Methods using lasers to produce deposition of diamond thin films on substrates

Infrared lasers are used to deposit diamond thin films onto a substrate. In one embodiment, the deposition of the film is from a gas mixture of CH.sub.4 and H.sub.2 that is introduced into a chemical vapor deposition chamber and caused to flow over the surface of the substrate to be coated while the laser is directed onto the surface. In another embodiment, pure carbon in the form of soot is delivered onto the surface to be coated and the laser beam is directed onto the surface in an atmosphere that prevents the carbon from being burned to CO.sub.2.

BACKGROUND OF THE INVENTION 
Diamond is potentially a very useful material for numerous scientific and 
engineering applications because it possesses high hardness, has the 
highest thermal conductivity of any known material, has excellent optical 
transparency throughout the ultraviolet to infrared range, is chemically 
inert, has high stiffness and superior acoustical properties, and is 
biocompatable. Moreover, diamond can be an excellent n- or p-type 
semiconductor if doped with appropriate impurities. 
Because of the unique characteristics of diamond, there are many possible 
applications of diamond thin films. Some of these are illustrated in the 
following Table 1: 
TABLE 1 
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Applications of diamond thin films 
Characteristic Applications 
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High thermal conductivity 
Heat sinks for semiconducting 
devices, heat dissipating 
printed circuit boards, and 
minimum laser damage 
High hole mobility, dopability 
n- and p-semiconducting 
materials, temperature and 
radiation-resistant power 
semiconducting devices, 
transistors in motors and jet 
engines, Schottky diodes 
Optical transparency 
Coatings for lenses and 
glasses, infrared windows, high 
density compact disks, X ray 
windows and lithography 
masks 
Wear resistance, hardness, 
Abrasives, drilling tools, 
low friction bearings for machines and 
motors, cutting tools, dies and 
molds, scratch proof coatings 
on watches, glasses, ceramics 
and jewelry 
Chemical inertness 
Chemical reactors, inert 
coatings on nuclear reactor 
walls, and corrosion-resistant 
watch cases 
High sound propagation velocity 
Loudspeaker diaphragms, 
tweeters, and midrangers 
Biocompatability Surface improvement of joints, 
teeth, implants, prosthetic 
materials, and biosensors 
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The most useful form of diamond for the electronics industry would be a 
film of single-crystal diamond on a nondiamond substrate. Since diamond is 
both a good thermal conductor and a good semiconductor, diamond microchips 
could be used successfully in high-speed devices 
At the present time, diamonds are synthetically produced for most 
industrial purposes by using one of two procedures. One method is to apply 
high temperatures and high pressures to carbon containing compounds. 
However, this method is not suitable for coating many of the substrate 
materials for which the full benefits of the unique properties of diamond 
can be best utilized. The second method is to deposit diamond thin films 
from the gas phase at low pressures using thermal, electric discharge and 
catalytic methods. Diamond thin films created in this manner on a 
substrate material use a variety of chemical vapor deposition (CVD) 
procedures that include the use of plasma, microwave, hot filament, ion 
beam, and electron beam as energy sources. Typically, a mixture of 0.5 to 
2% methane and the balance hydrogen gas is used as the carbon source. The 
mechanism of diamond deposition occurs because the hydrogen molecules in 
the gas decompose into atomic hydrogen which subsequently becomes 
deposited on the substrate as a monolayer. The more reactive forms of 
carbon such as graphite combine with the atomic hydrogen and are removed, 
thereby allowing diamond to condense on the substrate surface. 
Although the deposition of diamond films has been proven using the above 
methods, there are many disadvantages of the prior art procedures that 
prevent diamond films from being successfully and extensively utilized for 
commercial applications. Some of these disadvantages are: 
1. The deposition rates are generally low. A typical value is 1 micron per 
hour, although The Naval Research Laboratory claims to have formed "good 
quality, clear crystals" at 50 microns per hour while the Tokyo Institute 
of Technology claims rates as high as 930 microns per hour. 
2. The deposition area covered on the substrate is small. Current 
technology is limited to 4" diameter optical surfaces. 
