US3424909A - Straight parallel channel electron multipliers - Google Patents

Description

Jan. 28, 1969 H. ROUGEOT 3,424,909

STRAIG HT PARALLEL CHANNEL ELECTRON MULTIPLIERS Filed March 24, 1966 INVENTORZ H ROUGEOT a, 4 ,6 run BY nfirm United States Patent 0,462 US. Cl. 2s0 207 Int. (:1. H011 39/12 4 Claims ABSTRACT OF THE DISCLOSURE A straight parallel channel electron multiplier wherein channels are formed in a silicon diode biased in the reverse direction by a source establishing a longitudinal electric field in said channels.

The present invention relates to electron multipliers of the type in which a beam of primary electrons is multiplied through a series of secondary electron emissions within a set of straight, parallel channels, placed in a longitudinal electric field.

Such electron multipliers and the applications thereof in devices known as light amplifiers have been described in various publications, among others in the United States Patent 3,128,408 of July 7, 1964.

It may be recalled that electron multipliers of the type specified comprise an insulating body, pierced by canals of a very small diameter. The inner walls of the canals are coated with a very thin electrically resistive layer, having properties of secondary electron emission with acoeflicient 6 1. When an electric zfield is produced in the canals by establishing a suitable difference of potential between the extremities of the resistive coatings, a beam of primary electrons penetrating into the canals under different angles causes a series of secondary electron emissions on the internal coatings. The number of electrons at the output of the canals is then greatly increased as compared with the primary beam.

Unfortunately, the manufacture of the emissive coatings for those electron multipliers is rather difiicult because the diameter of the canals does not exceed a few tens of microns, while the thickness and the electric resistivity of the coating layers must be uniform over the entire length of the canals. Another difiioulty lies in the necessity of preventing the electric current that flows in the coatings from attaining an excessive value which could deteriorate the canals.

This invention has for its object an improved structure of electron multiplier in which the drawbacks and inconveniences of the prior art structures are avoided.

In accordance with the present invention, an electron multiplier of the type specified is characterized in that the canals are pierced in the body of a silicon diode, biased in the reverse direction.

Under these conditions there is no longer any need to provide the canals with an internal coating since the silicon, by itself, possesses a secondary electron emission coefficient greater than 1. Moreover, the electric current that traverses the silicon (leakage current) is zero or negligible since the diode is in the reverse or blocking condition.

The present invention will be best understood from the following decsription in connection with the accompanying drawing in which:

FIGURE 1 shows a light intensifier that utilizes an electron multiplier of the known art;

FIGURES 2 and 3 represent an electron multiplier in accordance with the present invention, FIGURE 3 being a section along line III-III of FIGURE 2; and

FIGURE 4 represents schematically a light intensifier utilizing an electron multiplier in accordance with the present invention.

The prior art light intensifier, represented in FIGURE 1, comprises within an air-evacuated enclosure aninsulating cylinder 2, for example of glass pierced bycanals 3 whose internal walls are coated with acoating 4 made of a resistive substance, deposited by surface treatment and capable of releasing secondary electrons with aratio 5 1 when submitted to the impact of primary electrons. On the opposite sides of cylinder 1 arephotocathode 5 and afluorescent screen 6, between which a source of current 7 establishes a DC. voltage of a few hundreds or a few thousands of volts. The extremities of thecanals 3 facing the photocathode are carried at a potential somewhat lower than the screen.

When a light image is projected ontophotocathode 5, electrons released therefrom penetrate into thecanals 3 under different angles. These primary electrons cause on theemissive coatings 4 emissions of secondary electrons which, in turn, strike the emissive walls and cause emissions of tertiary electrons, and so on. Since thecoefiicient 6 is greater than 1, the electrons are multiplied at the output of thecanals 3 and produce on thefluorescent screen 6 animage 9 of a brilliancy intensified with respect to that of theinitial image 8.

In accordance with the present invention, the electron multiplier of FIGURE 1, formed by thecylinder 2 and thecanals 3, is replaced by the improved electron multiplier shown in elevational view in FIGURE 2 and in sectional view in FIGURE 3. This novel electron multiplier comprises a thick diode of the surface barrier type, formed by a monocrystal ofsilicon 11 of high resistivity, bearing on one of its faces a rectifyinggold contact 12 and on the other face an aluminum layer 13- that provides an ohmic, i.e., non-rectifying contact. Canals I14 having a diameter of the order of 30 microns and spaced apart, for example, by one hundred microns from each other, are pierced into the diode whose thickness is about 1 millimeter.

A source of DC.voltage 15 sets ahe rectifyingcontact 12 at 1,000 volts With respect to the aluminum layer (reverse bias), whereby an accelerating electric field is produced along the entire length of the canals.

In operation, electron multiplication is obtained in the canals through secondary emissions merely on the walls of the canals which do not bear any complementary coating, since silicon has a secondaryelectron emission ratio 5 1.

Since the diode is biased in the reverse direction, the leakage current is negligible.

Thick diodes can be made in different manners. If a monocrystal of silicion of high resistivity, for example, 1 mm. thick, is available, canals of a few tens of microns are pierced in that sample by an electron beam, or by laser effect, or by any other process. In order to eliminate surface dislocations and, if necessary, to enlarge the "holes, the sample is soaked for a few minutes in an etching solution. The surface 'barrier type diode is then formed by evaporating obliquely on one of its faces a thin layer of gold and on the other face a layer of aluminium or indium.

If the resisitivity of the original monocrystal is sufficient, the space charge zone that defines the zone of the electric field will extend over the entire thickness of the crystal.

The diode may also be constructed in an n-i-p type rectifier, !well known to those skilled in the art.

Lithium (n impurity) is diffused in a monocrystal of p silicon, thus producing an n region (excess of lithium),

:n i region (compensated) and a p region (initial silicon).

The i region is enlarged by draining the lithium ions in an electric field obtained by biasing the diode in the reverse direction. The temperature must be comprised between 100 and 200 degrees.

The dead 11 and p portions are reduced by grinding and pickling, and the diodes thus obtained may have a thickness of a few millimeters.

A modification of the technique for manufacturing thick diodes, also known to those skilled in the art, consists in totally suppressing the n region and depositing on the concerned face a layer of gold. Thereafter, like previously, one proceeds with piercing the canals, pickling, and deposting the contacts.

FIGURE 4 represents schematically a light intensifier utilizing the electron multiplier of FIGURES 2 and 3. It comprises aphotocathode 21, the electron multiplier symbolized by theblock 22, and afluorescent screen 23. A source ofvoltage 24 and avoltage divider 25 permit to set the various elements at appropriate potentials.

The silicon diode of theelectron multiplier 22 is disposed in the reverse direction with the gold face (rectifying contact) facing thephotocathode 21 and the aluminium side facing thefluorescent screen 23.

Like in FIGURE 1, thephotocathode 21 here converts a light image into an electron image. The latter penetrates into themultiplier 22 from which it emerges intensified and strikes thefluorcesent screen 23 on which it produces an image having increased brilliancy as compared with the initial image.

While I have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as 'known to those skilled in the art, and I therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

I claim:

1. An electron multiplier comprising a diode constituted by a silicon wafer and two metal layers in rectifying and non-rectifying contact with the two faces of said wafer, respectively, said diode being pierced transversely with approximately straight, parallel canals, a source of direct current potential connected at its negative and positive terminals to said rectifying and non-rectifying layers, respectively, for biassing said diode in the reverse direction and simultaneously establishing a longitudinal electric field within said canals, means for injecting primary electrons into said canals at the negative potential end thereof at various angles with respect to the inner surfaces of the canals, thereby causing multiple secondary electron emissions from said surfaces within the canals, and means for picking up the multiplied electrons emerging from said canals at the positive potential end thereof.

2. A light intensifier including an electron multiplier as claimed in claim 1, further comprising a photocathode opposite the negative potential face of said silicon wafer, a fluorescent screen opposite the positive potential face of said silicon wafer, and means for setting said photocathode and said fluorescent screen at direct current potentials lower than said negative potential and higher than said positive potential, respectively.

3. An electron multiplier, comprising a photocathode, an anode and between said photocathode and anode a relatively thick diode structure of the barrier type having two faces and provided with relatively small canals extending through said diode structure from one face to the other, and rectifying contact means on one of said faces and non-rectifying contact means on the other of said faces.

4. An electron multiplier according toclaim 3, wherein said diode structure consists of a monocrystal of silicon.

References Cited UNITED STATES PATENTS 2,998,541 8/1961 Lempert 313--103 X 3,128,408 4/1964 Goodrich et al. 3l3--103 X 3,341,730 9/1967 Goodrich et a1. 250207 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

US. Cl. X.-R.

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