WO2010073189A1 - Radiation detector with improved charge collection and minimized leakage currents - Google Patents

Radiation detector with improved charge collection and minimized leakage currents Download PDF

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Publication number
WO2010073189A1
WO2010073189A1 PCT/IB2009/055784 IB2009055784W WO2010073189A1 WO 2010073189 A1 WO2010073189 A1 WO 2010073189A1 IB 2009055784 W IB2009055784 W IB 2009055784W WO 2010073189 A1 WO2010073189 A1 WO 2010073189A1
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Prior art keywords
anode
steering electrode
material layer
radiation detector
direct converter
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PCT/IB2009/055784
Other languages
French (fr)
Inventor
Klaus J. Engel
Christian Baeumer
Christoph Herrmann
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Koninklijke Philips Electronics N.V.
Philips Intellectual Property & Standards Gmbh
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Application filed by Koninklijke Philips Electronics N.V., Philips Intellectual Property & Standards Gmbh filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2010073189A1 publication Critical patent/WO2010073189A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14665—Imagers using a photoconductor layer
    • H01L27/14676—X-ray, gamma-ray or corpuscular radiation imagers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof
    • H01L27/14603—Special geometry or disposition of pixel-elements, address-lines or gate-electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof
    • H01L27/1462—Coatings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof
    • H01L27/14636—Interconnect structures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14643—Photodiode arrays; MOS imagers
    • H01L27/14658—X-ray, gamma-ray or corpuscular radiation imagers
    • H01L27/14659—Direct radiation imagers structures

Definitions

  • the present invention relates to a radiation detector with improved charge collection and minimized leakage currents.
  • the present invention further relates to an imaging device for imaging an object comprising the aforementioned radiation detector for detecting radiation after having passed through that object.
  • a direct converter detector generally consists of a block of semiconductor material between two electrodes, to which a (high) voltage is applied.
  • a geometry is chosen in which the photons are incident through the cathode side.
  • the photon transfers energy to a single electron which becomes a "fast electron”, and which therefore creates a number of electron/hole pairs in the direct vicinity of the original location of interaction.
  • the electrons drift to an array of anode pixels on the opposite side, while holes drift to the cathode.
  • the holes drift significantly more slowly than the electrons (for example, by a factor of about 20 in CZT).
  • the integral current pulse measured at an anode pixel corresponds to the total created charge, which is proportional to the original photon energy. It is important to note, that the anode current is induced by a capacitive coupling of the charge carriers to the anode and not at the time of arrival of the charges at the collecting anode. This means that both types of charge carriers - electrons, but also holes - contribute to the anode current not only of the underlying pixel, but also to all neighbor pixels. Thus, the slowly drifting holes still induce a charge offset, while the electrons are already collected by the anodes.
  • control electrode and "steering electrode” will be used synonymously.
  • the aim of this concept is to downsize the area of the collecting anode and to surround it with another electrode, i.e. the control electrode, which is generally on a highly more negative potential relative to the anode. In result, the electrons are guided to a small anode by an electric field, while the charge offset of the holes is reduced with decreasing anode size due to a more strongly confined weighting potential.
  • a concept of this kind is disclosed in US 6,333,504 Bl .
  • This document discloses a device and method for detecting ionizing radiation, and more particularly a semiconductor radiation detector with enhanced charge collection for reducing low-energy tailing effects.
  • This radiation detector comprises a single conductor crystal which is covered on one side by the cathode and on the opposite side by the control electrode and the anode.
  • the employed controlled electrode is thereby maintained at a voltage level that is negative with respect to the anode, but generally not more negative than the cathode.
  • the preferred value of the control electrode voltage V cont roi is in the range from (V ano de+V cat hode)/2 to Vcathode-
  • the ideal electrical potential of the control electrode for focusing electron charge to the anode is approximately the potential of the cathode.
  • the difference between the control electrode voltage potential and the anode voltage potential comes up to several hundred volts.
  • a radiation detector comprising a direct converter material layer for providing free electrons in response to an incident radiation, a cathode arranged on a first side of the direct converter material layer, an anode arranged on a second side of the direct converter material layer for detecting charges proportional to the energy level of the incident radiation, a steering electrode arranged on the second side of the direct converter material layer and at least partly surrounding the anode for guiding the free electrons, and an insulating material arranged between said anode and said steering electrode and between said steering electrode and said direct converter material layer for providing an electron barrier between said steering electrode and said direct converter material layer.
  • the steering electrode is at least partly insulated from the anode and especially from the direct converter material layer, so that at least a portion of the field lines could cross the insulating material in the absence of space charge.
  • electrons for example those inherently present from a dark current
  • drifting along that field lines are stopped by the insulating material, leading to a local concentration of electrons in the direct vicinity of the insulating material.
  • This concentration of negative space charge causes or amplifies, respectively, the electrical opposing field which is normally generated by the application of a relatively high potential difference between the steering electrode and the anode.
  • the present invention exhibits two major advantages.
  • the potential difference between the control electrode and the anode can be strongly reduced since the steering field is generated or amplified, respectively, by the negative space charge, leading to the elimination or at least a significant reduction of the leakage currents and additional noise.
  • the insulating material prevents at least a portion of the drifting electrons to be gathered by the steering electrode, thus additionally improving the spectral resolution of the radiation detector.
  • an imaging device for imaging an object comprising a radiation source for providing a radiation beam and a radiation detector according to the aforementioned radiation detector for detecting radiation after having passed through that object.
  • this imaging device can be used for applications which require very high and efficient imaging resolution and sensitivity, like e.g. at medical imaging to create images of the human body or parts thereof for clinical purposes or medical science.
  • the steering electrode is completely insulated from the direct converter material layer by the insulating material. That is, in other words, no electrons at all can be gathered by the steering electrode. Accordingly, the potential difference between the steering electrode and the anode can ideally be adjusted to zero, so that leakage currents and consequently additional noise are ideally completely eliminated.
  • the radiation detector comprises a bond pad connected to the anode and insulated from the steering electrode by the insulating material for providing a sufficient bonding area of the anode.
  • the anode spot is usually about 200-300 ⁇ m in size, since it is directly bonded to the application-specific integrated circuit (ASIC). Consequently, detector geometries show a minimum detector thickness of 600 to 1000 ⁇ m, since the aspect ratio, i.e. detector thickness to pixel width ratio, should not go below 3 for a good weighting potential. Thus, the fabrication of thin detector layers is not possible, without infringing said aspect ratio.
  • the integration of a bond pad allows for the minimization of the anode size on the detecting side (e.g. below 100 ⁇ m), while the anode size on the contacting side can be maximized, theoretically to an arbitrary size, since it is electrically shielded by the steering electrode.
  • This minimization of the anode size results in a significant reduction of the extension of the weighting potential attached to each single anode and thus the possibility of fabrication of very thin detector layer arrangements (e.g. below 500 ⁇ m) with arbitrary pixel size.
  • the cathode covers the entire first side of the direct converter material layer and the insulating layer covers the entire second side of the direct converter material layer not covered by the anode. This action increases the collection efficiency of the radiation detector in as much as no electrons are gathered by the steering electrode.
  • the direct converter material layer consists of CdTe or CZT.
  • Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CZT) are more and more applied as detector materials for photons from a few of keV to a few MeV.
  • Their relative advantages include high sensitivity for X-rays and gamma-rays, due to the high atomic numbers of Cd and Te, and superior spatial and energy resolution compared to scintillators. Further, they are more economical and compact compared to germanium (Ge), particularly considering that they do not require cryogenic cooling.
  • both materials provide a high mobility- lifetime ratio between electrons and holes, which also improves the efficiency of the radiation detector
  • the ratio of the direct converter material layer thickness to the anode size is greater than 3 to assure an effectively reduced weighting potential in the largest part of the detector volume (known as "small pixel effect").
  • the ratio of the direct converter material layer thickness to the pixel size is not limited to be greater than 3.
  • the radiation detector comprises at least two anodes surrounded by the steering electrode and connected in parallel thereby forming a multi-dot anode for providing a combined signal.
  • the spectral resolution of the direct converter material strongly relies upon the confinement of the weighting potential, wherein the weighting potential shrinks with decreasing anode sizes.
  • the idea is to connect several small dot-like collecting anodes in parallel, surrounded and shielded by the steering electrode, in order to keep the size of the anode dots as small as possible, thereby minimizing the extension of the weighting potential attached to each single dot.
  • the advantages are that on the one hand the total area to which charges are induced is reduced, and on the other hand a much lower potential difference between the steering electrode and the anode is required (due to an increased number of collecting anodes on the same area) to ensure the same collection efficiency compared to a single spot geometry.
  • the anodes of the multi-dot anode are connected through a bond pad, the bond pad being insulated from the steering electrode by the insulating material additionally arranged between said steering electrode and said bond pad for providing a sufficient bonding area of the multi-dot anode.
  • the bond pad having the afore-mentioned advantages, is additionally used for the parallel connection of the anodes of the multi-dot anode, thereby simplifying the detector structure.
  • the anodes, the steering electrode and the insulating material are formed on the second side of the direct converter material layer in a stacked layer arrangement of insulator and metal using standard lithographic techniques.
  • the benefit of the stacked layer arrangement and the use of lithographic techniques is that they provide an easy and cheap possibility of applying the electrodes and the insulation on the direct converter material layer, thus facilitating the manufacturing of the radiation detector.
  • the anodes of the multi-dot anode are arranged equidistantly to each other for providing a substantially homogenous electron collection leading to a path length being uniform and as short as possible for electrons collected in the anode.
  • the generated electric field shape improves the electron collection efficiency because of the optimal anode-dot distribution and thus the spectral resolution of the radiation detector.
  • the radiation detector comprises a plurality of equidistantly arranged multi-dot anodes, each defining together with its surrounding steering electrode a single pixel.
  • This detector array structure has the benefit that a large area can be examined and that the single pixels can be separately measured, so that regions of interest can be viewed at a customized resolution with a customized pixel size, without being dependent on the detector thickness or the ratio of detector thickness to anode size.
  • pixel arrays are particularly useful for radiation cameras, used in industrial or medical applications.
  • the detector provides a plurality of multi-dot anodes each multi-dot anode having at least two anodes, wherein the anode size is less than 100 ⁇ m and the distance between each anode is less than 500 ⁇ m.
  • This detector array provides a good compromise between the efficiency of the radiation detector, concerning the leakage currents and the electron collection, and the expenditure in manufacturing.
  • the very small dimensions amplify the advantageous effects of the present invention. Consequently, the present invention is preferably realized in small dimensions, wherein the anode size should be as small as possible but is not limited to any size limitations.
  • the imaging device comprises a voltage source for providing a bias voltage for the cathode, the steering electrode and/or the anode, so as to create a negative potential difference of the cathode relative to the steering electrode and a negative or no potential difference of the steering electrode relative to the anode thereby generating an electric field within the direct converter material.
  • This action provides a simple possibility to provide the appropriate voltage level for the cathode, the steering electrode and/or the anode.
  • FIG. 1 shows a side cross-sectional view of the preferred embodiment of a single-pixel configuration according to the present invention
  • Fig. 2 shows three cross-sectional views of different layers of the detector illustrated in Fig. 1
  • Fig. 3 shows a side cross-sectional view of a section of the preferred embodiment of a multi-pixel configuration with multi-dot anodes according to the present invention
  • Fig. 4 shows three cross-sectional views of different layers of the detector illustrated Fig. 3
  • Fig. 5 shows the multi-pixel detector of Figs. 3 and 4, emphasizing the influence of the insulating material with a potential difference between the steering electrode and the anode, and
  • Fig. 6 shows the multi-pixel detector of Figs. 3 and 4, emphasizing the influence of the insulating material with no potential difference between the steering electrode and the anode.
  • FIG. 1 is a cross-sectional side view of the preferred embodiment of a single- pixel radiation detector 10 of the invention, comprising a direct converter material layer 12, a cathode 14, an anode 16, a steering electrode 18, an insulating material 20 and a bond pad 22.
  • the direct converter material layer 12 is a square-shaped slap or wafer of semi-conductor material with high resistivity and suitable for fabrication in detectors, like e.g. CdTe or CZT.
  • the cathode 14 is arranged on a first side 24 of the direct converter material layer 12 and formed as a conductive layer covering the entire surface of the first side 24 of the direct converter material layer 12. Further, the cathode 14 is coupled to a cathode voltage source 26, biasing the cathode at a negative voltage potential Vc relative to the anode 16.
  • the anode 16 is arranged on a second side 28 of a direct converter material layer 12 and formed as a small conductive contact, centrally located on the surface of the second side 28 of the direct converter material 12. Further, the dot- like anode is connected to the bond pad 22 for providing a sufficient bonding area of the anode 16 to e.g. an application- specific integrated circuit (ASIC).
  • ASIC application- specific integrated circuit
  • An anode voltage source 30 biases the anode 16 at voltage level V A , which is more positive than the voltage level Vc of the cathode 14. However the anode 16 is preferably held at ground, while the cathode 14 is negatively biased. Further, a current measuring device 31 is coupled to the anode for detecting charges proportional to the energy level of an incident radiation.
  • the steering electrode 18 is also arranged on the second side 28 of the direct converter material layer 12 and formed as a conductive layer surrounding the anode 16 and covering the entire second side not covered by the anode 16 and the insulating material 20.
  • the steering electrode 18 is insulated from the direct converter material layer 12, the anode 16 and the bond pad 22 by an insulating material 20 comprising three insulating sections 32, 34, 36.
  • the first insulating section 32 lies between the direct converter material layer 12 and the steering electrode 18 and covers the entire surface of the second side 28 of the direct converter material layer 12 not covered by the anode 16, thereby providing a barrier for the electrons, drifting towards the steering electrode 18. That is, in other words, the first insulating section 32 blocks the path of the electrons drifting towards the steering electrode 18 and ensures that no electrons "drop away" at the steering electrode 18.
  • the second insulating section 34 lies between the steering electrode 18 and the anode 16 for providing good breakdown characteristics and for reducing the leakage currents from the steering electrode 18 to the anode 16.
