Abstract:
According to an exemplary embodiment of the invention a detector unit  301  for detecting electro-magnetic radiation may be provided. The detector unit  301  may comprise a conversion material  332  adapted for converting impinging electro-magnetic radiation into electric charge carriers. Moreover, the detector unit  301  may comprise a charge collection electrode  331  adapted for collecting the converted electric  321  charge carriers and an evaluation circuit  312, 313, 314  adapted for evaluating the electro-magnetic radiation based on the collected electric charge carriers. Moreover, the detector unit  301  may comprise a semiconductor  373  which may be electrically coupled between the charge  331  collection electrode  331  and the evaluation circuit  312, 313, 314.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a detector unit for detecting electromagnetic radiation, to a detector device and to a method of detecting electromagnetic radiation. Moreover, the invention relates to a computer-readable medium, in which a computer program of detecting electromagnetic radiation may be stored and to a program element of detecting electromagnetic radiation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Currently most solid-state digital X-ray detectors in the market may be built of a flat glass plate with amorphous silicon (a-Si) thin film electronics and an X-ray conversion layer on top of it. The X-ray detectors may be either of the indirect conversion type with a scintillator on top of an array of photodiodes or of the direct conversion type using a photoconductor on top of an array of electrodes. The impinging X-rays are absorbed in the conversion layer and, via the generated charges in each pixel of the array, create a digital image of the X-ray absorption. 
         [0003]    An alternative to thin film electronics on glass may be the use of wafers of monocrystalline silicon for the pixel electronics. As above, pixels with or without photodiodes can be built for either indirect or direct X-ray conversion. The use of standard CMOS processes in monocrystalline silicon may lead in general to electronic circuits with less noise and more functionality compared to a-Si pixel circuits. In case of an indirect conversion detector, the scintillator can either be glued or grown directly on the Si wafer. For direct X-ray conversion materials there might be also at least two possibilities: either connecting a separately fabricated layer, e.g. with bump balls or a direct deposition on silicon. 
         [0004]    Today the pixel pitch in flat X-ray detectors may reach from about 150 μm to about 200 μm except for mammography and dental imaging, where pixel sizes of less than 100 μm are common. A general trend can be observed in X-ray imaging, that the demand for higher spatial resolution also for cardiology, neurology and vascular applications is growing. The pixel size of a monocrystalline Si-detector may be reduced to values far below 100 μm because of the small feature sizes, which may be possible with this technology for transistors and other electronic elements. 
         [0005]    However, in the case of an indirect conversion detector, the spatial resolution may be limited by the light spread in the scintillator. In general the thickness of the scintillator may not be reduced to maintain a high X-ray absorption yield. To fully exploit the high spatial resolution of a detector with small pixels a direct X-ray conversion may be suited better. Direct conversion materials like selenium, mercury iodide, lead oxide or CdTe (Cadmium Telluride) can be easily made thick enough to absorb more than 80% of the X-rays with a beam quality typical for medical imaging. A very high spatial resolution may be usually achieved because the generated charge carriers which may be electrons and holes, may follow the field lines of the applied bias field, which may run perpendicular to the surface of the pixel electrode and the usually unstructured top electrode. 
         [0006]    Besides the spatial resolution another advantage of a direct conversion CMOS detector may be the possibility to overcome the limited fill factor of a photodiode in a small pixel. In a direct conversion detector a metal layer covering nearly the whole pixel area can serve as pixel electrode. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the invention to improve a detector, especially to provide a sufficiently sensitive detector. 
         [0008]    This object is achieved by the features of the independent claims relating to a detector unit, a detector device, a method of detecting electromagnetic radiation, a program element, and a computer-readable medium. 
         [0009]    According to an exemplary embodiment of the invention a detector unit for detecting electro-magnetic radiation may be provided. The detector unit may comprise a conversion material adapted for converting impinging electro-magnetic radiation into electric charge carriers. Moreover, the detector unit may comprise a charge collection electrode adapted for collecting the converted electric charge carriers and an evaluation circuit adapted for evaluating the electro-magnetic radiation based on the collected electric charge carriers. Moreover, the detector unit may comprise a semiconductor which may be electrically coupled between the charge collection electrode and the evaluation circuit. 
