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Deposition of pyrocarbon in a low temperature environmentA. Inspektor, U. Carmi, A. Raveh, Y. Khait, and R. Avni Citation: Journal of Vacuum Science & Technology A 4, 375 (1986); doi: 10.1116/1.573931 View online: http://dx.doi.org/10.1116/1.573931 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/4/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Low-field electron emission of diamond/pyrocarbon composites J. Vac. Sci. Technol. B 19, 965 (2001); 10.1116/1.1368669 Low temperature deposition for high performance photodetector J. Vac. Sci. Technol. B 18, 2627 (2000); 10.1116/1.1320802 Effect of oxygen in diamond deposition at low substrate temperatures Appl. Phys. Lett. 56, 437 (1990); 10.1063/1.102758 Lowtemperature silicon selective deposition and epitaxy on silicon using the thermal decomposition of silaneunder ultraclean environment Appl. Phys. Lett. 54, 1007 (1989); 10.1063/1.100781 Enhanced crystallinity of silicon film deposited at low temperature Appl. Phys. Lett. 26, 178 (1975); 10.1063/1.88106
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.76.6 On: Tue, 02 Dec 2014 17:57:50
Deposition of pyrocarbon in a low temperature environment A. inspektor, U. Carmi, A. Raveh, Y. Khait,a) and R. Avni Division of Chemistry, Nuclear Research Center~Negev, P. 0. Box 9001. Beer-Sheva. 84190 Israel
(Received 30 September 1985; accepted 22 November 1985)
This work presents aspects of the preparation ofpyrocarbon (PyC) depositions at relatively low temperatures (300-500 CC) and discusses its properties. The deposition process which is performed in a nonequiIibrium low pressure rf plasma of propylene-argon or methane argon mixtures coats thermally sensitive materials. The properties of the coatings were found to be similar to the properties of CVD-PyC films obtained at higher temperatures ( > 1000 CC) 0 The measured properties and their correlation to the experimental working parameters for the deposition of pyrocarbon are presented and discussed.
I. INTRODUCTION
Pyrocarbon (pye) is a material of relatively high mechanical properties and chemical resistance. 1
-3 As such, it is wen
suited to be used in environments where mechanical abrasion and chemical hostility prevails. This work presents the preparation of pye coatings at relatively low temperatures (300-500 ·C) and discusses its properties. This technique, which is based on the plasma-wall coupling phenomenon, enables the coating of thermally sensitive materials and involves processes occurring in the plasma-surface boundary region.4
•5 The deposition process is performed in a nonequi
Hbrium low pressure inductive rf and microwave plasma of propylene-argon mixtures. The coating is performed on a graphite (type ATJ) or metal substrate positioned in the plasma reactor. The purpose of this work is to analyze the effects of various experimental variable parameters on the formation and on the properties of the coating layer.
The research was performed in two complementary aspects: (a) Measurements of the deposition rate (weight and thickness growth), and characterization of the coating (structure analysis by x~ray diffraction and by scanning electron microscopy, composition analysis, microhardness, optical anisotropy, and sink float density).6--8 (b) Diagnostics of the plasma which was performed using a microwave plasma of propylene9
•10 or methanell as a model and the results
were correlated to the rf depositing plasma. This is following Avni et a/. n who demonstrated that the various plasmas, Le., microwave and radio-frequency initiated plasmas, can be correlated to each other using values such as electron temperature, electron density, and the collision cross section to define a reaction rate criteria. It was found 12 that by plotting the value of the log of the reaction rate constant versus the electron density divided by the plasma pressure, a straight line was obtained independent of the plasma generator type.
Plasma diagnostics included the following methods: (a) Plasma composition by mass spectrometry sampling9
.!1 of various regions along the plasma. (b) Ion density, electron density, and electron temperature using the electric double floating probes system (DFPS).lO.ll Also some extensive work has been done by optical emission spectroscopy of the plasma and the plasma layer engulfing the substrate13 but this will not be discussed here.
