A program to grow thin carbon films and investigate their physical and electrical properties is described. Characteristics of films grown by rf sputtering and vacuum arc deposition on p type, (100) silicon wafers are presented. Microhardness data were obtained from both the films and the silicon via the Vickers diamond indentation technique. These data show that the films are always harder than the silicon, even when the films are thin (of the order of 1000 A). Vacuum arc films were found to contain black carbon inclusions of the order of a few microns in size, and clusters of inclusions of the order of tens of microns. Transmission electron diffraction showed that the films being studied were amorphous in structure

Kolecki, J. C.

English

NASA Technical Reports Server

NASA Technical Memorandum 82980 
NASA-TM-82980 19830004898 
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Microhardness Studies on Thin Carbon Films 
Grown on P-Type, (100) Silicon 
Joseph C. Kolecki 
Lewis Research Center 
Cleveland, Ohio 
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MICROHARDNESS STUDIES ON THIN CARBON FILMS GROWN ON 
P-TYPE, (100) SILICON 
Joseph C. Kolecki 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 
ABSTRACT 
A program to grow thin carbon films and investigate their physical and 
electrical properties has been started at NASA Lewis Research Center. This 
paper presents characteristics of films grown by rf sputtering and vacuum 
arc deposition on p-type, (100) silicon wafers. Microhardness data were 
obtained from both the films and the silicon via the Vickers diamond inden-
tation technique. These data show that the films are always harder than the 
silicon, even when the films are thin (of the order of 1000 A). Vacuum arc 
films were found to contain black carbon inclusions of the order of a few 
microns in size, and clusters of inclusions of the order of tens of 
microns. Transmission electron diffraction showed that the films being 
studied were amorphous in structure. 
INTRODUCTION 
The growth of crysta 11 i ne carbon fi lms in vacuum has long been regarded 
as a theoretical possibility, though success in achieving it has been limit-
ed. Such films, referred to as diamondlike, would have practical applica-
tions as abrasion resistant coatings and, when doped with elements like 
boron, as semiconductors, where the high thermal conductivity associated 
with the diamondlike nature of the film would allow ready power dissipa-
tion. A program to ~row carbon films and investigate their physical and 
electrical properties has been started at NASA Lewis Research Center. This 
paper is concerned chiefly with Vickers microhardness studies of carbon 
films grown on crystalline silicon substrates. Gambino and Thompson, in a 
paper on spin resonance spectroscopy (ref. 1), include a curve of Vickers 
microhardness versus load, which differs somewhat in numeric values but 
shows trends very similar to those being reported here. In general, it 
appears that the microhardness of the carbon films exceeds that of the sili-
con over a film thickness range of 1200 to 4600 A. The films reported by 
Gambino and Thompson, and those reported in this paper, were all amorphous. 
EXPERIMENTAL 
Microhardness was measured by the Vickers diamond indentation method. 
This method consists of loading a small diamond-tipped stylus with a known 
weight and allowing it to penetrate freely the surface of the material being 
measured (fig. 1, ref. 2). The indentation, when viewed from above, is a 
square with well defined diagonals. 
Measurement of the diagonal length and use of a conversion table yields 
a numerical ~alue of microhardness with units of kilogram per square milli-
meter (kg/mm). Because the indent size is of the order of micrometers, a 
microscope with a reticle eyepiece is used to perform this measurement. The 
actual measurement of carbon film and silicon substrate microhardness was 
performed with a Buehler Micronmet Microhardness Tester (ref. 3). Stylus 
loads in these measurements ranged from 25 to 200 g. 
Carbon films were grown by two techniques: RF sputter deposition with a 
carbon target and 45- to 50-~m backpressure of methane gas and vacuum arc 
deposition using a carbon arc operating in a background pressure of 10-5 
to 10-6 torr at approximately 22 V and 50 A (ref. 4). The silicon 
substrate material was polished p-type silicon, with an electrical re-
sistivity of 2- to 7-n cm and with a (100) surface orientation. Silicon 
was chosen because of the readiness with which the carbon films grew and 
adhered (ref. 4). Glass and metal substrates were also tried, but with 
limited success. Films grown on glass and metal fractured and spalled much 
more frequently than those grown on silicon, and their adhesion was usually 
poor. Hardness measurements were performed on both the carbon films and on 
the silicon substrates. Additionally, film step heights and surface pro-
files were obtained with a Tencor Alpha-Step Surface Profiler (ref. 5). 
RESULTS 
Vickers microhardness measurements were performed on the (100) silicon 
