WO2004081604A2 - Direct-conversion x-ray imager and production method thereof - Google Patents

Abstract

The invention relates to an imager with direct photon-to-electron conversion for the detection of X-rays, and to the production method thereof. The inventive imager comprises an epitaxial layer (1) of semiconductor material having a high atomic number and a forbidden band of the order of 1.5 eV, said layer (1) being formed on a substrate n+ (or p+) (2), with individual detectors (4) each defining one pixel of the imager. According to the invention, each individual detector (4) comprises a first electrode (2) comprising the aforementioned substrate n+ (or p+), a second electrode (9) comprising a layer n+ (or p+) which is formed on the outer epitaxial layer and a third electrode (8) comprising a layer p+ (or n+) which is placed on the same face of the individual detector (4) as the second electrode (9). Moreover, said first (2) and second (9) electrodes are electrically connected such that they are at the same electrical potential.

Description

 Direct conversion imager and its manufacturing process

The present invention relates to an imager for the detection of X-rays with direct conversion of phoions to electrons and its manufacturing process.

There are two categories of X-ray photon detectors for image acquisition in digital form. The first uses a photosensitive layer (for example a phosphor) which transforms the X photons into visible photons, which are then detected by the techniques applicable to the latter. The second uses semiconductor materials which directly transform X photons into electrons. These latter detectors have many advantages over photosensitive layers. In principle, they allow images to be acquired at a faster rate and have higher performance (collection efficiency or sensitivity, noise, dynamics, acquisition speed).

The detectors of the second category currently manufactured with a solid semiconductor material, such as Silicon, Cadmium Telluride (CdTe) or CdZnTe (CZT) for example, use a so-called depletion zone in which there is an electric field allowing an electron-hole pair created by the absorption of a photon to be collected to thereby form an electrical signal. Such a depletion zone is obtained by using p / n, p / i / n structures or Schottky barriers. This active zone for the detection of particles penetrating the detector must have a large thickness to effectively absorb the X photons, of the order of several hundred micrometers depending on the energy of the photons and the material used.

In solid materials, the active area can have sufficient thickness due to the defects they contain, which compensate for the free carriers (linked to residual doping). However, solid materials do not have the optimum qualities for detection, in particular for imaging: the defects that they contain greatly reduce the lifetime of charge carriers and their electronic properties, due to the inhomogeneous distribution of faults, are not homogeneous on the scale of the detector. There are materials which do not contain (or very few) defects allowing the charge carriers to have an optimal lifetime and whose properties are homogeneous on large surfaces: these are layers obtained by epitaxial growth. In this case, the residual doping is not compensated for by the defects, which greatly limits the width of the depletion zone. The application of a reverse polarization can make it possible to increase the width of this zone but there is a threshold, the breakdown voltage. Tensions close to this value may even prove to be insufficient to obtain complete depletion of the structure. For example, a layer initially having 1 x 10 ¹³ cm ^"3 residual dopants (limit value rarely reached in epitaxial growth) can only undergo depletion in an area reaching a few tens of micrometers for an acceptable voltage of the order of 50 V The structures of these detectors may therefore prove to be insufficient to collect all of the charges created and reach the X-ray detection limits.

Furthermore, so-called Silicon Drift Detectors (SDDs) are known which allow the total depletion of charge carriers. An electric field having a strong component parallel to the surface of the detector sends the charge carriers, generated by the absorption of a photon, to a collection electrode placed for example at the heart of a set of rings. Carriers can drift a long distance because their lifespan is very long in this material. These drift chambers are only known for individual detectors whose active surfaces are of the order of ten mm ² , that is to say large compared to the typical dimensions of individual detectors defining the pixels of a imager. These detectors are therefore not suitable for imaging which implements an array of individual detectors, the number of which can range up to several hundreds of 1000.

