Fuse status detection circuit
BACKGROUND OF THE INVENTION 
     The Government has rights in this invention pursuant to Contract No. DNA001-83-C-0241 awarded by the Defense Nuclear Agency. 
    
    
     This application is a continuation-in-part of application Ser. No. 717,935, filed Mar. 29, 1985. 
    
    
     The present invention relates to the field of programmable fuse circuitry. More particularly, it relates to a circuit for detection of the status of a fuse. 
     It is common in the manufacture of large-are integrated circuit devices for defects to occur. To increase the yield of a circuit, it is common to include redundant circuit elements with which to replace defective circuit elements. For example, in memory devices, additional columns or rows may be included on the chip. During circuit test, fuses may then be conditioned to replace defective memory cells with non-defective redundant elements. 
     Another use for fuse status detection circuitry is as an input to a logic function circuit. Based on the state of a programmed fuse, the logic circuit will perform a selected logic function. 
     Common integrated circuit fuses are of either the laser blown type, laser annealed type, or of the electrically blown type. Regardless of the type of fuse used, the circuit to be programmed must be capable of detecting the status of its fuses in order to perform its intended function. Most conventional fuse detection circuitry includes pull-up or pull-down circuits to eliminate internal floating nodes. These circuits tend to be complex, sometimes requiring additional processing steps. Some of these conventional circuits allow a steady-state current to flow depending on the fuse status, and therefore compromise minimal power requirement. 
     SUMMARY OF THE INVENTION 
     The present inventio provides a fuse status detection circuit for use in reconfigurable integrated circuits. In the preferred embodiment, the fuse structure is incorporated into the circuit to produce a flip-flop, and CMOS technology is used to achieve low power consumption. The fuse status is inherently detected when the circuit is powered up, and no clocking pulses are necessary. The circuit is functionally stable and independent of the fuse technology used. The circuit is also inherently insensitive to the effects to globally or locally ionizing radiation. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a circuit schematic of a preferred embodiment of the present invention; 
     FIG. 2 shows a circuit schematic of an alternate embodiment of the present invention; and 
     FIG. 3 shows a circuit schematic of a third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a detailed schematic diagram of a programmable fuse circuit in accordance with a preferred embodiment of the present invention. A positive voltage supply terminal, V DD , is connected to one terminal of fuse 1, to the source S of a p channel transistor QP and to one terminal of an initializing element, shown as a capacitor C C . The other terminal of fuse 1, node A, is connected to the drain D of a transistor QN and to the gate of QP. The drain D of QP, node B, is connected to the gate of QN and to the other terminal of the initializing element (C C ) and is also V out . The source S of ON is connected to ground, V SS . Fuse 2 is connected in parallel between the gate of QN and ground. 
     The purpose of the initializing element is to turn on the transistors during power up. By providing a current path between the gate of QN and the power supply, or between the gate of QP and ground during power up, the transistors will reach their turn-on threshold voltage. Once the transistors turn on, the circuit will achieve a stable state depending on the fuse status because of a positive feedback loop which will be discussed later. The initializing element can be a capacitor or other device, such as a transistor, which provides a current path during power up of the circuit. As shown in FIG. 2, the initializing element can be placed in parallel with QN instead of in parallel with QP as in FIG. 1. Alternately, two initializing elements can be used as shown in FIG. 3. The three circuit embodiments shown are substantially equivalent. 
     Proper fuse status detection requires proper functioning of the initializing element(s) (as discussed further below). Therefore it is strongly preferred to use capacitors Cc of the type which offer the most stability under various environments as the initializing element(s). The thin film capacitor offers stability over the junction capacitor in this regard for two resons. 
     First, in an environment which can generate hole-electron pairs in, for example, silicon (e.g. due to radiation or heat), the resulting current across the p-n junction of a junction capacitor will reduce or even negate the net capacitance. This is not the case with the thin film capacitor since the thin film metal electrode is separated from the underlying doped semiconductor region by a dielectric layer. Second, the junction capacitor relies on a space charge or depletion region adjacent the junction to provide capacitance. The width of this depletion region is a function of the applied voltage and temperature. In contrast, the capacitance of the thin film capacitor is essentially independent of the applied voltage and the temperature. 
     The thin film capacitors can be trimmed to provide selected values of capacitance. 
     In the preferred embodment of the present invention, QN and QP are MOS transistors typically implemented in a CMOS technology. The fuses may be strips of conductive polysilicon which can be blown by a laser. When blown, the fuses change from a conductive state to a nonconductive state. For porper operation of the preferred circuit, both fuses must have the same state; that is, either both conductive or both nonconductive. 
     In a normal mode of operation, terminal V DD  is connected to a positive voltage supply of approximately 5 volts, and terminal V SS  is connected to a ground potential. When fuses 1 and 2 are conductive, node A is connected to the power supply through fuse 1 and is thereby charged to a positive voltage, causing transistor QP to become less conductive. As transistor QP becomes less conductive, the voltage at node B decreases through the discharging action of fuse 2, causing transistor QN to become less conductive. Likewise, as QN becomes less conductive, node A increases in voltage causing QP to become still less conductive, and so on. This positive feedback loop results in both QP and QN being turned fully off, reducing the static power supply current to the level of parasitic junction leakage, and producing a node B and node A voltage out equal to the ground potential, and power supply potential, respectively. 
     For the case when the fuses are conductive, if the fuse conductance is made large in comparison to the conductance of QP and QN, the only stable state for the circuit is node A equal to the power supply voltage, and node B equal to the ground potential. For example, if polysilicon, having a sheet resistivity of 100 ohms per square is used as the fuse material, the designer can choose the resistance of the fuse by varying the number of squares such that the fuse conductance is greater than the maximum conductance of the transistors. If the fuse is 5 squares long, the total fuse resistance is 500 ohms. The transistors need to exhibit a resistance roughly 2 to 4 times greater than 500 ohms. The typical transistor would then have a width to length ratio of less than 10. The exact ratio is dependent on process conditions and must be chosen to fit particular fabrication methods. 
     In the opposite case when the fuses are non-conductive, as the circuit is powered up and V DD  increases from zero volts, node B voltage increases due to the action of the initializing element (e.g., the capacitive coupling action of capacitor C C  to the power supply V DD ). By configuring the initialization element to produce a node B voltage sufficient to cause QN to turn-on, node A discharges which turns QP on, thereby raising node B to V DD  and completing the positive feedback loop. Both QP and QN will remain on with a drain current nearly equal to zero and node A and node B equal to the power supply potential and ground potential, respectively. Conversely, this same initialization could be performed by discharging node A, such that QP turns on. QP then charges node B, turning on QN and thereby completing the positive feedback loop. 
     A functional summary is given in Table 1. 
     
                       TABLE 1                                                     
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 Fuse 1 &amp; Fuse 2 Status                                                   
                 I.sub.DD   V.sub.out                                     
                                   ##STR1##                               
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conductive      0          V.sub.SS                                       
                                  V.sub.DD                                
nonconductive   0          V.sub.DD                                       
                                  V.sub.SS                                
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     The output signal is taken from the drain connection of either transistor. Either V out  or V out  or both can be used to control redundant circuitry. Thus, the flip-flop or bistable fuse detection circuits shown in the Figures produce an output level of ether ground or V DD , depending on the state of the fuses, without the need for clocking, refreshing, or d.c. power consumption. The disclosed circuits are stable and have a functionality that is independent of the fuse technology employed. 
     The circuits are also highly insensitive to upset by time dependent radiation phenomena, which produces charge that can be collected by circuit nodes. Due to the high conductivity of the fuses, a very large amount of radiation is required in order to significantly change the node voltages. When the fuses are nonconductive, the inherent circuit state is such that no semiconductor p-n junctions are reverse biased. Because the collection of charge due to radiation tends to reduce the reverse bias on a p-n junction, any charge collection resulting from a radiation phenomena merely reinforces the state of the circuit, and nodes A and B are therefore inherently stable. Thus, the circuits are inherently insensitive to transient ionizing radiation.