Patent Publication Number: US-2005134841-A1

Title: Sample inspection system

Description:
BACKGROUND OF THE INVENTION  
      This invention relates in general to sample inspection systems and, in particular, to an improved inspection system with good sensitivity for particles as well as crystal-originated-particles (COPs). COPs are surface breaking defects in semiconductor wafers which have been classified as particles due to inability of conventional inspection systems to distinguish them from real particles.  
      Systems for inspecting unpatterned wafers or bare wafers have been proposed. See for example, PCT Patent Application No. PCT/US96/15354, filed on Sep. 25, 1996, entitled “Improved System for Surface Inspection.” Systems such as those described in the above-referenced application are useful for many applications, including the inspection of bare or unpatterned semiconductor wafers. Nevertheless, it may be desirable to provide improved sample inspection tools which may be used for inspecting not only bare or unpatterned wafers but also rough films. Another issue which has great significance in wafer inspection is that of COPs. These are surface-breaking defects in the wafer. According to some opinions in the wafer inspection community, such defects can cause potential detriments to the performance of semiconductor chips made from wafers with such defects. It is, therefore, desirable to provide an improved sample inspection system capable of detecting COPs and distinguishing COPs from particles.  
     SUMMARY OF THE INVENTION  
      This invention is based on the observation that anomaly detection employing an oblique illumination beam is much more sensitive to particles than to COPs, whereas in anomaly detection employing an illumination beam normal to the surface, the difference in sensitivity to surface particles and COPs is not as pronounced. Anomaly detection employing both an oblique illumination beam and a normal illumination beam can then be used to distinguish between particles and COPs.  
      One aspect of the invention is directed towards an optical system for detecting anomalies of a sample, comprising first means for directing a first beam of radiation along a first path onto a surface of the sample; second means for directing a second beam of radiation along a second path onto a surface of the sample and a first detector. The system further comprises means including a mirrored surface for receiving scattered radiation from the sample surface and originating from the first and second beams and for focusing the scattered radiation to said first detector.  
      Another aspect of the invention is directed towards an optical system for detecting anomalies of a sample, comprising first means for directing a first beam of radiation along a first path onto a surface of a sample; second means for directing a second beam of radiation along a second path onto a surface of the sample, said first and second beams producing respectively a first and a second illuminated spot on the sample surface, said first and second illuminated spots separated by an offset. The system further comprises a detector and means for receiving scattered radiation from the first and second illuminated spots and for focusing the scattered radiation to said detector.  
      One more aspect of the invention is directed towards an optical system-for detecting anomalies of a sample, comprising a source supplying a beam of radiation at a first and a second wavelength; and means for converting the radiation beam supplied by the source into a first beam at a first wavelength along a first path and a second beam at a second wavelength along a second path onto a surface of a sample. The system further comprises a first detector detecting radiation at the first wavelength and a second detector detecting radiation at the second wavelength; and means for receiving scattered radiation from the sample surface and originating from the first and second beams and for focusing the scattered radiation to said detectors.  
      Yet another aspect of the invention is directed towards an optical system for detecting anomalies of a sample, comprising a source supplying a radiation beam; a switch that causes the radiation beam from the source to be transmitted towards the sample surface alternately along a first path and a second path; a detector and means for receiving scattered radiation from the sample surface and originating from the beam along the first and second paths and for focusing the scattered radiation to said detector.  
      Another aspect of the invention is directed towards an optical system for detecting anomalies of a sample, comprising means for directing at least one beam of radiation along a path onto a spot on a surface of the sample; a first detector and means for receiving scattered radiation from the sample surface and originating from the at least one beam and for focusing the scattered radiation to said first detector for sensing anomalies. The system further comprises a second, position sensitive, detector detecting a specular reflection of said at least one beam in order to detect any change in height of the surface at a spot; and means for altering the path of the at least one beam in response to the detected change in height of the surface of the spot to reduce position error of the spot caused by change in height of the surface of the spot.  
      Still another aspect of the invention is directed towards an optical system for detecting anomalies of a sample, comprising means for directing at least one beam of radiation along a path onto a spot on a surface of the sample; a first detector and means for collecting scattered radiation from the sample surface and originating from the at least one beam and for conveying the scattered radiation to said first detector for sensing anomalies. The system further comprises a spatial filter between the first detector and the collecting and conveying means blocking scattered radiation towards the detector except for at least one area having a wedge shape.  
      One more aspect of the invention is directed towards an optical method for detecting anomalies of a sample, comprising directing a first beam of radiation along a first path onto a surface of the sample; directing a second beam of radiation along a second path onto a sample of the surface; employing a mirrored surface for receiving scattered radiation from the sample surface and originating from the first and second beams and focusing the scattered radiation to a first detector.  
      Yet another aspect of the invention is directed towards an optical method for detecting anomalies of a sample, comprising directing a first beam of radiation along a first path onto a surface of the sample; directing a second beam of radiation along a second path onto a surface of the sample, said first and second beams producing respectively a first and a second illuminated spot on the sample surface, said first and second illuminated spots separated by an offset. The method further comprises receiving scattered radiation from the first and second illuminated spots and for focusing the scattered radiation to a detector.  
