ENCAPSULATION OF ORGANIC LIGHT EMITTING DIODES

Transcription

1 ENCAPSULATION OF ORGANIC LIGHT EMITTING DIODES Bhadri Visweswaran 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 1

2 Why do we need an encapsulation? Organic Light Emitting Diode on a plastic film Flexible permeation barrier films OLED Plastic substrate Lifetime of few minutes to few days. Required lifetime > 10 years Required barrier film water vapor transmission rate: 10-6 g / (m 2 day) Plastic substrate LG Display, SID 2013 Samsung, CES 2013 UDC, SID

3 Hybrid permeation barrier film Plasma Enhanced Chemical Vapor deposition hexamethyl disiloxane + oxygen The deposited film is a hybrid of a silicone polymer and inorganic silicon dioxide Hybrid film: Low water permeability Has no defects P. Mandlik, et al., APL 92, (2008). L. Han, et al., J. Electrochem. Soc., , H106 PECVD 3

4 Accelerated test of barrier performance Bottom Emission OLED 2 mm 2 pixel lit up barrier film OLED Glass substrate Permeation along interface 6 µm film at 65 C 85% RH Permeation along a particle 4 µm film at 65 C 85% RH t = h 1967 h 2692 h t = 0 17 h 115h 162 h Many questions 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?... P. Mandlik, et al., APL 93, (2008). 4

5 Modes of permeation through a barrier layer barrier film OLED Particle 3 Glass substrate Permeation along a particle 4 µm film at 65 C 85% RH Pin-hole 1 OLED Barrier 2 4 t = 0 17 h 115 h 162 h Permeation along interface 6 µm film at 65 C 85% RH Water permeates in four modes: 1. Through pin-holes 2. Through the bulk of the barrier layer 3. Along particles 4. Along interfaces t = h 1967 h 2692 h 5

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. Pin-hole barrier film OLED Particle Barrier OLED Glass substrate 4 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 This talk 6

7 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. PART 1 : TECHNIQUES FOR MEASURING BULK PERMEATION OF WATER PART 2 : PARTICLE ENCAPSULATION USING MULTILAYER FILMS PART 3 : OLED ENCAPSULATION WITH PARTICLES 7

8 Motivation for measuring bulk permeation Permeation along a particle 4 µm film at 65 C 85% RH t = 0 17 h 115 h 162 h Tests on OLEDs are not quantitative! We need new techniques! 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 8

9 concentration n x, t Evaluation of diffusion profiles Water concentration profile In an ideal barrier Water side: n = n(x=0) n(0) n x, t = n(x=0)erfc x Dt x OLED side: n h = 0 h time depth x 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: g / (m 2 day) Required OLED water vapor transmission rate: 10-6 g / (m 2 day) 9

10 Secondary Ion Mass Spectrometry, SIMS 100 D 2 O SIMS profile after 12 hours 100 D 2 O n x, t = n(x=0)erfc x Dt 1. A 660 nm thick barrier layer on a silicon wafer was boiled in heavy water, D 2 O for 12 hours. 2. Deuterium concentration was determined by sputter profiling using secondary ion mass spectroscopy atoms cm 3 Deuterium profile depth x (nm) The deuterium follows erfc function! Diffusion coefficient: D = cm 2 s Solubility of water: n 0 = molecules cm 3 = 4.8 mg cm 3 10

11 SIMS results SIMS Diffusion coefficient Area Barrier thickness D = cm 2 /s 0.5 mm x 0.5 mm sputter target 660 nm 100, 100% RH: 3 μm film WVTR = g/ m 2. day Secondary Ion Mass Spectroscopy Solubility Diffusion coefficient D Long lead time Expensive Heavy water testing Electrical Capacitance Diffusion coefficient Film stress Diffusion coefficient Simple, quick and immune to particles and defects 11

12 concentration n x, t N t 2 Extracting D from total dissolved water Water concentration profile x n(0) n x, t = n(0)erfc Dt Total number of dissolved molecules in the barrier time N t 2 = 2 4n x=0 D t π depth x time t Film capacitance C Film stress σ is proportional to N(t) Therefore C(t) and σ(t) can be used to determine D 12

