S h i m a d z u R e v i e w

1. IntroductionDue to their superior visibility, response time, power consumption, and other characteristics compared to conventional LCDs, OLEDs have been attracting attention as the next-generation in flat panel display technology. However, due to their vulnerability to humidity and oxygen, technology for encapsulating the cells is one of the most important issues for improving their reliability. In terms of specific moisture resistance requirements, it is commonly assumed that a high moisture barrier film with a water vapor transmission rate of about 10-6 g/m2/d is required.1) Furthermore, organic luminescent materials have poor heat resistance, which means they must be processed at low temperature below 100 °C. Due to the high aperture ratio, top emitting OLED cells2) require a transparent barrier film. Consequently, many materials or structures have been proposed for achieving both transparency and low vapor transmission, such as the atomic layer deposition method3) and SiO2/SiNx multilayer films4), but it has been difficult to simultaneously achieve transparency, low vapor transmission, and also high productivity.Therefore, the authors have been developing transparent barrier film technology for large areas through a NEDO Green IT Project entitled Development of Basic Technology for Next-Generation Large-Screen Organic Light-Emitting Diode Displays (H20 to H24).5),6) This paper presents an overview of developing a transparent and highly moisture-resistant SiNx-based barrier film applied using the surface wave plasma CVD method7), which is especially well suited for low-temperature processing with minimal damage.

Development of Transparent Encapsulation Films for OLEDs Using Surface Wave Plasma CVD System

by Kazufumi AZUMA, Ph.D.1, Satoko UENO2, and Yoshiyuki KONISHI2

（Received December 13, 2012）1 New Technology Unit, Technology Research Laboratory, Shimadzu Corporation,

Kanagawa, Japan2 Research & Development Department, Semiconductor Equipment Division,

Shimadzu Corporation, Kanagawa, Japan

Abstract

We have succeeded in developing a highly transparent, water impermeable SiNx encapsulation films for organic light-emitting displays using a surface-wave-plasma enhanced chemical vapor deposition (SWP-CVD) system. A highly reliable barrier performance (no deterioration for more than five years under the room temperature) was realized with above SiNx film as an encapsulation of OLEDs.

Keywords: Surface wave plasma (SWP), CVD, encapsulation, SiN, OLEDs

SR14_012E

2. Surface Wave Plasma CVD MethodThe surface-wave plasma (SWP) CVD method results in higher electron density and lower electron temperature levels near the substrate than the capacitively coupled plasma (CCP) enhanced CVD method (CCP-PECVD), which means it is less likely to damage the base layer even during high speed deposition.8) In addition, unlike the volumetric CCP process, the SWP CVD method allows substrates to be positioned farther away from the plasma, which helps inhibit temperature increases during film deposition. A diagram of the instrument7) used in the study is shown in Fig. 1.

Microwaves are transmitted from the waveguide through the slotted array antenna to the dielectric window. Given an angular frequency ω, dielectric window permittivity εs, light speed c, and wavenumber k, the microwave phase can be determined based on the following formula.

In addition, given a plasma frequency ωp, the permittivity can be determined as follows.

Fig. 1 Experimental apparatus of the SWP-CVD system

（1）

Therefore, (1) and (2) can be combined as follows.

（2）

If ω >> ωp (low electron density), then ω/k ≈ c.If electron density increases, so that ω = ωp, then k becomes zero.

（3）

3. Transparent SiNx Barrier Film

3.1 Designing the Transparent SiNx Barrier Film MaterialThe optical properties of the SiNx film are thought to be determined almost entirely by the ratio of Si and N atoms. Therefore, the transmission rate was measured from films formed using various ratios of the raw material SiH4 and NH3 gases and, at the same time, Rutherford backscattering spectrometry (RBS) was used to determine the ratio of Si and N atoms in those films. In addition, the Si-N bond status was analyzed using X-ray photoelectron spectroscopy (XPS).Fig. 3 shows the results from measuring the optical transmittance through 2 μm thick SiNx films formed using various ratios of NH3 and SiH4 gases. It shows that the optical transmittance increases as the NH3/(SiH4+NH3) ratio is increased, with optical transmittance reaching 95 % or more at mixture ratios over 0.8. Results from using RBS and XPS to investigate the film structure are shown in Fig. 4 and Table 1.