Finally, there is a device and a method of manufacturing, fast and non-polluting, an electronic GaAs detector having a high sensitivity for the detection of X-rays {Bourgoin J., WO 02061852}. According to this process, an n ⁺ (or p ⁺ ) substrate is formed on epitaxial material having very uniform electronic properties, the thickness of which is sufficient to efficiently absorb the X photons. The epitaxial material obtained has a certain residual doping, which can be reduced within a certain limit once the detector is finished, by uniform irradiation with electrons of approximately 1 MeV and at an adjusted dose. However, the qualities of this detector can be further improved by maximizing the collection of the signal generated by the radiation penetrating the detector.

The objective of the present invention is to provide a method and a device which is simple in its design and in its operating mode, rapid and economical, for producing imagers using epitaxial layers and having increased sensitivity for the detection of X-rays. using the drift chamber principle.

To this end, the invention relates to an imager with direct conversion of photons into electrons for the detection of X-rays comprising an epitaxial layer of an unintentionally doped semiconductor material having a high atomic number and a forbidden band of the order 1.5 eV, said layer being formed on an n ⁺ (or p ⁺ ) substrate, said imager comprising individual detectors each defining a pixel of the imager.

According to the invention,

each individual detector comprises a first electrode comprising said substrate n ⁺ (or p ⁺ ), a second electrode comprising a layer n ⁺ (or p ⁺ ) formed on the external epitaxial layer and a third electrode comprising a layer p ⁺ (or n ⁺ ) placed on the same face of the individual detector as the second electrode,

- The first and second electrodes are electrically connected to be at the same electrical potential.

In different particular embodiments of individual detectors on the external surface of the layer, each having its particular advantages and capable of numerous possible technical combinations, depending on the size of the pixel:

the second electrode covers at least 60% of the surface of said individual detector, the second electrode covers at least 90% of the surface of said individual detector,

the unintentionally doped epitaxial layer has a thickness of which depends on the energy of the X photons to be absorbed and on the material and which is between 100 μm and 1 mm,

the imager comprises at least one contact line for electrically connecting the second electrodes of said individual detectors to the substrate n ⁺ (or p ⁺ ),

- the epitaxial layer is made of GaAs, - the epitaxial layer is made of InP,

- the epitaxial layer is made of GaP,

- the epitaxial layer is in CdTe.

The invention also relates to a method for manufacturing an imager with direct conversion of photons into electrons for detecting X-rays, in which

- an epitaxial layer of an unintentionally doped semiconductor material having a high atomic number and a band gap of the order of 1.5 eV is formed on a substrate n ⁺ (or p ⁺ ), said epitaxial layer having a thickness d 'sufficient to efficiently absorb the X photons,

- the thickness of the substrate is reduced by chemical mechanical polishing.

According to the invention,

- n ⁺ (or p ⁺ ) ions are implanted on the external face of the epitaxial layer,

- said structure is annealed at a temperature θi for a time

- individual detectors are defined by masking and chemical pickling, - a first passivation is carried out,

- first ohmic contacts are made on both sides after masking and a contact line by metallic evaporation followed by annealing,

- a second passivation of the external surface is carried out between individual detectors, p ⁺ (or n ⁺ ) electrodes are produced by masking, chemical etching and implantation of p ⁺ (or n ⁺ ) ions on the external face of the epitaxial layer,

- said structure is annealed at a temperature θ ₂ for a time -2,

- Second ohmic contacts are made on said measurement electrodes after masking.

In different particular embodiments of individual detectors on the external surface of the layer, each having its particular advantages and capable of numerous possible technical combinations:

the thickness of the layer depends on the energy of the X photons to be absorbed and on the material and is between 100 μm and 1 mm,

- ohmic contacts are made by evaporation of a gold-based alloy followed by annealing,

- the epitaxial layer is made of GaAs,

- the epitaxial layer is in InP,

- the epitaxial layer is made of GaP,

- the epitaxial layer is in CdTe. The imager thus produced can be used for the detection of X photons with an energy between some keV and 200 keV. This imager can also be used for higher X photon energies, but the lower absorption coefficient for these energies reduces its sensitivity. It can advantageously be used for medical imaging (2 dimensions) and certain industrial applications (for example: examination of airport baggage (1 dimension)).

