Sensors and Actuators A, 41-42 (1994) 435-438 435

Ceramic pressure sensor based on tantalum thin film

I. Ayerdl, E. Castafio, A. Garcia-Alonso and F J. Graaa Mmvelectmmcs Department, Centro de Eshrdros e 1nveshgactone.s Thucas de Gmptizcoa (CEIT), PO Box 1555, San Sebashh E-2OlW9 (Spam)

Abstract

The propertIes of piezoresisme tantalum mtnde thm films on s&con substrates have been Investigated for use m a pressure-sensmg element The tiun-film deposition has been carried out by reactive sputtermg technrques under d c and r f power con&ions An r f reactwe sputtered tantalum mtrrde thm fdm m an Ar-9%N, depositIon atmosphere has been selected as the ideal plezoresistrve matenal for the stram gauge ‘Ilus plezoresisbve stram gauge combines a high resistw@, p=230 fi cm, a low temperature coeffinent of resistance, TCR= -80 ppm/ “C and a lugh temporal stabMy wxlth a good longrtudmal gauge factor, GF= 3 5

lntrodoction

In the last few years, the rapid progress m aerospace and mdustnal fields has reqmred the development of new plezoreslstlve pressure sensors intended to work m harsh envtromnents These pressure sensors demand strain gauges that must combme both high chermcal and mechanical resistances at high temperature Tan- talum-based ceramic matenals are good candidates for these apphcatlons Tantalum mtnde thm films are widely used as thin-film reslstorS m the nucroelectromcs m- dustry because of their high stab&y and good chemical resistance A large amount of experunental work con- cerning the electrical propeties of tantalum mtnde thin films 1s now available [l, 21 Nevertheless, the plezoreslstlve properties of tantalum mtnde thm fihns are less known, although they can be of great relevance m the field of pressure sensors

In this article we present the thm-fihn depositloo and maskmg methods to develop a sensmg element based on plezoreslstlve tantalum mtnde thm fihns The sensmg element consists of a sillcon substrate, thermal sibcon dloxlde as an msulatmg mterlayer, an optional tantalum oxide fihn deposited by sputtermg and, finally, a tantalum mtnde thm fihn as the sensmg plezoreslstlve layer This sensmg film 1s deposited using both d c and r f reactlve sputtermg The tantalum mtnde stolchl- ometry is controlled by the Nz partial pressure durmg the deposltlon DeposItion parameters and postde- position thermal treatments have been chosen m order to unprove the plezoreslstlve properties of the sensmg element a low temperature coefficient of resistance (TCR), high stability and high gauge factor (GF) The

results concemmg the Influence of temperature and stram on the electrical properties are also presented

Experimental

Tantalum mtnde thm films were deposlted onto thermally oxldlzed 500 pm thick Si wafers by d c and r f reactive magnetron sputtermg Pnor to the depo- sition, the SI substrates were cleaned m an ultrasonic degreasmg bath m a clean-room environment The purity of the metallic target was 99 95% The residual gas pressure was less than 8~ lo-’ Pa and the total gas pressure of the Ar-Nz mmre dunng TaN deposltlon was held at 0 3 Pa The N, gas flow was m the range 613% of the total gas flow Deposmon rates of 5-12 run/mm were achieved

In order to improve the adhesion between the d c - sputtered TaN thm film and the substrate, a tantalum oxide mtermedlate layer was produced by r f reactlve magnetron sputtermg m an Ar-50%0, atmosphere In addltlon, a 50 W r f bias was applied to the substrate durmg the deposition Thm-film thicknesses between 300 and 1500 nm were measured with a prof&nneter A post-deposition thermal treatment m air (300 “C, 3 h) was apphed to the r f sputtered layers m order to anneal the thin-fihn structure Heat treatment (800 “C, 2 h) m an Ar atmosphere was also carned out to investigate the composltlonal propertles of the thm films by X-ray dtiacuon Quahtatlve studies of thm-film stresses were performed usmg a substrate curvature technique [3]

