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Testing of Flexible Metamaterial RF Filters Implemented through
Micromachining LCP Substrates
Jonathan RichardRobert Dean
Michael Hamilton
Metamaterials Definition
• Metamaterials exhibit interesting properties not readily observed in nature
• These properties emerge due to specific interactions with electromagnetic fields or due to external electrical control.
Introduction to Metamaterials
• Double Negative materials a.k.a. Left handed
• Has negative index of refraction• Use of specially built structures• Special structures use normal
materials such as Rogers board,LCP, and Polyimides.
Combining Technologies
• Smaller feature size• More complex structures• Additional materials with useful properties• Rigid or flexible substrates• Integrated electronics
Metamaterials MEMS
EnhancedDevices
Common Uses
• Mirrors and lenses• Transmission Lines Filters• Invisibility cloaks
Cloak Structure
Physical Layout E‐field view with and without structure
Note: Only works at small frequency range and for one type of EM wave (i.e. TE01)
Common Filter Structures
• Split Ring Resonators (SRR)• Complementary Split Ring Resonators (CSRR)• Bandwidth is narrow• Must be electrically small for lumped element assumptions
• Lumped element parameters derived from empirical data
CSRR StructureLumped Element Representation• Cg = Gap Capacitance• Lc and Cc represent the
resonator as a tank circuit• L and C relate to the line
per unit length
Physical Layout• Orange = ground plane• White = etch off ground plane• Blue = metal on surface
CSRR Gap Capacitance EffectNo Gap – Band Stop Gap – Band Pass
Metamaterial Simulations• ADS Momentum uses method of moments
• CSRR arrays constructed in ADS layout
• Frequency sweep and calculate S‐parameters
Single Element
10 Element Array
Simulation Results (10 Element Array)
Expected bandpassfilter response
LCP Fabrication Overview
• Place mask on Liquid Crystal Polymer (LCP) with photolithography process
• Copper etching techniques• LCP etching techniques
Photolithography Process
• Inspect and clean substrate with HCl bath• Add HMDS to promote adhesion• Spin on photoresist• UV expose PR• Develop PR
Copper Etching
• Before etching, cover opposite side with tape or photoresist since LCP comes double clad with copper
• Wet etching is isotropic which is accounted for with photolithography mask
• Slower wet etching causes less variance across the substrate
• Therefore an LCP with thin Cu cladding is desirable
Copper Etching Continued
Over etched CSRR etched transmission gap
LCP Etching• After copper has been fully removed where desired, E‐beam Al onto substrate
• Add mask using photolithography process• Etch thin film Al mask with highly select etchant (PR developer)
• Use O2 RIE to remove LCP followed by chemical Al mask removal
• Realizes holes and vias in metamaterialsstructures
Testing Procedure Setup
• Since the T‐lines were impedance matched to 50Ω, sma connectors could be easily connected
• Provide extra T‐line on masks to help with soldering and providing more surface area between LCP and Cu
• End Launch Connectors simplified testing and provided accurate results
Testing Procedure
• Short/open/load procedure• Keep hands away from LCP and connectors during data capture to avoid stray capacitance
• Elevate LCP into the air to avoid affecting fringing capacitance for air calibration
• Test of flat PCB section• Use PVC pipe of various radii to perform flexibility tests
T‐Line Calibration
Air calibration PVC calibration
Flexibility Testing• Multiple filters were flexed over 6 different sized pvc pipes
• Tested with respect to T‐line orientation both up and down
• Larger radii approached being flat
Number 1 2 3 4 5 6
PVC Diameter 1/2" 3/4" 1" 1 1/4" 1 1/2" 2"
Radius of Curvaturein inches 0.542 0.65 0.804 0.984 1.115 1.3585
Testing Results
• Expected bandpassresponse observed
• Slightly lower frequency than simulation
• Flexing up or down had little effect on frequency response
• ½“ PVC pipe flex testing:
10 12 14 16 18 20 22-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Frequency (GHz)
S21
(dB
)
1/2" pvc pipe S21
S21 - Cal downS21 - Cal upS21 - down 1 aS21 - down 1 bS21 - up 1 aS21 - up 1 b
Applications
• Quality passive high order filters on flexible substrates– Wearable electronics– Flexible electronics
• RF cloaking– Conformal to airfoils or vehicle bodies– Reduction of radar cross‐section– EMI/EMC improvement
Conclusions• RF metamaterials possess interesting and useful properties
• Combining with MEMS technology enhances the usefulness of metamaterials
• LCP is a flexible substrate material with excellent RF properties
• RF metamaterials on flexible LCP substrates yield RF filters that can be conformably attached to nonplanar surfaces
Thanks