Sputter deposition, страница 2

Transition metal nitride (CrN) thin films were deposited by HIPIMS for the first time in February 2001 by A.P. Ehiasarian. The first thorough investigation of films deposited by HIPIMS by TEM demonstrated a dense microstructure, free of large scale defects.[8] The films had a high hardness, good corrosion resistance and low sliding wear coefficient.[8] The commercialisation of HIPIMS hardware that followed made the technology accessible to the wider scientific community and triggered developments in a number of areas.

The following materials have, among others, been deposited successfully by HIPIMS:

• Corrosion Resistant: CrN/NbN nanoscale multilayer

• Oxidation Resistant: CrAlYN/CrN nanoscale multilayer, Ti-Al-Si-N, Cr-Al-Si-N nanocomposite

• Optical: Ag, TiO2, ZnO, InSnO, ZrO2, CuInGaSe

• MAX phases: TiSiC

• Microelectronics: Cu, Ti, TiN, Ta, TaN

• Hard Coatings: carbon nitride CNx

Industrial application

HIPIMS has been successfully applied for the deposition of thin films in industry. The first HIPIMS coating units appeared on the market in 2006.

Advantages

The main advantages of HIPIMS coatings include a denser coating morphology[17] and an increased ratio of hardness to Young’s modulus compared to conventional PVD coatings. Whereas comparable conventional nano-structured (Ti,Al)N coatings have a hardness of 25 GPa and a Young’s modulus of 460 GPa, the hardness of the new HIPIMS coating is higher than 30 GPa with a Young’s modulus of 368 GPa. The ratio between hardness and Young’s modulus is a measure of the toughness properties of the coating. The desirable condition is high hardness with a relatively small Young’s modulus, such as can be found in HIPIMS coatings.

Gas flow sputtering

Gas flow sputtering makes use of the hollow cathode effect, the same effect by which hollow cathode lamps operate. In gas flow sputtering a working gas like argon is led through an opening in a metal subjected to a negative electrical potential.[3][4] Enhanced plasma densities occur in the hollow cathode, if the pressure in the chamber p and a characteristic dimension L of the hollow cathode obey the Paschen's law 0.5 Pa·m < p·L < 5 Pa·m. This causes a high flux of ions on the surrounding surfaces and a large sputter effect. The hollow-cathode based gas flow sputtering may thus be associated with large deposition rates up to values of a few µm/min.[5]

Structure and morphology

In 1974 J. A. Thornton applied the structure zone model for the description of thin film morphologies to sputter deposition. In a study on metallic layers prepared by DC sputtering, he extended the structure zone concept initially introduced by Movchan and Demchishin for evaporated films. Thornton introduced a further structure zone T, which was observed at low argon pressures and characterized by densely packed fibrous grains. The most important point of this extension was to emphasize the pressure p as a decisive process parameter. In particular, if hyperthermal techniques like sputtering etc. are used for the sublimation of source atoms, the pressure governs via the mean free path the energy distribution with which they impinge on the surface of the growing film. Next to the deposition temperature Td the chamber pressure or mean free path should thus always be specified when considering a deposition process.

Since sputter deposition belongs to the group of plasma-assisted processes, next to neutral atoms also charged species (like argon ions) hit the surface of the growing film, and this component may exert a large effect. Denoting the fluxes of the arriving ions and atoms by Ji and Ja, it turned out that the magnitude of the Ji/Ja ratio plays a decisive role on the microstructure and morphology obtained in the film.[8] The effect of ion bombardment may quantitatively be derived from structural parameters like preferred orientation of crystallites or texture and from the state of residual stress. It has been shown recently [9] that textures and residual stresses may arise in gas-flow sputtered Ti layers that compare to those obtained in macroscopic Ti work pieces subjected to a severe plastic deformation by shot peening.