  • the third insulating section 36 lies between the steering electrode 18 and the bond pad 22 and covers the entire second side 28 not covered by the anode 16. Thus, the bond pad 22 is insulated from the steering electrode 18, and can be easily applied on the third insulating section 36.
  • a steering electrode voltage source 38 biases the steering electrode 18 at positive potential Vs relative to the cathode 14 and a slightly negative potential relative to the anode 16. As will be explained below in detail, the potential difference between the steering electrode 18 and the anode 16 can even be adjusted to zero, so that e.g. both are held at ground.
  • the electrodes 14, 16, 18 generates an electric field within the direct converter material layer 12, wherein the field lines 40, representing the drifting paths of the free electrons within the direct converter material layer 12 in the absence of any space charges, extend from the entire surface of the cathode 14 to the steering electrode 18 and the anode 16.
  • the field lines 40 representing the drifting paths of the free electrons within the direct converter material layer 12 in the absence of any space charges, extend from the entire surface of the cathode 14 to the steering electrode 18 and the anode 16.
  • a respective portion of the field lines 40 (marked by dashed lines) is concentrated at the anode 16, while the remaining field lines still "end" at the steering electrode 18.
  • the negative space charge layer 44 amplifies the steering field generated by the steering electrode 18 and focusing the field lines 40 towards the anode, leading to a new shape of the electric field.
  • the field lines 40' (marked by solid lines) influenced by negative space charge layer 44 are stronger deflected and thus considerably focused on the anode 16, so that all field lines 40' "end” at the anode 16 and consequently all electrons are gathered by the anode 16.
  • the insulating material 20, more precisely the first insulating section 32 performs two important functions.
  • the insulating material 20 prevents that electrons might be gathered by the steering electrode 18, which results in a better electron collection efficiency and thus in an improvement of the spectral resolution of the radiation detector 10.
  • the electrons being blocked by the insulating material 20 and forming the negative space charge layer 44 amplify the steering field so that the potential difference between the voltage level Vs of the steering electrode and the voltage level V A of the anode can be reduced significantly to obtain the electric field, which was only influenced by the steering electrode 18.
  • the present invention offers significantly lower leakage currents with much better electron collection efficiency compared to the prior art, leading to an improved spectral resolution and higher sensitivity of the radiation detector 10.
  • Fig. 2 depicts the layer configuration of the second side 28 of the direct converter material layer 12.
  • Cross-sectional view A-A shows the centrally arranged anode 16 with the surrounding first insulating section 32 of the insulating material 20.
  • the first insulating section 32 preferably covers the entire surface of the second side 28 not covered by the anode 16, thereby providing an optimum insulation of the steering electrode 18.
  • Cross- sectional view B-B shows again the centrally arranged anode 16, insulated by the second insulating section 34 from the surrounding steering electrode 18.
  • the steering electrode 18 covers the entire second side 28 of the direct converter material layer 12 not covered by the anode 16 and the second insulating section 34, thereby providing a maximized area into which charges can be induced capacitively.
  • Cross-sectional view C-C shows the bond pad 22 which is formed on the third insulating section 36 and which has a circular shape.
  • the bond pad 22 can be of arbitrary shape and size suitable for bonding to e.g. an ASIC, since it is electrically shielded by the steering electrode 18, which results in that no charge generated in the direct converter material layer 12 can be induced on the bond pad 22.
  • Another preferred embodiment of the present invention is illustrated in Fig. 3 and 4.
  • the shown multiple-pixel radiation detector 10' has principally the same structure compared to the above described single-pixel radiation detector 10 (Fig. 1 and 2), deferring, however, in that it has a plurality of adjacent pixels (pixel boarders 46 indicated by dashed lines) each comprising a plurality of anodes 16 forming a multi-dot anode 48, 48'.
  • the connection of several small dot-like collecting anodes 16 in parallel, surrounded and shielded by the steering electrode 18, keeps the size of the anode dots 16 as small as possible, thereby minimizing the extension of the weighting potential attached to each single dot.
  • the advantages are that on the one hand the total area to which charges are induced is reduced, and on the other hand a much lower potential difference between the steering electrode 18 and the anode 16 is required (due to an increased number of collecting anodes 16 relative to the same area) to ensure the same collection efficiency compared to a single spot geometry.
  • Fig. 3 shows a cross-sectional side view of a section of the multiple-pixel radiation detector 10'.
  • two pixels are illustrated, it shall be understood that an arrangement of one to an arbitrary number of pixels, each comprising a multi-dot anode 48, 48', is possible, wherein the pixels can be arranged adjacent or in any other configuration according to requirements or circumstances.
  • the direct converter material layer 12 is sandwiched by the cathode 14, arranged on its first side 24, and a layer arrangement of anodes 16, a steering electrode 18, bond pad 22 and intermediate insulating material 20, arranged on its second side 28.
  • the cathode 18 covers the entire surface of the first side 24 of the direct converter material layer 12 and is coupled to a cathode voltage source 26, biasing the cathode 14 at a negative voltage potential Vc relative to the anode 16.
  • the multi-dot anodes 48, 48' are arranged on the second side 28 of a direct converter material layer 12, wherein each multi-dot anode 48, 48' comprises eight anodes 16 connected in parallel through the bond pad 22, for providing a combined signal of each pixel.
  • each multi-dot anode 48 can vary and differ from each other.
  • the anodes 16 of each multi-dot anode 48, 48' are formed as small conductive contacts, equidistantly arranged on the surface of the second side 28 of the direct converter material 12.
  • the bond pad 22 provides a sufficient bonding area of the multi- dot anode 48 to e.g. an ASIC, but also a connection means for the anodes 16 of each multi- dot anode 48 in each pixel.
  • the multi-dot anodes 48, 48' are coupled to different anode voltage sources 30, 30', which bias the multi-dot anodes 48, 48' at different voltage levels V A1 or V A2 , respectively, both being more positive than the voltage level Vc of the cathode 14.
  • the potential difference between the multi-dot anodes 48, 48' and the steering electrode 18 allows for influencing the shape of the steering field between the anodes 16 and the steering electrode, so as to modify the deflection of the field lines 40 according to the desired and designated object.
  • the multi-dot anodes 48, 48' are preferably equally biased by the same anode voltage source 30 or more preferably both are held at ground.
  • current measuring devices 31, 31' are coupled to the multi-dot anode 48 or 48', respectively, for detecting charges proportional to the energy level of an incident radiation in the respective pixel.
  • the steering electrode 18 is also arranged on the second side 28 of the direct converter material layer 12 and formed as a conductive layer surrounding the multi-dot anodes 48, 48'. Further, the steering electrode 18 is insulated from the direct converter material layer 12, the multi-dot anode 48, 48' and the bond pad 22 by the insulating material 20 comprising three insulating sections 32, 34, 36.
  • the insulating sections 32, 34, 36 perform the same task as in the above described single-pixel radiation detector 10 (Figs. 1 and 2).
  • the steering electrode voltage source 38 biases the steering electrode 18 at positive potential Vs relative to the cathode 14 and a slightly negative potential relative to the multi-dot anodes 48, 48'.
  • Vs positive potential
  • the potential difference between the steering electrode 18 and the multi-dot anodes 48, 48' can even be adjusted to zero, so that e.g. steering electrode 18 as well as multi-dot anodes 48, 48' are held at ground.
