Testing of Flexible Metamaterial RF Filters Implemented through - - PowerPoint PPT Presentation

Testing of Flexible Metamaterial RF Filters Implemented through Micromachining LCP Substrates

Jonathan Richard Robert 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 Enhanced Devices

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 Structure

Lumped 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 Effect

No Gap – Band Stop Gap – Band Pass

Metamaterial Simulations

• ADS Momentum uses method
• f moments
• CSRR arrays constructed in ADS

layout

• Frequency sweep and calculate

S‐parameters

Single Element 10 Element Array

Simulation Results (10 Element Array)

Expected bandpass filter 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
• r 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 metamaterials

structures

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 Curvature in inches 0.542 0.65 0.804 0.984 1.115 1.3585

Testing Results

• Expected bandpass

response 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

Frequency (GHz) S21 (dB) 1/2" pvc pipe S21 S21 - Cal down S21 - Cal up S21 - down 1 a S21 - down 1 b S21 - up 1 a S21 - 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