Structure and morphology of magnetron sputtered W films

The structural and morphological details of magnetron sputtered tungsten (W) thin films as a function of the Ar working gas pressure and the sputtering power are presented. The crystal structure of the W films was examined with grazing incidence x-ray diffraction (GIXRD), while the morphology characterization was performed by x-ray reflectivity (XRR) and grazing incidence small-angle x-ray scattering (GISAXS).

W films have an important role in a number of technological applications, including e.g. optic and near-infrared transition edge sensors, diffusion barriers in semiconductor interconnect layers, absorption masks in x-ray lithography or diffracting layers in x-ray mirrors. Recently, W films have also been used as coatings of plasma-facing surfaces in tokamaks. All these studies reported that the W film structure and morphology are crucial for the final properties of the device or functionality of the coating.

Tungsten thin films exhibit two crystalline modifications: a thermodynamically stable body centred cubic (bcc) phase (α-W) and a metastable A15 phase (β-W). These two phases exhibit different properties. The lattice parameters of α-W and β-W are 3.16Å and 5.04 Å, respectively. The electrical resistivity of α-W is always lower than the electrical resistivity of β-W. The superconducting transition temperatures of the two phases are 15mK for α-W and between 1 and 4K for β-W. Moreover, the hardness of α-W and β-W films is different. The occurrence and stability of β-W in physical vapour deposition (PVD)-produced thin films are associated with the presence of oxygen in the deposition chamber, either as a controlled admixture or as a residual gas. Beyond the conditions for β-W formation, an amorphous tungsten film is formed.

Tungsten films are often deposited by the magnetron sputtering technique which allows a high-rate, controlled, uniform deposition and thus lends itself to economic, large-area industrial applications. In addition, a variation of the sputtering conditions yields different film morphology which can be tailored to a number of physically and technologically interesting properties. With normal incidence sputter deposition, either compact W films with very smooth surface or W films with columnar morphology were reported previously. However, no comprehensive study of the relationship between phase composition and morphology of W thin films for normal incidence sputter-deposition geometry has been reported so far.

We employed now a normal incidence sputter-deposition geometry in order to elucidate the effects of two magnetron deposition parameters—working gas (Ar) pressure in the deposition chamber pAr and W sputtering power PW—on the thin tungsten film structure and morphology. Both parameters, pAr and PW, affect the growth mechanism, and by changing them we studied the effects of the kinetics and flux of the W atoms on the final morphology and structure of the prepared W films. Furthermore, the deposition rate could influence the rate at which the impurities are incorporated into the growing film, and thus the crystal structure of the W film.

We find that the crystalline properties and nanoporosity vary systematically with the deposition conditions. Depositions at low Ar pressures (<5mTorr) and high powers (>40 W) result in compact and smooth layers with only α-W crystallites. By reducing the sputtering power (20 W), along with stable α-W, a metastable β-W phase occurs. For an even lower power (10 W), the W film becomes amorphous and exhibits a columnar morphology accompanied by a 25% reduced layer’s mass density compared with that of bulk tungsten.

A similar columnar morphology was also found for films deposited at higher Ar pressures (>5mTorr), and a moderate sputtering power of 20W. However, the columns in high-pressure films consist of the metastable β-W phase. The cross-section diameter of the columnar voids in the amorphous sample is 3 nm, while that in the high-pressure samples is approximately 4–6 nm. The highest mass density reduction, of up to 50%, is observed for the highest pressure of 20mTorr. Moreover, the morphology of W films deposited at high Ar pressures exhibits a depth dependence: smaller columns (and voids) closer to the Si substrate tend to increase in size towards the surface. On the other hand, in the amorphous-like sample we observed a 3.5 nm surface layer, which we attributed to the oxygen-rich W or WO3 phase. The formation of a metastable β-W phase in some films can be related and understood based on the rate at which oxygen incorporates into the film during the growth process. We systematically found β-W in the films deposited with lower flux of W atoms. This is a strong argument that for lower fluxes the residual oxygen in the deposition chamber has a higher probability to affect the growth process and in that way to stabilize the β-W. The phase composition of the films is closely related to their columnar morphology or porosity. We found β-W or amorphous W, without stable α-W, in those films which develop columnar morphology. This type of morphology is due to the self-shadowing effects when deposited atoms arrive at non-normal directions to the substrate and with reduced kinetic energy.

This work shows the dramatic effects of the deposition conditions on the structure and morphology of tungsten films, and the powerfulness of small-angle x-ray methods in the characterization of thin films. The results presented here are useful in optimizing the process parameters to obtain W films with desired properties.

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Structure and morphology of magnetron sputtered W films studied by x-ray methods;
K. Salamon, O. Milat, N. Radic, P. Dubcek, M. Jercinovic and S. Bernstorff;
Journal of Physics D 46 (9), (2013) 095304.

Last Updated on Tuesday, 14 May 2019 17:05