Custom Product Papers
and Briefs
PECVD Diamond Films for Use in Silicon Microstructures
John A. Herb and Michael G. Peters—Crystallume, Menlo Park, CA
Stephen C. Terry and J. H. Jerman—IC Sensors, Milpitas, CA
ABSTRACT
and 3 times higher than silicon nitride. In addition, the dry
PECVD techniques have been used to deposit polycrystalline
coefficient of friction of diamond is between 0.05 and 0.15,
diamond films which have material properties nearly identical
similar to Teflon™. These properties make diamond very
to those of natural diamond. Diamond is the hardest known
useful in wear applications and indeed, the wear rate of
material and has the highest thermal conductivity at room
PECVD diamond films has been measured to be a thousand
temperature, while undoped material is an excellent insulator.
times less than that of SiC.1 In general, for those properties
These and other mechanical, chemical, and electrical
measured, the data for PECVD diamond matches those for
properties make diamond films attractive for use in
the natural material.
applications such as optics, electronic packaging, and parts
A number of other properties of natural diamond, along
with low wear rates. An x-ray window device has been
with a comparison with the other common diamond-type
fabricated using a diamond window and a micromachined
semiconductors, silicon and gallium arsenide, are given in
silicon substrate.
Table 1.2
INTRODUCTION
Table 1. Physical Properties of Common DiamondType Semiconductors
The vast majority of solid state sensors and actuators derive
much of their utility from the various thin films which are
part of the structure. The deposition of polycrystalline
diamond films by Plasma Enhanced Chemical Vapor
Deposition (PECVD) is an emerging technology of great
significance for a variety of these structures, due to the
superior material properties inherent in true diamond films.
Measurement
Si
Band Gap (eV)
1.10
Electrical Resistivity (Ω)
Hole Mobility (cm2/V-s)
Electron Mobility (cm2/V-s)
3
GaAs
Natural
Diamond
1.43
5.45
8
1x1016
1x10
1x10
600
400
1600
1500
8500
1900
3 x 105
3.5 x 105
1 x 107
2.5 x 10–3
10–8
10–10
1 x 107
2.5 x 107
2.7 x 107
These films can be deposited directly on silicon substrates,
Breakdown Field (V/cm)
which can be further processed using conventional micro-
Carrier Lifetime (s)
machining and processing techniques to form structures and
Electron Velocity (Max)(cm/s)
sensors which utilize the unique properties of diamond films.
Work Function (eV)
4.8
4.7
4.8
Dielectric Constant
11.0
12.5
5.5
PHYSICAL AND CHEMICAL PROPERTIES
Hardness (kg/mm2)
1000
600
10,000
The mechanical properties of diamond are equally attractive.
Thermal Conductivity at 25°C
(W/cmÞ°K)
1.45
0.46
20
Lattice Constant (Å)
5.43
5.65
3.57
3.5
3.4
2.41
2.6 x 10–6
5.9 x 10–6
0.8 x 10–6
1420
1238
≈3550
2
It has a hardness of about 10,000 kg/mm , the highest of any
known material, more than twice that of silicon carbide or
cubic boron nitride and approximately 10 times that of
silicon. Similarly, diamond has a very high Young’s modulus
of 10.4x1012 dynes/cm2, more than 5 times that of silicon
Refractive Index
Thermal Expansion Coeff.
(°K–1)
Melting Point (°C)
1.
Gardos, M.N. and Ravi, K.V., Tribological Behavior of CVD Diamond Films, presented at the Spring Meeting of the Electrochemical Society in Los Angeles, CA, May 7-12, 1989, pp. 153
2.
Albin, S., et. al., Laser Damage of Diamond Film Windows, SPIE Vol. 877, Micro-Opto Electronic Materials, 1988.
Custom Product Papers and Briefs
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PECVD Diamond Films for Use in Silicon Microstructures
DEPOSITION TECHNIQUES
Diamond films are produced using low pressure, plasmaenhanced CVD techniques which typically use a methane and
hydrogen feed gas stream at temperatures between 600°C and
1000°C in a proprietary reactor system. Details of similar
deposition techniques can be found in a number of
references.3,4 These process conditions are similar to those
used to produce other common CVD and PECVD films such
as silicon nitride and thus are compatible with typical
integrated circuit processing steps for sensor structures.
