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Introduction
Diamondlike carbon (DLC) is an amorphous, in most cases hydrogenated,
metastable material. The designation diamondlike has arisen because the
material is characterized by properties that are to a certain extent similar to
those of diamond: high hardness, high wear resistance and low friction
coefficient, chemical inertness, high electrical resistance, and optical
transparency in the visible and infrared. DLC films were first deposited
byAisenberg and Chabot in 1971 by quenching a beam of C+ ions accelerated
in a vacuum of 10-6 Torr to a negatively biased substrate
[1]. The attractive properties of the material
have stimulated a large amount of research on their deposition and characterization,
and on the development of relevant applications.
Diamond and graphite are stable forms of carbon with well-defined
crystallographic structures. Natural diamond is a crystalline material, and the
diamond films fabricated by various CVD methods are composed of diamond
microcrystallites up to tens of microns in size. Crystalline diamond is
composed entirely of tetrahedrally coordinated sp3-bonded carbon. Diamond
and diamond films are thus constituted of a well-defined material with fixed
properties. In contrast, DLC films lack any long-range order and contain a
mixture of sp3-, sp2-, and sometimes even sp1-coordinated carbon
atoms in a disordered network. The ratio between the carbon atoms in the
different coordinations depends on deposition conditions and, in hydrogenated
DLC films, has been found to be a strong function of the hydrogen content of
the films. While missing a long-range order, DLC may have medium-range
ordering, as is discussed later. The properties of DLC therefore cover a wide
range of values between those of diamond, graphite, and hydrocarbon polymers,
as illustrated in Figure 1.
 Figure 1
"Pure" DLC composed of carbon and/or hydrogen can be classified as follows:
hydrogenated amorphous carbon, a-C:H, and amorphous carbon, a-C. The first type
contains from less than 10% [3] to 60%
[4] hydrogen; incorporation of hydrogen
in this type of DLC is important for obtaining diamondlike properties. The
second type, amorphous carbon, contains less than 1% hydrogen.
In the quest to change and improve the properties of DLC films, various
materials derived from carbon films have been developed. Such materials are
similar in structure to DLC but, in addition to carbon and/or hydrogen, they
include nitrogen (NDLC or CNx films), silicon (SiDLC), fluorine (FDLC), and
metal atoms (MeDLC). Most modifications have been made to DLC to reduce its
(typically high) internal compressive stresses (N, Si, metal incorporation), or
to reduce its surface energy for further lowering of its already low friction
coefficients (F, Si-O incorporation) [5, 6]. In 1989 Liu and Cohen [7] made
theoretical predictions of the potential existence of a crystalline material
C3N4, similar to Si3N4. This has led to extensive studies
of CNx films; however, in spite of various claims, such a crystalline structure
has not yet been unequivocally identified.
PECVD-deposited DLC films grow as continuous films, even at a thickness less
than 50 nm; they are uniform over wide areas, and their roughness can be a
fraction of an angstrom [5]. Because of their attractive properties, DLC films
and their modifications have been technologically developed for a variety of
applications, and new potential applications are being investigated. The
following sections present a general review of the preparation techniques for
DLC and related films and their structure, physical, and tribological
properties, as well as an overview of the studies of such materials at the IBM
Thomas J. Watson Research Center. A description of earlier studies at IBM can
be found elsewhere [8].
Preparation techniques
Since diamondlike carbon is a metastable material, DLC and related films must
be deposited while their surfaces are continuously bombarded with energetic
ions in order to obtain diamondlike properties. The metastable structure of DLC
films most likely originates from the thermal and pressure spikes produced by
the impinging energetic species on the growth surface [9]. Since the deposition
by Aisenberg and Chabot, a variety of techniques based on beam and plasma
techniques have evolved for the preparation of DLC. Such methods include single
low-energy beams of carbon ions, dual ion beams of carbon and argon, ion
plating, rf sputtering or ion-beam sputtering from carbon/graphite target,
vacuum-arc discharges, or laser ablation. Details of the different deposition
methods and of specific references can be found elsewhere [5]. While some of
the mentioned methods are being extensively used in investigations of
nonhydrogenated carbon (taC = tetrahedral carbon), the main techniques for
depositing diamondlike carbon films are, however, based on plasma-enhanced
chemical vapor deposition (PECVD) reactors and magnetron sputtering.
PECVD deposition of DLC is generally performed in plasmas sustained by rf
excitation [2, 10-13], but dc discharge systems [14-16], microwave plasmas
[17], and ECR systems [18, 19] have also been used. High-density plasma (HDP)
tools have been used recently for deposition of modified DLC films [20, 21].
PECVD deposition of DLC and related films is preferably performed in systems
using parallel-plate reactors, as illustrated in Figure 2. Of the various
possible geometries, parallel-plate reactors are preferred because they allow
the deposition of uniform films over large areas and can be scaled up
relatively easily for coating large areas. In the rf PECVD reactors, the
substrates are placed on the rf-powered electrode (Figure 2), thus attaining a
negative dc self bias, which is dependent on the precursor gases used,
deposition parameters (e.g., power, pressure), and relative area of electrodes
[22]. In dc PECVD the plasma is sustained by applying the negative dc voltage
to the substrate electrode [16].
 Figure 2
Any hydrocarbon can be used as a precursor for DLC deposition by the PECVD
method, provided it has sufficient vapor pressure to be transported into the
reactor. Details on the use of specific precursors can be found elsewhere [5].
Hydrogen or argon is sometimes added to the hydrocarbon precursor used in the
PECVD reactors; however, the effect of such additions to the properties of DLC
is not obvious [23, 24].