substrates and on the deposited carbon films, making careful note of the 
orientation of the indent with respect to the (100) surface axes of the 
silicon in all cases (fig. 2). When the diagonals of the indent were par-
allel to the surface axes, the orientation was taken to be 0°; otherwise, 
the orientation was taken to be the smallest angle formed by the diagonals 
and the surface axes. This definition works well for the case of a (100) 
surface since the axes in this surface are orthogonal. When other surfaces 
are used (such as (111) in which the axes are trigonal), some other 
definition must be provided, or more practically, a different indenter, such 
as a Knoop indenter (ref. 2), substituted for the Vickers indenter. 
The measured microhardness of the silicon for a given load was found to 
vary with orientation, increasing in value with the angle (fig. 3). Corner 
cracking and concoidal edge fracturing also became more severe with in-
creasing angle. It was found that at 00 , a clean indent with some corner 
cracking and essentially no edge fracturing was obtained~ In this orienta-
tion, a 200-g load yielded ~ microhardness of ~930 kg/mm. When the 
indenter was rotated to ~20 , the amount of corner cracking and edge 
fract~ring increased, and the 200 9 load yielded a microhardness of ~980 
kg/mm. When the indenter was further rotated to 45 , the combined 
effects of cracking and fracturing became so severe that often no measure-
ment could be made. Where measurement was made, 200 g yielded a microhard-
ness ~1050 kg/mm2• Damage to the silicon substrate occurred in this 
orientation even when the indent was made in an overlying carbon film rather 
than directly on the silicon surface. 
Two sets of microhardness data are presented in figure 4. Figure 4(a) 
shows data for an rf sgutter-deposited film 3500 A thick, taken with the 
indenter oriented at 0°; figure 4(b) shows data for an rf sputter-deposited 
film 4100 A thick, taken with the indenter oriented at ~30°. Data for the 
p-type (100) silicon substrates at the same respective indenter orientations 
are also given. 
In the data of figure 4(a), the numerical value of microhardness is seen 
to increase with decreasing load below a load of 100 g for both the film and 
the substrate. This effect is discussed for brittle crystalline substrates 
by Brookes (ref. 6). Applying Brookes' argument, surface lattice damage, 
resulting from the (100) silicon surface being polished to a mirror-like 
2 
finish before deposition, can be assumed to result in the surface being 
somewhat harder than the underlying bulk material. Larger loads cause the 
indenter to penetrate more deeply into the bulk material so that more of the 
bulk characteristics are seen. Smaller loads, on the other hand, penetrate 
less deeply so that as the load is decreased, more and more of the surface 
characterisitics come into view. The bars in this figure represent standard 
deviations calculated from 18 to 24 measurements per datum. 
In the data of figure 4(b), the numerical value of microhardness is seen 
to increase with decrp.asing load for the carbon film, but not for the sili-
con substrate. The behavior of the carbon film is not significantly differ-
ent in this case than in the case of figure 4(a). Since the change of in-
denter orientation affects the silicon only, it may be inferred that the 
film behavior shown is indeed characteristic of the carbon film itself. The 
behavior of the silicon is also interesting because, if surface lattice dam-
age is indeed a factor in the measurements, its effects are not so readily 
apparent here as in figure 4(a). Since the indenter orientation has this 
effect, it might be that surface cleavage is also playing a part in these 
measurements. 
The shape of the film curves in figure 4 is explained by considering the 
depth-of-stylus-penetration to film-thickness ratio for a given stylus 
load. If this ratio is much greater than unity, the stylus has penetrated 
well into the substrate and the resultant measurement might be expected to 
be more characteristic of the substrate than of the film. If the ratio is 
small, on the other hand, the stylus has seen less of the substrate and, 
consequently, the measurement may be expected to be more characteristic of 
the film itself. For the geometry of the diamond indenter used here (ref. 
2), the ratio of stylus penetration to film thickness can be shown to vary 
from ~10:1 for a 1200 A film with a 200 g load to ~1.2:l for a 4600 A 
film with a 25 9 load. Thus, the hardness numbers obtained from thick films 
under light load are probably most characteristic of the carbon films 
alone. Nonetheless, even thin films under heavy load showed small 
differences in hardness from the underlying silicon always in the direction 
of greater film hardness. 
Film samples were subjected to electron diffraction and electron micro-
scopy. Preparation for electron diffraction involved dissolving the silicon 
substrate so that the carbon film would float freely. Transmission electron 
diffraction patterns could then be obtained. A typical pattern is shown in 
