680 µ m

Flexible and Printed Electronics for Flexible and Printed Electronics for Displays and Image SensorsDisplays and Image Sensors

Jurgen Daniel (daniel@parc.com)

A.C. Arias, W. Wong, S.E. Ready, T. Ng, B. Krusor, R. Lujan, R.A. Apte, J.P. Lu, M.L. Chabinyc, R.A. Street

Palo Alto Research CenterPalo Alto, CA

The Future Prospects and Obstacles of Organic ElectronicsSuncheon, Feb 25, 2008

OutlineOutline

A few words about PARC Motivation for flexible and printed electronics Required components for printed electronics

– Printer– Materials– TFT performance

Display and sensor applications Some of the challenges for printed pixel circuits

PARC OverviewPARC Overview

• Electronic Materials and Devices• Hardware and Systems

• Computer Sciences• Intelligent Systems

Palo Alto Research Center – incorporated in 2002 (formerly Xerox PARC)

~170 researchers in multiple disciplines

4 labs

www.parc.com

Large Area Electronics, MEMS, Optoelectronics, Printing Concepts/Systems,

Piezo Materials, Biomedical Systems,Clean Technology, ...

• Research with client companies• Partnerships with new ventures (startup @PARC) • New business creation • Licensing and technology transfer

Research as a business:

Large-Area Electronics at PARCLarge-Area Electronics at PARC

Low temperature a-Si on plastic

X-ray imagers

LTPS OLED backplane on steel

Jet-printed digital lithography of a-Si TFTs

displays

Technologies Amorphous silicon (a-Si) Low-temperature amorphous Si Low-temperature poly Si (LTPS) Organic semiconductor processes Jet-printed electronics

PARC R&D services:• Prototyping and System design• Testing and Characterization

(W. Wong, et al.)

(J.P. Lu, et al.)

Large-Area Electronics at PARCLarge-Area Electronics at PARC

Flexible Electronics ApplicationsFlexible Electronics Applications

Motivation for Printed ElectronicsMotivation for Printed Electronics

Printing can be a fast and inexpensive process (e.g. newspaper)

Therefore, printing processes may also enable inexpensive electronics for new applications (RFID tags, e-paper, disposable electronics, ...)

– Printing often simplifies conventional processes

Solna Web Presses

“inkjet, pad printing, screen, gravure, flexo, offset, ...”

Printable electronics market: ~$12 bn (2011) ~$30 bn (2013) ?

(Nanomarkets.net)

Printed Electronics ApplicationsPrinted Electronics Applications

Duracell: Battery tester

Cypak: Intelligent Pharmaceutical Packaging

PolyIC: RFID

CSG: via etching

Plastic Logic: e-paper

CDT: OLED displays

Konarka: Solar cellPelikon: EL displays

Some examples

Power Paper: battery

Jet-printing Jet-printing

X-y translation stages for positioning• 15” printing capability

Piezo printheads (single or multi ejector)• Xerox printhead (stainless steel)• ~1200 ejectors @ 20kHz• Potential linear print speed ~m/sec• +/- 5 µm print accuracy• Small drop size (~20-40 µm)• using also Spectra (~128 ejectors) and MicroFab (single ejector) print heads

Computer vision system and software for alignment and layer-to-layer registration

~10-20 pL

remark:with electrostatic jet-printing: ~3µm lines achieved, but only with single ejector (K. Murata, ICMENS'03)

PARC Jet-printing system

camera

printhead

control software

substrate holdertranslation

stage

Piezo ink-jetting:

Advantages (vs. offset, gravure, flexography):- variable pattern- on-the-fly error correction- non-contact- wide range of materials (incl. hot-melt)

Materials (Inks) for PrintingMaterials (Inks) for Printing

(ITO nanoparticles, SbO, ZnInO,... )

● Polymers (e.g. lens, 3D objects, dielectric, colour filters,polyimide alignment layers)

● Phosphors

● Biocoatings

● Etchants

● Silver, gold, nickel, solder

● Magnetic nanoparticles

● Quantum dots

● Metal halide precursors (e.g. SnCl)

● Transparent a-oxide semiconductors

● Electroluminescent materials

● CNTs, nanowires

● Microemulsions

● Liquid crystals● Spacer beads (e.g. for LCD)

● Chemicals (e.g. explosives, narcotics for standards)

● Biochemicals (e.g. for protein arrays)● Conductive/semiconductive polymers● Nanoparticles

● Adhesion promoter

● Wax / (photo)resist

● ...

viscosity range: ~1-40 cpsurface tension: ~20-70 dy/cm

For jet-printing:

Digital LithographyDigital Lithographysimplifies lithography

+ printed organic semiconductor(and PQT/PMMA blends)jet-printed wax

masking layer

conventional deposition:-sputterning-PECVD

All-additive Printed Organic ElectronicsAll-additive Printed Organic Electronics

s,d,g - contacts:• PEDOT:PSS• polyaniline• Ag/Cu-epoxy inks• nano-gold• Nano-silver• CNTs

