rf mems variable capacitors tae yong song micro wave devices term project snu 2004. 6. 22

RF MEMS variable capacitors

Tae Yong Song

Micro wave devices term project

SNU

2004. 6. 22

2June 21, 2004Tae Yong Song, Micro/Nano Systems & Controls Lab.

What are RF MEMS?

RF MEMS

Radio-Frequency: Transporting 9kHz ~ 300GHz signals

Micro: Feature size (1~500 um)

Electro-Mechanical: transferring

electrical energy into mechanical

motion

Systems


Applications – RF MEMS

• Integrated RF transceiver


Applications- Capacitor (1)

• In wireless communication systems– tunable capacitors and inductors often need a high quality factor an

d high self-resonant frequency.• Reduce power consumption• Better phase noise and power in voltage-controlled oscillators (V

COs)• Wide tuning range (VCOs)


Applications- Capacitor (1)

• Advantages over conventional varactor based filter– Increase dynamic range and dramatic part reduction– Two pole UHF filter : much smaller system

(a) Varactor and (b) MEMS capacitor-based tunable two-pole UHF filter.

Ref [3] :Robert R, et al. “Variable MEMS Capacitors Implmented into RF Filter Systems”


MEMS Capacitors-Type I (1)

University of Waterloo IEEE, Transaction on Electron Devices, Vol. 50, No.3, March 2003


Analysis (1)

• A schematic model of a conventional eletrostatically actuated two-parallel-plate tunable capacitor

0

DC

22

0

Neglecting the fringe effect

( )

when V is applied, an attractive electrostatic force ( ) is given by

1 1

2 2 ( )

An effective springs on constant for the electrostatic force

e

DCe DC

e

AC

h h

F

CVCF V

h h h

k

k

2

20

m

| |( )

The mechanical suspension of the top plate has a spring constant k

e DCe

m m

F CV

h h h

F k h

2

00

0

0

At equlibrium, the magnitude of and are equal,

1 1( )

2 ( ) 2

2

( )

The corresponding value of is called the pull-in voltage (

, 3

If , the two pl

)

e m

DCm e

me

m e

DC PI

DC PI

F F

CVk h k h h

h h

k hk

h h

hwhen h

V V

V V

k k

0 1

0 1

1

ates are snapped into contact (pull in effect)

C-C ( 1 )=> the tuning range =

C

Theoretically the maximum controllable tuning rage i %s 50

A Ah h h h

A h hh


Analysis (2)

• A schemetic model of the novel wide-tuning-range tunable capacitor

1 2 3

1

2

3

It consisted of three plates that are designated as E , E , and E .

E : movable top plate suspended by four cantilevel beams

E : fixed plate on the bottom substrate

E : the outer fixed bottom plate

An ac DC 3 1

1 2

0 1 1

0 1

1

2 21

tuation voltage (V ) is applied between plates E and E

h is designed to be smaller than h

C-C ( )Tuning range =

C

h h1) h > , then the maximum tuning range can be found at h=

3

A Ah h h h

A h hh

2max

1 2

21

max

2 1

31

=> Tuning range = 3 2

h2) h < , the pull-in effect will not occur at all

3=> Tuning range

(depend on other factors, such as surface roughness and curvature of E and E )

h

h h


Fabrication process

A schematic illustration of the fabrication process for the widetuning- range tunable capacitor.

Microscopic pictures of the novel wide-tuning-range tunable capacitor.


