Miniature Tunable Antennas for Power Efficient Wireless Communications

Darrin J. Young

Electrical Engineering and Computer Science

Case Western Reserve University

Cleveland, Ohio 44106

CWRU

Acknowledgement CWRU

NASA under Grant #: NAG3_2578

Collaborator: Professor Wen Ko

Graduate Student: Brian Quach

OutlineCWRU

• Motivation

• Proposed Power Efficient System

• MEMS Tunable Capacitors

• MEMS Fabrication Technology

• Conclusion

MotivationCWRU

3 dB Loss

2 W PA output 1 W @ antenna

Shortened battery life

Degraded receiver sensitivity

Conventional Radio Front-End ArchitectureReceive LNA Image Reject

Receive BPFReceive Mixer

Transmit PA Image RejectTransmit BPF

Transmit Mixer

Main LO

Offset LO

Antenna

Duplexer

Low Power Transceiver: Critical for Wireless Communication

Proposed Power Efficient Architecture CWRU

Receive Patch Antenna Receive LNA Image RejectReceive BPF

Receive Mixer

Transmit Patch Antenna Transmit PA Image RejectTransmit BPF

Transmit Mixer

Main LO

Offset LO

Tuning Capacitor

Tuning Capacitor

Eliminating Duplexer Power Saving!

Narrow-Band Antennas Frequency Tuning

Patch Antenna CWRU

1.6 GHz Patch Antenna S11 Measurement

Tuning Capacitor

Patch Antenna Tuning

Antenna Model

C ~ 26 pF

L ~ 0.4 nH

R ~ 50 Ω

Tuning Capacitor CWRU

• Nominal Capacitance: 1 ~ 2 pF

• Tuning Ratio: 100 % with 5 to 10 V (~100MHz)

• High Quality Factor (Q): ~ 100 at RF (GHz)

• High Voltage Handling: 1 W 20 V peak to peak

• Insensitive to RF Signals

MEMS Tunable Capacitor

Only Solution

MEMS Tunable Capacitor CWRU

Top View

Si

Cross-Section View

Tuning Voltage

Top Electrode

Isolation layer

Substrate Bottom Electrode

Vacuum

MEMS Capacitor Finite-Element Model

MEMS Capacitor Design CWRU

Diaphragm Radius

Insulator Thickness Electrode Radius

Gap

Diaphragm Thickness

Critical Design Parameters:

(1) Touch point pressure (TPP): 12 psi

Large TPP small initial touched area large tuning ratio

(2) Diaphragm thickness: 2 m (small initial capacitance & accurate process control)

(3) Gap: 1 m (accurate process control)

(4) Diaphragm Radius: 120 m (for TPP of 12 psi)

(5) Insulator (Oxide) Thickness: 300 Å (Thin Layer Large tuning, limited by BD)

(6) Bottom Electrode Radius: 80 m

MEMS Capacitor Simulation CWRU

MEMS Capacitor Under 0V MEMS Capacitor Under 10V

• Nominal Capacitance: 2 pF

• Tuning Ratio: 55% @ 5V and 120% @ 10V

• Estimated Q @ 1GHz: 340

• MEMS Large Voltage Swing

RF Insensitive

CWRUFabrication Technology

Silicon Substrate

Silicon Substrate

Silicon Substrate

Cavity and bottom electrode formation

Top electrode formation (P+ Si to oxide bonding & etching)

Vacuum seal & metalization

CWRUCurrent Status & Future Plan

• Current Status

Devices in fabrication

• Future Plan

Device characterization

Tunable antenna perform evaluation

CWRUConclusions

• Tunable patch antennas for low power wireless applications

• MEMS tunable capacitor provides:

High-Q

Large tunable range

Large voltage handling

Insensitive to RF signals

• MEMS capacitors for tuning patch antennas

transmitter output matching networks

high spectral purity RF oscillators