Abstract:
This invention is an electrostatic discharge testing circuit that can deliver current pulses to a component under test (CUT) with a custom amplitude versus time profile shape. Pulse generation with customized shapes is accomplished by discharging an energy storage network comprised of capacitor(s), transmission line(s) and other passive components. Current pulses compliant to the European International Electrotechnical Commission IEC 61000-4-2 standard can be so produced. These current pulses are delivered to the CUT with low distortion through a constant impedance electrical path, such as a combination of cables and controlled impedance conductors of printed wiring boards compatible with packaged IC devices, assemblies, and wafer probes. The current pulses can be delivered with various impedances, and measurements made that allow the CUT currents and voltages to be calculated.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/003,908, filed Nov. 23, 2007, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electrical testing circuitry for integrated circuits (ICs) and in particular to apparatus for electrostatic discharge (ESD) sensitivity testing that includes a pulse generator (pulser). 
     2. Description of the Related Art 
     Testing of ICs for ESD sensitivity involves applying stress pulses simulating electrostatic discharges to an IC or device under test to determine robustness. The use of this stress testing is expanding to help ensure reliability of products such as computers and cell phones when they are exposed to ESD. Equipment that must operate in harsh and/or high static environments, such as automotive electronics, also requires thorough ESD reliability testing. 
     All electrical equipment sold in the European Community is required to conform to government safety regulations. Products in compliance with such safety regulations may be signified by the CE Mark. One of the many tests for CE Mark compliance is the ability to withstand electrostatic discharges to the surfaces of the product without damage. The International Electrotechnical Commission test method IEC 61000-4-2: Electromagnetic compatibility (EMC)—Part 4-2: Testing and Measurement Techniques—Electrostatic Discharge Immunity Test Ed. 1.2 b: 2001 is a standard that prescribes the ESD test method. It is a purpose of this invention to provide testing similar to this standard but with advantages not found in prior art methods for ESD testing of ICs. 
     The International Electrotechnical Commission IEC 61000-4-2 testing method, herein termed the IEC Test, defines a testing environment and a discharge gun pulse generator for ESD testing. This equipment and procedure is commonly used for testing a finished product, which is generally termed equipment under test (EUT). This testing method was designed only for testing complete systems, and is not directly applicable to testing of subassemblies, such as circuit boards, or of individual IC components or devices, often collectively termed a component under test (CUT). IC design and/or manufacturing companies have begun testing their products using variations of the IEC Test. The desire to test CUTs with the IEC Test is motivated by the assumption that when a product is assembled from parts that each individually pass the IEC Test, the completed product or EUT will pass the IEC Test. 
     Testing a CUT with tools and methods designed for an EUT is difficult. A primary reason for difficulty in applying the IEC Test to circuit boards or IC devices is the inability of the IEC Test specified ESD pulse generator to be connected to the typical IC device. The apparatus for ESD event simulation described in the IEC Test standard is a hand-held discharge gun with a large metallic probe tip. These gun tips are shown in  FIG. 1 . Such prior art ESD pulse generators using a metallic probe tip are difficult to connect to the small dimensions of today&#39;s microelectronic circuitry. Gun testing is especially difficult with high density packaging of modern ICs, which may have pin-to-pin spacing of 0.5 millimeters or less. Improper connection leads to pulse distortions that can adversely affect the testing results. 
     Connection from the IEC Test discharge gun to a CUT has been attempted in a variety of ways, and often with inconsistent results. The ground connection of IEC Test guns are not made with wide bandwidth cables so the return path may be high impedance at pulse frequencies depending on electrical coupling between CUT and the gun. IEC Test guns are known to generate unwanted electromagnetic (EM) radiation pulses in addition to the desired conducted current pulse. These unwanted EM radiation pulses complicate the testing environment, and EM pulse shielding is sometimes done to help mitigate the effect. EM pulse variations may be exacerbated by the placement of conductive surfaces in the testing environment such as the horizontal, vertical and ground coupling planes described in the IEC Test specification (these surfaces can provide a low impedance radio frequency (RF) grounds and reflections of the gun&#39;s EM pulse). Given these factors, a good deal of uncertainty can be introduced into IC test results when using IEC Testing discharge guns. 
