Abstract:
A semiconductor device includes a semiconductor substrate; a buried insulator layer disposed on the semiconductor substrate, the buried insulator layer configured to retain an amount of charge in a plurality of charge traps in response to a radiation exposure by the semiconductor device; a semiconductor layer disposed on the buried insulating layer; a second insulator layer disposed on the semiconductor layer; a gate conducting layer disposed on the second insulator layer; and one or more side contacts electrically connected to the semiconductor layer. A method for radiation monitoring, the method includes applying a backgate voltage to a radiation monitor, the radiation monitor comprising a field effect transistor (FET); exposing the radiation monitor to radiation; determining a change in a threshold voltage of the radiation monitor; and determining an amount of radiation exposure based on the change in threshold voltage.

Description:
FIELD 
       [0001]    This disclosure relates generally to the field of radiation monitoring and dosimetry. 
       DESCRIPTION OF RELATED ART 
       [0002]    Radiation may come in various forms, including such as x-rays, γ-rays, or β-rays. There are various types of radiation monitors that may be used to determine an amount of radiation exposure, such as ionization detectors, Geiger counters, and thermoluminescent detectors (TLDs). Geiger counters and ionization detectors may determine and display a dose rate (for example, in mRad/hr) or an integrated dose (for example, in Rads) of radiation exposure in real time. Alarm set points may be programmed based on the dose rate or the integrated dose. A Geiger counter or ionization detector may communicate with a computer for data logging or firmware updates. However, Geiger counters and ionization detectors may be relatively expensive. TLDs allow determination of a dose of radiation based on emission of photons in response to application of heat. TLDs may be relatively inexpensive, but may only be read after a period of exposure time, typically between one and three months. A degree of radiation exposure may only be determined after-the-fact using a TLD; real time dose information is not available. 
         [0003]    A semiconductor radiation monitoring device may comprise a p-channel metal-oxide-semiconductor field effect transistor (MOSFET) transistor structure having a gate oxide layer fabricated on bulk silicon. Holes may be induced in the FET structure by ionizing radiation exposure and trapped in the gate oxide by a voltage applied to the gate, and the threshold voltage (V th ) of the transistor may change according to the amount of trapped holes. However, in order to measure the change in V th , a negative voltage is applied to the gate, which may trigger the release of the holes trapped in the gate oxide via direct or trap-assisted tunneling. Therefore, electrical readout of the V th  shift to determine the radiation dose in such a FET-type dosimeter may cause a loss of the radiation-induced charge, leading to incorrect long-term total dose data. Further, the trapping voltage and the readout voltage are both applied at the gate, they may not be applied at the same time, so real-time information regarding a dose of radiation may not be obtained. 
       SUMMARY 
       [0004]    In one aspect, a semiconductor device includes a semiconductor substrate; a buried insulator layer disposed on the semiconductor substrate, the buried insulator layer configured to retain an amount of charge in a plurality of charge traps in response to a radiation exposure by the semiconductor device; a semiconductor layer disposed on the buried insulating layer; a second insulator layer disposed on the semiconductor layer; a gate conducting layer disposed on the second insulator layer; and one or more side contacts electrically connected to the semiconductor layer. 
         [0005]    In one aspect, a method for radiation monitoring, the method includes applying a backgate voltage to a radiation monitor, the radiation monitor comprising a field effect transistor (FET); exposing the radiation monitor to radiation; determining a change in a threshold voltage of the radiation monitor; and determining an amount of radiation exposure based on the change in threshold voltage. 
         [0006]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0008]      FIG. 1  illustrates a cross section of an embodiment of a FET radiation monitor. 
           [0009]      FIG. 2  illustrates a top view of an embodiment of a radiation monitor configured as a ring FET. 
           [0010]      FIG. 3  illustrates a cross section of an embodiment of a FET radiation monitor. 
           [0011]      FIG. 4  illustrates a method for radiation monitoring using an FET radiation monitor. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Embodiments of systems and methods for a FET radiation monitor are provided, with exemplary embodiments being discussed below in detail. A FET radiation monitor may be fabricated using a fully depleted silicon-on-insulator (FDSOI) MOSFET that is capable of detecting doses of various types of ionizing radiation, and that exhibits long-term charge retention that enables long-term tracking of total radiation dosage. The FET radiation monitor may be made as small or large as desired using semiconductor wafer fabrication technology, and may have a relatively low power drain. 
