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Atomic oxygen in remote plasma of radio-frequency hollow cathode discharge source: Characterization and efficiency
Vacuum 85 (2010) 421e428
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Atomic oxygen in remote plasma of radio-frequency hollow cathode discharge source: Characterization and efficiency M. Naddaf*, S. Saloum, B. Alkhaled Atomic Energy Commission of Syria, Physics Department, P.O. Box 6091, Damascus, Syria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2010 Received in revised form 4 August 2010 Accepted 4 August 2010
Low-pressure, low-temperature remote oxygen plasma ignited by a radio-frequency (RF) hollow cathode discharge (HCD-L300) system is shown to be a powerful and effective source of neutral atomic oxygen (AO), useful in processing of polymeric materials. The density of AO was determined by a catalytic nickel probe as a function of pressure, RF power, oxygen flow rate and axial distance in the processing chamber. It was found to vary between w11019 to w11020 atom m3. The AO rich remote plasma-induced modification of wetting properties of polyimide (PI) and fluorinated ethylene propylene (FEP) surfaces has also been investigated. The wetting properties of the PI and FEP surfaces before and after exposure to the plasma were characterized by contact angle measurements and analysis. It was found that the influence of plasma surface treatment on wetting properties of FEP has an opposite effect to that of PI. On increasing the time of treatment, the surface of PI becomes more hydrophilic, whereas the hydrophobicity of FEP surface enhances. Moreover, a superhydrophobic FEP surface is produced at prolonged time of treatment. Changes in the surface morphology due to the plasma treatment were viewed under a scanning electron microscope (SEM). Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Atomic oxygen Catalytic nickel probe RF hollow cathode discharges Polyimide FEP Contact angle SEM
1. Introduction Atomic oxygen (AO) is one of the most important species generated in oxygen plasma reactor. It finds important applications in many areas of science, technology and industry. It has been successfully used for surface activation and cleaning in the electronic [1], chemical, automotive industries [2], ashing of organic materials in electronics [3], ashing of biological and medical samples [4], selective etching of composite materials [5], and plasma deposition of thin films [6]. One of the attractive sources of oxygen atom is electrodeless discharge, such as electron cyclotron resonance (ECR) and rf discharge. However, HCD remote plasma, in particular, has several advantages. It generates intense primary plasma with density approaching 1011 cm3 [7], allowing plasma jets to form and create remote plasma with a very homogeneous plasma density distribution. This can be utilized for treatment of large area planar substrate and deposition of thin films with good quality and high deposition rates. In addition, the low thermal load of the substrate during remote processing is very suitable for low temperature treatment of thermally sensitive substrates. Different methods exist for determining the density of AO. The oldest and most simple method is one involving catalytic probe, * Corresponding author. Tel.: þ963 11 2132580; fax: þ963 11 6112289. E-mail address: pscientifi[email protected] (M. Naddaf). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.08.004
which was first invented by Strutt [8]. It is based on the catalytic recombination of oxygen atoms on a metal surface. Although in principle any metal can be used in the construction of a catalytic probe, the choice of materials is limited for several reasons. For example, metals which form extremely stable oxides, such as aluminum and titanium, exhibit poor catalytic activity and therefore probe signals. However, it is found that nickel is one of the best metal for construction of a catalytic probe [9]. Nickel catalytic probe has been successfully utilized for determining the density of AO in a various plasma sources, such as RF, microwave and ECR oxygen plasmas [9e12]. Due to their intrinsic dielectric properties, good electrical insulation, good thermal stability, inertness to chemical attack and biocompatibility, polymeric materials find extensive use in a variety of applications in aerospace [13], electronic and other industries [14e16]. An important issue relevant to many of these applications is to control the wetting of polymer surface. For instance, the poor surface adhesion to other materials may result in the difficulty in depositing surface coatings, printings, dying, as well as poor protein absorption, platelet adhesion and cell attachment in biomedical applications. Surface modification techniques utilizing flame [17], chemical [18], grafting [19], corona discharge [20], low-pressure plasma [21,22], and ultraviolet exposure [23] have been proposed to improve the wetting properties of polymers. On the other hand, due to the wide applications of surfaces with hydrophobicity, many
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studies have been focused on surface modification to achieve such water-repellent property. Water-repellent or superhydrophobic surface is important in our daily lives, as well as, in many biological processes and industrial applications. These include the potential use of such a surface for satellite dishes, solar energy panels, photovoltaics, exterior architectural glass and green houses, heat transfer surfaces in air conditioning equipment, water-repellent coating on aerocrafts and microfluidics, piping and boat hulls [24e26]. The non-wettable character has been claimed in biomedical applications ranging from blood vessel replacement to wound management. Superhydrophobic surfaces can be achieved by various processes, such as, electrochemical reaction and deposition, phase separation, lithography, plasma etching and polymerization, chemical etching [24e30]. The materials being utilized in these processing can be polymers with intrinsic hydrophobic properties, nano-tube, nano-wire, or bulk metal substrates. In the present work, the density of AO in an HCD-L300 system induced remote oxygen plasma has been determined by using the nickel catalytic probe. The density of AO in the reaction zone of HCD eL300 reactor was found to be of the order of 1019e1020 atom cm3. In addition, a well know of excellent thermal, chemical and dielectric properties polymers; PI and FEP, were chosen to study the efficiency of the AO rich remote plasma for surface modification of materials. FEP is known as a polymer of low surface adhesion and low surface energy, the contact angle of water on smooth flat Teflon surfaces does not exceed 120 [31], whereas, PI is a hydrophilic polymer. Contact angle measurements and analysis showed that exposure of FEP surface to the plasma increases its hydrophobic nature, resulting in a superhydrophobic surface at exposure time 60 min or more. On contrary, plasma treatment of PI surface results in an enhancement of its wettability. 2. Experimental 2.1. Plasma system A schematic block diagram of the experimental set up of HCDL300 system (Plasma Consult GmbH PlasCon) used in the present
work is shown in Fig. 1. The plasma source is a hollow cathode configuration which consists of two coaxial tubes of 30 cm length. The inner tube is the hollow cathode and the outer-grounded tube forms the anode. Both cathode and anode are supplied with two rows of coaxial holes aligned with each other resulting in 30 plasma jets which form and create remote plasma with a very homogeneous plasma density distribution. The RF power at 13.56 MHz is applied to the cathode. The processing chamber, constructed of stainless steal, is cubic (50 50 50 cm3) and contains the HCDL300 plasma source and the sample holder. The system is provided with an adaptive pressure controller between the vacuum system and the plasma chamber, supplied with a stepping motor. This enables the variation of the flow rate at constant pressure and vice versa. 2.2. Nickel catalytic probe and AO density evaluation The catalytic probe, used in the study, was made up of nickel disk connected to a chromelealumel thermocouple (see Fig. 1). The thickness of the disk was 0.125 mm and its diameter was 4 mm. The diameter of the thermocouple wires was 0.2 mm and their lengths were 30 mm each. The wires were covered with narrow glass tubes. The probe was connected to a movable sample holder with a provision for scanning over the plasma region. High purity (99.999% Ar and O2) gas mixture was fed into the cathode through the mass flow controller from both ends in order to maintain a constant gas pressure over the entire cathode length. An RF power at 13.56 MHz (maximum power is 300 W) was applied to the cathode and the primary plasma was generated. In order to obtain the remote plasma of pure oxygen in the processing chamber, the flow of Ar gas was gradually decreases and then switched off. A suitable shutter was used to shield the probe assembly, during the stage of remote oxygen plasma ignition (10e30 s). Prior to the measurements, the probe was activated first, by degassing it under vacuum in the processing chamber for 2 h at a base pressure of w0.005 Pa. This is necessary for desorption of adsorbed gases or possible impurities on the probe surface. Secondly, the surface of the probe was oxidized by exposing it to oxygen remote plasma for
Fig. 1. A schematic block diagram of the experimental set up of HCD-L300 system.
