Fabrication and characterization of an Al₂O₃/OPSZ/Al₂O₃ barrier film on a PEN substrate via PEALD and spray-spin coating techniques

Abstract

One of the main drawbacks of the current flexible devices is the lack of a thin-film encapsulation with superior moisture barrier performance, light transmittance and mechanical flexibility. In this study, an inorganic/organic/inorganic composite film was fabricated on a 10 cm × 10 cm polyethylene naphthalate (PEN) substrate, which consisted of two inorganic layers of Al₂O₃ with an organic layer of organic polysilazane (OPSZ) in between. Al₂O₃ was deposited via plasma-enhanced atomic layer deposition (PEALD) with a thickness of about 40 nm. Approximately 200 nm thick OPSZ was prepared using the spray-spin coating method. The multilayer film exhibited outstanding optical transmittance (>90%) and an extremely low water vapor transmission rate (WVTR) (4.6 × 10⁻⁵ g per m² per day, 38 °C, 90% RH), which is two orders of magnitude lower than that of the corresponding single-layer Al₂O₃ film. Moreover, the introduction of an organic layer helped the Al₂O₃/OPSZ/Al₂O₃ film to exhibit good tensile and bending resistance. Thus, the multilayer film may find applications in the encapsulation of flexible devices.

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1. Introduction

Film encapsulation is an important technical assurance for the commercial application of flexible devices, such as flexible displays and flexible solar cells. Encapsulation barrier films play a crucial role in preventing the devices from being permeated by moisture and oxygen.^1–5 In particular, water causes more severe degradation than oxygen, and hence, effective protection from water vapor is crucial during the encapsulation of flexible electronic devices.^6–8

Metal foils are potential barrier materials owing to their good mechanical properties, low cost, ease of formability, and good barrier properties.⁹ However, barrier films for flexible electronic devices require transparency. Glass-based materials display good barrier properties and transparency, but their mechanical properties and processibility are undesirable.¹⁰ Alternatively, metal oxide deposited onto transparent and soft substrates affords nearly all requirements for flexible devices.¹¹ Metal oxides, such as Al₂O₃, MgO and ZrO₂, grown by atomic layer deposition (ALD) are reported to provide excellent moisture barrier properties.^12–14 Further, alternating inorganic laminates, such as Al₂O₃/MgO,¹⁵ Al₂O₃/ZrO₂ ¹⁶ and Al₂O₃/HfO₂,¹⁷ have shown superior barrier performance over corresponding single-layer films. However, during the fabrication process, microcracks or pinholes might be introduced, inevitably providing pathways for the penetration of oxygen and moisture.^18,19 To enhance the barrier performance, the total number of dyads and the thickness of the layers are often increased. However, shortcomings such as cracking occurring in films and low optical transmittance may still persist. Inserting an organic layer into neighboring inorganic layers can decouple defects or pinholes and prolong permeation pathways, and this appears to be a promising strategy.^20–23 Van de Weijer et al.²⁴ reported a thin-film encapsulation stack consisting of two inorganic barrier layers separated by an organic planarization layer. The 1.5-dyad structure exhibited a significant delay in the formation of black spots when applied on an OLED display. Duan et al.²⁵ fabricated multiple stacked layers of Al₂O₃/alucone film on a flexible PET substrate and delivered an extremely low water vapor transmission rate (WVTR) (9.94 × 10⁻⁵ g per m² per day). Previous studies have demonstrated the synergetic effect of hybrid films on the barrier properties. However, among the various fabrication protocols, achieving cost-effective manufacturing of multilayer architecture and realizing them as smooth, stretchable, durable barrier films are still challenging.

