FIELD 
     This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to a design for a protection circuit to protect the functional circuit from electrostatic discharge. 
     BACKGROUND 
     Most integrated circuits are designed to carry a given, relatively small amount of current, on the order of about ten milliamperes. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices. 
     When a current flow that is larger than that for which the integrated circuit is designed is passed through the circuit, the current flow tends to destroy various elements of the integrated circuit, such as gate dielectrics and junction, rendering it either unstable or inoperable. One source of excessive current flow is called electrostatic discharge. Electrostatic discharge is a general condition where a charge imbalance builds up over a period of time, caused by one or more of a variety of conditions, and then is suddenly released. The current flow, although extremely brief, can be quite high, such as on the order of about ten amperes, or about 10,000 times the normal operating current of an integrated circuit. Electrostatic discharge is not an uncommon occurrence in circuits. If unaccounted for in the design of integrated circuits, electrostatic discharge can potentially be a major cause of failure for integrated circuits. 
     Various methods and structures for the shunting of electrostatic discharge have been proposed. For example, shunting circuits operate to divert the current flow from an electrostatic discharge around the functional portions of the integrated circuit and through the shunt, which is designed to accommodate a larger current flow. Unfortunately, this method of protection requires the circuit designer to identify every circuit path that the discharge might follow, and insert an appropriate shunt around each such discharge path. Such shunts are placed in parallel to the circuits that they are designed to protect. 
     What is needed, therefore, is a system for inhibiting electrostatic discharge from damaging integrated circuits that overcomes problems such as those described above, at least in part. 
     SUMMARY 
     The above and other needs are met by an improvement to a digital integrated circuit of the type having a functional circuit that is susceptible to damage from an electrostatic discharge. An electrostatic discharge protection element is placed in series with the functional circuit and disposed upstream in a normal direction of current flow from the functional circuit. The electrostatic discharge protection element includes at least one of a resistive choke that exhibits thermal runaway and an inductive choke. 
     By using a direct series protection device in this manner, the many discharge paths do not need to be identified. Instead, The electrostatic discharge current is arrested where it enters the circuit to be protected. The protection point can be either very near the circuit to be protected, or very far upstream from the circuit. Because there tend to be fewer entrance paths than exit paths for electrostatic discharge, the implementation of these series protection devices can be much easier than that of parallel protection devices. 
     In various preferred embodiments, the electrostatic discharge protection element is only the resistive choke, only the inductive choke, or both the resistive choke and the inductive choke. The resistive choke preferably has at least one of (1) a material having a relatively high temperature coefficient of resistance, from which the resistive choke is formed, (2) a material having a relatively high specific heat capacity, within which the resistive choke is immediately disposed, and (3) a pinch point that creates a relatively high current density within the pinch point of the resistive choke. The inductive choke is preferably at least one of a spiral inductor, a relatively long electrically conductive line, a ferrite block, bonding wires between the integrated circuit and a package substrate on which the integrated circuit is packaged, and a transmission line with dead-end electrically conductive stubs disposed along its length. 
     The electrostatic discharge protection element may be disposed on a common monolithic substrate with the integrated circuit, on a package substrate on which the integrated circuit is packaged, or between the integrated circuit and the package substrate on which the integrated circuit is packaged. When the electrostatic discharge protection element is only the inductive choke, then it may include an additional parallel shunt impedance element. The resistive choke preferably includes at least one of a copper bottleneck, a polysilicon line, a salicided polysilicon resistor, a non-salicided polysilicon resistor, and a tungsten contact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIG. 1  is a functional block diagram of an integrated circuit mounted on a package substrate and electrically connected to the package substrate via a bonding wire. 
         FIG. 2  is a circuit diagram of a resistive choke electrostatic discharge protection device according to a preferred embodiment of the invention. 
         FIG. 3  is a circuit diagram of an inductive choke electrostatic discharge protection device according to a preferred embodiment of the invention. 
         FIG. 4  is a circuit diagram of a combined resistive choke inductive choke electrostatic discharge protection device according to a preferred embodiment of the invention. 
         FIG. 5  is a circuit diagram of a resistive choke electrostatic discharge protection device with a pinch point according to a preferred embodiment of the invention. 
         FIG. 6  is a circuit diagram of an inductive choke electrostatic discharge protection device formed from a transmission line with dead-end electrically conductive stubs along its length according to a preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Direct series protection devices, also referred to as blocking devices, impedance devices, or chokes herein, are beneficial because the discharge paths do not need to be identified. Instead, the onslaught of electrostatic discharge current is arrested at its entrance. Because there tend to be fewer entrance paths than exit paths for electrostatic discharge, the implementation of series protection devices can be much easier than that of parallel protection devices.  FIG. 1  is a functional block diagram of a packaged integrated circuit  10 , and provides a general overview of the elements of the present invention, as described below. 
