FR2629946A1 - Radiation sensor comprising a charge-transfer device and image intensifier tube and television picture tube which are fitted with such a sensor - Google Patents

Abstract

Radiation sensor comprising a slender charge-transfer device CTD connected by interconnection wires to metal tracks deposited on a support. This slender CTD is arranged inside an opening made in the support, the CTD and the interconnection wires being embedded in a sealing material which affords the sealing of the CTD to the support by its front face whilst leaving its rear face free. The latter is substantially flush with the face of the support which receives the radiation. It is covered with a thin conductive layer so as to constitute a uniform and substantially equipotential relief surface. This sensor is used in image intensifier tubes and television picture tubes. Application: image intensifier tubes and television picture tube.

Description

RADIATION SENSOR COMPRISING A TRANSFER DEVICE
CHARGE AND IMAGE ENHANCER TUBE AND TAKE TUBE
TELEVISION VIEWING EQUIPPED WITH SUCH A SENSOR
The invention relates to a radiation sensor comprising a thinned charge transfer device (DTC) fixed to a support, and interconnection wires ensuring the electrical connections between the front face of the DTC, which comprises contact pads, and metal tracks deposited on the support, the DTC receiving radiation on its rear face.

 It also relates to an image intensifier tube or a television shooting tube using such a sensor.

An invention of this kind is known from the document: "Large area CCD imager sensors for scientific applications",
MM BLOUKE, DL HEIDTMANN, B.CORRIE, M. LUST, JR JANESICK,
SPSE and IGC, Electronic Imaging'85, 7-10th October, BOSTON (MA), p. 160.

 There is described an imaging device using charge transfer devices (DTC). The sensor used in this device includes a DTC attached to a ceramic or other support which includes electrical tracks. The DTC is thinned until it forms a very thin membrane of the order of 10 microns which, in operation, is illuminated by the rear face of the DTC, that is to say that which is opposite to that which supports the contacts.

 An embodiment in accordance with this principle of use is known from US Pat. No. 4,422,091 which relates to a DTC produced on a GaAs substrate.

 It is a uniform thinning technology.

The DTC matrix is sealed over its entire surface on an insulating mechanical support by the face equipped with transfer electrodes. The entire matrix is thinned down to approximately 10 microns, then the periphery is super-thinned by practicing central masking to clear the area of the electrical contacts. The matrix is then placed on a support and the electrical connections are made in a conventional manner by thermocompression of gold or aluminum wires. This technology is characterized by the thinning of the active area before the establishment of electrical connections by metallic wires. Mechanical rigidity is ensured by the insulating support. As is apparent in Figure 1 of the document from
M.BLOUKE et al, in accordance with the state of the art on this problem, the connection wires must form a loop, so that they are not stiffened, which would have the consequence of weakening the connections or creating risks of short-circuits on the edges of the matrix. Because of these loops, the wires are raised, relative to the sensitive plane of the DTC on which the radiation to be detected arrives. This radiation can be photonic or electronic.

 In photonic radiation, these wires must be arranged so as not to produce shadows cast on the sensitive area so as not to degrade the quality of the image. On the other hand, the incident photons must not arrive outside of the matrix zone which constitute the image points, under penalty of creating parasitic charges which would disturb the detected image by coming to be superimposed thereon. This is why Figure 1 of this document shows a type of protective cover.

 When the radiation is electronic, the sensor is placed in a vacuum envelope and the distribution of potentials around the sensitive surface of the DTC have a primordial role. A sensor such as that of this document cannot fulfill this use since the connection wires distributed in high numbers all around the surface of the DTC disturb the potential distributions. In this operating mode the presence of a cover, in relief of the sensitive plane, is also essential. Indeed, the presence of insulating zones between the connections, imposes the use of a mask which suppresses the impact of the electrons in these zones, and avoids any disturbance of the equipotential distributions by electrostatic influence. The presence of the loops on the interconnection wires, and that of a metal cover requires the active area of the device to be set back. The depth of this setback forming a cavity is determined by the size of the loops and the size. - briefly unavoidable of the mask delimiting the active zone, reduced to the matrix zone.

 One of the aims of the invention is to remove any protuberance or cavity which would deteriorate the distribution of the potential, and to cause the surface formed by the DTC and the support to constitute a regular equipotential surface.

