Single and dual beam ion source systems are used to generate amorphous diamondlike carbon (DLC) films, which were evaluated for a variety of applications including protective coatings on transmitting materials, power electronics as insulated gates and corrosion resistant barriers. A list of the desirable properties of DLC films along with potential applications are presented

Banks, B. A.

Kussmaul, M. T.

Mirtich, M. J.

Sovey, J. S.

English

NASA Technical Reports Server

N91-23042
DUAL ION BEAM PROCESSED DIAMONDLIKE FILMS FOR INDUSTRIAL
APPLICATIONS
M.J. Mirtich
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
M.T. Kussmaul
Sverdrup Technology, Inc.
Lewis Research Center Group
Brook Park, Ohio 44142
and
B.A. Banks and J.S. Sovey
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
INTRODUCTION
Diamondlike carbon (DLC) filmshave generated interestdue to the unique propertiesof the
material. The durable,smooth, adherent filmsare quitetransparentand impervious to reagents
which dissolvegraphiticand polymeric carbon structures.These amorphous filmshave desirable
characteristicssimilarto those ofdiamond, such as extreme hardness,high transparency (both at
visibleand infraredwavelengths),high electricalresistivity,low coefficientof friction,and
chemical inertness.They do not,however, have the long range order ofthe diamond crystal
structure.Most importantly,DLC filmscan be depositedon surfacesat room temperature,
allowing a broad range of applications.At NASA Lewis Research Center,singleand dual beam
ion source systems are used to generate amorphous DLC films,which have been evaluated fora
varietyofapplicationsincludingprotectivecoatingson transmittingmaterials,power electronics
as insulatedgates and corrosionresistantbarriers.A listof the desirablepropertiesof DLC films
along with potentialapplicationsidentifiedat NASA Lewis and those focusedDLC film appli-
cationspresentlybeing developed at Diamonex, Inc.are presentedherein.
DUAL BEAM ION SOURCE SYSTEM AND DLC DEPOSITION PROCEDURE
A $0-cm diameter ion source with its extraction grids masked to 10 cm in diameter is used
to directly deposit DLC films. The ion source, developed for electric propulsion technology, uses
argon gas in the hollow cathode located in the main discharge chamber, as well as in the
neutraliser (ref. 1). After a discharge is established between the cathode and the anode, methane
(CH4) is introduced through a manifold into the discharge chamber. For the depositions
presented in this paper the molar ratio of CH 4 to argon was 0.28. This ratio was found to be
ideal for generating films at NASA Lewis. In these experiments the total ion beam energy is the
sum of the discharge voltage and the screen grid voltage and is around 100 eV. Typically current
densities at these conditions are 1 ma/cm 2 at a distance of 2.5 cm axially downstream of the grids
(ref. 2). Films are deposited at these conditions on Si and SiO 2 at deposition rates as high as
71 _/min to film thicknesses as great as 1.5 #m.
It is believed that the amorphous carbon films are produced under conditions where both
growth and sputtering occur simultaneously, increased sputtering may decrease the number of
graphite precursors incorporated in the films and hence improve film quality.
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In addition, Marinow and Dobrew (ref. 3) have found that active sites for nucleation are
created, and the growth and coalescence of the nuclei enhanced due to an increased mobility of
the condensing atoms when film structures are bombarded by inert gas beams. With these factors
in mind, a dual beam system was created by adding an 8-era diameter argon ion source. This
system, shown in figure 1, was used to generate another set of DLC films. The 8-era source, using
a filament cathode, was located at a 12° angle with respect to the 30-cm source and 25 cm from
the substrate. There was no observed interaction between the two sources or the ion beams
during operation.
The 8-cm ion source was used to direct a beam of energetic argon ions (200 to 600 eV) at a
current density of 25/_a/cm 2 on the substrates while the deposition from the $0-cm ion source
was taking place. The beams were approximately monoenergetic, however no mass selection was
attempted to determine species.
PROPERTIES OF DIAMONDLIKE FILMS
OPTICAL PROPERTIES OF FILMS
Shown in figure 2 is the spectral transmittance for DLC films generated using the single and
dual beam ion source systems. The 1500 ,/l thick dual beam film has greater transmittance at all
