Article pubs.acs.org/JPCC
Optimization of Al2O3 Films Deposited by ALD at Low Temperatures for OLED Encapsulation Yang Yong-Qiang and Duan Yu* State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Jilin 130012, China ABSTRACT: The properties of Al2O3 encapsulation layers prepared by atomic layer deposition were investigated as a function of film thickness. By using a short pumping gas time (PGT) and a low viscosity gas, highquality films for the encapsulation of organic light-emitting diodes were obtained. The encapsulation properties could be improved by decreasing the PGT and using a low viscosity gas as the precursor at a low temperature. The water vapor transmission rate of the film could reach up to 4 × 10−3g/m2/day. In addition, atomic force microscopy images and investigations of the refractive index showed that the film properties did not change substantially with increasing thickness. Furthermore, by optimizing the PGT, the total deposition time could be effectively reduced.
was explained by the viscosity of the fluid which increased the required PGT.12 These depositions performed at a low temperature increased the redundant precursors, which has a negative effect on the barrier performance of the encapsulated film. To minimize this effect, the PGT should be prolonged to a certain extent. Most of the previously reported studies on encapsulation by ALD focused on the thickness or type of material. Only a few reports have been published concerning the parameters of the ALD process.13 In particular, very little is known regarding the influence of PGT during the ALD process. Therefore, we investigated the influence of PGT and viscosity of the precursor gas on the film properties.14 This study describes how the properties of ultrathin Al2O3 film can be significantly improved by varying the PGT at a low temperature of only 80 °C. On the basis of the mass transport model for surface reactions, the thickness of the films was increased by reducing the PGT of the precursor gases and replacing H2O with the low viscosity O3. The surface roughness measured by atomic force microscopy (AFM) and the refractive index of the films deposited with a shorter PGT at a low temperature were identical to the corresponding values of films deposited by the conventional process at higher temperatures. Furthermore, the ultrathin films showed perfect encapsulation properties, and a low WVTR of 4 × 10−3 g/m2/day, tested at ambient conditions of 25 °C and 80% relative humidity (RH), could be achieved. Furthermore, by reducing the PGT, the total time of the encapsulation process could be decreased, improving the efficiency of the production process.
O
rganic light-emitting diodes (OLEDs) have attracted much attention due to their potential application in flat panel lightings and flat panel displays because of their high efficiency, fast response time, and wide viewing angle. But the highest value lies in their potential to fabricate flexible devices.1−5 However, the cathode of OLEDs may by damaged by water vapor and oxygen, and this may lead to the degradation of the devices.6 For a stable device operation, a water vapor transmission rate (WVTR) on the order of 10−6 g/ m2/day is mandatory.7 In order to obtain long lifetime OLEDs, various encapsulation technologies have been applied to protect the OLEDs. Among them, thin-film encapsulation (TFE) is considered to be indispensable for the fabrication of future flexible electronic devices. Comparing different TFE techniques, films deposited by atomic layer deposition (ALD) have been shown to provide superior protection from moisture degradation of organic devices. Recently, several research studies have focused on improving the encapsulation properties of Al2O3 films by using ZrO2/Al2O3 nanolaminates and changing the precursor gas to a gas with higher activation energy for ligand elimination than H2O.8 All of these studies were done at comparatively low temperatures to effectively increase the lifetime of OLEDs without having a significant impact on the properties of the devices.9 However, the reported temperature of these ALD deposition processes was still in the range from 180 to 400 °C.10 Since ALD is a deposition technique consisting of a series of self-limiting, surfacesaturated reactions, the lower temperature was expected to influence the parameters of the deposition process. The amount of absorbed precursor gas in the ALD reactor usually was the limiting factor for a fast film fabrication, and more pumping gas time (PGT) was required for purging the excess H2O.11 By utilizing computational fluid dynamics to calculate the amount of species absorbed on the surface of substrate, this behavior © 2014 American Chemical Society
Received: June 16, 2014 Revised: July 17, 2014 Published: July 20, 2014 18783
dx.doi.org/10.1021/jp505974j | J. Phys. Chem. C 2014, 118, 18783−18787
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For this study, Al2O3 ultrathin films were grown in a 12 in. ALD warm wall reactor (Ensure Nanotech Inc.) in a fabrication process consisting of 550 cycles at temperatures of 80 and 200 °C, respectively, using H2O, trimethylaluminum (TMA), and O3 as precursor gases. The O3 was produced by an ozone generator prior to the ALD process by mixing oxygen (400 sccm, 99.999%) and catalytic nitrogen (20 sccm) to generate a gas containing approximately 3.5 wt % O3 at a concentration of approximately 50 mg/L. High-purity N2 was used as carrier gas for the precursors, flowing from the reactant lines through the flow tube into the mechanical exhaust vacuum pump (Oerlikon Leybold D8C). The pumping speed was set to 2.25 L/s, and the PGT was varied by changing the settings of the pump. To determine the refractive index and the root-mean-square (RMS) of the surface roughness of the Al2O 3 films, corresponding Al2O3 test samples were prepared on precision-polished Si substrates (RMS: 0.6 ± 0.1 Å). The thickness and refractive index of the deposited Al2O3 were determined by J. A. Woollam M-2000VI variable-angle spectroscopic ellipsometer. The surface roughness of the test samples was measured by AFM (Vecco Dimension Icon). Due to a very smooth surface, the influence of the substrate on the surface roughness of the films could be neglected. ALD is a self-limiting vapor deposition process. The deposition process and corresponding experimental models have been described in many studies.8,10,12 In order to avoid the occurrence of chemical vapor deposition (CVD), the sample exposure to the different precursor gases is separated by inert gas purging. When the inert gas pressure is sufficiently high (>10 Pa), the gas will be in viscous flow.16 As shown in Figure 1, the precursors were alternately fed into the reactor. The
Figure 2. Illustration of the purging of TMA with N2.
