Sub-band infra-red irradiation for detector crystals

Abstract

The invention relates to radiation detection with a directly converting semiconductor layer for converting an incident radiation into electrical signals. Sub-band infra-red (IR) irradiation considerably reduces polarization in the directly converting semi-conductor material when irradiated, so that counting is possible at higher tube currents without any baseline shift. An IR irradiation device is integrated into the readout circuit to which the crystal is flip-chip bonded in order to enable 4-side-buttable crystals.

Claims

1. A radiation detector comprising: a) a directly converting semiconductor layer for converting an incident radiation into electrical signals; b) a substrate comprising readout electronics for receiving said electrical signals via pixel pads arranged at said directly converting semiconductor layer; and c) a plurality of radiation sources connected or integrated to said substrate and adapted to irradiate said directly converting semiconductor layer; wherein d) said plurality of infrared radiation sources are provided on an infrared source layer which is interposed between said directly converting semiconductor layer and a readout chip of said substrate and which is flip-chip bonded to said directly converting semiconductor layer via said pixel pads, wherein said plurality of infrared radiation sources adapted to irradiate said directly converting semiconductor layer are arranged at corners of the pixel pads of said directly converting semiconductor layer, wherein said infrared source layer comprises through connection portions for electrically connecting said pixel pads to related contact portions on said readout chip.

2. The radiation detector as defined in claim 1, wherein said plurality of radiation sources are adapted to irradiate said directly converting semiconductor layer with a sub-band infrared radiation having a photon energy smaller than the band gap of said directly converting semiconductor layer.

3. The radiation detector as defined in claim 1, wherein each of said plurality of infrared radiation sources is arranged at a gap portion of said pixel pads.

4. The radiation detector as defined in claim 2, wherein said plurality of infrared sources comprise a plurality of groups each consisting of infrared sources with different wavelengths of said sub-band infrared radiation.

5. The radiation detector as defined in claim 4, wherein each infrared source of one of said plurality of groups is arranged at a different corner of a respective pixel pad of said directly converting semiconductor layer.

6. The radiation detector as defined in claim 1, wherein said plurality of infrared sources are arranged to irradiate said directly converting semiconductor layer from an anode side.

7. The radiation detector as defined in claim 1, wherein each of said infrared sources is allocated to a subset of pixels of said radiation detector.

8. The radiation detector as defined in claim 1, wherein said directly converting semiconductor layer is made of a Cd[Zn]Te crystal.

9. A method of manufacturing a radiation detector, said method comprising: a) arranging a plurality of pixel pads on a directly converting semiconductor layer for converting an incident radiation into electrical signals; b) connecting a readout chip for receiving said electrical signals to said pixel pads; and c) connecting or integrating a plurality of infrared radiation sources to said readout chip; wherein interposing an infrared source layer with said plurality of infrared radiation sources between said directly converting semiconductor layer and said readout chip, wherein said plurality of infrared radiation sources are arranged at corners of the pixel pads of said directly converting semiconductor layer; wherein connecting a readout chip for receiving said electrical signals to said pixel pads includes flip-chip bonding though connection portions within and through the infrared source layer which connect the directly converting semiconductor layer via the pixel pads to the readout chip.

10. The method as defined in claim 9, further comprising using said infrared source layer as an interposer to test said directly converting semiconductor layer before mounting it to said readout chip.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following drawings:

(2) FIG. 1 shows schematically and exemplarily a cross-sectional side view of a radiation detector according to a first embodiment,

(3) FIG. 2 shows schematically and exemplarily a top view of the radiation detector according to the first embodiment,

(4) FIG. 3 shows schematically and exemplarily a top view of a radiation detector according to a second embodiment for different IR wavelengths; and

(5) FIGS. 4a and 4b show schematic diagrams of measured counts over short measurement periods without and with IR-LED irradiation, respectively.

DETAILED DESCRIPTION OF EMBODIMENTS

(6) FIG. 1 shows schematically and exemplarily a cross-sectional side view of a radiation detector according to a first embodiment with an IR LED layer 10 interposed between a 4-side-buttable CMOS (Complementary Metal Oxide Semiconductor) readout circuit or chip 50, e.g. and application specific integrated circuit (ASIC), and a CZT crystal 60. The CZT crystal 60 is flip-chip bonded via pixel pads 20 to a substrate (i.e. IR LED layer 10 and readout chip 50) which does the readout. More specifically, an IR irradiation device (i.e. the IR LED layer 10) is integrated into the readout chip 50 to which the CZT crystal 60 is flip-chip bonded, so that a 4-side-buttable crystal is achieved, in which electrical input/output circuits are arranged under the device rather than at its edge. This 3-D packaging allows extremely close spacing of radiation detectors.

(7) Irradiation with sub-band IR LED light, where the optical photons have an energy which is smaller than the band gap, reduces the tendency towards polarization within pixels of the CZT crystal 60 so that counting at higher X-ray fluxes becomes possible. However, IR irradiation from the side has the disadvantage that it only allows for 3-side-buttable detector modules. In order to support 4-side-buttable detectors, IR irradiation can be done from the cathode side (i.e. upper side in FIG. 1), where the cathode metallization (not shown) probably may not be an obstacle, since it is thin. Then, however, the IR LED layer 10 is within the X-ray beam. As an alternative option, IR irradiation can be done from the anode side (i.e. lower side in FIG. 1), which is advantageous since on the anode side, there are anyway non-metalized regions. The IR irradiation would further be improved, if these pixel gaps are irradiated.

