Abstract:
The present invention is a radiation detector that includes a crystalline substrate formed of a II-VI compound and a first electrode covering a substantial portion of one surface of the substrate. A plurality of second, segmented electrodes is provided in spaced relation on a surface of the substrate opposite the first electrode. A passivation layer is disposed between the second electrodes on the surface of the substrate opposite the first electrode. The passivation layer can also be positioned between the substrate and one or both of the first electrode and each second electrode. The present invention is also a method of forming the radiation detector.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is a high performance room-temperature semiconductor x-ray and gamma ray radiation detector and method of manufacture thereof. Although the invention will be described in connection with a semi-insulating Cd x Zn 1-x Te (0≦x≦1) radiation detector, the invention is applicable to any II-VI compound with semi-insulating properties. As such, the invention is applicable to any nonlinear or electro-optical device or application where semi-insulating or high resistivity semiconductor material is required. The 0≦x≦1 concentration, or mole fraction range, encompasses CdZnTe with any Zn percentage including CdTe (x=1) and ZnTe (x=0). 
         [0003]    2. Description of the Prior Art 
         [0004]    With reference to  FIG. 1 , a typical, prior art radiation detector  1  includes a substrate  2  formed from a suitable II-VI compound, such as a CdZnTe crystal, a continuous electrode  4  covering one surface of substrate  2 , a side electrode  6  forming an electrically conductive band around a side surface of substrate  2  and one or more segmented electrodes  8  on a surface of substrate  2  opposite continuous electrode  4 . For purpose of describing the prior art and the present invention, radiation detector  1  and radiation detector  1 ′ (described herein) will be described as having a plurality of segment electrodes  8 . However, this is not to be construed as limiting the invention since radiation detector  1  and/or radiation detector  1 ′ may include only a single electrode  8  if desired. 
         [0005]    In use, detector  1  is typically bonded to a carrier or substrate  12  that includes a suitable pattern of conductors (not shown) that facilitate the acquisition of radiation event signals from segmented electrodes  8 . More specifically, segmented electrodes  8  of detector  1  are bonded to electrode pads  14  of substrate  12 , that match the geometry of segmented electrodes  8  of detector  1 , via bonding bumps  16 . Each bonding bump  16  can be, without limitation, an In bump, a low-temperature solder bump, a bump of conductive adhesive, and the like. 
         [0006]    Segmented electrodes  8  can be pixels, strips, grids, steering grids, bars or rings of arbitrary size and geometry. Segmented electrodes  8  can be biased or unbiased relative to each other, to side electrode  6  and/or continuous electrode  4 . 
         [0007]    An exemplary embodiment of detector  1  includes 256 equal sized electrodes, like segmented electrodes  8 , arranged in a 16×16 two-dimensional array that is surrounded by a side electrode, like side electrode  6 , thereby defining a 257 th  electrode. 
         [0008]    Detector  1  is operated by applying one or more voltages between continuous electrode  4  and segmented electrodes  8  that cause charge carriers (electrons and holes) generated by radiation events in the volume of substrate  2  to drift toward continuous electrode  4  and segmented electrodes  8 . Segmented electrodes  8  are coupled to appropriate readout circuitry via substrate  12  to convert the charge or current generated in each segmented electrode  8  from the motion of the generated charge carriers to an electronic signal tailored by the readout circuitry for further processing. Desirably, side electrode  6  is biased to optimize the electric field distribution in the volume substrate  2  and, as a result, optimize the performance of detector  1 . 
         [0009]    Problems encountered with prior art detector  1  include unacceptably low breakdown voltages between pairs of segmented electrodes  8  and/or between continuous electrode  4  and one or more segmented electrodes  8 , with or without side electrode  6  present. Another problem with prior art detector  1  is that unacceptably high levels of leakage current may flow during operation thereby adversely effecting the performance of detector  1 . 
         [0010]    It would, therefore, be desirable to provide an improved detector that overcomes at least the above the problems and perhaps others. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is a high performance room-temperature semiconductor x-ray and gamma ray radiation detector and method of manufacture thereof. The present invention provides a detector having excellent performance and long-term stability. 
         [0012]    A detector in accordance with the present invention can include on a side surface thereof a passivation layer that exhibits very low side-surface leakage current, very high side surface breakdown voltage, excellent physical and chemical stability, and excellent long-term stability under continuous biasing conditions. 
