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Mechanism of dense silicon dioxide films deposited under 100 °C via inductively coupled plasma chemical vapor deposition
Accepted Manuscript Mechanism of dense silicon dioxide films deposited under 100 °C via inductively coupled plasma chemical vapor deposition
Yun-Shao Cho, Chia-Hsun Hsu, Kuo-Yao Shen, Sam Zhang, Wan-Yu Wu, Shui-Yang Lien PII: DOI: Reference:
S0257-8972(18)31391-4 https://doi.org/10.1016/j.surfcoat.2018.12.068 SCT 24136
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
31 August 2018 12 November 2018 17 December 2018
Please cite this article as: Yun-Shao Cho, Chia-Hsun Hsu, Kuo-Yao Shen, Sam Zhang, Wan-Yu Wu, Shui-Yang Lien , Mechanism of dense silicon dioxide films deposited under 100 °C via inductively coupled plasma chemical vapor deposition. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.12.068
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ACCEPTED MANUSCRIPT Mechanism of dense silicon dioxide films deposited under 100oC via inductively coupled plasma chemical vapor deposition Yun-Shao Cho1, Chia-Hsun Hsu1, Kuo-Yao Shen2, Sam Zhang3, Wan-Yu Wu1, Shui-Yang Lien1*
2
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Department of Materials Science and Engineering, Da-Yeh University, Taiwan Department of Electrical Engineering, Da-Yeh University, Taiwan Faculty of Materials and Energy, Southwest University, China
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3
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1
*Corresponding author’s email: [email protected]
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Abstract
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High density of the inductively coupled plasma chemical vapor deposition (ICPCVD) enables
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growth of films on soft substrate such as polyethylene terephthalate (PET) that otherwise curls at
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temperatures higher than 100°C. In this study, dense silicon oxide (SiOx) films are prepared on PET by ICPCVD. Intrusive optical emission spectroscopy is employed to monitor distribution of the
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plasma radicals during the deposition process. The study discovers a direct relationship between the
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dense structure of the deposited film and the CH/(CH+OH) plasma radical intensity ratio. The deposition plasma with the lowest CH radical ratio results in the SiOx film with the least CHx incorporation and densest film structure. The film exhibits the smallest etched area in hydrochloric acid etching tests and the minimal water-vapor transmission rate of about 1.5×10-1 g/m2/day. Keywords: optical emission spectroscopy, silicon oxide, polyethylene terephthalate, inductively coupled plasma, tetramethylsilane
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1. Introduction Transparent barrier coatings have recently gained great attention in industries for applications in electronic devices, especially in flexible organic light-emitting diodes. These coatings provide
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encapsulation against inward permeation of water and oxygen, both of which can oxidize the
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metallic cathode thus causing the device performances deteriorate [1]. Thin, glassy barrier coatings
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of silicon compounds such as silicon dioxide (SiO2) have been a favorite choice. SiO2 films deposited by plasma enhanced chemical vapor deposition (PECVD) at 200-450°C have a satisfactory
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film density [2]. It has been reported that the addition of oxygen to tetramethylsilane (TMS) vapors
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during PECVD deposition leads to the change in film structure which varies from organic to
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inorganic character close to SiO2 films [3]. However, this deposition temperature is still too high for
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flexible substrate such as polyethylene terephthalate (PET), which can easily curl at temperatures higher than 100°C. Decreasing temperature to below 150°C leads to high porosity in PECVD SiO2
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[4], resulting in encapsulation failure. The challenge is to prepare SiO2 film with high density at low
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temperatures. The gas barrier property of a SiOx film synthesized on PET from PECVD using TMS-oxygen gas mixture has been reported to have water vapor transmission rates of 0.34-1.6 g/m2/day [5,6]. In addition, SiOx-based multilayers on plastic substrates have been reported to have WVTR lower than 10-5 g/m2/day [7,8]. One possible solution for reducing deposition temperature lies in the high plasma density CVD technique with low process pressures. Inductively coupled plasma chemical vapor deposition (ICPCVD) is a good candidate as it has high plasma density of
ACCEPTED MANUSCRIPT 10-11-10-12 cm-3, nearly two orders of magnitude higher than what is possible in PECVD[9,10]. Although the properties of the SiO2 films prepared by ICPCVD have been studied, the plasma chemistry of the deposition process and its relationship with film properties are rarely reported.
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In this study, we prepare dense SiO2 films on PET via ICPCVD. During the deposition the
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plasma radical distribution is monitored with optical emission spectroscopy (OES), and the plasma
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radical distribution is analyzed and correlated to the structural property of the films deposited. A direct relationship is discovered between the radical density and the SiO2 film density, thus providing
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useful guide in encapsulation of flexible electronic devices.
