P.224 / Shiva Prakash

P-224: Damage-Free Cathode Coating Process for OLEDs
Shiva Prakash
DuPont Displays, 600 Ward Drive, Santa Barbara, CA 93117, USAAbstract
OLED displays require the growth of inorganic films
over organic films. The inorganic film sometimes manifests ascathode metal (bottom-emission displays), or as the opticalenhancement layer (OEL, in top emission displays). Sputteringis the first process of choice in both cases due to simplicity, lowcost and scalability. However, plasma damage of the organicunderlayers prevents sputtering from being routinely used. A
closed-drift ion beam sputtering process was found effective ingrowing inorganic films with negligible damage to the organicunderlayers.

1.

ion-beam system (figure 1b) are secondary electrons from thetarget. These can also be reduced considerably using a magnetic
trap at the wafer (not shown).

Introduction

In top-emitting OLED displays, the cathode metal ismade as thin as possible to allow maximum light transmission
while still serving as an electron injector. Overlaying the thincathode metal is an optical enhancement layer (OEL) whosefunction is to provide higher light extraction efficiency. Indium
Tin Oxide (ITO) or other oxides or nitrides are ideal candidates
for the OEL material, especially if some degree of thin film
encapsulation is also desired.

Figure 1a: Magnetron Sputtering

For an OEL material which cannot be evaporated (suchas ITO), a plasma-based deposition process is needed.
Sputtering is a robust manufacturing process, and is a process ofchoice for most industries. Conventional sputter deposition ofITO has been shown to degrade the efficiency and lifetime of
the OLED device due to the flux of charge particles and highenergy atoms impinging on the device [1-6]. Thick protection
layers are used to alleviate the damage but that puts a severe
constraint on the design of the OLED stack. Other groups usevery low deposition powers (and rates) that increase TAC time
considerably.
In bottom-emitting OLED displays, the cathode metal
itself is preferably deposited using sputtering. Here the damage
to the organic layers will be more severe, as there is noprotection for the electron transport layer (ETL), and minimal
protection of the emissive layer (EML) under the ETL.
Our objective was to develop a process for depositing
the inorganic layers, whether metal, nitride or oxide, on top oforganic layers without degrading device performance.

2.

Ion Beam vs. Magnetron Sputtering

In conventional Magnetron sputtering (MS), the wafer is
exposed to the plasma, allowing electrons and ions to bombard
the growing film. In addition, two other high energy species
bombard the growing film: reflected gas neutrals and thesputtered flux itself. For these reasons, organic films typicallyused in OLEDs are not compatible with sputter depositionprocesses, as they suffer damage from charge particles as well as
the high energy flux. In Ion Beam Sputtering (IBS), the wafer is
not exposed to the plasma, which is contained in the ion source.
Furthermore, the geometry of deposition can be used to avoid
high energy particles of any kind. These differences areillustrated in Figure 1. The only charges reaching the wafer in anFigure 1b: Ion Beam Sputtering
3.

The Iontron Ion Beam Gun

In the ion beam gun used in this study, the triangular
geometry of a conventional IBS system is combined into a unitthat simulates the parallel plate geometry of an MS system, toallow scaling up the gun for flat panel displays. The Iontron isshown in cross-section in Figure 2. The target is mounted in thecenter, and receives ion flux from all around it at roughly 45
degrees to its surface. The ion beams are generated from aclosed-drift hall source and extracted from a slit. The closeddrift ion source was supplied by Micron Surface Technology,
Inc. under the trade name IontronTM.
There are some important features that make this gun
ideally suited for OLEDs work. Firstly, the configuration isparallel-plate, which is easy to scale up. Second, the high energy
deposition flux and reflected neutrals are diverted away from theP.224 / Shiva Prakashwafer. Third, secondary electrons from the target are prevented
from reaching the wafer by means of a magnetic trap placednear the exit point of the gun. See Appendix for details.

0.5 Cd/A from 3 Cd/A, and the voltage became prohibitively
high. Devices did not reach 100 Cd/m2 even at 7V.

Substrate
Low energy particles
ejected from target

Secondaryelectrons arediverted by trapCharged Particle Trap
Cathode with self-regulated
charge neutralizer
AnodeGas Feed

TargetBase Flange

Accelerated
Ion BeamISOConnection

Figure 2: Schematic of the Iontron Ion Beam gun showing salient features
The resulting flux of depositing material is low energy
and charge-free, ideal for OLED organic layers that are sensitive
to any charge particle impingement.

4. Results and Discussion
For evaluating the effectiveness of the Iontron, blue
OLED devices with layers up to the EIL were built. The device
stacks were thus composed of Anode/ HIL/ HTL/ EML/ETL/EIL. The cathode metal was deposited to a thickness of1kÅ, making it opaque. Three types of samples were created:
one using conventional evaporation for the cathode metal, one
using conventional magnetron sputtering for the cathode metal,
and the third using Iontron IBS for the cathode metal.

The data thus indicate that the IBS process with anelectron trap is a far superior process to conventional magnetron
sputtering when applied to sensitive substrates. As far as theOEL in a top-emitting device is concerned, no damage wasobserved to the device with the presence of a thin metal cathodecomposed of 1nm Al/20nm Ag as protection (data not shown).
However, top-emitting devices could potentially suffer damage
if there is less metal protection or if rates higher than 1Å/sec aredesired for the sputtered layer. In the case of the more stringent
cathode metal deposition, less than 10% loss of each device
property was seen with no protection for the underlying organic
layers when using the Iontron gun.

