This paper reports on the design of a novel ultra low actuation voltage, low loss radio frequency micro-electro-mechanical system (RF MEMS) capacitive shunt switch. The concept of the switch relies on a mechanically unconstrained armature actuated over a coplanar waveguide using electrostatic forces. The minimum actuation voltage of the switch is &lt;2V, with an isolation of 40dB and insertion loss &lt;0.7dB at 78GHz

Chong, H. M. H.

Elgaid, K.

Hwan, C.J.

Kiang, K.S.

Kraft, M.

Lok, L.B.

English

Southampton (e-Prints Soton)

DEVELOPMENT OF A LOW ACTUATION VOLTAGE RF MEMS SWITCH K. S. KIANG1, H. M. H. Chong1, C. J. Hwang2, L. B. Lok2, K. Elgaid2 and M. KRAFT1 1Nano Group, School of Electronics and Computer Science,  University of Southampton, Highfield, Southampton, Hampshire, SO17 1BJ United Kingdom 2Ultrafast Systems Group, Department of Electronics and Electrical Engineering,  University of Glasgow, Rankine Building, Oakfield Avenue, Glasgow, G12 8LT United Kingdom  *Corresponding author: K. S. Kiang, Phone: +44 2380 593127, e mail: ksk05r@ecs.soton.ac.uk  Abstract: This paper reports on the design of a novel ultra low actuation voltage, low loss radio frequency micro electro mechanical system (RF MEMS) capacitive shunt switch. The concept of the switch relies on a mechanically  unconstrained  armature  actuated  over  a  coplanar  waveguide  using  electrostatic  forces.  The minimum actuation voltage of the switch is <2V, with an isolation of 40dB and insertion loss <0.7dB at 78GHz.  Keywords: Low Loss Switch, Low actuation voltage, Electrostatic actuation INTRODUCTION RF MEMS switches offer two distinct advantages in comparison  with  p i n  diodes  and  field  effect transistors: enhanced RF performances and almost zero  power  consumption.  However,  a  major disadvantage is the high operating voltage that is required  to  actuate  the  switch.  A  typical  MEMS switch requires between 20V 80V for operation [1]. A large variation of MEMS switches were studied and developed by many different research groups [1 3].  The  MEMS  switch  typically  use  actuation mechanisms  such  as  electrostatic,  thermal  and piezoelectric,  but  recently  hybrid  actuation mechanism,  such  as  magneto static  [4]  and thermal electrostatic [5] were investigated. Many of these  studies  were  conducted  with  the  aim  to reduce the actuation voltage of the switch. Several authors  [6 7],  reported  reduced  actuation  voltage using a serpentine folded suspension. Peroulis [6], achieved  a  reduction  of  80%  in  the  actuation voltage of his switch by increasing the number of meanders of the spring from 1 to 5. However, these structures are complex in design. This  paper  presents  an  extended  study  and  the fabrication process of the design of the spring less capacitive RF MEMS switch; the concept of which was first described in [8].  MECHANICAL MODELLING As  shown  in  Figure  1,  there  is  no  mechanical suspension connecting the switch armature to the substrate.  The  operation  of  the  switch  is determined by the application of actuation voltages either at the top or the bottom electrodes. The RF signal travels through the CPW during the up state or through the armature of switch into the ground planes during the down (shunt) state.   For  switching  operation  to  take  place,  an electrostatic force is defined by eq. 1 and must be greater  than  the  gravitational  force  acting  on  the armature.  As  the  armature  is  a  free  body,  it  is electrically  floating.  In  such  a  configuration,  an electrostatic actuation force can only be applied to the armature with at least two actuation electrodes, where  each  electrode  is  energised  with  opposite polarities but equal magnitude voltages [9]. ) 1 (2122oact otic electrosta gAVFε=  where εo is the permittivity of free space, A is the total  area  of  the  actuation  electrodes,  Vact    is  the applied  actuation  voltage  and  go  is  the  initial capacitive gap. The minimum actuation voltage can therefore be calculated as: ) 2 (22(min) AGmgVooact ε≅   where m is the mass of the armature, G is the earth gravity.   Effects of Damping Considerations The small displacement damping coefficient for a pair of parallel plates can be written as [6]: ) 3 (2332oarmgAb π=    where   is the air viscosity and Aarm is the area of the armature.  