Cubic Boron Nitride Prepared by an ECR Plasma
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CUBIC BORON NITRIDE PREPARED BY AN ECR PLASMA Y. OSAKA, M. OKAMOTO AND Y. UTSUMI Department of Electrical Engineering, Hiroshima University, Higashihiroshima 724, Japan ABSTRACT A number of deposition techniques for cubic BN films from the vapor phase at low pressures have been proposed. We show that the essential factor for creating cubic BN films is a negative self-bias applied to the substrate. The optical and mechanical properties of the deposited films are characterized by reflectance and stress measurements, respectively. INTRODUCTION Boron nitride (BN) is chemically and thermally stable and highly insulating. Cubic BN (c-BN) having the zincblende-type structure is an especially hard material comparable to diamond. Synthesis of c-BN in equilibrium requires very high pressures and temperatures[l]. Recently, a number of processes for the deposition of c-BN films from the vapor phase at lower pressures have been investigated[2-7]. In these techniques, it seems that energetic ions impinging on the growing surface play an essential role in the synthesis of c-BN. We have succeeded in the formation of microcrystalline cubic BN films on a Si substrate using an electron cyclotron resonance (ECR) plasma and RF substrate bias[8,9]. We have found that the formation of the cubic BN phase necessitates an appropriate self-bias. In this paper, the terminology applied self-bias means the net resultant bias due to the RF power. The deposition techniques and deposition conditions for creating c-BN films are discussed in details. The structure and physical properties of the deposited films on the Si substrate are summarized. The practical and important technique to prevent the peeling of c-BN films from the Si substrate are discussed. DEPOSITION TECHNIQUES Figure 1 and 2 shows the apparatus to deposit BN thin films from the vapor phase of B 2H 6 and N2 . It is evacuated by a diffusion pump through a throttling gate valve. The base pressure is 2x 10- 7 Torr. Figure 1 is the apparatus using both ECR plasma and RF substrate bias. Microwave power is introduced through a rectangular waveguide and a fused quartz window into the plasma chamber of water cooled stainless steel with inside diameter 11 cm and height 16 cm. The magnetic field intensity is 1000 G at the center of coils. In Figure 2, the arrangement using only RF power is shown. The magnetic field direction is changed to obtain uniform thin films. The substrate electrode is coupled to RF generator through an impedance-matching network with a blocking capacitor. The RF power is applied between the grounded chamber and the substrate electrode. A negative DC self-bias is generated basically due to the higher mobility of electrons as compared to ions in a megahertz glow discharge. The self-bias is measured using an oscilloscope connected to the substrate electrode through "achoke coil to block RF power. The temperature of the substrate holder is measured by "athermocouple behind the electrode. The deposition condition are summarized in Table I. We used single crystal silicon and
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