- Email: [email protected]
70 cm radio frequency hollow cathode plasma source for modification of foils and membranes
Surface and Coatings Technology 97 (1997) 759–767
70 cm radio frequency hollow cathode plasma source for modification of foils and membranes D. Korzec *, M. Mildner, F. Hillemann, J. Engemann Microstructure Research Center—fmt, University of Wuppertal, Obere Lichtenplatzer Strasse 336, 42 287 Wuppertal, Germany
Abstract For industrial use long linear plasma sources are needed. Basing on a radio frequency (13.56 MHz) hollow cathode discharge (HCD) concept proved for 30 cm long cathodes, an upscaled version of the plasma source with cathode length of 70 cm was developed. Additional hollow anode plasma chambers allow easier switching to an HCD discharge mode. Holes between anode and cathode plasmas cause a local increase in the ion concentration by a factor of 40 (up to 2×1011 cm−3). Homogeneity of the cathode plasma was examined by use of a double Langmuir probe technique. It improves with increasing number of holes distributed along the cathode trench. To characterize the process uniformity, polypropylene foil and cellulose membrane were used. Typical process conditions are RF power of 600 W, argon flow 60 sccm, pressure in the process chamber 0.1 Pa. The contact angle measured with water on the polypropylene surface decreases from 100° to 50° after 2 s of treatment in plasma with ion concentrations of more than 2×1010 cm−3. The distributions of the contact angle over the foil width show minima for places of maximum ion concentration and are almost identical on both foil sides. The intensity of the spectral line at 308.6 nm wavelength was measured as a function of time after discharge ignition for determination of the water outgasing from the cellulose. The amount of water in the discharge correlates with the ion concentration. The second emission intensity maximum after about 7 s of treatment occurs at the position with highest ion concentration. The warming up of the membrane is the most probable reason for this effect. © 1997 Elsevier Science S.A. Keywords: Plasma sources; Radio frequency; Hollow cathode discharge; Surface modification
1. Introduction Gas discharge treatments of plastic surfaces for improvement of wettability [1], paintability [2], adhesion [3], electrical conductivity [4] and biocompatibility [5,6 ] are broadly accepted techniques. A special interest of industry is focused on the continuous processing of ‘‘endless’’ materials. Discharge-based modifications of fibers [7], paper [8], textile products [9], membranes [10], foils [11], and many others, are known. The particular motivation of this work was the cellulose membrane used, for example, for dialysis [12]. Drawbacks of widely used atmospheric pressure discharges, such as corona discharge [13], spark or arc jets [14] and atmospheric pressure microwave discharge [15], are weak modification [15], generation of ozone [9], inhomogeneous treatment, and strong aging of modification effect ( loss of 50% after one day) [13]. They * Corresponding author. Tel: (49)-202-595096; Fax: (49)-202-595098; e-mail: [email protected] 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 32 4 - 1
motivate for application of low pressure plasma processing. The treatment of most plastic surfaces in a low temperature, low pressure plasma allows a very strong, durable and homogeneous modification [16 ]. Continuous processing is also possible when special airto-vacuum ports are applied [17]. Otherwise batch processing of continuous materials is an acceptable alternative for many applications. With reactive gases the microwave and the radio frequency discharges can be used. Even though the microwave discharges allow high ion concentrations [18] and efficient free radical production [19], they cause a high thermal load of materials processed in the direct discharge [20]. Consequently the application of radio frequency discharge is a good choice for thermally sensitive substrates. An especially efficient method of radio frequency plasma production is a hollow cathode discharge (HCD) [21]. Similar to that in the DC cold cathode HCD [22], in RF HCD the electrons are trapped in hollow cavities formed in the RF active electrode, allowing high plasma
760
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
densities. Very different shapes of hollow cathodes can be used for treatment of flat substrates [23]. For example the hills and valleys may be arranged in a concentric circular form, concentric square form, cylindrical hollows [24] etc. The discharge design avoiding the presence of the plasma outside of the plasma source is known [25]. A novel concept of RF HCD allowing a symmetrical, double-sided modification of foil-like materials has been developed [26 ]. The direct geometrical scaling of the concept, which was successful for 30 cm HCD plasma source [27], did not work for 70 cm. Difficulties with switching into the HCD mode and a strong plasma inhomogeneity were observed. Two measures were undertaken to improve the source performance: (i) an additional anode plasma chamber was incorporated; and (ii) transfer holes between anode and cathode plasma chambers were introduced. The characterization of the source with this constructional improvements is a main aim of this work.