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Table 1 Calculated values of oxygen recombination coefficient (g) as a function of RF power. Power (W)
g
50 100 150 200 250 300
0.2 0.17 0.19 0.17 0.18 0.18
1 h at a pressure of 30 Pa and RF power of 300 W. Repeating the measurement of the probe temperature (T) as a function of time (t), at same operating conditions, showed a stable T(t) response, indicating that the probe is well activated. The probe characteristic T(t) as a function of time (t) was then monitored at different operating parameters adjusted by varying the pressure, RF power and the flow rate of oxygen. The density of atomic oxygen (no) is calculated from the first time derivative of the probe characteristics just after the plasma source is turned off [9,10] as;
no ¼
4mcp
ngWD pr2
dT dt
(1)
where m is the mass of nickel disk, cp its specific thermal capacity, WD is the dissociation energy of oxygen molecule, n is the random velocity of oxygen atoms, r is the radius of the nickel disk, dT/dt is the derivative of the probe temperature just after turn-off of the oxygen source and g is the recombination coefficient of oxygen atoms. In present, the recombination coefficient was determined experimentally following similar procedures described by Sorli et al. [9]. For this purpose, a nickel tube, 5 mm in diameter and 1.2 cm in length was mounted onto the probe holder and fixed in the remote zone of the plasma in the chamber. The time dependence of the tube temperature was measured at different operating parameters inside the chamber and the recombination coefficient of OA was determined [9]. Table 1 shows the calculated values of recombination coefficient (g) as a function of RF power at constant pressure and flow rate, which assure a constant value of pumping speed. The resulting average value of g (0.18) is used in Eq. (1) for determining the density of AO. However, a value of 0.27 for recombination coefficient of polycrystalline nickel is reported [9]. This difference from the present obtained value can be attributed to two points. Firstly, the recombination coefficient may depend on
420
material purity and morphology of the probe. Secondly, the recombination coefficient is very sensitive to any change in the pressure through effective pumping speed of the vacuum system that included in the calculation process of g [9]. Therefore, the plasma operating parameters during the measurements of g should be taken into account. The previous reported value was obtained by averaging the value of g calculated at different pressures. The value of g was seen to noticeably vary as function of operating pressure, having a maximum change of 0.1 as the pressure varied from 10 Pa to 60 Pa. By comparison, it can be seen from Table 1 that the maximum variation in the calculated g value in the present report is only 0.03. 2.3. Plasma treatment The samples of commercially available of polyimide (Kapton) and fluorinated ethylene propylene (FEP) were obtained by cutting sheets of kapton (w100 mm thick) and FEP (w500 mm thick) into small pieces, each of size 15 mm 15 mm. These samples, after cleaning and drying, were stored under a dry and clean
50 W 100 W 150 W 200 W 250 W 300 W
Off
400 380
T(K)
Fig. 3. Variation of AO density (no) as a function of RF power. The inset shows the variation of no as a function of three different pressures.
360 340 320 300 280
On 0
100
200
300
400
Time (s) Fig. 2. Variation of nickel probe temperature versus the time during plasma exposure and after turning the plasma off for different RF powers.
Fig. 4. Variation of AO density (no) as a function of oxygen flow rate. The inset shows the variation of no as a function of axial distance from the plasma source (Z) inside the plasma processing chamber, as measured by using the nickel catalytic probe.
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Table 2 Calculated values of degree of dissociation of oxygen (a) as a function the studied operating parameters. Plasma operating parameter Pressure (Pa) 5 10
Dissociation degree a (%) 3.6 6.2
Power (W) 50 100 150 200 250 300
4.1 7.2 11.8 14.4 16.4 18.5
Flow rate (sccm) 25 50 100 200 300 400 500
34.6 18.5 5.6 3.4 2.0 0.91 0.67
Z (cm) 2 3 4 5 6
16.5 19.5 18.5 15.9 14.7
atmosphere. After loading the samples into the chamber, it was evacuated to a base pressure of w0.005 Pa. After ignition of the primary plasma using argon gas, the oxygen gas was fed into the hollow cathode chamber and the argon flow was set off. The shutter which was used to shield the samples from the effect of argon plasma during ignition was then put off. Samples were then exposed to the remote oxygen plasma, for varying periods of time. The operating pressure during the plasma exposure was 20 Pa. The RF power was 300 W, the flow rate of oxygen was 50 sccm and the sample holder was positioned at Z ¼ 4 cm. 2.4. Characterization techniques The effect of plasma treatment on surface wetting properties and surface energy was characterized by contact angle measurement. The contact angles were obtained using the sessile drop method with OCA 15 plus, SCA 20, Data Physics Instrument GmbH. This instrument consists of a CCD video camera with a resolution of 768 576 pixel and up to 50 images s1. The digital drop image was processed using an image analysis system that calculated both the left and right contact angles from the shape of the drop with an accuracy of 0.1. Contact angles were measured for each PI film
before and after plasma treatment using 3 ml drops of either double distilled water, ethylene glycol, glycerol or decane. Whereas, contact angle information in case of FEP was obtained by using liquids of double distilled water, ethylene glycol, glycerol and decane. The surface morphological changes due the plasma treatment are viewed under a TESCAN VIGA II XMU Scanning Electron microscope (SEM). 3. Results and discussion 3.1. AO density The nickel probe characteristics, T ¼ T(t) was monitored as a function of pressure, RF power, flow rate of oxygen and axial distance (Z) of the probe from the HCD plasma source. For example, a set of nickel probe characteristics, measured at the pressure of 20 Pa, flow rate of oxygen 50 sccm and for various RF powers is plotted in Fig. 2. The density of AO, no, was then calculated from the rate of change of the nickel probe temperature with time just after turning off the plasma, using the expression given by Equation (1) and the average calculated value of g ¼ 0.18. Fig. 3 shows the variation of no as a function of RF power at a pressure of 20 Pa, oxygen flow rate 50 sccm and Z ¼ 4 cm. The inset shows the variation of no as a function of three different pressures at RF power 300 W, oxygen flow rate 50 sccm and Z ¼ 4 cm. The variation of AO density as a function of oxygen flow rate is shown in Fig. 4, at constant pressure of 20 Pa, RF power 300 W and Z ¼ 4 cm. The oxygen flow rate is varied between 25 and 500 sccm and the pressure is kept at 20 Pa via the pressure controller valve. The inset in Fig. 4 shows the variation of no as a function of five axial distance Z ¼ 2, 3, 4, 5 and 6 cm, at pressure 20 Pa, RF power 300 W and oxygen flow rate 50 sccm. It can be seen that no increases on increasing both the pressure and RF power, reaching a value of w 9 1019 m3 at pressure of 20 Pa and RF power of 300 W. The degree of dissociation of oxygen molecules (a) was calculate from measured values of no, using the relation [32],
a ¼ no kT= 2po2
(2)
where k the Boltzmann constant, T the gas temperature and po2 oxygen partial pressure. Table 2 shows the calculated values of (a) as a function of the above studied operating parameters. As the pressure and the applied RF power increase, the degree of dissociation increases, this leads to increase the density of the plasma and in turn the density of AO, as it is observed from Fig. 2. In contrast, the degree of dissociation and hence the density of AO decrease on increasing the oxygen flow rate. However, the dependence of no on the axial distance (Z) does not exhibit simple trend of behavior. The density of AO is seen to initially increase and then start to decrease for Z > 3 cm.
