into two orthogonally polarized traveling beams, the beams being angularly displaced one from the other. The two beams are focused onto the surface being inspected, and the returning radiation is combined in the analyzer before being focused onto the detectors. Phase information of the surface is thus converted into different intensity levels at the detectors, and the different intensity levels enhance the visibility of aspects of the surface such as edges. 
   In a further alternative embodiment, the image enhancing element comprises an annular phase plate followed by an annular aperture or mask. The plate and the aperture (or mask) operate to form the microscope into a phase contrast microscope, and are positioned at appropriate entrance and exit pupils of the microscope so that their operation is not affected by the traveling beams. 
   In other disclosed embodiments of the present invention, two or more of the enhancing elements described above are incorporated into one scanning microscope. The advantages of each of the different enhancing elements are incorporated into the one microscope, without negatively affecting each other. 
   The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings, a brief description of which follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are schematic diagrams of a scanning microscope, according to an embodiment of the present invention; 
       FIG. 2  is an enlarged schematic diagram of detecting assemblies used in the microscope of  FIGS. 1A and 1B , according to an embodiment of the present invention; 
       FIG. 3  shows simulated graphs of intensity vs. amount of defocusing, according to an embodiment of the present invention 
       FIG. 4  is a schematic diagram of a scanning microscope, according to an alternative embodiment of the present invention; 
       FIG. 5  is a schematic diagram of a scanning microscope, according to a further alternative embodiment of the present invention; and 
       FIG. 6  is a schematic diagram of a scanning microscope, according to another alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Reference is now made to  FIG. 1A , which is a schematic diagram of a scanning microscope  21 , and to  FIG. 1B , which is a schematic side view of elements of the microscope, according to an embodiment of the present invention. Microscope  21  comprises a laser  20  which radiates a beam of coherent radiation  22  to a beam expander  24 . Typically, laser  20  is selected to emit optical radiation at a wavelength in a region of the electromagnetic spectrum between and including far infra-red and deep ultra-violet (DUV), although it will be understood that the principles of the present invention apply equally to other wavelengths of the spectrum. Beam expander  24  generates an expanded collimated radiation beam  26  which reflects from a plane mirror  28  as a collimated radiation beam  30 . A processor  29  operates elements of scanning microscope  21 . 
   An acousto-optic (AO) element  34  receives beam  30  at a first surface  31  of the AO element, the first surface acting as a radiation input surface. Processor  29  generates a radio-frequency (RF) signal, with which it drives AO element  34  via an RF input port  35  coupled to the AO element. The RF signal is in the form of variable frequency pulses, or “chirps,” each of which generates planar traveling acoustic waves in AO element  34 . The planar traveling waves have varying wavelengths corresponding to the variable frequencies of the chirp. The waves act as traveling diffraction gratings  32  having variable spacing, and the gratings operate as cylindrical lenses which focus incoming beam  30 , via a second surface  37  of the AO element acting as a radiation output surface. A cylindrical lens  23  focuses the converging beams from element  34  to a series of traveling focused spots  36 , which are approximately collinear. A wedge-shaped prism  25 , positioned at spots  36 , receives the focused beams, and diverts the beams to respective diverging conical beams  27 , the axes of each of the conical beams being approximately parallel to an axis  33  of microscope  21 . A scanning lens  38  converts diverging beams  27  to a series of collimated beams  40 . Collimated beams  40  pass through a pupil  41 , and traverse a beam splitter  42  to an objective  44 . Optionally, a stop  43  is positioned at pupil  41 , the pupil acting as an exit pupil for lens  38 , and as an entrance pupil for objective  44 . ( FIG. 1A  and  FIG. 1B  show front and side views of elements  34 ,  23 ,  25 ,  38 , and  43 , and beam paths through the elements.) 
   Radiation exiting from splitter  42  is focused by objective  44  to a series of traveling spots  46  on a surface  48 , the traveling spots typically traveling along an approximate straight line in the surface. In  FIG. 1A  the approximate straight line along which the points move is in the plane of the paper. Surface  48  is typically the upper surface of a wafer  49  which is being inspected by microscope  21 . Typically, the number of spots  46  on surface  48  at any one time is from approximately three to approximately ten. However, the number of spots may vary from this range; it will be appreciated that the actual number on the surface at any one time is set by the rate of repetition of the RF chirps and the speed of the traveling waves in AO element  34 . 
