Abstract:
A PIN diode-based monolithic Nuclear Event Detector and method of manufacturing same for use in detecting a desired level of gamma radiation, in which a PIN diode is integrated with signal processing circuitry, for example CMOS circuitry, in a single thin-film Silicon On Insulator (SOI) chip. The PIN diode is implemented in either a p-, intrinsic, or n-substrate layer. The signal processing circuitry is located in a thin semiconductor layer and is in electrical communication with the PIN diode. The PIN diode may be integrated with the signal processing circuitry onto a single chip, or may be fabricated stand alone using SOI methods according to the method of the invention.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a Continuation In Part (CIP) application claiming the benefit of utility application Ser. No. 12/154,212, filed with the USPTO on May 21, 2008, which is herein incorporated by reference in its entirety. This application also claims the benefit of provisional patent application Ser. 60/939,118 filed with the USPTO on May 21, 2007, which is herein incorporated by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The present invention was developed under SBIR program # N00030-06-00031, “Development of Nuclear Event Detectors and Circumvention Controller Technology”. 
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK 
       [0003]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The present invention relates, in general, to nuclear event detectors (NEDs), and is particularly directed to a new and improved semiconductor architectecture for such a detector, wherein a sensitive PIN diode and operate-through integrated circuitry are combined onto a single chip using a silicon-on-insulator (SOI) process that is effective to place the signal processing circuitry portion of the NED chip in CMOS circuitry located in a thin silicon layer. The use of a thin-film device minimizes generation volume and hence maximizes hardness in the transient gamma environment. The PIN diode used for sensing the transient gamma radiation is built into the structure, which maximizes generation volume, and hence maximizes detector sensitivity. 
         [0006]    2. Background Art 
         [0007]    Nuclear event detectors (NEDs) may be employed in a variety of systems, such as military electronic systems, whose components are susceptible to damage from transient gamma radiation. An effective nuclear event detector must first detect the transient radiation generated by the nuclear event. At a pre-determined level of transient radiation, the NED is generally employed to generate appropriate signals to either circumvent or shut down critical circuitry that might otherwise be damaged or destroyed as a result of the transient radiation. 
         [0008]    Several types of NEDs have been developed and are thus known in the art. The most common topologies used for detecting the transient gamma radiation associated with a nuclear event utilize PIN diodes to detect the rising gamma radiation. PIN diodes are well known in the electrical arts and are often used as radiation detectors and photo detectors. A PIN diode is generally a diode with a wide, lightly doped ‘near’ intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor regions. Such diodes experience a detectable change in current under bias as the level of gamma radiation rises. This change in current is then characterized with respect to the desired gamma radiation threshold, and electrical circuitry is employed to measure said current and provide a desired output signal for use by the protected system. The protected system may take such action as to power down sensitive circuitry, or some other action, when a nuclear event has been detected. 
         [0009]    A commonly used approach to transient gamma survivability uses electrical circuitry consisting of discrete and integrated components. This approach uses a PIN diode as the primary radiation detector, along with discrete packaged components, such as transistors, integrated circuit amplifiers, transistors, and resistors, to implement the circuitry necessary to perform the detection and control functions. As proper timing of the system&#39;s response to the event is critical, this approach can be quite complex, slow in response, and difficult to repeat with great certainty as to signal timing due to component tolerances, thus limiting the capabilities of the NED to accurately detect and provide the desired response to the radiation produced by a nuclear event. Furthermore, as it is also desirable to use a plurality of nuclear event detectors distributed around the system to address non-uniformities in the transient gamma radiation caused by such phenomena as airframe shadowing, the variabilities in circuit path delay and component tolerances mentioned above pose a significant risk to NED performance and repeatability. 
         [0010]    A second approach to the nuclear event detector problem uses a hybrid or multichip module assembly. This results in a more compact solution, but the multichip assembly is complex, labor intensive to produce, and expensive. Such hybrid or multichip NEDs are generally housed within a metal or ceramic housing containing a ceramic substrate upon which the above mentioned electrical components are placed, with electrically conductive paths connecting said components contained within or upon the substrate itself. Such a hybrid may typically contain a radiation-sensing element such as a PIN diode and a signal processing and timing chip. One example of a hybrid microcircuit approach to packaging an NED is the Matra BAe Dynamics (UK) NMC6419 product, which is offered in a Dual In Line package. 
