Outline

1
Micromachined Antennas for Integration with
Silicon Based Active Devices
Erik Öjefors Signals and Systems, Dep.of
Engineering Sciences Uppsala University, Sweden
2
Outline of talk

• 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

3
Introduction
Objective On-chip antenna integrated with a 24
GHz ISM band transceiver in SiGe HBT technology
for short range RADAR and communication devices
Integration
Antenna
Self-contained SiGe front-end
3x3 mm large chip
4
Introduction
One application RADAR for traffic surveillance
and anti-collision warning systems
5
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

6
Challenges of on-chip antenna integration
Antenna size can NOT be reduced without
consequences!
Minimum Q (quality factor) of small
antennas a is the radius of a sphere
enclosing the antenna. k 2p/l. High Q leads
to small bandwidth and can reduce the efficiency
McClean, " A Re-examination of the Fundamental
Limits on the Radiation Q of Electrically Small
Antennas," IEEE Trans AP, May 1996.
7
Challenges of on-chip antenna integration
Problem Size of antenna is an important
parameter due to the high cost of the processed
SiGe wafer Solution Chose antenna types which
offer compact integration with the active circuits
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Proposed integration with active devices
Slot antenna
Active devices
Active elements integrated within slot loop
3 mm
Top metallization
3 mm
Active devices
Si
p channel stopper
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Challenges of on-chip antenna integration
Problem Commercial silicon-germanium (SiGe)
semiconductor use low resisistivity (lt 20 Wcm)
substrates Solution Use of a low loss
interface material such as BCB polymer or
micromachining to reduce coupling between antenna
and lossy silicon substrate
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Micromachining
Micromachining mechanical shaping of silicon
wafers by semi-conductor processing techniques
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Micromachining BCB process flow
Post processing technique compatible with
pre-processed SiGe wafers from commercial
semiconductor foundaries
Active circuit
Pre-processed wafer from foundary
Si
BCB
10-20 um BCB layer applied and cured
Si
Gold
Top metallization evaporated and defined using
standard photolitho- graphic techniques
Si
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Surface micromachining of silicon
Micromachining

Top metali
zation

Slot

Optional micro
-

BCB,



machining

20 um


10 um


W
Si 11
-
15

cm

Surface micromachining applied to the substrate
before BCB-spin-on
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Bulk micromachining of silicon
Micromachining

Top metali
zation

Slot

10-20 um

BCB membrane,







Backside etching

Si


Back side of silicon substrate etched as last
step in processing
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Outline of talk

• 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

15
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

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Surfaced micromachined slot loop antenna
Micromachined 24 GHz antennas
BCB, Si
10, 20 um slot width
3000 um
CPW probe pad
BCB 10-20 um
Si 11-15 Wcm
2000 um
3000 um
Slot loop length corresponds to one guided
wavelength at 22 GHz
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Micromachined 24 GHz antennas
Surfaced micromachined slot loop antenna
Small return loss outside the the operating
frequency indicates that losses are present
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Results Radiation Pattern
Antenna on 20 um thick BCB interface layer on low
resistivity Si
H-plane
E-plane
Reasonably good agreement between simulated and
measured radiation pattern, (some shadowing in
E-plane caused by measurement setup)
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Micromachined 24 GHz antennas
Results Gain and efficiency
Reference horn antenna

• Measured gain -3.4 dBi
• Directivity (simulated) 3.2 dBi
• Calculated efficiency 20

Wafer probe station
80 cm
Foam material (low dielectric constant)
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Micromachined 24 GHz antennas
Bulk micromachining improving efficiency
Slot supported by BCB membrane
Si
200 ?m
No trenches
Trenches can be formed from the back side of the
wafer by chemical wet etching (KOH) or dry
etching (DRIE) methods
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Micromachined 24 GHz antennas
Bulk micromachining improving efficiency
Radiating slots

• DRIE
• gt100 um trench width can be etched

Radiating slots
Anisotropic etching (KOH, TMAH) Needs wafer
thinning (300 um)
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Micromachined 24 GHz antennas

• Bulk micromachining 3D-FEM simulations (HFSS)

By etching 200 um wide trenches in the silicon
wafer the simulated input impedance is increased
from 60 W to 210 W at the second resonance,
simulated efficiency increased from 20 to gt50
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Bulk Micromachining Slot Loop Antenna
Micromachined slot loop antenna
sa
Si
wt
Trench (membrane)
Slot