         [0010]    The principles of the invention may be applicable in different kinds of sensors, especially in image sensors, such as CMOS image sensors which may be used in X-ray devices and in X-ray detectors, especially in CMOS X-ray detectors. Thus, the principles of the invention may refer to an X-ray detector, which may use direct X-ray conversion combined with CMOS pixel circuits. The proposed pixel circuit may provide a very high sensitivity by means of an additional charge transfer step from the large pixel electrode to a dedicated small additional integration capacity. The effective input capacitance may be reduced in this case without the need of a permanent bias current like in other solutions. The main application of such a high sensitive direct conversion detector may be mammography, but it may be usable for many other X-ray imaging applications. It may also be foreseen that in front of the charge collection electrode or below the charge collection electrode there may be arranged a shielding electrode. This shielding electrode may be adapted to form a capacitance with the charge collection electrode. This may improve the capacitive characteristic of the detector unit. 
         [0011]    According to an exemplary embodiment, the semiconductor of the detector unit may be a transistor, comprising a gate connection, a drain connection and a source connection, wherein the source connection may be connected to the charge collection electrode and the drain connection may be connected to the evaluation circuit. 
         [0012]    The semiconductor may be of any type, for example a FET, especially a MOSFET. 
         [0013]    According to an exemplary embodiment, the gate connection may be held to a predetermined voltage wherein the predetermined voltage may be adapted to provide a current flow of a source drain current from the charge collection electrode to the evaluation circuit. 
         [0014]    It may be foreseen that the predetermined voltage is a timely constant voltage or permanent voltage of a predetermined value which may be applied during the whole operation time of the detector unit. It may also be possible that the applied voltage is a pulsed voltage, which may be applied in predetermined time intervals and which may be not present during the whole operating time due to the pulse characteristic. 
         [0015]    According to an exemplary embodiment, an integration capacitance may be electrically coupled to the semiconductor and to the evaluation circuit. 
         [0016]    The electrically coupling may be provided as a conducting connection between the integration capacitor and the semiconductor as well as between the semiconductor and the evaluation circuit. The integration capacitance may comprise a first connection and a second connection. The first connection may be electrically coupled to the semiconductor as well as to the evaluation circuit. The second connection may be connected to a reference potential, especially to a ground potential. 
         [0017]    According to an exemplary embodiment of the invention, the integration capacitance may comprise a first connection and a second connection wherein the first connection may be connected to the drain connection of the transistor and the second connection may be connected to a reference potential. 
         [0018]    The reference potential may be a ground potential. 
         [0019]    According to an exemplary embodiment of the invention, the semiconductor may be connected to a charge pump. 
         [0020]    It may also be possible that the charge pump may be connected to an input electrode, especially to the charge collection electrode of the detector unit. 
         [0021]    According to an exemplary embodiment, the charge pump may be adapted to be controlled by a first control line. 
         [0022]    The first control line may also be connectable to additional detector units in order to control different detector units with one control line. 
         [0023]    According to an exemplary embodiment, the semiconductor may be connected to a first charge transfer transistor which may be adapted to be controlled by a second control line. 
         [0024]    The semiconductor may comprise a gate connection which may be electrically connected to a control line. Furthermore, the semiconductor may comprise a drain connection which may be electrically connected to the first charge transfer transistor. The first charge transfer transistor may be a FET (field effect transistor), especially an n-channel transistor, which may comprise a gate connection, a drain connection and a source connection. The source connection of the first charge transfer transistor may be connected to the semiconductor. 
         [0025]    According to an exemplary embodiment, the first charge transfer transistor may be connected to a first charge storage capacitor. 
         [0026]    The first charge transfer transistor may function as a switch and may transfer in a closed status the charge from the integration capacitor to the first charge storage capacitor. 
         [0027]    According to an exemplary embodiment of the invention, the first charge transfer transistor may be connected to a second charge transfer transistor which second charge transfer transistor may be adapted to be controlled by a third control line. 
         [0028]    The second charge transfer transistor may function as a switch and may transfer in a closed status the charge from the first charge storage capacitor to the second charge storage capacitor. In addition it may be foreseen that further integration capacitors and further charge transfer transistors may be utilized in a chain like manner, similar as the first charge storage capacitor, the second charge storage capacitor, the first charge transfer transistor and the second transfer transistor are connected to each other. 
