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Characterization of microwave plasma for polymer surface modification using FTIR emission spectroscopy Maryam Mavadat 1,2,a , Stéphane Turgeon 2,b , André Ricard 3,c and Gaétan Laroche 1,2,d 1 Laboratoire d’ingénierie de surface, Centre de Recherche sur les Matériaux Avancés, Département de génie des mines, de la métallurgie et des matériaux, 1065 avenue de la médecine, Université Laval, Québec, G1V 0A6, Canada 2 Centre de recherche du CHUQ, Hôpital Saint-François d’Assise, 10 rue de l’Espinay, Québec, G1L 3L5, Canada 3 LAPLACE, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France a [email protected], b [email protected], c [email protected], d [email protected] Keywords: Plasma emission spectroscopy, Polymer surface treatment, Infrared spectroscopy, Plasma temperature Abstract. Infrared (IR) emission spectroscopy measurements were performed in N 2 microwave discharges at pressures ranging from 0.5 to 3 Torr and powers of 200 and 300 W. Although emission spectroscopy in the infrared region has rarely been investigated, this technique has nevertheless provided numerous key data. For instance, numerical generation of spectra to match experimental FTIR emission data allowed estimating the plasma temperature. 1. Introduction Non-equilibrium plasmas have a great impact on polymeric material science, in that they allow modifying the outermost surface layer, in ‘cold’ processes, conferring materials a tailored surface composition and properties such as adhesion, wettability, and biocompatibility. Different plasma external parameters (pressure, input power and gas flow) affect the characteristics of the plasma and consequently the surface chemistry of plasma modified polymers. In this context, the knowledge of fundamental processes occurring in the plasma is a prerequisite for further process control. The plasma temperature, which most of the time is estimated by rotational temperature (T r ), is one of the most important plasma parameters. As a matter of fact the chemistry of the discharge can be influenced by the gas temperature, since it governs the reaction rate of active species generation through dissociation, excitation, and ionization processes, and consequently, accurate knowledge of T r is required to understand and control plasma processes. In the present work in situ, non-intrusive diagnostic (optical emission spectroscopy) in the infrared spectral region were performed in a N 2 microwave plasma discharge using FTIR emission spectrometer. The data recorded from emission spectroscopy were used to determine the temperature of the N 2 microwave discharge as a function of gas pressure and microwave power by comparing the experimental and numerically generated spectra. The results of plasma temperature as a function of pressure are presented. Advanced Materials Research Vol. 409 (2012) pp 797-801 Online available since 2011/Nov/29 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.409.797 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 132.174.255.116, University of Pittsburgh, Pittsburgh, USA-02/12/14,04:13:46)

Characterization of Microwave Plasma for Polymer Surface Modification Using FTIR Emission Spectroscopy

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Page 1: Characterization of Microwave Plasma for Polymer Surface Modification Using FTIR Emission Spectroscopy

Characterization of microwave plasma for polymer surface modification

using FTIR emission spectroscopy

Maryam Mavadat1,2,a, Stéphane Turgeon2,b, André Ricard3,c and Gaétan Laroche1,2,d

1Laboratoire d’ingénierie de surface, Centre de Recherche sur les Matériaux Avancés,

Département de génie des mines, de la métallurgie et des matériaux, 1065 avenue de la médecine, Université Laval, Québec, G1V 0A6, Canada

2Centre de recherche du CHUQ, Hôpital Saint-François d’Assise, 10 rue de l’Espinay, Québec,

G1L 3L5, Canada

3 LAPLACE, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France

[email protected],

[email protected],

[email protected],

[email protected]

Keywords: Plasma emission spectroscopy, Polymer surface treatment, Infrared spectroscopy, Plasma temperature

Abstract. Infrared (IR) emission spectroscopy measurements were performed in N2 microwave

discharges at pressures ranging from 0.5 to 3 Torr and powers of 200 and 300 W. Although

emission spectroscopy in the infrared region has rarely been investigated, this technique has

nevertheless provided numerous key data. For instance, numerical generation of spectra to match

experimental FTIR emission data allowed estimating the plasma temperature.

1. Introduction

Non-equilibrium plasmas have a great impact on polymeric material science, in that they allow

modifying the outermost surface layer, in ‘cold’ processes, conferring materials a tailored surface

composition and properties such as adhesion, wettability, and biocompatibility. Different plasma

external parameters (pressure, input power and gas flow) affect the characteristics of the plasma and

consequently the surface chemistry of plasma modified polymers. In this context, the knowledge of

fundamental processes occurring in the plasma is a prerequisite for further process control. The

plasma temperature, which most of the time is estimated by rotational temperature (Tr), is one of the

most important plasma parameters. As a matter of fact the chemistry of the discharge can be

influenced by the gas temperature, since it governs the reaction rate of active species generation

through dissociation, excitation, and ionization processes, and consequently, accurate knowledge of

Tr is required to understand and control plasma processes.

In the present work in situ, non-intrusive diagnostic (optical emission spectroscopy) in the

infrared spectral region were performed in a N2 microwave plasma discharge using FTIR emission

spectrometer. The data recorded from emission spectroscopy were used to determine the

temperature of the N2 microwave discharge as a function of gas pressure and microwave power by

comparing the experimental and numerically generated spectra. The results of plasma temperature

as a function of pressure are presented.

Advanced Materials Research Vol. 409 (2012) pp 797-801Online available since 2011/Nov/29 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.409.797

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 132.174.255.116, University of Pittsburgh, Pittsburgh, USA-02/12/14,04:13:46)

Page 2: Characterization of Microwave Plasma for Polymer Surface Modification Using FTIR Emission Spectroscopy

2. Experimental set-up

Fig. 1 presents the experimental set-up.

Figure 1. Experimental set-up for MW discharge and emission spectroscopy.

The low pressure plasma reactor illustrated in Fig. 1 is described in detail elsewhere [1]. Briefly, the

experiments were performed in a microwave reactor purchased from Plasmionique Inc. (Varennes,

QC, Canada). The N2 gas (99.998% purity) used for the plasma characterization was introduced in

the discharge tube by means of a mass flow controller. The plasma column was created in N2 gas at

pressures ranging from 0.5 to 3 Torr. The power delivered to the launcher was 200 and 300 W.

As aforementioned, the plasma characterisation was performed using IR emission spectroscopy.

The near infrared spectra were recorded with a FTLA2000 FTIR (770-3300 nm) spectrometer

purchased from ABB-Bomem (Québec, QC, Canada) at a resolution of 2 cm-1

. For infrared

collection, the IR light exited through a ZnSe feedthrough window, was expanded through a

concave ZnSe lens then was redirected by a concave gold mirror into the FTIR’s entrance port. All

IR optics was purchased from International Scientific Products Corp. (Irvington, NY, USA). The

great advantage of near infrared emission spectroscopy is that it makes it possible to record features

of the N2 first positive (1st positive) system that are not observable through classical UV-VIS

emission spectroscopy.

The detector used during each experiment was a thermoelectrically cooled Indium Arsenide

(InAs) semiconductor diode which is sensitive in the range of 3000-14000 cm-1

(710-3300 nm). One

hundred interferograms were routinely co-added and Fourier-transformed, thereby enabling us to

record spectra with an acceptable signal-to-noise ratio and reasonable acquisition time.

3. Results and discussion

Infrared emission spectrum of the N2 microwave discharge at 3 Torr is shown in Fig. 2. In the

present study, the observed infrared spectra consisted mostly of N2 1st positive transitions. In

addition, spectra recorded at low gas pressures P <1 Torr allowed visualizing various NI atomic

transitions. In the illustrated spectrum, the infrared features are related to different vibrational

transitions originating from the 1st positive system of nitrogen.

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Page 3: Characterization of Microwave Plasma for Polymer Surface Modification Using FTIR Emission Spectroscopy

Figure 2. Nitrogen plasma emission spectrum in the 630-1830 nm range (300W, 3 Torr, 80 sccm).

Rotational temperature. In principle, the rotational temperature of N2 can be determined using

various bands from either the first negative [2] or second positive system [3] through a Boltzmann

plot, provided that the spectrometer resolution is high enough to separate the rotational structure of

the bands or by fitting numerical models to the band envelope when medium to low resolution

spectrometers are used [4]. However, despite the characteristics of the 1st positive system that make

it a more reliable tool to estimate gas temperature (low excitation threshold, long radiative lifetime,

higher predissociation level, and high emission intensity), its complex structure with 27 rotational

branches has hampered its widespread use [4-6].

