Abstract:
A gap conducting structure for an integrated electronic circuit that functions as an electronic fuse device and that is integrated as part of the semi-conductor chip wiring for providing over-current and thermal runaway protection. The gap conducting structure includes one or more air gap regions of predefined volume that fully or partially exposes a length of interlevel conductor layer in an IC. Alternately, the air gap region may wholly located within the dielectric region below a corresponding conductor and separated by insulator. When functioning as a fuse, the gap region acts to reduce thermal conductivity away from the exposed portion of the conductor enabling generation of higher heat currents in the conducting line with lower applied voltages sufficient to destruct a part of the partially exposed/fully exposed conducting line, thus preventing thermal runaway and over-current condition. The presence of gaps, and hence, the fuses, are scalable and may be tailored to the capacity of currents they must carry with the characteristics of the fuses defined by a circuit designer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to fuse structures in integrated circuits, and specifically, to a novel thermal fuse device and circuit architecture for providing thermal protection of semi-conductor chips and circuit structures. 
     2. Discussion of the Prior Art 
     Thermal runaway may occur in integrated circuit chips/devices due to the following reasons: 1) during a chip/device “burn-in ” performed at high temperature (e.g., 140° C.-150° C.); 2) voltages above 1.5× operating voltages; or 3) if the chips go into latchup. Also, chips can experience current runaway, with accompanying heating and destruction when operating in systems. 
     While current fuse devices are manufactured as elements in semiconductor devices, i.e., IC chips, they are predominantly large and cumbersome requiring application of a laser stimuli for their fuse blowing. Typically, these fuses are applied in circuit trimming applications and/or removing unwanted or defective circuits prior to chip packaging. Currently, there is no thermal fuse protection device that is designed to protect integrated circuit devices from thermal run-away, latch-up, and over-current conditions during normal operation, and, that is capable of being blown at a pre-determined operating condition, e.g., a specified current and/or voltage level, for instance. 
     U.S. Pat. No. 4,272,753 discusses a method for fabricating a discrete metallic fuse device using fusible material which is necked to a narrow region. The fusible material is separated from an insulative layer by an air gap which provides lower thermal conductivity environment. 
     U.S. Pat. No. 4,089,734 discusses a metallic fuse device for read only memory and memory reconfiguration applications which involves a complicated technique including a four layer deposition of metallizations, requires the etching at two sides of a fusible link layer to form the resultant fusible link, and etching of the silicon substrate material to form a mesa structure. 
     U.S. Pat. No. 4,879,587 discusses manufacture of a metallic fuse device that is provided on top of a supporting insulating bridge structure for connecting two conductive regions. 
     The manufacture of each of the fuse devices described in the above-mentioned prior art is generally complicated requiring many processing steps. Additionally, they are treated as discrete elements, and are not integral with the conducting line itself. Further, these prior art fuse devices are not configured to destruct according to a designed voltage and current operating condition, and, do not readily detect thermal runaway phenomena that may occur in integrated circuits. 
     Accordingly, it would be highly desirable to provide a novel electrical fuse structure for integrated circuits that comprises actual conductive chip wiring and that may be easily blown at a specified voltage and current level, and, that may be used to provide thermal runaway and over-current protection for integrated circuit devices and structures. 
     It would additionally be highly desirable to provide a novel electrical fuse structure for integrated circuits that are defined by the designer and tailored to the capacity of currents they must carry. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a novel electrical fuse structure for integrated circuits that comprises actual conductive chip wiring and, that may be used to provide thermal runaway protection and over-current protection for semi-conductor circuit devices and structures. 
     It is another object of the present invention to provide a novel high-performance signal line structure in integrated circuits having a locally minimized thermal conduction path below a conductor to the silicon substrate to enable a more controlled fusing activation at lower current and voltage levels. 