  • the functionality of the single-pixel and multi-pixel radiation detectors 10, 10' is principally the same.
  • the negative space charge layer 44 amplifies the steering field generated by the steering electrode 18 and focusing the field lines 40 towards the anode, leading to a new shape of the electric field.
  • the field lines 40' (marked by solid lines) influenced by negative space charge layer 44 are stronger deflected and thus considerably focused on the multi-dot anodes 48, 48', so that all field lines 40' "end" at the multi-dot anodes 48, 48' and consequently all electrons are gathered by multi-dot anodes 48, 48'.
  • an additional advantage of the last described embodiment is that the necessary deflection of the field lines 40, 40' for guiding the electrons to the multi-dot anodes 48, 48' is generally much smaller than in devices comprising pixels with a single anode. Consequently, the required potential difference between the steering electrode 18 and the multi-dot anodes 48, 48' is reduced, leading to significantly lower leakage currents. In addition, the reduced anode distances give rise to a much better electron collection efficiency.
  • Cross-sectional view A-A shows a section with two pixels, wherein multi-dot anodes 48, 48' are surrounded by the first insulating section 32 of the insulating material 20.
  • the first insulating section 32 preferably covers the entire surface of the second side 28 not covered by the multi-dot anodes 48, 48', thereby providing an optimum insulation of the steering electrode 18.
  • Cross-sectional view B-B shows again the two pixels, wherein the multi-dot anodes 48, 48' are insulated by the second insulating section 34 from the surrounding steering electrode 18.
  • the steering electrode 18 covers the entire second side 28 of the direct converter material layer 12 not covered by the anode 16 and the second insulating section 34, thereby providing a maximized area into which charges can be induced capacitively.
  • Cross-sectional view C-C shows the bond pad 22 which is formed on the third insulating section 36 and which connects the single anodes 16 of each multi-dot anode 48, 48' in parallel.
  • the circular shape of the bond pad 22 can be of arbitrary shape and size suitable for bonding to e.g. an ASIC, since it is electrically shielded by the steering electrode 18, which results in that no charge generated in the direct converter material layer 12 can be induced on the bond pad 22.
  • Figs. 5 and 6 show the described multi-pixel radiation detector 10' and point out the shape of the generated steering fields, wherein the field lines 40 (marked by dashed lines) represent the case without the insulating material 20 and thus the negative space charge layer 44, whereas the field lines 40' (marked by solid lines) illustrate the run of the field lines 40 in the presence of the insulating material 20 and thus after the development of the negative space charge layer 44.
  • the multi-dot anodes 48, 48' are held at ground, the steering electrode 18 is on a slightly more negative potential Vs relative to the multi-dot anodes 48, 48' and the cathode 14 is on a highly negative potential Vc relative to the steering electrode 18.
  • the field lines 40, 40' emphasize the advantageous influence of the negative space charge layer 44, developed due to the insulating material 20, which amplifies the initially generated steering field, generated by the steering electrode. Consequently, the potential difference between the steering electrode 18 and the multi-dot anodes 48, 48' can be strongly reduced to achieve e.g. the run of the field lines 40, leading to a significant reduction of the leakage currents from the steering electrode 18 to the multi-dot anodes 48, 48'.
  • Fig. 6 shows a further important advantage of the present invention.
  • the present invention allows for adjusting the potential difference between the steering electrode 18 and the anode 16 or multi-dot anode(s) 48, 48', respectively, to zero.
  • the steering electrode 18 and the multi-dot anodes 48, 48' are held at ground, so that they are on the same voltage level, whereas the cathode 14 is still negatively biased. In this case, no steering field would exist, so that the field lines 40 would generally run straight through the direct converter material layer 12, without being deflected by the steering electrode 18.
  • the negative space charge layer 44 adopts the function of the steering electrode and generates the steering field, so that the field lines 40' are focused at the anodes 16 of the multi-dot anodes 48, 48'.
  • the leakage currents are ideally completely eliminated and no additional noise is induced.
  • a computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.
  • a suitable medium such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

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  • Measurement Of Radiation (AREA)

Abstract

The present invention relates to a radiation detector with improved charge collection and minimized leakage currents. The present invention further relates to an imaging device for imaging an object comprising the aforementioned radiation detector for detecting radiation after having passed through that object. In particular, a radiation detector is presented that comprises a steering electrode (18) arranged on the a side of a direct converter material layer (12) for guiding free electrons to an anode (16), wherein the steering electrode (18) is insulated by an insulating material (20) from the direct converter material layer (12) for providing an electron barrier between said steering electrode (18) and said direct converter material layer (12). Thus, electrons drifting towards the steering electrode (18) are blocked by the insulating material (20) and concentrated in its direct vicinity. This concentration of negative space charge causes or amplifies, respectively, the electrical opposing field which is normally generated by the application of a relatively high potential difference between the steering electrode (18) and the anode (16). Consequently, the potential difference between the steering electrode (18) and the anode (16) can be strongly reduced, resulting in the elimination or at least a significant reduction of the leakage currents and additional noise as well as the enhancement of charge collection and thus additional improvement of the spectral resolution of the radiation detector.

Description

Radiation detector with improved charge collection and minimized leakage currents
FIELD OF THE INVENTION
The present invention relates to a radiation detector with improved charge collection and minimized leakage currents. The present invention further relates to an imaging device for imaging an object comprising the aforementioned radiation detector for detecting radiation after having passed through that object.
BACKGROUND OF THE INVENTION
Energy resolving detectors for e.g. X-ray and gamma radiation become increasingly important in research, industrial and medical imaging. Detectors based on direct converter materials, as for example CdTe or CZT, has been proven as an efficient way to measure photon energies. A direct converter detector generally consists of a block of semiconductor material between two electrodes, to which a (high) voltage is applied. Typically, a geometry is chosen in which the photons are incident through the cathode side. The photon transfers energy to a single electron which becomes a "fast electron", and which therefore creates a number of electron/hole pairs in the direct vicinity of the original location of interaction. The electrons drift to an array of anode pixels on the opposite side, while holes drift to the cathode. Typically, the holes drift significantly more slowly than the electrons (for example, by a factor of about 20 in CZT).
Ideally, the integral current pulse measured at an anode pixel corresponds to the total created charge, which is proportional to the original photon energy. It is important to note, that the anode current is induced by a capacitive coupling of the charge carriers to the anode and not at the time of arrival of the charges at the collecting anode. This means that both types of charge carriers - electrons, but also holes - contribute to the anode current not only of the underlying pixel, but also to all neighbor pixels. Thus, the slowly drifting holes still induce a charge offset, while the electrons are already collected by the anodes.