      An additional aspect of the invention is directed towards an optical method for detecting anomalies of a sample, comprising supplying a beam of radiation of a first and a second wavelength; converting the radiation beam into a first beam at a first wavelength along a first path and a second beam at a second wavelength along a second path, said two beams directed towards a surface of the sample. The method further comprises collecting scattered radiation from the sample surface and originating from the first and second beams, focusing the collected scattered radiation to one or more detectors, and detecting radiation at the first and second wavelengths by means of said detectors.  
      Yet another aspect of the invention is directed towards an optical method for detecting anomalies of a sample, comprising supplying a radiation beam, switching alternately the radiation beam between a first and a second path towards a surface of the sample, receiving scattered radiation from the sample surface and originating from the beam along the first and second paths, and focusing the scattered radiation to a detector.  
      Another aspect of the invention is directed towards an optical method for detecting anomalies of a sample, comprising directing at least one beam of radiation along a path onto a spot on the surface of the sample; collecting scattered radiation from the sample surface and originating from the at least one beam, and focusing the collected scattered radiation to a first detector for sensing anomalies. The method further comprises detecting a specular reflection of said at least one beam in order to detect any change in height of the surface at the spot and altering the path of the at least one beam in response to the detected change in height of the surface of the spot to reduce position error of the spot caused by change in height of the surface of the spot.  
      One more aspect of the invention is directed towards an optical method for detecting anomalies of a sample, comprising directing at least one beam of radiation along a path onto a spot on a surface of the sample; collecting scattered radiation from the sample surface and originating from the at least one beam, conveying the scattered radiation to a first detector for sensing anomalies, and blocking scattered radiation towards the detector except for at least one area having a wedge shape.  
      Still another aspect of the invention is directed towards an optical system for detecting anomalies of a sample, comprising means for directing a beam of radiation along a path at an oblique angle to a surface of the sample; a detector and means including a curved mirrored surface for collecting scattered radiation from the sample surface and originating from the beam and for focusing the scattered radiation to said detector.  
      One more aspect of the invention is directed towards an optical method for detecting anomalies of a sample, comprising directing a beam of radiation along a path at an oblique angle to a surface of the sample; providing a curved mirrored surface to collect scattered radiation from the sample surface and originating from the beam, and focusing the scattered radiation from the mirrored surface to a detector to detect anomalies of the sample.  
      Another aspect of the invention enables the distinction between COPs and particles on the surface. When the surface is illuminated by a P-polarized beam at an oblique angle of incidence, the radiation scattered by a particle has more energy in directions away from the normal direction of the surface compared to directions close to the normal direction to the surface. The radiation scattered by a COP from oblique incident P-polarized light is more uniform compared to that of the particle. Therefore, by detecting radiation scattered in directions away from the normal direction to the surface and comparing it to radiation scattered in directions close to the normal direction to the surface, it is possible to distinguish between COPs and particles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A, 1B  and  1 C are schematic views of normal or oblique illumination beams illuminating a surface with a particle thereon useful for illustrating the invention.  
       FIG. 2A  is a schematic view of a sample inspection system employing an ellipsoidal mirror for illustrating one embodiment of the invention.  
       FIG. 2B  is a schematic view of a sample inspection system employing a paraboloidal mirror to illustrate another embodiment of the invention.  
       FIG. 3  is an exploded simplified view of a portion of the system of  FIG. 2A  or  FIG. 2B  to illustrate another aspect of the invention.  
       FIG. 4  is a schematic view of a sample inspection system employing two different wavelengths for illumination to illustrate yet another embodiment of the invention.  
       FIGS. 5A and 5B  are schematic views of sample inspection systems illustrating two different embodiments employing switches for switching a radiation beam between a normal illumination path and an oblique illumination path to illustrate yet another aspect of the invention.  
       FIG. 6  is a schematic view of a beam illuminating a semiconductor wafer surface to illustrate the effect of a change in height of a wafer on the position of the spot illuminated by beam.  
       FIG. 7  is a schematic view of a portion of a sample inspection system inspecting a semiconductor wafer, employing three lenses, where the direction of the illumination beam is altered to reduce the error in the position of the illuminated spot caused by the change in height of the wafer.  
       FIG. 8  is a schematic view of a portion of a sample inspection system employing only one lens to compensate for a change in height of the wafer.  
       FIGS. 9A-9F  are schematic views of six different spatial filters useful for detecting anomalies of samples.  
       FIG. 10A  is a simplified partially schematic and partially cross-sectional view of a programmable spatial filter employing a layer of liquid crystal material sandwiched between an electrode and an array of electrodes in the shape of sectors of a circle and means for applying a potential difference across at least one sector in the array and the other electrode, so that the portion of the liquid crystal layer adjacent to the at least one sector is controlled to be radiation transparent or scattering.  
       FIG. 10B  is a top view of the filter of  FIG. 10A .  
       FIG. 11  is a schematic view of a sample inspection system employing an oblique illumination beam and two detectors for distinguishing between COPs and particles to illustrate another aspect of the invention.  