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2. Experimental methods
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The PET (50 µm, Nan Ya Plastics, Taiwan) squares of 10×10 cm2 in size were used as substrate,
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which were ultrasonically cleaned in deionized water, ethanol and acetone for 15 min each, followed by drying under flowing nitrogen. The O2 and tetramethylsilane (TMS) gas mixture was input into
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the ICPCVD system at flow rates of 2 and 60 sccm, respectively. The deposition temperature was set
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to 90°C, at which no curling of PET was observable. The radiofrequency power was varied from 300 to 1800 W to change the plasma radical distribution. The thickness of the SiO2 film was measured as 300 nm by an alpha-step profilometer. During the deposition, the plasma spectrum was monitored by OES via an optical fiber head placed inside the deposition chamber beside the substrate. The chemical bond configuration in the films was characterized by Fourier transform infrared spectroscopy (FTIR). The surface morphology of the films was observed with an atomic force
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microscope (AFM). The water-vapor transmission rate (WVTR) was measured using a Mocon Permatran-W 3/61 at 40°C and 100% relative humidity. The film stress was measured on a silicon substrate using a laser profilometer (FLX-2320, Tencor) by applying the Stone formula [11] E h2 (R −R2 )
s 1 σf t f = 6(1−V )R
(1)
1 R2
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s
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where σf is the in-plane stress component in the film, tf is the thickness of the film, Es is Young’s
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modulus of the substrate, Vs is Poisson’s ratio of the substrate, h denotes the thickness of the substrate, and R1 and R2 are the radii of curvature of the substrate before and after film deposition,
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respectively.
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3. Results and discussion
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Figure 1 shows the proposed growth mechanism of the SiOx films deposited by ICPCVD with
collision can be written as
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TMS and oxygen mixture. During plasma deposition, the decomposition of TMS by electron
(2)
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Si(CH3)4 + xe- →Si(CH3)4-x + xCH3 + xe-
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where x=1 to 4. CH3 can further be decomposed into CH2 and then CH by removing hydrogen atoms. The hydrogen atoms react with oxygen atoms, from dissociation of oxygen molecules, and forms OH radicals. It is reported that reactions between OH plasma radicals and methyl groups may cause carbon removal by eventually forming CO, CO2 or CH4 gas [12–14]. Once carbon is removed from methyl groups of TMS, OH radicals have more possibility to bond with silicon to form (OH)xSi(CH3)4-x, which has been reported to be as one of the most important intermediates for the
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SiOx films synthesized from TMS/O2 mixture [15]. Therefore, in this study OH radicals is considered to be favorable for silicon oxide growth as they can form Si-O bonds after reacting with decomposed TMS, as given by (3) (4)
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OH-Si(CH3)4-x + OH-Si(CH3)4-x → (CH3)4-x-Si-O-Si-(CH3)4-x + H2
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OH + Si(CH3)4-x → OH-Si(CH3)4-x
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The OH can also react with the methyl parts of TMS, where CH4 is released and a Si-O bond is generated as given by
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OH + Si-CH3 → Si-O + CH4
(5)
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In contrast, in eq. (5) if the TMS reacts with a CHn (n=1-3) radical instead of the OH radical, the
(6)
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CHn + Si-CH3 → Si-CHn-1 + CH4
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equation becomes
Therefore, it is assumed that the film structure would have large fraction of Si-O-Si bonds and come
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closer to ideal SiO2 when the plasma has large amount of OH radicals. Whereas the film deposited at
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significant amount of CHn radicals would have Si-CHn-1 bonds. The incorporation of Si-CHn-1 bonds in Si-O-Si network deforms film structure and increases the porosity of the deposited films [16,17]. Figure 2a shows the OES spectra in the wavelength range of 200-800 nm for the SiOx films deposited at different powers. All the peaks are very weak at 300 W, indicating that electrons in the plasma do not have sufficient energy to dissociate much of TMS molecules. As the power increases, more TMS is decomposed and several main peaks present, which are OH (A2Σ+→X2Π) at 309 nm
ACCEPTED MANUSCRIPT [18], CH (A2Δ→X2Π) at 431 nm [19], Hβ (4d→2d) at 486 nm, Hα (3d→2p) at 656 nm [20] and O (3p5P→3s5S0) at 777 nm [18]. There should be a peak at 252 nm associated with Si atomic radicals [21] dissociated from TMS, but this peak is almost unobservable because of its relatively weak
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intensity. CH3 and CH2 radicals cannot be observable in OES spectra, but their amounts can be
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positively related to CH peak intensity. As in the proposed film growth mechanism, in which OH and
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CH radicals affect the film structure, further analysis of the OES spectra is carried out by taking into account the OH and CH plasma radical intensity. An CH radical relative intensity ratio is defined as