     Prior art efforts to test CUTs to the IEC Test have focused on providing conductive paths from the discharge gun tip to one connection on the CUT and from another CUT connection to an earth connected ground plate so that the current produced by the pulse generator passes through the CUT. One prior art test method uses a test fixture board (TFB) to hold the CUT, either in a socket or by soldering the CUT directly to the TFB printed wiring, to make such connections. Electrical terminals, sometimes termed test points, are provided on a TFB that are compatible connections for the discharge gun tip. Printed wiring on the TFB conducts the discharge current from the gun&#39;s test point to the CUT and another electrical path for ground return current. This approach suffers from pulse shape distortion due to different electrical properties of the gun and the TFB. The gun is defined by the IEC Test specification to have 330 ohms of series resistance, as shown in  FIG. 2 . The wiring of TFBs can have impedances from about 30 to 150 ohms, but cannot be manufactured at 330 ohms due to physical printed wiring size limits. The mismatch of impedances will result in pulse reflections and waveform distortions to the fast current pulses. Such prior art testing also suffers from non-reproducibility due to TFB-to-TFB design variations in the conductive paths to the CUT. TFB wiring can act as an antenna picking up energy from the EM pulse, and any variations in the TFB wiring can change the EM pulse coupling from the gun to the TFB. Maintaining the proper waveform is required to simulate ESD events and meet the IEC Test requirements, and variations produce non-repeatable measurements. 
     Another prior art technique is to eliminate the intermediate conduction paths and antenna properties of the TFB by touching the gun tip directly to the CUT. With the miniature contacts on small microelectronic components, this can be very difficult. A prior art solution is to employ a robot that can position the discharge gun with accuracy that cannot be obtained from a human operator. However, since a complete electrical conduction path also requires a ground return, two connections must be made to the CUT. Even when more than one robot is used it can be impossible to make contacts to neighboring pins due to the size of the IEC Test gun&#39;s metallic tip. ESD testing requires specific characteristics from pulse generators to properly simulate an ESD event. Some pulse generators used for bench testing of electronic systems can generate arbitrary waveforms into 50-ohm coaxial cables. In theory, therefore, such pulse generators could generate the waveform that is needed for the IEC Test, and be an alternative to the hand-held gun. However, the IEC Test requires many amperes of current and hundreds, or even thousands, of volts to reach desired stress testing levels. These requirements for high power levels are far beyond the capabilities of common arbitrary waveform generators. A different type of pulse generator is needed for this application. 
     Prior art test methods lack the power, reproducibly and consistency in applying stress pulses to ICs or other devices that is needed for repeatability in testing. Therefore, there is a need for new equipment and methodologies to produce a repeatable and reliable test to evaluate the ESD sensitivity of ICs, devices, components and subassemblies with a test method similar to the EUT IEC Test but compatible with testing CUTs. 
     SUMMARY OF THE INVENTION 
     Electrostatic discharge (ESD) testing pulsers are characterized by an energy storage component, such as a capacitor, which is charged to a high voltage over a relatively long time period, and a switching device, such as a relay, that can isolate the storage component while it is being charged, and then connect the charged storage component to a conductive path to a component under test (CUT) whereby the stored energy is discharged into the CUT during a relatively short time period simulating an ESD event. The conductive path, which may include current limiting resistances, is important in producing the correct stress current waveform. The prior art ESD pulser, shown in  FIG. 2 , has a 150 pF capacitor storage element and a 330Ω resistor in the conductive path to the device under test or CUT. 
     The present invention employs multiple energy storage elements, combined with additional passive components that together will produce a desired waveform. The storage devices used in this invention are transmission lines combined with capacitors. Transmission lines can be synthesized by a sequence of inductors and capacitors, so this invention can be practiced with a combination of inductors and capacitors replacing a transmission line. The use of a single transmission line segment for energy storage would be recognized by those trained in the ESD testing art as being similar to a transmission line pulser (TLP) system. However, TLP systems are purposely designed to produce pulses of constant amplitude while the present invention produces a specific and non-constant amplitude current pulse. In particular, one application of this invention is to produce the current pulse waveform that is described in the IEC 61000-4-2 test specification (the IEC Test), which is diagrammed and summarized in  FIG. 3 . 
