liquid crystal polymer for mems application

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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liquid crystal polymer for mems application

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