2. Hollow cathode discharge In general the potential distributions and charge carrier currents in RF discharge are complicated functions of time and position. For calculating them, particle-incell (PIC ) simulation is the proper tool [28]. The very simplified formulae covered in the next two paragraphs are suitable for rough estimation only. 2.1. Electrons in hollow cathode The concentration of electrons directly at the cathode surface n , depends on the local voltage between cathode s and plasma bulk V and is described by the Boltzmann p relation:
A
B
eV n =n exp − p , (1) s e kT e where n is the concentration and T temperature of e e electrons in the plasma bulk. The physical meaning of Eq. (1) is that only electrons with sufficient energy arrive at the electrode surface. The condition for ignition of the hollow cathode discharge is the creation of a potential trap for electrons between two cathode surfaces positioned face-to-face. The high energy electrons behave as in an electrostatic trap, where their free path, given for monoenergetic electrons, as: 1 kT L = = (2) e n s ps n e e is comparable with the distance between trapping cathode surfaces. The s is the total cross section for electron e
collisions, which is energy and gas dependent. n is the n concentration and T the temperature of neutral particles in the plasma chamber. As an example, for argon at pressure of 0.1 mbar the free path for elastic collisions for 100 eV electrons is about 8 mm. According to Eq. (2), for decreasing pressure the optimal distance between electrodes for HCD increases. 2.2. Plasma inhomogeneity in long hollow cathode plasma columns For most of the RF cycle time the cathode is sufficiently negative polarized to keep the thermal electrons apart from its surface. To the cathode flows the ion current, J , given as: i,satt
A B S 1
kT e, (3) 2 m i where n is the mean ion concentration in the plasma 0 bulk, and m the ion mass. To fulfil the requirement of i quasineutrality, the sum of currents flowing out of the cathode plasma should be zero. Consequently in the cathodic part of the RF cycle, from the cathode to the anode flows an electron current, I , given as: e,− I =A J , (4) e,− c +,satt where A is the effective area of the cathode. The total c charge flowing to the cathode must be zero, because the cathode is capacitively coupled, and no net DC current can flow to it. In this case the electron current, I , e,+ flowing to the cathode in the short period of time, t, when the cathode is sufficiently weakly negative polarized to allow the thermal electrons from the bulk plasma to overcome the potential barrier, can be given as: J
I
i,satt
e,+
=exp −
=
A B 1
tf
−1 I
qn
e,−
0
,
(5)
where f is the RF excitation frequency. Assuming the hollow cathode as a long tube with an area of the cross section A , electron current flowing along the plasma t column is given as: =qn v A , (6) e,+ e e t where v is the drift velocity of electrons described as a e function of the E/p (mean electric field divided by pressure). The curves representing v =f(E/p) are availe able for different gases [29]. I
2.3. Influence of transfer hole on plasma density The density of electron current flowing through a transfer hole between cathode and anode plasmas (as shown in Fig. 1) during the anodic polarization of
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
761
Fig. 1. Operation principle of the hollow cathode discharge with hole connecting anode and cathode plasma.
cathode can be estimated by use of following expression:
A B
I A 1 J = e,+ = c (7) −1 J e,h i,satt A A tf h h A is the area of the hole cross section. Current densities h in transfer holes calculated from Eq. (7) are several orders of magnitude higher than ion saturation currents in the cathode plasma. This explains very high ion concentrations of 1013 cm−3 measured in such holes [30].
