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Plasma Chemistry and Plasma Processing, Vol. 13, No. 2, 199.t
Fluid Boundary Layer Effects in Atmospheric-Pressure
Plasma Diamond Film Deposi t ion
S. L. Girshick, t C. Li, ~ B. W. Yu, t and H. Han t
Received July 10, 1992; revised September 7, 1992
Diamond films were deposited in an atmospheric-pressure radio-frequent3' plasma reactor. Hydrogen and methane were injected coaxially into the plasma as a high-velocity jet which impinged on the molybdenum substrate. In some cases argon was added to the reactant jet to increase its momentum, thereby reducing the boundary layer thickness, In most cases argon addition substantially improved diamond growth. A numerical model was developed, which calculated two- dimensional reactor temperature and velocity distributions, and the chemical kinetics in the boundary layer. The calculations indicate that under the experimental condi- tions argon addition reduced the thickness of the hydrogen nonequilihrium boundary layer from 3.5 to 1.0 mm. In addition, the calculations suggest that monatomic carbon may be a key diamond growth species under thermal plasma conditions,
KEY WORDS: Thermal plasmas; chemical vapor deposition; diamond film; impinging jet; atomic carbon.
Thermal plasmas have been used to deposit diamond films at relatively high rates, but little is known about how the fluid mechanics of a hot plasma jet impinging on a cooled substrate affects diamond growth. The freestream of this jet is characterized by a pressure of - 1 atm, a temperature in excess of 5000 K, and hydrogen in a completely dissociated state. This jet impinges on a substrate, usually oriented normal to the flow, which is maintained for diamond growth at a temperature of -1000-1400 K. A fluid boundary layer with a steep temperature gradient exists above the substrate.
The thickness of this boundary layer has considerable significance for the transport to the substrate of chemically active species. For example, although hydrogen may be completely dissociated in the freestream, H
~Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455.
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170 Girshick, Li, Wu, and Han
atoms may recombine before reaching the substrate if the boundary layer is sufficiently thick. Alternatively, depending on species diffusion velocities, on mean free paths between collisions involving chemical reactions, and on the boundary layer thickness, the species responsible for diamond growth may be chemically destroyed in the boundary layer.
An analytical solution is available for constant-property stagnation- point flow, which shows that the boundary layer thickness is inversely correlated with the jet momentum. That is, the boundary layer thickness scales o n ( p V ) t 2, w h e r e p a n d V are the freestream values of mass density and velocity, respectively.' ~' We believe that this correlation explains why higher linear diamond growth rates have in general been reported using dc plasma jets '2 ~'~ than with rf thermal plasmasJ 7 t~ Core velocities in dc plasma jets are typically much higher ( - 5 0 0 m/s) than in rf thermal plasmas ( - l0 m/s), producing a thinner boundary layer and more effective transport to the substrate of diamond growth species. However, the higher velocity in a dc plasma jet comes at the expense of plasma uniformity, and hence of film uniformity. Another advantage of rf plasmas is that, being electrode- less, they do not suffer contamination from electrode erosion, which may be a significant impediment to the use of dc plasma jets in depositing diamond for electronic device applications. In our earlier work, we attempted to combine the advantages ofboth types of plasma by introducing reactants (methane and hydrogen) directly into an rf plasma in the form of a high-velocity je t (V, ..... > 100m/s). The results were promising in that good diamond film uniformity was obtained over 25-ram-diameter substrates at growth rates exceeding 10 ~m/h .
In the present work we investigate the effect of adding argon to the reactant jet. As argon atoms are 20 times as massive as hydrogen molecules, we would expect argon addition to increase the jet momentum substantially, reducing the boundary layer thickness. Presumably this should result in improved or at least modified film deposition. The reduction in boundary layer thickness by adding argon to the central jet is not completely decoupled from other effects--for example, the temperature profile downstream of the injection tube is affected--but within a certain range of conditions we hypothesized that the reduction in boundary layer thickness would be the dominant effect, which would have clear effects on film growth.
2. DESCRIPTION OF EXPERIMENTS
The experimental apparatus is shown schematically in Figure 1, and was the same as for the earlier experiments '~''~~ except that a small modification was made to the substrate holder, employing a ring clamp rather than a screw-down design.
Plasma Deposition of Diamond Film 171
Sheath ~Ias Ar
I Water Out
gv 0 Coil
Stainless Steel Plate
Reactant Gas CH4+H2+Ar
0 0 0 0 0
Water In T Water Out
Fig. I. Reactor schematic.
Table I. Operating Conditions in Common ['or the 16 Cases
Total pressure 1 atm Generator plate power 12-14 kW Frequency 2.9 MHz ('oil current 80-
85 A (rms) Main plasma gas (argon) flow rate 40slpm
172 Girshick, Li, Wu, and Han
Operating conditions are summarized in Table I. The plasma operated at atmospheric pressure and was driven by inductive coupling to a nominally 20-kW radio-frequency power generator. Our calculations indicate that typically - 4 - 6 kW of power was actually coupled to the plasma, and the measured frequency through the five-turn induction coil was 2.9 MHz. The plasma tube was made of water-cooled quartz, with an inner diameter of 44 mm. The main plasma gas, argon, was introduced at the top of the torch at a flow rate of 40 slpm. It entered the torch through injection tubes oriented at 45 with respect to the plasma tube axis, so as to impart swirl, which stabilizes the plasma.
A water-cooled stainless steel injection probe, with an inner diameter of 1.8 mm, was inserted coaxially directly into the plasma, terminating at the level of the midplane of the induction coil. Hydrogen, methane, and in some cases additional argon were injected through this tube into the plasma. For each experiment the separate flow rates of these three gases were maintained at either a "low" or a "high" value: for hydrogen, either 4 or 8 slpm; for methane, either 1 or 2% of the hydrogen flow rate; and for argon, either 0 or 4 slpm.
Table I1. Cond i t i ons of the 16 Cases Tes ted"
Case Ar (s lpm) H 2 (s lpm) C H 4 / H 2 (%) T.s (C) number
1 0 4 1 890
2 0 4 1 1014
3 0 4 2 901 4 0 4 2 1039
5 0 8 1 900 6 0 8 1 1070
7 0 8 2 845 8 1) 8 2 959
9 4 4 1 906
10 4 4 1 1027 11 4 4 2 884 12 4 4 2 1047
13 4 8 1 874 14 4 8 1 951
15 4 8 2 830 16 4 8 2 1005
"The argon flow rate refers to the flow rate through the central in ject ion tube,
not the main p lasma gas.
Plasma Deposition of Diamond Film 173
The reactant jet impinged on a molybdenum substrate which was mounted to a cooling assembly. Referring to Fig. 1, the distances y and z were respectively 25 mm and 45 mm, giving a total distance from the injec- tion tube exit to the substrate surface of 70 mm. The total diameter of this assembly was 27.4 mm; accounting for the 5-mm molybdenum ring which fastened the substrate, the actual deposition area of the substrate was 17.4 mm in diameter. (Typically diamond was also deposited on the ring.) The substrate surface temperature was controlled by inserting stainless steel disks of various thicknesses d between the substrate and a water-cooled stainless cylinder; d could range from 1 to 10 mm. Surface temperatures as measured by a two-color optical pyrometer (Ircon Modline Series R) ranged from 830-1070C. For each set of conditions we ran one case with a surface temperature which was relatively low within this range, and another case with a relatively high temperature.
We tested all 16 possible combinations of these four parameters (the three central jet flow rates and the surface temperature), with all other conditions held the same, as listed in Table I1. In each case we ran the deposition for 4 hours. The internal consistency of the results, together with several repeated runs, indicated a high degree of reproducibility.
The different cases produced significant differences in whether or not diamond was produced and in the film morphology. Morphologies were consistently uniform over the substrate surface. Cases with continuous diamond films had film thicknesses ranging from 20 to 70 ~m, indicating time-averaged linear growth rates of 5-18 ~m/h , which is about the same as has been reported for a dc plasma torch operating at atmospheric pressure/~-~
Figure 2 shows scanning electron microscope (SEM) photographs of the films produced in Cases 1-4, in which the hydrogen flow rate was 4 slpm and no argon was added to the central jet. Under these conditions the jet velocity and momentum were at a minimum, and the micrographs indicate that in all four cases nondiamond carbon was produced. In both of the high-temperature cases (Cases 2 and 4) one observes ball-like structures that are coated with what appear to b