Abstract:
An array of radiation sensors or detectors is integrated within a three-dimensional semiconductor IC. The sensor array is located relatively close to the device layer of a circuit (e.g., a microprocessor) to be protected from the adverse effects of the ionizing radiation particles. As such, the location where the radiation particles intersect the device layer can be calculated with coarse precision (e.g., to within 10 s of microns).

Description:
[0001]    Applicant claims the benefit of Provisional Application Ser. No. 61/159,830, Array of Alpha Particle Sensors, filed on Mar. 13, 2009. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to semiconductor devices, and, more particularly, to the sensing or detection of ionizing radiation in semiconductor devices. 
         [0003]    Ionizing radiation can cause single event upsets (“SEUs”) or “soft errors” in semiconductor integrated circuits (“ICs”). In general, there exist multiple radiation sources that affect the proper operation of ICs. These may comprise, for example, alpha particles from packaging materials (e.g., lead or lead-free solder bumps) or impurities introduced in wafer processing. Other radiation sources include daughter particles from terrestrial cosmic ray neutron collisions, energetic heavy ions in a space environment, and daughter particles from proton collisions in a space environment (e.g., trapped proton belts). 
         [0004]    Ionizing radiation can directly upset storage circuits, such as SRAMs, register files and flip-flops. Undesirable memory cell bit state flips or transitions between binary logic states have occurred for years, and error checking and correction (“ECC”) techniques are typically used to correct for any radiation-induced errors in the memory cells. More recently, as combinational logic has scaled down in size, radiation events create voltage glitches that can be latched by such logic circuits. In general, SEUs may cause the IC to perform incorrect or illegal operations. 
         [0005]    Methods to prevent SEUs include adding spatial and/or temporal redundancy within the semiconductor device, so that a single radiation event cannot cause an SEU therein. However, redundancy solutions incur area, power and performance penalties. It is known to use relatively small detectors or sensors as part of the IC to detect radiation that is relatively uniform. However, the radiation environment for commercial and space applications typically comprises discrete events, localized in time and space, rather than a uniform dose of radiation spread across the IC. Thus, the known devices cannot detect the more common radiation environment consisting of individual events. 
       BRIEF SUMMARY 
       [0006]    According to an embodiment of the invention, an array of radiation sensors or detectors is integrated within a three-dimensional semiconductor IC. The sensor array is located relatively close to the device layer of a circuit (e.g., a microprocessor) to be protected from the adverse effects of the ionizing radiation particles. As such, the location where the radiation particles intersect the device layer can be calculated with coarse precision (e.g., to within 10 s of microns). Various embodiments of the invention teach the sensing and localization of individual radiation events. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
           [0008]      FIG. 1  is a schematic diagram of a radiation sensor circuit utilized in various embodiments of the invention; 
           [0009]      FIG. 2  illustrates a layout of the radiation sensor circuit of  FIG. 1 ; 
           [0010]      FIG. 3  illustrates a layout of an arrangement of four of the radiation sensor circuits of  FIG. 1 ; 
           [0011]      FIG. 4  illustrates in cross-section a three-dimensional IC in which an array of a plurality of the radiation sensor cells of  FIGS. 1-3  are arranged together in a layer disposed on top of a circuit layer according to an embodiment of the invention; 
           [0012]      FIG. 5  illustrates the steps involved in fabricating the IC of  FIG. 4 : and 
           [0013]      FIG. 6  illustrates in cross-section two alternative embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring to  FIG. 1 , there illustrated is a schematic diagram of a radiation sensor circuit or cell  100  utilized in various embodiments of the invention. The radiation sensor cell  100  includes three transistors: a pre-charge NFET  102 , a detector NFET  104 , and an access NFET  106 . The pre-charge NFET  102  functions as a reset device, the detector NFET  104  functions as a detector pull-down device, and the access NFET  106  functions as an access device. An internal charge collection node  108  connects to all of the devices  102 - 106 , as discussed hereinafter in more detail. 