   Radiation scattered from spots  46  is collected by objective  44 , and traverses generally the same path as the incoming radiation from splitter  42 . In the specification and in the claims, scattered radiation is assumed to include radiation that is reflected. Objective  44  collimates the collected radiation, and the collimated beams from the objective are diverted by beam splitter  42  as a set of traveling collimated beams  52 . 
   Beams  52  pass through a pupil  53 , which is effectively a mirror image of pupil  41 , and enter a converging lens  54 . Optionally, a stop  55  is placed at pupil  53 , which acts as an exit pupil for objective  44  and as an entrance pupil for lens  54 . Lens  54  focuses traveling beams  52  to a linear series of substantially circular spots  58 , at the focal plane of lens  54 . A detecting assembly  60  includes a detector entry port  62 , which is a linear substantially one-dimensional port allowing passage of radiation, and which is positioned at the focal plane of lens  54 . Assembly  60  also includes a detector  64 . Different embodiments of assembly  60  are described in more detail with reference to  FIG. 2  below. 
   Radiation from spots  58  traverses port  62 , and is received by detector  64 . In response, detector  64 , typically comprising a linear array of charged coupled detectors (CCDs) or photo-multiplier tubes (PMTs), generates respective signals according to the intensity of the received radiation. Processor  29  receives the signals and analyzes them to determine characteristics of the regions of surface  48  generating the radiation. 
   Those with ordinary skill in the art will appreciate that the order of elements described above for microscope  21  is not unique, and that other arrangements of the elements may be made, giving substantially the same results as those described above; such rearrangements may require changes in elements used. For example, in one embodiment of the present invention, splitter  42  is positioned before pupil  41 , causing pupil  53  to substantially coincide with pupil  41 , so that one of stops  43  or  55  becomes superfluous. Other rearrangements of elements of microscope  21  will be apparent to those skilled in the art, and all such rearrangements are assumed to be included within the scope of the present invention. 
     FIG. 2  is a schematic diagram of three examples of detecting assembly  60 , according to embodiments of the present invention. The three examples are each shown with schematic front and side views, and except where otherwise stated, each example is assumed to use a substantially similar detector  64  comprising a linear array of rectangular detecting elements  63 . 
   In a first example  61 , assembly  60  comprises a linear slit  65  acting as port  62 , behind which is positioned detector  64 . A length L 1  of slit  65  is set so that no detector elements are shielded from radiation of spots  58 . Slit  65  is configured to have a width W approximately equal to a diameter D of spots  58 . As is explained in more detail below, the actual width W is typically set according to the performance required of microscope  21  and the confocal effects generated by slit  65 . As is known, spots  58  do not have sharp edges, and the diameter D referred to here is assumed to be the diameter of a circle within which approximately 90% of the energy of the radiation of the spot is found. Alternative methods known in the art for determining D, such as using a full width half maximum (FWHM) measure, or a value at which the spot intensity falls to a fraction such as 
           1     e   2           
of the central maximum intensity, may be used. All such methods are assumed to be included within the scope of the present invention.
 
   By configuring circular spots  58  to traverse linear slit  65 , rather than a circular aperture as in prior art confocal microscopes, embodiments of the present invention combine advantageously the properties of confocal and non-confocal systems. In a direction orthogonal to slit  65 , the confocal properties preponderate; in a direction parallel to the slit, the non-confocal properties preponderate. 
   Typical values for W are in a range between approximately 0.5D and approximately 2D. The value of W affects both the depth of field of microscope  21 , and its auto-focus requirements. A narrow slit provides high depth discrimination, and requires relatively restricted auto-focusing; a wide slit provides low depth discrimination, and requires relatively relaxed auto-focusing. Embodiments of the present invention typically set the value of W to take this dependence into account. Thus, by setting W to be approximately 0.5D, microscope  21  has a small depth of field and a correspondingly narrow range within which processor  29  is able to satisfactorily auto-focus; by setting W to be approximately 2D, microscope  21  has a relatively large depth of field and processor  29  has a correspondingly large range within which it is able to satisfactorily auto-focus. 
   In a second example  67 , assembly  60  comprises only detector  64 , which is configured to have a width W and a length L 2 . L 2  is set so that all radiation from spots  58  is received by the detector. Width W is set substantially as described for width W in example  61 , and the description of the properties of W given therein also applies. In example  67 , front faces  69  of elements  63  of detector  64  act as entry port  62 . 
