GB2180986A - Image intensifier - Google Patents

Abstract

An image intensifier includes a channel plate 1 consisting of a slice of silicon in which a very large number of hollow channels 10 extend through its thickness. The silicon slice is formed with its crystalline <110> direction approximately normal to the faces of the slice, and the channels are formed by the use of an etchant, which etches preferentially in this direction. The walls of the channels are coated with secondary electron emissive material which may be deposited from a vapour including lead oxide and silicon monoxide. A pn junction 5 is formed one micron below the top surface 3 of the plate by diffusing a p-type dopant such as boron into the n-type silicon. <IMAGE>

Description

SPECIFICATION Image intensifiers This invention relates two image intensifiers and is specificallyconcernedwith intensifiersofthe kind which utilise channel plates. In a channel plate intensifier an image in the form of a pattern of electrons is incident upon one face of an electron multiplier, and a much larger number of electrons is emitted from an opposite surface thereof so that a more intense electron image is obtained. Image intensifiers ofthis kind generally have a photocathode which receives an incident optical image (often a very weak image) and which emits in response thereto a corresponding pattern of electrons which is intensified by the channel plate image intensifier, the intensified electron pattern being received by a fluorescent screen which generates an intensified optical image.

A channel plate consists of a thin sheet of material having a very large number of hollow channels passing through the thickness ofthe material, and the wall surfaces of these channels are such as to emit secondary electrons in response to the incidence of a prima ry electron. By applying a su itable electric field across the thickness of the plate, large numbers of secondary electrons emerge from the output end of a channel in response to avery much smallernumberof primary electrons which enterthe input end of the channel. Electron multiplication ratios of 1000 are readily obtainable with current microchannel plates.

In order to produce a good image resolution, as many channels as possible are required per unit surface area ofthe channel plate, with the intervening solid land occupying as small an area as possible.

Typically, the thickness of a channel plate may be I mm, and the width of each channel may be ofthe order of 5 lim to 10 pom spaced apart on centres of 10 pm to20 pm. The conventional method of manufacturing a channel plate isto draw out a large bundle of glass tubes or rods into a long strand so as to form a unitary structure which is then cut into short lengths of only a millimetre or so. Such a process is difficult and expensive and results in an irregular array of channels. The present invention seeks to provide an improved image intensifier, and a method of manufacturing it.

Accordingto a firstaspect of this invention an image intensifier includes athin plate of crystalline silicon having narrow hollow channels extending through its thickness, the walls of the channels being secondary electron emissiveand channels extending in the crystalline < 110 > direction ofthesilicon.

According to a second aspect ofthis invention an image intensifier includesathin parallel sided plate of crystalline silicon having narrow-hollow channels extendingthrough the thickness ofthe plate, the walls of the channels being coated with a secondary electron emissive material andthe channels being parallel to each other and extending in the crystalline < 110 > direction ofthe crystalline silicon; and means for establishing an electricfieldwithin said channels so as to accelerate along the direction ofthe channels, secondary electrons generated therein.

Preferably the plate of crystalline silicon includes meansforminimisingthefiowofcurrnntthroughthe bulk of the material in responseto said electric field.

These meansconvenientlytaketheform ofa p-n junction which in operation is reverse biased. Silicon which is sliced normal to the crystalline < 110 > direction is capable of being selectively etched, such thatthe rate of removal of silicon in the crystalline < 110 > direction is many times greater than in transverse directions, so that by the application of a suitable chemical etchantthrough apertures in a mask at one surface of the silicon, very narrow deep recesses are formed. By continuing the etching process until the recesses breakthrough into the opposite surface ofthe plate, a matrix of very fine, filamentary channels results.Such a plate then has an operational structure very similarto a conventional microchannel plate image intensifier, but this plate is produced by a very simple and relatively inexpensive process which utilises the accurate processing techniques which have been developed for the manufacture of intricate semiconductor electronic devices. To give a high degree of electron magnification itwill generally be necessary to provide a coating of a suitable material on the walls of the channels so that when electrons are incident thereon, secondary electrons are copiously emitted. A suitable secondary electron emissive coating can be formed by the deposition from a gas of a material such as lead on to the interior walls ofthe channels.

