WO2004104635A1 - Arrangement for detecting x-ray radiation and method for producing the same - Google Patents

Abstract

The invention relates to an arrangement for detecting X-ray radiation and to a method for producing the same. Said arrangement can be used in a wide variety of domains such as non-destructive radiographic testing and medical diagnosis, but also in many security-related applications. The aim of the invention is to obtain a high resolving capacity, reliable use over long operating periods, and a high degree of sensitivity with low production costs. To this end, the inventive arrangement comprises a sensitive layer consisting of a semiconductive material that absorbs X-ray radiation. A covering electrode is electroconductively contacted on one side of the sensitive layer, and the opposite side comprises detector elements that are respectively electrically isolated from each other. Furthermore, the sensitive layer preferably consisting of a polycrystalline semiconductive material contains a barrier layer which, in turn, is preferably an amorphous carbon layer in one form of embodiment. Said barrier layer comprises electroconductive regions which are located on individual detector elements and are separated from each other by separating plates consisting of a dielectric material or having a high specific electrical resistance.

Description

Arrangement for the detection of X-rays and method for their production

The invention relates to an arrangement for the detection of X-rays and a method for producing such arrangements. It can be used in a wide variety of areas of technology, such as non-destructive testing, in medical diagnostics, but also for many safety-related applications.

X-ray technology has been used for a long time and in the recent past developments have been made to use X-rays in conjunction with electronic evaluation as so-called digital X-ray pixel systems.

As a result, by using relatively large-format X-ray detector elements, X-rays incident on them can be detected in a spatially resolved manner and a digital image can be made available in electronic form. The electronic signal values obtained in this way can be stored and evaluated. Of course, a visual representation is also immediately or later with a delay

With the help of electronic image reproduction technology (moni gates, displays) or a printout on paper.

Investigations have been carried out for the production of such x-ray detector arrays, in order in particular to be able to select suitable x-ray absorbing materials.

It has emerged that various polycrystalline, semiconducting materials are particularly suitable. This applies, for example, to CdTe, Pbl ₂ or TiBr. Mercury iodide Hgl ₂ has proven to be very particularly suitable, since this polycrystalline material has a further increased atomic number and requires a significantly lower electron-hole pair formation energy.

Direct detection is possible with the designated materials, so that the associated advantages in terms of increased sensitivity and simplified evaluation can be exploited.

Corresponding theoretical principles have been explained in EP 0 784 801 B1. It also describes ways in which, for example, a sensitive layer of polycrystalline mercury iodide can be obtained in a highly pure form.

This document also mentions possibilities of how such X-ray detection systems can be constructed.

Thus, a layer formed from polycrystalline mercury iodide can be coated on both sides, with semiconductor transistor elements as at least on one side of such a polycrystalline layer Thin film transistors have been formed.

Because of the chemical aggressiveness of mercury iodide but also of the other designated suitable polycrystalline materials, it is necessary to use a passivating intermediate layer.

EP 0 784 801 proposes using parals or humiseal for this purpose. However, these substances have disadvantages because of their electrical properties and their limited protective action with regard to the electrode materials used and the chemically aggressive polycrystalline semiconducting materials, so that the signal-to-noise ratio is deteriorated and there are failures of individual flat areas on a detector system designed in this way, e.g. the so called "dark lines".

This leads to a significantly reduced reliability and a correspondingly reduced service life of such X-ray detection systems.

In the case of these known solutions, it is also disadvantageous that at least one such passivation layer has to be formed by means of an additional manufacturing step that is different from other process technologies.

In the solutions known hitherto, restrictions on the achievable resolution must also be accepted, since the individual, electrically separated, detecting surface areas must have a minimum size for production reasons. It is therefore an object of the invention to propose an arrangement for the detection of X-rays which has a high resolving power, reliable use over long operating periods and can be produced inexpensively with increased sensitivity.

According to the invention, this object is achieved with an arrangement which has the features of claim 1 and a method according to claim 17. Advantageous embodiments and developments of the invention can be achieved with the features specified in the subordinate claims.

The arrangement according to the invention for the detection of X-rays has a sensitive layer made of a semiconducting material that absorbs X-rays. This sensitive layer can be in the form of a planar structure and is bordered on two opposite sides. Thus, on a surface of the sensitive layer, a cover electrode made of an electrically conductive material is electrically conductively contacted with this layer, which is exposed to the incident X-ray or gamma radiation. Such a cover electrode can also be designed as a closed flat element.

On the side of the sensitive layer opposite the cover electrode, a plurality of detector elements are arranged, discretely to one another and electrically insulated from one another. According to the invention, a barrier layer has been formed between the sensitive layer and the detector elements. The layer thickness of this barrier layer can be limited to a few nanometers and should not exceed a maximum layer thickness of 500 nm, preferably 100 nm. There- it should be ensured that the barrier layer forms at least one closed layer, whereby this can already be achieved with a minimum layer thickness of 2 to 3 nm.

With the barrier layer to be used according to the invention, the negative influence of aggressive semiconducting materials with respect to the electrodes of materials forming semiconductor transistor elements can be completely prevented.

In addition, the electrical properties of a suitable barrier layer have a very advantageous effect on the sensitivity. The extremely small required layer thickness of the barrier layer is particularly advantageous here, since a considerably increased electrical conductivity is achieved, starting from the sensitive layer in the direction of the detector elements.

Already when the barrier layer is formed, but possibly also with a subsequent treatment, it is possible to form barrier layer regions with increased electrical conductivity, which in turn are enclosed by separating webs. This

Separators are either formed from a dielectric material or have a significantly increased specific electrical resistance compared to the barrier layer regions. A corresponding barrier layer area is locally assigned to each detector element, and electrical separation is ensured by the separating webs and, if appropriate, additionally electrically insulating areas which are formed around the detector elements.

So barrier layer areas can be dimensioned this way be that the ratio of their respective layer thicknesses in relation to a maximum area extension along an axis of at least 1: 150 can be maintained. For example, the ratio of the layer thickness to a surface diagonal of a barrier layer region formed in a square or rectangular shape can easily adhere to this ratio, wherein ratios of 1: 500 or even 1: 1000 are also possible.

The individual barrier layer areas should be adapted in surface shape and in their dimensions to the respective shape and size of the electrodes of detector elements assigned to them.

The individual detector elements with the barrier layer regions forming a detector array should be electrically insulated from one another in such a way that local crosstalk to neighboring elements can be prevented.

The specific electrical resistance of the amorphous carbon in the barrier layer regions can be in the range between 10 and 1000 Ωcm, this being able to be influenced by influencing various parameters during the deposition of such layers in a vacuum. The proportions of graphitic carbon (Sp ₂ ) and the proportions of diamond-like carbon (Sp ₃ ) can be varied, the specific electrical resistance being able to be reduced as the Sp ₂ proportion increases. With an increased Sp ₃ content, the density of such a layer can be increased, which in turn means that a reduced layer thickness of such a barrier layer can be used.

However, the formation of a barrier layer can also by galvanic deposition of a suitable metal or a suitable metal alloy.

The deposition can take place directly on an array which is formed from detector elements. A particularly suitable metal for this is palladium, which can also be electrodeposited in combination with nickel as a palladium-nickel alloy.

However, there is also the possibility of forming a corresponding thin barrier layer made of nickel, gold or even gold / nickel.

When galvanically forming barrier layers, it is advantageous to subsequently planarize the surface of the barrier layer formed, to which the sensitive layer is then to be subsequently applied.

The planarization can be carried out, for example, by mechanical-chemical polishing.

It may be advantageous and possibly also necessary to form a thin coating, which consists of BCB, polyimide or another resistant polymer, on the galvanically deposited barrier layer.

Even in the case of such an electrodeposited metal layer, possibly with the intermediate layers mentioned above, only a layer thickness is required which forms the desired closed layer in order to achieve the required barrier effect to the sensitive layer arranged above it and ensures a sufficiently large electrical conductivity between the sensitive layer and detector elements.

