Etchable Core Glass Compositions and Method for Manufacturing a High Performance Microchannel Plate

These and other advantages of the invention will be readily apparent when considered in light of the detailed description hereinafter of the preferred embodiment and of the drawings which include the following figures:

FIG. 1 is a graph showing the plot of viscosity [log (poise)] plotted as a function of temperature (degree Celsius) for two samples of glass, one of which is based upon several analyses of the glass sample;

FIG. 2 is a perspective view of a glass clad tube formed of the glass composition of the present invention;

FIG. 3 is a perspective view of a core glass rod fabricated for use with the glass clad tubing;

FIG. 4 is a cross-sectional view taken along section lines 4--4 of FIG. 2;

FIG. 5 is a cross-sectional view taken along section line 5--5 of FIG. 3;

FIG. 6 is a pictorial representation, in cross-section, of a core glass rod of FIG. 3 inserted into the hollowed out central area of the glass clad tube of FIG. 2;

FIG. 7 is a pictorial cross-sectional representation showing the relationship between the glass clad tube, the core glass and a spacing therebetween forming a clad/core glass assembly;

FIG. 8 is a pictorial representation of a heating furnace utilized for fabricating a glass fiber from a glass clad/core glass assembly of FIG. 7;

FIG. 9 is a pictorial representation of a plurality of single fibers formed into a fiber bundle;

FIG. 10 is a pictorial representation of a multifiber form fabricated of the glass fibers illustrated in FIG. 9;

FIG. 11 is a pictorial representation of a multichannel plate formed from multifiber forms illustrated in FIG. 10;

FIG. 12 is a pictorial representation of a plurality of microchannels formed in a microchannel plate illustrated in FIG. 11 wherein the core glass has been removed to form a path or microchannel; and

FIG. 13 is a pictorial representation of a single microchannel wherein the inner surface thereof has been chemically and thermally treated to form an emissive material and where an electron (e.sup.-) received thereby is transported therethrough to produce a plurality of "N" electrons (Ne.sup.-) as an output.

Glass compositions used in the manufacture of microchannel plates determine the secondary emission properties and gain stability of the microchannel plate. In such microchannel plates, the count of electrons entering into a plurality of parallel microchannels is multiplied such that a plurality of electrons are emitted interiorly in the microchannel or path by the process of secondary emission. As such, each glass channel defines a microchannel for each electron that strikes it.

It is one teaching of the present invention that microchannel plates fabricated, assembled or manufactured from glasses having silicon dioxide (SiO.sub.2) in mole percent of 58 to 68, lead oxide (PbO) in mole percent from 10 to 15, barium oxide (BaO) having mole percent of 5.5 to 16.2 and, other components of bismuth oxide (Bi.sub.2 O.sub.3) having mole percent of 0.3 to 2.1, boron oxide (B.sub.2 O.sub.3) having a mole percent in the order of 2.0 to 3.8, aluminum oxide (Al.sub.2 O.sub.3) having a mole percentage of 0.9 through 1.2 and cesium oxide (Cs.sub.2 O) having a mole percent of 0.0 to 1.9 result in a microchannel having increased secondary emission properties and improved gain stability.

In order to increase the working temperatures, various additional components can be added to the glass composition to vary working temperature and other characteristics of the glass including its secondary emission properties.

Preferably, the glass composition of the present invention has a transformation temperature in a range of about 570.degree. C. to about 610.degree. C. The glass has a liquidus temperature below 1000.degree. C. A glass having the glass composition and property of the present invention has a linear thermal expansion coefficient in the order of 63.times.10.sup.-7 .degree.C. to about 82.times.10.sup.-7 .degree.C. at a temperature between 25.degree. C. and 300.degree. C. Preferably, the surface conductivity, after hydrogen firing of the microchannel, will be in the order of about 10.sup.12 to about 10.sup.13 ohms per square. The physical properties would include a firing temperature greater than 550.degree. C., a transformation temperature (Tg) of about 570.degree. C. to about 610.degree. C. and a Littleton softening temperature (T.sub.7.6) when .eta.=10.sup.7.6 is about 720.degree. C. to about 790.degree. C. The working temperature (when the temperature is equivalent to .eta.=10.sup.4 poise) is in the range of about 945.degree. C. to about 1000.degree. C.

