Abstract:
Production test of integrated circuit face thermal management challenges with higher power devices. Current production handlers do not have adequate thermal management characteristics. This invention employs thermal diodes on each device under test and a closed loop microprocessor controlled feedback system for thermal control during production test. The feedback system controls the open/close state of a valve supplying cooling fluid to bathe the integrated circuit based upon the difference between a temperature indicated by at least one thermal diode and a set point temperature.

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. 119(e) (1) to U.S. Provisional Application No. 61/429,848 filed Jan. 5, 2011 and U.S. Provisional Application No. 61/434,948 filed Jan. 21, 2011. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of this invention is integrated circuit testing. 
     BACKGROUND OF THE INVENTION 
     This invention controls the temperature of a self-heating, high power device during production test. 
     SUMMARY OF THE INVENTION 
     This invention places a microcontroller on the device under test (DUT) load board or on an external enclosure couple 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 pocketed nest and manifold system designed to create cooling fluid flow over much of the DUT top&#39;s surface area. This invention can be extended for use system with multiple die in one package (SIP), where each die under test can be individually read and thermally controlled independent of the other die in the package. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of this invention are illustrated in the drawings, in which: 
         FIG. 1  is a schematic illustration of the electronics of this invention; 
         FIG. 2  illustrates the Proportional-Integral-Derivative (PID) feedback control system of the microcontroller in schematic form; 
         FIG. 3  is a simplified schematic diagram of the solenoid drive circuit; 
         FIG. 4  is a simplified cross-sectional view of the handler interface of this invention; 
         FIGS. 5 ,  6  and  7  compare the thermal performance of this invention with the control program turned ON and OFF for several set point temperatures; 
         FIG. 8  illustrates a prior art handler interface; 
         FIGS. 9 and 10  are two views of the retrofitted handler according to an embodiment of this invention; and 
         FIG. 11  illustrates an alternative embodiment of the arrangement of parts of this invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     This invention is easy to implement and is a cost effective way to retrofit existing production handler to be able to test higher power devices having high self heating. 
     This invention is more cost effective than prior handlers that are single site and very expensive. This invention can retrofit to existing multisite handlers. 
     Existing handlers use airflow only thermal management and are not capable of maintaining a temperature guard band for products with this power dissipation. This current roadmap of products do not reach power dissipation ranges that warrant a more expensive handler solution used for 80+ watt range devices. These products to which this invention is applicable are in the mid power range. In this range standard airflow-only handlers are not adequate and the more costly handler lines (such as an external chiller, liquid cooled chuck) seem like overkill. 
     Thus it would be advantageous to develop an economical retrofit to existing of handlers that would allow for accurate temperature control. The DUTs suitable for this invention have multiple on-die thermal diodes. These permit development of an effective solution. This invention is called Cryogenic Temperature Control System (CTCS). 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 duty-cycled pulse to LN 2  solenoid drive circuitry. The LN 2  is directed through a cryogenic hose into a manifold on the back of the DUT handler. The manifold has an interface system to deliver LN 2  bursts into the DUT nest for circulation around the DUT lid and then recapture for expulsion to the ambient air outside the handler. This invention allows accurate temperature control on a per-die basis of SIP (stacked die) products that we may encounter. 
       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 pago 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 value 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 production electrical test of the DUT. 
     Prior art uses the following method 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: 
                     T   C     =         (       V   H     -     V   L       )       1.985   ×     10     -   4       ×   n       -   273.15             (   1   )               
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.
 
     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. 
     This invention is a solution to this problem. In this invention circuits are installed on the tester adapter boards to provide the CTCS with 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. For multi-site tester adapter boards this circuit is repeated for each site. The tester adapter boards preferably also has 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. 
     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 multiple 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. 
       FIG. 2  illustrates CICS 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 an actual measured temperature from sensor  208  from the set 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. 
     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. 
     Block  204  computes the Integral factor. This is the product of an integral constant K I  by an integral of the error e(t) 
               (       K   I     *       ∫   0   t     ⁢     e   ⁡     (   t   )           )     .         
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.
 
     Block  205  computes the Derivative term. This is the product of a derivative constant K D  and the derivative of the error signal 
               (       K   D     *       ⅆ               ⅆ   t       ⁢     e   ⁡     (   t   )         )     .         
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.
 