         [0013]    The detector can include between the segmented electrodes a passivation layer that exhibits very low surface leakage current, very high surface breakdown voltage, excellent physical and chemical stability, and excellent long-term stability under continuous biasing conditions. 
         [0014]    The detector can include conductive electrodes. Also or alternatively, the detector can include insulator-conductor electrodes with superior current blocking properties that enable the detector to exhibit very low bulk leakage current, very high bulk breakdown voltage, excellent physical and chemical stability, and excellent long-term stability under continuous biasing conditions. 
         [0015]    The detector can exhibit superior adhesion properties of the electrodes to the detector surface thereby eliminating electrode delamination due to surface contamination. 
         [0016]    The detector can be formed with thin, highly electrically insulating layers. 
         [0017]    The detector can be fabricated utilizing a unique combination of the following thin film deposition and surface modification techniques:
   The combination ultraviolet light and ozone surface etching and oxidation;   Atomic hydrogen surface etching;   Pulsed DC reactive sputtering of insulating nitride (AlN, Si 3 N 4  or similar) or oxide (Al 2 O 3 , SiO 2 , TeO 2 , CdO, CdTeO 3 , ZnO or similar), oxynitride (AlON or similar) and selenide (ZnSe or similar) films;   Sputtering or evaporation of single layer or multi-layer metal electrodes including Pt, Au, In, Ti, Ni, Fe, Ta, Pd, Al, Ag, Cr, Mo, W, Zn, Te or any combination of them in binary, ternary and quaternary form; and   Photolithography to form segmented electrodes.   
 
         [0023]    The detector includes a crystalline substrate formed of a II-VI compound and a first electrode covering a substantial portion of one surface of the substrate. A plurality of second, segmented electrodes is provided in spaced relation on a surface of the substrate opposite the first electrode. A passivation layer is disposed between the second electrodes on the surface of the substrate opposite the first electrode. 
         [0024]    The passivation layer can be an oxide film having a thickness that enables a tunneling current to flow therethrough. The thickness of the passivation layer can be #250 Angstroms, desirably #100 Angstroms and more desirably #25 Angstroms. The passivation layer can also be disposed between the substrate and each second electrode. 
         [0025]    The passivation layer can include a first insulating film formed of native oxides of the II-VI compound and a second insulating film overlaying the first film. The second insulating film can either be a nitride film, an oxynitride film or an oxide film. 
         [0026]    The passivation layer can also cover at least part of a side surface of the substrate. A side electrode can be disposed on the passivation layer covering the at least part of the side surface of the substrate. 
         [0027]    The passivation layer can also be disposed between the first electrode and the one surface of the substrate. 
         [0028]    A method of forming the detector includes (a) forming a passivation layer on a crystalline substrate formed of a II-VI compound; (b) forming an array of apertures in the passivation layer on a first surface of the substrate; (c) depositing conductive material in each aperture and over the passivation layer on the first surface of the substrate; and (d) selectively removing the conductive material deposited over the passivation layer on the first surface of the substrate, whereupon the conductive material remains in each aperture of the passivation layer and the conductive material in each aperture of the passivation layer is separated from the conductive material in each other aperture of the passivation layer on the first surface of the substrate. 
         [0029]    In the method, the conductive material deposited in each aperture can contact at least one of the first surface of the substrate and a thin oxide layer over the first surface of the substrate. 
         [0030]    The method can further include removing at least part of the passivation layer from a second surface of the substrate opposite the first surface thereby exposing at least a portion of the second surface of the substrate and depositing conductive material on the exposed portion of the second surface of the substrate. 
         [0031]    The method can further include depositing conductive material over the passivation layer on a side surface of the substrate. 
         [0032]    The passivation layer can include a first insulating film formed of native oxides of the II-VI compound and a second insulating film overlaying the first film. Step (b) can include forming the array of apertures in the second film and step (c) can include depositing the conductive material on the exposed surface of the first film in each aperture. 
         [0033]    At least a part of the second film can be removed from a second surface of the substrate opposite the first surface thereby exposing at least a portion of a surface of the first film on the second surface of the substrate. Conductive material can then be deposited on the exposed surface of the first film on the second surface of the substrate. Desirably, the first film has a thickness that enables a tunneling current to flow therethrough. The thickness of the first film can be #250 Angstroms, desirably #100 Angstroms and more desirably #25 Angstroms. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0034]      FIG. 1  is a cross-sectional view of a prior art radiation detector coupled to a substrate; 
           [0035]      FIGS. 2-7  are cross-sectional views of a method of forming radiation detector in accordance with the present invention; and 
           [0036]      FIG. 8  is a cross-section of another radiation detector in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    The present invention will be described with reference to  FIGS. 2-7  where like reference numbers correspond to like elements. 