Tantalum mtnde thm-film resistors, 150 pm wide with a 1000 pm long meandenng path, were patterned

0924-4247/‘94/$07 00 Q 1994 Elsewer Sequom All nghts reserved SSDI 0924-4247(93)00535-C

436

usmg photohthographlc techniques All electrical mea- surements were done by four-probe methods An au- tomatic data-acqmsltlon system controlled by a personal computer was used for the TCR measurement m the range 25-175 “C Accelerated hfe tests at 175 “C were made to study the temporal stabtity of the thm-film resistors Current-voltage-temperature characterlstlcs were used to analyse the conduction mechanism of the tantalum mtnde thm films Both the longtudmal (dl- rectlon of current and strain parallel) and transversal (dlrecfion of current and strain perpendicular) GF values were determined using the cantilever method [4] The hysteresis effect due to strain cychng was also studied

Results and discussion

Structural pq.wties X-ray d8raction showed that the as-deposited thm

films had an amorphous structure Even after the air- anneahng thermal treatment, the tantalum mtnde thm film mamtamed this amorphous structure As-deposited tantalum mtnde thm films with an amorphous structure or with a very fine gram structure have been reported by Petrovlc et al and Au er al [5, 61 After the Ar- thermal treatment a ddfractlon pattern was observed for the tantalum nitride thm film deposited m an Ar-9%N, atmosphere under r f power On comparison with the standard ddfractlon pattern [7], the poly- crystailme tantalum mtnde of our samples was ldentltied to be the orthorhomblc Ta.,N

Convex deflections of the substrates mdlcated com- pressive residual stresses m the fihn, which increases the crack resistance of the sensmg element when it is workmg under tensile stresses Thm-fihn roughnesses were about 15 nm The d c sputtered tantalum mtnde thm films required a tantalum onde mterlayer and an r f bias applied to the substrates in order to achieve good adhesion

Electrtcal propems Figure 1 shows the electrical resistlvlty p and the

temperature coeffiaent of reslstance, TCR, as a fun&on of Nz gas flow durmg the reactive magnetron sputtenng under d c and r f wndttions It can be observed how the reslstivlty mcreases from 200 @2 cm up to 320 j.& cm and the TCR decreases from 325 ppmPC down to -2500 ppn$C This behavlour is attnbuted to the tram&on from a highly doped Ta thin film to a tantalum mtnde thin film These curves are very snmlar to the typical curves reported by other authors [1, 23 The lowest TCR value of -80 ppn$‘C IS acbeved Hnth an Ar-9%N, deposltton atmosphere under r f wndltlons This deposltlon wndltlon urlll be called the ideal workmg

n p(rtRcm) 0 TCR (ppmfc)

n PWcm) 300

0 TCR (ppmfC)

c ml

(b) %N2 (Reachve Sputtermg)

Fig 1 tiectncal resrsttvlty p and temperature co&bent of res&mcc TCR of the tantalum mtnde thm-film resmtors as a fun&on of N2 gas flow, (a) d c -sputtered tantalum mtnde, (b) r f-sputtered tantalum mtnde

0+ 1

1110

Fig 2 Relative remstance change vs temperature at the Ideal workmg pomt A TCR = - 80 ppm/‘C IS deduced from this Figure

pomt from now on The reslstivlty in this situation IS p = 230 fl cm Figure 2 depicts the high lmeanty and low hysteresis of the TCR at the ideal workmg point

In Fig 3 the resistance vanations versus tune are reported from an accelerated life test at 175 “C for both d c and r f conditions The resBtance vanatlon rate 1s very small for r f sputtered thm films, AR/At = 3 ppm/h This lmphes that the temporal stab&y 1s good Meanwhde, the resistance vanatlon rate for as-deposited d c sputtered thm fihns 1s very high Therefore, the air-thermal treatment 1s necessary to stab&e the elec- tncal properties of the sputtered thm films

TIME (Hours)

