Abstract:
A radiation hardened memory device having static random access memory cells includes active gate isolation structures placed in series with oxide isolation regions between the active regions of a memory cell array. The active gate isolation structure includes a gate oxide and polycrystalline silicon gate layer electrically coupled to a supply terminal resulting in an active gate isolation structure that prevents a conductive channel extending from adjacent active regions from forming. The gate oxide of the active gate isolation structures is relatively thin compared to the conventional oxide isolation regions and thus, will be less susceptible to any adverse influence from trapped charges caused by radiation exposure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Divisional of U.S. patent application Ser. No. 09/393,125, filed Sep. 10, 1999 now U.S. Pat. No. 6,091,630. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to semiconductor memory devices, and more particularly, to a radiation hardened semiconductor static random access memory cell. 
     BACKGROUND OF THE INVENTION 
     Semiconductor memory devices used in outer space, for example, in a satellite, are subjected to severe environmental conditions that may compromise the integrity of the stored data, or cause the memory devices to fail. In many cases, the memory devices are part of a larger embedded system, where the memory device is just one of many devices sharing the same die. The integrity of the memory devices used in outer space applications is critical because the information stored by the memory devices may be related to critical functions, such as guidance, positioning, and transmitting and receiving data from a ground base station. Furthermore, semiconductor memory devices for use in space applications should remain functional for the lifetime of the satellite, which may be as long as several years. Contrast this with applications where the memory devices are also subjected to harsh operating conditions, such as guidance systems in missiles, but only for a relatively short time period. 
     One cause of errors in semiconductor memory devices that are used in outer space applications is due to high-energy particles impinging on the memory device. There are several forms of high energy particles in outer space. For example, there are alpha particles and gamma rays, to name a couple. These high-energy particles strike the semiconductor material on which the memory devices are formed with enough energy to cause the generation of electron-hole pairs. The resulting charge carriers are often trapped in the various oxide layers of the memory devices. In the case of metal oxide semiconductor (“MOS”) transistors, charges trapped in the gate oxide will shift the threshold voltage. Vt, of the transistor. As a result, leakage currents of the transistors, and consequently, of the memory devices may increase. Where the transistor is used as a transfer gate for a conventional memory cell, the increased leakage current may compromise the integrity of the data stored by the data storage node, such as a capacitor, by allowing the charge representing the data to dissipate. 
     The frequency or number of charges trapped in an oxide layer is proportional to the thickness of the oxide layer. Consequently, oxides having a greater thickness will, on the average, have a greater number of trapped charges. In the case where the oxide is relatively thick, for example, approximately 4000-5000 Å, charge trapped in the oxide will result in a much more dramatic shift in the leakage current characteristics than for an active transistor having a relatively thin gate oxide. Such an application of thick oxide is for isolating active transistor areas in which memory cells may be formed, for example, regions of local oxidation of silicon (“LOCOS”). The accumulating charge trapped in the LOCOS region may become great enough to cause a conduction channel to form below the LOCOS region, and consequently allow current to leak between neighboring active transistor regions. 
     As mentioned previously, transistors in the active region of a memory device, which typically have gate oxides that are much thinner than the oxides of the LOCOS region, will have shifting Vts as a result of the trapped charges in the gate oxide. A method that has been used to accommodate the shifting Vts is to raise the Vts of the active transistors so that the relative changes in the Vts due to the trapped charges are minor. Thus, the effect that trapped charges will have on the overall performance of the memory devices is minimized. However, the aforementioned technique is not as effective when applied to LOCOS isolation regions. Although the Vt of the incidental transistor of the LOCOS isolation regions may be adjusted so that the Vt shift due to charge trapped in the thick oxide is relatively small, the resulting isolation region will have an unacceptably low breakdown voltage. 
     Another technique used to accommodate charge trapped in a thick LOCOS isolation region is to create a two-layer oxide isolation structure. A relatively thin layer of oxide of approximately 500 Å is grown in the isolation region, followed by the formation of a relatively thick layer of phosphorous doped oxide of approximately 4000 Å over the relatively thin layer of oxide. The phosphorous doped oxide is subsequently densified by baking. The resulting structure minimizes the effect trapped positive charges have on the leakage characteristics of the isolation region by neutralizing the trapped positive charges with the abundance of electrons of the relatively thick phosphorous doped oxide. However, this approach is limited by the doping concentration of the relatively thick oxide layer. That is, the greater the level of impurities, the greater the diffusion of the impurities from the thick oxide into the adjacent layers. 
     Another approach taken in providing semiconductor memory devices suitable for use in space applications is through a method of trial and error. Samples of potentially applicable semiconductor memory devices are taken from several different process lots and subjected to severe radiation conditions that simulate the environment of outer space. The sample devices are subsequently tested for functionality and reliability. Those memory devices from the process lots having samples surviving the testing are judged to be acceptable for use in applications in outer space. However, this approach merely screens existing memory devices for those which may be sufficient for use in outer space applications, and consequently, the quantity of satisfactory memory devices produced by this method is subject to the unpredictable nature of semiconductor processing. 
