Metal working – Barrier layer or semiconductor device making – Barrier layer device making
Reexamination Certificate
1997-07-08
2001-12-18
Graybill, David E. (Department: 2814)
Metal working
Barrier layer or semiconductor device making
Barrier layer device making
C438S396000
Reexamination Certificate
active
06331194
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to insulator and dielectric material fabrication and more particularly to layered stacks of alternating bulk insulators and foil-like conductors, and bulk insulators with half-buried parallel rows of conductors, that exhibit very high voltage surface breakdown characteristics, and further to the diffusion braze furnace and optical photoresist deposition, irradiation and development methods for fabricating such structures.
2. Description of Related Art
Glass, ceramic, and other such materials are universally relied on as insulator materials in high voltage systems. But such materials allow the insulators they are used in to be damaged by avalanche and flashovers that occur when the insulator has been subjected to a voltage over-stressing. Fine tracks can develop that lower the insulator's breakdown voltage to successive exposures to stress voltages. Conventional bulk material insulators also tend to be very large, and the systems that incorporate them must necessarily provide enough room to accommodate them.
Many electronic devices depend on a pair of opposing high voltage electrodes contained in a vacuum. Antique vacuum tubes were once used in radios and TV's and comprised glass envelopes in which were disposed at least one cathode and anode. More complex vacuum tubes had one or more grids and control screens placed between the cathode and anode to control the plate current. Usually the plate voltages used did not exceed 200-300 volts, and so the cathode and anode connections could all be brought out together in a single base. But glass envelope surface flashover can occur with vacuum tube devices that use plate voltages over 100 kV. Some neutron tubes need to operate at well over 200 kV across an insulator only a few centimeters long.
Conventional dielectric materials for vacuum tube devices, capacitors, accelerators, and other high-voltage applications are typically made from glass, ceramics and other metal oxides, polymers, or other common bulk materials. Simple homogenized mixtures of such materials are also conventional. Polymer films used as dielectric layers and dielectric mixtures spread over a conductive surface are common ways to fabricate capacitors.
A widely held view of the process by which an insulator-vacuum interface breaks-down contends that there is an enhancement of the electric field at triple points, e.g., points where there is an intersection of a vacuum, a solid insulator and an electrode. Electrons that are field emitted from a triple point on a cathode initially drift in the electric field between the end plates of the insulator which is a dielectric and is polarized when the emitted electrons impact the surface and knock loose additional electrons in a kind of chain reaction. This results in an electric field which further attracts additional electrons into the surface of the insulator. The electron collisions with the surface can liberate a greater number of electrons than originally collided with the surface, depending upon the electron energy of the collisions. This can lead to a catastrophic event in which the emission of these electrons further charges the insulator surface, leads to more collisions with the surface, and the release of even more electrons. This growing electron bombardment desorbs gas molecules that are stuck to the insulator surface and ionizes them, creating a dense plasma which then electrically shorts out the surface of the insulator between the electrodes, e.g., secondary electron emission avalanche (SEEA).
The scale length for the electron hopping distance along a conventional insulator's surface can be on the order of a fraction of a millimeter to several millimeters. When isolated conductive lamination layers are alternated with insulator lamination layers, SEEA current is prevented such that no current amplification can take place. The electron current amplification due to secondary emission is stopped when the electrode spacing is comparable to the electron hopping distance. Direct bombardment of the surface by charged particles or photons can still liberate electrons from the insulator, but the current will not avalanche. Surface breakdown then requires the bombardment by charged particles or photons that is so intense that adsorbed gas is ionized or enough gas is released from the surface that an avalanche breakdown in the gas occur between the plates.
The microstack was assumed to act as a capacitive voltage divider, and the voltage between layers was assumed to be a constant on the time scale of streamer creation. Such microstack insulators were designed for specific pulse periods and for known residue gases in a system. Conductors such as copper and tungsten were either too soft or too hard, and dielectrics such as NYLON, TEFLON, and LEXAN (polycarbonate) were too unstable or melted during fabrication with a loose preassembled stack that was hydraulically pressed into a solid laminate block. So samples were made with 0.010″ sheets of MYLAR and stainless steel in conical stacks that were cured by twenty-hours of heating and then surface polished.
Some of the present inventors participated as authors in the preparation of a paper titled, “High Gradient Insulator Technology for the Dielectric Wall Accelerator”, for the 1995 Particle Accelerator Conference and International Conference of High-Energy Accelerators, May 1-5, 1995, in Dallas, Tex. Such paper mentions that insulators composed of finely spaced alternating layers of dielectric and metal are thought to minimize secondary electron emission avalanche (SEEA) growth. The structure was not described further, nor was the fabrication method used to produce high gradient insulators mentioned. In the published test results, pulses of 1.3 &mgr;S and up to 250 K volts were applied to small samples that included substrates of polycarbonate, fused silica, and alumina. A similar scenario was used to test the flashover strength of a high gradient insulator. The test results reported indicated that a 1× to 4× increase in the breakdown electric field stress is possible with this technology.
Prior art insulator structures and methods have proven to be impractical to use in the fabrication of certain vacuum barrier walls and envelopes, especially in microminiaturized systems.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a high gradient insulator with a hard seal characteristic suitable for vacuum applications.
A further object of the present invention is to provide an insulator with very high breakdown voltage that permits vacuum tube device microminiaturization.
A still further object of the present invention is to provide a method for fabricating high gradient hard seal insulator structures from stacks of metalized flat annular dielectric substrate rings.
Another object of the present invention is to provide a method for fabricating high gradient hard seal insulator structures with inlaid parallel rows of metal in the surface of dielectric substrates.
Briefly, a method embodiment of the present invention comprises fabricating a hollow insulator cylinder that can have each end closed off with a high voltage electrode to contain a vacuum. A series of fused-silica plates are fabricated from quartz and ground flat to simplify later stacking and bonding. The thickness of each of the fused-silica round flat plates is targeted to be about 0.25 millimeter. An adhesion layer of about 5,000 Å of chromium is sputter deposited onto each top and bottom surface of each of the fused-silica round flat plates. A 25,000 Å-35,000 Å layer of gold is next deposited on the chromium adhesion layer, to prohibit oxidation and to provide enough material to level imperfections in the surface. An alloy of gold and chromium forms at the interface. Once the gold is deposited, the metallized plates can be exposed to air. The metalized plates are aligned and then stacked in a diffusion braze furnace. About one to two pounds per square inch of pre
Davis Steven C.
Decker Derek E.
Elizondo-Decanini Juan M.
Krogh Michael L.
Rosenblum Ben Z.
Caress Virginia B.
Daubenspeck William C.
Gottlieb Paul A.
Graybill David E.
The United States of America as represented by the United States
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