Rapid determination of the temperature coefficient of thin-film resistors

Microelectronics and Reliability

Pergamon Press 1968. Vol. 7, pp. 181-184

Printed in Great Britain

RAPID D E T E R M I N A T I O N OF THE TEMPERATURE COEFFICIENT OF THIN-FILM RESISTORS A. ENGELTER and H. KAPPErIJN Solid State Electronics Division, Electrical Engineering Research Department, NRIMS, Council for Scientific and Industrial Research, Pretoria, South Africa

A l m t r a c t - - I n s t r u m e n t a t i o n for the rapid determination of the temperature coefficient of thin-film resistors is described; temperature coefficients between + 5 and 4000 x 10-s/°c can be measured in less than 8 min. Hysteresis and ageing effects are indicated.

1. INTRODUCTION

THIN-FILM circuits are used increasingly for precision applications where close tolerances and high stability over a wide temperature range arc of paramount importance. T h e temperature cocflicientof thin-film resistors is influenced by m a n y parameters of the manufacturing process, such as alloy composition, deposition rate, residual gas composition and pressure, etc. In order to optimize these conditions, a simple and rapid method for measuring the temperature coefficient of thin-film resistors was developed.

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2. PRINCIPLE OF OPERATION

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thermometer deposited on a similar substrate are clamped to a plate which can be electricallyheated or water cooled (Fig. I). The substratc under test is connected to a resistance meter, the output voltage, which is proportional to the relative change in resistance, is fed into the Y channel of an X Y recorder. Similarly, the output of the thermometer circuit is connected to the X channel of the recorder (Fig. 2). Thus, a resistance change vs. temperature graph is obtained, usually in the form of a straight line, the slope of which is proportional to the temperature coefficient. In addition, any permanent changes in the resistance value, as well as hysteresis or other unpredictable effects, show 181

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a = testaubstrate b = resistancethermometer c = contact probe d = aluminium

e = brass plate f ----heater element g ~ cooling tube plate

182

A. E N G E L T E R and H. K A P P E T I J N

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F1o. 2. Block diagram of the instrumentation.

up clearly if the pen is allowed to trace out a complete heating-cooling cycle.

In the temperature range AT, the temperature coefficient = of the resistance is defined as

3, DESIGN CONSIDERATIONS

3.1 Mechanical design The similarity in the dimensions and geometrical arrangement of the sample under test and the thermometer ensures that both acquire the same surface temperature, even under rapid heating or cooling conditions (+60°C/min and --40°/rain, respectively). As a precaution, the contact probes were also arranged identically. The hot-plate consists of two sheets (170 m m x 100 m m x 3 ram) of aluminium and brass, respectively, between which a coaxial heater element (1000 mm x 0-8 m m dia., 110 V, 750 W) is sandwiched. A 6 m m copper tube, hard-soldered to the brass plate, is used for watercooling. An ahminium lid prevents draughts affecting the measurements. All connexions are made by screwing, spotwelding or hardsoldering, so that the heater can be used up to 300°C. The probes consist of hemispherical gold contacts as used for low voltage relays. Contact pressures and spacings are individually adjustable. 3.2 Electronic circuitry Both the resistance and the temperature measuring circuits make use of the four-point probe method which eliminates the influence of contact resistance variations.

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R o AT, where R o is the resistance at the lower temperature and AR is the variation in resistance over the temperature range AT. Most resistance measuring bridges deliver an output proportional to 5R. If this is plotted against T, the tangent of the slope angle, divided by R o, will yield the temperature coefficient. In the circuit described below, however, the output is a measure for AR/Ro, so that the slope directly represents the value of the temperature coefficient, independent of the absolute value of the resistance Ro. To this end, a constant current I is passed through the resistor under test (10Q-100k~). This current is preset between 0.02 and 25 mA in order to make the voltage Vy between the (inner) voltage probes exactly 1 V at room temperature (see Fig. 3). This voltage is balanced out by a fixed, zener-stabilized voltage in order that the Y input of the recorder should sense changes in Vy only. I f resistance R 0 changes by AR, the voltage will vary by Av~ = 1AR.

(1)

RAPID DETERMINATION OF TEMPERATURE COEFFICIENT OF THIN-FILM RESISTORS

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Because the preset value of the current I is such that V~ = I R o = 1 V

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A V y = I A R . I R ° - - Ro

From this equation it is evident that the voltage A Vv is equal to the relative change of the resistance, independent of its absolute value. If the X voltage AVz is proportional to temperature, i.e. AVz = C A T , then the tangent of the slope angle ~ will be given by A Vy

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recorder or by changing the value of the reference voltage (variable between 0.25 and 2 V). The resistance thermometer consists of a glazed ceramic substrate coated with a pure nickel film of approximately 12 ta/[~ sheet resistivity. By spark erosion, the resistance has been trimmed to 100 ft. The temperature measuring circuit (Fig. 4) comprises a constant current supply (Tr2) a reference voltage source (Tr 1, Rx) and an emitterfollower impedance transformer (Trs, Tr4). For zeroing the output at 0°C, the current throughthe resistance thermometer is adjusted by /'1 until the voltage across the inner electrodes is equal to the constant voltage across R1 (1 V). Calibration of the meter and the output is effected with P~ at a higher temperature, say 150°C.

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tan ~3 = "AVx - - C R o A T - - C

(4)

where c~= A R / R o A T is the temperature coefficient of the resistor in the temperature range A T. For graphical evaluation, AT was chosen to be 50 °, the difference between 50 and 100°C, say. With a recorder sensitivity S v = 0-2 mV/mm, each millimeter difference in ordinate corresponds to 4 × 10-6]deg. Higher or lower sensitivities may be chosen either by using the range switch of the

4. PERFORMANCE After a warm-up period of 15 min, the substrate under test is clamped beneath the electrodes and the current is adjusted to zero reading on the recorder. Next, the heater current is switched on, heating the substrate to 150°C in approximately 110 sec. Heating is terminated at a preset temperature by means of a microswitch linked to recorder. Cooling from 150 to 50°C is effected within 150 sec of turning on the cooling water.

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Test substrates with highly conducting contact areas were used, but uncoated resistive films can also be investigated. Care must be taken, however, that the current is switched on only after the substrate has been damped underneath the probes, as spark erosion may destroy the thin contact area. In some cases, noisy graphs were obtained, presumably due to "current noise" generated in the contact resistance, but graphic evaluation was still possible.

With the current and voltage ranges mentioned above, resistors between 10~2 and 100 k~ can be investigated with full sensitivity.

Achnowledgements--The authors

are indebted to Mr. R. L. Magill, T S D , for constructing the oven and probe arrangement.