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RF MEMS devices
Prof. Dr. Wajiha Shah
OUTLINE
 Use of RF MEMS devices in wireless and satellite
communication system.
1. MEMS variable capacitor (tuning range : 280%)
2. MEMS tunable inductor
3. MEMS multiport switch (using range : > 20GHz)
Introduction (1)
 The MEMS technology has the potential of replacing
many RF components used in today’s mobile
communication and satellite system.
 Advantage of MEMS
1. can reduce the size, weight, power consumption and
component counts.
2. promise superior performance.
3. can be built with low cost, mass producibility and
high reliability.
4. new functionality and system capability.
MEMS variable capacitors (1)
 MEMS technology has the potential of realizing variable
capacitors with a performance that is superior to varactor
diodes in areas such as non-linearity and losses.
 Over the past, variable capacitors was theoretical tuning
range of 50% - 100%, in practice, the capacitor operate
over a smaller tuning range away from the collapse
voltage.
 Proposed MEMS capacitor
Fig. 1 illustrates a schematic diagram.
MEMS variable capacitors (2)
Fig. 1 A schematic diagram of the proposed capacitor
MEMS variable capacitors (3)
 It consists of two movable plates with an insulation
dielectric layer on top of the bottom plate.
 With the two plates being flexible, makes it possible for
the two plates to attract each other and decrease the
maximum distance before the pull-in voltage occurs.
 Construction of proposed capacitor
 Two structural layers, three sacrificial layers, and two
insulating layers of Nitride.
MEMS variable capacitors (4)
 The top plate is fabricated from nickel with a thickness
of 24㎛ covered by a gold layer of thickness 2㎛.
 The bottom plate is made of polysilicon covered by a
Nitride layer of a thickness of 0.35㎛.
============================================
 Making process  Metal MUMPs (Multi-User MEMS
Process)
 Simulation  Coventor Ware
MEMS variable capacitors (5)
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Fig. 2 The fabrication process of the Metal MUMPs that used to
build the proposed variable capacitor.
 First, a layer of 0.5-micron oxide is deposited and
patterned as illustrated in Fig. 2(a) – 2(b).
 This oxide layer outlines the area that will be used to
etch a trench in the silicon substrate.
MEMS variable capacitors (6)
 The first Nitride layer of 0.35-micron thickness is
deposited and patterned as illustrated in Fig. 2(c).
 This Nitride layer forms the bottom cover of the
polysilicon layer and is used as a part of the capacitor’s
bottom plate.
 On top of the first Nitride layer, a 0.7-micron layer of
polysilicon is deposited and patterned to form the
bottom conductive plate of the variable capacitor. 2(d)
 The last step in building the bottom plate of the variable
capacitor is to deposit the second Nitride layer on top
of the polysilicon layer to form the isolating area that
prevents any electrical contact between the two plates.
MEMS variable capacitors (7)
 A 1.1-micron layer of second oxide is then deposited as
illustrated in Fig. (f). The second oxide layer is etched
so that the metal layer is anchored on the Nitride and a
physical contact between the bottom electrode (Polysilicon) and the two outer pads is ensured.
 The last layer is metal layer, which is formed of a 24㎛
of Nickel with 2㎛ of gold on top of the Nickel layer.
 The last step is to etch out the sacrificial layers as well
as to etch a trench in the silicon substrate. The trench
etch of the substrate is determined by the first oxide
layer.
MEMS variable capacitors (8)
 Once the first oxide is etched away by opening holes
through the Nitride layer, the solvent will etch the
isolation layer underneath.
 The silicon substrate is then etched to form a trench of
a depth of 25㎛. The total depth from the bottom plate
of the variable capacitor is 27.5㎛.
 Fig. 3 shows
a SEM picture
of the proposed
MEMS variable
capacitor.
Fig. 3 An SEM picture of the fabricated variable capacitor
MEMS variable capacitors (9)
Fig. 4 Measured capacitance vs. frequency at different DC voltage
 When applied voltage : DC 0 – 39V
 At 1GHz, the achievable tuning of the proposed
capacitor is found to be 280%.
MEMS tunable inductors (1)
 High-Q inductors find widespread use in RF transceivers
circuits.
 The availability of tunable inductor
 circuits to circumvent, construct filter for frequency
agile applications
 Before variable inductors have been achieved by using
drive coil coupled to the RF inductor.
 use of mutual inductance and 100% tuning range
 However, this technique requires the use of an
additional drive circuit to change the phase of the
current in the drive coil.
MEMS tunable inductors (2)
 A λ/4 transmission line was used to construct the
inverter. The inverter has a limited bandwidth and would
relatively occupy a large area in low frequency
applications.
 MEMS tunable inductor is proposed using lumped
element inverters.
 advantage : wider bandwidth, more design flexibility
 The circuit representing the tunable inductor consists of
two inductors, two fixed capacitor and a shunt variable
capacitor.
 The fabricated variable inductor chip is given in Fig. 5.
MEMS tunable inductors (3)
Variable Capacitor
Fixed Capacitor
Pad
Inductor
Fig. 5 A MEMS Tunable Inductor chip
MEMS tunable inductors (4)
 The MUMPs process includes three layers of polysilicon
(poly0, poly1, poly2), two layers of oxide, one layer of
gold.
 The gold layer is deposited on the top polysilicon layer
(poly2).
 The poly0, first oxide, poly1, second oxide, poly2 and
gold have thickness of 0.5, 2.0, 2.0, 0.75, 1.5 and 0.5㎛
respectively.
 The two parallel plate variable capacitor
1. lower plate : poly1  air gap (0.75㎛)
 upper plate : poly2
2. C : 2.05pF, Area : 210*270㎛2
MEMS tunable inductors (5)
 The fixed capacitors are constructed using the same
concept except that no voltage source will be applied
to the plates. (C : 1.76pF, Area : 200*280㎛2)
 There are 8 pads used in this design : one for the DC
voltage, one for ground and 6 pads for the coplanar RF
input and output signals. (material : poly2, gold)
 There pads have a significant low parasitic capacitance
of 0.25pF. (Area : 86*86㎛2)
RF MEMS multiport switch (1)
 Microwave switch
1. Mechanical-type (coaxial & waveguide)
2. Semiconductor-type (PIN diode & FET)
 MEMS switches promise to combine the advantageous
properties of both mechanical and semiconductor
switches.
 Most of the research effort reported in literature has
been directed toward the development of Single-PoleSingle-Through (SPST) switches.
RF MEMS multiport switch (2)
 An integrated SP3T MEMS switch. Three beams with
narrow-width tips are integrated on top of a coplanar
transmission line.
 The junction where the three beams interact is
inherently a wide band junction, which make it
possible to design a wideband SP3T switch with 30dB
isolation up to 20GHz.
 The mechanical design of the switch is analyzed using
Coventor-Ware.
RF MEMS multiport switch (3)
 It is compact (500*500㎛)
coplanar series switch,
consisting of three actuating
beams.
 One end of each beam is
attached to a 50Ω coplanar
transmission line, while the
Fig. 6 The proposed MEMS SP3T switch
other end is suspended on top of another 50Ω
transmission coplanar line to form a series-type contact
switch.
 The pull down electrodes, which are parts of the RF ground,
are placed underneath the beams.
RF MEMS multiport switch (4)
 Alternatively, the SP3T switch can be implemented in a
hybrid-form where the beams are micro-machined
separately and then integrated on an Alumina substrate
using flip-chip technology.
 The beams are fabricated using the Multi-User MEMS
Process surface micro machining.
 Each beam is made of a Polyilicon layer of a thickness
of 1.5㎛ coverd by a gold layer of 0.5㎛ thick.
 Release holes are accommodated for HF accessibility
to the trapped oxide under the beams.
RF MEMS multiport switch (5)
 The coplanar line circuit is
fabricated on a 254㎛ thick
Alumina substrate.
 In order to improve the isolation
of the switch, the beams are
narrowed at the tip and the
contact is performed only
by small tips at the end of
Fig. 7 The fabrication process of the SP3T
the beams.
RF MEMS multiport switch (6)
 The RF performance of the SP3T switch has been
characterized over a wide range of frequency from DC
to 40GHz, using HFSS software.
 The results for the case that port 2 is in ON state and 3
and 4 are in OFF state are shown in Fig. 8.
The
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