Diamond films can be deposited with thickness variations of
only 10% and no distinguishable differences in composition
across a 100-mm diameter wafer. In addition, they are
Figure 1. Raman Spectra of Natural Diamond
The Raman spectrum of a PECVD diamond film is
conformal to the substrate; continuous diamond coatings
essentially the same, and an example from a high quality
have been achieved on geometries as complex as a twist drill.
sample is shown in Figure 2. DLC films on the other hand
have a broad peak around 1580 cm -1 indicative of sp2 bonded
Like natural diamond, PECVD diamond films are
crystalline and composed of sp3 covalently bonded carbon in a
carbon. In general, the DLC materials are softer and less
tetrahedral arrangement and contain little or no incorporated
transparent than diamond, and they have a very high
hydrogen. In contrast, diamond-like-carbon (DLC) coatings
proportion of hydrogen (up to 60 atomic percent). When
2
are amorphous in nature and have a preponderance of sp or
heated to about 400°C, DLC films lose their hydrogen and
graphitically bonded carbon in their microstructure. The
graphitize. PECVD diamond films are thermally stable up to
3
difficulty inherent in providing sp bonding in the thin films
650°C in air, and to over 1500°C in neutral, reducing, or
is that the formation of double bonds between carbon atoms
vacuum environments. PECVD diamond films oxidize in a
is slightly favored energetically, leading to the formation of
relatively complex fashion. Because the films are
graphite. Diamond films can easily be distinguished from
polycrystalline, and because the intergranular material
DLC by Raman Spectroscopy. The Raman spectrum of a
contains appreciable quantities of non-diamond carbon,
natural diamond shows a strong narrow peak at about 1333
oxidative attack occurs first at film grain boundaries.5 Onset
wavenumbers, characteristic of sp3 bonded carbon (Figure 1).
of oxidation can be detected at approximately 650°C at 76
Torr O2 partial pressure.
3.
Sokolowski, M., et. al., Reactive Pulse Plasma Crystallization of Diamond and Diamond-Like Carbon, J. Crystal Growth, vol. 52,
pp. 219-226, 1981.
4.
Sawabe, A. and Inuzuka, T., Growth of Diamond Thin Films by Electron-Assisted Chemical Vapor Deposition and Their Characterization, Thin Solid Films, vol. 137, pp. 89-99, 1986.
5.
Plano, Linda S., Yokota, S., and Ravi, K.V. Oxidation of DC PECVD Diamond Films, presented at the Spring Meeting of the
Electrochemical Society Extended Abstracts, Los Angeles, May 7-12, 1989, p. 153.
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Custom Product Papers and Briefs
PECVD Diamond Films for Use in Silicon Microstructures
Figure 2. Raman Spectra of PECVD Diamond Material
In addition to the nature of the bonding, it is also
necessary in most applications to produce films with uniform
thickness and composition, microstructure, grain size and
morphology, and adhesion to the substrate. By modifying the
surface preparation of the silicon substrate and the deposition
conditions, the grain size and morphology can be varied to
suit a particular application. Examples of these different films
are shown in Figure 3.
The chemical inertness of diamond films is beneficial in a
Figure 3. Grain Size and Morphology Variations
in Diamond Films
APPLICATIONS
The early applications for PECVD diamond films fall into
three categories, optics, tribology and wear, and electronics.
All of these areas offer opportunities in the sensor field.
Diamond films can be used either as an active part of a sensor
structure, or as a packaging or encapsulation material for its
thermal and wear properties. Of the many substrates possible
for diamond deposition, silicon is particularly interesting
number of applications and in the general processing of
because of the good diamond-silicon adhesion possible and
diamond coated microstructures. The films are impervious to
the opportunity to use silicon micromachining to form a
the usual concentrated acids and bases as well as organic
variety of structures utilizing the properties of diamond.
solvents, and they hold up well during oxygen and SF6 planar
plasma etching. These properties also make the films difficult
to pattern. Diamond films fabricated during this work have
been RF sputter-etched in an argon ambient at a rate about
100 Å/min. By using an appropriate mask, this allows the
films to be patterned. Plasma-activated etching of natural
diamond specimens has been developed to provide thin
diamond windows for X-ray transmission applications. Etch
rates of up to 1.9 µ/hour were demonstrated.6,7 It has also
been shown that ion-beam-assisted etching of natural
diamond can be used to produce grating structures tens of
micrometers deep.8
One such device, a diamond x-ray transmission window
on a silicon support grid, has been made which highlights the
advantages of diamond films in combination with silicon
micromachining techniques. By capitalizing on the high
transmissivity of diamond and its ability to form continuous
thin films on silicon, this device can replace standard
beryllium vacuum windows and greatly increase the sensitivity
to x-rays from light elements such as carbon and oxygen.