     In the present invention the current pulse shaping is largely accomplished by the energy storage network rather than the combined energy storage element and electrical path to the CUT. The energy storage in this invention is accomplished not by a single component such as used in prior art pulse generators, but a network comprised of two or more storage components, specifically transmission lines and capacitors, directly connected or connected with resistors. This invention can produce a wide variety of pulse shapes using specific combinations of storage components and resistors to generate the desired waveforms. A specific embodiment herein described is a set of components that will produce an IEC Test compliant current waveform. 
     One embodiment of the present invention includes a current pulse generator that produces ESD stress pulses and conducts these pulses to the CUT through 50-ohm coaxial cables. While this invention can be practiced with cables of impedance other than 50 ohms, this particular impedance is commonly used in the electronics industry, and therefore many 50-ohm connectors and conductors are available, making this impedance the preferred value. Herein the reference to “50-ohm” should be understood to be a convenient but not necessary impedance value as the invention can be practiced with transmission lines and cables of any impedance including cables of different impedances. Various means of pulse transmission can be employed to conduct the current pulse to the CUT instead of or in addition to cables. Herein the reference to “delivery cable” or “conduction path” should be understood to be encompassing all combinations of controlled impedance conduction paths including but not limited to cables, wire pairs, connectors, circuit boards with controlled impedances, relays, wafer probes, probe cards, sockets, and other components with approximately constant characteristic impedances that can be used alone or in combinations to provide the conductive path to the CUT. 
     An advantage of using controlled impedance delivery cables is that the current pulses are transmitted with little distortion, allowing fast pulse rise times to be delivered over a distance of several meters from the pulse generator to the CUT. Without the use of controlled impedances the parasitic wiring capacitances and/or inductances will attenuate the high frequencies of the current pulse causing the pulse to be modified as it is transmitted. Using delivery cables allows flexibility in applying ESD testing to a wide physical variety of CUT types. 
     CUTs, such as IC devices, can be in different physical forms, therefore, flexibility in connecting ESD testers to CUTs is an important feature of this invention. For example, if an IC is still in wafer form, as manufactured before being cut into individual dice, a wafer probing system featuring 50-ohm wafer probes may be used as a means of interfacing the pulse to the CUT. Wafer probes are also compatible with packaged CUTs and modules, as well as board assemblies and other component configurations. 
     In prior art ESD pulsers, the transmission path from the switching device to the CUT is often responsible for pulse shaping. It is not necessary, but possible to add shaping components compatible with 50-ohm cables forming the conduction path to the CUT of this invention. An example of this is a low pass filter that can be used to slow the generated pulse rise time. Another example is a 50-ohm signal attenuator that can be used to attenuate signals and reflections. 
     The production and delivery of current waveforms defined in the IEC Test specification is an application of the present invention. It is a purpose of this invention to remove problems of prior art IEC Test pulse generators. This invention makes improvements over prior art that will be described hereinafter, including: preventing strong EM pulses from impinging on the CUT and causing testing variations, generating pulses with high reproducibility, providing CUT current monitoring that can measure stress level and detect changes in CUT impedance, making testing easy due to integrated leakage measurement and automating test sequences and automatic report generation. 
     The stress level in the IEC Test specification is defined as a function of the pre-pulse voltage of a storage capacitor. However, the controlling waveform parameters of the IEC test are specified as currents from a discharge gun into a low impedance test load. Therefore, the pulses produced by the present invention will be herein described in terms of current waveform parameters, which can be related to IEC gun charge voltages as listed in  FIG. 3 . The IEC Test defines a current pulse waveform specified by four parameters: pulse rise time between 0.7 and 1.0 ns, peak current of 3.75 A per kV±10% (usually within 2 ns of the beginning of the pulse), current at 30 ns from the beginning of the pulse of 2 A±30% per kV, and current at 60 ns from the beginning of the pulse of 1 A±30% per kV, where kV is the IEC gun circuit storage element pre-pulse voltage in kilovolts. Current pulses compliant to the IEC specification should be generated at stress levels of up to at least 30 amperes peak current, which corresponds to an IEC Test gun pre-pulse charge voltage of 8 kV. According to Ohm&#39;s Law, the voltage of 30 A pulses is 1,500 volts when transmitted in a 50-ohm cable. This is a peak power of 45,000 watts according to Joules&#39; Law. Therefore, it is a purpose of the present invention to provide high power pulses to this level. 
