Abstract:
A method and assembly for a spark gap. A plurality of resistors are positioned adjacent the spark gap for reducing the energy in the gap during electrostatic discharge.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a spark gap and more particularly the present invention is directed to a spark gap having enhanced thermal dissipation for the prevention of open circuit formation in high voltage applications. 
     BACKGROUND OF THE INVENTION 
     Silicon integrated circuits are moving inexorably to finer geometries and higher frequencies, use of these circuits is also expanding into new areas requiring improved performance. One of the most important of these new areas is high voltage. 
     In the art presently, a problem exists, where circuit operating voltages are well below the required electrostatic discharge protection level and integrated diodes that have suitable breakdown voltages are often unavailable. 
     A further complication in higher voltage applications relates to the concomitant increase in the amount of energy that must be dissipated in the diode with increasing voltage. 
     In view of the inherent complications in the art, high voltage integrated circuits have not yet reached maturity. 
     Over the years, since the invention of integrated circuits, a greater number of high voltage circuit functions have been integrated into the silicon integrated circuit. Prior to the advent of this incorporation, the high voltage circuit functions were implemented with discrete components or designed into hybrid modules. These two technologies proved to be impractical from a cost effectiveness point of view for a given circuit function. 
     In principle, a simple spark gap could be used to provide protection for high voltage integrated circuits especially as it can hold off high voltages in either direction. A spark gap can be made to operate at less than 1000V. The two problems to be solved are how to safely absorb the increased energy dissipation and how to reduce the voltage. 
     The present invention seeks to overcome the limitations in the art with respect to voltage handling capacity and thermal dissipation. 
     By employing the technology of the present invention, it is possible to reduce the dissipation problem by making the active part of the spark gap large so that the heat is dissipated over a larger area, causing a much smaller rise in temperature. However, without other measures the size of the device needed would be inconveniently large. 
     The present invention solves this problem by making use of poly silicon as the spark gap material so that the dissipation problem is greatly reduced. The higher melting point of poly silicon suppresses open circuit formation and reduces the probability of conductive paths being formed out of evaporated material. 
     SUMMARY OF THE INVENTION 
     In one object of one embodiment of the present invention, there is provided a spark gap assembly suitable for use in an integrated circuit, comprising: 
     a first at least partially conductive electrode; 
     a second at least partially conductive electrode, the first electrode and the second electrode in spaced relation; 
     a first conductive layer overlying the first electrode and a second conductive layer overlying the second electrode; 
     a spark gap formed between the first conductive layer and the second conductive layer; 
     a plurality of discrete resistors in electrical communication with each layer proximate the gap; whereby the resistors absorb energy and limit the energy dissipated in a spark discharge in the gap. 
     The discrete resistors may comprise any suitable material capable of functioning as a resistor. Suitable examples include poly silicon, refractory metal, high melting point alloys. 
     According to another object of one embodiment of the present invention, there is provided a spark gap assembly suitable for use in an integrated circuit, comprising: 
     a hermetically sealed package, the package containing: 
     a first at least partially conductive electrode insulated portion having a conductive portion and an insulated portion; 
     a second at least partially conductive electrode having a conductive portion and an insulated portion, the first electrode and the second electrode in spaced relation and defining a spark gap therebetween; and 
     a plurality of resistors in electrical communication with each the electrode and the spark gap, whereby electrical current between electrodes is divided by the resistors for reducing the thermal energy dissipated in a spark discharged in the gap. 
     As alternatives, a metal with a higher melting point than aluminum instead of poly silicon could be used. 
     Any layer of poly silicon or any conductive layer with a sufficiently high melting point could be used for the spark gap structure. 
     The inventions disclosed herein may be applied to any integrated circuit which requires protection at a high voltage or to any sort of integrated circuit, particularly as it uses only conductive layers that are common to any integrated circuit (e.g. MOS, III/V, silicon carbide, bipolar). 
     It is possible that the application may be found outside integrated circuits, where very finely defined spark gaps are needed. 
     Micro mechanical integrated circuits is also emerging technology that is due to discover ESD damage. These tiny components will be very susceptible to ESD, but in many cases there will be no electronic circuitry to provide protection diodes. It would be simple and cost effective to integrate spark gaps into these devices and, if necessary, introduce an special gas at the required pressure. 
     Having thus generally described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a prior art spark gap assembly; 
     FIG. 2 is a section along line  2 - 2  of FIG. 1; 
     FIG. 3 is a schematic illustration of a further embodiment of the present invention; 
     FIG. 4 is a section along line  4 - 4  of FIG. 3; 
     FIG. 5 is a schematic illustration of a further embodiment of the present invention; and 
     FIG. 6 is a section along line  6 - 6  of FIG.  5 . 
    
    
     Similar numerals employed in the specification denote similar elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIGS. 1 and 2 illustrate a spark gap assembly, globally denoted by numeral  10  which is known in the art. The arrangement includes two isolated metal conductors  12  and  14  in spaced relation forming a spark gap  16 . 
