Download Study of Thermal Resistance Measurement Techniques

STUDY OF THERMAL RESISTANCE MEASUREMENT
TECHNIQUES
The Development of the Next Generation VLSI Technology for Very
High Speed Analog Chip Design
PROJECT UPDATE
Prepared for :
Dr. Ronald Carter
Dr. W.Alan Davis
Prof. Sungyong Jung
By
Swati Sahasrabudhe
[email protected]
September 13,2002
Modern high-speed logic devices consume appreciable amount of electrical energy. Also
device and interconnection sizes are becoming small. This leads to the generation of large
amount of self-heat in a device.
Thermal Characterization of a semiconductor device is the determination of the
temperature response of the semiconductor circuit junctions due to internal self-heating.
Device heat generation is concentrated in a small region in the semiconductor die from
which it diffuses outward into the package.
The measurement of component thermal resistance is a common approach to junction
temperature determination.
Thermal Resistance Rth is defined as the junction temperature rise above the temperature
of any reference point e.g. above the ambient temperature, related to the electrical power
P dissipated in the device. So the thermal resistance Rthja between the junction with the
temperature Tj and surroundings with the ambient temperature Ta is given by
Rthja
Tj - Ta
= ---------P
---------- (1)
The self-heating if a bipolar transistor changes the  and VBE according to the temperature
rise within the device. So  and VBE both are used to determine the thermal resistance.
Various methods to determine the Thermal Resistance of the semiconductor device:
A.
DC Pulse Measurement [1]:
This is the basic method used for the measurement of thermal resistance.
This method uses the VBE as a temperature sensitive parameter. First the device
calibration is performed. That is the voltage/temperature relation is individually
measured for the device.
Device Calibration: Device Calibration is performed under thermal equilibrium
conditions so that the junction and the case temperature are nearly equal.
Procedure used for calibration:
a. Selection of Sense Current Level
b. Establishment of thermal equilibrium between the component and a stable
temperature environment.
c. Measurement of component case temperature and VBE .
d. Alteration of environment temperature and repeat the steps b through d.
Sense Current Level Selection:
Sense current must be large enough to establish conduction through the device. Sense
current should not generate the self-heating within the device under test.
Calibration Environment:
Liquid baths and ovens are used as stable temperature environments. When the
device is immersed in the constant temperature environment, any point on the device
case will eventually reach the same temperature as the device junction.
Thermocouples Measurements: Thermocouples are generally used to measure thermal
resistance reference temperatures.
K type or T type thermocouples with 36 gauge are generally used.
Infrared temperature sensors are also used to measure the non-conductive surface
temperature.
Test Method:
Figure 1 [1]: D.C. Pulse – Measurement Setup.
Circuit used for measurement is shown in Figure 1[1]. Device Under Test (D.U.T.)
shown is diode for simplicity.
1. Heating Interval: Heating power is delivered to the device under test. Thermal
resistance is measured at thermal equilibrium condition. Heating intervals are
longer to satisfy the steady, fixed heat dissipation for the thermal junction
measurement. The end of heating interval corresponds to the beginning of
temperature sampling.
2. Temperature Sampling Interval: The sense current is applied to sense the junction
voltage. Temperature sampling interval is very small compared to the heating
interval. Temperature sampling is used to measure the junction temperature at the
instant the heating interval ends so as to avoid the cooling of the junction.
3. Measurement Delay Interval:
Switching from Heating to Temperature Sampling mode introduces the electrical
transients. If the junction voltage is measured as soon as the Heating Interval ends
then the voltage values measured will not be correct. So the sense voltage is
measured after the electrical transients have died out. This waiting interval is
called the “Measurement Daly Interval”.
Measurement interval if it is too short, the junction temperature will be spurious;
if too long the cooling of the junction will take place.
Measurement Delay
Temperature Sampling Interval
Heating Interval
Heating Power
Figure 2: Heat and Measurement Intervals – DC Pulse Thermal Resistance
Measurement.
B.
Frequency Domain Measurement [2]:
In the method using the DC Pulse Measurement the device needs to be operated
under two different bias conditions; at low power dissipation (Temperature
Measurement Interval) and at high power dissipation (Heating Interval). This
method needs strong variation of power dissipation for the extraction of thermal
resistance. Also it is not possible to measure the junction temperature as soon as
the heating is removed. Frequency Domain Measurement is used to over come
these shortcomings.
For the measurement, the variation of VBE or  with temperature is considered.
Figure 3 [2]. Frequency Domain – Measurement Setup.
Circuit used for Frequency Domain Thermal Resistance Measurement is shown in Figure
3[2]. Method explained in the circuit is based on the variation of  with temperature.
Initially only DC circuit consisting of DC voltage source (VCC) connected via a resistor
(R) to the collector of the transistor and a constant current source ( IB ) connected to the
base is considered. Then by choosing value for IB and VCC, a bias point in the active
region is found. Around this point collector-emitter voltage is varied by modulating VCC
with signal Vcc. Then voltage at vC is given as:
VCC * (f)
VC = ---------------------------- (2)
R + (f)
Where (f) is frequency dependant differential resistance between Collector and Emitter
of the transistor.
For higher frequencies the signal is shorter than T and junction temperature stays
essentially constant.
For small frequencies we can find the transition between steady state and transient mode.
Value of (f) can be negative in the small frequency range. Plotting a graph of vC vs.
Frequency in Bode diagram, as shown in Figure 4[2] allows extraction of T from higher
characteristics frequency by finding its 3dB-point. Thermal resistance R T and C T
need to be separated.
Instrumentation for Thermal Measurement [3][4][5]:
a) Thermal Measurement Systems:
Many times Computer Controlled Thermal Characterization custom systems
are used. Programmable Multimeters are used for the temperature sensitive
parameter voltage measurement and thermocouple voltage measurement. E.g.
HP 3455A, HP 3458A.
Control signals for switching device and programmable multimeter are
generated within arbitrary waveform generator like WAVETEK MODEL 75.
b) Thermometry:
Sometimes different thermocouple systems differed at certain temperatures
but agree at other temperatures. To choose a thermocouple system, it is
examined versus a precision thermometer.
c) Current Sources:
Making a perfect Current Source is difficult and can lead to oscillations due to
the small load the current source sees. Voltage sources do not exhibit this
problem. Therefore voltage source with large resistor in series used as a
Current Source. Resistor with low temperature coefficients (200PPM) is used.
C.
Direct Extraction Technique [3]:
Earlier we have studied two methods for the thermal resistance measurement.
These methods rely on fitting a linear relationship between either VBE or  with
the junction temperature. Linearity is within 4 % to 10%. Also expensive
instruments are needed for the measurement.
Steve Marsh’s [3] Direct Extraction Technique is a simple method to derive the
device junction temperature under Self-Heating Bias conditions.
This method uses three DC measurements of the device where the junction
temperature is known to be the same.
Figure 4.[3] Direct Extraction Technique – Measurement plot of Ic vs. Vce.
Figure 4[3] shows the curves where Collector current (Ic) is plotted vs. the collectoremitter voltage (Vce) for a fixed base current (Ib) and three different ambient
temperatures.
Normal bias point Z for the transistor is found for certain collector current (Ic) and base
current (Ib) at ambient temperature T1.
Power dissipation = P1 = V1*Ic.
Another bias point Y is at higher ambient temperature T2 with the same collector current
(Ic) and same base current (Ib).
Power dissipation = P2 = V2*Ic.
Bias point X is at still higher temperature. For X the collector and base currents are same
as those for Z and Y.
Power dissipation = P3 = V3*Ic.
Since all the three bias points are at the same Ic and Ib the current gain  is also identical.
As  is known to be the function of junction temperature and it is same at these three
points, the device mean junction temperature must also be the same.
Experimental results show that Vbe is also same for these three points.
Now, Tj = Tamb +  * Pdiss
---------- (3)
So we have the following three equations:
Tj = T1 +  * P1
---------- (4)
Tj = T2 +  * P2
---------- (5)
Tj = T3 +  * P3
---------- (6)
Thermal impedance from a fixed junction temperature to the base-plate is a linear
function of the temperature difference as follows:
(Tj) = A + B*Tamb
---------- (7)
Figure 5 [3] Plot of simulated thermal impedance verses ambient temperature.
Figure 5 [3] shows the simulation result, which indicates that the thermal impedance is a
linear function of ambient temperature.
Substituting equation (7) in (4),(5) and (6) we get,
Tj = T1 + (A+B*T1) * P1
---------- (8)
Tj = T2 + (A+B*T2) * P2
---------- (9)
Tj = T3 + (A+B*T3) * P3
---------- (10)
Also choosing T1,T2,T3 equally spaced we get, T3= T2 +T, T1= T2 - T . Then
solving the simultaneous equations we get,
Tj = T2 + T( P2/P3 – P2/P1) / ( P2/P3 + P2/P1 - 2)
---------- (11)
To minimize the effect of measurement errors, T should be as large as possible.
This technique of measurement is compared with other measurement techniques by the
author and the result agrees within 3 to 5%.
Only limitation of the technique is that the device under test should sufficient negative
slope in their I-V characteristics.
References
[1]
Dr. John W. Sofia, “Fundamentals of Thermal Resistance Measurement”, 1995
http://www.analysistech.com/PDF_Files/fundamen.pdf
[2] S.Bruce, A.Trasser, M.Birk, A.Rydberg and H.Schumacher, “Extraction of
thermal time constant in HBTs using small signal measurements”, Electronics
Letters, Volume: 33 Issue: 2, 16 Jan. 1997, Page(s): 165 -167
[3] Steve P. Marsh, Senior Member, IEEE, “Direct Extraction Technique to Drive
the Junction Temperature of HBT’s Under High Self-Heating Bias Conditions”,
Electron Devices, IEEE Transactions on, Volume: 47 Issue: 2, Feb. 2000
Page(s): 288 –291
[4] Shope, D.A.; Fahey, W.J.; Prince, J.L.; Staszak, Z.J. “Experimental thermal
characterization of VLSI packages ” Semiconductor Thermal and
Temperature Measurement Symposium, 1988. SEMI-THERM IV., Fourth
Annual IEEE , 1988 Page(s): 19 –24
[5]
Jakopovic, Z.; Bencic, Z.; Zunac, R. “A correction of measured power
MOSFET's normalized temperature response because of a case temperature rise
Power Electronics and Applications, 1993., Fifth European Conference on , 1993
Page(s): 143 -148 vol.2