Fig. 5. Variation of water contact angle (CA) of FEP surfaces: (a) virgin sample and (b)e(e) after exposure to the remote oxygen plasma for 15, 30, 45 and 60 min, respectively.
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Fig. 6. Three consecutive video photos exhibiting the water drop as it is rolling off the surface of 75 min treated sample.
Fig. 5 shows the variation of water contact angle (CA) of FEP surfaces: (a) virgin sample and (b)e(e) after exposure to the remote oxygen plasma for 15, 30, 45 and 60 min, respectively. It is seen that CA increases on increasing the time of treatment, reaching a value of about 151 after 60 min. However, for samples treated at time 75 and 90 min, a self-cleaned surface is obtained, where water drops have rolled off the surface without any sticking. Fig. 6 shows three consecutive video photos exhibiting the water drop as it is rolling off the surface of 75 min treated sample. Numerous events can occur when AO impacts a surface [33]. These include elastic scattering of AO from the surface in a specular manner, energy and momentum accommodation of AO to the surface followed with ejection process in a diffuse manner, AO attachment to the surface and reaction with other arriving species, and the chemical reactions of AO with the surface. A study of surface energy is found to be one of the best methods for estimating the nature of chemical change occurring on the polymer surfaces. The contact angle measurement is probably the most common method giving information on the surface energy of the outmost surface layer. Over the years, a number of models have been developed to predict the surface energy of a solid from contact angle data [34e38]. In the present, contact angle information, obtained using double distilled water, ethylene glycol, glycerol and Decane, was analyzed using Wu theory to calculate the surface free p energy (gs) as well as its polar ðgs Þ and dispersive ðgds Þ components [36]. Each contact angle is the average value of three measurements. Table 3 shows the variation of contact angles of probe liquids as a function of time of exposure to remote oxygen plasma. Fig. 7 shows the variation of the calculated surface free energy and its components as a function of time of plasma exposure. The total surface free energy decreases from w23 mN/m for untreated FEP to w5.6 mN/m after 90 min of plasma exposure. The total surface free energy gsof the untreated FEP surface is composed of a high dispersive component and a low polar component. However, the p contribution of polar component gs to the total surface free energy
Table 3 Contact angles of different probe liquids for FEP surface exposed to remote oxygen plasma for different time period. Oxygen plasma exposure (min)
0 15 30 45 60 75a 90a a
Contact angle ( ) Water
Ethylene glycol
Glycerol
Decane
99 105.2 116.4 131 150.8 164.4 170.6
73.4 76.3 82.7 89.9 111.1 124.4 127
95.4 98.6 104 115.5 130.3 134.9 140
23.2 17.9 14.7 11.7 29.3 36 42.4
Measured just after dropping on the surface (see Fig. 5).
vanishes completely after 30 min of plasma treatment. Surface free energy arises from the unbalance of the force between atoms or molecules inside and interfaces. Several types of Van der Waals interactions contribute to gs. In particular, the polar component results from these different intermolecular forces due to permanent and induced dipoles and hydrogen bonding, whereas the dispersive component of surface free energy is due to instantaneous dipole moments, like the Coulomb interaction between an electron and the nuclei in two molecules [35]. In reality, the actual surface free energy is affected by many factors, in particular, the surface chemistry and roughness. It is generally recognized that as the roughness increases, the contact angle increases. However, there are reports states that the surface roughness has no definite effect on the contact angle [39]. Fig. 8 shows the SEM micrograph of FEP surface for; (a) virgin (b)e(d) plasma treatment for 30, 60 and 90 min, respectively. A drastic change in the surface morphology after plasma treatment is clearly observed. The average of surface roughness (Rms) is found to increase after plasma treatment from about 8.6 nm for virgin FEP surface to about 18 nm for 90 min plasma treated FEP surface. This can be attributed to the well-known surface etching effect due to AO attack that causes erosion and surface roughening of polymeric materials [13]. However, the other species in the plasma, in particular, energetic positive and negative oxygen ions contribute also to the etching process via surface sputtering effect. The flux of oxygen positive ions produced in the remote plasma was monitored by a planar electric probe, described in details elsewhere [40]. The fluxes of positive oxygen ions at the same operating conditions during the plasma treatment are found to be of the order of 1014 ions cm2 s1. In comparison, the AO flux as calculated from
γs
20
γ scomponents [mN/m]
3.2. Effect of AO on hydrophobic properties of FEP
γ sP γ sD
15
10
5
0 0
20
40
60
80
100
time (min) Fig. 7. Variation of the calculated surface free energy and its components for FEP surface as a function of time of plasma exposure.
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Fig. 8. SEM micrograph of FEP surface for; (a) virgin (b)e(d) plasma treatment for 30, 60 and 90 min, respectively.
measured values of AO density is of the order of w1016 atom cm2 s1. Therefore, the above results with surface free energy calculation and SEM clearly point out that AO produced in the remote plasma has the major role in modifying the wetting properties of FEP surface.
40
In this case, contact angle information, obtained using double distilled water, ethylene glycol, glycerol and DFM, was analyzed Table 4 Contact angles of different probe liquids for PI surface exposed to remote oxygen plasma for different time period.
0 5 15 30 45 60
γsP γ sD
30 25 20 15
s
Contact angle ( )
components [mN/m]
3.3. Effect of AO on hydrophilicity of Kapton polyimide films
Oxygen plasma exposure (min)
γs
35
Water
Ethylene glycol
Glycerol
DFM
71.7 66.4 54.1 59.6 48.6 47.5
51 47 37.8 44 34.3 31.7
62.1 57.2 45.8 53.7 40.7 37.8
24.3 20.2 14.2 17 13.1 11
10 5 0
10
20
30
40
50
60
time (min) Fig. 9. Variation of the calculated surface free energy and its components for PI surface as a function of time of plasma exposure.
