Date of Award

Winter 2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Committee Director

Hani E. Elsayed-Ali

Committee Member

Helmut Baumbart

Committee Member

Gon Namkoong

Committee Member

Leposava Vuskovic

Abstract

A novel multicharged ion source, using laser ablation induced plasma coupled with spark discharge, has been investigated in this work. The designed and demonstrated ion source is cost-effective, compact and versatile. Experiments are described with the intention of demonstrating the practicability of ion implantation via laser ion source.

Multicharged aluminum ions are generated by a ns Q-switched Nd:YAG laser pulse ablation of an aluminum target in an ultrahigh vacuum. The experiments are conducted using laser pulse energies of 45–90 mJ focused on the Al target surface by a lens with an 80-cm focal length to 0.0024 cm2 spot area and incident at 45° with the Al target surface. With the increase in the laser pulse energy, a slow increase in the number of ions generated is observed. The generation of ions with a higher charge state is also observed with the increase in the laser pulse energy. For 5 kV accelerating voltage applied to the Al target and using laser energy of 90 mJ, up to Al4+ charge is delivered to the detector which is located 140 cm away from the Al target. Raising accelerating voltage increases the charge extraction from the laser plasma and the energy of multicharged ions.

The components of a transport line for a laser multicharged ion source are described. Aluminum and carbon multicharged ions are generated by a Q-switched, nanosecond Nd:YAG laser (wavelength λ = 1064 nm, pulse width τ = 7.4 ns, and pulse energy up to 82 mJ) ablation of a target in a vacuum chamber. Time-of-flight and three-grid retarding ion energy analyzers are used to determine the velocity and the charge state of the ions. A three-electrode cylindrical einzel lens is used to focus the ions. At 30 cm from the center of the focusing electrode of the einzel lens, Al1+ and Al2+ have a minimum beam diameter of ∼1.5 mm, while for Al3+ and Al4+ the minimum beam diameter is ∼2.5 mm. The simulation of the ion trajectories is done using SIMION 8.1. A high voltage pulse applied to a set of two parallel deflecting plates is used for the pickup of ions with different charge states according to their time-of-flight. An electrostatic cylindrical ion deflector is used for analysis and selection of charges with specific energy-to-charge ratio. The design of these transport line components and their operation are described.

A spark discharge is coupled to a laser multicharged ion source to enhance ion generation. The laser plasma triggers a spark discharge with electrodes located in front of the ablated target. For an aluminum target, the spark discharge results in significant enhancement in the generation of multicharged ions along with higher charge states than observed with the laser source alone. When a Nd:YAG laser pulse (wavelength 1064 nm, pulse width 7.4 ns, pulse energy 72 mJ, laser spot area on target 0.0024 cm2) is used, the total multicharged ions detected by a Faraday cup is 1.0 nC with charge state up to Al3+. When the spark amplification stage is used (0.1 μF capacitor charged to 5.0 kV), the total charge measured increases by a factor of ∼9 with up to Al6+ charge observed. Using laser pulse energy of 45 mJ, charge amplification by a factor of ∼13 was observed for a capacitor voltage of 4.5 kV. The spark discharge in-creases the multicharged ion generation without increasing target ablation, which solely results from the laser pulse. This allows for increased multicharged ion generation with relatively low laser energy pulses and less damage to the surface of the target.

Laser plasma generated by ablation of an Al target in vacuum is characterized by ion time-of-flight combined with optical emission spectroscopy. A Q-switched Nd:YAG laser (wavelength λ = 1064 nm, pulse width τ ∼ 7 ns, and fluence F ≤ 38 J/cm2) is used to ablate the Al target. Ions are accelerated according to their charge state by the double-layer potential developed at the plasma-vacuum interface. The ion energy distribution follows a shifted Coulomb-Boltzmann distribution. Optical emission spectroscopy of the Al plasma gives significantly lower plasma temperature than the ion temperature obtained from the ion time-of-flight, due to the difference in the temporal and spatial regions of the plasma plume probed by the two methods. Applying an external electric field in the plasma expansion region in a direction parallel to the plume expansion increases the line emission intensity. However, the plasma temperature and density, as measured by optical emission spectroscopy, remain unchanged.

Aluminum multicharged ion generation from femtosecond laser ablation is studied. A Ti:sapphire laser (wavelength 800 nm, pulse width ∼100 fs, and maximum laser fluence of 7.6 J/cm2) is used. Ion yield and energy distribution of each charge state are measured. A linear relationship between the ion charge state and the equivalent acceleration energy of the individual ion species is observed and is attributed to the presence of an electric field within the plasma-vacuum boundary that accelerates the ions. The ion energy distribution follows a shifted Coulomb-Boltzmann distribution. For Al1+ and Al2+, the ion energy distributions have two components; the faster one can be attributed to multiphoton laser ionization, while the slower one is possibly due to collisional processes. Ion extraction from the plasma is increased with an applied external electric field, which is interpreted to be due to the retrograde motion of the plasma edge because of the external electric field. Multicharged ion generation by femtosecond laser ablation is compared to previously reported ion generation with nanosecond laser ablation and is shown to require significantly lower laser fluence and generates higher charge states and more energetic ions.

Fully-stripped boron ions are generated by a nanosecond Nd:YAG laser (wave-length λ = 1064 nm, pulse width τ = 7 ns, and maximum laser pulse energy E = 175 mJ) ablation of a B target in vacuum. Higher charge states, along with the increase in the number of ions detected, are observed with the increase in the laser fluence. An external electric field between the end of the expansion chamber and a grounded grid is used to extract the ions and accelerate them according to their charge state. For 5 kV accelerating voltage applied to the B target and using a laser fluence of 115 J/cm2, ∼1.5 nC of total charge is delivered to the detector which is located ∼150 cm away from the B target. Ion deflection by an electrostatic field separates the ions from the neutrals and makes this geometry suitable for ion implantation.

The developed multicharged ion deposition and implantation system was used to per-form interfacial treatment of the SiC/SiO2 interface using boron and barium ions. SRIM simulation was used to estimate the ion penetration depth in the SiC substrate. The multicharged ions were used for shallow ion implantation in 4H SiC. The optical bandgap of the 4H SiC was reduced due to boron ion implantation. Several MOSCAP devices were fabricated with a combination of boron and barium shallow implantation. High-low C-V measurements were used to characterize the MOSCAPs. Boron implantation affects the flatband voltage significantly, while the effect of barium ion implantation is negligible. Shallow boron implantation in the SiC/SiO2 interface reduces the flatband voltage from 4.5 V to 0.04 V.

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DOI

10.25777/kh2s-5r68

ISBN

9780438900295

ORCID

0000-0003-0984-5619

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