DE19711849A1 - X=ray detector with semi-insulating semiconductor substrate - Google Patents

Abstract

An X-ray detector has a semi-insulating semiconductor (preferably GaAs) substrate (1) including an x-ray photon sensitive registering region with front face contacts (4) in the form of microstrip electrodes (3) and with a back face contact (5). The microstrip electrodes (3) and back face contact (5) are in the form of Schottky contacts and (i) the back face contact-bearing face (7) of the substrate (1) is prepared by ion implantation and/or (ii) the mean free paths of electrons and holes in the substrate material have a similar size to the substrate thickness between the contacted faces at similar electron and hole free paths. Also claimed is a similar detector consisting of a stack tightly adjoining (especially adhesively bonded) individual detectors which are individually contacted by front face contacts in the form of microstrip electrodes (3) and by a back face contact. Preferably, the detector is provided with a contact strip layer of metallised polyimide foil corresponding to the electrode shape. Also claimed is a method of producing the above detectors, in which the substrate back face is subjected to ion implantation within the Schottky zone so that subsequent annealing is not required.

Description

The invention relates to an X-ray detector with semi-isolie rendes semiconductor substrate, preferably made of gallium arsenide (GaAs), which is sensitive to X-ray photons Registration area with microstrip electrodes front contacts and a - preferably flat - Backside contact, the opposing surfaces of the substrate are arranged. The invention relates a method for producing an X-ray detector. Under the term "X-ray photons or rays" in Within the scope of the invention high-energy photons or electromagnetic table rays of all kinds, including gamma radiation, Understood.

DE 43 44 252 A1 describes an X-ray detector for use in computer tomography, in which a semi-iso insulating semiconductor body made of a compound semiconductor, like GaAs, a constant voltage source as X-ray sensitive cher photoresistor is operated. The well-known detector be however, there is an insufficient one for many applications Energy resolution regarding those to be registered X-ray photons.

The invention is based on the overarching object  To create an X-ray detector with the highest possible energy resolution which is able to efficiently capture single photons assign and specify their energy.

A solution according to the invention exists for the beginning benen X-ray detector in that the microstrip electrodes and the rear contact is designed as a Schottky contact and that the side of the sub strats through a - preferably annealing not required - ion implantation is prepared.

Alternatively, there is a parallel version of the above inventive idea to solve the same problem - al so also in the sense of achieving the highest possible Energy resolution also for single photons and their detection regardless of the type of conversion - in that the Microstrip electrodes and the rear contact as a Schottky contact are formed and that the mean free path of electrical at least comparable holes and holes in the substrate material large with that by the mutual distance of the contacted Substrate areas defined substrate thickness at approximately ver is the same free path length of the electrons and holes. in the All alternatives of the invention come first (Semi-insulating) GaAs substrates in question.

According to the invention, each on both sides of the substrate Single detector Schottky contacts provided. This measure serves the formation of a depletion zone and the reduction of dun kelstrom. They drift in the electrical field of the depletion zone radiation-generated charge carriers at low currents so that they can be demonstrated. After all, it should the back of the detector (back contact) by a Ion implantation - preferably without annealing - especially be prepared so that the hole injection and thus associated breakdown is only used at higher voltages Zen. An annealing should be dispensable to match one to avoid material damage. If - especially according to Al  alternative solution - the mean free path lengths of electrons and holes approximately the same size and comparable to the Are substrate thickness, the signals generated are independent from the conversion site of the incident photons.

The alternatively preferred feature, the middle free path lengths of electrons and holes comparable to that Making substrate thickness sets with relatively large substrate thick, e.g. B. 2 mm, a correspondingly high average life of electrons and holes ahead. Within the scope of the invention can preferably substrate thicknesses in the order of 0.25 to 3 mm can be provided. The with each other and with the Comparable substrate thickness - exceed the latter if possible the - mean free path lengths of electrons and holes in GaAs can be defined by setting third party Share of substances such as carbon, oxygen, boron and / or Chromium, already specified when growing crystals. The ent standing semi-insulating crystals with the desired ones excellent charge transport properties are also extreme radiation-proof, so that they have a correspondingly long service life to have.