     This invention is an ESD test system that forces a current pulse through the CUT. An application of the present invention is a pulser with output current pulses as described in the IEC Test that is compatible with coaxial cable delivery for ESD testing. This is different than the prior art IEC Test pulse generator, which is a hand-held discharge gun that sparks to or contacts directly to a surface of the EUT from its metallic tip. These metallic gun tips ( FIG. 1 ) are not compatible with constant impedance coaxial cable for mechanical and electrical reasons. Mechanically, the hand held gun discharge tip can make connection with some connectors but requires user skill and contact can be intermittent and therefore not always reproducible. Electrically, there is an impedance mismatch between the gun tip and a conductive path of a TFB. The gun has an impedance of approximately 330 ohms due to the output path resistor of the IEC Test specification (see  FIG. 2 ). Printed wiring board conductors have typical impedance ranging from 40 to 120 ohms. Cables are commonly available from 50 to 93 ohms. Coaxial cables and printed wiring boards cannot be manufactured with an impedance matching the IEC Test discharge gun. The impedance mismatch between the gun tip and a test fixture or cable will generate repeated reflections that distort the stress pulse and significantly reduce the power of the stress pulse applied to the CUT. This problem can be avoided if the pulse generator has an output impedance equal to the cable and/or circuit board used for a TFB. Furthermore, using a commonly available impedance, such as 50 ohms, allows the stress pulse to be conducted by a combination of cables and printed wiring boards connected in series. Since the IEC test specification prescribes 330 ohms series resistance for the hand-held gun, there could be testing differences between a current pulse delivered by the gun and a pulse delivered through lower impedance cables. Therefore, it is an additional purpose of this invention to provide a system test environment that is of higher impedances than are obtained by individual cables or printed wiring on circuit boards alone. This is accomplished by adding series resistance close to the CUT in its ground return current path. 
     Since the IEC Test is defined by a current waveform generated from a 150 pF storage capacitor, and the present invention uses different storage elements, the charging voltage will be different for the same stress current. To verify the stress level, it is useful to monitor the current that passes through the CUT, and therefore it is a purpose of the present invention to provide current pulse waveform measurement of the stress pulse applied to the CUT. In an embodiment of the present invention, the current return path is provided by an additional controlled impedance cable with radio frequency (RF) attenuators and an oscilloscope that can monitor the current pulse amplitude and waveform shape. In yet another embodiment, the current return path is made to have a resistance that when combined with the other system resistances will match the delivery resistance of prior art discharge guns. 
     The current that passes through the CUT is determined not only by the generated current pulse, but also the resistance of the CUT. When the current stress pulse reaches a level that does physical damage to the CUT, the resistance of the CUT may change. With current waveform analysis, it is possible to determine a change in CUT resistance and thereby detect the stress level that induces failure for some types of CUTs. 
     ESD testing is usually accomplished by the application of pulses of increasing stress until the CUT sustains physical damage from the ESD stress pulse. Those skilled in ESD testing will understand that control and reproducibility of applied stress pulses is important to such testing. The highest level of stress before damage is typically the reported testing results. Therefore, it is a further purpose of the present invention to provide repeatable and reproducible stress pulses of predefined magnitudes. Control of the high voltage supply will determine the pre-charge voltage of the storage network and the pulse current levels. Such voltage control to produce a sequence of stress pulses is typically done by computer control in ESD testing systems. 
     In order to determine when physical damage is sustained, electrical test instruments are sometimes connected to the CUT and functional or parametric testing is performed after stress pulse application. Equipment, such as a source-meter unit, can make CUT leakage measurements or curve traces between pulses to determine the failure level. Device failure is determined by changes in its electrical characteristics, such as leakage or curve traces, due to application of the ESD simulating stress pulse or pulses. Another feature of this invention is electrical switching between stress pulsing and electrical testing. The can be accomplished by adding a 50-ohm switch or relay to the delivery cable so the test equipment can be connected to the CUT by such a switch or relay. This is more convenient than a prior art gun discharge where leakage measurement equipment must be connected and disconnected manually between each pulse discharge. A further improvement over prior art is the automation of this electrical path switching and automatic coordination of a testing sequence allowing automatic stress of increasing levels, damage data collection and report generation by computer. 
