Abstract:
Integrated circuits and methods for reducing the Single Event Upset (SEU) susceptibility of a memory cell are disclosed. By using one or more Through Silicon Vias (TSVs) as capacitor(s) coupled to the Q and/or Qbar nodes of the memory cell, the critical charge (Qcrit) of the circuit is increased. In so doing, the memory cell has greater resistance to an SEU occurrence and reduced sensitivity to neutron and alpha or other charged particle events. The capacitor(s) can be coupled between the Q or Qbar node(s) and a silicon substrate, or between the Q and Qbar nodes, for example.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to integrated circuits, and more specifically to circuits that are resistant to single event upset occurrences. 
     BACKGROUND 
     A programmable device (PD) is a well-known type of integrated circuit (IC) that may be programmed by a user to perform specified logic functions. There are different types of programmable devices, such as programmable logic arrays (PLAs) and complex programmable logic devices (CPLDs). One type of programmable device, called a field programmable gate array (FPGA), is very popular because of a superior combination of capacity, flexibility, time-to-market, and cost. An FPGA typically includes an array of configurable logic blocks (CLBs) and programmable input/output blocks (IOBs). The CLBs and IOBs are interconnected by a programmable interconnect structure. The CLBs, IOBs, and interconnect structure are typically programmed by loading a stream of configuration data (bitstream) from an external source into internal configuration memory cells that define how the CLBs, IOBs, and interconnect structure are configured. Thus, the collective states of the individual configuration memory cells determine the function of the FPGA. 
     A well-studied occurrence in circuitry is called Single Event Upset (SEU). SEU is an inadvertent change in state of a circuit caused by external energy source such as, for example, cosmic rays, alpha particles, energetic neutrons, and the like. The energetic particles may randomly strike a semiconductor device and penetrate into the active regions (e.g., transistor source and drain regions) of the semiconductor device. These particle strikes create pairs of electrons and holes, which in turn cause undesirable transients that may upset circuit elements such as, for example, flipping the logic state of a latch or other memory element. Such an upset will result in data corruption and can lead to system corruption or failure. As fabrication geometries and supply voltages continue to decrease and circuit densities and transistor counts continue to increase, SEU problems become more severe. 
     The sensitivity of a static ram (SRAM) cell to incident neutron or alpha or other charged particles depends upon a number of factors, including the critical charge (Qcrit) and the SRAM cell target area. Qcrit is a measure of the charge required to flip the state of an SRAM cell or latch and depends upon many factors, including the capacitance on the cross-coupled nodes of the SRAM cell or latch. Efforts to maximize Qcrit and minimize target area can lead to lowered SEU susceptibility. However, these two factors are often working in opposition. While the scaling of technology will reduce the target area, it will also lead to significantly reduced capacitance, and hence reduced Qcrit. Hence, SEU rates may increase from one technology generation to the next (e.g., 45 nm to 32 nm). 
     It may be necessary to develop special non-standard processes to maximize the Qcrit for small area cells, in order to keep the SEU rate at an acceptable level. Such non-standard processes have been demonstrated to increase Qcrit and hence improve the SEU. However, a solution that uses commonly available processes utilized in standard CMOS technology (e.g., with few or no additional masks or process steps) would have a clear cost and time-to-market advantage. 
     There is a need to address the above identified issues. The present invention addresses such a need. 
     SUMMARY 
     Integrated circuits and methods for reducing the Single Event Upset (SEU) susceptibility of a memory cell are disclosed. (The term “memory cell” as used herein includes static random access memory (SRAM) cells, latches, and other circuits storing a logical value by using cross-coupled nodes.) By using one or more Through Silicon Vias (TSVs) as capacitor(s) coupled to the Q and/or Qbar nodes of the memory cell, the critical charge (Qcrit) of the circuit is increased. In so doing, the memory cell has greater resistance to an SEU occurrence and reduced sensitivity to neutron and alpha or other charged particle events. 
     In a first aspect, an integrated circuit comprises a silicon substrate with a plurality of nodes; and one or more through silicon vias (TSV) within the silicon substrate. The plurality of nodes includes Q and Qbar nodes cross-coupled to form a memory cell. The integrated circuit further includes an isolating member surrounding the one or more TSVs for isolating the TSVs from the silicon substrate, wherein one or more capacitors are formed. The one or more capacitors are coupled to at least one of the Q or Qbar nodes. 
     One or more capacitors may be formed, for example, between the silicon substrate and the at least one of the Q or Qbar nodes. In other embodiments, e.g., embodiments including at least one coaxial TSV, the one or more capacitors may be formed between the Q and Qbar nodes. 
