Plasma diagnostics of low temperature plasma during the thin film deposition process


The important research activity in the Department of Low-Temperature plasma was investigations of new approaches in low temperature plasma diagnostics during the process of thin film growth. We aim our investigation on plasma behaviour within various technological applications, mainly in a pulsed magnetron sputtering deposition. Our goal is to find relationships between plasma microscopic parameters and physical properties of deposited thin films. We have developed new diagnostic techniques that allow the discharge to be examined in situations where known diagnostics techniques fail. Based on the outlined research strategy, a new diagnostic method of low temperature plasma was developed, which we called the modified Katsumata probe. We started from the concept of the Katsumata probe used to study ion temperature infusion magnetic devices. By adding an external homogeneous magnetic field of a proper magnitude to a standard Katsumata probe, we have created a new probe that allows measuring the velocity distribution of ions in low temperature plasma even without a strong magnetic field, such as typical technological discharges [1].

Another important research goal was the study of velocity distribution functions of working gas ions and sputtered particles in a magnetron discharge. Here we used the energy resolved mass spectrometer (Hiden Analytical Ltd, UK). We focused our attention on the study of Ar+, Ti+, ArTi+, O+, O2+ ions and their possible dimers such as Ar2+ or Ti2+. The obtained velocity distribution functions of the various ions showed that the Ar+ distribution consists of a low-energy and a high-energy part, while the Ti+ distribution contains predominantly a high-energy part and the low-energy part of the distribution appears only with increasing working pressure. This shows the processes of formation of individual ions in the plasma and elementary processes taking place in the discharge. For example, the Ti2+ dimer is formed by ionizing the Ti2 dimer in the plasma volume and the Ti2+ density decreases with increasing working pressure due to the formation of Ti clusters [2]. We also applied a similar approach to the measurement of ion velocity distributions in HiPIMS discharge. It has been shown that the argon and titanium ion velocity distributions exhibit similar shapes as in a standard magnetron discharge, i.e. the presence of a low-energy and high-energy part of the distribution. However, the velocity distribution can be well approximated by the so-called shifted bi-Maxwellian distribution. Such velocity distribution suggests that in the region of formation of ions an electric field of the opposite direction that accelerates the ions towards the substrate must exist. This supports the existence of the so-called potential hump, which has been observed by other authors based on the plasma potential measurements [3].

We have further developed plasma diagnostic techniques that can be easily used during the technological process. An important technological parameter of the HiPIMS discharge is the degree of ionization of the sputtered particles. In our work, we have focused on this topic by studying ionized flux fractions of deposited particles using in our laboratory developed probe (so-called ion meter) and measured plasma parameters using a time-resolved Langmuir probe. Results have shown that by selecting a suitable model, the plasma parameters obtained from Langmuir probe measurements can be used to determine the ionization fraction of particles incident on the substrate. In this work, we investigated plasma parameters for various target materials, namely Ti, Al and C as it can be seen in Fig 3. It has been shown that the peak of the ionized fraction of deposited particles exists for Ti material (78%) [4, 5].


[1] M. Čada, Z. Hubička, P. Adámek, J. Olejníček, Š. Kment, J. Adámek, J. Stöckel: A modified Katsumata probe-Ion sensitive probe for measurement in non-magnetized plasmas. Rev. Sci. Instrum. 86 (2015) 073510-7.

[2] R. Hippler, M. Cada, V. Stranak, Z. Hubicka, C.A. Helm: Pressure dependence of Ar2+, ArTi+, and Ti2+ dimer formation in a magnetron sputtering discharge. J. Phys. D: Appl. Phys. 50 (2017) 445205-8.

[3] R. Hippler, M. Cada, V. Stranak, C. A. Helm, Z. Hubicka: Pressure dependence of singly and doubly charged ion formation in a HiPIMS discharge. J. Appl. Phys. 125 (2019) 013301-7.

[4] D. Lundin, M. Čada, Z. Hubička: Ionization of sputtered Ti, Al, and C coupled with plasma characterization in HiPIMS. Plasma Sources Sci. Technol. 24 (2015) 035018-11.

[5] M. Čada, D. Lundin, Z. Hubička: Measurement and modelling of plasma parameters in reactive high-power impulse magnetron sputtering of Ti in Ar/O2 mixtures. J. Appl. Phys. 121 (2017) 171913-7.

Figure: Ionized flux fraction of deposited particles Ti (top) and Al (bottom) as a function of average HiPIMS pulse current density. The left column displays the case of 100 μs length pulses and the right column the case of 400 μs length pulses.
Theme is contributed to by

Mgr. Martin Čada, Ph.D.

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