These windows are typically used in analytical instruments
associated with scanning electron microscopes to isolate the
x-ray detector from the sample chamber. Thus the diamond
window must withstand a one atmosphere pressure
differential and exhibit a very low diffusion rate through the
membrane.
6.
Garwin, E.L., Reactive Sputter-Thinning of Large Diamonds While Preserving Excellent Crystalline Perfection; SLAC Publication
No. 1933, May 1977, pp.1-4.
7.
Chu, Wei-Kan and Sandu, G.; Materials Processing of Diamond: Etching, Doping by Ion Implantation and Contact Formatin;
Annual Technical Report, Contract Number N00014-87-K-0243, Office of Naval Research, Sept. 30, 1988.
8.
Efremow, N.N., et. al., Ion-Beam-Assisted Etching of Diamond, J. Vac. Sci. Technol. B, vol.3, n. 1, Jan 1985, pp. 416-418.
Custom Product Papers and Briefs
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PECVD Diamond Films for Use in Silicon Microstructures
The windows are fabricated by growing a diamond film
silicon, but poorly or not at all to germanium or zinc sulfide).
less than 0.5 µm thick directly on a silicon wafer. The wafer is
The reduction of scattering requires better control of surface
then etched, using a previously defined pattern on the back
morphology and grain structure. The best PECVD diamond
side, until the etch stops against the diamond film on the
IR coating would probably be a single-crystal film, but that
front side. The resulting diamond window is 6 mm in
technology is not yet available.
diameter supported with a series of silicon bars permitting in
excess of 70% transmission, as shown in Figure 3. This
structure has a helium leak rate less than 1x10–9 STP cc/sec
Diamond’s optical properties suggest that PECVD
diamond will find increasing use in the optical industry as
ability to control its synthesis matures. Diamond’s 5.4 eV
and yet is capable of withstanding well over 5000 pressure
band gap results in a UV absorption edge of 225 nm. At the
cycles of a one atmosphere pressure differential. The thermal
other end of the spectrum, it is transparent to longer
expansion of diamond is similar to silicon dioxide, and since
wavelengths ranging from near IR to radio frequencies, with
the diamond is deposited at elevated temperatures, the
the exception of phonon-mediated absorption features in the
diamond film is under compressive stress at room
3µ - 5µ range. The natural material has been pumped to give
temperature. Depending on the extent and thickness of a
tunable coherent output at 540 nm. Refractive index of the
diamond diaphragm, buckling of the diaphragm can occur.
PECVD material varies from about 1.8 to 2.4 at 632.8 nm
depending on synthesis parameters. Other optical applications
for PECVD diamond range from near-term uses such X-ray
lithography mask membranes to longer-term applications like
lasers and diamond optical elements.
THERMAL APPLICATIONS
Diamond has the highest known thermal conductivity at
temperatures between 100°K and 1000°K. Near room
temperature it is nearly 3 times higher than the best metals
Figure 4. Silicon X-Ray Window with Diamond Film
The extreme hardness and low coefficient of friction of
and is over 10 times greater than some of the commonly used
electrical insulators (Table 2). The PECVD diamond films
diamond make the material attractive for applications where
have a measured thermal conductivity very close to that of
wear is important. In conventional applications, diamond
natural diamond.9,10,11 Its very high thermal conductivity in
coatings are being investigated for use in machine tools and
conjunction with its properties of chemical inertness, high
other similar applications. When combined with silicon
electrical resistivity and extreme hardness make diamond a
micromachining techniques numerous applications in the
very attractive material for a wide variety of applications
fields of thermal printing heads, thin film disk drive heads,
where thermal management is an important problem.
and slurry flow measurement devices are evident. Early wear
data on PECVD diamond films indicate that the material
shows nearly 3 orders of magnitude less wear than silicon
carbide under similar conditions, suggesting great utility as a
hard coating for IR optics, particularly in high-speed airborne
systems subject to raindrop and dust impact. Further work in
this area needs to focus on optical scattering and adhesion to
IR optical materials (PECVD diamond adheres well to
9.
A. Ono, T. Baba, H. Funamoto, and A. Nishikawa, Japanese Journal of Applied Physics, vol. 25, n. 10, Oct. 1986, pp. L803-L810.
Morelli, D.T., C.P. Beetz, and T.A. Perry, J. Appl.Phys. vol. 64, Sept. 15, 1988, pp.3063-3066.
11.
Herb, J.A., et al; Proceedings of the First International Conference on Diamond and Diamond-Like Films; Los Angeles, May 712, 1989, to be published.
10.
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Custom Product Papers and Briefs
PECVD Diamond Films for Use in Silicon Microstructures
Table 2. Thermal Conductivity of Selected Materials
at 100°C (W/cm-°K)
Natural Diamond (max.)
PECVD Diamond
ELECTRONICS APPLICATIONS
In electronics applications PECVD polycrystalline diamond is
14.0
attractive either as a passive or active material. As a passive
10.0-13.0
layer the high thermal conductivity and moderate dielectric
constant coupled with the high resistivity may offer
ELECTRICAL CONDUCTORS
Silver
4.3
advantages over materials such as silicon nitride. Since
Copper
4.0
diamond is a crystalline semiconductor, it is possible to
Graphite
2.1
fabricate active electronic devices in diamond, such as
Silicon
1.5
polydiamond FET’s, bipolar transistors, and electro-optical
switches.12,13 The advantages of diamond in these
ELECTRICAL INSULATORS
SiC
2.7
applications are due to the high band gap and high electron
Beryllium Oxide
2.2
and hole mobility, particularly at high electric field strength.
AlN
1.7
Boron doped PECVD diamond has been investigated 14 and
Al2O3
0.4
resistivities between 0.001 and 1 Ω cm were obtained. These
Sapphire
0.3
p-type resistors should be able to be used as piezoresistors in a
SiO2
0.014
With the ability to deposit high thermal conductivity
diamond over large areas and varied shapes, this technology
provides new solutions for a wide variety of electronic
packaging problems. The increased heat load being generated
by high performance electronic systems results in increased
operating temperatures and can result in unacceptably short
Mean Time To Failure (MTTF). Typically, semiconductor
manner similar to p-type polysilicon resistors.
Potential applications of active diamond electronic
devices include production of electronics which can operate
with sensor elements in hostile environments. Signal
processors which could operate at high temperatures,
integrated with sensor elements in jet engine combustion
chambers may be feasible with diamond electronics.
junction temperatures cannot exceed 120°C and still maintain
CONCLUSIONS
acceptable MTTF. Moreover, lower temperatures are highly
As the techniques for processing diamond films on silicon
desirable in order to increase the MTTF and may be necessary
continue to advance, the films will be incorporated in devices
in systems with very large numbers of devices. Cooler
for their mechanical, chemical, thermal, and electrical
operating temperatures also improve performance. Increased
properties. They represent valuable enhancements now
switching delays, reduced amplifier gain and increased diode
available in the design and fabrication of silicon
laser threshold current are only a few examples of the
microstructures.
deleterious effects of increased operating temperature. In
certain applications like those involving IMPATT diodes and
high power laser diode or laser diode arrays, heat sinks of
This paper presented at Transducers ‘89, Montreux,
Switzerland, June 28, 1989.
single crystal diamond are necessary to insure the best
performance. However expense and availability limit the use
of natural diamond to those applications having small size
requirements. The size and cost limitations encountered in the
use of natural diamond are not encountered with the use of
PECVD diamond films.
12.
Prins, J.F., Bipolar Transistor Action in Implanted Diamond, Appl. Phy. Letters, vol. 41, n. 10, pp. 950-952, 1987.
Panchhi, P.S., and Van Driel, H.M., Picosecond Optoelectronic Switching in Insulating Diamond, Optics Lett., vol. 10, n. 10, pp.
481-483, 1985.
14.
Fujimori, N, T. Imai, and A. Doi, Characterization of Conducting Diamond Films, Vacuum, vol. 36, n. 1-3, pp. 99-102, 1986.
13.
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PECVD Diamond Films for Use in Silicon Microstructures
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Custom Product Papers and Briefs