3. The substrate temperatures must be in the range from 800.degree. C. to 
around 1000.degree. C. These high temperatures restrict diamond from being 
deposited on low melting point substrates such as polymers. 
4. The process of diamond growth is not yet fully understood. 
5. Hydrogen impurity will be present if the substrate temperature is held 
below 400.degree. C. when using the ion beam technique. The properties of 
films containing hydrogen do not approach the desirable properties of 
diamond films. 
6. Adhesion of the film to the substrate is insufficient because of the 
thermal expansion mismatching of diamond with most of the substrates. 
7. A smooth surface finish is hard to achieve especially for optical 
applications. 
Because of the forgoing deficiencies in the prior art methods, new methods 
for producing diamond thin films have been explored in as effort to 
overcome at least some of the limitations of the existing methods and to 
improve the quality of diamond films. One such known method is 
laser-induced chemical vapor deposition (LCVD). In this method, a laser 
serves as an energy source to decompose the gases and raise the surface 
temperature of the substrate for deposition. LCVD offers several unique 
advantages over the other prior art methods: 
1. Laser is a clean source of energy which leads to less contamination for 
diamond growth. 
2. A relatively high deposition rate is achieved. 
3. A low substrate temperature can be used. 
4. The area of deposition can be controlled. 
5. Better surface integrity results. 
6. Fine microstructures are produced. 
7. The method has the capability for using different precursors. 
8. Little thermal damage occurs to the substrate. 
Lasers deposit thin films by pyrolysis and photolysis mechanisms of 
breaking the gaseous molecules. In laser pyrolysis, the substrate is 
heated by a directed laser beam up to the desired temperature by 
controlling the power and irradiation time of the beam, while the chemical 
gases decompose by the collisional excitation with the hot surface. The 
laser-driven reactions are different from those initiated by other CVD 
sources for the same heat input because of the higher temperatures 
obtainable in the smaller reaction volume defined by the directed laser 
beam. In the laser photolysis process, the photons break the chemical 
bonds of the gaseous molecules which allows the products to get deposited 
on the substrate. An important requirement in photolysis is the need to 
match the wave length of the laser beam with the bandgap of the reactants. 
The bond breaking may be caused by either single or multiphoton 
dissociation. In general, pyrolysis is more efficient than photolysis. 
Because of these capabilities and characteristics of lasers, some prior art 
researchers have attempted laser-induced CVD for diamond deposition, and 
the results initially indicated that diamond films were obtained using an 
ArF-excimer laser beam in a gas mixture of C.sub.2 H.sub.2 and H.sub.2 at 
a substrate temperature in the range 40.degree. to 800.degree. C. This 
seemed to be an important improvement in diamond technology because the 
deposition of the diamond film was claimed to occur at a temperature as 
low as 40.degree. C. However, the same researchers later indicated that 
their earlier findings were not correct and that the film observed was a 
"heat treated carbon black" instead of diamond. 
Other prior art researchers have experimented with a system using a mixture 
of CCl.sub.4 and H.sub.2 gases that are passed over the substrate and 
irradiated with a 193 nm ArF laser beam. This gas mixture was apparently 
chosen instead of CH.sub.4 because it absorbs the 193 nm wavelength more 
efficiently, the binding energy is more appropriate, and it forms reactive 
radicals. Using a laser power of 8 watts, it was determined that the 
maximum growth rate of 6 microns per hour occurred at a substrate 
temperature of 800.degree. C. It is believed that the surface of the film 
is photoreactive which activated the hydrogen which in turn increased the 
amount of carbon available for the diamond film. 
Amorphous carbon films have been deposited by excimer laser 
photodissociation of C.sub.2 H.sub.3 Cl and CCl.sub.4 gases at ambient 
temperature, and recently, researchers have claimed the deposition of 
carbon films containing a large fraction of diamond by using an excimer 
laser and carboxylic acids. 
Other than the foregoing examples, there are no known prior art teachings 
of the successful use of lasers to produce diamond thin films on a 
substrate, but in light of these prior art teachings, it is evident that 
laser CVD is a technology that has potential for diamond deposition. 
However, the prior art LCVD methods all require relatively long processing 
times and supplementary heating of the substrate in order to attain the 
substrate temperature necessary to produce diamond deposition. Moreover, 
the prior art processes must take place in an enclosure to confine the 
gases from which the diamond film is formed. These are obvious limitations 
on the use of the LCVD methods. In addition, the prior art teachings have 
not established optimum laser parameters and other process variables for 
growing diamond thin films using CH.sub.4 /H.sub.2 precursors, nor does 
the prior art contain any teaching on the use of lasers other than excimer 
lasers. There is therefore a definite need for improved methods of 
utilizing the obvious advantages of laser technology for diamond 
deposition. 
SUMMARY OF THE INVENTION 
In one embodiment of the invention, a 1200 Watt CO.sub.2 gas laser is used 
as an energy source to deposit a diamond thin film onto a substrate from a 
gas mixture of CH.sub.4 and H.sub.2 in a chemical vapor deposition 
chamber. Using an unfocused CO.sub.2 laser beam with a power density of 
3000 watts/cm.sup.2 supplied at normal incidence to the substrate surface, 
a diamond thin film can be deposited in approximately two hours, with the 
substrate temperature being raised to about 600.degree. C. In another 
embodiment, the process is performed without the use of a vapor deposition 
chamber. In this embodiment, pure solid carbon particles, in the form of 
soot, are delivered onto the surface of the substrate to be coated and a 
CO.sub.2 gas laser beam is directed at normal incidence onto the surface. 
Movement between the substrate and the laser beam causes momentary heating 
of a very small area of the substrate surface with minimun energy input, 
resulting in a diamond deposit on the surface with almost no disturbance 
of the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The invention has identified the process variables required for using laser 
chemical vapor deposition (LCVD) without the disadvantages of the prior 
art methods when depositing diamond on substrates. 
The specifications of the laser system used in the method of the first 
embodiment of the invention are set forth in the following Table 2: 
TABLE 2 
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Laser system 
______________________________________ 
Laser type CO.sub.2 
Power, watts 100-1200 
Pulse length millisec to continuous 
Pulse repetition rate 
continuous to 100 Hz 
Beam size 0.75" 
Energy distribution 
Gaussian 
Manufacturer Photon Sources Model V1200 
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The CO.sub.2 laser has a wavelength of 10,600 nm in the infrared range of 
the electromagnetic spectrum, and when used in the method of the 
invention, the laser is operated in pulsed and continuous wave modes. A 5" 
focal length lens and an optical integrator also are sometimes used. The 
chemical vapor deposition (CVD) chamber used in performing the method of 
the invention may be of any suitable type, but preferably is made of 
stainless steel. A schematic diagram of the CVD chamber is shown in FIG. 
1. The chamber includes sidewalls 10 and 12 and a top wall 14 and bottom 
wall 16 that provide a sealed chamber 18. A port (not shown) is provided 
in one of the walls for connection to a vacuum pump (not shown) so that a 
vacuum can be created in the chamber 18 prior to the diamond deposition 
process. A gas inlet port 20 is provided in sidewall 12 with a suitable 
valve 22 controlling the flow of gas through port 20 into the chamber 18. 
Similarly, a gas outlet port 24 is provided in sidewall 10 with a suitable 
valve 26 controlling the discharge of gas from the chamber 18 through 
outlet port 24. Ports 20 and 24 provide for the continuous flow of the 
desired gas through the chamber 18 during the deposition process. A 
thermocouple 28 is used to measure the temperature of the substrate 30 
that is positioned in chamber 18 for coating with diamond. Connection to 
thermocouple 28 inside of chamber 18 is provided through any one of the 
walls of the chamber 18 in any suitable manner. The laser beam from a 
laser (not shown) of any suitlable type is directed onto the surface of 
the substrate 30 to be coated through a window 32 in the top wall 14 so 
that the laser beam will be directed normally to the surface of the 
substrate 30. An example of a laser suitable for use in the method of the 
invention is set forth in Table 2, above. The window 32 is formed of any 
suitable material such as ZnSe. The gas is preferably directed through a 
tube 34 so as to impinge on the surface of substrate 30 where the laser 
beam is incident Thermocouple 28 is in contact with the substrate 30 and 
the pressure in the chamber is monitored in any suitable manner. 