surface, the influence of viscosity on the gas speed decreases. Then the purging N2 gas would carry the reaction products away, together with the redundant precursors on the surface of the substrate. The ALD process requires vacuum conditions in general. In our experiments, the base pressure inside the reaction chamber was approximately 2 Pa but changed with the amount of gas inserted into the chamber, as shown in Figure 3.
Figure 3. Chamber pressure measured for the different selected PGTs: (a) H2O and TMA purging process at 80 and 200 °C, (b) O3 purging process at 80 °C.
Figure 1. Illustration of the Al2O3 ALD deposition process (a) using H2O as the precursor and (b) using O3 as the precursor.
Depending on the pulse length, a certain amount of gas was let into the chamber, increasing the pressure. Due to the pump, the pressure would then decrease again. However, the pumping process was different for each precursor. At a high reaction chamber temperature of 200 °C, the thermal motion of H2O is relatively high, so that only a short PGT was needed to pump the precursor, as shown in Figure 3a. When the temperature was lowered to 80 °C, the H2O molecules are easier to absorb on the surface of the substrate due to hydrogen bonding, and thus the PGT increased to 20 s. Because of their lower viscosity, the PGT did not show this temperature sensitivity for TMA and O3. The optimal PGT for TMA and O3 were 5 s and 7 s, respectively.16 The process of O3 exposure is shown in Figure 2.
redundant precursors and reaction products were swept by N2 during the purging phase, with the duration of this phase depending on the viscosity of the precursor gas. Generally, the viscosity of a gas will increase with decreasing temperature.15 In addition, the distribution of the gas speed is not uniform, as shown in Figure 2. Focusing on a relatively thin space above the substrate surface, the gas speed near the surface of the substrate is almost the same for different gas viscosities but increases with increasing distance to the top surface.17 In general, more than one layer of molecules will usually remain near the surface for a relatively long time.18 However, further away from the substrate 18784
dx.doi.org/10.1021/jp505974j | J. Phys. Chem. C 2014, 118, 18783−18787
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Table 1. Comparison of the Thickness and Roughness of the Al2O3 Films Deposited by ALD Using Different Oxide Precursor Gases and Process Parametersa thickness (nm) roughness (nm)
film A
film B
film C
film D
film E
film F
71.727 ± 0.415 0.168 ± 0.041
70.246 ± 0.352 0.309 ± 0.073
null 0.427 ± 0.091
71.435 ± 0.766 0.132 ± 0.044
70.193 ± 0.485 0.154 ± 0.039
100.537 ± 1.071 0.264 ± 0.062
a Film A: H2O as precursor with PGT of 10 s at 200 °C. Film B: H2O as precursor with PGT of 30 s at 80 °C. Film C: H2O as precursor with PGT of 20 s at 80 °C. Film D: O3 as precursor with PGT of 10 s at 200 °C. Film E: O3 as precursor with PGT of 10 s at 80 °C. Film F: O3 as precursor with PGT of 3 s at 80 °C.