 Another object of the invention is to remove any element in relief which would have the consequence of forcing to increase the distance between the electronic source and the surface of the sensor, in order to place the sensor very close to the transmitter, to constitute a device. operating in proximity focusing.

 For this, the invention as defined in the preamble is remarkable in that the thinned DTC is disposed inside an opening made in the support, the DTC and the interconnection wires being embedded in a sealing material which ensures the sealing of the DTC to the support by the front face of the DTC leaving its rear face free, the latter being substantially flush with the face of the support which receives the radiation, the sensor being in this direction, covered with a thin conductive layer, excluding the active area of the DTC which detects radiation, in order to constitute a uniform relief surface that is substantially equipotential.

 It is also remarkable in that the face of the sensor facing the radiation has surface irregularities of less than about one micron.

 The support, which can be a ceramic material such as alumina, is pierced with an opening so that the DTC can be placed there. This is of conventional structure for thinned devices. It comprises an epitaxial layer p of approximately 10 microns, deposited on a silicon substrate p +.

This p / p + interface is intended to facilitate thickening by mechanical-chemical attack and to ensure a homogeneous thickness in the active layer. In addition, it makes it possible to achieve at the DTC / vacuum interface a doping gradient favorable to the collection of the charges by the image elements of the DTC. The structure of the transfer electrodes is produced according to the technology and the etching rules of the type of DTC chosen. Mechanical pre-thinning can be implemented to improve the selective mechanical-chemical attack. The thickness of silicon substrate p + conserved (100 to 150 microns) ensures the mechanical strength during the intermediate stages and the thermocompressions.

 The installation of the DTC in the opening of the support is carried out using shims, according to a temporary fixing respecting the desired geometry. The electrical connections between the matrix and the support are then made by thermocompression of gold or aluminum wires for example. Then proceed to the sealing by the front face of the matrix.

 The sealing materials are chosen to allow implementation with thermal cycles tolerable by DTC. In electron-sensitive mode, they must be compatible with the technology of electronic tubes.

 It is possible to use a glass with a low annealing temperature, for example a glass rich in lead oxide. The coefficient of expansion must be close to that of the materials of the matrix and close to 60.10-7 / OC. One and two-component insulating resins or polyimide organic compounds can also be used.

 In the case of sensors sensitive to electrons, where compatibility with vacuum is decisive, in particular with regard to degassing rates and tolerable temperatures, sealing glasses are preferably used at low temperature.

 The DTC is thus rigidly fixed to the support and exceeds the thickness of the substrate from the surface of the support.

The sensor is then thinned mechanochemically to eliminate the remaining thickness of substrate and to reveal the active epitaxial layer p intended to receive the radiation.

 At the end of the thinning, a metallic layer, for example aluminum, is deposited locally on the surface of the DTC and on its support to delimit the image window and constitute an equipotential surface. This layer eliminates the photons or electrons incident outside the field and avoids the creation of parasitic charges coming to be superimposed on the useful electronic image.

 A sensor is thus obtained in which the electrical connection wires are arranged on the side opposite to the detector surface. It has a planar structure in which no conductor is in relief from the detector surface. The connection wires are established on the face comprising the transfer electrodes before fixing and thinning the matrix. These connection wires are embedded in the sealing material during the fixing operation. The sensor is mechanically rigid and solid: the entire thinned surface of the matrix is sealed with mechanical support, no area of the matrix is super-thinned, the average thickness of the matrix is constant (around ten microns).

 A large part of the sensor is in direct contact with the mechanical support, which promotes heat exchanges to the outside while benefiting from the thermal conductivity of the support which can for example be made of ceramic. This type of mounting makes it possible to stabilize the temperature of the component in operation - which makes it possible to avoid the drift of the dark current - or else to lower the value by cooling.

 In electronosensitive mode, no disturbance of the equipotential surfaces can alter the quality or resolution of the electronic image and in photosensitive mode, no drop shadow or lateral parasitic image can disturb the image focused on the DTC.

 Such a sensor can be placed in a vacuum envelope in which an input photocathode receives light radiation and emits an electron beam towards the sensitive face of the DTC which thus operates in electronically sensitive mode.

 With a frame transfer DTC one can thus detect an electronic image and reproduce it in the form of a television signal. An image intensifier tube for shooting is thus produced which can be used in a television camera with low light level.

 Such a sensor can also operate in photosensitive mode to detect light radiation.

