CA1081371A

CA1081371A - Target for an x-ray image intensifier tube and itensifier tube comprising the target

Info

Publication number: CA1081371A
Application number: CA272,108A
Authority: CA
Inventors: Barry M. Singer; Allan I. Carlson
Original assignee: North American Philips Corp
Current assignee: Philips North America LLC
Priority date: 1976-02-23
Filing date: 1977-02-18
Publication date: 1980-07-08
Also published as: JPS5748825B2; NL183155C; NL183155B; IT1075810B; FR2341939A1; AU2250677A; DE2705487A1; AU502159B2; DE2705487C2; NL7701723A; FR2341939B1; JPS52102623A; GB1549146A

Abstract

ABSTRACT:
An X-ray image intensifier tube having a target and a number of regularly arranged diodes which, on the side bombarded by the electrons, has a deep diffused phosphorus n+ layer which is covered with a metal buffer layer, comprising a layer of a material having a low atomic number, for example beryllium, and a layer of a material having a comparatively large density, for example, niobium, is preferably present between said layer and the said n+ layer which, under the influence of the incident electrons, mainly emits L.alpha. X-ray radiation which is strongly absorbed in the n+ layer.

Description

~ 313~1 "T~rget for an X-ray image intensi~ier tube and intensifier tube comprisillg the target"

_____________ The invention relates to a semiconductor target for use in a image intensifier tube comprising a semi~conduc-tor plate ~or converting an electron image into electrical f signals, which plate consists mainly of a collcctor region 5 of one conductivity type and has a first side for receiving the said electron image, and, on the oppositely located second side, comprises means for storing charge carriers generated in the said collector region by electrons having a given high energy which are incident on the said first 10 side and thus form a charge pattern on the said second side~ a highly doped surface layer of the first conductivi-ty type being present on the first side and having a higher doping concentration than tlle adjoining collector region, said surface layer being covered with a metal layer which 15 is pervious to the incident electrons.
The invention also relates to an X-ray image inten-sifier tube having such a semi-conductor target.
A semi-conductor target as described above is known from the article "Silicon Electron Multiplication Camera 20 Tube" in IEEE Transactions on Electron Devices, vol.ED 18, - No. 11, November 1971, ~. 1023-1028.
A con~rentional target for image intensifier tubes which usually consists of silicon and is known as "sili-con intensified target (SIT)" operates with incident elec-25 trons accelerated to energies from 2.5 to 10 ~eV corres-ponding to target gains of approximately 1 to 2000.

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In practice the photocathode of the X-ray image inten-sifier tube is held at an negative potential and the photo-electrons strike the target which is substantially a-t ground potential. The drawback of the standard target is in the fact that in the range required for an X-ray image intensi-fier tube having photocathode voltages of -19 kV to -25 kV, the target gain is too high. Therefore the X-ray Plux in the image intensifier must be kept low to avoid saturation of tke output signal of the target. An X_ray image intensi-fier tube operated in this manner has a low signal-to-noise ratio.
It has been endeavoured to avoid this drawback by providing a metal buffer layer on the said first side of the target. However, such a buffer layer normally gives rise to local non-uniformities which result in a spotted image so that the choice of the material for the buff`er layer is restricted to very specific metals and thick-nesses so as to prevent the said formation of spots.
In addition, said solution has the important drawback that by inhibiting the incident high-energy electrons, X-ray radiation is generated in the metal layer which also generates charge carriers in the semi-conductor plate which contribute to an increase of the noise.
It is the object of the invention to avoid the said drawbacks and to provide a target for an X-ray image inten-sifier tube which has a controllable gain factor in the range -of 3 to 300 when the photocathode voltage varies from -19 kV
to -25 kV, respectively, the noise as a result of the X-ray radiation generated in the buffer layer being reduced to a minimum.

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Therefore a target as described in the preamble is charact~riæcd according to the invention in that the metal layer is a buf~er layer, the thickness of the highly doped surface layer and of the buffer layer being chosen to be 5 so that sufficient incident electron energy can be dissi-pated to obtain an optimum signal-to-noise ratio in the said electrical signal, and that the buffer layer comprises at least a layer of a metal having a low atomic number so that by inhibiting the incident electrons only a low K~ X-ray radiation is generated in æaid layer.
The thickness of the highly doped "dead" surf`ace layer and of the buffer layer is chosen to be so as to dissipate suffiecient incident electron energy to shift the gain vs.photocathode voltages curves of a conventional silicon intensified target to the range required for an X-ray in-tensifier tube so as to obtain optimum signal-to-noise ratio.
Although the buffer layer may be constructed from a single layer, a preferred embodiment is characterized in that a second metal layer of higher density than the first layer is situated between the first metal layer of low atomic number and the highly doped surface layer, in which - second layer the electrons incident ~rom the first layer generate an X-ray radiation consisting only for a small part of ~ radiation but mainly of L~ radiation, said L ~
radiation being strongly absorbed in the highly doped surface layer.
The semiconductor plate, at lea-st the collector re--~ gion, preferably consists of n type silicon. The buffer ~ layer preferably comprises a beryllium layer as a metal .~

