rf mems switch0

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RF MEMS SWITCHJonathan Hernandez Aguirre



Outline

� - Target Design Specifications

� - Design Methodology

� - Mathematical Model and Simulation

� - Design Process

� - Finite element Analysis

� - Comparison Results

� - Conclusions.



Target design specifications

Parameter Value

Spring Constant 5Nw/m to 40 Nw/m [1]

Cantilever beam length 110 µm to 300 µm [1]

Mass

Resonant frequency 50 Khz

Radio frequency (RF) is a rate of oscillation in the range

of about 30 kHz to 300 GHz.



Design Methodology

RF Switchrequired for

the class

Initial target

specifications

Mathematical modeling

with mathlab

Design for

resonant

frequency

Match

Design for spring

constant

Match Finite element

analysis

MATHLAB

and FEAmatch

Complete

No

Yes

No

Yes

No

Yes



Mathematical Model and simulationlc

ymax

lc

b

lc

l

x

Bottom electrode

A fixed-free beam with one free movable edge is subjected to transverse load qo distributed about the free end of thebeam. It can be easily seen from the above figure, Ymax is at the free end of the beam. Making use of the load-deflection properties of the beam, accounting for transverse and axial loading simultaneously, and applying

superposition principle, the effective stiffness (spring constant) K at the position of maximum deflection, Ymax [1] isgiven by :

(1)

qo

Fig.1



Mathematical Model and simulation

� Pull in Voltage

(2) (3)



Mathematical Model and simulation� Resonant Frequency

k = Spring constant Nw/m

m = Mass k g

We can calculate the mass of the RF Switch by adding the mass of the cantilever beam, the topelectrode and the tip (contact pad) [1]

We can find the mass by knowing the density of the material and the volume of every part of the

cantilever beam.

(4)

(5)



Mathematical model and simulation

Determining the spring

constant and the mass

of the cantilever beam.

Spring constant

7.014 Nw / m

Mass 35.024 pkg




Determining the

length and the

thickness of the

cantilever beam.

Length = 221 µm

Thickness = 4.4µm




Determining the

pull-in voltage

Pull in voltage

296.3 v



Parameters from the Mathematical Model

Parameter Value

Spring Constant 7.014 Nw / m

Resonant Frequency 50.333 kHz

Mass of the cantilever beam 35.024pkg

Length of the cantilever beam 221 µm

Width 15 µm

Thickness 4.4 µm

Pull-in Voltage 296.3 v

Effective electrode area




Capacitance up state

(6)



Design Process

� In order to

compare the

simulation result

with Mathlab, a

cantilever beam

was designed as

the 3D FEAmodel, shown as

followed.

� The model is

refined with

maximum meshsize being 8µm.



Design process

Level 0 , Bottom electrode, Si,Thickness 25 µm

Level 1 , Dielectric layer, Silicon

nitride, Thickness 5 µm

Level 2 , Contact Pad, Al,Thickness 1 µm

Level 3 , Air Gap, Height 2 µm

Level 4 , Contact Pad and top

electrode, Al, Thickness 1 µm

Level 5 , Cantilever beam, SiliconNitride, Thickness 4.4 µm

211 µm 10 µm

10 µm

26.52 µm

221 µm



Design dimensions

Level Material Thickness Length

Substrate Si 25µm 221µm

Dielectric layer Si3N4 5µm 221µm

Contact area Al 1µm 10µm

Air gap 2µm

Top electrode and

contact area

Al 1µm Electrode = 26.52µm

Contact Area = 10µm

Cantilever beam Si3N4 4.4µm 221µm



FEA using Intellisuite



Resonant frequency

f = 52.298 kHz



Deflection



Pull-in Voltage



Up-state capacitance

C = 8.228 fF



Stress distribution



Pressure distribution



Comparison with Mathlab

Mathlab calculation 3D FEA Intellisuite

Natural frequency 50.333 Khz 52.988 Khz

Pull in Voltage 296.3 V 300 V

Mathlab calculation 3D FEA

Intellisuite

Capacitance 8.3318 fF 8.228 fF



Insertion loss and isolation loss

Generally the contact resistance is around 1 ohm [2]

In order to calculate both, the insertion and isolation loss we need to know

the transmission-line impedance.

(7)

(8)



Switching time

Assuming aV

s voltage of 1.3V

p [2] and using the resonantfrequency of 50 kHz, the switching time is 9µsec.

(9)



Conclusions

� A RF MEMS switch was designed, a FEA was performed to find the value of

the natural frequency, pull-in voltage and up-state capacitance. After to run

this analysis, was found that the calculated and simulated values are quite

similar, which means the calculations were performed correctly.

� A problem of this design is the high value of the pull-in voltage needed to

activate the switch, this voltage can be optimized by increasing the effective

electrode area and reducing the thickness of the dielectric layer located over

the silicon substrate.



References

� [1] Abhijeet V. Kshirsagan ¶Design of MEMS Cantilever ± Hands

Calculation¶¶, Sensors and transducers Journal, Vol. 91, Issue 4, April

2008 pp 55-69.

� [2] Gabriel M. Rebeiz, Jeremy B. Muldavin, ¶RF MEMS Switches andswitch circuits¶¶, IEEE Microwave Magazine, December 2001, pp 59-

71.



Thank you!!

rf mems switch0

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