      For simplicity in description, identical components are labelled by the same numerals in this application. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 1A  is a schematic view of a surface  20  of a sample to be inspected and an illumination beam  22  directed in a direction normal to surface  20  to illuminate the surface and a particle  24  on the surface. Thus, the illumination beam  22  illuminates an area or spot  26  of surface  20  and a detection system (not shown) detects light scattered by particle  24  and by portion or spot  26  of the surface  20 . The ratio of the photon flux received by the detector from particle  24  to that from spot  26  indicates the sensitivity of the system to particle detection.  
      If an illumination beam  28  directed at an oblique angle to surface  20  is used to illuminate spot  26 ′ and particle  24  instead, as shown in  FIG. 1B , from a comparison between  FIGS. 1A and 1B , it will be evident that the ratio of the photon flux from the particle  24  to that from the illuminated spot will be greater in the case of the oblique illumination in  FIG. 1B  compared to that in  FIG. 1A . Therefore, for the same throughput (spots  26 ,  26 ′ having the same area), the sensitivity of the oblique incidence beam in detecting small particles is superior and is the method of choice in the detection of small particles.  
       FIG. 1C  illustrates an oblique beam  28 ′ illuminating a surface  30  having a pit  32  and particle  24 ′ thereon. As can be seen from  FIG. 1C , even though the pit  32  is of comparable size to particle  24 , it will scatter a much smaller amount of photon flux compared to particle  24  from oblique beam  28 ′. On the other hand, if the pit  32  and particle  24  are illuminated by a beam such as  22  directed in a direction normal to surface  30 , pit  32  and particle  24  would cause comparable amount of photon flux scattering. Almost regardless of the exact shape or orientation of COPs and particles, anomaly detection employing oblique illumination is much more sensitive to particles than COPs. In the case of anomaly detection with normal illumination, however, the differentiation between particles and COPs is less pronounced. Therefore, by means of a simultaneous, or sequential, comparison of feature signatures due to normal and oblique illumination will reveal whether the feature is a particle or a COP.  
      Azimuthal collection angle is defined as the angle made by the collection direction to the direction of oblique illumination when viewed from the top. By employing oblique illumination, together with a judicious choice of the azimuthal collection angle, rough films can be inspected with good sensitivity, such as when a spatial filter shown in any of  FIGS. 9A-9F ,  10 A and  10 B is used in any one of the embodiments as shown in  FIGS. 2A, 2B ,  3 ,  4 ,  5 A and  5 B, as explained below. By retaining the normal illumination beam for anomaly detection, all of the advantageous attributes of the system described in PCT Patent Application No. PCT/US96/15354 noted above, are retained, including its uniform scratch sensitivity and the possibility of adding a bright-field channel as described in PCT Patent Application No. PCT/US97/04134, filed Mar. 5, 1997, entitled “Single Laser Bright Field and Dark Field System for Detecting Anomalies of a Sample.” 
      Scanning a sample surface with oblique and normal illumination beams can be implemented in a number of ways.  FIG. 2A  is a schematic view of a sample inspection system to illustrate a general set up for implementing anomaly detection using both normal and oblique illumination beams. A radiation source that provides radiation at one or more wavelengths in a wide electromagnetic spectrum (including but not limited to ultraviolet, visible, infrared) may be used, such as a laser  52  providing a laser beam  54 . A lens  56  focuses the beam  54  through a spatial filter  58  and lens  60  collimates the beam and conveys it to a polarizing beamsplitter  62 . Beamsplitter  62  passes a first polarized component to the normal illumination channel and a second polarized component to the oblique illumination channel, where the first and second components are orthogonal. In the normal illumination channel  70 , the first polarized component is focused by optics  72  and reflected by mirror  74  towards a sample surface  76   a  of a semiconductor wafer  76 . The radiation scattered by surface  76   a  is collected and focused by an ellipsoidal mirror  78  to a photomultiplier tube  80 .  
      In the oblique illumination channel  90 , the second polarized component is reflected by beamsplitter  62  to a mirror  82  which reflects such beam through a half-wave plate  84  and focused by optics  86  to surface  76   a.  Radiation originating from the oblique illumination beam in the oblique channel  90  and scattered by surface  76   a  is collected by an ellipsoidal mirror and focused to photomultiplier tube  80 . Photomultiplier tube  80  has a pinhole entrance  80   a.  The pinhole  80   a  and the illuminated spot (from the normal and oblique illumination channels on surface  76   a ) are preferably at the foci of the ellipsoidal mirror  78 .  
      Wafer  76  is rotated by a motor  92  which is also moved linearly by transducer  94 , and both movements are controlled by a controller  96 , so that the normal and oblique illumination beams in channels  70  and  90  scan surface  76   a  along a spiral scan to cover the entire surface.  
