Abstract:
Fault analysis of high power integrated circuits face thermal management challenges. This invention employs thermal diodes incorporated in the device undergoing fault analysis, and a closed loop microprocessor controlled feedback system for thermal control during test and fault analysis.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The technical field of this invention is integrated circuit failure analysis. 
       BACKGROUND OF THE INVENTION 
       [0002]    This invention controls the temperature of a self-heating, high power device during failure analysis. 
       SUMMARY OF THE INVENTION 
       [0003]    This invention places a microcontroller on the device under test (DUT) load board or on an external enclosure coupled to the DUT load board. This microcontroller reads the DUT&#39;s thermal diode. The microcontroller controls a metering valve connected to an existing cooling fluid line (such as liquid nitrogen (LN 2 ) or compressed air) based on the reading. Based on the DUT&#39;s internal die temperature, the microcontroller will open or close the metering valve to regulate the device temperature. The cooling fluid will be injected to the top of the device with a special manifold system incorporated into the test socket designed to create cooling gas flow over much of the DUT top&#39;s surface area without blocking the access to the top of the DUT. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    These and other aspects of this invention are illustrated in the drawings, in which: 
           [0005]      FIG. 1  is a schematic illustration of the electronics of this invention; 
           [0006]      FIG. 2  illustrates the Proportional Integral Derivative (PID) feedback control system of the microcontroller in schematic form; 
           [0007]      FIG. 3  is a simplified schematic diagram of the solenoid drive circuit; and 
           [0008]      FIG. 4  is an illustration of the open top lid of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0009]    This invention addresses the problem of doing failure analysis on high power devices. During failure analysis, the top surface of the die must be exposed and accessible to the test apparatus, negating the use of conventional temperature control means. 
         [0010]    The DUTs suitable for this invention have one or more on-die thermal diodes. This invention uses the DUT thermal diodes for real time on-die temperature measurement. The system uses an I 2 C communications chip (on-board the tester adapter board) to read the DUT thermal diode(s). An 8-bit microcontroller running code to measure the temperature uses this information to calculate a third order control system response. This microcontroller sends a variable duty-cycled pulse to LN 2  solenoid drive circuitry. The LN 2  is directed through a cryogenic hose into an open lid covering the DUT. The lid has an interface system to deliver LN 2  bursts around the exposed DUT without blocking the top surface of the die. 
         [0011]      FIG. 1  is a schematic illustration of the electronics of this invention. This invention includes parts on the DUT board side  110  and on the handler side  120 . DUT board side  110  includes microcontroller  111 , I 2 C chip  112  and plural DUT wafers  113 . Handler side  120  includes solenoid drive circuitry  121 , cryogenic solenoid  122  and LN 2  flow  123 . Thermo diodes on wafers  113  supply signals corresponding to their current temperatures. I 2 C chip  112  conditions these signals for use by microcontroller  111 . In this embodiment I 2 C chip  112  is an LM9534 which is more fully explained below. Microcontroller  111  produces a solenoid drive signal for temperature control. A communications interface transfers signals from microcontroller  111  to solenoid drive circuitry  121 . Solenoid drive circuitry  121  controls the opening and closing of solenoid  122 . This controls a valve controlling LN 2  flow  123 . LN 2  flow  123  influences the temperature measured by the thermo diodes of wafers  113 . Microcontroller  111  operates upon the measured temperature to control solenoid  122  for thermal control during failure analysis of the DUT. 
         [0012]    Prior art to monitor DUT temperature during test was by reading a thermal diode during the test flow. This function uses the ideality factor algorithm (equation (1) below) to calculate temperature by forcing two different currents through the thermal diode and reading the voltage results from each forced current. The force currents typically differ by a factor of 10:1. The measured temperature T C  is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                     C 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             V 
                             H 
                           
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                         n 
                       
                     
                     - 
                     273.15 
                   
                 
               
               
                 
                   ( 
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         where: V H  is the voltage reading during the higher force current; V L  is the voltage reading during the lower force current; and n is an ideality factor of the thermal diode. 
       
