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Bhadri Visweswaran

ENCAPSULATION OF

ORGANIC LIGHT EMITTING DIODES

1

Sigurd Wagner, James Sturm,

Electrical Engineering, Princeton University, NJ

Siddharth Harikrishna Mohan, Prashant Mandlik,

Jeff Silvernail, William Quinn, Ray Ma,

Universal Display Corporation, Ewing, NJ

Plastic substrate

OLED

2

Why do we need an encapsulation?

Samsung, CES 2013 LG Display, SID 2013

UDC, SID 2012

Lifetime of few minutes to few days.

Required lifetime > 10 years

Required barrier film

water vapor transmission rate:

≤ 10-6 g / (m2 day)

Flexible permeation barrier films Organic Light Emitting Diode on a plastic film

Plastic substrate

Hybrid permeation barrier film

3

+

hexamethyl disiloxane

oxygen

Plasma Enhanced Chemical Vapor deposition

L. Han, et al., J. Electrochem. Soc., 2009. 156, H106

The deposited film is a hybrid of

a silicone polymer and inorganic silicon dioxide

PECVD P. Mandlik, et al., APL 92, 103309 (2008).

Low water permeability Has no defects

Hybrid film:

Accelerated test of barrier performance

4 P. Mandlik, et al., APL 93, 203306 (2008).

What does 2692 hours mean at room temperature? What is the maximum size of particle we can tolerate? How thin a barrier can we get?...

Many questions…

2 mm2 pixel lit up

Glass substrate

OLED barrier film

Bottom Emission OLED

t = 0 17 h 115h 162 h

Permeation along a particle

4 µm film at 65°C 85% RH

Permeation along interface


t = 0 863 h 1967 h 2692 h

Water permeates in four modes: 1. Through pin-holes 2. Through the bulk of the barrier layer 3. Along particles 4. Along interfaces

5

Modes of permeation through a barrier layer

Permeation along interface


t = 0 863 h 1967 h 2692 h

t = 0 17 h 115 h 162 h


4 µm film at 65°C 85% RH Glass substrate

OLED barrier film

1

3

4 OLED

Particle

Barrier Pin-hole

2

To achieve this target, we prevent water permeation through: 1. Through pin-holes 2. Through the bulk of the

barrier layer 3. Along particles 4. Along interfaces

6

Aim

• We need a sub-5 µm thickness barrier film that protects an OLED containing 5 µm size particles.

• The OLED must have a lifetime of greater than 10 years at 25 °C and 50% relative humidity.

This ta

lk

Glass substrate

OLED barrier film

1

3

4 OLED

Particle

Barrier Pin-hole

2

7

Aim

• We need a sub-5 µm thickness barrier film that protects an OLED containing 5 µm size particles.


PART 1 :

TECHNIQUES FOR MEASURING BULK PERMEATION OF WATER

PART 2 :

PARTICLE ENCAPSULATION USING MULTILAYER FILMS

PART 3 :

OLED ENCAPSULATION WITH PARTICLES

8

Motivation for measuring bulk permeation

How does quantitative evaluation of bulk permeation help? 1. Evaluate new permeation barrier materials 2. Design new single and multilayer barrier films 3. Extrapolate and predict room temperature condition performance

from accelerated tests

I quantitatively evaluate intrinsic water diffusion using 3 techniques: 1. Secondary Ion Mass Spectroscopy (SIMS) 2. Electrical capacitance 3. Film stress

Tests on OLEDs are not quantitative!