     FIG. 3 illustrates a first embodiment of the spark gap assembly  10 ′ according to one embodiment. In this arrangement, the electrodes comprise a pair of isolated metal electrodes  18  and  20  which are isolated from one another by spark gap  16 . The arrangement  10 ′ also includes a poly silicon layer under each bonding pad  22 . This replaces Al metal by poly silicon contact to metals at the bonding pad  22  to avoid small contacts between metal and aluminum , which may be damaged by temperature and current. 
     Referring now to FIG. 5, shown is a further embodiment of the spark gap assembly of the present invention. The assembly is denoted by numeral  10 ″ and includes electrodes  18  and  20 . Each of the electrodes further includes an array of ballast resistors  24  and  26  extending integrally therefrom. The assembly  10 ″ is very similar to the arrangement in FIG.  3 . The spark gap  16  itself is formed between the long strips of poly silicon connected to electrodes  18  and  20  by the resistors  24  and  26 . The resistors  24  and  26  distribute current generated during an electrostatic discharge along the strips. When an electrostatic discharge occurs between the two electrodes  18  and  20 , the spark gap  16  breaks down by avalanche ionization. The voltage is divided across the resistors  24  and  26  which absorb energy and limit the energy dissipation in the spark discharge. The resistors  24 ,  26  help spread the current evenly along the spark gap  16 . In terms of the breakdown voltage for the spark gap, the gas species, gap length and pressure can be chosen to produce the required voltage in accordance with Pascal&#39;s law. Suitable gases include the noble gases. Typically, low pressures would be used to attain low breakdown voltages in accordance with Panchen&#39;s law. 
     By providing the resistor arrays  24  and  26 , there has been significant progress with respect to heating and erosion in the spark gap  16 . 
     Although FIGS. 5 and 6 illustrate a plurality of resistors in a laterally spaced apart relation, it will be appreciated that other arrangements are possible and will be known to those skilled in the art. Furthermore, regarding the material for the resistors, any material capable of limiting electrical current may be employed. Suitable examples include poly silicon, refractory metals, nichrome and sichrome. 
     The metallization is kept well away from the spark gap because of the high temperatures generated by the discharge. One option is to remove metallization from the device by putting poly silicon under the bonding pad so that no metal/poly silicon contact is needed on the device (see FIG.  3 ). Heating and erosion of the spark gap is minimised by making the spark gap out of massive parallel stripes of poly silicon instead of the customary fine point, or points. The current is limited and spread evenly along the spark gap by building ballast resistors into the structure (FIG.  5 ). 
     Regardless of the materials used or the spark gap width (above about 1 micron), the breakdown voltage tends to remain at about 800V. This is because a certain minimum mean free path is required to initiate ionisation, and further reductions in the gap simply cause the discharge current to move further back on the conductor, maintaining an essentially constant spark length. It has long been known that the breakdown voltage of a spark gap will fall with reduced gas pressure in the gap and our experimental measurements show a reduction in breakdown voltage to about 400V in a commercial ceramic package due to the reduced pressure in the cavity caused by cooling from the high lid sealing temperature. 
     Lower break down voltage could be achieved by deliberately controlling the cavity pressure. A still lower break down voltage may be achieved by introducing special gases into the cavity (for example, noble gases). A device embodying the key structural features is shown in FIG.  5 . 
     FIG. 5, by way of example, shows a practical spark gap where poly silicon is incorporated under the bonding pads and connection to the spark gap is made by a continuous run of poly silicon. Aluminium metallization is used only on the bonding pad to connect the bonding wire through a large contact window to the poly silicon. The spark gap itself is formed between two long stripes of poly silicon connected to the bonding pads by a set of parallel resistors designed to distribute the current evenly along the contact strips. When ESD occurs between the two pads, the spark gap breaks down by avalanche ionization. The voltage is divided across the two resistors, which absorb energy and limit the energy dissipated in the spark discharge. The resistors also help spread the current evenly along the spark gap. The gas species and pressure can be chosen to give the required spark gap break down voltage. 
     Important design variables are the pressure and species of the gas in the hermetic package. 
     Other Applications 
     The ideas outlined in this application can be applied to any silicon integrated circuit that requires protection at a high voltage. It can also be applied to any sort of integrated circuit, particularly as it uses only conductive layers that are common to any integrated circuit (e.g. MOS, III/V, silicon carbide, bipolar). 
     It is possible that the application may be found outside integrated circuits, where very finely defined spark gaps are needed. Micro mechanical integrated circuits is an emerging technology that is due to discover ESD damage. These tiny components will be very susceptible to ESD, but in many cases there will be no electronic circuitry to provide protection diodes. It would be simple and cost effective to integrate spark gaps into these devices and, if necessary, introduce an special gas at the required pressure. 
     Although embodiments of the invention have been described above, it is not limited thereto and it will be apparent to those skilled in the art that numerous modifications form part of the present invention insofar as they do not depart from the spirit, nature and scope of the claimed and described invention.