Thin-film cryogenic resistors from aluminium alloys

Thin-film cryogenic resistors from aluminium alloys N.N. Tadros* and LB. Holdemant Electricity Division, NBS, Gaithersburg, M D 2 0 8 9 9 , USA

Received 15 March 1985 The temperature dependence of the resistances of thin films sputtered from three commercially available aluminium alloys (5052, 5086, 5456) has been measured in the temperature range 1.5-4.2 K. The 5 0 5 2 - a l l o y films had a positive temperature coefficient of resistance (TCR) throughout this temperature range, whereas films of the other two alloys had a negative TCR.

Keywords: cryogenic resistors; thin-film resistors; aluminium alloy

Although superconducting instrumentation for precision electrical metrology (e.g cryogenic current comparators, .Iosephson voltage standards) is now well established (see for example Reference 1), many national standards laboratories maintain ongoing programmes to develop such instrumentation further. The technology base for cryogenic electrical metrology would be broadened by the development of standard resistors that are reproducible upon thermal cycling between room and liquid helium temperatures: these resistors must also have a ~small temperature coefficient of resistance, TCR = R -~ ( d R / d T ) , at liquid helium temperatures. Numerous alloys have been studied in the search lk)r resistor materials having these two properties, and several promising alloys with TCR values typically of a few p p m K -1 at4.2 K have been described in the literature 2 s. In general, these alloys have been drawn into wires for constructing traditional wire-wound resistors, but thin-film resistors could replace bulk resistors in a variety of cryogenic applications. Thin films generally have higher resistivities than the same material in bulk. and lower TCR values 9. Thin-film resistors also have other advantages over bulk resistors, such as adjustment of resistor value by simple techniques (e.g. by anodization or thermal oxidationp. Microelectronics fabrication technology, adapted for superconducting devices during the development of the Josephson compute? °, is now being applied to the development of superconducting instrumentation for electrical metrologyn: stable, thermally cyclable thin-film resistors will be useful in the development of fully integrated monolithic circuits. In this Paper some results are reported from an investigation of resistors that were fabricated from sputtered films of three commercially available a l u m i n i u m - m a g n e s i u m alloys (5052, 5086, 5456). These particular AI-Mg alloys were selected for initial study, partly because of availability, but primarily because they *Present address: Electrical Measurement Laboratory, National

contain magnetic constituents. Sub-ppm TCR values have been reported for AI-Mg alloys with magnetic impuritiesL The Aluminum Association registered compositions for the three alloys specify permissible variations for their constituents, as well as limits on certain impurities~2: their nominal compositions ~3 are given in T a b l e 1.

Resistor fabrication Thin films were deposited onto glass substrates by rf sputtering( 13.56 MHz) from 15 cm diameter, 0.6 cm thick discs of the alloys. These disc targets were presputtered for 2 h in flowingAr at a pressure of 0.4 Pa(3 × 10 _3 tort), after which six substrates were sequentially sputter coated: presputter and sputter-deposition parameters were the same. The forward rf power was 250 W with reflected p o w e r ( 3 W: target self-bias was ~ - - 170 V d.c. The base pressure *br the deposition chamber was < 6 × 10 -7 Pa, achieved with a cryopump/ion-pump combination: during sputtering, this p u m p combination was used in parallel with a liquid-nitrogen cooled sorption pump. The sputtered films were photolithographically patterned, using photoresist and Keuler's etch(by volume, 10 H20:5 HNO3:3 HF:2 H C L diluted eight parts etchant to three parts H20). This aluminium etchant also etches glas~ which precluded accurate film-thickness measurements with a stylus profilometer on finished resistors. The

Table 1

Nominal compositions of wrought aluminium alloys 5052, 5086 and 5456 a Alloying elements (%) Alloy 5052 5086 5456

Magnesium

Manganese

Chromium

2.5 4.0 5.1

0.45 0.8

0.25 0.15 0.12

Institutefor Standards,Cairo, Egypt

tpresent address: Microelectronics Division, COMSAT Laboratories, MD 20871, USA 0011-2275/85/120709-04 $03.00 © 1985 Butterworth 8" Co (Publishers) Ltd

"~The percentages of the alloying elements are tabulated; aluminium and normal impurities constitute the remainder

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709

Thin-film resistors" N.N Tadros and L B Holdeman Table 2.

