Abstract:
A waveguide made of optical glass used as an detector in electron microscopy, having a beveled hole through which an electron beam passes and a phosphor coated region to detect secondary and back-scattered electrons. The photons generated by secondary and back-scattered electrons striking the phosphor coated region are directed to a photomultiplier detector mated to the waveguide by internal reflections which are further enhanced by reflective surfaces. Further, photon transmission from the waveguide to the photomultiplier is enhanced by providing a flared section at the mating end to reduce internal reflections.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a waveguide for the efficient detection and collection of back-scattered electrons in electron beam apparatuses such as electron microscopes. 
     2. Description of the Related Art 
     Back-scattered electrons are detected using scintillators. Typically electrons are directed to strike a phosphor coating on a piece of glass or plastic so that the light generated by the phosphor is directed onto a detector placed at one end of a light guide. A typical light guide is shown in FIG.  1 . The drawback with this type of detector is that it has a low collection efficiency, thus limiting the speed at which the system can be operated. Also, the collection efficiency is not very homogeneous throughout the phosphor coating, i.e. the collection efficiency is a strong function of where the electron strikes the phosphor. Electrons that strike the phosphor on the same side of the hole as the detector are much more efficiently collected than those that strike the opposite side of the hole from the detector. The collection inhomogeneity leads to a reduced contrast depending on how the electrons scatter from the sample. 
     SUMMARY OF THE INVENTION 
     A primary object of this invention is to provide a more efficient waveguide for detecting back-scattered and secondary electrons, particularly back-scattered or secondary electrons reflected from or emitted by a sample scanned by an electron beam from a minicolumn or microcolumn used in an electron microscope. A waveguide of optical glass is designed so that, when back-scattered or secondary electrons are detected by a phosphor deposited on the waveguide, the light emitted by the phosphor is reflected internally along the waveguide to a detector. The inventive design includes surfaces in the region of the phosphor deposition which are coated with a highly reflective material to direct the light towards the end of the waveguide to which the detector is coupled. Further, the waveguide may include a flared section at the detector end to ensure that all rays striking the output face of the waveguide strike the surface at angles less than the critical angle and are transmitted to the detector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and benefits of the invention will be readily appreciated in the light of the following detailed description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings wherein: 
     FIG. 1 shows a conventional design. 
     FIG. 2 shows a first embodiment of the inventive waveguide. 
     FIG. 3 shows a typical installation of the inventive waveguide. 
     FIG. 4 shows a second embodiment of the inventive waveguide. 
     FIG. 5 shows a further embodiment of the inventive waveguide. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described with reference to the attached drawings, wherein identical elements are designated with like numerals. 
     The inventive waveguide offers clear and distinct advantages over other waveguides used to detect secondary or back-scattered electrons. In a typical installation, referring to FIG. 3, a waveguide  20  used to detect back-scattered and secondary electrons  5  is positioned within an electron microscope  2  containing an electron beam source  3 . The electron beam  4  passes through a beveled hole  23  and strikes a sample under inspection  6 . The electron beam  4  generates back-scattered and secondary electrons  5  which in turn strike the phosphor coating  22 . The phosphor coating  22  acts as a Lambertian source directing most of the emitted light into the waveguide  20  perpendicular to the top  28  and bottom  27  faces, where the light is reflected off the interior surfaces and directed towards a detector  1 , which may be a photomultiplier tube detector. 
     In a conventional design, as shown in FIGS. 1 a  and  1   b , back-scattered and secondary electrons  5  strike a phosphor coating  22  which is deposited on an angled face  12  annularly about a hole  11  through which the electron beam  4  passes. Photons from the phosphor coating  22  are emitted into the waveguide  10  and are reflected by the internal surfaces of the waveguide  10 . Photons which strike the interior surface of the waveguide  10  at an angle of incidence (AOI) greater than the critical angle (critical angle=ArcSin(1/n), where n is the refractive index of the waveguide material) are internally reflected, while those that strike the inner surface at less than the critical angle are lost from the waveguide  10 . The angled sides  13  tend to reflect photons towards the rear face  17  either directly or off the side faces  14  or the top  18  and bottom faces  16 . A detector is then optically coupled to the rear face  17  to collect the reflected photons. 