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CH/(OH+CH) (“CH relative ratio” for short) and is plotted in Fig. 2b together with that of the OH
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and the CH. Below 1200 W, generation of OH radicals accelerates with increasing power while the
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increase of CH radicals seems stagnant. From 1200 to 1800 W, generation of CH increases abruptly
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at a much higher rate. The generation of CH radicals requires continuous electron collisions with CH3. As the power increases, the electron density increases, which increases the probability of
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collisions. The CH radical intensity has thus a sharp increase after 1200 W. Overall, the CH ratio
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reaches its minimum at the power of 1200 W. This predicts that the film deposited at 1200 W might have the least fraction of Si-C bonds and the highest fraction of Si-O-Si bonds. Figure 3a shows FTIR results for films prepared at different powers. The peak at about 447, 795, and 1058 cm-1 emerge from the Si-O-Si rocking, bending and stretching modes [22,23]. The peak at 1246 cm-1 corresponds to the Si-CH3 bonds [24], where the hydrogen-terminated structure can form voids and thus decrease film density. The peak intensity of Si-O-Si (filled square) and Si-CH3 (open
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square) for the films deposited at different powers are shown in Fig. 3b. At 300 W, the Si-O-Si peak intensity is low due to the low amount of the OH radicals. The high Si-CH3 bond intensity results from the non-reacted Si-CH3 parts in TMS. As the power increases, the increase of Si-O-Si peak
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intensity and the decreases of Si-CH3 peak intensity are due to the enhancement of eq. (4) and (5).
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However, the Si-CH3 peak intensity increases again when the power is larger than 1200 W. The
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significant amount of the CH radicals promotes eq. (6). This mitigates the generation of Si-O bonds and increases Si-C bonds. The ratio R of Si-C to Si-O-Si peak intensity is calculated for the different
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powers. The 1200 W has the lowest peak ratio of 0.97. As mentioned before that the
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hydrogen-terminated structure of Si-CH3 can represent as voids in the films, the film deposited at
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1200 W could be expected to have the least voids and thus the best density. Based on the correlation
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between OES and FTIR results, optimization of the density of silicon oxide deposited using TMS and O2 gas mixture could be done at the stage of plasma deposition by control the ratio of CH
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relative ratio.
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Figure 4 shows the AFM topographical images of the SiOx films deposited at different power. The corresponded average surface roughness (Ra) values are shown in Fig. 5. Ra decreases from 1.3 to 0.9 nm when the power increases from 300 to 1200 W, and then increases rapidly from 0.9 to 1.8 nm when the power further increases from 1200 to 1800 W. As compared to the case at 1200 W, the increased Ra at low power level and high power level. In FTIR it shows that both low and high power levels lead to a high content of Si-CH3 bonds in the deposited films. These
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hydrogen-terminated bonds in the film interrupt the network connectivity. Voids or grain boundary could be formed and surrounded by several terminated bonds [25]. The films with voids and grain boundaries become less dense and have non-smooth (rough) surface [26,27]. The film deposited at
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1200 W having the lowest Ra value indicates the densest structure.
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Figure 6 shows the residual stress and WVTR measurement results for the SiO2 films deposited
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at different power. The negative sign of the residual stress value indicates compress. It is reported that silicon oxide films are more compressive with increased film density [28]. Whereas the Si-CH3
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bonds can induce tensile stress, which reduces the compressive stress of SiOx films. Therefore, the
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SiOx film deposited at 1200 W has the most compressive stress due to the least Si-CH3 proportion.
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For the WVTR values of the films, the lowest value of 1.51×10-1 g/cm2/day can be obtained at 1200
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W. As water vapor penetrates the film mainly through pores or grain boundary, this measurement thus supports our earlier analysis that the film is the densest at deposition power of 1200W.
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The film density is also confirmed by performing wet etching at 55°C for 60 min on 50 nm
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aluminum (Al) films covered with the 300 nm SiO2 films. The etchant consists of HCl, H2O2 and HO2 with a ratio of 3:1:1. The SiO2 layers serves as barrier layers to protect the Al layer underneath from the etching process. The morphological images for the films after wet etching are shown in Fig. 7. The Al-etched area is presented as the black region, and its ratio is calculated using ImagePro computing software, as shown in Fig. 8. The minimal ∆A is 1.4% occurring at the power of 1200 W, and the least etched area confirms the densest structure.
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4. Conclusion This paper demonstrates the dense SiO2 films deposited by ICPCVD with instructive OES system. The OH and CH radicals are assumed to be important radicals determining the density of the
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films. The ratio of CH to (OH+CH) is the lowest at the power of 1200 W. The FTIR result confirms
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that OH plasma radical is responsible for generation of Si-O-Si bonds, while the CH radical will
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produce Si-C bonds in the deposited films. The power of 1200 W yields the highest Si-O-Si bond intensity ratio, indicating the densest film structure. The film deposited at the power of 1200 W also
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shows the lowest surface roughness of 0.9 nm, the most compressive residual stress of -354 Mpa,
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and the lowest WVTR of 1.51×10-1 g/m2/day. The OES data are in good agreement with the
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measurement results of the film properties. Therefore, this paper demonstrates the feasibility of the
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OES in-situ monitoring for the deposition of dense SiO2 films. Acknowledgments
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This work is sponsored by the Ministry of Science and Technology of the Republic of China under
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Figure captions Fig. 1: Growth mechanism of SiOx films deposited with TMS and oxygen mixture. Fig. 2: (a) Optical emission spectra, and (b) OH*, CH* radical intensities, and CH* radical
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intensity ratio for SiO2 films prepared at different powers.
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Fig. 3: (a) Fourier transform infrared spectra for SiO2 films deposited with different powers. (b)
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Si-O-Si (filled symbol) and Si-CH3 (open symbol) peak intensity with different power. R is the ratio of Si-O-Si intensity to Si-CH3 intensity.
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Fig. 4: Atomic force microscope images of SiOx films deposited with different powers (a) 300, (b)
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600, (c) 900, (d) 1200, (e) 1500 and (f) 1800 W.
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Fig. 5: Surface roughness of SiOx films deposited with different powers. Error bar indicates
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standard deviation for five samples in each case. Fig. 6: Residual stress and water vapor transmission rate for SiO2 films with different powers. Error
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bar indicates standard deviation for five samples in each case.
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Fig. 7: Optical microscope images for wet etching tests for different powers (a) 300, (b) 600, (c) 900, (d) 1200, (e) 1500 and (f) 1800 W. Fig. 8: Aluminum-etched area ratio for the wet etching tests. Error bar indicates standard deviation for five samples in each case.
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CH3 O
CH3
Si
CH3 O
CH3
Si
CH3 O
CH3
Si
CH3 O
CH3
Si
CH3 O
CH3
Si
CH3 O
CH3
OH+Si-CH3→Si-O+CH4
Si
CH3 O
CH3
Si CH3
CHn+Si-CH3→Si-CHn-1+CH4 Carbon incorporation
Si
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Si
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Figure 1
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O (777)
Hβ (486)
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Intensity (a. u.)
OH (309)
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Hα (656)
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OH*(309 nm)
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CH/(CH+OH)
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1500
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(
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Si-CH3
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0.4
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Dense ICPCVD SiOx layer is deposited on PET under 100°C. Optimization of film density can be achieved by monitoring plasma radicals. Intensity ratio of CH/(CH+OH) plasma radical is correlated to SiOx film density. Lowest CH radical ratio leads to the densest SiOx and the least carbon in films. The densest film with a WVTR of 1.5×10-1 g/m2/day is obtained.
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1. 2. 3. 4. 5.
Yun-Shao Cho, Chia-Hsun Hsu, Kuo-Yao Shen, Sam Zhang, Wan-Yu Wu, Shui-Yang Lien PII: DOI: Reference:
S0257-8972(18)31391-4 https://doi.org/10.1016/j.surfcoat.2018.12.068 SCT 24136
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
31 August 2018 12 November 2018 17 December 2018
Please cite this article as: Yun-Shao Cho, Chia-Hsun Hsu, Kuo-Yao Shen, Sam Zhang, Wan-Yu Wu, Shui-Yang Lien , Mechanism of dense silicon dioxide films deposited under 100 °C via inductively coupled plasma chemical vapor deposition. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.12.068
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ACCEPTED MANUSCRIPT Mechanism of dense silicon dioxide films deposited under 100oC via inductively coupled plasma chemical vapor deposition Yun-Shao Cho1, Chia-Hsun Hsu1, Kuo-Yao Shen2, Sam Zhang3, Wan-Yu Wu1, Shui-Yang Lien1*
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Department of Materials Science and Engineering, Da-Yeh University, Taiwan Department of Electrical Engineering, Da-Yeh University, Taiwan Faculty of Materials and Energy, Southwest University, China
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*Corresponding author’s email: [email protected]
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Abstract
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High density of the inductively coupled plasma chemical vapor deposition (ICPCVD) enables
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growth of films on soft substrate such as polyethylene terephthalate (PET) that otherwise curls at
