Characterization of an argon magnetron plasma by a cylindrical Langmuir probe

Abstract

Magnetic fields enhance the synthesis of high-quality thin films in plasma chemical vapour deposition (CVD) at low pressures. To understand this phenomenon, it is first required that we closely examine the role of the magnetic field in the generation of a highdensity plasma. For this purpose, the present study has been performed on the dependences of the plasma parameters such as the electron temperature, electron density and ionization current of the plasma on the magnetic field strength. It was found that the generation of high-density plasma in low-pressure CVD strongly depends on the magnetic field strength. High-density plasma CVD was reported to be appropriate for the synthesis of high-quality thin films [1]. However, the plasma CVD technique is usually used for a low-density plasma. In view of this, a magnetic field has been applied for the generation of high-density plasma since it can effectively raise the confinement of the secondary electrons [2]. So far, major parts of the investigation on high-density plasma deposition have been performed using a sputtering technique and only rarely using CVD [3, 4]. In 1988, Ohnishi et al. [5] reported the influence of a magnetic field on the CVD of hydrogenated amorphous Si films using silane gas. They found that the magnetic field can markedly improve the optical emission intensity of the plasma by the motion of the secondary electrons. This implies that the magnetic field plays a significant role in the generation of high-density plasma even in low-pressure CVD. In spite of such an important role of the magnetic field, however, it is still not clear how the lowpressure CVD process enables the synthesis of highquality thin films by a magnetron plasma. To solve this problem, it is first required to understand the generation mechanism of the high-density plasma in low-pressure magnetron plasma CVD. Therefore, we investigated the dependences of the plasma parameters such as the electron temperature, Te, electron density, ne, and saturation ion current, Isat i0 , of the plasma on the magnetic field strength. The generation mechanism of the high-density plasma was also discussed in terms of the plasma parameters. A schematic diagram of the radio-frequency (RF) planar magnetron plasma CVD system is shown in Fig. 1. In the figure, permanent magnets are attached perpendicular to the cathode. Fig. 2 shows the magnetic field lines formed by the magnetron of this study. Te, ne and Isat i0 were determined by a Langmuir probe measurement [6]. The single cylindrical probe, in which the copper wire core is insulated with a Pyrex glass tube [7] and stainless steel tube, was positioned 47 mm above the cathode surface. The schematic diagram for the cylindrical probe used is shown in Fig. 3. The system was then evacuated to 1:33 3 10y4 Pa before the generation of plasma. The plasma source was supplied by a 13.56 MHz RF generator with 100 W and low-pressure argon gas of 0.6 Pa. The magnetic field strength at the probe position was varied to 45, 74 and 82 G by the arbitrary configuration of the magnets in the chamber. Under these magnetic field strengths, the probe current was measured by applying the direct current probe potential in the range between y100 and ‡250 V. The electrical equivalent circuit for a Langmuir probe measurement is shown in Fig. 4.