Deposition and Characterization of Magnetron Sputtered Beta-Tungsten Thin Films

β-W is an A15 structured phase commonly found in tungsten thin films together with the bcc structured W, and it has been found that β-W has the strongest spin Hall effect among all metal thin films. Therefore, it is promising for application in spintronics as the source of spin-polarized current that can be easily manipulated by electric field. However, the deposition conditions and the formation mechanism of β-W in thin films are not fully understood. The existing deposition conditions for β-W make use of low deposition rate, high inert gas pressure, substrate bias, or oxygen impurity to stabilize the β-W over α-W, and these parameters are unfavorable for producing β-W films with high quality at reasonable yield. In order to optimize the deposition process and gain insight into the formation mechanism of β-W, a novel technique using nitrogen impurity in the pressure range of 10-5 to 10-6 torr in the deposition chamber is introduced. This techniques allows the deposition of pure β-W thin films with only incorporation of 0.4 at% nitrogen and 3.2 at% oxygen, and β-W films as thick as 1μm have been obtained.

The dependence of the volume fraction of β-W on the deposition parameters, including nitrogen pressure, substrate temperature, and deposition rate, has been investigated. The relationship can be modeled by the Langmuir-Freundlich isotherm, which indicates that the formation of β-W requires the adsorption of strongly interacting nitrogen molecules on the substrate. The dependence of β-W formation on the choice of underlayer materials has also been investigated. The β-W phase can only be obtained on the underlayer materials containing non-metallic elements. The dependence is explained by the existence of strong covalent bonds in β-W compared with that in α-W. The nickel and permalloy underlayers are the only exception to the above rule, and β-W has been successfully deposited on permalloy underlayer using very low deposition rate for spin-diffusion length measurement of β-W. The permalloy thin films usually take the (111) texture, since its (111) planes have the lowest surface energy. However, permalloy thin films deposited on β-W underlayer can achieve (002) texture using amorphous glass substrates.

Therefore, the permalloy/β-W bilayer system can work as a seed layer for the formation of (002) textured films with fcc or bcc structure. The mechanism of the (002) texture formation cannot be explained by the existing models. The β-W to α-W phase transition was characterized by differential scanning calorimetry. The enthalpy of transformation is measured to be 8.3±0.4 kJ/mol, consistent with the value calculated using density functional theory. The activation energy for the β-W to α-W phase transformation kinetics is 2.2 eV, which is extremely low compared with that of lattice and grain boundary diffusion in tungsten. The low activation energy might be attributed to a diffusionless shuffle transformation process.