U. EXPERIMENTAL
The experimental setup for both the plasma characterization and for the deposition procedure was described in detail elsewhere. B-l1 However, some aspects of the process are given below. The deposition (rf plasma, 0.5 MHz) and the plasma investigations (mw, 2.45 GHz) were conducted at low pressure (P < 10 Torr) in a flowing gaseous mixture of argon and the hydrocarbon (propylene or methane). The variable plasma parameters were the hydrocarbon concentration in the feed, the total gas pressure, and the net power input. Measurements were taken at three different regions along the plasma, relative to the gas flow direction and the location of the high frequency antenna: position H: upstream, the inlet region before the antenna; position G: the plasma center, at the center of the antenna; position F: downstream, beyond the antenna. Depositions were performed on a well defined graphite (industrial A TJ) substrate attached to a stretched feedthrough, at position F of the plasma.
m. RESULTS AND DISCUSSION
ft.. Characterization of the plasma
Table I shows the mass spectrometric species behavior along the propylene-argon microwave plasma indicating that the propylene concentration decreases immediately after the plasma initiation and new compounds appear. It is seen that in position G, the products concentration is twice their concentration compared to position H. This increase is accompanied by an equivalent decrease of 50% in the propylene concentration. In position F the propylene concentration is reduced to 60% of its concentration in position G, whereas the product concentration showed an increase of only 13% indicating the further propagation of the reaction to produce the deposit.
Figure 1 shows the electrical characteristics of the plasma. A DFPS 10. 11 was applied to a microwave plasma of methane and argon and the ion and electron densities together with the electron temperature along the plasma were recorded. Although not of the same hydrocarbon, both systems share a common behavior indicating similar concentration profiles along the plasma. Thus the plasma can be modeled into two regions according to the behavior of the compounds. The HG region, where a high decomposition rate of hydrocarbon
375 J. Vac. Sci. Techno!. A 4 (3), May/Jim 1986 0734-2101186/030375-04$01.00 @ 1986 American Vacuum Society 375
•• ' ••••• n .y ....... H ....... _ ••••• _ ........ " .-•••••••• ,_._._ .... 'n ••• Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.76.6 On: Tue, 02 Dec 2014 17:57:50
376 Inspektor et al. : Deposition of pyrocarbon
TABLE I. The concentration ratio of plasma constituents in positions G-H and positions F-G.
Concentration at G Concentration at F Mass
No. Mass assignment Concentration at H Concentration at G
1 2 H2 1.75 1.3 2 15 CH, 1.9 1.0 3 16 CH. 2.0 1.3 4 26 ~H2 2.2 1.2 5 28 C2R. 1.S 0.9 6 42 C,H. 0.5 0.6 7 50 C.H2 2.8 1.2
exists and an increase in the positive ion density is observed. The F region (downstream) where the concentration ofthe products stabilizes, a solid deposit is observed on the reactor walls and a decrease of positive ion density was measured. Addition of argon to the feed causes an increase in the electron temperature and of the positive ion density in this region, as shown in Fig. 2.
The pyrolytic carbon (PyC) deposit is the final product of polymerization and poiycondensation of the active particles of the plasma. These reactions occur through an ionic mechanism 10 and the process reaches its maximum conversion in the F region. The results of these findings lead to the conclusion that the substrates must be located in this region.
B. Characterization of the PVC
During the process the substrate was electrically grounded and its temperature was about 350 ·C. The electrically insulated reactor walls were covered by a thin layer of high hydrogen content (3%-10%) carbon deposit. This precipitate was found to be amorphous with a density of 1.1-1.5 gI
(n ) e 310rr ,
S? x 5.0 (n,)
1 410rr
Hl·
0.5
I I
0<R:3 I / C~14
1/ I I I j
I I i
l ( (
I I J
0,1 LI_---,, ___ " ---rl ~'I-....-----.-----'----H G F H G F
samptir:g positions -.._
FIG. 1. Ion density (n, ) and electron density (n, ) in various positions along the plasma. CR. flow rate 47 sccm; microwave power 100 W; R = (Ar]! ! CR.] ratio.