In this study, we developed an inorganic/organic/inorganic composite film. Al₂O₃, which has the best water vapor barrier performance among the inorganic oxides, was chosen as the inorganic sublayer. Organic polysilazane (OPSZ) was chosen as the organic sublayer owing to its high transparency and thermal stability. A multilayer film consisting of two inorganic Al₂O₃ layers and an intermediate OPSZ layer was fabricated. The first Al₂O₃ layer was prepared by plasma-enhanced atomic layer deposition (PEALD) operated at a low deposition temperature, allowing a high quality Al₂O₃ layer with excellent barrier performance. An adjacent OPSZ layer was prepared using the spray-spin coating method. In general, spin coating is suitable for the uniform coating of small samples. A process that combines spray coating and spin coating can produce nano-scale polymer films with highly uniform thickness over a large substrate area. Afterwards, the plasma treatment was used on the hydrophobic OPSZ surface to provide rich nucleation sites for the deposition of another Al₂O₃ layer. The elemental content and water contact angle (WCA) verified the changes in the surface characteristics after plasma treatment. A structural analysis was carried out, and the WVTR performance and optical transmittance of the developed multilayer film were evaluated. The obtained results demonstrated that multilayer film exhibits better performance than single-layer Al₂O₃ film. This work also provides insights into optimizing performance by modulating a hybrid structure.

2. Experimental section

2.1 Preparation of multilayer barrier films

The PEN/Al₂O₃/OPSZ/Al₂O₃ multilayer films were prepared on a polyethylene naphthalate (PEN) substrate with a thickness of 100 μm (DuPont Tejin films). The Al₂O₃ layer was grown in a Picosun R-200 ALD reactor using trimethylaluminium (TMA) and O₂ plasma at 120 °C. For each ALD cycle, 0.1 s exposure to TMA vapor, 6 s of N₂-purge, 26 s exposure to O₂ plasma, and 6 s of Ar-purge were applied in sequence. The intensity of the O₂ plasma generator was 2700 W. The growth rate of Al₂O₃ was approximately 0.124 nm per cycle. The spray-spin coating method²⁶ was used to prepare the nanoscale OPSZ (Durazane®1500, Merck) layer. In detail, 1 mL of OPSZ solution (20% solid content) was sprayed on the (ALD-deposited PEN/Al₂O₃) substrate at room temperature, followed by 60 s of spin coating. The rotation speed of the spin coating varied from 500 to 5000 rpm. The PEN/Al₂O₃/OPSZ film was cured at 80 °C for 2 hours. An 8 min plasma treatment was then performed to tune the wettability of the as-cured OPSZ film, in which the plasma power was set at 250 W. After that, the second layer of Al₂O₃ was deposited using the same ALD process conditions described above. Fig. 1 is a schematic of the multilayer film structure with information on the thickness of each layer.

Fig. 1 Schematic of the multilayer barrier film.

2.2 Characterization of the barrier films

The microstructures of the films at varying preparation steps were characterized by applying a Scanning Electron Microscope (SEM, Nova NanoSEM 450, FEI Company). The surface morphology of the OPSZ prepared at different rotation speeds was captured using a 3D laser scanning microscope (VK-X1000, Keyence). The surface contact angle of the films was determined using a contact angle meter (DSA100, KRUSS). The surface elemental composition was measured using energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) (Escalab 250Xi, Thermo Scientific). The cross-sectional morphology of the film was observed using a Transmission Electron Microscope (TEM, JEM ARM200F, JEOL). Before TEM imaging, site-specific milling was performed by applying the focused ion beam (FIB) technique. The surface morphology and roughness of the films were evaluated using atomic force microscopy (Dimension ICON, Bruker). The optical transmittance of the films was measured using a spectrophotometer (UV-3600, Shimadzu) in the wavelength range of 400–1200 nm. The WVTR of the films with a sample size of about 10 cm × 10 cm was obtained using MOCON Aquatran 3 (Aquatran model 3, MOCON Inc., USA) at 38 °C and 90% relative humidity (RH). The fatigue tensile test method was used to study the flexibility of the barrier films. The number of tensile cycles was 100, and the tensile strain increased from 1% to 5%. In the bending test, the bending strain is calculated from the bending radius of the film. The formula is as follows: ε = (t₁ + t₂)/2r, where t₁ and t₂ are the thicknesses of the substrate and coating, respectively, and r is the bending radius. An ultra-depth 3D microscope (DSX510, OLYMPUS) was used to observe the changes in the surface topography of the samples.

