Detection device

Abstract

An X-ray detection device is described which has a first pixel layer comprising a semiconductor body and a second pixel layer located adjacent to the first pixel layer. The first pixel layer comprises an array of pixel photodiodes each arranged to detect first pixel charge, and an array of first pixel circuits each arranged to store and read out a first signal representing the first pixel charge detected by a corresponding pixel photodiode, the first pixel charge arising from the interaction of X-rays, and / or the interaction of X-ray generated scintillation photons, within the semiconductor body proximally to that pixel photodiode. The second pixel layer comprises a detector material body and the device also comprises an array of pixel electrodes each arranged to detect second pixel charge generated by X-rays interacting with the detector material body proximal to a corresponding pixel electrode. The first pixel layer then also comprises, for each pixel electrode, a second pixel circuit arranged store and read out a second signal representing the second pixel charge detected by the corresponding pixel electrode.

Description

[0001] Detection device

[0002] The present invention relates to devices for the detection of X-rays or other radiation, for example to a detection device which can separately detect such radiation over two different energy ranges.

[0003] Introduction

[0004] Many known X-ray applications benefit from simultaneously imaging X-rays of two or more different energy ranges so as to provide additional information about a sample. For example, this can be achieved using multiple X-ray sources of different energy ranges, or a single X-ray source that can switch between different energy ranges, although these techniques tend to be complicated and relatively expensive to implement. Instead a single X-ray source can be used either with dual scintillator detectors for detecting different energy ranges, or with a spectral ASIC detector which is capable of detecting the number of photons or magnitude of charge arising from each X-ray detection event. Such spectral ASIC detectors tend to require compromise in the choice of sensor material, with some materials being better for lower energy X-rays and others for higher energies.

[0005] Similar issues arise in the field of detection of electromagnetic and / or particulate radiation in other energy ranges, including optical and infrared regions of the electromagnetic spectrum, gamma rays, and neutrons.

[0006] It would be desirable to address these and other limitations of the related prior art.

[0007] Summary of the invention

[0008] The invention provides a detection device comprising distinct first and second pixel layers each for detecting electromagnetic or other radiation of a different energy or energy ranges, noting that the energy ranges may be overlapping. The invention may be particularly useful for detecting X-rays across two such energy ranges, for example lower energy X-rays at around 1 - 30 keV, and higher energy X-rays at around 20 - 200 keV.

[0009] Embodiments of the invention may be constructed by adding a further detector material body onto a CMOS based monolithic active pixel sensor (MAPS) radiation detector base, the construction of which has been modified to comprise read out circuits both for the native detection of the MAPS detector base and for the added detector material body. This construction can be achieved in various ways, including by bringing together and electrically connecting through bump bonding or similar techniques a separately constructed unit comprising the detector material body with the modified MAPS detector base, or by depositing the detector material body onto the modified MAPS detector base. Such deposition can be for example by growing the detector material body from solution, using pressed powders, vapour phase deposition or similar, with any appropriate metallisation, passivation and other material layers as required.

[0010] For the purposes at least of X-ray detection, typically the modified MAPS detector base may capture lower energy X-rays in a semiconductor body of silicon which has an atomic number of 14. The detector material body may be formed of a higher atomic number material so as to capture higher energy X-rays, for example a material having an average atomic number by mass of the constituents of at least 30 or at least 40. Suitable such materials are GaAs, CdTe or CZT.

[0011] The construction of a single detection device in this way to detect two different radiation energy ranges provides advantages in terms of cost and simplicity of deployment over the use of separate serial detectors or multiple X-ray sources of different energy ranges, while retaining the advantages of known CMOS and MAPS radiation detectors.

[0012] In particular, the invention provides a detection device comprising: a first pixel layer comprising a semiconductor body, an array of pixel photodiodes each arranged to detect first pixel charge, and an array of first pixel circuits each arranged to store and read out a first signal representing the first pixel charge detected by a corresponding pixel photodiode, the first pixel charge arising from the interaction of radiation, and / or the interaction of radiation generated scintillation photons, within the semiconductor body proximally to that pixel photodiode; a second pixel layer located adjacent to the first pixel layer, the second pixel layer comprising a detector material body; an array of pixel electrodes each arranged to detect second pixel charge generated by radiation interacting with the detector material body proximal to a corresponding pixel electrode, wherein the first pixel layer further comprises, for each pixel electrode, a second pixel circuit arranged store and read out a second signal representing the second pixel charge detected by the corresponding pixel electrode.

[0013] The radiation giving rise to the detected pixel charges may typically be X-rays, although either or both of the pixel layers of the detection device may be configured to detect radiation of other types including for example optical and / or infrared light, or neutron radiation, gamma radiation, or radiation of other types, and the discussions below should be read in this light. For example, the second pixel layer may be arranged to detect neutrons by being formed of a semiconductor material that directly detects neutrons, or by further comprising a layer of a neutron sensitive material which generates neutron reaction products (such as alpha, beta and gamma radiation) which then interact with, and are detected by the second pixel layer.

[0014] The first pixel layer may in particular be configured to detect a lower energy range of X-rays or other radiation than the second pixel layer. For example, the semiconductor body of the first pixel layer may be formed of silicon or another material with lower atomic number or lower average atomic number for absorption of lower energy X-rays, and the detector material body of the second pixel layer map be formed of a material such as CdTe or CZT with a higher atomic number or average atomic number for absorption of higher energy X-rays.

