Abstract:
To verify robustness with respect to electrical overstresses of an electronic circuit under test, the latter is exposed to electrical overstresses, and the behavior thereof is monitored. In particular, both the testing of the electronic circuit in dynamic conditions is performed by causing it to be traversed by the currents that characterize operation thereof, and by exposing at least one supply line of the electronic circuit under test to electrical overstresses and the testing of the electronic circuit under test in static conditions, without causing it to be traversed by the currents that characterize operation thereof, and by exposing to electrical overstresses both the supply and the input and/or output lines of the electronic circuit under test. The device for generating the overstresses can be mounted on a circuit board, which can be coupled as daughter board to a mother board, on which the electronic circuit under test is mounted.

Description:
FIELD OF THE INVENTION 
     The present description relates to techniques for verifying the reliability of electronic devices. 
     Specifically, this description relates to verification of the robustness of electronic devices with respect to electrical stresses induced by system transients that can arise during operation of the device. 
     BACKGROUND OF THE INVENTION 
     Electrical over stresses (EOSs) are a constant cause of failure for integrated circuits. According to certain reports, approximately 40% of the failures found in integrated circuits can be attributed to EOS phenomena or events. 
     In particular, for semiconductor devices, EOS events comprise a wide range of stresses of an electrical nature, due, for example, to electro-magnetic pulses (EMPs), system transients of various nature (the so-called “overvoltage spikes” on the supply lines and input/output lines) and electro-static discharge (ESD). In particular, events belonging to the latter type of EOS have durations of between 1 ns and 1 μs, and occur prevalently in the production and handling stages, i.e., when the device is not subject to electrical biasing. 
     The failures linked to EOS events in semiconductor devices can be classified according to the respective primary failure mechanism: failures induced as a result of thermal or electrical effects (phenomena of thermomigration or electromigration of material, which involve the metallization layers), phenomena of latch-up, breakdown of the gate oxide, and other failures that can be correlated to electrical fields. 
     The EOS events considered herein belong to the family of system transients (on the supply lines and input/output lines). For these events, the sensitivity as a function of temperature depends upon the failure mode considered. For example, it is minimal for (thermo-electrical) damage of the metallizations and very serious for breakdown of the oxides. 
     These particular events can be defined as overvoltage or overcurrent phenomena which have a duration of between 1 μs and 1 ms, and which occur during operation of the device. 
     “Automation of electrical overstress characterization for semiconductor devices”, Diaz C. H., Hewlett-Packard Journal, Hewlett-Packard Co. Palo Alto Calif., US, vol. 45 no. 5, pages 106-111, describes an automatic test system developed to characterize semiconductor devices and interconnect failures caused by electrical overstress. Electrical stress in the form of current pulses of increasing amplitude is applied to a device under test in the testing mode. The test system was developed for monitoring EOS robustness in advanced CMOS processors under test. 
     “EOS/EDS; ADI Reliability Handbook”, 2000 Analog Devices Inc. Norwood, Mass. 02062, pages 1-22, describes Human Body Model and Machine Model test methods that simulates the application of EOS on ICs. 
     US 2007/0018670 A1 describes a system and method for electrostatic discharge (ESD) testing. The system includes a circuit that has a switch coupled to an input/output (I/O) circuit of a device under test (DUT), a charge source coupled to the switch, and a control circuit coupled to the switch. The control circuit turns on the switch to discharge an ESD current from the charge source to the I/O circuit, and the circuit is integrated into the DUT. The system provides on-chip ESD testing of a DUT without requiring expensive and specialized test equipment. 
     “Improvement of degradation detection in ESD test for semiconductor device”, Satoh S ED—Institute of Electrical and Electronics Engineers, conference record of the 2002 IEEE Industry applications conference 37th IAS annual meeting, Pittsburgh, Pa., Oct. 13-18, 2002 describes ESD test as evaluation tests for semiconductor products. In particular, the most popular and standardized test is the Human Body Model ESD test. In HBM ESD test some issues are experienced in the detection of device degradation as pass/fail judgment after ESD stress applied. Some kind of device degradation cannot be detected with the conventional test condition and criteria. Improved judgment test using effective combination of optimized DC leak test and device functional test was proposed. 
     US 2005/017745 A1 describes an electrostatic withstand voltage test method that enables semiconductor integrated circuit testing to be performed with a high degree of precision and at low cost. In this method, with one of ground pins VSS and VSSI of a semiconductor integrated circuit grounded, static electricity is applied from a static electricity discharge apparatus to all pins of semiconductor integrated circuit, after which, with power supply apparatus connected to power supply pin VDD of semiconductor integrated circuit and the other grounded, a leakage current test apparatus is connected to all signal pins and pin leakage current is tested, and with ground pin VSSI of the internal circuitry of semiconductor integrated circuit grounded and leakage current test apparatus connected to power supply pin VDDI, a pattern generator that supplies a digital signal is connected to signal input pins (IN, I/O), and power supply leakage current is tested. 
     U.S. Pat. No. 5,132,612 describes an apparatus for applying high current fast rise time pulses simulating electrostatic discharge (ESD) to various combinations of pins of a device under test (e.g., a microcircuit). The apparatus also provides for testing of the DUT after the performance of ESD stress testing. The apparatus establishes electrical connections between the terminals of a high voltage pulse generator (HVPG) and several different combinations of the DUT pins in sequence in order to apply ESD stresses. The apparatus further provides functional parameter tests whether the connection to the DUT pins during ESD stressing has caused the DUT to fail. 
     Further advances in testing the performance of electronic devices in the presence of electrical overstresses are, however, needed. 
     SUMMARY OF THE INVENTION 
     On the basis of the state of the art outlined above, there is the need to have available methods that enable evaluation of the robustness/sensitivity of electronic devices, such as integrated circuits, with respect to system transients (EOS) that occur during normal operation. 
     This need is felt, in particular, for devices obtained using BCD (Bipolar CMOS DMOS) technology; these devices are more sensitive to the events than, for example, system-on-chip (SOC) devices as a result of the differences in operation. Normally, in fact, BCD devices directly interface with inductive loads, which notoriously generate spikes. 
     An embodiment described herein envisages that a generator of stresses of a system transient type (SOS) is able to apply positive and negative EOS system transient stresses on the input-output (I/O) terminals of the Device Under Test (DUT), including the supply lines both in static conditions and in dynamic conditions. 
     An embodiment described herein is able to perform the verification outlined previously by taking into account the operational conditions of the device (both static and dynamic) and to perform an action of verification with respect to system transients of an EOS type applied in dynamic conditions. (i.e., with devices under test (DUTs) that are fully operative, with the possibility of control in current of ohmic-inductive loads), and in static conditions. (i.e., with electrically biased devices in a wait state or in standby state and not traversed by high currents). 
     An embodiment is able to operate in dynamic conditions (i.e., with the device functioning), for example in HTOL (High-Temperature Operating Life) conditions, with the alternative application of positive stresses to just the terminals of the supply lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, purely by way of non-limiting example, with reference to the annexed drawings, in which: 
         FIG. 1A  is a top view of a system for conducting HTOL reliability tests on an electronic device, in accordance with the present invention. 
         FIG. 1B  is a side view of the system of  FIG. 1A . 
         FIG. 2  is a perspective block diagram of a system for conducting electrical over stress tests on an electronic surface, in accordance with the present invention. 
         FIG. 3  is as more detailed block diagram of the system of  FIG. 2 . 
         FIG. 4  illustrates waveforms that can be applied to the devices under test using the system of  FIGS. 1A-3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the ensuing description, various specific items are illustrated aimed at providing a deep understanding of the embodiments. The embodiments can be obtained without one or more of the specific items, or with other methods, components, materials, etc. In other cases, known structures, materials or operations are not illustrated or described in detail in order not to obscure the various aspects of the embodiments. 