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` 1ai81371 layer o~ low atomic number; a double buffer lay~r pre~erably has a first outermost layer of beryllium and a second innermost layer of niobium. The L~ radiation of niobium is strongly absorbed by the highly doped silicon sur~ace layer. The means for storing charge carriers preferably comprise a number of regularly arranged islands of the second conductivity type which adjoin the second side of the semiconductor plate and form p-n diodes with the collec-tor region, although instead of this a homogeneous radiation-sensitive layer might alternatively be used.
The invention will now be described in greater detailwith reference to the accompanying drawings, in which Fig. 1 shows diagrammatically an X-ray image inten-sifier tube, 1g Fig. 2 is a cross-sectional view on an enlarged scale of a part of the target with silicon diode array according to the invention, and Fig. 3 shows the curve in which the intensification is plotted against the photocathode voltage of a standard "SIT" compared with the target according to the invention.
In Fig. 1, the X-ray radiation passes through the window 2 of an X-ray image intensifier tube 1 and impinges upon the scintillation screen 3 which in turn illuminates a photocathode 4 which is in ~ose proximity to the scin-tillation screen. Electrons emitted by the photocathodeare focused by a focusing cone 5 and projected onto one side of a target 6 comprising a large number of regularly arranged diodes 1~. The oppositely located side of the target 6 is scanned by an electron beam from an electron gun 7 which al90 comprises a cathode and grid electrode 9.

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1~81371 l`he emitted scanning electron beam is deflected in the usual manner by deflection coils 8.
In the X-ray image intensifier tube of this type, an incrased negative voltage has to be applied to the photocathode in comparison with a conventional tube to obtain good resolution in the electron optics of the intensifier tube. In order to adapt the target 6 to the above requirements ~ the X-ray image intensifier tube, it is essential that the intensification of the 10 target 6 be adjustable between approximately 3 and 300, when the photocathode voltage is varied between approxi-matoly ~19 kV and -25 kV, dependent on the particular design of the intensifier section. As shown in Fig. 2, , these high-energy electrons 14 which are incident on the 15 n-type silicon collector region 11 generate a large number of hole-electron pairs in the region 11. The holes diffuse and discharge reversely biased p-n diodes 15 (dep~etion zone 27) on the oppositely located surface of the target. The resulting charge stored on the diodes 20 produces a potential profile corresponding to the incident electron image. The potential profile is scanned and read-out by the scanning beam 18 generated by the electron gun 7. ~ resistive sea 16 is situated on the oxide and , on the diodes 15.
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In order to adapt the target 6 to the above require-- ments of the X-ray image intensifier tube where the photo-cathode voltage is varied between approximately -19 kV
and -25 kV, and the target intensification must be adjus-' table betueen approximately 3 to 300, the n-type collector ;: :
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~81371 region 11 on the side facing the incident electrons 14 is varied by a heavy phosphorus diffusion so as to provide a deep n~ "dead" layer 12 and by covering the surface thereof with a metal buffer layer 13. The thickness of the layer 12 ~is approximately 0.5 micron and the thickness of the metal layer is approximately 1 mic~on. The depth of the recombination layer 12 can be adjested so as to minimise defect~r and non- uniformities of the target response caused by defects in the metal buffer layer 13.
Fig. 3 shows typical intensification curves for a standard target and an X-ray target according to the inven-; tio in which the intensification factor is plotted on the vertical and the photocathode voltage (in kV) is plotted on the horizontal. Curve A relates to a standard target ., and curve ~ relates to a target according to the present invention. The plotted curves show that the incorporation of the target 6 according to the invention in an X~ray image intensifier tube will permit the operator to easily adjust the lntensification of the tube from 3 to 300 to give optimum signal-to-noise ratio for a specific diagnos-tic situation.
In Fig. 2 the collect~r region 11 consists of a monocrystalline silicon substrate. The phosphorus diffu-sion which prodeces the n+ "dead" layer 12 is adjusted so ~! 25 as to obtain a very slow rise in the collector efficiency ;~ ~ with the depth in the target. At the 5/~ collection efficien-cy point the rise ~n collection efficiency against the depth ~ in the target should be less than approximately 50~ per 0.1/um - with correspondingly low rise ratios in other points on the collection-efficiency curve.