(8) Hence, as indicated in FIG. 1, the pixels of the readout electronics of the readout chip 50 are bonded via respective solder bumps and through connection portions 40 directly to the pixel pads 20 of the CZT crystal 60. Additionally, a supply contact 42 for the IR LED layer 10 is provided. The substrate with the readout circuit also implementsbesides the readout electronicsone or more sub-band IR LED sources 30. Optionally, as described below in connection with the second embodiment, more than one IR LED source 30 per pixel may be implemented, if different IR light wavelengths are to be used, which may help to release even more charges of deep traps of different energies.

(9) Alternatively, it may suffice to implement the IR LED sources 30 only in a sub-set of pixels. Since the IR LED sources 30 may be manufactured from GaAs or AlGaAs material which differs from CMOS, the IR LED sources 30 can be implemented on a different substrate (i.e. the IR LED layer 10), which is then flip-chip bonded to the readout electronics of the readout chip 50 manufactured in CMOS technology, as discussed e.g. in McKendry et al.: Individually addressable AlInGaN micro-LED arrays with CMOS control and subnanosecond output pulses, IEEE Photonics Techn Let, Vol. 21, No. 12, Jun. 15, 2009. The CMOS substrate of the readout chip 50 implements the contact pad for each pixel and may also implement the power supply for the IR LED layer 10. The IR LED layer 10 through-contacts the pixel pads 20 by the through connection portions 40 so that the IR LED layer 10 can be flip-chip bonded to the CZT crystal 50, where the emitting dots of the IR LED sources 30 on the IR LED layer 10 illuminate pixel gaps or through the thin pad metallization of the pixel pads 20.

(10) Furthermore, during the manufacturing process the LED IR layer 10 can be used as an interposer to also test the CZT crystal 60 before mounting it to the readout electronics, so that after mounting the LED IR layer 10, the CZT crystal 60 can be tested as to its performance for energy-resolved X-ray photon detection together with IR light irradiation. Thus, it is possible to measure how far the considered CZT crystal 60 can actually improve its performance due to the added IR irradiation.

(11) FIG. 2 shows schematically and exemplarily a top view (towards the anode side) of the radiation detector according to the first embodiment with a 2D arrangement of the IR LED sources 30 on the IR LED layer 10 for a single IR wavelength. As can be gathered from FIG. 2, the IR LED sources 30 are arranged at the pixel gaps between the pixel pads 20 of the CZT crystal 60 (not shown in FIG. 2).

(12) FIG. 3 shows schematically and exemplarily a top view (towards the anode side) of a radiation detector according to a second embodiment with a 2D arrangement for IR LED sources 32 providing four different IR wavelengths. In the second embodiment, four IR LED sources 32 of different IR wavelengths are arranged at the edges of neighbouring pixel pads 20 of the CZT crystal 60 (not shown in FIG. 3).

(13) In the above first and second embodiments, the IR LED sources 30, 32 may be integrated into the CMOS readout chip 50. E.g., organic LEDs (OLEDs) may be integrated into a Si CMOS ASIC. With such an integration, further simplifications of the embodiments are possible.

(14) FIG. 4b shows a diagram similar to FIG. 4a and indicating measured photon counts over the number of short measurement periods of 100 s on 4 different energy channels Ch1 to Ch4 with four different thresholds (30 keV, 33 keV, 36 keV, 51 keV) with added IR-LED irradiation (880 nm). As can be gathered from FIG. 4b, the IR light causes the count rate drop to disappear, which can be explained by the deep traps being released so that no space charge comes into being. With the proposed sub-band IR radiation, the IR light is not simply absorbed by the CZT material and can generate electron-hole pairs (due to defect states). In the experiments underlying the diagram of FIG. 4b, IR irradiation was done from the side. Since the light is not absorbed, also pixels which are farthest away from the light source still receive sufficient sub-band IR light.

(15) In summary, radiation detection with a directly converting semiconductor layer for converting an incident radiation into electrical signals has been described. Sub-band infra-red (IR) irradiation considerably reduces polarization in the directly converting semiconductor material when irradiated, so that counting is possible at higher tube currents without any baseline shift. An IR irradiation device is integrated into the readout circuit to which the crystal is flip-chip bonded in order to enable 4-side-buttable crystals.

(16) Although in the above described embodiments the radiation to be detected is X-ray, in other embodiments also other types of radiation can be used. For instance, the radiation source can be a radiation source generating light within another wavelength range, for instance, in the visible wavelength range. The radiation source can also be a lasing device.

(17) Furthermore, although in the above embodiments, sub-band infra-red light is discussed, also other radiation wavelengths may allow for reducing polarization effects. Hence, the present invention is not intended to be limited to sub-band radiation. The described IR sources may be IR laser diodes instead of the IR LEDs 30, or other radiation sources with other wavelengths. Also, the readout chip 50 is not intended to be restricted to CMOS technology and can be implemented based on any other semiconductor technology. The direct conversion material is not restricted to CZT. Rather, any other suitable semiconductor material in the form of CdTe, CdTeSe, CdZnTeSe, CdMnTe, InP, TIBr2 or HGI2 can be used instead of the CZT crystal 60 for detecting X-ray or other radiation photons

(18) Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

(19) The figures are schematically only. For instance, they are not to scale, i.e., for example, the electrodes are thinner than shown in the figures.

(20) In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality.

(21) A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

(22) Any reference signs in the claims should not be construed as limiting the scope.