wavelengths when compared to the 1500 _ thick single beam films. Some thinner DLC f'dms (800
to 1500 _, thick) look clear to yellowlike in appearance and films thicker than 1500 _ usually look
brown. A DLC conformed complete cover coating can usually be obtained for films as thin as 500
_,. The 500 _ thick film presented in figure 2 has transmittance values greater than 90 percent at
wavelengths greater than 7000 _. It is the goal of a joint effort by NASA Lewis and Diamonex,
Inc. to Gbtain clear DLC films, 1000 _ thick, with a transmittance of 85 percent at a wavelength
of 5000 ,/k (the peak of the visible spectrum). Diamonex, Inc. (90 Winsor Drive, Allentown, Pa.
18106) has been in a DLC film technology transfer program with NASA Lewis.
Figure $ shows the results of some of the on-going studies at NASA Lewis using the dual
beam system to reduce the absorption at 5000 _ and hence improve the transmittance. Shown in
the figure is the transmittance at 0.5/_m (the peak of the visible) for a DLC film on quarts for
various dual beam gaseous deposition conditions. Figure 3 clearly shows that a higher trans-
mittance can be obtained by using the dual beam system where the second ion source uses
hydrogen gas. The role of energetic hydrogen in improving transmittance of the DLC film is still
under investigation.
The infrared transmittance of DLC films has been shown to be 100 percent transmitting in
the wavelength region between 2.5 and 20 _am. DLC films of controlled thickness have been
shown to improve the transmittance of Ge infrared optics (refs. 4 and 5). Because the index of
refraction (-,,2) of DLC films is intermediate between Ge and air, a DLC coating is capable of
improving overall transmittance by reducing interface reflection losses. A DLC coating can also
be used to reduce reflection loses of Si. The high hardness of these films also imparts abrasion
protection to these infrared optical materials.
Listed in table I are values of H/C ratio, optical band gap, index of refraction, absorption
coefficient and solar transmittance for DLC films generated using the single or dual beam ion
sources. Certain properties are enhanced by using a dual beam system, mainly the solar
transmittance which is higher, as is the index of refraction, and a lower absorption coefficient,
when compared to the single ion source films. The optical band gap and density for the single
and dual beam system films are similar. Also listed in table I are values of resistivity, density,
and adherence. Since all of the NASA Lewis DLC films were processed in oil diffusion-pumped
facilities without the benefit of liquid nitrogen cryo-traps, even higher quality films might be
obtained by using a helium cryopumped facility.
174
CHEMICAL AND PHYSICAL PROPERTIES
The DLC single and dual beam films were subjected to a solution of $ parts H2SO 4 ud I
part HNO 3 (concentrated acids, by volume) at 80 "C for periods up to 20 hr. The f'dms on
silicon were unaffected by the reagent. These results clearly indicate that the films are far more
resistant to chemical etching than normal polymeric hydrocarbons or graphite. Dr. J.C. Angus
(ref. 6) measured the diffusion coefficient of Argon, trapped at the DLC rdm/subetrate interface,
to be less than I0 "Is cm_/sec. This value is 10 orders of magnitude (10 billion) times less than
conventional hydrocarbons or polymers. Thus DLC f'dms are one of the best known hermetic
barriers. This suggests the potential use of the DLC films u chemical and/or a diffusion barrier
for microelectronic or optical components.
Transmission electron microscopy at SO 0OO X showed the f'dms to be smooth and essentially
free of features. No pinholes or other defects were observed.
The adherence of the films on quarts was measured following the procedure used by Mirtich
(ref. 7). The adherence of the films generated with either the single or dual beam systems were as
good as the maximum adherence of the Sebastian Adherence TesterO used in the measurement
(~5.5xi07 N/m 2 or 8000 psi). The film adherence was extremely high and frequently found to
exceed the cohesion of the substrate such that portions of quarts gave way with the film still
intact.
Some films deposited with the dual beam on Si have been stored for I0 years and show no
visible signs of deterioration. Figure 4 shows a plot of the coefficient of friction for a diamondlike
carbon fdm sliding on steel as a function of relative humidity. For a humidity less than one
