Hard carbon and hydrogenated amorphous carbon films, prepared by a hybrid ... elasticity and conductivity have been correlated against deposition conditions, ...
Surface and Coatings Technology, 47(1991) 315—326
315
Physical properties of carbon films produced using a hybrid physical vapour deposition technique P. Holiday, A. Dehbi-Alaoui and A. Matthews Research Centre in Surface Engineering, Department of Engineering Design and Manufacture, University of Hull, Hull HU6 7RX (U.K.)
Abstract Hard carbon and hydrogenated amorphous carbon films, prepared by a hybrid thermionically assisted electron beam physical vapour deposition system have been deposited at substrate temperatures between 100 and 540°Cand negative substrate bias voltages of 0—1000 V (d.c.) or 2500 V (r.f., d.c. offset). A variety of substrates have been used, including glass, tool steel (ASP23), mild steel and silicon. The nanoindentation hardness, adhesion, thermal stability, elasticity and conductivity have been correlated against deposition conditions, such as substrate bias voltage, current density, temperature, process gas, annealing temperature and hydrogen content. The stresses in the film have also been studied. Structural analysis has been undertaken using a scanning electron microscope to study the development of the film structure and the effect of the process gas (butane or argon) and this work has confirmed the crucial importance of hydrogen in the nucleation, growth conditions and its contribution to the physical properties.
1.
Introduction
After Aisenberg and Chabot [1] deposited amorphous diamond-like carbon (DLC) coatings in 1971, such films have been produced by a variety of deposition methods including plasma-assisted chemical vapour deposition (PACVD) or physical vapour deposition (PVD) and ion beam techniques from a variety of solid or gaseous carbonaceous sources. The concentration of hydrogen within these films is highly dependent upon the precursor materials and process conditions. This concentration is reported to have a significant effect on the residual stress, elastic behaviour and conductivity of the film. It is widely believed that the hydrogen is responsible for the linking of the unoccupied bonds, although hydrogen may be present in an unbonded or “free” form. The presence of hydrogen, although important for the nucleation and for increasing growth rate, is not entirely essential for the production of DLC. It is possible to grow hard carbon in the absence of hydrogen, e.g. by ion beam methods as demonstrated by Aisenberg and McKimock [2]. Depending upon the method, however, the inclusion of other unbonded gaseous elements, such as argon, can occur and remain in the films because of their closely packed nature [3]. The contribution of impurities, such as hydrogen and argon, on the mechanical, electrical and optical properties thus remains a matter of considerable interest. 0257-8972/91/$3.50
©
Elsevier Sequoia/Printed in The Netherlands
316 Substmt.
Fig. 1. Schematic layout of electron beam PVD equipment: EB, electron beam.
2. Experimental details 2.1. Film deposition Hard carbon films were prepared for this study by a hybrid thermionically assisted PVD technique (Fig. 1). This combines the evaporation of solid graphite from a differentially pumped, 225°bent beam electron gun into an r.f. or d.c. glow discharge at a pressure of typically 10 mTorr. The process gases used were argon for amorphous carbon (a-C) and butane for hydrogenated amorphous carbon (a-C:H) films. The substrates were mounted on the cathode and the substrate potential was controlled at a fixed value, during deposition, of 0 and 1000 V (d.c.) or 2500 V (r.f., d.c. offset). These produced maximum current densities of between 0.9 and 13.7 mA cm2 respectively. The substrates were suspended approximately 300 mm above the graphite source and about 100 mm offset to one side. Their temperature was measured using a NiCr—NiAI type K thermocouple. A filament made from tungsten wire ~0.5 mm in diameter and 380 mm long was positioned 150 mm below the substrate and was held at a temperature of 1750-4875 °Cduring d.c. deposition (as recorded with an optical pyrometer). It was not possible to run an r.f. discharge with thermionic assistance since the increase in electron activity caused breakdown in insulation, resulting in excessive arcing. Adhesion Scratch testing using a Leitz Miniload hardness tester was employed to assess the adhesion, with the diamond indenter locked in place. A series of scratches were produced, at increasing loads. By optical observation of the scratch, a critical’ load was identified which gave a relative indication of coating adhesion. 2.2.