     There are two basic types of series impedance devices  16  according to the present invention: (1) resistor devices  26  and (2) inductor devices  28 , as depicted in  FIGS. 2-6 . The main attributes of each type of serial device  16  is that the impedance of the protection device  16  preferably remains relatively small during normal integrated circuit  14  operation in relation to the rest of the circuit  18  in which it is disposed, so that the impedance of the protection device  16  does not significantly degrade the operation of the integrated circuit  14 , such as its speed. For the present purposes, “relatively small impedance” means that the series protection impedance is less than about 5% of the total impedance of the I/O net. However, the impedance of the protection device  16  becomes relatively large during an electrostatic discharge event, thereby blocking large currents from flowing through the integrated circuit  14 . In other words, the series protection device impedance  16 , after increasing its impedance, will be at least several times the normal I/O net  18  impedance, and large enough to reduce the electrostatic discharge current going into the system  18  by more than about twenty percent. 
     Resistive Choke 
     The operation of resistive chokes  26  is based on a phenomenon called joule heating. Ohm&#39;s law states that the voltage drop (measured in volts) across a resistor is equal to the resistance of the resistor (measured in ohms) multiplied by the current (measured in amperes) flowing through the resistor. Thus, the higher the electrostatic discharge current, the larger the drop in the voltage across the protection resistor  26 , which thereby reduces the electrostatic discharge voltage appearing at the end of the protection resistor  16 . 
     In order not to lower performance during normal chip operation, the resistance of the protection resistor  26  preferably remains small during the normal operation of the integrated circuit  14 , but becomes large when a large current is detected, such as during an electrostatic discharge event. One method of accomplishing this change in resistance is to use the temperature rise that is caused by joule heating within a resistor during an electrostatic discharge event. 
     The large current that starts to flow through the protection resistor  26  causes an increase in the current density within the resistor  26 . The increase in current density causes heating in the resistor  26 , due to the joule effect. For the present purposes, a “relatively high current density” means more than an order of magnitude increase in the normal operating mode current density. The heating of the resistor  26  increases the resistivity of the resistor material, which thereby increases the resistance of the protection resistor  26 . The increase in the resistance then causes more heating within the protection resistor  26 . Thus, as a large current occurs, such as from an electrostatic discharge event, the resistance of the protection resistor  26  rises, thereby reducing the output voltage on the protection resistor  26  and protecting all of the downstream circuits  18 . 
     This spiraling cycle of heating, which causes more resistance, which causes more heating, is generally called thermal runaway and is greatly disfavored in the resistors that are used in integrated circuits. The reason for this is that circuit designers want their circuits to operate predictably under a wide range of conditions. Therefore, great effort is made to design a resistor that will not experience this thermal runaway. According to the present invention, however, special protection resistors  26  are inserted into the circuit design, which are specifically designed to exhibit a relatively high degree of thermal runaway. For the present purposes, a “relatively high degree of thermal runaway” means a degree at which the current from an electrostatic discharge event is reduced to a point that equilibrium is achieved. For example, if the current density is increased by three times, the joule heating may increase by ten times, which causes the resistance to double—this positive feedback cycle fuels the thermal runaway. The resistance will keep on increasing until it becomes so big that it starts to reduce the current, at which point equilibrium is achieved. 
     There are a few different methods by which such a resistor  26  can be fabricated. For example, selecting a material for the resistor  26  that has a “relatively large temperature coefficient of resistance,” which is defined as a temperature coefficient of resistance of from about three thousand parts per million per centigrade to about six thousand parts per million per centigrade, within the temperature ranges anticipated for normal use of integrated circuits. Some materials have a very little change in resistance as they are heated, while other materials have a very large change in resistance as they are heated. The property of the material that controls this effect—whether it be large or small—is called the temperature coefficient of resistance of the material. Resistors in standard integrated circuit designs are fabricated with materials having low temperature coefficients of resistance, meaning that their resistance tends to change very little, or not at all, as they heat. 
     Another method by which such a protection resistor  26  can be fabricated is to thermally insulate the resistor  26 , such that any heat generated by the resistor tends to remain trapped within the resistor, thereby causing an elevation in the temperature of the resistor  26 , and thereby further exacerbating the thermal runaway of the protection resistor  26 . Low k dielectric materials tend to have a relatively low thermal conductivity, meaning about an order of magnitude lower than that of silicon dioxide. This means that they do not dissipate heat quickly to surrounding structures. Thus, immediately surrounding the protection resistor  26  with a material having a relative low thermal conductivity, with no heat absorbing materials between the two, tends to enhance the protection properties of the resistor  26 . 
     Yet another method of enhancing the desired properties of a choke resistor  26  is to fabricate the protection resistor  26  with a geometry or shape that enhances the joule heating effect. One way of accomplishing this is to reduce the cross sectional area of the resistor  26  at some point along its length, such as by either reducing its width, thickness, or both, as depicted in  FIG. 5 . In so doing, the current density within this pinch point  30  of the resistor  26  is naturally increased, regardless of the amount of current that is flowing through the protection resistor  26 . As described above, this increase in current density tends to trigger the joule heating effect, which is the basis for the choke. Increasing the length of the resistor  26  within this pinch point  30  may also increase the thermal runaway. 