M. Naddaf et al. / Vacuum 85 (2010) 421e428
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Fig. 10. SEM micrograph of PI surface for; (a) virgin PI and (b) and (c) after plasma treatment for 30 and 60 min, respectively.
using OWENS, WENDT, RABEL & AELBLE (WORK) theory to calculate the surface free energy and its components [38]. Table 4 shows the variation of contact angles of probe liquids as a function of time of exposure to remote oxygen plasma. Fig. 9 shows the variation of the calculated surface free energy and its components as a function of time of plasma exposure. In contrast to previous case, the total surface free energy increases from w34.2 mN/m for virgin PI (Kapton) to w41.2 mN/m after 60 min of plasma exposure. The total surface free energy gsof the PI films is composed of a high dispersive component and a low polar component. The polar component gps has the same trend with total gs, but the dispersive component gds is on the contrary exactly. The polar component is also lower than the dispersive component. Fig. 10 shows the SEM micrograph of PI surface for; (a) virgin PI and (b) and (c) after plasma treatment for 30 and 60 min, respectively. The effect of plasma treatment on surface morphology does not exhibit that drastic effect observed in case of FEP. The measured Rms value of virgin PI surface is found to be about 5.7 nm, as compared to about 7.6, 9.1, 8.7, 6.2, and 6 nm for the sample treated for 5, 15, 30, 45 and 60 min. It can be inferred from this result and results of contact angle that the change in surface roughness of PI does have not definite effect on contact angle. Therefore, in this case the changes in contact angle can be related to
two competition effects. The first is formation of hydrophilic groups on the surface of PI due to its interaction with AO, and the second is the increase in the surface roughness via etching and sputtering effect of AO and energetic oxygen ions. The results of surface free energy indicates that the first is the dominant in this case, in oppose to the case of FEP. This can be explained on the bases of well-known facts that PI has less resistance to chemical attack than FEP. However, more experimentations using surface sensitive techniques, such as XPS, is required for a getting more quantitative information regarding the chemical changes caused by the plasma treatment. 4. Conclusion Hollow cathode discharge (HCD-L300) induced remote oxygen plasma is shown to be an effective source of AO. The density of AO has been determined by nickel catalytic as a function of pressure, RF power, oxygen flow rate and axial position of the probe inside the reaction chamber. It is found to vary from w1 1019 m3 to w1 1020 m3, depending on the operating conditions and parameters. The effect of AO on surface wetting properties of PI and FEP is also investigated by using contact angle measurements and analysis. The results show that AO rich plasma of HCD-L300 is an
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effective source for achieving a superhydrophobic surface of FEP and for improving the surface wettability of PI. Acknowledgments The authors would like to thank the Director General of AECS, Professor I Othman for encouragement and support. References [1] Ruzyllo J. Solid State Technol 1990;18:211. [2] Hollahan JR. Techniques and application of plasma chemistry. New York: John Wiley & Sons; 1974. [3] Heilmann A, Haman C, Kampfarth G, Hopfe V. Vacuum 1990;41:1472. [4] Gleit GE. Am J Med Electron 1963;2:112. [5] Hartney MA, Hess DW, Soane DS. J Vac Sci Technol B 1989;7:1. [6] Nguyen AM, Murarka SP. J Vac Sci Technol B 1990;8:533. [7] Fedosenko G, Schwabedissen A, Korzec D, Engemann J. Surf Coat Technol 2001;142e144:693. [8] Strutt RJ. Proc Phys Soc 1910;23:147. [9] Sorli I, Rocak R. J Vac Sci Technol A 2000;18:338. [10] Mozetic M, Vesel A, Gaillard M, Ricard A. Plasma Polym 2001;6:71. [11] Mozetic M. Inform Midem 1998;28:175. [12] Naddaf M, Bhoraskar VN, Bhoraskar SV, Mandale AB, Sainkar SR. Plasma Sources Sci Technol 2002;11:1. [13] Houdayer A, Cerny G, Klemberg-Sapieha JE, Czeremuszkin G, Wertheimer MR. Nucl Instrum Meth Phys Res B 1997;131:335. [14] Flitsch R, Shih DY. J Vac Sci Technol A 1990;8:2376.
[15] Matienzo LJ, Zimmerman JA, Egitto FD. J Vac Sci Technol A 1994;12:2662. [16] Grondahi L, Cardona F, Chiem K, Wentrup-Byrne E. J Appl Polym Sci 2002;86: 2550. [17] Strobel M, Sullivan N, Branch MC, Park J, Ulsh M, Kapaun RS, et al. J Adhes Sci Technol 2000;14:1243. [18] Kim EY, Kong JS, An SK, Kim HD. J Adhes Sci Technol 2000;14:1119. [19] Mao C, Zhang C, Qiu Y, Zhu A, Shen J, Lin S. Appl Surf Sci 2004;228:26. [20] Bader MJ, OBrien JJ, Riddle KL. US Patent 1998;5:725e962. [21] Grace JM, Gerenser L. J Disp Sci Technol 2003;24:305. [22] Toanle Q, Pireaux JJ, Caudano R, Leclere P, Lazzaroni R. J Adhes Sci Technol 1998;12:999. [23] Gotoh K, Nakata Y, Tagawa M. Colloids Surf A 2003;224:163. [24] Minglin M, Randal MH. Curr Opin Colloid Interface Sci 2006;11:193. [25] Yabu H, Shimomura M. Chem Mater 2005;17:5231. [26] Shirtcliffe NJ, McHale G, Atherton S, Newton MI. Adv Colloid Interface Sci, in press. [27] Onda T, Shibuichi S, Satoh N, Tsujii K. Langmuir 1996;12:2125. [28] Zhai L, Cebeci FC, Cohen RE, Rubner MF. Nano Lett 2004;4:1349. [29] Nakajima A, Hashimoto K, Watanabe T. Monatsh Chem 2001;132:31. [30] Hosono E, Fujihara S, Honma I, Zhou HS. J Am Chem Soc 2005;127:13458. [31] Tzeng GS, Chen J, Wang YY, Wan CC. Surf Coat Technol 1997;89:108. [32] Cvelbar U, Mozetic M, Babic D, Pobera I, Ricard A. Vacuum 2006;80:904. [33] Banks BA, Rutledge SK. Low earth orbital atomic oxygen simulation for materials durability evaluation. In: Proceedings of fourth European symposium on spacecraft materials in space environment, CERT, Toulouse; 1988. [34] Young T. Philos Trans R Soc London 1805;95:65. [35] Van oss CJ, Chaudhury MK, Good RJ. Chem Rev 1988;88:927. [36] Wu S. Rev Macromol Chem 1974;11:1. [37] Good RJ, Girifalco LA. J Phys Chem 1960;64:561. [38] Owens DK, Wendt RC. J Appl Polym Sci 1969;13:1741. [39] Bikerman J. J Phys Colloid Chem 1950;54:653. [40] Saloum S, Akel M, Alkhaled B. J Phys D Appl Phys 2009;42:175206.