As  the  damping  force  for  a  MEMS  switch  is dependent on the displacement of the armature, a constant damping coefficient is insufficient to model the  behaviour  of  the  armature  when  in  switching mode.  In  Peroulis  [6]  case,  he  derived  a displacement  compensating  damping  coefficient equation for modelling larger displacement: ) 4 (638 . 9 1 1159 . 1 232 −   −−=x gx gQmkkboooλ  Where k is the spring constant, Qo is the nominal small  displacement  quality  factor  of  the  MEMS switch at x=go and λ is the mean free path. The last Pyrex Figure 1: Cross section of the RF MEMS switch. Dielectric Pyrex Silicon Actuation Electrode Coplanar Waveguide Armature term  in  the  denominator  accounts  for  slip flow effect.  As  there  is  no  mechanical  spring  in  this design,  the  squeeze  film  spring  constant  is substituted and is calculated as given in [10]: ( ) ( ) ( ) ( )) 5 (/ /1 642 1, 4 2 2 222122 18 ∑= + +=odd n n oal w n n n n gA Pkπ σ πσ   where  n1  and  n2  are  odd  integers,  Pa  is  the atmospheric  pressure  and  σ  is  the  squeeze number. The squeeze number, σ is given by: ) 6 (1222o aarmg PA ω  σ =  where ω is the frequency of the motion, and can be estimated  as  2π/period.  The  numerical  result  for the  given  design  show  that  the  spring  force  is negligible  compared  to  the  inertial  and  damping forces. Bao  et.  al  [11]  developed  a  modified  Reynold’s Equation,  where the damping force considers the effects of plates perforated with holes. ( )( ) ( ) 748 23 l w l l lgb m mo− − =  ( ) ( ) ( ) 83375 . 0 22 23oo a orr t x glβη π + −=  ( )( ) 91631 34x g tK ro ao−+ = η  where l is the characteristic length, η is a factor for the average damping pressure in a cell caused by air  flow  and  3   -   ln   4   -    - 4  4 2 β β β = K , where c o/r r        = β . The radius of the perforated hole is given by ro while rc is the pitch between adjacent holes.  The  subsequent  simulations  use  the  parameters given in table 1 with the diameter of the perforated hole  and  the  pitch  between  adjacent  holes  is assumed to be 5 m and 10 m respectively.   Table 1: Proposed dimensions of the MEMS switch optimised based on the actuation voltage and switching speed mentioned in [8]. ls  680µm  S  15µm ws  80µm  R  90µm go  5µm Armature mass, m  1.84e 9 kg td  0.2µm  Vact  1.9V Figure 3 illustrates the Matlab/Simulink simulation results  of  the  switch  using  the  abovementioned variable  and  constant  damping  conditions.  The results  show  that  the  switch  response  is  slower when  variable  damping  is  used.  The  variable damping effect is able to generate a more reliable result  as  the  damping  coefficient  of  the  device  is dependent on the displacement of the armature. If the perforated holes are considered, the actuation time is decreased to 14% of the original actuating time  that  used  the  solid  armature.  This  result indicates  that  the  effect  of  the  perforated  holes cannot  be  ignored  when  modelling  the  MEMS switch.  ELECTROMAGNETIC MODELLING A  conventional  RF  MEMS  shunt  switch  can  be electrically modelled as shown in Figure 4(a). The switching capacitance of the switch can be simply considered  as  a  parallel  plate  with  either  an insulating layer only or as an insulating layer and an air gap, as its dielectric layer, depending on the state of the switch.  Their values can be calculated as:  ( ) 10dt r odown tACε ε=  ( ) 11/ r d ot oup t gACεε+=  where εo is the permittivity of free space, go is the initial  capacitive  air  gap,  At  is  the  area  of  the armature contacting the transmission line, td is the thickness  of  the  dielectric  material  with  dielectric constant, εr.  However, in the case of the spring less RF MEMS switch, the armature is not physically attached to (a)      (b) Figure 4: Electrical equivalent circuit of (a) a conventional RF MEMS switch (b) the spring less RF MEMS Switch Zo  Zo Rs Ls Cs Zo  Zo Rs Ls Ct Cg  Cg Zb R  S  S ws ls Figure 2: Simplified plan view illustration of the RF MEMS shunt switch. RF Signal line RF ground plane Displacement, x ( m) 2 4 5 0 3 1 0.06  0.02  0.04 VD P CD 0.08  0.12 VD NP 0.14 Time (s) Figure 3: Comparison of actuation time and the release time of the MEMS switch, assuming the model using constant damping (CD), variable damping without considering perforated holes (VD NP) and variable damping with perforated holes (VD P). 