• Designed antenna
• Trench width wt 100 um
• Results
• Measured gain 0-1 dBi
• Single ended feed (CPW)
• Impedance 100 Ohm

Silicon space for active devices
wb
lg
wt
Slot
Top metallization (groundplane)
lg
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Micromachined 24 GHz antennas
Inverted F Antenna
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Micromachined 24 GHz antennas
Inverted F antenna on membrane
Ltr
LF
Ltr
Wtr

• 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

HF
Membrane
CPW feed
Space for circuits
LGP
WGP
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Micromachined 24 GHz antennas
Inverted F antenna on membrane

• Measured input impedance
• 50 W at 24 GHz
• Measured gain 0 dBi
• Antenna tuning sensitive to ground plane size

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Micromachined 24 GHz antennas
Wire loop antennas
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Micromachined 24 GHz antennas
Wire loop antenna on micromachined silicon
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Micromachined 24 GHz antennas
24 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 W at 24 GHz
• Measured gain 1-2 dBi

Lc
Si
Wtr
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Micromachined 24 GHz antennas
Meander dipole antenna
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Micromachined 24 GHz antennas
Meander Dipole on BCB membrane
3.3 mm

0.9 mm
Membrane
Silicon

• Membrane size 3.3 x 0.9 mm
• Differential feed
• Input impedance at 24 GHz 20W
• Measured antenna gain 0 dBi

Antenna
BCB
Silicon
Wtr
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Micromachined 24 GHz antennas
Patch antennas
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Micromachined 24 GHz antennas
Differentially fed patch antenna by University of
Ulm
Patch
3800 um
BCB
30 um
Polarization
Si
Ground-plane
SiGe

• Differential feed no ground connection
• Suitable for wafer scale packaging
• Disadvantages small bandwidth

Feed point
2000 um
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Micromachined 24 GHz antennas
Differentially fed patch antenna transmission
line model
Modelled return loss
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Comparison of 24 GHz Antennas
Slot 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 W Differential 75-100 W Differential 20-25 W Single ended 50 W Differential typically 50 W
Gain 0-1 dBi 1-2 dBi 0 dBi 0 dBi lt 7 dBi
Remark Circuits within antenna footprint Circuits within antenna footprint Sensitive to size of on-chip ground Wafer level integration
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Outline

• Introduction, applications
• Challenges of on-chip antenna integration
• Design and results for implemented antennas
• Crosstalk with on-chip circuits
• Micromachined antenna packaging
• Conclusions and future work

37
Crosstalk with active circuits

Slot mode E
-
field

Parallel
-
plate
BCB


mode




p layer, active


device area

W
Si 11
-
15

cm

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

BCB substrate
Slot mode E
-
field

contact

BCB





p layer, active


W
Si 11
-
15

cm

circuit ground

Parallel plate modes short circuited by BCB via
to substrate, crosstalk improvement of 30 dB
possible in some cases
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Outline of talk

• Introduction, applications
• Challenges of on-chip antenna integration
• Design and results for implemented antennas
• Crosstalk with on-chip circuits
• Micromachined antenna packaging
• Conclusions and future work

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Packaging of Micromachined Antennas

• 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

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Packaging of Micromachined Antennas
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Packaging - Evaluated Glob-tops
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Packaging glob top characterization
Measured resonator insertion loss single tape
(100 um dielectric)
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Packaging glob top characterization
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Packaging - Summary

• A low cost packaging method for 24 GHz MMICs 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

46
Packaging future and ongoing work
Membrane / glob-top compatibility Preliminary
results promising no membrane breakage for gt 10
mm2 membranes covered with BCB glob
tops Glob-top covered antennas electrical
performance Glop-top covered loop and dipole
antennas mounted on standard FR4 printed circuit
boards characterization pending
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Outline

• Introduction, applications
• Challenges of on-chip antenna integration
• Design and results for implemented antennas
• Crosstalk with on-chip circuits
• Micromachined antenna packaging
• Conclusions and future work

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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
  antennas
• with reasonable efficiency
• Simple glob-top packaging for micromachined
• antennas has been evaluated

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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

50
Future and ongoing work
Ring slot antenna integrated with 24 GHz
receiver being manufactured
Micromachined trenches to be inserted in silicon
Slot in metal 3
3 mm
Receiver
Substrate contacts
Transistor test structures
Receiver is designed by University of Ulm
3 mm
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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