CVD Diamond as Dielectric Material for Capacitor Applications
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ABSTRACT Diamond films synthesized using Microwave Plasma Chemical Vapor Deposition (MWCVD) were evaluated for use as dielectric material for high power and high temperature capacitors. The effect that the deposition
parameters and annealing have on the frequency and temperature stability of the electronic properties was investigated. Dielectric constants ranging between 8.0 and 4.2 and resistivities between Ix 10' ohm-cm and 5x 10'4 ohm-cm were obtained. Diamond produced using less than 6.6% methane had very stable dielectric constants over the frequency range of 100 Hz to IMHz, and the loss tangent was less than 0.01. Adding oxygen to the precursor
gas increased the dielectric constant and lowered the loss tangent of CVD diamond, but the resistivity was also lowered. As the temperature increased to 300TC, the dielectric constant and loss tangent increased. However, when
diamond was annealed to 700°C, there was less than a 5%change inthe dielectric constant from 230C to 300*C. INTRODUCTION Capacitors are essential components of power conversion and control subsystems in aircraft, automobiles and machinery. Their size and reliability are major concerns for developing improved power systems, particularly when operating at high voltages and at high temperatures.' A demand exists for cost-effective, dielectric materials for capacitors which can achieve high energy densities and withstand both high temperatures and high voltages. Diamond is a promising candidate for high power and high temperature capacitors because the dielectric constant and loss tangent of natural diamond are 5.7 and 0.0006, respectively, and the resistivity of type I and Ila diamond is between 101" and 1020 ohm-cm.2' 3 In addition, natural diamond has the highest thermal conductivity of all known materials (2000 W/m-°K). 2 However, high quality type I and type 11a diamond is very expensive. Chemical Vapor Deposited (CVD) diamond possesses many of the unique properties of natural diamond, and can be produced at a much lower cost.3 However, the properties of polycrystalline CVD diamond are strongly affected by their growth conditions. Dielectric constant's between 19.8 and 11.1 and loss tangents between 0.036 and 0.12 were reported by Alam & Lucero'. Fujimori measured the dielectric constant of diamond as 6.5 and the loss tangents between 0.006 and 0.16. In this paper, the deposition conditions of CVD diamond are investigated in order to optimize the dielectric properties for use in high power and high temperature capacitors. EXPERIMENTAL DETAILS Diamond films were produced by microwave plasma-assisted chemical vapor deposition (MWCVD) from a methane-hydrogen-oxygen precursor gas mixture using an ASTEX microwave reactor capable of delivering 5000 W power at 2.45 GHz frequency. The substrates were 3.175 cm x 3.175 cm square tungsten polished to a mirror finish. Samples were prepared using two different sets of deposition conditions which are specified in Table I, respectively. After deposition, the samples cooled and the diamond films popped off
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