      Instead of using an ellipsoidal mirror to collect the light scattered by surface  76   a,  it is also possible to use other curved mirrors, such as a paraboloidal mirror  78 ′ as shown in system  100  of  FIG. 2B . The paraboloidal mirror  78 ′ collimates the scattered radiation from surface  76   a  into a collimated beam  102  and the collimated beam  102  is then focused by an objective  104  and through an analyzer  98  to the photomultiplier tube  80 . Aside from such difference, the sample inspection system  100  is exactly the same as system  50  of  FIG. 2A . Curved mirrored surfaces having shapes other than ellipsoidal or paraboloidal shapes may also be used; preferably, each of such curved mirrored surfaces has an axis of symmetry substantially coaxial with the path of the normal illumination path, and defines an input aperture for receiving scattered radiation. All such variations are within the scope of the invention. For simplicity, the motor, transducer and control for moving the semiconductor wafer has been omitted from  FIG. 2B  and from  FIGS. 4, 5A ,  5 B described below.  
      The general arrangements shown in  FIGS. 2A and 2B  can be implemented in different embodiments. Thus, in one arrangement referred to below as the “GO and RETURN” option, a half-wave plate (not shown) is added between laser  52  and lens  56  in  FIGS. 2A and 2B  so that the polarization of the light reaching the beamsplitter  62  can be switched between P and S. Thus, during the Go cycle, the beamsplitter  62  passes radiation only into the normal channel  70  and no radiation will be directed towards the oblique channel  90 . Conversely, during the RETURN cycle, beamsplitter  62  passes radiation only into the oblique channel  90  and no radiation will be directed through the normal channel  70 . During the GO cycle, only the normal illumination beam  70  is in operation, so that the light collected by detector  80  is recorded as that from normal illumination. This is performed for the entire surface  76   a  where motor  92 , transducer  94  and control  96  are operated so that the normal illumination beam  70  scans the entire surface  76   a  along a spiral scan path.  
      After the surface  76   a  has been scanned using normal illumination, the half-wave plate between laser  52  and lens  56  causes radiation from laser  52  to be directed only along the oblique channel  90  and the scanning sequence by means of motor  92 , transducer  94  and control  96  is reversed and data at detector  80  is recorded in a RETURN cycle. As long as the forward scan in the Go cycle and the reverse scan in the RETURN cycle are exactly registered, the data set collected during the GO cycle and that collected during the return cycle may be compared to provide information concerning the nature of the defects detected. Instead of using a half-wave plate and a polarizing beamsplitter as in  FIG. 2A , the above-described operation may also be performed by replacing such components with a removable mirror placed in the position of beamsplitter  62 . If the mirror is not present, the radiation beam from laser  52  is directed along the normal channel  70 . When the mirror is present, the beam is then directed along the oblique channel  90 . Such mirror should be accurately positioned to ensure exact registration of the two scans during the GO and RETURN cycles. While simple, the above-described GO and RETURN option requires extra time expended in the RETURN cycle.  
      The normal illumination beam  70  illuminates a spot on surface  76   a.  The oblique illumination beam  90  also illuminates a spot on the surface  76   a.  In order for comparison of data collected during the two cycles to be meaningful, the two illuminated spots should have the same shape. Thus, if beam  90  has a circular cross-section, it would illuminate an elliptical spot on the surface. In one embodiment, focusing optics  72  comprises a cylindrical lens so that beam  70  has an elliptical cross-section and illuminates also an elliptical spot on surface  76   a.    
      To avoid having to scan surface  76   a  twice, it is possible to intentionally introduce a small offset between the illuminated spot  70   a  from normal illumination beam  70  (referred to herein as “normal illumination spot” for simplicity) and the illuminated spot  90   a  from oblique illumination beam  90  (referred to herein as “oblique illumination spot” for simplicity) as illustrated in  FIG. 3 .  FIG. 3  is an enlarged view of surface  76   a  and the normal and oblique illumination beams  70 ,  90  to illustrate an offset  120  between the normal and oblique illumination spots  70   a,    90   a.  In reference to  FIGS. 2A, 2B , radiation scattered from the two spots  70   a,    90   a  would be detected at different times and would be distinguished.  
      The method illustrated in  FIG. 3  causes a reduction in system resolution and increased background scattering due to the presence of both spots. In other words, in order that radiation scattered from both spots separated by an offset would be focused through pinhole  80   a,  the pinhole should be somewhat enlarged in the direction of the offset. As a consequence, detector  80  will sense an increased background scattering due to the enlargement of the pinhole  80   a.  Since the background is due to both beams whereas the particle scattered radiation is due to one or the other spot, the signal-to-noise ratio is decreased. Preferably, the offset is not greater than three times the spatial extent, or less than the spatial extent, of the point spread function of either the normal or oblique illumination beam. The method illustrated in  FIG. 3 , however, is advantageous since throughput is not adversely affected compared to that described in PCT Application No. PCT/US96/15354 and the Censor ANS series of inspection systems from KLA-Tencor Corporation of San Jose, Calif., the assignee of this application.  