The growth rates of the films may decrease with
dilution of the precursor with hydrogen, and in some cases the addition of
hydrogen can have a detrimental effect on the film properties. In other cases
it was found that under certain plasma conditions, the addition of argon to the
hydrocarbon precursor can improve DLC properties [25, 26].
In addition to the large variety of precursors, DLC deposition conditions also
spread over large ranges of parameters. DLC depositions have been carried out
in rf PECVD systems at an rf power of 15-1000 W, a negative substrate bias of
100-1500 V dc, a pressure of 6 mTorr-7.5 Torr, and a substrate temperature
between room temperature and 250°C. While power density is a more
significant deposition parameter than rf power, it is not always possible to
determine this value from published results, because reported depositions have
been performed in reactors of different sizes and geometries. Nevertheless, it
seems that power densities of the order of 0.1-0.5 W/cm2 are generally
used to obtain DLC films in PECVD systems. Deposition of DLC must be carried
out at substrate temperatures below 325°C to prevent graphitization
[14].
In the PECVD processes, DLC grows mainly by condensation of the radicals formed
in the plasma; however, while the concentration of the ions in the plasma is
several orders of magnitude smaller than that of the radicals, ion bombardment
is an important factor affecting the growth and properties of the films. With
use of the right conditions, the films deposited on the biased electrode of an
rf reactor are hard DLC. Under the same conditions, the films deposited on the
grounded electrode are soft and polymerlike. The deposition rate on the biased
electrode is also much higher than on the grounded electrode. This can be
explained by assuming that the ion bombardment leads to increased fragmentation
of the hydrocarbon species arriving at the surface to ever more
hydrogen-deficient radical species, resulting in an increased sticking
coefficient on the bombarded surface [27]. The near-surface excited region is
quenched by the underlying cold substrate. Angus and Jansen [28] have proposed
that the composition of a DLC film, and its average nearest-neighbor
coordination, adjust to a minimum possible energy state through the
incorporation of hydrogen.
Deposition of DLC films by PECVD is usually performed in rf-sustained plasmas,
although dc plasmas have also been used for PECVD of DLC films. It is generally
believed that because of the insulating characteristic of DLC, it may not be
possible to apply the dc voltage through the films in order to sustain the
plasma and grow them. Therefore, dc deposition of DLC has been performed in
PECVD systems with an additional screen in front of the sample electrode
[14, 15]
or by using a triode configuration [29].
However, it has been shown that
highly insulating DLC films can be deposited on electrically conductive
substrates using a dc-powered diode-type parallel-plate reactor [16]. The
deposition is possible at sufficiently high bias because, as shown later, the
resistivity of the DLC films is not constant and drops several orders of
magnitude at high electric fields, enabling bias transfer to their surface. DLC
films several micrometers thick have been grown by dc PECVD at high rates using
low power densities (~0.25 W/cm2) [16].
Modified DLC films can be prepared by PECVD by admixing volatile precursors of
the modifying elements to the main gas feed. Thus, NDLC (or CNx) films have
been prepared from mixtures of hydrocarbons with nitrogen [30], SiDLC films
have been prepared by adding diluted SiH4 to the hydrocarbon [31], and FDLC
films have been deposited from pure fluorocarbons or fluorocarbons diluted with
hydrogen [32]. Metal incorporation was achieved by sputtering a metal or metal
carbide target in a plasma of argon with hydrocarbon mixture
[33, 34].
Bias and temperature effects
The substrate bias appears to be the dominant factor determining the properties
of DLC films. Their hardness, density, and refractive index increase with
increasing bias, while their hydrogen content and optical gap usually decrease
with increasing bias
[5, 16,
25]. It should, however, be noted that in most
PECVD deposition cases the bias is not controlled directly as an independent
process variable. In rf PECVD deposition systems, the substrate bias is
frequently changed by changing the rf power (W) and/or the pressure in the
reactor (p). According to Catherine [29], the self-bias of the substrate
(VB) in a specific reactor is related to these two parameters by the
expression
The substrate bias in PECVD systems can thus be varied by adjusting the bias
directly, or by adjusting the rf power and/or the pressure in the reactor.
Varying the substrate bias by changing the rf power affects not only the
phenomena occurring at the surface of the growing film but also the
fragmentation of the precursors in the plasma. Reports about the "effect of
bias" on the properties of DLC films may therefore reflect the combined effects
of two independent plasma parameters, W and p, and thus variations in the
active source species produced in the plasma by changes in the degree of
fragmentation of the precursor gas. In addition, for identical gases and
identical values of pressure and rf power, the self-bias also depends on the
reactor geometry, making it difficult to extrapolate results from one reactor
to another.
At the relatively high pressures used in PECVD of DLC films, the ions are
scattered by collisions and reach the substrate with a distribution of energies
lower than the applied bias. The average impact energy (E) of ions bombarding
the growing film in the rf plasma therefore depends on the dc bias and on the
pressure in the reactor, according to the relation [35]
E 
| VB |
|
| .
| | p½ | (2)
|
It has been claimed [2] that a substrate bias higher than 100 V dc is required
to obtain hard DLC-type films. However, this lower bias threshold can depend on
the pressure during deposition, because operation at high pressure reduces the
average energy of species impinging on the growth surface. At sufficiently low
pressures, the threshold value is relaxed, and hard, wear-resistant films can
be obtained at negative bias smaller than 100 V dc
[8, 13].
The minimum bias required to obtain diamondlike characteristics can also be
affected by the precursor used to deposit the films. Thus, for films deposited
by dc PECVD, a higher bias is required to obtain films with DLC characteristics
from cyclohexane than is required for films deposited from acetylene [25].
