Abstract:
Position sensitive radiation detection is provided using a continuous electrode in a semiconductor radiation detector, as opposed to the conventional use of a segmented electrode. Time constants relating to AC coupling between the continuous electrode and segmented contacts to the electrode are selected to provide position resolution from the resulting configurations. The resulting detectors advantageously have a more uniform electric field than conventional detectors having segmented electrodes, and are expected to have much lower cost of production and of integration with readout electronics.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application 62/213,980, filed on Sep. 3, 2015, and hereby incorporated by reference in its entirety. 
    
    
     GOVERNMENT SPONSORSHIP 
     This invention was made with Government support under contract DESC0010107 awarded by the Department of Energy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to position sensitive semiconductor radiation detectors. 
     BACKGROUND 
     For semiconductor radiation detectors, it is often desired to provide a position readout for detected radiation. Conventionally, position sensitivity can be provided by using a segmented electrode. However, such segmented electrodes can be disadvantageous in practice because they increase device complexity and create undesirable non-uniform electric fields within the detector. 
       FIG. 1  shows an example of this conventional approach. Here the body of the detector is referenced as  102 ,  104  is a p++ back side electrode and the n++ top electrode is segmented (i.e., electrodes  106   a ,  106   b , and  106   c ) to provide position resolution. Electrical contact to the top electrodes can be made via aluminum contacts  108   a ,  108   b , and  108   c  through insulator  110 . Such contact is via AC coupling between the contacts and the corresponding electrode segments. 
       FIG. 2  shows typical electrical field modeling results for a configuration as in  FIG. 1 . It is apparent that the electric field is highly non-uniform. This has several drawbacks: (i) high field at the edge of the electrode can cause the detector to go into breakdown prematurely; (ii) the uneven electric field accelerates the charge carriers differently depending on their positions, producing very different signals as a function of the position of the impinging radiation (i.e., pulse time dispersion of collected charge); and (iii) the possibility of two adjacent readout electrodes becoming electrically connected due to radiation damage. 
     Accordingly, it would be an advance in the art to provide position sensitive radiation detectors that are free from such disadvantages. 
     SUMMARY 
     We have found, somewhat paradoxically, that position sensitive radiation detection can be provided using a continuous electrode, as opposed to the conventional use of a segmented electrode.  FIG. 3  schematically shows the concept. Here the segmented electrodes  106   a ,  106   b  and  106   c  of  FIG. 1  are replaced with a single continuous n++ electrode  112 . Surprisingly, it turns out to be sufficient, as described in greater detail below, to only have the contacts ( 108   a ,  108   b ,  108   c  on  FIG. 3 ) be segmented in order to provide position resolution. Since the electric field within the detector is determined by a bias applied to the electrodes, the resulting electric field is very uniform, as shown in the modeling results of  FIG. 4 . This electric field uniformity alleviates the above-described disadvantages of the conventional segmented electrode approach. 
     Three important features of this work are as follows: 
     1) We provide semiconductor sensors with uniform charge collection and high spatial resolution. Usually segmented semiconductor detectors have non-continuous electrodes to provide position resolution. These problems can be avoided by using a continuous electrode. The position is determined by coupling the collected charge through a capacitor from a resistive electrode into segmented metal contact pads that are read out; 
     2) The use of a continuous electrode allows cheaper production; and 
     3) The use of a continuous electrode allows a simplified implementation of charge multiplication on the electrode with the largest field strength. 
     Advantages in practice include being less prone to radiation damage, requiring less precision in fabrication (e.g., in mask alignment), and providing larger dynamic range due to a wider range of operating voltages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional position sensitive detector. 
         FIG. 2  shows electric field modeling results for a conventional position sensitive detector approach as in  FIG. 1 . 
         FIG. 3  schematically shows the position sensitive detector concept of the present invention. 
         FIG. 4  shows electric field modeling results for a position sensitive detector approach as in  FIG. 3 . 
