Abstract:
A vertical spark gap assembly for electronic circuits employing poly silicon. The assembly permits dissipation of higher voltages in spark discharge without shorting in the circuit. The spark gap assembly includes a first partially conductive layer and a second partially conductive layer and a non-conductive material positioned between the layers and maintaining a vertically spaced apart relationship therebetween. At least one opening is provided in the first layer and the second layer with the non-conductive material removed from the layer having at least one opening. As such, the arrangement provides a vertical gap formed between and communicating with each layer.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to an arrangement to alleviate the deleterious effects of electrostatic discharges in electronic circuits and more particularly, the present invention is directed to a vertical spark gap suitable for use in microelectronic circuits. 
     BACKGROUND OF THE INVENTION 
     Spark gaps have been proposed earlier in the art with the objective of counteracting electrostatic discharges on integrated circuits. Previous arrangements employed aluminum, however, due to the physical properties of the metal and especially its low melting point resulted in mass transport through and across the oxides and dielectrics and this was found to be problematic thus making aluminum an impractical choice. 
     Another limitation encountered in this field relates to the control of the breakdown voltage. Spark gaps are typically lateral and formed by photoengraving techniques. This process makes tolerances difficult to control leading to problems in forming short spark gaps. 
     Finally, limitations in successful operation of spark gaps in plastic packages are realized since the air in the gap is displaced by the plastic. 
     SUMMARY OF THE INVENTION 
     One object of one embodiment of the present invention is to provide a spark gap assembly suitable for use in electronic circuits, comprising: 
     a first at least partially conductive layer; 
     a second at least partially conductive layer; 
     nonconductive material positioned between the first layer and the second layer maintaining a vertically spaced relationship therebetween; 
     at least one opening in at least one of the first layer and the second layer, the nonconductive material removed from the layer having the at least one opening, whereby a vertical gap is formed between and communicates with each layer. 
     A further object of one embodiment of the present invention is to provide a spark gap assembly, comprising: 
     a first at least partially conductive layer; 
     a second at least partially conductive layer; 
     nonconductive material positioned between the first layer and the second layer maintaining a vertically spaced relationship therebetween, each layer in an overlying relationship with the material; and, 
     at least one opening in at least one of the first layer and the second layer, the nonconductive material removed from the layer having the at least one opening, the opening comprising a vertical spark gap for dissipating electrostatic charge. 
     As a further object of one embodiment of the present invention, there is provided a method of forming a vertical spark gap suitable for use in dissipating electrostatic buildup in an integrated circuit, comprising: 
     providing a first at least partially conductive layer and a second at least partially conductive layer; 
     positioning nonconductive material between the first layer and the second layer maintaining a vertically spaced relationship therebetween; and 
     forming at least one opening in one of the first or the second layer by etching insulating material associated with the first layer and the second layer to form a vertical gap therebetween. 
     Having thus described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a spark gap arrangement of the prior art; 
     FIG. 2 a  is a cross-section of an integrated circuit; 
     FIG. 2 b  is an end elevation view of FIG. 2 a;    
     FIG. 3 a  is a plan view of a first embodiment of the present invention; 
     FIG. 3 b  is a side view of FIG. 3 a;    
     FIG. 4 a  is a plan view of a second embodiment of the present invention; 
     FIG. 4 b  is a cross-section of FIG. 4 a;    
     FIG. 5 is a plan view of a further embodiment of the present invention; 
     FIG. 6 a  is a top plan view of a further embodiment of the present invention; 
     FIG. 6 b  is a cross-section of FIG. 6 a;    
     FIG. 7 a  is a top plan view of yet another embodiment of the present invention where a spark gap structure is shown to incorporate a metal to N+ arrangement; 
     FIG. 7 b  is a top plan view of yet another embodiment of the present invention where a spark gap structure is shown to incorporate a different metal to N+ arrangement; 
     FIG. 7 c  is a top plan view of the arrangement includes two conductive metals; 
     FIG. 7 d  is the arrangement includes a metal to a poly; 
     FIG. 7 e  is the arrangement includes a metal to gate poly; 
     FIG. 7 f  is a further metal to poly; 
     FIG. 7 g  is a gate poly to N-substrate; 
     FIG. 7 h  is a poly to an N-substrate; 
     FIG. 7 i  is a metal to a gate poly; and 
     FIG. 7 j  is poly to a P-Well. 
    
    
     Similar numerals employed in the text denote similar elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 illustrates a typical lateral spark gap assembly, globally denoted by numeral  10  in which strips of metal  12  and  14  are placed in close proximity and suitable for connection to nodal points in a circuit (not shown) to be protected by the gap. The gap is generally designated by numeral  16 . 
     FIG. 2 a  illustrates a cross-section of a typical integrated circuit having a passivation layer  18 , a metal layer  20 , dielectric layer  22 , a first level metal layer  24 , a pyroglass layer  26 , a second level poly silicon layer  28 , a capacitor oxide layer  30 , a first level poly silicon layer  32 , a thermal oxide layer  34 , a diffusion layer  36  and a substrate layer  38 . With reference to FIG. 2 b,  implementation of a vertical spark gap involves a lateral component broadly denoted by numeral  40  and this introduces alignment errors. 
