Abstract:
A test apparatus and method is provided for dynamic thermal and electrical fatigue testing of a semiconductor in an operating environment, such as air, that mimic thermal and electrical stress in the semiconductor during high power switching in the operating environment. Comparisons of pre- and post-testing electrical measurements, i.e., current, voltage and contact resistance, are combined to provide an indicator or long-term reliability.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an apparatus and method for combined thermal and electrical fatigue testing in an operating environment of the reliability of electrical devices. More particularly, this invention provides an apparatus and method that each mimics thermal and electrical stress in a semiconductor during high power switching in an operating environment, such as air. Most particularly, this invention provides and apparatus and method for assessing long term reliability of electronic devices through dynamic testing in an operating environment of semiconductors for voids and short circuits. 
   2. Discussion of the Related Art 
   In order to promote, design and realize reliable SiC power devices it is necessary to assess the performance of device components under the influence of their potential operational stress regimes. This is particularly critical for pulsed power device applications, namely, palpitated high power switching, in which the operational environment is dominated by acute cyclic pulsed power actions. The pulsed power action ultimately translates into severe thermal, electrical, and mechanical cyclic stresses in the device materials. 
   Prior art approaches to assessing the thermal stability for SiC high power devices at high temperatures employ static thermal testing. A usual testing procedure for SiC consists of inducing thermal fatigue by exposing a test structure to a temperature in the range of 300 to 500° C. for more than 100 hours. A typical vacuum furnace, as well known in the art, is employed for such testing. Pre- and post-testing electrical measurements, i.e., current-voltage and specific contact resistance, are compared to obtain an indicator of long-term device reliability. Prior art approaches to assessing pulsed-power thermal stability have consisted of inducing thermal fatigue by exposing the contact-SiC structure to high temperatures (in the range of 900-100° C.) for several minutes in a vacuum environment. A rapid thermal annealer (RTA) is the typical instrument employed for such testing. After exposure, electrical measurements, i.e., current-voltage, and specific contact resistance, are taken and compared to pre-thermally fatigued electrical measurements. 
   In order to reliably utilize SiC for pulsed power switching applications it is necessary to determine the effects of such cyclic stress regimes on the fundamental pulsed power device components. Another aspect of SiC reliability derives from the high current densities associated with SiC power devices that can cause failure due to electromigration. Electromigration refers to the transport of mass in metals under the influence of current. Electromigration occurs by the transfer of momentum from the electrons to the positive metal ions. When a high current passes through thin metal conductors (metal conductors/interconnects and contacts) metal ions in some regions may pile up whereas voids will form in other regions with resulting respective short-circuits of adjacent conductors and open circuits. Ultimately, electromigration limits device performance and reduces reliability in the long-term. 
   Prior art test approaches employed for detecting and measuring electromigration are similar to those prior art approaches for static thermal testing. That is, the contact-SiC structure is exposed to high fields in a vacuum for a static non-pulsed duration. Thermal vacuum furnaces do not provide results with respect to thermal stability and electromigration survivability that apply to survivability in air. In other words, a vacuum ambient is not representative of the typical environmental condition these devices typically operate in. 
   In addition to this shortcoming of prior art static thermal testing, this prior art testing has been directed to measuring individual effects and has not been directed to a combined effects test approach. 
   SUMMARY OF THE INVENTION 
   Thus, there is a need for a power device reliability test apparatus and method for the evaluation of non-static combined effects testing, i.e., combined thermal and electrical effects, in a device-compatible environment, such as air, or other ambient, i.e., nitrogen etc. 
   The present invention provides a combined effects “thermal and electrical” test apparatus and method which serves to determine the window of reliability for individual SiC device components, namely ohmic contact structures, or complete device structures. Using the apparatus and method of the present invention, unreliable ohmic contacts can be detected in the devices that cause degradation in device properties. It is this degradation that ultimately leads to system when unreliable ohmic contacts are present. 
   By performing both pre- and post-testing materials and electrical analyses and comparing results, the apparatus and method of the present invention enable assessment of the degradation effects resulting from the present invention&#39;s combined effects fatigue testing. Reliability concerns with respect to wide bandgap SiC-based semiconductor devices are exacerbated by the need for these devices to operate at high temperature and high power. Thus, the performance of SiC switching devices is limited by the electrical and materials integrity and reliability of their ohmic contacts. The apparatus and method of the present invention determine the reliability of the contact structure after exposure to combined effects thermal and electrical fatigue. 
   The present invention for combined thermal and electrical fatigue testing is an apparatus preferably comprising a laser in combination with a timed shutter for raising the temperature of a semiconductor under test to between 300° C. and 1000° C. over a pulse duration of three seconds. Using the pulse-forming electrical circuit of the present invention, a series of about ten pulses is applied to the semiconductor undergoing testing, each pulse followed by a cooling period of about sixty seconds in ambient air. Pre- and post-testing performance parameters are measured, e.g., current, voltage and contact resistance, in the semiconductor undergoing testing. Temperature is measured by a thermocouple or pyrometer. The pre- and post-testing measurements are compared to yield an indication of long-term reliability, e.g., the presence of short circuits and voids. 
   The present invention is a novel combined effects “thermal and electrical” apparatus and method which serves to determine a window of reliability for individual SiC device components, namely ohmic contact structures or complete device structures. It is an improvement over prior art static testing methods in which pre- and post-testing measurements are compared for a semiconductor under test that is heated in a vacuum furnace to a temperature of 500° C. for at least 100 hours. The improvement of the present invention derives from the greater accuracy of the resulting measurements because these pre- and post-testing measurements address the performance of a semiconductor device not in a vacuum environment, as in the prior art, but in the actual environment in which the device will be used. Results of testing with the device and method of the present invention can ensure optimum performance and long-term reliability for SiC high power pulsed switching devices. Such devices include Insulated Gate Bipolar Transistors (IGBTs) and MOS Controlled Thyristors (MCTs). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a preferred embodiment of the combined thermal and electrical fatigue effects test apparatus of the present invention. 
       FIG. 2   a  illustrates an example of a preferred thermal loading scheme demonstrating one thermal pulse with a 3 second duration at a temperature of 650° C. 
       FIG. 2   b  illustrates a preferred cyclic thermal scheme of 10 consecutive pulses, each with a 3 second duration at a temperature of 650° C. and a cooling interval between pulses of 60 seconds. 
       FIG. 3  illustrates a schematic of a preferred embodiment of an electromigration test setup of the present invention. 
       FIG. 4  illustrates a process flow chart of a preferred embodiment of the method of the present invention. 
   