An improvement of the spectral resolution is reached by the concept of control or streering electrodes. It should be mentioned, that in the following the terms "control electrode" and "steering electrode" will be used synonymously. The aim of this concept is to downsize the area of the collecting anode and to surround it with another electrode, i.e. the control electrode, which is generally on a highly more negative potential relative to the anode. In result, the electrons are guided to a small anode by an electric field, while the charge offset of the holes is reduced with decreasing anode size due to a more strongly confined weighting potential. A concept of this kind is disclosed in US 6,333,504 Bl . This document discloses a device and method for detecting ionizing radiation, and more particularly a semiconductor radiation detector with enhanced charge collection for reducing low-energy tailing effects. This radiation detector comprises a single conductor crystal which is covered on one side by the cathode and on the opposite side by the control electrode and the anode. The employed controlled electrode is thereby maintained at a voltage level that is negative with respect to the anode, but generally not more negative than the cathode. Typically, the preferred value of the control electrode voltage Vcontroi is in the range from (Vanode+Vcathode)/2 to Vcathode- However, the ideal electrical potential of the control electrode for focusing electron charge to the anode is approximately the potential of the cathode. Thus, the difference between the control electrode voltage potential and the anode voltage potential comes up to several hundred volts.
However, this detector still suffers from relative high leakage currents from the steering electrode to the anode as well as the gathering of electrons by the steering electrode instead of the collecting anode. Both result in a degradation of the spectral resolution and thus provide an unwanted effect. As further disclosed in US 6,333,504 Bl, the wrong-sided electron collection can be reduced by increasing the potential difference between anode and control electrode, leading to a stronger guidance of the electrons to the anode. However, this always conflicts with a much stronger injection of leakage currents from the control electrode to the anode, which induces additional noise.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a radiation detector and an imaging device of the kind mentioned at the outset, which show an improved charge collection and minimized leakage currents, leading to a better spectral resolution and higher sensitivity, and which obviate the disadvantages and drawbacks of conventional radiation detectors.
In a first aspect of the present invention a radiation detector is presented that comprises a direct converter material layer for providing free electrons in response to an incident radiation, a cathode arranged on a first side of the direct converter material layer, an anode arranged on a second side of the direct converter material layer for detecting charges proportional to the energy level of the incident radiation, a steering electrode arranged on the second side of the direct converter material layer and at least partly surrounding the anode for guiding the free electrons, and an insulating material arranged between said anode and said steering electrode and between said steering electrode and said direct converter material layer for providing an electron barrier between said steering electrode and said direct converter material layer.
That is, in other words, the steering electrode is at least partly insulated from the anode and especially from the direct converter material layer, so that at least a portion of the field lines could cross the insulating material in the absence of space charge. Thus, electrons (for example those inherently present from a dark current) drifting along that field lines are stopped by the insulating material, leading to a local concentration of electrons in the direct vicinity of the insulating material. This concentration of negative space charge causes or amplifies, respectively, the electrical opposing field which is normally generated by the application of a relatively high potential difference between the steering electrode and the anode.
Consequently, the present invention exhibits two major advantages. First of all, the potential difference between the control electrode and the anode can be strongly reduced since the steering field is generated or amplified, respectively, by the negative space charge, leading to the elimination or at least a significant reduction of the leakage currents and additional noise. Further, the insulating material prevents at least a portion of the drifting electrons to be gathered by the steering electrode, thus additionally improving the spectral resolution of the radiation detector.
In a further aspect of the present invention an imaging device for imaging an object is presented, comprising a radiation source for providing a radiation beam and a radiation detector according to the aforementioned radiation detector for detecting radiation after having passed through that object.
The benefit of this imaging device is that it can be used for applications which require very high and efficient imaging resolution and sensitivity, like e.g. at medical imaging to create images of the human body or parts thereof for clinical purposes or medical science.
Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method has similar and/or identical preferred embodiments as the claimed device and as defined in the dependent claims. In a further embodiment of the present invention, the steering electrode is completely insulated from the direct converter material layer by the insulating material. That is, in other words, no electrons at all can be gathered by the steering electrode. Accordingly, the potential difference between the steering electrode and the anode can ideally be adjusted to zero, so that leakage currents and consequently additional noise are ideally completely eliminated.
According to a further embodiment of the present invention the radiation detector comprises a bond pad connected to the anode and insulated from the steering electrode by the insulating material for providing a sufficient bonding area of the anode. In common single-spot technologies, the anode spot is usually about 200-300 μm in size, since it is directly bonded to the application- specific integrated circuit (ASIC). Consequently, detector geometries show a minimum detector thickness of 600 to 1000 μm, since the aspect ratio, i.e. detector thickness to pixel width ratio, should not go below 3 for a good weighting potential. Thus, the fabrication of thin detector layers is not possible, without infringing said aspect ratio. However, the integration of a bond pad allows for the minimization of the anode size on the detecting side (e.g. below 100 μm), while the anode size on the contacting side can be maximized, theoretically to an arbitrary size, since it is electrically shielded by the steering electrode. This minimization of the anode size results in a significant reduction of the extension of the weighting potential attached to each single anode and thus the possibility of fabrication of very thin detector layer arrangements (e.g. below 500 μm) with arbitrary pixel size.
According to a further embodiment of the present invention, the cathode covers the entire first side of the direct converter material layer and the insulating layer covers the entire second side of the direct converter material layer not covered by the anode. This action increases the collection efficiency of the radiation detector in as much as no electrons are gathered by the steering electrode.
In a further embodiment the direct converter material layer consists of CdTe or CZT. Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CZT) are more and more applied as detector materials for photons from a few of keV to a few MeV. Their relative advantages include high sensitivity for X-rays and gamma-rays, due to the high atomic numbers of Cd and Te, and superior spatial and energy resolution compared to scintillators. Further, they are more economical and compact compared to germanium (Ge), particularly considering that they do not require cryogenic cooling. In addition, both materials provide a high mobility- lifetime ratio between electrons and holes, which also improves the efficiency of the radiation detector
In a further embodiment of the present invention, the ratio of the direct converter material layer thickness to the anode size is greater than 3 to assure an effectively reduced weighting potential in the largest part of the detector volume (known as "small pixel effect"). The ratio of the direct converter material layer thickness to the pixel size is not limited to be greater than 3.
According to a further embodiment of the present invention the radiation detector comprises at least two anodes surrounded by the steering electrode and connected in parallel thereby forming a multi-dot anode for providing a combined signal. The spectral resolution of the direct converter material strongly relies upon the confinement of the weighting potential, wherein the weighting potential shrinks with decreasing anode sizes. Hence, the idea is to connect several small dot-like collecting anodes in parallel, surrounded and shielded by the steering electrode, in order to keep the size of the anode dots as small as possible, thereby minimizing the extension of the weighting potential attached to each single dot. The advantages are that on the one hand the total area to which charges are induced is reduced, and on the other hand a much lower potential difference between the steering electrode and the anode is required (due to an increased number of collecting anodes on the same area) to ensure the same collection efficiency compared to a single spot geometry. In a further embodiment of the present invention the anodes of the multi-dot anode are connected through a bond pad, the bond pad being insulated from the steering electrode by the insulating material additionally arranged between said steering electrode and said bond pad for providing a sufficient bonding area of the multi-dot anode. The benefit of this embodiment is that the bond pad, having the afore-mentioned advantages, is additionally used for the parallel connection of the anodes of the multi-dot anode, thereby simplifying the detector structure.
