- Email: [email protected]
SiOx plasma thin film deposition using a low-temperature cascade arc torch
Thin Solid Films 519 (2011) 4824–4829
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
SiOx plasma thin film deposition using a low-temperature cascade arc torch A.C. Ritts a, C.H. Liu b, Q.S. Yu a,⁎ a
Center for Surface Science and Plasma Technologies, Department of Mechanical and Aerospace Engineering, University of Missouri, E2403D Lafferre Hall, Columbia, MO 65211, USA Plasma Science and Application Department, Advanced Manufacturing Core Technology Div. M200, MSL/ITRI, Rm. 144, Bldg. 11, 195, Sec. 4, Chung Hsing Rd. Chutung, Hsinchu 310, Taiwan, ROC
b
a r t i c l e
i n f o
Available online 13 January 2011 Keywords: Hexamethyldisiloxane Refractive index Hardness X-ray photoelectron spectroscopy Plasma deposition Cascade arc torch
a b s t r a c t Plasma thin films were deposited from gas mixtures of hexamethyldisiloxane (HMDSO) and oxygen (O2) using a low-temperature cascade arc torch (LTCAT). Various properties of the deposited HMDSO plasma coatings, including refractive index (RI), surface contact angle, and hardness were evaluated. The characterization results using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, ellipsometry, and water contact angle measurements indicated that, with increased O2 addition, the deposited HMDSO plasma thin films were of inorganic SiOx nature. It was also found that, in the LTCAT plasma system, O2 addition significantly improves the hardness of the resulting HMDSO plasma coatings. The film hardness of the deposited HMDSO plasma coatings measured by a standard pencil test (ASTM D3363-05) reached 6H with increased O2 addition in the HMDSO/O2 gas mixture. Such hard plasma coatings could be potentially used for many important industrial applications, such as anti-scratch coatings on plastic glasses and various plastic lens materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Many materials with desirable bulk qualities lack specific surface properties required for specific industrial applications. To meet the application requirements, a variety of surface treatment methods are needed to modify the surface and improve the surface characteristics of these materials. In recent years plastic materials are alternatives to inorganic glasses due to their desirable bulk characteristics, light weight, low cost, and high toughness [1]. On the other hand, the poor surface characteristics of plastics, such as low hardness and low wear resistance, limit their use to mild applications [2]. Various surface treatment methods have been applied to plastic surfaces in order to improve their surface properties [3–7]. Wet chemical methods are commonly utilized because they are easily implemented. However wet chemical methods produce thick, non-uniform coatings and the chemicals required are generally deemed as environmentally hazardous [1]. As environmentally friendly processes, various plasma chemical vapor deposition techniques have been recognized as the suitable and promising alternatives to wet chemical methods for surface modification of plastic materials by depositing hard and controllable thin coating to improve their wear resistance and anti-scratch properties [1,2]. In comparison with the conventional plasma deposition techniques, the low-temperature cascade arc torch (LTCAT) technique offers a very different plasma deposition process in that the activation and
⁎ Corresponding author. Tel.: +1 573 882 8076; fax: +1 573 884 5090. E-mail addresses: [email protected] (C.H. Liu), [email protected] (Q.S. Yu). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.037
deactivation of the precursors (monomers) are decoupled [8–10]. The major feature of LTCAT plasma deposition is a two step activation process. First inert argon (Ar) carrier gas was activated to form Ar plasmas inside the arc column and then emanates from the arc column into the reaction chamber as luminous Ar gas plasma torch. During this process most charged plasma species were confined inside the arc column. Plasma diagnosis results showed that the main plasma species of the luminous Ar gas plasma torch observed in the chamber are electronically excited Ar neutrals [9,10]. Second precursor (monomer) gases, such as hexamethylsiloxane (HMDSO) and oxygen (O2) that were introduced into the Ar gas plasma torch, were activated mainly by the electronically excited Ar neutrals. This two step activation process provides better control of the reactive gas activation and minimizes the electron impact ionization. In traditional plasma processes, many undesirable plasma reactions such as plasma induced degradation of the polymer chains and etching of the surface materials could occur on a plastic substrate [3]. These undesirable reactions could result in significant detrimental effects on the plastic substrate and adversely affect their successful applications. One typical example of such an adverse effect is the adhesion failure due to the formation of weak-boundary-layer after plasma treatment [11]. In the LTCAT process, however, the Ar plasma torch emanated from the arc column enters the plasma chamber with gas phase temperature decreasing dramatically during expansion of the gas [12]. The decreased Ar plasma torch temperature and the reduced ionization reactions provide a safe and less harsh environment for the treated plastic substrates, which are sensitive to temperature and ion bombardment. This minimizes the detrimental effects on plastic surfaces usually suffered from the traditional plasma treatment process.
