ICMCTF2010 Session D3: Carbon and Nitrogen-Containing Nanostructured Composite and Nanolaminated Films

Thin films of MAX phase alloys, which are multilayer materials with chemical composition M_n+1AX_n, where M is a transition metal, A is an element from the A group in the periodic table and X is either C or N. These materials were first reported by Nowotny et al (1,2) and are stable at high temperatures but are relatively easy to machine. A remaining problem is to produce thin films of these materials at temperatures low enough to preserve the properties of the substrates. Hence, in the present study thin films of around 1µm thickness of Cr_n+1AlC_n, MAX phases have been produced using a dual ion beam system at substrate temperatures ranging from 300 to 750 K and ion beam assistance was used to mediate the film growth. Substrates used were silicon, silica, sapphire and steel. The areas of the component species were adjusted to maintain film composition under various deposition conditions. Some films were later annealed in vacuum at temperatures up to 1150 K. Material chemical composition was determined by Energy- and Wave-Dispersive X-Ray Spectroscopy (EDX and WDX). Chemical bonding was probed by X-Ray Photoelectron Spectroscopy (XPS) and micro-Raman. Structural analysis was conducted by X-Ray Diffraction (XRD) and cross- sectional Transmission Electron Microscopy (TEM). Mechanical properties have been determined by nano-indentation and tribological properties have been measured by nano-scratch testing.

The fully developed MAX phase is known to have a precisely layered atomic structure (3,4,5). As-grown films already exhibit some ordering which increases with the substrate temperature and is influenced by ion assistance at low bombarding ion energies which leads to more uniform local chemical bonding. Sample annealing reinforces this trend. Most films show good crack resistance during nano-scratch testing. The mechanical properties depend on deposition and annealing conditions and possibly can be explained on the basis of nanostructural hardening associated with the transition from the disordered state to a layered atomic arrangement.

Materials in electrical connectors should have low inherent resistivity and should be able to give a low contact resistance. It is also good if the friction against its mating surface is low for easy connection, and in case the contact is a sliding contact. Today, many electrical contacts have a surface made of noble metals such as gold or silver and in most of these cases both mating surfaces are made of the same material. The choice of noble metals is based on their ability to provide low contact resistance through a combination of softness that gives a large contact area, a low resistivity and a good oxidation resistance. The drawbacks with connectors coated with noble metal are their low wear resistance, high friction and the cost. In many cases there is also problem with cold welding of the surfaces giving low contact resistance but extreme forces for separation.

In this work carbon based nanocomposite coatings in the Ti-Ni-C system are considered as alternative coatings for electrical contacts. These have previously shown promising tribological performance, due to an ability to provide free carbon in the contact, but so far their combined tribological and electrical properties have not been studied in detail.

Nine different nanocomposite coatings with different compositions were deposited on copper cylinders with 20 μm nickel as an interlayer using sputtering. The tests where performed by having two cylinders, 10mm in diameter, sliding reciprocally in a crossed cylinder geometry. One cylinder was coated with nanocomposite coating and the other one by a thick silver layer. Since only one of the mating surfaces has a nanocomposite on top the contact area, determined by the hardness of the softest material and the load, will stay virtually the same as in the case of a silver-silver contact. This eliminates influences from contact area differences on the results. A load of 40N was applied and a current of 3A was fed through the contact to simulate a high current connector. The frequency of the sliding was 1 Hz and the amplidude was 1 mm. During the test the evolution of contact resistance and the coefficient of friction was measured and after the test the surfaces where closely investigated in SEM. The results show that the coatings give low contact resistance and coefficient of friction. The tests also show how the different amounts of carbon, dictated by the Ti to Ni ratio, influences the contact resistance, coefficient of friction and wear. Contact resistances as low as 100 micro Ohm was measured for the best coatings.

TiN, TiC and TiCN thin films with thicknesses below <100 nm were deposited using reactive magnetron sputtering on Si-wafer substrates. A PCS300 magnetron sputtering deposition unit developed by the Assembly Systems and Special Machinery department of Robert Bosch GmbH in Stuttgart Germany was used for the coating experiments. Different analytical techniques were used, such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) for crystalline phases, exact thin film thickness/surface topography and roughness evaluation, respectively. The XRD spectra demonstrated the formation of TiN, TiC and TiCN phases at the different films investigated with different preferred orientations. Cross sectional SEM images showed a thickness of about 85 nm. Laser acoustic waves (LAWAVE) technique was used to confirm the elastic modulus of the films, which was found to be between 100 and 240 GPa. Since miniature devices, such as MEMS and storage disks is the ultimate application of this work, nanomechanical properties of the thin films were also explored. Nanotribological performance was investigated using nanoscratch technique. Coefficient of friction in the nanoscale was found to be in the 0.08-0.1 range.

SiC is a promising semiconductor material for power and sensor applications and for high temperature operation in harsh and corrosive environments due to its large bandgap, thermal conductivity, and chemical inertness. The formation of high quality ohmic contacts to SiC is very important to device performance and is an issue remaining to be satisfactorily solved. Ti₃SiC₂, which is a member of the MAX-phase family, form at the interface between SiC and Ti-containing contacts when annealed at high temperature [1] . Ti₃SiC₂ is thermally and electrically conductive like a metal, while simultaneously displaying ceramic properties such as high decomposition temperature and low thermal expansion [2, 3] . We propose that epitaxially grown Ti₃SiC₂ layers may give low resistance ohmic contacts to SiC.

We have grown Ti₃SiC₂ films on n-type and p-type 4H-SiC through magnetron co-sputtering from three separate targets at a temperature of 850 °C and investigated the ohmic contact properties of these films. The films have been characterized with X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM), confirming the successful growth of Ti₃SiC₂ with a stacked plate-like structure, which follows the structure of the 8° off axis cut SiC substrates closely. The sheet resistance of the Ti₃SiC₂ films is ~1 Ω/cm². Since patterning methods are essential for processing ohmic contacts, different methods for patterning the Ti₃SiC₂ were investigated, including wet chemical etching as well as dry etching methods, where Inductively Coupled Plasma (ICP) show the most promising results and was employed to pattern TLM structures for measuring the specific contact resistivity of the material.

[1] Pécz, B., et al., /Ti_3 SiC_2 formed in annealed Al/Ti contacts to p-type SiC./ Applied Surface Science, 2003. *206*(1-4): p. 8-11.

[2] Barsoum, M.W. and T. El-Raghy, /The Max Phases: Unique New Carbide and Nitride Materials./ American Scientist, 2001. *89*: p. 334-343.

[3] Eklund, P., et al., /The Mn + 1AXn phases: Materials science and thin-film processing./ Thin Solid Films. *Published online, Corrected Proof*.