Optical cvd diamond
Diamond has high spectral transmission performance in a long
wavelength range from ultraviolet to far-infrared, and its excellent mechanical
and thermal properties make diamond film an excellent optical window material
that can be used in harsh environments.
Our company has fully possessed and mastered the hardware
conditions and core technology for the preparation of optically grade CVD
diamond film, and has made a breakthrough in the preparation of large area
optically grade diamond film by using the dc plasma jet technology since 1995.
The optical CVD diamond film is mainly used in military
applications such as the hood of a high-speed missile and the window of an
airborne infrared thermal imaging device.
The civil (or dual-use) targets our company is developing
include: high-power CO2 laser processing machine window;
High power microwave Windows and so on.
CO2 laser processing machine is widely used for cutting,
welding and material surface modification and surface alloying of use, and the
window of the currently used material (ZnSe), the thermal lens effect is
significant and low mechanical strength and hardness, chemical properties is
not stable, so the optical CVD diamond window may have a very good market
prospects in this field.
What is CVD diamond
CVD diamond film is a kind of film diamond artificially
prepared by low-pressure chemical vapor deposition technology, which is
different from powder diamond artificially prepared by high pressure and high
temperature (HPHT) technology.
Low pressure chemical vapor deposition (CVD) diamond film
technology is an emerging technology developed in the middle and late 1970s. It
is another major breakthrough in the field of synthetic diamond since the high
pressure and high temperature method was used to convert graphite into diamond
under the catalysis of catalyst in the mid-1950s.
It indicates that man can make full use of the excellent
physical properties of diamond.
As is known to all, diamond has excellent properties,
including the highest hardness, the highest thermal conductivity at room
temperature, small thermal expansion coefficient, full-band high optical
transmittance, fast sound propagation speed, good dielectric performance,
semiconductor property after doping, and excellent chemical stability.
It is because of these unique and excellent properties that
diamond has a broad potential application prospect in mechanical, electronic,
optical, acoustic and other fields.
CVD diamond film preparation technology
At present, there are many methods for low-pressure vapor
deposition of diamond film, which can be roughly divided into four categories
according to their differences in gas activation: (1) hot-wire CVD method;
(2) dc plasma CVD method;
(3) rf (or high-frequency) plasma CVD method;
(4) microwave plasma CVD method.
Our company mainly USES the following two methods in the
production and r&d departments:
1.DC Arc Plasma Jet
This method by dc arc discharge, make with CH4 and H2 as main
composition of mixed gases (also useful Ar - CO2 - CH4 - H2) form the high
temperature plasma, then sprayed on the substrate, the high energy of plasma
and its associated with chemical reaction, can make the carbon in methane is
firmly combined into diamond polycrystal, can reach a high growth rate, but the
sedimentary area is limited, cost is higher also.
With the adoption of magnetron and Arc expanding technology,
DC Arc Plasma Jet technology has become one of the main directions for high
speed deposition of large area diamond film.
2.Microwave plasma CVD method (MPCVD)
MPCVD is to enter the microwave waveguide generated by the
microwave generator into the reactor through the isolator and the mixture of
CH4 and H2, so as to generate ch4-h2 plasma and deposit diamond film on the
substrate.
The low temperature deposition of diamond film can be
realized by MPCVD.
The mechanism of nucleation and growth of diamond film lags
behind the development of preparation technology.
However, although the structures and working principles of
the devices of various CVD methods are quite different, they have the following
commonalities in the process of diamond film deposition
1. The carbon source.
Including various carbonaceous compounds and even graphite,
such as alkanes, hydrocarbons, alkenes, alkynol, ketones, CO, CO2 and CF4, etc.;
2. The hydrogen gas.
This is the main gas inhibiting graphite nucleation, growth
and etching of symbiotic graphite.
3. Suitable hydrocarbon ratio.
The C/H ratio in air source is generally less than 10%.
4. Suitable substrate temperature.
Besides the particularity of low temperature deposition,
substrate temperature limit in the range of 700 ℃ to 1200 ℃.
Low working pressure.
Usually less than one atmosphere.
6. Gas activation technology.
At least one (or several) gas activation techniques are required
to excite the carbon source and hydrogen to produce a diamond film deposition
atmosphere with active carbon groups and atomic hydrogen.
In order to obtain high-quality diamond film, it is often
necessary to reasonably select the substrate material, pretreat the substrate
to increase the nucleation density and speed of diamond, add a small amount of
O2 or F2 to the air source to further inhibit the growth of defects and reduce
the deposition temperature of the substrate, and posttreat the diamond film if
necessary.
Applications of CVD diamond film can be divided into tool
applications, electronic applications, thermal applications and optical
applications.
Among all applications, CVD diamond film tools have the
largest market capacity and the lowest requirement for diamond film quality,
which is the field most likely to form large-scale industrial applications at
present.
However, due to the quality, variety and price of products,
as well as the acceptance process of the market, it will take at least a few
years to really form a large-scale market.
In recent years, the electronic application of CVD diamond,
especially the application of field-induced CVD diamond electron emission in
energy-saving displays and other fields, has almost become a hot trend.
Heat sink application and optical application require much
higher quality of CVD diamond film than tool application, that is, diamond film
heat sink application requires high thermal conductivity, and diamond film
optical application requires good light transmittance.
High quality requirements will lead to high production costs,
but the technological content of the products has also been significantly
improved, and the added value of the products is also high.
Whether a mass market can be established also depends on the
economic lever of the overall production cost of heat sink and optical
application products.
The military optical application of CVD diamond film has
always been one of the main goals pursued by countries all over the world,
especially the United States. It is mainly used as the window of the missile
head cover and infrared focal plane array imaging device for high Mach number
flight, as well as the protective coating of various military optical devices.
The quality of diamond film required for military optical
applications is extremely high, as well as a large deposition area (the minimum
missile diameter is also about 100mm) and a suitable thickness (2-3mm), as well
as special shapes required for specific applications.
Large-area, high-rate and high-quality diamond film
deposition equipment technology is the key to the military optical application
research of CVD diamond film.
The application market of civil optics is also quite good,
but it also depends on large-area, high-rate and high-quality diamond film
deposition equipment and technology.
Without solving this key problem, there is no way to
industrialize the optical application of CVD diamond film.
The study shows that the high power DC Arc Plasma Jet is the
most promising technology to unify the large area, high rate and high quality,
thus greatly reducing the cost of diamond film preparation.
High power DC Arc Plasma Jet is very suitable for the
preparation of curved diamond films in addition to achieving large area, high
rate and high quality diamond film deposition.