     
         [0014]    There is a problem with this prior art method. With this prior art method temperature readings cannot be made in real time. In addition each reading causes an increase in test time. The prior art typically executes the thermal diode read function either before a test function or after the test function. As a result the prior art measurement is not an accurate temperature reading during pattern execution. Thus there is a need for an external method of reading of the thermal diode that does not use the test program. 
         [0015]    This invention is a solution to this problem. In this invention circuits are installed on the tester adapter boards to provide real-time DUT temperature readings. This invention preferably uses a National Semiconductor LM95234 device to read the on-chip thermal diodes. The LM95234 preferably is given direct access to the DUT thermal diode pins and is connected to our microcontroller via a Molex connector. The tester adapter boards preferably also have a Texas Instruments TMP100 (temperature monitor) mounted on the DUT side  110 . This temperature monitor is accessed by microcontroller  111 , allowing measurement of the handler ambient temperature. 
         [0016]    Microcontroller  111  controls the DUT temperature. Microcontroller  111  monitors the device temperature in real-time and controls a cooling device. This invention preferably includes an Arduino ATMEGA328 microcontroller because of its small size, low cost and ease of code development. The Arduino microcontroller includes the ability to communicate to other devices using an I 2 C link. In the preferred embodiment of this invention the tester adapter board uses a remote diode temperature sensor IC that communicates the temperature readings of one or more thermal diodes through an I 2 C channel. With this connected to our microcontroller, we have the ability to read the device temperature of multiple sites as well as the top and bottom side temperature of the tester adapter board. These temperature readings preferably are collected real-time and stored in a vector format for further analysis. The microcontroller controls the self heating of DUT by pulsing cryogenic solenoid  122  injecting boiled LN 2  gas directly on the device lid. Early experiments showed the need to develop a smart algorithm to calculate the LN 2  solenoid pulse duration in order to keep DUT die temperatures within the specified guard band. 
         [0017]      FIG. 2  illustrates the system software-based Proportional-Integral-Derivative (PID) feedback control system  200  in schematic form. Control system  200  receives an independent input  201  determining the desired temperature. Summer  202  subtracts a actual measured temperature from sensor  208  from the step point temperature generating an error signal e(t). According to the preferred embodiment of this invention the cryogenic valve is operated on a one-second period Pulse Width Modulation (PWM) scheme. Microcontroller  111  sets the duty cycle of the PWM by PID control. In order to achieve optimal temperature control, special consideration had to be given to this software implementation. 
         [0018]    Block  203  computes the proportional aspect of the PID from a product of error signal e(t) and a proportional constant K P  (K P *e(t)). This component increases the PWM duty cycle proportional to the error signal. 
         [0019]    Block  204  computes the Integral factor. This is the product of an integral constant K I  by an integral of the error e(t) 
         [0000]    
       
         
           
             
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         [0000]    In a discrete sampled system this integral is computed by multiplying the time elapsed since the last calculation by the error signal e(t). This portion of the PID control helps to eliminate any steady-state error in the DUT test temperature by summing the instantaneous error over time. 
         [0020]    Block  205  computes the Derivative term. This is the product of a derivative constant K D  and the derivative of the error signal 
         [0000]    
       
         
           
             
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         [0000]    In a discrete sampled system this derivative is computed by subtracting the error from the previous calculation by the present error and dividing this difference by the time elapsed between the two readings. This portion of the control system helps to control over-shoot and maintain system stability. 
         [0021]    Each of the three individual PID terms has an associated constant that is used to fine-tune the response of the system (K P , K I , K D ). The CTCS uses these constants to guard against system over-shoot. Summer  206  sums these three terms of the PID control calculation generating am overall PID result. Block  207  translates this PID result to a PWM duty cycle by dividing by a maxoutput constant. This constant gives yet another tool that can be used to adjust system response. This signal controls the cryogenic solenoid. The cryogenic solenoid controls the rate of supply of LN 2  to the DUT. This in turn controls the DUT temperature. Sensor  208  measures the DUT temperature and completes the feedback loop. 
         [0022]    The preferred cryogenic solenoid is a 24 Volt cryogenic solenoid specially manufactured for LN 2  service applications by GEMS Sensors and Controls. The specified drive current necessary to close this solenoid is 3 Amperes. Since the microcontroller drive current is only specified in the mA range, This invention includes a circuit to drive the solenoid, using a Texas Instruments OPA548 operational amplifier. 
         [0023]      FIG. 3  is a simplified schematic diagram of this solenoid drive circuit  300 . Operational amplifier  301  receives an input from the microcontroller on its inverting input. The non-inverting input of operational amplifier  301  is connected to the center node of a voltage divider formed of resistors  302  and  303 . In the preferred embodiment illustrated in  FIG. 3 , resistor  302  is 1 KΩ and resistor  303  4 KΩ. The voltage divider is connected between the output of operational amplifier  301  and ground. The output of operational amplifier  301  also connects to one terminal of capacitor  304 , whose other terminal is connected to ground. As illustrated in  FIG. 3  capacitor  304  is preferably 220 μf. 
         [0024]    This circuit is powered using an external power supply. The exemplary values of resistors  302  and  303  provide 5:1 non-inverting gain. This gain was selected to match the 22 V input requirement of the selected solenoid. 
         [0025]      FIG. 4  shows the lid used by this invention. Inlet port  401  is connected to the cooling medium source. A number of gas channels  402  distribute the cooling medium around the circumference of top opening  404 , and deliver the cooling medium to gas injection ports  403 . The geometry of the lid and the injection ports is such that the cooling medium will flow across the surface of the DUT.