Abstract:
A signal conduction channel having a first element that receives electrons at a first end from a vacuum environment, produces photons as the electrons are received, and propagates the photons along a length of the first element to a distal second end, and a second element that receives the photons from the second end of the first element, converts the photons to electrons, and multiplies the electrons, where no additional element is disposed between the second end of the first element and the second element, except optionally at least one of a photon-conductive epoxy, a lens, and an optical coupling plate that touches both the second end of the first element and the second element.

Description:
FIELD 
   This invention relates to the field of scintillation detection. More particularly, this invention relates to reducing signal to noise losses in the scintillation channel of an instrument. 
   BACKGROUND 
   Instruments such as x-ray detection devices and scanning electron microscopes make use of scintillation crystals to receive and detect high energy particles, such as secondary electrons. The particular case of scanning electron microscopes will be generally used throughout this disclosure, but it is appreciated that this is by way of example only, and that other devices that make use of scintillation detection channels are also comprehended hereunder. 
   A scanning electron microscope operates by directing a beam of electrons onto the surface of an object under investigation, such as a substrate upon which integrated circuits are fabricated. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices. 
   This primary beam of electrons is rastered across the surface of the object, and as it does so, so-called secondary electrons are emitted from the surface at characteristic angles and energies. These secondary electrons are received by one or more detection devices of some sort, which—in correlation with the scan pattern of the primary beam—are interpreted as various characteristics of the sample. 
   One such detector is comprised of one or more crystals of a material that scintillates when it is struck by an electron, such as yttrium aluminum perovskite, commonly referred to as YAP, or yttrium orthosilicate, commonly referred to as YSO, or any similar scintillating material. Thus, these scintillation crystals produce photons as they are struck by electrons. The photons are routed from the scintillation crystal to a photomultiplier tube via a light pipe. The photomultiplier tube receives the photons and multiplies—or in other words amplifies—the signal represented by the photons. This signal is received and interpreted by control and analysis electronics, and an image of the object is generated. 
   Unfortunately, there are many ways in which the primary signal, as represented by the original electron that strikes the scintillation crystal, can be diminished or lost. Further, there are many ways in which stray signals, otherwise known as noise, can be introduced into the signal path. Both of these effects tend to reduce the signal to noise ratio of the scintillation channel, which for the purposes of this disclosure is defined as those elements of an instrument from the scintillation crystal to the photomultiplier tube. 
   For example, it is difficult for photons generated within the scintillator to escape across the back surface, even if this surface is roughened and a coupling epoxy is present. There is also cross-channel signal exchange between light and dark field signals, light loss due to the acceptance angle of the fiber bundle, losses due to packing of the fibers and their cladding, absorption of light by the fiber, and losses due to poor coupling from the end of the fiber to the window and the photomultiplier tube. In all, only a few percent of the light generated in the scintillator actually makes it to the photomultiplier tube. 
   Further, the low efficiency of current detectors, at only several percent, necessitates the use of high beam currents and long exposures, increasing sample contamination (burn/carryover) and reducing throughput. In addition, cross-channel interactions causes a reduction in separation between the various detectors, mixing the bright field and dark field images, and reducing the effective dark field angle of the detector. 
   What is needed, therefore, is a system by which problems such as those described above are reduced, at least in part. 
   SUMMARY 
   The above and other needs are met by a signal conduction channel having a first element that receives electrons at a first end from a vacuum environment, produces photons as the electrons are received, and propagates the photons along a length of the first element to a distal second end, and a second element that receives the photons from the second end of the first element, converts the photons to electrons, and multiplies the electrons, where no additional element is disposed between the second end of the first element and the second element, except optionally at least one of a photon-conductive epoxy, a lens, and an optical coupling plate that touches both the second end of the first element and the second element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
       FIG. 1  is a diagram of a system according to a first embodiment of the present invention. 
       FIG. 2  is a diagram of a system according to a second embodiment of the present invention. 
       FIG. 3  is a diagram of a system according to a third embodiment of the present invention. 
       FIG. 4  is a diagram of a scintillation crystal channel configuration according to a first embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   With reference now to  FIG. 1 , there is depicted an embodiment of an instrument  10 . The instrument  10  as depicted in the figures is a very general embodiment of a scanning electron microscope. However, this is an exemplary embodiment, and other instruments also receive benefits from the improvements as described herein, which are not limited to the specific case of an electron microscope. 
   However, with continued reference to the case of an electron microscope, a primary electron beam  14   a  is generated by an electron beam source  12 , and directed toward a sample  16 , from which secondary electrons  14   b  are expelled. The secondary electrons  14   b  are received and sensed by one or more scintillators  18 .  FIG. 1  depicts a segmented scintillator  18 , having at least three parts,  18   a ,  18   b , and  18   c . However, the scintillator  18  could be a single block of crystal, or have more than three portions, such as is depicted in  FIG. 4 , where a bright field channel  18   b  of scintillator material is surrounded by four dark field channels  18   a ,  18   b ,  18   c , and  18   d  of scintillator material. The scintillator material is YAP or YSO in some embodiments, or some other scintillating material in other embodiments. 