The invention will be better understood using the following figures, given by way of nonlimiting examples, which represent
FIG. 1: a sectional view of a sensor, according to the invention, before the thinning operation,
FIG. 2: the same sensor after the thinning operation and coated with a conductive layer,
FIG. 3: a sectional view of an image intensifier tube with enlargement comprising a sensor according to the invention,
Figure 4: a sectional view of an image intensifier tube with proximity focusing.

 FIG. 1 represents the sensor 5 comprising a charge transfer device 10 comprising inter alia a pre-thinned substrate 11 of 130 to 150 microns thick for example, and an epitaxial layer 12 of 10 microns thick for example. It is in this last layer that the DTC circuits are made. The contact pads and control lines required for operation are deposited on the free face of the DTC located on the side of the epitaxial layer. This side of the DTC is called the front side of the DTC.

Connection wires, for example wire 13, provide the connections between the DTC and electrical tracks 14 deposited on a support 15, for example made of alumina. The electrical tracks can be produced by a deposit in the form of solid conductors or thick layers deposited by screen printing, but preferably a metal comb is used, of thickness for example 200 microns, which is fixed to the support 15 by sealing with materials. in vitro according to known art. In this case, each contact element of the comb protrudes from the support 15 in order to carry out the subsequent mounting.

This support 15 has an opening so that the DTC can be placed there.

 The DTC 10 is positioned in this opening so that the interface between the substrate 11 and the layer 12 is substantially at the level of the face 16 of the support 15. This is obtained using shims of appropriate thickness and a provisional base not shown in the figure.

 After this positioning, the DTC is temporarily fixed by gluing. This makes it possible to make the electrical connections between the studs of the DTC and the tracks of the support by thermocompression of gold or aluminum wires (diameter about 25 microns). A sealing material is placed around the connection wires so as to cover the whole of the DTC and part of the support. The assembly undergoes the appropriate heat treatment for the material used. The DTC 10 is thus fixed to the support 15 using a sealing material 17 which may be a low-annealing temperature glass, for example rich in lead oxide, or insulating resins with one or two components or polyimide organic compounds.

The DTC 10 and the connection wires 13 are thus rigidly fixed to the support by the sealing material. The temporary base, which has become unnecessary, can be eliminated.

To give the sealing material 17 a face
18 having a desired shape, for example planar, one can say
put on the sealing material during its treatment
thermal, suitable pressure parts. This has the advantage of allowing the face 18 to serve as support in the subsequent stages of production.

 The structure of FIG. 1 constitutes an intermediate stage in the production of the sensor. The DTC semiconductor is then thinned mechanochemically in order to make the substrate 10 disappear.

 The epitaxial layer thus becomes apparent and its surface is then in the extension of the face 16 of the support. The assembly is then covered with a thin conductive layer 21 excluding the sensitive zone of the DTC as it appears in FIG. 2. The radiation 22 therefore arrives on the sensor and excites the sensitive zone of the DTC by its so-called face back. The connection wires are thus placed behind the DTC in the direction of propagation of the radiation.

The face 23 of the sensor, which faces the radiation, is very regular and only presents microscopic irregularities, of the order of a micron, linked to the machining of the ceramic support. There is thus a face 23 which constitutes an equipotential surface thanks to the deposited metallic coating. This face is preferably planar, but other shapes are possible, in particular the face 16 of the support 15 can be concave.

 Such a sensor 5 is preferably used in electron-sensitive mode and for this is mounted in a reducing image intensifier tube, or in an image intensifier tube with proximity focusing.

 The reduction tube 30, shown in FIG. 3, comprises an inlet window 31 on which a photocathode 32 is deposited, a glass-metal vacuum envelope 33 according to the usual art, a focusing electrode 39, an anode 37 and the radiation sensor 5. The incident light radiation 24 arrives on the photocathode 32 and generates electrons 25 which are accelerated and focused according to a beam 38 which forms an electronic image on the sensor 5. The operating conditions are, for example, the following. The photocathode 32 is in electrical contact with the part 33 which is brought to a negative high voltage, for example -14 Kvolts.

The focusing electrode 39 located between the photocathode 32 and the anode 37 is brought to the adjustable focusing potential, for example -13.7 Kvolts. Anode 37 and the substrate of
DTC are joined to the mass.

 The tube produced is a roughener, and by acting on the distribution of the potentials and the dimensions of the electrodes, the roughening ratio can be varied, which can be adjusted from 3 to 7 approximately.

