Characteristics of Transparent SiN[sub x] Thin-Film Passivation Layer Grown by CECVD for...

Journal of The Electrochemical Society, 154 �12� J402-J407 �2007�J402

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Characteristics of Transparent SiNx Thin-Film PassivationLayer Grown by CECVD for Top-Emitting OLEDsJin-A Jeong,a Soon Wook Jeong,a No-Jin Park,a Jae-Wook Kang,b

Jong-Kuk Kim,b Do-Geun Kim,b and Han-Ki Kima,z

aSchool of Advanced Materials and Systems Engineering, Kumoh National Institute of Technology,Gumi, Gyeongbuk 730-701, South KoreabSurface Technology Research Center, Korea Institute of Machinery and Materials, Changwon-si,Gyeongnam 641-831, South Korea

The characteristics of a SiNx passivation layer grown by a specially designed catalyzer-enhanced chemical vapor deposition�CECVD� system for top-emitting organic light-emitting diodes �TOLEDs� were investigated. Using a tungsten catalyzer con-nected in series, a high-density SiNx passivation layer with high transmittance was deposited on TOLEDs at a substrate tempera-ture of 50°C. Even at low substrate temperature, 150 nm thick SiNx passivation layer prepared by CECVD exhibited a low watervapor transmission rate of 2–6 � 10−2 g/m2/day. In addition, it was found that the transmittance of SiNx film at 550 nm wassignificantly influenced by the hydrogen flow rate. Current density-voltage-luminescence results of TOLEDs passivated with a150 nm thick SiNx film indicated that the electrical and optical properties of TOLEDs were not affected by the high temperaturetungsten catalyzer during the SiNx deposition. The lifetime to half initial luminance of a TOLED passivated with a 150 nm thickSiNx layer was much longer than that of a nonpassivated reference sample. This shows that the CECVD is a promising plasma freethin-film passivation technique for high-performance TOLEDs and flexible displays.© 2007 The Electrochemical Society. �DOI: 10.1149/1.2789948� All rights reserved.

Manuscript submitted July 16, 2007; revised manuscript received August 23, 2007. Available electronically October 15, 2007.

0013-4651/2007/154�12�/J402/6/$20.00 © The Electrochemical Society

Top-emitting organic light-emitting diodes �TOLEDs� are of in-terest for their application in active matrix OLEDs due to their geo-metrical advantages, such as high aperture ratio, high pixel resolu-tion, and low power consumption.1-3 For those reasons, TOLEDsrecently came into the display market as main windows of cellularphones and MP3 players. However, the organic layers used in theTOLEDs degrade rapidly when they are exposed to ambient air dueto severe chemical reactions with oxygen and the moisture in air.4,5

Thus, the long-term stability of TOLEDs is still limited. In order toprevent the ingression of moisture and oxygen into organic layers,transparent glass lids with a transparent desiccant have been em-ployed in the encapsulation process of TOLEDs. Although transpar-ent glass lid-type encapsulation is the most commonly used inTOLEDs, it causes an increase of TOLED thickness and difficultiesin applying flexible displays and large area TOLEDs. To overcomethese problems, it is imperative to develop high quality thin-filmpassivation with long-term stability and a high degree of transpar-ency. Furthermore, a plasma-free process for thin-film passivationwould be desirable because the effect of the exposure of TOLEDs toplasma is critically harmful to organic layers in the TOLEDs. How-ever, most reported thin-film passivation techniques are plasma-based processes, such as plasma enhanced chemical vapor deposi-tion �PECVD�, inductively coupled chemical vapor depositionplasma enhanced atomic layer deposition, and radio frequency �rf�sputtering.6-11 Although plasma-based CVD or sputtering processare well recognized as a thin-film passivation process for OLEDsand flexible OLEDs, the development of plasma-free thin-film pas-sivation process for TOLEDs is still lacking.

In this work, we report the preparation and characteristics of atransparent SiNx thin film passivation layer grown by a speciallydesigned catalyzer enhanced chemical vapor deposition �CECVD�system for TOLEDs. In addition, the effect of hydrogen flow rate onstructural and optical properties of the SiNx passivation layer is in-vestigated in detail. An increased lifetime of TOLED with CECVDgrown SiNx passivation layer indicates that CECVD is a promisingplasma-free thin film passivation technique for high performanceTOLEDs.