We need new techniques! t = 0 17 h 115 h 162 h



9

Evaluation of diffusion profiles

𝑥 ℎ

Water side: 𝑛 = 𝑛(𝑥=0)

OLED side: 𝑛 ℎ = 0

In an ideal barrier

𝑑𝑒𝑝𝑡ℎ 𝑥

𝑡𝑖𝑚𝑒

𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑛

𝑥,𝑡

𝑛 𝑥, 𝑡 = 𝑛(𝑥=0)𝑒𝑟𝑓𝑐𝑥

𝐷𝑡 𝑛(0)

Water concentration profile

Permeability 𝑃 = 𝐷 × 𝑛

Water Vapor Transmission Rate WVTR = 𝑃/ℎ

Fundamental properties:

• Solubility of water, 𝑛(𝑥=0) • Diffusion coefficient, 𝐷

1mm thick plastic films water vapor transmission rate: 1-100 g / (m2 day)

Required OLED water vapor transmission rate: ≤ 10-6 g / (m2 day)

𝑑𝑒𝑝𝑡ℎ 𝑥 (nm) 𝑎𝑡𝑜𝑚𝑠𝑐𝑚

3

𝐷𝑒𝑢𝑡𝑒𝑟𝑖𝑢𝑚 𝑝𝑟𝑜𝑓𝑖𝑙𝑒

100℃ 𝐷2𝑂

SIMS profile after 12 hours

1. A 660 nm thick barrier layer on a silicon wafer was boiled in heavy water, 𝐷2𝑂 for 12 hours.

2. Deuterium concentration was determined by sputter profiling using secondary ion mass spectroscopy

𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡: 𝐷 = 4.2 × 10−15 𝑐𝑚2 𝑠

𝑆𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟: 𝑛 0 = 1.6 × 1020 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑐𝑚3 = 4.8 𝑚𝑔 𝑐𝑚3

10

Secondary Ion Mass Spectrometry, SIMS

The deuterium follows erfc function!

100 ℃ 𝐷2𝑂

𝑛 𝑥, 𝑡 = 𝑛(𝑥=0)𝑒𝑟𝑓𝑐𝑥

𝐷𝑡

11

SIMS results

Simple, quick and immune to particles and defects

Electrical Capacitance • Diffusion coefficient

Film stress • Diffusion coefficient

• Long lead time

• Expensive Heavy

water testing

Secondary Ion Mass Spectroscopy • Solubility

• Diffusion coefficient D

Diffusion coefficient Area Barrier thickness

SIMS 𝐷 = 4.2 × 10−15 𝑐𝑚2/𝑠 0.5 mm x 0.5 mm

sputter target 660 nm

100 ℃, 100% 𝑅𝐻: 3 𝜇𝑚 𝑓𝑖𝑙𝑚 𝑊𝑉𝑇𝑅 = 5.8 × 10−5 𝑔/ 𝑚2. 𝑑𝑎𝑦

12

Extracting D from total dissolved water

𝑑𝑒𝑝𝑡ℎ 𝑥

𝑡𝑖𝑚𝑒

𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑛

𝑥,𝑡

𝑛 𝑥, 𝑡 = 𝑛(0)𝑒𝑟𝑓𝑐𝑥

𝐷𝑡 𝑛(0)

Water concentration profile

Film capacitance C Film stress σ

is proportional to 𝑁(𝑡)

Therefore C(t) and σ(t) can be used to determine D

1 2

3

𝑡𝑖𝑚𝑒 𝑡

𝑁𝑡

2

Total number of dissolved molecules in the barrier

𝑁 𝑡 2 =4𝑛 𝑥=0 2

𝜋𝐷 × 𝑡

1

2

3

𝑑𝑒𝑝𝑡ℎ 𝑥 𝐷𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡,𝜖𝑥,𝑡

𝑡𝑖𝑚𝑒

𝜖(0)

𝜖𝑏𝑎𝑟𝑟𝑖𝑒𝑟

13

D from electrical capacitance


Compare D from SIMS: 4.2 × 10−15 𝑐𝑚2 𝑠

1

𝐶(𝑡)−

1

𝐶 0=

𝐶 ∞ − 𝐶 0

𝐶 0 2

2

ℎ 𝜋𝐷𝑡

𝐶 𝑡 = capacitance at time t 𝐶 0 = initial capacitance 𝐶(∞) = saturated final capacitance