       FIG. 4  is a schematic view of a sample inspection system employing a normal illumination beam comprising radiation at a first wavelength λ 1  and an oblique illumination beam of radiation of wavelength λ 2  to illustrate another embodiment of the invention. The laser  52  of  FIGS. 2A, 2B  may supply radiation at only one wavelength, such as 488 nm of argon. Laser  52 ′ of  FIG. 4  supplies radiation at at least two different wavelengths in beam  54 ′, such as at 488 and 514 nm, instead of radiation of only one wavelength, Such beam is split by a dichroic beamsplitter  162  into a first beam at a first wavelength λ 1  (488 nm) and a second beam of wavelength λ 2  (514 nm), by passing radiation at wavelength λ 1  and reflecting radiation at wavelength λ 2 , for example. After being focused by optics  72 , beam  70 ′ at wavelength λ 1  is reflected by mirror  74  towards surface  76   a  as the normal illumination beam. The reflected radiation of wavelength λ 2  at beamsplitter  162  is further reflected by mirror  82  and focused by optics  86  as the oblique illumination beam  90 ′ to illuminate the surface. The optics in both the normal and oblique illumination paths are such that the normal and oblique illuminated spots substantially overlap with no offset there between. The radiation scattered by surface  76   a  retains the wavelength characteristics of the beams from which the radiation originate, so that the radiation scattered by the surface originating from normal illumination beam  70 ′ can be separated from radiation scattered by the surface originating from oblique illumination beam  90 ′. Radiation scattered by surface  76   a  is again collected and focused by an ellipsoidal mirror  78  through a pinhole  164   a  of a spatial filter  164  to a dichroic beamsplitter  166 . In the embodiment of  FIG. 4 , beamsplitter  166  passes the scattered radiation at wavelength λ 1  to detector  80 ( 1 ) through a lens  168 . Dichroic beamsplitter  166  reflects scattered radiation at wavelength λ 2  through a lens  170  to photomultiplier tube  80 ( 2 ). Again, the mechanism for causing the wafer to rotate along a spiral path has been omitted from  FIG. 4  for simplicity.  
      Instead of using a laser that provides radiation at a single wavelength, the laser source  52 ′ should provide radiation at two distinct wavelengths. A commercially available multi-line laser source that may be used is the 2214-65-ML manufactured by Uniphase, San Jose, Calif. The amplitude stability of this laser at any given wavelength is around 3.5%. If such a laser is used, the scheme in  FIG. 4  will be useful for applications such as bare silicon inspection but may have diminished particle detection sensitivity when used to scan rough films.  
      Yet another option for implementing the arrangements generally shown in  FIGS. 2A and 2B  is illustrated in  FIGS. 5A and 5B . In such option, a radiation beam is switched between the normal and oblique illumination channels at a higher frequency than the data collection rate so that the data collected due to scattering from the normal illumination beam may be distinguished from data collected from scattering due to the oblique illumination channel. Thus as shown in  FIG. 5A , an electro-optic modulator (e.g. a Pockels cell)  182  is placed between laser  52  and beamsplitter  62  to modulate the radiation beam  54  at the half-wave voltage. This results in the beam being either transmitted or reflected by the polarizing beamsplitter  62  at the drive frequency of modulator  182  as controlled by a control  184 .  
      The electro-optic modulator may be replaced by a Bragg modulator  192  as shown in  FIG. 5B , which may be turned on and off at a high frequency as controlled. Modulator  192  is powered by block  193  at frequency ω b . This block is turned on and off at a frequency ω m . In the off condition, a zero order beam  194   a  passes through the Bragg modulator  192 , and becomes the normal illumination beam reflected to surface  76   a  by mirror  74 . In the on condition, cell  192  generates a deflected first order beam  194   b,  which is reflected by mirrors  196 ,  82  to surface  76   a.  However, even though most of the energy from cell  192  is directed to the oblique first order beam, a weak zero order normal illumination beam is still maintained, so that the arrangement in  FIG. 5B  is not as good as that in  FIG. 5A .  
      Preferably, the electro-optic modulator of  FIG. 5A  and the Bragg modulator of  FIG. 5B  are operated at a frequency higher than the data rate, and preferably, at a frequency at least about 3 or 5 times the data rate of tube  80 . As in  FIG. 4 , the optics in both the normal and oblique illumination paths of  FIGS. 5A, 5B  are such that the normal and oblique illuminated spots substantially overlap with no offset there between. The arrangements in  FIGS. 2A, 2B ,  4 ,  5 A,  5 B are advantageous in that the same radiation collector  78  and detector  80  are used for detecting scattered light originating from the normal illumination beam as well as from the oblique illumination beam. Furthermore, by employing a curved surface that collects radiation that is scattered within the range of at least 25 to 700 from a normal direction to surface  76   a  and focusing the collected radiation to the detector, the arrangements of  FIG. 2A, 2B ,  4 ,  5 A,  5 B maximize the sensitivity of detection.  
      In contrast to arrangements where multiple detectors are placed at different azimuthal collection angles relative to the oblique illumination beam, the arrangements of  FIG. 2A, 2B  has superior sensitivity and is simpler in arrangement and operation, since there is no need to synchronize or correlate the different detection channels that would be required in a multiple detector arrangement. The ellipsoidal mirror  78  collects radiation scattered within the range of at least 25 to 700 from the normal direction to the surface which accounts for most of the radiation that is scattered by surface  76   a  from an oblique illumination beam, and that contains information useful for particle and COPs detection.  
      The three dimensional intensity distribution of scattered radiation from small particles on the surface when the surface is illuminated by a P-polarized illumination beam at or near a grazing angle to the surface has the shape of a toroid. In the case of large particles, higher scattered intensity is detected in the forward direction compared to other directions. For this reason, the curved mirror collectors of  FIGS. 2A, 2B ,  4 ,  5 A,  5 B are particularly advantageous for collecting the scattered radiation from small and large particles and directing the scattered radiation towards a detector. In the case of normal illumination, however, the intensity distribution of radiation scattered from small particles on surfaces is in the shape of a sphere. The collectors in  FIGS. 2A, 2B ,  4 ,  5 A,  5 B are also advantageous for collecting such scattered radiation. Preferably, the illumination angle of beam  90  is within the range of 45 to 85° from a normal direction to the sample surface, and preferably at 70 or 75°, which is close to the principal angle of silicon at 488 and 514 nm, and would allow the beam passage to be unhindered by the walls of the collector. By operating at this shallow angle, the particle photon flux is enhanced as illustrated in  FIGS. 1A and 1B  and the discrimination against the pits is substantial.  
      Beam Position Correction  
      A prerequisite for the comparison of signals generated by two detection channels for a given defect is the ability to place the two spots on the same location. In general, semiconductor wafers or other sample surfaces are not completely flat, nor do they have the same thickness. Such imperfections are of little concern for anomaly detection employing a normal incidence beam, as long as the wafer surface remains within the depth of focus. In the case of the oblique illumination beam, however, wafer-height variation will cause the beam position and hence the position of the illuminated spot to be incorrect. In  FIG. 6 , θ is the oblique incidence angle between the beam and a normal direction N to the wafer surface. Thus, as shown in  FIG. 6 , if the height of the wafer surface moves from the dotted line position  76   a ′ to the solid line position  76   a  which is higher than the dotted line position by the height h, then the position of the illuminated spot on the wafer surface will be off by an error of w given by h.tan θ. One possible solution is to detect the change in height of the wafer at the illuminated spot and move the wafer in order to maintain the wafer at a constant height at the illuminated spot, as described in U.S. Pat. No. 5,530,550. In the embodiment described above, the wafer is rotated and translated to move along a spiral scan path so that it may be difficult to also correct the wafer height by moving the wafer while it is being rotated along such path. Another alternative is to move the light source and the detector when the height of the wafer changes so as to maintain a constant height between the light source and the detector on the one hand and the wafer surface at the illuminated spot on the other. This is obviously cumbersome and may be impractical. Another aspect of the invention is based on the observation that, by changing the direction of the illumination beam in response to a detected change in wafer height, it is possible to compensate for the change in wafer height to reduce beam position error caused thereby.  
      One scheme for implementing the above aspect is illustrated in  FIG. 7 . As shown in system  200  of  FIG. 7 , an illumination beam is reflected by a mirror  202  and focused through three lenses L 1 , L 2 , L 3  to the wafer surface  204   a.  The positions of the lenses are set in order to focus an oblique illumination beam  70 ″ to wafer surface  204   a  in dotted line in  FIG. 7 . Then a quad cell (or other type of position sensitive detector)  206  is positioned so that the specular reflection  70   a ″ of the beam  70 ″ from surface  204  reaches the cell at the null or zero position  206   a  of the cell. As the wafer surface moves from position  204   a  to  204   b  shown in solid line in  FIG. 7 , such change in height of the wafer causes the specular reflection to move to position  70   b ″, so that it reaches the cell  206  at a position on the cell offset from the null position  206   a.  Detector  206  may be constructed in the same manner as that described in U.S. Pat. No. 5,530,550. A position error signal output from detector  206  indicating the deviation from the null position in two orthogonal directions is sent by cell  206  to a control  208  which generates an error signal to a transducer  210  for rotating the mirror  202  so that the specular reflection  70   b ″ also reaches the cell at the null position  206   a.  In other words, the direction of the illumination beam is altered until the specular reflection reaches the cell at null position, at which point control  208  applies no error signal to the transducer  210 .  
      Instead of using three lenses, it is possible to employ a single lens as shown in  FIG. 8 , except that the correct placement of the illuminated spot on the wafer corresponds not to a null in the position sensing signal from the position sensitive detector, but corresponds to an output of the detector reduced by ½. This approach is shown in  FIG. 8 . Thus, controller  252  divides by 2 the amplitude of the position sensing signal at the output of quad cell detector  254  to derive a quotient signal and applies the quotient signal to transducer  210 . The transducer  210  rotates the mirror by an amount proportional to the amplitude of the quotient signal. The new position of the specular reflection corresponds to the correct location of the spot. The new error signal is now the new reference.  
      The above described feature of reducing beam position error of the oblique illumination beam in reference to  FIGS. 7 and 8  may be used in conjunction with any one of the inspection systems of  FIGS. 2A, 2B ,  3 ,  4 ,  5 A and  5 B, although only the quad cell ( 206  or  254 ) is shown in these figures.  