figure 5. The obvious lack of well-defined spots and/or sharp rings indi-
cates an amorphous structure. The actual surface structure is seen in the 
scanning electron micrograph (taken at 10,000X) of figure 6. 
More data of microhardness versus load for rf-sputtered carbon films, 
ranging in thickness from 1200 to 4600 A, are presented in figure 7(a). 
These data were all taken with the indenter oriented at 0°, and all show the 
same variation with decreasing load seen previously. Data for silicon at 0° 
are included for comparison. A crossplot showing microhardness versus film 
thickness (fig. 7(b)) shows no appreciable variation except for a slight 
increase of microhardness with increasinq film thickness. 
Carbon films qrown on silicon with the vacuum arc were also measured. 
(The arc configur~tion used is fully described in ref. 4). These films had 
many dark inclusions which appeared to be bits of black carbon or graphite 
and which probably resulted from particulate ejecta from the cathode. In-
clusions ranged in size from a few tenths to several micrometers. Clusters 
of inclusions also occurred, some being a loose aggregation of smaller in-
dividuals, others being large, tight, opaque islands. Clusters ranged in 
3 
size from 10 to 50 ~m. The rf sputter grown films, by comparison, contained 
no such inclusions or clusters. A trace of a vacuum arc film profile (fig. 
8) shows that individual inclusions often protruded several tenths of a 
micrometer above the surface of the film. 
Attempts to measure microhardness of clusters were not successful since 
the clusters were torn away by the indenter stylus, leaving a hole in the 
film. Data were successfully collected from the clear regions between in-
clusions and clusters, and typical data are presented in figure 9. These 
data were obtained from a film 2800 A thick. No essential differences in 
behavior from the RF sputter grown films of the ~revious figures are evi-
dent. As before, the indenter orientation was O. The corresponding curve 
for silicon is included for comparison. 
CONCLUSIONS 
Microhardness data were obtained from thin carbon films grown on p-type, 
(100) silicon of 2- to 7-n-cm electrical resistivity using rf sputter and 
vacuum-arc deposition. These data show that the film hardness is somewhat 
greater than that of the (100) silicon for a film thickness range of 1200 to 
4600 A and a load range of 25 to 200 g. Film hardness numbers were found to 
increase with decreasing indenter load for all orientations of the stylus 
with respect to the (100) surface axes of the silicon, and showed a weak 
tendency to increase with film thickness over these same ranges. Radio-
frequency sputter-deposited films were found to be considerably more uniform 
in structure than vacuum arc-deposited films in terms of black carbon and/or 
graphite inclusions, which occur in large quantities in the vacuum arc 
films. Both the rf and the vacuum arc films were found to be of comparable 
hardness, however. Transmission electron diffraction patterns, obtained 
after the silicon substrates were dissolved away, showed that the carbon 
films were amorphous in structure. 
REFEREMCES 
1. Gambino, R. J. and J. A. Thompson: Spin Resonance Spectroscopy of 
Amorohous Carbon Films. Solid State Commun., Vol. 34, No.1, Apr. 1980, 
pp. 15-18. 
2. Petty, E. R.: Hardness Testing. Techniques of Metals Research, Vol. V, 
R. F. Bunshah, ed., Interscience Publishers, 1971, pp. 158-215. 
3. Buehler Ltd.: Hardness/Physical Testing Equipment. Buehler Analyst 
Section 13, Buehler Ltd., 1980. 
4. Domitz, S. and J. Kolecki: Vaccuum Arc Deposited Diamondlike Films.1I 
to be published 
5 •. Tencor Instruments, 2426 Charleston Road, Mountain View, CA 94043, 
(415) 969-6767. 
6. Brookes, C. A.: Indentation Hardness. The Properties of Diamond, J. E. 
Field, ed., Academic Press, 1979, pp. 383-402. 
4 
////// 
.L. Corner 
cracking 
Load 
Figure 1. -Indentation hardness testing. 
Oi rection of noo) 
su rface axes 
Orientation at rP 
Concoldal 
edge 
fracturi ng .., 
I 
Figure 2. -I ndenter orientation effects. 
Indentation 
N 
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E 
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0 
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1400 
N 1300 E 
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~ 1200 
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e 
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1050 
1000 
950 
900 0 10 
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20 30 
Orientation, deg 
o 
o 
50 
Figure 3. - Variation of microhardness with indenter or-
ientation measured with a 200 gm load. 
Upper curve: carbon film on silicon 
Lower curve: (100) silicon 
Film thickness = 3500 A 
Indenter orientation = 00 
200 0 
Load, gm 
(b) 
Upper curve: Carbon film on silicon 
Lower curve: (m) silicon 
Fil m thickness = 4100 A 
I ndenter orientation = 300 
200 
Figure 4. - Plots of microhardness vs load. 
Figure 5, - Transmission electron diffraction pattern of carbon film, 
[lm 
Figure 6. - Scanning electron photo-micrograph of carbon film surface, 