In a further aspect of the present invention the anodes, the steering electrode and the insulating material are formed on the second side of the direct converter material layer in a stacked layer arrangement of insulator and metal using standard lithographic techniques. The benefit of the stacked layer arrangement and the use of lithographic techniques is that they provide an easy and cheap possibility of applying the electrodes and the insulation on the direct converter material layer, thus facilitating the manufacturing of the radiation detector. In a further embodiment of the present invention the anodes of the multi-dot anode are arranged equidistantly to each other for providing a substantially homogenous electron collection leading to a path length being uniform and as short as possible for electrons collected in the anode. The generated electric field shape improves the electron collection efficiency because of the optimal anode-dot distribution and thus the spectral resolution of the radiation detector.
According to a further embodiment of the present invention the radiation detector comprises a plurality of equidistantly arranged multi-dot anodes, each defining together with its surrounding steering electrode a single pixel. This detector array structure has the benefit that a large area can be examined and that the single pixels can be separately measured, so that regions of interest can be viewed at a customized resolution with a customized pixel size, without being dependent on the detector thickness or the ratio of detector thickness to anode size. Typically, such pixel arrays are particularly useful for radiation cameras, used in industrial or medical applications. In a further embodiment of the present invention, the detector provides a plurality of multi-dot anodes each multi-dot anode having at least two anodes, wherein the anode size is less than 100 μm and the distance between each anode is less than 500 μm. This detector array provides a good compromise between the efficiency of the radiation detector, concerning the leakage currents and the electron collection, and the expenditure in manufacturing. Moreover, the very small dimensions amplify the advantageous effects of the present invention. Consequently, the present invention is preferably realized in small dimensions, wherein the anode size should be as small as possible but is not limited to any size limitations.
In a further embodiment of the present invention the imaging device comprises a voltage source for providing a bias voltage for the cathode, the steering electrode and/or the anode, so as to create a negative potential difference of the cathode relative to the steering electrode and a negative or no potential difference of the steering electrode relative to the anode thereby generating an electric field within the direct converter material.
This action provides a simple possibility to provide the appropriate voltage level for the cathode, the steering electrode and/or the anode.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings Fig. 1 shows a side cross-sectional view of the preferred embodiment of a single-pixel configuration according to the present invention,
Fig. 2 shows three cross-sectional views of different layers of the detector illustrated in Fig. 1, Fig. 3 shows a side cross-sectional view of a section of the preferred embodiment of a multi-pixel configuration with multi-dot anodes according to the present invention,
Fig. 4 shows three cross-sectional views of different layers of the detector illustrated Fig. 3, Fig. 5 shows the multi-pixel detector of Figs. 3 and 4, emphasizing the influence of the insulating material with a potential difference between the steering electrode and the anode, and
Fig. 6 shows the multi-pixel detector of Figs. 3 and 4, emphasizing the influence of the insulating material with no potential difference between the steering electrode and the anode.
DETAILED DESCRIPTION OF THE INVENTION
In Figs. 1 and 2 a first embodiment of a detector 10 according to the present invention is shown. Fig. 1 is a cross-sectional side view of the preferred embodiment of a single- pixel radiation detector 10 of the invention, comprising a direct converter material layer 12, a cathode 14, an anode 16, a steering electrode 18, an insulating material 20 and a bond pad 22.
The direct converter material layer 12 is a square-shaped slap or wafer of semi-conductor material with high resistivity and suitable for fabrication in detectors, like e.g. CdTe or CZT.
The cathode 14 is arranged on a first side 24 of the direct converter material layer 12 and formed as a conductive layer covering the entire surface of the first side 24 of the direct converter material layer 12. Further, the cathode 14 is coupled to a cathode voltage source 26, biasing the cathode at a negative voltage potential Vc relative to the anode 16. The anode 16 is arranged on a second side 28 of a direct converter material layer 12 and formed as a small conductive contact, centrally located on the surface of the second side 28 of the direct converter material 12. Further, the dot- like anode is connected to the bond pad 22 for providing a sufficient bonding area of the anode 16 to e.g. an application- specific integrated circuit (ASIC). An anode voltage source 30 biases the anode 16 at voltage level VA, which is more positive than the voltage level Vc of the cathode 14. However the anode 16 is preferably held at ground, while the cathode 14 is negatively biased. Further, a current measuring device 31 is coupled to the anode for detecting charges proportional to the energy level of an incident radiation. The steering electrode 18 is also arranged on the second side 28 of the direct converter material layer 12 and formed as a conductive layer surrounding the anode 16 and covering the entire second side not covered by the anode 16 and the insulating material 20. Further, the steering electrode 18 is insulated from the direct converter material layer 12, the anode 16 and the bond pad 22 by an insulating material 20 comprising three insulating sections 32, 34, 36. The first insulating section 32 lies between the direct converter material layer 12 and the steering electrode 18 and covers the entire surface of the second side 28 of the direct converter material layer 12 not covered by the anode 16, thereby providing a barrier for the electrons, drifting towards the steering electrode 18. That is, in other words, the first insulating section 32 blocks the path of the electrons drifting towards the steering electrode 18 and ensures that no electrons "drop away" at the steering electrode 18. The second insulating section 34 lies between the steering electrode 18 and the anode 16 for providing good breakdown characteristics and for reducing the leakage currents from the steering electrode 18 to the anode 16. The third insulating section 36 lies between the steering electrode 18 and the bond pad 22 and covers the entire second side 28 not covered by the anode 16. Thus, the bond pad 22 is insulated from the steering electrode 18, and can be easily applied on the third insulating section 36.
A steering electrode voltage source 38 biases the steering electrode 18 at positive potential Vs relative to the cathode 14 and a slightly negative potential relative to the anode 16. As will be explained below in detail, the potential difference between the steering electrode 18 and the anode 16 can even be adjusted to zero, so that e.g. both are held at ground.
As mentioned before, appropriate biasing of the electrodes 14, 16, 18 generates an electric field within the direct converter material layer 12, wherein the field lines 40, representing the drifting paths of the free electrons within the direct converter material layer 12 in the absence of any space charges, extend from the entire surface of the cathode 14 to the steering electrode 18 and the anode 16. Depending on the potential difference between the steering electrode 18 and the anode 16, a respective portion of the field lines 40 (marked by dashed lines) is concentrated at the anode 16, while the remaining field lines still "end" at the steering electrode 18. As soon as free electrons of the direct converter material layer 12 are available, either thermally excited as a dark current, or by an incident radiation 42 having excited them by energy transfer, these free electrons drift along the field lines 40 towards the second side 28 of the direct converter material layer 12. The electrons, drifting along field lines 40 ending on the steering electrode 18 would be gathered from the steering electrode 18 in the absence of the first insulating section 32. However, these electrons are blocked from the first insulating section 32 of the insulating material 20 and stay in the direct vicinity of the surface of the second side 28 of the direct converter material layer 12, thereby establishing a negative space charge layer 44 located on the insulating section 32. The negative space charge layer 44 amplifies the steering field generated by the steering electrode 18 and focusing the field lines 40 towards the anode, leading to a new shape of the electric field. In result, the field lines 40' (marked by solid lines) influenced by negative space charge layer 44 are stronger deflected and thus considerably focused on the anode 16, so that all field lines 40' "end" at the anode 16 and consequently all electrons are gathered by the anode 16. Thus, the insulating material 20, more precisely the first insulating section 32, performs two important functions.