J. Vac. ScI. Techno!. A, Vol. 4, No.3, May/Jun 1986
376
~--.----
I
t
G
'"r Vi ,
co '"
!l)
f-
ft 30 QJ
'" R - 2
'" C ;,
'" R = 1 u
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I
101
CH4
_.L_-J __ ~._~. I
6 8
gas pTessuTc (torr)
FIG. 2. Electron temperature at position F VB gas pressure and the ratio R = [AI J! [ CH. J as a parameter. CH. flow rate 47 secm; microwave power 100 W.
cm3• The substrate, however, was coated by. a hard black deposit which was identified by x-ray diffraction as pyrocarbon having a graphitic imperfect hexagonal structure (4H) and unit cell a = 2.456 A, c = 6.696 A.
The variation in the formation rate of dense deposition with propylene concentration at constant pressure (5 Torr) and rfpower input (600 W) is shown in Fig. 3. The results reveal two maxima at 8 and 18 vol % propylene, with the general trend showing that admixing argon in the feed increases the deposition rate.
For an rf plasma of a gas mixture containing 16.6 vol % propylene and at 600 W power input, a parabolic dependency of the deposition rate versus gas pressure was found. A maximum deposition rate was found at a pressure of 6.5 Torr. Under similar conditions, the maximum precipitation
12.0C-
r-, .-< 10.0
1 ..c
E'l ;.1 8.0-'-'
30
/ .
• - FIe If == 7.6 cclSTPJ o m5.n- 1
3 6 At. - F1C3H6 '" 25.0 cc[STP}.min-
1
I I I----L-20 10)
Propylene conc. (vol%)
FIG. 3. Deposition rate vs propylene concentration in the feed. n power 600 W; total gas pressure 5 Torr.
·······f··· . Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.76.6 On: Tue, 02 Dec 2014 17:57:50
377 Inspektor et at: Deposition of pyrocarbon
:: ::r_-.,--,--,---,-----r--,---~~[ ~ 1
0.07
.... '.c 0.06
£; 0.05 '" '-;;:
.::: 0.04 - I .;.> I
'" I o I ~O.03 - I
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or i ,., L
~J ~.
gas pressure (Torr)
FIG, 4. Deposition rate of dense PyC and the precipitate vs gas pressure. rf power 600 W; [Ar]![CH4 ] ratio = S,
rate of carbonic material on the reactor walls was obtained at a pressure of2.S Torr. The two maxima are shown in Fig. 4.
The composition ofthe dense carbon deposit was found to consist of two components8 whose ratio depends on working parameters: a hard high qensity material (1.8-2.1 s/cm3
)
and a soft carbon-black-like component (1.2-1.6 g!cm'). A maximum of the hard material was obtained at a pressure of 7.0 Torr, whereas the maximum of the soft material was at 2.5 Torr, i.e., in the same pressure range as the carbonic precipitants. The density of the dense PyC (deposited on the grounded substrate) as a function of propylene concentration in the feed presented in Fig. 5 shows that the addition of argon increases the densities of the PyC up to 2.1 gl cm3
•
The density of the similar black and dense carbon obtained by chemical vapor deposition (CVD) is dependent on the process temperature l4
,15 and its structure consists of a mixture of tangled and layered components.
2,0 -
... 1.9 , "' u .,;,
1.8
'" p . r< if-f:: I.]
'" '0
1.6
:.5
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Propylene cone. (vol%)
FIG. S. Density of the PyC coatings vs propylene concentration in the feed. rfpower 600 W; tow gas pressure 5 Torr.
J. Vac. Sci. Technol. A, Vol. 4, No.3, May/Jun 1986
...... -.... ,.', ......... -.-... -." .... -~ .. ; ... -... , ..•................... -....•.•...•.......
N , fj
377
FiG. 6. Microhardness and OPTAF vs density. [C3H6l = 5-30 vol %; rf power 600 W; gas pressure 5 Torr.