Preferably, the chemical solubility is less than 0.05 milligrams per cm.sup.2 per hour in a 10% hydrochloric acid solution at 20.degree. C.

Typically, the chemical composition ranges of the various elements in the preferred embodiment are as follows:

The compositions and examples of compositions for practicing the invention is set forth in Chart III A and B hereinbelow.

The composition of the preferred embodiment of the invention is presented in the column under Example Ex-9 in the above described table.

The components utilized in the above described glass compositions is prepared from appropriate raw materials in proportions to supply to specific amounts of oxide components by standard melting practices.

In addition to the preferred glass composition identified as Ex-9 in the above Chart III(B), the compositions identified in examples Ex-1 through Ex-8, and Ex-10 and Ex-11 likewise result in a glass composition which, when manufactured into microchannel plates, have high performance characteristics in terms of improved secondary emission and gain stability.

The glass compositions described in Example 1 (Ex-1) through Example 11 (Ex-11) in the above chart result in a glass composition consisting essentially of the forming components present in the glass in the following low percent ranges:

The graph of FIG. 1 is a plot of the viscosity [(log n (poise)] plotted as a function of temperature in .degree.C. The solid line 20 is Example 11 (Ex-11) in the Chart III B set forth above. The viscosity-temperature curve is based upon various measured values of viscosity which were measured using three different melts of glass of the same composition. As such, the viscosity-temperature curve 20 depicts the desired characteristic of a glass composition of the present invention. The preferred embodiment of the glass composition is Example 9 (Ex-9) shown. The viscosity-temperature curve for Example 9 (Ex-9) is depicted by dashed line 22.

Based upon the viscosity-temperature curve of Examples 9 and 11, it is readily apparent that the viscosity at the liquidus temperature stage is less than 10.sup.4. At firing temperature which is greater than 550.degree. C., and the viscosity of the glass is well in excess of 10.sup.13 poise. At the Littleton softening point (T.sub.7.6), the temperature is in the range of about 720.degree. C. to about 790.degree. C. In the working temperature of about 945.degree. C. to about 1000.degree. C., the viscosity of the glass is in the range of about 10.sup.3 to about 10.sup.5 poise.

The perspective view of FIG. 2 shows a glass clad tube, shown generally as 28, formed of a glass composition of the present invention. The glass clad tube 28 has two ends which are shown as ends 30 and 32. The glass clad tube is cylindrical in shape and has a hollowed out central area 34 and an interior surface 36. The cross-sectional shape of the hollowed out central area 34 is preferably circular in cross-section. Also, the exterior surface 40 of the glass clad tube 28 is circular. As such, the outer surface 40 and the hollowed out central area 34 define a thick wall 38 to form the hollow glass tube 28. The hollowed out central area 34 extends axially through the center of the glass tube 28.

FIG. 3 is a perspective view of a core glass rod, shown generally as 46. The glass rod 46 can be formed from any one of several known glass core compositions which is capable of being chemically etched. Several examples of compatible core glass can have the following composition:

As noted hereinbefore, the preferred embodiment of clad glass composition for practicing this invention is Example 9 set forth on Chart III, Part B. The core glass examples set forth on Chart V are usable with the clad glass composition of Example 9.

However, an improved, more compatible etchable core glass composition of the preferred embodiment of the invention usable with the clad glass composition of Example 9 on Chart III is presented under Example Ex-F in the above described Chart V.

These glass core compositions have excellent etching characteristics and substantially the same lineal coefficient of thermal expansion as the glass composition used for the glass clad tube 28.

In the above examples, the total mole percent of SiO.sub.2 +B.sub.2 O.sub.3 +BaO is in the range of about 76 to about 96 and the glass has an average lineal coefficient of thermal expansion in the range of about 74.times.10.sup.-7 per .degree.C. to about 91.times.10.sup.-7 per .degree.C. at a temperature between 25.degree. C. and 300.degree. C.

