Toward smooth MWPECVD diamond films: Exploring the limits of the hydrogen percentage in Ar/H₂/CH₄ gas mixture

Abstract

In Ar-rich Ar–H₂–CH₄ gas mixture the presence of H₂ is found to be beneficial to the plasma stability. On the other hand, too high H₂ percentages lead to materials showing a high surface roughness. In the present work, diamond films were grown on p-type Si (100) substrates screening different quantities of H₂. The plasma phase and plasma–substrate interface were investigated by in-situ optical emission spectroscopy and pyrometric interferometry to determine the behavior of emitting species and the deposition rates, respectively. The obtained films were characterized by Raman micro-spectroscopy, AFM and SEM techniques. For H₂ percentages between 6.3 and 10%, the structure and morphology are characteristic of nanocrystalline films, affording low roughness values when a buffer layer was grown between the diamond coating and the treated silicon surface.

Introduction

The last two decades have witnessed an increasing interest in diamond film growth. This is essentially due to two reasons: firstly, the unique physical properties of diamond, such as extreme mechanical hardness, highest known thermal conductivity, broad optical transparency from the deep UV to the far IR radiations and chemical inertness, just to cite a few [1], [2]; secondly, the spectacular technical advancement achieved by chemical vapor deposition techniques [3]. Among them, the microwave plasma enhanced chemical vapor deposition (MWPECVD) has proved to be an efficient and convenient strategy for the coating of various substrates. Depending on the experimental conditions, the grain size can range from nanometers, < 10 nm in ultrananocrystalline diamond (UNCD) [4], [5], < 100 nm in nanocrystalline diamond (NCD) [6], [7] to micrometers, < 100 μm [8], [9] in polycrystalline diamond (PCD). Typical deposition conditions involve the use of a gas mixture containing a small quantity of hydrocarbon gas, e.g., 1% CH₄ in excess of hydrogen, which normally affords PCD films. Nanocrystalline diamond films have been produced in various ways: 1) by increasing the CH₄ percentage from 10% to 20% in the standard CH₄–H₂ mixture [10], 2) by biasing the untreated Si substrate on which the film is deposited during the growth in MWPECVD [11], [12] or 3) by replacing H₂ with inert gases [6], [13].

Specifically, in Ar-rich (> 95%) Ar–H₂–CH₄ gas mixtures, the grain size significantly decreases, becoming of the order of nanometers. As a consequence, the morphology of the film itself changes drastically at the nanometer scale. UNCD and NCD films obtained in this way result smoother than PCD films. In Ar-rich Ar–H₂–CH₄ gas mixtures, the presence of H₂ was found to confer stability to the microwave plasma. On the other hand, too high H₂ percentages would lead to materials showing a high surface roughness. In the present work, diamond films were grown on silicon substrates screening different quantities of H₂ from 15% down to 6.3%. The plasma phase and plasma–substrate interface were investigated by in-situ optical emission spectroscopy (OES) and pyrometric interferometry (PI) to determine the behavior of emitting species and the deposition rates, respectively. The obtained films were finally characterized by Raman spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. The films were grown on an initial buffer layer (BL) about 500 nm thick, which allowed the production of uniformly deposited samples, in contrast with the coatings obtained without BL that were non continuous and/or covered with pinholes. In particular, for H₂ percentages between 6.3 and 10%, the presence of a buffer BL afforded films having nanocrystalline structure and morphology, generally showing low root-mean-squared (rms) roughness values.