percent the coefficient of friction is very low, indicating a potential use of a DLC fdm as a wear
resistant surface for inert or vacuum environments.
Scratch tests were performed on DLC f'dms depicted on quarts by rubbing SiO 2 particles
(,--80 Ism) over the surface. Figure 5 shows the result of these tests. The DLC f'dm protected the
quarts from the abrasive effects of the SiO 2 particles.
Table II lists the properties of diamondlike fdms and compares them to those of natural
diamond and polycrystalline diamond films (ref. 8). In a couple of categories DLC films do not
quite measure up to those of polycrystalline or natural diamond. For instance, natural diamond
and polycrystaIline diamond have thermal conductivities of 900 and TOO W/mK respectively;
whereas, a DLC fdm is only 7 W/mK. Likewise, the resistivity of DLC films is not as high as
that of diamond. Although the microhardness of DLC films (5 000 kg/mN 2) is aoproximately
half (_) of that of diamond, it is twice as hard as herd corundum (2 085 kg/mN') and harder
than sapphire. This hardness value makes DLC fdms a good coating when a hard surface is
desirable (i.e., magnetic memory pro_ection, eyeglass lenses, etc.). The coefficient of friction (0.1)
for a diamondlike film at low humidity is similar to that of natural diamond. Thus any material
that is affected by wear has the potential to benefit from the reduced friction properties and
hardness of diamondlike coatings.
The distinguishing difference between DLC film and polycrystalline film is that DLC films
are smooth, adherent and are deposited at room temperature or less. This feature of DLC fibns
alone makes them a popular coating choice in a vast area of applications. Whereas, polycry-
stalline diamond films, which are rough, not very adherent and not specularly transmitting as yet,
have the limiting feature of having the substrate at temperatures greater than 600 "C during
deposition. This limits polycrystalline diamond f'dm's applications.
POTENTIAL APPLICATIONS
A feasibility study of commercial applications of diamondlike films was done at the NASA
Lewis. The objective of the study was to evaluate, rank, recommend sad plan commercial
applications for NASA-developed diamondlike film deposition technology. The approach was to
175
identifypotential users, prioritize potential applications and assess the feasibility of selected
applications. A questionnaire was mailed to 230 companies. The number of responses returned
was 39 with a total of 44 applications identified. Table III presents the top 12 potential
applications identified by the study that appear, at the moment, feasible using diamondlike films.
There are, of course, many other applications not listed here but bear mentioning like durable
luster enhancing coatings for gemstones, abrasion resistant coatings for plastics, etc.
Shown in table IV are the focused diamondlike film applications at Diamonex, Inc., is a new
corporation founded in 1990 whose principle technologies are polycrystalline diamond and
diamondlike films. They have purchased an exclusive license to the NASA Lewis dual ion beam
patent to make diamondlike films. Diamonex has the rights to also sublicense the patent.
Because of some of the properties of DLC films listed in table II, the technical feasibility and the
user need, they have focused their resources on making abrasion-resistant optical coatings their
number one priority. Other applications listed include the use of DLC films as a hermetic seal of
transparent substrates and wear protection of nonoptical substrates.
SUMMARY
A major advantage of DLC films is that they are smooth and adherent and can be deposited
on substrates at room temperature. These features allow DLC films to be used readily in many
applications where properties of DLC films such as hardness, scratch resistance, hermeticity and
transparency are important. Included in this paper are prioritised lists of many potential DLC
film applications.
Diamonex, Inc., which has a license to NASA Lewis dual ion beam DLC film patent, has
estimated the DLC film potential market to be several billion dollars. The abrasion-resistant
optical coatings market for eyeglass lenses alone is estimated at a half a billion dollars. Diamonex
is authorized to grant sub-licenses under their exclusive license arrangement from NASA.
REFERENCES
1. Sovey, J.S.: AIAA paper 76-1017, Key Biscayne, Fla., Nov., 1976.
2. Mirtich, M.J.: AIAA paper 81-0672, Las Vegas, Nev., April, 1981.
3. Marinow, M., and Dobrew, D.: Thin Solid Films, Vol. 42, 1977.
