Characterization of oxide barrier layers prepared by atomic layer deposition

Abstract

Atomic layer deposition has become an important thin-film growth technique for producing gas diffusion barriers because of its low process temperature and its ability to produce uniform films. In this work, atomic layer deposition was used to deposit various Al₂O₃ and ZnO thin films on polyethylene terephthalate substrates; subsequently, the physical properties and water vapor transmission rates of the films were characterized. Single and hybrid films (Al₂O₃, ZnO, Al₂O₃/ZnO, and ZnO/Al₂O₃) with thicknesses of 25, 50, and 100 nm at a deposition temperature of 60°C were investigated. The deposited films were characterized for surface roughness, optical transmittance, adhesion, water vapor transmission rate, and contact angle. The results showed that the double-layer structure provided a higher water vapor transmission rate and higher adhesion strength than those of the single-layer structure although both the surface roughness and optical transmittance of the single-layer structure were slightly better than those of the double-layer structure. The results revealed that the atomic layer deposition-grown hybrids could act as water vapor barriers.

Keywords

Oxide barrier atomic layer deposition water vapor transmission rate adhesion transmittance

Introduction

Atomic layer deposition (ALD) has become an important thin-film growth technique because of its precise control over the thin-film thickness, its ability to produce uniform films over large areas, and its low process temperature.^1–3 ALD can be employed at sufficiently low temperatures to deposit various high-quality thin films on thermally fragile or flexible substrates. ALD provides continuous and pinhole-free thin films that act as high-quality gas diffusion barriers. A key application of ALD is thin-film encapsulation, which is used in food packaging, microelectronics, flexible electronics, OLEDs, and biomaterials.⁴

Many ALD-grown gas diffusion barriers have been proposed for thin-film encapsulation. Generally, a thin Al₂O₃ film is used as a diffusion barrier because Al₂O₃ has low oxygen and water vapor permeability. Hirvikorpi et al.⁵ used ALD to deposit thin and highly uniform Al₂O₃ coatings on various polymeric materials. They found that the ALD-grown Al₂O₃ coatings significantly enhanced the oxygen and water vapor barrier performance of these materials. Similar reports^6,7 revealed that ALD-grown Al₂O₃ thin films on flexible substrates can be an excellent diffusion barrier to protect organic electronic devices from the permeation of oxygen and water vapors. Malm et al.⁸ used ALD to deposit ZnO thin films on Si substrates in the low temperature range from 23°C to 140°C. They observed highly oriented ZnO films at temperatures below 80°C. Mauro et al.⁹ deposited ZnO thin films on polymeric substrates by ALD at a low deposition temperature and revealed that the ALD-grown ZnO thin film could be a high-quality flexible photocatalyst. For composite films, Elam and collegues^2,10 studied the growth rate and surface roughness of ZnO/Al₂O₃ films. They found that the properties of the composite films can be tuned by changing the composition and interfacial density. ZnO/Al₂O₃ films allow the surface roughness to be controlled because ZnO is crystalline and Al₂O₃ is amorphous at low deposition temperatures. Furthermore, Choi et al.⁴ investigated hybrid ZnO/Al₂O₃ layers as thin-film encapsulants on polymer substrates. They found that the multi-laminated structures with thinner ZnO and Al₂O₃ had better barrier properties than single ZnO and Al₂O₃ layers. Gan et al.¹¹ also reported that the crystalline quality of ALD-grown ZnO films was improved by inserting an ultrathin Al₂O₃ buffer layer between the ZnO film and Si substrate, resulting in an increase in the intensity of the photoluminescence.

In this work, various thicknesses of Al₂O₃ thin films, ZnO thin films, and hybrid Al₂O₃/ZnO and ZnO/Al₂O₃ laminated thin films deposited on flexible polyethylene terephthalate (PET) substrates prepared by ALD were investigated. The adhesion of the ALD oxide films on PET substrates was discussed. Further observations of the water vapor transmission rate (WVTR), surface roughness, contact angle, and optical transmittance of the ALD films were also studied.