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temperatures higher than 100°C. In this study, dense silicon oxide (SiOx) films are prepared on PET by ICPCVD. Intrusive optical emission spectroscopy is employed to monitor distribution of the
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plasma radicals during the deposition process. The study discovers a direct relationship between the
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dense structure of the deposited film and the CH/(CH+OH) plasma radical intensity ratio. The deposition plasma with the lowest CH radical ratio results in the SiOx film with the least CHx incorporation and densest film structure. The film exhibits the smallest etched area in hydrochloric acid etching tests and the minimal water-vapor transmission rate of about 1.5×10-1 g/m2/day. Keywords: optical emission spectroscopy, silicon oxide, polyethylene terephthalate, inductively coupled plasma, tetramethylsilane
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1. Introduction Transparent barrier coatings have recently gained great attention in industries for applications in electronic devices, especially in flexible organic light-emitting diodes. These coatings provide
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encapsulation against inward permeation of water and oxygen, both of which can oxidize the
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metallic cathode thus causing the device performances deteriorate [1]. Thin, glassy barrier coatings
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of silicon compounds such as silicon dioxide (SiO2) have been a favorite choice. SiO2 films deposited by plasma enhanced chemical vapor deposition (PECVD) at 200-450°C have a satisfactory
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film density [2]. It has been reported that the addition of oxygen to tetramethylsilane (TMS) vapors
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during PECVD deposition leads to the change in film structure which varies from organic to
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inorganic character close to SiO2 films [3]. However, this deposition temperature is still too high for
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flexible substrate such as polyethylene terephthalate (PET), which can easily curl at temperatures higher than 100°C. Decreasing temperature to below 150°C leads to high porosity in PECVD SiO2
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[4], resulting in encapsulation failure. The challenge is to prepare SiO2 film with high density at low
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temperatures. The gas barrier property of a SiOx film synthesized on PET from PECVD using TMS-oxygen gas mixture has been reported to have water vapor transmission rates of 0.34-1.6 g/m2/day [5,6]. In addition, SiOx-based multilayers on plastic substrates have been reported to have WVTR lower than 10-5 g/m2/day [7,8]. One possible solution for reducing deposition temperature lies in the high plasma density CVD technique with low process pressures. Inductively coupled plasma chemical vapor deposition (ICPCVD) is a good candidate as it has high plasma density of
ACCEPTED MANUSCRIPT 10-11-10-12 cm-3, nearly two orders of magnitude higher than what is possible in PECVD[9,10]. Although the properties of the SiO2 films prepared by ICPCVD have been studied, the plasma chemistry of the deposition process and its relationship with film properties are rarely reported.
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In this study, we prepare dense SiO2 films on PET via ICPCVD. During the deposition the
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plasma radical distribution is monitored with optical emission spectroscopy (OES), and the plasma
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radical distribution is analyzed and correlated to the structural property of the films deposited. A direct relationship is discovered between the radical density and the SiO2 film density, thus providing
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useful guide in encapsulation of flexible electronic devices.
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2. Experimental methods
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The PET (50 µm, Nan Ya Plastics, Taiwan) squares of 10×10 cm2 in size were used as substrate,
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which were ultrasonically cleaned in deionized water, ethanol and acetone for 15 min each, followed by drying under flowing nitrogen. The O2 and tetramethylsilane (TMS) gas mixture was input into
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the ICPCVD system at flow rates of 2 and 60 sccm, respectively. The deposition temperature was set
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to 90°C, at which no curling of PET was observable. The radiofrequency power was varied from 300 to 1800 W to change the plasma radical distribution. The thickness of the SiO2 film was measured as 300 nm by an alpha-step profilometer. During the deposition, the plasma spectrum was monitored by OES via an optical fiber head placed inside the deposition chamber beside the substrate. The chemical bond configuration in the films was characterized by Fourier transform infrared spectroscopy (FTIR). The surface morphology of the films was observed with an atomic force
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microscope (AFM). The water-vapor transmission rate (WVTR) was measured using a Mocon Permatran-W 3/61 at 40°C and 100% relative humidity. The film stress was measured on a silicon substrate using a laser profilometer (FLX-2320, Tencor) by applying the Stone formula [11] E h2 (R −R2 )
s 1 σf t f = 6(1−V )R
(1)
1 R2
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s
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where σf is the in-plane stress component in the film, tf is the thickness of the film, Es is Young’s
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modulus of the substrate, Vs is Poisson’s ratio of the substrate, h denotes the thickness of the substrate, and R1 and R2 are the radii of curvature of the substrate before and after film deposition,
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respectively.