Fig 3 Relative resistance change vs time at 175 “C with 9% N2 gas flow for (a) d c -sputtered tantalum mtnde, (b) r f - sputtered tantalum mtnde

Mctnlhc Coaduchoo

Fig 4 Current-voltage characterlstlcs at five dtierent temper- atures at the Ideal workmg pomt A metalhc conductlon mechamsm IS deduced from this Figure

The Z/V/T charactenstlcs are illustrated m Fig 4 for the ideal workmg point The resistance is held constant dunng the test lks phenomenon 1s m agreement vvlth a metallic conduction mechamsm The high resistity and the negative TCR of this compound mdxate that the conduction electron mean free path IS very small It is beheved that the high scatter m this type of matenal IS caused by a large amount of disorder due to the amorphous structure of these tantalum mtnde thm films This behavlour has also been described by MOOIJ

for NlCr films [8]

hezommve proper&es Fwre 5 1s a plot of the longrtudmal gauge factor

evaluation test for the ideal workmg point A longl- tudmal GF of 3 5 is obtained for this compound This

/ 21

Fe 5 Relative resistance change vs longtudmal stram at the Ideal workmg pomt A GF-3 5 IS deduced from this Figure

I

7 I 9 10 11

%N2 Gas Flow

Fig 6 Gauge factor vs %NZ gas flow

4

I2 II

value 1s shghtly hq$er than the theoretxal predictions for a plezoresistwe metalhc thm fihn [4] A high hnearlty m the GF behavlour 1s observed and no appreciable hysteresis has been detected on cychng the sample twice These two charactenstics are sultable m a pres- sure-sensing element A smaller transversal GF of - 0 7 has been measured The lon@tudmal GF as a function of N2 gas flow is reported m Fig 6 under d c and r f power condlhons The plezoreslstwe behavlour of the tantalum nitride thin fdm does not depend on the N, gas flow under these deposition and test conditions

Conclusions

Piezoreslstive tantalum mtnde thm films have been deposlted onto &con substrates The sputtenng con- ditions have been defined in order to optnnlze the electrical and plezoresutrve properties of the sensmg film Rf reactlve sputtermg with an Ar-9%N, de- position atmosphere has been selected as the Ideal workmg pomt A high res&vlty, p=230 fl cm, a low temperature coefficient of resatance, TCR = - 80 ppm/ “C, a h& temporal stabdlty, dW&=3 ppm/h at T= 175 “C, and a good longtudmal gauge factor, GF = 3 5 are obtained at the Ideal workmg pomt A metalhc con- duction mecharusm has been ldentdied m the deposited tantalum mtnde thm films

438

References 5

L I Malssel and R Glang, Handbook of lhn Fdm Technology, McGraw-Hti1, New York, 1970, pp 18-12 W D Westwood, N Waterhouse and P S Wilcox, Tantalum Nltnde Thm Fdms, Academic Press, New York, 1975 ME Thomas and M P Hartnett, The use of surface pro- fdometer for the measurements of wafer curvature, J Vat Scr Tech&, 6 (1988) A Garcia-Alonso, J Garcia, E Castaiio, I Obleta and F J Gracta, Pressure senslbd@ and temperature influence on sputtered thin-film materials for piezoresistive sensors, Sensors and Actuators A, 37-38 (1993) 784-789

R Petrovlc, T Nemodovx, N Kraljevlc and T Dmntnjevlc, Electrical and structural propertles of tantalum mtnde thm films deposlted by sputtering, 7’hm Solzd Fti, 57 (1979) 333-336

CL Au, WA Anderson, DA Scbmltz, J C Flassayer and F M Colhns, Stabllrty of tantalum mtnde thm tilm resistors, J Mater Res, 5 (1990) 1224-1232

Jomt CommIttee on Powder DiffractIon Standards, Card 32- 1282, Phdadelphla, PA, USA (1987)

J H Mool~, Electrical conduction m concentrated dlsor- dered transitIon metal alloys, Phys Status Sobdr, 17 (1973) 521-529