Abstract:
The invention provides a Silicon Photomultiplier (SiPM). The SiPM includes a plurality of microcells, a nonlinear element integrated in each one of the plurality of microcells, and a trigger line for outputting a summated current of the plurality of microcells, wherein the nonlinear element provides for a separated timing and energy signal.

Description:
This application is the U.S. national phase application of PCT International Application No. PCT/EP2011/063966, filed Aug. 12, 2011, which claims priority to European Patent Application No. EP10009073.1, filed Sep. 1, 2010 and German Patent Application No. 102010041805.6, filed Sep. 30, 2010, the contents of such applications being incorporated by reference herein. 
     The invention relates to a device having a plurality of photosensitive microcells arranged in row or matrix form. 
     Silicon photomultipliers consist of detector cells or microcells each of which has an avalanche photodiode (APD) and a series resistor. When a feed voltage is applied, the photodiode, which is usually biased in the reverse direction, connects through to some extent upon the arrival of radiation, for example a photon. This effect is amplified by the avalanche effect in the photodiode. At the output of the microcell, i.e. at the node between the resistor and the photodiode, a voltage drop that is caused by the switching through of the photodiode can be measured and evaluated. 
     WO 2006/111883 A2 discloses an array of detector cells in which every detector cell is designed as an avalanche photodiode. An avalanche photodiode is integrated in a CMOS process. A digital circuit outputs a first value in a resting state and a further other value when the avalanche photodiode detects a photon. A circuit outputs a trigger signal at the start of the integration period as a response to the transition of the cell from one digital value to the other. 
     T. Frach et al., “Digital Silicon Photo Multiplier—Principle of Operation and Intrinsic Detector Performance”, IEEE Nuclear Science Symposium, Talk 29 May 2009 describes a fully digital silicon photomultiplier in which an avalanche photodiode is integrated in a CMOS process. This solution contains an active quenching and a fully digital readout with the disadvantage of higher costs due to a complex production process. Compared with an analog silicon photomultiplier, the timing resolution is improved since the capacitances of the individual microcells are not summed up with each other and the timing trigger is generated directly at cell level. 
     SUMMARY 
     The object of the invention consists in making a fast and inexpensive silicon photomultiplier available. 
     The object is achieved by the features of the independent claims. 
     Developments of the invention and embodiments are specified in the dependent claims. 
     It is suggested that an individual non-linear element be integrated in every microcell to obtain the time signal directly from the microcell and evaluate it. This non-linear element is preferably an active element or a bipolar or MOS transistor. It is preferably either an NMOS or a PMOS transistor. This component/device serves as an amplifier that activates a general trigger signal line. The energy or charge signal is evaluated by integrating the summated current of all microcells in the device, as in the prior art of analog silicon photomultiplier devices. 
     Most applications make use of an analog SiPM (silicon photomultiplier) and derive both the timing and the energy information signals from the same summated signal of all microcells. This is not the optimal configuration for the timing information. The triggering at the first detected photon or photons/quanta is especially difficult. Furthermore, the electronics for the timing trigger need a higher bandwidth than for the energy signal, and this is not optimal in terms of the power consumption. 
     A digital trigger line is added to the analog silicon photomultiplier so that the timing signal is separated from the energy signal or charge signal. The advantages comprise:
     1. Triggering at the first photon is made simpler and more precise than in a purely analog silicon photomultiplier;   2. The energy signal can be evaluated with less electronics, which results in a considerably lower current consumption/power requirement;   3. The technological effort is less than with a fully integrated CMOS process so that production is rendered less expensive.
       The invention provides a Silicon Photomultiplier (SiPM). The SiPM includes a plurality of microcells, a nonlinear element integrated in each one of the plurality of microcells, and a trigger line for outputting a summated current of the plurality of microcells, wherein the nonlinear element provides for a separated timing and energy signal.   
       

     The invention also provides for a device for detecting radiation. The device includes a plurality of photosensitive microcells arranged in a row or matrix for detecting one or more photons, a plurality of avalanche photodiodes each one of the plurality of photodiodes being associated with a respective photosensitive microcell, a plurality of quenching resistors wherein each quenching resistor is connected in series with a respective one of the plurality of avalanche photodiodes, a plurality of amplifier elements wherein each amplifier element is located between a respective avalanche photodiode and quenching resistor in each one of the plurality of microcells, a first trigger line for detecting at least one time point of an incidence of one or more photons within at least one of the plurality of photosensitive microcells, and a second trigger line for detecting charges that are generated within at least one of the plurality of photosensitive microcells by an incidence of one or more photons. 
    
