Probing the electron density in HiPIMS plasmas by target inserts

Abstract

High power impulse magnetron sputtering (HiPIMS) is a versatile technology to deposit thin films with superior properties. During HiPIMS, the power is applied in short pulses of the order of 100 μs at power densities of kW to a magnetron target creating a torus shaped dynamic high density plasma. This plasma torus is not homogeneous, but individual ionization zones become visible, which rotate along the torus with velocities of 10 km . Up to now, however, any direct measurement of the electron density inside these rotating ionization zones is missing. Here, we probe the electron density by measuring the target current locally by using small inserts embedded in an aluminium target facing the plasma torus. By applying simple sheath theory, a plasma density of the order of at the sheath edge can be inferred. The plasma density increases with increasing target current. In addition, the dynamics of the local target current variation is consistent with the dynamics of the traveling ionization zone causing a modulation of the local current density by 25%.

Spokes in high power impulse magnetron sputtering plasmas

“…The sheath thickness in HiPIMS discharge was also calculated using Child-Langmuir law, yielding values of about 100 µm for target voltages corresponding to a power density above 250 W/cm 2 (∼ 10 A at 2" planar target). This estimate is corroborated by a benchmark experiment using an optical inspection of the sheath thickness that yields values between 50 µm and 100 µm [42,44].…”

“…At low powers typical for dcMS, spokes are Ar dominated and the plasma densities are in the order of 10 16 m −3 , the ion energy distribution function (IEDF) of metal and Ar ions arriving at the substrate are similar, comprising low energy peak of thermalised species and a tail up to 10 eV [50]. At high powers typical for HiPIMS, spokes are dominated by sputtered metal species, and the plasma densities in the spoke reach up to 8×10 19 m −3 [44]. The IEDF of metal ions consists of two main peaks at low energies (LE) as well as at high energies (HE) [51]: if the power density is increases to very high values so that the plasma becomes homogeneous again, the HE part of the metal ion IEDF does not change, but the LE peak is shifted by several eV to higher energies.…”

“…• (ii) Measurement of the plasma density at the target sheath, and at 15 mm above the target surface: Measurements using circular inserts in the target showed that the plasma density increases linearly with power density, exhibiting a 25% modulation induced by the travelling spokes [44]. The average plasma density was in the range Figure 9.…”

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

“…Probe measurements are simple to undertake, but data interpretation is difficult when a magnetic field is present 12,13 and the relative abundance of ionic species is required for calculating ion density (n i ). Previous studies have used target inserts to detect localized ionization zones (spokes) and obtain n i at the target sheath edge; 14 used outside of the magnetic trap for measurements of n i , n e , T e and the electron energy distribution function (EEDF). 8,[15][16][17] There have been fewer Langmuir probe studies within the magnetic trap 18,19 because the presence of the probe stem and current drainage to the probe tip are expected to significantly perturb the main ionization region.…”

White paper on the future of plasma science for optics and glass

“…that occur due to the interaction of plasmas with other materials, and hence to focus on the fingerprints of plasma‐surface interactions . Characterization‐ and control‐related diagnostics are performed by means of Langmuir probes, laser‐based techniques, optical emission spectroscopy (OES), infrared and Fourier transform infrared absorption spectroscopy, mass spectrometry, microwave interferometry, and radio frequency diagnostics . Since low‐temperature, non‐equilibrium, and partially ionized plasmas are characterized by cold electrons (with a mean energy of bulk electrons of a few eV) dedicated methods are necessary for the qualitative/quantitative monitoring of neutral/charged atomic/molecular species and electric fields, over a wide range of pressures, electrode geometries, and gaseous mixtures .…”