Sample reproducibility after a 2 h presputter for samples of 45 min sputtering time Alloy 5456

Alloy 5086

Alloy 5052

Deposition sequence

R(4.2 K) (9~)

AR/R (ppm)

R(4.2 K) (~)

AR/R (ppm)

R(4.2 K) (1~)

AR/R (ppm)

1 2 3 4 5 6

30.50597 30.33836 30.89296 30.50617 30.50612

53.3 53.0 53.1 52.6 52.1

31.05130 30.86420 31.01243 31.25117 31.14316 30.94048

26.7 27.1 25.5 27.6 25.7 26.2

38.06923 38.17232 38.61093 38.86530

-18.4 -19.4 -18.8 -18.6

38.96709

-18.7

thickness for one film (5052 alloy) that was deposited through a stencil mask was measured to be 250 nm for 45 min sputtering time. Assuming similar deposition rates for the three alloys, the physical thicknesses of all the samples can be estimated from sputtering time using this 45 min/250 nm relationship.

55 50 45 40 35

Measurement technique The thin-film resistors were measured in a four-terminal configuration using a precision digital voltmeter, which in its resistance mode was capable of resolving 10-~ 1~. In this mode, the current through the resistor was 10 mA. The resistances of the samples located in the liquid helium were compared to the resistance of a 100 ~ standard resistor at room temperature. Readings were taken for both current directions to eliminate the effects of thermal e.m.f, values. The temperature of the resistors was varied by pumping the liquid helium in which they were immersed. The temperature of the liquid helium was measured with an uncertainty of 0.01 K by means of a 470 1~ carbon resistance thermometer, which had been calibrated using a germanium doped resistance thermometer that had in turn been calibrated according to the provisional 0.5-30 K temperature scale of 1976.

30 A

E Q.

25 2O

1 4~

~,

lO 5 o -5

-10 -15

Results

- 20

Under steady-state deposition conditions, the sputtered composition is the same as the bulk target composition ~4. For this preliminary investigation, the sputter targets rested on water-cooled backing plates, but were not bonded to these plates. In such a situation, target heating with attendant diffusion can increase the time required to reach steady state ~4. Table 2 gives the resistance at 4.2 K and the normalized total resistance change R(1.5 K) - R(4.2 K) R

R(4.2 K)

for three sets of resistors. The resistors in each set were fabricated from 45 min films that were deposited sequentially after a 2 h presputter, thus the first film was deposited after 1>2 h continuous sputtering and the last after at ~>6 h. In effect, the presputtering time was increased by ~ 50 min for successive films. The agreement of the values of R(4.2 K) and AR/R among the resistors of each set indicates that 2 h of continuous sputtering was sufficient to establish steady state deposition conditions for all three alloys. Other data indicated that a 1 h presputter was not sufficient. The temperature dependence between 1.5 and 4.2 K

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Cryogenics 1985 Vol 25 December

I 1.5

l 2.0

I 2.5

I 3.0

I 4.0

I 3.5

T(K) Figure 1 Temperature dependence of resistance between 1.5 and 4.2 K for thin-film resistors fabricated from three wrought alloys of aluminium. The sputtering time was 45 min for each sample, which corresponds to a thickness of ~ 2 5 0 nm. The resistance at 4.2 K for the samples is given by the first entries in Table 4. ©, Alloy 5456; A, alloy 5086; S, alloy 5052 Table 3 Resistance hicknesses

values

for

samples

with

different

Alloy

Sample identification

Deposition time (min)

R(4.2 K) (1"~)

R(1.5 K) (~)