Although the plate may be cut such that the channels are normal to the majorfaces of the plate of silicon, it is preferred to offsettheir longitudinal direction buy a small angle of 5"to 10 . Thus they are slightly oblique with respect to the major faces ofthe plate and this prevents the direction transmission of incident light th rough the channels, and also helps to preventthe unwanted transmission of ions in the reverse direction The invention is further described by way of example with reference to the accompanying drawings in which:: Figures 1 and 2 show sectional views of a channel plate at different stages of its manufacture, Figure 3 shows a plan view of part of a channel plate, and Figure 4 shows an optical image intensifier incorporating such a channel plate.

Referring to Figure 1, the channel plate is manufacturedfrom a plate 1 of n-type crystalline (110) silicon which has a fairly high resistivity, e.g. of the order of 100 QcmvThe plate is a carefully cut slice of a large crystal of monocrystalline silicon such that the crystalline < 110 > direction is in the direction of the arrows 2, i.e. so that the two majorsu rfaces3, 4, (which are mutually parallel) lie in the same crystalline direction, and are offset by a small angleof 5 & o 1 09from the normal to the < 110 > direction. Typically the plate 1 is about 1 mm thick and is circularwith diameter of about 20 to 30 mm.

A pn junction 5 is formed approximately one micron below the surface 3 by diffusing a p-type dopant such as boron into the n-type silicon to produce a large area semiconductor diode. The entire plate 1 is then coated with an etchant resistant coating 6 into which small windows 7 are cut above the surface 3. The size of the windows is very much exaggerated in the drawing for the sake of clarity. These windows correspond to the sites at which the channels will be formed, and so the size, shape and position ofthe windows are of importance. The windows 7 may be circular or square, but are preferably rhombic as is shown in the pattern in Figure 3, in which the length of each straightside, is about 5 ,um, and the distance between adjacent windows is also about 5 Mm.

The windows 7 can be formed by the use of conventional high quality optical photolithographic mask techniques ofthe kind very commonly em ployed in the manufacture ofsemiconductor devices.

The photolithographic mask can be prepared using electron beam or X-ray cutting processes to give a high degree of accuracy and consistancy. Indeed, the windows 7 could be formed directly by electron beam cutting.

An etchant is then applied to the plate 1 to locally remove the silicon atthe surface 3 underthe windows 7. A suitable etchant is diluted potassium hydroxide, at a temperature of about 80"C. Such an etchant cuts deeply into the crystal lattice ofthe silicon in the < 110 > direction with negligible lateral spreading in the < 111 > direction so that deep recesses of substantiallyconstant cross ection are formed, the longitudinal direction ofthe recesses being perpendicularto the surfaces 3 and 4. The cross sectional shape ofthe recess is of course initially determined by the profile of the windows but is largely determined bythecrystal- line properties ofthe silicon, and the natural profile is rhombic.It is forthis reason that the rhombic windows shown in Figure3 are adopted, The preferred angle of the rhombus being about 109'. The pattern of the recesses has a high degree of regularity which is determined by the accuracy ofthe mask. The etching is continued until the bottoms ofthe recess reach the surface 4-obviously the etchant does not etch through the coating 6 on thesurface 4. This stage can be determined by illuminating the top surface 3 ofthe plate, as when the recesses reach the lower surface 4, light will be visible through the thin coating 6. At this stage the recesses constitute the channels 10 of the channel plate.

The etch resistant coating 6 is then completely removed, afterwhichthe uppersurface 3 twhich in operation is the input surface) ofthe plate is subjected to an ion bombardmentwhich removes silicon so as to widen the top ends of each channelintoafunnel-like shape 12. This is achieved by irradiating the upper surface3 ofthe plate bya broad uniform beam of ions at an angle represented by the arrow 8 whilst the plate as a whole is slowly rotated about the axis 9 as shown in Figure 2. This achieves the outward taper ofthe channel shown in Figure 2, and considerably in creases the useful catchment area of incident eiec trons atthe input surface 3 when the plate isin use as an image intensifier.