         [0013]    A FET radiation monitor may be relatively small and inexpensive, and may be embedded in automobiles, buildings, air filters, or portable electronic devices such as computers, cell phones, music players, PDAs, or GPS, or in other items, including but not limited to passports, credit cards, or drivers licenses. The device may be in communication with a radiofrequency (RF) tag that may communicate a radiation dosage experienced by the FET radiation monitor to an RF tag reader. Integrated radiation dose information may be determined from the radiation monitor so that treatment decisions may be made quickly in an emergency situation. A radiation monitor may also be implanted into the body of a patient undergoing radiation therapy, in order to determine radiation dosage to a tumor, or an amount of radiation received during medical imaging. Real-time radiation dose information may be gathered from the implanted radiation monitor to confirm that a proper dose of radiation is delivered to a patient. The FET radiation monitor may be used in conjunction with a relatively small battery or precharged capacitor. The FET radiation monitor may also be electrically connected with one or more inductors, and be used in conjunction with an LC circuit such as is described in U.S. application Ser. No. 12/627,076 (Cabral et al.), filed Nov. 30, 2009, which is herein incorporated by reference in its entirety. 
         [0014]      FIG. 1  illustrates an embodiment of a cross section  100  of a radiation monitor. The radiation monitor comprises gate conductor layer  101 , insulator layer  102 , semiconductor layer  103 , buried insulator layer  104 , semiconductor substrate  105 , source  106 , drain  107 , side contacts  108   a - b , spacer material  109 , and back contact  110 . Gate conductor layer  101  may comprise one of polysilicon, a metal, or a silicide in some embodiments. Insulator layer  102  may comprise an oxide, such as silicon oxide or silicon oxide nitride, in some embodiments; the dielectric constant of the insulator layer  102  may be below about 4. Semiconductor layer  103  may comprise a silicon-on-insulator (SOI) layer in some embodiments, and may comprise undoped silicon, which may form fully-depleted SOI (FDSOI) under voltages used for normal operation of the FET radiation monitor. Buried insulator layer  104  may comprise buried silicon oxide (BOX), silicon oxide, or silicon nitride in some embodiments. Semiconductor substrate  105  may comprise one of silicon, germanium, silicon germanium, or gallium arsenide in some embodiments. Side contacts  108  may comprise implanted silicon, a metal, or a silicide in some embodiments. Spacer material  109  may comprise silicon nitride in some embodiments. Back contact  110  is electrically connected to semiconductor substrate  105 , and side contacts  108   a  and  108   b  are electrically connected to source  106  and drain  107 , respectively. The FET radiation monitor may comprise either an n-channel or a p-channel FET device. 
         [0015]    Insulator layer  102  may be between about 5 angstroms and about 50 angstroms thick in some embodiments. Semiconductor layer  103  may be less than about 40 nanometers (nm) thick in some embodiments. A relatively thin semiconductor layer  103  may facilitate effective detection of the radiation dosage through monitoring the change of threshold voltage of the FDSOI FET comprising gate conductor layer  101 , insulator layer  102 , and semiconductor layer  103 . The desired thickness of semiconductor layer  103  may be achieved by thermal oxidation of the surface of a SOI wafer. The oxidized silicon layer may then be removed, leaving the relatively thin layer of SOI. Standard semiconductor fabrication processes may be otherwise employed to fabricate the FET radiation monitor. Buried insulator layer  104  may be between about 500 and 2000 angstroms thick in some embodiments, and between about 1400 and 1600 angstroms thick in some exemplary embodiments. Semiconductor substrate  105  may be less than 800 microns thick in some embodiments. 
         [0016]      FIG. 2  illustrates an embodiment of a top view  200  of an embodiment of a radiation monitor configured as a ring FET.  FIG. 2  is discussed with reference to  FIG. 1 . The radiation monitor comprises source  106 , which is separated from gate conductor  101  by spacer material  109 . Gate conductor  101  is separated from drain  107  by spacer material  109 . Side contacts  108   a  and  108   b  are deposited on source and drain regions  106  and  107 , respectively. The ring FET radiation monitor configuration of  FIG. 2  is shown for illustrative purposes only; a radiation monitor may comprise any appropriate configuration of FET having a source, drain, and gate. 