Thin Film Transistor(TFT)

(bottom-gate)semiconductor:(polymers, oligomers, precursors, nanoparticles)

• polyfluorenes• porphyrins • TIPS pentacene• polythiophenes (P3HT, PQT12, PBTTT)• CNT, nanowires, nanosilicon • ....

e.g.:

e.g.:

substrate:• polyester• polyimide• PEN (polyethylene naphthalate)• ...

e.g.:

gds

inter-layer dielectrics:

• PMMA • BCB (benzocyclobutane)• polyimide• Spin-on-glass • PVP (polyvinylphenol)• epoxy (SU-8)• ...

e.g.:

(gate dielectric, top dielectric)

many new materials are introduced into the process -> performance of the materials has to be optimized and

process compatibility ensured

Requires solution-processable materials !

The Importance of the Gate Dielectric The Importance of the Gate Dielectric

too hydrophobic

too hydrophilic

e.g. compatibility with jet-printing:

• Composites (TiO2, BaTiO

4, ZrO

2) • Polymer + silver nanoparticles

• Cyano-based resins• Electrolytes

• Polyimide

possible gate dielectrics:

• SANDS• BCB

• PVA

• PVP • SU-8 • Glass-like resins

• PMMA

• Cytop

optimized

low leakage currents high C

i (thin layers, high k)

low pinhole density good film former compatible with large-area coating processes low-temperature process (< ~150-200 degC)

delivers good TFT performance process compatibility (surface energy, solvent resistance)

nanosilver

Jet-printable Organic Semiconductor Jet-printable Organic Semiconductor

hydrophobic self-assembled monolayers generally improve field effect mobility

“chemistry of gate dielectrics impacts TFT performance”

octadecyltrichlorosilane (OTS)

lamellar ordering

Ong, et al., JACS, 126 (2004) 3378

polythiophene PQT-12

potential problems:- crystallinity of film is affected by surface- interface traps in dielectric- dipolar disorder due to Si-OH groups

h+ transport

SiO2 'standard' dielectric:

All-additive Pixel-TFT All-additive Pixel-TFT PQT-based TFT with PVP gate dielectric and printed nanosilver G/S/D on PEN

L~28µmW/L ~8

C~15nF/cm2

on/off ~105-106

slope ~2.8 V/decµlin ~0.06 cm2/Vsµsat ~0.08 cm2/Vs

TFT performance is similar to the one with SiO2

TFTs d

g

Jet-Printed Active-Matrix BackplanesJet-Printed Active-Matrix Backplanes (a) (b)

(c)

680 µ m

680 µ m

(a) (b)

(c)

680 µ m

680 µ m

50x50 pixel array

• glass substrate • 680 µm pixels• metal printed with multi-nozzle printhead• spin-coated dielectric

gate-layer

data-layer

• pixel design

• printed pixels (PQT-TFT)

pixel pad

TFT

data-line

gate-line

Flexible Printed BackplanesFlexible Printed Backplanes

on flexible PEN polymer substrate (DuPont)

• PEN substrate• PVP gate dielectric• Printed Ag• PQT semiconductor

Pixel PerformancePixel Performance'All-additive printed pixels on PEN substrate'

• charging/discharging times : ~0.5ms• low gate-feedthrough (<7% of gate-swing)• sufficient charge storage

Pixel performance is sufficient to drive electrophoretic media

Picoprobe measurements

Testing of Display Backplanes Testing of Display Backplanes

poor contact

scratched data lines

shorted data lines

line defect

PARC test electronics

array

charge amplifiers

'charge mapping' enables visualization of defects and of array uniformity

Applications for Flexible and Applications for Flexible and Printed Pixel CircuitsPrinted Pixel Circuits

Displays

Sensors+

(image sensors, bio-sensors, MEMS sensors, ...)

Electrophoretic DisplaysElectrophoretic Displays

● conventional AM backplane with PQT semiconductor

● fixed electrode pattern● flexible substrates

● conventional AM backplane with amorphous Si semiconductor

Display media: E-ink electrophoretic imaging film

active-matrix active-matrix

segmented

(250 ppi)(75 ppi)

+-

sideview

electrophoretic media

Flex Displays with Low-T a-SiFlex Displays with Low-T a-Si

6” flexible display (E-ink media)

bistable image after ~1 month

• 6” digital lithography array

• 8” photolithography array on PEN

(printed mask layers)

TFT:

All-additive Backplane with All-additive Backplane with Electrophoretic MediaElectrophoretic Media

jet-printed silver S/G/D and PQT semiconductor polymer gate dielectric

• charge map:

• backplane is capable of writing images• image is consistent with charge map

50x50 pixel array

Sensor Applications Sensor Applications

X-ray imageX-ray image

B/W light imageB/W light image

• 180x180 pixel, 75dpi array• low-T a-Si:H TFTs and p-i-n sensors• flexible PEN substrate • reduced film stress

