Composite Interfaces 2012, 1–10, iFirst Article
Chemical synthesis and surface morphology of amorphous hydrogenated carbon nitride film deposited by N2/CH4 dielectric barrier discharge plasma Abhijit Majumdara*, Sadhan Chandra Dasa,b, T. Shripathib and Rainer Hipplera a
Institut für Physik, Ernst-Moritz-Arndt-Universität Greifswald, Felix-Hausdorff-Str. 6, 17489 Greifswald, Germany; bUniversity Grant Commission-Department of Atomic Energy Consortium for Scientific Research, Indore 452017, M.P. India (Received 17 February 2012; accepted 31 May 2012) A dielectric barrier discharge of N2:CH4 mixture was operated at 500 mbar was employed to deposit poly-amide (HCNx) film, under low input power and atmospheric pressure. The top surface layer of amorphous hydrogenated carbon nitride (a-HCNx) is analyzed by X-ray photoelectron spectroscopy (XPS). Bulk a-HCNx is analyzed by means of fourier transform infrared (FTIR) spectroscopy. The deposited polymer having triple bond (–C≡N/–C≡C–) at 2180 cm 1 and carbonyl-amide group –(C=O)NH– at 1680 cm 1, were synthesized by FTIR spectroscopy to elucidate the nature of the products formed from N2/CH4 mixture. The nitrile group (–C≡N/–) is decreasing with the elevated annealing temperature is obtained from FTIR spectrum. Shake-up satellite peaks are observed in XPS spectrum at room temperature. The shake-up peaks disappear gradually as the annealing temperature is increased from 50 to 300 °C. Simultaneously, the chemical shift of 1s electron shifted to its own advantageous position. The surface roughness (Root Mean Square, RMS) of the HCNx film is changing from 5.3 to 28.2 nm as the annealing temperature increases from room temperature to 300 °C. Keywords: dielectric barrier discharge; hydrogenated-CN film; polyamides; FTIR; XPS
1. Introduction Plasma deposition of polymer films and their characterization is a great challenge in the research of polymer chemistry and its application. Amorphous hydrogenated carbon nitride films are well-known for their excellent thermal and oxidation stability, as well as their excellent mechanical properties. On the other hand, such a polymer film can be used in sol-gel application as well as in medicine industry; as capsule coating which is easily soluble in water (or alcohol). The presence of acetylene units in the a-HCNx films raised the interest in the area of high-temperature adhesives. Acetylene units such as internal acetylenes, acetylene end cappers, and pendant acetylenes are known as excellent cross-linkable sites [1–7]. Polyamides containing ethynyl terminal groups to yield acetylene-terminated imide oligomers [8,9] and polyamides containing pendant ethynyl groups have been reported [10,11]. a-HCNx thin films have interesting applications as biosensors [12,13], infrared (IR) detectors [14], applications making usage of ultra-low dielectric properties [15], and anti-biomaterials [16,17]. *Corresponding author. Email:
[email protected] ISSN 1568-5543 online Ó 2012 Taylor & Francis http://dx.doi.org/10.1080/15685543.2012.699751 http://www.tandfonline.com
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Dielectric barrier discharge (DBD) plasma has the novel aspect that its reaction zone is confined to the electrodes area. DBD plasma is very effective for thermo-sensitive molecules like proteins, enzymes, or antibodies. DBD plasmas are filamentary and thermally nonequilibrium low temperature plasmas where the thermal energies of electrons are considerably higher than the thermal energies of ionic and neutral species [18]. During the discharge process the energetic electrons collide with the background gases, the plasmas cause dissociation, excitation, and ionization processes that assist plasma polymerization [19]. The properties of the deposited polymer films depend on the deposition parameters such as precursor gases, pressure, executing power, frequency, and so on. By controlling the plasma parameters one can produce soft as well as hard polymer films and liner polymer or highly cross-linked/oligomer polymer films. Such films are complicated for characterization due to the absence of a reference compound, the lack of long range order and the poor knowledge about their bonding structure. Presence of amino/amic acid group in the films deposited by N2/CH4 DBD plasma will open a new arena of research in the field of biophysics and biochemistry [16,17]. In this article, our work has been focused toward the chemical properties (bond structure), surface morphology, and annealing effect on the deposited film. Here, we made use of the DBD to deposit the polymer film at elevated pressures of N2:CH4 gas mixtures and investigated the changes of chemical composition and chemical bond structure of the deposited polymer films in correlation with deposition parameters, e.g. gas composition. 2.