         [0017]      FIG. 3  illustrates an embodiment of a cross section  300  of a FET radiation monitor after exposure to ionizing radiation  301 . Radiation  301  may include but is not limited to high-energy ionizing radiation, proton beam, X-ray, photons, gamma ray, or neutron beam radiation. Radiation  301  causes electron-hole pairs to be created in semiconductor layer  102  and buried insulator layer  104 , which causes a positive charge  302  to build up and be retained in buried insulator layer  104 . The amount of positive charge  302  is indicative of the amount of radiation to which the radiation monitor has been exposed. Positive charge  302  leads to device degradation and a change in the FET radiation monitor&#39;s threshold voltage (V th ). The dose of radiation may be determined based on the change in V th . Tracking the change in V th  allows measurement of the total dose of radiation. 
         [0018]    To prevent recombination of the electron-hole pairs created by radiation exposure  301 , a backgate voltage (V bg ) is applied to semiconductor substrate  105  at back contact  110  to form a positive bias across the insulating layer  102  and buried insulator layer  104 , so that electrons drift toward gate conductor  101  and semiconductor substrate  105  while holes move toward the interface between semiconductor layer  103  and buried insulator layer  104 , forming positive charge  302 . The buried insulator layer  104  may comprise a plurality of charge traps in which charge  302  is trapped; the number of charge traps per cm 3  of buried insulator material may be between about 1E17 and 1E18 in some embodiments. Positive charge  302  is thereby trapped in the buried insulator layer  104  by application of V bg , and the trapped charge  302  causes the change in V th . V bg  is applied at the semiconductor substrate  105  during radiation exposure to keep the holes that are created by the radiation  301  trapped in buried insulator layer  104 . This decouples the gate  101  of the FET radiation monitor from the trapped charge  302 . Determination of V th  may be performed by applying a readout voltage to gate conductor  101 . The readout voltage may comprise a negative bias across buried insulator layer  104  and semiconductor layer  102 . Application of the readout voltage at gate conductor  101  allows non-invasive real-time determination of V th , as the readout voltage does not interfere with positive charge  302 . The change in V th  may also be measured by wiring the FET radiation monitor in a capacitor configuration, and measuring the change in capacitance between the gate conductor  101  and one of side contacts  108   a  or  108   b  at a zero gate voltage or at a constant gate voltage. 
         [0019]    Because buried insulator layer  104  is electrically insulated from the ambient surroundings and from the electrodes (i.e., gate conductor  101 , source  106 , and drain  107 ) of the FET radiation monitor, the FET radiation monitor may exhibit good retention of charge  302  over time, with negligible degradation. For example, degradation of the radiation-induced V th  shift may be less than about 10% over a period of about 25 days in a PFET radiation monitor, and over a period of about 15 days in an NFET radiation monitor. Charge retention allows use of the radiation monitor for tracking of long-term cumulative radiation treatments. 
         [0020]      FIG. 4  illustrates an embodiment of a method  400  for radiation monitoring using a FET radiation monitor.  FIG. 4  is discussed with reference to  FIG. 3 . In block  401 , a backgate voltage (V bg ) comprising a positive bias is applied to a silicon substrate  105  of a FET radiation monitor. In block  402 , the FET radiation monitor is exposed to radiation. V bg  continues to be applied to the silicon substrate during the radiation exposure. In block  403 , a threshold voltage (V th ) of the FET radiation monitor changes based on the amount of radiation to which the FET radiation monitor has been exposed, due to positive charge  302  built up in buried insulator layer  104 . In block  404 , a readout voltage is applied to a gate conductor  101  of the FET radiation monitor in order to determine the change in V th . V bg  may continue to be applied to the silicon substrate during readout in some embodiments. In block  405 , the amount of radiation exposure is determined based on the change in V th . 
         [0021]    A single semiconductor substrate  105  may hold a plurality of FET radiation monitors, each FET radiation monitor comprising a separate gate conductor layer, insulator layer, semiconductor layer, buried insulator layer, source, drain, side contacts, spacer material, and back contact. A plurality of FET radiation monitors may also be arranged in an array, including but not limited a linear array, a 2-dimensional array, or a 3-dimensional array, in order to detect radiation doses in different areas and from different directions. The different directions may be orthogonal. In some embodiments, a filter may be disposed between a FET radiation monitor and the source of the radiation  301  to prevent some of radiation  301  from passing through the device, or to make the device more or less sensitive to the type of incident radiation. Another type of device may also be incorporated into the semiconductor substrate  105 , including but not limited to a memory cell, a clock, a microprocessor, a DNA sensor, a biological sensor, a hazardous material sensor, a glucose sensor, a red blood cell sensor, or a camera. 
         [0022]    The technical effects and benefits of exemplary embodiments include a relatively small, inexpensive radiation monitor that may be used to determine long-term or real-time radiation dosage information. 
         [0023]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0024]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.