US 3458745 A
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July 29, 1969 K. R. SHOULDERS THIN WAFER-CHANNEL MULTIPLIER Filed June 9, 1967 INVENTOR.
Kim/52W Ivar/am; q fwM A 770 RNE Y5 United States Patent U.S. Cl. 313ll04 9 Claims ABSTRACT OF THE DISCLOSURE An electron multiplier of the type employing narrow channels, comprising a plurality of layers of vacuumdeposited molybdenum separated by layers of a resistive molybdenum-alumina composition, all of the layers having microscopic holes which are aligned to provide narrow channels extending through the thickness of the multilayered device. Progressively higher voltages are connected to succeeding layers of molybdenum to establish a definite voltage gradient along the narrow channels. The device is manufactured by utilizing a base with microgrid holes of 0.2 to 0.5 micron diameter, and evaporatively deposit layers to cause them to acquire corresponding holes.
Background of the invention This invention relates to amplifying devices and more particularly to improved thin wafer channel electron multipliers and methods for producing them.
Microchannel multipliers are used to multiply, or intensify, electron beams. The multipliers comprise a wafer of resistive material in which are formed microscopic channels extending through the entire thickness of the wafer. One side of the wafer is connected to one potential such as ground while the other side is connected to a high potential such as 1500 volts, thereby creating a voltage gradient along the channels. An electron beam source is directed at one face of the Wafer. When an electron enters a channel and strikes the channel walls, one or more electrons may be ejected from the surface. These electrons continue down the channel, accelerated by the axial field developed by the applied voltage, and strike the channel Walls further down the channel Where additional electrons are ejected. This multiplication process continues along the channel to thus yield a very large number of electrons emerging from the last layer.
Electron beams to be multiplied often have a substantial cross sectional area and an intensity which varies at different points of the beam, so that the beam may contain a visual image. If such a beam is directed at a phosphor coating, it can cause the phosphor to emit light with intensities proportional to electron beam intensity, and the image contained in the electron beam can be viewed. Electron beam multipliers preferably have very narrow channels so that each small area of the beam is individually amplified and the image contained in the beam is maintained with considerable resolution.
Presently available microchannel Wafers for use in electron multipliers have channels of about 12 microns in inside diameter with the axes of the channels being about 18 microns apart. Such wafers sometimes have been constructed out of a bundle of small glass tubes by heating them to cause them to adhere to each other and drawing them down to a fine diameter, and then repeat ing the process until a tube with extremely tiny channels results. Small lengths of such a tube, constituting wafers, have been utilized by connecting a voltage difference between opposite faces, to establish an axial electric field down the length of each channel thereof. Such a device has several disadvantages including the fact that the field Patented July 29, 1969 "ice down each channel is not uniform because the resistivity of the material is not the same at every point of the wafer. Accordingly, equal amplification at each area of the beam cannot be assured. Furthermore, the appreciable diameter of the channels is not as small as would be desired, inasmuch as narrower channels would enable the maintenance of greater resolution of the electron beam.
Summary of the invention The device of this invention provides a thin wafer for an electron multiplier wherein the channels are extremely narrow, in the range of 0.2 to 0.5 micron, and enables the attainment of a uniform and predictable electric field gradient down every channel. The wafer comprises alternate layers of an electrically conductive material such as molybdenum, separated by layers of a resistive material such as molybdenum-alumina, all of the layers have corresponding small holes forming channels projecting through the thickness of the wafer. The wafer may be utilized by electrically grounding one of the conductive layers, and connecting successively higher voltages to each of the successive conductive layers. For example, a wafer having fifteen conductive layers separated by resistive layers may be utilized by connecting one conductive layer of the stack to zero potential, connecting the next conductive layer to volts, the next to 200 volts and so forth until the fifteenth conductive layer is connected to 1500 volts. The electric field gradient through each narrow channel is relatively uniform because fifteen evenly spaced points therealong have voltages which are uniformly progressive. Although the voltage between any two conductive layers may not be uniform due to variations in resistivity of the layer between them or buildup of charges, the overall uniformity of the wafer is greatly increased as compared to previously available devices.
