Aug 2004 Micromachined Antennas for Integration with Silicon Based Active Devices Erik Öjefors Signals and Systems, Dep.of Engineering Sciences Uppsala

Aug 2004 Micromachined Antennas for Integration with Silicon Based Active Devices

Micromachined Antennas for Integration with Silicon Based Active Devices

Erik Öjefors

Signals and Systems, Dep.of Engineering Sciences

Uppsala University, Sweden


• Introduction, applications

• Challenges of on-chip antenna integration

• Design of 24 GHz on-chip antennas

• Crosstalk with on-chip circuits

• Micromachined antenna packaging

• Conclusions and future work

Outline of talk


Introduction

ObjectiveOn-chip antenna integrated with a 24 GHz ISM bandtransceiver in SiGe HBT technology for short range RADAR and communication devices

PLL

1/8

VCO12 GHz

CrystalOscillator20 MHz

DC

IF

LO RF

RF

SHM actingas a frequencydoubler

PA

LNARFIC

Antenna

Self-contained SiGe front-end

Integration

3x3 mm large chip


Introduction

One applicationRADAR for traffic surveillance and anti-collision warning systems


Introduction

Advantages of integrated antenna:

• Simplified packaging (no high frequency interconnects)

• Lowered cost due to reduced number of components

• Omnidirectional radiation pattern often needed,

low gain on-chip antenna feasible


Challenges of on-chip antenna integration

Minimum Q (quality factor) of small antennas

“a” is the radius of a sphere enclosing the antenna. “k” = 2/

High Q leads to small bandwidth and can reduce the efficiency

BWkakaQ

1113 2a

McClean, " A Re-examination of the Fundamental Limits on the Radiation Q of Electrically Small Antennas," IEEE Trans AP, May 1996.

Antenna size can NOT be reduced without consequences!



Problem:Size of antenna is an important parameter due to the high cost of the processed SiGe wafer

Solution: Chose antenna types which offer compactintegration with the active circuits


Proposed integration with active devices

Si

Active devices

Slot antenna

Activedevices

Top metallization

3 mm

Active elements integratedwithin slot loop 3

mm

p+ channel stopper



Problem:Commercial silicon-germanium (SiGe) semiconductor uselow resisistivity (< 20 cm) substrates

Solution: Use of a low loss interface material such as BCB polymer or micromachining to reduce coupling between antenna and lossy silicon substrate


Micromachining – mechanical shaping of silicon wafers by semi-conductor processing techniques

Micromachining


Post processing technique compatible with pre-processedSiGe wafers from commercial semiconductor foundaries

Si

Active circuit

Si

Si

BCB

Gold

Pre-processed wafer fromfoundary

10-20 um BCB layer appliedand cured

Top metallization evaporated anddefined using standard photolitho-graphic techniques

Micromachining – BCB process flow


Surface micromachining of silicon

BCB, 20 um 10 um

Si 11 -15 cm

Slot

Optional micromachining

Top metalization

Surface micromachining applied to the substrate beforeBCB-spin-on

Micromachining


Bulk micromachining of silicon

BCB membrane, 10-20 um

Slot

Backsideetching

Top metalization

Back side of silicon substrate etched as last step in processing

Micromachining

Si


Outline of talk



• Design of 24 GHz on-chip antennas





Micromachined 24 GHz antennas

• Surface micromachined slot loop antenna• Bulk micromachined slot loop antenna• Inverted F antenna• Wire loop antenna• Meander dipole• Differential patch antenna• Comparison of designed antennas


Surfaced micromachined slot loop antennaC

PW

pro

be p

ad

Slot loop length corresponds to one guided wavelength at 22 GHz

2000 um

3000

um

3000 um

Si 11-15 cm

BCB 10-20 um

10, 20 um slot width

BCB, Si



Small return loss outside the the operating frequency indicates that losses are present

Surfaced micromachined slot loop antenna



Results – Radiation Pattern

-150 -100 -50 0 50 100 150-25

-20

-15

-10

-5

0

Angle [deg]

[dB

]

E-plane

MeasuredSimulated

Antenna on 20 um thick BCB interface layer on low resistivity Si

E-planeH-plane

Reasonably good agreement between simulated and measured radiation pattern,(some shadowing in E-plane caused by measurement setup)


• Measured gain: -3.4 dBi

• Directivity (simulated): 3.2 dBi

• Calculated efficiency: 20 %

80 cm

Reference hornantenna

Foam material (lowdielectric constant)

AUT

Wafer probe station

Results – Gain and efficiency



200 m

Si

Slot supportedby BCBmembrane

Trenches can be formed from the back side of the wafer bychemical wet etching (KOH) or dry etching (DRIE) methods

No trenches

Micromachined 24 GHz antennasBulk micromachining – improving efficiency


Bulk micromachining – improving efficiency

Anisotropic etching (KOH, TMAH) Needs wafer thinning (300 um)

DRIE>100 um trench width can be etched

Radiating slots

Radiating slots



Bulk micromachining 3D-FEM simulations (HFSS)

freq (22.00GHz to 30.00GHz)

tren

ches

..S

(1,1

)

freq (20.00GHz to 25.00GHz)

S(1

,1)

By etching 200 um wide trenches in the silicon wafer thesimulated input impedance is increased from 60 to 210 at the second resonance, simulated efficiency increased from 20% to >50%



Bulk Micromachining – Slot Loop Antenna

Designed antenna

• Trench width wt = 100 um

Results

• Measured gain 0-1 dBi

• Single ended feed (CPW)

• Impedance 100 Ohm

Slot

Top metallization (groundplane)

Slot

wb

lg

lg

Siwt

sa

Trench (membrane)

wt

Micromachined slot loop antenna

Silicon space for active devices



Inverted F Antenna


Si

Wtr

LF

WGP

Ltr

WtrHF

LGP

Ltr

CPW feed

Membrane

Space for circuits

Inverted F antenna on membrane

• Bent quarterwave radiator formed by offset fed inverted F

• Inverted F radiator placed on 2.6 x 0.9 mm BCB membrane

• Single ended feed



15 20 25 30 35

-20

-15

-10

-5

0

FANT

S1

1 [

dB

]

Frequency [GHz]

SimulatedMeasured


Inverted F antenna on membrane

• Measured input impedance50 at 24 GHz

• Measured gain 0 dBi

• Antenna tuning sensitive to ground plane size



Wire loop antennas


Micromachined 24 GHz antennasWire loop antenna on micromachined silicon


Micromachined 24 GHz antennas24 GHz wire loop antenna on micromachined silicon

• 3 x 3 mm wire loop• 360 um wide BCB trenches

covered with BCB membranes

• Chip size 3.6 x 3.6 mm

• Differential feed• Measured input impedance

75 at 24 GHz• Measured gain 1-2 dBi

Lc

Slot

Top metallization (ground-plane)

Trench

Wbr

Wtr

Wc

LL

Si space for active devices

SiWtr



Meander dipole antenna



Wtr

Antenna BCB

Silicon

Silicon Membrane

3.3 mm

0.9 mm

• Membrane size 3.3 x 0.9 mm

• Differential feed

• Input impedance at 24 GHz 20

• Measured antenna gain 0 dBi

Meander Dipole on BCB membrane


Patch antennas



Differentially fed patch antenna by University of Ulm

• Differential feed – no ground connection

• Suitable for wafer scale packaging

• Disadvantages – small bandwidth

Si

Feed point

BCB

Patch

Ground-plane30 um

3800

um

2000 um


Pol

ariz

atio

n

SiGe



Modelled return loss

Differentially fed patch antenna transmission line model


Comparison of 24 GHz AntennasSlot loop

antenna

Wire loop antenna

Meander

dipole

Inverted F

antenna

Patch

antenna

Size at 24 GHz

Trenches, die size 3.3 x 3.3 mm

Trenches, die size 3.6 x 3.6 mm

Membrane size 3.3 x 0.76 mm

Membrane size 2.6 x 0.9 mm

Thick BCB area of 3.8 x 1.9 mm

Feed type and impe-dance

Single ended 100-200

Differential

75-100 Differential 20-25

Single ended 50

Differential typically 50

Gain 0-1 dBi 1-2 dBi 0 dBi 0 dBi < 7 dBi

Remark Circuits within antenna footprint

Circuits within antenna footprint

Sensitive to size of on-chip ground

Wafer level integration




• Design and results for implemented antennas




Outline


Crosstalk with active circuits

BCB

Si 11 -15 cm

Slot mode E-field

Parallel-plate mode

p+ layer, active device area

Parallel plate modes can be excited between the antenna groundplane and conductive active device area