Experimental

For the deposition of high-density SiNx passivation layer at a lowsubstrate temperature, a specially designed CECVD system with a

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address. Redistribution subject to ECS term134.75.192.119aded on 2015-06-01 to IP

tungsten catalyzer and bipolar electrostatic chuck was employed.12

The CECVD system has been successfully applied to deposit a high-quality thin-film passivation layer on OLEDs and flexible OLEDs ata low substrate temperature with a relatively high growth rate. Toapply the CECVD system in the OLED and flexible OLEDs, the gasinjection system, tungsten catalyzer array, and substrate cooling sys-tem were designed to be adapted for square shaped glass substratesand flexible substrates. A schematic of the CECVD system is shownin Fig. 1. The ladder-shaped tungsten catalyzer with a diameter of0.7 mm and size of 45 � 50 cm is located below a showerhead.This tungsten catalyzer plays an important role in decomposingSiH4, by a catalytic reaction.13 To obtain optimal conditions and tomaintain a constant substrate temperature, we employed a verticallymovable glass chucking setup with a bipolar-type electrostaticchucking system and cooling system as shown in Fig. 1. By adjust-ing the distance between the substrate and tungsten catalyzer, wecan control the growth rate, film uniformity, growth steps, and sub-strate temperature. The bare glass substrate size was 200 � 200

Figure 1. �Color online� Schematic diagram of a catalyzer-enhanced chemi-cal vapor deposition �CECVD� system with a tungsten catalyzer and bipolarelectrostatic chuck system.

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cm. The catalyzer temperature was monitored based on resistance ofthe tungsten catalyzer. The CECVD system not only has severaladvantages, such as high deposition rate, low hydrogen concentra-tion of the films, simple apparatus, and low cost, but also obviatesthe plasma-damage effect on organic films.12,14 Using the CECVDsystem, a SiNx passivation layer was deposited on the bare glass andon TOLED samples at a substrate temperature of 50°C. A mixture ofSiH4, NH3, and H2 was used for the deposition of the SiNx passiva-tion layer. To optimize the SiNx growth condition, H2 flow rate �0,500, 1000, 1500 sccm�, NH3 flow rate �480, 800, 1120, 1440 sccm�,working pressure �80, 150, 220 mTorr� and distance of the catalyzersubstrate �100, 150, 200 cm� were varied. The temperature of thetungsten catalyzer was kept at 1800°C constantly during the SiNxdeposition. In addition, the substrate temperature was also main-tained constantly at 50°C by a movable electrostatic chuck systemand cooling system. Ellipsometry was used to measure the refractiveindex �RI� and thickness of SiNx films. Optical transmittancethrough SiNx films was measured by a UV/visible spectrometer inthe wavelength range of 220–800 nm. To investigate structural prop-erties of the SiNx film, an X-ray diffraction �XRD� examination wascarried out. The surface morphology of SiNx films was analyzed byfield emission scanning electron microscopy �SEM�. The water va-por transmission rate �WVTR� of the SiNx films grown on polycar-bonate substrates �10 � 10 cm� were determined at 38 ± 2°C,100% RH by a MOCON tester for 72 h. To investigate the thermalradiation effect of the tungsten catalyzer on electrical and opticalproperties of TOLEDs, a 150 nm thick SiNx passivation layer wasdeposited on the test TOLED sample with a structure of transparentcathode/organic layers/anode layer/reflective layer. A detailed cross-sectional structure of a test cell and stacking sequence of organicmaterials are shown in Fig. 2. All organic layers were grown on aglass substrate coated with a 150 mm thick indium tin oxide anodeand an Al reflective layer in the following order: hole injectionlayer �60 nm thick 4,4�,4�-tris�N-�2-naphthyl�-N-phenyl-amino�-triphenylamine: 2TNATA�/hole transporting layer �20 nm thicknapthylphenlylbipheny diamine: NPB�/emission layer �60 nm thicktris-�8-hydroxyquinoline� aluminum: Alq3�/electron injection layer�1 nm thick LiF�. Finally, a 10 nm thick transparent Mg–Ag �10:1�

Figure 2. �Color online� Cross-sectional structure of top emitting OLEDs�TOLEDs� and schematic organic layers in the TOLEDs.


cathode layer was patterned using a shadow mask. After preparationof the TOLED samples, they were passivated with a SiNx layergrown using a CECVD system. The current density-voltage-luminance �J-V-L� characteristics of TOLEDs were measured usinga Photo Research PR-650 spectrophotometer driven by a program-mable dc source. Driven at 2 mA/cm2, the device lifetime ofTOLEDs passivated with the SiNx film at an initial luminance of500 cd/m2 was measured.