     By combining elements such as these in the design and fabrication of a protection resistor  26 , a ten fold increase in the resistance of the protection resistor can be achieved. It is desirable, however, when using the methods described above, to construct a protection resistor  26  that does not have a significant amount of resistance at normal operating conditions, such that it would impair the normal operation of the circuit  14  in which it is placed. Further, it is desirable that the choke  26  does not exhibit any significant thermal runaway during normal operation of the circuit  14 , for the same reasons. In normal operation, the series protection resistor  26  should have no more than about two to three ohms. During the electrostatic discharge event, the desired resistance should be at least about sixty to one hundred ohms. 
     Resistors  26  such as those described above can be formed of a variety of different materials and in a variety of different locations, within the parameters described above. For example, resistive materials such as polysilicon and tungsten can be used. Materials with non-linear resistivity, such as non-salicided polysilicon, can also be used. The resistive choke  26  can be implemented either on the chip of the integrated circuit  14  itself, or in the package substrate  12  on which the integrated circuit  14  is packaged, or between the integrated circuit  14  and the package substrate  12 , all as depicted in  FIG. 1 . Possible implementation on the chip  14  includes copper bottlenecks, polysilicon lines, salicided and non-salicided polysilicon resistors, and tungsten contacts. Because these protection devices  16  work in series, they can be placed at the many fewer inlets for electrostatic discharge current, rather than in parallel with all of the many discharge paths. 
     Inductive Choke 
     The voltage drop across an inductor is equal to the inductance of the inductor multiplied by the time rate of change of the current flowing through the inductor. In other words, the faster the current through the inductor, the larger the voltage drop across the inductor, which reduces the voltage spike appearing at the end of the inductor. Because electrostatic discharge events tend to create very sudden current flows, an inductor  28  can work well as a serial electrostatic discharge event protection circuit  16 . Similar to that as describe above in regard to the resistive choke  26 , it is desired that the inductive choke  28  function in a manner where it doesn&#39;t significantly impede the normal operation of the circuit  14  that it protects. The inductance of the inductive choke  28  is preferably below about five to ten nanohenrys (defined as a “relatively small impedance”) during normal operation of the integrated circuit  14 , in order for its impedance to not alter the performance of the circuit  14 . 
     Therefore, in order to not lower the performance of the integrated circuit  14  during normal operation, the impedance of the inductor  28  preferably remains relatively small during normal operation of the circuit  14 , but becomes relatively large when an electrostatic discharge current transient is present. A “relatively large impedance” is defined to be at least about sixty to one hundred nanohenrys, which is sufficient to reduce the electrostatic discharge current. 
     The inductive choke  28  can be implanted on the chip with the integrated circuit  14  (depicted as  16   a  in  FIG. 1 ), such as a spiral inductor as depicted in  FIG. 3 , a long metal line, and a transmission line with stubs as depicted in  FIG. 6 . For the present purposes, “long” means a metal line that is at least about a few millimeters in length. It can also be implemented off of the chip  14 , such as in the package substrate  12  (depicted as  16   c  in  FIG. 1 ) or between the chip  14  and the package substrate  12  (depicted as  16   b  in  FIG. 1 ), such as a discrete inductor, a ferrite block, a transmission line with stubs, and bonding wires  22  between the pad  24   a  of the substrate and the pad  24   b  of the chip  14 . Use of inductors is typically limited to analog circuits, rather than the digital circuits of the present invention. 
     Inductor chokes  28  can be designed with an additional shunt (or parallel) impedance  20 , which design tends to provide better performance than either the shunt  20  or series inductor  28  alone. 
     The use of either a resistive choke  26  or an inductive choke  28  as depicted singly in  FIGS. 2-3 , or both together as depicted in  FIG. 4 , tends to require less space on the chip  14  than a shunt. Further, they block the electrostatic discharge at the entrance, without needing to know the many different discharge paths. Such chokes  16  do not affect the normal operation of the circuit  14 . Resistive chokes  26  tend to block high current, and inductive chokes  28  tend to block fast current. The use of both types of chokes  16  together can block both types of events. However, electrostatic discharge events tend to have both of these characteristics, and so these two types of chokes  16  can also be used individually to protect a circuit  14  against an electrostatic discharge event. 
     The foregoing description of preferred embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
An improvement to a digital integrated circuit of the type having a functional circuit that is susceptible to damage from an electrostatic discharge. An electrostatic discharge protection element is placed in series with the functional circuit and disposed upstream in a normal direction of current flow from the functional circuit. The electrostatic discharge protection element includes at least one of a resistive choke that exhibits thermal runaway and an inductive choke.