     Therefore, there is a need for a semiconductor memory device having reliable data retention over a sufficiently long period of time for use in applications subjected to severe radiation environments, such as outer space. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a semiconductor memory includes static random access memory (“SRAM”) cells formed on a substrate and arranged in a matrix where each memory cell has data storage nodes that are selectively coupled through transfer gates to a respective sense node. The data storage nodes are electrically isolated from each other, and the data storage nodes of adjacent memory cells by an active gate isolation structure that is formed in between the nodes and coupled to a voltage potential. The active gate isolation structure prevents a conductive channel extending between the data storage nodes of the memory cells from forming. The data storage nodes of the memory cells may be further isolated from one another by an oxide isolation structure formed in series with the active gate isolation structure and in between the data storage nodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic diagram of a conventional SRAM memory cell. 
     FIG. 2 illustrates a plan view of an SRAM memory cell according to an embodiment of the present invention. 
     FIG. 3 illustrates a cross-sectional view of the SRAM memory cell shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments according to the present invention use both a thick field isolation structure and an active gate isolation structure, either alone or in combination, to reduce leakage currents in a semiconductor memory cell array. The technique is described herein with respect to radiation hardened static random access memory (“SRAM”) cells. Data storage nodes of a memory device are isolated from one another using an active gate isolation structure formed adjacent to a thick field oxide isolation structure. The resulting isolation between the data storage nodes is formed from the active gate isolation structure being in series with the adjacent thick field oxide isolation structures. The active gate is tied to a voltage potential in order to prevent a conductive channel from forming under the active gate isolation structure. Embodiments of the memory cell structure described herein may be used in a variety of memory applications, such as embedded memories, as well as discrete memory devices, and the like. 
     The thickness of the oxide of the active gate isolation structure is relatively thin compared to the thickness of the thick field oxide isolation structures and will trap less charge when the semiconductor memory device is exposed to a radiation environment. Consequently, the probability of accumulating trapped charge in the gate oxide of the active gate isolation structure sufficient to create an inversion layer is reduced. Although charge trapped in the thick field oxide may nevertheless cause an inversion layer to form under the thick field oxide, the inversion layer will not extend under the active gate isolation structure. 
     The fabrication and use of active gate isolation structures in semiconductor integrated circuits has been previously described in U.S. Pat. No. 5,821,600 to Chan, issued Oct. 13, 1998, and U.S. Pat. No. 5,849,614 to Chan, issued Dec. 15, 1998, which are incorporated herein by reference. These patents describe using a first polysilicon layer for the active gate isolation, and a second polysilicon layer for the gate structures of the transistors. These patents further discuss the advantages of isolating active transistor regions using an active gate structure over using conventional thick field oxide isolation. 
     A conventional SRAM memory cell  4  is illustrated in FIG.  1 . The SRAM cell  4  is formed from two cross-coupled CMOS inverters  6  and  8 , coupled between a first supply terminal  10  and a second supply terminal  12 . The supply terminal  10  is typically the power supply voltage, VCC, and the supply terminal  12  is typically ground. A storage node  14  of the inverter  6  is coupled to a complementary bit line  16  through an NMOS transfer gate  18 . A storage node  20  of the inverter  8  is similarly coupled to a non-complementary bit line  22  through an NMOS transfer gate  24 . The bit lines  16  and  22  may also be viewed as respective sense nodes. The storage nodes  14  and  20 , or data storage nodes, store the complementary and non-complementary data written to the SRAM cell  4  until new data is written, or power is removed. The gates of the NMOS transfer gates  18  and  24  are coupled to a word line  28  so that when sufficient voltage is applied to the word line  28 , data stored by the SRAM cell  4  is provided to the complementary and non-complementary bit lines  16  and  22 , or new data may be written to the SRAM cell  4 . 
     Illustrated in FIG. 2 is a planar layout of an SRAM memory cell  50  in accordance with an embodiment of the present invention. As is conventional in the field of integrated circuit representation, the lateral sizes and thicknesses of the various layers are not drawn to scale and these various layers or layer portions are arbitrarily enlarged to improve drawing legibility. 
     An n-well region  54  and a p-well region  56  are provided on a substrate  60 . It can be appreciated that the n-well region  54  and the p-well region  56  may be also formed in a larger well or another layer, such as an epitaxial layer. However, in the interests of brevity, the description of the embodiments according to the present invention will refer to only the substrate  60 . Thick oxide isolation regions  62  are formed on the substrate  60 . The process of forming the thick oxide regions  62  is well known in the art and need not be described in detail herein. For example, the thick field oxide regions  62  may take the form of a trench oxide isolation structure, a LOCOS field oxide, a trench field oxide, or the like. Active n+ regions  64  and  65  are implanted in the p-well region  56 , and an active p+ region  66  is implanted in the n-well region  54  in the areas where the thick oxide isolation regions  62  have not been formed. The active n+ region  65  is coupled to a ground VSS representing the supply terminal  12 , and the active p+ region  66  is coupled to VCC representing the supply terminal  10 . 
