Ion-beam sputtering apparatus for deposition of multilayered films

Abstract

Multilayered films are,composed of repeated alternate deposition of ultrathin layers of different materials. These films are of interest for their unique properties not intrisically found in conventional monolithic substances. Multilayered films have been prepared by several methods: molecular beam epitaxy [1], thermal evaporation [2, 3], chemical vapour deposition [4], and sputtering [5-7]. In an attempt to deposit thin films of tin oxide, we used an ion-beam sputtering (IBS) apparatus with a simplified accelerator system [8]. The IBS method is particularly attractive for deposition because a high-purity thin film with an improved adhesion to the substrate can be obtained. In the present letter we describe the construction of the IBS apparatus specially designed for the synthesis of multilayered films. The constructed apparatus is simple, relatively inexpensive and convenient to control. The results of analysing periodic structures of Pt/SnOx multilayers deposited by this apparatus are also presented. The outline of the apparatus used to prepare multilayered films is shown in Fig. 1. The vacuum chamber is a vertical cylinder, 40 cm in diameter and 80 cm tall, of which only the upper-half is actually used. The base pressure is below 1 x 10 5 torr. Argon gas of 99.9995% purity is fed into the ionsource chamber and the chamber pressure during the run is maintained at (1 + 0.1) x 10 4torr. The target is placed at about 45 ° to both the axis of the beam and the substrate. The distance from the centre of the target to the ion-source chamber and to the substrate is 3.5 and 5 cm, respectively. The target assembly can contain four different targets and the targets are screwed to the aluminium target-holders of 8-cm diameter. In this experiment two targets, platinum foils and a sintered tin oxide disc, were used. The rotation of the targets was carried out using an ordinary stepping motor (Oriental motor, PH266-01) in the vacuum chamber. Generally the vacuum hinders the rapid heat transfer, thus heat tends to accumulate in the running motor and augments its temperature. To avoid this temperature increase of the stepping motor, the motor is mounted on a water-cooled copper plate; flexible hoses are used to conduct the cooling water. Although the placing of the stepping motor in the vacuum chamber induces a possible contamination from the gaseous species from the organic materials in the motor, it makes the overall design of the apparatus very simple. A microcomputer (NEC, PC-8801) and a driver/controller (Cosmotron, PMC-14C) together with the stepping motor control the rotation of the targets. When the sputtering time to form a layer reaches a preset value, the control system turns the target assembly by 90 ° to the other target. It takes about 1 sec for this rotation. The substrate assembly contains eight different substrate holders that can be selected manually via a d.c. motor. The shutter has a window (2 c m x 4 cm) through which sputtered particles deposite on a designated substrate. During the pre-sputtering of the targets, the window is closed. The substrate-shutter separation is held at 4 mm so that all particles passing through the window hit the planned substrate, thus contamination of other substrates by the sputtered particles is avoided. The electrical connections for the system are shown schematically in Fig. 2. It is similar to that used in our previous experiment [8], however the stability of the discharge current increased markedly by employing a d.c. current supply (Metronix, 411A-32) and an insulation transformer instead of a voltage slider. The electrical system is run typically at a discharge voltage of 50 V, a discharge current of 600 mA, a beam voltage of 1000 V, an accelerator voltage of 150 V and an ionbeam current (defined here as the current between the target and the ground) of 0.8 mA. The temperatures of the substrate and the target reached 90 and 130°C, respectively after 3h; they were measured by a CA-thermocouple during the sputtering of an