3. Results and discussion

3.1 Microscopic morphology of the Al₂O₃ layer

In this work, Al₂O₃ layers were deposited by PEALD at 120 °C.²⁷ Fig. 2(a) shows the surface SEM image of the first Al₂O₃ layer deposited on the PEN substrate. The Al₂O₃ layer shows a homogeneous, smooth surface. Fig. 2(b) shows a cross-sectional SEM image of the PEN/Al₂O₃ film obtained after 350 ALD cycles. The Al₂O₃ layer thickness is about 44.7 nm, corresponding to an ALD growth rate of 0.124 nm per cycle. The number of ALD cycles was fixed at 350 for the Al₂O₃ layer deposition in the subsequent preparation of the Al₂O₃/OPSZ/Al₂O₃ multilayer barrier films.

Fig. 2 Surface (a) and cross-sectional (b) SEM images of the Al₂O₃ film.

3.2 Microscopic morphology of the OPSZ layer

The spray-spin coating method was used to prepare the nano-scale OPSZ layer. To determine the optimum coating conditions, OPSZ layers were first prepared on a PEN substrate by varying the rotation speed in the range of 500–5000 rpm. A 3D laser scanning microscope was used to investigate the uniformity of the OPSZ films. As shown in Fig. 3(A)–(D), the OPSZ layer prepared with a rotation speed of 5000 rpm yields the smoothest surface. Besides, the thickness of the OPSZ layer decreases as the rotation speed increases, and the downward trend in film thickness levels off at rotation speeds above 3000 rpm. When the rotation speed reaches 5000 rpm, the thickness of the OPSZ layer is evaluated to be about 210 nm (Fig. 3(d)). A rotation speed of 5000 rpm was used in the subsequent spray-spin coating preparation of the OPSZ layer in the multilayer films.

Fig. 3 Optical microscope images (A)–(D) and SEM cross-sectional images (a)–(d) of OPSZ layers on PEN substrate.

3.3 Changes in PEN/Al₂O₃/OPSZ film surface with plasma treatment

OPSZ layer was coated on the surface of PEN/Al₂O₃ film. The prepared PEN/Al₂O₃/OPSZ film was then cured at 80 °C for 2 hours before plasma treatment. Water contact angle (WCA) measurements were taken on the as-cured and plasma treatment films. Fig. 4(a) shows that the OPSZ layer before plasma treatment shows a generally hydrophobic character with a contact angle of around 93°. In general, hydrophobic surfaces have low surface energy,²⁸ which is not conducive to nucleation during PEALD.^29,30 As shown in Fig. 4(b), a contact angle of around 0° is observed, indicating that a complete wetting surface has been achieved after O₂ plasma treatment. Fig. 4(c)–(f) further shows the effect of storage time on the WCA values on the OPSZ surface; the plasma-treated PEN/Al₂O₃/OPSZ film retains relative hydrophilicity after 7 days of storage time. It is thus demonstrated that plasma treatment is an effective protocol for rendering a hydrophilic surface and providing nucleation sites for the ALD process.

Fig. 4 Images of contact angle before (a) and after (b)–(f) plasma treatment: 0 day (b), 1 day (c), 3 days (d), 5 days (e), and 7 days (f) storage time after treatment.

Fig. 5 shows the surface elemental compositions of films containing as-cured OPSZ layers measured by Energy Dispersion Spectroscopy (EDS). After plasma treatment, the oxygen content increases as shown in Fig. 5, indicating that oxygen-containing groups were introduced onto the surface of the OPSZ layer. The introduced functionalities improved the surface hydrophilicity. However, it is found that the increase in the oxygen element is relatively low. This is mainly because the plasma treatment is confined to only a few nanometers at the surface.

Fig. 5 EDS results of the OPSZ surface before (a) and after (b) plasma treatment.

Fig. 6(a) and (c) show the C 1s high-resolution spectra of the OPSZ surface before and after plasma treatment, respectively. Compared with untreated OPSZ, plasma-treated OPSZ surface shows increased C–O and O–C=O/C–C ratios. Fig. 6(b) and (d) compare the O 1s high-resolution spectra of the OPSZ surface before and after treatment, respectively. A peak of O=C (530.7 eV) is detected in the treated OPSZ surface. Simultaneously, the binding energy of O 1s shifts from 532.2 eV to 532.6 eV, suggesting the presence of hydroxyl groups. The above analysis indicates that oxygen containing groups, such as O–C=O, C–O, and OH, have been successfully introduced on the OPSZ surface after the treatment.