3. Experimental details 3.1. Plasma source The following requirements were the base for the plasma source development: (i) up-scalable source construction; (ii) sufficient plasma density for modification speeds of several tens of meters per minute; (iii) the modification being homogeneous, and equal on both sides of the substrate material; (iv) closed source construction, to avoid the influence of the process chamber geometry on the source performance [31], ‘‘burn out’’ of lubricants used in foil transfer mechanisms and undefined plasma treatment time, which can occur in open HCDs [32]. Similar to our earlier work [26 ] two identical hollow cathodes, positioned face-to-face and enclosed in a grounded casing were used. Both of them were supplied from a common RF power source. The foil is guided between both hollow cathodes (see Fig. 2a). The hollow spaces in both cathodes are formed as straight 35×35 mm2 trenches parallel to each other at predetermined intervals, open on both ends, where the grounded wall of the plasma chamber plays the role of the RF
anode (see Fig. 2(a)). When up-scaling the cathode length from 30 to 70 cm, the ratio of anode-to-cathode area becomes so small that the bias of the cathode [31,33] is not sufficient for ignition the HCD. The required increase of the anode surface was realized by the introduction of two additional anode plasma chambers placed on the backs of both cathodes and separated from them by grounded dark space shields (see Fig. 2(b)). All parts were machined from aluminum. The first cathode was powered by a 1 kW RF generator (Dressler, type mpg 1310) via a standard gamma-type matchbox and through a coaxial connector. The RF power is supplied to the second cathode from the first one through the contact pads on ends of all hollow cathode trenches (see Fig. 2(c)). The second cathode is movable together with half of the grounded casing, to allow the opening the source for easy foil handling. For improvement of the source performance holes with diameter of 20 mm were introduced in the first plasma chamber, symmetrically on both sides of the treated material. Experiments with one hole couple in the center (see Fig. 2(b)), and with three hole couples distributed along the cathode plasma chamber (see Fig. 2(c)) were performed. 3.2. Process set up The process chamber was a cylinder of stainless steel with a diameter of 1 m, a length of 1.2 m and two ISO 1000 high vacuum flanges. The pumping system consists of turbomolecular pump (Balzers TPH 2200), a rotary pump ( TRIVAC D65 BCS, Leybold) with a pumping speed of 65 m3 h−1 and a Roots blower (RUVAC WSU 501, Leybold) with a pumping speed of 500 m3 h−1. The achievable base pressure was 2×10−4 Pa. Pressure in
762
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
Fig. 2. Cross section of radio frequency hollow cathode plasma source for double-sided membrane modification: (a) front view; (b) side view; (c) top view of the cathode.
the process chamber was controlled between 1 Pa and 600 Pa with a swing valve and a vacuum gage (MKS Baratron). The gas flow was controlled by an MKS flow controller.