Some improvements and further and refinements of the Er invention are described in the subclaims. Given if the measure of size and equality of the mean free path lengths of electrons and holes with regard to an optimization of the energy resolution can be set. The The values of the mean free path lengths can differ from each other deviate, but the deviation should preferably be less than about Be 10%. The energy resolution of a detector according to the invention tors gets better, the bigger and the same the free ones Path lengths of the two charge carriers are.

According to another invention, the parent is used to solve Task an X-ray detector from a stack of close to one another tight, especially glued or otherwise Single detector according to the invention, cohesively connected  ren each designed as microstrip electrodes Front contacts and with rear contacts, which are individually are contacted. Such a stack acts like an thick pixel detector, the sensitive area (which the spatial resolution is determined) by the width of the pixels Microstrip and the thickness of the substrate of the single detector and the substrate length measured in the direction of the microstrip (which determines the detection efficiency). The Contacting of the individual detectors in the stack is said to be pre be taken that the x-ray sensitive area portion of the right levante incidence edge of the overall detector as large as possible becomes.

Preferably, each of the microstrip electrode areas a stack, possibly also the back electrodes, a contact strip layer, especially one on both sides suitable metallized film made of insulating material, for individual ellen electrical contact of the individual micro strip electrodes and (e.g. as a contact layer) the backside electrodes assigned to troden. For example, there are two before Alternatives drawn: Either the respective contact dispute fenschicht (separating the entire surface) between two individual De tectors inserted in the stack or each individual detector of a stack is on its (the X-ray to be registered opposite edge to be exposed to radiation Contact edge tapers in a wedge shape so that the microstrip fen electrodes and the back contacts in the tapered Continue substrate area and can be contacted there nen. The substrate of each individual detector can be on the contact edge - possibly outside of that for the X-ray photons particularly sensitive area - (preferably one-sided) abge the area having the microstrip electrodes be inclined or graduated.

The contact strip layer for individual removal of the in of the single microstrip electrode registered signal can preferably be a plastic film, in particular made of polyimide,  with metal stripes on it in the pattern of the micro strip electrodes are formed. Such a contact strip layer can be between two detectors clamped, alternatively it can be (only) attached to the bevel Contacted th ends of the individual detectors, e.g. B. bonded and / or glued. In the latter case correspondingly less in between the individual detector positions active zones. This leads to a significantly higher x-ray sit area share on the respective incidence edge of the Ge velvet detector. For example, there is between two detectors instead of the contact strip foil (with conductor tracks) of 50 Micron thickness only one insulator layer with sizes properly required 10 microns thick.

The contact strip layer assigned to the individual detectors In principle, each can preferably consist of at least one Stacking channel arranged perpendicular to the contacted surfaces te, the so-called contact edge, for individual contacting each microstrip electrode of the front contacts as well if necessary, the rear contacts protrude like flags hen. The individual detectors can be advantageous here Arranged back-to-back, so that two individual detectors only a common connection of the rear contacts and thereby the inactive area of the area edge is reduced.

In principle, each individual detector preferably has one as - preferably vertical - connection of two contacted surfaces Chen oriented, essentially flat leading edge for closed registering x-ray photons, such that the on the one fall edge approximately parallel to the contacted surfaces X-ray radiation incident on the substrate is grasped. The leading edges of a stack of single detec In principle, goals should be on a common level of inspiration lie. The edge of incidence and the where appropriate standing contact strip layers having contact edge should oppose each other with respect to the substrate or the stack  overlap.