317
2.3. Residual stress Stress values for the films were obtained by measuring the curvature of a 4 in, single-crystal silicon Si( 100) wafer using a Form Talysurf 50 profllometer. Initially the silicon wafer is evaluated to establish the inherent curvature before deposition, which together with the deflection after deposition, establishes the net deflection. The stress was then calculated by employing the formula reported by Maissel and Glang [4] (5 E d~2 ar23(1)d
where (5 (cm) is the wafer deflection, r (cm) is the radius of scan, a (dyn cm-2) is the stress, E (1.33 x 1012 dyn cm-2) is Young’s modulus for silicon, v (=0.28) is Poisson’s ratio for silicon, df (cm) is the thickness of the film and d, (=0.05 cm) is the thickness of the substrate. 2.4. Hardness It has been reported that for thin films (tf < 1. ~.tm)the penetration depth during hardness testing should not exceed 10% of the film thicknese [5]. This is necessary in order to obtain a hardness value which is related to the film properties and not influenced by the substrate. To achieve this, the analysis of the films was performed using a Nanotest ultralow load hardness tester, made available by Micro Materials Ltd. This registers the displacement of a three-sided pyramidal diamond indenter, by means of a capacitive transducer. It gives a continuous record of depth against load, during both loading and unloading. From this, the elastic behaviour can be investigated by studying the hysteresis between the loading displacement and unloading displacement plots (Fig. 2). Conductivity Conductivity changes can give a useful indication of the nature of carbon films. Diamond-like films, which have a characteristic strongly covalent bonding, have an absence of free electrons which can give high resistivity and high optical transparency, since transparency is also closely linked to the absence of “free” electrons. Measurements of d.c. resistance were taken by recording the voltage— current characteristics of the film using the Van der Pauw technique. A constant current is applied across two adjacent contacts, whilst the resulting potential is detected across the two remaining contacts. The resistance is then measured in the other two possible configurations giving an average resistance. According to ASTM standards, the process was repeated for the other two remaining permutations. The values are then put into the Van der Pauw formulae, which takes account of geometrical irregularities and coating thickness, as follows: itt Ra+Rb (Ra ~ln2 2 1k.,Rb 2.5.
318 500
400
300
dp
~
7
Hardness GPa
\\\
Load /100 (mN)
/1 \~\
//
~
I
~
dh
..9 ~
,
100~
1
~-
/
,1
.-,&__
—,
~~1 I /
I
/
/ 0~
0
I.
.
50
100
~
ht
150
200
Depth (nm) Fig. 2. Hardness (El) and
_____________
(•)
load vs. indentation depth for r.f.-deposited a-C:H films.
~
~
(a)
~
(b)
Fig. 3. Scanning electron micrographs of fracture sections: (a) a-C:H, 1500 V (d.c.), evaporated graphite + butane; (b) a-C:H, 1500 V (r.f.), evaporated graphite + butane.
where f(Ra/Rb) is a geometrical constant, Ra is the (parallel) resistance, t is the film thickness, p is the resistivity and Rb is the (perpendicular) resistance. 2.6. Structure and thickness Structural examination of fracture sections of the films was undertaken with a Cambridge Stereoscan 200 scanning electron microscope (Fig. 3) and this provided a further check on thickness, in addition to a Talystep profilometer.
:
a,, a,, a,, a,, a,, a,, a,,
319
3. Results and discussion The a-C films (Table 1) produced in a d.c. argon discharge on glass were both soft and extremely thin (deposition rate, 0.2—0.8 g.tm h~).They had poor adhesion, low resistivity and poor transparency. No net deposit was produced on the tool or mild steel probably owing to excessive etching. The inclusion of butane, however, altered several parameters, most notably the deposition rate, adhesion and resistivity. The deposition rate increased dramatically to 4 ~tm h1, although at the same time the structure became brittle and very fragile. Only a slight increase in hardness as a result of increased substrate potential was noted. Again, it was not possible to deposit onto ASP23 or mild steel. The use of an r.f. plasma improved many film qualities. The films in argon remained very thin (deposition rate less than 1 ~tmh’) but adhered well to glass and silicon and were hard (24.4 GPa (about 2500 kgf mm-2)). They are dense and amorphous in structure but optically transparent [6]. Adhesion to tool or mild steel was extremely poor in these conditions. With the addition of butane into the plasma, the hardness once again improved dramatically (35.5 GPa (about 3600 kgf mm2)), as did the adhesion and deposition rate (3 j.tm h’). The most prominent change was the increase in elastic modulus of these films, from 72.4 to 126.9 GPa with the introduction of hydrogen atoms, making microhardness measurement very problematic. This elastic behaviour was also witnessed on tool and mild steel which could now be coated. The optical transparency decreased, however. In both r.f. deposition cases, the films were dense and amorphous and had a high resistivity. Figures 4 and 5 show how the inclusion of butane increases the adhesion of both d.c. and r.f. films. In addition, it can clearly be seen that the bombardment intensity of the arriving species influences the adhesion. With a greater overall impact energy, the interdiffusion at the interface will be enhanced. Increased substrate temperature can also provide another mechanism for improved adhesion. This is also related to the power, as well as to the pressure and the process gas. The temperature increase in this case, however, did not exceed 550 °C for any of the deposition processes. The introduction of thermionic emission during deposition increased the substrate temperature only slightly, since the dominant heating effect appeared to be from the plasma itself. Some films were so sensitive to delamination that this occurred within minutes of deposition. It has been found that these films have a low intrinsic stress (Fig. 4) and therefore it seems likely that delamination is caused by the diffusion of moisture into the film—substrate interface [7]. This is further confirmed by observation of debonding initiated at an edge or discontinuity. Residual stress measurements were performed on films deposited by the decomposition of butane in an r.f. plasma, with the addition of evaporated graphite. The deposition temperatures varied between 100 and 380 °C,for a d.c. offset voltage of 500—1500 V. The stress levels recorded appear lower, at
D.c.
