TF012 - High Temperature Sputtering of Barium StrontiumTitanate on Nickel Foils

Seymen M. Aygiin1, Patrick Daniels1

, William Borland2, and Jon-Paul Maria1

1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695 USA2 DuPont Electronic Technologies, Research Triangle Park, NC 27709 USA

Abstract - The effects of sputtering temperature onthe dielectric properties and the microstructure ofbarium strontium titanate thin films have beeninvestigated. (Bao.6Sro.4)Ti03 films were deposited on Nifoils by radio frequency magnetron sputtering attemperatures between 100-450 °C. Pt top electrodeswere sputtered and the metal-insulator-metalconfiguration was obtained. The capacitors weresubsequently co-fired at 900°C for crystallization anddensification in a reducing atmosphere to avoidoxidation of Ni. The permittivity and the tunability ofthe films were observed to increase with the sputteringtemperature, reaching a maximum at 400°C with apermittivity of 1800 and a tunability ratio of 10:1 at anapplied electric field of 45 V/~m. Loss tangents wereless than 1.5% at 10 kHz. The formation of NiO wasobserved in samples sputtered at temperatures above450°C and resulted in degraded dielectric properties.Atomic force microscopy images revealed increasinggrain size with the increasing sputtering temperature.This microstructural trend accounts for the increasingpermittivity values and sharper ferroelectric anomaliesfound in films processed with increasing sputtertemperature.

INTRODUCTION

The use of base metal foils as substrates forferroelectric thin films offers a viable solution forintegration as embedded passive components in aprinted wiring board (PWB). Base metal foils havemany advantages such as flexibility, low cost and highprocessing temperatures (typically> 650°C) which areneeded for the crystallization of the most commonlyused material BST [1-3]. There has been increasinginterest in preparing of both barium titanate and leadbased ferroelectric thin films directly on metalsubstrates to obtain high permittivity, low loss, and lowleakage current capacitors. The possibility of depositingBST on Ni foils was reported using the chemicalsolution route, however there has not been a systematicstudy to sputter deposit BST on Ni [4]. Ni is ofparticular interest given its wide firing window underreducing atmospheres which encompasses temperaturesin excess of 1300 °c, close to the sintering temperaturesof bulk BaTi03. This may facilitate substantial graingrowth in sputtered BSTs, which commonly exhibitvery fine grain structures. As such, permittivitiesreaching bulk values are an attractive possibility.

In this work, (Ba,Sr)Ti03 was RF magnetronsputtered directly on Ni foils. The effects of sputteringtemperature on the dielectric properties and themicrostructure were investigated. It was shown that thepermittivity and the tunability increase with theincreasing sputtering temperature, which is attributed tothe increase in grain size.

EXPERIMENTAL

(Bao.6Sr0.4)Ti03 thin films were RF magnetronsputtered on 10 Jlm thick nickel foils from a 10.16 cmdiameter stoichiometric (Bao.6Sro.4)Ti03 target. Filmswith 0.8 ~m thickness were sputtered at temperaturesbetween 100-450 °C. After BST deposition, 0.25 ~m

thick Pt top electrodes were DC magnetron sputteredthrough a shadow-mask to define 0.5 mm diametercapacitors, which were ready for high temperatureannealing. The samples were co-fired at 900°C atp02'-10-12 atm for 30 min. under atmospheric pressure.To compensate for the oxygen vacancies created byreducing atmospheres, a reoxidation anneal wasperformed at 500°C in a high vacuum tube furnace for30 min. under a background oxygen pressure of 10-8 atm.

X-Ray diffraction was used to confirm phaseformation and crystallinity. The surface and themicrostructural characterizations were performed with ascanning electron microscope (SEM) and an atomicforce microscope (AFM) using contact mode. Thedielectric measurements were made using an impedanceanalyzer and a Joule-Thomson cooler. Capacitance­voltage measurements were made under a DC biassweep (-35 to 35 V) with steps of 1 V and an oscillationvoltage of 0.05 V at 10 kHz. Temperature dependencewas measured while cooling from 450 to 100 K at zerobias with the same ac signal.

RESULTS AND DISCUSSION

The X-Ray diffraction patterns showed increasingcrystallinity with increasing sputtering temperature asevidenced by narrower peaks with higher relativeintensities. The fully crystallized BST was obtainedafter annealing.

The room temperature capacitance vs. field curvesfor the samples measured at 10kHz exhibited typicalparaelectric C-V behavior with maxima at zero bias andsaturation at high fields. The zero bias capacitanceincreased with increasing sputtering temperature, whilethe saturation values remained nearly constant.

The tunability is given as the ratio of Cmax

(capacitance at 0 V) to Cmin (capacitance at 35 V) and atunability of 10:1 was obtained from the samplesputtered at 400°C. The capacitors processed at 450 °cshowed very high dielectric losses and very lowtunability.

Due to the high surface roughness ofNi foils (30 nmon a 1 ~m x 1 ~m area), the BST layer was co-fired withthe top Pt electrode to increase capacitor yield. Asreported previously, this co-firing approach persuades Pttop metallization to retreat from the cracks to minimizeits interfacial energy, and hence decreasing thepossibility of shorts [5]. However, a consequence of co­firing is local de-wetting of the top electrodes, whichprecludes the use of simple capacitor area calculationsusing the shodowmask aperture diameter. To calculatethe accurate area, fraction mapping of the Pt wasconducted using the SEM images taken at variousmagnifications. In this manner, the actual contact area,hence capacitor dimensions could be computed withhigh confidence. The area ratio of Pt to the BST filmwas calculated using the secondary electron contrastbetween the Pt electrode and BST. The area wascalculated by averaging the values from the imagestaken at multiple magnifications and it was consistentlyfound to be --70% of the nominal electrode area. Thepermittivities were calculated using this value.

The 400°C sputtered sample showed a maximumpermittivity of 1800 and a loss tangent of 0.012 at 10kHz. This permittivity value with a 10: 1 tunability isremarkably high for a sputtered BST thin film [6-9]. Thepermittivity saturated at a value of --180 which is typicalfor Ba-based ferroelectric materials at fieldsapproaching 0.5 MVIcm.

The dielectric properties of BST thin films stronglydepend on the microstructure and the grain size. Usingatomic force microscopy the average grain size wascalculated to be --50 nm for the sample sputtered at 100°c and it increased to --80 nm with sputtering at 400°C(Fig. 1).

The temperature dependent permittivity data showedthat with higher final processing temperature, the BSTgrain size increases with a concomitant increase inpermittivity. The dielectric maxima at the phasetransition temperature also became substantially sharper.

SUMMARY

(Bao.6SroA)Ti03 thin films have been successfullydeposited on Ni foils. Metal-insulator-metal capacitorswere fabricated by co-firing the BST thin films with Pttop electrodes. The dielectric properties were shown toimprove substantially by increasing the sputteringtemperature up to 400°C. The increase in thepermittivity and the tunability was attributed to theincrease in grain size. The ferroelectric anomaly alsobecame sharper with the increased grain size. The bestsample of this work exhibited a room temperature

Fig. 1. Atomic force microscopy image of the sample sputtered at400°C and fired at 900 °C.

permittivity of 1800, a tunability ratio of 10: 1, and a losstangent less than 1.5% at 10 kHz.

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