Materials and methods

An ALD apparatus (Syskey Technology) was used to deposit Al₂O₃ and ZnO thin films on PET substrates; a schematic diagram of the ALD reaction chamber is shown in Figure 1. Each PET substrate with a size of 300 × 200 × 0.175 mm³ was treated by ultrasonic bath cleaning and oxygen plasma cleaning before performing the oxide layer deposition. The principle of Al₂O₃ ALD growth from H₂O and trimethyl aluminum (TMA) was based on the following two self-termination reactions^12,13

AlOH * + Al {(C H_{3})}_{3} \to AlOAl {(C H_{3})}_{2} * + C H_{4}

(1)

AlC H_{3} * + H_{2} O \to AlOH * + C H_{4}

(2)

Figure 1.

Schematic diagram of the ALD reaction chamber.

where the asterisks denote the surface species. The growth of thin films was started by the sequential adsorption of H₂O and TMA. As the surface is exposed to water vapor, hydroxyl groups will be absorbed on the surface because of the hydrophilicity. Next, the TMA reacted with the surface hydroxyl groups in a self-limiting reaction, resulting in Al₂O₃ film growth and the release of CH₄ gas. Similarly, the ZnO ALD films were deposited using alternating exposures to H₂O and diethylzinc (DEZ) as follows^14,15

\begin{matrix} ZnOH * + Zn {(C H_{2} C H_{3})}_{2} \to \\ ZnOZn (C H_{2} C H_{3}) * + C_{2} H_{6} \end{matrix}

(3)

Zn (C H_{2} C H_{3}) * + H_{2} O \to ZnOH * + C_{2} H_{6}

(4)

where the asterisks denote the surface species. Sequential adsorption of H₂O and DEZ resulted in the ZnO film growth and the release of C₂H₆ gas. The precursor and purging gas of the ALD reactor were controlled by switching valves and flowed past the substrate surface before entering into a mechanical pump. The ALD processing parameters are listed in Table 1. The substrate temperature was controlled at 60°C, and the chamber pressure was 9×10⁻³ Torr. The precursor H₂O, purging N₂ gas, precursor TMA, and purging N₂ gas were controlled to flow into the chamber in sequence. The Al₂O₃ and ZnO films sequentially grew with 10 ms of precursor H₂O, 6000 ms of N₂ purging, 6 ms precursor of TMA and DEZ, and 6000 ms of N₂ purging. The film thickness was controlled at 12.5, 25, 50, and 100 nm, where the thickness of each deposited film on the PET substrate was directly confirmed by a surface profiler (Kosaka ET4000).

Table 1.

ALD processing parameters.

Film type	Al₂O₃, ZnO
Temperature (°C)	60
Pressure (Torr)	9×10⁻³
H₂O pulse (ms)	10
N₂ pulse (ms)	6000
TMA, DEZ pulse (ms)	6
Film thickness (nm)	12.5, 25, 50, 100

ALD: atomic layer deposition; TMA: trimethyl aluminum; DEZ: diethylzinc.

Adhesion had a great influence on the performance and reliability of the ALD films. Interfacial failure was often the primary failure mechanism of the ALD films, limiting their applicability and lifetime. The conventional adhesion testing of thin films often involves the Scotch tape method, where a pressure-sensitive tape is pressed onto the film and then rapidly stripped off. This method provides qualitative information for practical applications. However, more accurate measurement methods are required. The scratch adhesion testing has been extensively used to determine the quantitative adhesion strength of coated surfaces.^16,17 In this work, the micro-scratch test was used to characterize the adhesion strength of ALD films. In the micro-scratch tests conducted using a Rhesca CSR-02A micro-scratcher, the specimen was mounted on the 3° tilt stage, which moved horizontally 5 µm/s to push up the 5 µm radius stylus on the top end of the sensor. A load was then applied to the surface of the specimen by the stiffness and strain of the cantilever stylus. The cartridge of the micro-scratch tester oscillated parallel to the specimen surface at a fixed 100-µm amplitude via the built-in actuator. The friction between the specimen and the stylus caused relative movement, resulting in the generation of electric output signals. As long as the specimen surface was not damaged, the sensor could generate output signals proportional to the stylus velocity. As the load reached the critical value, some scratching noises were generated via the destruction or rough surface of the sample film; these noises were reflected and observed as output signals.

