Materials Science and Engineering B77 (2000) 288 – 292 Strain sensitivity and temperature behavior of invar alloy ﬁlms K. Rajanna a, *, M.M. Nayak b a Department of Instrumentation, Indian Institute of Science, Bangalore 560 012, India b Adanced Transducers Diision, Liquid Propulsion Systems Centre, ISRO, 80 Feet Road, Bangalore 560 008, India Received 28 April 2000; accepted 12 July 2000 Abstract The electrical resistance – strain characteristics of Invar ﬁlms has been studied for their possible application as strain gauges. The Invar ﬁlms were prepared by sputtering technique and the necessary details of ﬁlm preparation are given. Temperature coefﬁcient of resistance (TCR) of the ﬁlms has been measured by systematic annealing of ﬁlms in vacuum. It has been found that TCR of ﬁlm is of the order of 10 -3 /°C. The gauge factor which is an index of the strain sensitivity, was measured for as-deposited and annealed ﬁlms. Films exhibited linear electrical resistance versus strain characteristics. The gauge factor for relatively thinner ﬁlms was found to be as high as 5.5 and for ﬁlms with thickness greater than 500 A  , it was lower. For ﬁlms annealed by heating up to temperature less than 300°C, the gauge factor value did not change. However, ﬁlms annealed at temperature above 300°C, showed reduction in gauge factor. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Strain gauges; Sputtering; Thin ﬁlms; Invar alloy ﬁlms www.elsevier.com/locate/mseb 1. Introduction Invar alloys are a family of nickel–iron alloys known to exhibit very low thermal expansion over a wide temperature range. Although nickel and iron are the common constituents of invar, addition of small amounts of materials such as carbon, manganese etc. is known to improve the characteristics of invar. In view of low thermal expansion and good dimensional stabil- ity, these alloys are increasingly becoming important for several applications [1,2] in shadow masks, frames and cathode ray tube gun parts, computer terminals, ﬁre safety cutoffs, household appliances and advanced electronic components. Invar alloys are interesting not only in bulk form but also in thin ﬁlm form [3,4]. In addition to general usage, in recent years invar materi- als in thin ﬁlm form are gaining importance for their applications in the area of sensors as well as MEMS [3]. An experimental study carried out on vacuum de- posited invar ﬁlms with respect to strain gauge applica- tion has been reported in this paper. An ideal strain gauge for measurement purposes should posses fairly good strain sensitivity, low temperature coefﬁcient of resistance (TCR) and excellent thermal and temporal stability [5–7]. Therefore, in the present work a system- atic study of the strain sensitivity (electrical resistance – strain characteristics) and the temperature coefﬁcient of resistance (TCR) of invar ﬁlm has been made. 2. Experimental The invar ﬁlms were prepared by DC sputtering technique using a Univex 300 vacuum system (M/S Leybold, FRG) comprising of a turbomolecular pump backed by a rotary pump. The substrates used were microscopic glass slides of 75 mm length, 25 mm wide and 1.4 mm thick. These were cleaned in a detergent, in an ultrasonic bath and then in an isopropyl alcohol vapor bath, with washing in distilled water after each stage. Copper contact pad ﬁlms of thickness 4000 A  were deposited using a manually prepared mechanical mask. Fig. 1 shows the schematic of the ﬁlm conﬁgura- tion employed. As can be seen, for the sensing ﬁlm (Invar ﬁlm) double rectangular conﬁguration of 10 × 0.8 mm has been employed. For this purpose, a preci- sion mechanical mask was prepared using a CNC * Corresponding author. Fax: +91-80-3600683. E-mail address: kraj@isu.iisc.ernet.in (K. Rajanna). 0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0921-5107(00)00503-1