XXVIIth ICPIG, Eindhoven, the Netherlands, 18-22 July, 2005 Determination of negative ion density in slightly electronegative plasma V. Straňák 1,2,3 , J. Blažek 1 , M. Tichý 2 , P. Špatenka 4 , H. Steffen 3 , S. Wrehde 3 , R. Hippler 3 1 University of South Bohemia, Dept. of Physics, Jeronýmova 10, 371 15 České Budějovice, Czech Republic 2 Charles University in Prague, Fac. of Math. and Phys., V Holešovičkách 2, 180 00 Praha, Czech Republic 3 University of Greifswald, Institute of Physics, Domstrasse 10a, 174 89 Greifswald, Germany 4 Technical University of Liberec, Faculty of Mech. Engineering, Hálkova 6, 461 17 Liberec, Czech Republic The negative-ion density as well as the density and temperature of electrons in electronegative plasmas significantly influence plasma etching or sputtering. Thus, determination of these parameters belongs to the most important subjects. In this paper we suggest a method suitable for low concentrations of negative ions in investigated plasma. The proposed method is based on the comparison of two (one in electropositive and second in electronegative plasma) Langmuir probe characteristics measured in rare gas and slightly electronegative discharge plasma. The method was applied for determination of atomic negative oxygen ion density in DC discharge plasma in Ar with small admixture of O 2 in planar magnetron working in balanced as well as in unbalanced mode. 1. Introduction Plasma, due to its unique chemical reactivity, is widely used for surface treatment applications. One of the applications is deposition of thin layers of different types and properties. These different properties can be reached by different consistence of plasma and by different sputtered material. Very often nitride layers (N 2 is added) or oxide layers (O 2 is added) are created because of their wide utilization range. Rare gas (mostly Ar) is used as carrier gas. The deposition of thin layers by DC magnetron sputtering is one of the mentioned methods. The magnetrons are known for a long time and they are mostly used in two basic configurations – cylindrical magnetron [1] and planar magnetron [2], used in our case. Planar magnetrons can be operated in two modes – balanced (BLM) and unbalanced (UNB) mode [3]. In the balanced mode the magnetic field lines are well confined around the cathode and the electron loss is reduced to minimum. The basic principle of the unbalanced magnetron is to allow release of electrons from the magnetic trap in order to create ionization away from the magnetron cathode and at the substrate. Our work was focused on determination of basic parameters, mainly negative ion density, of plasma used for TiO x layers deposition. Magnetron plasma was created from Ar and O 2 mixture (both of 5.9 purity grade) and Ti as a sputtered target material was used. The Ar+O 2 plasma discharge was investigated by Langmuir probes [18]. However, Langmuir probe diagnostic of plasmas burning in mixtures with O 2 is coupled with difficulties. Oxide layers deposited on the probe surface cause deformation of probe VA characteristics, and presence of negative ions changes the electron density. The described problems complicate interpretation of Langmuir probe data. Simple method of negative ion density estimation from probe measurements in electronegative plasmas has been suggested in [4]. The approach is based on the comparison of two probe characteristics, the first measured in pure electropositive (Argon) plasma and the second measured in slightly electronegative (Ar+O 2 ) plasma. 2. Determination of negative-ion densities – theoretical considerations One possible way to estimate the negative-ion density is based on the charge neutrality condition n e p n n n + = , where the subscripts p,e and n indicate quantities concerning positive ions, electrons and negative ions, respectively. The electron and the positive-ion densities can be determined from the Langmuir probe characteristics, but the evaluation of the positive-ion density from the positive-ion saturation current is rather questionable task as the ion current theories do not provide sufficiently reliable results. To avoid difficulties concerning correct interpretation of positive-ion current we have taken into consideration two characteristics. The characteristics measured in electronegative mixture of Ar with trace of O 2 was evaluated with the aid of the reference characteristics measured in the pure electropositive Argon plasma. Similar idea was employed in [4, 5]. The total current normalized to the unit probe area consists of the electron and positive-ion part, p e j j j − = . The presence of negative ions, e.g. O - , Topic 8