However, for the same deposition system, enhancement of the ion bombardment of
the films by addition of argon to the precursor enables the preparation of hard
DLC films from cyclohexane at the lower bias.
The substrate temperature during deposition is another parameter which can
affect the properties and deposition rate of DLC films. The deposition rate of
DLC increases with decreasing temperature and increasing ion bombardment. The
decrease of the rate of deposition with increasing substrate temperature can be
explained by assuming that growth takes place through the interaction between
the surface and a physisorbed layer of hydrocarbon. This assumption is
consistent with the low values of growth activation energies (0.05-0.5 eV) and
with the increase of the deposition rate with increasing ion bombardment [29].
It was reported that the optical gap of DLC films decreases with increasing
substrate temperature, with a rapid decrease taking place above 250°C
[14, 36].
While some properties of DLC films, such as their index of refraction
[37] or wear resistance, are a weak function of the deposition temperature,
substrate temperature can have a significant impact on their hydrogen content
and thermal stability [38]. Deposition at temperatures above 325°C has
been found to result in a sharp decrease of their electrical resistivity and in
significant film softening [14]. DLC deposition is therefore performed at
substrate temperatures well below 300°C. Low deposition temperature may
lead in certain conditions to the formation of a polymerlike film, or a
polymerlike fraction embedded in a hard DLC layer [13].
Structure and composition of DLC and related materials
The properties of DLC films are determined by the bonding hybridization of the
carbon atoms and the relative concentration of the different bonds, i.e.,
sp3:sp2:sp1, as well as by the hydrogen distribution among the
different types of bonds. In hydrogenated DLC films, some of the carbon bonds
are terminated by hydrogen. A fraction of the hydrogen can be incorporated as
single atoms, as well as molecules in voids. Voids have been assumed to explain
the hardness or densities of DLC films [39, 40]. While direct evidence of such
voids has not been reported, it has been shown that depending on deposition
conditions, more than 50% of the hydrogen incorporated in DLC films can indeed
be unbounded hydrogen [16, 41].
The structure of hydrogenated DLC films can be described as a random network of
covalently bonded carbon in the different coordinations. Robertson [42] claimed
that in addition to the short-range order defined by carbon hybridization and
hydrogen content, a substantial degree of medium-range order on the ~1-nm scale
also exists in the films. The material can accordingly be described as a
network of graphitic clusters of fused sixfold rings, linked into islands by
sp3 bonds. The formation of the graphitic clusters relieves the strain in
the DLC structure. According to Robertson's model, the existence of the optical
gap in DLC films is the result of the correlation of hexagonal rings on a very
fine scale and does not necessarily prove a preponderance of diamondlike
sp3 bonding [42]. The width of the optical gap was claimed to depend on
the degree of medium-range order and was found to vary inversely with the
square root of the size of the clusters, being proportional to 6N-1/2, N being
the number of aromatic rings in a cluster.
Angus and Jansen have described the structure of hydrogenated DLC (a-C:H) by a
random covalent fully constrained network (FCN) model [28]. The model was based
on the assumption that a random covalent network is mechanically constrained
when the number of constraints equals the number of mechanical degrees of
freedom. The authors found that in a covalent network composed solely of
sp2 and sp3 carbon sites, in which some clustering of aromatic rings
occurs, the optimal ratio of the number of sites for each coordination,
designated in [28] as sp3/sp2, is determined by the atomic fraction
of hydrogen incorporated in the film, XH, and is given by
The DLC structure can be described according to the FCN model as a
three-dimensional array of rings, containing mostly six-membered rings but also
five- and seven-membered rings. Such a cross-linked structure lacks long-range
order, and the material appears amorphous [28]. According to Equation (3), the
optimal coordinated carbon network can exist only for bound hydrogen
concentrations 0.167 < XH < 0.615. The FCN model thus defines the range of
hydrogen concentrations for which it is possible to obtain amorphous
hydrogenated carbon with diamondlike properties, but does not indicate the
distribution of hydrogen among the different carbon sites. The model takes into
account only the concentrations of hydrogen bound to carbon atoms. Since some
hydrogen can be unbound, the total hydrogen content in DLC films may extend
beyond the upper limit.
• Bonding characterization
The structural characterization of DLC films is complicated by their amorphous
nature. A variety of spectroscopic techniques, such as Fourier transform
infrared absorption (FTIR), Raman, NMR, and electron energy loss spectroscopy
(EELS), have been used to characterize the structure and composition of DLC
films. While each of these techniques provides some qualitative information
about their structure, the interpretation of the results is often indirect and
sometimes even misleading.
FTIR is often used to characterize hydrogenated DLC and related films. Typical
for the FTIR spectra of hydrogenated diamondlike carbon is the wide absorption
band centered at about 2900 cm-1. This band is a superposition of
absorption peaks of stretching vibrations from different spm CHn
configurations, with n, m = 1-3 [5]. FDLC films are characterized by a large
absorption band centered at about 1300 cm-1, corresponding to the
superposition of CFn peaks (n = 1-3), and a smaller peak centered at about
1750 cm-1, corresponding to superposition of C-CF2 and FC=CF2
absorption peaks [32]. The incorporation of nitrogen in DLC results in a strong
increase in the IR absorption band at 1700-1100 cm-1, a broad absorption
band at about 3380 cm-1, and a narrower peak at 2210 cm-1 [30].
Typical FTIR spectra are illustrated in Figure 3, and the interpretation of the
absorption peaks can be found in the papers referenced above.