Results and Discussion

Figure 3 shows the deposition rate and RI value of the SiNx filmson glass as a function of the catalyzer-substrate distance at a con-stant SiH4/NH3/H2 ratio �32/480/1500 sccm�, working pressure of150 mTorr, and catalyzer temperature of 1800°C. It is shown thatthe deposition rate monotonically decreased with an increase of thecatalyzer-substrate distance. At a distance of 100 mm, the maximumdeposition rate of 45 nm/min was obtained. However, the RI valueof the SiNx films was kept constant regardless of the catalyzer-substrate distance. This indicates that the stoichiometry of CECVDgrown SiNx film is not critically influenced by the catalyzer-substrate distance.

Figure 4 shows the deposition rate and RI value of the SiNx filmson glass as a function of the NH3 and H2 flow rate at constantworking pressure and a catalyzer-substrate distance of 100 mm. Theincrease of NH3 flow rate at a constant SiH4/H2 �32/1500 sccm�flow ratio resulted in a slight increase of the deposition rate and adecrease of the RI as shown in Fig. 4a. Osono et al., investigatinghigh-rate deposition of SiNx film by Cat-CVD, also reported a RIdecrease by increasing NH3 flow rate, since N-rich SiNx films arelikely to show a low RI value.15 Figure 4b shows the deposition rateand RI value as a function of H2 flow rate at a constant SiH4/NH3flow rate, working pressure, and catalyzer-substrate distance. It isshown that the deposition rate of the SiNx film decreased with in-creasing H2flow rate. However, the RI value increased slightly withan increase in H2 flow rate. Mahan et al. recently suggested that Hatoms produced by the catalytic decomposition of H2 play an im-portant role in dissociating NH3.16 However, the decrease in depo-sition rate in CECVD with increasing H2 flow rate demonstrates thatthe effect of H2 addition is less efficient when the NH3 flow rate ishigh as previously reported by Ansari et al.17

The deposition rate and RI value as a function of working pres-sure at a constant SiH4/NH3/H2 flow ratio and catalyzer-substratedistance is shown in Fig. 5. Keeping SiH4/NH3/H2 flow ratio con-stant at 32/480/1500 sccm, a decrease in deposition rate from 52.3to 40.5 nm/min was observed with increasing working pressure.This could be attributed to the etching of the SiNx film by hydrogenatoms because an increase in working pressure leads to an increase

Figure 3. �Color online� Deposition rate and refractive index of the SiNxfilms grown by CECVD as a function of catalyzer-substrate distance.



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in hydrogen atoms in the ambient. Osono et al. also reported that thedeposition rate of SiNx films starts to decrease over 20 Pa due to anincrease in the number of hydrogen atoms in the gas phase.15 How-ever, the RI value of the SiNx films was kept constant regardless ofworking pressure.

Figure 6 shows the optical transmittance spectra of a 150 nmthick SiHx passivation film grown on glass substrate as a function ofH2 flow rate with an inset of the optimized sample picture. Trans-

Figure 4. �Color online� Deposition rate and refractive index of the SiNxfilms grown by CECVD as a function of �a� NH3 flow rate and �b� H2 flowrate, respectively.

Figure 5. �Color online� Deposition rate and refractive index of the SiNxfilms grown by CECVD as a function of working pressure at a constantSiH /NH /H ratio.
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mittance of the thin-film passivation layer in the TOLEDs is veryimportant because light is emitted through the passivation layer inTOLEDs.18 It is worth noting that the transmittance of the CECVDgrown SiNx film at 550 nm increases with an increasing H2 flowratio from 78–88%. In addition, transmittance in the UV regionincreases with increasing hydrogen flow rate. Even though the SiNxfilm is prepared at a substrate temperature of 50°C, it shows hightransmittance applicable to TOLEDs due to the high density of SiNxfilm.