The device is constructed by utilizing a sheet or disc of material having very fine holes which constitute a high proportion of the area of the material. Self-perforated microplastic grid material is available which has very small perforations on the order of a fraction of a micron in diameter, with such perforations comprising a high percentage of the area of the grid material such as 50%. The microgrid material is placed in a vacuum chamber and alternate layers of molybdenum and molybdenumdoped alumina are applied to the grid material by vacuum deposition.
In one embodiment of the invention a support ring having a central hole is utilized, with a disc of self-perforated microplastic grid material disposed on one face of the support over the central hole. The support ring is placed in a vacuum chamber and molybdenum is applied in layers of approximately micron in thickness, each layer covering the round area of the disc or a slightly smaller area. Resistive layers of molybdenum-doped alumina of approximately /2 to 1 micron in thickness are deposited after each molybdenum layer to separate the molybdenum layers. Each layer of molybdenum is formed with a tab extending onto the perimeter of the support ring for enabling the connection of electrical leads to the layer. A total of perhaps 16 sets of layers is deposited on top of the wafer of microgrid material. The vacuum deposited layers have surfaces which match the surface of the microgrid material, including the very small holes therein, and as a result, the multilayered structure has channels aligned with the holes in the microgrid material.
Brief description of the drawings FIGURE 1 is a sectional pictorial view of a portion of a thin water channel multiplier constructed in accordance with the invention;
FIGURE 2 is a pictorial view of a complete channel multiplier constructed in accordance with the invention, which utilizes the portion of a multiplier shown in FIG- URE 1;
FIGURE 3 is a sectional side elevation view of the channel multiplier of FIGURE 2; and
FIGURE 4 is a representational view of a portion of the channel multiplier material of the invention, showing its connection to a power source and the multiplication phenomena involved in its operation.
Description of the preferred embodiments FIGURE 3 of the drawings shows the channel multiplier which comprises a support ring of electrically insulative material, and a multichannel wafer 12 disposed on one face of the ring over its hole. A multiplicity of tabs 14 extend from the edge of the wafer 12 and onto the ring 10. The device is used by placing it in a low intensity electron beam 16, to generate a high intensity electron beam 18. The pictorial view of FIGURE 2 shows the arrangement of the wafer 12 and tabs 14. As can be seen, the tabs 14 extend from the wafer at different angles around its 360 degree circumference, so as to electrically separate the tabs from each other. The wafer 12 is extremely thin and the tabs 14 actually lie on top of the ring 10.
The construction of the multichannel wafer 12 is illustrated in the pictorial view of FIGURE 1 which shows a cross section of a portion of the wafer. The wafer 12 comprises a base of self-perforated microplastic grid material which has holes of several microns diameter or less. Such material can be obtained with holes of very small diameter, or narrowest dimension, such as 0.2 to 0.5 micron and with a high density of holes such as 50% of the area. Disposed on the base 20 is a layer of material 22 such as alumina which has been doped with molybdenum to form a high resistance material. A thin layer or film of conductive material 24 such as molybdenum is disposed on the layer 22 of resistive material. Successive layers of conductive and resistive material are disposed on top of the base 20, each layer of conductive material, such as layer 24 being separated from the succeeding and preceding conductive layers by a layer of resistive material such as layer 22. The multichannel wafer 12 shown in the drawings comprises sixteen layers of conductive material on top of the base 20.
In order for the wafer to multiply an electron beam it must be connected to a source of high voltage, by connecting the various layers 22 of conductive material to various potentials. FIGURE 4 illustrates the multiplication or amplification process of the device. A channel is shown, which is formed by the walls of successive layers of conductive material 24A and resistive material 22A. A voltage source V provides a high potential such as 1500 volts, which is stepped down through resistors 32 in 100 volt steps; for example, junction 34, may be at 400 volts so that the layer of conductive material to which it is connected is at this voltage, junction 36 may be at 300 volts, and junction 38 may be at 200 volts. An electron indicated at 40, from a beam of electrons to be amplified or multiplied enters the channel 30. The channel is narrow, having a length-to-diameter ratio of about 50 to 100, and the electron is likely to strike the channel walls. When the electron 40 strikes the material of the wafer it may cause the emission of two electrons which are accelerated down the channel by reason of the higher positive voltages further down the channel. Each of these electrons may strike a succeeding layer of conductive material and cause the emission of electrons therefrom. As a result, many electrons are finally emitted from the channel for each electron 40 which initially enters the channel.