 Figure 3
Because each individual spm CHn configuration is characterized by a
specific IR absorption peak, it is often assumed that one can use the
deconvoluted FTIR peaks to analyze the relative hybridization ratio,
sp3/sp2, of the carbon atoms and use the total intensity of the broad
peak centered at 2900 cm-1 to quantify the hydrogen content of the DLC
films
[12, 43,
44]. Such analyses have been performed assuming a constant
oscillator strength and no occurrence of preferential hydrogen bonding to a
particular type of carbon. However, the proportionality coefficients may differ
by an order of magnitude for the -CH bond and the -CH2 or -CH3 bonds
[45]. Therefore, FTIR cannot be used for determination of the carbon
hybridization or hydrogen content of the films [41]. It may, however, be used
for qualitative comparisons of films deposited in the same reactor, and can be
used together with forward recoil elastic scattering (FRES) measurements of
hydrogen content in the films to determine relative fractions of unbound
hydrogen in similar films [41], or to evaluate the general types of bonding
between different elements in DLC films or modified films
[30, 32].
FTIR spectroscopy may also be used for qualitative identification of changes in a
film undergoing postdeposition treatments, such as thermal annealing
[11, 12,
26, 37].
EELS is another technique used in determining the sp2 fraction in
nonhydrogenated DLC, especially in the so-called tetrahedral amorphous carbon
(taC), and then estimating the fraction of sp3 hybridized carbon (sp3
fraction = 1 - sp2 fraction). The method has also been attempted on
hydrogenated DLC [46], and it has been found that the films are sensitive to
the electron dose exposure. By using low electron-beam doses to minimize
irradiation damage, it was found that the sp3/sp2 ratio increases
with increasing hydrogen content in the films. However, these findings are only
qualitative for hydrogenated DLC films.
High-resolution 13C NMR spectroscopy appears to be the most reliable
method for determining the relative concentrations of sp2- and
sp3-hybridized carbon, as well as the local atomic environment of carbon
in each state in hydrogenated DLC
[13, 47-49].
Taking the spectra both with and
without the use of proton decoupling, one can identify the carbon atoms to
which at least one hydrogen atom is bound [13]. Because the C-H spin
interaction causes the carbon peak to be broadened into the baseline of the
spectrum, only the signal from carbon atoms with no bound hydrogen is obtained
when the proton signal is not decoupled. With the natural abundance of about 1%
13C, about 100 mg of DLC is required in order to obtain a
significant 13C NMR signal. Such studies have shown that, within
detectable limits ( 2%),
no "pure diamond" (tetrahedral carbon-to-carbon
bonding) was found in the DLC films, indicating that hydrogen is required to
obtain and stabilize the sp3 coordination.
• Hydrogen in DLC
Incorporation of hydrogen in the carbon films deposited by PECVD or sputtering
appears to be generally required in order to obtain "diamondlike" optical and
electrical properties. The hydrogen content in DLC films and its depth profile
with a resolution of 4 nm can be measured by the nuclear reaction
H(15N ,  )C:
15N + 1H  12C + 4He+.
| (4)
|
The measurement is done by bombarding the films with 15N ions and
counting the 4.33-MeV rays produced by the reaction, which is resonant
at 6.4 MeV [13]. A more accessible and generally used method for measurement of
the total hydrogen content of DLC films is the forward recoil elastic
scattering (FRES) or elastic recoil detection (ERD) technique
[38, 50].
While FRES is simpler than the nuclear reaction method, it has far inferior depth
resolution (about 50 nm vs. 4 nm for nuclear reaction).
The total hydrogen content critically determines film structure at the atomic
level (the ratio between sp3- and sp2-coordinated carbon atoms), and
therefore the physical properties of the films. Hydrogen content is also key to
obtaining a wide optical gap and high electrical resistivity, because it
passivates the dangling bonds in the amorphous structure [51]. DLC films
deposited from hydrocarbons must include hydrogen in concentrations from 17% to
61% in order to obtain diamondlike properties [28]. The loss of hydrogen
through annealing at high temperatures generally causes a collapse of the
structure to a graphitelike phase dominated by sp2 bonds
[37, 52].
As is discussed later, the hydrogen content of DLC films is also an important factor
controlling their tribological properties [53]. It was suggested in 1981 that
significant amounts of hydrogen in DLC films may not be bonded to carbon atoms,
but may be trapped interstitially [40]. The existence of unbound hydrogen was
proven by comparing FTIR and FRES data of different DLC films, and it was found
that in some cases the fraction of unbound hydrogen in DLC can be up to about
50% of the total hydrogen incorporated in DLC films
[16, 41].
• Optical properties
DLC films are typically transparent in the infrared, with the exception of the
CH absorbing bands mentioned earlier, are weakly absorbing in the visible
spectrum, and are increasingly absorbing with decreasing wave length in the UV.
Their hydrogen content is critical in controlling their optical properties, and
removal of hydrogen from hydrogenated DLC films causes the loss of IR
transparency
[54, 55].
A wide range of optical gap values (Eopt), spanning the range from 0.38 to
2.7 [2, 56-58], were reported for DLC films prepared under presumably similar
conditions, indicating the dependence of the property on the deposition system.
For otherwise similar deposition conditions, Eopt was found to decrease
strongly for DLC films deposited above 250°C
[5, 14]. This behavior
reflects the role of hydrogen in stabilizing the structure of DLC layers. Films
deposited at lower temperatures (in the range 25-250°C) contain
significant concentrations of hydrogen; most of the sp3-coordinated carbon
atoms, as well as a substantial fraction of the sp2-coordinated carbons,
are bound to at least one hydrogen atom. When the release of hydrogen is
induced in these materials, they revert to the configuration of lowest energy,
i.e., graphite.