     The inventive waveguide, FIGS. 2 a  and  2   b , provided improved efficiency of the transmission of photons from one end of the waveguide  20  to the other end. As back-scattered and secondary electrons  5  strike the phosphor coating  22 , photons are emitted into the waveguide  20 . The photons that strike the interior surface of the beveled hole  23  are reflected radially outward at shallow angles nearly parallel to the top  28  and bottom  27  faces. To further enhance photon reflection, a reflective coating  25  is deposited on the inside surface of the beveled hole  23 . The material of the reflective coating  25  is selected according to the wavelength of the light produced by the phosphor coating  22 , and is typically aluminum or silver, though other suitable reflective coatings also may be used. Those photons which are reflected along the longitudinal axis will arrive at the rear face  29  either directly or after one or more reflections off the side faces  26 , the top face  28  or the bottom face  27 . Those photons reflected towards the front of the waveguide  20  are further reflected off the angled sides  24  and directed towards the rear face  29 . The angled sides  24  also have a reflective coating  25 . 
     Further to minimize reflected loss at the rear face  29 , the waveguide  20  preferably has a flared section  21 . If the waveguide  20  were manufactured without the flared section  21 , a fraction of the light striking the rear face of the straight section would hit the surface at angles greater than the critical angle and would not reach a coupled detector. Including the flared section  21  ensures that all rays that strike the rear face  29  of the waveguide  20  are transmitted through the rear face  29  and reach the detector. The flared section  21  increases the collection efficiency of the waveguide  20  by approximately 30%. The flared section  21  can be manufactured integrally with the waveguide  20 , or can be manufactured separately and joined to the straight section in a manner that provides minimal optical losses at the interface. 
     The beveled hole  23  preferably has a bevel angle of approximately 45° and the angled sides  24  meet at an angle of approximately 90°, though other angles also may be used. Also, the taper of each of the four sides of the flared section  21  preferably is about 10°. The dimensions of the inventive waveguide are such that the waveguide  20  is usable in an electron microscope with a microcolumn as an electron source  3  having a cross-sectional area of about one inch square. The dimensions of the waveguide are approximately 22 mm in length of which approximately 16 mm is a straight section and 6 mm is the flared section. The flared section is 6 mm wide and 1.5 mm thick at one end, and 8.1 mm wide and 3.6 mm thick at the other. Of course, the dimensions of the waveguide  20  are variable to meet the needs of particular situations. All surfaces of the waveguide  20  are polished to specularly reflect light impinging on the surface. 
     In a further embodiment as shown in FIGS. 4 a  and  4   b , the dimensions of the flared section  41  have been changed to allow the waveguide  40  to be placed closer to the sample  6 . In this embodiment, the bottom face  47  has not been flared, and to compensate, the length of the flared section has been increased to 8 mm, and the flare angle of the top face  48  has been increased to approximately 15° with corresponding increases in the dimensions of the rear face  49 . 
     In a still further embodiment as shown in FIG. 5, the waveguide  40  as shown in FIGS. 4 a  and  4   b  is coupled with a first end of a cylindrical light guide  57  for the purpose of placing the detector  51  in a more mechanically advantageous location. The cylindrical light guide  57  is composed of two optical cylinders  61  of a refractive index close to that of the flared waveguide  40  and of identical diameters to each other. The cylinders  61  are aligned coaxially and a tight fitting sleeve  58  with a reflective coating disposed on its interior surface surrounds the gap between the cylinders  61 . The lower cylinder is engineered to have a face  59  at an angle of 45° to the axis of the cylinder  61 , and further having a notch  60  with a flat surface that is able to mate with the rear face  49  of the flared waveguide  40 . The angled face  59  is further coated with a reflective coating  45 . A detector  51  is coupled to the second end of the cylindrical light guide  57 .