     The IEC Test describes a testing table with horizontal and vertical metallic coupling planes upon which the equipment under test (EUT) is placed. Waveform variations with the IEC Test discharge gun are observed in different testing environments due to the gun&#39;s EM pulse that interacts with such coupling planes. An EM pulse is generated from the rapid change in current that occurs in an ESD simulator during pulse generation. Some prior art gun pulse generators purposely produce additional EM pulse radiation as part of their waveform shaping methods. A purpose of the delivery cable is to allow separation between the pulse source that will generate an EM pulse, and the CUT. The distance of separation can be large enough to effectively attenuate the generated EM pulse to a negligible level. EM pulses are attenuated by the square of the distance between the pulse source and CUT. Adding a meter of cable can reduce the EM pulse to only 3% of the strength from a gun discharge. It is a purpose of the present invention to not require special testing environments such as metallic covered tables or coupling planes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be better understood with reference to the accompanying drawings, which are incorporated in and form a part of this specification, and, together with the description, serve to explain the principles of the present invention. 
         FIG. 1  is a drawing of the prior art IEC Test discharge gun tips. 
         FIG. 2  is a prior art circuit diagram of the IEC Test ESD generator. 
         FIG. 3  is the IEC Test pulse current waveform definition shown graphically and in a parameter table. 
         FIG. 4  is a circuit diagram example of the present invention. 
         FIG. 5  is a drawing of the current waveform from the present invention. 
         FIG. 6  is a preferred embodiment of the present invention with 50 ohm impedance at the CUT. 
         FIG. 7  is a preferred embodiment of the present invention with pulse current monitoring and 100 ohm impedance at the CUT. 
         FIG. 8  is a preferred embodiment of the present invention with 330 ohm impedance at the CUT. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, numerous specific details are provided, such as the identification of various system components, to provide a thorough understanding of embodiments of the invention. One skilled in the art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In still other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     In one embodiment of the present invention a current pulse is generated by pre-charging a network that includes energy storage elements and subsequently discharging this network by relay closure into a controlled impedance delivery cable that conducts the stress pulse to the CUT. In general, the energy storage network is composed of one or more transmission lines plus, one or more capacitors, and optionally one or more resistors.  FIG. 4  is an example of the technique of this invention. In this embodiment one transmission line and two capacitors are the energy storage elements and two resistors control the charging and discharging rates. This ensemble will generate the desired waveform,  FIG. 5 , which has three distinct current pulse sections, each covering a specific time period, in order to meet the IEC Test waveform requirements as defined the IEC 61000-4-2 specification. It will be understood by those trained in the art of pulser designs that other waveforms can be generated using this invention. A coaxial cable provides the current pulse delivery and also the current return path. The system diagrammed in  FIG. 4  produces the waveform shown in  FIG. 5 , which satisfies the IEC current pulse waveform and its defining parameters shown in  FIG. 3 . 
     Referring to  FIG. 4 , the operation of this pulser begins when the high voltage power supply  110  charges the energy storage network  100  while the 50-ohm switch  170  is open. In this preferred embodiment the energy storage network  100  is composed of a current limiting resistor  120  with resistance R 1 , a decaying waveform capacitor  130  of capacitance C D , a transmission-line-matching resistor  140  of resistance R M , a transmission line  150 , and an initial spike generating capacitor  160  of capacitance C 1 . When switch  170  is closed the energy stored in  100  is transmitted to the pulse delivery cable  190  to the CUT  200 . When the switch  170  is subsequently opened the storage network is again recharged. 
     Resistor  120  is of a relatively large resistance R 1 , such as 100 kΩ to 10 MΩ, to provide a slow charging of the storage network. During the short pulse discharge time period the resistor  120  will not transfer significant current from the high voltage power supply due to its large resistance value so it also provides isolation. In  FIG. 4  resistor  120  is shown connected to storage capacitor  130 , however, it could also be connected to either end of transmission line  150 . In practicing this invention the exact connection point of the resistor  120  is not critical. The charge can be delivered to any ungrounded point in the storage network, providing flexibility of the storage network input. Alternately, the high voltage supply  110  can be connected through a switch or relay in addition to or in place of resistor  120  to provide increased isolation of the high voltage supply during the discharge. It should be understood that there are many equivalent ways to charge the storage network by various connections to a high voltage supply as will be know to those trained in electrical engineering. 