Films were deposited under different conditions, and their properties were tested. At first, films were deposited using the commonly reported PGTs, i.e., 10 s at 200 °C and 30 s at 80 °C when using H2O as oxidant. When using O3, the PGT was set to 10 s at 200 °C and also 10 s at 80 °C, respectively. The PGT for TMA was set to 10 s. In addition, films with a shorter PGT were deposited at the low temperature of 80 °C. As described above, a longer PGT could more effectively purge the precursor in the cross-flow ALD reactor. The thickness of the resulting Al2O3 films was measured using an ellipsometer. As shown in Table 1, the thickness of film B deposited using H2O at 80 °C and a long PGT was the same as the thickness of film A. However, when the PGT was changed to 20 s, the homogeneity of the resulting Al2O3 film was decreased. The H2O could more easily interact because of hydrogen bonding and could not react with TMA in the form of layer by layer. This process decreased the homogeneity of the film. Due to the irregularity of the sample surface, the thickness of film C is not listed in Table 1. When the PGT for the removal of H2O was decreased to 10 s, no film was formed on the substrate. For the films E and F deposited using O3 as the precursor, the thickness increased with decreasing PGT. The larger thickness of the film was caused by more than one layer of molecules becoming involved in the reaction, and thus, the deposition rate increased. The difference between H2O and O3 is that the O3 molecules would enter the reaction individually as a single molecule instead of interacting and initiating a CVD process. However, when the PGT was less than 2 s, the mass precursor could not be purged away sufficiently, so that little Al2O3 deposited on the substrate. In addition, the refractive index of the resulting Al2O3 films was measured. The mean square error (MSE) of each data fitting is less than 2, indicating that the calculated refractive indexes for the films deposited at each condition, respectively, were in good agreement with the experimental data. This suggested that the Cauchy model is reliable. Furthermore, the refractive index depends on the structure of the film.19 As shown in Figure 4, there are obvious differences when using H2O or O3, which have been discussed in our previous publication. When using H2O, the byproduct methane cannot be effectively purged at low temperatures, resulting in incomplete chain-reactions and an increasing number of voids. This also leads to the growth of pinholes in the films deposited using H2O.20 As shown in Figure 4, there was no obvious difference of the refractive index between the films with different thickness when O3 was used as the precursor, indicating that the structure of the films deposited using O3 was the same, and it can be claimed that the film intrinsic properties did not change with increasing thickness. For further analysis, the surface of corresponding test samples deposited on clean silicon (Si) substrates was investigated by AFM. As shown in Figure 5a−c, the homogeneity of the Al2O3 films deposited on the Si substrate using H2O as precursor gas decreased with
Figure 4. Comparison of the calculated refraction properties with experimental data. (a) Calculated and experimental data for the H2Obased Al2O3 ALD with a PGT of 10 s at 200 °C. (b) Calculated and experimental data for the O3-based Al2O3 ALD with a PGT of 3 s at 80 °C. (c) Measured refractive index for H2O-based Al2O3 ALD with a PGT of 10 s at 200 °C and 30 s at 80 °C, respectively, and for O3‑based Al2O3 ALD with a PGT of 10 s at 200 °C, and 10 and 3 s at 80 °C, respectively.
Figure 5. Three-dimensional atomic force microscopy (AFM) images of Al2O3 thin films deposited on a Si substrate: (a) H2O-based ALD with a PGT of 10 s at 200 °C; (b) H2O-based ALD with a PGT of 30 s at 80 °C; (c) H2O-based ALD with a PGT of 20 s at 80 °C; (d) O3based ALD with a PGT of 10 s at 200 °C; (e) O3-based ALD with a PGT of 10 s at 80 °C; (f) O3‑based ALD with a PGT of 3 s at 80 °C.
decreasing temperature and for shorter PGTs. As demonstrated by Figure 5c, granular products could be observed on the 3D AFM images, which were caused by a CVD process, as mentioned above. However, the homogeneity of the films deposited using O3 did not change significantly with increasing film thickness and a reduction of temperature, as shown in Figure 5d−f. In addition, the quality of the encapsulation was tested by calculating the WVTR of the films.21 The WVTR was 18785
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determined by the electrical Ca test using a 200 nm-thick Ca layer with an area of 1 × 1 cm2. For the test, the Ca layer was deposited on a glass substrate, and the Ca was connected to two aluminum electrical leads. This measurement was performed using an Agilent 2920 source meter. As shown in Figure 6, the WVTR of the Al2O3 film fabricated by using O3 as
Figure 7. Plot of the normalized luminance vs time obtained by tests performed at 25 °C and a relative humidity of 80% on bare OLEDs and OLEDs encapsulated with an Al2O3 thin film deposited by ALD at 80 °C using different PGTs and either H2O or O3 as oxide precursor gas.
showed a slightly increased lifetime of up to 38 h, which may be attributed to the increased thickness of TFE.
Figure 6. Plot of the normalized change in electrical conductance vs time obtained by Ca corrosion tests performed at 25 °C and a relative humidity of 80% on Al2O3 thin films deposited by ALD at 80 and 200 °C, respectively, using different PGTs and either H2O or O3 as oxide precursor gas.
