ICMCTF2000 Session B5: Ion Assisted Deposition and Ion Beam Technologies

Titanium oxide films have a wide range of applications, due to their outstanding optical, electrical and chemical properties. Another promising application, for their biocompatibility and hemocompatibility, is as a protective surface coating for biomedical implants. Vacuum arc deposition is a promising method as it allows high deposition rates, combined with a high ionization rate and, hence, easily controlled deposition energy.

In this report we present results of vacuum arc deposition of titanium oxides on silicon (100) and (111), without and with applying high voltage pulses of 5 and 10 kV to the target. A titanium cathode in oxygen atmosphere was used to produce the titanium and oxygen ions. Stoichiometric TiO2 or slightly understoichiometric TiO_2-x (x < 0.5) was obtained, depending on the oxygen pressure, at growth rates of more than 1 nm/s, as determined with Rutherford backscattering spectroscopy. For understoichiometric films a mixture of Ti₃O₅ and rutile (TiO₂) was observed with X-ray diffraction, whereas pure rutile was found for TiO₂ films. Additionally, a strong texture, with the rutile and Ti₃O₅ <110> direction parallel to the Si <111>, facilitated by a small lattice mismatch of 3%. This texture was observed for both substrate orientations, indicating a predominance of substrate influence over the surface free energy. While applying a negative d.c. bias or high voltage pulses to the substrate, a loss of texture was observed. No intermediate film orientation, minimising the radiation damage correlated with the ion bombardment was observed. Apparently, for the rutile on silicon system, the substrate influence is the only factor to influence the thin film orientation.

The use of physical vapor deposition (PVD), specifically ion beam assisted cathodic arc evaporation, historically has deposited thin films of superior quality onto substrates at temperatures above 200°C. For that reason, the substrate must have a high-tempering temperature otherwise the deposition process will deteriorate the mechanical properties of the substrate. The need for highly adhering, wear resistant coatings on low-tempering temperature substrates is the next logical step in the evolution of thin film deposition. In addition, the deposition of wear resistant coatings onto soft substrates such as aluminum alloys requires deposition temperatures below 200°C.

Using the technique of high-energy ion beam assisted cathodic arc evaporation of Cr_xN on 7075-T6 aluminum substrates, it has been determined that thin films of Cr_xN, with high adherence and good wear characteristics, are possible to be deposited at low temperatures (i.e. below 150°C for 4 hrs.) The thin films of Cr_xN appear dense under examination in the scanning electron microscope (SEM), but have a definite crystalline texture using x-ray diffraction analysis. This technique has been shown to lower the residual stresses in the thin film.

The paper will discuss the properties of these wear resistant Cr_xN films deposited onto 7075-T6 Al substrates.

Ion Beam Assisted Deposition (IBAD) was used to produce SiO₂ layers with a lateral thickness gradient on a gas sensor microarray, which was developed at the Forschungszentrum Karlsruhe. The microarray is based on some 100 nm thick SnO₂ or WO₃ layers deposited by HF-magnetron sputtering. Semiconducting metal oxides are used for gas sensors as their electrical conductivity is highly sensitive to the composition of the ambient atmosphere. By depositing a set of parallel electrode strips on top of the 4 by 8 mm wide sputtered metal oxide layer, the latter is currently subdivided into 38 sensor elements. In the last production step the microarray is coated with an inhomogeneous SiO₂ layer by IBAD using phenyl-triethoxy-silane as precursor. Owing to the thickness variation of the gas permeable SiO₂ coating from one side of the microarray to the other, the initially identical sensor elements are differentiated with respect to their gas response. Thereby the selectivity of gas detection for each sensor element becomes different and hence gas characteristic conductivity patterns are obtained with the microarray.

The inhomogeneous SiO₂ layers were produced by IBAD, since this method allows structuring of films during deposition because the film growth rate depends on the current density of the ion bombardment. With the aim of optimizing the selectivity variation of the sensor elements two different set-ups were tested. Two types of ion sources were used to obtain different shapes of ion beams in order to vary the ion current density across the microarray resulting in SiO₂ membranes with different thickness gradients. Furthermore, different positions of the microarray with respect to the ion beam were tested. The influence of the energy of the bombarding ions and of the precursor partial pressure on the properties of the deposited layers was investigated. The layers were examined with different surface analytical methods in order to determine the chemical composition and thickness of the layers, as well as the thickness gradient. SiO₂ layers with different thickness gradients from approx. 5 to 50 nm across the 8 mm wide microarray were produced and their influence on the sensing properties upon exposure to various gases was investigated.