For the electrical insulation of the barrier layer elements and possibly also simultaneously of the respective semiconductor transistor elements, webs made of a dielectric material can be used or formed. Such webs can for example consist of Si0 ₂ or organic materials that have been obtained by means of photolithographic processes.

The webs can advantageously be formed from dielectric material when producing an array of a plurality of detector elements.

Such detector elements can have been designed in the form of conventional thin-film transistors.

However, it is more advantageous to design the detector elements as complementary metal oxide semiconductor elements (CMOS). These are available as standard components, the individual CMOSs being able to be dimensioned very small and, accordingly, a very high spatial resolution capability can be achieved in an arrangement according to the invention. For example, maximum dimensions of individually detectable areas below an area size of 50 x 50 μm can be achieved. Each individual COMS can deliver a discrete measurement signal in direct form, so that high-resolution area detection can be achieved. However, higher functionality can also be achieved. In this way, an integrating measurement, a recording of individual events or an energy-dependent detection can be carried out. Other semiconductor transistor elements, such as thin-film ASICs, can also be used as detector elements. The suitable detector elements are not limited to the designated semiconductor transistor elements.

In addition to this high spatial resolution, a large-area configuration of an arrangement according to the invention can also be achieved, several arrangements of this type being used in the form of a row and column arrangement, and a further increased area for X-ray radiation detection can thereby be made available.

It can also be advantageous if an intermediate coating has been formed between the barrier layer and the detector elements. For example, indium tin oxide (ITO) can be used to reduce the electrical contact resistance.

Before the barrier layer elements made of diamond-like carbon are applied, the surface of an array arrangement of semiconductor transistor elements can be leveled by means of BGB (bencocyclobutene).

The cover electrode already mentioned can have been formed directly on the surface of the sensitive layer as a metallic thin film. However, there is also the possibility of forming such a cover electrode from graphitic carbon. For this purpose, an appropriate dispersion (Aquadag) is applied to the surface and, after drying, a closed cover electrode made of graphitic carbon is obtained. This graphitic carbon can additionally covered with a thin metal layer, e.g. gold.

The sensitive layer can be formed from amorphous selenium or silicon, but also from mono- or polycrystalline materials.

In particular, due to the advantageous properties of the polycrystalline semiconducting materials for the sensitive layer, and here in particular of mercury iodide or lead iodide, it is possible to work with significantly reduced powers and in particular radiation intensities, so that the radiation exposure in the environment is significantly reduced.

A targeted selection of the layer thickness of the sensitive layer can also have been made for the various application possibilities of an arrangement according to the invention.

For example, reduced layer thicknesses on sensitive layers which are used on arrangements according to the invention for medical diagnostics can be much smaller than in the case of arrangements which e.g. used in non-destructive material testing.

For example, with mercury iodide layers with a density of -6.3 g / cm ³ , when an absorption of more than 90% of the X-rays used in medical diagnostics is achieved, the sensitive layer can have a thickness of 10 to 20 μm at E = 5 up to 30 KeV, for safety-relevant applications and the non-destructive testing of suitable materials, a thickness of 80 to 650 μm E = 20 to 60 KeV and for non-destructive material testing, thicknesses in the range of 400 to 1200 μm at E = 50 to 150 KeV can be selected.

The arrangements according to the invention can be produced in such a way that surfaces of detector elements arranged discretely to one another, which if possible form a continuous array, are coated with a barrier layer made of diamond-like carbon in a vacuum. A sensitive layer, which consists of a polycrystalline, semiconducting material that absorbs X-ray or gamma radiation, is in turn applied.

In turn, an electrically conductive layer is formed on the surface of this sensitive layer as a cover electrode, which covers the entire surface of the sensitive layer. It is not necessary to structure such a cover electrode in a plurality of surface areas which are electrically insulated from one another.

The individual metallic barrier layer areas are separated from one another by separators made of dielectric material. Such dielectric separators can be formed between the individual detector elements already during the manufacture of an array consisting of a plurality of detector elements.

When the barrier layer is formed from diamond-like carbon in a vacuum, a pulsed plasma can be used, and graphite can be used as the target material.