   In a third example  70 , assembly  60  comprises detector elements  71  acting as detector  64 , and fiber optic cables  72 . Elements  71  are generally similar to elements  63 , but may, as shown in  FIG. 2 , be separate elements. Elements  71  are coupled to cables  72  at first ends  74  of the cables, and second ends  76  of cables act as entry port  62 . Second ends  76  are arranged in a line, which is configured to have a width W and a length L 3 . L 3  is set so that all radiation from spots  58  is incident on ends  76 . Width W is set substantially as described for width W in example  61 , and the description of the properties of W given therein also applies. 
   Those having ordinary skill in the art will be aware of techniques other than the three examples given here for forming detecting assembly  60 , and all such techniques are assumed to be included within the scope of the present invention. 
     FIG. 3  shows simulated graphs of intensity vs. amount of defocusing, for different values of width W of entry port  62  ( FIGS. 1A and 2 ), according to an embodiment of the present invention. The vertical axis of the graphs plots normalized intensities of radiation received at entry port  62 . The horizontal axis plots normalized distances of entry port  62  from the theoretical focus plane of lens  54 , i.e., a defocus distance. The graphs illustrate the effects described above with respect to  FIG. 2 , i.e., that for a given defocus distance, as the width W of port  62  decreases, the intensity ratio at detector  64  also decreases, so that microscope  21  has a smaller depth of field, and consequently an increased depth discrimination. 
     FIG. 4  is a schematic diagram of a scanning microscope  100 , according to an alternative embodiment of the present invention. Apart from the differences described below, the operation of microscope  100  is generally similar to that of microscope  21  ( FIGS. 1A and 1B ), such that elements indicated by the same reference numerals in both microscopes  100  and  21  are generally identical in construction and in operation. In place of detecting assembly  60 , a detector  101  is used. Detector  101  is typically generally similar to detector  64  described above with reference to example  61  ( FIG. 2 ), although it may not necessarily have dimensions that give the detector confocal properties. In microscope  100  a Wollaston, Nomarski, or equivalent prism  102 , herein termed a polarizing beam splitter prism, is positioned between beam splitter  42  and objective  44 . Polarizing beam splitter prisms are well known in the optical art, and descriptions of them are provided in more detail in references given in the Background of the Invention. As described therein, a polarizing beam splitter prism separates an incoming radiation beam into two plane polarized beams having orthogonal planes of polarization. There is an angular separation between the two beams. In microscope  100 , beams  40  are plane polarized if laser  20  emits beam  22  as plane polarized radiation. If beam  22  is not plane polarized, then one or more elements before prism  102  are adapted to plane polarize beams  40 ; alternatively, a polarizer is added to microscope  100  to form beams  40  into plane polarized beams. 
   Prism  102  is aligned so that its axis is 45° to the plane of polarization of beams  40 , in order that the two plane polarized beams emitted by the prism have approximately equal intensities. Thus, prism  102  separates each of incoming beams  40  into a pair of orthogonal plane polarized beams. Each pair of beams is focused by objective  44  to a pair of spots  104 , in contrast to microscope  21 , wherein one beam  40  is focused to one spot  46 . Prism  102  is constructed so that the two beams it outputs subtend an angle of the order of 100 μrad with each other, typically so that each pair of spots  104  is separated by approximately one spot diameter. 
   Objective  44  collects radiation from pairs of spots  104 , and conveys the collected radiation to prism  102 , along paths which are substantially the reverse of the paths of the beams exiting from the prism. Prism  102  consequently combines the returning radiation from each pair of spots  104  into single beams  106  (only the central lines of the beams are shown), each single beam consisting of a pair of orthogonally polarized overlapping beams. An analyzer  108 , aligned at 45° to the two polarization directions of the radiation of each of the beams  106 , acts to coherently interfere between the two polarizations to form sets of coherent beams  110 , each beam  110  being the result of the interference of its pair of beams. 
   Substantially as described above for microscope  21 , lens  54  focuses beams  110  to a set of traveling spots  112  on detector  101 . It will be understood that the intensity variations registered by detector  101  are the result of the beam interference generated by analyzer  108 . 
   It will also be appreciated that microscope  100  functions as a scanning differential interference contrast microscope, so that, for example, slope changes in surface  48  that may be poorly detected, or not detected at all, using a normal microscope are typically well detected in microscope  100 . Such slope changes may be caused by a variety of features on surface  48 , including edges of such features, as well as defects on the surface. 