The invention will be described in more detail with reference to the accompanying drawings in which:

- Figure 1 is a schematic representation of the successive steps 1a), 1b), 1c), 1d), 1e), 1f), 1g) and 1h) leading to the production of an X-ray imager, according to one mode of particular embodiment of the invention in which the measurement electrode at least partially covers the second electrode;

- Figure 2 schematically shows an imager during manufacture comprising a matrix (4x2) of individual detectors (Fig. 2a) and a sectional view along the axis AA of the imager with the measurement electrode covering at least partially the second electrode (Fig. 2b);

- Figure 3 is a schematic representation of an individual detector formed by a p / i / n structure showing the potential lines guiding the measurement signal generated in the depletion zone (Fig. 3a) and of an individual detector with its potential lines of an imager X, according to an embodiment of the invention (Fig. 3b);

One of the conditions required for the application of electronic detectors to imaging is that the size of the semiconductor material on which the detectors are made is sufficiently large and that its properties are homogeneous. However, obtaining large epitaxial layers is linked to the existence of monocrystalline bars of large diameter in which substrates of good crystallographic quality (but whose electronic properties are generally not homogeneous) can be cut. In addition, these layers must be made with materials with a large atomic number Z (ie those for which the absorption of X photons is important). The imager therefore consists of a semiconductor material having a high atomic number and a band gap of the order of 1.5 eV. The semiconductor material is then chosen from one of the materials having the following required characteristics: GaAs, InP and CdTe, GaP because the free electrons in said material have a mobility greater than 1000 cmVV ¹ and a long lifetime.

FIG. 1 schematically represents in the particular case of GaAs, the successive stages of the process leading to the manufacture of an X-ray imager. The first stage 1a) consists in the growth of epitaxial layers 1 of thickness dependent on l energy of the photons to be detected on an n ⁺ (or p ⁺ ) substrate. The method of growing epitaxial layers implemented in the present invention was the subject, in a particular embodiment, of a published patent application WO 02061852 on the part of the inventor of the present application. This application describes a method and a device for manufacturing an electronic GaAs detector for detecting X-rays. The growth of the epitaxial material 1 results from a method aimed at suppressing the transport of gases to the substrate 2 in order to obtain a rapid growth rate. For this, a source material having the same chemical composition as the material to be grown (GaAs) is brought to a temperature Ti> 600 ° C (preferably between 750 and 850 ° C) by a heater. Said heater consists, for example, of a metal resistor covered with boron nitride. The reactive gas H ₂ O, under a well-defined partial pressure, interacts chemically with the source material brought to the temperature Ti, creating volatile products. These volatile compounds are transported by the pressure gradient to a substrate n ⁺ (or p ⁺ ) brought to a temperature T ₂ <T ₁ (advantageously at least 50 ° C.) by a second heater identical to the first . The volatile products then recompose on the surface of the substrate n ⁺ (or p ⁺ ) to reform GaAs and H ₂ O gas. The latter is then available again to participate in the reaction process. It is a closed system: there is therefore no need to have a continuous supply of reactive gas.

Because of the relatively small temperature difference between the source material and the substrate, imposed by the short distance separating said elements, the growth of epitaxial layer 1 takes place under conditions close to the reaction equilibrium between the crystal and the gas. reagent.

The growth rate of the epitaxial layers 1 is only a function of the respective temperatures, Ti and T ₂ , of the source material and of the substrate n ⁺ (or p ⁺ ), of the partial pressure p of the reactive gas.

The source material is the unintentionally doped semi-insulating standard material: it contains p and n type dopants in a concentration of 10 ¹⁵ - 10 ¹⁶ cm ^"3 which are respectively mainly C and Si according to the literature. The material obtained contains therefore also these doping impurities but in different proportions because their transport depends on the nature of these.