[0015] Note that the radiation to be detected may arrive at the detection device from either the side of the first pixel layer or of the second pixel layers or both, depending on the particular design of the device and the intended detection properties.

[0016] In some embodiments, the second pixel layer comprises the array of pixel electrodes and is mounted in confrontation with the first pixel layer, and the detection device comprises a separate electrical connection between each pixel electrode and the corresponding second pixel circuit. For example, the electrical connections between the pixel electrodes and the corresponding second pixel circuits may be bump bond connections.

[0017] In other embodiments, the second pixel layer, and in particular the detection material body, is grown on, or deposited, on the first pixel layer, rather than being first constructed separately and then being joined or connected with the first pixel layer.

[0018] In either case, the second pixel layer may further comprise one or more second electrodes provided on an opposite side of the detector material body from the pixel electrodes, for applying an electric field across the detector material body.

[0019] The semiconductor body of the first pixel layer may typically be formed of silicon, for example comprising one or more of: an epitaxial silicon layer; and a depleted silicon substrate. However other materials such as germanium could in principle be used. If the first pixel layer is to be used to detect lower energy X-rays than the second pixel layer, then the semiconductor body is preferably formed of a material having a lower atomic number, or lower average atomic number by weight of constituents, which is lower than the atomic number or average atomic number by weight of constituents than the material of the detection material body.

[0020] The first pixel layer may further comprise a scintillation layer, wherein at least some of the first pixel charge for each pixel arises from the interaction of scintillation photons with the semiconductor body proximally to that pixel, and the scintillation photons arise from interaction of the X-rays or other radiation to be detected with the scintillation layer. In some embodiments the second pixel layer may also or instead further comprise a scintillation layer or other conversion layer such as a neutron conversion layer.

[0021] The detector material body of the second pixel layer may comprise or be formed of one or more of: CZT, CdTe, CdZnSe, CdZnTeSe, GaAs, TIBr, Hgls, a perovskite material (for example CsPbBrs ), BiOl, (Bismuth Oxi-iodide), Si (for example where the second pixel layer is provided by a partial or complete second CMOS sensor), amorphous silicon, Se, amorphous selenium, SiC, Si(Li), and Ge.

[0022] From another viewpoint, the detector material body of the second pixel layer may be formed of a material having an average atomic number by weight of the constituents of at least 30, for example so as to absorb and detect X-rays having higher energies than those absorbed and detected by the first pixel layer.

[0023] The detection device may further comprise one or more spectral filters arranged to modify the spectrum of X-rays or other radiation arriving at and detected by either or both of the first and second pixel layers, or at particular pixels of either or both of the first and second pixel layers. Such spectral filters may be disposed between the first and second pixel layers. So as to provide a different spectral response by different pixels, for either the first or second pixel layer, at least two different combinations of zero, one, or more than one spectral filter may be provided for each of two different discrete sets of pixels.

[0024] Typically, the first and second pixel circuits may be implemented as CMOS circuits formed within a circuitry layer of the first pixel layer. In some embodiments, the detection device may comprise two CMOS and / or silicon based detectors brought into confrontation with each other to provide the first and second pixel layers, with suitable electrical interconnections between the two, for example using bump bonding. Typically in this case the first CMOS sensor providing the first pixel layer may also provide the second pixel circuits for storing and reading out second signals representing the second pixel charges detected by the corresponding pixel electrodes of the second CMOS sensor.

[0025] The invention also provides methods of operating the above apparatus, for example a method comprising: directing X-rays or other radiation at the above detection device from either of the sides, or from both sides; receiving the first and second signals from the respective arrays of first and second pixel circuits; and forming first and second images of the radiation from the corresponding first and second signals. Typically then, each of the first and second images will represent different energy ranges of the X-rays or other radiation directed at the detection device. In particular, the X-rays or other radiation may be incident on the detection device from the side of the first pixel layer, and the second image may then represent a higher range of X-ray or other radiation energies than the first image.

[0026] The method may further comprise disposing a sample, such as a human or animal subject or some other object, between an X-ray source and one or more detection devices as described herein; and determining a structure or property of the sample, subject or object from the first and second X-ray images.

[0027] The invention also provides methods of manufacturing or constructing a described detection device. In some embodiments such a method comprises: providing, on a semiconductor body, an array of pixel photodiodes each arranged to detect first pixel charge arising from the interaction of X-rays or other radiation, and / or the interaction of scintillation photons generated by such X-rays or other radiation, within the semiconductor body proximally to that pixel photodiode, and a circuitry layer comprising an array of first pixel circuits each arranged to store and read out a first signal representing the first pixel charge detected by a corresponding pixel photodiode and an array of second pixel circuits; providing, on a detector material body, an array of pixel electrodes, each pixel electrode being arranged to detect second pixel charge generated by X-rays or other radiation interacting with the detector material body proximal to that pixel electrode, each pixel electrode corresponding to one of the second pixel circuits; and bringing together the semiconductor body and the detector material body to form electrical connections between each pixel electrode and each corresponding second pixel circuit, such that each second pixel circuit is configured to store and read out a second signal representing the second pixel charge detected by the corresponding pixel electrode.

[0028] In such embodiments, the pixel electrodes of the detector material body and the corresponding second pixel circuits may be electrically connected together through bump bonding or some other electrical connection only when the semiconductor body and the detector material body are brought together.

