Abstract:
An integrated circuit having improved soft error protection and a method improving the soft error protection of an integrated circuit are disclosed. The integrated circuit comprises a substrate  72 , a transistor formed in the substrate  72 , a first region  74  (e.g. a well) formed in the substrate having a first conductivity type, a second region  84  below the first region  74  having a second conductivity type, and a trench formed in the substrate having a depth at least substantially as deep as the well. The trench  70  is filled with a non-conductive material  71  that forms a frame around the transistor, whereby soft errors due to electron-hole pairs caused by ionizing radiation in the frame are substantially eliminated.

Description:
This application is a division of Ser. No. 09/353,478, filed Jul. 13, 1999, now U.S. Pat. No. 6,486,525, which claims priority under 35 U.S.C. 119(e)(1) based upon Provisional Application Ser. No. 60/092,729, filed Jul. 14, 1998. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to the field of integrated circuit manufacturing, and more particularly, to utilizing deep trench isolation for reducing soft errors in integrated circuits. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with the manufacture of integrated circuits for use in the creation of metal oxide semiconductor (MOS) memory devices, as an example. 
     The growing demand for increasingly smaller and thus more cost effective semiconductor devices, e.g., with large memory capacities, has pushed the development of miniaturized structures in sub-micron technologies. The development of dynamic random access memory (DRAMs) has made possible the storage capability of several million bits of information in a single integrated circuit chip. DRAMs are memory devices in which the presence or absence of a capacitive charge represents the state of binary storage element. DRAMs, which are capable of inputting and outputting data at random, generally comprise an array of memory cells for storing data and peripheral circuits for controlling data in the memory cells. Within the array, each memory cell is electrically isolated from adjacent cells. 
     Typically, a MOS DRAM cell includes a single transistor and a single capacitor for storing the electrical charge corresponding to one bit of information; the cell operates by storing a charge on the capacitor for a logic 1 and storing no charge for a logic 0. With such a construction, each cell of the memory array is required to be periodically refreshed so as to maintain the logic level stored on the cell capacitor. The greater the current leakage, the more frequent the refresh cycle. 
     SUMMARY OF THE INVENTION 
     It has been found, however, that present methods for the development of large monolithic circuits have encountered numerous difficulties. One such difficulty is the problem of shrinking size in order to pack more circuitry on a chip without increasing the soft error rate. As the size of DRAM arrays, for example, is decreased, the density of the integrated circuits within the DRAM arrays is correspondingly increased. Therefore, the potential grows larger for the occurrence of soft errors caused by charges injected from the surroundings, making the device less reliable. 
     Such soft errors in DRAM cells have also been attributed to the vulnerability of MOS capacitors to charges generated in the substrate by ionizing radiation such as cosmic rays, noise injected from the substrate, p-n junction leakage over the entire area of the capacitor, and sub-threshold leakage of the cell transistor. A cosmic ray, for example may directly or indirectly produce an ionization path. In fact, a 5 MeV alpha particle can produce more than 200 femtocoulombs of hazardous electrons. 
     What is needed is a high density integrated circuit that reduces the incidence of soft errors. The present invention disclosed herein provides an integrated circuit having improved soft error protection and a method improving the soft error protection of an integrated circuit. The integrated circuit can comprise a substrate, a transistor formed in the substrate, a first region formed in the substrate having a first conductivity type, a second region below the first region having a second conductivity type, and a trench formed in the substrate having a depth at least substantially as deep as the first region. The trench is filled with a non-conductive material that forms a frame around the transistor, whereby soft errors due to electron hole pairs caused by ionizing radiation in the frame area are substantially eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
     FIG. 1 is a schematic diagram of a DRAM cell; 
     FIG. 2 is a depiction of the introduction of particles into a semiconductor device; 
     FIG. 3 is plan view of a portion of a DRAM array; 
     FIG. 4 is a cross-sectional view of a portion of the DRAM array taken along line  4 — 4  of FIG. 3; 
     FIG. 5 is a cross-sectional view of a portion of the DRAM array taken along line  5 — 5  of FIG. 3; 
     FIG. 6 is a cross-sectional view of a portion of the DRAM array; 
     FIG. 7 is a cross-sectional view of a portion of a DRAM array in accordance with one embodiment of the present invention; 
     FIG. 8 is a cross-sectional view of a portion of the DRAM array of FIG. 7 in accordance with one embodiment of the present invention; 
     FIG. 9 is a cross-sectional view of a portion of the DRAM array of FIG. 7 in accordance with one embodiment of the present invention; 
     FIG. 10 is a cross-sectional view of a portion of a DRAM array in accordance with one embodiment of the present invention; 
     FIG. 11 is a cross-sectional view of a portion of the DRAM array of FIG. 10 in accordance with one embodiment of the present invention; and 
     FIG. 12 is a cross-sectional view of a portion of the DRAM array of FIG. 10 in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     Turning now to the schematic diagram of FIG. 1, a typical prior art DRAM cell  10  includes a transistor  12  and a capacitor  14 . Transistor  12  includes a source  16 , a drain  18  and a gate  20 . Source  16  is connected to capacitor  14 . Drain  18  is connected to a bit line  22 . Gate  20  is connected to a word line  24 . Transistor  12  is suitable for use as a metal oxide semiconductor field effect transistor (MOSFET). 