4. Results 4.1. Ion concentration distributions The properties of the plasma in the cathode chambers were characterized by use of a double Langmuir probe (DLP). Two probe tips are identical, 7 mm spaced, tungsten wires (0.5 mm diameter and 5 mm exposed length) in two-channel alumina tubes. The measurements were made by use of a DLP plasma diagnostics system developed by Brockhaus et al. [34]. It consists of a power supply, which both generates the probe voltage and measures the probe current. The calculation of ion concentration and electron temperature is based on an iterative method described by Peterson and Talbot [35]. The dense plasma limit for determination of the Laframbois factor can be assumed [36 ]. At low pressures and powers the plasma source works in the ‘‘normal’’ mode, which means not in hollow cathode operation mode. Characteristic for such working conditions are low ion concentrations. They are below 3×109 cm−3 for oxygen (Fig. 3(a)) and below 5×109 cm−3 for helium ( Fig. 3(b)). The ion concen-
tration distribution along the cathode is more homogeneous for helium (maximum-to-minimum ratio of two) than for oxygen (maximum-to-minimum ratio of four). The reason for the better result in helium is a much higher free path for electron impact ionization than in oxygen for comparable pressure. Switching from normal to hollow cathode operation mode can be initiated by increase of RF power (see Fig. 3(a)) or increase of gas flow (see Fig. 3(b)). Both electron temperature and ion concentration are typically much higher for HCD operation mode than for ‘‘normal’’ mode. For oxygen the ion concentration improves by a factor of 5 close to the anode and a factor of 2 in the central cathode location. The ion concentration improvement factors for helium are 200 and 50, respectively. Close to the anode a maximum ion concentration of more than 1012 cm−3 was measured. Both discharge operation modes show a high symmetry of the plasma parameter distributions, despite an asymmetric power coupling (see Fig. 2(a)). The simple up-scaling of the 40 cm prototype by making the cathode 70 cm long failed. The problem with such an up-scaled version of the HCD plasma source was a very poor homogeneity of the ion concentration along the cathode chamber. The ion concentration in the middle of the source was for the conditions in Fig. 4 a factor of 3.5 lower than in the vicinity of the anode. Voltage drop over the plasma column along the
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
(a)
(b) Fig. 3. Ion concentration distributions along a single cathode chamber measured in normal and hollow cathode operation mode for (a) oxygen flow of 20 sccm at pressure of 0.06 Pa, and for (b) helium at RF power of 600 W and pressure of 2 Pa.
763
From the drift velocity curves for oxygen the voltage drop along the cathode column of 3.5 V can be read [29]. Such a voltage drop causes, according to Eq. (1), the electron concentration in the cathode center to be a factor of 5 lower than at the anode. Fewer electrons arriving the cathode surface means fewer secondary electrons emitted from the cathode surface, and consequently less efficient HCD excitation in the center of the source in comparison with the location in the neighborhood of the anode. As a matter of fact much lower voltage drop over the hollow cathode can be expected, because the largest part of electron current will flow to the cathode part most adjacent to the anode. These corresponds to even more inhomogeneous plasma density distribution. To improve the ion concentration homogeneity, the reduction of the distance between the anode plasma and a most distant cathode plasma site is needed. It can be achieved by use of a hole connecting the anode and the cathode plasma chambers (see Fig. 2(b)). In Fig. 4 the ion concentration distributions in oxygen plasma along the first cathode chamber with and without such a hole are compared. Because of the hole, the ion concentration in the cathode chamber center is elevated by a factor of six. At anode ends of the cathode chamber the ion concentration remains almost without change. The influence of the hole on the ion concentration can be observed over a much longer distance than the diameter of the hole. 50% of the maximum value is exceeded at a distance of 8 cm from the hole center. The next logical step in the direction of optimal plasma homogeneity is the introduction of several transfer holes along each cathode chamber. In Fig. 5 the results for three holes are displayed. Each of these holes causes similar enhancement of the discharge in its vicinity. With increasing number of holes the effective cathode area per hole gets smaller. Consequently (see Eq. (7)), lower electron currents flow through a single hole and lower plasma density can be expected. Such
Fig. 4. Ion concentration distributions along a single cathode chamber with and without hole for oxygen flow of 20 sccm at 0.06 Pa and RF power of 400 W.
cathode plasma chamber explains this effect. Using a typical ion concentration of 5×1010 cm−3, an electron temperature of 5 eV, pressure of 0.1 mbar, length of the cathode plasma chamber of 35 cm, and assuming the time of the electron current flow to the cathode of onethird of the RF cycle [37], the drift velocity for electrons of 1 cm ms−1 can be calculated from Eqs. (5) and (6).
Fig. 5. Ion concentration distributions along a single cathode chamber with three holes for argon flow of 60 sccm at 0.15 Pa.