The following are the basic requirements for etchable core glass for microchannel plates:

1. Etchable by acid solution. The core glass should be etchable by an acid solution such as for example hydrochloric acid. The core glass should be able to be completely dissolved in the acid solution, preferably at room temperature. At the time of etching of the core glass, the clad glass sealed with the core glass should remain essentially unetched. The ratio of etchability between the core glass and the clad glass should be selected such that the etching bath etches away the core glass but does very little damage to the clad glass. A ratio of etchability of core glass-to-clad glass could be in range of about 1:500 to about 1:2000 in order to retain the configuration of the microchannel plate after etching. The glass should not contain any chemical ingredient which has a tendency to deposit after being dissolved in the etching medium. In the meantime, silica gel formation should be neglectable to maintain a clean channel surface after etching.

2. Viscosity matching with clad glass. The viscosity curve of core glass should preferably be close to clad glass in the fiber drawing temperature region which is about 750.degree. C. to about 860.degree. C. At the pressing temperature of MCP fiber boules it should be rigid to maintain the configuration of channels.

3. Matching in coefficient of thermal expansion with clad glass. The difference of coefficients of thermal expansion should be within about 10% to eliminate stress in the fiber boule.

4. Without devitrication during fiberizing and further processing. The core glass should be in a vitreous state throughout all thermal treatment of MCP production. It should be chemically compatible with the clad glass, i.e. without major reaction between two glass to keep a clean interface between clad and core.

Thus, an etchable core glass composition having a viscosity and lineal coefficient of thermal expansion and which is less expensive to fabricate than the core glass of Example A through Example D for core glass for use in manufacturing a high performance microchannel plate comprising the following compositions in mole percent ranges is as follows:

where the Re.sub.2 O.sub.3 comprises any one or more of the lanthanum group rare earth oxides, the total mole percent of which is within the above mole percent ranges for Re.sub.2 O.sub.3.

The preferred shape of the core glass before fabrication of microchannel plates is cylindrically shaped, non-porous rods as described hereinbelow.

The core glass rod 46 is solid in cross-section as depicted by the solid end 50. The exterior shape of the glass rod 46, defined by the exterior surface 48, is generally circular in cross-section. The diameter of the glass rod 46 is less than the diameter of the hollowed out central area 34 as defined in the glass tube 28 of FIG. 2. FIG. 2 shows that the hollowed out central area 34 defines an interior surface 36 which would be spaced from the exterior surface 48 of the glass rod of FIG. 3. FIG. 4 is a cross-sectional view taken along section lines 4--4 of FIG. 2 showing the details of the cross-section of the glass clad tube 28. The glass clad tube 28 has an exterior surface 40, a hollowed out central area 34, an interior surface 36 and a thick wall 38.

FIG. 5 is a cross-sectional view taken along section line 5--5 of FIG. 3 showing that the cross-section 50 of glass rod 46 is circular in cross-section and has an exterior surface 48.

FIG. 6 is a pictorial representation, in cross-section, of the core glass rod 46 of FIG. 3 inserted into the hollowed out central area 34 of the glass clad tube 28 of FIG. 2. FIG. 6 illustrates that the exterior surface 48 of the glass rod 46 is spaced a predetermined distance from the inner surface 36 of thick wall 38 of the glass clad tube 28. The distance therebetween is referred to as space 60 and provides a means for permitting relative movement of the glass rod 46 within the interior of the glass clad tube 28. The width of the spacing could be in the order of about 0.001" to about 0.010". The spacing widths determined by a number of factors such as, for example, the bow of the glass rod 46, the tolerances of the hollowed out central area 34 of the glass clad tube 28 and the like.

FIG. 7 is a pictorial cross-section representation showing the relationship between the glass clad tube 28 and the glass rod 46. The combination is referred to as a clad/core glass assembly 52. Typically, the glass clad tube 28 and the glass rod 46 are fabricated of clear glass, with the glass composition of the glass rod 46 being susceptible to chemical etching by an appropriate etching solution.

In order to produce the beginning fiber preform which is utilized in manufacturing a microchannel plate, the clad/core glass assembly 52 is drawn into a glass fiber in accordance with the pictorial representation of a heating furnace illustrated in FIG. 8.

To form a glass fiber, the clad/core glass assembly 52 is slowly fed downwards into furnace 56. In FIG. 8, the core glass assembly 52 has a holder 54 on top thereof to support and transport the subassembly 52 downward into the furnace 56. In the example of FIG. 8, the furnace 56 includes heating coils 58 which are heated by applying electrical energy through input terminals 64. The heating coils 58 within the furnace 56 heat the lower end of the clad/core glass assembly 52 at point 62. As the subassembly is fed downward, the glass begins a viscous low. The glass exhibits viscous flow to form a cone 66 as illustrated in FIG. 8.