     Such fuses prevent excessive current/thermal build-up in the chip, even in case of a catastrophic event such as latchup, and enables immediate corrective action in case such an excessive current/thermal build-up condition is detected. The chip is self protected, and cannot damage the board, power supply, other chips, etc. 
     According to the principles of the invention, there is provided a current overload protection system for an integrated circuit chip having a power source comprising a current monitoring device for monitoring current between the power source and a node of an integrated circuit device in the chip, the current monitoring device existing in a first operating state when the current is at or below a predetermined value and existing in a second operating state when the current exceeds the predetermined value indicating current overload condition; and, a voltage detection circuit responsive to the first operating state of the current monitoring device for deactivating implementation of a second integrated circuit device in the chip that permits corrective action of the overload condition, and, responsive to the second operating state for activating the second integrated circuit device for enabling continuous operation of the integrated circuit chip. 
     Preferably, the invention employs a gap conducting structure for the integrated electronic circuit that functions as an electronic fuse device and that is integrated as part of the semi-conductor chip wiring for providing over-current and thermal runaway protection. The gap conducting structure includes one or more air gap regions of predefined volume that fully or partially exposes a length of interlevel conductor layer in an IC. Alternately, the air gap region may be wholly located within the dielectric region below a corresponding conductor and separated by insulator. When functioning as a fuse, the gap region acts to reduce thermal conductivity away from the exposed portion of the conductor enabling generation of higher heat currents in the conducting line with lower applied voltages sufficient to destruct a part of the partially exposed/fully exposed conducting line, thus preventing thermal runaway and over-current condition. The presence of gaps, and hence, the fuses, are scalable and may be defined by the designer and tailored to the capacity of currents they must carry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features, aspects and advantages of the apparatus and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG.  1 ( a ) illustrates a top-view illustration of the electronic fuse circuit of the invention. 
     FIG.  1 ( b ) illustrates a side-view illustration of the electronic fuse circuit having a conducting element partially exposed in a gap region. 
     FIG.  1 ( c ) illustrates a side-view illustration of the electronic fuse circuit having a conducting element fully exposed in a gap region. 
     FIG. 2 illustrates a dual regulator power supply system for powering an integrated circuit (“chip”) and employing the overcurrent protection capability provided by the novel thermal fuse device. 
     FIG. 3 illustrates a dual-/multi-processor (CPU) system employing overcurrent protection circuitry including the novel thermal fuse device. 
     FIG. 4 illustrates an OCD drive amplifier stage employing overcurrent protection capability. 
     FIG. 5 illustrates the provision of the OCD drive amplifier stage coupled with a second identical drive amplifier stage having a common output and implementing over-current protection for PMOS and NMOS devices. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a novel thermal fuse structure that is integrated as part of the semi-conductor chip wiring and designed to prevent excessive current/thermal build-up in the chip. As will be described in greater detail, the thermal fuse structures themselves may be defined by the designer and tailored to the capacity of currents they must carry. Any chip incorporating the novel thermal fuse is self-protected, thus minimizing damage to the board, power supply, or other chips, etc. Furthermore, it is possible to recover from a failure and re-enable the chip for all types of fails. 
     A method for manufacturing the novel thermal fuse structure is described in commonly-owned, co-pending U.S. patent application Ser. Nos. 09/302,902 and 09/303,509 the whole contents and disclosure of each of which is incorporated by reference as if fully set forth herein. Basically, the design of the fuse is based on the principles described in the following reference: H. A. Schafft, “Thermal Analysis of Electromigration Test Structures”, IEEE Transactions on Electron Devices, Vol. ED-34, No. 3, March 1987, the contents and disclosure of which is incorporated by reference as if fully set forth herein. The reference shows that the temperature of a doped polysilicon, silicided polysilicon line, or a metal line is given by equation (1) as follows:              T   =       T   ambient     +         (     J   2     )     ·   ρ         (       (         K     therm                 cond       ·   Ratio                   of                 dimensions     )     -       (     J   2     )     ·   ρ   ·   B       )     )                 (   1   )                                
     where: ρ=resistivity of the metal in Ω-cm; 
     J=current density in the metal in Amperes/cm 2 ; 
     K thermal coefficient =thermal conductivity of the insulator in Watts/cm°K; and, 
     B=temperature coefficient of resistivity. 
     As metal lines, doped polysilicon and silicided polysilicon in semi-conductor structures are thermally coupled to a silicon wafer by an insulator, and, as a silicon substrate itself is an excellent heat conductor and conducts sufficiently well to prevent the metal from heating, reliability problems associated with current densities are minimized and the integrity of the lines is maintained. 
     According to the invention, if segments of metal conductors are decoupled from the insulating material in order to make the K thermal coefficient  coefficient of the insulator very small, the conductors may heat up. That is, the thermal coefficient in equation (1) may be controlled by design. The denominator of equation (1) comprises a thermal conduction portion minus a heating portion. If the thermal coefficient of conduction is reduced, it is possible to produce heating in the metal conductor with a small current, and cause rapid heating and evaporation of a line, e.g., polysilicon or metal, such as tungsten, aluminum, copper, etc. In fact, the current necessary to provide fusing may be adjusted by the structure. 
     FIG.  1 ( a ) shows the top view of a metal line segment  10  including a metal layer or conductor  12  comprises of aluminum or copper, etc. FIG.  1 ( b ) illustrates a cross section of the line segment  10  including the presence of first insulating layer  13 , e.g., SiO 2  or like oxide, disposed atop of the conducting metal layer  12 , second insulating layer  15 , e.g., SiO 2  or like oxide, a third insulating layer  18 , and, finally the silicon substrate  20 . It should be understood that alternately, insulator  15  may be directly on the silicon substrate  20  obviating need for the insulator layer  18 . As shown in FIG.  1 ( b ), according to the invention, the thermal fuse element  50  comprises one or more gap(s) regions  65  that is/are created under a segment of the conductor  12 , between the line and the silicon substrate, which have the effect of reducing the thermal conduction between the conductor  12  and the silicon substrate  20 . As shown in FIG.  1 ( c ), the thermal fuse conducting portion may be completely exposed within the gap  65 . By decreasing the thermal conductivity between the conductor and the substrate, the line conductor  12  at the fuse location may be designed to “blow” at smaller currents, e.g., depending upon the application. That is, the thermal melting point of the conducting line  12  is reached faster, as a greater amount of heat is generated in the line at the fuse due to the presence of the gaps regions. It should be understood that there may be smaller insulating regions, such as insulator region  15 ′ designed into the fuse  50  for separating gap regions and tailoring the thermal conductivity profile. It should also be understood that, if desired, the gap regions may comprise a dielectric material of low-level thermal conductivity. Additionally, a small layer of insulating material may separate a conductive layer from an air gap region, i.e, the gap does not have to touch the metal or reach it, but, rather may be below the metal with some insulative separation. 
     The air gaps  65  created under the conducting layer  12  and between the substrate  20  also provide a place for the “blown” melted conductor (metal, for example) to collect without stressing other parts of the structure. The first insulator layer  13  shown above the conductor line  12  provides structure for other metal layers, etc., but does not provide a significant thermal path to the silicon substrate. 
     As described in greater detail in commonly-owned, co-pending U.S. patent application Ser. No. 09/302.9023, the structure may be fabricated using a photo layer to open the gap  65  regions in insulator layer  15 ; the opening being temporarily filled with paralene and planarized. Then, metal is deposited over the insulator and paralene regions and the conductor is defined by etching. Then, the insulator  13  is deposited; a hole is etched to the paralene; the wafer is heated a temperature sufficient to vaporize the paralene (100 to 150°); the hole is filled and the surface is planarized. 
     Thus, given the design parameters according to equation (1), the line  12  may be adjusted to heat rapidly at five times (5×) the normal current, for example, to protect a circuit chip in the power supply path, or in a high current OCD path, etc. 
