MicroElectroMechanical System (MEMS) RF Systems
The projects are to develop MEMS RF/wireless architectures and
components for microwave/millimeterwave
applications. Components will be developed to establish key building blocks
for high-frequency radar/sensors and high-speed communication systems.
Components include high-performance MEMS RF switches, variable
capacitors/inductors, tunable filters, impedance matching circuits, phase
shifters, and nonlinear transmission lines.
Proceedings of SPIE Volume: 4592,
and Process Technologies
for MEMS and Microelectronics II
MEMS Millimeterwave Components
New microelectromechanical system
(MEMS) components that enable reconfigurable MEMS millimeterwave transceivers
were presented with a focus on the device architectures for system integration.
The architectures for reconfigurable antennas, planar impedance tuners,
microswitches and variable capacitors were demonstrated using the
Recent progress in monolithic
millimeterwave active devices has made it possible for implementation of
chip-scale integrated millimeterwave systems. However, due to the low output
power of solid-state sources and high losses in tuning and switching elements,
achievement of high-power or high-sensitivity systems is still a challenge.
Efforts toward reduction of insertion losses in transmission lines and
increasing of Q-factors in passive elements have shown significant improvement
and great promise [1-3]. To further develop a complete millimeterwave system,
reconfigurable components with low losses and high Q-factors are also needed.
Microelectromechanical system (MEMS) devices, which can provide the advantages
of fast actuation, low losses and high Qs, become an attractive option for RF
systems . As frequencies increase to millimeterwaves, the antenna and
component sizes reduce to the scales of MEMS architectures, which provide for
the possibility of fast actuation with low power dissipation by microactuators
[5,6]. Two- and three-dimensional reconfigurable radiation and wave-guiding
structures can be constructed by using metal-coated polysilicon, metal-to-metal
contacts and sacrificial materials on the surface of the devices. This allows
the waves to propagate and be manipulated in the free space and reduces the
dielectric losses and parasitic reactance in the substrate. It also increases
the Q-factors due to the low series resistance of metal.
In this work, we investigated several key
components for a reconfigurable transceiver. Fig. 1 shows a system-level
schematic drawing including a reconfigurable Vee-antenna, planar impedance
tuners, microswitches and variable capacitors. In the architecture, flip-chip
integration of monolithic millimeterwave integrated-circuits with MEMS devices
is proposed. The flip-chip approach separates the fabrication of the MEMS chips
and electronic circuits (RF and IF), allowing for integration of many different
substrate materials, increasing of yield rates and reduction of costs. The main
structures of all components were fabricated using the three-polysilicon-layer
surface micromachining process offered by the Microelectronics Center of North
Carolina (MCNC). Post-processing including deposition and etching of dielectric
material, metal and silicon was done at the University of Hawaii - Manoa.
Figure 1: The proposed architecture of a MEMS reconfigurable transceiver.
Figure 2 shows the concept and a photo of a MEMS reconfigurable Vee-antenna. The arms of the
Vee-antenna are moveable through pulling or pushing by microactuators. Each
antenna arm can be controlled independently with forward- or backward-moving
biases. When both antenna arms move in the same direction with a fixed
Vee-angle, the antenna can be used to steer the radiation beam. When the
Vee-angle changes, the radiation beam shape can be adjusted. The
reconfigurability and antenna performance have been demonstrated .
Beam-steering at 30° and 45° and beam-shaping for Vee-angles of 75°, 90° and
120° have been demonstrated for a 17.5-GHz MEMS Vee-antenna.
Figure 2: (a) The concept and (b) top view of a MEMS reconfigurable
Figure 3: (a) The concept and (b) a prototype for a sliding backshort
Figure 3 shows the concept and a photo of the planar impedance tuner. The
operation principle is similar to the one for mechanical waveguide backshort
tuners used in metal waveguides . A sliding planar backshort plate on top
of a planar transmission line forms a moveable short-circuit over a useful
bandwidth. It allows a variation for the electrical length of the
transmission line by varying the position of the sliding planar backshort.
The planar backshort has a cascade of several sections of short-circuit
separated by quarter wavelengths on the sliding plate. Lubecke demonstrated a
3-dB improvement of detector response over the untuned response at 100GHz
using a quasi-optical approach and a 5-section planar backshort without
electrically-controlled actuators . Planar tuners on coplanar strips
(CPS), shown in Fig. 3(b), and coplanar waveguide (CPW) transmission lines
have been designed and fabricated to investigate the tuning characteristics
and the integration of sliding planar backshort plates with microactuators.
Low insertion losses and low power dissipation for micromachining
membrane-based switches, such as single-pole cantilever switches and air-gap
switches, have been demonstrated. The results are very promising and have
initiated many millimeterwave applications. Practical application issues for
the membrane-based switches include stiction of metal or membrane, stress
control and reliability of the membrane . Therefore, we demonstrated new
MEMS architectures for microswitches using polysilicon to construct rigid
structures and microhinges  for mechanical movement to eliminate the
bending of membrane. Figure 4 shows the concept and a photo of a see-saw-bar
switch. A see-saw bar, with bias electrodes on both ends, is held by hinges
on the sides of the bar. One end of the bar is attached to a metal contact
pad to make connection between contacts. With biases on different bias
electrodes, the metal contact can be pulled up or pushed down to open or
close the connection. The rigid polysilicon bar helps prevent stiction and
degradation associated with the thin-film membrane. A 4-mm air-gap was
achieved with an actuation voltage of 20v. The DC series resistance is
negligible and RF performance is currently under investigation. For a
transceiver, it is necessary to have switches in the T/R module with high RF
isolation and low insertion losses. For a membrane-based switch, it requires
a tall air gap when the switch is open and a strong bending of the membrane
diaphragm when the switch is closed. To achieve high isolation and a good
connection, we proposed a derrick-type microswitch. Figure 5 shows the
concept and a photo for a derrick-type switch. The microactuators pull away
or push forward the pull/push arms which are connected on the sides of the
support bar with moveable hinges. With one end of the support bar held by
fixed hinges on the substrate, the moveable hinges on both ends of the
pull/push arms translate the lateral movement of the actuators to a vertical
movement of the support bar. A metal contact pad, attached in the end of the
support bar, will be pulled up to open, or pushed down to close the
interrupted transmission line. In the design, the air gap between the metal
contact and the transmission line can be as high as 100mm. The measured
switching time for a 100-mm gap is 150ms. The strong actuation force of
scratch drive actuators  can provide a firm contact between metal pads.
The RF performance and reduction of switching time is currently under study.
Figure 4: (a) The concept and (b) a photo for a see-saw-bar switch.
Figure 5: (a) The concept, (b) the cross section and (c) a photo of a
Figure 6: (a) The architecture of a parallel-plate variable capacitor. (b) A
photo of a parallel-plate variable capacitor with 1mmx1mm parallel plates.
Figure 7: SEM photos for a parallel-plate variable capacitor with a gap
spacing of (a) 100microns and (b) 1mm.
Figure 8: (a) The concept and (b) a photo of a circular parallel-plate
Figure 6(a) shows the concept of a MEMS parallel-plate variable capacitor
consisting of two parallel metal plates with variable gap spacings. The
microactuators pull or push the support arms. The microhinges on both ends of
the support arms translate the lateral movement of the microactuators to a
vertical motion in order to vary the gap spacing between two metal plates.
With the actuators on both sides moving toward the plate at the same time
pushing the support arms up, the gap spacing is increased. With the actuators
moving away from the plate, the gap spacing is reduced. The form of the
device is similar to the ones in [12,13] but with different mechanical and RF
design. The variable capacitance should follow the formula, C(x) = eA/x + Cp,
where A is the plate size, x is the gap spacing, e is the dielectric constant
of air and Cp is the parasitic capacitance. The actuators can be programmed
so that not only the height of the gap is variable, but the coupling area (A)
between two metal plates can also be varied for a linear operation of
variable capacitance. Several parallel-plate capacitors, with plate sizes in
the ranges from 100mmx100mm to 2mmx2mm, were designed and fabricated, as
shown in Fig 6(b). The gap spacings can be varied between 1mm and 100microns,
with an increment of 20nm, giving a dynamic range of 1:100. Fig. 7 shows the
SEM pictures for the gap spacings. Preliminary results found a maximum
capacitance of 35pF and a minimum capacitance of 0.5pF, measured by a
low-frequency capacitance meter. The breakdown voltages of the MEMS variable
capacitors should be much higher than diode varactors. The measured breakdown
voltage of the MEMS capacitor is more than 200v, where the measurement was
limited by the availability of high-voltage sources. A different architecture
for MEMS parallel-plate capacitors is also under investigation. In some
applications, the variable capacitors are not required to have a large
dynamic range, but a linear operation and compact configuration. Fig. 8 shows
a circular parallel-plate variable capacitor. The coupling area between the top
and bottom plates can be varied by the microactuators moving in a circular
motion. The top plate is held by a rotation hinge, which is electrically
isolated from the bottom plate, and actuated by the circular scratch drive
actuators on the edges . The actuators on the opposite sides move in the
opposite directions (clockwise or counter-clockwise movements), which allow
the top plate to rotate ±90°. The sizes of the parallel plates are reduced to
eliminate parasitic capacitance. The gap between the parallel plates is 2mm.
The coupling area can be varied by an amount corresponding to a 0.5°
increment in rotation angle.
We investigated several MEMS
millimeterwave components for a reconfigurable transceiver, including
reconfigurable Vee-antennas, planar impedance tuners, microswitches and
variable capacitors. In this paper, we focused on demonstration of
architectures with emphasis on integration issues. A layout database for
designing various components is being established. Many of these components,
such as MEMS microswitches and variable capacitors, can also be used at lower
frequencies such as microwaves and UHF.
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