matic stress relief is accomplished by ther-
mal annealing for a period of minutes at a
temperature of 600ꢂC (a similar annealing
step is required to reduce the stress in poly-
silicon used for MEMS, but typically a
temperature of over 1000ꢂC is required).13
The resulting stress-free films are hard, opti-
cally transparent, moderately conductive,
and atomically smooth (0.1 nm root-mean-
square roughness on Si, 1 nm roughness
on SiO2).3
Control of film stress is one of the
most critical issues facing any potential
MEMS material, particularly for surface-
micromachined structures, whose structural
elements can have large aspect ratios (beam
length/beam thickness can exceed 1000).
Generally, a prospective MEMS material
should exhibit a residual stress of ꢃ10 MPa
(ideally, just a few megapascals). For a high-
modulus material, this is an extreme re-
quirement; for example, for diamond or
amorphous diamond this stress level cor-
responds to an elastic strain of about
0.00001 (or about 1 Å of displacement for
every 10 ꢄm of beam length, assuming uni-
axial loading). Equally important is the con-
trol of stress gradients through the
thickness of the material. These stress gra-
dients give rise to out-of-plane distortion
of the MEMS structural element and can
prevent proper meshing of gears or other
more serious alignment errors. Ideally, the
radius of curvature of a MEMS beam
should approach or exceed 1 m (this ra-
dius of curvature would cause a 100-ꢄm-
long beam to deflect about 50 Å out of the
film plane).
For surface-micromachined structures,
smooth structural layers deposited as thin
films (1–2 ꢄm thick) are required. Mainte-
nance of layer planarity becomes increas-
ingly important as the number of structural
layers increases. For single-structural-level
surface-micromachined amorphous dia-
mond MEMS, fabrication is straightfor-
ward: (1) a sacrificial layer, typically 2 ꢄm
of SiO2, is deposited on Si; (2) stress-free
amorphous diamond is deposited on top of
this sacrificial layer to a thickness of 1–2 ꢄm;
(3) the diamond layer is patterned using
photolithography and an oxygen plasma
etch; and (4) the sacrificial layer is partially
removed beneath the diamond layer by a
timed wet chemical etch, leaving pedestals
of SiO2 supporting completely undercut
sections of diamond film. Figure 2 is an
example of an amorphous diamond electro-
static motor (a comb-drive structure) fabri-
cated in this manner.
Diamond and
Amorphous
Carbon MEMS
J.P. Sullivan, T.A. Friedmann, and K. Hjort
Introduction
The designer of microelectromechanical
approach crystalline diamond in hardness
(up to ꢀ90 GPa) and modulus (800ꢀ GPa).
The main appeal of these materials for the
MEMS designer, however, lies in their ex-
treme wear resistance (up to 10,000 times
greater wear resistance than Si),1 their hy-
drophobic surfaces with inherent stiction
resistance (parts do not stick together due
to capillary forces from entrapped water),2
and their chemical inertness (which allows
their use in aggressive chemical environ-
ments). Recently, researchers have made
considerable progress in the fabrication of
MEMS structures from these materials, both
in the area of surface micromachining and
in mold-based processes.3–10
systems (MEMS) can increase MEMS per-
formance either by improved mechanical
design or by the selection of a MEMS
material with improved mechanical per-
formance. In the quest to identify high-
performance MEMS materials, diamond
and amorphous carbon have recently
emerged as a promising class of materials.
These materials offer excellent tribological
properties, low-stiction (hydrophobic) sur-
faces, chemical inertness, and high elastic
moduli. The primary challenge with these
materials lies not in improving the mate-
rials’ performance, but rather in integrating
their relatively new deposition processes
with the well-established processes of the
silicon microelectronics industry.
Diamond has the highest hardness
(ꢀ100 GPa) and elastic modulus
(ꢀ1100 GPa) of all materials (see Figure 1).
Amorphous forms of carbon, specifically
the hard carbons, amorphous diamond
(a-D), tetrahedral amorphous carbon (ta-C),
and diamond-like carbon (DLC), can also
Materials Requirements for
Surface-Micromachined MEMS
and Amorphous Diamond MEMS
On the most highly disordered (i.e., amor-
phous) end of the range of carbon materials
(diamond being the most highly ordered),
one new class of carbon MEMS materials is
a stress-relieved form of hard amorphous
carbon, colloquially referred to as stress-free
amorphous diamond.3 Technically, the
material is best described as a mixture of
amorphous nanophases of tetrahedrally co-
ordinated carbon, comprising about 70%
of the total, with threefold-coordinated car-
bon comprising the remaining 30%. The
threefold-coordinated carbon is not ran-
domly distributed but is instead clustered
as conjugated chain-like or, perhaps, sheet-
like structures.11 The material is deposited
at room temperature using an energetic
pure-carbon beam that contains a signifi-
cant fraction of carbon ions with energies
peaked near 100 eV.12 Because of subsur-
face implantation of the energetic carbon
species, these films typically exhibit ex-
tremely high levels of compressive stress
(8 GPa); surprisingly, with suitable control
of film deposition, 100% stress relief (down
to 0 ꢁ 10 MPa) can be achieved. Such dra-
The MEMS structures themselves have
proven to be particularly valuable for as-
sessing film mechanical properties, particu-
larly because some of these materials have
no bulk analogue in nature and therefore
Figure 1. Hardness and elastic modulus
of a variety of hard materials.
MRS BULLETIN/APRIL 2001
309