     The current pulse is produced when the 50-ohm switch  170  connects the charged storage network  100  to the conduction path constant impedance cable  190 . The total delivery path is composed of the constant impedance conductors represented as cable  190 , the test fixture board  210 , and a ground return conductor  195 . The constant impedance of these transmission components deliver the pulse to the CUT largely unchanged. The delivery cable  190  has a grounded outer conductor that provides the current return path from conductor  195  that completes the electrical circuit from the energy storage network  100  through the CUT  200 . 
     Referring to the energy storage network  100 , the transmission line segment  150  and storage capacitors  130  and  160  become initially charged by current from the high voltage supply  110  through the current limited resistor  120  while switch  170  is open. When switch  170  is closed, it connects  150  to  190 , and the charge stored in both  150  and  160  begins to flow into  190  and then through the CUT  250 . The components  130 ,  140 ,  150  and  160  of the storage network  100  produce three specific features of the current pulse waveform, as depicted in the three time periods of  FIG. 5 . The initial spike generating capacitor C 1    160 , along with the transmission line  150 , produce the initial current spike that peaks with IEC waveform parameter I P  during period one. The value of capacitor C 1  determines the decay time from I P  to the plateau current. Period two is the current plateau from approximately 10 to 30 ns. The equation 
               i   ⁡     (   t   )       =       I   30     +       (       I   P     -     I   30       )     ·     ⅇ     -     t   τ                   
for 1 ns&lt;t&lt;30 ns describes the decay from Ip to a plateau current level, called I 30 , with the time constant τ=25·C 1  (where 25 is derived from the parallel impedance of two 50Ω components, the transmission line  150  and the switch  170 ). The initial current rise to I P  and decay from I P  to the plateau current I 30  defines the current waveform during time period one.
 
     Current from storage elements  150  and  160  combine to produce the peak current, which is sent into the delivery cable  190 . The current from transmission line  150  is 
                 i   TL     ⁡     (   t   )       =     Vo     2   ⁢   Zo             
for 0&lt;t&lt;2t L  where Vo is the initial storage network output voltage, Zo is the common impedance of  150 ,  170  and  190 , t=0 when the switch  170  closes, and t L  is the end-to-end signal propagation time of the transmission line  150  (as determined by its length multiplied by its signal propagation velocity). In this preferred embodiment the transmission line is selected to have a length so that t L  is approximately 15 ns in order to produce the current waveform of time periods one and two. During time period one, this i TL  current is augmented by the current
 
                 i     C   ⁢           ⁢   1       ⁡     (   t   )       =       Vo     2   ⁢     Z   0         ⁢     ⅇ     -       2   ⁢   t         C   1     ⁢     Z   0                     
from capacitor  160  of value C 1 . The peak current is set by the initial storage network voltage that is about 1.5 kV for equivalent current of an IEC Test 8 kV contact discharge. After time t′, when
 
             ⅇ     -       2   ⁢     t   ′           C   1     ⁢     Z   0                 
becomes very small, which is about 10 ns for a 100 pF capacitor C 1 , then C 1  no longer adds significant current to the output pulse. This defines the beginning of time period two.
 
     Relatively constant current characterizes the waveform during time period two, which lasts from t=t′ until t=2t L . The current at t=30 ns is 
                   i   TL     ⁡     (     30   ⁢           ⁢   ns     )       =     Vo     2   ⁢   Zo         ,         
which defines the I 30  current. When t&gt;2t L , the current waveform is in time period three, where current output from the storage network is
 
                 i   CD     ⁡     (   t   )       =     ⅇ     -       t   -     2   ⁢     t   L             C   D     ⁡     (       R   M     +     Z   0       )                   
for t&gt;2t L  from the capacitor C D    130 . Time period three is a decaying exponential set by the time constant of C D ·(R M +Zo), where Zo is the impedance of transmission line  150 , C D  is the capacitance of capacitor  130  and R M  is the resistance of resistor  140 . The transmission line matching resistor  140  is selected to equal the impedance of the transmission line  150 . This is required to produce a smooth current waveform transition from time period two to time period three. The value C D  of capacitor  130  sets the decaying waveform&#39;s time constant during time period three, and is selected to have the correct current at 60 ns according to the IEC Test parameter definition. This decay reduces the current to approximately half the current in 30 ns ( 160  is one-half of 130 after 30 ns of decay). For obtaining the current at 60 ns defined in the IEC Test,
 
     
       
         
           
             
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     It should now be understood that the multi-element energy storage network is central to this invention. Energy stored in capacitors and transmission lines is delivered in time sequences as determined by transmission line lengths. By proper component selection, as exemplified above, the defined parameters of the current waveform of the IEC test can be produced. While it is a purpose of this embodiment that this particular waveform be produced, this invention is not limited to this waveform alone. With simple modification of the number of components, their connections and values, other waveforms can be produced. 