         [0038]    With reference to  FIG. 2 , a method of forming a radiation detector  1 ′ in accordance with the present invention includes etching substrate  2 , such as a substrate of CdZnTe, in a suitable manner to remove cutting, lapping and mechanical polishing damage from the surface(s) thereof. For the purpose of describing the present invention, hereinafter, it will be assumed that substrate  2  is made from CdZnTe. However, this is not to be construed as limiting the invention. 
         [0039]    Substrate  2  can be etched utilizing any suitable wet or dry etching technique. Examples of suitable wet chemical etching solutions include a bromine methanol solution or a bromine ethanol solution. During etching of substrate  2 , a thin, slightly oxidized amorphous Te film  20  typically forms on substrate  2 . 
         [0040]    With reference to  FIG. 3  and with continuing reference to  FIG. 2 , using a suitable technique, the oxidized Te film  20 , if present, and any hydrocarbon contamination is removed from substrate  2 . Thereafter, a thin oxide film  22  of native oxides of CdZnTe, such as Cd(Zn)TeO x , TeO x , CdO or ZnO, is formed on substrate  2  by UV/Ozone oxidation. This film  22  is highly insulating and provides low leakage current, high breakdown voltage and superior long term stability. However, film  22  is typically thin, e.g., #25 Angstroms, and, therefore, desirably needs further protection in the final embodiment of detector  1 ′. 
         [0041]    An electrically insulating film  24  (500 to 5000 Angstrom), such as a nitride (AlN, Si 3 N 4  or similar), oxynitride or oxide film, is deposited atop of film  22  to protect it from damage during further processing and during operation of detector  1 ′. Desirably, insulating film  24  is deposited by pulsed DC reactive sputtering under conditions to provide a highly electrically insulating, low-stress film. 
         [0042]    Either one of film  22  and film  24  can be omitted from a top surface  30  of substrate  2  and/or around a side surface  28  of substrate  2  if the other film is deemed sufficient. For example, film  24  can be omitted on one or both of top surface  30  and around side surface  28  of substrate  2  whereupon film  22  is the sole insulating film. Alternatively, film  22  can be omitted on one or both of top surface  30  and around side surface  28  of substrate  2  whereupon film  24  is the sole insulating film. In yet another alternative, any combination of film  22  and/or film  24  can be utilized on top surface  30 , side surface  28  and/or bottom surface  32  of substrate  2  as desired. For purpose of describing the present invention, films  22  and  24  will be described as being deposited on substrate  2 . However, this is not to be construed as limiting the invention. 
         [0043]    With reference to  FIG. 4  and with continuing reference to  FIG. 3 , a protective film  26 , such as a photoresist, is deposited atop the portion of insulating film  24  that resides on side surface  28  and top surface  30  of substrate  2 . Thereafter, films  22  and  24  are removed (via chemo-mechanical polishing, wet or dry chemical etching, dry (ion or plasma) etching, or any other suitable and/or desirable etching technique) from bottom surface  32  of substrate  2  and continuous electrode  4  is deposited on bottom surface  32  by sputtering, evaporation or any other suitable and/or desirable deposition technique. If desired, prior to deposition of continuous electrode  4 , bottom surface  32  may be cleaned via UV ozone oxidation either alone or followed by atomic hydrogen cleaning. 
         [0044]    With reference to  FIG. 5  and with continuing reference to  FIG. 4 , next, an array of apertures  34  is formed in protective film  26  residing atop top surface  30  of substrate  2  in a manner known in the art, such as by photolithographic chemical processing, and films  22  and  24  in alignment with each aperture  34  are removed by one or more suitable solvents. If protective film  26  is a photoresist, apertures  34  are formed therein by selectively etching soluble portions of the photoresist. A positive or negative photoresist can be used for this purpose. 
         [0045]    Generally, a positive photoresist is one where each portion of the photoresist that is exposed to light, such as ultraviolet (UV) light, becomes soluble to a photoresist developer and the portion of the photoresist that is unexposed remains insoluble to the photoresist developer. A negative resist is one where each portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer and the portion of the photoresist that is unexposed is soluble to the photoresist developer. 