     The reference to “one embodiment” in the scope of this description is intended to indicate that a particular configuration, structure or characteristic described in relation to the embodiment is present in at least one embodiment. Hence, phrases such as “in one embodiment”, present in different points of this description, do not necessarily refer to the same embodiment. Furthermore, particular conformations, structures or characteristics can be combined in any adequate way in one or more embodiments. The references used herein are only for reasons of convenience and hence do not define the sphere of protection or the scope of the embodiments. 
       FIGS. 1A and 1B  represents, respectively in plan view and in a vertical median section, a mother board  10  that can be used for conducting reliability tests of an HTOL type on one or more electronic devices under test (DUTs). 
     This embodiment includes a mother board  10  that can be used for testing two devices alternatively, designated by DUT A and DUT B. 
     Also visible in  FIG. 1  is the OMI (OLT Module Interface) that communicates with the testing apparatus of which the mother board  10  forms part. 
     In the specific case of  FIG. 1 , the device for conducting EOS tests comprises a daughter board  12  that can be coupled to the mother board  10 . Mounted on the daughter board  12  is a set of circuits  14 , designed to generate pulses that simulate EOS system transient events according to the modalities described more fully in what follows. 
     The circuits  14  are configured typically as oscillator circuits that can generate pulses having the characteristics described more fully in what follows. 
     As may be seen more clearly in  FIG. 1B , the daughter board  12  is coupled to the mother board  10  on the face opposite to the face on which the devices under test OUT A and DUT B and the interface OMI are mounted. 
     This arrangement enables a dual advantage to be achieved. In the first place, no appreciable modification of the mother board  10  is necessary; moreover, the mother board  10  and the EOS daughter board  12  can be located directly in the testing apparatus. 
       FIG. 2  illustrates schematically the EOS test board configured as a stand-alone board  16  within a dedicated apparatus including a monitor  18  that can be managed by a personal computer integrated in the system.  FIG. 2  highlights the presence of the supply sources  20 , with the consequent possibility of applying EOS events also to the supply sources. 
       FIG. 3  highlights the possibility of using the system described herein within a testing apparatus  100 . 
     Highlighted schematically in  FIG. 3  is the presence of the device under test, to which—in a normal test cycle—“stimuli” of various nature are applied for the operation of the DUT. The monitor  18  is used as a terminal of a personal computer (programming of the stimulation pattern and data collection). 
     In the embodiment illustrated herein, there is moreover envisaged the possibility of intervening on the supply lines  20  (for example, 12 V, 5 V, 3.3 V nominal) applying thereto—in dynamic conditions, i.e., whilst the DUT is being tested in operating conditions—system transients (EOSs). Added to this is the possibility of observing/recording on the monitor  18  the outcomes produced by the EOSs on the device. 
     The evaluation of the robustness of the DUT will be obtained based upon a definitive parametric control by means of ATE (Automatic Testing Equipment) at the end of the EOS stimulations. 
     The methods described herein hence enable the DUT (typically an integrated circuit) to be subjected to stresses of an EOS system transient type during normal operation (i.e., while the circuits are handling currents), i.e., while the mother board is operating in a normal HTOL configuration. 
     The EOS events can be applied in a way that is synchronous or asynchronous with respect to the pattern of stimulation of the DUT  22 , with the added possibility of programming both the amplitude and the pulse widths. 
     For example, the diagram of  FIG. 4  shows the plot of a pulse train (bottom right part of  FIG. 4 ), the characteristics of which can be partially set, in particular with respect to the following characteristics:
         absolute amplitude a—settable;   rise time rt—in itself not settable, but a consequence of the amplitude a;   pulse width pw—settable;   fall time ft—in itself not settable, but a consequence of the amplitude a;   overshoot voltage (negative) uv—in itself not settable, but a consequence of the amplitude a.       