The surface roughness, water contact angle, and water vapor transmission rate (WVTR) of the specimens were measured using an atomic force microscope (AFM, Thermo Microscopes AP-0100), a contact angle meter (Magic Droplet Model 100SL), and WVTR measurement of sheet materials (ASTM E398), respectively. The optical transmittance was measured by an UV/VIS/NIR spectrophotometer (BWTEK BTC112) with normal incidence in the visible wavelength range from 400 to 800 nm, taking the ambient air as the reference.

In this work, six types of single-oxide layers, that is, Al₂O₃/PET and ZnO/PET films, each of thicknesses of 25, 50, and 100 nm, and six types of double-oxide layers, that is, ZnO/Al₂O₃/PET and Al₂O₃/ZnO/PET films, each of thicknesses of 25, 50, and 100 nm, were considered.

Results and discussion

Figure 2 shows the scratch results of the single ZnO and Al₂O₃ films on PET substrates; the adhesion strength of the ZnO thin films was found to range from 16.9 to 37.9 mN as the film thickness increased from 25 to 100 nm, and the strength of the Al₂O₃ film was found to range from 14 to 36.6 mN when the thickness increased from 25 to 100 nm. The results indicated that the adhesion strength of the ZnO film was higher than that of the same thickness Al₂O₃ film; it also indicated that thicker films had higher adhesion strength.

Figure 2.

Scratch results of single-oxide layers.

Figure 3 shows the scratch results of the double ZnO and Al₂O₃ films on PET substrates; the adhesion strength of the double-layer structures was found to range from 30.6 to 85.6 mN. The results indicated that the strength increased as the film thickness increased. As seen by comparing Figures 2 and 3, the adhesion strength of the double-layer structure was higher than that of the single-layer.

Figure 3.

Scratch results of double-oxide layers.

The single-layer adhesion strength carried by scratch is shown in Figure 4, where the error bars indicate the smallest and highest values of five parallel measurements; the adhesion strength of ZnO/PET was found to be slightly higher than that of Al₂O₃/PET with the same thickness. Figure 5 shows the adhesion strength of double layers. Note that no obvious difference was observed between the use of Al₂O₃ or ZnO as the first layer in the double-layer structure. The adhesion strengths for all the samples are listed in Table 2.

Figure 4.

Adhesion strength of single-oxide layers. The error bars indicate the lowest and highest values of the measurement.

Figure 5.

Adhesion strength of double-oxide layers. The error bars indicate the lowest and highest values of the measurement.

Table 2.

Result summary.

Film/substrate	AS (mN)	R (nm)	WVTR (g/m²/day)	CA (°)	T (%)
PET		1.46 ± 0.20	11.08 ± 1.0	72.5 ± 2.0	88.16 ± 1.5
Al₂O₃ 25 nm/PET	14.0 ± 0.2	1.78 ± 0.21	5.42 ± 0.71	89.5 ± 3.2	85.49 ± 3.1
ZnO 25 nm/PET	16.9 ± 0.1	1.82 ± 0.30	5.38 ± 0.51	88.7 ± 2.1	85.25 ± 2.0
Al₂O₃ 50 nm/PET	19.7 ± 0.5	1.91 ± 0.33	4.93 ± 0.73	84.3 ± 1.5	83.96 ± 3.2
ZnO 50 nm/PET	21.7 ± 0.3	1.98 ± 0.15	4.64 ± 0.56	87.0 ± 2.4	84.04 ± 2.5
Al₂O₃ 100 nm/PET	36.6 ± 0.7	2.59 ± 0.33	3.71 ± 0.51	83.1 ± 3.1	80.46 ± 3.2
ZnO 100 nm/PET	37.9 ± 0.5	2.76 ± 0.15	3.54 ± 0.53	89.1 ± 4.2	80.27 ± 2.8
ZnO 12.5 nm/Al₂O₃ 12.5 nm/PET	30.6 ± 1.2	2.19 ± 0.27	3.72 ± 0.45	87.0 ± 3.2	83.04 ± 3.0
Al₂O₃ 12.5 nm/ZnO 12.5 nm/PET	32.7 ± 0.7	2.05 ± 0.32	3.56 ± 0.54	86.9 ± 2.6	83.47 ± 3.0
ZnO 25 nm/Al₂O₃ 25 nm/PET	56.4 ± 0.5	2.18 ± 0.30	3.42 ± 0.51	89.9 ± 3.2	82.43 ± 3.1
Al₂O₃ 25 nm/ZnO 25 nm/PET	55.4 ± 0.9	2.19 ± 0.30	3.39 ± 0.46	84.6 ± 3.5	82.46 ± 3.1
ZnO 50 nm/Al₂O₃ 50 nm/PET	84.4 ± 1.5	2.75 ± 0.40	3.14 ± 0.46	86.6 ± 2.1	79.89 ± 3.0
Al₂O₃ 50 nm/ ZnO 50 nm/PET	85.6 ± 1.3	2.92 ± 0.30	3.04 ± 0.33	84.3 ± 1.4	79.93 ± 2.8