A.C. Ritts et al. / Thin Solid Films 519 (2011) 4824–4829
In this study, SiOx plasma thin films were deposited from HMDSO and O2 gas mixtures by the LTCAT and the plasma coating properties were investigated in terms of O2 addition amount in the gas mixture. In contrast to traditional plasma processes, the LTCAT deposition technique provides a very promising method for large-scale industrial applications because the LTCAT plasma torch can be simply applied to a large surface through a scanning process. 2. Experimental details The LTCAT plasma system used in this study has been described elsewhere [8–10]. The cathode assembly and cascade arc column depicted in Fig. 1 were fabricated by PlasmaCarb, Inc (Bedford, NH). As deposition substrates, Si wafers cleaned with acetone were attached to a 3 × 6 inch aluminum panel using double side tapes. The aluminum panel was then loaded inside the LTCAT plasma chamber at a position of 20 cm from the plasma torch outlet from the arc column. The chamber was then evacuated to a base pressure of 5 mTorr or less. Ar gas with 99.9995% purity purchased from Airgas (St. Louis, US) was introduced into the LTCAT plasma system through the arc column at a flow rate fixed at 3000 sccm, which was chosen by considering the arc stability and achieving reasonable deposition rate at a 6.0 A arc current. A mixture of HDMSO vapor and O2 gas was introduced directly into the Ar plasma torch at the position shown in Fig. 2. The flow rate of HMDSO was fixed at 5 sccm and the O2 flow rate was varied at 0, 5, 10, 15, 20, and 25 sccm. The LTCAT Ar plasma was ignited with 6 A of arc current which was drawn from an MDX-5K dc power supply (Advanced Energy Corporation). The thickness and refractive indices (RIs) of the LTCAT plasma thin films were measured using an AutoEL-II automatic ellipsometer (Rudolph Research Corp., Flanders, NJ) with a 632.8 nm helium–neon laser light source. The coating thickness of the LTCAT plasma thin films for surface analysis and mechanical characterization was controlled by varying deposition time and confirmed by ellipsometry. The chemical structure of the deposited plasma thin films was characterized by a Fourier transform infrared (FTIR) spectrometer (Spectrum100, Perkin-Elmer, Waltham, Massachusetts). The FTIR spectra were obtained from an average of 32 scans in the range of Insulating Material
Cathode Metal Disks
Coolant Channel
4825
Reactive gas
A+ _ Argon gas
+
A
-
A*
A
A* e-
e
A
*
A*
A* A* A A+
A*
Vacuum chamber
A+
-
A*
e A*
A A* A A*
Substrate A*
A : ground state atomt A*: excited state atom A +: positive ion -
e : negative electron Fig. 2. Depiction of energy transfer of Ar to monomers (A) in a LTCAT plasma process.
400–4000 cm−1 at a resolution of 4 cm−1. Elemental analysis was determined by X-ray photoelectron spectroscopy (XPS) (Thermo VG/ ESCAlab 250) with Mg Kα source (1253.6 eV). The surface contact angles of the LTCAT plasma thin films were determined by the sessile droplet method using the VCA 2500XE contact angle measurement system (Advanced Surface Technologies, Inc., Billerica, MA). The coating thickness of the LTCAT thin films prepared for surface analysis was in the range of 80–100 nm. The plasma coating hardness was determined by a standard pencil by following the testing and rating procedures described in ASTM D3363-05. The pencils with rated hardness from 6H to 6B (hard to soft) were purchased from Derwent, Inc. (Cambria, UK). In this method, the flat lead (graphite) head was placed against the coated Si substrates at a 45° angle and pushed forward to make a scratch on the LTCAT plasma coatings which were deposited on silicone wafer substrates. The pencils were tested, starting with hardness 6H and proceeding down one level of hardness, until one pencil was found that could not make a scratch on the substrate. The hardness rating of the final test pencil was defined as the hardness of the plasma coatings. For hardness measurements, the coating thickness of the LTCAT plasma thin films was controlled between 160 and 180 nm. 3. Results and discussion 3.1. Uniformity of the LTCAT plasma coating
Anode Plasma Jet Fig. 1. Depiction of the typical electrode configuration of the arc generator column of a LTCAT.
Fig. 3 shows the thickness profiles of LTCAT plasma thin films with varying oxygen flow rates. It can be seen that the center of the LTCAT plasma torch gave the highest deposition rate. It was also noted that an increase in O2 addition into the plasma, in which the HMDSO flow rate was fixed at 5 sccm, reduced the deposition rate of HMDSO plasma coatings. As seen from Fig. 4, the glow intensity and the length/volume of the LTCAT plasma torch were weakened and shrunk respectively with the increased addition of O2 into the plasma system in which other operation parameters were kept unchanged. The increase of O2 addition into the LTCAT could increase the plasma ashing effect during the HMDSO plasma deposition through removing organic hydrocarbon contents out of the LTCAT plasma deposition system. Our previous study on the deposition of pure HMDSO in the LTCAT system revealed a dependence of the deposition rate on the Ar flow rate and power input [10]. The maximum deposition rates observed in that study were around 7 nm/s at 2000 sccm Ar flow rate and 8.0 A arc current. As seen in Fig. 3, the operating condition used in this study with an Ar flow rate of 3000 sccm and 6.0 A arc current gave rise to the
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Fig. 5. Refractive index profile of the LTCAT plasma thin films deposited from HMDSO with varying O2 flow rate. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate. Fig. 3. Changes in the deposition profile of LTCAT plasma thin coatings from HMDSO with varying O2 flow rate. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
similar deposition rate while pure HMDSO was used without adding O2. These deposition rates shown in Fig. 3 are higher than that reported in typical low pressure radio frequency plasma deposition systems [13,14]. From Fig. 3, it was also noted that the deposition rate appears to saturate with O2 addition into the HMDSO LTCAT deposition system at a flow rate of 15 sccm.
