Comb-drive is an electromechanical actuator, having set of fixed and movable
comb fingers which are interdigitated with each other and they are fixed and
movable electrodes respectively as shown in Fig. 1. When external
power supply is given to actuator, due to generated electrostatic attraction
force, a micro-tilting movement occurs as shown in Fig. 2
and it is used to do switching or scanning functions. A reflective layer is
deposited on the moving top plate electrode where the light path is getting
tilted due the tilt movement of the actuator. Large aspect ratio provides large
range of actuation motion; here aspect ratio, defined as the ratio between comb
finger height and the finger separation gap. In the microfabrication process,
substrate with functional layers are formed by a combination of deposition,
lithographic patterning and etching operations. MEMS actuator elements are beams
or trench/cavity structures. These thin film structures are made for applications
requiring large flat surfaces and large actuation displacement. Uinn
et al. (2006) discussed these fabricated micro structures which can
be used as microactuators applications such as optical switches, scanning mirrors
of micro-optical systems. In microfabrication process, substrate with functional
layers are formed by a combination of deposition, lithographic patterning and
etching operations. MEMS actuator elements are beams or trench/cavity structures.
These thin film structures are made for applications requiring large flat surfaces
and large actuation displacements. These fabricated micro structures can be
used as microactuators applications such as optical switches, scanning mirrors
of micro-optical systems.
Bhusan (2007a) discussed that, a structure under tension
will contract and a under compression will expand. For all of these passive
structures, the forces acting on them come from the internal residual stresses
of the structural material. When a voltage is applied across either set of the
interdigitated comb fingers shown an electrostatic attraction is generated due
to the increase in capacitance as the overlap between the comb fingers increases.
This tensile stress can be thought of as a tensile load being applied at the
ends of the beam.
|| Micro mirror driven by vertically staggered comb-drive actuator
|| A vertically staggered comb driven micromirror implemented
in the surface micromachined MUMPS process
Microfabrication techniques and comb-drive types: The state of art of
technologies available for the microfabrication are PolyMUMPs (Poly-Multi-User-MEMS-Process),
DRIE (Deep Reactive ion beam etching, SOI (silicon On Insulator), Soft-lithographic
Lift-off and Grafting (SLLOG), LIGA, Nano-imprint lithography (NIL), SUMMiT
(Sandia Ultra-planar, Multi-level MEMS Technology). The X-Ray and UV-LIGA fabrication
techniques are used to realize three dimensional structures.
Different types of comb-drive systems such as vertical comb-drive, angular
vertical comb-drive, pre-stress comb-drive, self-aligned vertical comb-drive,
staggered vertical comb-drive.
Chiou and Lin (2005) discussed that it is highly desirable
to fabricate an actuator with no pull-in and no hysteresis characteristics.
It is realized through Pre-stress comb-drive Actuator (PCA) which consists of
a set of comb fingers fabricated along the laminated beam and substrate. One
end of the laminated beam is fixed to the anchor and the other end is raised
vertically by the residual stress. The fringe field induces electrostatic actuation
force which in turn pulls the laminated beam towards the base substrate. To
obtain large stroke of the PCA by increasing the initial lift height, post annealing
process was carried out.
Edwin et al. (2005) discussed that high aspect
ratio comb-drives need optimization in their fabrication steps. Various fabrication
steps are followed according to the actuator-specific requirements. Anisotropic
etching is controlled selectively in fabricating self-aligned comb-drives. The
offset type comb-drive generates actuation forces five times stronger than self-aligned
type. Self aligned comb-drive type is fabricated from SOI multiple layers.
Kwon et al. (2004) discussed that in order to
achieve the bidirectional actuation with larger actuation range can be realized
through defining static and movable comb fingers in the SOI layer and they are
isolated by trenches.
|| Micromirror with single-sided actuations
With low voltage the high-aspect SOI structures are capable of generating high
force density for larger actuation displacement.
Stress related issues in microfabrication: Thermally induced stress
and intrinsic film stress are two main sources of residual stress. The first
one is due to the mismatch of thermal expansion coefficients between the thin-film
layer and the substrate. The second one is originating from nucleation, grain
growth and impurities added to the thin-film during deposition step. The stable
deflection range can be significantly improved if the suspension structures
are fabricated in a pre-bent configuration.
Edwin et al. (2005) discussed that, stress gradient
is equally important as the stress itself. Due to stress gradients in the layer
structure, they try to bend upward or downward which is not desirable; since
the structures are designed to be completely planar. High aspect ratio comb-drives
need optimization in their fabrication steps. Various fabrication steps are
followed according to the actuator-specific requirements. Anisotropic etching
is controlled selectively in fabricating self-aligned comb-drives. The offset
type comb-drive generates actuation forces five times stronger than self-aligned
type (Fig. 3). Self aligned comb-drive type is fabricated
from SOI multiple layers.
Kim et al. (2006a) discussed that, in a stress-strain
curve of silicon it is shown that where σm is maximum yield
stress, σf is flow stress, the stress needed to continue plastic
deformation as shown in Fig. 4. At higher temperature (900°
C) annealing step changes the initial elastic deformations of the torsion hinges
that join the two comb finger electrode sets into permanent plastic deformations.
||The permanent plastic deformation and elastic recovery
Tabata et al. (2008) discussed that, the actuator
performance is influenced by the elastic properties such as Youngs modulus,
Poissons ratio and shear modulus. The stiffness of laminated thin-film
structural layer is proportional to the above elastic properties and further
it depends on the internal stress.
Pull-in, hysteresis, stiction are the limiting parameters and they are the
causes for instability of the performance of the actuators and these parameters
can be prior-controlled by managing the stress level during the fabrication
Edwin et al. (2005) discussed that, vertical
comb-drive actuation is generated parallel to the displacement direction and
the lateral pull-in instability causes misalignment in finger gap results in
poor performance of the vertical comb-drive. Vertical actuation which is generated
parallel to the direction as it moves towards.
John et al. (2003) discussed that, the side
suspension spring stiffness is nonlinear. The spring constant is a combined
value of the axial stiffness of the individual suspension beams and the geometric
stiffness of the suspension as a whole. The side spring constant of the suspension
is found to decrease with the square of the forward movement. The stiffness
in the side direction decreases initially and it goes up as the suspension beams
becoming straight with forward deflection and when side stiffness increases
in tune with side forces increase.
A brief summary of the different fabrication process
Gimbaled comb-drive: Kim et al. (2002) discussed
that, the new idea in this fabrication by introducing built-in air gap and large
rotation angle for gimbal structure self aligned comb-drive is achieved.
||The fabrication process of the vertical comb-based gimbaled
micromirror. The backside island is incorporated into the fabrication process
for the vertical comb drives
||Plastically deformed and tilted mirror and precisely aligned
DRIE technique is used to fabricate these structures as shown in Fig.
5. Over etching of Silicon Nitride (Si3N4) and Aluminum
(Al) took place during releasing process. High degree of flatness and high optical
reflectivity on the top surface of the actuator is achieved through a low-stress
It is suggested that the cross-coupling effect between the gimbals can be minimized
by controlling the stress level around the inner and outer hinges.
Pre bent multilayered comb-drive: Chiou et al.
(2007) discussed that, a pre-stress comb-drive system fabricated using PolyMUMPs
fabrication sequence summarizes as follows. Starting with nitride isolation
layer, three polysilicon structural layers (Poly0-2), two phosphosilicate glass
sacrificial layers (PSG1 and PSG2) and a gold metal layer (Au) for optical reflection
and electronic circuit interconnection embedded in between layers.
Edwin et al. (2005) discussed that, the internal
stress of the hinge decides the height of the upper finger of the pre-stress
Self aligned vertical comb-drive: Vertical comb-drive actuators are
suitable for requirement smaller actuator size, low driving voltage and large
vertical displacements. A single step lithography fabrication technology which
employs multiple thin layers of conductors separated by thin layers of dielectrics.
This technology is used to achieve the high aspect ratio self-aligned vertical
comb-drives with smaller gaps between finger electrodes. Small finger gaps resulting
in high force densities.
Edwin et al. (2005) discussed that, when surface
micromachining technique is used to fabricate vertical comb-drive, normally
it yields a small self-aligned finger gaps and due to low aspect ratio larger
displacement can not be obtained from it.
Angular vertical comb-drive: The torsion bars are permanently deformed
and actuated. the precisely aligned vertical-comb sets.
|| Working principle of vertical comb actuators
Figure 6 is a close-up view of a plastically deformed torsion
bar showing that the deformation appears to be uniform along the torsion bar.
By using the method of plastic deformation , the maximum value of initial tilt
angle is limited by the fracture strength of the Silicon at room temperature.
Stiction related Performance issues of comb-drives: Stiction selectively
used in positive way to fabricate self -aligned angular vertical comb-drive
made of single-crystal silicon as shown in Fig. 7. A new structure
called stiction plate with mechanical spring used to reduce the unwanted compliances
on the actuators there by the uncontrolled motion is prevented and stability
is ensured during operation.
Kim et al. (2006a) discussed that, the critical
fabrication step is summarized as follows as shown in the Fig.
8. The critical point CO2 drying process involves two important
steps. In the first step solvent such as methanol or isopropyl alcohol is dispensed
with high-pressure liquid CO2 . In the next step temperature is raised
above the critical point of CO2. During releasing of the structure
there is no liquid-solid interface is formed during the process. This step is
used to prevent the stiction which is not desirable during releasing of the
Kim et al. (2006b) discussed that, the moving-
combs and static-combs are coplanar provided they are fabricated in the same
device layer of SOI wafer. Coplanar configuration cannot induce vertical displacement,
when voltages applied between the comb fingers. In the next step, the combs
attached to mirror top are deformed from their default orientations. It is done
by pressing on them with a separately formed silicon substrate. This substrate
is processed separately so that it has a precisely located array of pillar structures.
The pillar structures are located such that each of them deforms one pair of
torsion bars. It is done by pushing the corresponding mirror edge towards down
from its initial default position. In the next step high-temperature at 900°C
annealing process changes the initial elastic deformations of the torsion bars.
|| The fabrication sequence of comb-drive, (a) Pattern on front
and back side of SOL (50 μm device layer), (b) DRIE on front and back
side, (c) Oxide wet etching in HF to release Si structures, (d) Drying and
These bars connect the two comb finger sets into permanent plastic deformation
portions. With the comb pairs in this angled configuration, driving voltages
applied between them there by inducing torque in the torsion-bar support structure
that connect them.
Edwin et al. (2005) discussed that, with a single
DRIE step where the upper and lower finger-electrodes are separated by the internal
stress of an attached hinge part. Angular vertical comb structures can be etched
from an SOI layer which are capable of producing high aspect ratio comb finger-electrodes.
The offset comb finger-electrodes are formed using a combination of Deep Reactive
Ion beam Etching (DRIE), growth of barrier layer oxide and a dry isotropic selective
etch of the upper comb finger electrode layer.
Compound comb-drive: By using a bulk-and-surface mixed silicon micromachining
process the compound comb-drive is fabricated in SOI substrate. The compound
driving structure is combination of a planar plate drive and a vertical comb
drive. This structure can actuate the mirror to achieve large-range analog rotation
as well as binary 90° rotation induced by the pull-in effect where this
effect is used in positive way in a voltage control fashion for its actuation.
VCD section is used in analog mode of operation where rotational movement is
obtained and the PPD section is used in binary mode of actuation where spontaneous
90° rotation induced by the pull-in effect.
Wu et al. (2007) discussed that the elastic
torsion springs were fabricated thinner than the micromirrors for low driving
voltage. The length of movable comb fingers and the thickness of fixed comb
fingers are the deciding factors of actual maximum rotation angle.
Elman et al. (2008) discussed that the MASIS
process (Multiple aspect ratio structural integration in single-crystal-silicon)
a novel fabrication process was especially introduced for fabrication of MOEMS
(Micro-Opto-Electro-Mechanical-Systems) actuators which are driven by long-stroke
comb-drive actuators. This process is implemented for the fabrication of single
crystal silicon structures having distinct aspect ratios in the same actuator
Residual stresses in polysilicon: The formation of residual stresses
of LPCVD polysilicon summarized here and the residual stress as a function of
deposition temperature taken from five different investigations from literatures.
||Typical results for residual stress as a function of temperature
for an aluminum film on a silicon substrate, Thickness 590 nm
All five sets of data show the same trend. The stress undergoes a change from
compressive nature at the lower deposition temperature to tensile nature at
intermediate temperature and returning back to compressive nature at the higher
temperature. In each data set the transitions are easily perceptible. The microstructure
of the LPCVD polysilicon films influences the origin of these residual stress
changes. Deposition conditions dictate the microstructure of LPCVD polysilicon
films. The thin films are grown at temperature slightly lower than 570°C
are in amorphous nature and they show fine grain structure at the temperature
range of 570 to 610°C. The grain structure is columnar at the 610 to 700°C
and also a fine-grained layer is formed at substrate and layer interface.
The slope of the heating curve for an aluminum film on a silicon substrate.
gives the difference in thermal expansion between the film and the substrate.
When the heating curve changes slope and becomes nearly horizontal, the yield
strength of the film is reached as shown in the Fig. 9. The
deposition rate is faster than crystallization rate during the homogeneous nucleation
and growth of grains within the regime as-deposited film which is amorphous
in nature and results in fine-grained microstructure. The as-deposited films
are crystalline at the substrate interface and amorphous at the free surface.
The amorphous part can be crystallized by annealing process which is done at
At higher growth temperature the columnar grain structure is formed and the
growth rate is faster in <110> directions. The crystallization of the
as-deposited amorphous material is accompanied with the formation of tensile
stress due to volume decrease in fine-grained structure. During the film growth
in amorphous and columnar films can increase the surface chemical potential.
It occurs due to increase in surface chemical potential which is caused by the
atoms deposited from the vapor. The increase in surface chemical potential further
promotes atoms to flow into newly generated grain boundaries, inducing a compressive
stress in the film.
Bhusan (2007b) discussed that the perfect bonding between
the two films and the total strain must be continuous across the interface.
The elastic moduli are different for the two films, so the stresses are not
continuous at the interface.
The PECVD oxide (on a silicon substrate) curvature stress measurement result
for a temperature cycling. It is also easy to distinct the thermal and non-thermal
stresses from the stress-temperature diagram as shown in Fig.
10. As the substrate thickness increases, the stress present in the layer
decreases with the higher value occurring at the interface. The stress in the
substrate increases at the interface but decreases on the bottom surface. The
stress distribution in the substrate that is compressive nature on the bottom
surface and tensile nature at the substrate-interface. Depending on thickness,
the stresses in the layer are either tensile in whole or partly tensile on the
interface and partly compressive on the free surface. With the increase of thickness,
the stress difference between the interface and the surface also increases and
it is a measure of stress gradient in each thin-film as shown in Fig.
|| Decomposition of thin-film stress into its components
||Characteristics of the normal and shear stress distribution
at the film/substrate interface near the free edge
It is suggested that in order to reduce the residual stress the Radio Frequency
plasma enhanced chemical vapor deposition (RF-PECVD) which supports the processing
temperature in the range of 100-250°C can be considered.
The causes and effect of residual stress formation during the microfabrication
process is studied from the literatures. This knowledge can be used to correlate
the performance of MEMS actuators with the fabrication critical parameters.
The critical steps that are followed to control the residual stress are presented
from literatures. Pull-in and stiction phenomena are positively used to achieve
the featured actuations in comb-drive system. Application and performance specific
fabrication methods for MEMS comb-drive actuators are briefly summarized.