D.c.
R.f.
R.f.
R.f.
Ar + graphite
Butane + graphite
Ar + graphite
Butane + graphite
Butane + graphite
of film properties
Plasma
1
Process
Summary
TABLE
and mild steel
Tool steel
and mild steel
Tool steel
and mild steel
Tool steel
Glass and Si
Glass and si
Substrate
elastic (E = 56.4 GPa)
Hard (14.5 GPa);
elastic (E = 126.9 GPa) tool eteel, poor
Mild steel, excellent;
Good
Transparent
10-‘-W
Poor
Hard (24.4 GPa); ductile Very hard (35.5 GPa);
Yellow-brown
IO-102
Good
10s
10-104 (prey) Brown-black
Semi-transparent
Grey-black
10-2-W
Poor
Soft; brittle-fragile
Appearance
Soft; ductile
Resistivity Rem
Adhesion
Hardness
1
3
1
4
0.2-0.8
(w
h-l)
Deposition rate
Dense, amorphous
Dense, amorphous
Dense, amorphous
Dense, amorphous
Dense, amorphous
Structure
321 600
500
400~
Critical Nonnal
‘a
• DC + butane
Force (mN) 300
S
DC+aigon
200
a
1°:
~
0
a
Current Density mA/cm Fig. 4. Adhesion vs. current density for d.c. films on glass:
•, butane;
El, argon.
2000 1750 1500 1250
Critical Normal
__________
Force (mN) 1000
C
RF + butane
S
RF+argon
750 500 250 S
0
•
0
2
•
4
6
8
10
12
14
16
18
20
2
Current DensIty mA/cm
Fig. 5. Adhesion vs. current density for r.f. films on glass:
•, butane;
El, argon.
room temperature, than those values published by Grill et al. [8], whose (r.f.) PACVD a-C:H films were approximately 60% higher in stress magnitude (1.6 dyn cm-2). The stresses have only a slight dependence on deposition temperature. Figure 6 presents the stresses of one of these a-C:H films at 1500 V bias, in both the as-deposited and the pre-annealed state. It is clear
322 2.0
1.5
[a] 1.0
•
0
[b]
100
200
300
Annealing Temperature
400
500
C
Fig. 6. Internal stress vs. annealing temperature: El, curve [a], data from Grill et al. [8]; curve [b], a-C:H, r.f., electron beam PVD, 1500 V.
•,
that this film, upon annealing, follows the same stress-relieving profile for increasing annealing temperatures as that reported by Grill et al. That is to say, at approximately 450 °Cthe stress has reduced by 80% of that of the room temperature value, whilst between 450 and 500 “C the film has become fully annealed. This stress reduction can be understood as the structural reordering through bonding rearrangements caused by the onset of graphitization. It was expected that some stress reduction would result, at intermediate temperatures, from the effusion of unbonded or “free” hydrogen but this was not detected. Figures 7—10 show the conductivity or resistivity against deposition temperature, annealing temperature and substrate bias. Figures 7 and 8 show a small difference between the magnitudes for a-C and a-C:H films, depending upon the deposition conditions. It is reported that generally a-C:H films have a higher resistance than a-C by 12 orders of magnitude [9]. The results also indicate that for all films, the conductivity increases with increased substrate bias (Fig. 10) and increased deposition temperature (Figs. 7 and 8). During annealing of these films (Fig 9), the conductivity increased dramatically as the temperature approached 350—400 °C,similar to behaviour predicted by Koidl [10]. Conversely the optical gap was observed to drop dramatically between 250 and 300 “C. The high relative resistivity and associated increased optical transparency could be due to the dominance of polymeric and/or sp3 components. Alternatively, upon annealing the growth
0
0~~
a),, •0)a)
323
:
io2. 10~
a,,’
‘a
100.