In other words, if the electron density is high, the plasma behaves like a metal. If the electron density increases further, so that ω << ωp, then microwaves can no longer penetrate the plasma and they form a standing wave in the surface wave mode propagating along the dielectric window.Fig. 2(b) is modified from a report by Suzuki et al.9) so that vertical distances correspond to Fig. 2(a). In other words, at a location a few millimeters from the dielectric plate, the electron temperature (Te) and electron density (ne) are both high, but at 20 mm or more from the plate, there is a region referred to as the bulk layer where the electron temperature is low and the electron density is high. Consequently, a low electron temperature-high electron density environment can be achieved by separating the substrate sufficiently from the dielectric plate, which makes it possible to deposit films at high speeds while minimizing substrate damage.

The ratio of elements contained in the films was determined by RBS and the bond status was determined by XPS. Near a NH3/(SiH4+NH3) ratio of about 0.75, RBS indicates a Si3N4 ratio, but XPS indicates a Si3N3.63 ratio. This is evidence that not all bonds around the Si atoms are those with N atoms.

(a)Fig. 2 Generation of SWP

NH3/(SiH4+NH3)

NH3/(NH3+SiH4) gas flowrate

Opt

ical

tran

smitt

ance

(%)

(a)

Fig. 3 Optical transmittance dependence on NH3, SiH4 ratio

Fig. 4 Relationship between NH3, SiH4 ratio and SiNx structure

3.2 Increasing the Moisture Barrier Performance of the SiNx Film

In general, SiNx films formed at high temperatures above 350 °C contain low hydrogen levels and have a dense film structure, but OLEDs require processing at low temperatures below 100 °C. Consequently, SiNx films formed at low temperatures tend to have low densities containing higher hydrogen levels. Therefore, we studied processes that would inhibit hydrogen entering the film even when formed at low temperatures, which resulted in determining that the hydrogen content in films could be reduced by increasing the distance (gap) between the dielectric and substrate.

All the bonds around the Si finally become bonds with N when the NH3/(SiH4+NH3) ratio increases above about 0.88. Given the RBS results of Si3N5 at that point, it is unclear what happened to the excess N atoms. Fig.5 shows data analysis results from separating the Si2p XPS spectral peaks. Almost no oxygen exists in films with Si3N4 formed by thermal decomposition of SiH4 and NH3. However, this shows that SiNx films formed at low temperatures and contain O-Si-N and SiO2

bonds in addition to Si-N bonds. Presumably the extra N atoms that did not bond with the Si3N5 structure indicated in RBS results bonded with the O atoms in the film. Fig. 5(a) also shows peaks presumably caused by the Si2O (sub-oxide) not present in (b). Based on these factors, SiNx films in XPS results that are not Si3N4 contain Si-Si bonds that might be increasing the light absorption of the film. If the NH3

ratio is increased, these might react with the SiH4 in the concentrated NH3 atmosphere, resulting in breaking the Si-Si bonds and forming new N bonds.

Fig. 5 Deconvolution of XPS spectra of SiNx (a): NH3/(NH3+SiH4) ratio 0.5625, (b): NH3/(NH3+SiH4) ratio 0.8772

Table 1 NH3, SiH4 ratio vs. SiNx structures analyzed by RBS and XPS, respectively

NH3/(NH3+SiH4) ratio RBS SixNy XPSSixNy

0.5625 Si3N3.2 Si3N3.39

0.7537 Si3N4 Si3N3.63

0.8772 Si3N5 Si3N3.95

(a) (b)

3.3 Improving Moisture Barrier Performance by Applying Multiple Layers

3.3.1 Difference in Moisture Barrier Performance of Single and Double Layers of SiNx

To evaluate the moisture barrier performance of SiNx films by the Ca test, samples were prepared with SiNx films deposited on one or both sides of a polyimide film. Polyimide film is used as the substrate because it has minimal moisture barrier performance, with a WVTR value of about 100 g/m2/d. However, the results (Fig. 9) show that there is big difference in moisture barrier performance between samples with a 2 µm SiNx film applied to only one side versus samples with a 1 µm film applied to both sides.