13 Dielectric constant, ε x, t D from electrical capacitance ε(0) time h ε barrier depth x Assumption: ε barrier with H2 O = ε barrier + K ε n(t) 1 C(t) 1 C 0 = C C 0 C h π Dt C t = capacitance at time t C 0 = initial capacitance C( ) = saturated final capacitance Diffusion coefficient: D = cm 2 s Compare D from SIMS: cm 2 s 13

14 Capacitance results SIMS Electrical Capacitance Diffusion coefficient Area Barrier thickness D = cm 2 /s D = cm 2 /s 0.5 mm x 0.5 mm sputter target 1 mm x 1 mm capacitor size 660 nm 200 nm Electrical Capacitance Diffusion coefficient Is there a simpler way? Film stress Diffusion coefficient Even simpler, quick and immune to particles and defects 14

15 Stress Measurement In-diffusing water causes film expansion of the barrier layer Barrier layer adheres to substrate Compressive stress Water uptake Film under stress Average film stress: R E W H h Stress: σ t σ = E W 6R H 2 h - Bending radius - Wafer elastic constant - Substrate thickness - Barrier thickness N(t) 18 = 2 10 h σ t - stress at time t σ - saturated final stress MPa Diffusion coefficient: D = cm 2 Compare D from SIMS : cm 2 /s Capacitance : cm 2 /s Advantages: 1. Extremely simple fabrication: 1 step! 2. Particles and defects have no impact! 15 s

16 Salient points of new techniques SIMS Electrical Capacitance Film stress Diffusion coefficient Area Barrier thickness D = cm 2 /s D = cm 2 /s D = cm 2 /s 0.5 mm x 0.5 mm sputter target 1 mm x 1 mm capacitor size 4 inch silicon wafer 660 nm 200 nm 1500 nm Uniform D over different area and thickness Measured at 100 C boiling water (100 C 100% RH) What about performance at room temperature? 16

17 Solubility and Diffusion coefficient activation energies Solubility Obtained from film stress measurements Diffusion coefficient n T = n 0 e 0.20 kt D T = D 0 e 0.71 kt E a = 0.20 ev E a = 0.71 ev + Tomozawa, M., Am. Ceram. Soc. Bull., 1337,

18 Extrapolating barrier performance to room temperature Number of monolayers of permeated water At 100 C and 100% Relative Humidity Solubility molecules cm 3 atm At 38 C and 90% Relative Humidity Diffusion coefficient cm 2 s Solubility molecules cm 3 atm Solubility activation energy Diffusion coefficient activation energy 0.20 ev 0.71 ev Diffusion coefficient cm 2 s 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 Water vapor transmission rate g m 2 day 1 monolayer of water Permeation time for 1 monolayer 13.4 years time t (years) 18

19 time τ ML (years) Acceleration factor Barrier design and testing At 38 C and 90% Relative Humidity Solubility molecules cm 3 atm Diffusion coefficient cm 2 s 1 monolayer permeation time at 38 C 90% RH Acceleration factor from 38 C 90% RH to 100% RH at higher temperatures τ ML = 2.41h µm, τ ML = 13.4 years Barrier thickness h (μm) Temperature ( ) Barrier film lifetime is not linear with thickness! 19

20 Diffusion coefficient vs RF deposition power The stress measurements can be repeated for different deposition conditions Diffusion coefficient vs RF power at 100 C Radio frequency deposition power Pressure HMDSO Oxygen 30 to 150 W 110 mtorr 1.1 sccm 33 sccm 20

21 Summary Introduced simple techniques to measure diffusion coefficient of water Electrical Capacitance 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 21

22 PART 1 : MEASURING BULK PERMEATION IN BARRIER FILMS PART 2 : PARTICLE ENCAPSULATION USING MULTILAYER FILMS PART 3 : OLED ENCAPSULATION WITH PARTICLES The aim is to encapsulate 5 µm size particles with a sub - 5 µm thick barrier film. The OLED must have a lifetime of greater than 10 years at 25 C and 50% relative humidity. 22

23 Need for studying particle encapsulation Permeation along a particle 4 µm film at 65 C 85% RH t = 0 17 h 115h 162h 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. 23

24 T-Shaped particle 1 cm x 1 cm substrate containing T-shaped structures SEM cross section of a T 500 nm polysilicon hat 1 µm silicon dioxide stalk Cross-section Silicon substrate 24

