Patent Publication Number: US-11393636-B2

Title: Ceramic overvoltage protection device having low capacitance and improved durability

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of pending U.S. patent application Ser. No. 16/195,159 filed Nov. 19, 2018 which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention is related to a ceramic overvoltage protection device which provides improved control over the trigger voltage, has reduced leakage current and which does not break down with repeated overvoltage pulses. 
     There is an ongoing need in modern electronics for increased protection of semiconductors, integrated circuits and other components from Electro-Static Discharge (ESD) and fast electronic transients. ESD can reach over 30 kV which is well beyond that which a core processor is capable of withstanding. ESD is a primary cause for the failure of integrated circuits and this is a particular problem with respect to miniaturization of electronics wherein suitable protection has proven difficult to provide reliably. 
     The necessity for ESD protection has increased that portion of the available space in a device devoted to ESD protection. This utilization of space, for other than device functionality, is contrary to the constant desire for miniaturization of components and devices. In addition to the space consumption of on-chip ESD protection devices they also inhibit the speed and quantity of data that can be processed. Therefore, there is a desire for off-chip protection and, particularly, discrete components which can provide ESD protection, without loss of processing speed, and which do not consume valuable space within the circuit design or in the device. 
     Multilayer Ceramic Capacitor (MLCC) devices have an established role in ESD protection. The higher capacitance levels of an MLCC absorb charge from an ESD or transient event. Unfortunately, high capacitance compromises signal integrity in high speed data applications. If the capacitance of the MLCC is reduced its ability to absorb the ESD or transient event is lowered because the charge absorbed, Q is dependent on the capacitance C, and voltage V; Q=CV. It can therefore be seen that if capacitance is lowered by 50% the voltage that the capacitor has to withstand has to increase by 50% to maintain the same capability to absorb the coulombic charge, Q. In practice it is not possible to increase the voltage withstanding capability of low capacitance MLCC to this extent so their capability is reduced. Furthermore, when protective devices such as varistors are employed the leakage current to ground, while the device is inactive, can be a major drain on the standby life of battery powered devices. Any parasitic drain on a battery is undesirable and therefore protection devices with high leakage and high capacitance are less desirable. 
     To meet the demands of modern circuitry an ideal ESD protection device would preferably have a low capacitance with negligible leakage current to ground in its inactive state. When exposed to elevated voltage or current, due to an ESD event, the ESD protection device should respond rapidly, such as &lt;1 ns, to divert potentially harmful transient energy to ground via a low resistance path. After the ESD event has subsided the ESD protection device should revert to its previous inactive state. Furthermore, the ESD protection device should be able to withstand multiple ESD or transient events while returning to near the pre-event capacitance and near pre-event leakage current characteristics of its initial inactive mode. Meeting all of these criteria, particularly with high voltage applications, has proven to be difficult as evidenced by the lack of a suitable ESD protection devices available to designers of modern circuitry. 
     Provided herein is an improved ESD protection device which meets the advanced demands of modern circuitry. 
     SUMMARY OF THE INVENTION 
     The invention is related to an improved overvoltage protection device and a method for forming the improved overvoltage protection device. 
     It is an object of the invention to provide an overvoltage protection device with low capacitance, low leakage current and the ability to withstand many ESD pulses. 
     A particular feature of the invention is the ability to manufacture the overvoltage protection device using standard manufacturing techniques used for the manufacture of MLCC capacitors. 
     Another feature of this invention is to allow high ESD protection to be combined with a low capacitance multilayer ceramic capacitor such that the combined device can protect from high transient voltages whilst retaining capacitance for signal filtering while occupying only a minimum amount of available space. 
     These and other embodiments, as will be realized, are provided in an overvoltage protection device. The overvoltage protection devices comprises at least one ESD protection couple comprising discharge electrodes in a plane, a gap insulator between the discharge electrodes, an overvoltage protection element parallel to the planar discharge electrodes wherein the overvoltage protection element comprises a conductor and an secondary material. The overvoltage protection element also comprises a primary insulator layer between the discharge electrodes and overvoltage protection element. 
     Yet another embodiment is provided in a method for forming an overvoltage protection device. The method comprises the steps:
     forming at least one first layer comprising a secondary insulator precursor and an overvoltage protection element precursor on a primary insulator precursor;   forming at least one second layer on an internal insulator precursor comprising a pair of discharge electrodes and a gap insulator precursor between the discharge electrodes;   forming a stack comprising at least one first layer on at least one second layer in registration with the over overvoltage protection element precursor overlaying the gap insulator precursor; and   heating the stack to form a laminated stack comprising:   discharge electrodes in a plane;   a gap insulator between the discharge electrodes in the plane;   an overvoltage protection element parallel to the discharge electrodes; and   a primary insulator layer between the discharge electrodes and overvoltage protection element.   

     Yet another embodiment is provided in a dual-function overvoltage protection device. The overvoltage protection device comprises at least one ESD protection couple comprising discharge electrodes in a plane, a gap insulator between discharge electrodes, an overvoltage protection element parallel to the discharge electrodes and a primary insulator layer between the discharge electrodes and overvoltage protection element. The dual-function overvoltage protection device further comprises a capacitive couple. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  is a cross-sectional schematic view of an embodiment of the invention. 
         FIG. 2  is a cross-sectional schematic view of an embodiment of the invention. 
         FIG. 3  is a cross-sectional schematic expanded view of an embodiment of the invention. 
         FIG. 4  is a cross-sectional schematic expanded view of an embodiment of the invention. 
         FIG. 5  is a top schematic view of an embodiment of the invention. 
         FIG. 6  is a flow-chart representation of an embodiment of the invention. 
         FIGS. 7 and 8  are graphical representations illustrating the advantages of the invention. 
     
    
    
     DESCRIPTION 
     The instant invention is related to an improved ESD protection device which has a low capacitance and low leakage current in the inactive state, has a rapid response time in the presence of ESD events, and which can withstand many ESD events without compromise of capacitance, leakage current or functionality. More specifically, the present invention is related to a layered structure comprising a discrete insulator layer as a laminate between discharge electrodes and an overvoltage protection element. The structure provides for optimization of the trigger voltage which is the threshold voltage at which transient energy is diverted to electrical ground. The instant invention is also related to an ESD protection device which also comprises a capacitive couple suitable for signal filtering below the threshold voltage wherein above the threshold voltage the transient energy is diverted to electrical ground. 
     During normal operation the ESD protection couple of the ESD protection device is passive, or does not provide any functionality to the circuit, so by design the ESD protection couple is expected to exhibit low capacitance so that high speed signal distortion can be minimized. However, during an ESD event the ESD protection couple effectively acts as a switch allowing excess voltage to be dispersed to electrical ground. 
     The invention will be described with reference to the figures forming an integral, non-limiting, component of the disclosure. Throughout the description similar elements will be numbered accordingly. 
     An embodiment of the invention will be described with reference to  FIG. 1  wherein a ESD protection couple,  2 , is illustrated in cross-sectional schematic view. In  FIG. 1 , discharge electrodes,  12 , which are preferably coplanar, are separated in the plane by a gap insulator,  14 . A primary insulator layer,  18 , extends in a planar manner over the discharge electrodes forming a laminate structure. An overvoltage protection element,  16 , is separated from each discharge electrode by the primary insulator layer,  18 . A secondary insulator,  20 , preferably surrounds the overvoltage protection element within the layer comprising the overvoltage protection element. In a preferred, but non-limiting embodiment, the gap insulator, primary insulator layer and secondary insulator are the same material for manufacturing convenience. The ESD protection couple functions by conducting the pulse energy between the discharge electrodes, via the overvoltage protection element, during the transient event wherein the pulse energy passes through the primary insulator layer at each transit between the overvoltage protection element and discharge electrode. In a particularly preferred embodiment the width of the overvoltage protection element,  16 , is no more than the width of the gap insulator,  14 , thereby minimizing the overlap of the overvoltage protection element and the discharge electrodes,  12 . 
     An embodiment of the invention is illustrated in cross-sectional schematic view in  FIG. 2 . In  FIG. 2 , the ESD protection device,  10 , comprises multiple ESD protection couples,  2 , in a layered or laminate arrangement. An optional but preferred internal insulator,  24 , may be between adjacent ESD protection couples. An external insulator,  22 , is external to all ESD protection couples. The external insulator and internal insulator are preferably the same for manufacturing conveniences without limit thereto. External terminations,  26 , in electrical contact with the discharge electrodes,  12 , allow the ESD protection device to be electrically connected to a circuit as would be realized by those of skill in the art. 
     In a further embodiment of this invention the aforementioned ESD protection device may be combined with a capacitive couple to form a dual-function ESD protection device. The capacitive couple may consist of at least 2 overlapping electrodes of opposite polarity or the capacitive couple may employ a floating electrode. When a capacitive couple is incorporated it is desirable to minimize the overlap area between the discharge electrode and overvoltage protection element to reduce the stray capacitance thereby maintaining a low capacitance for the combined component. For convenience of manufacture the primary insulator and dielectric of the capacitor element is the same. In this case it is essential that the thickness of the dielectric separating the electrodes of the capacitor exceeds the primary insulator thickness. 