Fig. 6 shows the relationship between the substrate temperature during film formation and the hydrogen content in the film, determined by the hydrogen forward scattering (HFS) method. If the film is formed at a gap of 150 mm, the hydrogen content in the film increases with decreasing substrate temperature, so that the content is nearly 40 at% at 100 °C . In contrast, measurement results from SiNx film formed at 100 °C by the CCP-PECVD method indicated a hydrogen content of about 33 at%. However, when the gap was increased to 200 mm, the hydrogen content in the film remains at 100 °C is at about 16 at% regardless of the substrate temperature (even at a film formation temperature of 100 °C ). This showed that the hydrogen content in films could be decreased by increasing the gap distance.Fig. 8 shows the results from using actinometry11) to investigate the relationship between the gap distance and hydrogen radical concentration. It shows that the hydrogen radical concentration drops sharply as the gap is increased. Unfortunately, it is not possible to also estimate the concentration of precursors to film formation, such as SiH3, at the same time. However, based on the reactions indicated below, SiH3 radicals are predicted to have a sufficiently long life, so that the number of hydrogen radicals relative to the SiH3 radicals decreases as the gap distance increases, which presumably results in fewer hydrogens in the film.

The Ca reaction rate values on the vertical axis of Fig. 9 were calculated as follows.

This calculation results in WVTR values of 2.5×10-4 g/m2/d and 1.2×10-4 g/m2/d for the single-sided 2 µm film samples, which are not only high WVTR values, but also indicate high variability as well.In contrast, the polyimide film with 1 �m thick SiNx applied to both sides indicated high moisture barrier performance, with a WVTR value of 2.5×10-5 g/m2/d or less.As reported by Schaepkens et al.12), the reason for this result is that inserting an intermediate layer not only embeds particles, but presumably also has the effect of separating defects (Fig. 10). In other words, inserting an intermediate layer that is thicker than the horizontal distance (offset) between defects in the upper and lower barrier films tends to have a greater effect in retarding the penetration of moisture. Furthermore, an organic intermediate layer, such as twisted wool, is more effective in retarding the penetration of moisture than an inorganic material.

H+SiH4→ SiH3+H2↑ (H quenched)

SiH3+SiH4→ SiH4+SiH3↑ (exchange)

Ca reaction rate ∞

WVTR

Ca(OH)2 density × Ca(OH)2 thickness× Processing

time

Fig. 7 Dependence of SiNx hydrogen content on gap distance

Fig. 8 Relationship between gap and hydrogen radical concentration

Fig. 6 Dependence of SiNx hydrogen content on substrate temperature

3.3.2 Searching for an Intermediate LayerThe fact that the moisture resistance improved significantly when the SiNx film was applied to both edges of the polyimide film in Fig. 9 may be due to the polyimide film functioning as a defect separating layer, as described above. Since a barrier film must be applied during the thin film deposition process for actual OLED cells, there is a need to search for an intermediate layer that can substitute for the polyimide film. Available organic films, such as Parylene, which includes para-xylene as its major ingredient, require heating and vapor deposition chambers and complicated film deposition processes. Therefore, we searched for a material that could be applied successively in the same chamber as used for SiNx film deposition. We discovered that inexpensive and easy-to-handle hexamethyldisiloxane (HMDSO), which has a chemical equation of O[Si(CH3)3]2, was commonly used as an ingredient in plasma polymerized SiOC films, such as those used as a moisture barrier film for food products.13),14)

Therefore, we studied using a film of this material, plasma-polymerized by SWP-CVD, as the intermediate layer.HMDSO is normally used mixed with oxygen, but the film structure differs significantly depending on the mixture ratio of oxygen. Fig. 11 shows a collection of FTIR spectra for SiOC films formed by varying the ratio of HMDSO and oxygen. At a HMDSO:O2 ratio of 1:10, the structure is mostly similar to SiO2, but as the proportion of oxygen decreases, the structure becomes a SiOC structure that contains organic -CHx radicals. Reducing the oxygen dilution rate as much as possible increases the film deposition rate and results in more organic film characteristics, but it also increases the HMDSO concentration, which causes problems with increased particles from gas phase collisions. Therefore, an HMDSO:O2 ratio near 1:2 was used so that particles would not be a problem and a high film deposition rate of 40 nm/s could be maintained.