N 
E 
E 
en 
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c: 
"0 
.... 
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.... 
u 
~ 
4000 
3000 
2000 
-1000 '---_--L_--' __ -'--_---'-_---" 
o 15 30 45 60 75 
1300 
1200 
llOO 
1000 
900 
0 
~m 
Figure 8. - Surface profile of vacuum arc 
film showing inclusions. 
T = 2800 II 
0 0 Film on silicon 0 Silicon 
0 0 
0 
0 
0 
0 0 
0 0 § 
0 
0 
100 200 
load, gm 
Figure 9. - Microhardness vs load for typical 
vacuum arc film. 
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 
NASA TM-82980 
4. Title and Subtitle 5. Report Date 
MICROHARDNESS STUDIES ON TmN CARBON FILMS November 1982 
GROWN ON P -TYPE, (100) SILICON 6. Performing Organization Code 
506 -55 -72 
7. Author(s) 8. Performing Organization Report No. 
Joseph C. Kolecki E-1410 
10. Work Unit No. 
9. Performing Organization Name and Address 
National Aeronautics and Space Administration 
11. Contract or Grant No. Lewis Research Center 
Cleveland, Ohio 44135 
13. Type of Report and Period Covered 
12. Sponsoring Agency Name and Address Technical Memorandum 
National Aeronautics and Space Administration 14. Sponsoring Agency Code Washington, D, C. 20546 
15. Supplementary Notes 
16. Abstract 
A program to grow thin carbon films and investigate their physical and electrical properties 
has been started at NASA Lewis Research Center. This paper presents characteristics of 
films grown by rf sputtering and vacuum arc-deposition on p-type, (100) silicon wafers. 
Microhardness data were obtained from both the films and the silicon via the Vickers 
diamond indentation technique. These data show that the films are always harder than the 
silicon, even when the films are thin (of the order of 1000 A). Vacuum arc films were 
found to contain black carbon inclusions of the order of a few microns in size, and clusters 
of inclusions of the order of tens of microns. Transmission electron diffraction showed that 
the films being studied were amorphous in structure. 
17. Key Words (Suggested by Author(s)) 18. Distribution Statement 
Carbon film; Vickers microhardness; Unclassified - unlimited 
Indent microhardness; RF sputter STAR Category 23 
deposited; Vacuum arc deposited 
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price' 
Unclassified Unclassified 
* For s.ale by the National Technical Information Service, Springfield, Virginia 22161 
End of Document 


Microhardness studies on thin carbon films grown on P-type, (100) silicon

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