The thickness d 'of the layer 1 produced depends on the energy of the X photons to be absorbed and in a preferred embodiment, it is between 100 μm and 1 mm. After growth of the epitaxial layer 1, the thickness of the substrate 2 is reduced by chemical mechanical polishing (step 2, Fig. 1b)) to uniformly reduce its thickness to less than 10 μm. In a third step (Fig. 1c)), ions are implanted to form a doped controlled surface layer 3 (greater than 10 ¹⁸ cm ^"3 ) of type n ⁺ , respectively p ⁺ . In one embodiment and for a GaAs epitaxial layer, Si ⁺ ions, respectively Mg ⁺ or Be ⁺ , are implanted with energies between 10 and 200 keV (advantageously around 40 keV) and typical doses (of the order of 5 × 10 ¹³ atoms / cm ² ). This implantation is followed by rapid thermal annealing at a temperature θi (approximately 850 ° C.) and for a short determined time t _t (10 s). Masking is then carried out by photolithography and chemical etching (step 4, Fig. 1d)) to form individual detectors 4, each of said detectors 4 defining a pixel of the imager. This step is followed by the deposition of a first insulating layer 5 for the passivation of the etched surfaces. passivation ", the deposition of an insulator on surfaces pickled to make them non-conductive, for example, by deposition of silicon nitride (Si ₃ N ₄ ) or silica (SiO ₂ ) (step 5, Fig. 1e)). Masking is then carried out by photolithography to make an opening on the pixels 4 in order to form first ohmic contacts 6 (step 6, Fig. 1f)) on both sides by evaporation of a metallic alloy (multilayer) based on gold, followed by annealing at a temperature of the order of 400 ° C and for a determined time (of the order of 300s). This alloy is for example AuGe or NiAu for the n ⁺ contacts and TiPtAu or AuZn for the p ⁺ contacts. This step is followed by the deposition of a second insulating layer 7 by deposition of silicon nitride (Si ₃ N ₄ ) or silica (SiO ₂ ) for protection against implantation p ⁺ , made after opening the measurement electrode. . A measurement electrode 8 is then produced for each individual detector 4 by masking (step 7, FIG. 1g)) and implantations of p ⁺ (or n ⁺ ) ions on the external face of the epitaxial layer 1. In a mode of particular embodiment, this measurement electrode 8 is arranged on one of the lateral sides of the individual detector 4. The effective dimensions of this measurement electrode 8 are small compared to the dimensions of the contact 9 n ⁺ (or p ⁺ ) formed on the same face of Individual detector 4. Advantageously, said contact 9 n ⁺ (or p ⁺ ) covers at least 60% of the surface of the individual detector 4 and preferably at least 90%. To form this measurement electrode 8, Mg ⁺ or Be ⁺ ions, respectively Si ⁺ , are implanted with energies between 10 and 200 keV (advantageously around 40 keV) and typical doses (of the order of 5 x 10 ¹³ atoms / cm ² ). This implantation is followed by rapid thermal annealing at a temperature θ ₂ (approximately 850 ° C) and for a short time t ₂ (10s). Masking is then carried out after revelation and after revelation of the resin, a chemical etching to form second ohmic contacts 10 (step 8, FIG. 1h)) on said measuring electrodes 8 by evaporation of a gold-based alloy, followed by annealing at a temperature of the order of 400 ° C and for a determined time (300s). This alloy is for example AuGe or NiAu for the n ⁺ contacts and TiPtAu or AuZn for the p ⁺ contacts. These second ohmic contacts 10 can cover the entire surface of the pixel (FIG. 1h), which allows an apparent surface for the measurement electrode 8 much greater than its effective surface.

This embodiment is particularly advantageous in the case where the second electrode 9 covers at least 90% of the surface of the individual detector 4.

The electrical contact 9 n ⁺ (or p ⁺ ) formed on the outer layer of the epitaxial material 1 and the substrate n ⁺ (or p ⁺ ) 2 are electrically connected. These two electrodes 2, 9 are thus at the same electrical potential. The term “same electrical potential” is understood to mean an electrical potential which is substantially equal and at least very different from the potential applied to the measuring electrode 8. In a preferred embodiment, after or during the formation of the first ohmic contacts 6 (step 6, Fig. 1f)) but before the deposition of a second insulating layer 7, a contact line 11 is therefore formed by metallic evaporation to electrically connect these two electrodes 2, 9. This contact line 11 can be closed by evaporation metallic based on gold.

