Abstract:
An electron flux amplifier is provided wherein a microchannel plate (MCP) is monolithically formed with, or bonded to, a semiconductor amplifier. In a preferred embodiment, microchannels are formed to extend into a semiconductor substrate to a predetermined depth from the surface, and a collection diode is formed in the substrate beneath the channels. The collection diode may comprise a single planar diode, or a plurality of electrically isolated diodes to provide for imaging of the electron flux. The electron flux amplifier may be used as a detector in a photomultiplier tube (PMT) having a photoelectronically responsive input surface and one or more accelerating electrodes for directing a photoelectron flux toward the electron flux amplifier.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electronic current amplification and collection structure for photomultiplier tubes and to a photomultiplier tube incorporating such a structure. In particular the current amplification and collection structure includes a micro-channel plate multiplier and a reverse-biased semiconductor diode. 
     BACKGROUND 
     Photomultiplier tubes are known for detection or imaging of electromagnetic signals including signals of particular spectral characteristics such as infra-red signals, visible light signals, ultra-violet, x-rays, and gamma rays. In a typical photomultiplier tube, photons of such signals are incident upon a biased conductive surface, a photocathode, which emits electrons via the photoelectric effect. These primary electrons are then accelerated toward a biased conductor, or dynode, which emits further electrons, i.e., secondary electrons. Amplification is achieved within a photomultiplier tube by arranging several dynodes to receive incident electrons and to emit secondary electrons, and by configuring the biasing electric fields among the dynodes to guide the emitted electrons along paths between successive dynodes. Ultimately, the cascading stream of electrons is collected to provide an electrical current proportional to the incident photon flux. The degree of amplification provided between the initial photon flux and the collected electron current is determined by factors including the electron emission characteristics of the dynodes, the number of dynode stages, and the voltage applied between successive dynodes for accelerating the electrons. 
     It is desirable for photomultiplier tubes to provide as high an amplification as possible for a given applied voltage. It is also desirable for photomultiplier tubes to be compact and mechanically reliable. For imaging purposes, it is also desirable for the two-dimensional cross section of the amplified electron stream to accurately represent the two-dimensional distribution of incident photons. 
     One device for amplifying an electron beam while maintaining the two-dimensional distribution of the beam is a microchannel plate. For example, U.S. Pat. No. 5,086,248 to Horton et al. describes methods for producing a variety of microchannel plate structures formed from semiconductor wafers. A typical microchannel plate includes a body of secondary electron emissive material having a number of pores extending through the body. Electrodes formed on respective sides of the body allow application of a bias voltage parallel to the direction of the pores. In operation, incident electrons collide with the walls of the pores, thus causing a cascade of secondary electrons which further collide with the pore walls to provide amplification of the incident photon flux. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a device for amplifying and collecting electron current in a photomultiplier tube is provided. The device combines a microchannel plate (MCP) formed of a semiconductor material and a planar, reverse-biased semiconductor diode for collecting electrons emitted from the microchannel plate. The MCP and reverse-biased diode may be provided as a monolithic structure by forming the MCP in a semiconductor substrate such that the channels of the MCP extend into the substrate to a predetermined depth, and by forming the diode to be located beneath the bottom of the channels. 
     In accordance with another aspect of the present invention, amplification and collection of an electron flux is enhanced by a structure incorporating a microchannel plate and a planar diode. The microchannel plate and diode are preferably formed monolithically. The microchannel plate amplifies an incident electron flux by emission of secondary electrons. The diode is configured to provide solid-state amplification by mechanisms of electron bombardment induced current (EBIC) and/or by avalanche generation of excess carriers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description, will be best understood in connection with the attached drawings in which: 
     FIG. 1 is a perspective view in partial cross-section of an electron flux amplification and collection device according to one embodiment of the present invention; 
     FIG. 1A is a partial sectional view of an alternative arrangement of the microchannel formed in the device of FIG. 1; 
     FIG. 2 is a sectional view of a device according to this invention that is configured for an imaging application; 
     FIG. 2A is a sectional view of an alternative embodiment of a device configured for imaging applications; 
     FIG. 3 is a schematic diagram of a photomultiplier tube employing an electron flux amplification and collection device according to the present invention; and 
     FIG. 4 is a sectional view of an alternative embodiment of the device wherein electron flux amplification and collection are provided by an assembly of two discrete components. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG.  1 . there is shown an electron current multiplication and collection device  20 . The device  20  is formed of a substrate of p-type semiconductor material in which a pn-junction  23  has been formed by providing an n-type semiconductor region  22  in or on one side of the substrate  21 , hereinafter referred to as the back side of the substrate  21 . The semiconductor material forming the substrate  21  is preferably silicon but may also be a semiconductor material in which a pn-junction can be formed by such techniques as diffusion, epitaxy, ion implantation, and the like. 