         [0046]    Next, UV/Ozone oxidation is applied to the top surface  30  of substrate  2  exposed in each aperture  34  to remove trace residues of photoresist therefrom. During UV/Ozone oxidation, a thin oxide layer  35  (shown in phantom) forms on the top surface  30  exposed in each aperture  34 . If desired, thin oxide layer  35  can be removed utilizing any suitable etching technique, such as, without limitation, atomic hydrogen etching, desirably done in-situ in an electrode deposition chamber, such as a sputtering chamber, to avoid re-oxidation of the surface due to contact with ambient air. 
         [0047]    With reference to  FIG. 6  and with continuing reference to  FIG. 5 , next a conductor  36 , such as a conductive metal, is deposited atop the portion of protective film  26  overlaying top surface  30  and in each aperture  34  such that said conductor  36  contacts thin oxide layer  35  or, when thin oxide layer  35  is not present, the portion of the top surface  30  exposed in each aperture  34 . Desirably, conductor  36  is deposited via sputtering or any other suitable vacuum deposition technique such as thermal evaporation or similar. 
         [0048]    With reference to  FIG. 7  and with continuing reference to  FIG. 6 , lastly, protective film  26 , and any portion of conductor  36  thereon, is removed to form detector  1 ′ where each conductor  36  on thin oxide layer  35  or surface  30  defines a corresponding segmented electrode  8 . Each segmented electrode  8  and/or continuous electrode  4  can be made of metal, metallic alloy or any suitable electrically conductive material or alloy. Each segmented electrode  8  and/or continuous electrode  4  can be a single conductor or a multi-layer stack of conductors. 
         [0049]    Detector  1 ′ shown in  FIG. 7  includes continuous electrode  4  on surface  32 , segmented electrodes  8  on surface  30  (or thin oxide layer  35 ), film  22  and/or film  24  on surface  30  acting as a passivation layer between segmented electrodes  8 , and film  22  and/or film  24  on side surface  28  of substrate  2 , also acting as a passivation layer. 
         [0050]    In an alternative configuration of detector  1 ′, a side electrode  40  (shown in phantom in  FIG. 7 ) can be deposited atop the passivation layer on side surface  28  of substrate  2  to ensure that such electrode is electrically insulated from substrate  2 . Side electrode  40  can be biased in any suitable manner relative to substrate  2  to adjust the electric field distribution in the volume of substrate  2  so that charge collection is optimized and optimum performance is achieved. The height and/or location of side electrode  40  on side surface  28  of substrate  2  can also be optimized to achieve the best possible detector performance. 
         [0051]    With reference to  FIG. 8  and with continuing reference to  FIGS. 2-7 , in an alternate configuration of detector  1 ′, thin oxide film  22  can be retained on substrate  2 . Thereafter, segmented electrodes  8  can be deposited atop the portion of film  22  overlaying top surface  30  of substrate  2  via apertures formed in film  24 , if present. Desirably, each segmented electrode  8  is formed by depositing conductor  36  in each aperture  34  in protective film  26  in the manner discussed above in connection with  FIG. 6 . Thereafter, protective film  26 , and any portion of conductor  36  thereon, is removed. The embodiment of detector  1 ′ with segment electrodes  8  deposited atop film  22  overlaying surface  30  of substrate  2  via apertures in film  24  is shown in  FIG. 8 . The embodiment of detector  1 ′ shown in  FIG. 8  can also or alternatively include continuous electrode  4  deposited atop of the portion of film  22  overlaying bottom surface  32 . 
         [0052]    Provided film  22  is not too thick (e.g., #250 Angstroms, desirably #100 Angstroms, and more desirably #25 Angstroms) electrical current can flow between substrate  2  and continuous electrode  4  and/or between substrate  2  and each segmented electrode  8  by way of so-called tunneling current. If desired, the embodiment of detector  1 ′ shown in  FIG. 8  can also include side detector  40  (shown in phantom) deposited atop the passivation layer on side surface  28  of substrate  2 . 
         [0053]    The present invention has been described with reference to the preferred embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. For example, while the present invention has been described with reference to segmented electrodes on top surface  30  of substrate  2 , it is envisioned that the foregoing technique can be adapted and modified as necessary in order to form segmented electrodes on top surface  30  and bottom surface  32  of substrate  2 . It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.