   The scintillator  18  is, in one embodiment, ground to a rounded lens-like feature at the distal end of the scintillator  18  from that which receives the secondary electrons  14   b . In this manner, a separate lens between the scintillator  18  and the photomultiplier  22  is not required, which tends to decrease the loss of photons between the scintillator  18  and the photomultiplier  22 . 
   Another embodiment that tends to reduce photon loss is the use of a coupling plate  20  that fits against the distal end of the scintillator  18  and couples the scintillator  18  to the photomultiplier  22 , as depicted in  FIGS. 2 and 3  ( 18   a  in  FIG. 3 ). A light-conductive epoxy is used between the elements in various embodiments. In the embodiment depicted in  FIG. 1 , the photomultiplier  22  is preferably a multi anode photomultiplier  22 , such that several scintillators  18   a ,  18   b , and  18   c  can be coupled to the multi anode photomultiplier  22  without confounding the bright field and dark field channels that are established by the several scintillators  18 . 
   In some embodiments the receiving ends of the scintillators  18  are coated with aluminum  26 , to aid in the reflection of photons down along the scintillators  18  toward the photomultiplier  22 , and for other benefits. If used, the aluminum coating  26  is preferably about fifty nanometers in thickness. In some embodiments, the sides of the scintillators  18  are coated with aluminum, at a thickness that is preferably much more than about fifty nanometers in thickness. 
   In these embodiments, the scintillator  18  acts as a light pipe and lens, and thus additional light pipes, fibers, and lenses are not required. This configuration uses the high index of refraction of the scintillator  18  to an advantage, since the focal point of this built-in lens is extremely short, and allows the photomultiplier tube  22  to be placed right up to the surface of the scintillator  18   a , as depicted in  FIG. 3 . It also allows more of the photons within the scintillator  18  to be captured by the photomultiplier tube  22 , since there is a surface at the receiving end of the scintillator  18 , normal to all of the angles that are transmitted through the scintillator  18 . 
   If necessary, a light pipe could be inserted between the scintillator  18  and the photomultiplier tube  22 . However, light absorption within the scintillator  18  is about one percent per five meters. This makes a nominal one-inch piece of scintillator  18  extremely efficient for conducting light. However, if absorption within the scintillator  18  is much greater than this, then a transition from the scintillator  18  to a light pipe can be made at a position that is closer to the receiving end of the scintillator  18 . 
   Additional light collection efficiency is obtained in some embodiments by placing a reflector  28  around the ground lens portion of the scintillator  18   b , or placing a powerful lens  30  (such as a spherical lens) between the scintillator  18   c  ground end and the photomultiplier tube  22 , both as depicted in  FIG. 3 . This is especially useful when a compact photomultiplier tube  22  with reduced active area is used. 
   Advantages of the proposed system over existing systems are several-fold. Improved light collection efficiency results in reduced sample exposure to the electron beam  14   a , and a proportionately lower rate of sample  16  carbonization. In addition, sample  16  charging or throughput could be greatly improved by utilizing either reduced current for the same exposure time (less charging), or similar beam  14   a  current for a shorter period of time (greater throughput). 
   In various embodiments according to the present invention, a thick scintillator  18  stands off the applied bias between the secondary electrons  14   b  and the photomultiplier tube  22 . The scintillator  18  is also segmented to maximize dark field separation, such as depicted in  FIG. 4 . The space between each scintillator  18  segment or channel  18   a - 18   d  is preferably at vacuum, which produces an excellent numerical aperture for each channel, due to the high index change along the vacuum/scintillator  18  interface. There is preferably only one glued interface, at the window between the scintillator  18  and the photomultiplier tube  22 , as depicted in  FIG. 2 , which depicts separate photomultiplier tubes  22   a ,  22   b , and  22   c  in this embodiment. Even allowing for the reduced numerical aperture of the coupling epoxy and the photomultiplier tube  22  window, both a simple analytical estimate and mathematical modeling indicate a transmission of more than about thirty percent of the light from the scintillator  18  to the photomultiplier tube  22 . Dark field separation is also excellent. 
   In other variations, chamfering is provided along the edge of the aluminized side of the scintillator  18 , which further increases the amount of light that is directed along the scintillator  18  to the photomultiplier tube  22 . Roughening at the non-aluminized side of the scintillator  18  increases transmission through the coupling epoxy to the photomultiplier tube  22 , and does not detrimentally reduce dark field separation, since the scattering occurs at the very end of the optical chain. Introducing a non-contacting absorbing material between the scintillator segments  18   a - 18   d  reduces cross talk between the channels. The surface of this absorbing material is preferably quite rough, to prevent coupling to the scintillator, and to reduce the total internal reflectance along the channel edges. Making this separator out of a white reflective material tends to scatter the light back into each channel  18   a - 18   d , improving transmission efficiency. Utilizing a segmented scintillator  18 , with each section  18   a - 18   d  bonded to an individual channel of sapphire tends to eliminate cross-talk, but introduces an additional interface along the detector light chain. The risk is that light self-absorption will occur along the length of the scintillator  18 . However, this risk is relatively low, as the absorption is only a few percent for a scintillator channel  18   a - 18   d  that is less than about one centimeter in length. The foregoing description of preferred embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.