         [0029]    According to an exemplary embodiment of the invention, the second charge transfer transistor may be connected to a second charge storage capacitor. 
         [0030]    The second charge transfer transistor may be a FET (field effect transistor), especially an n-channel transistor, which may comprise a gate connection, a drain connection and a source connection. The gate connection of the second charge transfer transistor may be connected to a further control line. 
         [0031]    According to an exemplary embodiment of the invention, a detector device for detecting electro-magnetic radiation may be provided. The detector device may comprise a plurality of interconnected detector units, according to an exemplary embodiment of the invention. 
         [0032]    The detector device may comprise a matrix of detector units, which may be connected to each other with vertical control lines and horizontal control lines. 
         [0033]    According to an exemplary embodiment of the invention, a method of detecting electro-magnetic radiation may be provided. The method may comprise converting impinging electro-magnetic radiation into electric charge carriers, collecting the converted electric charge carriers at the charge collection electrode. The method may further comprise providing a current flow from the charge collection electrode to an evaluation circuit and evaluating by an evaluation circuit the electro-magnetic radiation based on the collected electric charge carriers. 
         [0034]    Providing a current flow from the charge collection electrode to the evaluation circuit may be provided by a semiconductor and/or a charge pump. Moreover, it may be foreseen to provide a shielding electrode adapted to form a capacitance with the charge collection electrode. Such a shielding electrode may provide an improved capacitance characteristic of the X-ray apparatus comprising several detector units. An improved capacitance may result in an improved control of picture evaluation of the X-ray apparatus using a plurality of detector units. 
         [0035]    According to an exemplary embodiment of the present invention, a computer-readable medium may be provided in which a computer program of detecting electro-magnetic radiation may be stored, and which, when being executed by a processor may be adapted to control or carry out a method according to the invention. 
         [0036]    A computer readable medium may be a floppy disk, a hard disk, an USB (Universal Serial Bus) storage device, a RAM (Random Access Memory), a ROM (Read Only Memory), an EPROM (Erasable Programmable Read Only Memory) or the like. 
         [0037]    According to an exemplary embodiment of the invention, a program element of detecting electro-magnetic radiation may be provided. The program element when being executed by a processor may be adapted to control or carry out a method according to the invention. 
         [0038]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereafter. 
         [0039]    It has also to be noted that exemplary embodiments of the present invention and aspects of the invention have been described with reference to different subject-matters. In particular, some embodiments have been described with reference to apparatus type claims whereas other embodiments have been described with reference to method type claims. 
         [0040]    However, a person skilled in the art may gather from the above and the following description that unless other notified in addition to any combination between features belonging to one type of subject-matter also any combination between features relating to different subject-matters in particular between features of the apparatus claims and the features of the method claims may be considered to be disclosed with this application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]      FIG. 1  shows schematically an exemplary embodiment of a solid state X-ray detector. 
           [0042]      FIG. 2  shows schematically an exemplary embodiment of a circuit of an indirect X-conversion detector. 
           [0043]      FIG. 3  shows schematically an exemplary embodiment of a circuit of a direct conversion X-ray detector. 
           [0044]      FIG. 4  shows schematically a first exemplary embodiment of a circuit according to the invention. 
           [0045]      FIG. 5  shows schematically a second exemplary embodiment of a circuit according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0046]    The illustration in the figures is schematic. In the following description of  FIGS. 1 to 5 , the same reference characters may be used for identical or corresponding elements. 
         [0047]      FIG. 1  shows an exemplary embodiment of a solid state X-ray detector  101 . The solid state X-ray detector  101  comprises an array  201  of pixel cells  301  and associated line driver circuits  202  and readout amplifiers and/or multiplexers  203 . 
         [0048]      FIG. 2  shows an exemplary embodiment of a circuit of an indirect X-conversion detector. The circuit of  FIG. 2  comprises a photodiode  311  which can be reset to a supply voltage by means of a switching device  312  which is controlled by reset line  321 . This connection is also referred to an input node  337 . The X-ray or light exposure reduces the voltage on the input node  337 . During readout, the voltage on this node is copied by a buffer, usually a source follower  313 , and placed on the readout line  323  by means of the readout switch  314  which is actuated by the control line  322 . It is worth noting that the usual n-channel source follower in a standard CMOS process on a p-epitaxial layer has a gain of approximately 0.8, hence the signal from the input node  337  is copied only in reduced form to the readout line, affecting the achievable signal to noise ratio. 
