Amorphous silicon carbonitride films modified by plasma immersion ion implantation

Amorphous silicon carbonitride films modified by plasma immersion ion implantation

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Vacuum xxx (2014) 1e4

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Amorphous silicon carbonitride films modified by plasma immersion ion implantation R.G.S. Batocki a, R.P. Mota a, R.Y. Honda a, D.C.R. Santos b, * a b

Plasma Laboratory, College of Engineering, UNESP, 12516-410 Guaratinguetá, SP, Brazil FATEC, College of Technology, 12455-010 Pindamonhangaba, SP, Brazil

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 September 2013 Accepted 3 January 2014

Amorphous silicon carbonitride (a-SiCN:H) films were deposited from hexamethyldisilazane (HMDSN) organic compounds via radio-frequency (RF) glow discharges. Afterwards the films were bombarded, from 15 to 60 min, with nitrogen ions using Plasma Immersion Ion Implantation (PIII) technique. X-ray photoelectron spectroscopy (XPS) showed that Oecontaining groups increased, while CeC and/or CeH groups decreased with treatment time. This result indicates chemical alterations of the polymeric films with the introduction of polar groups on the surface, which changes the surface wettability. In fact, the hydrophobic nature of a-SiCN:H films (contact angle of 100 ) was changed by nitrogen ion implantation and, and after aging in atmosphere air, all samples preserved the hydrophilic character (contact angle <80 ) independently of treatment time. The exposure of the films to oxygen plasma was performed to evaluate the etching rate, which dropped from 24% to 6% while the implantation time increased from 15 to 60 min. This data suggests that PIII increased the film structure strength, probably due to crosslinking enhancement of polymeric chains. Therefore, the treatment with nitrogen ions via PIII process was effective to modify the wettability and oxidation resistance of a-SiCN:H films. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: HMDSN PIII XPS Etching rate Wettability

1. Introduction Plasma Process is a well established technique to synthesize solid materials as powders or thin films from organic compounds, and is performed using electrical discharges at low or high pressures, as well as at the atmospheric pressure [1e4]. Plasma polymerization is a nonequilibrium process and consequently, the different temperature of the electrons and ions influence the polymerization mechanism. Therefore, molecular structure and chemical composition of plasma-polymerized (PP) films can be completely different compared with those of conventional polymers synthesized by chemical processes. PP films present an amorphous structure, their chains are highly branched and crosslinked, and present good adhesion on different substrates, such as metal, polymer, ceramic and wood [1,2]. All these features make the PP films highly attractive for many technological applications, such as protective coatings, optoelectronic devices, and biomedical coatings, among others [5e9]. However, PP films can present poor

* Corresponding author. FATEC, College of Technology, Rod. Vereador Fabrício Dias, 4010, Pindamonhangaba 12455-010, SP, Brazil. Tel.: þ55 012 3648 8756. E-mail addresses: [email protected], [email protected] com (D.C.R. Santos).

mechanical and tribological properties, mainly during operations of sliding conditions against harder materials, which limit their application. In this sense, surface modification processes can be used to improve their mechanical and wear properties, such as ion implantation using high energy ion beam [10,11]. In this technique, the bombardment alters the chemical composition and molecular structure of the target surface, as well as the shelter layers, altering its surface properties. However, this is an expensive technique because the equipment is a line-of-sight ion gun, which treat the surface locally and demands time and energy to treat large surfaces or complex shape components [12,13]. An alternative technique is Plasma Immersion Ion Implantation (PIII), in which the samples biased periodically with higher negative pulses are bombarded by plasma ions. This is a clean and dry process, which can carry out one-step treatments at lower time and with higher ion implantation rate [12,13]. In this technique, the pulse characteristics (amplitude, frequency, duration), the discharge parameters (chemical specie, pressure, power, etc.) and the type of target material determine the final surface properties of the treated materials [12,13]. PIII has been applied for surface modification of metals, semiconductors and conventional polymers, but it is an innovative technique for treatment of PP films. For that reason, this work investigated the chemical structure and surface properties of the PP HMDSN films treated by PIII using nitrogen ions.

