Abstract:
One embodiment disclosed relates to an apparatus for inspecting a substrate using charged particles. The apparatus includes an illumination subsystem, an objective subsystem, a projection subsystem, and a beam separator interconnecting those subsystems. Advantageously, the illumination subsystem includes a tilt deflector configured to controllably tilt the incident beam. The tilt of the incident beam caused by the tilt deflector is magnified prior to the incident beam impinging onto the substrate. This technique allows for achieving large beam tilts at the substrate without lens aberrations caused by introducing tilt at the objective lens and without complications due to using a tiltable stage. Other embodiments are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims the benefit of U.S. Provisional Patent Application No. 60/633,955, entitled “Apparatus and Method for Tilted Particle-Beam Illumination,” filed Dec. 7, 2004 by inventors Marian Mankos, Luca Grella and David L. Adler, the disclosure of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to apparatus and methods for inspection or review of substrates, such as semiconductor wafers and masks. 
   2. Description of the Background Art 
   Low energy electron microscopy (LEEM) imaging systems which utilize electrons reflecting or mirroring off of the surface of a flat substrate are complicated when compared to conventional straight-axis electron beam (e-beam) systems. Additional complications are presented because the electron beam passes twice through one or more electron lenses, once upon incidence and a second time upon reflection. 
   Due to these complications, a design including a plurality of lenses arranged along one straight axis is not practically feasible, and a beam separator is needed to split the incident and reflected beams. One implementation of a beam separator uses a prism with a single shaped magnetic field as a beam separator. For example, see E. Bauer, “Low energy electron microscopy,” Rep. Prog. Phys. 57 (1994), p. 895. 
   It is desirable to improve LEEM systems, including those utilized for the automated inspection or review of substrate surfaces. More particularly, it is desirable to improve LEEM systems with a tilt capability. 
   SUMMARY 
   One embodiment of the invention relates to an apparatus for inspecting a substrate using charged particles. The apparatus includes an illumination subsystem, an objective subsystem, a projection subsystem, and a beam separator interconnecting those subsystems. Advantageously, the illumination subsystem includes a tilt deflector configured to controllably tilt the incident beam. The tilt of the incident beam caused by the tilt deflector is magnified prior to the incident beam impinging onto the substrate. This technique allows for achieving large beam tilts at the substrate without lens aberrations caused by introducing tilt at the objective lens and without complications due to using a tiltable stage. 
   Other embodiments are also disclosed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram depicting an apparatus for inspecting a substrate using charged particles in accordance with an embodiment of the invention. 
       FIG. 2A  is a diagram illustrating an untilted beam scattering from a feature on a substrate and a corresponding detected intensity profile. 
       FIG. 2B  illustrating a tilted beam scattering from a feature on the substrate and a corresponding detected intensity profile in accordance with an embodiment of the invention. 
       FIG. 3  is a diagram illustrating an electron beam trajectory being tilt deflected using magnetic fields in accordance with an embodiment of the invention. 
       FIG. 4  is a diagram illustrating an electron beam trajectory being tilt deflected using electrostatic fields in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   One conventional technique and apparatus for providing a tilt capability in an electron microscope uses a complex sample stage with tilt capability. However, the use of a tilting stage is very slow and timing consuming to operate. Moreover, it is difficult to maintain or find the same location on the substrate before and after the tilting. Furthermore, the tilting stage is a complex mechanical assembly that is typically prone to failure due to frequently moving parts. Such failures are especially time consuming due to the vacuum environment. 
   Another conventional technique and apparatus for providing a tilt capability in an electron microscope tilts the incident beam when it passes through the final objective lens. Unfortunately, a beam tilt in the final objective lens increases significantly the lens aberrations. This disadvantageously reduces the resolving power of the microscope tool. 
     FIG. 1  is a schematic diagram depicting an apparatus  100  for inspecting a substrate using charged particles in accordance with an embodiment of the invention. The apparatus  100  includes an illumination subsystem  102 , an objective subsystem  104 , a projection subsystem  106 , and a beam separator  108 . The beam separator  108  is coupled to and interconnects the illumination subsystem  102 , the objective subsystem  104 , and the projection subsystem  106 . 
   The illumination subsystem (illumination optics)  102  is configured to receive and collimate charged particles from a charged-particle source. The charged particles may comprise electrons, and the source may comprise an electron gun  110 . 
   In accordance with an embodiment of the invention, the illumination optics  102  advantageously includes a set of tilt deflectors  130 . The tilt deflectors  130  are configured to tilt the incident beam within the illumination subsystem  102 . In one configuration, the beam is tilted off the normal axis such that the tilted beam  132  enters the beam separator  108  at a slight, but controlled angle. The angle is such that the beam direction is slightly towards the objective optics  104  of the system  100 . The tilt angle is preferably small to maintain low aberrations. In  FIG. 1 , the tilt angle depicted is exaggerated for purposes of illustration. 
   The beam separator  108  is configured to receive the tilted incident beam  132  from the illumination subsystem  102  and to deflect or bend  133  the incident beam by approximately 90 degrees into the objective subsystem  104 . One specific embodiment of the beam separator  108  is disclosed in U.S. patent application Ser. No. 10/775,646, entitled “Prism array for electron beam inspection and defect review,” filed Feb. 10, 2004, by Marian Mankos. The disclosure of the aforementioned patent application is hereby incorporated by reference. Per that disclosure, the beam separator  108  may comprise a magnetic prism array including a central magnetic section, an inner magnetic section outside the central section, and an outer magnetic section outside the inner section. 
   In accordance with an embodiment of the invention, since the incident beam is tilted with respect to the normal axis when entering the beam separator  108 , the incident beam is similarly tilted  134  with respect to the normal axis when entering the objective optics  104 . The objective subsystem (objective optics)  104  is configured to receive the tilted incident beam  134  from the beam separator  108  and to decelerate and focus the beam  136  onto the substrate  112 . The beam  136  impinges upon the substrate  112  with an azimuthal angle magnified by the deceleration action of the objective lens optics  104  and further determined by the amount of beam tilt induced at the illumination optics  102  and other specific configuration aspects of the system  100 . 
   The incident beam impinging at a tilt angle onto the substrate  108  that is typically biased at or near the gun voltage. The electrons are scattered by the specimen, thus forming a two-dimensional image of emitted electrons. The emitted electrons are re-accelerated and refocused by the objective lens  104 , and then deflected by the beam separator  108  into the projection optics  106 . In order to accomplish the deceleration and re-acceleration by the objective subsystem  104 , the substrate is maintained at a negative high voltage potential close to that of the incident beam source while the objective optics is at ground potential. In an alternative arrangement, the substrate (and source) may be at ground potential and the objective optics (and other components) at a high voltage. Further specific details of the arrangement of lenses depend on specific parameters of the apparatus and may be determined by one of skill in the pertinent art. 
   The projection subsystem (optics)  106  may be configured to receive the emitted beam from the beam separator  108  and to magnify and project the emitted beam onto a detector  116 . In this way, a magnified two-dimensional image of the illuminated substrate area is obtained. In one embodiment, the detector  116  may comprise a phosphorescent screen  118  and a camera  120  as depicted. In another embodiment, the detector  116  may include a charge-coupled device (CCD) array. 
   In accordance with an embodiment of the invention, only a very small tilt angle is introduced in the illumination optics  102  so as to maintain low aberrations. Despite the tilt angle at the illumination optics  102  being very small, a large tilt angle may be advantageously achieved as the beam impinges upon the substrate  112 . 
   Applicant has determined that the magnification of the tilt angle at the substrate  112  appears to be due to the strong decelerating field as the electron beam approaches the substrate in such a LEEM system  100 . Due to the strong decelerating field near the substrate  112 , the tilt angle should be magnified by approximately the square root of the ratio of the beam energy and the landing energy. For example, for a 30 keV (30,000 electron-volts) beam energy and a landing energy of 3 eV (3 electron-volts), the ratio would be 10,000, and the square root of 10,000 is 100. Hence, in this example, the illumination angle as the beam  136  impinges at the biased substrate  112  should be about one hundred times (100×) magnified compared with the tilt angle introduced in the illumination optics  102 . As such, a small and easily achievable tilt angle of 0.45 degrees (7.85 mrad) by the tilt deflectors  130  should be magnified at landing to about a 45 degree angle. Therefore, this technique advantageously provides a full range of landing angles from 0 degrees to nearly 90 degrees. 
   The azimuthal direction of the tilted beam at landing is determined by the amount of beam tilt induced and other specific configuration aspects of the system  100 . In accordance with an embodiment of the invention, the resultant image may be analyzed to automatically determine the azimuthal angle of the tilted illuminating beam. 
     FIG. 2A  is a diagram illustrating an untilted beam  210  scattering from a feature  202  on a substrate and a corresponding detected intensity profile. In this example, for purposes of illustration, the feature  202  shown in the top portion of  FIG. 2A  rises above the surrounding surface of the substrate. 
   The untilted beam  210  of incident electrons is shown landing at a zero degree tilt at or near the surface of the substrate. The untilted beam  210  causes a relatively uniform distribution of emitted electrons  212  that are scattered from the substrate. 
   The intensity distribution of emitted electrons is illustrated in the bottom portion of  FIG. 2A . The intensity I is shown as relatively uniform with a slight increase or bup  216  in the intensity at the position Z of the feature  210  due to a greater number of electrons being emitted from the feature rising above the surrounding surface. 
     FIG. 2B  illustrating a tilted beam  220  scattering from the feature  202  on a substrate and a corresponding detected intensity profile in accordance with an embodiment of the invention. The feature  202  shown in  FIG. 2B  is the same as shown in  FIG. 2A . In  FIG. 2B , however, the incident beam  220  is tilted or angled as it approaches the substrate surface. 
   The resultant emitted electrons  222  have a distinctive distribution caused by the tilted incident beam  220 . The intensity distribution of the emitted electrons  222  is shown in the bottom portion of  FIG. 2B . Here, the intensity “I” is shown with a substantially nonuniform profile caused by the presence of the feature  202 . 
   As the tilted beam  220  impinges upon the sidewall of the feature  202  that faces the incoming beam  220 , a corresponding peak  226  in the distribution of emitted electrons  222  is generated. The strong peak  226  is caused, in this instance, by the strongly enhanced emission due to the capture of illuminating electrons on the side wall. 
   Furthermore, a shadowing effect occurs because the feature  202  blocks a portion of electrons from the tilted incident beam  220 . This results in a shadow  228  of the defect  202  in the intensity distribution shown at the bottom of  FIG. 2B . 
   With increasing tilt angle, the shadow  228  extends further, and the difference signal becomes stronger (i.e. the intensity in the shadow area  228  becomes lower relative to the surrounding area). This means that for a given defect size, the inspection can be carried out with a larger pixel size and still find the defect. By using a larger pixel size, the throughput of the inspection may be increased. 
   Advantageously, this technique provides a means for large beam tilts to be achieved at the sample without tilting the beam in the objective lens and without using a tilting stage. This allows for the achievement of high spatial resolution in the tilted mode while using a non-tilting stage, such as a more simple x-y stage. 
   The tilt deflectors  130  in the illumination subsystem  102  discussed above may be implemented in various ways. The implementations may use magnetic fields, electrostatic fields, or a combination of both. The fields may vary in strength and are controllably configured so as to achieve a desired tilted trajectory for the incident beam. 
   One implementation of the tilt deflectors  130  (as suggested by the coils in  FIG. 1 ) may run electric current through coils to generate magnetic fields so as to tilt the incident beam.  FIG. 3  is a diagram illustrating an electron beam trajectory  310  (traveling from right to left) being tilt deflected using magnetic fields in accordance with an embodiment of the invention. In this example, the e-beam trajectory  310  is bent in one direction (upwards in the drawing) by a first pair of coils  313  generating a first magnetic field  314 , then is bent in the other direction (downwards in the drawing) by a second pair of coils  315  generating a second magnetic field  316 . The strengths and distances covered by the magnetic fields are configured so as to achieve a desired tilted trajectory  318  (going from right to left but tilted downwards in the drawing) after the fields have been traveled across by the e-beam. Like in  FIG. 1 , the magnitude of the tilt in the e-beam trajectory  318  is exaggerated in  FIG. 3  for purposes of illustration. 
   In one implementation of the tilt deflectors  130 , electrostatic fields may be generated by charging plates or other electrodes so as to tilt the incident beam.  FIG. 4  is a diagram illustrating an electron beam trajectory  410  (traveling from right to left) being tilt deflected using electrostatic fields in accordance with an embodiment of the invention. In this example, the e-beam trajectory  410  is bent in one direction (towards the far plate in the drawing) by a first pair of plates  413  generating a first electrostatic field  414 , then is bent in the other direction (away from the far plate in the drawing) by a second pair of plates  415  generating a second electrostatic  416 . The strengths and distances covered by the electrostatic fields are configured so as to achieve a desired tilted trajectory  418  (going from right to left but tilted away from the far plate in the drawing) after the fields have been traveled across by the e-beam. Like in  FIGS. 1 and 3 , the magnitude of the tilt in the e-beam trajectory  418  is exaggerated in  FIG. 4  for purposes of illustration. 
   In another implementation, both electrostatic and magnetic fields may be used. For example, the beam may be first bent in one by a magnetic field, then bent in another direction by an electrostatic field. Or, the beam may be first bent in one by an electrostatic field, then bent in another direction by a magnetic field. The fields would be configured to achieve the desired tilt in the final trajectory. 
   The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. The above-described invention may be used in an automatic inspection or review system and applied to the inspection or review of optical or X-ray masks and similar substrates in a production environment. 
   In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
   These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.