170 degC process on PEN (photolithography)

p-i-n images sensor arrays on flexible substrate

digital lithography + we are also exploring organic photo sensor materials

LTPS on Flexible Stainless SteelLTPS on Flexible Stainless Steel• LTPS: Laser recrystallized a-Si

PMOS mobilities up to 59 cm2/Vs

• QVGA full color OLED display demonstrated (collaboration with UDC and L3)

• TFT performance similar to TFTs on glass

UDC'sphosphorescent

OLED

New Hemispherical ArraysNew Hemispherical Arrays

Geodesic dome

• Eye-like imagers• Spherical displays

W.S. Wong, T.N. Ng, R. A. Lujan, S. Sambandan, B. Krusor, R.A. Street

Applications:

Challenges for Printed BackplanesChallenges for Printed Backplanes

Higher display resolution

Device stability (environmental, Vt shift)

Circuit speed (e.g. for full video or for printed driver circuitry)

Cost of driver electronics and battery, versus printed backplane (transferred ICs, printed batteries ?)

Printing yield

Large-scale manufacturing (Roll-to Roll)

Challenge: Higher ResolutionChallenge: Higher Resolution

a) b) c)340 µ m680 µ m 170 µ m

a.r. ~72% a.r. ~58% a.r. ~32%

decreasing aperture ratio

• improve printing process (line width / spaces)• multi-layer pixel design

Higher display resolution requires smaller pixels

increasing pixel count

solutions

Improved Printing Process Improved Printing Process Printing of narrow lines Printing of narrow spaces

10 µm

• short TFT channel length• increased pixel aperture ratio

minimum value is determined by drop placement accuracy

53 µm

printed nano-silver

decreasing surface energy

~53 µm

57 µm

minimum value depends on surfaces and printhead

Multi-layer Pixel Via Formation Multi-layer Pixel Via Formation

• via formation by jet-printing a mask

process steps

silver mask

via interconnect via bank structure

via

Multi-layer Pixel with Molded DielectricMulti-layer Pixel with Molded Dielectric(for higher resolution)(for higher resolution)

• side view

• top view

Multi-layer pixel

printed wax pillar

~30 µm

polymer dielectric

Ag interconnect

• metal interconnect in molded UV polymer

• sacrificial wax pillarvias in molded UV polymer

Bias Stress Effects in PVP-based TFTs Bias Stress Effects in PVP-based TFTs PVP/PSSQ

SiO2-like :Salleo, et al., Appl. Phys. Lett., 86 (2005) 263505

- Bias stress behavior seems to depend on sample history (exposure to air / moisture)- in vacuum mainly negative Vt shift- samples with less x-linked dielectric show positive Vt shift at higher measurement temperatures

Vgs=0VVgs=-30V

recovery

-30V (~420nC/cm2)

recovery

∆Vt

observed behaviour

negative Vt shiftcaused by negative

gate bias

positive Vt shiftcaused by negative

gate bias

moisture !

Bias Stress in All-additive TFTsBias Stress in All-additive TFTs

PQT semiconductor

Speed of Display – video rate ?Speed of Display – video rate ?

190x64 µm, Cst ~0.2pF, Cp ~0.03pF

680x680 µm, Cst ~pF, Cp ~pF Cst

CpCd

Vft= Vs

Cp

Cst+Cp

·

• high mobility semiconductor is desirable (+ high W/L)• good control over printing process to reduce parasitics

• ~ 30 fps may be sufficient for many applications• for e-paper, faster display media is under development (E-ink, SID'05)

• Conventional LCD display pixels:

• Printed pixels:

'Fast pixel response requires careful balancing of feedthrough voltage and pixel capacitances'

Pixel capacitances and feedthrough voltage

Pixel capacitance (Cp+Cst)

Pixe

l RC

tim

e

<100µs

high R TFT

low R TFT

Pixel charging speed

typically larger feature size !

Roll-to-Roll ProcessingRoll-to-Roll Processing

Mekoprint Electronics

Konarka

PolyIC

CIT/ Xennia

Would be most adequate method for printing processes

Is used already for some organic electronics applications

Still requires materials improvement for short processing times

Layer to layer registration is challenging Is it the best method for display

backplanes ?

Prof G. Cho(Sunchon National Univ.)

Summary and OutlookSummary and Outlook PARC has a wide portfolio of flexible backplane

technologies All-additive printed electronics promises ultimate cost

savings Many new materials are introduced in the process -

performance/lifetime of the materials has to be investigated

Low resolution pixel circuits e.g. for signage-type applications

Higher resolution -> multilayer pixel design or narrower printed lines

Prototype electrophoretic displays have been fabricated

AcknowledgmentAcknowledgment

PARC: processing line underJackson Ho

Xerox Research Centre of Canada: Ping Liu, Yiliang Wu

NIST: partial funding

Dupont Teijin: PEN substrates

E-Ink: collaboration on electrophoretic media