25 2.3 µm Growth features Substrate growth front chimney Particle growth front Hat Stalk Hat Particle growth front Substrate growth front Stalk Substrate Substrate 1.6 µm barrier film 3.2 µm barrier film Radio frequency deposition power Pressure HMDSO Oxygen 70 W 110 mtorr 1.1 sccm 33 sccm 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 25

26 Low power, high pressure layer stops chimney growth Sealed Chimney Particle growth front Sealed Chimney Substrate growth front Substrate 1.3 µm Hat Stalk W W Substrate 1.7 µm single layer barrier film 2.5 µm bilayer film Radio frequency deposition power Pressure HMDSO Oxygen 70 W 30 W 110 mtorr 500 mtorr 1.1 sccm 33 sccm Radio frequency deposition power 1.2 µm bottom layer 1.3 µm top layer 70 W 30 W Pressure 110 mtorr 500 mtorr 26

27 RF deposition power High water permeability Tensile stress Good particle encapsulation Low permeability Compressive stress Poor particle encapsulation 30 W 50 W 70 W 90 W 110 W 130 W 150 W RF Deposition power 27

28 3 layer film for particle encapsulation High water permeability Tensile stress Good particle encapsulation Low permeability Compressive stress Poor particle encapsulation 30 W 50 W 70 W 90 W 110 W 130 W 150 W RF Deposition power We need: Low water permeability Zero stress Good particle encapsulation Particle Low D: High power Conformal: Low power Stress compensation: High power OLED Substrate with rough surface 28

29 3 layer film for particle encapsulation Particle Low D: High power Conformal: Low power Stress compensation: High power OLED Substrate with rough surface 2.6 µm three layer film Chimney stops Hat Stalk W W W Substrate 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 29

30 3 layer film for particle encapsulation Bottom layer Middle layer Top layer Chimney stops 3.7 µm 700 nm 1.2 µm 70 W 30 W 70 W 3.4 µm glass fiber 700 nm 70W µm 30W 110 mtorr 500 mtorr 110 mtorr 70W Substrate 5.6 µm three layer film Glass fibers on silicon wafer were encapsulated. The fibers have thicknesses of 2 to 8 µm. 3.4 µm glass fiber Silicon substrate Break stops at 30W middle layer 70W top layer 30W middle layer 70W bottom layer Break in encapsulation 30

31 PART 1 : MEASURING BULK PERMEATION IN BARRIER FILMS PART 2 : PARTICLE ENCAPSULATION USING MULTILAYER FILMS PART 3 : OLED ENCAPSULATION WITH PARTICLES The aim is to encapsulate 5 µm size particles with a sub - 5 µm thick barrier film. The OLED must have a lifetime of greater than 10 years at 25 C and 50% relative humidity. 31

32 Silica glass beads on OLED 1. OLED is fabricated. 5 µm diameter silica glass beads are used as control particles µm silica beads are spread. OLED pixel photograph Individual 5 µm glass beads 32

33 Silica glass beads on OLED 3. A 3.6 µm 3 layer barrier is deposited Low water permeability Zero stress Good particle encapsulation Layer Deposition condition Thickness Lower 80W 200mT 1.7 µm Middle 40W 300mT 1.1 µm Top 80W 200mT 0.8 µm Top views of the OLED sample after encapsulation to 30 silica glass beads are observed per pixel 33

34 Time in hours 85 C and 85% relative humidity testing No glass bead induced degradation is observed at 500 hours at 85 C and 85% relative humidity. 306 From diffusion measurements, the lifetime is 392 1) 19 years at 25 C and 50% relative humidity and, 508 2) 6 years at 30 C and 100% relative humidity. 3.6 µm thick barrier film protects an OLED containing 5 µm size particles. The lifetime is 19 years at room temperature. 34

35 Conclusion Introduced quantitative particle insensitive techniques to measure diffusion coefficient of water Electrical Capacitance 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. 35

36 Future work Improving OLED reliability: Preventing interface diffusion essential preventing shrinkage. Reducing shrinkage is essential for improving the reliability of lifetime prediction. 508 hours at 85 C and 85% RH 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. Top barrier film OLED Bottom permeation barrier film Glass substrate Plastic substrate 36

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 37