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What is UNCD®?
UNCD (ultrananocrystalline diamond) captures many of the best
properties of natural diamond in a scalable thin film technology
enabling diamond to be integrated into a wide range of products.
UNCD encompasses a proprietary family of materials manufactured
using patented chemical vapor deposition processes. UNCD coatings
are not diamond-like carbon films, but phase-pure crystalline
diamond materials.
What are the typical growth temperatures for UNCD films?
600-800 °C. New processes are under development enabling the
deposition of UNCD films at temperatures as low as 400 °C.
How thin/thick can a UNCD film be deposited?
UNCD films can be as thin as 100 nanometers (nm) or as thick as
20 microns (µm). The standard UNCD thickness on DoSi™
(diamond on silicon) and DOI™ (diamond on insulator) Wafers
are 300 nm and 1 µm.
On what materials can UNCD be deposited?
UNCD is routinely deposited on silicon, SiO2,
W, Mo, TiAl4V6,
and alpha “self-sintered” (binderless) SiC as well
as Ta and Si3N4.
Materials that readily form carbides tend to make the best substrates.
Please contact us to inquire about additional materials.
On how large of an area can UNCD be deposited?
UNCD is routinely deposited on silicon wafers up to 200 mm in
diameter and can be processed on substrates with complex geometries,
including large pump seal faces.
Can UNCD protect the substrate material from chemical
attack or oxidation?
Yes. Continuous pin-free films can be grown to render the substrate
impervious to chemical attack from strong acid solutions such
as HF and HNO3.
How smooth are the UNCD films?
UNCD films are typically very smooth as deposited on silicon (Si)
wafers, because standard Si wafers are nanometer-level smooth.
For instance, UNCD Aqua 25 films have RMS roughness values around
10 nm on standard Si wafers. The roughness of the film depends
on a number of factors: the roughness of the substrate, the thickness
of the UNCD layer and the chosen UNCD product.
How is UNCD different than diamond-like carbon (DLC)?
UNCD consists of small phase-pure diamond grains. UNCD is crystalline
diamond. DLCs are amorphous materials consisting of carbon bonded
locally with a combination of sp3
(diamond) and sp2 (graphitic) hybridized
bonding.
How do I integrate UNCD into a MEMS process?
The UNCD technology is appropriate for integration into a foundry
environment. After deposition, UNCD may be etched using a number
of hard mask materials (e.g., SiO2,
Aluminum, or photo resist) using oxygen-based reactive ion etching
(RIE). SiO2 works very well as
a sacrificial layer with nearly 100% differential etch rates between
diamond and SiO2 during the O2
RIE process. Ohmic contacts can be formed using W/Cr stacks and
almost any metal deposited on the UNCD surface. Please contact
ADT for more details in how to integrate UNCD into your MEMS process.
What is the thermal conductivity of UNCD?
The thermal conductivity of UNCD can be tuned. In the Aqua family
of UNCD products, Aqua 100 is the best thermal conductor due to
its large average grain sizes, whereas Aqua 25 is a poor thermal
conductor due to its small grain sizes. Aqua 25 films in the 1-10
micron thickness range have demonstrated values as low as 10-20
W/mK.
How would my MEMS device benefit from UNCD?
Most MEMS/NEMS devices currently under development are mainly
based on silicon because of the available surface machining technology
adapted from the silicon-based microelectronics batch fabrication
technology.
However, compared to diamond, silicon has:
- relatively poor mechanical (low Young’s modulus of
130 GPa, low hardness of 1,000 kg/mm2, and fracture strength
of 1.3 GPa)
- poor tribological properties (high adhesion energy or stiction
of 106 mJ/m2)
For an application like RF MEMS, diamond has a combination of
bulk properties that are extremely attractive for such an application:
- high acoustic velocity
- low dissipation
- low temperature coefficient of frequency
- low charge-trap dielectric
- linear electromechanical response
- lower sensitivity to environmental conditions, including:
- low adhesion energy
- resistance to tribo-electrical foiling in active electrical
contacts
This table compares the unique material properties making diamond
an attractive candidate for MEMS/NEMS devices:
| Property |
Silicon |
Silicon Carbide |
Diamond |
| Lattice Constant |
5.43 |
4.35 |
3.57 |
| Cohesive Energy |
4.64 |
6.34 |
7.36 |
| Young's Modulus |
130 |
450 |
1200 |
| Shear Modulus |
80 |
149 |
577 |
| Hardness |
1000 |
3500 |
10000 |
| Fracture Strength |
1.0 |
5.2 |
5.3 |
| Flexural Strength |
127.6 |
670 |
2944 |
| Friction Coefficient |
0.4-0.6 |
0.2-0.5 |
0.01-0.04 |
| Relative Wear Life |
1.0 |
|
10000 |
| Thermal Conductivity |
149 |
|
900-2320 |
What MEMS products have been built with UNCD?
| RF MEMS Resonator |
 |
This UNCD MEMS resonator was made from ADT's DoSi (Diamond
on Silicon) wafers.
Photo courtesy of Innovative Micro Technology (IMT), Santa
Barbara, CA |
| RF MEMS Switch |
 |
In collaboration with MEMtronics Corporation, this RF MEMS
Switch was fabricated using UNCD Aqua 25 as the low-trap dielectric
(i.e., the green area in the center of the switch). This switch
achieved 1 billion cycles in dry air. |
| AFM Probes |
 |
NaDiaProbes™ bring the power of diamond to AFM cantilevers.
NaDiaProbes are made entirely of UNCD (both the tip and cantilever)
in a single monolithic process. These are not diamond-coated
probes; they are entirely made out of diamond and demonstrate
the astonishing control and precision that is available with
diamond machines. |
| Molded 3D MEMS Structures |
 |
The award-winning UNCD technology has been molded into complex
3D MEMS structures, such as the tip array shown to the left,
and deposited onto processed MEMS wafers. |
Which UNCD product is right for my application?
UNCD captures, in a thin film, many of the extreme properties
of diamond: hardness, Young’s modulus, heat conductivity,
low friction, electronic field emission, and many others. In addition,
UNCD can be tuned to be conductive or insulating, IR or visibly
transparent and low friction with a smooth or a rougher surface.
Because of this ability to engineer diamond, UNCD can solve many
MEMS issues. All varieties of UNCD are available on DoSi and DOI
wafers.
Depending on your application, ADT has the right thin diamond
for your needs, available through our Aqua family of UNCD products:
|
Need |
Applications |
UNCD Solution |
| High Thermal Conductivity |
Heat spreader
Thermal Management |
Diamond has the highest thermal conductivity
of any material. For extreme heat transfer Aqua 100 will meet
your thermal transport needs. Aqua 100 has large grains of
diamond (200-300 nm on average) and deposits with an RMS of
< 100 nm RMS.
If you need heat transfer capabilities and a very smooth surface,
try Aqua 50. While retaining many of the heat transfer qualities
of bulk diamond, smaller grain sizes enable a mirror-smooth
finish (RMS < 50 nm).
|
| Optical Transparency |
Wear resistance optical coatings
Windows of diamond thin film |
For photonic applications that need transparency
in the visible spectrum, try Aqua 100, as its large grains
mimic bulk diamond best in a thin film. If you need transparency
in the IR, try our Aqua 100 DOI wafers. DOI wafers provide
diamond on SiO2 on a Si substrate,
which can be reactive ion etched to create windows of diamond
and SiO2. |
| Low friction and wear resistance |
Mechanical seals
Bearings |
All of the UNCD products mimic the low coefficient of friction
of diamond, so you might choose your variety of UNCD partnered
with another attractive property. Aqua 50 is optimized specifically
to provide the best wear resistance for demanding wear applications. |
| Corrosion resistance |
Electrochemical electrodes
Food & pharmaceutical processing |
All of the UNCD products are phase-pure diamond grains and
are thus as inert as carbon and resistant to corrosion and
harsh environments. For a pinhole-free thin diamond smooth
film, use Aqua 25. Although geometries will affect capability,
on a standard Si wafer, Aqua 25’s small grain sizes
allow for pinhole-free films as thin as 400 nm. |
| Mirror-smooth surface |
Low stiction coatings, MEMS
AFM Probes, RF electronics |
For the smoothest surface with all the
benefits of diamond, use Aqua 25. Its grain sizes between
3- 10 nm result in smooth films, with an RMS of less than
15 nm. |
| Biocompatibility |
Orthopedic implants
Implantable biosensors |
All of the UNCD products are phase-pure diamond grains,
and are thus as inert as carbon and resistant to corrosion
and harsh environments. For a pinhole-free thin diamond smooth
film, use Aqua 25. Although geometries will affect capability,
on a standard Si wafer, Aqua 25’s small grain sizes
allow for pinhole-free films as thin as 400 nm. |
Field Emission
Cold Cathode Emission |
Cold cathode devices
Field-emitter arrays |
UNCD has been investigated for years for application to
field emission sources. UNCD films consistently exhibit very
low threshold fields for field electron emission. In addition,
the field electron emission is very stable even when the surface
is exposed to 10-4 Torr of oxygen or hydrogen. Emission currents
as high as 100 µA from a single UNCD-coated silicon
microtip have been observed. (Krauss, 2001, Journal of Applied
Physics, Volume 89, Number 5, Page 2958). Emission currents
as high as 1 mA have been achieved from conformally-coated
arrays of silicon microtips. All UNCD varieties are available
now, on both Si and SiO2, for testing with your emission applications. |
Low stiction
Minimal wafer bow, Low stress |
MEMS
Other wafer-scale products |
Thick films, by nature, are high-stress with their substrate,
but all UNCD products deposit as a thin diamond film, which
reduces stress and wafer bow. |
| Foundry compatibility |
Mass production of MEMS |
All UNCD varieties are now offered on 200 mm wafers for
direct integration into a MEMS foundry process. |
| Electrical conduction |
Conducting AFM probes
Electrochemical electrodes
Electronics (high power, high temperature) |
Diamond is unique as a material because
it is extremely heat conductive, yet (due to its wide band
gap of 5.5 eV) effectively an electronic insulator. However,
UNCD can be tuned, both through its use of grain boundaries
and dopants, to be conductive.
A series of conductive UNCD products is planned. By altering
the deposition process, the electrical conductivity of UNCD
films can be changed over eight orders of magnitude. UNCD
films in development have exhibited the highest N-type conductivity
reported for a phase-pure diamond film and are more conductive
than any doped microcrystalline diamond film or diamond-like
carbon film.
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