     Since this pulser is based on passive components it is a linear system, and pulse amplitude control is achieved by varying the high voltage supply output to produce a current pulse corresponding to any voltage of the IEC contact test method. Using computer control of the high voltage supply voltage and operation of switch  170 , a sequence of increasing stress levels can be generated under software control. 
     All transmission lines and cables used in the herein embodiments are 50Ω for convenience and compatibility with readily available components such as connectors, filters, attenuators and oscilloscopes. As shown in  FIG. 6 , a low pass filter  180  can be used to increase the initial current pulse rise time to the IEC Test parameter range of 0.7 to 1 ns if needed. 
     The conduction path  190  can be a combination of cables, connectors, and printed wiring boards, all with 50Ω impedance. Maintaining a 50-ohm transmission path with RF cables and connectors, and strip-line or microstrip printed wiring board construction, removes impedance variations that degrade pulse quality, which can reduce the peak current stress of the pulse. This transmission path may include a test fixture board (TFB). It is a purpose of this invention to allow the CUT  200  to be placed on a TFB  210 . Such TFBs are common in the testing of radio frequency (RF) or microwave components. TFBs may be composed of connectors and conduction paths that can all be 50 ohm impedances, which allow the controlled impedance to extend uninterrupted from the pulser to the CUT. The ground return path on such TFBs may be a conductive trace or a plane that is commonly referred to as a ground plane. The electrical circuit for the current pulse is completed by connecting the ground return path  195  to the outer conductor, or shield, of the pulse delivery cable  190 . This technique is also compatible with 50-ohm wafer probes that are commonly used for RF testing of IC wafers (the form of ICs before they are cut into individual dice). 
     In order to activate the switch  170 , and to control the voltage of high voltage supply  110 , and to coordinate similar activities that are needed for a complete test system, a control system such as a computer is needed. This computer is not shown in  FIGS. 4 ,  6 ,  7  and  8  for clarity. However, as such controlling circuitry is required and is a common part of ESD testing systems, it should be understood as a part of this invention. A test plan can be executed by computer that would consist of a set of stress current levels, a number of pulses at each stress level, a time delay between pulses, and other testing parameters. The computer system would use this definition data to set the voltage of the high voltage supply  110 , to activate the 50-ohm switch  170  and to control the other components as may be part of the testing system, often including a status display and test report printer. 
     Another preferred embodiment of the present invention is diagrammed in  FIG. 6 . This embodiment adds a low pass filter  180  in the 50-ohm pulse delivery path to the CUT  200 . At high voltages, a spark discharge may occur within the 50-ohm switch  170  that can produce a current pulse rise time faster than 0.7 ns. This low pass filter  180  will remove unwanted high frequencies from the pulse. A low pass filter with a −3 dB high frequency cutoff of approximately f=0.35/tr, where tr is the desired minimum rise time, can be selected. In this embodiment, a 0.7 ns tr is made with a 500 MHz low pass filter. 
     The embodiment diagrammed in  FIG. 6  also has a source-meter unit  240  that can test the electrical properties of the CUT  200 . A 50Ω switch  220  and another section of transmission line  230  is added to the current pulse path. With the switch  220  in its inactive state the transmission path is unchanged from the previous embodiment description. When switch  220  is activated, the conductive path to CUT  200  is no longer connected to the pulse generator, but instead the CUT  200  is electrically connected to a source-meter unit (SMU)  240 . It is a purpose of this invention to have the ability to switch from stress pulsing to electrical testing in order to determine if the applied stress has damaged the CUT. The use of an SMU is common in ESD testing, but the SMU is just an example of test equipment. Other test equipment can be used including, but not limited to, semiconductor parametric analyzers, curve tracers, vector volt meters, network analyzers, time domain reflectometers and automated test equipment. This list is not meant to be exhaustive nor limit the application of this invention to the equipment named. 