First, the insulating material 20 prevents that electrons might be gathered by the steering electrode 18, which results in a better electron collection efficiency and thus in an improvement of the spectral resolution of the radiation detector 10. Second, the electrons being blocked by the insulating material 20 and forming the negative space charge layer 44 amplify the steering field so that the potential difference between the voltage level Vs of the steering electrode and the voltage level VA of the anode can be reduced significantly to obtain the electric field, which was only influenced by the steering electrode 18. As a result, the present invention offers significantly lower leakage currents with much better electron collection efficiency compared to the prior art, leading to an improved spectral resolution and higher sensitivity of the radiation detector 10.
Fig. 2 depicts the layer configuration of the second side 28 of the direct converter material layer 12. Cross-sectional view A-A shows the centrally arranged anode 16 with the surrounding first insulating section 32 of the insulating material 20. The first insulating section 32 preferably covers the entire surface of the second side 28 not covered by the anode 16, thereby providing an optimum insulation of the steering electrode 18. Cross- sectional view B-B shows again the centrally arranged anode 16, insulated by the second insulating section 34 from the surrounding steering electrode 18. The steering electrode 18 covers the entire second side 28 of the direct converter material layer 12 not covered by the anode 16 and the second insulating section 34, thereby providing a maximized area into which charges can be induced capacitively. Cross-sectional view C-C shows the bond pad 22 which is formed on the third insulating section 36 and which has a circular shape. It shall be understood that the bond pad 22 can be of arbitrary shape and size suitable for bonding to e.g. an ASIC, since it is electrically shielded by the steering electrode 18, which results in that no charge generated in the direct converter material layer 12 can be induced on the bond pad 22. Another preferred embodiment of the present invention is illustrated in Fig. 3 and 4. The shown multiple-pixel radiation detector 10' has principally the same structure compared to the above described single-pixel radiation detector 10 (Fig. 1 and 2), deferring, however, in that it has a plurality of adjacent pixels (pixel boarders 46 indicated by dashed lines) each comprising a plurality of anodes 16 forming a multi-dot anode 48, 48'.
Since the spectral resolution of the direct converter material layer 12 strongly relies upon the confinement of the weighting potential it is advantageous to minimize the extension of the weighting potential. In this case, charge carriers do not induce a significant amount of charge before they approach the direct vicinity around the pixel, i.e. just briefly before they are collected by the anode 16, resulting in a well-detectable short current pulse. Furthermore, the charge offset which is induced by the remaining holes is reduced because of a lower probability of presence of holes within the relevant volume of the weighting potential. Hence, the connection of several small dot-like collecting anodes 16 in parallel, surrounded and shielded by the steering electrode 18, keeps the size of the anode dots 16 as small as possible, thereby minimizing the extension of the weighting potential attached to each single dot. The advantages are that on the one hand the total area to which charges are induced is reduced, and on the other hand a much lower potential difference between the steering electrode 18 and the anode 16 is required (due to an increased number of collecting anodes 16 relative to the same area) to ensure the same collection efficiency compared to a single spot geometry.
Fig. 3 shows a cross-sectional side view of a section of the multiple-pixel radiation detector 10'. Although two pixels are illustrated, it shall be understood that an arrangement of one to an arbitrary number of pixels, each comprising a multi-dot anode 48, 48', is possible, wherein the pixels can be arranged adjacent or in any other configuration according to requirements or circumstances.
The direct converter material layer 12 is sandwiched by the cathode 14, arranged on its first side 24, and a layer arrangement of anodes 16, a steering electrode 18, bond pad 22 and intermediate insulating material 20, arranged on its second side 28. The cathode 18 covers the entire surface of the first side 24 of the direct converter material layer 12 and is coupled to a cathode voltage source 26, biasing the cathode 14 at a negative voltage potential Vc relative to the anode 16. The multi-dot anodes 48, 48' are arranged on the second side 28 of a direct converter material layer 12, wherein each multi-dot anode 48, 48' comprises eight anodes 16 connected in parallel through the bond pad 22, for providing a combined signal of each pixel. It shall be understood that the number of anodes 16 comprised in each multi-dot anode 48 can vary and differ from each other. The anodes 16 of each multi-dot anode 48, 48' are formed as small conductive contacts, equidistantly arranged on the surface of the second side 28 of the direct converter material 12. The bond pad 22 provides a sufficient bonding area of the multi- dot anode 48 to e.g. an ASIC, but also a connection means for the anodes 16 of each multi- dot anode 48 in each pixel.
The multi-dot anodes 48, 48' are coupled to different anode voltage sources 30, 30', which bias the multi-dot anodes 48, 48' at different voltage levels VA1 or VA2, respectively, both being more positive than the voltage level Vc of the cathode 14. The potential difference between the multi-dot anodes 48, 48' and the steering electrode 18 allows for influencing the shape of the steering field between the anodes 16 and the steering electrode, so as to modify the deflection of the field lines 40 according to the desired and designated object. However, the multi-dot anodes 48, 48' are preferably equally biased by the same anode voltage source 30 or more preferably both are held at ground. Further, current measuring devices 31, 31' are coupled to the multi-dot anode 48 or 48', respectively, for detecting charges proportional to the energy level of an incident radiation in the respective pixel. The steering electrode 18 is also arranged on the second side 28 of the direct converter material layer 12 and formed as a conductive layer surrounding the multi-dot anodes 48, 48'. Further, the steering electrode 18 is insulated from the direct converter material layer 12, the multi-dot anode 48, 48' and the bond pad 22 by the insulating material 20 comprising three insulating sections 32, 34, 36. The insulating sections 32, 34, 36 perform the same task as in the above described single-pixel radiation detector 10 (Figs. 1 and 2). The steering electrode voltage source 38 biases the steering electrode 18 at positive potential Vs relative to the cathode 14 and a slightly negative potential relative to the multi-dot anodes 48, 48'. As will be explained below in detail, the potential difference between the steering electrode 18 and the multi-dot anodes 48, 48' can even be adjusted to zero, so that e.g. steering electrode 18 as well as multi-dot anodes 48, 48' are held at ground. The functionality of the single-pixel and multi-pixel radiation detectors 10, 10' is principally the same. Appropriate biasing of the electrodes 14, 18, 48, 48' generates an electric field within the direct converter material layer 12, wherein the field lines 40 (marked by dashed lines), representing the drifting paths of the free electrons within the direct converter material layer 12 in the absence of any space charges, extend from the entire surface of the cathode 14 to the steering electrode 18 and the multi-dot anodes 48, 48'. As soon as free electrons are available from a dark current or excited by an incident radiation 42, these free electrons drift along field lines 40 wherein the electrons drifting towards the steering electrode 18 are blocked by the first insulating section 32 and stay in the direct vicinity of the surface of the second side 28 of the direct converter material layer 12, thereby establishing a negative space charge layer 44 located on the insulating section 32.
