DE2705487C2 - X-ray image intensifier tube - Google Patents

Description

el) a silicon wafer, which is essentially consists of an N-conductive collector region (11) and that of the photocathode (4) facing away from a matrix of P-conductive islands (15) for storage of charge images contains the one in the collector region (11) of the one from the photocathode (4) emitted electron beams (14) are generated,

c2) an electron-permeable buffer layer (13) arranged on the side facing the photocathode (4), which has a first, outer metal layer {Ϊ3Α) and a second, inner metal layer (UB) , the density of which is higher than the density of the first , outer metal layer (13/4 /

d) means for scanning the side of the semiconductor target facing away from the photocathode (4) (6) generated charge image to convert the charge image into electrical signals,

characterized by the following features:

e) the silicon wafer has a highly doped one on the side facing the photocathode (4) N + -conducting surface layer (12) with a higher doping than the adjacent collector area (11) on,

f) the first, outer metal layer (i3A) consists of beryllium,

g) the second, inner metal layer [13B) consists of niobium.

2. X-ray image intensifier tube according to claim 1, characterized in that the highly doped surface layer (12) is a layer diffused with phosphorus, the doping profile of which is selected in this way is that at the depth at which the collector effect is 5%, the increase in the collector effect with the Depth is less than 50% per 0.1 μίτι.

3. X-ray image intensifier tube according to one of the preceding claims, characterized in that that the thickness of the outer, first metal layer (13A ^ is chosen so that about half of the incident electron energy is absorbed.

The invention relates to an X-ray image intensifier tube according to the preamble of the patent claim 1.

An X-ray image intensifier tube of this type is known from US Pat. No. 3,761,762.

A semiconductor target for an X-ray intensifier tube is also in the article "Silicon Electron Multiplication Camera Tube" in "LEE E. Transactions on Electron Devices, "Volume ED 18. No. 11, November 1971, pp. 1023-1028.

A common target for image intensifier tubes, which is mostly made of silicon, works with incident tubes Electrons, which are at energies of 2.5 bir lOkeV are accelerated, the impact plate reinforcements

ίο correspond from about 1 to 2000, in practice the The photocathode of the X-ray image intensifier tube is held at a negative potential while the photoelectrons hit the target, which is practically at ground potential. The disadvantage of the standard target is that in the area suitable for an X-ray image intensifier tube with photocathode voltages -19 kV to -25 kV is required, the target gain is too high. Therefore must the x-ray current in the image intensifier is kept low so that saturation of the output signal of the target is avoided.

An X-ray image intensifier tube operated in this manner has a low signal-to-noise ratio a>. 'f.

Attempts have been made to eliminate this disadvantage by placing a metal buffer layer on said first side of the target is generated (US-PS 37 61 762). However, such a buffer layer leads usually to local irregularities that result in a blotchy image, so that the choice of Material for the buffer layer is limited to very special metals and thicknesses in order to prevent the formation of spots to avoid. In addition, this solution has the major disadvantage. that by braking of the incident electrons with high energy in the metal layer X-rays are generated, which in the semiconductor wafer also generates charge carriers, which contribute to an increase in the noise.

To avoid these difficulties, it is already known from US-PS 37 61 762, the buffer layer in a ground, outer, low-density metal layer (e.g. aluminum) and a second, inner, metal layer with a higher density (e.g. chromium) to be divided.

Finally, in the earlier patent application P 26 05 965.4-33, an X-ray image intensifier tube was developed proposed to the preamble of claim 1, which also has feature e) of claim 1.

The invention is based on the object of the X-ray image intensifier tube to develop according to the preamble of claim 1 so that the impact plate a adjustable gain factor, for example in the range between 3 and 300, if the photocathode voltage varies between -19 kV and -25 kV, with that generated in the buffer layer Noise caused by X-rays is further reduced.

This problem is solved by the features specified in the characterizing part of claim 1.

Advantageous further developments of the invention contain the dependent claims 2 and 3. It is from the "IEEE Transactions on Electron Devices" mentioned at the beginning is already known, the highly endowed N + -conducting surface layer to be generated by phosphorus diffusion.

The invention is explained in more detail below, for example with reference to the drawing. It shows

F i g. 1 schematically an X-ray image intensifier tube,

2 shows a cross-section on an enlarged scale through part of the target with silicon diode row of an X-ray image intensifier tube according to a Embodiment of the invention, and

3 shows the curve in which the gain over the Photocathode voltage of a known target is plotted, and comparatively such Curve for a target in an X-ray image intensifier tube 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 is incident a scintillation screen 3, which in turn illuminates a photocathode 4, which is in the immediate Near the scintillation screen. Electrons emitted from the photocathode are captured by a focusing cone 5 focused and on one side of a target 6, which is a plurality of regularly arranged Contains diodes 15, projected as follows with reference to FIG. 2 will be described. Between; η the diodes 15 are each isolating areas 19.: The opposite side of the target 6 is scanned by an electron beam originating from an electron gun 7, that too a cathode and a grid electrode 9 contains. The emitted scanning electron beam is on usual Way deflected by deflection coils 8.

