A study on the characteristics of plasma polymer thin film with controlled nitrogen flow rate
© Cho and Boo; licensee Springer. 2012
Received: 8 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Nitrogen-doped thiophene plasma polymer [N-ThioPP] thin films were deposited by radio frequency (13.56 MHz) plasma-enhanced chemical vapor deposition method. Thiophene was used as organic precursor (carbon source) with hydrogen gas as the precursor bubbler gas. Additionally, nitrogen gas [N2] was used as nitrogen dopant. Furthermore, additional argon was used as a carrier gas. The as-grown polymerized thin films were analyzed using ellipsometry, Fourier-transform infrared [FT-IR] spectroscopy, Raman spectroscopy, and water contact angle measurement. The ellipsometry results showed the refractive index change of the N-ThioPP film. The FT-IR spectra showed that the N-ThioPP films were completely fragmented and polymerized from thiophene.
The existing semiconductor technology lets a silicone material make integrations by a top-down form developed by nano- or molecule technology merged with nanotechnology, biotechniques, and information technology and by a bottom-up method to constitute a device and a circuit with self-alignment of atoms and molecules. Those are common opinions of a majority of experts. In spite of the basic consensus by such experts, the progress in the nano and molecule device research field is very slow, and it is much worse now. There are many causes as possible reasons, but it is recognized that the following are still in question: 'the choice of the stable molecule and design technology,' 'self-alignment technology of atoms and molecules,' and 'technology to form a molecule and contact between the metal electrode for stability' [1, 2]. The realization of nanoscale electronics expects the development of bottom-up strategies such as chemical synthesis, self alignment of atoms and molecules, and self-assembled supramolecule. In fabricating the bio-application material, the diamond-like carbon [DLC] films have been a good candidate for some applications such as blood-contacting devices  and cell-contacting materials  due to their excellent mechanical properties [5–7].
In this work, nitrogen-doped plasma polymer was deposited by nitrogen injection during the plasma-enhanced chemical vapor deposition [PECVD] process without ammonia gas. Also, N-ThioPP thin films were investigated on the surface properties such as surface energy and structural effects.
The chemical bonding type of plasma polymer thin films was investigated by FT-IR spectroscopy (Vertex 70, Bruker Optik Gmbh, Ettlingen, Germany). Moreover, Raman shift of each thin film was investigated by FT-Raman spectroscopy (Vertex 70 with RAM-II, Bruker Optik Gmbh, Ettlingen, Germany). Surface wettability was measured according to water contact angle measurements (Attension, KSV Instruments, Ltd., Helsinki, Finland). The ex-situ ellipsometry data of all investigated films were produced by an ellipsometer (GC5A automatic ellipsometry, Gaertner Scientific Corporation, Skokie, IL, USA) at 632 nm to investigate the relationship of film density with the doping amount of nitrogen. Transmittance and bandgap energy of the N-ThioPP thin film were investigated by a UV-Vis spectrophotometer (Optizen 2120UV Plus, Mecasys Co., Ltd., Yuseong-gu, Daejeon, South Korea).
Results and discussion
N-ThioPP thin films were deposited on Si(100) by the PECVD method. IR spectra (nitrile band, 2,025 and 2,200 cm-1; and the difference of the fingerprint region between ThioPP and N-ThioPP) show that the N-ThioPP thin film was fabricated by nitrogen gas injection during the PECVD process. Moreover, the increasing NHx species was definitely shown in the Raman spectra. NHx species was increased by increasing the N2 flow rate. Also, decreasing the contact angle shows the increasing surface energy of the N-ThioPP thin film with increasing N2 flow rate. Additionally, decreasing the contact angle indicates the indirect cause of the increasing nitrogen amounts in the N-ThioPP thin film. Nitrogen atoms bonded with thiophene molecules during the PECVD process. Also, nitrogen disturbs the strong bond between thiophene molecules. Thus, the refractive index of the N-ThioPP thin film was decreased by increasing the nitrogen amount. It indicates that the hardness of the N-ThioPP thin film was controlled by nitrogen amounts in the thin film. UV-Vis spectra of all samples show 80% of transmittance in the infrared region. However, transmittance in the visible region was dramatically changed by increasing the nitrogen amounts. Thus, the energy bandgap of N-ThioPP was increased by increasing the nitrogen amounts.
From those results, nitrogen-doped plasma polymer thin films could be fabricated easily by nitrogen injection during the PECVD process without ammonia as toxic gas. Also, we can control the optical, physical, and chemical properties of the N-ThioPP thin film by controlling of nitrogen flow rate.
This work was supported by NRF-20110027123 (Basic Science Research Program) and NRF-20110000849 (Plasma Bioscience Research Center, SRC Program).
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