Experiment
The a-HCNx films are deposited at pressures of 500 mbar (half of atmospheric pressure) and with varying N2:CH4 gas ratios. The experimental set up employing a DBD has been explained in details in Majumdar et al. [20–22]. Both Cu electrodes are covered by dielectrics: the upper (powered) electrode is covered with aluminum oxide (ɛ 10); the lower (grounded) electrode with a glass plate (ɛ 3.8). Both electrodes are separated by 0.15 cm from each other. The upper electrode is connected to a home-built high voltage power supply, while the lower electrode is grounded. The chamber is pumped by a membrane pump down to a base pressure of about 10 mbar. Pressure inside the plasma chamber was controlled by two gas flow controllers methane and nitrogen and by an adjustable needle valve between the chamber and the membrane pump. The high voltage power supply consists of a frequency generator delivering a sinusoidal output that is fed into an audio amplifier. The amplifier can be operated at up to 500 W. Experiments were performed at 10.5 kV (peak-to-peak) and at 5.5 kHz. The electrical power under these conditions was 5 W. The deposition time was typically about 2 h. The deposited films were investigated by means of: Fourier transform infrared (FTIR) transmission spectra were obtained by means of FTIR spectrometer Bruker (Vector 22). The plain sample was placed in a vacuum chamber built inside the spectrometer to minimize the IR signal of water vapor, CO2 content, and noise. The measuring signal passed the optical way with an aperture diameter of 3 mm with a spectral resolution 4 cm 1. For optimal signal-to-noise ratio 50 scans were averaged per sample spectrum and apodized by applying of the Norton Beer apodization function for Fourier transformation. Interferograms were zero-filled using a zero-filling factor of two. The background spectrum was independently measured on a pure silicon substrate [22]. X-ray photoelectron spectroscopy (XPS) measurements of the CNx films were performed on a VG Microtech (CLAM2: Multi-technique 100 mm hemispherical electron analyzer) XPS, using Mg Kα radiation (photon energy 1253 eV) as the excitation source, and the binding energy (BE) of Au (Au 4f7/2:84.00 eV) as the reference. The XPS spectra were collected in a
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constant analyzer energy mode, at a chamber pressure 10 8 mbar and pass energy of 23.5 eV [21,22]. Atomic force microscopy (AFM) is a powerful tool to characterize the surface morphology, and quantitative surface roughness in a nanometer scale can be measured. Surface topographies of the CN films were investigated by Nanoscope IV AFM (M/S Veeco, USA) in the tapping mode at ambient conditions using a silicon cantilever with a sharp silicon tip. The radius of curvature of the Si tip is around 10 nm. We varied the scan areas from 1 1 to 5 5 μm2 with a resolution of 512 pixels. In order to avoid an overestimation of the surface roughness resulting from the presence of a tilted plane when examining the film surface by AFM, a line-by-line flattening was made in the fast direction using Nanoscope data processing software to remove such a tilted plane [22]. The deposited films have been annealing by Ar annealed set up that annealed at slightly above than atmospheric pressure. Every sample has been annealed 10 min under Ar gas environment [23]. 3. Results and discussion 3.1. Fourier transform infrared spectroscopy The general strategy of the data evaluation was identical with standard spectroscopic techniques. The nature of the deposited films required a spectral allowance for an extinction inhomogeneity across the surface and long wave interference effects in the bulk. Therefore, base line correction of the recorded spectra was performed by the concave elastic band method. Typical IR transmission spectra are shown in Figure 1, within the range from 4000 to 500 cm 1. The spectra were recorded ex-situ for the films prepared in N2:CH4 (1:1 and 3:1) DBD plasma (Figures 1(a) and 2(b)). Spectroscopic properties of the polymerized films deposited at different mixture concentrations of the reactive gases N2:CH4 are presented in Figures 1 and 2. The transmission spectrum of the deposited a-HCNx films is characterized by several typical spectral regions. The band between 3100 and 3700 cm 1 is attributed to stretching vibrations of NH and OH functionally groups [21–25]. More precisely, the region from 3300 to 3450 cm 1 is referred to as the anti-symmetric NH2 stretching band and at 3200 cm 1 is the NH2 symmetric stretching bond. However, the separation of the overlapping bands is not possible due to intermolecular interactions as H-bridges, which are very intensive in this region and cause the broadening of the bands. The second