Regular Article Plasma parameters of RF capacitively coupled discharge: comparative study between a plane cathode and a large hole dimensions multi-hollow cathode Samira Djerourou 1,2,* , Mourad Djebli 2 , and Mohamed Ouchabane 1 1 Equipe Plasma et Applications, Centre de Développement des Technologies Avancées, Cité du 20 août 1956, BP17 Baba Hassen, Alger, Algérie 2 Laboratoire de Physique Théorique, Faculté de Physique, USTHB, BP 32 Bab Ezzouar 16079, Alger, Algérie Received: 28 June 2018 / Received in final form: 8 November 2018 / Accepted: 17 January 2019 Abstract. This work deals with a comparative study of plasma discharge generated by two geometrical configurations of cathodes through an investigation of their plasma parameters. A large hole diameter and depth (D = 40 mm, W = 50 mm) multi-hollow (MH) cathode compared with a plane (PL) cathode are presented for argon capacitively coupled radiofrequency discharge. The electrical characteristics of MH and PL cathodes have been measured in terms of the self-bias voltage (V dc ) while the Langmuir probe was used to measure electron density (n e ) and electron temperature (T e ) for a wide range of gas pressure (60–400 mTorr) and incident power (50–300 W). It is found that the hollow cathode effect (HCE) is optimum at 60 mTorr with 220 mTorr as a critical gas pressure for which a transition from HCE to insufficient HCE is seen. The electron temperature varies from 3 to 5 eV in the case of MH and PL cathodes with respect to incident power and gas pressure. 1 Introduction Nowadays plasma discharges have become widely used as a common tool for material processing such as surface activation, surface cleaning, sterilization, etching and thin films deposition [1–5]. In each of these processes, the characteristics of the generated electrical discharge are of great importance in terms of providing the best surface properties. It is obvious that performances of surface treatment processes are closely related to the plasma parameters (self-bias voltage, electron density, ion density, electron temperature, ion temperature, etc.) that cannot be controlled independently by external system operating conditions such as gas pressure and incident power. Besides, regardless of whether these plasma parameters are advantageous or not for a given process, it is confirmed that they can be monitored simply by making a suitable selection of excitation discharge type (DC, RF, MW, etc.) and plasma source (ICP, ECR, Magnetron, HWS, etc.) in addition to geometrical effects provided by electrodes when setting up the plasma reactor [6]. In this context, the capacitively coupled radiofrequency (CCRF) discharge with parallel plate electrodes has been largely used for plasma processing applications [7–9]. However, although the experimental setup of the CCRF discharge is simple and whatever the system operating conditions, the parallel plate electrode geometry yields systematically a low electron density (less than 10 9 cm 3 ), while high speed processings need higher electron densities [10, 11]. In such situation, the cathode geometry configuration is one of the possibilities that can improve the plasma parameters in terms of self-bias voltage (V dc ), electron density (n e ) and electron temperature (T e ), etc. [12–14]. In this sense, hollow and multi-hollow (H and MH) cathodes were proposed to be utilized as a light source [15], active media for gas lasers [16], sources of electrons [17], or for thin films deposition and large area surface treatment [18–20], and recently for the treatment of nanoparticles, plasma polymerization and nanocomposite fabrication [21]. This kind of discharges, produced from such specific geometries, is characterized by a high electron density and a low electron temperature. These latter are due to the hollow cathode effect (HCE) governed by the so-called pendulum effect within the holes where electrons are trapped, resulting in a number of secondary electrons, and hence in a high electron density [22–24]. The HCE is obtained for an appropriate combination of gas pressure and hole diameter [25–27]. This condition is expressed by the p.D product, where p is the gas pressure and D the hole diameter. The HCE was found to be established when the product p.D falls within the range of 0.026–37 Torr cm [25–27]. This wide range of reported values necessitates more investigations on various configurations of MH cathodes depending on the diameter, depth and number of holes [28–31]. For example, the effects of the hole diameter (ranging from 0 to 10 mm), the driving frequency and gas composition on the RF MH cathode discharge have been studied by several authors [29,30,32,33]. Ohtsu et al. [34] have used MH cathode discharges with small diameter * e-mail: sdjerourou@cdta.dz Eur. Phys. J. Appl. Phys. 85, 10801 (2019) © EDP Sciences, 2019 https://doi.org/10.1051/epjap/2019180193 THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS 10801-p1