      Spatial Filter  
      In reference to the embodiments of  FIGS. 2A, 2B ,  4 ,  5 A and  5 B, it is noted that the radiation collection and detection schemes in such embodiments retain the information concerning the direction of scattering of the radiation from surface  76   a  relative to the oblique illumination channel  90  or  90 ′. This can be exploited for some applications such as rough surface inspection. This can be done by employing a spatial filter which blocks the scattered radiation collected by the curved mirrored surface towards the detector except for at least one area have a wedge shape. With respect to the normal illumination channel, there is no directional information since both the illumination and scattering are symmetrical about a normal to the surface. In other words, if the normal illumination channel is omitted in the embodiments of  FIGS. 2A, 2B ,  4 ,  5 A and  5 B, the curved mirrored collector  78  or  78 ′ advantageously collects most of the radiation scattered within the toroidal intensity distribution caused by particle scattering to provide an inspection tool of high particle sensitivity. At the same time, the use of a curved mirrored collector retains the directional scattering information, where such information can be retrieved by employing a spatial filter as described below.  
       FIGS. 9A-9F  illustrate six different embodiments of such spatial filters in the shape of butterflies each with two wings. The dark or shaded areas (wings) in these figures represent areas that are opaque to or scatters radiation, and the white or unshaded areas represent areas that transmit such radiation. The size(s) of the radiation transmissive (white or unshaded) area(s) are determined in each of the filters in  FIGS. 9A-9F  by the wedge angle α. Thus, in  FIG. 9A , the wedge angle is 10°, whereas in  FIG. 9B , it is 20°.  
      Thus, if the filter in  FIG. 9B  is placed at position  300  of  FIG. 2A, 2B ,  4 ,  5 A or  5 B where the  200  wedge-shaped area of radiation collection is centered at approximately 90° and 270° azimuthal collection angles relative to the oblique illumination direction, this has the effect of generating a combined output from two detectors, each with a collection angle of 20°, one detector placed to collect radiation between 80 to 100° azimuthal angles as in U.S. Pat. No. 4,898,471, and the other detector to collect radiation between 260 and 280° azimuthal angles. The detection scheme of U.S. Pat. No. 4,898,471 can be simulated by blocking out also the wedge area between 260 and 280 azimuthal angles. The arrangement of this application has the advantage over U.S. Pat. No. 4,898,471 of higher sensitivity since more of the scattered radiation is collected than in such patent, by means of the curved mirror collector  78 ,  78 ′. Furthermore, the azimuthal collection angle can be dynamically changed by programming the filter at position  300  in  FIGS. 2A, 2B ,  4 ,  5 A,  5 B without having to move any detectors, as described below.  
      It is possible to enlarge or reduce the solid angle of collection of the detector by changing α. It is also possible to alter the azimuthal angles of the wedge areas. These can be accomplished by having ready at hand a number of different filters with different wedge angles such as those shown in  FIGS. 9A-9F , as well as filters with other wedge shaped radiation transmissive areas, and picking the desired filter and the desired position of the filter for use at position  300  in  FIGS. 2A, 2B ,  4 ,  5 A,  5 B. The spatial filters in  FIGS. 9A-9E  are all in the shape of butterflies with two wings, where the wings are opaque to, or scatter, radiation and the spaces between the wings transmit radiation between the mirrored surfaces and detector  80 . In some applications, however, it may be desirable to employ a spatial filter of the shape shown in  FIG. 9F  having a single radiation transmissive wedge-shaped area. Obviously, spatial filters having any number of wedge shaped areas that are radiation transmissive dispersed around a center at various different angles may also be used and are within the scope of the invention.  
      Instead of storing a number of filters having different wedge angles, different numbers of wedges and distributed in various configurations, it is possible to employ a programmable spatial filter where the opaque or scattering and transparent or transmissive areas may be altered. For example, the spatial filter may be constructed using corrugated material where the wedge angle α can be reduced by flattening the corrugated material. Or, two or more filters such as those in  FIGS. 9A-9F  may be superimposed upon one another to alter the opaque or scattering and transparent or transmissive areas.  
      Alternatively, a liquid crystal spatial filter may be advantageously used, one embodiment of which is shown in  FIGS. 10A and 10B . A liquid crystal material can be made radiation transmissive or scattering by changing an electrical potential applied across the layer. The liquid crystal layer may be placed between a circular electrode  352  and an electrode array  354  in the shape of n sectors of a circle arranged around a center  356 , where n is a positive integer. The sectors are shown in  FIG. 10B  which is a top view of one embodiment of filter  350  in  FIG. 10A  Adjacent electrode sectors  354 ( i ) and  354 ( i+ 1), i ranging from 1 to n−1, are electrically insulated from each other.  