interval (3010–2810 cm 1) is the characteristic for CH2 and CH3 groups [24,25]. All vibration modes (antisymmetric as well as symmetric stretch) are present in the spectrum. The intensity of the bands is low (absorption up to 1%) and is related to the concentration of methane in the gas discharge. In addition to this, we note a fine absorption band around 1600 cm 1 that is due to the sp2 carbon and is usually IR forbidden [24]. The appearance of this feature suggests that the incorporation of nitrogen breaks the sp2 symmetry and makes this feature IR active [26,27]. The broad absorption of single peak at 2165 cm 1 is attributed to C≡N triple bond stretching vibration (so-called nitrile group). The region from 1580 to 800 cm 1 is characterized by a large number of bonding vibrations and intermolecular interactions. Absorption in this region can be interpreted as the quality of cross-linking of the deposited structure; however, the rigorous analysis of fingerprint region requires additional diagnostics, such as Raman spectroscopy. The broadband from 1645 to 1665 cm 1 is attributed to C=C and C=N stretching mode [21–25], 28. The absorption band due to trace absorption of carbonyl of amide group –(C=O)NH– (coming from amic acid group) at 1682 cm 1. The absorption band observed in the interval from 1350 to 1480 cm 1 corresponds to the C–N single bond stretch [22,28]. Finally, the absorp-
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Figure 1. (a) FTIR transmission spectrum of a-HCNx film deposited in mixture of N2:CH4 = 1:1. (b). Rapid thermal annealing effect (100–400 °C) on a-HCNx at Ar environment. Red circle shows the decrease of the corresponding peak with respect to annealing temperature. Each annealing treatment time is 10 min.
tion peak in the zone 780–1000 cm 1 corresponds to the CHO (aldehyde group) stretching mode [22]. The absorption peaks appear in a regular manner with different intensities in the IR spectrum. The peaks appear at the same position in all the measurements, but the peak intensities change with the N2:CH4 ratio. Figure 2(b) shows the fast thermal annealing treatment on the deposited polymer film. We see that the nitrile (C≡N) peak is gradually decreasing as the temperature increases from 100 to 400 °C. Not only nitrile peak but NH and OH overlapping region also reduces to the lower value. At 400 °C the nitrile group has disappeared but the amide group (NH) is still
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Figure 2. (a) FTIR transmission spectrum of a-HCNx film deposited in mixture of N2:CH4 = 3:1. (b) Rapid thermal annealing effect (100–400 °C) on a-HCNx at Ar environment. Red circle shows the decrease of the corresponding peak with respect to annealing temperature. Each annealing treatment time is 10 min.
present in the film. It seems that due to heating the absorbed water molecules and hydrogen bridge bonds they brake up and form a linear chain molecule. Moreover, the region between 1700 and 1500 cm 1 is mostly referred as sp2 region also flattens at higher annealing temperatures. 3.2. X-ray photo electron spectroscopy In XPS spectrum, the surface charging effect take place due to the presence of oxide or nitride layer at the top surface of the deposited film. The shake-up line appears due to the
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anomalous surface charging effect (mostly in case of insulating or organic materials films). Here, we present the properties of two different polyamide films deposited with a N2/CH4 ratio of 1:1 and 3:1. Shake-up satellites are not observed in XPS spectrum at lower nitrogen (N2/CH4 = 1:1) concentrated film (Figure 3(a) and (b)), whereas the scenario is different for the higher nitrogen (N2/CH4 = 3:1) concentrated film and we observed that there is a pronounced shake-up satellite peak appearing at higher BE of the corresponding C1s, N1s, and O1s spectrum (Figure 4(c) and (d)). Due to the presence of nitrogen conjugate bonds, the C 1s peak broadens and also becomes more asymmetric in nature which indicate that the nitrogen is involved in chemical bonds with carbon in three possible chemical states: C–N, C=N, and C≡N bonds. The best Gaussian fits to the XPS lines resulted into four different peaks for the C1s line and three peaks for the N1s line. The observed chemical shift is caused by an anomalous behavior of the surface charge distribution of the silicon substrate covered by the a-HCNx film. Si (2p) with a BE = 99.3 eV was taken as a reference. The calibration details about this chemical shift are discussed in our previous work [21,22]. The results
Figure 3. Full-scale XPS spectrum of the deposited polymer films (a-HCNx) where (a) N2/CH4 = 1:1 and (b) N2/CH4 = 3:1, Each Rapid thermal annealing treatment time is 10 min.