    
     
       DRAWINGS 
       The invention is explained in detail on the basis of the following figures which show, respectively: 
         FIG. 1  is a diagram of a silicon photomultiplier in accordance with the prior art 
         FIG. 2  is a diagram of a silicon photomultiplier with an amplifier in accordance with an embodiment of the present invention; 
         FIG. 3  is a diagram of the silicon photomultiplier in  FIG. 2  with hierarchically structured amplifier electronics in accordance with an embodiment of the present invention; and 
         FIGS. 4A and 4B  are graphs of the results of simulations of the operation of a circuit in accordance with  FIG. 2  or  3  in accordance with an embodiment of the present invention. 
     
    
    
     In the following figures, identical or functionally corresponding units have the same reference symbols. The figures are described jointly in a group. 
     DETAILED DESCRIPTION 
       FIG. 1  shows the electrical circuit of a silicon photomultiplier (SiPM) F available in the prior art. A quantity of detector cells or microcells Mi,j, which have an avalanche photodiode (APD) Di,j and a quenching resistor Ri,j connected in series with it, is arranged in a matrix in order to form a photosensitive surface F or a photoelectric device F. 
     A supply voltage U s  is applied to the detector cells Mi,j in the reverse direction, i.e. the positive supply voltage is connected via the quenching resistor Ri,j to the cathode of the avalanche photodiode Di,j. The detector cells Mi,j operate in Geiger mode, i.e. the supply voltage Us is a dc voltage (Bias-Voltage_Bias, operating point/dc voltage point), which is somewhat higher than the breakdown voltage of the avalanche photodiodes Di,j. The avalanche effect generated upon the detection of a photon Ph 1  is only terminated by the lowering of the bias voltage UDi,j at the avalanche photodiode Di,j, which takes place with the aid of the quenching resistor Ri,j. This event, of the avalanche breakdown, takes place very rapidly. The resetting, or re-charging, of an avalanche photodiode Di,j takes around 100 to 200 μs in this case at a capacitance of the avalanche photodiode of 100 femtoFarad and a quenching resistance of 100 kOhm. This is especially suitable for individual photon counting. 
     The detector cells Mi,j are all connected to each other in parallel electrically. If one or more microcells Mi,j detects a photon Ph 1 , a trigger electron is triggered in the avalanche photodiode Di,j so that the avalanche effect is triggered. Due to the increasing cell current Ii,j and the thereby increasing voltage drop at the respective quenching resistor Ri,j, the bias voltage Ui,j at the avalanche photodiode(s) Di,j is reduced so strongly that the avalanche photodiode is quenched, i.e. blocks the current flow again. At the common terminal A at the base of the respective anode of the avalanche photodiodes Di,j, it is possible to measure the total current I of all cell currents Ii,j flowing in the individual microcells Mi,j, by means of which the charge avalanches triggered by the photons Phi in the avalanche photodiodes Di,j can be determined. For this reason, the common terminal or the output A is connected, for example, via a measuring or load resistor R 1  to ground. Although the operation of the individual/single detector cell Mi,j proceeds in a purely digital manner, the overall silicon photomultiplier works in an analog mode due to the parallel connections of the microcells Mi,j to each other. 
     The embodiment in accordance with  FIG. 1  represents an analog silicon photomultiplier (SiPM), at the output A of which energy and timing information is available. In an application of the silicon photomultiplier F (SiPM) in positron emission tomography (PET) detectors for evaluating scintillation events, two different signals have to be evaluated:
     a) the timing information, i.e. especially the detection of the first photon(s) Ph 1 , and   b) the energy (amplitude information), i.e. the quantity of incident photons Ph 1 , Ph 2 .   