Microhardness measurements and optical anisotropy factor (OPT AF) of the produced coatings exhibit a linear increase with the density increase. The relationships between the density and the microhardness and between the density and the OPT AF of the coating are illustrated in Fig. 6. Higher values of microhardness correspond to higher densities and are in agreement with the observation reported by Koizlik et al. on CVD PyC. 16 The coating's characterization of the plasma PyC coatings, i.e., density, structure, and anisotropy, show that they are similar to those received at CVD systems at temperatures higher than 1000 °C and to those obtained by glow discharge systems with a substrate potential of 700 V and higher, although in this research the sub~ strate was grounded and its temperature reached only 350°C.8
IV. CONCLUSION
The rf plasma formed PyC was found to have similar properties to those obtained by the CVD technique. The advantage of the rf plasma technique is its ability to produce an equally good quality ofPyC deposits at moderately low processing temperature, thus enabling the coating of thermally sensitive materials which cannot be processed by the CVD technique. Furthermore, the higher deposition rate of the rf plasma technique, which is at the beginning of its research is already higher than that obtained by the other technique . Both the above advantages, lower working temperature, and higher deposition rate give the ability to produce PyC coat-ings at lower cost (energy and time). .
oj Ben Gurion University of the Negev, p, O. BOll 653, Beer Sheva 84120, IsraeL
lR. W. Kidd, D. V. Seifert, and M, F. Browning, in Emergent Process Methods for High-Technology CeramiCS, edited by R. F, Davis, H, PalmOUf III, and R. L. Porter (Plenum, New York, 1984), ppo 381-396.
2W. V. Kotlensky, Chern. Phys. Carbon 9,173 (1973).
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378 Inspektor et at: Deposition of pyrocarbon
3R. J. Diefendorf, J. Chern. Phys. 57, 815 (1960). 4y' L. Khait, A. Inspektor, and R. Avni, Thin Solid Films 72, 249 ( 1980). 'y. L. Khait, U. Carmi. A Inspektor, and R. Avni, Symposium Proceedings ISPC-7, 7th International Symposium on Plasma Chemistry, edited by C. J. Timmermans (Eindhoven University of Technology, Eindhoven, The Netherlands, 1985). .
6A. Raveh, M. Eldan, A. Inspektor, U. Carmi, and R. Avni, Thin Solid Films 105,39 (1983).
7 A. Raveh, M. Eldan, A. Inspektor, and R. Avni, Carbon 23, 179 (1985), 8A. Inspektor, Y. Hornik, U. Carmi, R. Avni, E. Wallura, H. Hoven, K. Koizlik, and H. Nickel, Thin Solid Films 72, 195 (1980).
9U. Carmi, A, Inspektor, and R. Avni, Plasma Chern. Plasma Process. 1, 233 (1981).
!OA Inspektor, U. Carmi, R. Avni, and H. Nickel, Plasma Chern. Plasma
J, Vac. Sci. Technol. A, Vol. 4, No.3, May/Jun 1986
378
Process. 1. 377 (1981). "u. Carmi. A. Inspektor, R. Avni, and H. Nickel, Kernforschungsanlage
Jiilich Report No. 1499, Jiilich, 1978. 12R. Avni, D. Carmi, and A. Inspektor, Symposium Proceedings ISPC-7,
7th International Symposium on Plasma Chemistry, edited by C. J. Timmerrnans (Eindhoven University of Technology, Eindhoven. The Netherlands, 1985), Vol. 4, p. 1389.
ny. L. Khait, U. Carmi, and R. Avni, Symposium Proeeedings ISPC-6, edited by M. I. Boulos and R. J. Munz (1983), Vol. 3, pp. 729-736.
14J. L. Kaae, Carbon 13,55 (1975). "C. S. Yust and H. P. Krautwasser, Carbon 13, 125 (1975). l6K. Koizlik, H. Luhleich, and E. Wallura, KFA Report No. 1269, Jiilich,
1976.
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