764
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
decrease of the maximum ion concentration with increasing number of holes can be observed, when comparing the one-hole-curve from Fig. 4 with 400 W curve from Fig. 5. The maximum ion concentration does not scale linearly with cathode area per hole. In our example the area per hole changes from 368 cm2 for one hole to 184 cm2 for three holes. At the same time the maximum ion concentration reduces only from 3.5×1010 cm−3 to 2.5×1010 cm−3. The effective cathode area is determined not only by geometrical dimensions, but also by the current density distributions over the cathode surface. When the distances between adjacent holes are larger than the zones of high density plasma, low sensitivity of ion concentration on changes of cathode area per hole can be expected.
Fig. 6. Advancing angle of water vs time measured on PP foil treated in HCD plasma source operated with argon flow of 60 sccm at pressure of 0.2 Pa and RF power of 600 W.
4.2. Contact angle distribution Contact angle measurement is an easy and important method for determination of the surface modification quality. Because the receding contact angle is to a large extent the measure of the mechanical surface condition, the advancing angle was chosen for modification process ¨ characterization [38]. For measurements a Kruss Goniometer Type G1 with computerized data collecting system G40 was used. The volume of distilled water drop is increased until moving the three-phase boundary over the surface. Each point shown in plots represents the mean value of 60 contact angle values. They are collected automatically 10 times for the left and right sides of three droplets at each place. The contact angles were determined accordingly to the method of Owens and Wendt [39]. The measurements have to be performed directly after processing, otherwise an aging effect of the modified surface (30% increase of the contact angle after 7 days by treatment with oxygen) [2] falsifies the results. Treatment homogeneity on PP foil with width of 60 cm and thickness of 300 mm was characterized. For the best homogeneity characterization a treatment time should be chosen allowing the highest sensitivity of the contact angle on the variations of the plasma density. The time dependence of the advancing contact angle of distilled water was measured for three positions in the source to choose such critical treatment time (see Fig. 6). The contact angle measured on an untreated surface is 100°. This value is in agreement with cited data ranging for PP from 96° [40] to 103° [2]. For each curve in Fig. 6 the corresponding ion concentration is specified. Contact angle measured after 2 s treatment reduces more for higher ion concentration. Contact angle reduction is as strong as 50% for ion concentration higher than 1010 cm−3. The following 2 s of treatment cause a deterioration of the modification effect, documented as an increase of the contact angle
from 55° up to 80° and more. Then after longer treatment time (more than 15 s) for all curves, a low value of contact angle (45° for ion concentration of 5×1010 cm−3) stabilizes. The largest contact angle spread, allowing the best characterization of process homogeneity, occurs for treatment time of 2 s or 8 s. Because short processing times are of technological interest, 2 s were chosen as a treatment time for the following experiments. Strong correlation of the lateral contact angle distribution with construction marks and hence with ion concentration distribution can be observed. In Fig. 7 the influence of the holes between anode and cathode plasma chambers on the advancing contact angle distribution is displayed. At the positions of the holes the contact angle after 2 s of plasma treatment was 50°. Much weaker modification of the foil, correlating with higher contact angle of 80°, is observed at places which are most distant from the holes in the case of the source with holes, and
Fig. 7. Advancing contact angle measured across the foil modified in the plasma source without and with three holes. 300 mm thick PP foil was treated 2 s in argon plasma at flow of 60 sccm, pressure of 0.2 Pa and RF power of 600 W.