         [0049]    In the case of a direct conversion X-ray detector as shown in  FIG. 3 , the photodiode  311  is replaced by a charge collection electrode  331  and the shielding electrode  334  which is in first instance connected to a reference potential  336 . Further components of the circuit may also be connected to the reference potential  336 . The charge collection electrode  331  could be made in the top metal of the backend stack, the reference electrode in the next lower metal layer. The direct conversion material  332  is connected to the charge collection electrode  331  and has also a top contact  333  which is connected to a high voltage supply  335 . 
         [0050]    The electrodes  331  and  334  form a large part of the input capacitance (C_in), the rest being allocated in the connections, the reset switch  312  and the source follower  313 . 
         [0051]    The function of the circuit in  FIG. 3  is similar to function described for  FIG. 2 . A difference being that in  FIG. 3  the charges collected from the direct conversion material fill the pixel capacitance and hence this may change the voltage on the input node  337 . 
         [0052]      FIG. 4  shows a first exemplary embodiment of a circuit according to the invention. In comparison to  FIG. 3 , an additional transistor  371  and an integration capacitor  373  are placed between the charge collection electrode  331  and the source follower  313  in the exemplary embodiment of  FIG. 4 . The gate of the transistor  371  is held by line  372  permanently at such a voltage that a source-drain current can flow if the gate-source voltage exceeds a certain threshold. In case of an X-ray or light exposure of the detector the charge collected at the electrode  331  will be transferred to the integration capacitor  373  and reduces its voltage. The integration capacity is reset after the exposure. To avoid a long term charge accumulation on the charge collection electrode  331 , an injection of a small charge may be necessary from time to time, preferably once per X-ray exposure frame, via a charge pump  374 , which is controlled by control line  375 . This additional charge may be well-known and can be subtracted later from the real signal. In  FIG. 4  the charge pump  374 , the integration capacitor  373  and the shielding electrode  334  are connected to the reference potential  336 , respectively. 
         [0053]    The rest of the circuit in  FIG. 4  remains the same as in  FIG. 3 : the voltage on the integration capacity  373  is transferred via a source follower  313  and a readout switch  314  to a readout line  323 . The integration capacity  373  can be chosen as small as needed for a specific application leading to a very high sensitivity of the circuit. 
         [0054]      FIG. 5  shows a second exemplary embodiment of a circuit according to the invention.  FIG. 5  shows a circuit combined with means to increase the dynamic range of the pixel. One or more charge transfer transistors  360 ,  361  and one or more additional charge storage capacitors  351 ,  352  are added to the integration capacitor  373 .  FIG. 5  shows two additional stages, but changing that to one or more than two stages is easily done by one skilled in the art. The gate voltages of transistors  360 ,  361  are set by the respective control lines  340 ,  341  such that the first transistor  360  turns on when the voltage of the integration capacitor  373  has reached a certain lower limit. Further charge arriving through transistor  371  is now transferred to the additional capacitor  351 . When the voltage in this capacitor  351  reaches a certain lower limit, the next transistor  361  turns on and transfers further incoming charges to capacitor  352 . During readout, a first sub-image is formed by reading the first the capacitor  373  alone. This is achieved by fully turning off charge transfer transistors  360 ,  361  via their control lines  340 ,  341 . Then a second sub-image is formed by with transistor  360  turned fully on, thus reading the collective charges on  373  and  351 . Then a next sub-image is formed by fully turning on both transistors  360 ,  361 , thus the collective charges of  373 ,  351  and  352  are read. The final image is formed from those sub-images that have valid image information, i.e. those images where no charge has been transferred to a next stage. Thus, the final image can be formed with the smallest integration capacitor which also gives the smallest noise contribution and best signal to noise ratio. All additional capacitors  351 ,  352  are reset together with  373  by applying a sufficiently high gate voltage over the control lines  340 ,  341 , thus fully activating the transistors  360 ,  361 . 