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3. Results and discussion
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Figure 1 shows the proposed growth mechanism of the SiOx films deposited by ICPCVD with
collision can be written as
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TMS and oxygen mixture. During plasma deposition, the decomposition of TMS by electron
(2)
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Si(CH3)4 + xe- →Si(CH3)4-x + xCH3 + xe-
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where x=1 to 4. CH3 can further be decomposed into CH2 and then CH by removing hydrogen atoms. The hydrogen atoms react with oxygen atoms, from dissociation of oxygen molecules, and forms OH radicals. It is reported that reactions between OH plasma radicals and methyl groups may cause carbon removal by eventually forming CO, CO2 or CH4 gas [12–14]. Once carbon is removed from methyl groups of TMS, OH radicals have more possibility to bond with silicon to form (OH)xSi(CH3)4-x, which has been reported to be as one of the most important intermediates for the
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SiOx films synthesized from TMS/O2 mixture [15]. Therefore, in this study OH radicals is considered to be favorable for silicon oxide growth as they can form Si-O bonds after reacting with decomposed TMS, as given by (3) (4)
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OH-Si(CH3)4-x + OH-Si(CH3)4-x → (CH3)4-x-Si-O-Si-(CH3)4-x + H2
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OH + Si(CH3)4-x → OH-Si(CH3)4-x
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The OH can also react with the methyl parts of TMS, where CH4 is released and a Si-O bond is generated as given by
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OH + Si-CH3 → Si-O + CH4
(5)
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In contrast, in eq. (5) if the TMS reacts with a CHn (n=1-3) radical instead of the OH radical, the
(6)
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CHn + Si-CH3 → Si-CHn-1 + CH4
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equation becomes
Therefore, it is assumed that the film structure would have large fraction of Si-O-Si bonds and come
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closer to ideal SiO2 when the plasma has large amount of OH radicals. Whereas the film deposited at
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significant amount of CHn radicals would have Si-CHn-1 bonds. The incorporation of Si-CHn-1 bonds in Si-O-Si network deforms film structure and increases the porosity of the deposited films [16,17]. Figure 2a shows the OES spectra in the wavelength range of 200-800 nm for the SiOx films deposited at different powers. All the peaks are very weak at 300 W, indicating that electrons in the plasma do not have sufficient energy to dissociate much of TMS molecules. As the power increases, more TMS is decomposed and several main peaks present, which are OH (A2Σ+→X2Π) at 309 nm
ACCEPTED MANUSCRIPT [18], CH (A2Δ→X2Π) at 431 nm [19], Hβ (4d→2d) at 486 nm, Hα (3d→2p) at 656 nm [20] and O (3p5P→3s5S0) at 777 nm [18]. There should be a peak at 252 nm associated with Si atomic radicals [21] dissociated from TMS, but this peak is almost unobservable because of its relatively weak
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intensity. CH3 and CH2 radicals cannot be observable in OES spectra, but their amounts can be
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positively related to CH peak intensity. As in the proposed film growth mechanism, in which OH and
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CH radicals affect the film structure, further analysis of the OES spectra is carried out by taking into account the OH and CH plasma radical intensity. An CH radical relative intensity ratio is defined as
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CH/(OH+CH) (“CH relative ratio” for short) and is plotted in Fig. 2b together with that of the OH
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and the CH. Below 1200 W, generation of OH radicals accelerates with increasing power while the
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increase of CH radicals seems stagnant. From 1200 to 1800 W, generation of CH increases abruptly
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at a much higher rate. The generation of CH radicals requires continuous electron collisions with CH3. As the power increases, the electron density increases, which increases the probability of
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collisions. The CH radical intensity has thus a sharp increase after 1200 W. Overall, the CH ratio
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reaches its minimum at the power of 1200 W. This predicts that the film deposited at 1200 W might have the least fraction of Si-C bonds and the highest fraction of Si-O-Si bonds. Figure 3a shows FTIR results for films prepared at different powers. The peak at about 447, 795, and 1058 cm-1 emerge from the Si-O-Si rocking, bending and stretching modes [22,23]. The peak at 1246 cm-1 corresponds to the Si-CH3 bonds [24], where the hydrogen-terminated structure can form voids and thus decrease film density. The peak intensity of Si-O-Si (filled square) and Si-CH3 (open
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square) for the films deposited at different powers are shown in Fig. 3b. At 300 W, the Si-O-Si peak intensity is low due to the low amount of the OH radicals. The high Si-CH3 bond intensity results from the non-reacted Si-CH3 parts in TMS. As the power increases, the increase of Si-O-Si peak
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intensity and the decreases of Si-CH3 peak intensity are due to the enhancement of eq. (4) and (5).
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However, the Si-CH3 peak intensity increases again when the power is larger than 1200 W. The
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significant amount of the CH radicals promotes eq. (6). This mitigates the generation of Si-O bonds and increases Si-C bonds. The ratio R of Si-C to Si-O-Si peak intensity is calculated for the different
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powers. The 1200 W has the lowest peak ratio of 0.97. As mentioned before that the
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hydrogen-terminated structure of Si-CH3 can represent as voids in the films, the film deposited at
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1200 W could be expected to have the least voids and thus the best density. Based on the correlation
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between OES and FTIR results, optimization of the density of silicon oxide deposited using TMS and O2 gas mixture could be done at the stage of plasma deposition by control the ratio of CH
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relative ratio.