[0029] In other embodiments, such a method comprises: providing, on a semiconductor body, an array of pixel photodiodes each arranged to detect first pixel charge arising from the interaction of X-rays or other radiation, and / or the interaction of scintillation photons generated by such X-rays or other radiation, with the semiconductor body proximally to that pixel photodiode, and a circuitry layer comprising an array of first pixel circuits each arranged to store and read out a first signal representing the first pixel charge detected by a corresponding pixel photodiode and an array of second pixel circuits; providing, on the circuitry layer, an array of pixel electrodes, each pixel electrode being electrically connected to a corresponding one of the second pixel circuits; and depositing onto the array of pixel electrodes a detector material body arranged to detect second pixel charge generated by X-rays or other radiation interacting with the detector material body proximal to a corresponding pixel electrode, such that each second pixel circuit is configured to store and read out a second signal representing the second pixel charge detected by the corresponding pixel electrode.

[0030] In either case, the method may further comprise depositing or otherwise forming, on an opposite side of the semiconductor body to the circuitry layer, a scintillation layer arranged to generate the scintillation photons by the interaction of X-rays or other radiation with the scintillation layer.

[0031] Brief description of the drawings

[0032] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings of which:

[0033] Figure 1 is a schematic cross section through a detection device for detecting X- rays and / or other radiation typically arriving from below the device in the orientation depicted in the drawing, and in which a (upper) second pixel layer is bump bonded to a (lower) first pixel layer;

[0034] Figure 2 is a schematic cross section through a similar detection device to that of figure 1 but in which the (upper) second pixel layer is instead grown or deposited onto the (lower) first pixel layer;

[0035] Figure 3 shows how the first and second pixel layers of the detection device of figure 1 or 2 may be used to detect radiation of two different energy ranges, and to generate images or other data for each of the two different energy ranges;

[0036] Figure 4 is a graph of X-ray absorption characteristics as a function of X-ray energy for various materials which can be used for X-ray absorption in each of the first and second pixel layers of the detection device of figures 1 and 2;

[0037] Figure 5 illustrates how a scintillation layer can be added to the first pixel layer of the detection device to provide altered response characteristics to the incident radiation;

[0038] Figure 6 is a graph of calculated response characteristics of two different constructions of the second pixel layer in terms of material type and thickness of the detector material body;

[0039] Figure 7 shows one way in which spectral filters can be added to the detection device to alter the spectral response characteristics of either or both of the first and second pixel layers, or to alter spectral response characteristics of individual pixels; and Figure 8 shows how one or more of the detection devices can be used in combination with an X-ray or other radiation source to provide imaging of a sample, for example transmission imaging or more complex computed tomography X-ray imaging in a medical or security application.

[0040] Detailed description of embodiments

[0041] Referring now to figure 1 , there is illustrated an X-ray detection device 10 comprising a first pixel layer 100, and a second pixel layer 200 which is adjacent and parallel to the first pixel layer.

[0042] The first pixel layer 100 comprises a semiconductor body 110 within which an array of first pixels 120 (or voxels) is provided. In particular, the first pixel layer comprises an array of pixel photodiodes 130 each of which is arranged to collect or detect electrical charge generated by X-rays interacting with the semiconductor body 110 proximally to that pixel photodiode. The first pixel layer also comprises an array of first pixel circuits 140 each of which is arranged to store, and provide read out of, a signal representing the charge detected by a corresponding pixel photodiode. The first pixel circuits 140 is implemented within a circuitry layer 150 of the first pixel layer 100 which is also used for other purposes as described below.

[0043] The first pixel layer 100, and in particular the pixel photodiodes 130 and first pixel circuits 140, and more generally the circuitry layer 150, may typically be implemented using CMOS circuitry on a semiconductor substrate 160. The first pixel layer part of the device may be described as an active pixel sensor, or more particularly a monolithic active pixel sensor or a CMOS active pixel sensor. Typically, each pixel photodiode 130 may be a pinned photodiode. The circuitry layer 150 will typically provide various supra-pixel readout and management functions for both the first pixel circuits, and the second pixel circuits discussed below.

[0044] The second pixel layer 200 comprises a detector material body 210 within which an array of second pixels 220 (or voxels) is provided. The detector material body 210 is distinct from the semiconductor body 110, and the two are typically formed from different materials for reasons discussed below. An array of pixel electrodes 230 is associated with the detector material body 210. Each pixel electrode 230 is arranged to detect or collect electrical signal generated by X-rays interacting with the detector material body 210 proximally to that particular pixel electrode 230. In addition to the first pixel circuits 140, the first pixel layer also comprises an array of second pixel circuits 240 each of which is arranged to store, and provide read out of, a signal representing the charge detected by a corresponding pixel electrode. As for the first pixel circuits, the second pixel circuits 240 are implemented within the circuitry layer 150 of the first pixel layer 100, and therefore are typically also implemented using CMOS circuitry on the semiconductor substate 160. In the arrangement of figure 1 , each second pixel circuit 240 comprises a charge collection diode 270 for collection of charge detected or collected by the corresponding pixel electrode 230, for storage and subsequent readout.