         [0015]    More specifically, the pre-charge NFET  102  has a source connected to the charge collection node  108 , a drain connected to Vdd, and a gate controlled by a pre-charge signal (“Reset”) on a line  110 . The detector NFET  104  has a drain connected to the charge collection node  108 , a source connected to ground, and a grounded gate. The access NFET  106  has a source connected to the charge collection node  108 , a drain connected to a readout node  112  (i.e., the bitline), and a gate controlled by a Read Enable signal on a line  114 . 
         [0016]    Asserting the Reset signal  110  charges the internal charge collection node  108  through the pre-charge device  102 , which, when turned on by the Reset signal  110 , charges the node  108 . A radiation event on the detector pull-down device  106  discharges the node  108 ; that is, a radiation event will turn on the detector pull-down device  106 , which, in turn, discharges the charge collection node  108  to ground. When the Read Enable signal  114  is asserted, the access device  106  writes the state of the internal charge collection node  108  to the bitline  112 , where a logic high indicates that no radiation event has occurred and a logic low indicates the occurrence of a radiation event. As such, when there is no radiation event, the charge collection node  108  remains charged by the pre-charge device  102 , and the output of the node  108  is high. When a radiation event is sensed, the node  108  will discharge to ground and the output of the node  108  is low. 
         [0017]    To increase the sensitive area in each sensor cell  100 , multiple detector pull-down NFET devices  104  may be connected in parallel, since the occurrence of a radiation event on any one of the detector pull-down devices  104  will discharge the node  108 . The sensitive area of the radiation sensor cell  100  comprises a relatively larger portion of the sensor cell area as compared to an SRAM cell, such that the radiation sensor cell  100  will have relatively larger detection efficiency. The radiation sensor cell  100  does not enter a high current state when it detects radiation and will use less power than a PNPN structure. 
         [0018]    Referring to  FIG. 2 , there illustrated is an exemplary layout of the radiation sensor cell  100  of  FIG. 1 . To increase the sensitive area in each cell  100 , multiple detector pull-down devices  104  may be connected in parallel as shown in  FIG. 2 , since the occurrence of a radiation event on any one of the detector pull-down devices  104  will discharge the node  108 . To increase the fraction of the sensor cell  100  sensitive to radiation, the pre-charge device  102  and the access device  106  may have a smaller gate width than the detector pull-down device(s)  104 . 
         [0019]    In particular, the layout of the radiation sensor cell  100  illustrated in  FIG. 2  shows six detector pull-down devices  104  connected in parallel. In  FIG. 2 , the radiation sensor cell  100  may be fabricated in a silicon-on-insulator (“SOI”) wafer. The wafer may have a relatively thick SOI layer (250 nm) to increase the charge collection volume. The source and drain implants may not extend to the buried oxide (“BOX”) layer, such that holes generated in the source/drain diffusions are collected in the body, thereby charging the body and causing a parasitic bipolar response. 
         [0020]    The radiation detector shown in  FIG. 2  comprises the three transistors  102 - 106 . The reset signal  110  is used to charge the internal charge collection node(s)  108 , which are connected together via a metal wire (not shown). A radiation event on the detector pull-down device  104  discharges the charge collection node(s)  108 . When the Read Enable signal  114  is asserted, the access device  106  writes the state of the charge collection node  108  to the bitline (“BL”)  112 , where a logic high indicates that no radiation event has occurred and a logic low indicates the occurrence of a radiation event. Due to the parallel arrangement of the multiple pull-down devices  104  in  FIG. 2 , an SEU on any one of the pull-down devices  104  will discharge the charge collection node  108  to ground. 
         [0021]    For SOI devices with a relatively thick SOI layer and source and drain implants that do not extend to the BOX layer, the body, source and drain serve as collection areas. SOI circuits can be designed with Qcrit of approximately 1 fC. Depending on their energy, alpha particles generate 4-13 fC/um in silicon, such that the alpha particle path length through the SOI layer needs to be about 0.08-0.25 um. Having an SOI layer thickness of 0.25 um allows detection of alpha particles of any energy and angle that hits the sensitive region of the sensor cell  100 . 