Crosstalk with active circuits

Slot mode E-field

BCB

Si 11 -15 cm

BCB substrate contact

p+ layer, active circuit ground

Parallel plate modes short circuited by BCB via to substrate, crosstalk improvement of 30 dB possible insome cases


Outline of talk








Si

Glob top

Active devices

LTCC carrier

• LTCC (Low Termperature Co-fired Ceramic) used as a carrier for

flip-chip or wire-bonded device

• Glob-top encapsulation obviates the need for a packaging lid

Packaging of Micromachined Antennas


Packaging of Micromachined Antennas

Glob-top Type Loss tangent

Dielectric constant

Amicon S 7503 Silicone 0.0005 / 1 kHz

3.1

Semicosil 900LT Silicone 0.005 / 50 Hz

3.0

Lord CircuitSaf TM ME-455

Epoxy cavity fill

0.006 / 1 MHz

3.37

Lord CircuitSaf TM ME-430

Epoxy glob top

0.006 / 1 MHz

3.7

Namics XV6841-0209

Side fill 0.008 / 1 MHz

3.5


Packaging - Evaluated Glob-tops


Packaging – glob top characterization

22 22.5 23 23.5 24 24.5 25 25.5 26

-80

-70

-60

-50

-40

-30

-20

f [GHz]

S2

1 [

dB

]

AirAmicon siliconeSemicosil siliconeNamics side-fillME430 epoxyME455 epoxy

Measured resonator insertion loss – single tape (100 um dielectric)


Packaging – glob top characterization

Glob-top Single layer fr

[GHz]

Double layer fr

[GHz]

Single layer Q0

Double layer Q0

No glob-top / Air 24.67 24.85 95 75

Amicon S 7503 23.14 23.44 75 50

Semicosil 900LT 23.41 23.98 67 65

Lord CircuitSaf ME-455

22.84 23.26 95 72

Lord CircuitSaf ME-430

22.66 22.87 95 67

Namics XV6841-0209

22.78 22.96 87 71


Packaging - Summary

• A low cost packaging method for 24 GHz MMIC’s is proposed

• Ferro A6-S ceramic LTCC evaluated at 24 GHz

• Glob-top, cavity fill and side fill polymers characterized - epoxy based materials better than silicone ones


Packaging – future and ongoing work

Membrane / glob-top compatibilityPreliminary results promising – no membrane breakage for > 10 mm2 membranes covered with BCB glob tops

Glob-top covered antennas – electrical performanceGlop-top covered loop and dipole antennas mounted on standard FR4 printed circuit boards – characterization pending








Outline


Conclusions

• Integration of an on-chip antenna with a 24 GHz circuits in SiGe technology has been proposed

• 24 GHz on-chip antennas, suitable for integration, have been manufactured and evaluated

• Micromachining of the silicon substrate yields antennaswith reasonable efficiency

• Simple glob-top packaging for micromachinedantennas has been evaluated


Future and ongoing work

• Characterization and modeling of the manufactured antennas

• Improve antenna measurement techniques

• Integrate the antenna with SiGe receiver/transmitter

• Demonstrate packaging of micromachined antennas

• Integrate opto-electronic devices with antennas


Ring slot antenna integrated with 24 GHz receiver* being manufactured

3 mm

3 mm

Receiver

Transistor test structures

Slot in metal 3

Micromachined trenches

to be inserted in silicon

Substrate contacts

*Receiver is designed by University of Ulm

Future and ongoing work


Acknowledgements

• The entire ARTEMIS consortium: Staff at University of Ulm, CNRS/LAAS Toulouse, Atmel GmbH, Sensys Traffic, VTT Electronics

• Klas Hjort and Mikael Lindeberg at Ångström Laboratory

This work was financially supported by the European Commision through the IST-program

Documents

Aug 2004 Micromachined Antennas for Integration with Silicon Based Active Devices Erik Öjefors Signals and Systems, Dep.of Engineering Sciences Uppsala