[0045] Each pixel electrode 230 associated with a corresponding pixel of the second pixel layer 200 may be electrically connected to the corresponding second pixel circuit in the first pixel layer 100 in various ways. In the arrangement of figure 1 , the first and second pixel layers are formed separately from each other, and then are coupled or mounted to each other, with a physically separate electrical connection 250 being provided between each pixel electrode 230 and a corresponding connection pad 260 of the first pixel layer which provides the required ongoing electrical connection to the corresponding second pixel circuit 240. As illustrated in figure 1 , this can conveniently be achieved through bump bonding or other mechanical or thermo-mechanical techniques, where the electrical connection 250 is typically provided by a ball or body of a solder or other deformable connection material. A variety of different materials and structures can be used for this purpose, including Indium, Snln, SnPb, silver epoxy, copper pillars, conductive polymers, conductive films, conductive tapes, or gold studs. If bump bonding or a similar technique is used then this can be used to physically fix the first and second pixel layers together as a single device.

[0046] Also as shown in figure 1 , additional protection of the underlying circuitry layer 150 of the first pixel layer may be provided using one or more protection layer(s) 155, typically in the form of metallisation, passivation or photoresist, to provide protection of the circuitry layer 150 during and after the bump bonding or other connection process between the first and second pixel layers.

[0047] In other arrangements as discussed below in connection with figure 2, the second pixel layer can be grown or deposited directly onto the first pixel layer, rather than being formed separately and subsequently connected.

[0048] Depending on the materials, thicknesses and other aspects of the semiconductor body 160 and detector material body 210, each of the first and second pixel layers can be used to detect various different energy ranges of X-rays, and can in particular be used to detect different energy ranges to each other so that the device as a whole can provide pixellated detection of X-rays over a wider X-ray energy range. The X-rays to be detected can be incident on the first and second layers from either side of the X-ray detection device 10, depending for example on details of construction of the first and second layers.

[0049] In practice, the semiconductor body 110 of the first pixel layer 100 will usually be made of silicon. The pixel photodiodes 130 and the circuitry layer 150 can be readily formed on silicon, typically on an epitaxial silicon layer grown on a silicon substrate. Silicon, with an atomic number of 14, is relatively effective in absorbing X-rays with energies of up to about 10 keV, with a sharp drop off in absorption above that energy level although with a reasonable level of absorption remaining at about 20 keV. Other materials which could in principle be used for the semiconductor body include germanium, and with an atomic number of 32 this would lead to detection by the first pixel layer of somewhat higher energy X-rays.

[0050] In a typical configuration of the device of figure 1 or 2, the semiconductor substrate 160 of the first pixel layer 100 may be a silicon substrate with a thickness of around 100 - 750 pm. On this is typically formed an epitaxial layer 170 with a thickness of around 5 - 50 pm. The pixel photodiodes 130 and charge collection diodes 270 are then formed through usual techniques within this epitaxial layer, and the circuitry layer 150 is formed on top of, and optionally partly within, the epitaxial layer. If a low resistivity epitaxial layer 170 is used then this might typically have a thickness of up to about 30 pm, or a depleted epitaxial layer could have a thickness of up to about 50 pm. Either arrangement is suitable for detection of low energy X-rays, for example up to about 10 keV. Charge carriers generated by interaction of these X-rays with the epitaxial layer 170 then drift or diffuse to the pixel photodiodes 130 for detection, and the detected charge signal is then sampled, amplified and read out through using the first pixel circuits 140 and other parts of the circuitry layer 150 of the first pixel layer 100.

[0051] If the semiconductor substrate 160 is provided as a high resistivity or depleted substrate then charge carriers generated in the substrate 160 deep beneath the epitaxial layer 170 can drift or diffuse to the pixel photodiodes so as to contribute significantly to the first pixel charge detected by the pixel photodiodes, so that the first pixel layer can then detect higher X-ray energies up to around 20 keV or more. Consequently, if a depleted or high resistivity substrate is used then substantially the whole of the epitaxial layer and the depleted substrate in the vicinity of the pixel photodiodes will provide the semiconductor body 110 within which interaction of X-rays is detected. If a non-depleted or low resistivity substrate is used then the epitaxial layer but not the substrate will provide the semiconductor body 110. In some embodiments, the semiconductor substrate may be removed partially or completely from beneath the epitaxial layer 170 for example using etching or grinding. This can be used to increase sensitivity of the epitaxial layer to X-rays in the low energy range or for other purposes. As discussed further below, other layers and components interacting with the X-rays to be detected, such as one or more X-ray filters and / or scintillation layers may also be provided within the first pixel layer for various purposes. In some examples, the semiconductor substrate may be removed partially or completely so as to enable photons from a scintillation layer of the first pixel layer to reach the epitaxial layer.

[0052] Again in a typical configuration of the device of figure 1 or 2, the detector material body 210 of the second pixel layer could be formed of any of a wide variety of materials within which charge is generated by incident X-rays, with examples being CZT (CdZnTe), CdTe, CdZnSe, CdZnTeSe, GaAs, TIBr, Hgl2, perovskite materials such as CsPbBr3, Si and amorphous silicon a-Si, Se and amorphous selenium a-Se, SiC, Si(Li), Ge, and various other semiconductors. Some of these materials can readily detect X-rays of much higher energies than can be achieved using the epitaxial layer 170 or a depleted substrate 160 of the first pixel layer 100, for example up to about 150 keV or more in the case of CZT.

[0053] By way of example only, for higher energy X-ray detection, if CZT is used for the detector material body then a suitable thickness could be around 2000 pm. If a perovskite layer such as CsPbBr3is used then a thickness of around 300 pm may be appropriate. As noted above in respect of the first pixel layer, other layers and components interacting with the X-rays to be detected, such as one or more X-ray filters and / or scintillation layers may also be provided within the second pixel layer for various purposes.