     Capacitor  14  is also connected to ground  26  on the end opposite source  16 . Capacitor  14  stores charge to represent a bit of information. For example, if no charge is stored by capacitor  14 , this could represent a logic 0. Similarly, if charge is stored by capacitor  14  corresponding to a potential of, for example, 5 V across the capacitor plates, this could represent a logic 1. 
     The bit of information is accessed to read or write by applying a voltage on word line  24  to turn on transistor  12 . Once turned on, transistor  12  connects capacitor  14  to bit line  22  for the read and write operations. Word line  24  is then generally returned to a ground level voltage to turn off transistor  12 . 
     Charge on capacitor  14  may, however, slowly leak away due to inherent leakage currents. In operating DRAMs, it is therefore necessary to periodically refresh the device by rewriting the stored data on a bit by bit basis to each of the DRAM cells such as DRAM cell  10 . The greater the current leakage, the higher the frequency of such rewriting. 
     Additionally, capacitor  14  is vulnerable to charges generated in the substrate by cosmic rays, noise injected from the substrate, p-n junction leakage over the entire area of capacitor  14 , and sub-threshold leakage of transistor  12 . As depicted in FIG. 2 (prior art), a particle  30 , e.g., generated by a cosmic ray can be introduced into the device. For example, as a result of particle  30 , such as an alpha particle, transistor source (“storage node”)  16  may be partially or completely shorted to ground, and the capacitor  14  (of FIG. 1) at least partially discharged. 
     Electron-hole pairs  36  are created as particles  30  traverse the silicon lattice. Typically, there exists approximately one electron-hole pair  36  per 3.6 eV. In a positively charged source  16 , for example, the electrons, being attracted by the electric field, go to such a source, thus depleting the charge. Similarly, if a source/drain is negatively charged, holes will migrate to such a source/drain and at least partially deplete its charge. Such depletion effects transient errors, called soft errors, and may result in a false logic 0 rather than a logic 1. 
     Turning now to FIG. 3, a plan view of a portion of a DRAM array is shown. Bit lines  40 ,  42 , and  44  pass under word lines  46 ,  48 ,  50 , and  52 . While bit lines  40 ,  42 , and  44  and word lines  46 ,  48 ,  50 , and  52  are shown as perpendicular lines for ease of illustration, they may be curved or angled or take on various other configurations. Region  54 , which may sometimes be referred to as a moat, encompasses a pair of DRAM cells sharing a common bit line contact  64 . In such a configuration, region  54  comprises storage nodes  56  and  58 , gates  60  and  62 , and a bit line contact  64 . Gates  60  and  62  are word line contacts to word lines  48  and  50 , respectively. 
     The features of one embodiment of the present invention may be best understood with reference to FIGS. 4-6. FIGS. 4 and 5 depict cross-sectional views of portions of the DRAM array of FIG. 3 taken along lines  4 — 4  and  5 — 5 , respectively. It should be noted that the features of the present invention may be used for either n-channel, p-channel, or both types of devices. For the convenience of illustration, however, the following description refers to deep trench isolation with respect to a dual n-channel circuit with n+doped sources and drains. It should nevertheless be appreciated by one skilled in the art that the features of the present invention are not limited to devices of any one particular conductivity. 
     As shown in FIGS. 4 and 5, trench  70  is formed in substrate  72  for physically and electrically isolating p-region  74 , which may be lightly doped. The depth of p-region  74  may be, for example, a shallow depth of approximately 1.5μ to approximately 2.0μ. As shown in FIGS. 4 and 5, trench  70  is vertically deeper than p-region  74 , and penetrates deep n well  84 . Below deep n well  84  is p substrate  86 . 
     Memory cells  76  and  78  comprise storage nodes  56  and  58 , gates  60  and  62 , a shared drain (common bit line contact  64 ), and capacitors (not shown). Storage nodes  56  and  58  and bit line contact  64  are each heavily doped (n + ). A capacitor (not shown) is formed over and electrically connected to each of the storage nodes  56  and  58 . 