The substrate 30 can be of any desired material as long as it is capable of 
withstanding the surface temperature created during the deposition 
process. Since the method of the invention creates much lower temperatures 
in the substrate itself, many materials not previously capable of being 
coated with diamond can be coated using the principles of the invention. 
We have used successfully gallium arsenide (GaAs) and silicon (Si) for 
substrate materials. GaAs of a thickness of 0.0155" and with an 
orientation of (100) can be used. The GaAs substrate may also be doped 
with small amounts of silicon if desired. Silicon with about the same 
thickness and (111) orientation may also be used as the substrate 30 
because it has been proven by other CVD techniques that silicon is a 
suitable substrate for diamond deposition. The substrates are preferably 
polished with 3 to 6 microns diamond powder prior to commencement of the 
laser deposition process. 
The method of the invention is commenced by first evacuating the chamber 18 
to 10.sup.-6 torr and then introducing a mixture of 2% CH.sub.4 and 98% 
H.sub.2 gas through the inlet port 20 into the reaction area above the 
substrate 30. The gas pressure and the flow rate are preferably in the 
ranges of 50-100 torr and 2-5 l/hr. The other process variables used in 
practicing the process of the invention should be within the following 
parameters: 
Laser power: 100 to 1200 watts with CO.sub.2 laser 
Beam size: unfocused CO.sub.2 beam=0.75" unfocused CO.sub.2 
beam=0.25".times.0.25" square focused CO.sub.2 beam=0.004" using a 5" 
F.L.lens 
Beam mode: continuous wave and pulsed (1 to 100 Hz) 
Resistance heating of the substrate: 20.degree. to 800.degree. C. 
Deposition time: 15 minutes to 2 hours 
With the gas flowing into the reaction area above the substrate 30, the 
laser beam is then directed perpendicular to the surface of the substrate 
30, thus producing the diamond film on the surface of the substrate 30. 
The deposited film thus produced can be confirmed to be diamond by use of 
a scanning electron microscope, X-ray analysis, scratch test or Raman 
microprobe spectroscopy. 
In some instances and with certain substrates, it may be necessary to 
gradually increase the laser power from 0 to 1200 watts over a period of 
time, such as an hour, in order to prevent the shattering of the substrate 
30. 
One of the major problems in using a CO.sub.2 laser is that the chamber 18 
becomes very hot in 30 minutes of deposition time. Since diamond 
nucleation requires longer time than 30 minutes when using the method of 
this first embodiment, the walls of chamber 18 may be fitted with vertical 
aluminum fins for heat dissipation. Also, air jets may be directed onto 
the chamber walls to assist in reducing the chamber temperature. With such 
physical modifications to chamber 18, high quality, fine crystalline 
diamond deposits on the substrate 30 can be produced. 
In the laser thermal deposition process of the invention, the laser beam 
serves to heat the surface of substrate 30 and to decompose the gaseous 
molecules, thus leading to the necessary chemical reactions and physical 
processes required for the deposition of the diamond film on the 
substrate. The diamond growth mechanism using laser-induced CVD is 
therefore probably a result of the laser pyrolytic processes. 