     The requirement for these two signals is different. While the timing information needs fast electronics and a high bandwidth, the energy information is evaluated by means of integration of the signal, which does not need a high bandwidth. In  FIG. 1 , both the timing information and the energy information are sampled from the same signal source at output A. This is not the optimal configuration for the timing information. The triggering at the first detected photon Ph 1  or photons Ph 1 , Ph 2  is especially difficult. Furthermore, the electronics for the timing trigger need a higher bandwidth than for the energy signal, which does not correspond to an optimal setting for the power consumption. 
       FIG. 2  shows an expansion of the silicon photomultiplier device in  FIG. 1 . Every microcell Mi,j in  FIG. 1  is assigned an amplifier element Ti,j, all amplifier elements Ti,j preferably being constructed in one technology, for example in Complementary Metal Oxide Semiconductor (CMOS), N-type Complementary Metal Oxide Semiconductor (NMOS), P-type Complementary Metal Oxide Semiconductor (PMOS) or bipolar technology. 
     The high-impedance input Bi,j of the amplifier element Vi,j is connected to the node between the quenching resistor Ri,j and the blocking terminal of the avalanche photodiode Di,j and captures the falling bias voltage UDi,j at the avalanche photodiode Di,j. The respective amplifier elements Vi,j are fed by means of a supply voltage Up via a common pull-up resistor RTrig via a trigger signal line TRIG, preferably in each case via the drain terminal Di,j of the amplifier element Vi,j designed as a MOS transistor. An OR connection (“wired OR”) of the states of all connected microcells is therefore available at the trigger signal line TRIG. The “source” output Si,j of the respective amplifier element Vi,j is connected to ground. 
     The bases of the respective anodes of the avalanche photodiodes Di,j continue to be connected to the output A. As a result, the currents Ii,j flowing through the respective avalanche photodiodes Di,j are summed up into the total current I. For the purpose of measuring the total current I, the output A is connected to ground via the measuring or load resistor R 1  so that the total current I can be determined indirectly via the voltage drop UA at the load resistor R 1 . 
     The trigger voltage UTrig at the trigger signal line TRIG is reduced (“lower level”) as soon as an amplifier element Vi,j becomes at least partly conducting due to photon incidence on an avalanche photodiode Di,j, which is explained in detail below. 
     The speed of the trigger signal line TRIG is primarily dictated by parasitic capacitive loads, which are proportional to the quantity of microcells Mi,j that are connected to this trigger line TRIG. 
     For large-scale sensor arrangements, a hierarchical triggering scheme with mutually decoupled trigger signal lines TRIGi is employed for the purpose of reducing the parasitic capacitive loads on the trigger signal line TRIG, as represented in detail in  FIG. 3 .  FIG. 3  shows a silicon photomultiplier device F as in  FIG. 2 , wherein one trigger signal line TRIGi is provided for each microcell row Mi, 1 ; Mi, 2 ; . . . ; Mi,n. The amplifier elements Vi,j (drain) of a row i are supplied with a supply voltage Up via a respective amplifier stage Ti, 1 ; Ti, 2 ; Ti via a trigger signal line TRIG assigned in common to all rows  1 , . . . i, . . . , m via a common pull-up resistor RTrig. An amplifier stage Ti, 1 ; Ti, 2 ; Ti has three amplifier modules Ti, 1 ; Ti, 2 ; Ti, preferably designed as MOS transistors. Two amplifier modules Ti, 1 ; Ti, 2  are constructed as a current mirror circuit, while the third amplifier module Ti undertakes current impression in the common source branch of the two amplifier modules Ti, 1 ; Ti, 2 , which is dependent on the control current IBias. The base of the first mirror module Ti, 1  is in each case connected to the trigger signal line TRIGi serving the purpose of supplying the respective amplifier element Mi, 1 , . . . , Mi,j, . . . , Mi,n, and is supplied as necessary via the drain of the first mirror module Ti, 1  that is connected to the feed voltage US via the pull-up resistor RTrigi. The drain of the second mirror transistor Ti, 2  is connected to the common trigger signal line TRIG, which is connected to the (bias) feed voltage Up via a common pull-up resistor RTrig. The ground voltage level of the trigger signal lines TRIGi can be adjusted via the threshold voltage UThr, which is applied in common at the respective base of the second mirror modules Ti, 2 . 
     Functionally, the circuit in  FIG. 3  works like the circuit in  FIG. 2 , the additional amplifier elements with current mirror circuit and current impression reducing the parasitic capacitances of the trigger signal line TRIG by means of subdivision into individual trigger signal lines TRIGi and decoupling. Usually, all the microcells in a row M 1 , 1 , . . . , M 1 ,n or a column M 1 , 1 , . . . , Mm,M in the matrix arrangement F are typically connected to an amplifier stage Ti, 1 ; Ti, 2 ; Ti. 