As a result, the following advantages are achieved by the invention, in particular when used in the stack detector, compared to the prior art:

• a) Higher possible operating voltages and thus higher night Efficiency for photons, especially X-ray photons in the narrower sense and minimally ionizing particles.
• b) Due to targeted material breeding and contact technology are active substrate thicknesses of up to several mm - because of the large mean free path lengths of electrons and holes - easily possible.
• c) Intrinsic energy resolution E (FWHM) / E better than about 5% for photon energies greater than 50 keV.
• d) High detection efficiency, greater than about 90% for Pho toning with energies greater than 50 keV.
• e) High spatial resolution in both dimensions x, y lower right to the direction z of the incident X-rays in on the order of 10 microns.
• f) Extremely good time resolution (single signal analysis times) in on the order of 10 nanoseconds.
• g) Variable modularity of the stack: face area and The shape of the stack can almost be chosen by choosing the modules big given geometries.
• h) Reliable operation up to extremely high rates (about 1 GHz / mm ² ).
• i) Extremely high radiation hardness compared to ionizing radiation (energy dose greater than about 1 MGy) and non-ionizing radiation (flux greater than about 10 ¹⁵ neutrons / cm ² ).
• j) Operation at room temperature.
• k) The X-ray sensitive area of the incidence edge of a he inventive single or stack detector is depending depending on the type, between about 90 and about 98%. The proof probabilities lie with the appropriate substrate length of the single detector in the same order of magnitude.

The detection sought by the invention is more energy-intensive electromagnetic radiation (in the order of 10 keV to 1 MeV) with high efficiency and high spatial resolution requires high-Z materials (Z = atomic number), which also allow fine segmentation of the detectors. With room Suitable candidates for this are high-Z half-lead ter compounds such as GaAs. This also includes CdTe and HgJ.

To provide an X-ray detector with optimal energy resolution create that is able to efficiently even single photons to demonstrate and to indicate their energy will be in a before drafted embodiment of the invention provided the Mi stainless steel electrodes and the back contact of a single or stack detector with semi-insulating semiconductor sub to form strat from gallium arsenide as Schottky contacts, the side of the substrate carrying the rear-side contact a - preferably annealing-free - ion implantation prepare and / or the mean free path of electrical NEN and holes in the substrate material about the same and at least to make it comparable in size to the substrate thickness.

Based on the schematic representation of exemplary embodiments some details of the invention are explained. Show it:

Fig. 1 shows a single-detector according to the invention;

Figure 2 is a stack-detector according to the invention with a view to photons incident edge.

Fig. 3 shows a stack detector of Figure 2 seen from the side with signal readout via polyimide strip film. and

Fig. 4 shows a stack detector with signal readout from bevelled contact edges.

Fig. 1 shows an X-ray detector with semi-insulating GaAs substrate 1 , which has a sensitive to X-ray photons registration area 2 ( Figs. 3 and 4) with as Mikrostrei fen electrodes 3 - generally: structured metallization - formed front contacts 4 and flat back contact 5 has. The front side contact 4 and the rear side contact 5 are arranged on mutually opposite surfaces 6 and 7 of the substrate 1 . Passivations 8 are provided between two microstrip electrodes 3 or the same structured metallization of the front substrate side 6 .

Fig. 2 to illustrate a stack, generally designated 9 of - in the embodiment, three pieces -. Individual detectors 10 of FIG 1. Between the adjoining in the stack 9 front-side contacts 4 and back contacts 5 of adjacent individual detectors 10 can according to FIG 4, respectively. an electrical insulator layer 11 are located. The insulator layer 11 can, according to FIG. 3, be matched on both sides to the contacts 4 , 5, metallized contact strip layer 12 , for. B. polyimide film are formed. Both the individual microstrip electrodes 3 and - isolated therefrom - the rear sides or contacts 5 may need to be contacted. In order to simplify the contacting, in particular on the rear sides, it may be reasonable to stack the individual detectors 10 - in contrast to FIG. 2 - back-to-back, so that the rear-side contacts 5 of two individual detectors 10 are to be contacted at the same time.