　Fig. 9 Time-dependent change in Ca reaction rate under 40 °C and 90 % RH conditions

Fig. 10 Role of intermediate layer12)

4. Evaluating OLED CellsSection 3 described obtaining a transparent high-barrier film by combining SiNx and SiOC. Therefore, that film was applied as a barrier film on actual OLED cells to evaluate whether or not the barrier film deposition process damaged the cells and to evaluate the reliability of the film for use as a barrier film on OLED cells. The structure of the standard cell for the NEDO project, as shown in Fig. 13, was used for the cell structure.

4.1 Evaluating Cell DamageSiNx (1 µm)/SiOC (3 µm)/SiNx (1 µm) films were deposited on the cell shown in Figure 13 and a clear desiccant coating under development by JSR Corporation was applied on top. Then the cell's voltage versus brightness characteristics were measured. The measurement results are shown in Fig. 14. These results confirmed that there was no cell damage incurred from depositing the films or applying the desiccant.

Next, using an SiOC film obtained by plasma polymerization and containing the above HMDSO/O2 as the intermediate layer, the moisture barrier performance of the SiNx (1 µm)/SiOC (3 µm)/SiNx (1 µm) structure was evaluated using the same Ca test as described in 3.3.1. The results are shown in Fig. 12. Because the moisture barrier performance of the SiOC film itself is too low to be measured by the Ca test, it was measured separately by the MOCON method15). This confirmed that it was at 100 g/m2/d, comparable to the polyimide film. Even though the SiOC used for the intermediate layer has negligible moisture barrier performance, the results show that simply sandwiching it between 1 µm layers of SiNx film provided equally high moisture barrier performance as indicated in Fig. 9 for the polyimide film sandwiched between SiNx (1 µm) layers.

Fig. 11 Dependence of plasma-polymerized SiOC structure on gas ratio of HMDSO and oxygen

Fig. 12 Results from Ca test of SiNx/SiOC/SiNx multilayer films

Fig. 13 Standard OLED cell for evaluation

Fig. 14 OLED damage evaluation

Fig. 15 Status of OLEDs after at 85 °C 85 % RH acceleration test

4.2 Accelerated Cell Reliability TestingCell reliability was evaluated as follows. The acceleration factor was calculated by using samples consisting of Ca vapor-deposited on a glass substrate and coated with desiccant to determine the reaction speed of the non-transparent metal Ca reacting with water to produce transparent Ca(OH)2, given the conditions 85 °C and 85 % RH, 60 °C and 90 % RH, and 40 °C 90 % RH and then extrapolating the value at "25 °C 60 % RH" (ambient temperature and humidity)16) from the Arrhenius plot. The acceleration factor relative to "25 °C 60 % RH" obtained by this method was 265 for "85 °C 85 % RH" and 40 for "60 °C 90 % RH." In other words, accelerated testing for 190 hours at 85 °C and 85 % RH is equivalent to testing at ambient conditions for 50,000 hours.

5. Accommodating Large Substrateshe preceding discusses the barrier performance of SiNx and SiOC films formed using the SWP-CVD method. Currently, OLEDs are mainly used in mobile applications for fifth generation substrates (1100 mm × 1300 mm). However, they may be used for larger sixth generation (1850 mm × 1500 mm or larger) substrates being commercialized for large screen OLED televisions. Up to this point, the study evaluated films deposited on 100 mm square substrates, but for practical applications, the films must be applied uniformly over large substrates that exceed 2 m in size. Therefore, the authors decided to study using a reciprocating substrate film disposition method, commonly used in sputtering, as one means of forming a film uniformly across a large area. Typical CCP-PECVD methods involve placing the substrate between parallel flat electrodes. Consequently, moving the substrate can cause plasma instability. However, the SWP-CVD method involves electrodeless discharge, which allows moving the substrate to average out the film deposition. A diagram of the reciprocating substrate film deposition method is shown in Fig. 17. A metal belt moves a substrate holder located 200 mm below the plasma source back and forth at a speed of 100 mm/min. A Langmuir probe was then used to measure the plasma status during this movement. Fig. 18 shows the results from measuring the electron density near the substrate position when an Ar plasma is generated. This confirms that plasma is stable during the reciprocating motion, remaining at about the same level as when stationary.