The connection of the voltage source to the layers of emissive material is made through the tabs 14. As shown in FIGURE 2, the tabs are separated so that different voltages can be connected to them. The' tab connected to the layer adjacent the base 20 is indicated at 42 and the tab connected to the layer of emissive material furthest from the base 20 is indicated at 44. A wide angular separation is made between the tabs 42 and 44 of the first and last layers because a high voltage such as 1500 volts exists between them. Only about volts potential difference exists between any other two adjacent tabs.
In the fabrication of the device, a disc of self-perforated microplastic grid material with a high density such as 50%, of 0.2 to 0.5 micron diameter holes is positioned on a support ring of insulative material such as quartz, sapphire, or alumina and attached thereto by an epoxy or other attaching means. A transition coating such as silicon oxide may be applied immediately over the disc of grid material. The assembly is then placed in a vacuum chamber and alternate layers of a conductive material and a resistive material are deposited. Molybdenum may be used as the conductive material and may be applied in thicknesses such as one hundredth of a micron (100 angstroms), while alumina or molydoped-alumina may be used as the resistive material in thicknesses such as one-half to one micron.
The resistive layers of alumina are vacuum deposited through a mask with a circular hole of approximately the same diameter as the base of grid material. The layers of conductive material are deposited through a mask with a circular hole slightly smaller than the circular hole through which the insulative materials are deposited. The conductive-layer mask also has a slot extending radially out from the circular hole to allow for the deposition of a tab on the support ring. After each vacuum deposition of a layer of resistive material, the mask for the conductive layer is positioned, a conductive layer is deposited and the resistive layer mask is repositioned. Each time the conductive material mask is repositioned it is turned 20 so that the tabs are angularly separated from each other. Each tab subtends an angle of approximately 5, and the tabs are 20 apart from each other with the tabs of the first and last conductive layers being separated by 60 The thickness of the resistive layers is chosen so that they can withstand the potential diiference between the layers of conductive material they separate. Alumina in thicknesses of /2 to 1 micron withstands potentials of more than 100 volts. The use of molybdenum-doped alumina results in small currents flowing through the resistive material to provide for a more gradient of voltage through the layers instead of unpredictable abrupt differences. Resistance values ranging from l0- to over 10 ohm-cm. of the molybdenum and alumina mixture can be obtained by controlling the proportion of molybdenum and alumina. For a wafer with the geometry described above a resistance on the order of 10 ohm-cm. will provide uniformity with very small power dissipation. A low resistivity is permissible so long as it is more than about 10 ohm-cm. so that power dissipation is kept to a reasonable level. A controlled and fairly uniform low conductivity is desirable in the resistive layer also to avoid large charge buildup on the channel walls which interfers with the maintenance of a uniform voltage gradient.
The thin wafer channel multiplier of the invention is useful for amplifying electron beams to enable their display on an image tube. A wafer of approximately 0.3 centimeter is typical of the size required and such wafers constructed in accordance with this invention readily provide output currents of 10- to 10- amperes over the approximately .07 square centimeter area. The beam emerging from the thin wafer channel multiplier may be directed against a phosphor screen of an image tube for enabling visual display. The small wafer diameter permits good depth of focus with-relatively small focusing apparatus in the image tube. In presently used channel multipliers an electron-transparent opaque membrane is placed over the channel multiplier to prevent the passage of light through the channels which interfers with the display generated by the focused electron beam. However, for very small diameter channels, those substantially below visible light wavelengths, i.e. below approximately .4 mircon, the need for an electron transparent opaque membrane may be eliminated.
The narrow channel multipliers of the invention may be constructed in a variety of forms. For example, the microgrid structure on which successive layers are deposited may be removed by chemical etching or other means after formation of a few layers of conductive and resistive material. As another example, instead of evaporative deposition, other methods such as electroplating may sometimes be used to provide microchannels on a microgrid structure. The resulting electron multiplier, with large length to diameter ratio channels and with a multilayered structure which assures good uniformity of electric field, provides an electron multiplier of very good performance characteristics.