4. Pellicore, S.F., Peterson, C.M., and Henson, T.P.: J. Vac Sci. Technology, A4, 2350 (1986).
5. Koidl, P., Bubenzer, A., Dischler, B., Pro. Soc. Photo-opt. Instrum. Eng. 381, 186 (1983).
6. Angus, J.C., Mirtich, M.J., and Wintucky, E.G.: Metastable Materials Formation by Ion
Implantation, Elsevier Science Publishing Co., Inc., 1982.
7. Mirtich, M.J.: J. Vac. Sci. and Tech., 18, 2, March, 1981.
8. Kempfer, L.: Diamond: A Gem of a Coating. M.E. May, 1990, pp. 26-29.
176
TABLE I - OPTICAL,
OF DLC FILMS
SINGLE OR
CHEMICAL AND PHYSICAL PROPERTIES
GENERATED USING CH, WITH
DUAL BEAM ION SOURCES
Single beam
CH,/A ,, 28 percent
Dual beam
CH,/A = 28 percent
H/C ratio -- 1.0
Resistivity, _2-cm 8.66x 106 3.35x 106
Optical band gap (eV) 0.382 0.343
Density, gm/cm 3 1.8 1.8
Index of refraction 2.0 2.46
Absorption coefficlent/cm at 5000A 5.15x10' 4.26x10'
Solar transmittance
t = ./O(k)T(k)d(_.)
./Q(_.)dk
Adherence (Quartz)
Where: Q(X) = 6000°K black body
T(_.) = transmittance at a given wavelength
0.519
Film = (1500 A thick)
>5.5x 107 N/m 2
>8000 psi
0.648
(15ooA)
>5.5xlO _ N/m 2
>8000 psi
177
TABLE II
Properties of Diamondlike Films Compared to Natural
Diamond and Polycrystalline Diamond Films
Properties Diamond
Microhardness
(kg/mm 2)
Resistivity ('ohm cm)
Thermal Conductivity
('W/mK)
Refractive Index
Density ('g/cm 3)
Friction Coefficient
Chemical Resistance
Visible and Infrared
Transparency
Visible and Infrared
Specular Transparency
Adherence
Substrate Temperature
During Deposition
Film Roughness
10,000
Polycrystalline
Diamond Films
8-10,000
Diamond-Like
Carbon Films
3-5,000
10,3- 10,6 10'0- 10,s 10 _
2,000
2.4
3.5
0.1
High
7OO
2.3 - 2.4
3.2 - 3.4
High
High High
High
(7) Low
2.4
1.8
<0.1
('at low humidity)
Low
High
High
>600 o C
High
-- Low High
m_
Room temperature
or lower
Rough Smooth
178
Diamondlike
TABLE III
Film Potential Applications Identified at LeRC
DLC Film Ai0plication Desirable DLC Film Properties
1. Protective coating for sunglass
lenses
Hardness, scratch resistance
2. Protective coating for eyeglass
lenses
Hardness, scratch resistance,
transmittance
3. Hermetic coating for eyewear
Hermeticity, hardness, scratch
resistance, transmittance
o Abrasion, moisture resistent coating
for optical surfaces ('visible and
infrared)
5. Magnetic recording head
Hermeticity, hardness, scratch
resistance, transmittance
Hermeticity, hardness, scratch
resistance
6. Coating computer hard disk
Hermeticity, hardness, scratch
resistance
o Abrasion resistant coating for optical
windows in bar code scanners
Hardness, scratch resistance
8. Biomedical applications Biocompatibility, hermeticity
go
10.
Chemically resistant protective
coating
Enhanced IR transmittance of
Ge and Si infrared optics
11. Cutting blades
Hermeticity, hardness
IR transmittance, index of
refraction, abrasion protection
Smoothness, hardness, hermeticity
12. Abrasion resistant non-stick
coating for cookware
Hardness, scratch resistance
179
TABLE IV
Focused Diamondlike Film Applications at Diamonex, Inc.
(1) Abrasion-resistant optical coatings
• Plastic sunglass lenses
• Plastic eyeglass lenses
• Other optical substrates such as glass
(2) Chemical/environmental protection ("hermetic sealing') of transparent substrates
• Quartz
• Glass
• Plastics
(3) Wear protection of non-optical substrates
• Magnetic disks
• Cutting surfaces
• Other wear parts
180
ORIGINAL PAGE
BLACK AND WHITE PHOTOGBAPH
,;r,!C.,_;_t. PAGE IS
eoo_ QUAUTY
C-81-4380
Figure 1 .--Dual beam ion source for deposition of films with
diamondlike properties.
8
I'--
1.0
.8 --
.4
,2 --
0
2
1500 A thick
0 500 A thick film,
30 cm only
I I I I I I
4 6 8 10 12 14x103
Wavelenglh, A
Figure 2.--Transmittance versus wavelength for DLC
films using CH 4 in dual beam or single ion sources.
181
100_
80
i °
2O
0
30cm source:
8 onnsource:
iiiiiiiiiii!iiiiiiiiiiiiii!ii!i!iiii!i
Argon Argon
Off Argon 25% H2
75% Ar
_ A d
M_
50%H 2
50%_
DLC depo_ilk_n condilions
H2
"S'S'S'J-,
%%%%%
JJSS,
%%%%%
SSSf,
%%%_%
_SSI,
%%_%%
%%%%%
%%%%%
%%%%%
SS_S,
%%%%%
%%%%%
SSIS,
%%%%%
_SSS_
%%_%%
%%%%%
%%%_%
X_on
75% Ar
_% H2
Figure 3.--Transmittance at 0.5 Im_for 1000 A thick DLC films deposited on quartz under
various duel beam oondilions.
182
.2 m
0
10--6
I I I
10-4 10 -2 1 100%
h
Figure 4.J-,,oefficient of friction p. for a diamondlike
carbon film sliding on steel, as a function of the relative
humidity h.
DLC film Quartz
Figure 5.-- Scratch test of DLC film using SiC2 particles.
183


Dual ion beam processed diamondlike films for industrial applications

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