To investigate the effect of H2 flow ratio on the structure of theSiNx film grown by CECVD, XRD examination was performed.Figure 7 exhibits XRD plots of a CECVD grown SiNx film as afunction of H2 flow rate at a constant SiH4/NH3 flow ratio. Regard-less of H2 flow rate, all XRD plots of the SiNx film exhibit a broadpeak, which is characteristic of an amorphous structure. The amor-phous structure of the SiNx film grown by CECVD indicates that theeffect of the tungsten catalyzer ��1800°C� on the glass substrateduring SiHx growth was low due to effective substrate cooling bythe bipolar chuck system. In general, the thermal CVD using a tung-sten or tantalum heating source has a fairly high substrate tempera-ture due to radiation from heating sources. However, the CECVD

Figure 6. �Color online� Optical transmission spectra for the SiNx filmgrown by CECVD on glass substrate as a function of H2 flow rate with insetof picture of transparent SiNx/glass.

Figure 7. �Color online� XRD examination result of the SiNx film grown byCECVD on glass substrate as a function of the H flow rate.
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system using a tungsten catalyzer with temperature of 1800°C canmaintain low substrate temperature with the contribution of the bi-polar electrostatic chuck as shown in Fig. 1.

Figure 8 shows SEM surface images of the SiNx film grown onglass substrate as a function of H2 flow rate at constant SiH4/NH3ratio. An increase of H2 flow rate from 0 to 1500 sccm resulted in arough surface of the SiNx film. The rough surface of the SiNx filmgrown at a high H2 flow rate could be attributed to etching of theSiNx films by hydrogen atom. Matsuda reported that atomic hydro-gen could break the Si–Si bonds in the amorphous network structureand lead to removal of weakly bonded Si atoms when the hydrogenatom density in the environment is high.19 Therefore, the high H2flow rate during the SiNx deposition process in the CECVD leads toetching of the SiNx films. This may affect the surface roughness ofthe SiNx film which is prepared at a low substrate temperature.

Figure 9 shows the WVTR of a 150 nm thick SiNx film grown onPC substrate as a function of H2 flow rate at a constant SiH4/NH3ratio. To compare the WVTR value, the PECVD grown SiNx filmwas prepared on the PC substrate at optimized conditions�SiH4/NH3/N2 = 45/170/500 sccm, working pressure�1 Torr, rfpower�350 W�. When compared to the WVTR �0.2–0.8 g/day m2�of a 150 nm thick SiNx film grown by conventional PECVD at

Figure 8. Surface SEM images of the SiNx film grown by CECVD on glasssubstrate as a function of the H2 flow rate.

Figure 9. WVTR of a 150 nm thick SiNx film grown on the PC substrate byCECVD as a function of H flow rate.
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optimized conditions, CECVD-grown SiNx films exhibit a muchlower WVTR �0.02–0.06 g/day-m2� value. Furthermore, WVTR ofthe CECVD-grown SiNx film is much lower than previously re-ported WVTR �5–16 g/day m2� of the PECVD-grown SiNx films.20

Considering only the use of a single SiNx layer, the WVTR of theCECVD grown SiNx film has a very low value. In addition, it isworth noting that the increase in H2 flow rate during the CECVDprocess decreases the WVTR value of the SiNx film on the PCsubstrate. The low WVTR of the SiNx film grown at a high H2 flowrate could be explained by the chemical annealing effect of hydro-gen atoms.19 Most of the hydrogen atoms incorporated in the SiNxlayer could react with hydrogen atoms in the environment and formhydrogen molecules. The surface temperature of the SiNx layer canbe locally increased by hydrogen-recombination reactions on theSiNx surface. Therefore, the increased surface temperature by hydro-gen recombination could lead to greater film density, which is ben-eficial for barrier coating in TOLEDs.

To investigate the hydrogen and tungsten concentration in a SiNxfilm grown by CECVD, a secondary ion mass spectroscopy �SIMS�depth profile analysis was employed. Figure 10 exhibits the SIMSdepth profile of the SiNx film grown on a Si substrate by CECVDat optimized condition �SiH4/NH3/H2 = 32/480/1500 sccm�. It isshown that the hydrogen concentration in a SiNx film is very lowdue to the extraction of hydrogen atoms from the SiNx film throughrecombination with hydrogen in environment as reported byMatsuda.19 In addition, the concentration of tungsten atoms is lowindicating no contamination in the SiNx film by the evaporation oftungsten catalyzer during SiNx growth. This result demonstrates thata high-quality SiNx passivation film with low a hydrogen concentra-tion can be grown by CECVD without additional dehydrogenationprocesses.

