Theoretical and experimental investigation of enhanced glow discharge plasma immersion ion implantation

Abstract

In enhanced glow discharge plasma immersion ion implantation (EGD-PIII) that involves a small pointed anode and large area tabular cathode, the high negative substrate bias not only acts as the plasma producer but also supplies the implantation voltage. Consequently, an electric field is created to focus the electrons and the electron focusing field in turn enhances the glow discharge process. Previous results have demonstrated that EGD-PIII is an effective ion implantation method, and features many advantages compared with conventional plasma immersion ion implantation (PIII). However, the principles as well as discharge characteristics such as plasma distribution, sheath dynamics, implantation fluency, and so on are not clear, and industrial application of EGD-PIII requires further investigation. In the work described in this thesis, the discharge characteristics of EGD-PIII are investigated experimentally. The discharge initiation and extinction characteristics during pulsed biasing are discussed. The duration after pulse-off is explained from the viewpoint of particle motion and experimentally verified by employing an auxiliary. The plasma distribution is measured using a Langmuir probe to obtain the plasma density and numerical interpolation is performed to obtain the plasma density distribution throughout the entire discharge region. The effects of different distances between the anode and cathode on the glow discharge characteristics and the influence of the plasma electron density are also evaluated. The experimental results verify the electron focusing phenomenon and suggest the optimal processing windows for enhanced ionization rates and efficiency in EGD-PIII. The sheath physics is theoretically investigated using numerical simulation based on the multiple-grid particle-in-cell (PIC) code. Electron focusing is corroborated and the plasma sheath has enough expansion when t = 40 μs, so that a uniform distribution of the incident ion fluence is attained. In addition, the implantation energy and retained dose uniformity in enhanced glow discharge plasma immersion ion implantation (EGD-PIII) are investigated numerically and experimentally. Depth profiles obtained from different samples processed by EGD-PIII and traditional PIII are compared. The retained doses obtained using different pulse widths are calculated by integrating the area under the depth profiles. Our results indicate that the improvement in the impact energy and retained dose uniformity by this technique is remarkable. Ion focusing in enhanced glow discharge plasma immersion ion implantation (EGD-PIII) of hydrogen into silicon affects the lateral ion fluence uniformity. The phenomenon and its effects are investigated experimentally and theoretically under different conditions and compared to those in nitrogen EGD-PIII. Consistent results are obtained from experiments and numerical simulation disclosing that the lower the plasma density, the more severe is the ion focusing effect. The influence of the negative high voltage on the ion focusing effect is small compared to that of the plasma density. To extend the application of EGD-PIII to plasma gases with low ionization rates, an insulating tube is used to increase the interaction path for electrons and neutrals enhance the discharge near the anode. Results obtained from the finite element method and experiments show that this configuration enhances the ionization rate and subsequently ion implant fluence. The process is especially suitable for gases that have low ionization rates such as hydrogen and helium. In nitrogen enhanced glow discharge plasma immersion ion implantation, the implantation current increases sharply at a certain gas pressure when exceeding a threshold value, contrary to the normal observation that it tends to decrease as the plasma is depleted. The glow discharge dynamics changes gradually with increasing diatomic gas flow rate. The voltage drop rendered by the anode glow produces noticeable N+ dissociation and electron-atom ionization in the positive column which expands to the cathode reducing the plasma sheath propagation time. The larger plasma density and N+/N2+ ratio in the positive column lead to the higher current and X-ray photoelectron spectroscopy confirms that the N+/N2+ ratio increases with gas flow rate.

Date of Award	16 Jul 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Paul Kim Ho CHU (Supervisor)