A Laser Ion Source for Thin Film Deposition: Characterization of Source and Growth Conditions

A LASER ION SOURCE FOR THIN FILM DEPOSITION: CHARACTERIZATION OF SOURCE AND GROWTH CONDITIONS Md. Mahmudur Rahman Old Dominion University, 2019 Director: Dr. Hani Elsayed-Ali Multicharged aluminum and carbon ions are generated by a laser-arc ion source. The design, construction, and testing of a compact laser ion source is demonstrated where the laser plasma is amplified by a high voltage spark-discharge. Optical emission and ion time-of-flight are measured for a spark-coupled laser aluminum plasma. A Q-switched Nd:YAG laser (wavelength λ = 1064 nm, pulse width τ ~7 ns, pulse energy Ep ≤ 260 mJ, intensity I ≤ 15 × 10 9 W/cm) generates the Al plasma, while a synchronized spark-discharge enhances the ion flux and charge state. Time-integrated, spatially resolved optical spectra are used to obtain the plasma excitation temperature Te and density ne. The coupling of 2.4 J of spark-discharge to the laser plasma enhances the optical emission line intensity. The effective ion temperature Ti is calculated from a shifted Maxwell-Boltzmann distribution fit of the time-of-flight signal deconvolved for each ion charge. For I = 3.5 × 10 W/cm, Ti is ~15 eV. For spark energy of 2.4 J coupled to the laser plasma, Ti increases to ~50 eV, and up to Al 8+ is identified from the ion time-of-flight signal. The Ti obtained from the ion time-of-flight is much larger than Te obtained from optical spectroscopy, although the plasma is considered to be in local thermodynamic equilibrium. This result is explained in view of the temporal development of the ablation plume and the different plasma regions probed by the two methods. Multicharged carbon ions are also generated by a different laser-assisted spark-discharge (laser-arc) ion source configuration. A Q-switched Nd:YAG laser pulse (1064 nm, 7 ns, ≤ 4.5 × 10 W/cm) focused onto the surface of a glassy carbon target results in its ablation. The sparkdischarge (~1.2 J energy, ~1 μs duration) is initiated along the direction of the plume propagation between the target surface and a grounded mesh that is parallel to the target surface. Ions emitted from the laser-spark plasma are detected by their time-of-flight using a Faraday cup. The ion energy-to-charge ratio is analyzed by a three-mesh retarding field analyzer. In one set of experiments, the laser plasma is generated by target ablation using a 50 mJ laser pulse. In another set of experiments, ~1.2 J spark-discharge energy is coupled to the expanding plasma to increase the plasma density and temperature that results in the generation of carbon multicharged ions up to C. A delay-generator is used to control the time delay between the laser pulse and the thyratron trigger. The highest charge amplification is recorded at ~0.9 μs time delay between the laser pulse and spark-discharge. Ion generation from a laser pulse when a DC high-voltage is applied to the target is compared to that when a spark-discharge with equivalent pulsed voltage is applied to the target. The laser-coupled spark-discharge (7 kV peak voltage, 810 A peak current) increases the maximum detected ion charge state from C to C, accompanied by an increase in the ion yield by a factor of ~6 compared to applying 7.0 kV DC voltage to the target. Pulse laser deposition is used to deposit Al thin film on Si substrate. The growth conditions of the Al thin-film are investigated using a femtosecond pump-probe setup. The thermomodulation response from the thin film is measured. The goal is to measure the thin-film heat transfer as well as the thickness of the thin film. A femtosecond (800 nm, 100 fs FWHM, 0.15 nJ/pulse) laser pulse creates acoustic-strain pulse in the Al thin film. The time of flight of the acoustic pulse shows that the echoes of the acoustic pulse reflected from the metal/substrate interface change the optical reflectivity at the film surface. This information can be used to determine the thin-film thickness. The sensitivity of the femtosecond pump-probe setup is in the range of 10. On-line thickness measurement of Al thin-film during pulse laser deposition is not successful due to the roughness of the thin film; instead, thermal evaporation on Si substrate is demonstrated by femtosecond optical pump-probe spectroscopy. A femtosecond Ti:sapphire laser pulse (wavelength λ = 800 nm, pulse width τ ∼ 100 fs, pulse energy Ep = 1.25 nJ) is used to instantaneously heat (pump) the surface of Al thin-film that changes the temperature profile of the target surface. The delayed probe pulse, also with λ = 800 nm, is used to investigate the change in transient thermoreflectance ΔR/R in order of 10 for the Al thin-film. The thermal expansion creates isotropic thermal stress in the Al surface that generates an acoustic wave of ultrasonic frequency. The travel time of the optically induced strain normal to the surface of the sample is measured to evaluate in-situ thickness measurement of Al in the range of 30 to 450 nm during the film deposition.