Mechanisms of suppressing secondary nucleation for low-power and low-temperature microwave plasma self-bias-enhanced growth of diamond films in argon diluted methane

Ji-Heng Jiang,Yonhua Tzeng

doi:10.1063/1.3656241

Abstract

We report on mechanisms for suppressing diamond secondary nucleation in microwave plasma self-bias-enhanced growth (SBEG) of diamond films in methane diluted by argon. High-density plasma at a small distance from the substrate induces a floating potential which promotes high-flux, low-energy ion bombardment on diamond growing surfaces along with an equal flux of electrons. Increased atomic hydrogen generated by electron impact dissociation of methane and low-energy ion bombardment help remove hydrocarbon coatings on diamond grains in favor of continuous grain growth and, therefore, the suppression of secondary diamond nucleation. Energetic meta-stable excited argon, abundant C2 dimers, and enhanced effective surface temperature due to low-energy ion bombardment further promote the diamond grain growth resulting in the deposition of a diamond film with columnar diamond grains of much larger grain sizes and a much lower density of grain boundaries than ultrananocrystalline diamond (UNCD) films grown under similar conditions without optimized plasma-substrate interactions. SEM, XRD, PL, and Raman scattering help confirm the deposition of diamond films with columnar grains.

Highlights

Among diamond Chemically vapor deposited (CVD) processes,[3,4,5,6,7,8,9,10,11,12] microwave plasma enhanced chemical vapor deposition (MPECVD) in methane diluted by argon with neither hydrogen nor oxygen additives has attracted a lot of attention due to its capability of low-power and low-temperature deposition of ultra smooth, ultra-nano-crystalline diamond (UNCD) films with diamond grains of 3-5nm, in comparison with microcrystalline diamond films deposited by hydrogen-rich plasma.[9,10,11,12,13,14]
The high C2 concentration and low atomic hydrogen concentration lead to the coating on diamond growing surfaces with non-diamond carbons and hydro-carbons which block the further growth of crystalline diamond grains and provide favorable conditions for secondary nucleation of diamond.[15]
Instead of the commonly expected enhancement in secondary nucleation of diamond, suppressing of secondary diamond nucleation is pursued while retaining the advantages of low-power and lowtemperature MPECVD in argon diluted methane without hydrogen or oxygen additives

Summary

INTRODUCTION

Vapor deposited (CVD) diamond possesses many excellent properties for practical applications.[1,2] Among diamond CVD processes,[3,4,5,6,7,8,9,10,11,12] microwave plasma enhanced chemical vapor deposition (MPECVD) in methane diluted by argon with neither hydrogen nor oxygen additives has attracted a lot of attention due to its capability of low-power and low-temperature deposition of ultra smooth, ultra-nano-crystalline diamond (UNCD) films with diamond grains of 3-5nm, in comparison with microcrystalline diamond films deposited by hydrogen-rich plasma.[9,10,11,12,13,14] High concentration of C2 dimers is generated when the gas pressure increases to 100 Torr and above. 5% hydrogen was added to the gas mixture of 1% methane and 94% argon in order to increase the supply of atomic hydrogen for the suppression of secondary nucleation of diamond Simultaneous deposition of both large-grain microcrystalline and small-grain nanocrystalline diamond on different parts of a silicon substrate has been demonstrated by plasma generated by 1kW, 2.45GHz microwave power at 170 Torr gas pressure and at the 800-900◦C substrate temperature.[11,12] In that case, only the central part of the substrate is in direct contact with the CVD processing plasma. Due to the non-uniform nature of the small plasma ball generated in hydrogen-containing plasma at a high gas pressure of 170 Torr, it was not clear which physical and/or chemical interactions are the most important for determining the rate of secondary nucleation and the growth of large diamond grains. Instead of the commonly expected enhancement in secondary nucleation of diamond, suppressing of secondary diamond nucleation is pursued while retaining the advantages of low-power and lowtemperature MPECVD in argon diluted methane without hydrogen or oxygen additives

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSIONS