Design, Modeling, and Reliability

of Flexible Electronics

TAU 2011

K.-T. Tim Cheng and T.-C. Huang

Electrical and Computer Engineering

University of California, Santa Barbara

2

Introduction

Key element: thin-film transistors (TFTs)

Reliability modeling and simulation

Robust circuit design

Printed Pressure Sensors and Applications

Conclusion and future works

Outline

Si MOFET

TFT

Performance

Cost

What is Flexible Electronics

Thin-film, light-weight, and low-cost

Bendable, durable, and large-area

Flexible substrates

Plastics and metal foils

Non-photolithography manufacturing

• Ink-jet printing

• Roll-to-roll imprinting

3

[1]

[2]

[1] Roll-to-roll process, PolyIC;

[2] Ink-jet printed electronics,

Phillips

4

Applications: Large Area Electronics

Polymer Vision

RFIDPOLY IC

E-paper

Solar Cell

Contactless

Power

Flexible Display

E-Skin

Plastic Logic

H-Alpha

Univ. TokyoUniv. Tokyo

TFT

Si MOSFET

Performance

Cost

Flexible Electronics Market

Source: IDTechEx, 2008Display, Photovoltaics dominate the market share,

but new flexible module will change the segment

$B

Low-Cost

Large-Area Manufacturing

6Ref: ISSCC’07 Tutorial, T. Sakurai

Ink-Jet Printing

LG-Phillips©

Fujifilm-Dimatix©

PolyIC©

Roll-to-roll printing

Roll-to-roll imprinting

Screen

Printing

Someya group, Univ. Tokyo

Someya group, Univ. Tokyo

Screen printing

Thin-Film Transistors (TFTs)

The Key Element of Flexible Electronics

7

Hydrogenated Amorphous Silicon

(a-Si:H) TFTs

8Ref: Y.H. Yeh et al, SID 2007

Process Lithography

Min. Length 10 μm

Dev. Type N-type only

Mobility 1 cm2/Vs

Substrates Glass or plastics

Degradation Bias Stress

Self-Assembly-Monolayer (SAM)

Organic TFTs

9

Process Shadow Mask

Min. Length 50 μm

Dev. Type Complementary

Mobility P:0.5 / N:0.01 cm2/Vs

Substrates Rigid wafer or plastics

Degradation Bias Stress, ChemicalCourtesy: K. Fukuda, Univ. of Tokyo

Transparent InGaZnO (IGZO) TFTs

10

6-inch glass substrate

12 x 7 cm2 Polyimide

6-inch glass substrate

12 x 7 cm2 Polyimide

6-inch glass substrate

12 x 7 cm2 Polyimide

6-inch glass substrate

12 x 7 cm2 Polyimide

Courtesy: Y.H. Yeh, ITRI-FETD

Process Magnetron Sputtering

Min. Length 10 μm

Dev. Type N-type only

Mobility 6 cm2/Vs

Substrates Rigid wafer or plastics

Degradation (Mild) Bias Stress

Transparency 86 % Visible spectrum

Technology Comparison

11

Device TypeCrystalline-Si

MOSFET

Amorphous Si

TFT

Metal-Oxide

TFT

Organic

TFT

Process

Temperature1000 °C 200 °C ~ 150 °C room temp.

Process

Technologylithography lithography

roll-to-roll

lithography

Ink-jet printing

/shadow mask

Feature size 32 nm 10 μm 8 μm 20 μm

Dielectrics

Thickness~ 1.5 nm 350 nm 300 nm

300 nm /

7 nm (SAM)

Substrate wafer glass/plastics wafer/plastics wafer/plastics

Device Type Complementary N-type N-type P-type/ CMOS

Supply

Voltage1V 20V 40V

40V (Ink-jet)

/ 2V (SAM)

Mobility (cm2/Vs)

~1000 1 10P:0.5 / N:0.01

(SAM)

Cost/Area High Medium Low Low

Lifetime Years Months ~ Year weeks

Challenges in TFT Circuits Design

Material instabilities Electrical degradation (ex. bias-stress)

Chemical degradation (ex. oxygen, water vapor)

Process variations More than 50% variations in key device parameters (ex. VTH)

Device mismatches are common

Unable to have high quality VIAs for multiple interconnects

Device limitations Mono-type only (either p- or n-type, depending on materials)