     FIG. 5  is a schematic diagram of a scanning microscope  150 , according to a further alternative embodiment of the present invention. Apart from the differences described below, the operation of microscope  150  is generally similar to that of microscope  21  ( FIGS. 1A and 1B ), such that elements indicated by the same reference numerals in both microscopes  150  and  21  are generally identical in construction and in operation. In place of detecting assembly  60 , a detector  151  is used. Detector  151  is typically generally similar to detector  64  described above with reference to example  61  ( FIG. 2 ), although it may not necessarily have dimensions that give the detector confocal properties. In microscope  150  a phase plate  152  is positioned at exit pupil  41  of lens  38 . For clarity, phase plate  152  is shown enlarged and displaced from pupil  41  in  FIG. 5 , in an inset  157 . Phase plate  152  has a circular ring  153  which is configured to generate a phase shift of 
           π   2         
between radiation passing through the ring and radiation passing through the remainder of the plate. It will be understood that the phase shift introduced may be by ring  153  acting as a phase retarder, or as a phase advancer. By way of example, hereinbelow ring  153  is assumed to produce a phase retardation of
 
             π   2     .         
In some embodiments of the present invention, phase plate  152  includes an attenuating region  149  which attenuates a portion of the beam traversing the phase plate. Region  149  may be positioned in ring  153 , and/or in a remaining portion of the plate, as described in more detail below.
 
   The radiation having the two phases, shown schematically in  FIG. 5  as beams  155 , traverses beam splitter  42  and is focused by objective  44  to form traveling spots  158  on surface  48 . The operation of beam splitter  42  and objective  44  is generally as described above with reference to  FIG. 1A , and except for the phase shift introduced by plate  152 , spots  158  are generally similar to spots  46 . 
   Returning radiation from spots  158  is collected by objective  44  and is reflected by beam splitter  42  as radiation  154 , substantially as described above for the operation of microscope  21 . An annular aperture mask  156  is positioned so that it and phase plate  152  are at conjugate foci of objective  44 , and also so that the annular aperture is approximately at entrance pupil  53  of lens  54 , between the lens and splitter  42 . (For clarity, mask  156  is shown enlarged and displaced from pupil  53  in inset  157 .) The width of an annulus  160  of aperture mask  156  is configured to be consistent with that of the width of ring  153 . Annulus  160  constrains retarded and non-retarded radiation from spots  158  into the same path, to form beams  162 . Beams  162  are focused by lens  54  onto detector  151 , to form spots  164 , generally as described above for spots  58 . Alternatively, instead of mask  156  being configured as an annular aperture, the mask may be configured as an annular stop. 
   It will be understood that microscope  150  operates as a scanning phase contrast microscope, so that slope or phase features on surface  48  that would normally be undetectable with a normal microscope become detectable. In embodiments having attenuating region  149  in plate  152 , the region is positioned with respect to mask  156 , and the amount of attenuation the region provides is chosen, so that the attenuation enhances the phase contrast image generated by the plate-mask combination. 
   Returning to  FIGS. 1A and 1B , it will be appreciated that scanning microscopes generally similar to microscope  21  may be implemented using scanning mechanisms other than the specific system described with reference to acousto-optic element  34 . For example, instead of beam expander  24 , mirror  28 , AO element  34 , lens  23 , wedge  25 , and lens  38 , a scanning mechanism may comprise a beam multiplexer followed by a rotating mirror, the multiplexer and mirror being configured to provide multiple beams substantially similar to the series of parallel beams  40  described above. 
   It will also be appreciated that elements of the microscopes described above may be combined to form compound scanning microscopes. An example of such a combination is described in more detail below with reference to  FIG. 6 ; all such combinations are assumed to be included within the scope of the present invention. 
     FIG. 6  is a schematic diagram of a scanning microscope  200 , according to another alternative embodiment of the present invention. Apart from the differences described below, the operation of microscope  200  is generally similar to that of microscopes  21  and  150  ( FIGS. 1A ,  1 B and  5 ), such that elements indicated by the same reference numerals in microscopes  200 ,  150 , and  21  are generally identical in construction and in operation. Microscope  200  uses phase element  152  and mask element  156  as used by microscope  150 , as well as detecting assembly  60  as used by microscope  21 . Thus, in the case of microscope  200 , spots  164  are formed substantially as described above for microscope  150 . However, assembly  60  acts on spots  164  as described above with reference to  FIGS. 1A ,  1 B, and  2 . It will be appreciated that the effects of the phase and mask elements, and of the slit assembly, are substantially independent of each other. The image detected by detector  64  is thus a phase contrast image, showing the slope or phase features described above for microscope  150 ; in addition, the image has the confocal advantages described above for microscope  21 . 
   It will be appreciated that the 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 subcombinations 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.