Texture and microstructure of Cr2O3 and (Cr,Al)2O3 thin films deposited by reactive inductively coupled plasma magnetron sputtering
Introduction
Low-temperature physical vapor deposition (PVD) of crystalline Al2O3 (alumina), and especially the thermodynamically stable α phase, has been approached from two main directions. One is to use ionized PVD (I-PVD) techniques [1], an approach that has yielded low-temperature-deposited θ-, κ-, and γ-alumina films [2], [3], [4], [5], [6], [7] with higher crystallinity than for conventional PVD, and has permitted growth of α-Al2O3 at temperatures below 700 °C [8], [9], [10]. The second approach is to promote the nucleation of α-Al2O3 either by a crystallographic template such as α-Cr2O3 [11], [12], [13] or in solid solution α-(Cr,Al)2O3 [14], [15], [16], [17]. We have previously investigated [5], [6] amorphous and γ-alumina thin films deposited by the I-PVD technique inductively coupled plasma magnetron sputtering (ICP-MS), which uses an rf coil to increase the degree of ionization in the deposition flux [18]. Subsequently, we employed chromia template layers and demonstrated that the texture of the template strongly influenced the nucleation of α-Al2O3 [13]. This background motivates the present work, where we have investigated the texture effects in pure chromia films and the deposition of α-(Cr,Al)2O3 thin films with and without chromia template layers, using dc magnetron sputtering and ICP-MS.
Section snippets
Experimental details
Cr2O3 and Cr–Al–O thin films were deposited by reactive dc magnetron sputtering and ICP-MS with 25-mm Cr (99.995%, operated in dc power control mode, 40 W unless stated otherwise) and Al (99.995%, operated in current control mode with pulsed dc) targets. Details can be found elsewhere [5], [13]. Si(100) substrates (15 × 15 mm2) with native oxide were used. When a negative bias was applied to the substrate, an rf power supply was used (the bias voltages stated in this paper are the dc-equivalent
Initial observations
During all depositions, the Ar flow was set at 5 sccm, corresponding to a partial pressure of 0.7 Pa (5 mTorr). The hysteresis effect was investigated by running the Cr target at 40 W dc power, varying the O2 flow from 0 to 4 sccm and back (graphs not shown). When the rf coil was not used, metallic-mode sputtering occurred in the range 0–1 sccm, after which the expected transition region from metallic- to poisoned-mode sputtering was observed. At O2 flows above 1.4 sccm, the target was in poisoned
Concluding remarks
Chromia films with fiber texture were grown; the out-of-plane texture could be controlled from a < 0001> texture, due to competitive growth, to a < 101̅4> texture, probably due to recrystallization driven by ion-bombardment-induced strain. The latter films apparently have a large number of stacking faults and other defects as indicated by the asymmetric peak broadening. This is relevant for the use of chromia templates to promote α-alumina nucleation. Both the introduction of a large number of
Contributors
This paper is based mainly on K. Pedersen's M. Sc. Thesis, for which P. Eklund served as supervisor. K. P. performed the majority of the deposition experiments, XRD and SEM analysis and interpretation. P. E. also contributed to planning, analysis, and interpretation, deposition experiments, and wrote the paper based on K.P.'s thesis. J. Bøttiger served as examiner and contributed to planning and interpretation. M. Sridharan contributed to the design, planning, and setup of the experiments. M.
Acknowledgments
Funding from the Danish NABIIT program is acknowledged. P. E. acknowledges support from the Carlsberg Foundation and the Swedish Agency for Innovation Systems (VINNOVA) VINN Excellence Center in Research & Innovation on Functional Nanostructured Materials (FunMat).
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Present address: Center for Nanotechnology and Advanced Biomaterials (CeNTAB), SASTRA University, Thanjavur – 613 401, Tamil Nadu, India.