     Channels  24  are formed to extend into the top side of the substrate  21 . The bottoms of the channels  24  terminate within the substrate. The channels  24  are preferably formed by selective chemical or physical etching, such as plasma etching, or by other techniques such as laser-assisted drilling. The interior walls of the channels  24  are preferably formed of or coated with a layer of secondary emission material  26 , that is selected to emit secondary electrons in response to electron bombardment when the device is appropriately biased. The secondary emission layer  26  extends as shown along the front side of the substrate. The secondary emission layer  26  is preferably applied by known thin-film deposition methods or may be formed of an appropriate semiconductor material. The secondary emission layer  26  may also include an emission enhancing layer for providing additional secondary electron emission. The emission enhancing layer may be formed in-situ of the same material as the substrate by, for example, thermal oxidation. 
     A conductive, preferably metallic, contact  28  is formed on the front side of the device  20  to provide electrical contact to the secondary emission layer  26 . Another contact  30  is formed on the back side of the substrate to provide electrical contact to the n-type semiconductor region  22 . In operation, the device  20  is biased by connection of a voltage source  32  with the respective contacts  28  and  30  such that the pn-junction is reverse biased, and the secondary emission layer  26  is subjected to a gradient bias extending from the top of the channels  24  to the bottoms thereof. The relative doping of the p- and n-type regions of the substrate is selected so that the depletion region  31  preferably extends to a position at least adjacent to the bottoms of the respective channels  24  when the operative bias is applied. 
     As illustrated in FIG. 1, when an incident electron  34  enters a channel  24  and collides with a side wall thereof, the secondary emission layer  26  emits secondary electrons, which are accelerated toward the bottom of the channel. The secondary electrons collide with the wall of the channel, producing an amplification of electron current as they traverse along the length of the channel. At the bottom of the channel, the resulting electrons  36  are injected into the substrate in the depletion region  31  of the pn-junction  23 . Alternatively, if the depletion region does not extend as far into the p-type region, as shown in FIG. 1, the electrons  36  would diffuse within the p-type semiconductor to the edge of the depletion region. In such an alternative arrangement, the depletion region  31  preferably extends at least to within the minority carrier diffusion length for the p type semiconductor of which the substrate is formed. 
     Once the electrons  36  enter the depletion region, the electric field therein sweeps the electrons  36  across the junction  23  into the n-type region, for collection by the contact  30 . An electrical current is thereby produced that can be measured by, for example, an ammeter  40 . Additionally, the electrical current produced can be further amplified and/or subjected to various electronic manipulation and analysis for providing useful indicia regarding the incident photon flux. 
     It will be appreciated that alternative device configurations can be formed for providing a depletion region to collect minority carrier electrons from the p-type semiconductor substrate. In one such alternative embodiment, the backside conductive contact is selected to form a Schottky barrier with the substrate. The width of the depletion region will then depend on the relative work functions of the substrate and the conductive contact, and on the bias voltage applied to the contact. Such an alternative arrangement, which provides an electron collector, is particularly desirable where the substrate is a compound semiconductor, including III-V semiconductors such as GaAs and alloys thereof. Further alternative structures, such as metal-insulator-semiconductors (MIS), are also suitable for providing a depletion region within the substrate for collecting the injected electrons. These alternative structures can be patterned, as discussed below, for imaging applications. 
     The device  20  is capable of providing amplification of electric current in excess of the amplification that would otherwise be provided by a known microchannel plate configured of the same substrate and having the same geometry and secondary emission layer. This result is due to amplification effects that may occur after the resulting electrons are injected into the substrate. For example, electrons that have been accelerated within the channel to an energy of about 3.6 eV in excess of the thermal energy of electrons in the substrate are capable of generating electron-hole pairs in the substrate upon injection therein, as shown at  42 . Such electron-hole pair generation adds an electron bombardment induced current (EBIC) component to the overall current generated by the device. Additionally, the doping of the substrate  21 , or at least the depletion region  31 , may be selected so that electrons are accelerated within the depletion region to an energy sufficient to cause interaction with the crystal lattice, i.e., an avalanching effect, resulting in further generation of electron-hole pairs, such as shown at  44 . Such avalanche current may add a further component to the overall amplification. 
     As can be appreciated, the relative conductivity of the p-type semiconductor substrate  21  should be lower than that of the secondary emission layer  26  in order to maintain a suitable bias along the length of the channel walls. Suitable materials for the secondary emission layer  26  include silicates; doped glasses, such as lead glass (PbO—SiO 2 ); metal-alkali coatings, such as alkali-ant imonides, including metal oxides, such as MgO or Al 2 O 3 ; doped polycrystalline diamond; or other secondary emitters known in the art. Where the substrate  21  is silicon, the secondary emission layer  26  may be formed by doping or evaporating suitable material onto a thermal oxide layer composed of the substrate material. Where significantly resistive secondary emission layers are used, the p-type substrate should be lightly doped (e.g., less than about 10 18  cm −3  for a silicon substrate), and may include intrinsic or compensated semiconductor material (i.e., undoped material or material that has been doped to compensate for excess impurities). The relatively light doping of the p-type material enhances the extent of the depletion region in the substrate, and it may be desirable in some embodiments to provide a depletion region which extends beyond the bottoms of the channels, or even along the entire length of the channels, during operation. Although the channels  24  are shown to be vertically-oriented in FIG. 1, it is recognized that the channels may be formed to increase the likelihood of electron collisions by tapering the channels from top to bottom. Such a tapered profile can be obtained by using an isotropic etch to form the channels to be wider at the top or front surface of the device than at the bottom or rear ends thereof. 
     In a further alternative embodiment, the channels may be formed at an angle relative to the surface in order to increase the likelihood of electron collisions with the walls of the channels. Such an angled channel structure can be formed of known crystallographic etching techniques. 
     In order to make ohmic contact to the p-type material in embodiments where light doping is utilized, a more heavily doped p +  region is provided in the upper surface of the semiconductor substrate as shown in FIG.  1 A. The diode structure thus provided vertically through the substrate then resembles a p + -p-n diode or a p-i-n diode. The doping gradient near the upper surface region of the device also serves to produce an internal field that aids in the collection of electrons injected or generated in the more lightly doped p-type region of the device. In such an embodiment, electrical contact to the p +  material is made through vias formed in the secondary emission layer  26 . Alternatively, discrete p +  regions may be formed in the upper surface region of substrate  21  to provide ohmic contact with the metallic layer  28 . 
     Referring now to FIG. 2, there is shown a structure  220  suitable for electron amplification and collection wherein imaging of the incident flux is desired. The device  220  is formed of a p-type semiconductor substrate  21 , and has a plurality of channels formed therein. The channels  224   a  and  224   b  which are representative of the channels formed in substrate  221  are lined with a secondary emission layer  226 . A metallic contact  228  is provided on the front side of the device  220 , as described above in connection with the device  20 . On the back side of the substrate, discrete n or n +  regions  222   a  and  222   b  are formed beneath the respective channels  224   a  and  224   b.  The n +  regions  222   a  and  222   b  are aligned centrally with the bottoms of respective channels  224   a  and  224   b.  Discrete metallic contacts  230   a  and  230   b  are formed in contact with the respective n +  regions  222   a  and  222   b.  Electrons received and amplified along channel  224   a  will drift into depletion region  231   a  for collection at n +  region  222   a.  Electrons received and amplified along channel  224   b  will drift into depletion region  231   b  for collection at n +  region  222   b.    
     The device  220  of FIG. 2 functions similarly to the device  20  with respect to amplification and collection of an incident electron flux. However, the arrangement of discrete n +  regions  222   a  and  222   b  and corresponding contacts  230   a  and  230   b  allows electrical current from each of the n +  region to be measured, for example by ammeters  240   a  and  240   b,  in a manner that provides a two-dimensional image of the incident flux. 