     An embodiment of the invention is illustrated in cross-sectional expanded view in  FIG. 3 . In  FIG. 3 , a dual-function ESD device,  11 , is illustrated. The dual-function ESD device comprises up to n ESD protection couples,  2 , wherein n is the number of ESD protection couples in the device. At least m capacitive couples,  4 , are illustrated wherein m is the number of capacitive couples in the dual-function ESD device. The capacitive couples comprise parallel internal electrodes,  25 , of alternating polarity wherein adjacent internal electrodes terminate at different external terminations,  26 , and adjacent internal electrodes are separated by a capacitor ceramic,  23 . The internal insulator,  24 , may be between adjacent ESD protection couples,  2 , which is preferred. It is preferable that capacitor ceramic,  23 , is in alternating layers between adjacent internal electrodes,  25 , such that each set of adjacent internal electrodes forms a capacitive couple and therefore the number of capacitive couples functions as a single capacitor. The capacitor ceramic,  23 , external insulator,  22 , internal insulator,  24 , gap insulator,  14 , and primary insulator layer,  18 , are preferable the same material for manufacturing convenience. When the primary insulator layer and capacitor ceramic are the same composition it is preferable that the separation of the overvoltage protection element,  16 , from the discharge electrodes,  12 , represented as T OVP , is less than the separation distance between adjacent internal electrodes,  25 , represented as T C . A smaller separation distance in the ESD protection couples,  2 , than in the capacitive couples,  4 , prevents permanent electrical breakdown of the capacitative couple upon exposure to an ESD event. More preferably, T C  is over two times larger that T OVP . 
     An embodiment of the invention is illustrated in cross-sectional expanded view in  FIG. 4 . In  FIG. 4 , a dual-function ESD device,  11 , is illustrated which is similar to the device illustrated in, and described relative to,  FIG. 3 . In  FIG. 4  the dual-function ESD device comprises m floating electrode capacitive couples,  41 . The floating electrode capacitive couple comprises coplanar internal electrodes,  25 , of opposite polarity with each terminating to an opposite external termination,  26 . A floating electrode,  31 , is parallel to the coplanar internal electrodes and separated from the plane of the coplanar internal electrodes by capacitor ceramic,  23 . When the primary insulator layer and capacitor ceramic are the same composition it is preferable that the separation of the overvoltage protection element,  16 , from the discharge electrodes,  12 , represented as T OVP , is less than the separation distance between the plane of the coplanar internal electrodes,  25 , and the floating electrode represented as T FLO . A smaller separation distance in the ESD couples,  2 , than in the capacitive couples,  41 , promotes temporary electrical breakdown of the ESD couples in preference to permanent electrical breakdown of the capacitative couples. 
     It is preferred that the capacitance of the ESD protection device is at least 0.1 to no more than 23,000 pF. When the ESD protection devices further comprises at least one capacitive couple it is preferable that the ESD protection device have a capacitance of at least 100 pF and more preferably at least 1000 pF. When the ESD protection device does not comprise a separate capacitive couple it is preferable that the capacitance of the ESD protection device be no more than 100 pF, preferably no more than 10 pF and even more preferably no more than 2 pF. 
     The ESD protection device may be a 2-terminal device wherein a separate external terminal is in electrical contact with adjacent co-planer discharge electrodes, as illustrated in  FIG. 2 . Alternatively, the overvoltage protection elements may terminate and be in electrical contact with secondary external terminations collectively, or independently, to form multi-terminal devices such as a 4-terminal device, as illustrated, without limit thereto, in  FIG. 5 . In  FIG. 5 , secondary external terminations,  28 , may be in electrical contact with the same ESD protection couple, separate ESD protection couples, or a capacitive couple as described relative to  FIGS. 3 and 4 . 
     The number of ESD protection couples in an ESD protection device is not particularly limited. At least one ESD protection couple is necessary for functionality with many hundreds being within the scope of the invention. Above about 20 ESD protection couples the benefit is insufficient to justify the cost and manufacturing complexities associated with the large number of layers. Below about 3 ESD protection couples provides insufficient redundancy. It is preferable to have about 3 to about 10 ESD protection couples which is a balance between manufacturing efficiencies, cost, accumulated device capacitance and effectiveness. 
     The number of capacitive couples in a dual function ESD protection device is not particularly limited. At least one capacitive couple is necessary for functionality with many hundreds being within the scope of the invention. When the number of capacitive couples exceeds about 100 the benefit is insufficient to justify the cost and manufacturing complexities associated with the large number of layers. Below about 3 capacitive couples provides insufficient capacitance in the space allotted. It is preferable to have about 10 to about 20 capacitive couples which is a balance between manufacturing efficiencies, cost and effectiveness, although the capacitance can be tailored for specific application requirements. 
     Incorporation of an insulating material as the primary insulator layer, which is preferably an insulating dielectric material, between the discharge electrodes and overvoltage protection element provides an ESD protection device capable of maintaining a low effective capacitance for minimal signal distortion and low leakage to electrical ground in the passive mode. The primary insulating material is preferably sufficiently insulating to minimize leakage current. It is particularly preferred that leakage current of the ESD protection couple is no more than 5000 nA, more preferably no more than 2000 nA and most preferably less than 1000 nA. Furthermore, the primary insulator layer is preferably capable of withstanding the operating voltage without degradation thereby allowing the ESD protection component to return to a passive mode, after the transient event has subsided, without degradation. The lack of degradation, particularly in the primary insulator layer, improves capacitance stability and leakage current stability even after many transient events. 
     Using conventional MLCC manufacturing techniques an ESD protection device can be manufactured by depositing layers of discharge electrode and overvoltage protection element precursors directly onto a carrier film of ceramic dielectric precursor material. The layers can then be stacked as registered sheets followed by pressing and sintering to create a unified ceramic monolithic component. In this way the overvoltage protection element is separated from the discharge electrodes by a predetermined thickness of insulating material thereby allowing the thickness and composition to be controlled through the selection of the carrier film material. The ability to control the thickness of the primary insulator layer, coupled with the ability to control the composition, allows the trigger voltage to be predictably controlled. Alternatively, an insulating layer may be applied as a liquid, or film, to form an insulating film layer such as a polyimide or other insulating, preferably polymeric, laminated layer. 
     The thickness of the primary insulator layer, or distance between the discharge electrodes and overvoltage protection element, represented as T OVP  in  FIGS. 3 and 4 , can be much less than the typical electrode spacing in spark gap type ESD devices. Conventional spark type ESD devices have a separation between electrodes which is typically at least about 6 μm and can be in excess of 50 μm. With the inventive ESD protection device the distance between the discharge electrodes and overvoltage protection element is preferably no more than 10 μm in order to keep the trigger voltage at which the transient is diverted to &lt;5000V based on an 8 kV pulse. A thickness of the primary insulator of about 1 μm is suitable for demonstration of the invention. 
     The trigger voltage is the voltage below which the capacitive couple, if present, functions as a filter capacitor and the ESD protection couple is passive. At, or above, the trigger voltage the ESD protection couple shunts the excess current to electrical ground. The trigger voltage is based on the composition and thickness of the primary insulator and the composition of the overvoltage protection element. As the thickness of the primary insulator increases the trigger voltage increases with a given primary insulator and overvoltage protection element. It would be understood by one of skill in the art that the desired trigger voltage can be obtained for a preferred primary insulator composition and overvoltage protection element by the intial preparation of multiple ESD protection devices with a series of primary insulator thicknesses, followed by testing, to determine the optimum primary insulator layer thickness. It is desirable that the trigger voltage is at least 20% higher than the working voltage of the part wherein the working voltage is a design choice for the application as well known to those of skill in the art. 
     By eliminating the direct contact between the discharge electrodes and overvoltage protection element the insulation resistance is several orders of magnitude higher than ESD devices made with an overvoltage protection material in direct contact with the discharge electrodes. The presence of the discrete insulator layer, as a laminate, is instrumental in maintaining low leakage current performance in the inactive mode over repeated ESD pulses. The modular, multilayer nature also provides an additional advantage in that many ESD protection couples can be stacked to form the protective element of the component thereby increasing the ability of a device to withstand multiple ESD pulses before performance is degraded. 
     In another embodiment of this device the discharge electrode layers shown can be alternately stacked to provide a capacitive couple within the component as described above. It should also be noted that it may be desirable to use a higher permittivity (“K”) dielectric different from that used for the spark gap element capacitor between the terminals in order to add capacitance to the component dependent on the speed of signal transmission. By controlling the capacitance of the capacitive couple the ESD protection device can provide some noise suppression for slower transmission speeds. 
     To keep the capacitance low in the ESD protection couple it is desirable to reduce the overlap area between the overvoltage protection element and the discharge electrodes. This is because the primary insulating layer is relatively thin. Also for the same reason it is desirable for this insulating layer to have a low dielectric constant, preferably less than 100. This can be explained by applying the universal capacitance equation to the discharge electrode coupling through the primary insulator wherein:
 