         FIG. 5  shows a first embodiment of the invention. 
         FIG. 6  shows a second embodiment of the invention. 
         FIG. 7  schematically shows operation of the embodiment of  FIG. 5 . 
         FIG. 8  shows an electrical model relating to  FIG. 7 . 
         FIG. 9  shows a third embodiment of the invention. 
         FIG. 10  shows a fourth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  shows a first exemplary embodiment of the invention. In this example,  504  is a p++ electrode,  502  is a p− body layer,  506  is a p+ gain layer,  508  is an n++ electrode that is continuous as described above,  510  is an insulating layer, and  512   a ,  512   b ,  512   c  are segmented contacts that are AC coupled to electrode  508 . Here the doping of p+ gain layer  506  and n++ electrode  508  determines the gain of the detector according to known principles of radiation detector design. The resulting overall layer sequence is n++/p+/p−/p++ with the n++ electrode having segmented contacts (n-on-p). 
     It is convenient to define n-on-p as referring to a detector whose bulk is p-doped and the diode is formed by adding n-doping on the surface. Similarly, p-on-n refers to a detector whose bulk is n-doped and the diode is formed by adding p-doping on the surface. p-on-p refers to a detector whose bulk is p-doped and having a buried p-n junction where the segmented electrode is p++ at the surface. Similarly n-on-n refers to a detector whose bulk is n-doped and having a buried p-n junction where the segmented electrode is n++ at the surface. 
     The doping types can be interchanged in the embodiment of  FIG. 5 . In this case,  504  is an n++ electrode,  502  is an n− body layer,  506  is an n+ gain layer,  508  is a p++ electrode that is continuous as described above,  510  is an insulating layer, and  512   a ,  512   b ,  512   c  are segmented contacts that are AC coupled to electrode  508 . Here the doping of n+ gain layer  506  and p++ electrode  508  determines the gain of the detector according to known principles of radiation detector design. The resulting overall layer sequence is p++/n+/n−/n++ with the p++ electrode having segmented contacts (p-on-n). 
     Designs as in  FIG. 5  have the disadvantage of reduced design flexibility. The reason for this is that the resistance of the continuous electrode is an important parameter for providing spatial resolution as described below, and also affects the detector gain as indicated above because the doping determines the resistance. 
     Accordingly, designs as in  FIG. 6  are preferred. In this example,  504  is an n++ electrode,  502  is a p− body layer,  506  is a p+ gain layer,  508  is a p++ electrode that is continuous as described above,  510  is an insulating layer, and  512   a ,  512   b ,  512   c  are segmented contacts that are AC coupled to electrode  508 . Here the doping of p+ gain layer  506  and n++ electrode  504  determines the gain of the detector according to known principles of radiation detector design, and the doping/resistance of p++ electrode  508  only affects the spatial resolution of the detector. Thus the detector gain and electrode design for spatial resolution are decoupled from each other. The resulting overall layer sequence is p++/p−/p+/n++ with the p++ electrode having segmented contacts (p-on-p). 
     The doping types can also be interchanged in the embodiment of  FIG. 6 . In this case,  504  is a p++ electrode,  502  is an n− body layer,  506  is an n+ gain layer,  508  is an n++ electrode that is continuous as described above,  510  is an insulating layer, and  512   a ,  512   b ,  512   c  are segmented contacts that are AC coupled to electrode  508 . Here the doping of n+ gain layer  506  and p++ electrode  504  determines the gain of the detector according to known principles of radiation detector design, and the doping/resistance of n++ electrode  508  only affects the spatial resolution of the detector. The resulting overall layer sequence is n++/n−/n+/p++ with the n++ electrode having segmented contacts (n-on-n). 
