X-ray image detector.

The invention relates to an X-ray image detector having a plurality of photosensitive sensors for x-ray radiation having the following features:<ul><li>Each sensor comprises a collecting electrode and a switching element which connects the collecting electrode to an output lead;</li><li>There is a photoconductor layer between the collector electrodes and a bias electrode;</li><li>the collecting electrode together with reference electrode capacitances that are charged by generated in the photoconductor charge carriers.</li></ul>

With such X-ray image detectors are to be carried out, inter alia, fluoroscopy, in which X-ray images in rapid succession, for example, 60 frames / sec traffic.. Such an X-ray image detector, which is 444 720 (PHD 90-016 EP) known from EP-OS is shown in FIG. 1, schematically. For each image point (pixel), a sensor is provided which comprises a switching element 1, a capacitor 2 and a photosensor. The typically 2000 x 2000 switching elements are implemented on a common dielectric substrate (glass) in thin-film technology (for example in the form of thin film field effect transistors 1, as shown in Fig. 1 indicated.) The photosensors are, by a solid, the entire image field covering photoconductor layer 3 a bias electrode 4, which is applied to the photoconductor layer and formed a separate, located on the other side of the photoconductor layer collector electrode 11 for each pixel. When it is made in the operating state of the image detector of X-radiation, three carriers are generated in the photoconductor layer, which flow under the influence of an electric field which is generated by means of a line connected to the bias electrode 4 bias voltage source 40 through the photoconductor to the collecting electrodes eleventh Characterized associated with the collecting electrodes 2 are charged capacities 11, whose other electrode is connected to an electrode 10 to reference potential.

The sensors are in rows in the manner of a matrix and columns, wherein the spacing of the rows and columns is equal to each other. This distance determines the spatial resolution. To read the gates of the switching elements forming the thin film field effect transistors are connected in line by means of a drive circuit. 6 For this purpose, the gate electrodes of all the switching elements of a row are each connected to a common switching line. 5 The source electrodes of the thin film field effect transistors are connected to their associated capacity, while their drain electrodes are column-wise connected to a common output line. 7

In FIGS. 2a and 2b show a plan view and a cross section of a part of the image detector is shown, wherein the photoconductor layer 3 and the bias electrode 4 are omitted. The presentation is simplistic, but it shows the essential elements. On a substrate 15, the ground line 10 and the switch lines 5 are applied. The circuit lines 5 are provided with vertical taps 17, which form the gate electrodes of the thin film field effect transistors. Above the gate electrode 17 is a multi-layer structure 12 of semiconductor and insulating layers that forms a thin film field effect transistor together with the electrodes 7 (drain) and 11 (source). The electrode 11 thus has the function of collecting and the source electrode, and also it forms along with an area covered by their ground line 10 and an intervening dielectric storage capacity. 2

. When implemented in thin-film technology arrangement of Figure 2, the layers between the electrodes is very thin (in the order of 0.2 - 1 micron). It is therefore important that the Sammelektroden 11 the switch lines 5, but in particular the output lines 7 do not overlap, as this large parasitic capacitance between the collector electrode 11 and the respective electrodes would form. This would in the case of the output lines 7 lead to a capacitive signal extraction and to a reduction of the readable signal and to increased noise of devices connected to the output lines 7 output amplifier 8, since these larger input capacitance "seen". The collector electrodes 11 on the one hand and the lines 5, 7 on the other hand must be placed next to each other so, that is, the collector electrode may be gem in plan view. FIG. 2a does not overlap with the conduits 5 and 7. So you need to be limited to between two neighboring switching leads 5 or two adjacent output lines 7 remaining area.

Since all the conduits 5, 7 and 10 must have a width between 10 and 25 microns in order to achieve a sufficiently high conductivity, this means that the proportion of the collecting electrodes to the total area of ​​the X-ray image detector is the smaller, the smaller the image points (pixels) or their distance from one another, ie, the greater the resolution is.

Object of the present invention is to provide an X-ray image detector which has good sensitivity even at high spatial resolution at the same time low-capacity structure as possible.

This object is based on an X-ray image detector of the type mentioned in the invention achieves that the collecting electrode electrode parts per two located together in electrical contact comprise that of the first electrode member is disposed in an area adjacent to the output line that the second electrode portion has a larger area when the first electrode portion and located between this and the bias, and that an insulating layer is between the second electrode member and the output line.

