Characteristics of capacitively coupled helium plasma driven by various frequencies under constant power conditions E. Abdel-Fattah a, b, * a Physics Department, Faculty of Science, Zagazig University, Zagazig 44519, Egypt b Physics Department, College of Science, Salman Bin Abdul-Aziz University, Al-Kharj, P.O. 83, Al-Kharj 11942, Saudi Arabia article info Article history: Received 18 November 2012 Received in revised form 28 March 2013 Accepted 31 March 2013 Keywords: Fixed dissipated power Helium plasma Very high frequency Electron energy distribution function Electromagnetic effect abstract The influence of excitation frequency (13.56e96 MHz) on the characteristics of capacitively coupled helium plasma is investigated by means of Langumir probe and CCD camera. Measurements are per- formed in helium pressure of 10.66 and 33.3 Pascal (Pa) under fixed dissipated power of 10 W. With increasing the driving frequency, the RF/HF voltage and dc-self bias markedly decrease. Meanwhile, the plasma density and electron temperature peak in the frequency range 27e56 MHz, beyond which they decrease as exciting frequency increase. A different feature of the electron energy probability function EEPF is observed with exciting frequency; Maxwellian type EEPF at low frequency of 13.56 MHz evolves into a bi-Maxwellian type with a hump/beamlike in the frequency range 27e56 and eventually comes back to Maxwellian distribution at frequency 76 MHz. The observed results are explained in terms of electromagnetic wave effect and capacitive to inductive heating transition induced by exciting frequency. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Recently, very high frequency (VHF) capacitive coupled glow discharges have attracted a considerable attention due to its great utilizes over the conventional 13.56 MHz discharges. For example, the higher growth rate and better uniformity of hydrogenated amorphous silicon (a -Si:H) film deposition [1,2] and a high etch rate and less micro-loading effect in the self-aligned contact hole process [3]. These benefits have motivated researchers to investi- gate the role of the excitation frequency on the capacitively coupled discharges. In particular, knowledge of the electron energy proba- bility function (EEPF) evolution with the exciting frequency is crucial because radicals and positive ions are created by inelastic collisions between electrons and neutral parent molecules. At constant discharge voltage and gas pressure, the increase of the excitation frequency (13.56e50 MHz) causes a dramatic increase in the discharge power. This change induces major effects in most of the microscopic discharge parameters such as, electron density n e , electron temperatures T e and the electron energy probability function (EEPF) [4e6]. Though much understanding of the role of exciting frequency on the discharge characteristics have been achieved through these studies, however, Moisan et al. [7] reported that for reliable dependences of excitation frequency on the discharge parameters in capacitive discharge, one should ensure constant dissipated power into the bulk plasma. On the other hand, from plasma technology point of view, increasing the exciting frequency to an arbitrary higher value in pursuit of higher electron density was found to affect the unifor- mity of the produced plasma [8]. Namely, at very high frequency when the corresponding excitation wavelength l becomes com- parable to the electrode radius and the plasma skin depth d be- comes comparable to half of electrode spacing, electromagnetic effects and/or surface waves SW start to have a profound influence on the plasma properties [9,10]. Two of the most significant effects are the standing wave effect, which enhances power deposition at the discharge center, and the plasma skin effect; the latter accounts for an increased inductive heating component at high electron densities in the VHF regime. In addition, the discharge experiences a capacitive-to-inductive (E- to- H) transition at certain exciting frequency and discharge voltage [11,12]. Though, through theoret- ical and experimental studies, electromagnetic induces phenome- non such as non-uniform capacitive and inductive electric fields were observed [13,14]. However, measurements of electron energy probability function (EEPF) in a discharge conditions whereas the conditions of electromagnetic effects may applicable in a capacitive discharge working under dissipated power conditions have not yet been observed. It is important to understand this influence on the plasma parameters in particular the EEPF, in order to control the discharge parameters and optimize the plasma technique. * Physics Department, College of Science, Salman Bin Abdul-Aziz University, Al- Kharj, P.O. 83, Al-Kharj 11942, Saudi Arabia. Tel.: þ966 0533203227. E-mail address: essam29@hotmail.com. Contents lists available at SciVerse ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.03.022 Vacuum 97 (2013) 65e70