(Photochemical reactions are ruled out with CO.sub.2 lasers because the 
decomposition of CH.sub.4 is possible only with photons whose wavelength 
is less than 144 nm.) In pyrolytic LCVD, lasers directed normally onto the 
surface of the substrate induce chemical reactions heterogeneously at the 
gas-solid interface. It is believed that with normal incidence of laser 
beams, diamond nucleation and growth require heterogeneously activated 
reactions in which the reaction products are diffusion-driven towards the 
substrate surface giving rise to the thin films. In heterogeneously 
activated LCVD, the laser light is absorbed by the CH.sub.4 /H.sub.2 gases 
and by the substrate resulting in high heat at the interface of the gas 
and substrate surface. The absorption spectra of H.sub.2 molecules shows 
that H.sub.2 molecules are capable of absorbing large amounts of 10.6 
micron IR radiation. If the gas mixture is absent, the temperature rise of 
the substrate will exceed 1000.degree. C. within a few milliseconds of 
laser irradiation time. However, with continuous gas flow over the 
substrate, the temperature rise of the substrate will reach only about 
580.degree. C., even after continuous radiation by the CO.sub.2 laser beam 
at 1200 watts power for more than 2 hours. 
The laser energy absorbed by the CH.sub.4 /H.sub.2 gas mixture causes the 
gas to be in the excited, high-energy state. When the excited gases come 
in contact with the hot substrate, the adsorption of a CH.sub.4 /H.sub.2 
gaseous layer occurs on the substrate surface. This is followed by a 
reaction to form adsorbed layers of atomic hydrogen. The adsorbed hydrogen 
atoms then form clusters which grow and coalesce to form a continuous 
film. Once a monolayer of atomic hydrogen is formed, nucleation of carbon 
takes place as a result of the decomposition of CH. The free carbon atoms 
form clusters and provide nucleation centers for further film growth. 
Although the carbon can be deposited as diamond or graphite or other forms 
of carbon such as chaoite, ionsdaleite and carbyne, the probability of 
diamond deposition is more likely because the atomic hydrogen etches away 
other forms of carbon. 
Using the method of this first embodiment, we have found that an unfocused 
CO.sub.2 laser beam with a power density of 3000 watts/cm.sub.2 is capable 
of depositing a diamond film on the substrate surface in 2 hours of time 
if the laser beam is continuously applied at normal incidence to the 
substrate surface in the presence of CH.sub.4 and H.sub.2 gases. This is a 
major improvement over prior art methods which require considerably longer 
processing times. Also, it is significant that the substrate temperature 
will reach only about 600.degree. C., thereby allowing diamond deposition 
to be used with many substrate materials never before thought possible. 
Thus, the deposition of superior quality, fine crystals of diamond on a 
variety of non-diamond substrates using laser induced CVD of CH.sub.4 
/H.sub.2 mixture can be achieved more quickly and at a lower cost using 
the principles of the invention. 
FIG. 2 illustrates the second embodiment of the invention, in which the 
method of laser-induced diamond deposition is performed without the 
necessity of an environmental chamber. In this embodiment, rather than 
flowing a gas over the upper surface of substrate 30 in a confined 
environment, particles of pure carbon, in the form of soot, are delivered 
to the surface of the substrate 30, which can be positioned in the open 
atmosphere. The laser beam is directed normally toward the substrate 
surface, and relative movement is created between the laser beam and the 
substrate 30. This produces momentary heating of a very small area (0.05 
mm.sup.2 for 0.01 sec) on the substrate surface with a minimum energy 
input (as low as 110 watts). The method of this embodiment results in a 
diamond deposit with almost no disturbance of the substrate. Since the 
method is performed at atmospheric conditions, and no supplemental heating 
or pressurization of the substrate is required, the method is a relatively 
easy to perform and clearly a significant advance over the prior art 
methods. Moreover, the method of this embodiment eliminates the 
requirement for using hydrogen gas, is very much faster and requires a 
minimum energy input. However, in some instances, it may be necessary to 
conduct the deposition process of this embodiment in an atmosphere that 
prevents the carbon from being burned to CO.sub.2 by using a cover gas of 
either CO.sub.2 or helium. Even if such a cover gas is used, the method 
can be performed in the open atmosphere. 
Having thus described our invention in connection with preferred 
embodiments thereof, it will be evident to those skilled in the art that 
various revisions and modifications can be made to the preferred 
embodiments disclosed herein without departing from the spirit and scope 
of the invention. It is our intention, however, that all such revisions 
and modifications that are obvious to those skilled in the art will be 
included within the scope of the following claims.