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Organic/inorganic multilayer nano-thin-film, due to its flexibility, high moisture ... with a capability of water vapour transmission rate (WVTR) of 10-4g/m2/day, ...
Proceedings of the 4th International Conference on Nanomanufacturing (nanoMan2014) 8 – 10 July, 2014,Germany

Multilayer nano-thin-film encapsulation for flexible and printable electronics Zhen Shi1,2, Jianjun Zhang1, Mengmeng Deng1,2, Feng Li1* 1

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China

2

Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China *Corresponding author. (email: [email protected])

ABSTRACT: Flexible and printable electronics manufacturing use organic materials which are very moisture and oxygen sensitive and, therefore, require high performance encapsulation technique to obtain devices with long term stability and reliability. Organic/inorganic multilayer nano-thin-film, due to its flexibility, high moisture barrier property, and good transparency, is a potential enabling encapsulation technology. Here, we report our development of an ICP-CVD system and its high density low temperature thin film deposition. Using this system, flexible and transparent organic/inorganic multilayer nano-thin-film was deposited on PET substrates with a capability of water vapour transmission rate (WVTR) of 10-4g/m2/day, indicating this encapsulation technology is viable for flexible and printable electronics industrial applications.

Keywords: nano-thin-film, encapsulation, ICP-CVD, WVTR

1

Introduction

Today novel semiconductor devices and optoelectronics are rapidly developing in research and commercial fields. Flexible and printable electronics such as mobile devices, electronic papers, wearable and foldable displays manufactured on flexible plastic substrates have attracted great interest recently [1]. In particular, organic light emitting devices (OLEDs) with advantages of low power consumption, low cost, and superior viewing ability have been demonstrated. These devices can not only be built on glass substrate [2, 3], but can also be built on plastic substrate [4-6] using flexible molecule [7] and polymer [5] materials. For OLEDs fabrication, high performance encapsulation is the key to ensure the devices long term stability and reliability. OLEDs based on glass substrates utilize a glass lid and perimeter epoxy seal for encapsulation and their lifetime can reach up to thousands of hours [3]. However, it is difficult to get long-lasting lifetime for plastic substrate based OLEDs due to the permeation of moisture and oxygen through the plastic substrate causing metal cathode oxidation and chemical changes in organic layers, shorting the OLEDs lifetime [8-9]. To overcome the issue, thin film barrier coating or thin film encapsulation was proposed. Thin film barrier coating or encapsulation is a potential technique for flexible electronics manufacturing. However, a deposited single inorganic layer of silicon dioxide (SiO2), silicon oxide (SiOx), or silicon nitride (SiNx) thin film usually has pinholes and micro channels, which result in moisture and oxygen permeation

through the inorganic thin film and damage the organic devices. Multilayer thin films encapsulation uses a structure of two or more pairs of organic/inorganic thin films for devices encapsulation. This organic/inorganic structure greatly reduces the through pinholes and micro channels in the multilayer, deters moisture and oxygen permeation, and provides better encapsulation for the organic devices. Moreover, the thickness of the inorganic thin films in the organic/inorganic structure is less than 200 nm which makes the inorganic thin films more flexible and less stressed and, therefore, makes the multilayer thin film encapsulation maintain flexible with the flexible electronics. Silicon dioxide (SiO2) or silicon oxide (SiOx) film produced by chemical vapor deposition (CVD) has been used as barrier films for food packaging [10]. Compared with single layer inorganic silicon dioxide film, organic/inorganic multilayer nanothin-film by plasma enhanced chemical vapor deposition (PECVD) has various advantages. PECVD process temperature is lower than CVD process temperature, and hence the polymer substrate and organic materials can be protected from high temperatures. Furthermore, toxic and explosive precursor Silane (SiH4) of CVD is replaced by Hexamethyldisiloxan (HMDSO) or Tetraethoxysilane (TEOS) organosilicon precursors with chemical stability. The PECVD film’s properties can be tuned from organic (polymer-like) to the inorganic (silica-like) by varying the ratio of organosilicon precursors/O2 in the plasma [11]. Traditional low-density plasma deposited thin films are porous, resulting in a low barrier property. To overcome this problem, high-density plasma

Proceedings of the 4th International Conference on Nanomanufacturing (nanoMan2014) 8 – 10 July, 2014,Germany chemical vapor deposition technique with different plasma sources, such as electron cyclotron resonance (ECR), helicon resonator, and inductively coupled plasma (ICP) were developed [12]. By using ICP-CVD technique, high quality films can be deposited with high density plasma at low pressures and low temperatures (as low as room temperature). These advantages of ICP-CVD result in minimizing film contamination and promoting film stoichiometry. In this study, we developed an ICP-CVD system for thin film barrier and encapsulation. We used HMDSO, which has the advantages of being Fig. 1b Working schematic of the ICP-CVD nontoxic and higher vapor pressure, as precursor to deposit organic/inorganic films on flexible plastic 3 Materials and Methods substrates. This work focuses on the development of high performance nano-thin-film barrier coating, The organic/inorganic multilayer nano-thinincluding examining the effect of deposition films were deposited onto 125 μm thick parameters on the deposition rate, the chemical polyethylene terephthalate (PET) and (400±10) μm bonding, chemical composition and surface thick 4-inch silicon substrates by the ICP-CVD morphology to optimize the barrier coating system, respectively. HMDSO was used as structure. precursor and heated to 60 °C with a gas line, then