     In order to provide for independent detection of electron flux within each of the channels  224   a  and  224   b,  the n +  regions  222   a  and  222   b  are electrically isolated by virtue of the series-opposing diodes formed thereby. The material parameters of the substrate are chosen to prevent the depletion regions  231   a  and  231   b  from overlapping. To further enhance isolation between depletion regions  231   a  and  231   b,  or to provide such isolation in a lightly doped substrate, it may be desirable to form physical barriers between adjacent n +  regions in the imaging device  220 . For example, in FIG. 2A, there is shown an embodiment wherein insulating regions  250  (e.g. of SiO 2  or SiN) are formed between adjacent n +  regions  260 . The insulating regions  250  serve to confine collection of electrons from the respective channels to the corresponding n +  regions formed in the bottom surface of the substrate. In other alternative embodiments, such isolation may be provided by etched grooves or trenches formed in the substrate between adjacent n +  regions. In a further alternative embodiment, individual collection regions are established to collect electrons from groups of two or more channels as desired to obtain a specified spatial resolution and gain per image element. 
     Referring now to FIG. 3, there is shown a photomultiplier tube  300 . The photomultiplier tube  300  includes an evacuated glass envelope  302  having a photocathode  304  located at a forward interior portion of the envelope  302 . An electron amplification and collection device  320  of any of the configurations described above is positioned at the rear of the envelope  302 . Focus electrodes  306  are positioned along the length of the envelope  302  to accelerate and direct electrons within the interior of the envelope toward the amplification and collection device. 
     In operation, the photocathode end of photomultiplier tube  300  is directed at a source of photons. An incident photon  308 , upon colliding with the photocathode  304 , generates a photoelectron  310  which is released from the photocathode  304  into the interior of the envelope  302 . Appropriate voltage biases applied to the photocathode  304  and to the focus electrodes  306 , cause the photoelectron  310  to accelerate toward the amplification and collection device  320 . The resulting current generated by the collection device  30 , including current components generated by secondary emission amplification, electron bombardment induced current, and avalanching, is provided to external instrumentation (not shown) through electrical leads  330  connected with the device  320  and leading through the envelope  302  to the exterior of the photomultiplier tube. 
     The device  320  is constructed in accordance with any of the embodiments described above in which a single collection layer on the bottom side of the device is provided for collecting the total current generated in the device, or wherein discrete collection regions are provided for imaging purposes. The photomultiplier tube  300  may be of the type shown wherein the device  320  provides substantially all of the amplification available. Alternatively, one or more dynodes may be positioned within the envelope to provide further amplification of the electron flux within the photomultiplier as desired in accordance with known techniques. 
     For certain applications it may be desirable to allow independent optimization of the respective microchannel plate and EBIC diode components of the amplification and collection device of the present invention. Such optimization is provided in the device structure shown in FIG. 4, wherein the device is composed of two discrete parts that are held in a mechanically fixed relationship to accomplish the functions of secondary emission amplification in one part, and collection of electrons in the other part (along with solid-state amplification of current by EBIC and/or avalanche mechanisms). Such a structure would be suitable for use in a photomultiplier tube of the type described in FIG. 3, or in a photomultiplier tube employing a series of intermediate dynodes. 
     In the device shown in FIG. 4, a microchannel plate  402  and a planar diode  404  are held together by a fixture  406  for aligning the plate  402  and the planar diode  404 . In an alternative embodiment, the function of holding the plate  402  and diode in alignment may comprise a suitable adhesive for directly bonding the two parts together. The planar diode  404  has a front contact  410 , a single rear contact  430  and a single n doped collection layer  422 . In an alternative embodiment, a plurality of such contacts and corresponding discrete collection regions may be provided in order to obtain imaging of the incident electron flux. 
     In the structure shown in FIG. 4, the EBIC component of electronic current generated in the planar diode  404  may be enhanced during operation of the device by applying a voltage bias between the rear contact  408  of the microchannel plate  402  and the front contact  410  of the planar diode  404 . Such a bias accelerates electrons emitted from the rear of the microchannel plate  402 , and thus increases the energy of the electrons incident upon the planar diode  404 . Such increased energy enhances production of electron hole pairs within the planar diode  404  upon absorption of the incident electrons. 
     The terms and expressions which have been employed are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or any portions thereof. It is recognized, therefore, that various modifications are possible within the scope of the invention as claimed.