[0054] In order for the first and second pixel layers to detect different energy ranges of X- rays, the detector material body may in particular be a “high-Z” material having an average atomic number by weight of the atomic constituents of at least 30, or at least 40. In this way, if the semiconductor layer is of silicon (atomic number Z = 14) then the second pixel layer would be expected to detect X-rays of a considerable higher energy range or average energy than the first pixel layer.

[0055] In order to provide a suitable electrical circuit for the second pixel layer of figures 1 or 2, one or more second electrodes 280 may be provided on an opposite surface or side of the detector material body 210 to that of the pixel electrodes 230. These second electrodes 280 can then be used to create an electric field across the detector material body 210 to cause charge generated by the interaction of X-rays to drift within the detector material body 210 for detection by the pixel electrodes. However, absent any such electric field, charge detection may still be carried out through diffusion of such charge. The charge carriers detected by the pixel electrodes 230 could be either electrons or holes.

[0056] Whereas in figure 1 the first and second pixel layers are formed separately and only then are brought into confrontation with each other and electrically connected using bump bonding or a similar technique, in figure 2 the detector material body 210 of the second pixel layer 200 is grown, formed, or otherwise deposited onto the first pixel layer. Some suitable techniques for such growth or deposition include growing from solution, forming from pressed powders, liquid or vapour phase deposition, thermal evaporation or sputtering, and printing.

[0057] Note that in this arrangement, the use of separate pixel electrodes 230 and connection pads 260 as shown in figure 1 is not required for forming an electrical connection between a pixel of the second pixel layer and the corresponding second pixel circuitry 240 provided within the first pixel layer, so instead only pixel electrodes 230 are shown in figure 2. Since, in the arrangement of figure 2, these are formed on the first pixel layer 100 before the subsequent deposition of the detection material body, they might be considered to form a part either of the first or the second pixel layers, or indeed both.

[0058] Although the detection material body of figure 2 may be formed directly on the pixel electrodes and other exposed surfaces of the first pixel body, it may be desirable to first provide additional metallisation of the pixel electrodes to help match the work function of the detection material to be deposited for mechanical, electrical and chemical compatibility. Such metallisation could be deposited using a photo-lithographical, printing or shadow mask process with a suitable physical deposition, thermal, electrochemical, electroless or sputter coating technique.

[0059] As illustrated in figure 2, the second pixel layer 200 may be provided with additional layers to provide additional functions, beyond that of generating charge in response to the interaction of X-rays with the body. For example, if the detector material body 210 is formed of a perovskite material, then an additional electron extraction layer 290 may be provided between the perovskite layer and the pixel electrodes 230, and an additional hole extraction layer 295 may be provided between the perovskite layer and the second electrode(s) 280, thereby providing a diode like performance of the second pixel layer for the purposes of charge detection.

[0060] Each of the array of first pixels and the array of second pixels may be a one dimension or a two dimensional array, although typically both will be two dimensional arrays. Both may typically be of the same geometrical layout, usually rectilinear, although other array forms such as hexagonal arrays may be used. Although both arrays may have pixels spaced at the same pitch on one or more dimensions, the first and second arrays may have pixels spaced at different pitches, with the second array typically having wider spaced pixels that the first array. The number of pixels in each of the arrays, and the physical dimensions of each array may vary according to need, but typically the size of each array in any particular direction may be of the order of 102- 104pixels across, with adjacent pixel spacings of the order of 10 - 1000 pm.

[0061] Figure 3 illustrates how the device of figure 1 or 2 may be used in order to detect X- rays over an extended energy range. In this figure, X-rays are shown as incident from the first layer side of the device, passing first through the first pixel layer and then through the second pixel layer. The energy spectrum of the incident X-rays is depicted using the graph at the left of the figure, with the large arrows demonstrating how the lower energy X-rays are largely absorbed and detected by the first pixel layer, and the higher energy X-rays largely pass through the first pixel layer to be absorbed and detected by the second pixel layer.

[0062] The first signals 310 representing first pixel charge arising from interaction of X-rays or other radiation within the first pixel layer, and the second signals 320 representing first pixel charge arising from interaction of X-rays or other radiation within the first pixel layer, may then be processed by a suitable analyser 330 typically implemented using a suitable computer system. The analyser 330 may typically be used to generate a first image 340 representing a first energy range of the radiation directed at the detection device, for example of X-rays over a lower energy range, and to generate a second image 350 representing a second energy range of the radiation directed at the detection device, for example of X-rays over a higher energy range. Of course, in many applications it may be desirable to combine the first and second images 340, 350 together in some way, and optionally with further information about the sample which may be available from other imaging or analysis techniques, for example to form a multi-colour image providing a range of insights about the sample to a user.

[0063] Figure 4 shows how this may be achieved through selecting suitable materials and structures for the first and second pixel layers, and in particular shows a graph of absorption efficiency of each of four pixel layer options plotted against X-ray energy.

[0064] Within the particular options shown in figure 4, the semiconductor body 110 of the first pixel layer, for detecting low energy X-rays, is either either a 20 pm thick epitaxial layer 170 deposited on a silicon substrate, or a 750 pm thick depleted silicon substrate 160. As can be seen in the graph, the first is effective for detecting X-rays with energies up to about 10 keV, and the second to about 40 keV. The detector material body 210 of the second pixel layer, for detecting higher energy X-rays, is either a 0.3 mm thick CsPbBr3layer or a 2.0 mm thick CZT layer. The first is effective for detecting X-rays with energies up to about 100 keV or more, and the second for detecting X-rays with energies up to about 150 keV or more. The first pixel layer 100 is relatively transparent to the higher energy X-rays, which therefore make little contribution to the pixel signals read out from the first pixel circuits 140 even if the device is oriented so that the X-rays pass through the first pixel layer before the second. Although the second pixel layer 200 is in principle sensitive to low energy as well as high energy X-rays, in a configuration where the X-rays pass through the first pixel layer before the second, the low energy X-rays are absorbed by the first pixel layer before reaching the second, so do not make a significant contribution to the pixel signals read out from the second pixel circuits 240.

