Measured density of copper atoms in the ground and metastable states in argon magnetron discharge correlated with the deposition rate

H Naghshara (1) , S Sobhanian (1) , S Khorram (1) , N Sadeghi (2)

1 Faculty of Physics
2 IMRB - Institut Mondor de Recherche Biomédicale

In a dc-magnetron discharge with argon feed gas, densities of copper atoms in the ground state Cu(2 S 1/2) and metastable state Cu*(2 D 5/2) were measured by resonance absorption technique, using a commercial hollow cathode lamp as light source. The operating conditions were 0.3- 14 µbar argon pressure and 10- 200 W magnetron discharge power. The deposition rate of copper in a substrate positioned at 18 cm from the target was also measured with a quartz microbalance. The gas temperature, in the range of 300 to 380 K, was deduced from the emission spectral profile of N 2 (C 3 Π u − B 3 Π g) 0-0 band at 337 nm when trace of nitrogen was added to the argon feed gas. The isotope-shifts and hyperfine structures of electronic states of Cu have been taken into account to deduce the emission and absorption line profiles, and hence for the determination of atoms densities from the measured absorption rates. To prevent error in evaluation of Cu density, attributed to the line profile distortion by autoabsorption inside the lamp, the lamp current was limited to 5 mA. Density of Cu(2 S 1/2) atoms and deposition rate both increased with the enhanced magnetron discharge power but at fixed power the copper density augmented with argon pressure whereas the deposition rate followed the opposite trend. Whatever the gas pressure, the density of Cu*(2 D 5/2) metastable atoms remained below the detection limit of 1 x 10 10 cm -3 for magnetron discharge powers below 50 W and hence increased much rapidly than the density of Cu(2 S 1/2) atoms, over passing this later at some discharge power, whose value decreases with increasing argon pressure. This behavior is believed to result from the enhancement of plasma density with increasing discharge power and argon pressure, which would increase the excitation rate of copper into metastable states. At fixed pressure, the deposition rate followed the same trend than the total density of copper atoms in the ground and metastable states. Two important conclusions of this work are: i) copper atoms sputtered from the target under ion bombardment are almost all in the ground state Cu(2 S 1/2) and hence in the plasma volume they can be excited into the metastable states ; ii) all atoms in the long-lived ground and metastable states contributes to the deposition of copper layer on the substrate. 2 1- Introduction Magnetron discharges are widely used for thin film deposition in surface treatment 1, 2, magnetic and superconducting devices 3, semiconductor industry 4 and in surface coating engineering of materials under low temperature conditions 5, 6. In these systems, the magnetic field, usually produced by placing permanent magnets behind the cathode, confines the plasma in the near cathode region, reducing the electron loss to the sidewalls. By increasing the ionization rate near the cathode, which is also the target to be sputtered, its confinement helps to maintain the plasma at much lower buffer gas pressure than in a normal dc discharges. Bombardment of the target by ions leads to the sputtering of metal atoms, which then diffuse in the plasma volume and are partly deposited in the substrate positioned at about tens of cm from the target. So, in spite of small current densities, high metal atom production rates are achieved by magnetron discharges. Also, the high stability and uniformity of these plasmas are another factor for the success of these systems. But uniform, high quality and damage-free layer deposition requires the deep understanding of the gas phase processes in magnetron plasma, sputtering processes from the target, and deposition processes on the substrate. In particular, it is of prime importance to identify different electronic states of the metal, copper in the present case, which are highly populated. Beside the ground state, metastable states of copper, from which optical transition to the ground state is forbidden, must therefore be considered. The most popular diagnostic tools to characterize the plasma volume and control the discharge parameter are optical emission and absorption techniques. Britum et al have used a Fabry-Perot interferometer to deduce the gas temperature in argon magnetron discharges with different target materials 7, or to record the velocity distribution function of sputtered metal atoms 8. The same group characterized a titanium magnetron discharge by using the resonance absorption technique, with a hollow cathode lamp 9 or the laser induced fluorescence technique 10. Using the space and time resolved laser induced fluorescence technique in magnetron plasmas of different target materials (Fe, Ti, Cu) and rare gases (Ar, Kr, Xe), Sasaki et al have suggested that dimmers and trimers of metal atoms could also be sputtered from the target and get dissociated in the plasma volume 11. The space resolved electron density and energy distribution function of electrons have also been obtained with a Langmuir probe in several magnetron systems 12,

Mots clés: : 52.2.Hv; 52.70. Kz; 52.80.Sm; 32.70.Jz

H Naghshara, S Sobhanian, S Khorram, N Sadeghi. Measured density of copper atoms in the ground and metastable states in argon magnetron discharge correlated with the deposition rate. Journal of Physics D: Applied Physics, 2011, 44 (2), pp.25202. ⟨10.1088/0022-3727/44/2/025202⟩. ⟨hal-00597851⟩