By using a two-electrode parts collecting electrode also their functions share on: The first electrode member, together with one of the reference or ground electrode 10 has a capacity while providing an electrode of the switching element, whereas the second electrode portion generated in the photoconductor layer carriers accumulated. the restrictions do not apply for this second electrode part, which is subject to the first electrode portion, ie, the second electrode portion, the lines, in particular the output lines, overlap at least partially, so as to give a relatively high sensitivity. The between this second electrode portion and the lines resulting parasitic capacitances can be characterized small hold that the insulating layer is made sufficiently thick.

At this point, reference is made 4,471,371, relate to a thin film image detector for visible light to JP-OS 61-1177 or the US Patent. Image detectors for visible light are different from X-ray image detectors in that they have only a thin semiconductor layer instead of a relatively thick photoconductor layer, such as amorphous silicon. This results in a relatively large capacity, so that a separate capacity - as the capacity 2 - is not required. In the known image detectors the electrodes of the switching elements and the output line are in a plane that is separated from the plane in which there are the collecting electrodes by a produced in thin-film technology insulation. This results in between the electrodes of the switching element and the collecting electrode very high parasitic capacitances, which can not be tolerated in an X-ray image detector, which should be read with a high frame rate.

A second solution of the problem underlying the invention is proceeding from a radiographic image detector of the aforementioned kind characterized by the following features:<ul><li>The collecting electrode is arranged adjacent to the output line;</li><li>the switching element and the output line are covered by an insulating layer;</li><li>the insulating layer and the collecting electrode are covered by a semiconducting layer;</li><li>the semiconducting layer is doped so that it has a as compared to conductivity for charge carriers of opposite polarity is large-conductivity for the current flowing in the direction of the collecting electrode charge carriers.</li></ul>

This solution formed above the passivated by the insulating regions in the semiconductive layer space charges distort the electric field in the photoconductor layer so that also charge carriers, which are not generated above the in contact with the semiconducting layer collector electrodes, can achieve these , Despite relatively small areas of the collector electrodes, this solution provides good sensitivity. In another embodiment of this solution is that an additional semi-conductive layer located between the semi-conductive layer and the collecting electrodes, which has a low conductivity for both positive and negative charge carriers. This additional semiconducting layer is used to build a space charge in the field next to the collector electrodes. The space charge distorted the electric field such that the signal-generating charge carriers are transported in the overlying semi-conductive layer having a good conductivity for these to the collecting electrode - even if they were not generated in the region above the collecting electrode.

Suitable for both solutions of the invention provides that both sides of the photoconductor layer layers are provided with a small compared to the photoconductor thickness, which are doped so that they, for the carrier, based on the flow to the photoconductor them adjacent electrodes have as compared to conductivity for charge carriers of the opposite polarity large conductivity. The two layers on both sides of the photoconductor layer blocklieren from the collecting electrodes or biasing of the injected carriers, whereby the dark discharge rates are reduced. In a further embodiment it is provided that between the bias electrode and its adjacent layer a layer is applied, which consists of the same material as the photoconductor layer but is substantially thinner than this. Thus, the dark discharge rates are again reduced significantly.

The invention is explained hereinafter with reference to the drawings. Show it:<ul><li>Fig. 1 is a circuit diagram of the invention or of the known X-ray image detector.</li><li>Fig. 2 is a thin film structure of such a detector in the plan view (Fig. 2a) and in cross section (Fig. 2b).</li><li>Fig. 3 shows a first embodiment of a detector according to the invention in plan view (Fig. 3a) and in cross section (Fig. 3b).</li><li>Fig. 4 is an improved embodiment of such a detector in cross section.</li><li>Fig. 5 shows another embodiment of a detector according to the invention in plan view (Fig. 5a) and in cross section (Fig. 5b).</li><li>Fig. 6 shows the electric field lines in the arrangement of Fig. 5b and</li><li>Fig. 7 shows an improved embodiment in cross section.</li><li>Fig. 8 shows an improved embodiment in cross section.</li></ul>

When in Fig. 3 in the plan view and in cross-section embodiment of an X-ray image detector shown the same reference numerals are used for like parts as in Fig. 2. The section 3b illustrated in Fig. Not to scale and also the relations between the thicknesses of the individual layers do not correspond to the actual conditions that result from the following description. On the thin film structure illustrated in FIG. 2, an insulating layer 13 is first applied with a flat end surface. In this end surface are mounted above the collecting electrodes 11 by means of photolithographic process, contact holes that reach to the electrodes eleventh

Thereafter, a metallic layer is applied, for example by deposition from the vapor phase. This continuous layer of preferably aluminum is then patterned by a photolithographic process that the individual pixels largest possible electrode 14 are formed, the possible complete cover for each one pixel area available and through the contact holes through the electrodes underneath 11 have electrical contact. The distance between the facing edges of adjacent collecting electrodes can be between 5 and 15 microns, so that the ratio between the collecting electrode surface and available for a pixel area even with a pixel size of 100 microns may still be up to 90%. The electrodes 14 can therefore absorb a much larger portion of the carriers generated in the photoconductor as the electrodes 11, thus resulting in improved sensitivity.