The negative space charge layer 44 amplifies the steering field generated by the steering electrode 18 and focusing the field lines 40 towards the anode, leading to a new shape of the electric field. In result, the field lines 40' (marked by solid lines) influenced by negative space charge layer 44 are stronger deflected and thus considerably focused on the multi-dot anodes 48, 48', so that all field lines 40' "end" at the multi-dot anodes 48, 48' and consequently all electrons are gathered by multi-dot anodes 48, 48'. However, an additional advantage of the last described embodiment is that the necessary deflection of the field lines 40, 40' for guiding the electrons to the multi-dot anodes 48, 48' is generally much smaller than in devices comprising pixels with a single anode. Consequently, the required potential difference between the steering electrode 18 and the multi-dot anodes 48, 48' is reduced, leading to significantly lower leakage currents. In addition, the reduced anode distances give rise to a much better electron collection efficiency.
Fig. 4 points out the layer configuration of the second side 28 of the direct converter material layer 12. Cross-sectional view A-A shows a section with two pixels, wherein multi-dot anodes 48, 48' are surrounded by the first insulating section 32 of the insulating material 20. The first insulating section 32 preferably covers the entire surface of the second side 28 not covered by the multi-dot anodes 48, 48', thereby providing an optimum insulation of the steering electrode 18. Cross-sectional view B-B shows again the two pixels, wherein the multi-dot anodes 48, 48' are insulated by the second insulating section 34 from the surrounding steering electrode 18. The steering electrode 18 covers the entire second side 28 of the direct converter material layer 12 not covered by the anode 16 and the second insulating section 34, thereby providing a maximized area into which charges can be induced capacitively. Cross-sectional view C-C shows the bond pad 22 which is formed on the third insulating section 36 and which connects the single anodes 16 of each multi-dot anode 48, 48' in parallel. It shall be understood that the circular shape of the bond pad 22 can be of arbitrary shape and size suitable for bonding to e.g. an ASIC, since it is electrically shielded by the steering electrode 18, which results in that no charge generated in the direct converter material layer 12 can be induced on the bond pad 22.
Figs. 5 and 6 show the described multi-pixel radiation detector 10' and point out the shape of the generated steering fields, wherein the field lines 40 (marked by dashed lines) represent the case without the insulating material 20 and thus the negative space charge layer 44, whereas the field lines 40' (marked by solid lines) illustrate the run of the field lines 40 in the presence of the insulating material 20 and thus after the development of the negative space charge layer 44.
In Fig. 5 the multi-dot anodes 48, 48' are held at ground, the steering electrode 18 is on a slightly more negative potential Vs relative to the multi-dot anodes 48, 48' and the cathode 14 is on a highly negative potential Vc relative to the steering electrode 18. The field lines 40, 40' emphasize the advantageous influence of the negative space charge layer 44, developed due to the insulating material 20, which amplifies the initially generated steering field, generated by the steering electrode. Consequently, the potential difference between the steering electrode 18 and the multi-dot anodes 48, 48' can be strongly reduced to achieve e.g. the run of the field lines 40, leading to a significant reduction of the leakage currents from the steering electrode 18 to the multi-dot anodes 48, 48'.
Fig. 6 shows a further important advantage of the present invention. As aforementioned, the present invention allows for adjusting the potential difference between the steering electrode 18 and the anode 16 or multi-dot anode(s) 48, 48', respectively, to zero. As Fig. 6 illustrates, the steering electrode 18 and the multi-dot anodes 48, 48' are held at ground, so that they are on the same voltage level, whereas the cathode 14 is still negatively biased. In this case, no steering field would exist, so that the field lines 40 would generally run straight through the direct converter material layer 12, without being deflected by the steering electrode 18. However, the negative space charge layer 44 adopts the function of the steering electrode and generates the steering field, so that the field lines 40' are focused at the anodes 16 of the multi-dot anodes 48, 48'. Thus, the leakage currents are ideally completely eliminated and no additional noise is induced. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 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.
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. A single element or other unit 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 measured cannot be used to advantage.
A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.
Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:
1. A radiation detector comprising a direct converter material layer (12) for providing free electrons in response to an incident radiation, a cathode (14) arranged on a first side (24) of the direct converter material layer (12), an anode (16) arranged on a second side (28) of the direct converter material layer (12) for detecting charges proportional to the energy level of the incident radiation (42), a steering electrode (18) arranged on the second side (28) of the direct converter material layer (12) and at least partly surrounding the anode (16) for guiding the free electrons, and an insulating material (20) arranged between said anode (16) and said steering electrode (18) and between said steering electrode (18) and said direct converter material layer (12) for providing an electron barrier between said steering electrode (18) and said direct converter material layer (12).
2. A radiation detector as claimed in claim 1, wherein the steering electrode (18) is completely insulated from the direct converter material layer (12) by the insulating material (20).
3. A radiation detector as claimed in claim 1, comprising a bond pad (22) connected to the anode (16) and insulated from the steering electrode (18) by the insulating material (20) for providing a sufficient bonding area of the anode (16).
4. A radiation detector as claimed in claim 1, wherein the cathode (14) covers the entire first side (24) of the direct converter material layer (12) and the insulating layer covers the entire second side (28) of the direct converter material layer (12) not covered by the anode (16).
5. A radiation detector as claimed in claim 1, wherein the direct converter material layer (12) consists of CdTe or CZT.
6. A radiation detector as claimed in claim 1, wherein the ratio of the direct converter material layer (12) thickness to the anode (16) size in greater than 3.
7. A radiation detector as claimed in claim 1, comprising at least two anodes (16) surrounded by the steering electrode (18) and connected in parallel thereby forming a multi- dot anode (48) for providing a combined signal.
8. A radiation detector as claimed in claim 7, wherein the anodes (16) of the multi-dot anode (48) are connected through a bond pad (22), the bond pad (22) being insulated from the steering electrode (18) by the insulating material (20) additionally arranged between said steering electrode (18) and said bond pad (22) for providing a sufficient bonding area of the multi-dot anode (48).
9. A radiation detector as claimed in claim 8, wherein the anodes (16), the steering electrode (18) and the insulating material (20) are formed on the second side (28) of the direct converter material layer (12) in a stacked layer arrangement of insulator and metal using standard lithographic techniques.
10. A radiation detector as claimed in claim 7, wherein the anodes (16) of the multi-dot anode (48) are arranged equidistantly to each other for providing a substantially homogenous electron collection.
11. A radiation detector as claimed in claim 7, comprising a plurality of equidistantly arranged multi-dot anodes (48, 48'), each defining together with its surrounding steering electrode (18) a single pixel.
12. A radiation detector as claimed in claim 7, comprising a plurality of multi-dot anodes (48, 48') each multi-dot anode (48, 48') having at least two anodes (16), wherein the anode size is less than 100 μm and the distance between each anode (16) is less than 500 μm.
13. An imaging device for imaging an object, comprising a radiation source for providing a radiation beam and a radiation detector according to claim 1 for detecting radiation after having passed through that object.
14. An imaging device as claimed in claim 13, comprising a voltage source for providing a bias voltage for the cathode (14), the steering electrode (18) and the anode (16), so as to create a negative potential difference of the cathode (14) relative to the steering electrode (18) and a negative or no potential difference of the steering electrode (18) relative to the anode (16) thereby generating an electric field within the direct converter material layer (12).
PCT/IB2009/055784 2008-12-22 2009-12-16 Radiation detector with improved charge collection and minimized leakage currents WO2010073189A1 (en)

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