The refractive index (RI) of a plasma coating is a useful indication of the quality and uniformity of the deposited plasma thin films. The RI value of a plasma coating was determined by the mass density and the chemical composition of the coating [15,16]. Fig. 5 shows the RI
Fig. 6. Change of the average refractive index of LTCAT plasma thin films deposited from HMDSO with various O2 flow rates. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
Fig. 4. Visual comparison of the LTCAT plasma glow (a) before and (b) after 5 sccm HDMSO addition. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
Fig. 7. FTIR spectra of the LTCAT plasma thin films deposited from HMDSO with various O2 flow rates. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
A.C. Ritts et al. / Thin Solid Films 519 (2011) 4824–4829 Table 1 The elemental composition of the LTCAT plasma thin films obtained by XPS measurements. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate. O2 flow rate
Si2p
O1s
C1s
N1s
O/Si
C/Si
0 sccm 5 sccm 15 sccm 25 sccm
25.2 27.2 30.8 33.6
22.6 38.8 54.1 53.4
47.6 31.1 14.0 12.1
4.9 2.9 1.1 0.9
0.9 1.43 1.76 1.59
1.89 1.14 0.46 0.36
profile of the deposited HMDSO LTCAT plasma coatings. The RI profiles shown in Fig. 5 suggested relatively uniform RIs of the LTCAT plasma coatings. Such uniform RI distribution implies consistency in the LTCAT plasma coating quality. Fig. 6 shows the RI change of HMDSO LTCAT plasma coatings with addition of O2. Addition of O2 into the LTCAT system with a flow rate of 15 sccm and higher produced plasma coatings with RI between 1.470 and 1.490 which is the typical RI value of amorphous SiOx thin films [14–16]. This result indicates that the addition of O2 into the HMDSO LTCAT plasma deposition system was very effective in reducing or eliminating the hydrocarbon organic contents in the deposited HMDSO LTCAT plasma thin films. 3.2. Chemical analysis of the LTCAT plasma coatings FTIR was used to characterize the chemical structures of the LTACT plasma thin films deposited from gas mixtures of HMDSO and O2. Fig. 7 shows the FTIR spectra of the deposited HMDSO plasma thin films with different amounts of O2 addition in the gas mixtures. From the FTIR spectra, it was noted that an increase in the Si–O–Si main and shoulder peak intensities (1100–1000 cm−1) was observed with
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increasing O2 content in the HMDSO/O2 gas mixtures. Absorbance peaks due to Si–(CH3)x at wave numbers of 842–700 cm−1 were also observed in all the deposited LTCAT plasma thin films. Previous studies show that the plasma coatings deposited by LTCAT do not completely represent the molecular structure of the precursor or monomer molecules, but of its elemental composition [17]. The Si–(CH3)x structures observed in the HMDSO LTCAT plasma coatings could be caused by both the incomplete fragmentation of the HMDSO molecular structure and the plasma phase reactions of the molecular fragments [18]. An absorbance peak due to Si–OH stretching was observed at 930 cm−1 with 25 sccm O2 addition. From Fig. 7, it can be seen that no absorbance peak was observed at 1730 cm−1 and 2900 cm−1, which represent absorbance due to C–H and C_O stretching vibrations. Based on the FTIR results, therefore, it can be concluded that the LTCAT plasma coatings deposited from the HMDSO + O2 mixture are of inorganic nature. The qualitative information obtained from FTIR analysis complements the results achieved from XPS characterization of these LTCAT plasma thin films. The elemental composition results of the LTCAT plasma thin films obtained by XPS surface analysis are summarized in Table 1. It was noted that, with the increase of O2 addition into the LTCAT plasma deposition system, the organic carbon (C) content in the deposited LTCAT thin films was significantly reduced while the oxygen (O) content was significantly increased. As the oxygen flow rate increased to 15 sccm, the oxygen content in the film was almost doubled as compared with the plasma thin films deposited from pure HMDSO without O2 addition. These trends have also been observed in traditional radio frequency plasma chemical vapor deposition with HMDSO and O2 [19]. The reduction of the C content in the deposited LTCAT plasma thin films through O2 addition could be due to the following two mechanisms. The oxygen plasmas could preferentially etch the carbon content from the already deposited film surface by
Fig. 8. The deconvoluted Si2p peaks of the XPS spectra measured from the LTCAT plasma thin films deposited from HMDSO/O2 mixtures. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
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Table 2 Chemical composition of LTCAT thin films deposited at various O2 flow rates as determined by Si2p peak deconvolution of the XPS spectra. O2 flow rate
(CH3)3SiO
(CH3)2SiO2
(CH3)1 SiO3
SiO4
(CH3)X/SiOX
0 sccm 5 sccm 15 sccm 25 sccm
64.7 7.6 0 0
12.2 19.6 6.5 3.3
5.9 32.1 23.7 26.9
17.2 41.0 69.8 69.8
1.28 0.31 0.10 0.09