     This problem has been alleviated by the invention and with reference now to FIG. 3 a,  shown is a conductor in a first layer  42  which surrounds and overlies the second conductor  44 . This vertical arrangement avoids the alignment problem set forth with respect to FIG. 2 b.  FIG. 3 b  illustrates the arrangement in FIG. 3 a  in cross-section for greater detail and illustrates the vertical spark gap  46  formed between conductive layer  42  and conductive layer  44 . In each case, the conductive layers  42  and  44  have an insulating material  48  positioned therebetween and by etching (to be discussed in greater detail hereinafter), the insulator material around the opening between the two layers can be removed to create an open gap between the two layers  42  and  44 . 
     FIG. 4 a  illustrates a further embodiment of the invention in which conductive layer  44  includes an opening  50 . The insulator material  48  is removed about hole  50  thus providing the vertical air gap shown more clearly in the cross-section of FIG. 4 b.  In this manner, the air gap  50  is formed between the underside of the hole  50  and the lower plate and conductive layer  44 . It will be apparent to those skilled that this arrangement could easily be reversed. This structure provides a vertical spark gap with advantage of providing a well controlled, and if required, extremely small air gap which may be of the order of nanometers. The thickness of the insulating layer  48  can be used to set the spark gap voltage depending upon the intended use for the spark gap. The opening  50  is used to expose the underlying insulator to a process for removing the insulator from the region of the hole  50  to form an air gap between the two conductors  42  and  44 . Suitable methods such as etching or other known procedures can be employed to effect this result. The opening  50  can also serve to exclude packaging material from the gap if it is made narrow enough (&lt;1 μm). 
     As an alternative, as illustrated in FIG. 5, in the context of an integrated circuit (not shown), the bottom plate could be a first level of metal  52  separated from a second layer of metal  54  by dielectric  56  shown in chain line. Double dielectrics will be readily apparent to those skilled in the art. In this arrangement, the spark gap or opening, represented by numeral  58 , is in the form of a narrow slot. 
     Many implementations are possible and the one selected will depend upon the application intended with the primary factor being the vertical spark gap dimension. In, for example, high voltage discharge applications in a plastic package, reference will be made to FIGS. 6 a  and  6   b.  In the embodiment shown, the arrangement includes a poly silicon layer  58  having a narrow slot  60  graved through it to the underlying oxide, generally denoted by numeral  62  and comprising the second conductive layer in this example. The arrangement is exposed to etchant to remove insulation material  64  between the slot  60  of membrane  58  and layer  62  to thus form the spark gap  66 . In an electrostatic discharge, an electric field is developed between the periphery of the slot  60  and the lower plate  64 . Avalanche or dielectric breakdown of the gas in the spark gap  66  will occur (depending upon the spark gap dimension) leading to a low electric discharge between plates  58  and  62 . The breakdown voltage is made lower than the damage threshold of the component to be protected (not shown), no damage to the circuit will result. Either or both of the plates  58 ,  62  can be designed to limit the energy dissipated in the spark gap region  66 . 
     It has been found that a vertical spark gap can be constructed between any two conductive or semi-conductive layers on an integrated circuit. The availability of conductive layers and the spacings will vary from process to process. FIGS. 7 a  through  7   j  show alternative examples for the use of double metal, double poly silicon integrated circuit processes. In the embodiments of  7   a  through  7   j,  the conductive layer is represented by numeral  52 , conductive layer  2  is represented by numeral  54 , the spark gap by numeral  66 , the N+ active represented by numeral  70 , the cap poly silicon by numeral  72 , the gate poly silicon by numeral  74 , the contact points by numeral  76 , the P+ active by numeral  78 , the N-Well by numeral  80 , and the P-Well by numeral  82 . 
     Suitable materials which can be employed for the spark gap assembly according to the present invention can include the refractory metals and single crystal silicon, poly silicon and high melting point alloys. 
     An important feature in this invention is that by making the slot in the top conductor small enough, plastic material can be excluded from the gap thereby allowing application to integrated circuits packaged in plastic. 
     A second extremely important feature is that these devices can be made with very low parasitic capacitance thereby allowing applications to the radio frequency market where input protection has, to date, not been feasible. 
     Applications are also possible in micro mechanical devices where junction diodes are, typically, not present. 
     The invention can be applied to any variation of an integrated circuit as set forth herein previously and is particularly well suited for materials that are most suited for high voltage applications such as silicon carbide and diamond, both of which have large band gaps and high thermal conductivities. 
     In view of the fact that extremely short spark gaps are possible according to the present invention, the electrostatic discharge will be due to gas dielectric breakdown within the gap rather than by avalanche breakdown. Accordingly, this will extend the breakdown voltage to the range of values previously realized by making use of junction diodes. The energy dissipated in these low voltage discharges will be low enough to permit a very small spark gap device to be used. 
     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.