   DETAILED DESCRIPTION 
   Apparatus 
   Referring to  FIG. 1 , a schematic is illustrated of an apparatus for combined thermal and electrical fatigue effects testing, according to a preferred embodiment of the present invention. The apparatus shown in  FIG. 1  serves to determine a window of reliability for individual SiC device components, namely ohmic contact structures, or complete device structures. Through the use of the device shown in  FIG. 1  according to the method of  FIG. 4  it is possible to mimic the potential operational thermal and electrical stress regimes of high power switches and high power palpitated switching. 
   In  FIG. 1  a carbon dioxide CO 2  (10.1 micron) laser  10  provides the thermal loading source. A ZnSe beam spreader  11  is shown and can be used to spread the laser beam over an area of ˜5 mm. The power level of the laser  10  can be tailored to give the desired test temperatures. A pulse forming network (PFN)  13  is connected to the electronic device  14 . 
   In the device illustrated in  FIG. 1 , a thermocouple  12  or pyrometer is used to accurately measure temperature and a timed shutter (not shown) is used to create a cycle of heating-cooling events. That is, a shutter is employed that is timed to create the desired duration heating and cooling intervals. In this configuration, the shutter can also be set up to repeat the heating-cooling events to create any number of exposure cycles. 
   Thus, the thermal loading section of a preferred embodiment of a combined effects test apparatus illustrated in  FIG. 1 , allows complete tuning of temperature, cycle duration, number of cycles and area of exposure such that it is possible to mimic the operational environment of a pulsed power device with reasonable accuracy. 
     FIG. 2   a  illustrates the operation of the device of  FIG. 1  for one cycle  20  of thermal fatigue.  FIG. 2   b  illustrates the operation of the device of  FIG. 1  for ten cycles  21  of thermal fatigue. 
   By way of example only, in a preferred embodiment of a test for a particular electronic device using the apparatus of  FIG. 1 , a laser positioned 18 inches from the electronic device to result in a spot size on the electronic device of 5 mm and a temperature of 600° C. was achieved using a power level of 40 watts. The thermal and electrical regime imposed in this test is shown in the following table: 
   
