JINST Instrumentation Theses Archive

Ph.D. degree thesis
accepted by University of Lecce, Italy, in 2006

Fabio Belloni

Supervisor: Vincenzo Nassisi

Ion implantation via a laser ion source

 Keywords:

• Ion sources (positive ions, negative ions, electron cyclotron resonance (ECR), electron beam (EBIS))
• Plasma generation (laser-produced, RF, x ray-produced)
• Plasma diagnostics - charged-particle spectroscopy
• Plasma diagnostics - probes

 Abstract:
A novel ion implantation technique, using laser ablation induced plasma as ion source, has been investigated in the present work. A compact and versatile laser ion source (LIS) was set up, characterized and tested. Experiments are reported, aimed at demonstrating the feasibility of ion implantation via LIS, at tens - hundreds keV energies.

An excimer laser (KrF, 248 nm wavelength, 70 mJ/pulse and 23 ns FWHM pulse duration) was pulsed on solid targets, in order to allow the generation of an expanding plasma bunch. Typically, the laser beam was focused in a 1 mm^2 spot, resulting in an irradiance of about 3.5 x 10^8 W/cm^2. By applying a high voltage on the LIS accelerating gap, ions were extracted from the ablated plasma and accelerated towards a substrate. The occurrence of arcs during the extraction stage was a major challenge that could be overcome employing a specifically-designed plasma expansion chamber. Such a set-up allowed to apply the high voltage in DC mode. Moreover, it is also proposed the use of a novel pulser to apply the accelerating voltage in AC mode, generating single or repeated high voltage fast pulses. Implantations of Al, Cu and Ge on Si substrates were carried out, operating at tens kV DC acceleration voltage and 1 Hz laser repetition rate.

Assuming a phenomenological model for the plasma plume, useful equations were derived for several physical quantities of interest in ion diagnostics, extraction and implantation. The abundance and the energy distribution of the various ion charge states produced in the laser plasma were characterized by means of electrostatic mass-to-charge spectrometers. Charge states up to +3 were revealed. The ion fluence was estimated using Faraday cup diagnostics, both on the free expanding plasma and on the accelerated beam, and was in the order of 10^10 - 10^12 ions/cm^2 per pulse, depending on the ablated material. Ion doses ranging from 10^13 to 10^14 ions/cm^2 were implanted. The implanted samples were characterized by means of several analytical techniques, commissioned to various national and foreign laboratories: SEM-EDX, Rutherford backscattering spectrometry (RBS), X-ray photo-electron spectrometry (XPS) and laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS). Analyses showed that neutral atoms were deposited and ions were implanted at energies of tens keV per charge state unit. An implantation range larger than 100 nm was typically measured for all the implants, in good agreement with Monte Carlo simulations of the depth profiles. Furthermore, a parallel study on the LA-ICP-MS performances in the sub-μm depth range was carried out, with a view to implementing on-site analysis of the implanted samples.