         [0022]    As an alternate embodiment, the pre-charge device  102  may comprise a PMOS transistor. Using a PMOS transistor charges the internal charge collection node  108  to Vdd, without the diode drop of an NMOS pre-charge device. However, a PMOS pre-charge transistor  102  requires more area than an NMOS pre-charge transistor when placed in its own silicon island. The area penalty of a PMOS pre-charge transistor can be minimized by using butted junctions and mirroring sensors. 
         [0023]    Referring to  FIG. 3 , there illustrated is a layout of four of the radiation sensor cells  100  of  FIG. 1 . Each sensor cell  100  includes six detector pull-down devices in parallel. The PMOS pre-charge devices are in a shared Nwell with their drains abutting drains of NMOS pre-charge devices. 
         [0024]    Referring to  FIG. 4 , there illustrated in cross-section is a three-dimensional IC  400  in which an embodiment of the invention is implemented. Specifically, an array of a plurality of the radiation sensor cells  100  of  FIGS. 1-3  are arranged together in a layer  402 , which may be a SOI layer. The layer  402  may be disposed or located on top of a circuit layer  404 . The circuit layer  404  may comprise, for example, a microprocessor or SRAM memory or other logic device in which ionizing radiation normally causes an SEU to occur. The two layers  402 - 404  are relatively closely spaced (for example, within 15 microns of each other) such that the radiation sensor layer  402  will detect, e.g., alpha particles and cosmic ray daughter particles, which will hit the IC layer  404  in a localized spot or location on that layer  404 . 
         [0025]    The IC  400  of  FIG. 4  may be fabricated or constructed using a three-dimensional process to incorporate the radiation sensor array layer  402  together with the circuit layer  404  into the final IC  400 . Referring to  FIG. 5 , using a layer transfer method, the radiation sensor array  402  may be processed on a first wafer  500 , which may comprise a SOI wafer. The first wafer  500  may then be attached to a glass handler wafer or substrate  502 . Then the substrate  505  of the IC  400  is removed by etching, exposing or stopping at the buried oxide layer (“BOX”). The radiation sensor array  402  device layer  500  may then be aligned and bonded to the microprocessor circuit layer  404 , which has been processed in a second wafer  504 . Finally, the glass handler wafer  502  and adhesives are removed, and vertical interconnects can be made to the microprocessor  404 . 
         [0026]    Since the radiation sensor array  402  may be within 15 um of the microprocessor device layer  404 , the radiation sensor cells  100  in the array  402  can detect alpha particles from packaging materials (e.g. solder bumps and underfill) and daughter particles from cosmic ray neutron collisions that will hit the microprocessor layer  404  and can then convey this information to other components for further processing. The location of the radiation event in the microprocessor device layer  404  can be located with coarse granularity. For example, if the typical alpha particle range is 25 um and the radiation sensor array  402  is located 10 microns above the microprocessor device layer  404 , the radiation event will be within 28 um of the triggered radiation sensor cell  100 . In an embodiment of the invention, the radiation sensor array  402  may be located in a range of 10-15 microns from the microprocessor device layer  404 . 
         [0027]    Referring to  FIG. 6 , in an alternative embodiment of the invention, the radiation sensor array  600  is located below the microprocessor device layer  602 . The microprocessor  602  connects to the package through high resistance, lower density inter-device-layer vias. Placing the microprocessor  602  above the radiation sensor layer  600  allow greater contact density. Some alpha particles from packaging materials that cross the microprocessor device layer may not reach the radiation sensor array, so the detection efficiency would be reduced. 
         [0028]    Still referring to  FIG. 6 , in another alternative embodiment of the invention, three device layers  600 - 604  are stacked. A radiation sensor array  604  comprises the top layer; the microprocessor  602  comprises the middle layer; and a second radiation sensor array  600  comprises the third layer, located, e.g., 10-15 microns from the middle layer  602 . This embodiment of the invention is appropriate for high-energy ions in a space environment, which have sufficient range to cross both radiation sensor arrays  600 ,  604 . The location of the radiation event in the microprocessor device layer  602  can be calculated from the intersection points in the two radiation sensor arrays  600 ,  604  above and below it. 
         [0029]    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. 
         [0030]    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.