Fig. 6 C 1s and O 1s spectra of the OPSZ surface before (a) and (b) and after (c) and (d) plasma treatment.

3.4 Structure and properties of Al₂O₃/OPSZ/Al₂O₃ film

Fig. 7 shows a HRTEM image of the Al₂O₃/OPSZ/Al₂O₃ film on the PEN substrate. The film consists of a 44.76 nm-thick Al₂O₃ layer, a 207.58 nm-thick OPSZ layer and another 43.12 nm-thick Al₂O₃ layer. The interfaces of PEN/Al₂O₃ and OPSZ/Al₂O₃ layers are well-defined. It is known that grain boundaries in crystalline layers are potential pathways for water vapor diffusion. Notably, the Al₂O₃ thin layer displays a continuous, homogeneous, and amorphous structure, minimizing the pathways for water permeating. From the mapping images, it is clearly observed that the Al and Si elements are uniformly distributed within the Al₂O₃ and OPSZ layers, respectively. The O element is distributed throughout the whole barrier film, while the C element is mainly distributed in the OPSZ layer. Both Au and Pt elements are artificially plated to protect the sample surface.

Fig. 7 HRTEM cross-sectional image and EDS mapping of the Al₂O₃/OPSZ/Al₂O₃ three-layer barrier film.

The surface smoothness of the bare PEN and different barrier films is measured by AFM over a scanned area of 5 μm × 5 μm. As shown in Fig. 8, the average surface roughness (R_a) for bare PEN is 1.25 nm. Upon deposition of the first Al₂O₃ layer, the surface roughness of the PEN/Al₂O₃ film decreases noticeably to 0.59 nm. Coating the OPSZ layer onto the above film results in further surface smoothing (R_a = 0.46 nm) (Fig. 8(c)). Thus, both the Al₂O₃ and OPSZ layers contribute to roughness reduction, which is conducive to the growth of a dense and pinhole-free Al₂O₃ surface layer of the multilayer barrier film. As expected, the subsequent Al₂O₃/OPSZ/Al₂O₃ barrier film shows a desired minimal R_a of 0.38 nm (Fig. 8(d)).

Fig. 8 AFM images and surface roughness values of (a) PEN, (b) PEN/Al₂O₃, (c) PEN/Al₂O₃/OPSZ, and (d) PEN/Al₂O₃/OPSZ/Al₂O₃.

The WVTR was measured using MOCON Aquatran 3 to investigate the barrier properties of the films with regards to moisture. Unlike the small sample size of the Ca test, the size of the sample tested on MOCON equipment is as large as 10 cm × 10 cm. The WVTR for the pristine PEN substrate film is measured to be 1.3 g per m² per day, which is consistent with the reported results.³¹ As shown in Fig. 9(a), the 350-cycle deposited PEN/Al₂O₃ thin film exhibits a WVTR value of 1.26 × 10⁻³ g per m² per day. As the deposition cycles increase to 700, the WVTR value of the PEN/Al₂O₃ film increases to 2.54 × 10⁻² g per m² per day. The increased WVTR value could be ascribed to the propagation of defects. The introduction of an OPSZ layer on PEN/Al₂O₃ leads to a sharply decreased WVTR value (3.79 × 10⁻⁴ g per m² per day). Finally, after the ALD deposition of an additional layer of Al₂O₃, the multilayer film exhibits the lowest WVTR value of 4.6 × 10⁻⁵ g per m² per day, which is 2–3 orders of magnitude lower than that of a single Al₂O₃ film.

Fig. 9 WVTR values (a) and optical transmittance (b) of different barrier films.