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Atomic oxygen in remote plasma of radio-frequency hollow cathode discharge source: Characterization and efficiency M. Naddaf*, S. Saloum, B. Alkhaled Atomic Energy Commission of Syria, Physics Department, P.O. Box 6091, Damascus, Syria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2010 Received in revised form 4 August 2010 Accepted 4 August 2010
Low-pressure, low-temperature remote oxygen plasma ignited by a radio-frequency (RF) hollow cathode discharge (HCD-L300) system is shown to be a powerful and effective source of neutral atomic oxygen (AO), useful in processing of polymeric materials. The density of AO was determined by a catalytic nickel probe as a function of pressure, RF power, oxygen flow rate and axial distance in the processing chamber. It was found to vary between w11019 to w11020 atom m3. The AO rich remote plasma-induced modification of wetting properties of polyimide (PI) and fluorinated ethylene propylene (FEP) surfaces has also been investigated. The wetting properties of the PI and FEP surfaces before and after exposure to the plasma were characterized by contact angle measurements and analysis. It was found that the influence of plasma surface treatment on wetting properties of FEP has an opposite effect to that of PI. On increasing the time of treatment, the surface of PI becomes more hydrophilic, whereas the hydrophobicity of FEP surface enhances. Moreover, a superhydrophobic FEP surface is produced at prolonged time of treatment. Changes in the surface morphology due to the plasma treatment were viewed under a scanning electron microscope (SEM). Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Atomic oxygen Catalytic nickel probe RF hollow cathode discharges Polyimide FEP Contact angle SEM
1. Introduction Atomic oxygen (AO) is one of the most important species generated in oxygen plasma reactor. It finds important applications in many areas of science, technology and industry. It has been successfully used for surface activation and cleaning in the electronic [1], chemical, automotive industries [2], ashing of organic materials in electronics [3], ashing of biological and medical samples [4], selective etching of composite materials [5], and plasma deposition of thin films [6]. One of the attractive sources of oxygen atom is electrodeless discharge, such as electron cyclotron resonance (ECR) and rf discharge. However, HCD remote plasma, in particular, has several advantages. It generates intense primary plasma with density approaching 1011 cm3 [7], allowing plasma jets to form and create remote plasma with a very homogeneous plasma density distribution. This can be utilized for treatment of large area planar substrate and deposition of thin films with good quality and high deposition rates. In addition, the low thermal load of the substrate during remote processing is very suitable for low temperature treatment of thermally sensitive substrates. Different methods exist for determining the density of AO. The oldest and most simple method is one involving catalytic probe, * Corresponding author. Tel.: þ963 11 2132580; fax: þ963 11 6112289. E-mail address: pscientifi[email protected] (M. Naddaf). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.08.004
which was first invented by Strutt [8]. It is based on the catalytic recombination of oxygen atoms on a metal surface. Although in principle any metal can be used in the construction of a catalytic probe, the choice of materials is limited for several reasons. For example, metals which form extremely stable oxides, such as aluminum and titanium, exhibit poor catalytic activity and therefore probe signals. However, it is found that nickel is one of the best metal for construction of a catalytic probe [9]. Nickel catalytic probe has been successfully utilized for determining the density of AO in a various plasma sources, such as RF, microwave and ECR oxygen plasmas [9e12]. Due to their intrinsic dielectric properties, good electrical insulation, good thermal stability, inertness to chemical attack and biocompatibility, polymeric materials find extensive use in a variety of applications in aerospace [13], electronic and other industries [14e16]. An important issue relevant to many of these applications is to control the wetting of polymer surface. For instance, the poor surface adhesion to other materials may result in the difficulty in depositing surface coatings, printings, dying, as well as poor protein absorption, platelet adhesion and cell attachment in biomedical applications. Surface modification techniques utilizing flame [17], chemical [18], grafting [19], corona discharge [20], low-pressure plasma [21,22], and ultraviolet exposure [23] have been proposed to improve the wetting properties of polymers. On the other hand, due to the wide applications of surfaces with hydrophobicity, many
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studies have been focused on surface modification to achieve such water-repellent property. Water-repellent or superhydrophobic surface is important in our daily lives, as well as, in many biological processes and industrial applications. These include the potential use of such a surface for satellite dishes, solar energy panels, photovoltaics, exterior architectural glass and green houses, heat transfer surfaces in air conditioning equipment, water-repellent coating on aerocrafts and microfluidics, piping and boat hulls [24e26]. The non-wettable character has been claimed in biomedical applications ranging from blood vessel replacement to wound management. Superhydrophobic surfaces can be achieved by various processes, such as, electrochemical reaction and deposition, phase separation, lithography, plasma etching and polymerization, chemical etching [24e30]. The materials being utilized in these processing can be polymers with intrinsic hydrophobic properties, nano-tube, nano-wire, or bulk metal substrates. In the present work, the density of AO in an HCD-L300 system induced remote oxygen plasma has been determined by using the nickel catalytic probe. The density of AO in the reaction zone of HCD eL300 reactor was found to be of the order of 1019e1020 atom cm3. In addition, a well know of excellent thermal, chemical and dielectric properties polymers; PI and FEP, were chosen to study the efficiency of the AO rich remote plasma for surface modification of materials. FEP is known as a polymer of low surface adhesion and low surface energy, the contact angle of water on smooth flat Teflon surfaces does not exceed 120 [31], whereas, PI is a hydrophilic polymer. Contact angle measurements and analysis showed that exposure of FEP surface to the plasma increases its hydrophobic nature, resulting in a superhydrophobic surface at exposure time 60 min or more. On contrary, plasma treatment of PI surface results in an enhancement of its wettability. 2. Experimental 2.1. Plasma system A schematic block diagram of the experimental set up of HCDL300 system (Plasma Consult GmbH PlasCon) used in the present
work is shown in Fig. 1. The plasma source is a hollow cathode configuration which consists of two coaxial tubes of 30 cm length. The inner tube is the hollow cathode and the outer-grounded tube forms the anode. Both cathode and anode are supplied with two rows of coaxial holes aligned with each other resulting in 30 plasma jets which form and create remote plasma with a very homogeneous plasma density distribution. The RF power at 13.56 MHz is applied to the cathode. The processing chamber, constructed of stainless steal, is cubic (50 50 50 cm3) and contains the HCDL300 plasma source and the sample holder. The system is provided with an adaptive pressure controller between the vacuum system and the plasma chamber, supplied with a stepping motor. This enables the variation of the flow rate at constant pressure and vice versa. 2.2. Nickel catalytic probe and AO density evaluation The catalytic probe, used in the study, was made up of nickel disk connected to a chromelealumel thermocouple (see Fig. 1). The thickness of the disk was 0.125 mm and its diameter was 4 mm. The diameter of the thermocouple wires was 0.2 mm and their lengths were 30 mm each. The wires were covered with narrow glass tubes. The probe was connected to a movable sample holder with a provision for scanning over the plasma region. High purity (99.999% Ar and O2) gas mixture was fed into the cathode through the mass flow controller from both ends in order to maintain a constant gas pressure over the entire cathode length. An RF power at 13.56 MHz (maximum power is 300 W) was applied to the cathode and the primary plasma was generated. In order to obtain the remote plasma of pure oxygen in the processing chamber, the flow of Ar gas was gradually decreases and then switched off. A suitable shutter was used to shield the probe assembly, during the stage of remote oxygen plasma ignition (10e30 s). Prior to the measurements, the probe was activated first, by degassing it under vacuum in the processing chamber for 2 h at a base pressure of w0.005 Pa. This is necessary for desorption of adsorbed gases or possible impurities on the probe surface. Secondly, the surface of the probe was oxidized by exposing it to oxygen remote plasma for
Fig. 1. A schematic block diagram of the experimental set up of HCD-L300 system.