     The circuit in  FIG. 3  preferably uses only NMOS transistors and resistors for all amplifier stages, i.e. the same devices that are already being used in the microcells Mi,j. By this means, these stages can also be integrated in monolithic fashion without raising the technological complexity. 
     Other embodiments of the amplifier circuits have only CMOS transistors or bipolar transistors or complete CMOS processes that are constructed in a similar manner to that specified in  FIG. 3 . 
     The avalanche effect generated upon the detection of a photon is only terminated by the lowering of the bias voltage UDi,j at the avalanche photodiode Di,j, which takes place with the aid of the quenching resistor Ri,j. The resetting, or re-charging, of an avalanche photodiode Di,j takes around 100 to 200 μs in this case at a capacitance of the avalanche photodiode Di,j of 100 fF (femtoFarad) and a quenching resistance Ri,j of 100 kOhm. This is especially suitable for individual photon counting. 
       FIGS. 4   a  and  b  show a general simulation carried out with the program SPICE of the voltage values at the common trigger signal line TRIG and the common output A of the photoelectric device F in  FIG. 2  or  3  with circuit parameters that are used by an existing analog silicon photomultiplier device and are based on a 0.5 μm CMOS process. Parasitic capacitive effects on the trigger line TRIG in  FIGS. 2 and 3  do not visibly influence the measurement curves in this example. It is assumed that a microcell Mi,j is struck by a photon Ph 1  at the time point t 1 =1 ns (nanosecond) so that a corresponding electron avalanche is triggered. 
     The charge quantity Q transferred by the relevant avalanche photodiode Di,j can be determined by way of integration of the current I or the voltage UA at the load resistor R 1 . The associated measurement curves for the voltage UA measured/simulated at different load resistors R 1  in the case of employing various load resistors R 1 =1 Ohm (square), 20 Ohm (diamond), 50 Ohm (triangle), and 100 Ohm (inverted triangle) are represented in  FIG. 4 . The area from t=1 ns to the end of the dead time and after the decay of Ri,j—C_Di,j —time constant (C_Di,j=capacitance of the avalanche photodiode Di,j) under the respective curves is proportional to the charge quantity transferred Q 1 . The rise time of the signal voltages UA comprises around 2 ns in each case. Furthermore, the influence of the optional arrival of a further photon Ph 2  on a further avalanche photodiode Dx,y at the time point t=5 ns is represented in the measurement curve indicated by a “double diamond” at R=20 Ohm. A further charge quantity Q 2  is transferred by the further avalanche photodiode Dx,y so that the total charge quantity Qges=Q 1 +Q 2  is given by the aforementioned integration. 
     The simulation curve in  FIG. 4   b  shows the trigger voltage/trigger signal UTrig of the trigger line TRIG as a function of the time t. The trigger signal UTrig shows a clear step change after the arrival of the first photon Ph 1  with a rise time or fall time of around 0.2 ns and a voltage drop of around 0.5 V. The shape of the curve UTrig is essentially independent of the charge quantity Q transferred by the relevant avalanche photodiode Di,j and the load resistor R 1 . The step change can simply be used to cause triggering with a high degree of timing resolution at the first photon Ph 1 . After the end of the dead time of &gt;100 ns of the relevant avalanche photodiode (which cannot be represented true to scale), the original voltage value of approx. 3.3 is restored. As an option, the influence of the arrival of at least one further photon Ph 2  on a further avalanche photodiode D 1 , 2  at the time point t=5 ns is represented in the measurement curve indicated by a “double diamond” in  FIG. 4   b . A further voltage drop of around 0.5 V occurs with a step duration of around 0.2 ns. Correspondingly, further photons Phi can also strike further avalanche photodiodes Dxi,yi, the precise arrival time points of which can be read off from the measurement curve. 
     Excellent conclusions about events can be made from the determined arrival time points t 1 , t 2 , . . . , in  FIG. 4   b  and the total charge quantity Qges in  FIG. 4   a , for example for evaluating scintillator events in scintillation crystals that are connected upstream of the silicon photomultiplier F. It is thus possible to distinguish between the direct incidence of one quantum of radiation with a high quantum energy on a scintillator crystal and Compton events with a plurality of radiation quanta with lower energies. 
     As can be seen from the measurement curves in  FIG. 4   a, b , the analog signal UA responds much more slowly (with a rise time of approx 2 ns)—by around a factor of 10 in this case—than the digital trigger signal UTrig (with a rise time/fall time of approx. 0.2 ns), but it is quite suitable for evaluating the total energy. The duration of capture of the charge quantity Q corresponds essentially to the dead time of the avalanche photodiode due to the integration of the current over time. The digital trigger signal line TRIG is not disturbed by the addition of external load resistors R 1  for evaluating the transferred charge Qges and the analog charge signal UA is not disturbed by the digital trigger line TRIG.