forming volatile byproduct gases. The oxygen plasmas could also oxidize the organic carbon fragments of HMDSO in the plasma states by forming non-depositing CO or CO2 gases, which were subsequently pumped out of the LTCAT system. Deconvoluting the Si2p XPS spectra gave a better insight into the quality of the LTCAT plasma thin films deposited with different amounts of O2 addition into the plasma system. The deconvolution of the Si2p XPS spectra illustrated in Fig. 8 shows that, with increasing O2 addition, an overall increase in the SiO4 structure and a continuous decrease in (CH3)3SiO2 and (CH3)2SiO2 structures were observed in the resulting LTCAT plasma thin films. Table 2 summarizes the chemical composition of the LTCAT plasma thin films using the peak area percentage of each substituent as well as the ratio of (CH3)x structure to SiOx structure. It can be seen in Table 2 that the ratio of CH3 to SiOx decreased significantly as the O2 flow rate increased. This leaves behind a film with primarily inorganic SiOx composition with O2 addition above 15 sccm. It appears that, based on the XPS data, the SiO4 content of the film was saturated at 69.8% with 15 sccm and higher O2 addition. The XPS results were well consistent with the FTIR data discussed in the previous paragraph. 3.3. Surface properties Water contact angles showed that the surface of the LTCAT plasma thin films was relatively uniform. Fig. 9 shows the change of the water contact angle of the LTCAT plasma thin films with O2 addition into the plasma system. It can be seen that the resulting coatings were hydrophobic without O2 addition and became more hydrophilic after O2 addition. This is consistent with the XPS data, which shows that the oxygen content in the resulting plasma coatings steadily increased with O2 addition. After O2 addition with a flow rate of 20 sccm, the water contact angle of the coating surface decreased to 31.7°, which is close to that of ~ 30° as measured on a SiO2 surface of inorganic glass [20]. The decreased water contact angle of the LTCAT plasma thin films deposited with increasing O2 addition is also consistent with the
Table 3 Hardness ratings of the LTCAT plasma thin films deposited at various O2 flow rates. O2 flow rate, sccm Coating hardness
0 2B
5 F
10 HB
15 6H
20 4H
more inorganic coating nature revealed by FTIR and XPS surface analysis results described in previous paragraphs. The pencil test as detailed in ASTM D3363-05 was used to evaluate the hardness of the deposited LTCAT plasma thin films and the test results were summarized in Table 3. The LTCAT plasma coatings deposited from pure HMDSO without O2 addition gave hardness of only 2B, which was rated based on the pencil hardness. In contrast, with 15 sccm O2 addition, the hardness of the deposited LTCAT plasma thin films increased to the highest pencil rating of 6H. This corresponds to the low organic C content and the high inorganic SiO4 structures of the XPS data obtained with O2 addition at 15 sccm and higher. The hardness rating of 6H obtained with HMDSO LTCAT plasma coatings is higher than that of the typical plasma coatings deposited by low temperature atmospheric plasma techniques [2], and is similar to other vacuum plasma deposited coatings [13,14]. 4. Conclusions SiOx plasma thin film coatings were successfully produced from HMDSO/O2 mixtures by using a low-temperature cascade arc torch (LTCAT) spray process. The property change of the resulting plasma thin films with O2 addition into the LTCAT plasma system was characterized and investigated by surface analysis and hardness measurements. It was found that the addition amount of O2 in the HMDSO/O2 gas mixtures has a significant influence on both the chemical structure and mechanical properties of the deposited LTCAT plasma thin films. Surface analysis results by FTIR, XPS, and water contact angle measurements showed that, with increased O2 addition, the resulting LTCAT plasma thin films were of inorganic nature. Hardness measurement demonstrated that, by appropriately adjusting operating parameters, LTCAT plasma deposition from HMDSO/O2 gas mixtures produced plasma thin films with reasonable hardness ratings. While further evaluation of these LTCAT plasma thin films is required for industrial applications, the present study concludes that the LTCAT is a promising means of producing SiOx type hard coatings. Acknowledgements This study was financially supported by the Industrial Technology Research Institute (ITRI), Taiwan, through an international research collaboration program. The authors would also like to thank the US Department of Education (DOE) Graduate Assistance in Areas of National Need (GAANN) Fellowship for the financial support to Mr. Andy Charles Ritts, who is a GAANN fellow. References
Fig. 9. Change in the water contact angle of the LTCAT plasma thin film coatings with oxygen addition at various flow rates. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
[1] Y.B. Guo, F. Chau-Nan Hong, Diamond Relat. Mater. 12 (3–7) (2003) 946. [2] C. Huang, C.H. Liu, C.H. Su, W.T. Hsu, S.Y. Wu, Thin Solid Films 517 (17) (2009) 5141. [3] F. Arefi, V. Andre, P. Montazer-Rahmati, J. Amouroux, Pure Appl. Chem. 64 (5) (1992) 715. [4] T. Hirotsu, P. Nugroho, J. Appl. Polym. Sci. 66 (6) (1997) 1049. [5] D. Rats, V. Hajek, L. Martinu, Thin Solid Films 340 (1–2) (1999) 33. [6] M. Arakawa, K. Sukata, M. Shimada, Y. Agari, J. Appl. Polym. Sci. 100 (6) (2006) 4273. [7] J. Bouchet, G. Rochat, Y. Leterrier, J.A.E. Månson, P. Fayet, Surf. Coat. Technol. 200 (14–15) (2006) 4305. [8] S.P. Fusselman, H.K. Yasuda, Plasma Chem. Plasma Process. 14 (3) (1994) 251. [9] S.P. Fusselman, H.K. Yasuda, Plasma Chem. Plasma Process. 14 (3) (1994) 277. [10] Q.S. Yu, H.K. Yasuda, J. Polym. Sci. A Polym. Chem. 37/7 (1999) 967. [11] D.J. Bettge, G. Hinrichsen, Compos. Sci. Technol. 47 (2) (1993) 131.