Lower mobility

Higher supply voltage

EDA supports Lack of trustworthy and compact device models

Lack of design verification supports (ex. LVS/DRC)12

Design/EDA Research Opportunities

for Reliable Flexible Electronics

Reliability simulation platform

Reliability analysis, modeling, and simulation

System solutions for reliability enhancement

Robust design for unreliable devices

Post-manufacturing self-test and self-tunable

design

Design-for-printability for roll-to-roll process

Substrate-aware physical design methodology

Self-aligned layer-to-layer patterning

Reliability Modeling and

Simulation*

Predicting the degraded circuits performance

14

Refs: 1. T.-C. Huang, et al, Design Auto. Conf. (DAC) 07

2. T.-C. Huang, et al, ACM J. Emerging Technologies in

Computing Systems (JETC), Aug. 2008

Electrical Instability in a-Si:H TFTs

ΔVTH results from prolonged bias-stress

Charge-trapping is the major mechanism

a-Si:H

(Semiconductor)

Substrate

SiNx (Insulator)

SiNx

Gate

Drain Source

n+ a-Si:H

15

10 100 1000

0.0

0.4

0.8

1.2

15V

Th

res

ho

ld V

olt

ag

e S

hif

t (V

)

Stress Time (s)

9V

22V

25V

W/L=50/8

T=25oC

Vstress

=32V

VGS > 0

10 100 1000

-3

-2

-1

0-5V

-10V

-25V

-30V

W/L=50/8

T=25o

Th

resh

old

Vo

ltag

e S

hif

t (V

)

Stress Time (s)

Vstress

=-35V

VGS < 0

Analytical Model for ΔVTHDuty Ratio VDS

0.1

1

10

0 10 20 30 40

VGS > 0

VGS < 0

Slope = 2.1

Slope = 1.2

|VGS-VTH0| (V)

|∆V

TH| (V

)

VGS

Stress Time = 1000 s

T = 25ºC

a-Si:H

(Semiconductor)

Substrate

SiNx (Insulator)

SiNx

Gate

Drain Source

n+ a-Si:H

)()1(

exp1

)(exp1

),,,(

0

0

DSR

THGS

DSR

THGS

RDSGSTH

VfDt

VV

VfDt

VV

tDVVV

α,β : process

parameters

16

a-Si:H TFT-based Scan Driver

17

Data Driver

Scan

Dri

ve

r

T1T2

VDD

Data Line

Scan

Line

CS

OLED

G(N)

CLK

CLKB

G(N-1)

G(N+1)

Scan

Driver

G(N+1)

CLK

CLKB

G(N)

G(N+2)

Scan

Driver

Ref: Y.H. Yeh et al, SID 2007

Reliability Simulation Methodology

18

Purpose: to find out

reliability problems in

early design stage

Validation with a-Si:H TFT Scan Drivers

Measured

19

Simulated

Successfully predicted a-Si TFT scan driver

degraded performance (85℃ / 9.2 hrs burn-in time)

Simulation time: ~ 20 mins

Robust Circuit Design

Pseudo-CMOS:Realizing CMOS performance and

reliability using only mono-type TFTs

Refs: 1. T.-C. Huang, et al, ISFED ‘07

2. T.-C. Huang, et al, IEEE J. Display Tech. (JDT) ‘09

3. T.-C. Huang, et al, DATE ‘10

4. T.-C. Huang, et al, IEEE Trans. Electron Devices, ‘11

Cell-Library for Flexible Electronics

21

Existing

VLSI Flow

Reusable

Existing

VLSI Flow

Not-reusable

22

Pseudo-CMOS Inverter

MUP

MDOWN

MDIODE

MININ

OUT

VDD

VSS

Level

Shifter

GND

PMOS-based

MUP

MDOWN

MDIODE

MIN

IN

OUT

VDDVDP

GND

NMOS-based

Ratioless-logic, 3 voltage levels, 1 direct path

“Pseudo-CMOS” Cell-Library

based on 2V p-type SAM OTFTs

23

Die

Photo

Assembly method

SAM OTFT Specifications

Dev. Type P-type only (no dev. model)

Shadow

Mask 4-Layer

Via Process Mechanical drill

Layout toolAdobe Illustrator

(no LVS/ DRC available)

Ref: H. Klauk et al, Nature 2007; R. Blache et al, ISSCC’09; E. Cantatore et al, JSSC’07