Inspired by Biloiu et al. [7] the plasma gas temperature was evaluated by calculating and

generating theoretical spectra closely matching the bands originating from 0-0 transition in the N2

1st positive system which is observed in the range of 1025-1055 nm. To that end, a synthetic

spectrum which was automated in MATLABTM

[7] has been used. However, such a procedure

requires the knowledge of the FTIR spectrometer apparatus function which, in turn, has to be

convolved with the calculated spectrum. To that end, the shape parameters of the 826 nm ArI line

taken from the spectrum of an Ar-Hg source were used, considering that the instrumental

broadening was modeled by a pseudo-Voight function:

( ) ( ) ( )( )

202 22

0

4ln 2 4ln 2 2, exp 1 .

4

wf p w p p

w w wλ λ

ππ λ λ

= − − + − + −

where p and 1−p are the relative magnitudes of the Gaussian and Lorentzian functions contributions,

respectively, w is the full width at half maximum of the line (FWHM), and λ0 is the central

wavelength. This allowed determining that p value was 0.5 while w was 0.2 nm for the

experimental setup used in the present study.

It is worth mentioning that the value of p has an error of ± 0.1. However, this has minimal effect on

the temperature calculation as can be seen from the error bars on the curves presented in Fig. 4.

Advanced Materials Research Vol. 409 799

Page 4: Characterization of Microwave Plasma for Polymer Surface Modification Using FTIR Emission Spectroscopy

Fig. 3 shows an example of a measured ro-vibrational N2 spectrum along with the corresponding

spectrum calculated using the aforementioned protocol. As can be seen, an excellent match between

both calculated and experimental spectra was observed, with a confidence level better than 95%.

Figure 3. Experimental spectrum (solid line) and corresponding numerically generated (dotted line) spectrum of the 0-0

band for the best fit gas temperature. In this example, the evaluated rotational temperature (Tr) was 1009 K

This method was thereafter used to characterize the pressure-dependence of the rotational

temperature in nitrogen plasmas generated at powers of 200 W and 300 W, respectively. As seen in

Fig. 4, Tr increased with pressure and power due to the more frequent collisions occurring between

the plasma species which lead to a decrease in electron temperature with concomitant increase of

gas temperature. These results are in good agreement with previous results on pure N2 discharges [1,

4].

Figure 4. Rotational temperature as a function of pressure for plasmas generated at 200 W and 300W.

The present results clearly highlighted that the numerical simulation of the spectrum of the 0-0

transition of the nitrogen first positive system allows calculating rotational temperatures with better

sensitivity as compared to using the P1/P2 ratio from the 2-0 transition [1, 4]. On one hand, the 0-0

transition gives rise to spectra with higher emission intensity as compared to the 2-0 transition. On

the other hand, the P1/P2 ratio from 2-0 transition revealed to be not sensitive enough to discriminate

subtle temperature changes.

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Page 5: Characterization of Microwave Plasma for Polymer Surface Modification Using FTIR Emission Spectroscopy

Conclusion

Optical emission spectroscopy in the near IR spectral region was used for the diagnostic of low

pressure nitrogen microwave discharges currently used for modifying the surface of biomedical

polymers. The experimental infrared spectra consisted mostly of N2 1st positive system transitions.

Rotational temperatures were accurately measured by fitting the N2 1st positive system spectra

recorded from plasmas generated at 0.5 to 3 Torr of N2 with powers of 200 W and 300 W. Under

these conditions, the N2 rotational temperatures ranged from 646 to 1012 K.

References

[1] Mavadat M, Ricard A, Sarra-Bournet C and Laroche G 2011 Journal of Physics D: Applied

Physics Accepted

[2] Williamson J M and DeJoseph C A 2003 Journal of Applied Physics 93 1893-8

[3] Tonnis E J and Graves D B 2002 Journal of Vacuum Science & Technology A: Vacuum,

Surfaces, and Films 20 1787-95

[4] Britun N, Gaillard M, Ricard A, Kim Y M, Kim K S and Han J G 2007 Journal of Physics D:

Applied Physics 40 1022-9

[5] Biloiu C, Sun X, Harvey Z and Scime E 2006 Determination of rotational and vibrational

temperatures of a nitrogen helicon plasma. Review of Scientific Instruments) pp 10F117-4

[6] Ricard A, Nouvellon C, Konstantinidis S, Dauchot J P, Wautelet M and Hecq M 2002 Journal

of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 20 1488-91

[7] Biloiu C, Sun X, Harvey Z and Scime E 2007 Journal of Applied Physics 101 073303-11

Advanced Materials Research Vol. 409 801

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THERMEC 2011 Supplement 10.4028/www.scientific.net/AMR.409 Characterization of Microwave Plasma for Polymer Surface Modification Using FTIR Emission

Spectroscopy 10.4028/www.scientific.net/AMR.409.797