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8/3/2019 Liquid Crystal Polymer for Mems Application
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B.ANUSHA
(2011H140039H)
LIQUID CRYSTAL POLYMER IN
MEMS APPLICATIONS
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OUTLINE
y Overview of Liquid Crystal Polymer(LCP)
y Microfabrication techniques
y Applications
Packaging
Substrate material
Microfluid application
Sensor applications
PCB application
y Conclusion
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INTRODUCTION
y MEMS field evolved from the semiconductor industry
y Initially,silicon had been the predominant material
y Emergence of polymers in MEMS industry due to:
(i) Lower cost
(ii) Flexible fabrication & packaging techniques
(iii) Unique physical & chemical properties such as
biocompatibilty & high mechanical fracture limit
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LIQUID CRYSTAL POLYMER(LCP)
y Molecules mutually aligned & organised (as in a crystal)
y Bulk of LCP can flow in molten state(as in a liquid)
y Rigid flexible monomers that align in shear flow direction
y
Persistence of orientation even below melting pointtemperature
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LCP PROPERTIES
Barrier to gases like O2,N2,CO2
Good chemical resistance
Flexible & capable of multilayer
laminationLow coefficient of hygroscopic
expansion
Mechanical properties are
anisotropic and dependent onpolymer orientation
Low temperature thermal
bonding
Biocompatibility
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MICROFABRICATION TECHNIQUES
OXYGEN PLASMA REACTIVE ION ETCHING
Deposition & patterning of Al on LCP film
Partial oxygen plasma etch of LCP
Second patterning of Al etch mask
Plasma etch through LCP leaving a flap
Removal of Al mask
Fabrication of a suspended flap supported by cantilevers
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SEM micrograph showing a flat flap supported by two
fixed-free beams in the LCP film.
(Etch rate of 25 m /min at 500 mTorr)
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LAMINATION PROCESS USING SURFACE ACTIVATED
BONDING
Lamination of Cu foil & LCP film
y Sputter cleaning of Cu & LCP surfaces with argon radio
frequency plasma etching in vacuum
y
Removal of inactive layers of native oxide and contaminantsachieved
y Deposition of Cu on the LCP film followed by direct bonding
at room temperature
y Subsequent heating of Cu/LCP sample for better interface
adhesion
y Bonded interface 15 times smoother than that produced by
heated lamination method
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Mean peel strength dependence on the sequential heating in Ar,
N2, Air, and O2.
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Fine pattern fabricated by chemical etching on
SAB processed LCP/Cu specimen
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PACKAGING
y Packaging critical in bringing MEMS devices into
application
y Passivation difficult due to the presence of moving
parts & chemically sensitive sensorsy Advantages of LCP packaging:
(i)Negligible outgassing
(ii)Low temperature processing that causes minimal
damage to MEMS device
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LCP PACKAGE FOR RF MEMS SWITCH
Lamination process of LCP on Si
Cu sputtering on LCP
Cavity formation by laser ablation to the
Cu lid
Lamination of LCP onto Si switch
After lamination
Pattern Cu & form vias
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Diagram of a packaged RF MEMS switch
in an LCP enclosure
Prototype of the packaged RF
MEMS switch
Insertion loss:0.5 dB
Return loss :25 dB
Isolation loss :14 dB
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LCP PACKAGED MEMS PRESSURE SENSOR
FOR UNDERWATER APPLICATIONS
Diaphragm -based pressure sensor
Packaging should prevent mass loading of the diaphragm
LCP-based diaphragm exhibits enhanced sensitivity to pressure variations due
to low elastic modulus
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ADVANTAGES
y Flexible; applicable on curved surfaces
y Exceptional hermetic properties
y Allows array arrangement and high-sensitive 2D
pressure mappingy Good pressure resolution due to closely-spaced sensors
y High durability and chemical resistance
y Low cost production
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LCP AS SUBSTRATE
Fabrication process flow of switch
(i)Polyimide spun on LCP to minimizethe roughness
(ii)Fabrication of CPW signal lines
(iii)Patterning of dielectric layer between
membrane & signal line
(i)Spin-coating & patterning of
photoresist to create air gap
(ii)Evaporation,patterning &
electroplating of seed layer
(i)Removal of sacrificial photoresist
(ii)Releasing of switch by a drying process
RF MEMS SWITCH ON LCP SUBSTRATE
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PERFORMANCE OF SWITCH
