Comparison of Langmuir probe and laser Thomson scattering for plasma density and electron temperature measurements in HiPIMS plasma

Abstract

The temporal evolution of plasma density and electron temperature in HiPIMS discharges has been measured using the Langmuir probe and laser Thomson scattering techniques. Measurements were performed (non-simultaneously) at two positions within the plasma; in the low magnetic field strength region on the discharge axis and in the high magnetic field strength region of the magnetic trap, for peak power densities of 450 Wcm −2 and 900 Wcm −2 respectively. The maximum plasma densities and temperatures were 6.9 × 10 19 m −3 and 3.7 eV in the pulse-on time, and values decayed to 4.5 × 10 17 m −3 and 0.1 eV at times up to 250 µs into the afterglow. The results indicate that although intrusive, the Langmuir probe can provide a good indication of electron properties in regions of different electron magnetization in the discharge.High power impulse magnetron sputtering (HiPIMS) is a novel ionized physical vapour deposition (IPVD) technique 1 in which high metal ionization fractions (∼ 70% in Kouznetsov et al 2 ) are obtained through the creation of a dense plasma (∼ 10 19 m −3 ). This necessitates high target power densities (0.5-10 kWcm −2 ) and these are generated using short high-voltage pulses (width ∼ 10 − 100 µs) with low duty cycle (∼ 1%) to prevent target melting. The main advantage of IPVD is that the energy and directionality of the film forming ions can be controlled by biasing the substrate, in contrast to line of sight deposition using neutrals. 3 This results in HiPIMS producing films with superior properties (e.g. denser, harder and smoother) compared to DC magnetron operation. [4][5][6] A schematic of an unbalanced planar magnetron is shown in figure 1. Electrons perform a closed E × B drift inside the last closed flux surface boundary, which confines electrons and produces a dense plasma. This magnetic trap region produces the most intense ionization in the discharge, predominately through the process of electron impact ionization. 7 There are also open flux surfaces that guide electrons which have escaped the trap (and consequently ions through ambipolar diffusion) from the target vicinity to the substrate position. A detailed understanding of the plasma dynamics inside the magnetic trap and at the substrate is required in order to optimize the deposition process. The most fundamental plasma properties for any physics investigation are electron density (n e ) and temperature (T e ).Unfortunately HiPIMS poses a difficult environment for diagnostic operation due to the high sputter flux; the relative abundance of species has a dynamic evolution; n e between pulses spans several orders of magnitude and T e at least an order of magnitude; and the plasma is spatially heterogeneous. These effects are most pronounced inside the magnetic trap. Non-intrusive diagnostics are obviously desirable and examples of such techniques applied to HiPIMS include optical emission spectroscopy 8-10 and recently THz time-domain a) Electronic mail: ryanp@liverpool.ac.uk FIG. 1. Schematic of the magnetic field from an unbalanced p...

Plasma Diagnostics in Reactive High-Power Impulse Magnetron Sputtering System Working in Ar + H2S Gas Mixture

“…Very similar behavior of the time evolution of the electron density and electron temperature after discharge switch-on was observed in an argon HiPIMS with a titanium target in a recent work [53]. The authors explained the local minimum of the T e by cooling of the EEDF due to increasing density of metallic species.…”

Origin of large-amplitude oscillations of dust particles in a plasma sheath

“…IV B, the collisionality of ions can have a non-negligible effect on the sheath's electric field. Lastly, the maximum electron collisional cross section for the pressure and electron energy regimes in our experiments is ∼2 × 10 −19 m 2 [32], implying an electron-neutral mean-free path λ en in the range of 1-5 cm. Thus, as discussed previously, λ en, in are larger than λ D and r p , and the use of "low-pressure theory" is a reasonable approximation for our Langmuir probe experiments.…”

“…The potential difference between the biased probe and plasma produces a sheath around the probe, resulting in a current flow through the probe which carries information regarding the plasma environment. Our probe design makes use of "low pressure theory," which implicitly assumes that r P is much smaller than the characteristic Debye length λ D and the inverse plasma Knudsen numbers K −1 i,e = r P /λ in,en 1, where λ in,en are the ion-neutral and electron-neutral mean-free paths [30][31][32]. For the plasma system described above, the electron Debye length λ D ≈ 1-2 mm.…”

Simulation Inference of Plasma Parameters From Langmuir Probe Measurements

“…(1971), Szuszczewicz and Takacs (1979), and Ryan et al. (2019). Niyogi and Cohen developed a theoretical model for magnetized, weekly ionized and collisional plasma (Niyogi & Cohen, 1973).…”

“…Following this early work, many authors have further refined and extended the theory of Langmuir probe current collection, in order to account for more general conditions than assumed in OML. For examples, the effect of magnetic fields was studied theoretically by Sanmartin (1970) and Rubinstein and Laframboise (1983); and experimentally by Brown et al (1971), Szuszczewicz and Takacs (1979), and Ryan et al (2019). Niyogi and Cohen developed a theoretical model for magnetized, weekly ionized and collisional plasma (Niyogi & Cohen, 1973).…”