2

Equipment

Fig. 1a shows the ICP-CVD system which we developed for organic/inorganic multilayer nanothin-film encapsulation and Fig. 1b shows the system working schematic. During the deposition process, the device substrate was placed on the wafer chuck in the chamber and a mask was fixed to control the encapsulation area. The inorganic layer, polymer layer and the graded composition layer were deposited by using the HMDSO precursor under different plasma conditions, and the reagent gases were O2, N2, and Ar for SiOx, SiNx, and polymer layer depositions, respectively. The system equipped an inductively coupled plasma (ICP) power source for high density plasma generation and a RF power source for chuck DC bias, respectively. Both of the power sources were operating at 13.56 MHz. The process temperature can be controlled in a range of 50 to 300 °C by controlling the temperature of the chuck through cooling or heating.

flowed into the chamber through side nozzles, while other reagent gases flowed into the chamber through the top shower head. Inorganic SiOx films were deposited using HMDSO/O2/Ar mixture, while organic layers were deposited using HMDSO/Ar mixture. The ICP power ranged from 0 to 800 W and the RF bias power ranged from 0 to 200 W. The silicon substrates were cleaned with isopropyl alcohol and acetone in an ultrasonic washer for 5 min respectively and then washed with DI water and dried before film deposition. The film’s thickness was measured using a Spectroscopic Ellipsometer (J. A. Woollam MD2000D, USA). The chemical bonding status was characterized using a Fourier Transform Infrared Spectrometer (Thermo Scientific NICOLET 6700, USA). The film’s chemical composition was investigated on silicon substrates by X-ray photoelectron spectroscopy (Kratos AXIS Ultra DLD, Kratos Analytical Ltd., UK) using Mg Kα radiation as the X-ray source. The surface morphology and root-mean-square surface roughness (Rq) were measured using a Tappingmode atomic force microscopy (AFM; Veeco Dimension 3100, USA). The water vapor transmission rate (WVTR) was measured using MOCON “Aquatran 2” instrument at a temperature of 37.8 °C with a relative humidity of 100%.

4

Results and Discussion

4.1 Effect of ICP power on deposition rate

Fig. 1a ICP-CVD system for organic/inorganic multilayer nano-thin-film encapsulation

In this study, we investigated the effect of ICP power on the SiOx film deposition rate. The deposition rate was observed while varying ICP power from 200 W to 800 W and fixing RF power at 100 W; other conditions such as temperature, pressure, etc. were fixed.

Proceedings of the 4th International Conference on Nanomanufacturing (nanoMan2014) 8 – 10 July, 2014,Germany Fig. 2 shows the deposition rate decreases 4.3 XPS analysis with increasing ICP power at a fixed RF power. This decrease should be attributed to the The XPS spectra shown in Fig. 4 correspond enhancement of plasma etching. The plasma to (a) organic film and (b) SiOx film. The spectra density increased with the increasing ICP power, indicate that the films are mainly composed of Si, while the RF power provided enough energy for C, O and N, the relative atomic concentration is the particles. Thus, more particles bombarded the shown in Table 1. In organic film, there is a high C film with etching effect, leading to the deposition concentration and a relatively low O concentration. rate decreasing. When more oxygen is added into the reagent gas, the precursor is further dissociated, reacting with the oxygen to generate more proportion SiOx in the film. Simultaneously, the carbon reacts with the oxygen, reducing the carbon concentration and generating more gas (e.g. CO, CO2, etc) which is pumped out of the chamber. We can conclude that a silica-like film with good barrier properties can be obtained at a high oxygen concentration. These results are also consistent with the FTIR analysis.

Fig. 2 Deposition rate as a function of ICP power 4.2 Fourier transform infrared spectrometer analysis Here, we studied the effect of ICP power on the film microstructure. The FTIR spectra of SiOx films obtained under different ICP power is shown in Fig. 3. The absorbance peaks located at approximately 1260cm-1, 1070cm-1 and 805cm-1 emerged from the Si-(CH3)3 rocking mode, Si-O-Si stretching mode and Si-O-Si bending mode, respectively. We can also see that Si-O stretching is the main vibration mode among the films, which indicates that the films are mainly composed of inorganic SiOx with Si-containing molecules, such as Si-(CH3)3. The intensity of Si-O-Si bonds increased with the increasing ICP power, making the film more silica-like with better barrier properties. At lower ICP power, the precursor structure was better preserved due to incomplete dissociation, resulting in a polymer-like film. Thus, a high ICP power was used to obtain inorganic layers as barrier films.