[0065] In practice of course, different materials and thicknesses for the semiconductor body and detector material body may be adopted to tune the sensitivities of each of the first and second pixel layers to different ranges of X-ray energies. However, typically, the first pixel layer will be adapted to detect X-rays within a lower energy range than the second pixel layer.

[0066] Due to general commercial availability of suitable silicon substrates for construction of the first pixel layer, and the X-ray absorption properties of silicon, with typical silicon substrates being readily available with thicknesses up to about 700 pm, a practical upper limit to the X-ray energies detectable by the first pixel layer in the arrangements shown in figures 1 and 2 may be considered to be about 20 - 30 keV. One way in which the energy range of the first pixel layer can be extended upwards is to add a scintillation layer 180 to the first pixel layer as illustrated in figure 5, the scintillation layer being of a material which generates optical scintillation photons when X-rays of a suitable energy range are absorbed in that material. Note that the scintillation layer 180 can readily be used either if the first and second pixel layers are formed separately and then joined as in figure 1 , or if the second pixel layer is grown or deposited on the first as in figure 2.

[0067] Suitable materials for the scintillation layer may for example be caesium iodide Csl, or Csl :TI, or some other scintillation material preferably which has an X-ray absorption as a function of energy which is different to the material used for the material of the detector material body 210. Suitable thicknesses for the scintillation layer may be around 100 pm, for example from about 10 to 1000 pm. To ensure efficient detection of the scintillation photons by the first pixel layer, and especially where the semiconductor substrate is not a depleted substrate, some or all of the semiconductor substrate 160 may be removed prior to addition of the scintillation layer as illustrated in figure 5. The scintillation layer may typically be added to semiconductor body of the first pixel layer by sputtering or some other deposition technique, or by being physically mounted adjacent to, and optical coupled to, the semiconductor body 110.

[0068] In one particular configuration, the scintillation layer may be a 100 pm thick scintillation layer of Csl adjacent to a 20 pm thick epitaxially grown layer forming the semiconductor body for detecting the scintillation photons, with the detector material body 210 of the second pixel layer being provided by a 2.0 mm thick layer of CZT. This is calculated by the inventors to provide a device where the average X-ray energies detected by the first and second pixel layers is about 32 keV and 46 keV respectively, with a roughly 50:50 balance in detection sensitivity. Note that the scintillation layer will typically have the effect of reducing or blocking the flux of lower energy X-rays to the second pixel layer. Figure 6 is a graph showing calculated spectra for a detector with these characteristics in terms of normalised counts per keV, with the light curve showing the spectrum of incident X-rays, the dark solid curve the expected detection by the first pixel layer, and the dashed curve the expected detection by the second pixel layer.

[0069] The large arrow A in figure 5 illustrates how X-rays of a suitable, and typically lower energy range give rise to scintillation in the scintillation layer 180, and the resulting scintillation photons give rise to first pixel charge within the semiconductor body 110 (in this case provided by an epitaxial layer) with corresponding read out by the first pixel circuits 140. The large arrow B illustrates how X-rays of a suitable, typically higher energy range largely pass through the first pixel layer to be absorbed and give rise to second pixel charge within the detector material body of the second pixel layer with corresponding read out by the second pixel circuits 240.

[0070] Although in figure 5 the direction of X-rays is shown such as to meet the first pixel layer before the second pixel layer, an X-ray detection device incorporating a scintillation layer as described could be used with X-rays arriving from the other side to meet the second pixel layer before the first. In that case, the X-rays arriving at the scintillation layer would generally be of even higher energy that those absorbed and detected by the second pixel layer, and so the range of detection energies of the first pixel layer could then be generally higher than the range of detection energies of the second layer.

[0071] As illustrated in figure 7, any of the above arrangements of the detector device can be modified to include one or more spectral filters 410 arranged to modify the X-ray energy spectrum arriving at particular parts of the detection device. For example, such spectral filters could modify the energy spectrum of X-rays arriving at either or both of the first and second pixel layers for detection. Such spectral filters could also modify the X-ray energy spectrum arriving at particular pixels of either or both pixel layers. For example, if a single spectral filter 410 is used across all pixels of either or both pixel layers of the device then the spectral response of the device as a whole can be adjusted. If different spectral filters, or different combinations of zero, one, or more than one spectral filter, are used for different pixels of either the first or second pixel layer, then different pixels can be used to detect different energy ranges. In figure 7, alternate pixels of the second pixel layer 200 are provided with each of two different spectral filters 410-1 and 410-2. If the X-rays to be detected arrive at the first pixel layer before the second pixel layer then these spectral filters are effective to modify the X-ray spectra arriving and detected at each of the alternate pixels of the second pixel layer.