     For example, the amplitude a can reach a value of 27 V with a rise time (rt) and fall time (ft) having a typical value of 5 μs. Furthermore, for example, the pulse width pw is programmable from 10 μs to 470 μs, and the negative voltage uv can reach a value of 10% of the maximum value of the pulse set. The pulse-repetition time can be set with a minimum of approximately 1 s. 
     The aforesaid values are of course to be understood purely as an example and are not to be read in a sense that might in any way limit the scope of the present description. Similar considerations apply to the values exemplified in the tables reproduced below. 
     The ensuing table makes reference to EOS events that can be applied on three DC voltages V 1 , V 2 , and V 3 —representing three supply voltages (for example, the voltages  20  of  FIG. 3 ). 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                 Voltage range 
                 Min. 
                 Max. 
               
               
                   
               
             
             
               
                   
                 DC voltage V1 
                 5 V 
                 20 V 
               
               
                   
                   
                 (min. spike 7 V) 
                 (max. spike 7 V) 
               
               
                   
                   
                 (max. spike 22 V) 
                 (min. spike 0 V) 
               
               
                   
                 DC voltage V2 
                 3 V 
                 8 V 
               
               
                   
                   
                 (min. spike 3 V) 
                 (max. spike 2 V) 
               
               
                   
                   
                 (max. spike 7 V) 
                 (min. spike 0 V) 
               
               
                   
                 DC voltage V3 
                 3 V 
                 8 V 
               
               
                   
                   
                 (min. spike 1 V) 
                 (max. spike 2 V) 
               
               
                   
                   
                 (max. spike 7 V) 
                 (min. spike 0 V) 
               
               
                   
               
             
          
         
       
     
     The next table relates to programmable spikes that can be superimposed on the aforesaid voltages V 1 , V 2 , V 3 . 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                 V1 Min.; 
                 V2 Min.; 
                 V3 Min.; 
               
               
                   
                 Max.. 
                 Max. 
                 Max. 
               
               
                   
               
             
             
               
                 Programmable spikes 
                 12 V; 27 V 
                 6 V; 10 V 
                 4 V; 10 V 
               
               
                 (absolute voltage 
                   
                   
                   
               
               
                 amplitude) 
               
               
                   
               
             
          
         
       
     
     During operation as a generator of EOSs of a static type (i.e., for verifying the behavior of a device with an integrated circuit when exposed to sudden spikes applied on the supply lines), the system described herein envisages that the circuit under test, or DUT, is biased without being traversed by the currents that characterize operation thereof with the application of spikes one at a time. 
     During static operation, the device described herein enables production of EOS system-transient events in an asynchronous way, with there not being a stimulation pattern. 
     The EOS events can be applied on as many as three supply lines and/or on all the inputs and outputs (both positive and negative). Both the amplitude and the duration of the spikes are programmable (in a single or multiple way). 
     In a possible experimental configuration, the embodiment to which  FIG. 3  refers has been used for applying EOS events to an integrated circuit with two supply lines V 1  and V 2  with nominal values equal to 13.2 and 5.5 V, respectively. 
     The system described herein enables validation, from the point of view of the designer, of the circuit protection adopted in the integrated circuit DUT with respect to the EOS system transients, with the possibility of seeking the limits or margins of application with respect to the EOS events. The system described herein can also be included in a program of control of production in real time (RTC—Real Time Control) with the possibility of intervening during verification on the production lots of the product itself. 
     Embodiments of the system described herein envisage the implementation of the stresses in static conditions, i.e., with the device biased, without the latter being traversed by the currents that characterize operation thereof, for example in HTRB (High-Temperature Reverse Bias) conditions, with the application of positive and/or negative stresses on the I/O terminals and on the terminals of the supply lines. 
     Embodiments of the system described herein envisage implementation of the stress in dynamic conditions (i.e., with the device functioning), for example in HTOL conditions, with application of the positive and/or negative stresses both on the terminals of the supply lines and on the I/O terminals, alternatively. 
     Embodiments of the system described herein can involve extension of the number of supplies of the DUT on which to apply the EOS system transients, one at a time, and/or increase in the maximum nominal value of the voltage of the DUT, for example, to +60 V. 
     For example, the maximum amplitude of the positive voltage applied to the DUT can be +100 V (maximum nominal value +60 V plus EOS spike), with a maximum amplitude of the negative voltage applied to the DUT of −60 V (nominal value minus EOS spike).
         Other possible values are the following:   max. undervoltage: 10%;   typical rise and fall slew rate: 2 μs;   time durations of the pulses that can be applied to the supply voltages: from a minimum of 5 μs to a maximum of 200 μs; and   time durations of the pulses that can be applied to the I/O terminals: from a minimum of 5 μs to a maximum of 500 μs.       

     In one embodiment, the electrical circuits for stimulation of the EOS system-transient events are integrated in a specific board and are independent of the type of the DUT. 
     In one embodiment, the specific configuration of HTOL mode of the DUT can be integrated in a mother board dedicated to the functionality of the DUT. 
     In one embodiment, the EOS system transients can be applied to the DUT, both to the I/O terminals and to the supply lines in an alternative way in both static configuration and dynamic configuration, as described previously. 
     One embodiment uses a personal computer for acquisition and management of the data of the OUT, detecting operation thereof in real time. 
     It follows that, without prejudice to the principle of the invention, the details of construction and the embodiments may vary, even significantly, with respect to what is described and illustrated herein purely by way of non-limiting example, without thereby departing from the scope of the invention, as defined by the annexed claims.