Avant-propos 93

Le chapitre 5 présente les résultats préliminaires de l’étude de dépôt de couches minces aminées en atmosphère N2-C2H4. Comme il a été mentionné plus tôt, l’approche initiale était d’obtenir des dépôts dans une décharge par barrière diélectrique de Townsend (DBDT) puisqu’elle présente des caractéristiques intéressantes au niveau de l’homogénéité des dépôts obtenus. Il s’agissait également d’une approche différente de ce que l’on pouvait trouver dans la littérature. Donc, les résultats présentés dans ce chapitre sont d’une nature prospective au niveau des conditions expérimentales permettant l’obtention d’une DBDT en atmosphère N2-C2H4 et des mécanismes de croissance de ces couches. Ces résultats ont été initialement présentés sous forme de proceeding à la conférence HAKONE XI à l’automne 2008. Cependant, ce travail a été sélectionné lors de la conférence pour faire partie d’une édition spéciale du journal European Physical Journal of Applied Physics (EPJAP).

5.2 Résumé

Ce travail est une investigation de la composition chimique et des profils de croissances de couches minces de nitrure de carbone hydrogéné amorphes (a-C:N:H) déposée par décharge de Townsend à la pression atmosphérique en atmosphère C2H4/N2. Plusieurs techniques de caractérisations de surface ont été employées pour évaluer les propriétés des dépôts (XPS, FTIR, MEB, profilométrie). Les dépôts obtenus présentent un ratio N/C et une concentration de fonctionnalités azotées élevées. Les résultats ont révélé deux mécanismes de croissances différents qui dépendent du temps de résidence des molécules de précurseurs; initialement, la croissance est principalement due aux radicaux et ensuite, un mécanisme de formation de poudre apparaît, menant à une composition chimique et des propriétés de surface différentes.

5.3 Abstract

The present work is an investigation of the chemical composition and growth profile of an hydrogenated amorphous carbon nitride film (a-C:N:H) deposited by atmospheric pressure Townsend discharge in C2H4/N2. Various surface characterization techniques were used to evaluate the coatings properties (X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, Profilometry, Scanning Electron Microscopy). The coating obtained presented a high N/C ratio and a high concentration of N-functionalities. The results revealed two different growth mechanisms depending on the residence time of the precursor molecules; at first, the growth is mainly due to radicals then a powder formation mechanism appears, therefore leading to different chemical composition and surface properties.

5.4 Introduction

Plasma surface treatments are commonly used to modify the surface properties of polymers such as adhesion, wettability, biocompatibility, etc. [6, 148]. In some cases, such as biomaterial or biosensor applications, they are used to insert chemically reactive functionalities onto otherwise non-reactive substrates [8, 32]. Compared to simple surface functionalization, where hydrophobic recovery and ageing limits the shelf life of the surface obtained, plasma coatings can provide a high density of chemical functionalities with good stability. In recent years, coatings obtained by dielectric barrier discharge (DBD) have received a lot of attention since they can operate at atmospheric pressure and have the potential for high density of active species. In the last decade, there have been many studies on amorphous carbon coatings (a-C:H) obtained in atmosphere pressure plasma processes for growing polymer-like materials for protection, lubrication, biomedical and other applications. Most of those studies were performed with various hydrocarbon precursors (CH4, C2H2, C2H4, C3H6, etc.) in rare gases such as helium [124, 125, 158, 202, 203] or argon [124, 204, 205]. A few groups have reported the use of DBD to obtain amorphous carbon nitride coatings (a-CN:H) using hydrocarbon precursors diluted in nitrogen such as CH4 [86, 87, 205], and very recently C2H2 and C2H4 [86]. Here, the DBD works in a specific mode, namely the Townsend one, which is homogeneous contrary to the more classical filamentary mode. The objective is to obtain an a-CN:H film with NH2- functionalities on the surface, from a N2/C2H4 mixture, with an emphasis put on the evaluation of the surface chemistry and growth mechanisms.

5.4.1 Experimental set-up

5.4.1.1 Plasma Deposition

The DBD configuration used in this work has already been described elsewhere [206]. Briefly, the dielectric barrier discharge is obtained between two parallel metalized alumina plates. The glass substrate (2mm thickness) on which the coating is realized covers the lower plate and maintains a gas gap of 1mm with the upper plate. A longitudinal gas injection is used in order to study both the plasma and the coating properties as a function of the mean gas residence time in the discharge. The total gas flow rates (N2 + C2H4) can be

varied between 1 and 6 L/min and are controlled via electronic mass flow controllers. In a first step, the Townsend regime domain was determined as a function of the frequency, voltage and for different C2H4 rate in N2. Then, plasma deposition parameters (power, gas flow, deposition time) were varied.

0 2 4 6 8 10 10 12 14 16 18 20

FD

TD

FD

TD

Figure 5-1 : Domain of the Townsend Regime in an atmosphere of N2 with different concentration of C2H4 („ 0 ppm, { 10 ppm, S 15 ppm, … 20 ppm, ¡ 50 ppm ) (FD = Filamentary discharge, TD = Townsend discharge).

Figure 5-1 shows the Townsend discharge domain as a function of voltage and frequency for different concentrations of C2H4. For a fixed frequency and C2H4 concentration, the Townsend discharge is delimited by the lower voltage limit (i.e. 11.5 kVpk-pk for 3 kHz and 10 ppm) where there is no discharge, and a upper voltage limit where the discharge switches to a filamentary discharge (i.e. 18kVpk-pk for 3 kHz and 10 ppm). Thus, for frequency > 2 kHz, a very limited working domain of the Townsend regime as a function of the C2H4 concentration is observed (between 0 and 10 ppm) in which a coating can be obtained in this configuration. In light of this observation, in order to keep the discharge in the Townsend regime, coatings were performed at a frequency of 3 kHz with a ratio of 10 ppm of C2H4 for various powers, between 2 and 6 W/cm3, corresponding to voltage of 12.5 to 18 kVpk-pk respectively. This choice is justified by previous studies showing, that higher the excitation frequency is, higher power and growth rate are obtained [207]. Compared to experimental conditions in the literature for DBDs with CH4 in N2 (10.5 kVpk-pk at 5.5 kHz, CH4/N2 ratio ≥ 1:20) [87] or with CH4, C2H2 or C2H4 in N2 (18 kVpk-pk at 10 kHz, CxHy/N2

ratio ≥ 1:1000) [86], precursor concentration in this experiment is very low (1:100000). Study of Figure 5-1 would suggest that their DBDs were probably filamentary discharges. 5.4.2 2.2 Surface Characterization Techniques

Film thickness was measured with profilometry using a TENCOR P2 stylus profilometer with a vertical resolution of 25 Å on glass samples. Chemical composition was determined with X-Ray Photoelectron Spectroscopy (XPS) using a PHI 5600-ci spectrometer (Physical Electronics, Eden Prairie, MN). A monochromatic aluminum X-Ray source (1486 eV) was used to record the survey spectra, whereas high-resolution spectra were acquired with a monochromatic magnesium X-ray source (1200 eV). The detection was performed at 45° with respect to the normal of the surface and the analyzed area was approximately 0.005 cm2. Some samples were also investigated with a Nicolet Magna 550 Fourier transform infrared spectrometer (Thermo-Nicolet, Madison, WI) in the attenuated total reflectance technique (ATR-FTIR) using a Split Pea attachment (Harrick Scientific Corp., Ossining, NY, USA) equipped with a silicon hemispherical 3 mm-diameter internal reflection element. One hundred scans were routinely acquired with a spectral resolution of 4 cm-1. Coatings on alumina substrate were cleaved in two to look at the slice and observe the coating growth mechanisms with a Field Emission Gun Scanning Electron microscope (FEG-SEM) using a JEOL JSM 6700F.

5.5 Results and discussion