Plasma immersion ion implantation with lithium atoms

A new method was developed with the aim to creating plasma of lithium for plasma immersion ion implantation. First an argon glow discharge with operation pressure ranging from 2 × 10⁻² mbar to 1 mbar is generated by negatively polarizing an electrode from −400 V to −1500 V. Small pieces of metallic lithium, 99.9% pure; fill the top of a conic crucible, with 2 cm depth, in electric contact with the electrode. Argon ions from the plasma are used to bombard this target where heat is created by the momentum transfer between the impacting ions and the crucible. By controlling the operation pressure and the electrode voltage polarization it is possible to easily heat the crucible to temperatures above the lithium melting point (180 °C), causing its evaporation. Lithium atoms are then ionized due to collisions with plasma particles. Double Langmuir probe measurements indicated variation on the density of the discharge from 4×109 cm⁻³ to 10¹⁰ cm⁻³, after lithium evaporation. Silicon wafer pieces immersed into this mixed plasma were submitted to repetitive negative high voltage pulses (3 kV/6 μs/3 kHz), in order to attract plasma ions. High resolution X-Ray diffraction performed in these samples revealed the presence of high strain on the analyzed layers, in comparison with samples submitted just to the argon discharge.