 C=K*K   0   *A*n/t  
     where:   C=Capacitance;   K=Dielectric constant of primary insulator;   K 0 =Permittivity of free space (8.854×10 −12  F/m);   A=Overlap Area of discharge electrode and overvoltage protection element;   n=Number of layers of discharge electrode and overvoltage protection element; and   t=Thickness of primary insulator.   Therefore, at a given overlap as the primary insulator thickness decreases the capacitance increases which can be overcome by decreasing overlap area. The overlap protection element may itself contain materials such as barium titanate that have relatively high permittivity so making it desirable to minimize the overlap area.   

     An advantageous attribute of this inventive ESD protection device is that the properties of the materials used in the device have an outstanding ability to function at high operating temperatures and voltages. An ESD protection device can be prepared which is capable of withstanding continuous operation at a high voltage, such as 500V and a high temperature such as 200° C. 
     The primary insulator layer, secondary insulator and gap insulator are independently selected from those material having a low dielectric constant and preferably each independently is, or contains, an insulating ceramic or glass. Low permittivity dielectrics are preferred and preferably the insulating ceramic has a permittivity of no more than 100, and preferably no more than 50. C0G dielectrics are particularly preferred. Particularly preferred materials for the primary insulating layer, secondary insulating layer, gap insulator and capacitor ceramic include calcium zirconate, non-stoichiometric barium titanium oxides such as Ba 2 Ti 9 O 20 ; BaTi 4 O 9 ; barium rare-earth oxides containing neodymium or praseodymium, titania doped with various additives, calcium titanate, strontium titanate, zinc magnesium titanate, zirconium tin titanate and combinations thereof. As would be known to those of skill in the art the material for the primary insulating layer, secondary insulating layer, gap insulator and capacitor ceramic must be thermally compatible with the discharge electrodes to avoid degradation of the discharge electrodes during sintering of the ceramic. 
     The internal insulator and external insulator are not particularly limited herein as these materials can be selected based on cost and compatibility with the other materials. In one embodiment the internal insulator and external insulator are the same as at least one of the primary insulator layer, secondary insulator layer or gap insulator for manufacturing conveniences. 
     The overvoltage protection element comprises a conductor preferably selected from a metal and secondary material wherein the secondary material is not a conductor. The secondary material preferable comprises at least one of a ceramic, a glass or a semi-conductor. The insulating material decreases conduction and therefore minimizes leakage current in the ESD protection device. In the case where it is desirable to minimize capacitance it is desirable that the overvoltage protection element does not appreciably overlaps the discharge electrodes. In some embodiments the overvoltage protection element is porous. The overvoltage protection element may include at least one of La, Ni, Co, Cu, Zn, Ru, Ag, Pd, Pt, W, Fe or Bi. Particularly preferred insulative ceramics include barium titanate or tantalum nitride. For the demonstration of this invention an overvoltage protection element consisting of 75 vol % Ni combined with 25 vol % barium titanate is suitable. The metal content must be above 50 vol % to no more than 90 vol %. Below 50 vol % the conductivity is insufficient to function as an overvoltage protection element and above 90 vol % the conductivity is too high to achieve adequate low leakage. It is preferred that the metal content be at least 70 vol % to no more than 80 vol % and the secondary material represents at least 20 vol % to 30 vol %. 
     The discharge electrodes and internal electrodes can be prepared from any noble metal or base metal with the preference for base metals which can be fired in air. Preferable base metals are selected from the group consisting of nickel, tungsten, molybdenum, aluminum, chromium, copper, palladium, silver or an alloy thereof. Most preferable the discharge electrodes comprise nickel. 