With these dimensions of the electrodes 11 mutually overlapping electrodes 14, it is inevitable that they also read lines and the control lines - at least partially - cover, bringing 7 and 5 forming additional parasitic capacitances between the electrode 11 and the lines. In order to keep these parasitic capacitances as low as possible, the insulating layer 13 must have a thickness of at least 3 micrometers, preferably 5 to 10 microns. This is based on a relative dielectric constant of 4-5, (at a higher dielectric constant, the insulating layer must be even larger). Suitable materials are silicon oxide, silicon or polyimide in question.

To achieve these relatively large film thickness, the insulating layer is preferably applied in several individual layers each reduced thickness. This is illustrated in Fig. 4, wherein the insulating layer is formed by the sub-layers 131 and 132. The purpose of making contact with the electrode parts 11 and 14 provided for contact holes have preferably an increasing upward size, so that steps give that guarantee a good metallization layer by the 14th It may be of advantage for the different sub-layers of different materials to be used (for example, the layer 131 of silicon oxide and the layer 132 made of polyimide). Using, then where required for the production of the contact holes etching etchant, each attack only one of the partial layers, then it is easily possible to stop the etching process at a defined depth for the individual layers.

the actual photoconductor layer 3 is then applied to the resulting after application of the electrodes 14 structure. The photoconductor layer 3 is finally provided with a metallic cover electrode 4 of gold or aluminum. It may prove advantageous to construct the photoconductor as a multilayer structure as follows:

First, a semi-conductive layer 31 is applied, the negative charge carrier practically not conducting, this positive charge carrier, the better. This layer may consist of different materials, such as HgI₂, CdSe, CdTe, PbO or Se, of which conductivity is set by certain additives in the sense described above. This is, for example, with a 1 - achieved 5 micron thick layer of selenium doped with 20 to 200 ppm Cl. Then the actual photoconductor layer 32 is deposited from amorphous selenium with an addition of 0.1 to 1% arsenic. This layer must be between 200 and 800 microns thick, to the X-ray quanta that arise in a medical examination, can sufficiently absorb. On the layer 32, a semiconductor layer 33 is applied, which is doped such that it does not conduct positive charge carriers, ie holes (holes), but the better the negative charge carriers (electrons). This layer may consist of selenium with 20 to 200 ppm of alkali metal (Li, Na, K, Cs) are made and have a thickness between 0.5 and 2 microns.

During the operation of the image detector, a positive voltage 1 to 10 kV is applied to the bias electrode 4th The semiconductor layers 31 and 33 will then block the charge carriers, which may be injected by the collecting electrodes 14 or the top electrode 4, so that the dark discharge rates are substantially reduced. Amazingly, this function will only be fulfilled satisfactorily if between the doped layer 33 and the bias electrode 4 of gold or aluminum a thin semiconducting layer that is hardly turned negative and positive charge carriers, for example a selenium layer with in the same way arsenic is doped as the actual photoconductive layer 32. Thus, 32 can be from 0.3 mm up to 5 kV applied to the bias electrode at a thickness of the layer, without it (with a density of more than 1 pA / cm²) are appreciable dark currents ,

Reading an X-ray image with such an X-ray image detector is as described in a similar manner as described in EP-OS 444 720. Before the X-ray is turned on, the switching elements 1 are closed (conducting), so that the capacity 2 can not charge. The X-ray exposure can be carried out at a frame rate of, for example 60 frames / sec, and the dose for the images between 10 and 50 nGy μGy should lie.

During the X-ray exposure the switches 1 are normally open. For reading out the X-ray image in each case all the switching elements of one line are simultaneously closed for a short time (10 to 20 microseconds) by a corresponding potential is applied to the associated shift line 5th During this time, from the cape flowing 2 collected charges azitäten via the output lines 7 on the inputs of the amplifier 8. The amplifiers are connected as current integrators, so that its output corresponds to the total outflow from the capacitor charge. The collector electrodes 11, 14 are maintained virtually at ground potential. The outputs of the amplifier are taken from an analog multiplexer 9, the deformed a serial signal stream with a correspondingly higher bandwidth from the parallel pending signals. The entire process is then repeated for the next and successively for all the other picture lines.