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
most distant from the cathode edge, for the source construction without holes. One of the important requirements for the development of large area plasma sources was the treatment symmetry for both sides of the foil. The contact angle distributions were measured as a function of lateral position by use of distilled water and ethylene glycol. The comparison of results collected on both sides of the membrane is shown in Fig. 8. For both test fluids the differences of the contact angles on both sides A and B of the foil are less than 5% for lateral positions form 17 to 52 cm. Accordingly to Fig. 7 the region of optimal modification (contact angle close to 50°) and also the region of maximum ion concentration, extents over a distance of 6 cm from the transfer hole. To achieve the optimal treatment (variation of the contact angle of less than 5%) of moving substrates, about 10 holes per cathode chamber are needed. To avoid too strong reduction of maximum ion concentration, five holes per cathode chamber could be enough, if the hole patterns in each cathode chamber are slightly shifted in respect to other chambers. In this case treatment homogeneity much better than the ion concentration homogeneity in a single cathode plasma chamber can be achieved. 4.3. Release of water from the treated membrane The cellulose membrane was the second material used for plasma source performance characterization. Because a strong influence of the winding speed and the RF power on the plasma properties was observed, a release of water from the membrane surface was suspected. Optical emission was used for determination of the water present in the plasma. An intensity of the spectral line at 308.6 nm wavelength was measured as a
Fig. 8. Advancing contact angle measured across both sides of the foil modified in the plasma source with three holes. 300 mm thick PP foil was treated for 2 s in argon plasma at flow of 60 sccm, pressure of 0.2 Pa and RF power of 600 W. Distilled water and ethylene glycol were used for determination of the contact angle.
765
function of time after discharge ignition. During the first treatment phase a strong water burst was observed. The emission intensities reach their maxima in all examined positions. The values of these maxima correlate with the local ion concentrations (see Fig. 9). In the second treatment phase the amount of water in plasma decreases exponentially. Only at the position with highest ion concentration does the emission intensity start to increase again after 2 s of treatment, and it achieves the second maximum after about 7 s of treatment. The warming up of the membrane is the most probable reason for this effect. Because the losses of water from the internal structure of the membrane causes its deformation, processing times of more than 2 s should be avoided. For the studied plasma source geometry this means the minimum required speed of the membrane of 0.125 m s−1.
5. Conclusions (1) A novel 70 cm radio frequency hollow cathode plasma source allows double-sided modification of membranes and foils. (2) A maximum ion concentration of 1012 cm−3 for helium was measured. (3) An easy switching in the hollow cathode mode was possible due to anode plasma chambers. The power and gas flow should exceed some threshold values, i.e. 20 sccm and 200 W for oxygen for ignition the HCD. (4) The required homogeneity of the ion concentration can be achieved by use of transfer holes connecting the anode and cathode plasma chambers. The influence of the 20 mm hole is present over a range of about 20 cm. (5) Advancing contact angle measured with water reduces from 100° before treatment to about 50° after 2 s treatment by ion concentration of
Fig. 9. Time resolved optical emission intensity for wavelength of 308.6 nm (OH ) measured in a chamber loaded with the membrane, for argon flow of 60 sccm at pressure of 0.2 Pa and RF power of 400 W.
766
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
2×1010 cm−3. The positions of minima in lateral distributions of contact angle overlap with positions of maxima in ion concentration distributions. (6) With properly designed pattern of holes and sufficient number of cathode chambers, homogeneous treatment (contact angle variation after critical treatment time less than 5%) is possible. (7) The cellulose membranes release water vapour into the plasma during plasma treatment. The amount of released water is proportional to the local ion concentration. The treatment should be not longer than 2 s to avoid drying out the membrane causing its deformation. Even though the application examples shown in this work concern surface modification, the RF HCD plasma source can be used also for surface cleaning [41], etching [42–45] or film deposition [46–48] as well.
Acknowledgement ¨ The authors wish to thank Peter Stolzle and Hans¨ Jorg Schmidt for construction of the source. The project was supported by Federal Ministry for Education, Research and Technology, Germany, under grant 426-4013-13N6243-0.