     The overcurrent protection capability employing the thermal fuse is demonstrated by the dual regulator power supply system  112  which powers an integrated circuit (“chip”) as shown in FIG.  2 . In FIG. 2, the dual regulator circuit  112  employs two regulator devices  102  and  104  each connected to chip power (not shown) for regulating Vps (power supply) power applied to the chip. The thermal fuse device  50  of FIG.  1 ( b ), for instance, is connected in series between the power supply Vps and the regulator  102 . A differential comparator device  110  is provided with a terminal coupled to either end of the thermal fuse device  50  for monitoring the voltage difference across the terminals. As long as the thermal fuse is intact and passing current in normal operation, there is a minimum voltage drop across it. Thus, with proper bias, the comparator device  110  will output a logic ‘0’ signal on line  111 , which is used to provide a non-fault output indication  120  at the output of latch circuit  115 . In the event of an over-current condition, e.g., a current demand surge due to a chip failure, latchup, etc., the thermal fuse  50  opens and the current conduction ceases. This condition results in the comparator device  110  sensing a significant voltage drop at its inputs which results in the flipping of its output signal, e.g., to a logic ‘1’, on line  111 , thus signaling a fault to latch  115 . This process deactivates regulator device  102  and sets the stage for activation of regulator device  104 . That is, some intervention, be it human, circuit, or otherwise, is needed to correct the over-voltage/current problem and toggle the enable line  125  to generate activate signal  118  which is used to fully activate the regulator  2 . As shown in FIG. 2, an AND gate  122  is provided to receive the latch output  120  input and the enable input  125  in order to generate activate signal  118  for activating the regulator device  104 . 
     Though the application of FIG. 2 depicts one stage of operation, it is understood that multiple stages of the circuit  100  may be connected such that several regulators may be added as back-up in a more sophisticated fault tolerant system. This concept may additionally be applied to other circuit functions such as receivers, memory controllers, disk drives, modems, whole computer systems, communication systems, satellite systems, etc. 
     The overcurrent protection capability employing the thermal fuse is now demonstrated by a dual-/multi-processor (CPU) system  130  of FIG. 3 which includes a controller device  140  for controlling operation of two (or more) parallel operating processors  142  and  144 . As shown in FIG. 3, the controller  140  additionally controls parallel data bus lines  132   a,b , parallel instruction bus lines  134   a,b  for controlling data and instruction flow to each respective parallel processor. In this application, the power supply Vps provides power to each processor  142 ,  144  via a respective thermal fuse device  52 ,  54 . A respective differential comparator device  152 ,  154  is provided across each thermal fuse  52 ,  54  having respective input terminals coupled to either end of its corresponding thermal fuse device for monitoring the voltage difference across the terminals. As long as the thermal fuse is intact and passing current in normal operation, there is a minimum voltage drop across it. Thus, with proper bias, if operation is normal, the voltage drop is zero, and each respective comparator device  152 ,  154  will output a logic ‘0’ signal on lines  162 ,  164  respectively, which is used to provide a non-fault output indication to the controller device  140 . In the event the processor overheats, or an over current condition exists in the processor, the thermal fuse device will blow and its respective comparator device will toggle, as herein described, and send a signal to the controller  140  indicating that the processor is faulty. The controller may thereafter take appropriate action, e.g., re-direct the task instructions sets and data intended for receipt by the faulty processor to a properly functioning processor. It should be understood that, although only two processors are shown in the example of FIG. 3, the principles described herein may apply to multi-processing systems having six or more parallel processors. 