The amplifier 8 can not currently be implemented with the necessary sensitivity and low noise in thin-film technology. You must therefore be outside the Dünfilmsubstrats and be implemented in a conventional integrated circuit technology. In this case, approximately 32-256 amplifier with the associated analog multiplexer can be integrated on one chip, respectively. For the total of 2000 image columns thereafter ie 8-64 such chips would be needed, the inputs would be connected with those located on the thin film substrate output lines. The outputs of the analog multiplexer are connected to analog-to-digital converters, after which the digital data are further processed.

In FIG. 5, a second embodiment of an image detector is shown, which can be manufactured easier. The insulating layer 13 is made here with a conventional thin-film technique layer thickness (between 0.5 microns and a maximum of 2 microns), wherein the top of each of the collecting electrodes each one reaching up to the collecting electrode 11 contact hole is provided.

In the following a possible implementation will be discussed using the example of the photoconductor Se. Analogous structures are to be realized also in other semiconductor materials such as HgI₂, CdTe, CdSe or PbO. In the event that - contrary to this example - is applied a negative voltage to the bias, so that when X-ray exposure negative charge carriers (electrons) migrate to the collecting electrode, the layers having a good conductivity for positive charge carriers (holes) and poor conductivity for electrons to exchange and vice versa. On the insulating layer 13, a semiconductive layer 35 is applied, which does not conduct the negative charge carriers, but having a good conductivity for positive charge carriers. In this example, there is a selenium layer, which - like the layer 31 - is 20 to 200 ppm CI doped, the thicker but as the layer 31 of Figure 3, for example 5 to 40 microns.. For the subsequent layers 32, 33, 34 and 4, which was carried out in conjunction with FIG. 3.

The operation of this embodiment will be explained below with reference to Fig. 6 in detail. Fig. 6 corresponds to Fig. 5b, except that the electric field lines and the paths of the electrical charge carriers are located.

If the bias electrode 4, a positive voltage of, for example 3 kV applied and doped by X-ray exposure charge carriers in the photoconductive layer 32, for example, here selenium with 0.1 to 1% arsenic, generated, then positive space charges formed above the insulating through the layer 13 passivated areas. Characterized the electric field is deformed as indicated in Fig. 6. Because of the good conductivity of layer 35 for holes can now even charge carriers that have not produced above the collecting electrode, pass into this layer at a high speed to the collecting electrode. Possibly injected by the collecting electrodes 11, electrons are held in the layer 35; in this point, the layer acts analogously to the layer 31 in Figures 3 and 4. FIG.

The good conductivity of layer 35 for positive charge carriers causes but at the same time that the space charge can easily flow apart in this layer, as outlined in FIG. 6 Thus, the desired field distortion will be relatively small in the equilibrium state. In addition, the field distortion can enter extend into the actual photoconductor layer 32nd However, this has a low conductivity, what inhibits the transport to the collecting electrode for the positive charge carriers.

A further development of this detector is shown in Fig. 7 Here's another semiconductor layer 36 is inserted under the well for holes and electrons poorly conductive layer having both polarities of charge carriers in approximately equally low conductivity. This layer is in the example about 1-40 microns, thick and consists of selenium, doped with 0.1 to 1% arsenic. Due to their low conductivity for positive charge carriers here, the space charge is based on more efficient because they can not flow parallel to the substrate apart. Thus Feldverbiegung is promoted and be localized to a large extent in the highly conductive layer for holes 35th The transfer of charge carriers to the collecting electrode, which now takes place predominantly in the conductive layer for holes 35, thereby enabling such a way that a maximum signal is obtained in a short time. Again injected by the collecting electrodes, electrons are held in the layer 35th That they have previously undergone layer 36 is irrelevant to the functionality of the detector.

The space charges in the semiconducting layer 35 above the thin film transistors may have on the function of these switching elements may influence. In order to eliminate this influence, the collecting electrodes may each additional electrode member 16 include that - as shown in Fig. 8 - above the insulating layer 13 covers the associated thin film transistor and is in electrical contact with the electrode portion 11. These electrode portions are formed by metallization of the surface, but the 35 produced after the application of the insulating layer and the attaching of the contact holes before the application of the semiconducting layer.