     In more general terms, one embodiment of the invention is a position-sensitive radiation detector including: 
     i) a semiconductor region (e.g.,  502  and  506  on  FIG. 5 ) configured to absorb radiation and to provide electrical charge carriers in response to absorbed radiation, where the semiconductor region has opposing first and second surfaces; 
     ii) a first electrode (e.g.,  508  on  FIG. 5 ) disposed on the first surface, where the first electrode is configured as a continuous first layer; 
     iii) a second electrode (e.g.,  504  on  FIG. 5 ) disposed on the second surface, where the second electrode is configured as a continuous second layer, and where electrical bias is provided to the semiconductor region by providing a voltage to the first and second electrodes; 
     iv) an insulating layer (e.g.,  510  on  FIG. 5 ) disposed on the first electrode; 
     v) two or more contacts (e.g.,  512   a ,  512   b ,  512   c  on  FIG. 5 ) disposed on the insulating layer and AC-coupled to the first electrode, where signals from the two or more contacts provide position information for the absorbed radiation. 
     Practice of the invention does not depend critically on the type of radiation to be detected (e.g., particles and/or energetic electromagnetic radiation), or on the specific conducting, insulating and semiconducting materials employed (e.g., aluminum, silicon oxide, silicon etc.), or on the specific thicknesses of the semiconductor layers. In the above examples, n++ and p++ refer to doping levels of about 1e19 cm −3  (e.g., 3e18 to 3e19 cm −3 ) for n-type and p-type respectively, n+ and p+ refer to doping levels of about 1e16 cm −3  (e.g., 3e15 to 3e16 cm −3 ) for n-type and p-type respectively, and n− and p− refer to doping levels of less than about 1e13 cm −3  for n-type and p-type respectively. Doping and thicknesses of semiconductor layers can be selected to provide suitable detector gain according to known design principles. Novel design principles relating to providing position resolution using continuous electrodes are described in greater detail below. 
       FIGS. 7 and 8  shows operating principles relating to embodiments of the invention, with reference to the example of  FIG. 5 . Here electrode  508  has a distributed sheet resistance R s  as shown and the detector as a whole has a distributed capacitance C D , also as shown. When radiation  704  interacts with the detector, an electrical pulse  702  can be received by amplifier  708 . With appropriate design as described below, we can ensure that each contact (e.g., contact  512   b ) only sees a small part of the detector (e.g., region  706  on  FIG. 7 ), thereby effectively providing position resolution. Having the AC read-out only see a small part of the detector also advantageously reduces leakage current and effective detector capacitance. 
     A resistive implant for the continuous n++(or p++) electrode of sheet resistivity R □ ˜ 1 kOhm, combined with an appropriate thickness of the insulator  510  can be used to produce a suitable capacitance C AC  between the contact pads and the continuous electrode. An important point of the design is that the combination of the electrode resistivity R s  and the contact pad capacitance C AC  form an RC circuit with time constant τ=R s C AC  that allows the signal to be transmitted to the segmented aluminum pads as if the electrode was segmented. The resistivity of the electrode and the value of capacitance created by the AC coupling oxide can be determined by simulation of the sensor. 
     For this design to efficiently transmit the signal to the aluminum pads, the time constant τ should be (i) longer than the typical duration of a signal in the semiconductor detector under consideration, otherwise the signal disappears too soon and the AC coupling mechanism would not work and (ii) shorter than the typical repetition time of the signal otherwise the signal charge is trapped under the oxide for too much time. More specifically, the time constant τ is preferably greater than 1 ns and is more preferably greater than 5 ns. The time constant τ is preferably less than 25 ns. 
     Amplifier  708  on  FIG. 7  can be regarded as having an input resistance R A . The additional rise time given by R A C D  is roughly 0.1 ns for typical circuit parameters, which is effectively negligible. 
     The present approach may also facilitate integration of detectors with detector electronics. The examples of  FIGS. 9 and 10  correspond to the examples of  FIGS. 5 and 6 , respectively, except that insulating layer  510  is instead an adhesive layer  906 , and read-out chip  902  including amplifiers  904   a ,  904   b ,  904   c  is shown. This bonding approach can be used instead of conventional bump bonding.