Moisture barrier properties of thin organic-inorganic multilayers prepared by plasma-enhanced ALD and CVD in one reactor
© Bülow et al.; licensee Springer. 2014
Received: 7 January 2014
Accepted: 11 April 2014
Published: 7 May 2014
A widely used application of the atomic layer deposition (ALD) and chemical vapour deposition (CVD) methods is the preparation of permeation barrier layers against water vapour. Especially in the field of organic electronics, these films are highly demanded as such devices are very sensitive to moisture and oxygen. In this work, multilayers of aluminium oxide (AlO x ) and plasma polymer (PP) were coated on polyethylene naphthalate substrates by plasma-enhanced ALD and plasma-enhanced CVD at 80â„ƒ in the same reactor, respectively. As precursor, trimethylaluminium was used together with oxygen radicals in order to prepare AlO x , and benzene served as precursor to deposit the PP. This hybrid structure allows the decoupling of defects between the single AlO x layers and extends the permeation path for water molecules towards the entire barrier film. Furthermore, the combination of two plasma techniques in a single reactor system enables short process times without vacuum breaks. Single aluminium oxide films by plasma-enhanced ALD were compared to thermally grown layers and showed a significantly better barrier performance. The water vapour transmission rate (WVTR) was determined by means of electrical calcium tests. For a multilayer with 3.5 dyads of 25-nm AlO x and 125-nm PP, a WVTR of 1.2 × 10 −3 gm−2d−1 at 60â„ƒ and 90% relative humidity could be observed.
The occurrence of hydrogen bubbles around the defects leads to a delamination of the electrode. The emerging hollow space furthermore accelerates the diffusion of water vapour. To suppress the described deteriorations, a reliable encapsulation of organic devices is absolutely necessary for long-term applications. In particular, OLEDs require very low permeation rates as the defects become visible as dark spots at a certain size. In the past, a water vapour transmission rate (WVTR) in the range of 10 −6 gm−2d−1 was postulated as an upper limit . This shall ensure a device lifetime of at least 10,000 operating hours. For organic solar cells, the degradation mechanisms are quite similar. However, since the local defects stay invisible as the device does not emit light, the barrier requirements can differ from that of OLEDs. In some cases, a WVTR of 10 −3 gm−2d−1 may already be sufficient .
A common way to encapsulate a device is to use a glass or metal lid, mounted with an ultraviolet-cured epoxy. Additionally, a desiccant can be used to absorb moisture which can diffuse only through the glue. However, this also implicates some drawbacks. The employment of a glass lid on a flexible OLED, for instance, is not reasonable due to the inelasticity of glass. In addition, the heat accumulation, arising from the poor thermal conductivity of glass, causes a reduced lifetime of the device . If utilised on a top-emitting OLED, which emits its light through the lid, the appearing waveguide losses reduce the external quantum efficiency without special treatments . The prementioned issues are serious reasons to replace this encapsulation approach by thin film barrier layers.
For this purpose, atomic layer deposition (ALD) turned out to be an appropriate tool for fabricating nearly defect-free thin films with excellent gas barrier properties . First and foremost, aluminium oxide (AlO x ) layers have emerged as a suitable thin film encapsulation [14, 15]. To deposit ALD films, an alternating inlet of precursors into the reactor chamber takes place. Between the single injections, the reactor is purged with an inert gas to remove redundant precursor molecules and by-products. This inhibits vapour phase reactions and allows a very homogeneous and self-limiting film growth within one reaction cycle . Additionally, plasma-enhanced atomic layer deposition (PEALD) reduces the process time at temperatures below 100â„ƒ since there is no need to remove residual water molecules. Furthermore, for AlO x , a higher growth per cycle (GPC) can be achieved compared to the thermal ALD (TALD) process. A benefit of hybrid multilayers (ML) is that the separation into several oxide layers leads to a decoupling of morphological defects, e. g. caused by particles, which prolongs the permeation path trough the barrier . A more detailed introduction into moisture barrier layers is given elsewhere .
An oxidation caused by molecular oxygen can be neglected [21–23]. Whereas pure calcium has a good electrical conductivity, Ca(O H)2 is an insulator. If a current is applied to a thin calcium film, its corrosion can easily be detected as a change of the resistance which allows an immediate calculation of the WVTR.
Since the deposition of hybrid multilayers by TALD/plasma-enhanced chemical vapour deposition (PECVD) has already been shown , in this paper, the preparation of MLs by PEALD/PECVD, carried out in one reactor, will be demonstrated. The WVTRs of moisture barrier layers were measured with electrical Ca tests. A correlation of the barrier performance of aluminium oxide layers and their impurity content will also be discussed.
In order to determine the WVTR, the thin film of interest was coated on a 200- μ m-thick polyethylene naphthalate substrate (Teonex Q65, DuPont Teijin Films, Luxembourg) with a size of 25 × 25 mm2. The polymer foils were cleaned before with acetone, isopropanol and ultrasonic treatments. Prior to deposition, the substrates were stored in the reactor for 72 h at 120â„ƒ to remove residual water in the polymer.