5456 5086 5052 5052

3183-1 5183-5 7183-1 7183-4

30 35.5 68 60

41.24634 46.79840 22.89483 26.97790

41.24799 46.79925 22.89419 26.97723

of the normalized resistances for representative samples from Table2 is shown inFigurel. As can be seen from the slopes of the curves, the 5052 alloy has a positive TCR throughout this temperature range, whereas the 5086 and

Thin-film resistors: N.N. Tadros and L B. Holdeman Table 4

Changes induced by thermal cycling a Alloy 5 4 5 6

Alloy 5 0 8 6

Alloy 5 0 5 2

Cycle number

R(4.2 K) (~)

AR/R (ppm)

R(4.2 K) (f~)

AR/R (ppm)

R(4.2 K) (~)

1 2 3 4 5 6 7 8 9

30.89296 30.89351 30.89460 30.94665 30.94722 30.94753 30.94775 30.94783 30.94793

53.1 51.9 50.3 46.7 44.4 41.0 40.1 40.1

31.01243 31.01348 31.04428 31.04550 31.04615 31.04636 31.04646 31.04654

25.5 24.5 23.8 20.5 19.0 17.9 17.1 16.4

38.61093 38.61769 38.92501 38.92631 38.92539 38.92546 38.92557 38.92561

AR/R

(ppm) -18.8 -16.9 -12.2 -11.3

-9.8 -8.7 -8.5

q-he resistors were cycled together for their last seven cycles, which took place during a 12-day test period that began = 3 weeks after :he resistors were fabricated

545(~ alloys have negative TCR values. The relatively sharp increase ira slope near3 K lot the 5052 alloy curve is probably a consequence of superconductive inclusions throughout the film. The negative TCR values of the 5086 and 5456 alloys, which contain higher percentages of magnetic constituents, are a consequence of the Kondo effect (see for example Reference 15). A limited nulnber of samples with different thickncsses (30-70 rain sputtering times) were teste& at least one different thickness for each alloy (see Table 3). The temperature dependence of these samples was similar to the corresponding45 rain samples: the magnitude of AR/ R increased with increasing fihn thickness. The three samples that provided the data forFigurel were tested for changes with thermal cycling After preliminary testing(two runs lot the 5456 resistor, one run lot thc other two), the resistors were cycled together seven times during a 12-day test periocL The values of R(4.2 K) and AR/R tot all runs are listed in Table 4. After the first of these seven runs, the resistors were removed from the liquid helium environment and placed directly into the [aborato U cnvironmenl, which resulted in increases ira the wdues of R(4.2 K) of 1-8 parts in 103. Although part of the increase could result t'rom moisture condensation and oxidation, resistance increases of the samc magnitude(parts in 103) have occurred in precision bulk resistors that were thermally shocked ira the same way3. After subsequent runs, the resistors were gradually warmed to room temperature in vacuum ira the cryostak and the ensuing resistance changes were at least an order of magnitude smaller(only a 12-wppm between the final cycles). The temperature dependence of the normalized ,esislances, between 1.5 and 4.2 K, measured in the last cycle, is shown in I"i~,,ure2. Discussion The magnitude of AR/R obse~,cd for the 5456 alloy thinfilm resistors is much larger than the values that were reported for bulk resistors with similar composition by Warnccke and Kose7, who measured the bulk resistivity' of AI-Mg-Mn alloys with 5% Mg and 0-1% Mn in the temperature range 1.5-4.2 K. These authors found that for Mn concentrations > 0.4%, the TCR values did not become more negative with increasing Mn concentration, indicating that the additional Mn had not alloyed but had precipitated. Such precipitation in a sputter target if dispersed reasonably uniformly, would not prevent steady state sputter conditions with the sputtered material having the nominal target composition. Judging from observed TCR values values, manganese precipitation appears to be inhibited in our sputtered films, but such precipitation