[0072] Typically, and as shown in figure 7, such spectral filters 410 may be provided between the first pixel layer 100 and the second pixel layer 200. If electrically conductive filter materials are used with suitable electrical isolation between adjacent second layer pixels then no further arrangements need be made for conduction of second pixel charge to the second pixel circuits 240. If insulating materials are used then suitable conductive pathways through the spectral filter materials may be required. In either case, the pixel electrodes 230 could be provided on either side of the spectral filters.

[0073] Alternatively or additionally, suitable spectral filters may be provided on the side of the first pixel layer further from the second pixel layer, in the region labelled X in figure 7, to apply spectral filtering for X-rays arriving in a direction arriving first at the first pixel layer, or on the side of the second pixel layer further from the first pixel layer, in the region labelled Y in figure 7, to apply spectral filtering for X-rays arriving in a direction arriving first at the second pixel layer

[0074] Suitable materials for the spectral filters include Al, Si, C, Fe, Co, Zn, Ni, Cu, Mo, Ag, Ba, W, Au, Pt, Pb, Bi and others materials and composites which can be deposited using a variety of techniques such as epitaxy, sputter coating, chemical, vapour or liquid deposition, printing or mounting foils. Suitable thicknesses for these materials, for example if disposed between the first and second pixel layers, may be 1 - 100 pm. One or more the spectral filters may be a Ross filter, in which pairs of materials with complimentary absorption edges, for example Cu and Zn, are be used in combination to effectively provide an X-ray band pass filter, to increase the absorption of a particular X-ray energy of interest.

[0075] The described X-ray detection device can be used in a variety of practical applications. Two particular areas of interest are for medical X-ray imaging and security related X-ray imaging, including for CT (computed tomography) techniques. For CT techniques a large number of images of a sample such as an object or a human or animal subject are taken from different angles, and this may be achieved either as shown in figure 8 in which an X-ray source 500and one or more X-ray detection devices 10 are rotated around a sample to be imaged, or the sample can be rotated and while the X-ray source 500 and one ore more X-ray detection devices 10 are kept stationary. For a typical X-ray spectrum used in medical or security imaging, having strong peaks around 10 keV and 60 keV, a broad intensity peak around 30 keV, but with significant X-ray flux remaining beyond 100 keV, the inventors calculate that a described detection device with the first pixel layer having a depleted silicon substrate of 700 pm thickness and a second pixel layer having a CZT layer of 2.0 mm thickness, about 25% of the X-ray flux is absorbed in the first pixel layer, and about 75% in the second pixel layer.

[0076] For security applications where objects are carried along a conveyor belt, the object may typically be illuminated using an X-ray slit beam source, and several of the described detection devices may be disposed around the object to be imaged as it passes along the conveyor belt past the X-ray source.

[0077] For mammography and similar medical applications, a described detection device with the first pixel layer being provided by a silicon epitaxial layer having about 20 pm thickness with the silicon substrate removed through back thinning, and a second pixel layer having a perovskite layer of about 300 pm thickness, such that about 10% of the X- ray flux is absorbed in the first pixel layer, and about 90% in the second pixel layer. For low dose DEXA bone density scanning, the first layer might instead be provided using a depleted silicon substrate layer having a thickness of about 700 pm, so that about 30% of the X-ray flux is absorbed in the first pixel layer, and about 70% in the second pixel layer.

[0078] Although the described detection device 10 may particularly be used for the detection of X-rays, it may also or instead be used for detection of other wavelengths of light or electromagnetic radiation, or for ionising particulate radiation such as high energy electrons, neutrons, protons or muons, and the above description should be understood accordingly, with suitable modifications being made to provide appropriate materials and dimensions such as thickness especially of the semiconductor body and detector material body, but also of any spectral filters, scintillators or other functional elements.

[0079] In one such example, the device 10 could be used to detect visible and infrared radiation arriving first at the first pixel layer, with the first pixel layer being used to detect visible light, and the second pixel layer at the same time being used to detect infrared light. The infrared light will largely pass through the first pixel layer especially if the semiconductor body and / or substrate are formed of silicon which is relatively transparent to infrared light, but will then be detected at the second pixel layer by virtue of the detector material body being formed of a suitable material such as HgCdTe, InAs, InGaAs, PbS and others.

[0080] In other examples, the first pixel layer 100 may be used to detect X-rays and / or gamma rays, for example by coupling with a scintillator, and the second pixel layer may be formed of a material to detect neutrons, for example by forming the second layer of a semiconductor material that directly detects neutrons or a detector material coated in a neutron sensitive material (B, Zn, Li and Gd for example). The neutron reaction products (beta, alpha and gamma radiation) are then detected in the second detector layer. Similarly, neutron detection may be achieved using a neutron sensitive scintillator, for example EJ-270 or EJ-290, with optical light from this scintillator being detected by the first or second pixel layer.

[0081] Various further options for modification and variation of the described embodiments will be apparent to the skilled person without departing from the scope of the invention as defined in the claims.

Claims

CLAIMS:1 . An X-ray detection device comprising: a first pixel layer comprising a semiconductor body (110), an array of pixel photodiodes (130) each arranged to detect first pixel charge, and an array of first pixel circuits (140) each arranged to store and read out a first signal representing the first pixel charge detected by a corresponding pixel photodiode, the first pixel charge arising from the interaction of X-rays, and / or the interaction of X-ray generated scintillation photons, within the semiconductor body proximally to that pixel photodiode; a second pixel layer (200) located adjacent to the first pixel layer, the second pixel layer comprising a detector material body (210); an array of pixel electrodes (230) each arranged to detect second pixel charge generated by X-rays interacting with the detector material body proximal to a corresponding pixel electrode, wherein the first pixel layer (100) further comprises, for each pixel electrode, a second pixel circuit (260) arranged store and read out a second signal representing the second pixel charge detected by the corresponding pixel electrode (230).