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2. Experimental The films investigated in this work were deposited on glass and polished silicon substrates, both previously cleaned in ultrasonic baths. Initially, the substrates were washed in a mixture of deionized water and detergent, and then rinsed in tap water. After this stage, the substrates were washed in petrol ether to remove any organic contamination from their surface, and finally they dried in an oven. Thin films were produced via Plasma Enhanced Chemical Vapor Deposition (PECVD). In order to do that, it was adjusted the gas pressure in the chamber. A turbomolecular pump was used to evacuate the reactor to the base pressure of 1.0  103 Pa. Then, argon gas was introduced into the reactor to reach the minimum operating pressure of rotary pump around 4.0  101 Pa. Finally, HMDSN vapor was added to reach the working pressure of 5.3 Pa. The system employed to perform both PECVD and PIII treatments present parallel-plate electrodes capacitively connected to RF power supply of 13.56 MHz, as described elsewhere [14]. The deposition discharge was established at 50 W of power applied to the upper electrode, while the lower electrode (substrate holder) was grounded. PECVD process lasted 30 min. After the deposition, the samples were submitted to PIII treatment. In this case, after the procedure to achieve the base pressure as described above, nitrogen was introduced into the chamber until reaching the working pressure of 5.3 Pa. The bombardment discharge was established at 50 W of RF power, while the substrate holder was connected to pulse supply. The pulse parameters were: 30 KV amplitude, 100 Hz of frequency, and 5 ms of duration. The PIII treatment ranged from 15 to 60 min. After nitrogen PIII, in the same plasma system, the PP films were exposed to etching discharge. In this process, oxygen was introduced in the reactor to reach the work pressure of 5.3 Pa, the discharge was ignited at 50 W of RF power, and the exposure time was 30 min. The thickness of the etched layer from the samples was measured in a Tencor Alpha-Step 500 profilemeter (INPE-São José dos Campos). Atomic chemical composition of the film surface was analyzed by X-ray photoelectron spectroscopy (XPS) using an XSAM HS Kratos spectromicroscope (UFSCAR e São Carlos). This analysis was performed in an ultra high vacuum environment, employing Mg Ka radiation fixed at 1253.6 eV of energy and 30 W of power. The spectra were fitted using a Gauss function, and the Shirley background was subtracted before peak fitting. All peak binding energies were referenced to C 1s peak at 284.8 eV related to CeC, CeH carbon bonds. 3. Results and discussion XPS survey spectra of the PP HMDSN films revealed the presence of four bands of binding energies recognized as carbon (C 1s), oxygen (O 1s), nitrogen (N 1s) and silicon (Si 2p). Their atomic concentrations are shown in Fig. 1 as a function of PIII treatment time. Although the HMDSN, (CH3)3eSieNHeSie(CH3)3, does not contain O atoms in its structure, and the O2 gas was not added in the plasma processes, this element appears on the as-deposited HMDSN film (t ¼ 0 s) structure and its concentration increases with PIII treatment time. This is usually observed in plasma processes and it is explained by post-reactions between free radicals trapped in the film structure with oxygen and water vapor from the environment [1e3]. When ions penetrate the polymer, the collisions along the ion tracks induce bond scissions, excitation, and ionization, among other effects [15,16]. Ionizations are responsible for the creation of free radicals, which can readily recombine via chain cross-linking and carbon unsaturated bonds [15,16].

Fig. 1. C, O, Si and N atomic concentration as a function of implantation time.

Incomplete chemical reactions left free radicals in the polymer structure, allowing post-reactions with environment reactive species, such as oxygen and nitrogen. Considering that plasma system consists of a turbomolecular pump, which allows reaching a base pressure of 104 Pa, and that there is no leak in the system, postreactions must be occurring when the films are removed from the reactor. As can be observed in Fig. 1, O atomic concentration increases rapidly for 15 min of implantation, while C and N atomic concentration decay at the same time. It is believed that free radicals are sparsely produced at the initial stage of the treatment and the average distance between radicals does not permit the chain cross-linking. Therefore, recombination reactions occur within polymeric chains through unsaturated bonds or, on the other hand, the bonds can be broken [15,16]. The bond breakage usually results in the emission of small molecules, such as CHx, NO, CO, H2, C, and H atoms, as verified by many researchers [17]. Consequently, C, H and N atom concentration may be diminished after the implantation. It is important to remember that XPS analysis cannot evaluate the hydrogen concentration, but it is known that these atoms are taken from the polymer surface because they are weakly bonded to polymeric chain termination [15e17]. Therefore, for a shorter treatment time, species are removed from the polymeric film due to bond scissions and free radicals and unsaturated bonds can be created in the polymeric structure, but the cross-linking may not be favored in this initial stage of implantation. Nevertheless, longer treatments allow successive cross-linking, which enhances the structural stiffness of the polymeric films and avoids the creation of free radicals. It is perceived by the saturation behavior observed in O, C and N concentration after 30 min of the process. Si atomic content it seems to increase after implantation but, if Si atoms were not introduced in the process, this amount is statistically constant upon treatment. Therefore, silicon atoms are not removed from PP HMDSN films by the implantation. From literature data [18], according to XPS analysis, HMDSN monomer presents 67% carbon, 22% silicon and 11% nitrogen. In our work, Si concentration in the polymeric film is around 20% and, therefore, it is practically the same proportion reported for the monomer. The deconvolution of C 1s high-resolution spectra resulted in three components located at 284.6 eV, 286.5 eV and 288.4 eV, respectively identified as CeC/Ce H, CeO/NeO and C]O bonds. The molecular concentration of these species is depicted in Fig. 2. As can be seen, CeC/CeH bond