     A gate oxide layer and polysilicon layer are deposited over the surface and etched to form polysilicon gates for the inverters  6  and  8 , and the transfer gates  18  and  24  (FIG.  1 ). Polysilicon gates  70   p  and  70   n , and  74   p  and  74   n  correspond to the gates of PMOS and NMOS transistors of the inverters  6  and  8 , respectively. Polysilicon gates  76  and  78  correspond to the gates of the transfer gates  18  and  24 . respectively. A polysilicon gate  80  is provided for the purposes of an active gate isolation structure. The gate  80  is coupled to the ground supply terminal VSS to ensure that a conduction layer does not form between the active regions of the transfer gates  18  (poly gate  76 ), the transfer gate  24  (poly gate  78 ), and the drains of the NMOS transistors of the inverters  6  and  8  (poly gates  70   n  and  74   n , respectively). The gates of the transistors and the active gate isolation structure are formed contemporaneously, and as a result, the gate oxide of the active gate isolation structures will have a thickness similar to the gate oxide of the PMOS and NMOS transistors. For example, the gate oxide thickness may be between 20 to 300 angstroms thick. Consequently, less charge may be trapped in the thin gate oxide compared to the thick oxide isolation regions  62  resulting in the active gate isolation structures being be less susceptible to adverse influences caused by trapped charges. 
     As shown in FIG. 2, contacts  84 - 98  and vias  100 - 110  are formed through a dielectric layer deposited subsequent to the polysilicon gate formation. A first metal layer is then deposited over the dielectric layer and etched to form interconnects  116  and  118 . The interconnect  116  couples together the drains of the PMOS and NMOS transistors of the inverter  6  (through contacts  84  and  86 ), and the gates of the PMOS and NMOS transistors (through vias  104  and  106 ) of the inverter  8 , to the active region of the transfer gate  18  (through via  96 ). With respect to FIG. 1, the active region of the transfer gate  18  represents the data storage node  14 , which is coupled to the drain regions of the PMOS and NMOS transistors of the inverter  6  by the interconnect  116 . Similarly, the interconnect  118  couples together the drains of the PMOS and NMOS transistors of the inverter  8  (through contacts  90  and  92 ), and the gates of the PMOS and NMOS transistors (through vias  100  and  102 ) of the inverter  6 , to the active region of the transfer gate  24  (through via  98 ). The active region of the transfer gate  24  represents the data storage node  20 , which is coupled to the drain regions of the PMOS and NMOS transistors of the inverter  8  by the interconnect  118 . First metal regions  120  and  122  are provided for a subsequent metal layer that will form the word line  28 . Similarly, first metal regions  124  and  126 , which represent the sense nodes, are provided for a subsequent metal layer that will form the bit lines  16  and  22 , respectively. 
     A cross-sectional view of the SRAM cell  10  of FIG. 2 is provided in FIG.  3 . The view is taken through the contacts  96  and  98 , that is the storage nodes  14  and  20 , respectively. Separating the active n+ regions  64  in which the data storage nodes  14  and  20  are formed are thick field oxide isolation regions  62  and an active gate isolation structure represented by the polysilicon gate  80 . As mentioned previously, the polysilicon gate  80  is connected to a voltage potential to ensure that a conduction channel will not form in the region under the polysilicon gate  80 . Thus, even if enough trapped charge accumulates in the thick field oxide isolation regions  62  to create a conduction channel under each respective thick field oxide, leakage currents cannot flow between the contacts  96  and  98  because the region under the polysilicon gate  80  remains nonconductive. 
     It will be appreciated that various methods for connecting the polysilicon gate  80  to the appropriate voltage potential are well known in the art. Thus, the specific form of the connection should not limit the scope of the present invention. For example, in the present example, the active n+ regions  64  in which the data storage nodes  14  and  20  are formed is implanted in the p-well  56 . Consequently, the polysilicon gate  80  of the active isolation structure should be coupled to ground. Where the active n+ region  65  is also connected to ground, the polysilicon gate  80  may be properly biased by being directly connected to the active n+ region  65  by metallization layers and/or buried contacts. For example, a conductive strap may be used to connect the polysilicon gate  80  to the active n+ region  65  at regular intervals, perhaps every  16  or  32  memory cells. 
     Further isolating the contacts  96  and  98  are polysilicon gates  76  and  78 , which surround the respective contact. As mentioned previously, polysilicon gates  76  and  78  represent the gates of the transfer gates  18  and  24 , respectively. While the word line  28  remains deactivated, the area under the polysilicon gates  76  and  78  are nonconductive, and consequently, provides greater isolation of the contacts  96  and  98 . The polysilicon gates  70   p ,  70   n ,  74   p , and  74   n , which have a similar shape as polysilicon gates  76  and  78 , also provide additional isolation for the contacts  84 - 92 , respectively. 
     As will be appreciated, the main steps according to the present invention are shown and described herein, however, some steps in the process have not been shown because such standard processing steps are commonly used. Additional processing steps such as threshold adjustment implants, epitaxial growth, well formation, and the like, may be carried out as desired. For example, the polysilicon gates are illustrated in FIG. 3 as having oxide sidewall formations and the active n+ regions  64  as having a profile where additional implant steps were made following the formation of the polysilicon gate layer. However, these additional process steps for the formation of these structures are well known in the art. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.