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CONCLUSION In summary, we demonstrate that decreasing the PGT can effectively improve the properties of Al2O3 films deposited by ALD at a low temperature of 80 °C when using a low viscosity gas such as O3. The films deposited at a shorter PGT possess a low surface roughness, a larger thickness, and a low WVTR. The observed properties are very similar to the properties of films deposited at high temperatures. Furthermore, by decreasing the PGT, the overall deposition time was reduced ensuring that less damage would be caused to the OLED devices by the high chamber temperature, thereby increasing the efficiency of the encapsulation process. OLEDs encapsulated with the optimized ultrathin Al2O3 films showed an increased lifetime. Each ALD process is unique, in the future, we attempt to fabricate TFE using various materials. This study of PGT represents a critical first step in the realization of fast and low temperature ALD.
the precursor increased with decreasing PGT. The value of WVTR decreased from 1.4 × 10−2g/m2/day to 4 × 10−3g/m2/ day. We suggest that the improvement of the WVTR can be attributed to the larger thickness of the films. However, a slight difference of the WVTR could be observed between the films deposited at low temperature and at high temperature. These results also imply that a PGT of 3 s is suitable for ALD when using O3 as the precursor gas. The characteristic properties of OLEDs encapsulated with Al2O3 by ALD were compared to the corresponding properties of bare devices. The device setup was prepared as follows: On top of the ITO, a 2 nm thick MoO3 layer, a 30 nm thick 4,4′,4″-tris(N-3(3-methylphenyl)-Nphenylamino) triphenylamine (m-MTDATA) as a hole injection layer, and a 20 nm N,N′-bis(1-naphthl)-diphenyl1,1′-diphenyl-4,4′-diamine (NPB) as the hole transport layer were deposited. A 30 nm tris(8-quinolinolato) aluminum (Alq3) layer with 1 wt % 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino-[9,9a-1gh] coumarin (C545T) was used as the light-emitting layer, and 20 nm of Alq3 and 0.5 nm of LiF were deposited as the electron transport and as the electron injection layer, respectively. To verify the quality of the encapsulation layers, lifetime tests were performed in constant-voltage mode at a starting luminance of 1000 cd/m2, as shown in Figure 7. The luminance was measured in air using a Minolta luminance meter LS-110. For this study, the lifetime is defined as the decay time of the luminance to L/L0 = 0.5; i.e., the time elapsed until the instantaneous luminance of the OLEDs only reached 50% of its initial value.22 As shown in Figure 7, the luminance of the bare device measured in air degraded more rapidly than the luminance of the devices with the encapsulation, indicating that the degradation was caused by the permeation of oxygen and water vapor. Compared to the films deposited using H2O as the precursor with a PGT of 30 s, and using O3 as the precursor with a PGT of 10 s at a low temperature of 80 °C, the OLEDs sealed with Al2O3 films deposited with a PGT of 3 s
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AUTHOR INFORMATION
Corresponding Author
*Office phone: 86 + 0431-85168243-8217. Mobile phone: 13756531922. Fax: 86 + 0431-85168270. E-mail:duanyu@mail. jlu.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Program of International Science and Technology Cooperation (2014DFG12390), National High Technology Research and Development Program of China (Grant 2011AA03A110), Ministry of Science and Technology of China (Grant 2010CB327701, 2013CB834802), National Natural Science Foundation of China (Grants 61275024, 61274002, 61275033, 61377206, and 41001302), Scientific and Technological Developing Scheme of Jilin Province (Grants 20140101204JC, 20130206020GX, 20140520071JH), Scientific and Technological Developing Scheme of Changchun (Grant 13GH02), and Opened Fund of the State Key Laboratory on Integrated Optoelectronics no. IOSKL2012KF01. 18786
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(20) Yang, Y. Q.; Duan, Y.; Chen, P.; Sun, F. B.; Duan, Y. H.; Wang, X.; Yang, D. Realization of Thin Film Encapsulation by Atomic Layer Deposition of Al2O3 at Low Temperature. J. Phys. Chem. C 2013, 117, 20308−20312. (21) Paetzold, R.; Winnacker, A.; Henseler, D.; Cesari, V.; Heuser, K. Permeation rate measurements by electrical analysis of calcium corrosion. Rev. Sci. Instrum. 2003, 74, 5147−5150. (22) Popovic, Z. D.; Aziz, H.; Hu, N. X.; Hor, A. M.; Xu, G. Longterm degradation mechanism of tris(8-hydroxyquinoline) aluminumbased organic light-emitting devices. Synth. Met. 2000, 111, 229−232.
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