2. The detection device of claim 1 wherein the second pixel layer comprises the array of pixel electrodes and is mounted in confrontation with the first pixel layer, and the detection device comprises a separate electrical connection between each pixel electrode and the corresponding second pixel circuit.

3. The detection device of claim 2 wherein the electrical connections between the pixel electrodes and the corresponding second pixel circuits are bump bond connections.

4. The detection device of claim 1 wherein the second pixel layer is grown on or deposited on the first pixel layer.

5. The detection device of any preceding claim wherein the second pixel layer further comprises one or more second electrodes (270) provided on an opposite side of the detector material body to the pixel electrodes (230) for applying an electric field across the detector material body.

6. The detection device of any preceding claim wherein the semiconductor body of the first pixel layer is formed of silicon.

7. The detection device of claim 6 wherein the semiconductor body of the first pixel layer comprises one or more of: an epitaxial silicon layer; and a depleted silicon substrate.

8. The detection device of any preceding claim wherein the first pixel layer further comprises a scintillation layer, at least some of the first pixel charge for each pixel arises from the interaction of X-ray generated scintillation photons with the semiconductor body proximally to that pixel, and the scintillation photons arise from interaction of the X-rays with the scintillation layer.

9. The detection device of any preceding claim wherein the detector material body of the second pixel layer is formed of one or more of: CZT, CdTe, CdZnSe, CdZnTeSe, GaAs, TIBr, Hgl2, a perovskite material such as CsPbBr3, Si, amorphous silicon, BiOl, Se, amorphous selenium, SiC, Si(Li), and Ge.

10. The detection device of any of claims 1 to 9 wherein the detector material body of the second pixel layer is formed of a material having an average atomic number by weight of the constituents of at least 30 or at least 40.11 . The detection device of any preceding claim further comprising one or more spectral filters arranged to modify the spectrum of X-rays arriving at pixels of either or both of the first and second pixel layers.

12. The detection device of claim 11 wherein the one or more spectral filters are disposed between the first and second pixel layers.

13. The detection device of claim 11 or 12 wherein, for either the first or second pixel layer, at least two different combinations of zero, one, or more than one spectral filter are provided for each of two different sets of pixels.

14. The detection device of any preceding claim wherein the first and second pixel circuits are CMOS circuits formed within a circuitry layer of the first pixel layer.

15. A method comprising: directing X-rays at the detection device of any preceding claim; receiving the first and second signals from the respective arrays of first and second pixel circuits; and forming first and second X-ray images from the corresponding first and second signals, wherein each of the first and second X-ray images represent different energy ranges of the X-rays directed at the detection device.

16. The method of claim 15 wherein the X-rays are incident on the detection device from the side of the first pixel layer, and the second X-ray image represents a higher range of X-ray energies than the first X-ray image.

17. The method of claim 15 or 16 further comprising disposing a subject or object between an X-ray source and one or more detection devices of any preceding claim; and determining a structure or property of the subject or object from the first and second X-ray images.

18. A method of constructing an X-ray detection device comprising: providing, on a semiconductor body (110), an array of pixel photodiodes (130) each arranged to detect first pixel charge arising from the interaction of X-rays, and / or the interaction of X-ray generated scintillation photons, within the semiconductor body proximally to that pixel photodiode, a circuitry layer comprising an array of first pixel circuits (140) each arranged to store and read out a first signal representing the first pixel charge detected by a corresponding pixel photodiode and an array of second pixel circuits (240); providing, on a detector material body, an array of pixel electrodes, each pixel electrode being arranged to detect second pixel charge generated by X-rays interacting with the detector material body proximal to that pixel electrode, each pixel electrode corresponding to one of the second pixel circuits; and bringing together the semiconductor body and the detector material body to form electrical connections between each pixel electrode and each corresponding second pixel circuit, such that each second pixel circuit (260) is configured to store and read out a second signal representing the second pixel charge detected by the corresponding pixel electrode.

19. The method of claim 18 wherein the pixel electrodes of the detector material body and the corresponding second pixel circuits are electrically connected through bump bonding when the semiconductor body and the detector material body are brought together.

20. A method of constructing an X-ray detection device comprising: providing, on a semiconductor body (110), an array of pixel photodiodes (130) each arranged to detect first pixel charge arising from the interaction of X-rays, and / or the interaction of X-ray generated scintillation photons, with the semiconductor body proximally to that pixel photodiode, and a circuitry layer comprising an array of first pixel circuits (140) each arranged to store and read out a first signal representing the first pixel charge detected by a corresponding pixel photodiode and an array of second pixel circuits (240); providing, on the circuitry layer, an array of pixel electrodes, each pixel electrode being electrically connected to a corresponding one of the second pixel circuits; and depositing onto the array of pixel electrodes a detector material body arranged to detect second pixel charge generated by X-rays interacting with the detector material body proximal to a corresponding pixel electrode, such that each second pixel circuit (260) is configured to store and read out a second signal representing the second pixel charge detected by the corresponding pixel electrode.21 . The method of any of claims 18 to 20 further comprising depositing, on an opposite side of the semiconductor body to the circuitry layer, a scintillation layer arranged to generate the scintillation photons by the interaction of X-rays with the scintillation layer.

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