Journal of the Korean Physical Society, Vol. 38, No. 3, March 2001, pp. 259∼263 Characterization of an Oxygen Plasma by Using a Langmuir Probe in an Inductively Coupled Plasma Jong-Sik Kim * and Gon-Ho Kim Department of Physics, Hanyang University, Ansan 425-795 Tae-Hun Chung Department of Physics, Dong-A University, Pusan 604-714 Geun-Young Yeom Department of Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746 Kwang-Ho Kwon Department of Electronic Engineering, Hanseo University, Seosan 356-820 (Received 12 April 2000) Oxygen plasmas were investigated by using a Langmuir probe for an inductively coupled plasma with various rf powers, 100 ∼ 400 W, and operating pressures, 0.1 ∼ 100 mTorr. The probe current ratio α of the positive and negative currents (I * + /[I * e + I * - ]) increased with the generation of negative ions and had its maximum value in the pressure region of 40 ∼ 70 mTorr. Also, the operating pressure to achieve the maximum α shifted from the low-pressure region to the high- pressure region with increasing input power because enhanced ion loss through positive-negative ion recombination. I. INTRODUCTION Oxygen plasmas have been widely used in many pro- cesses for semiconductor manufacturing. In the etch- ing process using CF4, for example, the additive oxy- gen enhances the liberation of fluorine atoms in the CF4 plasma, resulting in an increased etch rate for the sili- con wafer. The oxygen plasma also plays an important role in the ashing and cleaning processes. When oxygen gases are used, negative ions change the plasma poten- tial and the behaviours of the electrons and the ions near the substrate, so it may cause dislocations on the surface during film formation [1,2]. In spite of the need to un- derstand the scaling of the oxygen plasma constituents with the control parameters, investigations have not been progressed sufficiently since the parameter space is much enlarged from that of electropositive plasmas; also, the diagnostics is very difficult in the presence of negative ions. Unlike electropositive plasma with ions and electrons, an electronegative plasma is composed of three different charges, positive ions, electrons, and negative ions, which * E-mail: jongsik@newton.hanyang.ac.kr were formed from the attachment reaction of electron to the neutral atoms or molecules [3,4]. Kouznetosov et al. considered the electronegative discharge plasma to be distributed over three regions in the chamber: the electronegative bulk region, the electropositive edge re- gion, and the sheath region. Hence, the plasma in the electronegative bulk region is composed of negative ions, positive ions, and electrons, so the plasma quasineutral- ity condition satisfies the charge balance n i ≈ n e + n - , where n - is negative ion density, n i the positive ion density, and n e the electron density. In comparison, the plasma in the electropositive edge region is composed of positive ions and electrons. Finally, the sheath region has mostly ions [3,5]. Here are the important reactions in oxygen plasmas: The ionization reaction between an oxygen molecule and an energetic electron results in a positive ion (O 2 + e → O + 2 +2 e ). Negative ions are produced by the dissoci- ation reactions of an oxygen molecule and the attach- ment reactions between an oxygen atom and an elec- tron (O 2 + e → O - + O). The typical loss mecha- nism in the electronegative oxygen plasma is the recom- bination reaction between positive and negative ions, O + 2 + O - → O 2 + O. Note that we considered only -259-