From NEXAFS Spectroscopy to New Flat Panel Displays: A Story of Basic Science with Technological Impact

J. Stöhr1,2, M. Samant 2, J. Lüning1, Y. Momoi 3, S. Odahara 3, P. Chaudhari 4, and S.A. Lien 4

1 Stanford Synchrotron Radiation Laboratory, Stanford University, Stanford CA 943092 IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose CA 95120

4 IBM Display Business Unit, Yamato, Japan4 IBM Research Division, T.J. Watson Research Center,

Today’s Technology: Liquid Crystal Alignment by Rubbed Polymer Films

N

CO

CC C C C

Polyimide chains and monomer

Radius of gyration ~ 10nm

Surface layer ~ 1nm

Plate movement

Rotatingrubbing wheel

The Established Rubbing Technology

Pretilt eliminates multidomain boundary lines

Flat Panel DisplaysA $20 billion world-wide business

Twisted liquid crystals modulate light transmission

In-plane Alignment and Pretilt of Liquid Crystals

Pretilt Angle

Rubbing Direction

Liquid Crystals are magically aligned by rubbed polymer films

Liquid Crystal Display Production

Rubbing process:- is ineffective and costly- introduces debris and streaks- produces electrostatic discharging- allows only single domains – limited viewing angles - process is environmentally unfriendly

NEXAFS Spectroscopy

Unique properties of NEXAFS Spectroscopyrender it “technique of choice” for surface analysis

X-raysE

Metal Grid

Flux Monitor

Sample

Cylindrical Mirror Analyzer

(Variable) Surface Sensitivity

e -C

N

O

F

Elemental Composition

Polystyrene

Polyimide

284 286 288 290 292

NC

Chemical Bonding Orientation

Finding a replacement process has been the holy grail of the flat panel industry. This is the story of how it was accomplished.

NEXAFS Provides Key to Ion Beam Alignment Puzzle

o

Ion beam

AEY = Surface TEY = Bulk

Ion beam treatment converts polymer surfaceinto a thin layer of amorphous carbon

2 to 6 nm Chemical bonds in amorphous carbon surface layer on polyimide are preferentially oriented

IBPoly-imide

285 295

Photon Energy (eV)

in-plane

285 295

Photon Energy (eV)

+45o

-45o

out-of-plane

rubbedPoly-imide

Substituting Polymer by A-Carbon

It works! Ion beam treatment introduces preferential orientation of a-Carbon and liquid crystals align beautifully

z

y

IonBeam

-x x

x

lessdestroyed

in-plane

+45o

-45o

out-of-plane

2 8 5 2 9 5P h o to n E n e rg y (eV )

Nor

m. A

uger

Yie

ld

2 8 5 2 9 5

The same in-plane but opposite out-of-plane molecular asymmetry found by NEXAFS for rubbed and ion beam treated polyimide explains different liquid crystal orientations on two surfaces

The breakthrough suggested by NEXAFS: Do away with the polymer altogether,start with a-Carbon and irradiate it with ion beam

Molecular orientation in a-Carbon arises from preferentialbond breaking by directional ion beam

NEXAFS solves LC Alignment Mechanism on Polymer Surfaces

morey

x

Before After

out-of-plane

Before After

z

x

z

x

x

y

in-plane

Liquid crystal orientation

Polymer surface orientationz’

y’x’

y

z

x

a

c

b

fb

fc

fa

Pretilt

fy ’

fx ’

fz ’

Molecular tilt

Auger Yield (AEY)probes top 2 nm

Total Yield (TEY)probes top 10 nm

Polyimide

Rubbing orients polymer groups preferentially- can be viewed as pulling on polymer chains

y

Ion Beam

x

z

yPretilt

x

z

x

y

z

RubbingDirection

x

yz

Pretilt

Rubbing

Ion Beam Irradiation

A remaining puzzle: Liquid crystals also align on ion beam irradiated polymers - but the opposite way!

Chaudhari et al. Jpn. J. Appl. Phys. 37, L55 (1998)

Polarization dependence of NEXAFS spectra reveals molecular orientation

The rubbed polymer surface can be viewed as an oriented liquid crystaland the alignment mimics the parallel alignment of two liquid crystals

Summary

• Work solves old scientific puzzle

• Scientific understanding redirects technology and leads to new materials and processes for manufacturing of liquid crystal displays

Chaudhari et al., Nature 411, 56 (2001)

• Understanding of alignment origin and discovery of a-carbon surface layer leads to development of new materials and processes

Stöhr et al., Science 292, 2299 (2001)

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