Pulsed plasmas study of linear antennas microwave CVD system for nanocrystalline diamond film growth
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Frantisek Fendrych,a) Andrew Taylor, and Michal Novotny Institute of Physics, Academy of Sciences of the Czech Republic, CZ-18221 Prague 8, Czech Republic
Michael Liehr Technical Consulting, D-63654 Buedingen, Germany (Received 17 August 2011; accepted 6 October 2011)
Optical emission spectroscopy (OES) was used to study plasmas generated by a novel plasmaenhanced linear antennas microwave chemical vapor deposition system for nanocrystalline diamond (NCD) growth in gas mixtures of H2 + CH4 + CO2. Atomic hydrogen intensities were investigated for pulsed plasmas and continuous wave (CW) mode plasmas. OES was used to study the effect of pressure (0.38–2 mbar), microwave pulse frequency (3.8–25 kHz), and total gas flow (125–1000 sccm). By using the Boltzmann plot for atomic hydrogen line intensities, plasma electron temperatures for pulsed and CW plasmas were calculated. During experiments, NCD films were deposited, which were investigated by secondary electron microscopy and Raman spectroscopy in terms of surface crystalline morphology and nondiamond carbon content. NCD films produced in high pulse frequency plasmas show low sp2 content (less than 5%) and homogenous crystalline structure with only a small amount of crystalline defects.
I. INTRODUCTION
In recent years, diamond has become a widely investigated material.1 Ultrananocrystalline diamond (UNCD) and nanocrystalline diamond (NCD) films have attracted increasing interest due to their unique electrical, optical, and mechanical properties,2 which make them widely suitable for different applications, such as MEMS devices, lateral field emission diodes, biosensors, thermoelectrics, etc. Additionally, nanodiamond offers properties such as biocompatibility and nontoxicity. Many of these applications of NCD films require special preparation parameters, such as large area growth or low-temperature deposition. For the growth of NCD, many constructions of microwave (MW) cavities chambers exist, such as classical ellipsoidal systems,3 circumferential antenna plasma systems,4 or linear antenna systems.5,6 One system capable of meeting the requirements of large area and low-temperature growth is the plasma-enhanced linear antennas microwave chemical vapor deposition (PELAMWCVD).1 This special reactor type (see Fig. 1) works with different conditions compared to classical MW chemical vapor deposition (CVD) systems used for the growth of NCD films. By the use
of a pulsed MW power supply, this type of system has been further improved.5,6 Typical condition for MW plasma-assisted growth is a mixture of hydrocarbon and hydrogen with a very low proportion of hydrocarbon. To improve plasma conditions by chemistry during deposition, it is suitable to add carbon dioxide to the gas mixture.6,7 Plasma emission spectroscopic studies of plasma deposition systems for NCD growth have been published, mainly for either simple cavity MW systems8 or hot filament systems.9 Also, plasma studies using timeresolved spectroscopic diagnostics of a classical CVD system with pulsed plasmas for NCD growth,
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