References ¨ [1] J. Behnisch, A. Hollander, H. Zimmermann, Factors influencing the hydrophobic recovery of oxygen-plasma-treated polyethylene, Surf. Coat. Technol. 59 (1993) 356–358. [2] Y. Takahashi, K. Fukuta, T. Kaneko, Plasma treatment for painting of polypropylene bumper, Toyota Motor Corporation International Congress, Detroit, 1995, pp. 65–72. [3] P. Mapleston, Plasma technology progress improve options in surface treatment, Mod. Plastic Int. 20 (1990) 74–79. [4] H. Hayashi, T. Nakano, Y. Ohki, Effect of plasma surface modification on electrical conduction in polyethylene, Elect. Engng Jap. 111 (1991) 17–24. [5] T.G. van Kooten, J.M. Schakenraad, H.C. van der Mei, A. Dekker, C.J. Kirkpatrick, M. Walter, D. Korzec, J. Engemann, H.J. Busscher, Fluid shear induced endothelial cell detachment from modified polystyrene substrata, Colloids Surf. B: Biointerfaces 3 (1994) 147–158. [6 ] E. Poncin-Epaillard, G. Legeay, J.C. Brosse, Plasma modification of cellulose derivates as biomaterials, J. Appl. Polym. Sci. 44 (1992) 1513–1522. [7] S. Mujin, H. Baorong, W. Yisheng, T. Ying, H. Weiqiu, D. Youxian, The surface of carbon fibres continuously treated by cold plasma, Compos. Sci. Technol. 34 (1989) 353–364. [8] M.F. Bottin, H.P. Schreiber, J. Klemberg-Sapieha, M.R. Wertheimer, Modifiaction of paper surface properties by microwave plasmas, J. Appl. Polym. Sci. 38 (1984) 193–200. [9] Y. Sando, T. Goto, I. Tanaka, H. Ishidoshiro, M. Minakata, Anlage zum kontinuierlichen Behandeln von Textilprodukten mit Hilfe von Niedertemperatur-Plasma, German Patent DE 3300095 A1, 1983. [10] N.K. Man, G. Legeay, G. Jehenne, D. Tiberghien, D. de La Faye,
Improved biocompatibility by plasma modification of surface for cellulosic dialysis membrane, Artif. Organs 14 (1990) 44–50. [11] H.J. Griesser, Gas plasma techniques for the preparation of hydrophilic polymer surfaces, Mater. Forum 14 (1990) 192–202. [12] N.K. Man, G. Legeay, G. Jehenne, D. Tiberghien, D. de La Faye, Improved biocompatibility by plasma modification of surface for cellulosic dialysis membrane, Artif. Organs 14 (1990) 44–50. [13] R. Van der Linden, Die Corona-Behandlung von PE-Folien: Beurteilung der Vorgiinge und ihrer Auswirkungen, Kunststoffe 69 (1979) 71–75. [14] J.F. Friedrich, P. Rohrer, Th. Gross, A. Lippitz, W. Unger, Improvement in polymer adhesivity by low and normal pressure plasma surface modification, Surf. Coat. Technol. 59 (1993) 371–378. [15] R. Winter, D. Korzee, N. Sprang, D. Theirich, J. Engemann, PTFE surface modification with broad pressure range slot antenna microwave discharge, Surf. Coat. Technol. 74–75 (1995) 618–624. [16 ] N. Sprang, D. Theirich, J. Engemann, Plasma and ion beam surface treatment of polyethylene, Surf. Coat. Technol. 74–75 (1995) 689–695. [17] Y. Sando, T. Goto, I. Tanaka, H. Ishidoshiro, M. Minakata, Anlagezum kontinuierlichen Behandeln von Stoff mit einem Niedertemperatur-Plasma, German Patent DE 3300096 A1, 1983. [18] F. Werner, D. Korzec, J. Engemann, Slot antenna 2.45 GHz microwave plasma source SLAN, Plasma Sources Sci. Technol. 3 (1994) 473–481. [19] D. Korzec, D. Theirich, E. Werner, K. Traub, J. Engemann, Remote and direct microwave plasma deposition of HMDSO films: comparison study, Surf. Coat. Technol. 7475 (1995) 67–74. [20] D. Korzec, F. Werner, A. Brockhaus, J. Engemann, T.P. Schneider, R.J. Nemanich, Characterization of a slot antenna microwave plasma source for hydrogen plasma cleaning, J. