Experiments and kinetic modeling of the ion energy distribution function at the substrate surface during magnetron sputtering of silver targets in radio frequency argon plasmas

Abstract

The quality of the films obtained by magnetron sputtering depends on numerous parameters, including the energy of the ions impinging on the substrate. The energy distribution functions of Ar and Ag ions during magnetron sputtering of a silver target in rf argon plasmas are hereby reported. Measurements were carried out by plasma sampling mass spectrometry at (i) various bias voltages on the surface of the target at constant pressure and (ii) various operating pressures at constant bias voltage. A distinct high-energy tail is observed for the sputtered silver ions (ionized in the plasma) in comparison to the argon gas ions. The results indicate that the sputtered Ag atoms are not completely thermalized by collisions with background gas atoms over the range of experimental conditions investigated. To confirm such assertion, a model has been developed for the ejection of Ag atoms from the target, their transport in the gas phase, and their acceleration in the sheath at the surface of the mass spectrometer. Since sputtering occurs at low impinging ion energies, the energy distribution function of the extracted atoms cannot be represented by the usual Sigmund–Thomson distribution. It is rather assumed to be characterized by a bi-Maxwellian distribution, with one population related to the direct “classical” sputtering and the other one to indirect “2-step etching.” During the transport of Ag neutrals, both ionization and thermalization processes are considered. Finally, the rf sheath near the entry of the mass spectrometer oscillates at a period close to the transit time of the ions passing through it. This induces a complex energy gain also implemented in the model. An excellent agreement between the latter and experimental measurements is obtained. The results are used to probe the effect of the bias voltage and pressure on the fitting parameters, namely, the dc and rf components of the voltage drop in the sheath, the mean energy of the sputtered atoms, and the relative importance of the sputtered populations.

Effect of gas pressure on ion energy at substrate side of Ag target radio-frequency and very-high-frequency magnetron sputtering discharge

“…During deposition of film using magnetron sputtering, the energy of an ion incident on a substrate surface has a strong influence on the growth and properties of the film, such as the microstructure, crystallography, phase composition, and mechanical and optical properties. Therefore, to understand the mechanism of film growth, the ion energy at the substrate surface is of great interest [1][2][3][4][5][6][7][8][9][10][11][12][13][14].…”

“…According to the model of the ion energy distribution at the substrate surface in radio-frequency (RF) magnetron sputtering developed by Garofano et al [1], the energy of an ion incident on the substrate surface was related to the ejection of atoms from the target, their transport in the gas phase, and their acceleration in the sheath near the surface of the substrate [1]. However, for magnetron sputtering, near the target surface, there existed an electric field pointing toward the target, which resulted from the self-bias voltage [2,14].…”

“…The ionization and thermalization processes in the bulk plasma are closely related to the operating conditions, such as the working pressure, discharge power, excitation frequency and electrode spacing [1]. The effect of the working pressure, discharge power and electrode spacing on the ion energy has been paid more attention [1,6,9,10], but the effect of excitation frequency, especially at different working pressures, has seldom been reported.…”

“…If the ion transit sheath time τ i is comparable with the RF period τ RF of excitation frequency, namely τ i <τ RF or τ i ∼τ RF , the ions can pass through the sheath in one RF period or less [13,15]. The ion energy at the substrate side mainly depends on the initial energy of the ions and the time at which they enter the sheath [1,16]. If τ i >τ RF , the ions cannot pass through the sheath quickly [13,15].…”

Large-grained Ag thin films with low electrical resistivity produced by sputtering in Kr gas

“…However, Ar ions impinging the target can be neutralized and reflected with high kinetic energies. [1][2][3][4][5][6][7][8][9][10] These neutral atoms can become entrapped in the deposited film. It has been reported that the amount of gas entrapped in a deposited film increases with increasing targetto-gas atomic mass ratio.…”