In the X-ray image intensifier tube of this type, an increased negative must be increased as compared with a conventional tube Voltage must be applied to the photocathode in order to have a good resolving power of the electron optics the amplifier tube. In order to meet the requirements placed on the X-ray image intensifier tube by the target plate 6 adapt, it is necessary that the reinforcement of the target plate 6 between about 3 and 300 is adjustable if the photocathode voltage is changed between about -19 kV and -25 kV, depending on the special design of the amplifier section. As in Fig. 2, these generate electrons 14 with high energy applied to the N-type silicon collector area 11 occur a variety of hole-electron pairs in area 11. The holes diffuse and discharge reverse biased pn diodes 15 (depletion zone 17) on the opposite one Surface of the target. The thus obtained charge stored on the diodes gives a charge image, which corresponds to the incident electron image. The charge image is different from that of the electron gun 7 generated scanning beam 18 is scanned and read out. On the diodes 15 and the A resistance layer (“resistive sea”) 16 is located in the insulating areas 19.

To the target to the aforementioned of the X-ray image intensifier tube set requirements, where the photocathode voltage between about - 19 kV and -25 kV is changed and the target reinforcement must be adjustable between about 3 and 300, the N-conductive collector region 11 in as is known, on the side facing the incident electrons 14 due to heavy phosphorus diffusion changed to a deep N + -conductive surface layer 12, the surface of which is coated with a metal buffer layer 13. The thickness of the surface layer 12 is approximately 0.5 μm and that of the buffer layer 13 about 1 μm. The depth of the the surface layer 12 serving for recombination can be adjusted in such a way that defects and irregularities the target sensitivity due to defects in the metal buffer layer 13 be reduced to a minimum.

The buffer layer 13 consists of two superposed metal layers, namely a first, outer one Metal layer 13Λ and a second, inner metal layer 135, the density of the second, inner metal layer 135 is greater than the density of the first, outer metal layer 13Λ (US-PS 37 61 762). In contrast to the X-ray image intensifier tube known from this US Pat. No. 3,761,762 the tubes according to the invention have a first, outer metal layer made of beryllium and a second, inner metal layer made of niobium, the side facing the photocathode 4 of the highly doped N + -conducting Surface layer 12 are arranged.

3 shows characteristic amplification curves for a standard target and a target according to the invention, the ordinate being the gain factor and the abscissa being the photocathode voltage (in kV). Curve A relates to a standard target and curve B relates to a target in an X-ray image intensifier tube according to the invention. The curves shown show that the installation of the target 6 in an X-ray image intensifier tube enables the operator to easily adjust the amplification of the tube between 3 and 300 so that an optimal signal-to-noise ratio is obtained for special diagnostic situations.

In Figure 2, the collector region 11 consists of a single-crystal silicon substrate. The phosphorus diffusion, which generates the N + -conductive surface layer 12 is set in such a way that a very slow increase in the Collector effect is obtained with the depth in the target. At the 5% collector action point should the increase in the collector effect with the depth in the target is less than about 50% per 0.1 μίτι with corresponding to low increase ratios at other points on the collector action curve.

The very slow increase in the collector effect with the depth in the target results in two favorable ones Effects:

Non-uniformities in target sensitivity caused by non-uniformities in the metal buffer layer are brought about are reduced;

2. Excessive target noise caused by absorption in the collector area of high-energy X-ray quanta is brought about by the incident on the metal buffer layer Photoelectrons are generated is significantly reduced.

As described above, it is made by appropriately choosing the combined thickness of the surface layer 12 and the metal buffer layer 13 possible, the target reinforcement for a certain photocathode voltage range to be changed so that this is within what is required for X-ray image intensifier tubes Area falls. The combined thickness is e.g. B. chosen so that a Auftreffplattenv receive reinforcement which varies from 3 to about 300 as the photocathode voltage varies from about -19 kV to -25 kV (Fig. 3).

The thickness of the first outer metal layer 13Λ becomes chosen so that about half of the incident electron energy is absorbed, the density of the second inner metal layer 135 is sufficiently high

is to the lateral diffusion of electrons with high
Energy through the first outer metal layer 13/4
penetrate through, to be limited to less than 1 μπι,
thus a deterioration in the resolving power
the target is avoided.

For this purpose 2 sheets of drawings

m 65

Claims (1)

Patent claims:
1. X-ray image intensifier tube with: a) a scintillator (3) for converting an X-ray image into an optical radiation image, b) a photocathode (4) for converting the optical radiation image into electron beams (14), c) a semiconductor target (6) containing: