Plasma-chemical processes in microwave plasma-enhanced chemical vapor deposition reactors operating with C/H/Ar gas mixtures

Abstract

Microwave ͑MW͒ plasma-enhanced chemical vapor deposition ͑PECVD͒ reactors are widely used for growing diamond films with grain sizes spanning the range from nanometers through microns to millimeters. This paper presents a detailed description of a two-dimensional model of the plasma-chemical activation, transport, and deposition processes occurring in MW activated H/C/Ar mixtures, focusing particularly on the following base conditions: 4.4%CH 4 / 7%Ar/balance H 2 , pressure p = 150 Torr, and input power P = 1.5 kW. The model results are verified and compared with a range of complementary experimental data in the companion papers. These comparators include measured ͑by cavity ring down spectroscopy͒ C 2 ͑a͒, CH͑X͒, and H͑n =2͒ column densities and C 2 ͑a͒ rotational temperatures, and infrared ͑quantum cascade laser͒ measurements of C 2 H 2 and CH 4 column densities under a wide range of process conditions. The model allows identification of spatially distinct regions within the reactor that support net CH 4 → C 2 H 2 and C 2 H 2 → CH 4 conversions, and provide a detailed mechanistic picture of the plasma-chemical transformations occurring both in the hot plasma and in the outer regions. Semianalytical expressions for estimating relative concentrations of the various C 1 H x species under typical MW PECVD conditions are presented, which support the consensus view regarding the dominant role of CH 3 radicals in diamond growth under such conditions.

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

“…67 Experimental validation of these conclusions is provided by Figure 5, which shows two sets of z-resolved absolute column density profiles for C2(a, v=0) and CH(X, v=0) radicals, and for electronically excited H(n = 2) atoms, measured (by CRDS) under process conditions that differ in just one important detail. 44,78 The data shown by the solid symbols were obtained for the same p, P and gas mixture as used for the model outputs displayed in Figure 4 (i.e.…”

“…46 We find it helpful to picture the reactor volume in terms of three nested regions. 67 Each supports H-shifting reactions within and between the CHx and C2Hy families illustrated in Figure 2, but with rates that are sensitive to Tgas, so that the dominant species vary between them. The outer region having Tgas < 1500 K (labelled C in Figure 4 and B is important in maintaining the balance between them, and the extent of gas processing in these regions ensures that this result holds irrespective of the choice of input hydrocarbon.…”

“…For typical process pressures, p >100 Torr, Tgas becomes sufficient for thermal dissociation of H2 to be the main route to producing H atoms. 63,67,71,74 The primary ionization mechanism also depends on the process conditions. Direct EI ionization of H2 (1), the sets of reactions (2) and (3) H2 + e  H2 + + 2e (1)…”

“…This value is both two orders of magnitude larger than that of any other CHx species and relatively constant across the substrate surface. 44,67 In many regards, understanding MW plasma-activated H/C gas mixtures under conditions relevant to diamond CVD is a solved problem. Combined experimental and modelling approaches such as ours have provided full descriptions of the plasma processing, the spatially resolved variation of Tgas and the gas-phase composition, the influence of process conditions (P, p, carbon input fraction, etc.…”

On the Role of Carbon Radical Insertion Reactions in the Growth of Diamond by Chemical Vapor Deposition Methods

“…Spatially resolved optical emission studies have provided further (relative) measures of these (and other) species, and insights into the electronic properties of the plasma (e.g., the electron number density, n e , and temperature T e ). 6 The measurements have been complemented by sophisticated 2-D (r, z for the cylindrically symmetric reactors of interest) modeling of the relevant plasma-chemical processes, 7 with the experimental measurements and the model outputs serving to test and tension each other in a symbiotic manner.…”