         [0011]    A more desirable approach would be to integrate the electronics associated with the event detection and circumvention control functions on one single integrated circuit contained on a single semiconductor chip, preferably using a high-performance analog process. As used herein, the term “semiconductor chip” means the multilayer semiconductor structure prior to encapsulation or packaging. This would enable moving nuclear event detection from an exotic application such as a discrete component NED, or a complex multichip module NED, to a much simpler single chip. 
         [0012]    However, in order to be successful, such an integrated single semiconductor chip NED must meet two conflicting requirements. The chip design must provide a circuit element that is highly sensitive to transient gamma radiation (such as the PIN diode used as a detector in the multichip module approach) while simultaneously providing analog and digital functions (the “signal processing circuitry”) that are insensitive to transient gamma radiation. 
         [0013]    With regard to said signal processing circuitry, it has been shown and is well known in the art that the use of thin-film SOI processing will provide “operate-through” capability (meaning that the circuitry continues to operate) at high transient gamma levels due to its very small generation volume. However, implementing the desired PIN diode detector in a thin SOI layer (i.e. creating a “monolithic detector”) is historically problematic due to the extremely small generation volume which is inherent in the SOI process: a higher generation volume is required in order for the PIN diode to operate effectively as a detector. Thus, such a monolithic detector would not likely provide a sufficiently strong signal at the transient gamma levels of interest to function as an NED. Unfortunately, the thin film layer of silicon inherent in the SOI process is simply too thin to produce an efficient PIN diode for NED purposes. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention provides an improved, simple, and cost effective NED in a monolithic device, and the method of the present invention overcomes the aforementioned obstacles in producing such a monolithic NED. In accordance with the present invention, the desire for a fully integrated, monolithic nuclear event detector, wherein the PIN diode and integrated circuit are contained within a monolithic semiconductor structure (i.e., “a semiconductor chip”) that contains such components as operational amplifiers and comparators are integrated into a common chip is successfully achieved by the use of a commercially available SOI process. Such a process provides a thin single-crystal layer on an insulating silicon dioxide layer, both of which are fabricated on a single-crystal ‘handle wafer’ substrate. 
         [0015]    In accordance with the invention, the signal processing circuitry portion of the NED semiconductor chip is implemented in CMOS circuitry located in the thin (single-crystal) silicon layer. The use of a thin-film device minimizes generation volume and maximizes hardness in the transient gamma environment. The PIN diode used for sensing transient gamma radiation is built into the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  diagrammatically illustrates the cross-section of a semiconductor structure, wherein a thin silicon layer is separated from an underlying substrate by an intervening layer of silicon dioxide; 
           [0017]      FIG. 2  is a diagrammatic cross-sectional view of an NED integrated circuit structure in the course of its manufacture according to the present invention in which a layer of silicon dioxide has been deposited on the top layer of active silicon; 
           [0018]      FIG. 3  is a diagrammatic cross-sectional view of an NED integrated circuit structure in the course of its manufacture according to the present invention in which photoresist has been applied, etching of the silicon dioxide layer has been performed, followed by selective etch of the active silicon layer; 
           [0019]      FIG. 4  is a diagrammatic cross-sectional view of an NED integrated circuit structure in the course of its manufacture according to the present invention in which additional silicon dioxide has been applied; 
           [0020]      FIG. 5  is a diagrammatic cross-sectional view of an NED integrated circuit structure in the course of its manufacture according to the present invention showing apertures etched into the silicon dioxide for use in forming p+, n+, and metalized contacts for the diode; 
           [0021]      FIG. 6  is a diagrammatic cross-sectional view of an NED integrated circuit structure in the course of its manufacture according to the present invention in which photoresist has been deposited, etched, and the p+ region has been formed; 
           [0022]      FIG. 7  is a diagrammatic cross-sectional view of an NED integrated circuit structure in the course of its manufacture according to the present invention in which photoresist has been deposited, etched, and the n+ region has been formed; 
           [0023]      FIG. 8  is a diagrammatic cross-sectional view of an NED integrated circuit structure in the course of its manufacture according to the present invention in which the photoresist material has been stripped from the structure; 
           [0024]      FIG. 8   a  is a diagrammatic cross-sectional view of an alternate embodiment of the NED integrated circuit structure, in which the p-substrate of  FIG. 8  has been replaced by an intrinsic substrate, and is shown in the course of its manufacture according to the present invention in which the photoresist material has been stripped from the structure; 
           [0025]      FIG. 8   b  is a diagrammatic cross-sectional view of an alternate embodiment of the NED integrated circuit structure, in which the p-substrate of  FIG. 8  has been replaced by an n-substrate, and is shown in the course of its manufacture according to the present invention in which the photoresist material has been stripped from the structure; 
           [0026]      FIG. 9  is a diagrammatic cross-sectional view of an NED integrated circuit structure in the course of its manufacture according to the present invention in which has undergone metallization in order to form metallic contacts to the PIN diode; 
           [0027]      FIG. 10  is a diagrammatic cross-sectional view of an NED integrated PIN diode circuit structure according to the present invention; 
           [0028]      FIG. 10   a  is a diagrammatic cross-sectional view of an alternate embodiment of the NED integrated PIN diode circuit structure according to the present invention in which the p-substrate of  FIG. 10  has been replaced by an intrinsic substrate; 
           [0029]      FIG. 10   b  is a diagrammatic cross-sectional view of an alternate embodiment of the NED integrated PIN diode circuit structure according to the present invention in which the p-substrate of  FIG. 8  has been replaced by an n-substrate substrate; and 
           [0030]      FIG. 11  is a plan view of an interdigitated pattern of the p+ and the n+ doping regions formed in the p-substrate of the PIN diode structure of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The basic SOI material employed to produce the improved NED in accordance with the present invention is shown in  FIG. 1  as comprising a thin active silicon layer  10  separated from an underlying semiconductor (silicon) support substrate  12 ,  80 , or  81  by a layer of silicon dioxide  14  therebetween. This basic SOI material may be produced by any of the well known methods for producing such material, such as, for example, the process well known as “Separation by IMplantation of OXygen” (SIMOX); the process well known as “bonded wafer” processing; or any of the other well known process for producing SOI material. The method of producing the basic SOI material is not a limitation of the present invention. All of the individual microelectronic devices that are used in the signal processing circuitry of the NED reside in the active layer of silicon  10  as shown in the accompanying Figures. Said signal processing circuitry is well known in the electrical arts for processing the signal current generated in the PIN diode in the presence of radiation, and may be comprised of circuit elements such as amplifiers, transistors, resistive elements, filters, capacitive elements, and other circuit elements. The techniques for fabricating said signal processing circuit elements within the active silicon layer, and the circuit topologies utilized in said signal processing circuits, are well known in the art. 
         [0032]    The PIN diode is integrated into the silicon support substrate  12  (p-),  80  (intrinsic), or  81  (n-) as described herein. 
         [0033]    While the structure disclosed herein discusses an integrated chip in which the PIN diode and signal processing circuitry are integrated in to one chip, the PIN diode of the present invention may be fabricated “stand alone” according to the method of the invention using the SOI process. 
         [0034]    In general, the substrate used in SOI processes is electronically inactive and not electronically connected. However, the substrate may be connected to supply voltage to de-bias the bottom of the active Si layer. Depending on wafer size and final wafer thickness, the SOI substrate may be on the order of, for example, 200-400 microns thick. This is a suitable thickness for a PIN diode. As an example, an article by Marczewski, J. et al, entitled “SOI Active Pixel Detectors of Ionizing Radiation-Technology and Design Development”, IEEE Trans. Nuc. Sci., vol. 51, no. 3, pp 1025-1028 (June 2004), describes a 300 um thick detector of float zone (FZ) silicon material, with a resistivity of up to 4000 ohm-cm. The detector is used for particle detection in high-energy physics applications. Clearly, a similar high-resistivity material can be used as an SOI substrate. Given that the substrate is p-type (as in Honeywell&#39;s 0.8 micron SOI process), or intrinsic or n-type, all that remains is to create the diode structure, consisting of the PIN structure, a suitable p+ contact region to the high resistivity p-type substrate material, and an n+ contact that also forms the n-side of the PIN diode. The substrate may be intrinsic as depicted in  FIG. 10   a  or may be n-type as depicted in  FIG. 10   b ; again, all that remains is to create the diode structure, consisting of the PIN structure, a suitable p+ contact region to the high resistivity p-type substrate material, and an n+ contact that also forms the n-side of the PIN diode. 
         [0035]    The process employed to form the NED in accordance with the present invention may be understood with the reference to  FIGS. 1 through 11 . Here, both the n+ and the p+ contact regions are on the top surface of the wafer. This is an excellent fit with current manufacturing methods, as opposed to backside processing operations which have been shown to be detrimental to product yield. Using, for example, a process such as the Metal Topside Contact process option in the Honeywell 0.8 micron SOI process will allow the fabrication of a substrate PIN detector in the substrate without any process modifications. The use of a metal topside contact etch combines a standard reverse field etch, a topside contact etch, and a standard contact etch to allow the first metal to make contact through the buried oxide directly to the silicon substrate. 
         [0036]    Referring now to  FIG. 1 , the SOI starting material comprises a p-type substrate  12  having a resistive property, or alternatively an intrinsic substrate  80  or an n-substrate  81 , a top surface, and a bottom surface, a buried oxide (“BOX”) layer  14  having a top surface and a bottom surface in which said bottom surface of said BOX layer  14  is in contact with said top surface of said p-type substrate  12 , and an active silicon layer  10  having a top surface and a bottom surface in which said bottom layer of said active silicon layer  10  is in contact with said top surface of said BOX layer  14 . 
         [0037]    Referring now to  FIG. 2 , a layer of silicon dioxide  16  having a top surface and a bottom surface is grown onto said top surface of said active silicon layer  10  by a first oxidation step such that said bottom surface of silicon dioxide layer  16  is in contact with said top surface of said active silicon layer  10 . 
         [0038]    A first lithography step, comprising applying photoresist then etching, occurs after the first oxidation step. A first application of photoresist is performed in which photoresist is deposited upon said top surface of said active silicon layer resulting in a first photoresist layer, and said first photoresist layer is then patterned in a desired pattern using techniques well known in the art to define anode and cathode regions of the substrate PIN diode. Said desired pattern may be shaped in any footprint that fits upon the area of the monolithic device such as, for example, serpentine or interdigitated, but is preferably interdigitated as shown in  FIG. 11 . Following the first application of photoresist, two selective etches are performed. A first etch is selective of the oxide layer  16 ; a second etch is a selective etch of the silicon active layer  10 . Following said second etch step, said photoresist is then stripped using techniques well known in the art. The resulting structure is shown in  FIG. 3 . 
         [0039]    A second oxidation step is next performed to electrically isolate said silicon active layer  10  by means of a grown oxide layer  18 , resulting in the structure shown in  FIG. 4 . 
         [0040]    To define the actual contact regions, a second lithography step is now performed. A second application of photoresist performed in which photoresist is deposited onto the top surface of said structure forming a second photoresist layer and said second photoresist layer is patterned to define locations of the anode and cathode of the substrate implanted PIN diode, utilizing said desired pattern as utilized in the first lithography step. This is followed by a third etch step, this time of the oxide layer  14 , to open a first implant aperture  22  for subsequent p+ anode implant and a second implant aperture  24  for subsequent n+ cathode region implant. Said second photoresist layer is then stripped. The resulting structure is shown in  FIG. 5 . 
         [0041]    Referring now to  FIG. 6 , the p+ anode is next created. To create the p+ anode contact to the PIN diode, a third lithography step is performed in which a third application of photoresist is performed in which photoresist is deposited on the upper surface of the structure, creating a third photoresist layer  34  as shown in  FIG. 6 . Said third photoresist layer  34  is patterned as shown in  FIG. 6  leaving the anode aperture  22  exposed. A fourth etch step is then performed to open an anode implant aperture  32  in photoresist layer  34  for the p+ implant, resulting in the structure shown in  FIG. 6 . A p+ implant is then performed through the aperture  32  using techniques well known in the art, thus creating a p+ anode region  42  in a first portion of the top surface of the p-substrate  12 , or alternatively intrinsic substrate  80  or n-substrate  81 , as shown in  FIG. 6 . 
