Spectroscopic Characterization of Laser-induced Titanium Plume

A titanium target was ablated by a KrF excimer laser with fluences varying from 4 to 8 J/cm in an argon filled environment with pressures ranging from vacuum to 1 torr. The effects of laser fluence and background gas pressure on the kinetic energies of the ablated species were investigated by temporally and spatially resolved emission spectroscopy. The maximum surface temperatures were calculated by an onedimensional conduction model. Experimentally obtained surface temperatures from the kinetic energy of the ejected plume were one order higher than the calculated temperatures due to plasma absorption of the laser. This discrepancy is most likely due to the absorption of laser energy by the plasma that is formed early in the pulse. Temporally resolved imaging with 10 ns gate width was also employed to reveal the evolution of the ablated plume against the background gas. Separation of slower and faster components were observed, and angular concentration of titanium in the plume was determined. INTRODUCTION Since its beginning, Pulsed laser deposition (PLD) of thin films has evolved into a well recognized technique for a wide range of materials and in a variety of devices. Among other factors that affect the quality of the deposited film, a key parameter is the kinetic energy of the vaporized atoms. Adatoms with greater kinetic energy have higher mobility once they reach the substrate and consequently form a more crystalline film (Ohring, 1992.) Titanium has been a material of great research interest since Titanium based thin films have a great range of uses in microelectronic devices, optics, and tribology (Sundgren et al., 1985; Hems et al., 1990; Narayan et al., 1992; Valkonen et al., 1983.) Optical methods such as emission time-of-flight (TOF) measurements are attractive for determining the energy and the extent of ionization of the ablation plume for several reasons. First, these methods are non-intrusive. With fast detectors, it is possible to obtain time-resolved spectroscopic information on the nanosecond scale. Furthermore, emission spectroscopy can effectively distinguish between different neutral and ionized species. This work employed emission time-of-flight and imaging techniques to investigate laser ablated titanium plumes. NOMENCLATURE C = constant Cp = heat capacity f = full-range Maxwellian distribution I = excimer laser intensity k = imaginary part of the refractive index K = thermal conductivity kB = Boltzmann constant m = mass of titanium atom M = Mach number n = real part of the refractive index Q = laser energy absorbed by the target R = reflectivity of the target surface t = time T = temperature v = velocity x = distance above the target surface z = distance into target from the surface Greek Letters α = absorption coefficient