STUDY OF THERMAL RESISTANCE MEASUREMENT TECHNIQUES

Transcription

1 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 swat_vs@hotmail.com September 13,2002

2 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 R th 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 R thja between the junction with the temperature T j and surroundings with the ambient temperature T a is given by T j - T a R thja = (1) P The self-heating if a bipolar transistor changes the β and V BE according to the temperature rise within the device. So β and V BE 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 V BE 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 V BE. 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.

3 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 Ð 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

4 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 ÒMeasurement Daly IntervalÓ. Measurement interval if it is too short, the junction temperature will be spurious; if too long the cooling of the junction will take place. Ê Measurement Delay Ê Temperature Sampling Interval Ê Heating Interval Ê Ê Ê Ê Ê Ê Ê Ê Ê Heating Power Ê Ê Ê Ê Ê Ê Ê Ê Ê Ê Ê Ê Ê Ê Ê Figure 2: Heat and Measurement Intervals Ð 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 V BE or β with temperature is considered.

5 Figure 3 [2]. Frequency Domain Ð 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 β 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 ( I B ) connected to the base is considered. Then by choosing value for I B and VCC, a bias point in the active region is found. Around this point collector-emitter voltage is varied by modulating VCC with signal V cc. Then voltage at vc is given as: VCC * R(f) VC = (2) R + R(f) Where R(f) is frequency dependant differential resistance between Collector and Emitter of the transistor. For higher frequencies the signal is shorter than τ T and junction temperature stays essentially constant. For small frequencies we can find the transition between steady state and transient mode. Value of R(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 τ T from higher characteristics frequency by finding its 3dB-point. Thermal resistance R T and C T need to be separated.

6 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 V BE or β with the junction temperature. Linearity is within 4 % to 10%. Also expensive instruments are needed for the measurement. Steve MarshÕs [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.

7 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 β is also identical. As β 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 + θ * Pdiss (3) So we have the following three equations: Tj = T1 + θ * P (4) Tj = T2 + θ * P (5) Tj = T3 + θ * P (6) Thermal impedance from a fixed junction temperature to the base-plate is a linear function of the temperature difference as follows:

8 θ(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) * P (8) Tj = T2 + (A+B*T2) * P (9) Tj = T3 + (A+B*T3) * P (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 Ð 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.

9 References [1] Dr. John W. Sofia, ÒFundamentals of Thermal Resistance MeasurementÓ, [2] S.Bruce, A.Trasser, M.Birk, A.Rydberg and H.Schumacher, ÒExtraction of thermal time constant in HBTs using small signal measurementsó, Electronics Letters, Volume: 33 Issue: 2, 16 Jan. 1997, Page(s): [3] Steve P. Marsh, Senior Member, IEEE, ÒDirect Extraction Technique to Drive the Junction Temperature of HBTÕs Under High Self-Heating Bias ConditionsÓ, Electron Devices, IEEE Transactions on, Volume: 47 Issue: 2, Feb Page(s): 288 Ð291 [4] Shope, D.A.; Fahey, W.J.; Prince, J.L.; Staszak, Z.J. ÒExperimental thermal characterization of VLSI packages Ó Semiconductor Thermal and Temperature Measurement Symposium, SEMI-THERM IV., Fourth Annual IEEE, 1988 Page(s): 19 Ð24 [5] Jakopovic, Z.; Bencic, Z.; Zunac, R. ÒA 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): vol.2