     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 which might result in under-testing the DUT. 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. 
     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. 
       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. 
     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. 
       FIG. 4  is a simplified cross-sectional view of the handler interface  400  of this invention. Handler interface  400  includes a DUT side and a solenoid side. DUT side includes DUT board  411 , contactor  412 , a holding space for the DUT with the internal thermal diode  413 , and a handler wall  414  (shown in shadow) that surrounds the DUT. The solenoid side includes handler chuck  421 , a special pocketed nest  422  and a LN 2  port  423  in chuck  421 . Electrical lines from the DUT side connect to lines of the solenoid control circuit via a pogo pin interface. 
     Handler interface  400  uses an National Pipe Fitting (NPT) connection to perform delivery to the lid of the DUT and expel the boiled N2 gas outside of the handler. Handler interface  400  implements a stationary manifold in the adapter plate in order to limit the number of moving parts. LN 2  is piped from inlet pipe  415  via the NPT fitting through that plate to inlet  424  including specially designed nozzles that protrude into the handler chamber. The chuck/nest assembly have mating nozzles in a larger ID that meet the manifold nozzles and make a connection as the chuck and nest assembly plunge towards the tester adapter board. The chuck nozzles then route the LN 2  through ports to the nest, where the LN 2  is circulated over the DUT lid. The LN 2  is captured by a second port  425  and coupled to outlet  416  by a second NPT fitting. The captured LN 2  is expelled through another set of plumbing to the outside air. 
       FIGS. 5 ,  6  and  7  compare the thermal performance of this invention with the CTCS system turned ON and OFF for several set point temperatures.  FIG. 5  illustrates a set point temperature of 105 C.  FIG. 5  shows a deviation of about ±2 C with CTCS ON and a maximum deviation of over 20 C with CTCS OFF.  FIG. 6  illustrates a set point temperature of −5 C. FIG. 6 shows a deviation of about +5 C with CTCS ON and a maximum deviation of over 25 C with CTCS OFF.  FIG. 7  illustrates a set point temperature of −45 C.  FIG. 7  shows a deviation of about +4 C with CTCS ON and a maximum deviation of over 50 C with CTCS OFF. 
       FIGS. 8 to 10  illustrate a retrofit of elements of this invention into an existing handler interface.  FIG. 8  illustrates one view of a prior art handler interface  800  which does not include the cooling control of this invention. Prior art handler interface  800  includes a DUT board side  810  and a handler side  820 . When closed to enclose the DUT prior art handler interface  800  includes a cavity  825  accommodating the DUT. 
       FIGS. 9 and 10  are two views of the retrofitted handler according to an embodiment of this invention.  FIG. 9  illustrates a first cut away view of a cooling fluid inlet. DUT board side  810  is modified to include a inlet  911 .  FIG. 9  further illustrates a large exposed device area  915  receiving the cooling fluid over the back of the DUT.  FIG. 10  is another cut away view illustrating a cooling fluid outlet.  FIG. 10  shows that DUT board side  810  includes a smaller diameter exhaust area  1015  feeding an exhaust  1011 . 
       FIG. 11  illustrates an alternative embodiment of the arrangement of parts of this invention. Handler  1100  includes test board  1110  holding plural DUTs  1113 . I 2 C chip  1112  is connected to each DUT  1113  generating a temperature signal corresponding to a temperature sensed by thermal diodes on each DUT  1113 . These temperature signals are supplied to controller box  1121 . Controller box  1121  includes a microcontroller similar to microcontroller  111  and a solenoid drive box similar to solenoid drive circuitry  121  for each DUT. Controller box  1121  supplies PWM drive signals for the solenoids. 
     Solenoid box  1122  receives input cooling fluid on line  1131 . Solenoid box  1122  individually controls cooling fluid in lines  1132  and  1133  supplied to the plural DUTs  1113 .  FIG. 11  does omits illustration of the exhaust system. 
     This invention is an external system that would control DUT thermal heating using our existing production handlers (Delta Castle series). This invention uses an externally controlled solenoid system, the DUT thermal diode and a microcontroller. This invention uses a retrofit fixture for an existing production handler.