      Therefore, by applying appropriate electrical potentials across one or more of the sector electrodes  354 ( i ), where (i) ranges from 1 to n, on one side, and electrode  352  on the other side, by means of voltage control  360 , it is possible to programmably change the wedge angle α by increments equal to the wedge angle β of each of the sector electrodes  354 ( 1 ) through  354 ( n ). By applying the potentials across electrode  352  and the appropriate sector electrodes, it is also possible to achieve filters having different numbers of radiation transmissive wedge-shaped areas disposed in different configurations around center  356 , again with the constraint of the value of β. To simplify the drawings, the electrical connection between the voltage control  360  and only one of the sector electrodes is shown in  FIGS. 10A and 10B . Instead of being in the shape of sectors of a circle, electrodes  354  can also be in the shape of triangles. Where electrodes  354  are shaped as isosceles triangles, the array of electrodes  354  arranged around center  356  has the shape of a polygon. Still other shapes for the array  354  are possible.  
      If the wedge angle β is chosen to be too small, this means that an inordinate amount of space must be devoted to the separation between adjacent sector electrodes to avoid electrical shorting. Too large a value for β means that the wedge angle α can only be changed by large increments. Preferably β is at least about 5°.  
      For the normal illumination beam, the polarization state of the beam does not, to first order, affect detection. For the oblique illumination beam, the polarization state of the beam can significantly affect detection sensitivity. Thus, for rough film inspection, it may be desirable to employ S polarized radiation, whereas for smooth surface inspection, S or P polarized radiation may be preferable. After the scattered radiation from the sample surface originating from each of the two channels have been detected, the results may be compared to yield information for distinguishing between particles and COPs. For example, the intensity of the scattered radiation originating from the oblique channel (e.g., in ppm) may be plotted against that originating from the normal channel, and the plot is analyzed. Or a ratio between the two intensities is obtained for each of one or more locations on the sample surface. Such operations may be performed by a processor  400  in  FIGS. 2A, 2B ,  4 ,  5 A,  5 B.  
      As noted above in connection with  FIG. 1C , a pit  32  of comparable size to a particle  24  will scatter a smaller amount of photon flux compared to particle  24  from an oblique beam  28 ′. Moreover, if the oblique incidence beam is P-polarized, the scattering caused by the particle is much stronger in directions at large angles to the normal direction to the surface compared to the scattering in directions close to the normal direction. This is not the case with a COP whose scattering pattern for an oblique incident P-polarized beam is more uniform in three-dimensional space. This feature can be exploited as illustrated in  FIG. 11 .  
      In reference to  FIGS. 2A and 11 , the sample inspection system  500  of  FIG. 11  differs from system  50  of  FIG. 2A  in that an additional detector  502  is employed with its corresponding pinhole  504 . Direction  510  is normal to the surface  76   a  of the wafer  76 . The radiation scattered in directions close to the normal direction  510  are reflected by a mirror  512  through the pinhole  504  to a photomultiplier tube  502  for detection. The radiation that is scattered by surface  76   a  in directions away from the normal direction  510  are collected by mirror  78  and focused to pinhole  80   a  and photomultiplier tube  80 . Thus, detector  80  detects radiation scattered by surface  76   a  in directions at large angles to the normal direction  510  whereas detector  502  detects radiation scattered by the surface along directions close to the normal direction  510 .  
      For the purpose of distinguished particles and COPs, the oblique illumination beam in the oblique channel  90  is preferably P-polarized. In such event, a particle on surface  76   a  illuminated by the oblique illumination beam will scatter radiation in a three-dimensional pattern similar to a toroid, which is relatively devoid of energy in a normal direction  510  and in directions close to the normal direction. A COP, on the other hand, would scatter such beam in a more uniform manner in three-dimensional space. Therefore, if the signal detected by detector  502  differs by a large factor from that detected by detector  80 , the anomaly on surface  76   a  is more likely to be a particle, whereas if the signals detected by the two detectors differ by a smaller factor the anomaly present on surface  76   a  is more likely to be a COP.  
      The P-polarized oblique illumination beam in channel  90  may be provided by a laser  52  in the same manner as that described above in reference to  FIG. 2A . The S-polarized beam reflected towards mirror  82  by polarizing beam splitter  62  may be altered into a P-polarized beam by a half-wave plate  84 . Since the illumination beam in the normal channel is not used in system  500 , it may simply be blocked (not shown in  FIG. 11 ). A comparison of the outputs of the two detectors  80 ,  502  may be performed by a processor  400 . Mirror  78  may be ellipsoidal in shape, or paraboloidal in shape (in which case an additional objective similar to objective  104  of  FIG. 2B  is also employed) or may have other suitable shapes.  
      While the invention has been described by reference to a normal and an oblique illumination beam, it will be understood that the normal illumination beam may be replaced by one that is not exactly normal to the surface, while retaining most of the advantages of the invention described above. Thus, such beam may be at a small angle to the normal direction, where the small angle is no more than 10° to the normal direction.  
      While the invention has been described above by reference to various embodiments, it will be understood that changes and modifications may be made without departing from the scope of the invention, which is to be defined only by the appended claims and their equivalents. For example, while only two illuminating beams or paths are shown in  FIGS. 2A, 2B ,  4 ,  5 A,  5 B, it will be understood that three or more illuminating beams or paths may be employed and are within the scope of the invention.