In FIG. 8, the glass melting begins by forming or pulling a glass rod from cone 66 and extending the rod past the bottom of the furnace 46 where the rod then begins to harden. The rod is removed and the glass is then drawn downward forming a glass fiber 68. The fiber 68 is continuously drawn off of the viscous cone 66.

The clad/core glass subassembly 52 is fed slowly into the furnace such that the lower end thereof is continuously heated and maintained at a constant temperature. The glass fiber 68 is drawn off the continuously heated subassembly at a continuous rate of speed forming a uniform diameter fiber. Further, the clad tube and the glass core are fused together so that the fiber consists of a solid center core with a fused outer coating. The center core is of uniform diameter as is the outer core and the outer core is completely coated with no breaks, cracks or the like.

FIG. 9 is a pictorial representation of a plurality of drawn fibers 68 depicted as 68 formed into a multi fiber assembly. As illustrated in FIG. 9, the fibers are individual cylindrical rods which have been reduced to a desired size, say in the order of 4,000 to 1. Each fiber 68 includes a glass core whose outer periphery is surrounded by a glass clad layer.

As noted hereinbefore, the core of each of the fibers 68 is fabricated of material such that the core glass is etchable in an appropriate etching solution such that the core glass can be removed during subsequent processing to form a microchannel plate. The glass clad tube is made from a glass composition which has a softening temperature substantially the same as that of the glass core. Typically, the glass material of the clad tube is different from the glass material of the core glass in that the glass clad tube has a higher lead content which renders it non-etchable under those conditions used for etching the core glass material. As such, the clad tube remains after the chemical etching of the glass core and becomes a means defining a pathway for electrons in the form of a chemically and thermally treated channel which is defined within the hollowed out central area of the clad glass.

Several thousand of the fibers 68, illustrated in FIG. 9, are cut into lengths of single fibers which are then clamped together and then heated and drawn, at an appropriate drawing temperature of the glass in order to form a hexagonal array 70 illustrated in FIG. 10.

A plurality of hexagonal arrays 70, each having several thousand single fibers, are assembled into a multi-assembly. The multi-assembly is fused together in a furnace at appropriate temperatures to maintain the hexagonal configuration within a annular outer ring. The fused multi-assembly is ultimately cut into individual wafers to form microchannel plates 80.

FIG. 11 is a pictorial representation of a wafer 80 having a plurality of multi-assemblies 70 formed therein which is utilized for forming a multichannel plate. Subsequently, the wafer is subject to further processing to form the microchannels.

As illustrated in FIG. 12, the microchannels are formed by chemically etching the core glass from area 76, preferably by etching with a dilute acid. After etching, the clad glass 74 remains to form a microchannel which is illustrated in FIG. 13. Also, the clad glass 74 remains solid and provides a mechanical support for the wafer 80 illustrated in FIG. 11.

After etching, the microchannel plates are heat treated in an atmosphere of hydrogen gas whereby certain reducible oxides, such as lead oxide, are reduced to render the clad tube inner channel surface electronically conductive. In this way, a semiconductor layer is formed in each of the glass clad interior surfaces and the latter extends inwardly from the surface which defines a microchannel 76.

FIG. 13 is a pictorial representation of a microchannel 76 which has the interior surface thereof chemically and thermally treated to make the same subject to secondary emission. When electron e.sup.- enters into the microchannel 76, it collides with the secondary emissive interior surface causing other electrons, such as electrons shown by lines 80, to be emitted from the secondary emission surface. The electrons e.sup.- emitted by the first collision again collide with the secondary emissive surface producing additional electrons shown by lines 84 which results in the amplification of the electron e.sup.- as it is transported through the microchannel 76. The electron e.sup.- is amplified to produce Ne.sup.- as depicted by electrons being emitted in the form of Ne.sup.- electrons shown as 86 in FIG. 13.

It is envisioned that the microchannel plates could be used in a wide variety of applications, such as for detecting charged particles, UV-Photon detection, X-ray and ion detection and for night vision glasses.