5. Acknowledgments

In Figure 3, various graphs are shown that capture thefull L-I-V characteristics of the devices after the deposition of1kÅ cathode metal film at 1Å/sec over the EIL of the OLEDdevices. The organic emissive layer was covered with a 10nm
Alq3/1nm LiF bilayer.

The author would like to acknowledge the help of Michael
Gutkin of Micron Surface Technology for providing the Iontron
Ion Beam gun and schematics.

From the graphs it can be seen that the devices with ion
beam sputtered metal were very similar to the controls. Only aslight loss of efficiency and a concomitant rise in voltage wereseen.
The magnetron sputtered devices however were
essentially destroyed by the process: the efficiency dropped toIon beam sputtering allows for a wider choice of materials
for the growth of inorganic materials on top of organic layers,
with negligible degradation. Top-emitting as well as bottomemitting displays would improve, and the cost burden would beless for manufacturing as the Iontron is a scalable gun. The

6. Conclusions

P.224 / Shiva Prakashprocess can also be used in other applications where sensitive
organic layers need to be protected.

Log Luminance vs Voltage
10000
Log Luminance (Cd/m 2)

Efficiency vs Current Density

Efficiency (Cd/A)

43.532.52
1.5
11010.102468

Voltage (V)0200

400600

800

1000

1200

1400

Current Density (A/m2)
BL9_ION

BL6_EXP

BL5_EXP

BL11_ION

BL11-MAG2

BL10-MAG2

BL12-MAG2

Figure 3a: Efficiency vs. current density. EXP: evaporation
controls; MAG2: magnetron sputtered; ION: ion beam sputtered

Luminance vs Current Density

Luminance (Cd)

100

0.01

0.504500
4000
3500
3000
2500
2000
1500
1000
500
0

BL9_ION

BL6_EXP

BL5_EXP

BL11_ION

BL11-MAG2

BL10-MAG2

BL12-MAG2

Figure 3d: Luminance vs. voltage. EXP: evaporation controls;
MAG2: magnetron sputtered; ION: ion beam sputtereda 2 order of magnitude lowering of ion and secondary electron
current density to the wafer with a magnetic trap present. This isdue to the Lorentz force acting on the electrons by the magneticfield.
IONTRON DATA

150mm wafer
Average
EnergyCurrent

Current Density

4-7 mA
3-5 mA

20-40 uA/cm
215-30 uA/cm

260 eV
300 eV

10-30 uA
30-50 uA

50-170 nA/cm
2170-300 nA/cm

210 eV
300 eVNo magnetic trapElectronsIonsWith magnetic trap
0200

400

600

800

1000

1200

1400

Current Density (A/m 2)
BL9_ION

BL6_EXP

BL5_EXP

BL11_ION

BL11-MAG2

BL10-MAG2

BL12-MAG2

Figure 3b: Luminance vs. current density. EXP: evaporation
controls; MAG2: magnetron sputtered; ION: ion beam sputtered

Efficiency vs Log LuminanceElectrons
Ions

Table 1: Data for charge particle impingement on thewafer using an Iontron gun with and without a magnetic trap.

References[1] G.Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, M.E.
Thompson, “Transparent Organic Light-Emitting Devices”,Applied Physics Letters, 68 (19), 6 May 1996.
[2] H. Suzuki, M. Hikita, “Organic Light-Emitting Diodes with
Radio-Frequency
Sputter-Deposited
Electron-Injecting
Electrodes”, Applied Physics Letters, 68 (16), 15 April 1996.

4
3.5

Efficiency (Cd/A)

1000

3

[3] G. Parthasarathy, P.E. Burrows, V. Khalfin, V.G. Kozlov,
S.R. Forrest, “A Metal-Free Cathode for Organic Semiconductor
Devices”, Applied Physics Letters, 72 (17), 27 April 1998.

2.5
21.5
10.5
0110100

1000

10000

Log Luminance (Cd)
BL9_ION

BL6_EXP

BL5_EXPBL11_ION

BL11-MAG2

BL10-MAG2

BL12-MAG2

Figure 3c: Efficiency vs. luminance. EXP: evaporation controls;
MAG2: magnetron sputtered; ION: ion beam sputtered

7. Appendix
The effectiveness of a strong magnetic trap is seen in Table 1.
Electron and ion energies were also measured using a cylindricalenergy analyzer through a 15mm opening in the wafer. There is[4] L.S. Hung, L.S. Liao, C.S. Lee, S.T. Lee, “Sputter
Deposition of Cathodes in Organic Light Emitting Diodes”,
Journal of Applied Physics, 86 (8), 15 October, 1999.
[5] L.S. Liao, L.S. Hung, W.C. Chan, X.M. Ding, T.K. Sham, I.
Bello, C.S. Lee, S.T. Lee, “Ion-Beam Induced Surface Damages
on tris-(8-hydroxyquinoline) aluminum”, Applied Physics
Letters, 75 (11), 13 September 1999.
[6] S. Han, X. Feng, Z.H. Lu, D. Johnson, R. Wood,“Transparent Cathode for Top-Emission Organic Light-EmittingDiodes”, Applied Physics Letters, 82 (16), 21 April 2003.

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P.224 / Shiva Prakash