Thick platinum films as low temperature thermometers

Thick platinum films as low temperature thermometers I. B a t ' k o , M. Somora*, D. Vanick#*, K. F l a c h b a r t , V . M a t e j a n d V . P a v h ' k Institute of Experimental Physics, Slovak A c a d e m y of Sciences, 0 4 3 53 Ko~ice, Czechoslovakia * I n s t i t u t e of Hybrid Microelectronics, Faculty of Electrical Engineering, Technical University, 0 4 3 89 Ko~ice, Czechoslovakia

Received 2 7 March 1991; revised 6 February 1992 Resistance measurements of thick platinum films b e t w e e n 4 . 2 and 3 0 0 K are reported. The thick films were prepared by standard screen-printing techniques. The properties of the films make t h e m suitable for l o w temperature t h e r m o m e t r y . In the temperature range 7 7 - 3 0 0 K experimental data are fitted using a quadratic fit w i t h an error in temperature of less than 0 . 2 % .

Keywords: thick films; resistance thermometers

thermometers;

platinum

Platinum resistance thermometers are one of the most suitable for use above = 20 K. Their excellent reproducibility, high sensitivity and easy handling are advantages that have prompted development of new types of platinum based resistance thermometers L2. We prepared thick platinum films (TPFs) by standard screen-printing techniques• TPFs were printed from home-made platinum paste which consists of commercial platinum powder (SAFINA, Jesenice, Czechoslovakia), lead-boron-silicon glass and a 3 - 5 % solution of ethylcellulose in terpineol. Using printing through a net (#250 mesh) on a 0.635 mm thick A1203 substrate, a meander with 10-15 #m thick and 250 #m wide lines placed on a 10 x 12 mm 2 area was formed. After deposition the films were dried for 15 min at 150°C and then fired for 1 h in a standard thermal profile which is used in thick film fabrication. The highest temperature in profile was 930°C with l0 min duration. The filling volume of the TPFs so prepared was approximately 70%. The d.c. resistance measurements were made using the four-probe method. As a reference thermometer a calibrated Rosemount platinum thermometer was used. The experimental data were recorded between 4.2 K and 300 K in 0.5 K steps. Typical resistance ratio data W = R(T)/R(273.16) are shown in Figure 1. For comparison the reference function Wr(Tgo) for standard platinum resistance thermometers defined by the International Temperature Scale of 1990 (ITS-90) 3 is plotted. As can be seen in Figure 1, the slopes of both curves above T --~ 50 K change only slightly. The greatest differences are 0011 ©

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Figure 1 Temperature dependence of the TPF resistance ratio ) and the reference function Wr (T9o) ( - - - ) R/R273.16 (

observed in the residual resistivity temperature region because of the high contribution of residual resistance (about 10% of resistance at 273• 16 K). The high value of residual resistance R0 is due to grain boundaries, dislocations and impurities, which originate from the technology• Figure 2 shows sensitivity S = d W / W d T of TPFs and the sensitivity calculated from the reference function. Both curves behave similarly above ---50 K. The TPF sensitivity curve has a maximum at T = 45 K and then sharply drops with decreasing temperature due to the high contribution of the residual resistance. Some characteristic data of one of the TPF resistors compared with calculated values from Wr(Tgo) are given in Table 1. The experimental data were fitted in the temperature range 7 7 - 3 0 0 K by the expressions R = ao + ai T + a2 T 2

(la)

and T = b o + b , R + b2R 2

(lb)

Both expressions give similar results and fit the experimental data with an error in temperature less than 0.2%. Note that due to the imponderabilities of the Pt film preparation process every thermometer needs its own calibration to yield the quoted resolution• If in Equations (la) and (lb) the resistance R is replaced by N \

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1O0 200 300 Temperature(K) Figure 2 Temperature dependence of the sensitivity dWlWdT of the TPF resistors ( ) and that calculated from the reference function Wr(T90 ) ( - - - } between 15 and 300 K

2275/92/070683-02 Butterworth-

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Research and technical note Table 1 Characteristic resistance ratio data of one of the TPF resistors compared with calculated values from reference function Wr(Tgo ) Parameter

R(273.16 K) (fi)

W14.2 K)

W(40 K)

W(77 K)

dW/dT (l/K) at 273.16 K

TPF resistor Calculated

99.5

0.0998 -

O. 147 0.041

0.277 O. 186

0.003458 0.003989

the resistance ratio W, then calibration equations W = W(T) and T = T(W) of one thermometer may be used for others (prepared in the same run of the preparation process); however, the error in the temperature rises to 2.5% between 77 and 100 K and 1% for temperatures above 200 K. The first motivation for studying TPF resistors was to use them as low cost, easy-to-manufacture and easy-touse industrial thermometers at room temperature and above. However, the observed properties of TPFs and the ability to prepare even smaller ones than those described in this paper on substrates not thicker than 0.2 m m suggests their use as very low heat capacity sensors. This enables one to prepare small calorimeters or

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thermometers with short relaxation times for the temperature range at least above 77 K. Additional work will be needed to optimize the technological process with the aim to obtain T P F resistance sensors useful even at lower temperatures.

References 1 Dimitrov, D.A., Terzijska, B.M., Guevezov, V. and Kovachev, V.T. Cryogenics (1990) 30 348-350 2 lye, Y. Cryogenics (1988) 28 164-168 3 Mangum, B.W. and Furukawa, G.T. Guidelinesfor Realizing the International Temperature Scale of 1990 (ITS-90) NIST Technical

Note 1265