While a multilayered structure provides many points along the length of the channels of defined potential, many steps are required to deposit the many layers. The multilayer structure can be eliminated by providing a wafer of uniform moderate resistivity. The control of resistivity is difficult, but a one-layered wafer is inherently simpler to produce and use. The extremely small channels provided by this invention can be obtained for a single layered device just as they can for a multilayered device. For example, a wafer may be produced by establishing a disc of microgrid material with holes of 0.2 to 0.5 micron diameter in a vacuum chamber, and depositing a single layer of perhaps 30 microns thickness of molybdenum-doped alumina on the disc. Such depositions to thicknesses of more than 30 microns have been found to retain the perforations of the base without substantial narrowing of the channels.
While a particular embodiment of the invention has been illustrated and described, it should be understood that many modifications and variations may be resorted to by those skilled in the art, and the scope of the invention is limited only by a just interpretation of the following claims.
What is claimed is: 1. A channel multiplier for intensifying electron beams comprising:
at least one layer of resistive material capable of producing secondary emissions, with a multiplicity of channels formed therethrough, a majority of said multiplicity of channels having a narrowest crosssectional dimension of less than 0.4 micron; and
means for applying an electrical potential difference between opposite ends of said channels of said layer of resistive material, to produce an electric field along said channels.
2. A channel multiplier as defined in claim 1 wherein:
said layer has a thickness ranging between one micron and one hundred microns and the resistivity of said resistive material ranges between and 10 ohmcentimeter, whereby a current flow occurs therethrough which provides a relatively uniform voltage gradient along said channels upon application of a stack having a plurality of sets of layers, every other layer constructed of electrically conductive material and the remaining layers being constructed of electrically resistive material, said layers having a plurality of holes having a maximum cross-sectional dimension of less than several microns and the holes in each layer being aligned with the holes in the other layers for forming elongated channels; and
conductor means for applying operating potentials to said electrically conductive layers.
5. A channel multiplier as defined in claim 4 wherein:
a majority of said holes in said layers have a narrowest cross-sectional dimension of less than 0.5 micron, to suppress the passage of visible light therethrough.
6. A channel multiplier as defined in claim 4 wherein:
the thickness of said insulative layers is on the order of 1 micron and the resistivity thereof ranges between 10 ohm-centimeter and 10 ohm-centimeter, whereby to provide a definite voltage gradient between adjacent layers of conductive material when they are connected to potential differences on the order of volts.
7. A channel multiplier as defined in claim 4 wherein:
said conductive layers are constructed of molybdenum of a thickness on the order of 1() microns, said insulative layers are constructed of molybdenumdoped alumina with a thickness on the order of 1 micron, and said holes have a narrowest cross-sectional dimension on the order of one-half mircon.
8. A channel multiplier comprising:
a support with an aperture therein:
a multiplicity of layers of resistive material capable of secondary emissions disposed over said aperture in said support, said layers all having corresponding holes, a majority of which have a narrowest crosssectional dimension of less than one micron; and
a multiplicity of layers of electrically conductive material disposed between said layers of resistive material having a tab extending outwardly therefrom, the tabs of the different conductive layers being spaced from each other about the circumference of said layers.
9. A channel multiplier as defined in claim 8 wherein:
the circumferential separation between the tab of the conductive layer nearest said base and the tab of the conductive layer furthest from said base is greater than the circumferential separation between the tabs of any two adjacent conductive layers.
References Cited UNITED STATES PATENTS 2,881,384 4/1959 Durant 313-346 X 2,931,914 4/1960 Orthuber et al. 250-213 3,179,834 4/1965 Ochs 313329 X 3,202,854 8/1965 Ochs 313329 X 3,207,937 9/1965 Hannam 313-329 X FOREIGN PATENTS 965,044 5/ 1962 England.
JAMES W. LAWRENCE, Primary Examiner DAVID OREILLY, Assistant Examiner US. Cl. X.R. 313-405, 329, 346, 348