     Trench  70  is formed utilizing conventional lithographic techniques. Trench  70  is first defined by forming a photoresist layer on substrate  72 , and utilizing a photomask, which forms the pattern of apertures for trenches  70 . The substrate  72  is then subjected to an anisotropic etch, such as a plasma reactive ion etch, to remove semiconductor material to form trench  70 . The anisotropic etch is preferably continued until such time that trench  70  penetrates deep n well  84 . After the photoresist layer is removed, a plasma deposition can be used to fill trench  70  with an electrically non-conductive material such as SiO 2 , Si 3 N 4  or silicon oxynitride. 
     Trench  70  is etched to a depth so as to penetrate deep n well  84 , thus completely isolating p-region  74  when filled with a non-conductive material  71 , as shown in FIG.  6 . Trench  70  thus frames p-region  74  and acts as an insulative barrier against unwanted charges in either the trench or outside the frame. The use of deep trench isolation therefore provides improved immunity against soft errors. Additionally, the use of deep trench isolation limits the (e.g. electron) diffusion to storage nodes  56  and  58 . 
     Turning now to FIGS. 7-9, another embodiment of the present invention is depicted. As shown in FIGS. 7 and 8, trench  88  is etched into p-region  74  to meet deep n well  84 . In such a configuration, trench  88  is etched to a depth equal to the depth of p-region  74  to frame p-region  74  and serve as an effective insulative barrier against carriers generated by alpha and other ionizing particles. 
     Trench  88  is formed in substrate  72  for physically and electrically isolating p-region  74  which may be lightly doped. The depth of p-region  74  may be, for example, a shallow depth of approximately 1.5μ to approximately 2.0μ. As shown in FIGS. 7 and 8, trench  88  is as deep as the bottom of p-region  74 . 
     Memory cells  76  and  78  comprise storage nodes  56  and  58 , gates  60  and  62 , a common bit line contact  64 , and capacitors (not shown). Storage nodes  56  and  58  and bit line contact  64  are each heavily doped (n + ). Capacitors would again be formed over storage nodes  56  and  58 . 
     Trench  88  is formed utilizing conventional lithographic techniques. Trench  88  is first defined by forming a photoresist layer on substrate  72 , and utilizing a photomask to pattern trench  88 . The substrate is then subjected to an anisotropic etch, such as a plasma reactive ion etch, to remove the semiconductor material and form trench  88 . The anisotropic etch is continued until such time that trench  88  is as deep as p-region  74  and meets deep n well  84 . A thin field oxide  90  can then be grown on substrate the walls of trench  88 , as shown in FIG. 9. A plasma deposition can be used to fill trench  88  with an electrically non-conductive material. 
     Turning now to FIGS. 10-12, yet another embodiment of the present invention is depicted. As shown in FIGS. 10 and 11, trench  98  is etched into p-region  74  leaving a narrow gap  96  of p-region  74  between the floor of trench  98  and deep n well  84 . In such a configuration, trench  98  is etched to a depth substantially near the depth of p-region  74  to frame p-region  74  and serve as an effective insulative barrier against carriers generated by alpha and other ionizing particles. 
     Trench  98  is formed in substrate  72  for physically and electrically isolating p-region  74  which may be lightly doped. The depth of p-region  74  may be, for example, a shallow depth of approximately 1.5μ to approximately 2.0μ. As shown in FIGS. 10 and 11, trench  98  is nearly as deep as the bottom of p-region  74 . 
     Memory cells  76  and  78  comprise storage nodes  56  and  58 , gates  60  and  62 , a common bit line contact  64 , and capacitors (not shown). Storage nodes  56  and  58  and bit line contact  64  are each heavily doped (n + ). Capacitors would again be formed over storage nodes  56  and  58 . 
     Trench  98  is formed utilizing conventional lithographic techniques. Trench  98  is first defined by forming a photoresist layer on substrate  72 , and utilizing a photomask to pattern trench  98 . The substrate is then subjected to an anisotropic etch, such as a plasma reactive ion etch, to remove the semiconductor material and form trench  98 . The anisotropic etch is continued until such time that trench  98  is nearly as deep as p-region  74  and approaches deep n well  84 . A thin field oxide  100  can then be grown on substrate the walls of trench  98 , as shown in FIG. 12. A plasma deposition can be used to fill trench  98  with an electrically non-conductive material. 
     It should be noted that the DRAM cells of the present invention may be implemented using either p-channel or n-channel transistors. The conductivity type of the source and drain regions governs the conductivity type of the polycrystalline silicon used as the capacitor electrode. In addition, while DRAMs have been used herein to illustrate the features of the present invention, it should be appreciated by one skilled in the art that soft errors occur in many types of semiconductor devices, and that the principles of the present invention are thus wholly applicable to many other types of circuits including an embedded memory in a logic device, for example. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.