._ .7 _ ~131371 The very slow rise in the collection efficiency with the depth in the target provides two advantageous - effects: 1) non-uniformities in target response caused by non-uniformities in the metal buffer layer are reduced.
2) excessive target noise caused by the absorption in the collector region of energetic X-ray quanta generated by photoelectrons incident on the metal buffcr layer is strongly reduced.
As described above, a correct choice of the combined thickness of the n~ "dead" layer and the metal buffer layer permits of varying the target intensification for a given photocathode voltage range so that this falls within the intensification range which is necessary for X-ray image intensifier tubes. The combined thickness is, for example, chosen to be so that a target intensification ;~ is obtained which varies from approximately 3 to approximately 300 when the photocathode voltage varies from approximately -19 kV to -25 kV (Fi~. 3).
The buffer layer 13 is made ei~her from a single material or from two superimposed layers 13a and 13b of different materials (Fig. 2). In the former case the material, for example beryllium, has a low atomic number so that therein the i~cident high-energy electrons only give rise to a low X-ray radiation level. In the latter , case the outer layer on which the high-energy electrons are incident, also has a low atomic number as in the case of the buffer layer from a single material~ whereas the :
- inner layer consists of a material which has a comparatively ~ -high density, for example niobium, and in which in addition the high-energy electrons which penetrate through ., , "~ , ~081371 the outcr layer generate only a low K~ X-ray radiation but mainly a L~ radiation which is strongly absorbed by the n+ "dead" layer. The thickness of the outer layer is chosen to be so that approximately half of the inc~dent electron energy is absorbed and the density of the inner layer is high enough to restrict the lateral dlffusion of the high-energy electrons penetrating through the outer layer to less than 1 micron so as to avoid degradation of the resolution of the target.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
CLAIMS:

1. A semiconductor target for use in an image intensifier tube comprising a semiconductor plate for converting an electron image into electrical signals, which plate consists mainly of a collector region of one conductivity type and has a first side for receiving the said electron image, and, on the oppositely located second side, comprises means for storing charge carriers generated in the said collector region by electrons having a given high energy which are incident on the said first side and thus from a charge pattern on the said second side, a highly doped surface layer of the first conductivity type being present on the first side and having a higher doping concentration than the adjoining collector region, said surface layer being covered with a metal layer which is previous to the in-cident electrons, characterized in that the metal layer is a buffer layer, the thickness of the highly doped surface layer and of the buffer layer being chosen to be so that sufficient incident electron energy can be dissipated to obtain an optimum signal-to-noise ratio in the said electrical signal, and that the buffer layer comprises at least a layer of a metal having a low atomic number so that by inhibiting the incident electrons only a low K.alpha. X-ray radiation is generated in said layer.

2. A semiconductor target as claimed in Claim 1, characterized in that a second metal layer of higher density than the first layer is situated between the first metal layer of low atomic number and the highly -?0-doped surface layer, in which second layer the electrons incident from the first layer generate an X-ray radiation consisting only for a small part of K.alpha. radiation but mainly of L.alpha. radiation, said L.alpha. radiation being strongly absorbed in the highly doped surface layer.

3. A semiconductor target as claimed in Claim 1, characterized in that the semiconductor target consists of silicon and the collector region is of the n-conductivity type.

4. A semiconductor target as claimed in Claim 3, characterized in that the metal layer of low atomic number is beryllium.

5. A semiconductor target as claimed in Claim 3, characterized in that the buffer layer consists of a first layer of beryllium and a second layer of niobium.

6. A semiconductor target as claimed in Claim 3, 4 or 5, characterized in that the highly doped surface layer is a phosphorus-diffused layer of which the doping profile is chosen to be so that at the depth where the collector efficiency is 5%, the rise in the collector efficiency with the depth is less than 50% per 0.1 micron.

7. A semiconductor target as claimed in Claim 2, characterized in that the thickness of the outer, first metal layer is chosen to be so that herein approximately half of the incident electron energy is absorbed.

8. A semiconductor target as claimed in Claim 2, characterized in that the density of the second metal layer is so high that the lateral diffusion of the electrons penetrating into the second metal layer is less than 1 micron.

9. A semiconductor target as claimed in Claim 1, characterized in that the said means for storing charge carriers comprises a number of regularly arranged islands of the second conductivity type adjoining the second side of the semiconductor plate and forming p-n diodes with the collector region.

10. An X-ray image intensifier tube having a target as claimed in Claim 1, characterized in that the tube com-prises a photocathode for converting an X-ray image into an electron image, means to project the electron image on the first side of the semiconductor plate, and means to read out the charge pattern on the second side of the plate so as to convert the charge pattern into electrical signals.

11. An X-ray image intensifier tube as claimed in Claim 10, characterized in that the thickness of the highly doped surface layer and of the metal layer are chosen to be so that sufficient incident electron energy can be dissi-pated to provide an intensification factor which varies between 3 and 300 when the voltage of the photocathode with respect to the target varies between -19 kV and -25 kV.