The index of refraction (both the real part n and imaginary part k) and its
spectroscopic variation have been found to be dependent on the preparation
conditions and hydrogen content of the films. Its value at 632.8 nm has been
found to increase from 1.7 to 2.25, with an increasing rf power/pressure ratio
during deposition [59]. The dependence of the index of refraction on the
wavelength is strongly connected to the preparation conditions and can show
opposite behaviors
[60, 61].
The index of refraction at 632.8 nm of DLC films
deposited from acetylene by rf PECVD at a range of substrate temperatures, a
bias of -80 V, and 0.1 W/cm2 was found to be insensitive to the deposition
temperature and to have a value of n = 1.9 ± 0.2 [37]. It is, however,
dependent on the ion bombardment of the growing film and increases with
increasing ion bombardment. Ion bombardment increases with increasing bias,
decreasing pressure, and increasing Ar dilution, when Ar is added to the
precursor [25], resulting in an increasing index of refraction, as illustrated
in Figure 4.
 Figure 4
The index of refraction is also affected by the hydrogen concentration in the
DLC films, as determined by the deposition condition mentioned above. It
generally increases with decreasing hydrogen content, as shown elsewhere
[25, 38, 62].
It is, however, dependent on the concentration of bound hydrogen and
not total hydrogen content in the film [16]. A higher index of refraction
usually indicates DLC with stronger cross-linking, higher hardness, and better
wear resistance [25].
• Electrical properties
Owing to the range of properties of the naturally occurring forms of carbon,
i.e., graphite and diamond, the electrical properties of diamondlike carbon
films can vary from that of a semimetal to that of a wide-bandgap insulator.
Electronic transport itself is quite complex in DLC as a result of its
disordered structure. Diamond is characterized by high electron mobilities and,
since it is a wide-bandgap semiconductor, diamond-based devices such as diodes
can continue to function properly at temperatures in excess of 500°C; a
wide variety of applications for such high-temperature electronics are being
considered. In contrast, DLC has neither the wide energy gap nor the thermal
stability to operate in such an environment. The electronic and optical
properties of hydrogenated DLC behave to a certain extent in a fashion similar
to those of hydrogenated amorphous silicon.
The electrical properties of DLC films have been modeled assuming that the band
structure consists of only a mobility gap and that carriers residing in gap
states are localized [63]. This mobility gap produces semiconductor behavior,
but the existence of a high density of such gap states significantly degrades
the otherwise desirable properties of semiconducting materials. A more
quantitative description of the band structure of diamondlike carbon layers can
be found elsewhere
[4, 63].
Hydrogenated DLC can be doped similarly to
amorphous silicon; however, the doping is inefficient, since high levels of
doping are required to produce significant changes in its electrical properties
[63, 64].
Diamondlike carbon films are generally characterized by high electrical
resistivities spanning a large range of values, from 102 to 1016
 -cm, depending on the deposition conditions
[14, 36, 64].
Their electrical resistivity has been found to be strongly reduced by up to seven
orders of magnitude by incorporation of metals
[33, 65].
Nitrogen incorporation also reduces the electrical resistivity of the films.
The values of electrical resistivities of the DLC films discussed above have
generally been reported at low electric fields, and in most cases no indication
has been given of the dependence of the resistivity on the electric field.
Electrical resistivities in fields up to several MV/cm have been measured by
Grill et al. for DLC and nitrogen-doped DLC films deposited from several
precursors at a variety of conditions [64]. The electrical resistivities are
shown in Figure 5 as a function of the electric field E. As can be seen, the
resistivities vary strongly with E, indicating that the DLC films do not
exhibit ohmic behavior. The resistivities of these DLC films change only
slightly up to a field of 3 x 105 V/cm, but decrease sharply at higher
fields. Furthermore, the resistivities are strongly dependent on deposition
conditions, and the resistivities of dc PECVD films deposited at identical
conditions are strongly dependent on the precursor used [see Figure 5(a)]. As
can also be seen in Figure 5(b), the incorporation of nitrogen into the DLC
films reduced their resistivity over the whole range of electric fields.
Incorporation of 11% nitrogen (film N2) decreased the resistivity by about five
orders of magnitude compared to the undoped film (CY1) [64].
 Figure 5
Hydrogen, which stabilizes the sp3 bonds and determines the carbon
hybridization ratio, is therefore required for obtaining a high electrical
resistivity. However, electrical resistivity differences of several orders of
magnitude have been found between films having small or no differences in total
hydrogen content [64]. Because the electrical properties of DLC films depend on
the carbon hybridization, which in turn is affected by C-H bonding, the
electrical conductivity is determined by the bound and not by the total
hydrogen concentration in the films and, for films deposited under similar
conditions, the conductivity decreases with increasing concentration of bound
hydrogen in the films [64].
Special consideration has recently been given to the dielectric constant (k) of
DLC and FDLC films [66]. Low-k materials are needed for the "back-end-of-line"
(BEOL) multilevel interconnect structures of ULSI circuits to improve their
performance. It was found that by adjusting deposition conditions, it is
possible to change the dielectric constant of DLC films in the range 2.7-3.8,
as illustrated in Figure 6. The integration of such films in BEOL structures,
however, imposes further requirements on the low-k material, such as low
stresses and thermal stability at 400°C. It has been found that for DLC
films, the intrinsic stresses decrease with decreasing k values, while the
thickness changes increase with decreasing k values, as illustrated in
Figure 7. The increased changes reflect the reduced thermal stability of films
with low k values.
 Figure 6
 Figure 7
FDLC films with dielectric constants k < 2.8 (see Figure 8) have been prepared
having intrinsic stresses below 200 MPa. The FDLC films seem to be stabilized
after a first annealing at 400°C in an inert ambient. Such films are
therefore potential candidates as low-k insulators in BEOL interconnect
structures; however, integration issues, especially those related to
potentially reactive fluorine, have yet to be effectively addressed.