FIGS. 3 and 4 show two embodiments for contactless orientation of the various electrodes of the individual detectors 10. In the case of FIG. 3, it is provided to insert at least one polyimide strip - contact strip layer 12 - between the (essentially total) contact surfaces of two adjacent individual detectors 10 . At least one edge, especially at the to be registered Rönt-radiation provided incident edge 13 of the contact-strip layers are 12 connect smoothly with the local substrate edges, so that a to be turned towards to be registered photon 16 incident face or plane of results. At this one edge 13 (with respect to the substrate 1 or the stack 9 of individual detectors 10 ) opposite contact edge 14 of the detector according to the invention, the contact strips 12 should look out of FIG. 3 as flags 15 . The corresponding inputs of associated readout electronics, generally individually to each microstrip and to each rear contact, are to be connected to the flags 15 .

In the exemplary embodiment according to FIG. 4, a stack 9 of individual detectors 10 is shown, which (each individually) on the contact edge 14 opposite the incident edge 13 of the photons 16 - that is, outside the registration area 2 - on one side on the microstrip electrodes 3 bearing side beveled. In this case, between two individual detectors 10 - at least in the registration area 2 - no polyimide strip films or the like need to be arranged. Rather, the microstrip electrodes 3 can be connected directly - in particular individually bonded - to a downstream readout electronics.

For sufficient detection sensitivity, a substrate length of about 2 cm, measured between the incident edge 13 and the contact edge 14, is often sufficient. Depending on the energy of the incident radiation, substrate lengths of 5 cm and more can also be provided. According to the invention, the individual detectors 10 or the detector stack 9 can be combined to form total detector arrangements (possibly also with a combination of a plurality of stacks 9 ) which have an X-ray-sensitive area of the order of 100 cm ² .

Detectors according to the invention are extremely suitable for X-ray computer tomograph with low radiation exposure (for the patient in medical applications), high space resolution and faster online analysis. Also suitable net are such detectors for non-destructive material testing tests, especially with very high X-ray energies and high In intensities, in particular the high efficiency, the high spatial resolution and the radiation resistance of the he make detector according to the invention advantageously noticeable.  

Reference list

1

Substrate

2nd

Registration area

3rd

Microstrip electrode

4th

Front contact

5

Rear contact

6

front substrate surface

7

rear substrate surface

8th

Passivation

9

stack

10th

Single detector

11

Insulator layer

12th

Contact (stripe) layer

13

Leading edge

14

Contact edge

15

flag

16

Photons

17th

bevel

Claims (16)