 This intensifier tube 30 is provided with means 361, 362 enabling it to be mounted and the photocathode to be produced. This assembly is carried out according to the usual techniques in this field. This mounting must be carried out with great rigor, because the depth of field of such a tube is very small.

 The photocathode is produced using alkaline sources 34 and antimony sources 35, in order to obtain the photocathode with the required characteristics.

The radiation sensor 5 is mounted on a tube bottom 46 which can be ceramic or glass, provided with n insulated electrical bushings 471 to 47n The sensor 5 is rigorously mounted on these insulated electrical bushings which ensure both the electrical contacts and the mechanical strength of the sensor. For this the sensor is positioned using mounting shims so that the sensor is correctly centered on the axis of the tube, the face of the DTC being perpendicular to the axis of the tube better than about 0.05 rad. To achieve this objective, the plane of the lips 361 362 is correctly machined relative to the axis of the tube, then the sensor is mounted so that the plane of the DTC and the plane of the lips 361, 362 are parallel. The ends of the metal tracks of the
sensor are then welded to insulated electrical bushings
471 to 47n for example by laser or electric welding.

When the conductive tracks of the support 15
of sensor 5 consist of thick layers, they are
welded on the periphery of the support 15 to cone pins
tact which are also used for electrical and mechanical mounting
of the sensor.

Figure 4 shows an intensifier tube
proximity focusing images according to a preferred application of the radiation sensor according to the invention. The
tube 50 includes an entry window 51 made of glass or fibers
optics on which photocathode 52 is placed, one in
veloppe 54 glass-metal or ceramic-metal and the
radiation 5. The operating conditions correspond
to the type of focus by proximity. The photocathode is in
electrical contact with the tube 53 closing means.

It is brought to a potential of around -5 to -10 kV compared to
port to the sensor during operation. This intensified tube
Cateur 50 is preferably produced by trans technique
fert to obtain a photocathode of great sensitivity and
homogeneous and ensure a reduced inter-electrode distance,
1 to 2 mm, imposed by the principle of professional focusing
ximited. The radiation sensor 5 is mounted on a bottom of
ceramic or glass tube 56 with n bushings 571 to
57n Mechanical assembly is rigorously ensured at
using shims. This parallelism between the photo
cathode and the sensor face is assured to the tenth of a milli
metre. To ensure this objective, the geometry and the plani
The sealing means are checked by machining.
means of. closure 53, 55 and mounting 54 ensure the quality
mechanical integrity and tightness of the tube. In this type of struc
the image resolution is linked to the quality of the optics
electronic and varies according to the relation R = k.V0,5.d-1, it is
ie as the square root of the potential difference
inter-electrode and the inverse of the inter-electrode distance.

 On the one hand the face of the DTC must be perpendicular to the axis of the tube. On the other hand, no zone of the sensor must be in relief from the active surface under penalty, for a given operating voltage limited by the breakdown voltage, of being forced to increase the distance photocathode-active zone of the sensor and per channel. consequently result in a decrease in the resolution of the system.

 The radiation sensor can also be used to capture a photonic image in the field of short wavelengths. This requires leaving the photosensitive surface of the silicon free from any absorption constraint.

Claims (6)

1. Radiation sensor comprising a thinned charge transfer device (DTC) fixed to a support, and interconnection wires ensuring the electrical connections between the front face of the DTC, which includes contact pads, and deposited metal tracks on the support, the DTC receiving the radiation on its rear face, characterized in that the thinned DTC is placed inside an opening made in the support, the DTC and the interconnection wires being embedded in a sealing material which seals the DTC to the support by the front face of the DTC, leaving its rear face free, this latter being substantially flush with the face of the support which receives the radiation, the sensor being, in this direction, covered with 'a thin conductive layer, excluding the active area of DTC which detects radiation, in order to constitute a uniform relief surface that is substantially equipotential. 2. Sensor according to claim 1, characterized in that the face directed towards the radiation has surface irregularities of less than about one micron. 3. Sensor according to claims 1 or 2, characterized in that the sealing material is a glass with low annealing temperature. 4. Sensor according to claims 1 or 2, characterized in that, the sealing material is an epoxy resin or an organic polyimide compound. 5. Image intensifier tube, characterized in that it comprises a radiation sensor according to one of claims 1 to 4. 6. Television shooting tube characterized in that it comprises at least one radiation sensor according to one of claims 1 to # 4.