A 77GHz on-chip Microstrip patch antenna with suppressed surface wave using EBG substrate

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A 77GHz on-chip Microstrip patch antenna with
suppressed surface wave using EBG substrate
Mohammad Hossein Nemati, Ibrahim Tekin
Electronics Engineering, Sabanci University,
34956 Istanbul, Turkey tekin_at_sabanciuniv.edu

APS , Orlando, July 2013
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Outline

• Motivation
• Patch antenna with improved performance
• Measurement setup for antenna at 77GHz
• Conclusion and future work

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Motivation

• Millimeter wave systems are promising
• high speed comunication
• less interference
• Single chip solution including the antenna
  (antenna size is comparable to the chip size and
  integration of chip with antenna is feasible)
• We encounter more civilian use of millimeter wave
  radars especially in
• navigation
• road traffic control
• safety for highway driving (short range
  automotive radar at 77 GHZ band)
• Measurement at millimeter wave frequency is
  challenging. Minimizing the measurement
  uncertainty is critical in the development of new
  mm-wave applications.
• High precision devices
• High skills needed for measurement and
  calibration of devices

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Integrated antenna LNA RF MEMS phase shifter
Patch antennas
Two microstrip patch antenna Two LNA with 15 dB
gain Two RF MEMS 4 bit phase shifter Less than 10
mm2 chip area (2.6 mm X 3.9 mm)
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IHP technology for antenna and EBG structure
SiO2

• Metal5(patch)

11.4um
Metal1(EBG structure)
250um
Silicon substrate 20ohm-cm
Etched part

• 5 metal layers for antenna and EBG structure
• Metal5 layer is used for Antenna and Metal1 line
  is used for implementing EBG structure.
• Localized back-side etching (LBE) module to etch
  the lossy substrate under the antenna to increase
  the gain.

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Microstrip Antenna on High Dieletric
Substrate(Silicon)
Patch size 1.1mm1mm
SW diffraction from edge
GSG Probe
Surface wave

• On-chip microstrip patch antenna (Integration of
  the antenna with active circuitry)
• Compact antenna size due to small wavelength at W
  band and silicon substrate
• However, the substrate will cause gain and
  efficiency loss and also distort the radiation
  pattern due to surface wave.
• Surface waves can easily be excited on thick and
  high dielectric substrates(Silicon)
• pattern distortion, gain drop,
  cross-polarization increase

Silicon(e12, lossy)
h250um
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Surface wave

• Propagating electromagnetic waves that occur on
  the interface between two dissimilar
  materials(Both TM TE nature)
• metal and free space
• dielectric coated conductor

a) Dielectric Coated Substrate
b) TM0 mode pattern for coated substrate
C) Patch Antenna Mode(E field)
TM Surface wave mode has same polarization with
patch mode
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Solutions to improve gain and radiation pattern

• Substrate can be etched
• Etching establish a low effective
  dielectric-constant environment
• Less localized EM fields
• Increase the antenna gain and efficiency
• EBG structures can be patterned close to the
  antenna to stop the SW propagation.
• EBGs are sub-class of Meta-Material
• Creates band-gap for surface wave
• Different type of EBG structure are available
• Uni-planar Electromagnetic Band-Gap is chosen due
  to construction simplicity(no need for via)

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Etching of the microstrip patch antenna
holding walls

• Localized back-side etching (LBE) is used.
• Different substrate height by mechanical polish
  of Silicon
• Removing silicon right under the patch reduce
  loss and increase gain
• Max etching size is 700umx and Min. is100um

Etched regions
Patch
Etching size 600500um
Silicon(e12, lossy)
Different substrate height by polishing
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Uni-planar EBG structure for Supressing Surface
Wave
Patch(Metal5)

• TM10 is Patch fundamental mode (Radiating mode)
• But patch supports unwanted surface waves of TM
  TE nature
• Electromagnetic bandgap structure(EBG) can filter
  SW
• A type of Photonic Bandgap structure that creates
  bandgap
• Block unwanted surface mode around antennas
  operative frequency
• Increase coupling efficiency from patch mode to
  space mode

EBG structure (printed at Metal1)
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Modeling EBG structure at HFSS

• Unit cell of EBG modeled at HFSS to derive its
  propagation constant
• Dispersion diagram for EBG structure
• Propagation at first Brillouin zone
• propagation constant of surface wave at different
  frequency and directions
• Only TM nature SW can cause problem(gain,
  cross-pol, pattern distortion)

Antenna operation Freq
650um
300um
Unit cell of EBG (HFSS)
EBG structure
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Microstrip Antenna with improved
performance(etched and surrounded by EBG)

• Presence of the EBG drops the resonance frequency
  which can be removed easily by tuning the length
  of the patch.

a) Patch antenna surrounded by EBG
b) Return Loss vs. Frequency
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Microstrip Antenna with improved
performance(etched and surrounded by EBG)

• EBG increase gain by 3dB and remove pattern
  distortion
• Etching also decrease losses and increase gain
  and efficiency

Pattern with EBG Pattern without EBG
Distortion mainly exist in E plane After
construction distortion can shift anywhere
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Antenna Measurement Setup at 77GHz

• Setup enables reflection coefficient, gain and
  far-field radiation pattern measurement
• E and H Plane measurement
• Both co- and cross-polarization
• Calibration procedure
• Corrects different errors
• Unwanted ambient reflection
• Absorbing material
• Time domain filtering

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Indoor Antenna measurement setup
Extender
Network Analyzer 50 GHz PNA 5245A
Horn antenna and bent WG
Table for Extender, cascade probe , probe
positioner and AUT
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W-Band Antenna Measurement Setup
Network Analyzer 50 GHz PNA 5245A
Extender
AUT GSG probe Two type of GSG probe are
available with 90 degree spatial difference(to
switch from E-plane to H-plane)
Rotating Arm
Standard Horn
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Antenna S-parameter measurement at 77GHz

• S parameter of a dipole antenna measured by our
  setup
• S parameter of a sample dipole antenna from
  previous work is measured
• Due to delay in delivery of patch antenna we were
  not able to measure the result for patch

Freq(GHz)
a) Dipole antenna measured by our setup
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Conclusion and future work

• Patch antenna with EBG structure is introduced.
• S-parameter and radiation pattern will be
  measured for the EBG patch antenna
• EBG structure can be used to reduce mutual
  coupling between array elements.

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