     Before the first stress pulse, the SMU  240  can measure current passing through the CUT under one or more applied voltages when the 50-ohm relay or switch  220  connects the SMU  240  to the CUT  200  through connecting cables  230  and test fixture board  210 . After each stress pulse the measurement can be repeated for detection of electrical changes in the CUT by comparing the before and after data. Switches connecting the CUT to test equipment can be a single switch as shown in  FIG. 7  or a matrix of switches connected to a multiplicity of CUT electrical connections. 
       FIG. 7  shows another embodiment of the current invention that includes current pulse measurement by use of an oscilloscope (scope)  290 . Electrical measurements of the current pulse traveling to the CUT  200  is made with voltage probe  250  and current probe  260  inserted into the delivery path. In order to insert such electrical probes the delivery cable is made in two sections  190  and  230  and the probes are connected to the signal conductor in the delivery path. Voltage probe  250  and/or current probe  260  may be commercial measurement probes, or other measurement transducers. The outputs of these transducers are connected the inputs of scope  290  to record the pulse waveforms. It should be noted that the recorded pulse signals can be distorted from the actual voltage and current pulses delivered to the CUT due to time separation between the generated pulse and reflections from the CUT. When the measurements are made very close to the CUT, these distortions are minimized. The current that passes through the CUT  200  can also be measured in the current return path. In  FIG. 7  the return path of the current pulse is not connected to the delivery transmission line outer conductor shield as in previously described embodiments, but is connected to another transmission line cable  270 . All current flowing through the CUT  200  is forced to also flow through the center conductor of cable  270 . This current pulse is reduced to a level that can be measured by scope  290  using signal attenuator  280 . The purpose of attenuator  280  is to reduce the current pulse level to a range that can be delivered to the scope without damage to the scope&#39;s 50-ohm input. The required signal attenuation may require a series of discrete attenuators to be used. One or more of the three scope signals of  FIG. 7  may be used in implementing this invention while the signal labeled  13  is the preferred current signal because it is a direct measurement of the pulse. Measurement of the voltage across the CUT can be done by measuring the voltage at both ends of the CUT and calculating the difference. 
     Except for the unusual case where the CUT has the same impedance as the pulse delivery cable, not all the energy of the pulse will be absorbed by the CUT. The unabsorbed energy will be reflected. Reflections will travel back over the transmission path to the storage network and may cause a re-reflection which will distort the pulse. Attenuators can is inserted into the delivery path which will reduce a significant portion of these unwanted reflections, but will also reduce the power of the original current pulse. An improved method of reducing reflections is to add a terminating resistance between the CUT and the ground return of the delivery path. Most components that will be tested will exhibit a low resistance when conducting the large current pulse, for if this was not the case then the large heating power (equal to the current squared times the resistance) will overheat the CUT and induce destruction. If we add a resistor of resistance approximately equal to the impedance of the delivery cable, then the sum of the CUT impedance and this new resistance will approximate a matched load to the delivery cable, and not produce a significant reflection. 
     Implied in  FIG. 7  is a control system, such as a computer, which is not shown for clarity, but that can coordinate the current pulsing (activation of switch  170 ) with the waveform recording of scope  290 . The computer or control system could also gather data from scope  290  and use such data to generate test reports. The recording of the actual current pulse waveform delivered to the CUT is an advantage over prior art ESD test systems as the exact stress applied to the CUT can be determined. 
     The prior art discharge gun has a 330Ω resistance in its delivery path. The embodiments of the present invention previously described have 50Ω impedance or 100Ω impedance (when two cables connect to the CUT). The result of the different impedances is that the current flowing through the CUT can be different in the test methods of this invention compared to the current from the discharge gun for the same pre-charge voltage. To correct this possible problem, a resistor may be inserted in the ground path of the CUT to increase the delivery impedance. This resistor could be placed on the TFB  210  or in the ground path  195  shown in  FIG. 4 . This test method controls the pulse delivery impedance seen at the CUT. In another preferred embodiment, shown in  FIG. 8 , resistor  300  is added to the current return path on the TFB  210  or at the end of the cable  270 . The value of R A  of resistor  300  is set approximately to 330−2Zo=230Ω in order to match the impedance of the IEC Test gun. It should be noted that the signal seen at the scope  290  will be reduced by 15 dB, or the factor of 
               50       R   A     +   Zo       ,         
due to the attenuation action of R A , the 50Ω scope input, and cable  270  impedance Zo. Therefore, the required attenuation of attenuator  280  may be reduced accordingly from the previous embodiment.