Diamond films made by a new low-pressure vapor process
are making diamond a viable choice for engineers.
Patterned silicon wafer coated with a 1 micron film of
UNCD ready for subsequent fabrication into MEMS devices and probe arrays (below).
If natural diamond were inexpensive and could be
integrated into products as easily as other materials, it could be used in
countless applications. A solution to this decades-old problem was recently discovered-diamond
films made by a new low-pressure vapor process make diamond a viable choice for
engineers. In particular, thin film diamond can now be integrated with ceramics
for friction/wear applications, thermal applications and microsystems.
Thin film diamond chemical vapor deposition (CVD)
technology was developed about 30 years ago. Despite advances over the years,
CVD diamond has failed to meet lofty industry expectations based on the
perfection of natural diamond. Instead of becoming the material of choice for
demanding applications, thin film diamond has been commonly referred to as the
material of last resort; engineers consider CVD diamond only if every other
commercial material fails to do the job.
In addition to cost, a number of technological problems,
such as poor reproducibility, lack of mature deposition technology, relatively
small deposition areas and poor film properties, have restricted the
application of thin film diamond to cutting tools, heat sinks and other niche
Over the past two decades, research at Argonne National
Laboratory led to the discovery of ultrananocrystalline diamond (UNCD®
a new technology that overcomes previous limitations related to thin film
diamond.* The UNCD innovation is rooted in the chemistry used to synthesize the
*Advanced Diamond Technologies (ADT) was formed in 2003
to commercialize the ultrananocrystalline diamond technology developed at
Argonne National Laboratory. ADT is the exclusive licensee to Argonne's
portfolio of patents for synthesizing and using UNCD, and has received generous
support from the National Science Foundation's Small Business Innovation
Research program, the U.S. Department of Energy, the Defense Advanced Research
Projects Agency, and the State of Illinois'
Department of Commerce and Economic Opportunity.
Figure 1. High-resolution transmission electron microscopy image of UNCD thin film.
Developing a Smoother Alternative
In the past, the growth chemistries used to synthesize
pure CVD diamond resulted in rough films characterized by very large diamond
grains (microns in size) and weak, low-angle grain boundaries. These
large-grained, microcrystalline diamond (MCD) films were nearly impossible to
integrate with other materials, and the palette of compatible substrates was
limited. Applications for such films tended to require thick, freestanding
pieces of diamond that took days or weeks to synthesize and were extremely
expensive. They were also rough on one side and consisted of graphitic carbon
on the other, requiring extensive post-processing to be usable.
Efforts to develop smoother films led to work on
nanocrystalline diamond (NCD). The straightforward modification of the
chemistries used in microcrystalline material to make NCD resulted in films
that were combinations of the diamond and graphitic phases of carbon, but these
films exhibited inferior properties, such as greatly reduced hardness, modulus
and fracture strength. Consequently, NCD developed a poor reputation in the diamond
Overcoming the inferior qualities of MCD and NCD demanded
a new deposition technology. Growing diamond requires techniques to generate
chemical species that are unstable under normal conditions. The role of the
excitation source (i.e., plasma) is to heat up the reactants to temperatures
where the decomposition of methane (CH4
) and hydrogen (H2
can form reactive species.
discovered that by adding argon gas, along with methane and hydrogen, to the
vapor mixture, the radicals generated during growth changed and yielded diamond
films consisting of approximately 5 nanometer grains without any graphitic
phases (i.e., phase-pure, diamond-bonded carbon with high-energy, atomically
abrupt grain boundaries).
Ultrananocrystalline diamond was the name given to this
film to distinguish it from its predecessor, NCD film. Because carbon atoms are
bonded together in grains that are about 20 carbon atoms in size, UNCD is as
hard and stiff as natural diamond while tougher and stronger than typical MCD
films. UNCD's film stress is also significantly lower, and by adding impurities
such as nitrogen, boron or hydrogen to the grain boundaries, the properties of
UNCD can be adjusted depending on its application (see Figure 1).
Since 5 nm grain UNCD is phase-pure, diamond-bonded
carbon, it retains the desirable characteristics of natural diamond, including
hardness, modulus, refractive index, acoustic velocity and surface chemistry.
UNCD's electronic, thermal and optical transport properties, however, are
different from natural diamond. It also has a low thermal conductivity compared
to single crystal diamond (10-20 vs. ~2200 W/m°K) and is not optically
From an electronic structure point of view, UNCD has
diamond's bandgap (5.5 eV), but the bonding states within the grain boundaries
create a material that is more electrically conductive with lower electron
mobility. UNCD features a number of unique properties, including low
as-deposited film roughness, low stress, higher fracture toughness and higher
strength compared to natural diamond. Unlike previous NCD technologies, UNCD
films can be grown at lower temperatures (400°C compared to 600-900°C). All of
these differences result in a thin film diamond that can be easily integrated
with other materials, such as other thin film technologies that are the basis
for microelectronics and microelectromechanical systems (MEMS).
