Compound solid state switches such as
GaAs MESFETs and PIN diodes are widely used in microwave and millimeter wave
integrated circuits (MMICs) for telecommunications applications including
signal routing, impedance matching networks, and adjustable gain amplifiers.
However, these solid-state switches have a large insertion loss (typically 1
dB) in the on state and poor electrical isolation in the off state. The recent
developments of micro-electro-mechanical systems (MEMS) have been continuously
providing new and improved paradigms in the field of microwave applications.
Different configured micro machined miniature switches have been reported. Among these switches, capacitive membrane
microwave switching devices present lower insertion loss, higher isolation,
better nonlinearity and zero static power consumption. In this presentation, we
describe the design, fabrication and performance of a surface micro machined
capacitive microwave switch on glass substrate using electroplating techniques.
The switch is built on coplanar wave-guide (CPW) transmission lines, which have an impedance of 50 that matches the impedance of the system. The width of the transmission line is 160 µm and the gap between the ground line and signal line is 30 µm. The insertion loss is dominated by the resistive loss of the signal line and the coupling between the signal line and the membrane when the membrane is in the up position. To minimize the resistive loss, a thick layer of metal needs be used to build the transmission line. The thicker metal layer result in a bigger gap that reduces the coupling between signal and ground yet also requires higher voltage to actuate the switch. To achieve a reasonable actuation voltage, a 4-µm-thick copper is used as the transmission line. The glass wafer is chosen for the RF switch over a semi-conductive silicon substrate since typical silicon wafer is too lossy for RF signal. When the membrane is in the down position, the electrical isolation of the switch mainly depends on the capacitive coupling between the signal line and ground lines. The dielectric layer plays a key role for the electrical isolation. The smaller the thickness and the smoother the surface of the dielectric layer, the better isolation of the switch is. But there is another trade-off here. When the membrane is pulled down, the biased voltage is directly applied across the dielectric layer. Since this layer is very thin, the electric field within the dielectric layer is very high. The thickness of the dielectric layer should be chosen such that the electric field will never exceed the breakdown electric field of the dielectric material. The silicon nitride film has breakdown electric field as high as several mega-volts per centimeter and can be utilized as dc block dielectric layer. In this project, the thickness of the silicon nitride layer is chosen as 0.2 µm to accomplish the dc block and RF coupling purpose.
SWITCH DESIGN AND OPERATION
The geometry of a capacitive MEMS
switch is shown in Fig. 1. The switch consists of a lower electrode fabricated
on the surface of the glass wafer and a thin aluminum membrane suspended over
the electrode. The membrane is connected directly to grounds on either side of
the electrode while a thin dielectric layer covers the lower electrode. The air
gap between the two conductors determines the switch off-capacitance. With no
applied actuation potential, the residual tensile stress of the membrane keeps
it suspended above the RF path. Application of a DC electrostatic field to the
lower electrode causes the formation of positive and negative charges on the
electrode and membrane conductor surfaces. These charges exhibit an attractive
force, which, when strong enough, causes the suspended metal membrane to snap
down onto the lower electrode and dielectric surface, forming a low impedance
RF path to ground.
ADVANTAGES
OF MEMS SWITCHES
(1)
Small
size :
Semiconductor manufacturing
techniques used in the batch fabrication of micro systems, these systems
process sizes ranging from micro meters to a few milli meters.
(2)
Low
Cost :
Mems technology allows complex
electromechanical systems to be manufactured using batch fabrication
techniques, allowing cost of switches to be put in party with that of
integrated circuits. Much of labour involved in packing and assembly of such a
system would simply disappear.
(3)
Low
power consumption :
The mems switches are power
efficient. The power losses in data transmission and also the time lag are
eliminated because the switches are made next to control circuitory in the same
chip.
(4)
High
isolation :
Isolation of mems switches in the range
1-40GHz is very high than the other switches.
(5)
Ability
to be integrated with other electronic devices with excellent linearity.
CONCLUSION
MEMS capacitive switches of RF
applications show low insertion losses in the OFF state and high isolation in
the ON state. The micro machine switches have applications in phased antenna
arrays, in MEMS impedance matching networks, and in communication applications.