Optical properties are extremely important for barrier film for flexible devices. Normally, a light transmittance of higher than 90% is required for flexible electronic device encapsulation.³² The optical transmittance as a function of wavelength for different barrier structures was measured by applying a UV-visible spectrophotometer, and the transmittance spectra are shown in Fig. 9(b). PEN/Al₂O₃/OPSZ and multilayer films show average transmittance values of 91.4% and 90.1% between 400 and 1200 nm wavelengths, respectively, which are more transparent than those of bare PEN substrate (Tr 87.5%) and PEN/Al₂O₃ film (Tr 88.5%). This demonstrates that the introduction of the OPSZ polymer layer can effectively prevent light scattering owing to its lower surface roughness, resulting in a higher transmittance of the multilayer films.

A schematic illustration of the reduced WVTR of the multilayer film compared to the neat Al₂O₃ film is presented in Fig. 10. First, the introduction of the OPSZ layer can help to achieve a flattened surface.³³ Meanwhile, the organic layer is known to decouple defects in Al₂O₃ layers, thereby preventing direct penetration through defect sites. Moreover, diffusion lag times are remarkably extended with a prolonged diffusion path of water vapor permeation owing to the presence of an organic layer sandwiched between the two Al₂O₃ inorganic layers.³⁴ Consequently, the Al₂O₃/OPSZ/Al₂O₃ multilayer film achieves significant barrier properties compared to the film made solely of Al₂O₃.

Fig. 10 Schematic of the diffusion of water vapor through the Al₂O₃ single-layer (a) and Al₂O₃/OPSZ/Al₂O₃ multilayer (b) films.

Tensile and bending strain measurements for 100 cycles are used to test the flexibility of barrier films. Fig. 11 shows that obvious cracks can be observed in the Al₂O₃ single-layer film when stretched at 5% strain, while no cracks are observed in the Al₂O₃/OPSZ/Al₂O₃ multilayer film with the applied strain from 1% to 5%. A bending test is performed with strains of 1.3%, 1.8%, 2.5%, and 3%. From Fig. 12, it can be observed that neither Al₂O₃ single-layer nor Al₂O₃/OPSZ/Al₂O₃ multilayer films show cracks at a low bending strain. Until the strain reaches 3%, cracks appear in the Al₂O₃ single-layer film. The above analysis indicates that the organic layer is advantageous for promoting the flexibility of barrier films. Therefore, the tensile and bending resistance of the Al₂O₃/OPSZ/Al₂O₃ multilayer film is better than that of the Al₂O₃ single-layer film.

Fig. 11 Microscopic images for Al₂O₃ single-layer (a) and Al₂O₃/OPSZ/Al₂O₃ multilayer (b) films after 1%, 2%, 3%, 4%, and 5% tensile strain.

Fig. 12 Microscopic images for Al₂O₃ single-layer (a) and Al₂O₃/OPSZ/Al₂O₃ multilayer (b) films after 1.3%, 1.8%, 2.5%, and 3% bending strain.

The Al₂O₃/OPSZ/Al₂O₃ barrier film has potential value and wide application prospects in many industries, especially OLED and solar photovoltaic module encapsulation, which require a very high barrier grade for the encapsulation film. Furthermore, Al₂O₃ coatings have been used in a wide range of food packaging and bio-applications owing to their antibacterial and biocompatibility properties.^35,36 The Al₂O₃/OPSZ/Al₂O₃ barrier film, owing to its superior barrier performance, has greater potential for application in these fields.

4. Conclusions

In summary, Al₂O₃/OPSZ/Al₂O₃ multilayer barrier film on a 10 cm × 10 cm large-area PEN substrate is prepared using PEALD and spray-spin coating processes. The effects of fabrication parameters, layer number, and layer interaction on optical properties and microstructure are investigated systematically. Consequently, the fabricated multilayer film shows a smooth surface characteristic with a minimal R_a of 0.38 nm. Ultra-low WVTR value (4.6 × 10⁻⁵ g per m² per day, at 38 °C, 90% RH), high optical transmission (>90%), and good tensile and bending resistance are achieved. Thus, this work demonstrates that the fabricated multilayer structure barrier films possess great potential for the encapsulation of flexible devices.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors upon request.

Author contributions

X. J. Sun: conceptualization, methodology, validation, formal analysis, investigation, writing—original draft preparation; W. Feng: writing-review and editing, supervision, project administration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 52327802).

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