To investigate the radiation damage effect caused by the tungstencatalyzer at 1800°C on the electrical and optical properties ofTOLEDs, a 150 nm thick SiNx passivation layer was deposited overa thin Mg–Ag cathode layer of a test sample. Figure 11a shows theJ-V characteristics of the TOLEDs with a CECVD grown SiNx pas-sivation layer and TOLED without a passivation layer �for refer-ence�. Both TOLEDs with SiNx passivation layers grown at differentH2 flow rates show similar forward bias current densities to those ofTOLED with only a Mg–Ag cathode �for reference�. In addition,TOLEDs with a SiNx passivation layer show a very low leakagecurrent density at a reverse bias, which is comparable to the refer-ence sample. Figure 11b shows L-V curves of TOLEDs with a SiNxpassivation layer and a reference sample. As expected from the J-Vcurve, TOLEDs with a SiNx thin passivation layer show an identicalturn-on voltage and luminance to the reference sample regardless of

Figure 10. �Color online� SIMS depth profile of the SiNx film grown byCECVD on Si substrate at optimized conditions.



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H2 flow rate. This identical J-V-L curve of the TOLED with a SiNxpassivation layer to that of the reference TOLED indicates that theelectrical and optical characteristics of TOLEDs are not criticallyaffected by the tungsten catalyzer exposure during SiNx deposition.

Figure 12 shows lifetime data for TOLEDs passivated with a 150nm thick SiNx film grown at optimized conditions �SiH4/NH3/H2= 32/480/1500 sccm� and the reference sample �nonpassivatedTOLED�. The operating lifetime of TOLEDs passivated with a SiNxfilm was measured at a dc drive of 2 mA/cm2 with an initial lumi-nance of 500 cd/m2 at room temperature. Curves of normalized lu-minance vs operating time are shown in Fig. 12. The lifetime of thereference sample with an initial luminance of 500 cd/m2 was ap-proximately 155 h. However, the lifetime of a TOLED passivatedwith 150 nm thick SiNx film is much longer than that of the refer-ence sample. It is worth noting that the lifetime of TOLED could beincreased using a single layer SiNx film unlike other reportedmultilayer thin-film passivation processes with a thickness above300 nm. The prolonged lifetime of TOLED verifies that the SiNxfilm grown by CECVD is a good transparent passivation layer. Al-though direct comparison of lifetime is difficult in TOLEDs withother types of TOLEDs, the increasing lifetime of TOLED by theCECVD-grown SiNxpassivation layer indicates that thin SiNx filmprepared by CECVD is a promising thin film passivation layer forhigh-quality TOLEDs.

Figure 11. �Color online� �a� Current-voltage and �b� luminance-voltagecurves of the TOLED passivated with 150 nm thick SiNx films grown at H2flow rate of 1000 and 1500 sccm, respectively, and nonpassivated TOLED�reference sample�.


Conclusion

In summary, the optical, structural, and surface properties ofSiNx films grown by CECVD and passivation properties of TOLEDswith a transparent SiNx passivation layer were investigated. Using aCECVD equipped with a tungsten catalyzer and bipolar electrostaticchuck system, a high-quality SiNx film was deposited on glass, PCsubstrate, and test cell at a substrate temperature of 50°C. Even at alow substrate temperature, the 150 nm thick SiNx film grown at ahigh H2 flow rate showed superior barrier and optical properties.Due to local substrate heating by hydrogen recombination duringSiNx film formation, we can obtain a high density SiNx passivationlayer without additional substrate heating. J-V-L results of a TOLEDpassivated with a 150 nm SiNx film indicated that the electrical andoptical properties of TOLED were not affected by the high tempera-ture tungsten catalyzer during the SiNx deposition. The lifetime tohalf initial luminance of a TOLED passivated with the 150 nm thickSiNx layer was much longer than that of a nonpassivated referencesample. These indicate that CECVD is a plasma free thin film pas-sivation technique for TOLEDs and flexible OLEDs.

Acknowledgments

This work was supported by a Korea Research Foundation grantfunded by the Korean Government �MOEHRD: Basic Research Pro-motion Fund� �KRF-2006-331-D00243�.

Kumoh National Institute of Technology assisted in meeting the publica-tion costs of this article.

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Figure 12. �Color online� Typical curves of normalized luminance vs oper-ating time of TOLEDs passivated with 150 nm thick SiNx film and referencesample.




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