The plasma can be produced in a manner known per se pulsed laser radiation, pulsed arc discharge or in combination, can be generated as a so-called laser-are process.

It is advantageous to use a filter within the vacuum chamber with which so-called “droplets” within the barrier layer elements or the negative influence of debris can be prevented.

The sensitive layer, and here in particular a sensitive layer, is composed of mercury iodide or lead iodide, as described in EP 0 784 801 B1.

It is advantageous to form such a mercury iodide layer or lead iodide layer directly on the barrier layer using a PVD or CVD method, it being possible to infer an appropriately suitable method from this prior art.

In the latter case in particular, the cover electrode can subsequently also be formed in vacuum as a metallic thin layer.

However, the sensitive layer can also be formed by a screen printing process.

The individual detector elements can then each individually, optionally with the interposition of

Gain stages can be connected to an electronic evaluation unit and in turn the individual signals can be stored, processed or displayed as an image for a visual evaluation. The arrangement according to the invention is characterized in that a high sensitivity can be achieved with a simultaneously reduced spatial resolution, even with a significantly reduced X-ray intensity. In addition, reliable long-term operation can be ensured without failure of individual detector elements or detection areas. It has a particularly favorable effect here that corresponding contacting methods, such as bonding or flip-chip technology, can be dispensed with.

The invention will be explained in more detail below by way of example.

It shows:

Figure 1 shows in schematic form a structure of an example of an arrangement according to the invention.

Here, detector elements 2 are formed discretely on one another as CMOS. These have a contact surface 2 'made of aluminum. The barrier layer 4, which is formed from diamond-like carbon, is in turn arranged on this. The barrier layer 4 has a thickness of 20 nm. The aluminum contact surfaces 2 'are separated here by webs 3 made of silicon dioxide, so that electrical insulation of the individual detector elements 2 is achieved.

In the example shown here, a full-surface formation of a barrier layer 4 made of amorphous carbon was carried out and subsequently, using a laser beam, as an example of a suitable energy beam, a locally targeted influencing of the amorphous carbon of the barrier layer 4 performed. In this way, electrically conductive barrier layer regions 4 ′ with an increased sp ₂ component and separating webs 5 surrounding them with an increased sp ₃ component can be formed. At the separating webs 5, the specific resistance of the carbon was increased to approx. 10 ⁹ Ωcm and thus reached at least 10 ⁵ Ωcm.

A sensitive layer 1 of mercury iodide was then formed. A layer thickness of approx. 200 μm can be selected for medium X-ray energies.

Aquadag was applied to the sensitive layer 1 as a graphite dispersion, which can be achieved, for example, by screen printing or spin coating. After drying, the top electrode 7 was then formed from graphitic carbon.

In initial tests, such arrangements could be formed with a total area size of approx. 110 x 110 mm and an area resolution for the detection for individual pixels or pitches below 50 μm could be achieved.

Sufficient detection sensitivities have already been achieved with X-ray radiation energies of 30 keV.