0.16  0.10 any  structure.  The  shunt  signal  will  have  to propagate  from  the  CPW  signal  line  to  ground plane through i) the air gap between the signal line and  armature  and  ii)  the  air  gap  between  the armature  and  ground  plane.  The  formula  for  the down  and  up  state  capacitance  needs  to  be adjusted  to  account  for  the  additional  series capacitive path.  ( ) ( ) 12g t dg t r odown A A tA AC+=ε ε ( )( ) 13g trdog t oupA A t gA AC+   +=εε where  Ag  is  the  area  contacting  the  ground  line. This  capacitance  computation  is  important  as  it affects  the  scattering  parameters,  which determines  the  RF  performance  of  the  MEMS switch. The  scattering  (S)  parameters  determine  the  RF performance and characteristics of the switch. The switch can be modelled as a two port network with a shunt network. This will give rise to insertion loss when the switch is in the  up state, return  loss in both  states  and  the  isolation  when  the  switch  is actuated.  The  S11  parameter  denotes  the  return loss of the switch in both operating states, while the S21  parameter  represents  the  insertion  loss  and isolation of the switch when in the off and on state respectively. In general, S11 and S21 can be defined as [12]: ( ) 142log 20 11o boZ ZZS+−− =( ) 1522log 20 21o bbZ ZZS+− =  The return loss is defined as the power reflected over the incident power  in a transmission line. In the  case  of  a  RF  MEMS  switch,  Zb  is  the impedance of the armature. When the armature is not  actuated,  the  resistance  and  inductance  are negligible  as  the  armature  does  not  have  any physical contact with the dielectric layer, hence no current passes through the armature. The up state return  loss  of  a  capacitive  shunt  switch  can  be expressed as: ( ) 162log 20 11o upo upZ C jZ C jSωω+−− ≈  where Zb was substituted according to figure 4(b) and the inductive and resistive terms are neglected, ω is the angular frequency of the RF signal, Zo is the  characteristic  impedance  of  the  transmission line and Cu is up state capacitance of the MEMS switch.  The isolation and insertion loss are defined as the transmitted  power  over  the  incident  power  in  a transmission  line.  By  substituting  the  correct armature  impedance  expression  into  eq.15 (depending  on down or up state), the isolation or insertion loss can be defined. Eq. 17 19 is defined as  the  isolation  of  the  capacitive  shunt  MEMS switch  depending  on  the  frequency  of  the  RF signal. ( )( )( ) >>+−=+−<<+−≈1922log 20182log 201722log 2021ossoo ssoo downf fL jL jf fZ RRf fZ C jSωωω where S21 is the magnitude of the isolation and C is capacitance;  fo  is  the  LC  series  resonating frequency of the switch given by: ( ) 2021s so C Lfπ=   Figure 5 shows the simulated response of the RF MEMS  switch  characteristics  over  a  50   CPW transmission  line.  The  EM  simulation  was implemented from 1 80 GHz using Sonnet Lite. The switch off  resonance  frequency  is  approximately 78GHz  with  an  isolation  of  40dB.  The  switch insertion  loss  is  <0.7dB  at  a  similar  frequency.   Figure  6  shows  the  simulated  response  of  the same structure using Ansoft HFSS for a range from 1 120GHz.  The  simulation  shows  that  the  switch has a rejection of more than 16dB when the switch is down and it increases with the frequency of the RF signal. The simulation results of both software tools are comparable for frequencies below 40GHz. Figure 6: Simulated down state isolation (S21) for a 50  RF MEMS switch using Ansoft HFSS  Frequency (GHz) S Parameter (dB) Figure 5: Simulated down state return loss (S11) and isolation (S21) for a 50  RF MEMS switch using SonnertLite TM. Frequency (GHz) S Parameter (dB) S11 S21 However,  for  higher  frequencies,  SonnertLite TM decreases  rapidly,  showing  some  form  of resonance while the HFSS simulation continues to decrease  gradually.  The  difference  between  the two  simulated  results  at  higher  frequencies  is currently  being  investigated.  This  may  be  due  to inductive  and  capacitive  elements  in  the  switch structure. FABRICATION The fabrication process requires 9 photolithography steps  and  will  make  used  of  one  SOI  wafer sandwiched between two Pyrex wafers to form the device. The process can be split into three different parts,  mainly  Pyrex  wafer  fabrication,  silicon  on insulation (SOI) wafer fabrication and assembly of the device. 1. Pyrex Wafer Fabrication The  Pyrex  wafers  are  initially  prepared  using fuming  nitric  acid.  The  top  and  the  bottom  Pyrex wafer  have  a  3  µm  cavity  trench  etched respectively, to create the operating cavity for the armature and provide the space for the bond pads, using  Rockwell  7:1  buffered  hydrofluoric  acid (BHF).  A layer of chrome gold (CrAu) is deposited and  etched  to  form  the  coplanar  waveguide,  the actuation  electrodes  and  the  bond  pads  of  the device. Tantalum pentoxide (Ta2O5) is used as the material  for  the  dielectric,  which  covers  the coplanar waveguide and the actuation electrodes. Figure  7  shows  the  process  of  the  bottom  Pyrex wafer. Laser micromaching is subsequently used to create access holes on the top side wafers. These holes act as the windows for the etchant to access the wafer stack during the final release step of the fabrication. 2. SOI Wafer Fabrication The armature will be fabricated using SOI wafers. Firstly, a layer of silicon dioxide is deposited using PECVD  on  the  surface  of  the  active  layer.  This oxide  layer  is  used  as  a  sacrificial  layer  for  the armature.  Subsequently,  a  layer  of  CrAu  is sputtered  to  form  bond  pads  for  the  top  Pyrex wafer.  3. Device Assembly The assembly of the device will require the use of anodic bonding twice. The partially processed SOI wafer is firstly anodically bonded to the top Pyrex wafer. This bonding step is required to electrically connect the metal tracks on the top Pyrex wafer to the bond pads on the SOI wafer. The holding wafer and  buried  oxide  are  subsequently  removed, exposing the back of the active layer. The armature is then patterned on the active layer using deep reactive ion etching (DRIE). A second anodic bonding process is employed to attach the bottom Pyrex wafer to the Silicon/Pyrex stack. The wafer  can  then  be  diced.  The  final  stage  of  the fabrication  process  is  to  release  the  armature through  an  etching  process  before  proceeding  to device packaging. CONCLUSIONS A Matlab/Simulink model was used to simulate the dynamic  response  of  a  novel  spring less  MEMS switch.  The  variable  damping  model,  which accounts for a more realistic damping force with the variation  of  the  capacitive  gap,  requires  a  longer switching  time  when  compared  to  the  constant damping  effect  model.  The  effect  of  perforated holes on the damping force cannot be ignored.  The  RF  characteristic  and  performance  was presented using two EM simulation software tools, both showing good agreement up to 40 GHz The  device  is  currently  being  fabricated  and prototypes are expected to be available for testing in due course. ACKNOWLEGMENT The author would like to acknowledge the financial supported  rendered  by  the  Commonwealth Scholarship Commission, U.K.  REFERENCES 1.  G.  M.  Rebeiz  et.  al,  IEEE  Microwave Magazine, 2 (2001), pp 59 71. 2.  J.  J.  Yao,  Journal  of  Micromechanics  and Microengineering, vol. 10, p. 9, 2000. 3.  C.  T.  C.  Nguyen,  et.  al,  Proceedings  of  the IEEE, vol. 86, p. 1756, 1998. 4.  I.  J.  Cho,  et.  al,  IEEE  Transactions  on Microwave Theory and Techniques, 53 (2005), pp. 2450 57. 5.  D.  Saias,  et.  al,  IEEE  Journal  of  Solid State Circuits, vol. 38, p. 2318, 2003. 6.  D.  Peroulis,  et.  al,  Microwave  Theory  and Techniques, IEEE Transactions on, vol. 51, pp. 259 270, 2003 7.  R.  Chan,  et.  al,    J.  Microelectromechanical Systems, vol. 12, pp. 713, 2003 8.  K.  S.  Kiang,  et.al,  Proc.  MME  2006 Conference, pp. 105 108, UK, Sept. 2006 9.  S. J. Woo, et. al, Proceedings of the 34th SICE Annual  Conference.  International  Session Papers, (1995), pp. 1347 52. 10. J. J. Blech, J. Lubrication Technology, vol. 105, pp. 615 620, 1983. 11. M. Bao, et. al, Journal of Micromechanics and Microengineering, vol. 13, pp. 795 800, 2003. 12. D.  M.  Pozar,  Microwave  Engineering,  2  ed.: Artech House, 1998. Figure 7: Microscopic picture of structures of a processed bottom Pyrex wafer with a 50  CPW. CPW Actuation Electrodes Dielectric Layer 

Microwave Theory and Techniques,

Development of a low actuation voltage RF MEMS switch

Development of a low actuation voltage RF MEMS switch

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