Plasma Diagnostics in KrF Excimer Laser Deposition of Ti Thin Films

Pulsed laser deposition of Ti thin films for wafer metallization is investigated by optical spectroscopy. Ti plasma emission spectra are captured to study plasma generation, expansion and recombination during the thin film deposition. Ti plasma has a very rich emission spectrum in a region from I 80 to 650 nm. There exist two phases of plasma characterization. The first phase is plasma ionization by the laser heating near the target surface. Gate delay is up to 200 ns at this phase. There is continuum emission resulted from free-free and free-bound transitions. The second phase is plasma recombination in its propagation process. The dependence of Ti spectral line intensities on gate delay, laser fluence and chamber pressure is also studied. The intensities decrease with gate delay, but increase with laser fluence and tend to saturate at laser fluence higher than 4.6 J/cm2 due to the plasma shielding effect. The spectral line intensities do not vary with chamber pressure up to 0.1 Ton. At the higher pressure, plasma is confined by air molecules and the intensities increase greatly with the chamber pressure. Plasma electron density is calculated as the functions of gate delay, laser fluence and chamber pressure from the Stark broadening of the spectral lines. Electron temperature is also estimated as 1 .5 eV from the local thermodynamic equilibrium (LTE) assumption. Fast time-resolved photography is applied to analyze the plasma plume evolution in its propagation. The plasma expands outwards and flies from target to substrate at speeds up to 106 cm/s.