OTFT Inverters Comparison

24

MUP

VOUT

GND

VDD

VIN

MDN

Complementary

MUP

VOUT

GND

VDD

VIN

MDN

Phillips RFID

MUP

MDP

VOUT

GND

VDD

VIM

VSS

VIN

M1

M2

Pseudo-CMOS

0.5V 1.0V 1.5V 2.0 V

Max-

PlanckPolyIC Phillips

0

0.5

1

1.5

2 -20

-15

-10

-5

00 0.5 1 1.5 2

VO

UT (

V)

VIN (V)

GA

IN

VDD=2 V

D type

VSS=-2 VUCSB

PerformanceMax-

PlanckPolyIC Phillips

Pseudo-

CMOS

VDD/ SNM (Convt. 2V) 3V/ 0.53V 20V/ 0.67V 30V/ 0.14V 2V/ 0.91V

Static Power (μW) 0.03 1.4 (Est.) 3 (Est.) 0.1

Mobility (P/N-type)

(cm2/Vs)0.6 / 0.02 0.03 / 0.05 0.01 0.5

Lifetime (ambient) Days Months Months Months

MaterialPentacene

/F16CuPC

P3HT

/ N2000

Soluble

Pentacene

Evaporated

Pentacene

Post-Fabrication Tunability

0

0.5

1

1.5

2 -20

-15

-10

-5

00 0.5 1 1.5 2

VO

UT (

V)

VIN (V)

GA

INVDD=2 V

D type

VSS=-1 V

0

0.5

1

1.5

2 -20

-15

-10

-5

00 0.5 1 1.5 2

VO

UT (

V)

VIN (V)

GA

IN

VDD=2 V

D type

VSS=-1.5 V

0

0.5

1

1.5

2 -20

-15

-10

-5

00 0.5 1 1.5 2

VO

UT (

V)

VIN (V)

GA

IN

VDD=2 V

D type

VSS=-0.5 V

0

0.5

1

1.5

2 -20

-15

-10

-5

00 0.5 1 1.5 2

VO

UT (

V)

VIN (V)

GA

IN

VDD=2 V

D type

VSS=-2 V

0

0.5

1

1.5

2 -20

-15

-10

-5

00 0.5 1 1.5 2

VO

UT (

V)

VIN (V)

GA

INVDD=2 V

D type

VSS=0 V

0

0.5

1

1.5

2 -20

-15

-10

-5

00 0.5 1 1.5 2

VO

UT (

V)

VIN (V)

GA

IN

VDD=2 V

D type

VSS=1 V

25

A Portable Multi-Pitch

e-Drum Based on Printed

Pressure Sensors

Ref: C.-M. Lo, et al, DATE 2010 (joint project of UCSB

and ITRI)

27

Application: Rollable, Bendable

Percussion Instrument

Critical component of

percussion instrument

Human-instrument interface

Physical resonator

Amplifier

Elements of a rollable

percussion instrument

Linear pressure sensor

Linear transimpedance

amplifier

External DSP ICs

28

Demo Video Clip

Ref: C.-M. Lo, et al, DATE 2010.

Printed Flexible Sensors

Manufacturing process: Screen-printing piezo-

resistive material on plastic substrate

Can fabricate versatile shapes

Special requirements:Rollable and reliable

Functioning correctly under 5cm-radius bending

29Ref: C.M. Lo, et al, DATE 2010.

Printed Flexible Sensors - Principles

30

L / A

R

Ring-Shaped Sensors

Sensor array requires extra scan circuitry - not

ideal for low-cost sensors

Ring-shaped arrangement results in a low-cost

sensor structure - in terms of both fabrication

and reading operation

31

Sensor Performance

• High linearity

• measured conductivity vs applied pressure: <2.5% error

from an ideal straight line)

• High uniformity

• High robustness

32

3 different locations: After pressing 1M

and 10M times:

<2.5% from an

ideal straight line

Possibilities in Large-Area Applications

Displays

Rollable displays with integrated drivers (ex. scan driver)

E-poster for public information display

Full-color wall-size displays with multiple-touch capability

Sensors and detectors

Motion detectors for security and energy use controls

Artificial-skin for robots

Solar-powered wireless sensor networks

Invisible surveillance sensors

Flexible music instrument

Energy

DC-AC or DC-DC converters for flexible solar cells (ex.

amorphous Si, organic)

Electroluminescent (EL) lighting for efficient energy use33

Summary

Flexible circuit design ≠ Silicon ckt design

Significantly larger area

Significantly lower cost

Significantly slower

Larger process variations

Lower device reliability

Mono-type devices

Many new applications in large-area

electronics (including displays, sensors,

energy, etc.)34

35

Q & A

Thank you for your attention !

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