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J.H. Lee, D.S. Kim, Y.H. Lee, J. Electrochem. Soc. 143 (4) (1996) 1443. Q.S. Yu, H.K. Yasuda, J. Polym. Sci. A Polym. Chem. 37/11 (1999) 1577. Y. Sawada, S. Ogawa, M. Kogoma, J. Phys. D Appl. Phys. 28 (8) (1995) 1661. S. Sahli, Y. Segui, S. Ramdani, Z. Takkouk, Thin Solid Films 250 (1994) 206. Y. Wu, H. Suginura, Y. Inoue, Thin Solid Films 435 (2003) 161.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
SiOx plasma thin film deposition using a low-temperature cascade arc torch A.C. Ritts a, C.H. Liu b, Q.S. Yu a,⁎ a
Center for Surface Science and Plasma Technologies, Department of Mechanical and Aerospace Engineering, University of Missouri, E2403D Lafferre Hall, Columbia, MO 65211, USA Plasma Science and Application Department, Advanced Manufacturing Core Technology Div. M200, MSL/ITRI, Rm. 144, Bldg. 11, 195, Sec. 4, Chung Hsing Rd. Chutung, Hsinchu 310, Taiwan, ROC
b
a r t i c l e
i n f o
Available online 13 January 2011 Keywords: Hexamethyldisiloxane Refractive index Hardness X-ray photoelectron spectroscopy Plasma deposition Cascade arc torch
a b s t r a c t Plasma thin films were deposited from gas mixtures of hexamethyldisiloxane (HMDSO) and oxygen (O2) using a low-temperature cascade arc torch (LTCAT). Various properties of the deposited HMDSO plasma coatings, including refractive index (RI), surface contact angle, and hardness were evaluated. The characterization results using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, ellipsometry, and water contact angle measurements indicated that, with increased O2 addition, the deposited HMDSO plasma thin films were of inorganic SiOx nature. It was also found that, in the LTCAT plasma system, O2 addition significantly improves the hardness of the resulting HMDSO plasma coatings. The film hardness of the deposited HMDSO plasma coatings measured by a standard pencil test (ASTM D3363-05) reached 6H with increased O2 addition in the HMDSO/O2 gas mixture. Such hard plasma coatings could be potentially used for many important industrial applications, such as anti-scratch coatings on plastic glasses and various plastic lens materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Many materials with desirable bulk qualities lack specific surface properties required for specific industrial applications. To meet the application requirements, a variety of surface treatment methods are needed to modify the surface and improve the surface characteristics of these materials. In recent years plastic materials are alternatives to inorganic glasses due to their desirable bulk characteristics, light weight, low cost, and high toughness [1]. On the other hand, the poor surface characteristics of plastics, such as low hardness and low wear resistance, limit their use to mild applications [2]. Various surface treatment methods have been applied to plastic surfaces in order to improve their surface properties [3–7]. Wet chemical methods are commonly utilized because they are easily implemented. However wet chemical methods produce thick, non-uniform coatings and the chemicals required are generally deemed as environmentally hazardous [1]. As environmentally friendly processes, various plasma chemical vapor deposition techniques have been recognized as the suitable and promising alternatives to wet chemical methods for surface modification of plastic materials by depositing hard and controllable thin coating to improve their wear resistance and anti-scratch properties [1,2]. In comparison with the conventional plasma deposition techniques, the low-temperature cascade arc torch (LTCAT) technique offers a very different plasma deposition process in that the activation and
⁎ Corresponding author. Tel.: +1 573 882 8076; fax: +1 573 884 5090. E-mail addresses: [email protected] (C.H. Liu), [email protected] (Q.S. Yu). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.037
deactivation of the precursors (monomers) are decoupled [8–10]. The major feature of LTCAT plasma deposition is a two step activation process. First inert argon (Ar) carrier gas was activated to form Ar plasmas inside the arc column and then emanates from the arc column into the reaction chamber as luminous Ar gas plasma torch. During this process most charged plasma species were confined inside the arc column. Plasma diagnosis results showed that the main plasma species of the luminous Ar gas plasma torch observed in the chamber are electronically excited Ar neutrals [9,10]. Second precursor (monomer) gases, such as hexamethylsiloxane (HMDSO) and oxygen (O2) that were introduced into the Ar gas plasma torch, were activated mainly by the electronically excited Ar neutrals. This two step activation process provides better control of the reactive gas activation and minimizes the electron impact ionization. In traditional plasma processes, many undesirable plasma reactions such as plasma induced degradation of the polymer chains and etching of the surface materials could occur on a plastic substrate [3]. These undesirable reactions could result in significant detrimental effects on the plastic substrate and adversely affect their successful applications. One typical example of such an adverse effect is the adhesion failure due to the formation of weak-boundary-layer after plasma treatment [11]. In the LTCAT process, however, the Ar plasma torch emanated from the arc column enters the plasma chamber with gas phase temperature decreasing dramatically during expansion of the gas [12]. The decreased Ar plasma torch temperature and the reduced ionization reactions provide a safe and less harsh environment for the treated plastic substrates, which are sensitive to temperature and ion bombardment. This minimizes the detrimental effects on plastic surfaces usually suffered from the traditional plasma treatment process.