1. X-ray detector with semi-insulating semiconductor substrate ( 1 ), preferably made of gallium arsenide (GaAs), which has a ge compared to X-ray photons sensitive registration area ( 2 ) with microstrip electrodes ( 3 ) formed on the front contacts ( 4 ) and has a rear side contact ( 5 ), which are arranged on opposite surfaces of the substrate ( 1 ), characterized in that the microstrip electrodes ( 3 ) and the rear side contact ( 5 ) are designed as Schottky contacts and that the Back side contact ( 5 ) carrying side ( 7 ) of the substrate ( 1 ) is prepared by an ion implantation. 2. X-ray detector with semi-insulating semiconductor substrate ( 1 ), preferably made of gallium arsenide (GaAs), which has a registration area ( 2 ) which is sensitive to X-ray photons and is formed with microstrip electrodes ( 3 ), front contacts ( 4 ) and has a rear side contact ( 5 ) which are arranged on opposite surfaces of the substrate ( 1 ), in particular according to claim 1, characterized in that the microstrip electrodes ( 3 ) and the rear side contact ( 5 ) are designed as Schottky contacts and that the mean free path length of electrons and holes in the substrate material is comparatively large with the substrate thickness defined by the mutual spacing of the contacted substrate surfaces ( 6 , 7 ) with an approximately comparable free path length of the electrons and holes. 3. X-ray detector according to claim 2, characterized, that the measure of size and equality is the mean free path lengths of electrons and holes with a view to opti the energy resolution is set.   4. X-ray detector according to at least one of claims 1 to 3, characterized in that the rear side contact ( 5 ) carrying side ( 7 ) of the substrate ( 1 ) is prepared by an ion implantation which does not require healing. 5. X-ray detector according to at least one of claims 1, 3 and 4, characterized in that an ion implantation with silicon or oxygen of the side contact ( 5 ) carrying side ( 7 ) of the substrate ( 1 ) is provided. 6. X-ray detector with semi-insulating semiconductor substrate ( 1 ), preferably made of gallium arsenide (GaAs), as a single detector, which has a registration area ( 2 ) sensitive to X-ray photons with front contacts formed as microstrip electrodes ( 3 ) ( 4 ) and a rear side contact ( 5 ), which are arranged on mutually opposite surfaces of the substrate ( 1 ), in particular according to at least one of claims 1 to 5, characterized by a stack ( 9 ) from close to close, in particular glued or otherwise cohesively connected, individual detectors ( 10 ) each with the microstrip electro ( 3 ) formed front contacts ( 4 ) and with rear side contacts ( 5 ), which are individually contacted. 7. X-ray detector according to claim 6, characterized in that the individual detectors ( 10 ) in pairs ( 9 ) lie back to back in pairs. 8. X-ray detector according to claim 6 or 7, characterized in that each of the microstrip electrode surfaces of a stack ( 9 ), in particular also on the rear electrodes ( 5 ), a contact strip layer ( 12 ) for individual electrical contacting of the individual microstrip Electrodes ( 3 ) and optionally a contact layer for the rear side ( 5 ) is assigned. 9. X-ray detector according to claim 6 or 7, characterized in that the respective contact strip layer ( 12 ) between two individual detectors ( 10 ) in the stack ( 9 ) is inserted. 10. X-ray detector according to at least one of claims 6 to 8, characterized in that the substrate thickness of each individual detector ( 10 ) of a stack ( 9 ) on the contact edge ( 14 ) - optionally outside the registration areas ( 2 ) - continuously or gradually decreases and that the microstrip electrodes ( 3 ) and the rear contacts ( 5 ) continue into the tapered substrate area and are only contacted there. 11. X-ray detector according to claim 10, characterized in that the individual substrate ( 1 ) at the contact edge is tapered by a lateral bevel or gradation of only the front surface ( 6 ) carrying the microstrip electrodes ( 3 ). 12. X-ray detector according to at least one of claims 8 to 11, characterized in that a polyimide film corresponding to the electrode shapes is provided as the contact strip layer ( 12 ). 13. X-ray detector according to at least one of claims 8 to 12, characterized in that the individual detectors ( 10 ) associated individual contact strip layers ( 12 ) each from at least one stack edge arranged in principle perpendicular to the contacted surfaces - the so-called contact edge ( 14 ) - for an individual contact of each microstrip electrode ( 3 ) of the front side contacts ( 4 ) and optionally the rear side contacts ( 5 ) protrude like flags ( 15 ). 14. X-ray detector according to at least one of claims 8 to 13, characterized in that when the individual detectors ( 10 ) are arranged back-to-back in the stack ( 9 ), two individual detectors are assigned a common or short-circuited rear-side contact ( 5 ) is. 15. X-ray detector according to at least one of claims 8 to 14, characterized in that each individual detector ( 10 ) is a - in principle as a - preferably vertical - connection of two contacted surfaces orien tiert, substantially flat incidence edge ( 13 ) for X-ray regi strieren -Photons has, in such a way that specifically the X-radiation incident on the incident edge ( 13 ) parallel to the contacted surfaces in the substrate can be detected, that the incident edges ( 13 ) of a stack ( 9 ) of individual detectors ( 10 ) in principle a common plane of incidence bil and that the incidence edges ( 13 ) opposite the contact edges ( 14 ) with respect to the substrate ( 1 ) or stack ( 9 ). 16. A method for producing an X-ray detector according to claim 1, 2 or 6, characterized in that in the provided for the rear contact ( 5 ) side of the substrate ( 1 ) within the Schottky zone an ion implantation is carried out so that a subsequent Annealing is not required.