     
       
             
             
           
         
             
                 
             
             
               Thermal Parameters 
               Electrical Parameters 
             
             
                 
             
           
           
             
               Pulse width*: 3 second thermal 
               Pulse width*: 400:sec. 
             
             
               pulse with 60 second cool 
             
             
               Rise time: nanosec. 
               di/dt: 25 kA/:sec. 
             
             
               Single pulse or repeated pulses 
               Single pulse or repeated to 5 
             
             
                 
               shots/min. 
             
             
               Temperature*: 300° C. to 1000° C. 
               Current density ∃ 2.5 kA/cm 2   
             
             
                 
             
             
               *variable parameters  
             
           
        
       
     
   
   A preferred embodiment of a pulse-forming network (PFN) for electromigration stress testing, is illustrated in FIG.  3 . The PFN comprises an inductor (L)  30  and a capacitor (C)  32  which form an RLC (resistance-inductance-capacitance) network that is used to first store a prescribed amount of electrical energy and then provide a fixed amount of electrical power and energy to the SiC electronic device being tested. Both the amount of electrical energy as well as the method of delivery to the SiC electronic device are tunable and depend on the type of electronic device being tested. In the configuration shown in  FIG. 3 , the magnitude and duration of delivery of electrical power to the SiC electronic device under test is a function of the initial capacitor voltage, the discrete values of L and C, the size and resistance characteristics of the SiC, and the repetition rate of charge/discharge cycles. 
   The PFN of the preferred embodiment illustrated in  FIG. 3  provides a flexible design that can accommodate the test conditions needed to approximate the magnitude and pulsed power wave fronts necessary for device operation that are relevant to palpitated high power switching architectures. 
   Method 
   A process flow for an acute cyclical electrical-thermal test method applying the test apparatus of  FIGS. 1 and 3  to an electronic device, is illustrated in FIG.  4 . The steps of this method are:
         Taking pre-testing electrical and materials measurements (structural, microstructural, and chemical);   Mounting an electronic device to be tested on an insulated holder;   Providing test application requirements;   Determining the distance from the laser to the electronic device from the size of the area to irradiated, the target temperature, and the power capabilities of the laser;   Positioning the electronic device the determined distance from the laser;   Tailoring the thermal and electrical parameters to the provided test application requirements:       

   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Thermal Parameters 
               Electrical Parameters 
             
             
                 
                 
             
           
           
             
                 
               pulse width 
               pulse width 
             
             
                 
               rise time 
               di/dt 
             
             
                 
               single or repeated 
               shots/min. 
             
             
                 
               temperature 
               current density 
             
             
                 
                 
             
           
        
       
     
       
       
         
           Constructing a pulse forming network having the tailored thermal and electrical parameters; 
           Applying the constructed PFN to the mounted electronic device; 
           Taking post-testing electrical and materials measurements; and 
           Comparing pre- and post-testing electrical and material measurements to assess degradation of the provided electronic device. 
         
       
     
  
   The contacts  21  and  22  of the pulse forming network  13  of  FIG. 3  are arranged to provide an electrical pulse to the SiC device  14  as shown in FIG.  1 . 
   The embodiments and modifications discussed herein are by way of example only, are not to be construed as limiting in any sense, and various other embodiments and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention. For example, another type of laser as a thermal loading source may be used at a different distance from the electronic device depending on the desired temperature and power capabilities of the laser.