A.C. Ritts et al. / Thin Solid Films 519 (2011) 4824–4829
In this study, SiOx plasma thin films were deposited from HMDSO and O2 gas mixtures by the LTCAT and the plasma coating properties were investigated in terms of O2 addition amount in the gas mixture. In contrast to traditional plasma processes, the LTCAT deposition technique provides a very promising method for large-scale industrial applications because the LTCAT plasma torch can be simply applied to a large surface through a scanning process. 2. Experimental details The LTCAT plasma system used in this study has been described elsewhere [8–10]. The cathode assembly and cascade arc column depicted in Fig. 1 were fabricated by PlasmaCarb, Inc (Bedford, NH). As deposition substrates, Si wafers cleaned with acetone were attached to a 3 × 6 inch aluminum panel using double side tapes. The aluminum panel was then loaded inside the LTCAT plasma chamber at a position of 20 cm from the plasma torch outlet from the arc column. The chamber was then evacuated to a base pressure of 5 mTorr or less. Ar gas with 99.9995% purity purchased from Airgas (St. Louis, US) was introduced into the LTCAT plasma system through the arc column at a flow rate fixed at 3000 sccm, which was chosen by considering the arc stability and achieving reasonable deposition rate at a 6.0 A arc current. A mixture of HDMSO vapor and O2 gas was introduced directly into the Ar plasma torch at the position shown in Fig. 2. The flow rate of HMDSO was fixed at 5 sccm and the O2 flow rate was varied at 0, 5, 10, 15, 20, and 25 sccm. The LTCAT Ar plasma was ignited with 6 A of arc current which was drawn from an MDX-5K dc power supply (Advanced Energy Corporation). The thickness and refractive indices (RIs) of the LTCAT plasma thin films were measured using an AutoEL-II automatic ellipsometer (Rudolph Research Corp., Flanders, NJ) with a 632.8 nm helium–neon laser light source. The coating thickness of the LTCAT plasma thin films for surface analysis and mechanical characterization was controlled by varying deposition time and confirmed by ellipsometry. The chemical structure of the deposited plasma thin films was characterized by a Fourier transform infrared (FTIR) spectrometer (Spectrum100, Perkin-Elmer, Waltham, Massachusetts). The FTIR spectra were obtained from an average of 32 scans in the range of Insulating Material
Cathode Metal Disks
Coolant Channel
4825
Reactive gas
A+ _ Argon gas
+
A
-
A*
A
A* e-
e
A
*
A*
A* A* A A+
A*
Vacuum chamber
A+
-
A*
e A*
A A* A A*
Substrate A*
A : ground state atomt A*: excited state atom A +: positive ion -
e : negative electron Fig. 2. Depiction of energy transfer of Ar to monomers (A) in a LTCAT plasma process.
400–4000 cm−1 at a resolution of 4 cm−1. Elemental analysis was determined by X-ray photoelectron spectroscopy (XPS) (Thermo VG/ ESCAlab 250) with Mg Kα source (1253.6 eV). The surface contact angles of the LTCAT plasma thin films were determined by the sessile droplet method using the VCA 2500XE contact angle measurement system (Advanced Surface Technologies, Inc., Billerica, MA). The coating thickness of the LTCAT thin films prepared for surface analysis was in the range of 80–100 nm. The plasma coating hardness was determined by a standard pencil by following the testing and rating procedures described in ASTM D3363-05. The pencils with rated hardness from 6H to 6B (hard to soft) were purchased from Derwent, Inc. (Cambria, UK). In this method, the flat lead (graphite) head was placed against the coated Si substrates at a 45° angle and pushed forward to make a scratch on the LTCAT plasma coatings which were deposited on silicone wafer substrates. The pencils were tested, starting with hardness 6H and proceeding down one level of hardness, until one pencil was found that could not make a scratch on the substrate. The hardness rating of the final test pencil was defined as the hardness of the plasma coatings. For hardness measurements, the coating thickness of the LTCAT plasma thin films was controlled between 160 and 180 nm. 3. Results and discussion 3.1. Uniformity of the LTCAT plasma coating
Anode Plasma Jet Fig. 1. Depiction of the typical electrode configuration of the arc generator column of a LTCAT.