The AlO x and the plasma polymer (PP) films were deposited in a newly developed plasma system from SENTECH Instruments (patent pending), placed in an ISO class 6 clean room environment. The system was developed and designed for both inductively coupled plasma-enhanced chemical vapour deposition (ICPECVD) and ALD in the same reactor using flexible system architecture. The used plasma source is an inductively coupled planar triple spiral antenna (ICP PTSA 200). A high radio-frequency current flows from the centre through the three arms to the periphery and induces the electric field for generating the high-density plasma . Besides the deposition of AlO x and PP layers, the system enables ICPECVD of high-quality oxides and nitride films at low temperatures (80â„ƒ to 130â„ƒ) on different substrate types and sizes. This combination allows the deposition of layer stacks from ALD with low growth rate and ICPECVD with high growth rate in the same chamber.
Reactor walls as well as the substrates were heated to 80â„ƒ, and nitrogen (40 sccm) was applied as carrier and purge gas for trimethylaluminium (TMA) and benzene. The process pressure for the coating of AlO x and PP was 12 and 3 Pa, respectively. During the AlO x process, the oxygen flow was set to 150 sccm. One AlO x deposition cycle included the following steps: 10-s plasma pulse (400 W), 1-s purge time, 0.08-s TMA pulse time and 20-s purge time. The recipe for the PP worked as follows: 0.02-s benzene pulse time, instantly followed by 4-s plasma pulse (200 W) and 6-s purge time. In order to improve the smoothness of PP films, a mass flow of 40 sccm argon was applied. PP-benzene as spacer layer was chosen simply because it allows a rapid film growth. Because of the high vapour pressure of benzene, neither active bubbling nor heating is necessary. One ML dyad is composed of 25-nm PEALD aluminium oxide, which is deposited at first, and 125-nm PECVD PP. x.5 dyads means that the ML is covered with 25-nm PEALD AlO x on top. The precursor containers for TMA and benzene were kept at room temperature.
The factor 2 takes into account that water is the only species in our setup Ca reacts with . k includes the fact that the Ca sensor overlaps the electrodes a little. These areas absorb humidity, but their corrosion does not affect the measured voltage. A is the area of the aperture, given by the glass lid, and l is the length as well as the width of the Ca sensor. M is the molar mass of calcium and water, and δ and ρ are the density and conductivity of calcium, respectively. R is the resistance of the sensor. The additional impact of the PEN and Ag electrodes on the total WVTR is insignificant and therefore neglected in the calculation. The resulting steady-state WVTRs were composed of the average of four samples. To accelerate the measurement, the tests were performed in a climate cabinet (Binder KBF 115, BINDER GmbH, Tuttlingen, Germany) at 60â„ƒand 90% relative humidity (RH). These conditions naturally lead to higher permeation rates than measurements at room temperature.
The carbon (C) content of different AlO x layers was detected with energy-dispersive X-ray spectroscopy (JEOL JSM 6400, JEOL Ltd., Tokyo, Japan) at a beam energy of 7 kV. In order to control the growth per cycle, the total thickness as well as the refractive index of the films, deposited on silicon substrates with native oxide, was measured with spectroscopic ellipsometry (GES5, Semilab Semiconductor Physics Laboratory Co. Ltd., Budapest, Hungary) and then divided by the number of process cycles. The surface roughness was determined by atomic force microscopy (AFM) with a DME DualScope DS 45-40 (Danish Micro Engineering A/S DME, Herlev, Denmark).
Results and discussion
WVTRs with mean deviation of several AlO x /PP multilayers and single AlO x films, measured at 60â„ƒ and 90% RH
(6 ± 2) × 10 −4
(1.2 ± 0.7) × 10 −3
(2 ± 0.9) × 10 −3
(3.6 ± 1.3) × 10 −3
50-nm PEALD aluminium oxide (400 W, 10 s)
(4.4 ± 0.8) × 10 −3
50-nm PEALD aluminium oxide (100 W, 1 s)
(8.5 ± 2.4) × 10 −3
50-nm TALD aluminium oxide
(7.7 ±2.3) × 10 −3
WVTRs with mean deviation of TALD aluminium oxide films with layer thicknesses from 25 to 100 nm, measured at 60â„ƒ and 90% RH
(8.5 ± 2.2) × 10 −2
(7.7 ± 2.3) × 10 −3
(6.4 ±1.2) × 10 −3
Carbon content and refractive index at 633 nm of aluminium oxide films at different process conditions, deposited at 80â„ƒ
Plasma power [W]
Plasma pulse time [s]
A combination of a PEALD and PECVD process in one reactor chamber was demonstrated in order to accelerate the fabrication of thin moisture barrier layers with a high film quality. For hybrid multilayers of 3.5 dyads, a steady-state WVTR of 1.2 × 10 −3 gm−2d−1 at 60â„ƒ and 90% RH could be achieved, which is nearby the value of a glass lid encapsulation. At optimised process conditions, a single PEALD aluminium oxide layer revealed a better barrier performance than a thermally grown one, which is probably associated with a lower incorporation of hydrocarbons and hydroxyl groups, respectively.
This work was funded by the Federal Ministry of Economics and Technology, Germany. Support code: KF 200 5003 CK9. The polyethylene naphthalate substrates were kindly provided by DuPont Teijin Films.
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