Claims

claims 1. Arrangement for the detection of X-rays with a sensitive layer made of a semiconducting material that absorbs X-rays, this sensitive layer is electrically conductively contacted on one side with a cover electrode and detector elements on the opposite side of the sensitive layer, which are each electrically insulated from one another are arranged because a barrier layer (4) is formed between the sensitive layer (1) and the detector elements (2), which has electrically conductive barrier layer areas (4 ') assigned locally to the individual detector elements (2), which are separated from one another by separators (5) made of dielectric material or with increased specific electrical resistance. 2. Arrangement according to claim 1, characterized in that the barrier layer (4) is formed from amorphous carbon. 3. Arrangement according to claim 1, characterized in that the barrier layers (4) is an electrodeposited metal layer and electrically conductive barrier layer regions (4 ') are electrically isolated from one another by separating webs (5) formed around detector elements (2). 4. Arrangement according to claim 1 or 2, characterized in that electrically conductive barrier layer regions (4 ') made of amorphous carbon are enclosed by separating webs (5) made of amorphous carbon with increased specific electrical resistance. 5. Arrangement according to one of claims 1 to 4, characterized in that the barrier layer (4) has a maximum thickness of 500 nm. 6. Arrangement according to one of claims 1 to 5, characterized in that the barrier layer regions (4 ') maintain a ratio of their respective layer thickness to the maximum surface area along an axis of at least 1: 150. 7. Arrangement according to one of the preceding claims, characterized in that the specific electrical resistance of the separating webs (5) is greater than 10 ⁵ Ωcm. 8. Arrangement according to one of the preceding claims, characterized in that galvanically formed barrier layer regions (4 ') are formed from palladium, palladium / nickel, tungsten, gold or gold / nickel. 9. Arrangement according to one of the preceding claims, characterized in that a metallic electrically conductive contact surface (2 ') is present between barrier layer regions (4') and detector elements (2). 10. Arrangement according to one of the preceding claims, characterized in that between the sensitive layer (1) and barrier layer (4) the intermediate layer influencing the electrical transition resistance is formed. 11. Arrangement according to one of the preceding claims, characterized in that barrier layer regions (4 ') and contact surfaces (2') of Detector elements (2) are formed from indium tin oxide. 12. Arrangement according to one of the preceding claims, characterized in that the sensitive layer (1) has been formed from Hgl ₂ or Pbl ₂ . 13. Arrangement according to one of the preceding claims, characterized in that the cover electrode (7) is formed from a metallic thin layer and / or graphitic carbon. 14. Arrangement according to one of the preceding claims, characterized in that the detector elements (2) are designed as complementary metal oxide semiconductor transistor elements (CMOS). 15. Arrangement according to one of the preceding claims, characterized in that the detector elements (2) are designed as thin-film transistors. 16. Arrangement according to one of the preceding claims, characterized in that the detector elements (2) are each individually connected to an electronic evaluation unit (6). 17. A method for producing an arrangement for the detection of X-rays, in which Surfaces of discrete to each other Detector elements (2) a barrier layer (4) is formed; in this case, electrically conductive barrier layer regions (4 '), which are electrically separated from one another by separating webs (5), are locally assigned to the individual detector elements and on the surface of the barrier layer (4) a sensitive layer (1) consisting of a semiconducting material that absorbs X-rays, applied and an electrically conductive layer is formed as a cover electrode (7) on the surface of the sensitive layer (1). 18. The method according to claim 17, characterized in that electrically conductive barrier layer regions (4 ') and insulating Separators (5) are formed by influencing the barrier layer (4) made of amorphous carbon by means of an energy beam. 19. The method according to claim 17 or 18, characterized in that the barrier layer (4) is formed from a pulsed plasma. 20. The method according to claim 19, characterized in that the plasma is generated by means of pulsed laser radiation and / or pulsed arc discharge. 21. The method according to claim 17, characterized in that barrier layer regions (4 ') on one Carriers for detector elements (2) which are electrically insulated by insulating separating webs (5) are formed by electrodeposition of a metal. 22. The method according to claim 21, characterized in that palladium, palladium / nickel, tungsten, gold, or gold / nickel are deposited. 23. The method according to any one of claims 17 to 22, characterized in that the sensitive layer (1) is also formed in a vacuum by a CVD or PVD method. 24. The method according to any one of claims 17 to 23, characterized in that the sensitive layer (1) is produced from Hgl ₂ or Pbl ₂ . 25. The method according to any one of claims 17 to 24, that an intermediate layer influencing the electrical contact resistance between barrier layer (4) and sensitive layer (1) is formed. 26. The method according to any one of claims 17 to 25, characterized in that the cover electrode (7) is formed from a metallic thin layer and / or graphitic carbon. 27. The method according to any one of claims 17 to 26, characterized in that the cover electrode (7) is formed from graphitic carbon by application and subsequent drying of a dispersion. 28. The method according to any one of claims 17 to 27, characterized in that the detector elements te (2) are each individually connected to an electronic evaluation unit (6). 29. The method according to any one of claims 17 to 28, characterized in that the surface of the barrier layer (4) before the application of the sensitive layer (1) is planarized by chemical-mechanical polishing and / or the application of an intermediate layer.