Fig. 3 shows the thickness profiles of LTCAT plasma thin films with varying oxygen flow rates. It can be seen that the center of the LTCAT plasma torch gave the highest deposition rate. It was also noted that an increase in O2 addition into the plasma, in which the HMDSO flow rate was fixed at 5 sccm, reduced the deposition rate of HMDSO plasma coatings. As seen from Fig. 4, the glow intensity and the length/volume of the LTCAT plasma torch were weakened and shrunk respectively with the increased addition of O2 into the plasma system in which other operation parameters were kept unchanged. The increase of O2 addition into the LTCAT could increase the plasma ashing effect during the HMDSO plasma deposition through removing organic hydrocarbon contents out of the LTCAT plasma deposition system. Our previous study on the deposition of pure HMDSO in the LTCAT system revealed a dependence of the deposition rate on the Ar flow rate and power input [10]. The maximum deposition rates observed in that study were around 7 nm/s at 2000 sccm Ar flow rate and 8.0 A arc current. As seen in Fig. 3, the operating condition used in this study with an Ar flow rate of 3000 sccm and 6.0 A arc current gave rise to the
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Fig. 5. Refractive index profile of the LTCAT plasma thin films deposited from HMDSO with varying O2 flow rate. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate. Fig. 3. Changes in the deposition profile of LTCAT plasma thin coatings from HMDSO with varying O2 flow rate. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
similar deposition rate while pure HMDSO was used without adding O2. These deposition rates shown in Fig. 3 are higher than that reported in typical low pressure radio frequency plasma deposition systems [13,14]. From Fig. 3, it was also noted that the deposition rate appears to saturate with O2 addition into the HMDSO LTCAT deposition system at a flow rate of 15 sccm.
The refractive index (RI) of a plasma coating is a useful indication of the quality and uniformity of the deposited plasma thin films. The RI value of a plasma coating was determined by the mass density and the chemical composition of the coating [15,16]. Fig. 5 shows the RI
Fig. 6. Change of the average refractive index of LTCAT plasma thin films deposited from HMDSO with various O2 flow rates. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
Fig. 4. Visual comparison of the LTCAT plasma glow (a) before and (b) after 5 sccm HDMSO addition. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
Fig. 7. FTIR spectra of the LTCAT plasma thin films deposited from HMDSO with various O2 flow rates. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
A.C. Ritts et al. / Thin Solid Films 519 (2011) 4824–4829 Table 1 The elemental composition of the LTCAT plasma thin films obtained by XPS measurements. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate. O2 flow rate
Si2p
O1s
C1s
N1s
O/Si
C/Si
0 sccm 5 sccm 15 sccm 25 sccm
25.2 27.2 30.8 33.6
22.6 38.8 54.1 53.4
47.6 31.1 14.0 12.1
4.9 2.9 1.1 0.9
0.9 1.43 1.76 1.59
1.89 1.14 0.46 0.36
profile of the deposited HMDSO LTCAT plasma coatings. The RI profiles shown in Fig. 5 suggested relatively uniform RIs of the LTCAT plasma coatings. Such uniform RI distribution implies consistency in the LTCAT plasma coating quality. Fig. 6 shows the RI change of HMDSO LTCAT plasma coatings with addition of O2. Addition of O2 into the LTCAT system with a flow rate of 15 sccm and higher produced plasma coatings with RI between 1.470 and 1.490 which is the typical RI value of amorphous SiOx thin films [14–16]. This result indicates that the addition of O2 into the HMDSO LTCAT plasma deposition system was very effective in reducing or eliminating the hydrocarbon organic contents in the deposited HMDSO LTCAT plasma thin films. 3.2. Chemical analysis of the LTCAT plasma coatings FTIR was used to characterize the chemical structures of the LTACT plasma thin films deposited from gas mixtures of HMDSO and O2. Fig. 7 shows the FTIR spectra of the deposited HMDSO plasma thin films with different amounts of O2 addition in the gas mixtures. From the FTIR spectra, it was noted that an increase in the Si–O–Si main and shoulder peak intensities (1100–1000 cm−1) was observed with
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increasing O2 content in the HMDSO/O2 gas mixtures. Absorbance peaks due to Si–(CH3)x at wave numbers of 842–700 cm−1 were also observed in all the deposited LTCAT plasma thin films. Previous studies show that the plasma coatings deposited by LTCAT do not completely represent the molecular structure of the precursor or monomer molecules, but of its elemental composition [17]. The Si–(CH3)x structures observed in the HMDSO LTCAT plasma coatings could be caused by both the incomplete fragmentation of the HMDSO molecular structure and the plasma phase reactions of the molecular fragments [18]. An absorbance peak due to Si–OH stretching was observed at 930 cm−1 with 25 sccm O2 addition. From Fig. 7, it can be seen that no absorbance peak was observed at 1730 cm−1 and 2900 cm−1, which represent absorbance due to C–H and C_O stretching vibrations. Based on the FTIR results, therefore, it can be concluded that the LTCAT plasma coatings deposited from the HMDSO + O2 mixture are of inorganic nature. The qualitative information obtained from FTIR analysis complements the results achieved from XPS characterization of these LTCAT plasma thin films. The elemental composition results of the LTCAT plasma thin films obtained by XPS surface analysis are summarized in Table 1. It was noted that, with the increase of O2 addition into the LTCAT plasma deposition system, the organic carbon (C) content in the deposited LTCAT thin films was significantly reduced while the oxygen (O) content was significantly increased. As the oxygen flow rate increased to 15 sccm, the oxygen content in the film was almost doubled as compared with the plasma thin films deposited from pure HMDSO without O2 addition. These trends have also been observed in traditional radio frequency plasma chemical vapor deposition with HMDSO and O2 [19]. The reduction of the C content in the deposited LTCAT plasma thin films through O2 addition could be due to the following two mechanisms. The oxygen plasmas could preferentially etch the carbon content from the already deposited film surface by
