Abstract:
In order to maintain a semiconductor device under test at a generally constant temperature, the temperature change of the device under test is characterized as the device under test undergoes changes in power level in response to an electrical testing sequence. Additionally, the temperature change of the device under test is characterized in response to changes in power level of a thermal head associated with the device under test. This information is used to select power levels of the thermal head during the electrical testing sequence so that the device under test remains at a substantially constant temperature during the electrical testing sequence.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to semiconductor technology, and more particularly, to maintaining substantially constant temperature of a semiconductor device under test. 
     2. Discussion of the Related Art 
     Semiconductor devices typically undergo a variety of electrical test procedures, including short-circuit tests, burn-in tests, and device functional tests to insure their proper operation. During for example functional testing, it is important that the temperature of the device under test be held at a chosen, substantially constant value. However, during such functional testing, the power level of the device may vary greatly, causing the temperature of the device to fluctuate. The most important parameter is junction temperature, or the temperature of active regions in the device (there may be some temperature non-uniformity within the device). In dealing with this problem, it is well known to provide a thermal head  10  a surface  11  which may be brought into contact with the lid  12  of a device  14  under test, for example, a flip-chip mounted on a printed circuit board  16  (FIG.  1 ), or in the case of an unlidded device, in contact with the device  14  itself (FIG.  2 ). The thermal head  10  typically includes an electric heating element  18  along surface  11  the output of which can be increased and decreased by respectively increasing and decreasing electrical current flow therethrough, and a passage  20  through which coolant  22 , for example, water, may flow. By changing electrical current flow and/or providing or cutting off coolant flow, the temperature of the thermal head  10 , and thus the temperature of the device under test  14  adjacent thereto, can be adjusted or varied. As the temperature of the device under test  14  varies due to changes in power level thereof as described above, the temperature of the thermal head  10  is caused to change to compensate for the changing temperature of the device  14 , in order to attempt to maintain the device under test  14  at a constant, chosen temperature. 
     One approach in attempting to maintain the device under test  14  at a substantially constant temperature is to compare the temperature of the device under test  14  with a desired temperature as the temperature of the device under test  14  varies due to fluctuation of power level thereof. A PID (Proportional Integral Derivative) controller is used to sense that difference and vary the temperature of the thermal head  10  in order to bring the temperature of the device under test  14  back to the chosen value. However, this approach requires an accurate measurement of the temperature of the device under test  14 , which cannot realistically be achieved with a lidded device if a temperature sensor is not incorporated in the device, and is also difficult even with an unlidded device. Additionally, in the case of a lidded device, because of the thermal capacitance of the lid, a substantial delay occurs in change of temperature of the device under test through change in the temperature of the thermal head. Thus, this approach has not proven entirely satisfactory. 
     Another approach, currently practiced by Schlumberger, Ltd. for unlidded devices uses an algorithm as follows: 
     
       
         
           T 
           c 
           =T 
           d 
           −K 
           θ 
           P 
           d 
         
       
     