The manufacturing process of an X-ray imager of the invention cannot be limited to the above description and is subject to modification with the evolution of technologies. Substitutions and / or modifications in the steps of the present process can be performed by a person skilled in the art without departing from the spirit of the present invention. Thus certain steps of the process can be modified so that the implantation anneals are carried out at the same time and before the formation of the first and second ohmic contacts.

Similarly, the individual detectors can be manufactured on the face of the substrate 2, after its thickness has been reduced by chemical mechanical polishing, instead of the external face of the epitaxial layer 1. The steps of the method of the invention leading to the manufacturing an imager remain the same. However, the steps leading to the formation of the individual detectors are applied to the external face of the substrate and therefore the thickness of the layer to be chemically etched is greater. It is in fact slightly greater than the residual thickness of the substrate 2 in order to reach the epitaxial layer 1. The imager thus produced advantageously allows irradiation on the side of the external face of the epitaxial layer 1 in which n ⁺ ions (or p ⁺ ) 3 have been implanted. The thickness of this implanted and inactive layer for detection, being much less than 1 μm and therefore much less than the thickness of the substrate n ⁺ (or p ⁺ ) 2 of the order of 10 μm, the fraction of charges induced by the absorption of incident photons in the epitaxial material is thus increased compared to an imager in which the individual detectors would be produced on the external face of the epitaxial layer.

The invention also relates to an imager with direct conversion of photons into electrons for the detection of X-rays comprising an epitaxial layer 1 of a semiconductor material having a high atomic number and a band gap of the order of 1.5 eV, said layer 1 being formed on a substrate n ⁺ (or p ⁺ ) 2. The semiconductor material is then chosen from one of the materials having the required characteristics, following: GaAs, InP, CdZnTe, CdTe and GaP. In a preferred embodiment, the thickness of the epitaxial layer 1 depends on the energy of the X photons to be absorbed and it is generally between 100 μm and 1 mm.

This imager includes individual detectors 4 each defining a pixel. Each individual detector 4 comprises a first electrode comprising said substrate n ⁺ (or p ⁺ ) 2, a second electrode comprising a layer n ⁺ (or p ⁺ ) 9 formed on the external epitaxial layer and a third electrode comprising a layer p ⁺ (or n ⁺ ) 8 placed on the same face of the individual detector 4 as the second electrode 9. The first 2 and second 9 electrodes are electrically connected to be at the same electrical potential.

In a particular embodiment, the measurement electrode 8 is arranged on one of the lateral sides of the individual detector 4. The effective dimensions of this measurement electrode 8 are small compared to the dimensions of the second electrode 9 placed on the same face of the individual detector. Advantageously, the second electrode 9 comprising the contact n ⁺ (or p ⁺ ) covers at least 60% of the surface of the individual detector 4 and preferably at least 90%. The apparent surface of the measurement electrode 8 can be increased relative to the effective dimensions of said electrode by forming the second ohmic contacts 10 on said measurement electrode 8 and on the insulator 7 covering the second electrode 9 (FIG. 2b).

Figure 3a schematically shows an individual detector 12 formed by a p / i / n junction. By applying reverse polarization to the electrodes, the width of the depletion zone 13 is W. The electric field lines 14 created in the structure p / i / n are shown in dotted lines. The absorption of a photon in the depletion zone 13 creates an induced charge 15 which under the influence of the potential existing in said zone is rapidly collected to form a basic electrical signal. The electric field lines 14 guide said induced charge 15 towards the electrodes. If the width W of the depletion zone does not extend completely through the structure p / i / n, for example due to residual doping, the induced charge may be lost for detection. However, a fraction is collected by diffusion.

The collection of the charges induced 15 in the depletion zone 13 can however be improved with the imager X according to the present invention. Figure 3b shows the electric field lines created in the structure of an individual detector 4 of the imager X. The field lines 14 start from the first 2 and second 9 electrodes which are at same electrical potential for joining the third electrode placed, for example, laterally on the surface of the individual detector 4. At the heart of the depletion zone the field lines 14 direct the induced charges 15 towards the third electrode 8 parallel to the surface of the detector All of the induced charges are therefore guided through the depletion zone 13 towards the measurement electrode.