AS: adhesion strength; R: roughness; WVTR: water vapor transmission rate; CA: contact angle; T: transmittance.

It is obvious that for the same film thickness, the adhesion strength of the double layer was much higher than that of the single layer. The adhesion strength of ZnO 12.5 nm/Al₂O₃ 12.5 nm/PET was 2.19 times that of Al₂O₃ 25 nm/PET, and Al₂O₃ 12.5 nm/ ZnO 12.5 nm/PET was 2.34 times that of Al₂O₃ 25 nm/PET. A similar ratio was found for the other cases. On average, the adhesion strength of the 25-nm-thick double layer was 2.07 times that of the 25-nm single layer. The adhesion strength of the 50-nm double layer was 2.64 times that of the 50-nm single layer, and the adhesion strength of the 100-nm double layer was 2.28 times that of the 100-nm single layer.

The average surface roughness of the single- and double-layer oxide films deposited on PET substrates determined using AFM are listed in Table 2. For single-oxide layers, the results indicated that the average surface roughness of 25-nm Al₂O₃/PET was 1.78 nm and increased to 2.57 nm as the film thickness increased to 100 nm. The roughness of 25-nm ZnO/PET was 1.82 nm and increased to 2.76 nm as the film thickness increased to 100 nm. These results illustrated that the roughness increased as the film thickness increased. It also indicated that the roughness of ZnO was rougher than that of Al₂O₃; this finding is consistent with the report that the low-temperature ALD-grown ZnO was conducting, crystalline, and rough and that Al₂O₃ was insulating, amorphous, and smooth.¹⁰ For double-oxide layers, the average roughness of 12.5-nm ZnO/12.5-nm Al₂O₃/PET was 2.19 nm and then increased to 2.75 nm as the film thickness increased to 50 nm ZnO/50 nm Al₂O₃/PET. It also indicated that for the same film thickness, the roughness of the single layer was lower than that of the double layer.

The WVTRs of the single and double-oxide layers deposited on PET substrates are listed in Table 2. For single-oxide layers, the WVTR of Al₂O₃/PET was found to decrease from 5.42 to 3.71 g/m²/day as the film thickness increased from 25 to 100 nm. Similarly, the WVTR of ZnO/PET decreased from 5.38 to 3.54 g/m²/day as the film thickness increased from 25 to 100 nm. The WVTR of ZnO/PET was smaller than that of Al₂O₃/PET. For double-oxide layers, the WVTR of Al₂O₃/ZnO/PET decreased from 3.56 to 3.04 g/m²/day as the film thickness increased from 25 to 100 nm. In addition, the WVTR of ZnO/Al₂O₃/PET decreased from 3.56 to 3.14 g/m²/day as the film thickness increased from 25 to 100 nm. For the same oxide film thickness, the WVTR of the double layers was better than that of the single layer. The WVTR of ZnO 12.5 nm/Al₂O₃ 12.5 nm/PET decreases by 52% relative to that of Al₂O₃ 25 nm/PET, and ZnO 50 nm/Al₂O₃ 50 nm/PET decreased by 18% compared to Al₂O₃ 100 nm/PET. On average, the WVTR of the double layer decreased 37% compared to that of the single layer.

The water contact angles of the single- and double-layer oxide films deposited on PET substrates are listed in Table 2. All the contact angles ranged from 83° to 90°, showing that both Al₂O₃ and ZnO films are between hydrophilic and hydrophobic in character. This result indicated that the contact angle of Al₂O₃ films decreased as the film thickness increased, in agreement with the result that a lower surface roughness corresponds to a higher contact angle. However, the contact angle of ZnO films only had a small change as the film thickness increased.

The transmittance values of single-oxide layers are shown in Figure 6, where the average transmittance of 25-nm-thick layers of Al₂O₃/PET and ZnO/PET were 85.49% and 85.25%, respectively. The figure indicates that the transmittance decreased with the increase in film thickness, where 100-nm-thick layers of Al₂O₃/PET and ZnO/PET have transmittance values of 80.46% and 80.27%, respectively. The transmittance decreased by approximately 5% as the film thickness increased from 25 to 100 nm. The figure also indicated that the transmittance of Al₂O₃ and ZnO thin films on the PET substrates were similar. Figure 7 shows the transmittance of double-oxide layers, where the average transmittance value of 12.5 nm ZnO/12.5 nm Al₂O₃/PET and 12.5 nm Al₂O₃/12.5 nm ZnO/PET was 83.04% and 83.47%, respectively. Similarly, the transmittance decreased as the film thickness increased. For the same film thickness, the transmittance of double layers was lower than that of single layers. On average, the transmittance of the double layer decreased by 1.38% relative to that of the single layer. The film with ZnO as the first layer was found to have higher transmittance than the film with Al₂O₃ in the middle.

Figure 6.

Transmittance of single-oxide layers.

Figure 7.

Transmittance of double-oxide layers.

All the results, that is, adhesion strength, surface roughness, WVTR, contact angle, and optical transmittance, of the various thicknesses of Al₂O₃/PET, ZnO/PET, Al₂O₃/ZnO/PET, and ZnO/Al₂O₃/PET are listed in Table 2. The characteristics of the thin-film/substrate structures were found to strongly depend on the deposition parameters, thin-film properties, and substrate bonding conditions. Table 2 provides a comparison of the various thin films for the same deposition parameters and substrate conditions. For the single-layer case, the adhesion strength and WVTR of ZnO/PET were better than those of Al₂O₃/PET. However, there was no obvious difference between the use of Al₂O₃ and the use of ZnO as the first layer in the double-layer structures. Although the roughness, contact angle, and transmittance of the double layer were slightly worse than those of the single layer, the results emphasized that the adhesion and WVTR of the double layer were much better than those of the single layer with the same thickness. The results revealed that the ALD-grown hybrid layers improved the water vapor permeability, in agreement with other studies.^4,11

Conclusion

In this article, the parameters of surface roughness, optical transmittance, adhesion, water vapor transmission rate, and contact angle of various thicknesses of ALD-grown Al₂O₃, ZnO thin films, and hybrid Al₂O₃/ZnO and ZnO/Al₂O₃ laminated thin films deposited on flexible PET substrates were investigated. The results showed that, for the same film thickness, the adhesion strength of the double layer increased 2.33 times that of the single layer. The WVTR of the double layer increased 37% compared to that of the single layer. Furthermore, the optical transmittance of the double layer only decreased by 1.38% compared to that of the single layer. In summary, the hybrid layer was better than the single layer in the prevention of water vapor penetration and the enhancement of the adhesion strength. The improvement indicates the potential of ALD-grown hybrid laminates to be the water vapor barriers used in thin-film encapsulation.

Footnotes

Academic Editor: Artde Lin

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the Ministry of Science and Technology through grant MOST 104-2221-E-129-006-MY3.

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