Both faces 3, and 4 are polished flat, and a conductive coating 11 is deposited on to all of the surfaces ofthe plate, in particular so asto coat the interior surfaces of the channels 10. This coating II I is chosen so as to be secondary electron emissive to a high degree, and preferably includes lead. The coating 11 is conveniently deposited from a vapourwhich includes lead oxide and silicon monoxide so that an appreciable thickness of coating is produced along the whole length of all channels. Subsequentlyconductive electrodes 13,14 are formed on the flat surfaces 3, 4 of the'plate. It is between these electrodes that a potential difference is present in operation so asto directthe secondary electrons along the channels towards the output surface 4.This potential difference also serves to reverse bias the pn junction to minimise the currentflow by free carriers through the bulk ofthe semiconductor material ofwhich the plate 1 is composed.

To form an optical image intensifier, the plate as so far described is mounted between a photocathode 20 and a phospherscreen 21 within an evacuated chamberwhich is divided into two sections 22, 23, by the plate 1, as is shown in Figure 4. A light image which is incident upon the photocathode 20 generates electrons which are attracted to the upper surface 3 of the plate by means of a suitable accelerating potential difference. Many of the primary electrons enter the channels 10, and as a result of multiple collisions with the coating 11 a very much largernumberof secondary electrons emerge from the far end of the channels and are accelerated by a further electric field to the phospherscreen,which fluorescesto produce a more intense replica of the incident light image. As the channels are regularly spaced in a uniform array as a consequence of using the etch-resistant mask to determine their position, the image resolution is entirely constant across the whole area of the image intensifier.

Claims (13)

1. An image intensifier including a thin plate of crystalline silicon having narrow hollow channels extending through its thickness, the walls of the channels being secondary electron emissive andthe channels extending in the crystalline < 110 > direction ofthe material.

2. An image intensifier including a thin parallel sided plate of crystalline silicon having narrow hollow channelsextending through the thickness ofthe plate, the walls of the channels being coated with a secondary electron emissive material andthe channels being parallel to each otherand extending in the crystalline < 110 > direction of the crystalline silicon; and means for establishing an electric field within said channels so as to accelerate along the direction of the channels, secondary electrons generated therein.

3. An image intensifier as claimed in claim 2 and wherein the plate of crystalline silicon includes means for minimisingtheflow of currentthroughthe bulk of the material in response to said electric field.

4. An image intensifier as claimed in claim 3 and wherein said meansfor minimising the flow of current includes a pn junction, which in normal operation is reverse biassed.

5. An image intensifier as claimed in claim 2,3 or 4 and wherein the direction ofthe channels is angularly offset between 5'and 10'from the normal to the outer facesofsaid plateofsilicon.

6. An image intensifier as claimed in any of claims 2to 5 and wherein the surfaces of the channels are coated with a secondary electron emissive material.

7. An image intensifieras claimed in any of claims 2to 6 and whereinthe input ends ofthe channels are flared outwardly.

8. A method of manufacturing an image intensifier which is in accordance with any ofthe preceding claims including the steps offorming a large area pn junction in a slice of silicon, andforming the narrow hollow channels through the thickness of the slice by means of a chemical etchant which is applied via windows in an etch-resistant mask.

9. A method as claimed in claim 8, and wherein the cross-sectional profile of each channel is rhombic.

10. A method as claimed in claim 8 or9 and wherein a coating of secondary electron emissive material is deposited from a vapour on to the walls of the channels.

11. A method as claimed in claim 10 and wherein said coating contains lead.

12. An image intensifier substantially as illustrated in and described with referenceto the accompanying drawing.

13. A method of making an image intensifier substantially illustrated in and described with reference to the accompanying drawing.