Measurement (1)

• When Vdc is greater than 19V, h1 changes directly from 1 um to zero– Pull in voltage is about 19V from simulation

• The measured data is not zero over pull in voltage– Surface roughness– Curvature of the plates– The damping between the plates have an effect on the space


Measurement (2)

• Quality factor as a function of frequency• Quality factor of about 30 at 5 GHz• Self-resonant frequency far beyond 10 GHz


MEMS Capacitor-type II

• 40 um thick single-crystal silicon actuator suspended over a glass substrate : Long operational lifetimes

• Through the use of a large throw electrostatic comb drive and a separate area tuning capacitor– Wide tuning range and high Q can be achieved

• Only added to aluminum conductive layer – reduce series resistance

SEM of a 40-m-thick tunable capacitor


Fabrication

a) Aling marker is etched and 2 um Al is deposited and patterned.

b) Epoxy bondingc) The handle wafer side and burie

d oxide are removedd) Silicon etching using the Bosch

processe) Released by isotropically etchin

g the epoxy in an oxygen plasma

After released, additional metal can be sputtered onto the structure to increase device Quality factor


Measurement (1)

• Exceptionally large tuning range, larger than 8:1• Resistance of the MEMS tunable capacitor is less than 1Ω

– Lead to a high Q

Tuning range of the device measured at 500 MHz.

Q factor of the device from 100MHz to 3 GHz


Measurement (2)

• Some filter applications require lifetimes of over 10-billion cycles.

• MEMS tunable capacitor is fabricated from single-crystal silicon– A material that has a large elastic deformation region and

shows very little fatigue

Change in series resistance after 11.1-billion cycle

Change in capacitance versus tuning voltage


Conclusion

• Capacitor 의 양 평판에 bias 전압을 가하여 발생하는 정전력을 이용하는 일반적으로 방식은 이론적으로는 1.5:1 로 작은 단접이 있지만 구조적 변화를 통해 향상시킬 수 있다 .

• MEMS device 는 high Q-factor, wide tuning range, low phase noise 가 가능하다 .

• MEMS 기술로 제작된 capacitor 는 저 전압 , 빠른 tuning 및 작은 크기로 구현이 가능하다 .


Reference

[1] Jinghong Chen, Member, IEEE, Jun Zou, Chang Liu, Senior Member, IEEE, José E. Schutt-Ainé, Senior Member, IEEE, and Sung-Mo (Steve) Kang, Fellow, IEEE “Design and Modeling of a Micromachined High-Q Tunable Capacitor With Large Tuning Range and a Vertical Planar Spiral Inductor”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 3, MARCH 2003.

[2] Robert L. Borwick, III, Philip A. Stupar, Jeffrey F. DeNatale, Member, IEEE, Robert Anderson, and Robert Erlandson, Member, IEEE “Variable MEMS Capacitors Implemented Into RF Filter Systems”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 51, NO. 1, JANUARY 2003

[3] Borwick, R.L. III, Stupar, P.A., DeNatale, J., Anderson, R., Chialun Tsai, Garrett, K., ” A high Q, large tuning range, tunable capacitor for RF applications”, Micro Electro Mechanical Systems, 2002. The Fifteenth IEEE International Conference on , 20-24 Jan. 2002 Pages:669 – 672

[4] Mansour, R.R., Bakri-Kassem, M., Daneshmand, M., Messiha, N.,” RF MEMS devices”, MEMS, NANO and Smart Systems, 2003. Proceedings. International Conference on , 20-23 July 2003 Pages:103 - 107

[5] Zhiping Feng; Wenge Zhang, Bingzhi Su, Harsh, K.F., Gupta, K.C., Bright, V., Lee, Y.C.,” Design and modeling of RF MEMS tunable capacitors using electro-thermal actuators”, Microwave Symposium Digest, 1999 IEEE MTT-S International , Volume: 4 , 13-19 June 1999 Pages:1507 - 1510 vol.4

[6] De Coster, J., Puers, R., Tilmans, H.A.C., van Beek, J.T.M., Rijks, T.G.S.M.,” Variable RF MEMS capacitors with extended tuning range “, TRANSDUCERS, Solid-State Sensors, Actuators and Microsystems, 12th Innational Conference on, 2003 , Volume: 2 , 8-12 June 2003 Pages:1784 - 1787 vol.2

rf mems variable capacitors tae yong song micro wave devices term project snu 2004. 6. 22

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