Microwave Plasma-Activated Chemical Vapor Deposition of Nitrogen-Doped Diamond. I. N₂/H₂ and NH₃/H₂ Plasmas

We report a combined experimental/modeling study of microwave activated dilute N₂/H₂ and NH₃/H₂ plasmas as a precursor to diagnosis of the CH₄/N₂/H₂ plasmas used for the chemical vapor deposition (CVD) of N-doped diamond. Absolute column densities of H(n = 2) atoms and NH(X³Σ^–, v = 0) radicals have been determined by cavity ring down spectroscopy, as a function of height (z) above a molybdenum substrate and of the plasma process conditions, i.e., total gas pressure p, input power P, and the nitrogen/hydrogen atom ratio in the source gas. Optical emission spectroscopy has been used to investigate variations in the relative number densities of H(n = 3) atoms, NH(A³Π) radicals, and N₂(C³Π_u) molecules as functions of the same process conditions. These experimental data are complemented by 2-D (r, z) coupled kinetic and transport modeling for the same process conditions, which consider variations in both the overall chemistry and plasma parameters, including the electron (T_e) and gas (T) temperatures, the electron density (n_e), and the plasma power density (Q). Comparisons between experiment and theory allow refinement of prior understanding of N/H plasma-chemical reactivity, and its variation with process conditions and with location within the CVD reactor, and serve to highlight the essential role of metastable N₂(A³Σ⁺_u) molecules (formed by electron impact excitation) and their hitherto underappreciated reactivity with H atoms, in converting N₂ process gas into reactive NH_x (x = 0–3) radical species.