40 35 30

20 Ol

~

15

,~

lO

-lO[1.5

2.0

2.5

3.0

3.5

4.0

T(K) Figure 2 Temperature dependence of the resistance of the samples of Figure 1 after repeated thermal cycling. These data were taken approximately 30 days after resistor fabrication. The resistance at 4.2 K for the samples is given by the final entries in Table 4. ©, Alloy 5 4 5 6 ; G, alloy 5 0 8 6 ; e , alloy 5 0 5 2

might be partly responsible for the decrease with cycling o f the magnitude of zXR/R for the 5086 and 5456 samples. Warnecke and Kose obtained a sub-ppm TCR for a Mn concentration of 0.078%. The commercial alloy 5056 has a nominal composition of5.1% Mg 0.12% Mn, and {).12'!',, Cr (the balance being AI and normal impurities) and should therefore yield sputtered-film resistors with lower TCR values than the 5086 or 5456 alloys. Of the alloys tested the 5052 alloy films had the lowest total AR/R, but their relatively rapid change in resistance near 3 K is undesirable for precision resistors. Conclusions A prelinainary' investigation has been made of thin-film cryogenic resistors sputtered from three commercially available aluminium alloys. The changes in R(4.2 K) and zXR/R with thermal cycling were greatly reduced after

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Thin-film resistor~" N.N Tadros and LB. Holdeman repeated cycling and room-temperature ageing of these resistors. However, evaluation of their long-term stability requires more extensive study. The results of the present authors' preliminary work establish the viability of sputtered films as an alternative to bulk resistors for cryogenic applications It seems possible that with a suitable choice of composition for the sputter target (e.g. 5056 or the DIN-A1 Mg 5 alloy studied by Warnecke and Kose 7) and a suitable choice of film thickness, sputtered film resistors with sub-ppm TCR values between 1.5 and 4.2 K could be achieved.

Acknowledgements The authors acknowledge with appreciation the assistance of R E Dziuba~ J. Toots, N.B. Belecki and B.N. Taylor of the Electricity Division of NBS, and RJ. Soulen Jr, of the Temperature and Pressure Division. N.N. Tadros gratefully acknowledges receipt of a US Agency for International Development Peace Fellowship, which made a visit to NBS as a guest scientist possible.

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Cryogenics 1985 Vol 25 December

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Proc 1974 ConflEEE Trans lnstrum Meas (1974) 1M-23 {4) Sullivan, D.B. Rev Sci Instrum (1971) 42 612 Gallop, J.C. and Petley, B.W. IEEE Trans lnstrum Meas (1974) IM-23 267 Warneeke, P. and Kose, V. Rev Sci Instrum (1975) 46 1108 March, J.F. and Thurley, F. PTB-Mitteilungen (1976) 86 430 Cimberle, M.R., Michi, U., Mori, F., Rizzuto, C., Siri, A. and Vaccarone, R. Proc ICEC6 (Ed Mendelssohn, K.) IPC Science and Techology Press. Guildford. UK (1976) 190 Warnecke, P. and Kose, V. Co'ogeni{w (1977) 17 635 Macfarlane, J.C. and Collins, H.C. Cryogenics (1978) 18 668 Berry, R.W., Hall, P.M. and Harris, M.T. Thin Film Technology Van Nostrand Co. Inc., Princeton. USA (1968) I B M J R e s Develop (1980) 24 (2) Niemeyer, J., Hinken, J.H. and Kautz, R.L. Appl Phys Lett (1984) 45 478 Gibbons, ICC. (Ed) Woldman's Engineering Alloys 6th Edn, American Society for Metals. Metals Park, OH, USA (1979) 1765 Soldering Alcoa Aluminum Aluminum Company of America, Pittsburg, USA (1972) 116 Vossen, J.L, and Cuomo, J.J. ln Thin Film Processes ( Eds Vossen. J.L. and Kern. W.) Academic Press. New York (1978) 11 Harrison, W.A. Solid State Theory Dover Publications. Inc., New York (1980) 490