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Table 1 Calculated values of oxygen recombination coefficient (g) as a function of RF power. Power (W)
g
50 100 150 200 250 300
0.2 0.17 0.19 0.17 0.18 0.18
1 h at a pressure of 30 Pa and RF power of 300 W. Repeating the measurement of the probe temperature (T) as a function of time (t), at same operating conditions, showed a stable T(t) response, indicating that the probe is well activated. The probe characteristic T(t) as a function of time (t) was then monitored at different operating parameters adjusted by varying the pressure, RF power and the flow rate of oxygen. The density of atomic oxygen (no) is calculated from the first time derivative of the probe characteristics just after the plasma source is turned off [9,10] as;
no ¼
4mcp
ngWD pr2
dT dt
(1)
where m is the mass of nickel disk, cp its specific thermal capacity, WD is the dissociation energy of oxygen molecule, n is the random velocity of oxygen atoms, r is the radius of the nickel disk, dT/dt is the derivative of the probe temperature just after turn-off of the oxygen source and g is the recombination coefficient of oxygen atoms. In present, the recombination coefficient was determined experimentally following similar procedures described by Sorli et al. [9]. For this purpose, a nickel tube, 5 mm in diameter and 1.2 cm in length was mounted onto the probe holder and fixed in the remote zone of the plasma in the chamber. The time dependence of the tube temperature was measured at different operating parameters inside the chamber and the recombination coefficient of OA was determined [9]. Table 1 shows the calculated values of recombination coefficient (g) as a function of RF power at constant pressure and flow rate, which assure a constant value of pumping speed. The resulting average value of g (0.18) is used in Eq. (1) for determining the density of AO. However, a value of 0.27 for recombination coefficient of polycrystalline nickel is reported [9]. This difference from the present obtained value can be attributed to two points. Firstly, the recombination coefficient may depend on
420
material purity and morphology of the probe. Secondly, the recombination coefficient is very sensitive to any change in the pressure through effective pumping speed of the vacuum system that included in the calculation process of g [9]. Therefore, the plasma operating parameters during the measurements of g should be taken into account. The previous reported value was obtained by averaging the value of g calculated at different pressures. The value of g was seen to noticeably vary as function of operating pressure, having a maximum change of 0.1 as the pressure varied from 10 Pa to 60 Pa. By comparison, it can be seen from Table 1 that the maximum variation in the calculated g value in the present report is only 0.03. 2.3. Plasma treatment The samples of commercially available of polyimide (Kapton) and fluorinated ethylene propylene (FEP) were obtained by cutting sheets of kapton (w100 mm thick) and FEP (w500 mm thick) into small pieces, each of size 15 mm 15 mm. These samples, after cleaning and drying, were stored under a dry and clean
50 W 100 W 150 W 200 W 250 W 300 W
Off
400 380
T(K)
Fig. 3. Variation of AO density (no) as a function of RF power. The inset shows the variation of no as a function of three different pressures.
360 340 320 300 280
On 0
100
200
300
400
Time (s) Fig. 2. Variation of nickel probe temperature versus the time during plasma exposure and after turning the plasma off for different RF powers.
Fig. 4. Variation of AO density (no) as a function of oxygen flow rate. The inset shows the variation of no as a function of axial distance from the plasma source (Z) inside the plasma processing chamber, as measured by using the nickel catalytic probe.
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Table 2 Calculated values of degree of dissociation of oxygen (a) as a function the studied operating parameters. Plasma operating parameter Pressure (Pa) 5 10
Dissociation degree a (%) 3.6 6.2
Power (W) 50 100 150 200 250 300
4.1 7.2 11.8 14.4 16.4 18.5
Flow rate (sccm) 25 50 100 200 300 400 500
34.6 18.5 5.6 3.4 2.0 0.91 0.67
Z (cm) 2 3 4 5 6
16.5 19.5 18.5 15.9 14.7
atmosphere. After loading the samples into the chamber, it was evacuated to a base pressure of w0.005 Pa. After ignition of the primary plasma using argon gas, the oxygen gas was fed into the hollow cathode chamber and the argon flow was set off. The shutter which was used to shield the samples from the effect of argon plasma during ignition was then put off. Samples were then exposed to the remote oxygen plasma, for varying periods of time. The operating pressure during the plasma exposure was 20 Pa. The RF power was 300 W, the flow rate of oxygen was 50 sccm and the sample holder was positioned at Z ¼ 4 cm. 2.4. Characterization techniques The effect of plasma treatment on surface wetting properties and surface energy was characterized by contact angle measurement. The contact angles were obtained using the sessile drop method with OCA 15 plus, SCA 20, Data Physics Instrument GmbH. This instrument consists of a CCD video camera with a resolution of 768 576 pixel and up to 50 images s1. The digital drop image was processed using an image analysis system that calculated both the left and right contact angles from the shape of the drop with an accuracy of 0.1. Contact angles were measured for each PI film
before and after plasma treatment using 3 ml drops of either double distilled water, ethylene glycol, glycerol or decane. Whereas, contact angle information in case of FEP was obtained by using liquids of double distilled water, ethylene glycol, glycerol and decane. The surface morphological changes due the plasma treatment are viewed under a TESCAN VIGA II XMU Scanning Electron microscope (SEM). 3. Results and discussion 3.1. AO density The nickel probe characteristics, T ¼ T(t) was monitored as a function of pressure, RF power, flow rate of oxygen and axial distance (Z) of the probe from the HCD plasma source. For example, a set of nickel probe characteristics, measured at the pressure of 20 Pa, flow rate of oxygen 50 sccm and for various RF powers is plotted in Fig. 2. The density of AO, no, was then calculated from the rate of change of the nickel probe temperature with time just after turning off the plasma, using the expression given by Equation (1) and the average calculated value of g ¼ 0.18. Fig. 3 shows the variation of no as a function of RF power at a pressure of 20 Pa, oxygen flow rate 50 sccm and Z ¼ 4 cm. The inset shows the variation of no as a function of three different pressures at RF power 300 W, oxygen flow rate 50 sccm and Z ¼ 4 cm. The variation of AO density as a function of oxygen flow rate is shown in Fig. 4, at constant pressure of 20 Pa, RF power 300 W and Z ¼ 4 cm. The oxygen flow rate is varied between 25 and 500 sccm and the pressure is kept at 20 Pa via the pressure controller valve. The inset in Fig. 4 shows the variation of no as a function of five axial distance Z ¼ 2, 3, 4, 5 and 6 cm, at pressure 20 Pa, RF power 300 W and oxygen flow rate 50 sccm. It can be seen that no increases on increasing both the pressure and RF power, reaching a value of w 9 1019 m3 at pressure of 20 Pa and RF power of 300 W. The degree of dissociation of oxygen molecules (a) was calculate from measured values of no, using the relation [32],
a ¼ no kT= 2po2
(2)
where k the Boltzmann constant, T the gas temperature and po2 oxygen partial pressure. Table 2 shows the calculated values of (a) as a function of the above studied operating parameters. As the pressure and the applied RF power increase, the degree of dissociation increases, this leads to increase the density of the plasma and in turn the density of AO, as it is observed from Fig. 2. In contrast, the degree of dissociation and hence the density of AO decrease on increasing the oxygen flow rate. However, the dependence of no on the axial distance (Z) does not exhibit simple trend of behavior. The density of AO is seen to initially increase and then start to decrease for Z > 3 cm.