     The ESD protection device can be manufactured in a similar manner to the manufacture of multilayered ceramic capacitors, which is well documented and well known to those of skill in the art, wherein large discrete layers comprising printed patterns of the active layers are overlayed in registration followed by pressing, dicing, firing and terminating to form discrete monolithic components. In the instant application the active layers are the overvoltage protection element, discharge electrodes, internal electrodes and floating electrodes. It will be realized by those skilled in the art of MLCC manufacture that it is desirable to provide cover layers consisting of blank dielectric at the top and bottom of the device to form the external insulators described herein thereby insulating the elements of the component from the external surface. The external terminals can be plated using conventional techniques and materials and the component surface mounted. 
     An embodiment of the invention will be described with reference to  FIG. 6  wherein the process of manufacturing a ESD protection device is illustrated in flow-chart representation. In  FIG. 6 , a series of layers are prepared at  100 . Layers  102  and  102 ′ comprise a precursor to the external insulator and are prepared in accordance with standard manufacturing procedures for ceramic layers in MLCC capacitors as well known in the art. Layers  104  are those layers which, when  104 A and  104 B are taken together, form the ESD protection couples after sintering. In a preferred embodiment a layer is formed at  104 A comprising an overvoltage projection element precursor and a secondary insulator precursor, as a coating, on a layer of primary insulator precursor. A layer is formed at  104 B comprising the precursor to the discharge electrodes and the gap insulator on a precursor to an internal insulator. If capacitive couples are included a layer is formed at  105  comprising alternating layers of internal electrode on ceramic dielectric precusors. A floating electrode can be formed by registration of the same layers used to form the altering layers of internal electrode and ceramic or it may include a different print pattern on a ceramic dielectric precursor as well known in the art. The layers are stacked in registration at  106  such that discharge electrodes, insulator layer precursors and overvoltage protection elements are in registration as discussed and illustrated elsewhere herein, as are the precursors to the internal electrodes, to form a registered stack. In registration the overvoltage protection element precursor will overlay the gap insulator as would be realized from  FIGS. 1 and 2 . The registered stack is pressed and heated at  108  to the extent necessary to bond the insulator precursors, thereby forming insulator, and to adhere adjacent layers together thereby forming a composite laminate sheet. The composite laminate sheet is diced at  110  to provide discrete ESD protection device precursors followed by thermal processing at  112 . The ESD protection device precursors are finished at  114  including the addition of external terminations, overcoating if desired, to form the ESD protection device which may be tested and packaged as part of the finishing step. 
     EXAMPLES 
     A series of 0603 EIA case size ESD protection devices were produced using class I, C0G dielectric consisting primarily of calcium zirconate having a dielectric constant of about 32 as the gap insulator, primary insulator, secondary insulator, internal insulator and external insulator. The ESD protection devices were produced having varying thicknesses of primary insulator layer. The overvoltage protection elements comprised primarily nickel and barium titanate in a 3:1 volume ratio. The overvoltage protection elements were made with either 3 or 10 coplanar pairs of discharge electrodes with each coplanar pair of discharge electrodes having an overvoltage protection element separated from the discharge electrodes by the primary insulator layer. The overvoltage protection elements were co-sintered within the devices. The ESD protection devices were subjected to 8 kV ESD pulses in accordance with International Electrotechnical Commission test procedure IEC 61000-4-2 and the response to the pulses were analyzed. A typical test setup consists of a Noiseken ESS S3011/GT30R ESD simulator (150 pF 330Ω combination) and Keysight MSOS 804A high definition oscilloscope with appropriate high bandwidth attenuation. 
     As discussed above, it is desirable to achieve a low trigger voltage whilst retaining stability after exposure to multiple pulses with continued low leakage. To test the effectiveness of the primary insulating layer a multiplicity of ESD protection devices were manufactured with 3 pairs of nickel discharge electrodes with differing thicknesses of the primary insulator layer. In Table 1 the trigger voltage after ten ESD pulses is reported as is the leakage current after one thousand pulses. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Effect of ceramic separation thickness on capacitance, 
               