ℎ

𝜀𝑏𝑎𝑟𝑟𝑖𝑒𝑟 𝑤𝑖𝑡ℎ 𝐻2𝑂 = 𝜀𝑏𝑎𝑟𝑟𝑖𝑒𝑟 + 𝐾𝜀 𝑛(𝑡) Assumption:

14

Capacitance results


SIMS 𝐷 = 4.2 × 10−15 𝑐𝑚2/𝑠 0.5 mm x 0.5 mm


Electrical Capacitance

𝐷 = 5.6 × 10−15 𝑐𝑚2/𝑠 1 mm x 1 mm capacitor size

200 nm

Even simpler, quick and immune to particles and defects

Film stress • Diffusion coefficient

Electrical Capacitance • Diffusion coefficient

Is there a simpler way?

Water uptake Film under stress

In-diffusing water causes film expansion of the barrier layer

Barrier layer adheres to substrate Compressive stress

15

Stress Measurement


SIMS : 4.2 × 10−15 𝑐𝑚2/𝑠 Capacitance : 5.6 × 10−15 𝑐𝑚2/𝑠

𝐶𝑜𝑚𝑝𝑎𝑟𝑒 𝐷 𝑓𝑟𝑜𝑚

𝜎 𝑡 = 2 × 10−18𝑁(𝑡)

ℎ 𝑀𝑃𝑎 Stress:

𝜎 𝑡 - stress at time t 𝜎 ∞ - saturated final stress

Advantages: 1. Extremely simple fabrication: 1 step! 2. Particles and defects have no impact!

Average film stress:

𝜎 = 𝐸𝑊6𝑅

𝐻2

ℎ

𝑅 - Bending radius 𝐸𝑊 - Wafer elastic constant 𝐻 - Substrate thickness ℎ - Barrier thickness

16

Salient points of new techniques


SIMS 𝐷 = 4.2 × 10−15 𝑐𝑚2/𝑠 0.5 mm x 0.5 mm



𝐷 = 5.6 × 10−15 𝑐𝑚2/𝑠 1 mm x 1 mm capacitor size

200 nm

Film stress 𝐷 = 4.4 × 10−15 𝑐𝑚2/𝑠 4 inch

silicon wafer 1500 nm

Uniform D over different area and thickness

What about performance at room temperature?

Measured at 100 °C boiling water (100 °C 100% RH)

17

Solubility and Diffusion coefficient activation energies

Solubility

𝑛 𝑇 = 𝑛0𝑒0.20

𝑘𝑇

Obtained from film stress measurements

Diffusion coefficient

𝐷 𝑇 = 𝐷0𝑒−0.71

𝑘𝑇

+Tomozawa, M., Am. Ceram. Soc. Bull., 1337, 1985.

𝐸𝑎 = 0.71 𝑒𝑉 𝐸𝑎 = −0.20 𝑒𝑉

18

Extrapolating barrier performance to room temperature

At 100 °C and 100% Relative Humidity

Solubility 1.6 × 1020 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑐𝑚3𝑎𝑡𝑚

Diffusion coefficient 4.2 × 10−15 𝑐𝑚2 𝑠

Solubility activation energy −0.20 𝑒𝑉

Diffusion coefficient activation energy 0.71 𝑒𝑉


Solubility 3.2 × 1019 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑐𝑚3𝑎𝑡𝑚


Water vapor transmission rate

1.5 × 10−7 𝑔 𝑚2𝑑𝑎𝑦

𝑡𝑖𝑚𝑒 𝑡 (𝑦𝑒𝑎𝑟𝑠)

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑛𝑜𝑙𝑎𝑦𝑒𝑟𝑠

𝑜𝑓 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑑 𝑤𝑎𝑡𝑒𝑟

Total quantity of permeated water

Performance of a 3 µm barrier at 38°C and 90% Relative Humidity

3 µm, 38 °C and 90% RH

1 monolayer of water

Permeation time for 1 monolayer

13.4 𝑦𝑒𝑎𝑟𝑠

19

Barrier design and testing


Solubility 3.2 × 1019𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑐𝑚3𝑎𝑡𝑚