         [0055]    The pixel shown in  FIG. 5  can also be used to reduce the sensitivity in fixed steps by fully activating one or more of the transistors  360 ,  361 . This puts capacitors  351  and possibly  352  in parallel to capacitor  373  already during the exposure phase or integration phase. The circuit shown in  FIG. 5  is partially self protecting against leakage currents. If the n-MOS reset switch  312  is used with a negative high voltage on the direct conversion material, a high leakage current will turn on the reset switch and the current will be drained to the supply voltage. If positive high voltage is used, a p-MOS reset switch will likewise drain the excessive current and protect the buffer. 
         [0056]    With other words, according to an exemplary embodiment of the invention it is provided an additional transistor between the existing large pixel electrode and an additional dedicated and almost smaller integration capacity. The gate of this transistor may be held at a certain intermediate voltage, so that a source-drain current can flow from the pixel electrode to the integration capacity as long as the voltage is above a certain threshold. This charge transfer step may reduce the effective input capacitance, which may be then only determined by the choice of a small integration capacity and the gate of the subsequent source follower amplifier. 
         [0057]    In the case of direct conversion solid state X-ray detector, nearly the complete pixel surface may need to act as collection electrode. This electrode is part of the pixel capacitance and is very sensitive to both the input charge and disturbing signals from the underlying electronics. Hence a shielding electrode connected to a reference potential may need to be implemented below the collection electrode to provide a stable second electrode for the pixel capacitor and to keep unwanted disturbing signals from reaching the charge collection electrode. 
         [0058]    The arrangement of a charge collection electrode and a shielding electrode forms an input capacitance. The value of this capacitance may be dictated by the pixel size and the actual fabrication process used to build the pixel and is frequently larger than wished for, hence resulting in a low sensitivity of the circuit. 
         [0059]    Other possibilities to reduce the input capacitance are to use either bootstrapping circuit as it is proposed in EP2006117527 or a dedicated operational amplifier (OpAmp) in the pixel. In both cases a permanent bias current that is fed in every pixel may be needed, which may be difficult to realize in a large sensor with a higher number of rows. 
         [0060]    The invention can be applied to all sorts of X-ray detectors using direct X-ray conversion and pixel electronics using CMOS electronics. The invention may also be applied for photo diodes of optical imagers, using indirect X-ray conversion. 
         [0061]    The applications may comprise cardio-vascular X-ray, general X-ray, neurology, orthopaedics, mammography and dental imaging. It may be foreseen to utilize a conversion material reacting to a wavelength of about 1 μm to about 15 μm or infrared radiation on the sensor or the detector unit in order to provide a thermal imaging device. 
         [0062]    The invention is not limited to the disclosed embodiments, and gives examples of as many alternatives as possible for the features included in the embodiments discussed. 
         [0063]    In addition, it should be pointed out that “comprising” does not exclude other elements or steps, and “a”, “an” or “one” does not exclude a plural number. 
         [0064]    Moreover, features cited in separate dependent claims may be advantageously combined. 
         [0065]    Furthermore, it should be pointed out that characteristics or steps which have been described with reference to one of the above exemplary embodiments can also be used in combination with other characteristics or steps of other exemplary embodiments described above. Reference characters in the claims are not to be interpreted as limitations. 
       LIST OF REFERENCE SIGNS 
       [0066]      101  X-ray detector 
         [0067]      201  Array 
         [0068]      202  Line drive circuit 
         [0069]      203  Read out amplifiers/multiplexers 
         [0070]      301  Detector unit or pixel cell 
         [0071]      311  Photo diode 
         [0072]      312  Switching device 
         [0073]      313  Source follower, buffer 
         [0074]      314  Read out switch 
         [0075]      321  Reset line 
         [0076]      322  Control line 
         [0077]      323  Read out line 
         [0078]      331  Charge collection electrode 
         [0079]      332  Direct conversion material 
         [0080]      333  Top contact 
         [0081]      334  Shielding electrode 
         [0082]      335  High voltage supply 
         [0083]      336  Reference potential 
         [0084]      337  Input note 
         [0085]      340  Second control line 
         [0086]      341  Third control line 
         [0087]      351  First charge storage capacitor 
         [0088]      352  Second charge storage capacitor 
         [0089]      360  First charge transfer transistor 
         [0090]      361  Second charge transfer transistor 
         [0091]      371  Transistor 
         [0092]      372  Fourth control line 
         [0093]      373  Integration capacitor 
         [0094]      374  Charge pump 
         [0095]      375  First control line