 Figure 8
• Chemical resistance
At room temperature, DLC films are chemically inert to practically any solvent
and are not attacked by acids, alkalis, or organic solvents. The films are
inert even to strong acid mixtures, such as the "acid etch" (HNO3:HF =
7:2) and to exposure to alkali solutions at 85°C for several hours. As a
result of their chemical resistance, DLC films can be used as
corrosion-resistant coatings. The films and their modifications can be removed
from a substrate by exposure to atomic oxygen or fluorine species generated in
a plasma, which react with the carbonaceous films to produce volatile COx
and CFy species which are pumped out of the system. Reactive ion etching
in oxygen- or fluorine-containing plasmas can be used to pattern DLC films. The
rate of etching can be strongly modified by incorporating Si in the films, and
SiDLC can be used as an etch stop during the etching of DLC or FDLC films [31].
• Mechanical properties
DLC films are characterized by high hardness, spanning the range from 10 to 30
GPa [68], associated with high intrinsic compressive stresses in the range from
0.5 to 7 GPa
[9, 10,
13, 38,
39, 69].
The high internal stresses limit the
thickness of films that can be used for various applications, often to less
than 1 µm. Adhesive forces must overcome the high stresses in order to
prevent delamination of the film from the substrate. Some of the mechanical
properties of DLC have been explained by Robertson [68] using the constraint
counting model of Phillips and Thorpe
[69, 70],
taking into consideration the
elasticity of the individual bonds and the connectivity and coordination of the
network.
The stresses in DLC films are generally directly correlated to the fraction of
sp3 carbon [71]. For rf PECVD films, the intrinsic stresses therefore
depend on the combination of deposition parameters which affect the sp3 C
hybridization. Because of the interdependence of the plasma parameters, stress
dependency reported with regard to one specific parameter is valid for only a
specific plasma system. Stresses were found to decrease with increasing
deposition pressure
[10, 39]
and to have a bias relation which depends on other
deposition parameters
[10, 70, 71].
Of the factors affecting the sp3 hybridization, the more important one is
the average energy of the ions bombarding the film, which accounts for both
substrate bias and pressure in the plasma and is given by Equation (2).
Figure 9,
which presents a plot of the stresses in DLC films deposited by dc PECVD
from several precursors as a function of the average ion energy, shows that the
stresses in the films deposited from one precursor do increase monotonically
with increasing average ion energy. However, for identical average ion
energies, the stresses are strongly dependent on the gases used to deposit the
films. The stresses in films deposited from acetylene by rf PECVD at similar
power densities [38] are also shown in
Figure 9. Although the rf films were
deposited at conditions corresponding to much lower ion energies (E values),
the stresses are much higher in these films than in the dc films. The results
shown in Figure 9 thus indicate that the factors determining the internal
stresses in DLC films are much more complex than just the direct effects of
bias or even of average ion energy.
 Figure 9
The intrinsic stresses in DLC films are also correlated with changes in
hydrogen content obtained by changing deposition parameters
[39, 72].
Strong variations in stresses have been observed in DLC films in which the differences
between the total hydrogen contents were too small to predict or explain the
large differences in the stresses
[16, 24].
In addition, it was observed that
films containing smaller amounts of hydrogen had higher stresses than DLC films
containing higher amounts of hydrogen [24].
As illustrated in Figure 10 for
dc-deposited DLC films, the stresses increase with an increasing fraction of
unbound hydrogen, without reflecting a direct precursor dependence.
 Figure 10
Stresses in DLC films have been decreased by doping the films with nitrogen
[24, 30,
73] to values as low as 0.22 GPa
[24]. Such NDLC films contain
significant numbers of NH bonds, which increase with increasing concentrations
of nitrogen [30]. The stress reduction in nitrogen-doped DLC films was
explained
[24, 73]
by using the overconstraining model of DLC [28]. According
to this model, replacement of CH with NH bonds in NDLC reduces the average
coordination number and the overconstraining in the NDLC films, resulting in
decreased internal stresses. However, other effects may contribute to the
stress reduction in DLC as a result of nitrogen incorporation.
• Tribological properties
Wear protection is needed for surfaces in kinetic contact, in situations which
generally also require low friction between the moving surfaces. As a result of
their high hardness, DLC films are utilized as wear-resistant protective
coatings for metals and for optical or electronic components. The use of DLC is
especially attractive in applications where the thickness of the protective
film is limited to less than 50 nm, as for example in the case of magnetic
recording media, where the trend toward higher-density data storage has led to
the requirement of very low flying heights between a disk and a recording head.
The coating must be able to protect the magnetic media against wear and
corrosion but must also be thin enough not to impede the achievement of high
recording density.
Tribological (friction and wear) characterization is usually performed on a
pin-on-disk apparatus [8]. The disk coated with the DLC film is rotated at a
fixed speed, while a pin with a spherical tip is loaded against it with a
chosen normal load. After a number of rotations, the wear of the disk is
calculated from the cross section of the profile of the produced wear track.
The friction force between the pin and the disk can generally also be measured
in "pin-on-disk" tools, thus enabling the determination of the friction
coefficient. Because detailed reviews of the tribology of DLC and related
materials can be found elsewhere
[6, 74],
the following contains only a summary
of the tribological properties of DLC films.
