Thin-film Deposition Process by Pulsed Ion-Beam Generated Ablation Plasma

Thin-film Deposition Process by Pulsed Ion-Beam Generated Ablation Plasma

W. Jiang, H. Kawahara, H. Shishido, H. Suematsu, T. Yunogami, K. Yatsui

Extreme Energy-Density Research Institute Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan

Abstract. Pulsed ion-beam evaporation (IBE) is a practical technique of thin film deposition.  In this paper, the process of IBE is studied with concentration on the importance of physical properties of the target material. The experiments were carried out by using different kinds of metal targets. The results have helped us in understanding the physical relation between the ablation plasma behavior and the thin-film surface morphology.

INTRODUCTION

An ion beam carries a high power-level directed energy flow. For example, an ion beam of ion-energy 1 MeV and current density 1 kA/cm² carries a power flow of 1 GW/cm². The beam energy density, even for a very short pulse width of 50 ns, can reach 50 J/cm². By using such a pulsed ion beam, one can easily produce ablation plasma at the surface of any kind of solid material.

When the ablation plasma touches a solid surface, it is cooled by the surface, leaving a thin film on it. Many kinds of thin films have been successfully prepared by using the ion-beam generated ablation plasmas. This technique, called «pulsed ion-beam evaporation» (IBE), has been demonstrated to be a unique tool for novel material development [1–3].

Compared with other techniques of thin-film deposition, the IBE is advantageous in the following respects. First, the pulsed ion beam produces high-density plasma, which leads to a very high thin-film deposition rate. Second, the high-density ablation plasma carries a large amount of thermal energy, which also deposits on the substrate during thin film deposition. This instantaneous energy deposition heats the substrate upon which the film deposits, playing an important in forming the crystalline structure of the thin film. Third, the energy efficiency of ion-beam ablation is higher than other heating methods.

Figure 1. Basic processes of IBE: a) ion-beam energy deposition; b) ablation plasma expansion; c) thin film deposition

The IBE basically consists of three processes, namely target ablation process, plasma expansion process, and thin-film deposition process, as shown in Figure 1. These processes are studied separately in order to understand the behavior of the ablation plasma and its effects on the properties of the deposited thin film [4–6]. Figure 2 shows the typical experimental setup of IBE. The pulsed high-voltage generated by pulsed power generator «ETIGO-II» is applied between the electrodes of the magnetically insulated ion-beam diode. The ion beam is geometrically focused and extracted to the target cell where the ablation plasma is produced and the thin film is deposited. Typical experimental parameters are shown in Table I.

experimental results

Ion beam ablation removes a part of the target material from the target surface. This can be observed by comparing target weights before and after ion-beam irradiation. Figure 3 compares the ablated target mass with the melting and boiling temperature of the target material. It is clearly seen that there is a relation between these parameters. For example, high boiling and melting temperature material such as tungsten has a low ablated mass and, oppositely, low boiling and melting temperature material such as lead has a high ablated mass. In other words, the target melting and boiling temperatures are important factors in determining the target ablation depth.

Table. Typical experimental parameters for thin-film deposition by IBE