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Figure 4 shows the AFM topographical images of the SiOx films deposited at different power. The corresponded average surface roughness (Ra) values are shown in Fig. 5. Ra decreases from 1.3 to 0.9 nm when the power increases from 300 to 1200 W, and then increases rapidly from 0.9 to 1.8 nm when the power further increases from 1200 to 1800 W. As compared to the case at 1200 W, the increased Ra at low power level and high power level. In FTIR it shows that both low and high power levels lead to a high content of Si-CH3 bonds in the deposited films. These
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hydrogen-terminated bonds in the film interrupt the network connectivity. Voids or grain boundary could be formed and surrounded by several terminated bonds [25]. The films with voids and grain boundaries become less dense and have non-smooth (rough) surface [26,27]. The film deposited at
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1200 W having the lowest Ra value indicates the densest structure.
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Figure 6 shows the residual stress and WVTR measurement results for the SiO2 films deposited
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at different power. The negative sign of the residual stress value indicates compress. It is reported that silicon oxide films are more compressive with increased film density [28]. Whereas the Si-CH3
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bonds can induce tensile stress, which reduces the compressive stress of SiOx films. Therefore, the
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SiOx film deposited at 1200 W has the most compressive stress due to the least Si-CH3 proportion.
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For the WVTR values of the films, the lowest value of 1.51×10-1 g/cm2/day can be obtained at 1200
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W. As water vapor penetrates the film mainly through pores or grain boundary, this measurement thus supports our earlier analysis that the film is the densest at deposition power of 1200W.
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The film density is also confirmed by performing wet etching at 55°C for 60 min on 50 nm
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aluminum (Al) films covered with the 300 nm SiO2 films. The etchant consists of HCl, H2O2 and HO2 with a ratio of 3:1:1. The SiO2 layers serves as barrier layers to protect the Al layer underneath from the etching process. The morphological images for the films after wet etching are shown in Fig. 7. The Al-etched area is presented as the black region, and its ratio is calculated using ImagePro computing software, as shown in Fig. 8. The minimal ∆A is 1.4% occurring at the power of 1200 W, and the least etched area confirms the densest structure.
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4. Conclusion This paper demonstrates the dense SiO2 films deposited by ICPCVD with instructive OES system. The OH and CH radicals are assumed to be important radicals determining the density of the
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films. The ratio of CH to (OH+CH) is the lowest at the power of 1200 W. The FTIR result confirms
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that OH plasma radical is responsible for generation of Si-O-Si bonds, while the CH radical will
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produce Si-C bonds in the deposited films. The power of 1200 W yields the highest Si-O-Si bond intensity ratio, indicating the densest film structure. The film deposited at the power of 1200 W also
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shows the lowest surface roughness of 0.9 nm, the most compressive residual stress of -354 Mpa,
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and the lowest WVTR of 1.51×10-1 g/m2/day. The OES data are in good agreement with the
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measurement results of the film properties. Therefore, this paper demonstrates the feasibility of the
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OES in-situ monitoring for the deposition of dense SiO2 films. Acknowledgments
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This work is sponsored by the Ministry of Science and Technology of the Republic of China under
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Figure captions Fig. 1: Growth mechanism of SiOx films deposited with TMS and oxygen mixture. Fig. 2: (a) Optical emission spectra, and (b) OH*, CH* radical intensities, and CH* radical
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intensity ratio for SiO2 films prepared at different powers.
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Fig. 3: (a) Fourier transform infrared spectra for SiO2 films deposited with different powers. (b)
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Si-O-Si (filled symbol) and Si-CH3 (open symbol) peak intensity with different power. R is the ratio of Si-O-Si intensity to Si-CH3 intensity.
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Fig. 4: Atomic force microscope images of SiOx films deposited with different powers (a) 300, (b)
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600, (c) 900, (d) 1200, (e) 1500 and (f) 1800 W.
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Fig. 5: Surface roughness of SiOx films deposited with different powers. Error bar indicates
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standard deviation for five samples in each case. Fig. 6: Residual stress and water vapor transmission rate for SiO2 films with different powers. Error
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bar indicates standard deviation for five samples in each case.
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Fig. 7: Optical microscope images for wet etching tests for different powers (a) 300, (b) 600, (c) 900, (d) 1200, (e) 1500 and (f) 1800 W. Fig. 8: Aluminum-etched area ratio for the wet etching tests. Error bar indicates standard deviation for five samples in each case.