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concentration is reduced drastically after shorter ion implantation, which confirms that CeC and CeH bonds are broken during the process. Consequently, such scissions resulted in loss of C and H atoms, as verified in atomic concentration. At the same time, CeO and C]O bond content increased, indicating recombination processes through unsaturation and oxidation. Similar results were verified in other work about PIII-treated PP acetylene films [19]. Observing Fig. 2, CeO and C]O molecular concentrations show a trend to saturation at longer implantation, which is interpreted as a consequence of the saturation of free radicals in the samples, as well as of the crosslinking saturation. Fig. 3 depicts the contact-angle measurements as a function of the aging time in the air. As can be seen, PP HMDSN films present a typically hydrophobic character [20] which is evidenced by contact angle of 98 . Immediately after ion implantation, this measure decreased drastically to values lower than 20 , which is explained by the incorporation of oxygen, as discussed before. However, upon aging under atmospheric conditions, the contact-angle of the films tends to increase. This behavior is commonly seen after plasma

processes, and it is interpreted as a reorganization of the polymeric structure [1e3]. The incorporation of oxygen species in the sample surface generates a gradient of polar group concentration between the surface and the bulk of the polymeric film, altering its enthalpy and entropy [21,22]. As a result, the mobile polymeric chains tend to bury the polar groups of the surface into the bulk, which diminishes the surface wettability of the samples [23]. Observing Fig. 3, the hydrophobic recovery occurred at the same rate for all the samples and, therefore, regardless of the PIII treatment time. Notwithstanding, after 50 days of aging, the samples still show a hydrophilicity character indicated by a contact angle around 78 . Therefore, the wettability of the films was modified by nitrogen PIII treatment. Considering that ion implantation also promotes alteration in the surface roughness of the samples, the wettability of the PP HMDSN films probably occurred due to chemical and morphological alterations of the surface [22,23]. Fig. 4 shows the etching rate of the PP HMDSN films as a function of treatment time. As can be noticed, etching rate diminished from 0.55 to 0.42  A/s after PIII treatment and, therefore, there was a slight increase of the oxidation resistance of the implanted samples. Similar results were found in another work about PP acetylene films treated by helium and nitrogen PIII [19]. Considering that RF oxygen plasmas produce a large amount of atomic oxygen, which react with C and H atoms on the polymer surface and creates CO, CO2 and H2O gaseous species [1e3], such species are removed from the surface, diminishing the thickness of the films. This ablation process is, of course, highly dependent of the polymeric structure. It is known that the crosslinking degree is related to the hardness of the samples and, therefore, harder surfaces offer higher resistance to oxygen plasmas. Thus, the decrease of the etching rate of the HMDSN films is interpreted as the improvement of crosslinking and hardness. Many papers report the crosslinking becomes the polymeric structure more cohesive, which is responsible by extreme hardness and chemical inertness. The hardness of PIII-treated PP HMDSN films was reported in another work [24], and it increased almost fivefold within 60 min of processing. In this work, the oxidation resistance increased around 25% at the same range, and it suggests there is not a linear relation between the crosslinking degree and the etching rate. But, anyway, nitrogen PIII improved the oxidation resistance of PP HMDSN films.

Fig. 3. Contact angles of the films as a function of aging time.

Fig. 4. Etching rate of the films as a function of implantation time.

Fig. 2. CeC/CeH, CeO and C]O molecular proportion as a function of treatment time.

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4. Conclusion PP HMDSN films treated by nitrogen PIII presented significant alterations in their surface properties. The effect of PIII treatment usually depends of process time, but the wettability of the samples was altered independently of the implantation time. PP HMDSN films presented hydrophilic character after the treatment, and showed stability after 30 days of aging. There was an improvement of oxygen resistance of the films, probably due to structural modification, which promoted the hardening of the sample surface. Thus, the PIII treatment shows an effective technique to modify plasma polymers, and it is a promising treatment to enhance the tribological properties of the PP HMDSN films, expanding their technological application. Acknowledgments The authors thanks to FAPESP for financial support and Professor Pedro Augusto de Paula Nascente (UFSCAR) for technical assistance. References [1] Yasuda H. Plasma polymerization. Orlando: Academic Press; 1985. [2] Biederman H, editor. Plasma polymer films. London: Imperial College Press; 2004. [3] d’Agostino R, Favia P, Kawai Y, Ikegami H, Sato N, Arefi-Khonsari F, editors. Advanced plasma technology. Weinheim: Wiley-VCH; 2008. [4] Harry J. Introduction to plasma technology: science, engineering and applications. Weinheim: Wiley-VCH; 2010.

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