A variety of factors contribute to friction and wear, yet friction is
controlled mainly by the formation and breaking of chemical bonds at the
interface between the sliding parts, while breaking of bonds within a part
controls the wear [75]. A low friction coefficient and low wear are two
attractive properties of diamond. However, the friction coefficient of diamond
depends strongly on the environment and is mainly controlled by surface
dangling bonds. Diamond has a low friction coefficient in humid air or dry
nitrogen because of the presence of a contaminant layer of low shear strength
on its surface. Clean diamond surfaces can interact strongly and result in high
friction coefficients. The friction coefficient of diamond against diamond in
vacuum can increase to values as high as 1.0 when an increase of the
temperature causes the desorption of surface hydrogen, leaving interacting
dangling bonds [75]. Saturation of dangling bonds by atomic hydrogen or other
adsorbates reduces the friction coefficient to µ = 0.02. There is also
evidence that the debris formed during sliding is contributing to the low
friction in diamond [76]. Note that water [77-79] and atomic hydrogen [80] act
as lubricants for graphite and improve its wear and lubricating behavior.
In spite of the high chemical inertness of DLC films, their tribological
behavior is controlled to a large extent by their surface chemistry. The latter
is dependent on the method used for the preparation of the films, but can also
be affected by the testing environment. The tribochemical behavior of DLC films
(i.e., the effect of environment on their friction and wear) is well accepted
for DLC film tribology [74].
As summarized in other reviews
[6, 74],
DLC films deposited by PECVD have
several common tribological characteristics:
-
DLC films display a behavior similar to that of diamond in UHV but not in a
humid atmosphere.
-
The friction coefficient of hydrogenated DLC is low in humid nitrogen or
oxygen, extremely low in dry nitrogen or ultrahigh vacuum (UHV), and very high
in dry oxygen.
-
Loss of hydrogen through annealing at high temperature causes a marked
increase in the friction coefficient in UHV, but not in a humid environment.
-
Both friction and wear can be affected by a transfer layer which forms in
most cases during friction. The transfer layer in the wear scars was observed
directly with microlaser Raman spectroscopy, which showed that the layer has a
disordered graphite structure that is different from that of the original DLC
film [81]. Raman spectra taken from the wear track of the DLC film also revealed evidence of graphitization.
Initial explanations of the tribological behavior of DLC materials were based
on their mechanical properties and surface energy [34]. The films attain
hardness values (H) typical of inorganic ceramics, but have Young's modulus
values (E) significantly lower than those of ceramics. The corresponding H/E
ratios are relatively high, corresponding to those of polymers. The surface
energy (S) of DLC is also more like that of polymers than metals or ceramics.
The combination of the high H/E with low S/H ratios (S/H values being smallest
among the other type of materials) was used to explain the low adhesion of
other materials to DLC films, their low friction coefficient, and low wear
[34]. The excellent tribological performance of these materials was attributed
to a combination of favorable properties which are ceramiclike on the one hand
(high hardness) and polymerlike on the other (high elasticity, H/E, and low
surface energy).
However, it has become clear in recent years that in all environments the
tribological behavior of DLC films is controlled by an interfacial transfer
layer formed during friction. The transfer layer is formed by a
friction-induced transformation of the top layer of a DLC film into a material
of low shear strength. This transformation may be brought about by
friction-induced annealing caused by thermal and strain effects generated
during sliding [81]. The shear strength of the transfer layer and its adhesion
to sliding surfaces can be affected by the environment and by contact load and
sliding speed. The composition of the transfer layer can also be affected by
the material of the sliding counterpart. The low friction and ultralow wear of
DLC and counterparts can be explained by the low shear strength of the transfer
layer [6], which can also be affected by the testing environment [82].
A recent compilation of friction coefficients of DLC films [83] shows that
their friction coefficients span a range of µ = 0.007-0.4 in vacuum, below
10-4 Pa, while in ambient air at relative humidities of 20% < RH < 60%
they span a range of µ = 0.05-1.00. Most typical, however, are the ranges
µ = 0.007-0.02 in vacuum and µ = 0.1-0.4 in ambient air. The large
spread in the values of the friction coefficient are caused by variations in
the structure and composition of the films. The transfer layer described above
has a lubricating effect, and its formation can be enhanced by hydrogen but may
be restricted in the presence of water or oxygen. The increase of the friction
coefficient of hydrogenated DLC films in humid air was attributed by Gardos to
the increase in the van der Waals bond strength of hydrogen bonding to adsorbed
water molecules (~5 kcal/mol) compared to the bonding of hydrocarbons (~2
kcal/mol) [75].
Hydrogen passivates the dangling bonds in hydrogenated DLC films and permits
only weak interactions between the DLC and the sliding partner. When hydrogen
is lost from a hydrogenated DLC film by annealing, the formed dangling bonds
cause strong interactions between the surfaces in contact, resulting in
increased friction in UHV or dry nitrogen, similar to that reported for both
diamond or graphite
[80, 84].
It was shown that removal of hydrogen from DLC by
annealing the films above 550°C caused an increase in the friction
coefficient to µ = 0.68 in UHV or dry nitrogen, indicating that the
presence of hydrogen in these films is essential for obtaining very low
friction coefficients [76]. It has recently also been shown that the
achievement of a low friction coefficient in UHV requires a hydrogen content of
more than 40% [53].
Certain applications may require modification to preserve wear resistance but
change other characteristics (e.g., increase electrical conductivity or reduce
surface energy). It has been shown that incorporation of F, O, N, and Si in DLC
could modify water wetting angles of the modified films [85]. The incorporation
of F or Si increased the contact angle of water, while the incorporation of O
and N decreased the contact angle. The effect of F and Si on the surface energy
of DLC films was attributed to the reduction primarily of the polar part of the
surface energy, due to the loss of sp2 C hybridization and dangling bonds
[85]. Yet DLC films containing F and Si can be prepared to be as wear-resistant
as the pure hydrogenated DLC
[32, 86],
as illustrated in Figure 11 for FDLC
films deposited by dc PECVD. Similar results have been obtained for FDLC films
deposited by rf PECVD [86]. The incorporation of Si in DLC films has been
reported to render their friction against steel and the wear of the steel
counterpart insensitive to moisture [87]. Me-DLC films containing 15% Ta, W,
Ti, or Nb, having wear resistance similar to that of DLC and friction
coefficients <0.2, but higher conductivity (up to 0.005 -cm), have been
deposited by dc magnetron sputtering of the metals in acetylene [85]. Modified
DLC films incorporating different elements such as N, Si, and metals can
display tribological properties similar to those of pure DLC films, and these
properties are affected more by preparation conditions and testing environments
than by the incorporated element.