This imager can advantageously be used for example when the epitaxial material is made of GaAs for medical imaging.

(mammography, dental care, etc.), certain industrial applications (e.g. examination of airport baggage), diffraction in synchrotron radiation, etc.

Claims

1. Imager with direct conversion of photons into electrons for X-ray detection comprising an epitaxial layer (1) of an unintentionally doped semiconductor material having a high atomic number and a band gap of the order of 1.5 eV, said layer being formed on an n ⁺ (or p ⁺ ) substrate (2), said imager comprising individual detectors (4) each defining a pixel of the imager, characterized in that each individual detector (4) comprises a first electrode comprising said n ⁺ (or p ⁺ ) substrate (2), a second electrode comprising an n ⁺ (or p ⁺ ) layer (9) formed on the outer epitaxial layer and a third electrode comprising a p ⁺ (or n ⁺ ) layer (8) placed on the same face of the individual detector as the second electrode (9), and in that the first (2) and second electrodes (9) are electrically connected to be at the same electrical potential. 2. An imager according to claim 1, characterized in that the second electrode (9) covers at least 60% of the surface of said individual detector (4). 3. Imager according to claim 2, characterized in that the second electrode (9) covers at least 90% of the surface of said individual detector (4). 4. Imager according to one of claims 1 to 3, characterized in that the unintentionally doped epitaxial layer (1) has a thickness of which depends on the energy of the X photons to be absorbed and on the material and which is between 100 μm and 1 mm. 5. Imager according to one of claims 1 to 4, characterized in that the imager comprises at least one contact line (10) for electrically connecting the second electrodes (9) of said individual detectors (4) to the substrate n ⁺ ( or p ⁺ ). 6. Imager according to one of claims 1 to 5, characterized in that the epitaxial layer (1) is made of GaAs. 7. Imager according to one of claims 1 to 5, characterized in that the epitaxial layer (1) is made of InP. 8. Imager according to one of claims 1 to 5, characterized in that the epitaxial layer (1) is made of GaP. 9. Imager according to one of claims 1 to 5, characterized in that the epitaxial layer (1) is made of CdTe. 10. A method of manufacturing an imager with direct conversion of photons into electrons for the detection of X-rays in which: - an epitaxial layer of a non-intentionally doped non-intentiated semiconductor material (1) having a high atomic number and a forbidden band of the order of 1.5 eV is formed on an n ⁺ (or p ⁺ ) substrate (2) the said epitaxial layer (1) having a thickness of sufficient to efficiently absorb the X photons, the thickness of the substrate (2) is reduced by chemical-mechanical polishing, characterized in that - n ⁺ (or p ⁺ ) ions (3) are implanted on the external face of the epitaxial layer, - said structure is annealed at a temperature θi for a time - individual detectors (4) are defined by masking and chemical etching, - a first passivation is carried out, - first ohmic contacts (6) are made on both sides after masking and a contact line (11) by metallic evaporation followed by annealing, - a second passivation of the external surface is carried out between individual detectors, - electrodes (8) are produced by masking, chemical etching and implantation of p ⁺ (or n ⁺ ) ions on the external face of the epitaxial layer, - said structure is annealed at a temperature θ ₂ for a time t ₂ , - Second ohmic contacts (10) are made on said measurement electrodes (8) after masking. 11. Method according to claim 10, characterized in that the thickness d 'of the layer (1) depends on the energy of the X photons to be absorbed and on the material and that it is between 100 μm and 1 mm. 12. Method according to claims 10 or 11, characterized in that the ohmic contacts (6, 10) are produced by evaporation of a gold-based alloy followed by annealing. 13. Method according to any one of claims 10 to 12, characterized in that the epitaxial layer (1) is made of GaAs. 14. Method according to any one of claims 10 to 12, characterized in that the epitaxial layer (1) is in InP. 15. Method according to any one of claims 10 to 12, characterized in that the epitaxial layer (1) is made of CdTe.