Results from accelerated testing of barrier films consisting of a single SiNx layer 500 nm thick and a SiNx (100 nm)/SiOC (1 µm)/SiNx (100 nm) multilayer film at 85 °C and 85 % RH are shown in Figure 15. In both cases, deterioration started after 670 hours, with no deterioration observed after 190 hours, which corresponds to the target 50,000 hours under ambient conditions. In addition, results from repeating the tests with a larger number of samples indicate that more cells stop illuminating with the single layer SiNx film than with the multilayer film, as shown in Fig. 16. The plasma polymerized intermediate layer used in the multilayer films can be formed extremely quickly (40 nm/s). Furthermore, the SiNx film on both sides of the intermediate layer can be made very thin 100 nm. Therefore, given an intermediate layer thickness of 1 µm, for example, the cycle time is only 67 seconds. The cycle time for single layer SiNx films that require a minimum film thickness of 500 nm is 110 seconds. Therefore, the multilayered barrier film has more advantages.

Fig. 16 Ratio of luminescent cells after 85 °C 85 % RH accelerated test (a): SiN single layer, (b): SiN/SiON/SiN multilayer

Fig. 17 Schematic view of reciprocating substrate film deposition system

Fig. 18 Fluctuation of electron density near substrate during reciprocation

(* Transfer, evacuate, deposit film, leak time)(a)

(** Transfer, evacuate, deposit film, switch gases, leak time)(b)

To verify the potential of the reciprocating film deposition method for sixth-generation substrates, a system was created based on the above results that moves 1500 mm × 100 mm substrates in the direction of their shorter dimension during film deposition (see Fig. 19). The system deposits a film by moving the 1500 mm × 100 mm substrate up and down, where the dielectric plate that contains the SWP-CVD

plasma source consists of four 200 mm × 450 mm plates arranged in a linear source configuration.Results from evaluating the uniformity are summarized in Fig. 20. The in-plane film thickness distribution was within ±3 % and the refractive index distribution was within ±0.75 %.

Fig. 19 SWP-CVD apparatus for verification of Gen 6 size substrate uniformity

Fig. 20 In-plane uniformity of 1500 mm × 100 mm area for reciprocating deposition system

6. ConclusionThis paper discusses film deposition methods, transparency of barrier films, guidelines for achieving adequate barrier performance, and accelerated testing of the reliability of the resulting films if they are used as a barrier film on OLEDs as they relate to large-area transparent barrier film technology being developed as part of the NEDO Project entitled Development of Basic Technology for Next-Generation Large-Screen Organic Light-Emitting Diode Displays (H20 to H24). The SiNx-based barrier film developed offers both superior moisture barrier performance and productivity. Therefore, we anticipate its use in OLED devices that are expected to proliferate in post-LCD markets. However, the application technology for using the film on large-area substrates is just starting to be studied. Consequently, several issues still need to be resolved, such as improving film uniformity and ease-of-maintenance, before a deposition system can be developed for sixth generation or higher substrates. Therefore, based on the core technologies we've obtained from the NEDO project, we intend to continue making additional technological breakthroughs as we move toward quantity production.

AcknowledgementsWe would like to thank Kazuhiro Takahashi at KRATOS XPS Section, Analytical & Measuring Instruments Division, Shimadzu Corporation, for his help with XPS measurements and analysis involved in this research. We would like to thank Mr. Tao and Mr. Aoe at Choshu Industry for their effort in supplying the OLED panels. Also, we would like to thank Dr. Konno and Dr. Arai from JSR Corporation for their assistance with applying desiccant coatings. A portion of this research was performed as part of the Green IT Project entitled Development of Basic Technology for Next-Generation Large-Screen Organic Light-Emitting Diode Displays, funded by New Energy and Industrial Technology Development Organization (NEDO). We also are grateful to all the others involved in this research.

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