 Figure 11
The dependency of the tribological behavior of DLC materials on preparation
parameters and tribotesting conditions is summarized in Figure 12.
 Figure 12
Applications
The unique properties of DLC films and their modifications, together with the
possibility of adjusting the properties by choosing appropriate deposition
parameters, make them suitable for a variety of applications. The exploited
properties include high wear resistance and low friction coefficient, chemical
inertness, infrared transparency, high electrical resistivity, and,
potentially, low dielectric constant. Owing to their IR transparency, DLC films
can be used as antireflective and scratch-resistant wear-protective coatings
for IR windows or lenses made of Ge, ZnS, or ZnSe, where they also act as an
antireflective coating at the used wavelength of 8-13 µm
[88, 89].
Aluminum mirrors used in optical imaging deteriorate with time as a result of
exposure to atmosphere. This deterioration can be prevented by coating the
surface of the mirrors with DLC films [90]. Because of their absorbance in
visible light, DLC coatings can be applied as scratch-resistant coatings on
sunglasses. The low deposition temperatures of DLC films allow their use as a
wear-protective layer on products made of plastic. Currently, DLC films are
used for protection against abrasion of sunglass lenses made of polycarbonate
[91].
The most widespread use of DLC films is in wear and corrosion protection of
magnetic storage media. Nanosmooth DLC films are especially useful where the
protective coating must be very thin (<50 nm), such as in magnetic hard drives.
When the storage disk is started or stopped, sliding contact occurs between the
disk and slider, and this can cause mechanical damage and wear unless a
suitable protective coating is present between the surfaces in contact. The
protective coating must be resistant to wear and corrosion, but also thin
enough so as not to impede the achievement of high recording density; DLC
coatings are ideal for such applications. DLC is used as a corrosion- and
wear-protective coating for both magnetic disks and magnetic heads, and is also
used for corrosion protection of metal films during the manufacturing of
magnetic heads. Tapes for video recording or magnetic data storage, using
ferromagnetic metal as a recording medium, as well as the metallic capstans in
contact with the tapes, are also protected with DLC coatings to reduce wear and
friction, thus extending the life of the tapes.
Diamondlike carbon films appear to be biocompatible, and applications are being
developed for their use in biological environments. One major reason for the
failure of metallic implants in the body is their corrosion by body fluids.
In vitro tests have shown that DLC coatings allow cells to grow without
inflammatory response [92]. Because of their
chemical inertness and
impermeability to liquids, DLC coatings could protect the implants against
corrosion and serve as diffusion barriers. DLC films are being considered for
use as coatings of metallic as well as polymeric (e.g., polyurethane,
polycarbonate, and polyethylene) biocomponents in order to improve their
compatibility with body tissues
[90, 93].
Their wear resistance is another
useful property for bioapplications. Diamondlike carbon films, deposited on
stainless steel and titanium alloys used for components of artificial heart
valves, have been found to satisfy both mechanical and biological requirements
and to be capable of improving the performance of these components
[94]. The same properties may make DLC films
useful as protective coatings for joint implants.
As already indicated, DLC films and their modifications are currently being
considered for use as low-dielectric insulators for the interconnect structures
of ULSI chips. A better understanding of the means to control their thermal
stability and other integration problems should improve their potential for
this application.
Summary
Diamondlike carbon films can be prepared from a large variety of precursors
using the PECVD technique. Film deposition is carried out at substrate
temperatures between room temperature and 250°C under ion bombardment of
the substrate and of the growing film. Although the term DLC was adopted to
describe these films and their modifications, the films consist essentially of
amorphous networks of sp3- and sp2-bonded carbon, containing hydrogen
in amounts between 16 and 61%. The films are characterized by infrared
transparency, a significant optical gap, high electrical resistivity, low
dielectric constant, high hardness and internal compressive stresses, low
friction coefficients, and chemical inertness.
The properties of the films are affected by preparation conditions, and, for a
given preparation system, depend on the amount of hydrogen incorporated in the
films. Some of that hydrogen can be incorporated as unbound hydrogen, and the
relative fraction of bound or unbound hydrogen also affects the properties of
the films. Although the type of hydrocarbon used as a precursor often appears
to have only an insignificant effect on film properties, this is not always
true. The properties of DLC can be tailored by adjusting deposition parameters
such as substrate bias and temperature, power density, and precursor.
The wear resistance of DLC films appears to be strongly dependent upon their
deposition conditions. It can be maintained while modifying the material
through incorporation of different elements. Both the wear resistance and the
friction coefficients of DLC films and their modifications are strongly
affected by the testing environment and are controlled by a transfer layer
formed between the surfaces in kinetic contact.
Because of their attractive properties and the ease of preparation of extremely
smooth films at relatively low substrate temperatures, DLC films have found a
large variety of practical applications, and it is expected that the range of
applications will expand further. Although currently limited to applications at
temperatures below film deposition temperatures, their applicability may
eventually extend to higher temperatures.
Received December 2, 1997; accepted for publication March 16, 1998
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