Transition from ballistic to thermalized transport of metal-sputtered species in a DC magnetron

Abstract

Tunable Diode Laser Induced Fluorescence (TD-LIF) technique has been optimized to accurately measure the titanium (Ti) sputtered Atoms Velocity Distribution Functions (AVDF) in a magnetron discharge operating in Direct Current (DC) mode. The high spatial and spectral resolution achieved unveils some features of the transport of the metal sputtered atoms and their thermalization. The two groups of thermalized and energetic atoms have been very well separated compared to previous works. Hence, the fitting of the energetic atoms group shows dumping from modified Thompson to Gauss distribution when the product pressure-distance from the target increases. In parallel, sputtered metal transport from the target has been simulated using the Monte Carlo collision (MCC) approach. The direct comparison between numerical and experimental results led to an improved cross section for Titanium - Argon momentum transfer, based on the \textit{ab initio} formulae of the interaction potential derived from noble gases interaction. The numerical parametric study of the angular distribution and cut-off energy for the initial distribution of sputtered atoms steered to a precise characterization of the initial conditions, allowed by the accuracy of experimental data. A very good overall agreement is obtained for measured and calculated AVDFs. The confrontation between measured and modeling results emphasized the major role played by the argon ions not only in the sputtering process but also in the neutral metal transport, by the gas rarefaction near the target. The microscopic description provided by the MCC model clearly reveals different transport regimes: ballistic, diffusive, and back-scattering and brings new insights on the thermalization of sputtered species in the intermediate pressure range.

Saturation pressure of nonequilibrium titanium evaporation during additive manufacturing by electron powder bed fusion

“…The probing laser source was a Toptica Phototonics DL 100 15 mW tunable diode blue-laser system operating in single mode with the line centered at λ 0 (Ti) = 398.176 nm. This wavelength corresponds to the ground state of titanium atoms and its absorption can excite atoms from the ground state 3d 2 4s 2 to the upper state 3d 2 ( 3 F)4s4p( 1 P°) [26,27]. The diode temperature was kept constant while the wavelength was tuned by controlling the current.…”

On the correlation between the TiN thin film properties and the energy flux of neutral sputtered atoms in direct current magnetron discharge

“…Figure 2 shows the neutral Ti ( 3 F 2 ) AVDFs in dcMS discharge at 0.4 and 1.3 Pa, for the different gas mixtures (N 2 /Ar). We used the resonance fluorescence signal (λ laser = λ lif = 398.17 nm), which significantly improved the TD-LIF signal compared with previous studies [5,16]. Then, the AVDF profiles and the atoms populations are accurately described: thermalized (centered at v = 0 km s −1 ) and energetic (v > 1 km s −1 ≈ 1 eV for titanium atoms) [5] are separated at relatively low and high magnetron pressures (p = 0.4 and 1.3 Pa).…”

“…Only the measurements performed at the substrate position are shown at a distance z = 5 cm from the target. For a detailed description of the AVDFs' fitting and titanium sputtered atoms transport characterization combined with PIC Monte Carlo modeling in argon gas, the reader is referred to Revel et al [5].…”

“…the number of collision events between the sputtered species and the processing gas. The number of collisions can be directly controlled by the deposition pressure and the target-to-substrate distance [5]. Their energy ranges from the energy they gain in the sputtering process and at virtually no collisions (∼3.2 eV in average) to fully thermalized species at a larger number of collisions (∼0.1 eV in average).…”

“…Tunable diode laser-induced fluorescence (TD-LIF) is a powerful technique for diagnosing neutral sputtered atoms, and the plasma remains practically unperturbed in terms of temperature and density [5,[13][14][15][16]. Moreover, these measurements have numerous advantages compared to other optical diagnostics: (a) they allow probing of the ground state of the sputtered atoms in dcMS, (b) they are highly space and spectrally resolved, and (c) they allow measurement of the flux and energy of sputtered atoms at the substrate position in front of the target.…”