     In a second aspect, a method comprises providing one or more through silicon vias (TSVs) within a silicon substrate, the silicon substrate including a plurality of nodes; and isolating the one or more TSVs from the silicon substrate, wherein a capacitor is formed. The plurality of nodes includes Q and Qbar nodes cross-coupled to form a memory cell, and the one or more capacitors are coupled to at least one of the Q or Qbar nodes. 
     In another aspect, an SRAM cell includes a silicon substrate having a Q node and a Qbar node cross-coupled one to another, one or more TSVs within the silicon substrate, and one or more dielectric liners. Each of the dielectric liners surrounds one of the one or more TSVs for isolating the one or more TSVs from the silicon substrate. The one or more TSVs are placed under a metal cross-coupling of the Q node and the Qbar node to form individual capacitors therebetween. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an SRAM cell. 
         FIG. 2  is an embodiment of the SRAM cell. 
         FIG. 3-1  is a first embodiment of an SRAM cell. 
         FIG. 3-2  shows a cross-section of  FIG. 3-1  through the plane X-X′. 
         FIG. 4-1  is a second embodiment of an SRAM cell. 
         FIG. 4-2  shows a cross-section of  FIG. 4-1  through the plane Y-Y′. 
         FIG. 5  is a third embodiment of an SRAM cell. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to integrated circuits, and more specifically to a circuit that is resistant to single event upset occurrences. The following description is presented to enable one of ordinary skill in the art to make and use the invention, and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
     The embodiments presented herein are described in the context of SRAM cells. However, one of ordinary skill in the art will readily recognize that the systems and methods described herein can be utilized in a variety of circuits, and that use is within the spirit and scope of the present invention. 
       FIG. 1  shows an SRAM cell  100 . The SEU FIT (Failures in Time, or the number of failures in one billion hours of operation) rate of a circuit is determined from a number of factors. The SRAM cell  100  includes six transistors  102 ,  104 ,  106 ,  108 ,  110  and  112  (single port example shown here). NMOS transistors N1, N2, N5, and N6 ( 102 ,  104 ,  110  and  112 , respectively) are typically placed in a single PWell  114 , and the PMOS transistors P3 and P4 ( 106  and  108 , respectively) are placed in an adjacent Nwell  116 . A neutron or alpha or other charged particle incident upon the SRAM cell  100  will create a charge cloud in the Pwell  114  or Nwell  116  regions. The critical charge (Qcrit) required to flip the state of the SRAM cell  100  is calculated for the four possible states of the SRAM cell  100  (‘I’ state with upset to the ‘off’ PMOS transistor; ‘I’ state with the upset to the ‘off’ NMOS′ transistor; ‘0’ state with upset to the ‘off’ PMOS′ transistor; ‘I’ state with upset to the ‘off’ NMOS transistor). 
     In each case, the relevant Qcrit can be found from a simulation of the dynamic switching response of the SRAM cell  100 . Qcrit depends upon the capacitances of the nodes of the SRAM cell  100  (plus other factors), especially on the Q and Qbar nodes  118  and  120  capacitances (and notably on the Q to Qbar capacitance). Adding additional capacitance to the SRAM cell  100  nodes  118  and  120 , especially between the cross-coupled Q and Qbar nodes, can increase the Qcrit value and hence reduce the sensitivity to a Single Event Upset (SEU). This approach is illustrated in  FIG. 2 . 
       FIG. 2  is an embodiment of the SRAM cell  100 ′. This embodiment has similar elements to the SRAM cell  100  of  FIG. 1  and has similar reference numerals. In this embodiment, capacitors C1, C2, and C3 ( 200   a ,  200   b , and  200   c , respectively) are added to Q and Qbar nodes  118  and  120  to minimize the effect of SEUs on the cell  100 ′. In this embodiment, capacitor  200   a  is added between power (Vdd) and Qbar node  120 , capacitor  200   b  is added between Q and Qbar nodes  118  and  120 , and capacitor  200   c  is added between the Q node  118  and ground (Vss). Each capacitor can be of a value typically of 5 to 10 fF (femtoFarads), for example, using conventional silicon TSV processing techniques. Such extra capacitance placed on Q or Qbar of an SRAM cell or latch can significantly improve the SEU resistance (i.e., lower the SEU FIT rate). 
     Each of the capacitors  200   a - 200   c  increases Qcrit and hence reduces sensitivity to a SEU. Although it is known that adding capacitance will minimize the SRAM cell&#39;s sensitivity to SEUs, simply adding capacitors to the SRAM cell will usually add to overall SRAM cell area &amp; device size. This approach is unacceptable for many applications, as it will increase the cost of the product. 
     Through Silicon Vias (TSVs) are utilized in 3-dimensional technologies to provide interconnects through silicon devices to allow vertical die stacking. TSVs are usually built to provide electrical connections through the silicon device from front to back of the silicon wafer. Fine pitch TSVs down to 5 micron pitch or below are well known. 
     According to the methods described herein, to add additional capacitance to a circuit, one or more TSVs are connected to nodes of the circuit to minimize the circuit&#39;s sensitivity to an SEU. In an embodiment, the TSVs are used to form capacitors connected to the Q and Qbar nodes  118  and  120 . The remaining figures provide detailed exemplary circuits having such structures. Note that while 6-transistor SRAM cells are illustrated, the described techniques can be similarly applied to 8-transistor SRAM cells, for example. 
       FIG. 3-1  is a first embodiment of an SRAM cell  300 . This embodiment has similar elements to the SRAM cell  100  of  FIG. 1 , and has similar reference numerals. In the embodiment, TSVs TSV1 and TSV2 ( 302   a  and  302   b , respectively) are utilized as capacitors by placing them under the metal cross coupling connections to the Q and Qbar nodes  118  and  120 . Thus, capacitor C1 is added between TSV1 and node Qbar, and capacitor C2 is added between TSV2 and node Q. Thus, TSV1 and TSV2 can be used to provide capacitance to ground in the body of the wafer, for example, as shown in  FIG. 3-2 . 
       FIG. 3-2  shows a cross-section of  FIG. 3-1  through the plane X-X′. As is seen, TSV1  302   a  is located within the silicon substrate  304  and is coupled to metal-1 layer  306  (which in this example forms a portion of node Qbar). A liner  310  of dielectric material is utilized to isolate the TSV1  302   a  from the silicon substrate  304  to form a capacitor to the ground plane in the body of the silicon substrate  304 . The dielectric material can be a variety of types, including but not limited to silicon dioxide, a high k dielectric or an organic dielectric. A shallow trench isolation layer or STI, also a dielectric such as silicon dioxide, isolates the wafer from the structures formed on the upper surface thereof. 
       FIG. 4-1  is a second embodiment of an SRAM cell  400  in accordance with another embodiment. This embodiment has similar elements to the SRAM cell  100  of  FIG. 1 , and has similar reference numerals. In this embodiment, one coaxial TSV  402 , comprising a core portion and a coaxial outer portion  404  surrounding the core portion, is placed between the Q and Qbar nodes  118  and  120 . The coaxial TSV  402  is placed under the metal cross-coupling, with connections to the Q and Qbar nodes  118  and  120 , forming a capacitor directly between the Q and Qbar nodes  118  and  120 . 
       FIG. 4-2  shows a cross-section of  FIG. 4-1  through the plane Y-Y′. In this embodiment, a first portion  406   a  of the metal  1  layer  306  is coupled to the Qbar node  120 , and a second portion  406   b  of the metal layer  306  is coupled to the Q node  118 . In this embodiment, the TSV capacitance is provided directly between Q and Qbar. Liner layers  410  of dielectric material isolate the Q and Qbar nodes from one another within the coaxial TSV  402 , and from the body of the wafer. The dielectric material can be a variety of types, including but not limited to silicon dioxide, a high k dielectric or an organic dielectric. 
       FIG. 5  is a third embodiment of an SRAM cell  500 . This embodiment has similar elements to the SRAM cell  100  of  FIG. 1 , and has similar reference numerals. In this embodiment, SEUs are minimized through the addition of a plurality of arrays of TSVs  502  beneath the Q and Qbar nodes  118  and  120 . TSVs  502  can be similar, for example, to TSV1 and TSV2 shown in  FIGS. 3-1  and  3 - 2 , or TSV  402  shown in  FIGS. 4-1  and  4 - 2 . The arrays of TSVs  502  are placed under the metal cross-coupling connections to the Q and Qbar nodes  118  and  120 . Accordingly, the array  502  maximizes the capacitance between the Q and Qbar nodes  118  and  120  and/or between these nodes and the silicon substrate  304 . 
     It should be understood that the one or more TSVs can be utilized in an integrated circuit both for its normal purpose, that is to allow for the vertical stacking of a plurality of dies, as well as adding capacitance to minimize failure rates due to SEU events when utilized on a stacked die. However, the invention is not limited to implementation in stacked die structures. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations are within the spirit and scope of the present invention. For example one or more TSVs can be utilized in a variety of areas on a device, and that use is within the spirit and scope of the present invention. In addition, the present invention is not limited to SRAM cells or to programmable devices, but can be utilized in any latch or other device where a single event update (SEU) is possible and could cause a failure of the device. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.