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Figure 4. Typical (a) C1s and (b) N1s XPS spectra of a-HCNx film with a mixture of N2:CH4 = 3:1, (c) C1s and shake-up satellites and (d) N1s and shake-up satellites peaks of the film deposited at N2: CH4 = 1:1.
shown below were corrected by subtracting the experimentally observed shift for all the analyses and a clear image of the possible chemical bonds between nitrogen and carbon can be deduced from the deconvolution of the individual C1s and N1s lines into Gaussian-shaped lines [29]. The general strategy of the data evaluation was identical to those for standard spectroscopic techniques. Full-scale XPS spectra of the deposited a-HCNx films (N2/CH4 = 3:1) are presented in Figure 3(a) and (b) with the ratio N2/CH4 = 1:1 and 3:1, respectively. The BE varies from 200 to 600 eV, supporting the presence of C1s, N1s, and O1s bands in the deposited polymer films. Figure 3(a) does not exhibit any shake-up satellite and on the other hand in Figure 3(b) it shows a pronounced peak. As the annealing temperature increases the shake-up satellites peaks are shifting toward lower binding energies (Figure 3(b)). Moreover, at higher annealing temperature the C1s, N1s, and O1s peak intensity increased with gradual decrease of shakeup satellites of the corresponding spectrum. These shake-up lines appear due to the excited electrons at the top of the valence band. When a core electron is photo-ejected, the Columbic potential experienced by the outer shell electrons is suddenly altered. This sudden perturbation may induce a shake-up transition, involving the excitation of a valence electron to a higher, previously unoccupied orbital. The shake-up process is thought of as taking place simultaneously with core electron photo-ejection. If shake-up occurs, the kinetic energy of the ejected core electron will be less than that of an electron ejected from a corresponding core
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orbital in another molecule where shake up has not occurred. Consequently, shake-up satellites always appear on the higher BE side of the main peaks. The typical C1s and N1s spectra of the polymer film are presented in Figure 4(a). The C1s spectrum exhibits four peaks at 284.43, 285.38, 286.68, and 288.15 eV, which are attributed to C=C, C=N, C–N or C≡N, and C–O bonds, respectively [21,22,29–31]. Similarly, from Figure 4(b) the deconvoluted N1s spectrum shows three peaks at 397.90, 399.25, and 400.40 eV which are assigned to C–N or C≡N, C=N, and N–O bonds, respectively [21,22,29–31]. On the other hand, Figure 4(c) and (d) shows XPS spectra of C1s and N1s peak of N2/CH4 = 3:1 film. In this case, the shake-up satellites lines are dominating compared to the main peaks (C1s and N1s). The reason behind this shake-up lines is explained later on. The C–O bond appears with widely shifted peak distribution area and energy ranging from 288.50 to 289.5 eV range in accordance with different deposition parameters. In some cases, the peak appears at 286.4 eV and is attributed to the nitrile group (C≡N) [21,22,31]. The energy of the sp2 C–N peak falls within 285.5–285.9 eV and that of the sp3 C–N peak is assigned in the range 287.0–287.8 eV. Due to these wide spectral ranges a clear understanding of the differences between C–N and C≡N bonds [32–34] in the analysis of C1s XPS spectra is quite difficult. The N-sp3 C and N-sp C are assigned the energy ranges 398.5–399.12 and 400.0–400.7 eV, respectively. This strong interference of the energy levels makes it difficult to distinguish between C–N and C≡N spectral lines in N1s XPS analysis. It can be deduced from Figure 4, that carbon-nitrogen bonds (C=N and C–N) irrespectively of the nitrogen content of the N2/CH4 gas mixture (1:1) at the expense of carbon–carbon and carbon–oxygen bonds. The C1s BE of 286.4 eV and the N1s BE of 399.10 eV referring nitrile group compound as Polyacronitrile (C≡N, sp hybridization). Since the corresponding BE of 287.8 is much closer to that of materials with sp3 configuration (286.9 and 287.8 eV), except for that of polyacrylonitrile, which has sp configuration (286.4 eV), it can be inferred that sp3 C–N bonds, instead of sp C≡N bonds, exist in the carbon–nitrogen hybridization phase. From XPS analysis, it observed that the oxygen content in the polymer films is about 9–10%. The symmetrical O1s peak at 532.1 eV may be a signature of adsorbed oxygen on the film surface, which is due to contamination of the sample in open air [28–31]. The oxygen peak configuration is not discussed here. A drastic change observed in XPS spectrum with respect to nitrogen concentration. Figure 3(a) does not have any shake-up satellites peak, where as Figure 3(b) shows a drastic change in the XPS spectrum with respect to the shake-up peaks. Anomalous surface charging effect is happening due to higher nitrogen concentration. The charging effect reduces at higher annealed temperature and simultaneously the shake–up peak disappears gradually. Due to annealing the surface impurities (oxygen layer) evaporates and further annealing helps to diffuse these impurities through the top surface (oxide, nitride, or unsaturated oxygen or bubbles). As a result, the shake-up satellites disappear gradually as the annealing temperature is increased from 50 to 500 °C. Simultaneously, the chemical shift of 1s electron shifted to its own advantageous position. 3.3.