Fig. 5. Variation of water contact angle (CA) of FEP surfaces: (a) virgin sample and (b)e(e) after exposure to the remote oxygen plasma for 15, 30, 45 and 60 min, respectively.
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Fig. 6. Three consecutive video photos exhibiting the water drop as it is rolling off the surface of 75 min treated sample.
Fig. 5 shows the variation of water contact angle (CA) of FEP surfaces: (a) virgin sample and (b)e(e) after exposure to the remote oxygen plasma for 15, 30, 45 and 60 min, respectively. It is seen that CA increases on increasing the time of treatment, reaching a value of about 151 after 60 min. However, for samples treated at time 75 and 90 min, a self-cleaned surface is obtained, where water drops have rolled off the surface without any sticking. Fig. 6 shows three consecutive video photos exhibiting the water drop as it is rolling off the surface of 75 min treated sample. Numerous events can occur when AO impacts a surface [33]. These include elastic scattering of AO from the surface in a specular manner, energy and momentum accommodation of AO to the surface followed with ejection process in a diffuse manner, AO attachment to the surface and reaction with other arriving species, and the chemical reactions of AO with the surface. A study of surface energy is found to be one of the best methods for estimating the nature of chemical change occurring on the polymer surfaces. The contact angle measurement is probably the most common method giving information on the surface energy of the outmost surface layer. Over the years, a number of models have been developed to predict the surface energy of a solid from contact angle data [34e38]. In the present, contact angle information, obtained using double distilled water, ethylene glycol, glycerol and Decane, was analyzed using Wu theory to calculate the surface free p energy (gs) as well as its polar ðgs Þ and dispersive ðgds Þ components [36]. Each contact angle is the average value of three measurements. Table 3 shows the variation of contact angles of probe liquids as a function of time of exposure to remote oxygen plasma. Fig. 7 shows the variation of the calculated surface free energy and its components as a function of time of plasma exposure. The total surface free energy decreases from w23 mN/m for untreated FEP to w5.6 mN/m after 90 min of plasma exposure. The total surface free energy gsof the untreated FEP surface is composed of a high dispersive component and a low polar component. However, the p contribution of polar component gs to the total surface free energy
Table 3 Contact angles of different probe liquids for FEP surface exposed to remote oxygen plasma for different time period. Oxygen plasma exposure (min)
0 15 30 45 60 75a 90a a
Contact angle ( ) Water
Ethylene glycol
Glycerol
Decane
99 105.2 116.4 131 150.8 164.4 170.6
73.4 76.3 82.7 89.9 111.1 124.4 127
95.4 98.6 104 115.5 130.3 134.9 140
23.2 17.9 14.7 11.7 29.3 36 42.4
Measured just after dropping on the surface (see Fig. 5).
vanishes completely after 30 min of plasma treatment. Surface free energy arises from the unbalance of the force between atoms or molecules inside and interfaces. Several types of Van der Waals interactions contribute to gs. In particular, the polar component results from these different intermolecular forces due to permanent and induced dipoles and hydrogen bonding, whereas the dispersive component of surface free energy is due to instantaneous dipole moments, like the Coulomb interaction between an electron and the nuclei in two molecules [35]. In reality, the actual surface free energy is affected by many factors, in particular, the surface chemistry and roughness. It is generally recognized that as the roughness increases, the contact angle increases. However, there are reports states that the surface roughness has no definite effect on the contact angle [39]. Fig. 8 shows the SEM micrograph of FEP surface for; (a) virgin (b)e(d) plasma treatment for 30, 60 and 90 min, respectively. A drastic change in the surface morphology after plasma treatment is clearly observed. The average of surface roughness (Rms) is found to increase after plasma treatment from about 8.6 nm for virgin FEP surface to about 18 nm for 90 min plasma treated FEP surface. This can be attributed to the well-known surface etching effect due to AO attack that causes erosion and surface roughening of polymeric materials [13]. However, the other species in the plasma, in particular, energetic positive and negative oxygen ions contribute also to the etching process via surface sputtering effect. The flux of oxygen positive ions produced in the remote plasma was monitored by a planar electric probe, described in details elsewhere [40]. The fluxes of positive oxygen ions at the same operating conditions during the plasma treatment are found to be of the order of 1014 ions cm2 s1. In comparison, the AO flux as calculated from
γs
20
γ scomponents [mN/m]
3.2. Effect of AO on hydrophobic properties of FEP
γ sP γ sD
15
10
5
0 0
20
40
60
80
100
time (min) Fig. 7. Variation of the calculated surface free energy and its components for FEP surface as a function of time of plasma exposure.
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Fig. 8. SEM micrograph of FEP surface for; (a) virgin (b)e(d) plasma treatment for 30, 60 and 90 min, respectively.
measured values of AO density is of the order of w1016 atom cm2 s1. Therefore, the above results with surface free energy calculation and SEM clearly point out that AO produced in the remote plasma has the major role in modifying the wetting properties of FEP surface.
40
In this case, contact angle information, obtained using double distilled water, ethylene glycol, glycerol and DFM, was analyzed Table 4 Contact angles of different probe liquids for PI surface exposed to remote oxygen plasma for different time period.
0 5 15 30 45 60
γsP γ sD
30 25 20 15
s
Contact angle ( )
components [mN/m]
3.3. Effect of AO on hydrophilicity of Kapton polyimide films
Oxygen plasma exposure (min)
γs
35
Water
Ethylene glycol
Glycerol
DFM
71.7 66.4 54.1 59.6 48.6 47.5
51 47 37.8 44 34.3 31.7
62.1 57.2 45.8 53.7 40.7 37.8
24.3 20.2 14.2 17 13.1 11
10 5 0
10
20
30
40
50
60
time (min) Fig. 9. Variation of the calculated surface free energy and its components for PI surface as a function of time of plasma exposure.