ACCEPTED MANUSCRIPT CH3 Si
CH3 O
CH3
Si
CH3 O
CH3
Si
CH3 O
CH3
Si
CH3 O
CH3
Si
CH3 O
CH3
Si
CH3 O
CH3
OH+Si-CH3→Si-O+CH4
Si
CH3 O
CH3
Si CH3
CHn+Si-CH3→Si-CHn-1+CH4 Carbon incorporation
Si
O
Si
O
O
O
O
Si
Si
O
O
O
Si O
O
Si
O
Figure 1
PT
Si
CH3
NU
O
O
Si
MA
O
O
D
O
O
Si
O
PT E
Si
O
CE
O
Si
O
O
CHn-1
O
O
Si
O
RI
O
O
AC
Si
O
O
SC
O
Si O
O
O
Si
Si O
1800 W
400
500
600
RI
300
700
800
SC
200
PT
1500 W 1200 W 900 W 600 W 300 W
O (777)
Hβ (486)
CH (431)
Intensity (a. u.)
OH (309)
(a)
Hα (656)
ACCEPTED MANUSCRIPT
MA
OH*(309 nm)
60
CH*(431 nm)
2000
CH/(CH+OH)
D
1500 1000
CE
500
30
15
0
AC
300
600
45
900
1200
Power (W)
Figure 2
1500
0 1800
CH*/(OH*+CH*) (%)
(b)
PT E
Intensity (a. u.)
2500
NU
Wavelength (nm)
(
Si-O-Si
(
Si-O-Si
Si-O-Si
Peak intensity (a. u.)
Si-CH3
ACCEPTED MANUSCRIPT
1800 W 1500 W
PT
1200 W 900 W
RI
600 W
1400
1200
SC
300 W
1000
800
600
400
NU
Wavenumber (cm-1) 50
MA
30 20
5 0 4
CE
3
300
R=0.11
D
R=0.14 R=0.12.
6 10
R=0.09
R=0.13
R=0.18
600
900 1200 1500 1800 2100
600
900 1200 1500 1800 2100
PT E
Intensity (a. u.)
40
Si-O-Si Si-CH3
2
AC
0
300
Power (W)
Figure 3
ACCEPTED MANUSCRIPT
0.4
(μm)
0.2
0.6 0.4 0.2 (μm)
0.4
0.2
0.2
0.4
0.6
0.8
(µm)
0.6
0.4
0.2
0.2
0.4
0.6
0.8
(μm)
(nm) 0.6
(μm)
0.4
0.2
0.8 0.6 0.4 0.2 (μm)
D
MA
Figure 4
1.0
0 0.8 0.6
RI
(μm)
0.8
NU
0.2
0.8
0.8
(μm)
SC
(nm) 0.2
0.6
1.0 0
0
(f)
PT E
(μm)
0.4
0.4
CE
0.6
AC
(nm)
0.6
1.0
(e)
1.0 0.8
0.8
(μm)
(d)
0
0
(nm)
1.0
PT
0.6
0.8
(nm)
(nm)
1.0 0 0.8
(c)
(b)
(a)
(μm)
0.4
0.2
0.2
0.4
0.6
0.8
(μm)
ACCEPTED MANUSCRIPT
2
1.4
PT
1.1
RI
0.8 0.5 300
600
900
SC
Roughness (nm)
1.7
1200
AC
CE
PT E
D
MA
NU
Power (W)
Figure 5
1500
1800
ACCEPTED MANUSCRIPT
Residual stress WVTR
6
4
4 2
PT
2
0 600
900
1200
1500
SC
300
NU
Power (W)
AC
CE
PT E
D
MA
Figure 6
0 1800
WVTR (10-1 g/m2/day)
8
RI
Residual stress (-102 MPa)
6
ACCEPTED MANUSCRIPT
(a)
(b)
(c)
Etched area
100 μm (e)
100 μm (f)
100 μm
NU
SC
100 μm
RI
PT
(d)
100 μm
AC
CE
PT E
D
MA
Figure 7
100 μm
ACCEPTED MANUSCRIPT
25
15
PT
10
RI
5
0 300
600
900
SC
Etched area (%)
20
1200
AC
CE
PT E
D
MA
NU
Power (W)
Figure 8
1500
1800
ACCEPTED MANUSCRIPT Highlights
CE
PT E
D
MA
NU
SC
RI
PT
Dense ICPCVD SiOx layer is deposited on PET under 100°C. Optimization of film density can be achieved by monitoring plasma radicals. Intensity ratio of CH/(CH+OH) plasma radical is correlated to SiOx film density. Lowest CH radical ratio leads to the densest SiOx and the least carbon in films. The densest film with a WVTR of 1.5×10-1 g/m2/day is obtained.
AC
1. 2. 3. 4. 5.