y Insertion loss of about 1 dB
y Isolation loss of about 20 dB
y Return loss of about 20 dB
y Enables integration of reconfigurable architecture on
LCP for 3D RF front ends
y Flexible in nature
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LCP IN MICROFLUID APPLICATIONS
MICROCHANNEL IN LCP FILM
Deposition and patterning of Al on LCP film
Defining fluid channels (trenches) by oxygen plasma
etching
Removal of A1 mask and thermal bonding LCP
to glass base
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LCP-LCP bonding can form flexible 3D multi-layer fluid circuits
SEM micrograpgh showing channel cross-
section in LCP microchannel
Flow channel under epi-
flouroscence microscope
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LCP IN SENSORS
(a) When there is no flow rate, the cantilever is straight
(b)Flow imparts momentum on the cantilever and causes bending
(c)Bending induces strain at the base of the cantilever,which is sensed using a
piezoresistive sensor
FLOW SENSOR
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Deposition & patterning NiCr strain gauges on LCP
Deposition & patterning of Au/Cr wiring
Physical cutting of LCP to form sensor beam &bonding to glass carrier
Fabrication steps
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Sensor beam
Strain gauge
Optical micrograph of a
cantilever flow sensor
Experimentally measured output
characteristics as a function of
flow rate
Sensitivity comparable to Si- based devices
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TACTILE SENSORS
y Converts normal applied load into change in resistance
y Maximum stress at the centre of edge of a square plate with fixed sides
is linear with applied deflection
y Tactile bump height ensures that stress doesnt exceed max value
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Double sided alignment,deposition & patterning of
NiCr strain gauges & Al mask on LCP
Patterning of Al followed by RIE on backside of
cavity & removal of Al
Deposition & patterning of Au interconnects
Spin & pattern polymide tactile bumps
Fabrication process flow
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Optical micrograph of an
array of tactile sensors
Tactile membrane displacement
measured using precision LVDT
Linear response over 20m range, with
0.86 Ohm/m sensitivity
Array of tactile sensors can image tactile contact
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LCP IN PRINTED CIRCUIT BOARDS
Cross-section of circuit traces on LCP
LCP used as base laminate in PCBs
Good adhesion between the copper & LCP
Good resistance to etching chemistry
Negligible undercutting at interface
LCP circuitry traces & pads
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Laser drilling used for hole making in LCP
Cleaning using plasma treatment for clearing debris prior to metallisation
LCP as drilled (left) after plasma desmear (right)
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CONCLUSION
y Usage of polymers in MEMS applications has become an
emerging trend
y LCP properties make it useful and suitable for MEMS industry
y Microfabrication methods developed for use of LCP
y Wide range of applications including packaging,sensors,PCB
processing ,substrate material makes LCP a versatile polymer
y Performance of LCP based devices comparable,if not higher, to
that of conventional Si based devices
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REFERENCESy Morgan Jikang Chen, Anh-Vu H. Pham, Nicole Andrea Evers,ChrisKapusta, Joseph Iannotti, William Kornrumpf,
John J. Maciel, Nafiz Karabudak, Design and Development of a Package Using LCP for RF/Microwave MEMS
Switches, IEEE Transactions On Microwave Theory And Techniques, Vol. 54, No. 11, November 2006
y Tan Zhang & Wayne Johnson,Auburn University, Brian Farrell,Foster Miller, Inc, Michael St. Lawrence,Rogers
Corporation, The Processing and Assembly of Liquid Crystalline Polymer Printed Circuits
y XuefengWang, Jonathan Engel and Chang Liu, Liquid crystal polymer (LCP) for MEMS:processes and applications,
Journal Of Micromechanics And Microengineering,vol13 (2003) 628633
y
M. M. R. Howladera and T. Sugab, Surface Activated Bonding Method for Flexible Lamination, EngineeringPhysics Department and Electrical and Computer Engineering Department,McMaster University
y Guoan Wang, Dane Thompson, EmmanouilM. Tentzeris and John Papapolymerou , Low Cost RF MEMS Switches
Using LCP Substrate,School of Electrical and Computer Engineering, Georgia Institute of Technology
y Xuefeng Wang, Liang-Hsuan Lu, and Chang Liu,MicroelectronicsLaboratory, Micromachining Techniques For
Liquid Crystal Polymer ,Department of Electrical and Computer Engineering,Universityof Jllinois at Urbana-
Champaign
y http://ilo.technologypublisher.com/technology/5203
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QUESTIONS?