Fig. 8. The deconvoluted Si2p peaks of the XPS spectra measured from the LTCAT plasma thin films deposited from HMDSO/O2 mixtures. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
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Table 2 Chemical composition of LTCAT thin films deposited at various O2 flow rates as determined by Si2p peak deconvolution of the XPS spectra. O2 flow rate
(CH3)3SiO
(CH3)2SiO2
(CH3)1 SiO3
SiO4
(CH3)X/SiOX
0 sccm 5 sccm 15 sccm 25 sccm
64.7 7.6 0 0
12.2 19.6 6.5 3.3
5.9 32.1 23.7 26.9
17.2 41.0 69.8 69.8
1.28 0.31 0.10 0.09
forming volatile byproduct gases. The oxygen plasmas could also oxidize the organic carbon fragments of HMDSO in the plasma states by forming non-depositing CO or CO2 gases, which were subsequently pumped out of the LTCAT system. Deconvoluting the Si2p XPS spectra gave a better insight into the quality of the LTCAT plasma thin films deposited with different amounts of O2 addition into the plasma system. The deconvolution of the Si2p XPS spectra illustrated in Fig. 8 shows that, with increasing O2 addition, an overall increase in the SiO4 structure and a continuous decrease in (CH3)3SiO2 and (CH3)2SiO2 structures were observed in the resulting LTCAT plasma thin films. Table 2 summarizes the chemical composition of the LTCAT plasma thin films using the peak area percentage of each substituent as well as the ratio of (CH3)x structure to SiOx structure. It can be seen in Table 2 that the ratio of CH3 to SiOx decreased significantly as the O2 flow rate increased. This leaves behind a film with primarily inorganic SiOx composition with O2 addition above 15 sccm. It appears that, based on the XPS data, the SiO4 content of the film was saturated at 69.8% with 15 sccm and higher O2 addition. The XPS results were well consistent with the FTIR data discussed in the previous paragraph. 3.3. Surface properties Water contact angles showed that the surface of the LTCAT plasma thin films was relatively uniform. Fig. 9 shows the change of the water contact angle of the LTCAT plasma thin films with O2 addition into the plasma system. It can be seen that the resulting coatings were hydrophobic without O2 addition and became more hydrophilic after O2 addition. This is consistent with the XPS data, which shows that the oxygen content in the resulting plasma coatings steadily increased with O2 addition. After O2 addition with a flow rate of 20 sccm, the water contact angle of the coating surface decreased to 31.7°, which is close to that of ~ 30° as measured on a SiO2 surface of inorganic glass [20]. The decreased water contact angle of the LTCAT plasma thin films deposited with increasing O2 addition is also consistent with the
Table 3 Hardness ratings of the LTCAT plasma thin films deposited at various O2 flow rates. O2 flow rate, sccm Coating hardness
0 2B
5 F
10 HB
15 6H
20 4H
more inorganic coating nature revealed by FTIR and XPS surface analysis results described in previous paragraphs. The pencil test as detailed in ASTM D3363-05 was used to evaluate the hardness of the deposited LTCAT plasma thin films and the test results were summarized in Table 3. The LTCAT plasma coatings deposited from pure HMDSO without O2 addition gave hardness of only 2B, which was rated based on the pencil hardness. In contrast, with 15 sccm O2 addition, the hardness of the deposited LTCAT plasma thin films increased to the highest pencil rating of 6H. This corresponds to the low organic C content and the high inorganic SiO4 structures of the XPS data obtained with O2 addition at 15 sccm and higher. The hardness rating of 6H obtained with HMDSO LTCAT plasma coatings is higher than that of the typical plasma coatings deposited by low temperature atmospheric plasma techniques [2], and is similar to other vacuum plasma deposited coatings [13,14]. 4. Conclusions SiOx plasma thin film coatings were successfully produced from HMDSO/O2 mixtures by using a low-temperature cascade arc torch (LTCAT) spray process. The property change of the resulting plasma thin films with O2 addition into the LTCAT plasma system was characterized and investigated by surface analysis and hardness measurements. It was found that the addition amount of O2 in the HMDSO/O2 gas mixtures has a significant influence on both the chemical structure and mechanical properties of the deposited LTCAT plasma thin films. Surface analysis results by FTIR, XPS, and water contact angle measurements showed that, with increased O2 addition, the resulting LTCAT plasma thin films were of inorganic nature. Hardness measurement demonstrated that, by appropriately adjusting operating parameters, LTCAT plasma deposition from HMDSO/O2 gas mixtures produced plasma thin films with reasonable hardness ratings. While further evaluation of these LTCAT plasma thin films is required for industrial applications, the present study concludes that the LTCAT is a promising means of producing SiOx type hard coatings. Acknowledgements This study was financially supported by the Industrial Technology Research Institute (ITRI), Taiwan, through an international research collaboration program. The authors would also like to thank the US Department of Education (DOE) Graduate Assistance in Areas of National Need (GAANN) Fellowship for the financial support to Mr. Andy Charles Ritts, who is a GAANN fellow. References
Fig. 9. Change in the water contact angle of the LTCAT plasma thin film coatings with oxygen addition at various flow rates. Plasma conditions were: 3000 sccm Ar flow rate, 6.0 A arc current, and 5 sccm HMDSO flow rate.
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