Figure 2. UNCD deposited onto a 200 mm silicon wafer (left) compared with an uncoated wafer (right).
Products and Commercialization
A family of materials based on UNCD has been developed that offers an array of
adjustable properties to make it suitable for a range of thin film diamond
applications. For example, very smooth films can have transport properties
close to natural diamond, yet retain the superior ability of UNCD to integrate
with other materials. It is now possible to make electrically or thermally
conductive films that are extremely smooth with low stress for use as thermal
Furthermore, advances in deposition technologies have
enabled the production of these films on substrates of up to 300 mm. One
commercially available UNCD series of films on silicon wafers** allows researchers
and industrial developers to evaluate these materials for potential
applications (see Figure 2). In addition, the first UNCD-based
were developed specifically
for MEMS and electronics application development. UNCD also integrates with
many other thin film ceramic materials, including SiC, SiN, AlN, ZnO and PZT.
The list of diamond-related products is endless, but the
market potential varies dramatically, ranging from niche applications like
synchrotron X-ray detectors to the billion-dollar market of radio frequency
(RF) electronics. Key components of the UNCD technology will be advanced to
address the major opportunities in MEMS, biomedical applications and other
industries while allowing UNCD, both as a material and a technology, to gain
acceptance and to disprove the previous mindset that diamond is both expensive
Currently, three specific applications enabled by UNCD
are under development. The first is wear-resistant, low friction coatings for
mechanical components, including mechanical seals for fluid pumps. UNCD films
as thin as 1 micron can change the performance of state-of-the-art silicon
carbide seals and dramatically reduce the friction and wear at the seal face,
increasing the lifetime of the pump for applications in chemical refineries,
ethanol production, petroleum exploration and pharmaceutical processing. Since
mechanical seals are found in most fluid pumps, it is estimated that reducing
friction could save trillions of BTUs of energy annually. The same UNCD films
can be used as a tribological coating in other industrial settings.
**UNCD Aqua series of films deposited on silicon wafers
Figure 3. Scanning electron micrograph (SEM) image of a
released and suspended fixed-fixed 2nd overtone mode RF resonator made from
UNCD Aqua 25. Image courtesy Innovative Micro Technology (IMT), Santa Barbara, Calif.
The second important application area for UNCD is as a
structural material in MEMS, including AFM probes, RF MEMS filters, oscillators
and switches. These applications leverage the greatest number of diamond's
superlative bulk and surface properties, since the performance and long-term
stability of MEMS devices depend on the chemical stability of the exposed
For RF MEMS, such as resonators (see Figure 3), UNCD acts
like a tuning fork, vibrating at a set frequency that cannot vary with time,
temperature or other environmental conditions. UNCD, like natural diamond, has
a chemically inert, hydrophobic, low stiction surface that allows devices to
function without the need for expensive die-level hermetic packaging.
Figure 4. Scanning electron microscopy image of an UNCD atomic force microscopy probe.
By leveraging the high acoustic velocity and surface
stability of UNCD, devices for X- and Ka-band (2-20 GHz) wireless communication
systems can be developed that allow for smaller, more energy efficient and less
expensive RF front-ends for radios in mobile phones, base stations and military
applications. UNCD-based atomic force microscopy (AFM) probes,·
which are simple forms of MEMS devices, will enter the market in late 2007 (see
Figure 5. Schematic of a UNCD-based MEMS biosensor.
The third application area includes bio-implants and
sensors, with the goal of creating functional devices that integrate both
passive and active UNCD elements combining diamond's bio-inertness and
bio-compatibility with the ability to covalently immobilize biomolecules on the
surface (see Figure 5). Active electrochemical-based sensors using conductive
UNCD thin films can enable implantable devices that conduct real-time
monitoring of blood chemistry (e.g., glucose, alcohol, cholesterol). This
advancement will enable a new generation of biosensors that work in real-time
in devices that are both compact and light enough to wear as jewelry. Imagine
the life-changing and potential life-saving impact of wearing a
"watch" that automatically monitors and administers insulin
continuously via a wireless link to an implanted UNCD-based biosensor.
Such biomedical applications will take additional effort
to overcome many fundamental technical challenges. However, diamond has finally
come of age in a platform technology suitable for broad integration into
numerous applications, and UNCD is being developed into commercially available
products to turn the idea of diamond for use as an engineering material into a
For additional information regarding UNCD,
contact Advanced Diamond Technologies, Inc., 429 B Weber Rd. #286, Romeoville,
IL 60446; (815) 293-0900; e-mail email@example.com;
or visit www.thindiamond.com. Argonne's
website is located at www.anl.gov.