         [0042]    Following the p+ region implant, said third photoresist layer  34  is stripped using techniques well known in the art, and a fourth application of photoresist is performed in which photoresist is deposited on the upper surface of the structure, creating fourth photoresist layer  44 . Said fourth photoresist layer  44  is then patterned as shown in  FIG. 7  leaving said cathode aperture exposed to define a cathode aperture  46  for implanting an n+ cathode. An n+ implant is then preformed through said aperture  46  using techniques well known in the art, realizing an n+cathode region  48  in a second portion of the top surface of the p-substrate  12 , or, alternatively the top surface of intrinsic substrate  80  or n-substrate  81 . Said fourth photoresist layer  44  is next stripped, again using techniques well known in the art. The resulting structure is shown in  FIG. 8 . The resulting structure shown in  FIG. 8  has an upper surface  60  and anode aperture  32  and cathode aperture  46 . 
         [0043]    The remainder of the method deals with metallization and passivation. A metal layer  75  having a top surface and a bottom surface is non-selectively deposited onto said upper surface  60  and into said anode aperture  32  and cathode aperture  46 . A fifth application of photoresist is performed in which photoresist layer  70  is deposited onto said top surface of metal layer  75  forming a fifth photoresist layer. Said fifth photoresist layer is then patterned to define metallic contacts to the anode and cathode regions as shown in cross section in  FIG. 9 . The metal not covered by the said fifth photoresist layer is unwanted metal: said unwanted metal is then etched in a fifth etch step, leaving a first metallic anode contact  66  which is in electrical communication with anode region  62 , and a second metallic cathode contact  68  which is in electrical communication with cathode region  64 , as shown in  FIGS. 10 ,  10   a , and  10   b . Said fifth photoresist layer  70  is then stripped using techniques well known in the art. 
         [0044]    As a final optional step, the resulting integrated PIN diode and signal processing chip may be passivated as is currently done as standard practice in the semiconductor integrated circuit art. 
         [0045]    The resulting substrate diode is a lateral device, and the depletion layer will spread horizontally from the p/n+ junction  56  through the p-substrate  12  as shown in  FIGS. 9 and 10  (or, alternatively, through the intrinsic substrate  65  shown in  FIG. 10   a  or through the n-substrate  66  shown in  FIG. 10   b ). To create the substrate diode, no process modifications to any standard SOI process are necessary. It will be understood that the dimensions of said desired pattern used to create said PIN diode as described herein are a function primarily of said resistive property of said silicon substrate  12  (or in the alternative embodiments, substrate  80  or substrate  81 ), and it is well within the understanding of a person of average skill in the art to determine the dimensions of said desired pattern without undue experimentation. 
         [0046]    Three layout designs (topologies) for the silicon substrate PIN diode have been developed. Each of the three designs utilizes the standard source/drain implant process used in the SOI silicon active layer, combined with a silicon dioxide etch used to make contact with the substrate. 
         [0047]    Referring now to  FIG. 11 , the interdigitated source region  62  and drain region  64  are then formed by a silicon dioxide etch and source implant and drain implant. 
         [0048]    Referring to  FIG. 10 , the p+ and n+, regions  62  and  64  and the p-substrate  12 , respectively, make up the substrate PIN diode structure. Referring to  FIG. 10   a , the p+and n+regions  62  and  64  and the intrinsic substrate  80 , respectively, make up the substrate PIN diode structure. Referring to  FIG. 10   b , the p+ and n+ regions  62  and  64  and the n-substrate  81 , respectively, make up the substrate PIN diode structure. To create increased carrier generation volume, the source and drain regions  62  and  64  may have and interdigitated topology, as shown in the plan view of  FIG. 11 . 
         [0049]    If a high resistivity substrate silicon material is available the same techniques used to create the structure shown in  FIG. 10  may be used. However, the resulting PIN diode would produce higher currents, due to a greater collection volume, as a result of the high resistivity substrate material. 
         [0050]    Where bonded wafer fabrication technology is employed, the starting material of the handle wafer may be processed prior to being oxidized and bonded. This allows more elaborate doping profiles in the substrate material, eliminating the need for serpentine topology shown in  FIG. 11  and thus reducing required chip size. 
         [0051]    While a specific embodiment of the semiconductor structure and method of fabrication are disclosed herein, it will be understood that there exist equivalent embodiments of the structure, and equivalent steps of the method, and that such equivalents are within the intended scope of the present invention.