𝐵𝑎𝑟𝑟𝑖𝑒𝑟 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 ℎ (𝜇𝑚)

𝑡𝑖𝑚𝑒 𝜏𝑀𝐿 (𝑦𝑒𝑎𝑟𝑠)

1 monolayer permeation time at 38 °C 90% RH

𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (℃)

𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟

Acceleration factor from 38 °C 90% RH to 100% RH at higher temperatures

Barrier film lifetime is not linear with thickness!

𝜏𝑀𝐿 = 2.41ℎ1.57

3 µm, 𝜏𝑀𝐿 = 13.4 𝑦𝑒𝑎𝑟𝑠

20

Diffusion coefficient vs RF deposition power

Radio frequency deposition power

30 to 150 W

Pressure 110 mTorr

HMDSO 1.1 sccm

Oxygen 33 sccm

The stress measurements can be repeated for different deposition conditions

Diffusion coefficient vs RF power at 100 °C

21

Summary

Introduced simple techniques to measure diffusion coefficient of water


Film stress

Determined the concentration of water with SIMS, used to calibrate capacitance and film stress

The techniques are

Simple: fabrication & testing

Immune to particles and defects

With the techniques we can:

Rapidly evaluate barrier materials and films

Predict room temperature performance

PART 1 :

MEASURING BULK PERMEATION IN BARRIER FILMS

22

PART 2 :


PART 3 :


• The aim is to encapsulate 5 µm size particles with a sub - 5 µm thick barrier film.


23

Need for studying particle encapsulation

Problem: • Randomness in size and shape of

the particles have prevented a systematic study.

We use two standard particles: 1. Micro fabricated T-shaped particles. 2. Dispersed Glass micro-fibers as particles.