     FIG. 4 illustrates a further application of the overcurrent protection capability provided by the thermal fuse employed in an off-chip device (OCD) drive amplifier stage  170 . In operation, a pre-driver gain stage  173  is provided having outputs  174   a ,  174   b  for driving a bus output  175 . Respective outputs  174   a ,  174   b  are input to respective transistor devices  180   a  (e.g., P-FET), and  180   b  (e.g., N-FET), capable of handling large currents (100 ma, for example) for driving the output bus  175 . A first thermal fuse device  50   a  is in series with a power source Vps, and connected between the power source and a PMOS “drive” transistor  180   a ; second thermal fuse device  50   b  is in series between the NMOS “sink” transistor  180   b  and ground. Given a tendency of increasingly smaller and scaled P-MOS transistors devices to exhibit a thermal runaway condition and consequent heating and current increase, the fuse  50   a  in series with Vps is designed to open, thus, protecting the PMOS drive device  180   a . Fuse device  50   b  is similarly provided for to NMOS transistor  180   b . Thus, not only is the transistor device itself protected, but so is the board that the chip is mounted on. 
     FIG. 5 illustrates an OCD pre-drive amplifier stage  170  of FIG. 4 coupled with a second identical pre-drive amplifier stage  190  that is also fused, whose output is common to the same node  175 . As in FIG. 4, each stage circuit implements fuse monitoring devices  50   a ,  50   b  for protecting respective PMOS “drive” transistor  180   a  and NMOS “sink” transistor  180   b . When the first pre-driver stage  170  switches off, e.g., due to an over-current/thermal runaway condition, for example, then the gain stage  193  of second pre-driver stage  190  automatically turns on. Thus, as described herein with respect to the overcurrent protection capability employing the thermal fuse as demonstrated by the dual regulator system of  112  of FIG. 2, a voltage detection circuit  185  comprising the differential comparator device (not shown) having terminals coupled to either end of the thermal fuse device  50   a  for monitoring the voltage difference across the terminals is provided along with a latch circuit (not shown) for latching the comparator output signal indicating existence of an over-current/thermal runaway condition. As long as the thermal fuse is intact and passing current in normal operation, there is no voltage drop across it. Thus, with proper bias, the comparator device of the voltage detection circuit  185  will output a logic ‘0’ signal on line  161 , which during normal operation, is used to provide a non-fault output indication. In the event of an over-current condition, e.g., a current demand surge due to a chip failure, latchup, etc., the thermal fuse  50   a  opens and the current conduction ceases. This condition results in the voltage detection circuit  185  sensing a significant voltage drop at its inputs which results in the toggling of its output signal, e.g., to a logic ‘1’ signal on line  161 , thus signaling a fault. This process deactivates the driver stage  173  for the PMOS transistor  180   a  via signal line  161   a  and simultaneously activates PMOS driver stage provided in pre-drive amplifier stage  193  through inverter  165 . Likewise, a voltage detection circuit  185 ′ comprising the differential comparator device (not shown) having terminals coupled to either end of the thermal fuse device  50   b  for monitoring the voltage difference across the terminals is provided along with a latch circuit (not shown) for latching the comparator output signal indicating existence of an over-current/thermal runaway condition. Again, as long as the thermal fuse is intact and passing current in normal operation, there is no voltage drop across it. Thus, with proper bias, the comparator device of the voltage detection circuit  185 ′ will output a logic ‘0’ signal on line  161 ′, which during normal operation, is used to provide a non-fault output indication. In the event of an over-current condition, e.g., a current demand surge due to a chip failure, latchup, etc., the thermal fuse  50   b  opens and the current conduction ceases. This condition results in the voltage detection circuit  185 ′ sensing a significant voltage drop at its inputs which results in the toggling of its output signal, e.g., to a logic ‘1’ signal on line  161 ′, thus signaling a fault. This process deactivates the signal to the NMOS device in driver stage  173  via signal line  161   a ′ and simultaneously activates the driver for NMOS device of gain stage  193  of pre-driver stage  190  through an inverter  165 ′. 
     While the invention has been particularly shown and described with respect to illustrative and preformed embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention which should be limited only by the scope of the appended claims.