               
                 trigger voltage and leakage current 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Ceramic 
                   
                 Trigger 
                 Leakage 
               
               
                   
                 separation 
                 Capaci- 
                 voltage 10 
                 current 1000 
               
               
                   
                 layer/μm 
                 tance/pF 
                 pulses/V 
                 pulse/nA 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 0 
                 0.44 
                 750 
                 Short 
               
               
                   
                 1 
                 0.45 
                 750 
                 Short 
               
               
                   
                 2 
                 0.54 
                 931 
                 12500 
               
               
                   
                 3 
                 0.52 
                 1045 
                 20 
               
               
                   
                 4 
                 1.43 
                 1264 
                 7.2 
               
               
                   
                 5 
                 0.77 
                 1564 
                 1 
               
               
                   
                   
               
            
           
         
       
     
     The trigger voltage of the ESD protection devices produced was evaluated as the peak voltage recorded during repeated 8 kV point discharges. In parts manufactured with no primary insulator layer between the discharge electrodes and overvoltage protection element the trigger voltage was observed to increase on average 25% between pulses 1-10 and 91-100. With a primary insulator layer having a thickness of 2 μm the trigger voltage increased an average of 13%. As the thickness of the primary insulator layer was increased to 5 μm the performance degradation in the trigger voltage was observed to be only 2% 
     ESD protection devices with 3 and 10 pairs of nickel-based discharge electrodes using 9 μm insulation layer thicknesses were prepared and evaluated. ESD protection devices with 10 protective layers reduced the trigger voltage by approximately 20% whereas with 3 pairs of discharge electrodes leakage currents were maintained below 1 nA as shown in Table 2. Furthermore, increasing the number of ESD protection couples reduced the trigger voltage degradation over 1000 8 kV pulses from 1.7 kV to 1.3 kV. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Number of electrode/OVP element pairs vs Trigger 
               
               
                 voltage, leakage current and capacitance 
               
            
           
           
               
               
               
               
               
            
               
                 Number of 
                 V trigger after 
                 V trigger after 
                 1000 pulse 
                   
               
               
                 electrode/OVP 
                 10 8 kV 
                 1000 8 kV 
                 leakage 
               
               
                 element pairs 
                 pulses/kV 
                 pulses/kV 
                 current/nA 
                 Cap/pF 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 3 
                 2.9 
                 4.6 
                 0.016 
                 0.30 
               
               
                 10 
                 2.4 
                 3.7 
                 0.21 
                 0.47 
               
               
                   
               
            
           
         
       
     
     Voltage versus time for response is illustrated graphically in  FIG. 7  for an 8 kV ESD pulse. In addition to reduced peak voltage, parts with additional ESD protection couples demonstrated improved durability with respect to voltage response after 1000 repeated 8 kV ESD pulses, as illustrated in  FIG. 8  where the average voltage versus time after 1000 8 kV ESD pulsed is reported graphically. 
     The invention has been described with reference to the preferred embodiments without limit thereto.