Fig. 3 FTIR spectra as a function of the ICP power

Fig. 4 The XPS spectra of (a) organic film and (b) SiOx film Table 1 The atomic concentration of organic film and SiOx film Sample Si (at.%) C (at.%) O (at.%)

N(at.%)

Organic

32.47

50.98

15.27

0.49

SiOx

36.96

7.21

54.99

0.83

4.4 AFM surface morphology It is known that RF power has a great influence on the DC bias and film density [13]. In this study we also examined the effect of different RF powers on the surface morphologies, since they are deeply correlated with the coating barrier properties. Fig. 5a-c illustrates the surface morphologies of single organic film on silicon substrates. The different films roughness of 0.99 nm, 0.78 nm and 0.89 nm were obtained under different RF power, respectively. All of the films were smooth and there was no significant difference in the surface roughness. However, we observed that when a RF power of 150W was used, many holes were produced on the film surface (see Fig. 5c) due to the higher energy plasma radicals bombarding the substrates under higher RF power applied. The damage would provide permeation paths for water vapour to diffuse during WVTR test, causing the poor barrier properties. Thus, although increasing RF power could increase the

Proceedings of the 4th International Conference on Nanomanufacturing (nanoMan2014) 8 – 10 July, 2014,Germany film density, too high RF power can result in a thick SiOx layer. More experiments should be done damaged barrier film. with this turbo pump-contained vacuum system and promising results should be gained in the future.

Fig. 5 Three-dimensional AFM images of organic films under different RF power 4.5 WVTR analysis For WVTR analysis, we measured the WVTR for varying pairs of encapsulation structures and examined the effect of using a turbo pump on the WVTR. To measure the WVTR for varying pairs of encapsulation structures, we first optimized the deposition parameters with HMDSO/Ar = 10/20 (sccm), ICP/RF = 0/200 (W) and SiOx films at HMDSO/Ar/O2 = 10/10/100 (sccm), ICP/RF = 800/100 (W). Then we deposited different pairs of encapsulation structures (organic/inorganic) and studied the relationship between dyad units and WVTR. As shown in Fig. 6, the WVTR decreased from 0.663 g/m2/day with single pair to 0.016 g/m2/day with 3 pairs, while the WVTR increased to 0.067 g/m2/day with 4 pairs. It is known that water vapour permeates through the inorganic and organic films via either micro-size defects such as micro channels or nano-size defects such as grain boundaries, so when the number of the coating pairs increases, the micro channels can be reduced and the permeation paths become longer, thus the water permeation lag time increases and the WVTR decreases. However, when 4 pairs of barrier structure are fabricated on the PET substrate, the substrate becomes warped due to the high internal stress of the total structure and some defects might be produced, leading to an increase in WVTR.

Fig. 6 WVTR of the PET substrate coated with different pairs of barrier structure After adding a turbo pump to the vacuum system, the lowest WVTR 3.66×10-4 g/m2/day (correspond to 10-5 g/m2/day at atmospheric environment) was achieved with just single pair barrier structure (as shown in Fig. 7). Here, we deposited 800 nm thick organic layer and 150 nm

Fig. 7 The lowest WVTR with single pair barrier structure

5

Conclusion

We have developed an ICP-CVD thin film deposition system, with which organic/inorganic multilayer nano-thin-film was deposited onto the flexible PET substrate as a barrier using HMDSO precursor. The system can provide simple and efficient single-chamber processes of thin film encapsulation for flexible electronics. We studied the effect of ICP power on deposition rate and found that the deposition rate decreased with increasing ICP power. In the FTIR analysis, we found that high ICP power could make the film more silica-like. In the XPS analysis, we found that increased oxygen concentration could change the film property and chemical composition, obtaining a silica-like film at high oxygen concentrations. Through AFM surface morphology analysis, we found that the deposited films could be damaged by high RF power, limiting the maximum RF power. Finally, as part of WVTR analysis, we found that we could achieve the lowest WVTR of 0.016 g/m2/day by coating with 3-pairs of barrier structure without using turbo pump for vacuum. After adding a turbo pump into the vacuum system, the lowest WVTR of 3.66×10-4 g/m2/day with a single-pair coating was achieved, indicating that the better vacuum could generate the better barrier. Further work on multilayer nano-thin-film barrier and device encapsulation are currently under investigation. It is expected that the ICPCVD system can be a great potential tool for flexible and printable electronics industrial applications.

6

Acknowledgments

This work was supported by The Instrument & Equipment Development Project, Chinese Academy of Sciences, Grant No.YZ201155.

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