:
~-..
10
\•
~
\
(~Tcm)1
\
10
\
~
[12]
a
~T~--
\\ io~
:
—-...__lC._____.—-
10 -7. __________________________________________________ 0 1 2 3 4 5 6 7
K Fig. 7. Conductivity vs. deposition temperature: El, d.c. argon; from Morgan [11]; 6, evaporated, data from Hauser [12].
•, d.c., butane; a, sputtered data
101. C
10_i .
I0_
\
\ S
-1
io~
~
~
((1cm)
---_~
—~
10
—
1o~ 7 io 1o8
\
10_9. ______________________________________________________ • 0
1
2
3
4
5
6
7
K 1 Fig. 8. Conductivity vs. deposition temperature: El, r.f., argon; from Kalish et al. [i3].
•,
r.f. butane; 0, a-C:H, data
324 0~
I
,,// ~
In
1
-10.
4
-12
~
-i4~ -16 -18 -20 0
100
200
300
400
Temperature
500
600
C
Fig. 9. Conductivity vs. annealing temperature for films produced at 500 V d.c. with butane (thickness, 3.2 run) (El) in comparison with the data obtained by Koidl [10] (•).
2,
io
1 10•—
——
~0~
N
0,
N \
~0.
10_i
\
.
\
((1cm)
5
io2,
\
1
~
_____________
DC+argon • oc + butane •\
a a
10
C.
‘a
io6
‘a
: ‘a
-100
400
900
1400
1900
:
substrate bias (volts) Fig. 10. Resistivity vs. substrate bias: El, d.c., argon;
•, d.c., butane.
325
io8
“a
__________________________________________
io~
acm
‘a
io~ io2 101 100 10_i 0
5
10
15
integrated C:H Stretched Absorption
20
3 .2 x 10 cm
Fig. ii. Resistivity vs. hydrogen content. 108
io~
10430
~0
510
6
7~8~
90
Filament Bias (volts) Fig. 12. Resistivity vs. filament bias.
of more graphitic sp2 components causes a transition to softer, more conducting and less transparent films. It is reported that hydrogen content can have a relationship with resistivity depending upon deposition conditions. Figures 11 and 12 show very weak dependences on hydrogen content and filament bias.
326
4. Conclusions a-C and a-C:H films have been produced using a thermionically assisted, d.c. or r.f. PVD technique. It was found that the introduction of butane influenced many properties, particularly adhesion, residual stress and resistivity. Equally the deposition process was also important in influencing these properties, particularly bias voltage and current density.
Acknowledgments The authors would like to thank Micro Materials Ltd. for the use of a Nanotest ultralow load indenter, Peter Holiday thanks the Science and Engineering Research Council and the Electricity Development and Research Centre, Capenhurst and Azzeddine Dehbi-Alaoui thanks Ion Tech Ltd. We are also grateful to John Hebden and David Wright as well as to colleagues in the Research Centre in Surface Engineering for their advice and help.
References 1 2 3 4 5 6 7 8 9 10 ii 12 13
S. Aisenberg and R. Chabot, J. Appl. Phys., 42 (1971) 2953. S. Aisenberg and F. McKimock, Mater. 3d. Forum, 52—53 (1989) 1—40. J. C. Angus and F. Jansen, J. Vac. Sci. Technol. A, 6 (1988) 1778. L. I. Maissel and R. Glang, Handbook of Thin Film Technology, McGraw-Hill, New York, 1970, pp. 12—23. H. K. Pulker, Coatings On Glass, Elsevier, Amsterdam, 1984, p. 353. A. Dehbi-Alaoui, P. Holiday and A. Matthews, Surf Coatings Technol., SCT 1839. H. Tsai and D. B. Bogy, J. Vac. Sci. Technol. A, 5 (6) (1987) 3287—331i. A. Grill, V. Pate! and B. S. Meyerson, Diamond and Diamond-like Films and Coatings, Italy, July-August 1990, in NATO Adv. Study Inst. Ser., (1990). H. Tsai, Mater. Sci. Forum, 52—53 (1989) 71—102. P. Koidl, Proc. 1st ml. Symp. on Diamond and Diamond-like Films, in Electrochem. Soc. Conf. Proc., 89—12(1989) 237—249, M. Morgan, Thin Solid Films, 7 (1971) 313. J. J. Hauser, J. Non-Cryst. Solids, 23 (1977) 2i. R. Kalish and M. E. Adel, Mater. Sci. Forum, 52—53 (1989) 427—474.
