Validating optical emission spectroscopy as a diagnostic of microwave activated CH4/Ar/H2 plasmas used for diamond chemical vapor deposition

Abstract

Spatially resolved optical emission spectroscopy ͑OES͒ has been used to investigate the gas phase chemistry and composition in a microwave activated CH 4 / Ar/ H 2 plasma operating at moderate power densities ͑ϳ30 W cm −3 ͒ and pressures ͑Յ175 Torr͒ during chemical vapor deposition of polycrystalline diamond. Several tracer species are monitored in order to gain information about the plasma. Relative concentrations of ground state H ͑n =1͒ atoms have been determined by actinometry, and the validity of this method have been demonstrated for the present experimental conditions. Electronically excited H ͑n = 3 and 4͒ atoms, Ar ͑4p͒ atoms, and C 2 and CH radicals have been studied also, by monitoring their emissions as functions of process parameters ͑Ar and CH 4 flow rates, input power, and pressure͒ and of distance above the substrate. These various species exhibit distinctive behaviors, reflecting their different formation mechanisms. Relative trends identified by OES are found to be in very good agreement with those revealed by complementary absolute absorption measurements ͑using cavity ring down spectroscopy͒ and with the results of complementary two-dimensional modeling of the plasma chemistry prevailing within this reactor.

Property mapping of polycrystalline diamond coatings over large area

“…The growth rate across the 100-mm wafer ranges from 2 µm/h for sample I up to 2.57 µm/h for sample H. Different parameters, like pressure, substrate temperature, methane percentage, hydrogen flow rate, etc., are known to affect the growth rate [41]. Axial and radial distributions of gas species and temperatures inside an MPCVD reactor were elaborately discussed by Ma in his thesis [42]; he showed that there is a wide variation in the species concentration and gas temperature away from the discharge core and the substrate. However, even though the hemispherical plasma symmetrically covers the whole 100-mmdiameter substrate surface, the temperature distribution is not radial.…”

What [plasma used for growing] diamond can shine like flame?

“…[44][45][46] Figure 3 shows an image of the operating reactor, along with a schematic cross-section. MW radiation is delivered through a rectangular waveguide, converted to the TM01 mode and coupled into the cylindrical chamber.…”

“…Thus the emission intensities are intimately linked to the electron energy distribution function (EEDF), and can provide information about how the electron density (ne) and temperature (Te) vary with position and changes in process conditions. 45,60 High-resolution OES measurements can provide estimates of the temperature of the emitting species (through, for example, the Doppler broadening of lines in the Balmer series of atomic hydrogen or from the relative intensities of series of lines in the emission spectrum of H2 or of the C2 radical), which is often used as a proxy for the local Tgas. Actinometry, a variant of OES in which emission from a target species is measured in parallel with that from a small quantity of an inert tracer species (usually Ar), offers a route to determining the densities of, for example, ground state H atoms (a quantity that can be difficult to measure by laser based methods).…”

“…Actinometry, a variant of OES in which emission from a target species is measured in parallel with that from a small quantity of an inert tracer species (usually Ar), offers a route to determining the densities of, for example, ground state H atoms (a quantity that can be difficult to measure by laser based methods). 45,60 Any thorough description of diamond-growing plasmas requires experimental measurements and theoretical modelling. Many inter-related phenomena must be accounted for to build a complete model of diamond growth in a MW CVD reactor.…”

“…As seen in Figure 3, the plasma is luminous, and is thus amenable to analysis by optical emission spectroscopy (OES)-a technique that has long been employed for monitoring and optimizing plasma processes. 45,[58][59][60] OES measures the emission from electronically excited species, which can be populated in a number of ways (chemiluminescent reactions, dissociative excitation, electron-ion recombination) but, most typically, by electron impact (EI) excitation of the ground-state species. Thus the emission intensities are intimately linked to the electron energy distribution function (EEDF), and can provide information about how the electron density (ne) and temperature (Te) vary with position and changes in process conditions.…”