Solid chemical vapor deposition diamond microchannel plate

Plastic and nonmetallic article shaping or treating: processes – Gas or vapor deposition of article forming material onto...

Reexamination Certificate

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C216S024000, C216S033000

Reexamination Certificate

active

06521149

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to microchannel plate (MCP) technology. It finds particular application in conjunction with microchannel plates constructed of solid diamond grown using a chemical vapor deposition (CVD) process and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other like applications.
Microchannel plates are the electron producing section of photomultiplier tubes (“PMT”) used for night vision devices and general infrared (“IR”) detection equipment. Light enters the PMT through a photocathode, which converts the light signal into a very weak electronic signal. The electronic signal is amplified using an electron multiplier, which operates based on secondary electron emission. Secondary electron emission is the electronic property by which secondary electrons are produced as a result of primary electrons impinging on the material surface. Secondary electron coefficients (i.e., the ratio of secondary electrons produced from a single primary electron) range from two to four in most materials used for PMT applications, and as high as 25 for some exotic materials used for specialized PMT applications.
It is well known that chemical vapor deposition (“CVD”) diamond, doped with specific impurities, is electrically conductive and produces copious amounts of secondary electrons. A cesiated diamond surface will produce more than 40 electrons at 1000 eV primary electron energy. Since 40 electrons are produced from each collision, and the process is multiplicative, a diamond MCP can produce millions of times more electrons than state-of-the-art MCPs. CVD Diamond growth processes, however, would likely be unable to coat prefabricated MCP structures since the “pores” or microchannels are typically about 6 &mgr;m in diameter and about 360 &mgr;m deep. Diamond deposition processes require background pressures, which would likely prevent source gasses from filling these pores.
A microchannel plate may be used as a secondary electron multiplier. Typically, the MCP consists of millions of glass microchannels in the form of capillary tubes, which are assembled and fused together to form a two-dimensional array in the shape of a disk. The capillary tubes are formed by drawing down glass-filled, glass-jacketed rods, arid then etching out the glass filling. Typical microchannel diameters range from about 40 &mgr;m to about 10 &mgr;m, with the corresponding channel pitch being such that the channel cross-sectional areas constitute about 50% of the total MCP face area. Metal films, deposited on both faces of the disk, serve as electrodes for applying an electric field across each channel and also electrically connect the multitude of channels together in parallel. Each channel behaves as a sort of continuous-dynode electron multiplier. The input end of an MCP-based detector includes a suitable photocathode optimized for the spectral characteristics of the incident radiation. The photocathode receives the incident photons and generates the primary photoelectrons, which then enter the glass capillary channels. The capillary material is specially chosen (the most common being a lead-oxide glass) such that when electrons impinge on the channel walls, secondary electrons are generated. These secondary electrons are accelerated by the voltage applied across the electrodes and travel in a parabolic trajectory along the length of the channels, until, due to the transverse component of their motion, they collide with the channel walls and dislodge additional secondary electrons with each impact, thus producing electron multiplication, or gain.
Detection of low-level signals—optical (infrared, visible, ultraviolet and X-ray) as well as particle (electrons and ions)—is a critical requirement in a wide variety of applications, both military and civilian. A good example of devices whose performance de pends on their ability to amplify very low-level input signals with large gain (≧10
4
) are night vision systems, which constitute an important part of the increasingly complex and technologically-intensive equipment employed in modem warfare. Currently available high-gain detectors include numerous types of photomultiplier tubes (PMTs) and image-intensifier tubes (IITs), many of which incorporate microchannel plates as the primary amplifying device.
Microchannel plates are almost always geometrically designed so that the channel axes are at a small angle (“bias angle”) to the perpendicular of the input and output faces.
Conventional MCP Manufacturing Technology
One process used in the industry for manufacturing microchannel plates is primarily based on the technology of drawing glass fibers and fiber bundles. The fabrication process begins with tubes of a specially formulated glass, usually a lead-oxide composition, that is optimized for secondary-electron emission characteristics. In the tubes are inserted soleid cores of a different glass with differential chemical etching characteristics. The filled tube is softened and drawn to form a monofiber. Millions of such fibers are now stacked together in a bundle in a hexagonal-close-packed format. The bundle is fused together at a temperature of about 500° C. to 800° C. and again drawn until the solid core diameters become approximately equal to the required channel diameter, which ranges from about 40 &mgr;m down to <10 &mgr;m. Now individual microchannel plates are cut from this billet by slicing at the appropriate bias angle to the billet axis. The thickness of the slices is generally such that the channels have a length-to-diameter ratio of about 40-80.
The individual plates are ground and polished to an optical finish. The solid cores are removed by chemical etching in an etchant that does not attack the lead-oxide glass walls, thus producing the hollow channels. Further processing steps lead to the formation of a thin, slightly conductive layer beneath the electron-emissive surface of the channel walls. Electrodes, in the form of thin metal films, are then deposited on both faces of the finished wafer. Finally, a thin membrane of SiO
2
(formed on a substrate which is subsequently removed) is deposited on the input face to serve as an ion barrier film, and the plate is secured in one of several different types of flanges. The finished MCP may now be incorporated in various image-intensifying detection systems.
DETAILED DESCRIPTION OF THE PRIOR ART
For best understanding of the microchannel plate, it is best to review in detail the manufacturing process for conventional MCPs.
FIGS. 1-3
illustrate the prior art MCP.
FIGS. 4 and 5
show the conventional MCP production process of drawing and etching.
FIG. 1
shows an MCP. Microchannels
101
are held in a flange
102
. Electrodes and connections, plus ion-barrier film and other items necessary for operation, are not shown. The function of the MCP is to augment the strength of the optical signal input without distortion.
FIG. 2
shows how electron amplification takes place. An electron
103
arrives from the left-to-right direction, and enters the microchannel tube
104
. A voltage V
D
from a voltage source
105
supplies the high voltage necessary for the electron acceleration. Each electron, whether introduced as the electron
103
or generated within the tube
104
, may dislodge one or more other electrons, which may proliferate to hundreds, or even hundreds of thousands, of electrons.
FIG. 3
shows how a photon striking electrode
106
dislodges an electron (e

), which passes through chevron configuration microchannel plates
107
,
108
with electron amplification and emerges as a great number of electrons (Ge

), which strike a phosphor layer
109
and result in phosphorescent light output P.
FIG. 4
shows how a tiny glass-clad glass rod is drawn down to a capillary size, essentially by heating it to a softened state and then pulling. This leaves an inner glass
111
still clad in an outer glass
112
, with both drawn down to a miniature capillary size. Additional step

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