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					 RF MEMS devices

R.R. Mansour, M. Bakri-Kassem,
  M. Daneshmand and Messiha

   University of Waterloo
 This paper addresses the use of RF MEMS devices in
  wireless and satellite communication system.

 Main theme of this paper

 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.
                 Introduction (2)
 The feasibility of developing RF MEMS components
  with superior performance has been demonstrated in
  literature and is now well documented in several books.

 Over the past three years, the emphasis of RF MEMS
  research has been shifted to system integration,
  reliability and the development of new device
         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

 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
 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 (Poly-
  silicon) 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
        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
 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
 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
 The fabricated variable inductor chip is given in Fig. 5.
MEMS tunable inductors (3)
        Variable Capacitor   Fixed Capacitor



    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
 The gold layer is deposited on the top polysilicon layer
 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㎛
 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

 Most of the research effort reported in literature has
  been directed toward the development of Single-Pole-
  Single-Through (SPST) switches.
      RF MEMS multiport switch (2)
 In this paper, we present 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
          RF MEMS multiport switch (3)
 It is compact (500*500㎛)
  coplanar series switch,
  consisting of three actuating
 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
 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   end