t = 0 17 h 115h 162h



24

T-Shaped particle

SEM cross section of a T 1 cm x 1 cm substrate containing T-shaped structures

500 nm polysilicon hat

1 µm silicon dioxide stalk

Silicon substrate

Cross-section

25

Growth features

1. Particle growth front and Substrate growth front do no merge until 2.3 µm height.

2. There is a chimney separating the two. 3. Chimney height > particle height


70 W

Pressure 110 mTorr

HMDSO 1.1 sccm

Oxygen 33 sccm

1.6 µm barrier film 3.2 µm barrier film

Hat

Stalk

Substrate

Particle growth front

Substrate growth front

Hat

Stalk

Substrate



2.3

µm

chimney

26

Low power, high pressure layer stops chimney growth


70 W 30 W

Pressure 110 mTorr 500 mTorr

HMDSO 1.1 sccm

Oxygen 33 sccm

Hat

Stalk



Substrate

Sealed Chimney

1.7 µm single layer barrier film

1.2 µm @ 70W

1.3 µm @ 30W

Substrate

1.2 µm bottom layer

1.3 µm top layer


70 W 30 W

Pressure 110 mTorr 500 mTorr

Sealed Chimney

2.5 µm bilayer film

1.3

µm

27

RF deposition power

RF Deposition power

30 W 150 W

Low permeability Compressive stress Poor particle encapsulation

High water permeability Tensile stress Good particle encapsulation

50 W 70 W 90 W 110 W 130 W

28

3 layer film for particle encapsulation

We need: Low water permeability Zero stress Good particle encapsulation

Low D: High power

Conformal: Low power

Stress compensation: High power

OLED

Substrate with rough surface

Particle

RF Deposition power

30 W 150 W

Low permeability Compressive stress Poor particle encapsulation

High water permeability Tensile stress Good particle encapsulation

50 W 70 W 90 W 110 W 130 W

29


Bottom layer

Middle layer

Top layer

Thickness 930 nm 1.2 µm 450 nm

Radio frequency deposition

power 70 W 30 W 70 W

Pressure 110 mTorr 500 mTorr 110 mTorr

930 nm @ 70W

450 nm @ 70W

Hat

Stalk

Substrate

Chimney stops

1.2 µm @ 30W

2.6 µm three layer film

Low D: High power

Conformal: Low power

Stress compensation: High power

OLED

Substrate with rough surface

Particle

30


Substrate

3.4 µm glass fiber

3.7µm @ 70W

700 nm 70W + 1.2 µm 30W

Chimney stops

Bottom layer

Middle layer Top layer

3.7 µm 700 nm 1.2 µm

70 W 30 W 70 W

110 mTorr 500 mTorr 110 mTorr

70W bottom layer

3.4 µm glass fiber

70W top layer 30W middle layer

Break in encapsulation

Break stops at 30W middle layer

Silicon substrate

5.6 µm three layer film

• Glass fibers on silicon wafer were encapsulated.

• The fibers have thicknesses of 2 to 8 µm.

PART 1 :

MEASURING BULK PERMEATION IN BARRIER FILMS

31

PART 2 :


PART 3 :


• The aim is to encapsulate 5 µm size particles with a sub - 5 µm thick barrier film.


32

Silica glass beads on OLED

2. 5 µm silica beads are spread.

Individual 5 µm glass beads

1. OLED is fabricated.

OLED pixel photograph

5 µm diameter silica glass beads are used as control particles.

4 3 2 1

33

3. A 3.6 µm 3 layer barrier is deposited

Layer Deposition condition Thickness

Lower 80W 200mT 1.7 µm

Middle 40W 300mT 1.1 µm

Top 80W 200mT 0.8 µm

Silica glass beads on OLED

Low water permeability Zero stress Good particle encapsulation

Top views of the OLED sample after encapsulation.

10 to 30 silica glass beads are observed per pixel

34

Tim

e in h

ou

rs

1 2 3 4

96

163

306

392

508

0

85 °C and 85% relative humidity testing

No glass bead induced degradation is observed at 500 hours at 85 °C and 85% relative humidity. From diffusion measurements, the lifetime is 1) 19 years at 25 °C and 50% relative humidity and, 2) 6 years at 30 °C and 100% relative humidity.

4 3 2 1

3.6 µm thick barrier film protects an OLED containing 5 µm size particles.

The lifetime is 19 years at room temperature.

35

Conclusion Introduced quantitative particle insensitive techniques to measure diffusion

coefficient of water


Film stress

Secondary Ion Mass Spectrometry.

Performed a systematic study of particle encapsulation with control particles

Micro fabricated T-shaped particle

Glass fibers

Designed a three layer barrier system to encapsulate particles of a given size.

A 3.6 µm barrier film protects an OLED contaminated with 5 µm glass beads to give a lifetime of >19 years at 25 °C and 50% relative humidity.

36

Future work

• Improving OLED reliability: – Preventing interface diffusion essential preventing shrinkage.

– Reducing shrinkage is essential for improving the reliability of lifetime prediction.

• Flexible Encapsulation – Characterize the critical strain in the barrier film.

– Design a barrier film on a plastic substrate that would be interposed between substrate and the OLED.

508 hours at 85 °C and 85% RH

Bottom permeation barrier film

Plastic substrate Glass substrate

OLED Top barrier film

37

Acknowledgements

Prof. Sigurd Wagner and group Sushobhan Avasthi, Warren Rieutort-Louis, Josh Sanz-Robinson,

Lin Han, Prashant Mandlik, Yifei Huang, Ting Liu.

Prof. James Sturm

Members of Universal Display Corporation: Siddharth Harikrishna Mohan, Jeff Silvernail, William Quinn, Ray Ma.

Princeton Program in Plasma Science and Technology

Clean room staff

Barbara Fruhling

powerpoint presentation · powerpoint presentation author: bhadri lalgudi visweswaran created date:...

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