     where: 
     T d =temperature of device under test; 
     T c =temperature of thermal head 
     P d =power dissipated by device under test; 
     K θ =thermal stack up coefficient of device (overall thermal resistance between the die and the thermal head). 
     In this approach, the device under test temperature T d  is chosen and thermal head temperature T c  is set in accordance with this formula. The power dissipated by the device under test  14  is monitored. Through use of this formula, the temperature of the thermal head  10  can be varied in an attempt to hold the device under test  14  at a substantially constant temperature. However, it has been found that while ideally K θ is a constant, this has proven not to be the case, that is K θ may vary from one test run to another, causing inaccuracies in the attempt to hold the device under test  14  at a substantially constant temperature. Additionally, for functioning of this system, substantial, rapid swings in thermal head temperature are required, in turn requiring expensive and complicated hardware. 
     Therefore, what is needed is an approach in maintaining a device under test at a substantially constant temperature which overcomes the problems set forth above, meanwhile being simple, inexpensive and effective, and is equally effective in the case of lidded and unlidded devices. 
     SUMMARY OF THE INVENTION 
     In maintaining a device under test at a generally constant temperature, the temperature change of the device under test is characterized as the device under test undergoes changes in power level in response to an electrical testing sequence. Additionally, the temperature change of the device under test is characterized in response to changes in power level of a thermal head. Using this information, power levels at the thermal head are selected for use during the electrical testing sequence, based at least in part on the characterization of the temperature change of the device under test in response to the electrical testing sequence, so that the device under test remains at a substantially constant temperature during the electrical testing sequence. 
     The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as said preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a sectional view of apparatus for illustrating a typical prior art process, with a lidded device under test; 
     FIG. 2 is a sectional view similar to that shown in FIG. 1, but with an unlidded device under test; 
     FIG. 3 is a sectional view of apparatus used in furtherance of the present invention; 
     FIG. 4 is a schematic view of a portion of the apparatus used in furtherance of a part of the present invention; 
     FIG. 5 is a schematic view of a portion of the apparatus used in furtherance of another part of the present invention; 
     FIG. 6 is a graphical view of die temperature response vs. frequency in the present invention; 
     FIG. 7 is a graphical view of current draw vs. time in the present invention; 
     FIG. 8 is a graphical view of predictive control of power dissipated in the thermal head; and 
     FIG. 9 is a graphical view illustrating temperature control of the device under test over a period of time. 
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to a specific embodiment of the present invention which illustrates the best mode presently contemplated by the inventor for practicing the invention. 
     With reference to FIG. 3, a thermal head assembly  30  includes a thermal head  32  having a passage  34  through which coolant  35  may flow, and a plate  36  mounted thereto and having a recess  38  which houses an electric heating element  40 . In use, a surface  39  of the plate  36  is in contact with the lid  12  of a device under test  14 , for example, a flip chip mounted on a printed circuit board  16 . 
     Initially, and with reference to FIG. 4, the thermal characteristics of the device  14  are determined upon application of various levels of power thereto. The device  14  is characterized using harmonic Joule heating and a temperature sensor in the form of a diode  46  in the same package as the device  14 . The temperature response to self heating (and heating control by heating element  40  as later described) are determined at a spectrum of frequencies. A function generator  42  provides a source of current  43  I=I o cos(ωt) to the device  14  (resistance of the device  14  indicated by resistor  44 ). Application of this harmonic current to the device  14  causes the temperature of the device  14  to fluctuate. The temperature sensitive diode  46  is included as part of the device  14 , and a current source  48  provides a constant current through the diode  46 . As a given level of power is supplied to the device  14  (P=I 2 R), the temperature of the device  14  has periodic components (in this case at twice the frequency of the electrical current due to I 2 R law)in response to harmonic Joule heating which is detected using thermal diode  46 . The voltage difference from one side of the diode  46  to the other, which is proportional to the temperature of the device  14 , is provided to a differential input of a lock-in amplifier  50  which is also supplied a reference signal  52  from the function generator  42 . It will be seen that upon a given level of power being supplied to the device  14 , a corresponding device  14  temperature can be noted by reading the voltage drop across the diode  46 . The device  14  temperature response (FIG. 6) is noted for each of a large range of frequencies of signal applied to the device  14  by the function generator  42 . The device  14  temperature response has two components, due to “In-Phase Self-Heating”, as shown in FIG. 6, and “Out-Of-Phase Self-Heating”(out of phase with the input power) as also shown in FIG.  6 . Both of these components are detected by the lock in amplifier  50 . 
     With reference to FIG.  3  and FIG. 5, surface  39  of the plate  36  of the thermal head assembly  30  is in contact with the lid  12  of a device  14  under test, as shown in FIG.  3 . Then, the thermal characteristics of the device  14  are determined without application of power to the device  14 , but with variations in frequency (through application of signals of different frequencies) applied to the heating element  40  of the thermal head assembly  30  (resistance of the heating element  40  indicated by resistor  54 ). The diode  46  of the device  14  is used in the same manner as above, i.e., the diode  46  is temperature sensitive, and a current source  48  provides a given current through the diode  46 . Application of current to the heating element  40 , without application of power to the device  14 , causes the temperature of the device  14  to increase. As a power level at a given frequency is supplied to the heating element  40 , temperature of the device  14  changes and is detected by the diode  46 . Again, this voltage difference from one side of the diode  46  to the other is proportional to the temperature of the device  14 , and is provided to a lock-in amplifier  50 . It will be seen that upon a given frequency supplied to the heating element  40 , a corresponding device  14  temperature can be noted by reading voltage across the diode  46 . The device  14  temperature response has two components, “In-Phase control Heating” and “Out-Of-Phase Control Heating”(FIG.  6 ). 
     FIG. 6 illustrates how the temperature of the device  14  responds independently to (1) functional testing thereof (without functioning of the heating element  40 ) and (2) operation of the heating element  40  of the thermal head assembly  30  in close proximity thereto, i.e., with the surface  39  of the plate  36  in contact with the lid  12 , without functioning of the device  14 . With the thermal head assembly  30  removed from the lid  12 , since the functional testing of the device under test  12  is specified and all characteristics of the testing are known prior to actual test, the temperature of the device  14  through self heating can accurately be predicted by means of the above characterization. Then, with the surface  39  of the plate  36  in contact with the lid  14 , at any given portion of the functional test, a power level of the heating element  40  can be provided, determined by the state of self heating of the device under test  14  as described above, to keep the device  14  at a substantially constant, chosen temperature. 
     For example, and again with reference to FIG. 6, assuming that the device  14  has provided thereto a signal of 0.01 Hz during a portion of the functional testing thereof, a device  14  temperature response of 0.18 K/W is provided for “in phase self heating” of the device  14 . Meanwhile, with that same signal provided to the heating element  40 , a device temperature response of 0.13 K/W is provided for “in phase control heating”. At a given frequency, therefore, the device  14  exhibits a greater temperature response in self heating than in heating by the heating element  40 . An adjustment must be made for this difference for maintenance of substantially constant temperature of the device  14 . The ratio of sensitivity is 0.18/0.13=1.38, so it will be seen that the power level of the heating element  40  must be adjusted by this factor, i.e., the current through the heating element  40  must be sufficiently lowered to properly compensate for the increase in temperature which would occur due to device  14  self heating. 
     A mathematical analysis is provided further on. 
     The above operation is carried outer for the entire spectrum of power levels applied in the complete testing sequence of the device  14 . Thus, the desired power levels of the heating element  40  for keeping the device  14  at a substantially constant temperature during this testing sequence can be arrived at. Using Fourier transform, the desired level of power applied to the device  14  and the heating element  40  in the time domain can be arrived at (FIG.  7 ). In fact, and with reference to FIG. 8, showing an enlarged portion of FIG. 7, the changes in heating element  40  current draw are shown to slightly precede in time the changes in device under test  14  current draw, so as to anticipate by a small amount of time the changes in power of the device  14 . Power matching is indicated in FIG.  8 . 
     FIG. 9 illustrates results of the present invention in use. As shown therein, over a period of time, with the device  14  undergoing functional tests at a variety of power levels, the device  14  temperature is maintained at close to the desired 40° C., through anticipative adjustment of the power level of the heating element  40  of the thermal head assembly  30 . 
     It will be seen that the present system overcomes the problems of the prior art in keeping a device under test, particularly a lidded device under test, at a substantially constant temperature. Additionally, the system can readily be applied to current thermal head apparatus, avoiding the expense and complication of prior art systems. 
     The foregoing description of the embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Other modifications or variations are possible in light of the above teachings. 
     Analysis 
     The transform operator establishes relationship between the time and frequency domains for both temperature and heating power. 
     
