Diamond Particles Deposited among Nickel/Copper Particles in Energy Controlled CH4/H2 RF Discharge Plasmas ()
1. Introduction
Self-organization process is quite interesting phenomenon because an ordered structure will evolve even in a chaotic background. Here, it is also quite interesting that the growth of diamond happens even under a presence of many other obstacle atomic species that have been generated in plasmas by the physical sputtering, thermal evaporation and chemical dissociation. Diamond has only sp3 carbon bonding. However, during its growth, the site of carbon bonding might be attacked by the other atomic species such as Mo, Cu, and Ni. This reaction may result in a formation of metallic carbon with different bonding except for diamond. Concerning to the growth of diamond on metallic substrates, an intermediate layer or an incubation layer, combined with carbon and metals, will be usually produced before the start of diamond growth. In any case, in order to construct pure crystal structure of diamond, reactions of carbons with the other atomic species should be restricted even when they are contained simultaneously in the plasma during the deposition. Carbon has to exclude those atoms during the growth of diamond.
Reactions of carbons with such metallic atoms were usually preceded on the metal surface. Diamond films grown on Cu substrate were studied since Cu was an excellent substrate candidate for hetero-epitaxial growth owing to its cubic structure and lattice mismatch of only 1.3%. However, the nucleation density of diamond growth on Cu surface was very low, so it was a problem for further application in the industry. Moreover, less adhesion of diamond on Cu substrate was attributed to an insufficient formation of interfacial carbide layer [1,2]. Nucleation and growth of diamond film were also investigated by using substrate of Mo [3]. Since diamond had superior characteristics for the heat conductivity, the growth of diamond on Cu had attracted considerable attention for the heat dissipation for the electronic and power devices [4,5]. The problem was that a large thermal stress caused poor adhesives between Cu and diamond boundaries [6]. On the other hand, the growth of diamond on Mo was relatively easy because of a formation of Mo2C layer, which could provide a favorable intermediate layer for diamond nucleation. The growth of diamond on Ni substrate was also studied [2,7]
Here, high quality diamond could be produced in a low electron temperature plasma, where a novel method for the control of electron energy distribution function has been developed in order to reduce higher order dissociation of CH4 [8]. This technique was applied to CH4/ H2 plasma for diamond deposition. High quality diamonds and nanocrystalline diamonds were produced in the low electron temperature plasma by employing a grid method [9,10].
In the present study, we investigated the growth of diamond on Ni substrate under an influence of Cu vapor evaporated from Cu wire connected to a heating system for Ni substrate in CH4/H2 plasma. The growth of diamond was carried out in the lower electron temperature plasma by employing the grid method. The properties of the depositions were expected to depend on the electron temperature in the CH4/H2 plasma.
2. Experimental
Figure 1 shows a schematic of the experimental apparatus. There were two regions in a vacuum chamber, i.e., regions I and II. Separation mesh grid was made of stainless wires, to which dc voltage VG was biased. The plasma was generated in region I by a 13.56 MHz radio-frequency (RF) power source. Here, a hollow-type magnetron (HTM) RF plasma source was adopted, because it was expected that the plasma could be created by a synergy between the hollow-cathode effect and the magnetron effect. The magnetic field for providing the magnetron effect was supplied by two magnet rings wound around the plasma source. The plasma produced diffused in the axial z direction across the diverging magnetic field. Here, a cylindrical RF electrode of 16 cm or 9 cm in diameter was employed, to which RF power of 300 W was supplied through an impedance matching circuit with a blocking condenser for an efficient plasma production. By using the HTM plasma source, a highdensity plasma was efficiently produced.
A nickel substrate, which was kept at a floating potential, was set up in region II and was heated up to about 700˚C - 800˚C. In the heating section a tungsten W wire coil was placed in the ceramic holder and connected to Cu wires to feed the electric heating power. The temperature of W coil was increased up to about 1300˚C, therefore the Cu wires were heated to at least 500˚C - 600˚C, which was enough to sublimate Cu though lower than its melting point of 1083˚C. Nickel, of course, could deform its structure though its temperature was lower
Figure 1. Schematic of the experimental apparatus.
than its melting point of 1453˚C. Before the deposition experiments, the surface of Ni substrate was not scratched artificially. Total flow rate of CH4 and H2 was fixed at 200 sccm. Total pressure was kept at 100 mTorr throughout the whole deposition experiment. The properties of the deposited films were characterized by a scanning electron microscopy (SEM) and Raman spectroscopy.
3. Experimental Results
The low electron temperature plasma was produced when the grid was kept at its floating potential. It was because that in our case the floating potential was decreased to about –20 V, which was sufficiently low enough for the electrons to be expelled towards the plasma source. Hence, those high-energetic electrons passing over such potential barriers could ionize neutral gas in downstream region. So, the low electron temperature plasma was produced by such ionization. In this case, the electron temperature Te in region II was kept constant, being almost independent of the contamination deposited on the mesh grid. Therefore, the plasma parameters were kept almost constant in time during the depositions. The electron temperature was an important key factor to produce diamond particles. When the electron temperature was high, disordered graphite and/or diamond-like carbon with broad D and G bands in Raman spectrum were simply obtained. On the other hand, when the electron temperature was decreased to be 1 eV or less, by the floating grid, it was possible to produce diamond particles.
Figure 2 shows a typical scanning electron microscopy image (SEI: secondary electron imaging) of the surface of Ni substrate, where many microparticles were observed. Though the shape of almost all particles was irregular, some of them were small spherical particles. Henceforth, the labels “a” and “c” were given to the bigger particles with irregular shape, and the label “b” was given to the small spherical particles, as shown in Figure 2.