Atomic force microscopy (AFM)
Figure 5(a)–(e) presents typical 3-dimensional 2 μm 2 μm topographies of several a-HCNx polymer films with two different mixtures of N2:CH4 (3:1). For smaller ratios of the nitrogen contents, the film shows a 3-dimensional island growth. The average size of the Islands as measured from the sectional profile typically ranges between 30 and 40 nm in this case. As the nitrogen incorporation increases the Islands gradually flatten down and the surface becomes smoother. The surface roughness of the underlying Si substrate is about 0.65 nm,
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Figure 5. Three dimensional AFM image of a-HCNx (N2/CH4 = 3:1) film on Si substrate. The Root Mean Square (RMS) roughness changes with annealing temperature as: (a) room temperature (25 °C), (b) 50 °C, (c) 100 °C, (d) 200 °C and (e) 300 °C.
whereas the roughness of the film in the plateau region is about 0.35–4.0 nm, which indicates that the present deposited polyimide film is much smoother compared to a-HCNx films deposited by vapor deposition, such as radio-frequency plasma enhanced pulsed laser deposition (RMS roughness below 1.0 nm) [35]. Apparently, more deposition occurs in the valleys of the Si surface than on the crests. It may also indicate that plasma ions injected into the growing films tend to grow atomically smooth films. Figure 5 shows the evolution of RMS roughness for a 2 2 μm2 sample area as function of annealing temperature. Annealing causes the significant increase in the RMS roughness. The RMS roughness increases from 5.3 to 28.2 nm as the annealing temperature increased from 50 to 300 °C. The surface of the deposited aHCNx film is not homogeneous; hence, the distribution of surface roughness is not uniform through out the entire film surface. There are some Islands growth observed in the deposited sample. As the surface is annealed the Island became sharper and took a new shape but the RMS roughness is increased due to surface deformation. From Figure 5(d) and (e) we saw that there are huge bumps but the height of the Island is reduced. It is a momentary effect due to high temperature. But at 300 °C we observe (Figure 5(e)) few random Islands growth on the surface area and it is took place due to the surface diffusion to the upward direction. It indicates that the annealing of the bulk film increases the surface RMS roughness of a-HCNx film. 4. Summary Amorphous hydrogenated carbon nitride film is deposited by N2/CH4 DBD plasma with different nitrogen ratio. FTIR reveals that there is NH/OH overlapped region, amine/amide (–(C=O)NH– present in the film. FTIR spectra suggest that the majority of the incorporated nitrogen atoms are in the form of amide group (–(C=O)NH at 1680 cm 1), C=N double bonds and C≡N triple bonds (nitrile group). XPS analysis supports the presence of pyridine (C=N bond, sp2 hybridization), C–N bond, sp3 hybridization), and polyacrylonitrile (C≡N, sp hybridization). The shake-up satellites appear due to the anomalous surface charging effect (mostly in case of insulating or organic materials films). Shake-up satellite is observed in the higher nitrogen concentrated film (N2/CH4 = 3:1). Fast thermal annealing at Ar environment shows the removal of nitrile group from the bulk and removal of shake-up satellite from XPS spectrum. The surface roughness (RMS) of the HCNx film is changing from 5.3 to 28.2 nm as the annealing temperature increases from room temperature to 300 °C. Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through Sonderfors chungsbereich SFB/TR 24 ‘Fundamentals of Complex Plasmas’.
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