       
         θ(ω)=Φ[ T ( t )] 
       
     
     
       
           p (ω)=Φ[ P ( t )] 
       
     
     where T(t) is the desired temperature of the device. 
     Assuming linearity of the temperature response we obtain 
     
       
         θ(ω)= P   H (ω)τ H (ω)+ P   c (ω)τ c (ω) 
       
     
     From this relation we can extract the desired power at the heater            P   H          (   ω   )       =         f        (   ω   )           τ   H          (   ω   )              [       θ        (   ω   )       -         p   C          (   ω   )              τ   C          (   ω   )           ]                              
     where f is a filter function, which may be chosen to avoid convolution of high frequencies into the control sequence, not always necessary due to diminished responses at this frequency range. The input to the heater in time domain is obtained using inverse Fourier transform. 
     
       
           P   H ( t )=Φ −1   [p   H (ω)] 
       
     
     In operator form the heater input can be written as 
     
       
           P   H ( t )= A{circle around (x)}P   c ( t )+B{circle around (x)} T ( t ) 
       
     
     where        A   =       -     Φ     -   1         ⊗     {       f        (   ω   )                τ   C          (   ω   )           τ   H          (   ω   )            Φ     }               B   =       -     Φ     -   1         ⊗     {       f        (   ω   )            1       τ   H          (   ω   )            Φ     }                              
     Where: 
     t: time, s 
     ω: angular frequency, rad s −1    
     T: temperature in time domain, K 
     θ: temperature in frequency domain, K 
     P c : dissipated power due to self-heating (time domain), W 
     Pc: dissipated power due to self-heating (frequency domain), W 
     P H : dissipated power due to control heating (time domain), W 
     PH: dissipated power due to control heating (frequency domain), W 
     θ: temperature in frequency domain, K 
     τ c : reduced temperature response due to self-heating (frequency domain), K W −1    
     τ H : reduced temperature response due to control heating (frequency domain), K W −1    
     f: filter function (e.g. Butterworth filter), dimensionless 
     A: operator defined in text 
     B: operator defined in text 
     {circle around (x)}: operation symbol 
     The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill of the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.