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Fig. 10. SEM micrograph of PI surface for; (a) virgin PI and (b) and (c) after plasma treatment for 30 and 60 min, respectively.
using OWENS, WENDT, RABEL & AELBLE (WORK) theory to calculate the surface free energy and its components [38]. Table 4 shows the variation of contact angles of probe liquids as a function of time of exposure to remote oxygen plasma. Fig. 9 shows the variation of the calculated surface free energy and its components as a function of time of plasma exposure. In contrast to previous case, the total surface free energy increases from w34.2 mN/m for virgin PI (Kapton) to w41.2 mN/m after 60 min of plasma exposure. The total surface free energy gsof the PI films is composed of a high dispersive component and a low polar component. The polar component gps has the same trend with total gs, but the dispersive component gds is on the contrary exactly. The polar component is also lower than the dispersive component. Fig. 10 shows the SEM micrograph of PI surface for; (a) virgin PI and (b) and (c) after plasma treatment for 30 and 60 min, respectively. The effect of plasma treatment on surface morphology does not exhibit that drastic effect observed in case of FEP. The measured Rms value of virgin PI surface is found to be about 5.7 nm, as compared to about 7.6, 9.1, 8.7, 6.2, and 6 nm for the sample treated for 5, 15, 30, 45 and 60 min. It can be inferred from this result and results of contact angle that the change in surface roughness of PI does have not definite effect on contact angle. Therefore, in this case the changes in contact angle can be related to
two competition effects. The first is formation of hydrophilic groups on the surface of PI due to its interaction with AO, and the second is the increase in the surface roughness via etching and sputtering effect of AO and energetic oxygen ions. The results of surface free energy indicates that the first is the dominant in this case, in oppose to the case of FEP. This can be explained on the bases of well-known facts that PI has less resistance to chemical attack than FEP. However, more experimentations using surface sensitive techniques, such as XPS, is required for a getting more quantitative information regarding the chemical changes caused by the plasma treatment. 4. Conclusion Hollow cathode discharge (HCD-L300) induced remote oxygen plasma is shown to be an effective source of AO. The density of AO has been determined by nickel catalytic as a function of pressure, RF power, oxygen flow rate and axial position of the probe inside the reaction chamber. It is found to vary from w1 1019 m3 to w1 1020 m3, depending on the operating conditions and parameters. The effect of AO on surface wetting properties of PI and FEP is also investigated by using contact angle measurements and analysis. The results show that AO rich plasma of HCD-L300 is an
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effective source for achieving a superhydrophobic surface of FEP and for improving the surface wettability of PI. Acknowledgments The authors would like to thank the Director General of AECS, Professor I Othman for encouragement and support. References [1] Ruzyllo J. Solid State Technol 1990;18:211. [2] Hollahan JR. Techniques and application of plasma chemistry. New York: John Wiley & Sons; 1974. [3] Heilmann A, Haman C, Kampfarth G, Hopfe V. Vacuum 1990;41:1472. [4] Gleit GE. Am J Med Electron 1963;2:112. [5] Hartney MA, Hess DW, Soane DS. J Vac Sci Technol B 1989;7:1. [6] Nguyen AM, Murarka SP. J Vac Sci Technol B 1990;8:533. [7] Fedosenko G, Schwabedissen A, Korzec D, Engemann J. Surf Coat Technol 2001;142e144:693. [8] Strutt RJ. Proc Phys Soc 1910;23:147. [9] Sorli I, Rocak R. J Vac Sci Technol A 2000;18:338. [10] Mozetic M, Vesel A, Gaillard M, Ricard A. Plasma Polym 2001;6:71. [11] Mozetic M. Inform Midem 1998;28:175. [12] Naddaf M, Bhoraskar VN, Bhoraskar SV, Mandale AB, Sainkar SR. Plasma Sources Sci Technol 2002;11:1. [13] Houdayer A, Cerny G, Klemberg-Sapieha JE, Czeremuszkin G, Wertheimer MR. Nucl Instrum Meth Phys Res B 1997;131:335. [14] Flitsch R, Shih DY. J Vac Sci Technol A 1990;8:2376.
[15] Matienzo LJ, Zimmerman JA, Egitto FD. J Vac Sci Technol A 1994;12:2662. [16] Grondahi L, Cardona F, Chiem K, Wentrup-Byrne E. J Appl Polym Sci 2002;86: 2550. [17] Strobel M, Sullivan N, Branch MC, Park J, Ulsh M, Kapaun RS, et al. J Adhes Sci Technol 2000;14:1243. [18] Kim EY, Kong JS, An SK, Kim HD. J Adhes Sci Technol 2000;14:1119. [19] Mao C, Zhang C, Qiu Y, Zhu A, Shen J, Lin S. Appl Surf Sci 2004;228:26. [20] Bader MJ, OBrien JJ, Riddle KL. US Patent 1998;5:725e962. [21] Grace JM, Gerenser L. J Disp Sci Technol 2003;24:305. [22] Toanle Q, Pireaux JJ, Caudano R, Leclere P, Lazzaroni R. J Adhes Sci Technol 1998;12:999. [23] Gotoh K, Nakata Y, Tagawa M. Colloids Surf A 2003;224:163. [24] Minglin M, Randal MH. Curr Opin Colloid Interface Sci 2006;11:193. [25] Yabu H, Shimomura M. Chem Mater 2005;17:5231. [26] Shirtcliffe NJ, McHale G, Atherton S, Newton MI. Adv Colloid Interface Sci, in press. [27] Onda T, Shibuichi S, Satoh N, Tsujii K. Langmuir 1996;12:2125. [28] Zhai L, Cebeci FC, Cohen RE, Rubner MF. Nano Lett 2004;4:1349. [29] Nakajima A, Hashimoto K, Watanabe T. Monatsh Chem 2001;132:31. [30] Hosono E, Fujihara S, Honma I, Zhou HS. J Am Chem Soc 2005;127:13458. [31] Tzeng GS, Chen J, Wang YY, Wan CC. Surf Coat Technol 1997;89:108. [32] Cvelbar U, Mozetic M, Babic D, Pobera I, Ricard A. Vacuum 2006;80:904. [33] Banks BA, Rutledge SK. Low earth orbital atomic oxygen simulation for materials durability evaluation. In: Proceedings of fourth European symposium on spacecraft materials in space environment, CERT, Toulouse; 1988. [34] Young T. Philos Trans R Soc London 1805;95:65. [35] Van oss CJ, Chaudhury MK, Good RJ. Chem Rev 1988;88:927. [36] Wu S. Rev Macromol Chem 1974;11:1. [37] Good RJ, Girifalco LA. J Phys Chem 1960;64:561. [38] Owens DK, Wendt RC. J Appl Polym Sci 1969;13:1741. [39] Bikerman J. J Phys Colloid Chem 1950;54:653. [40] Saloum S, Akel M, Alkhaled B. J Phys D Appl Phys 2009;42:175206.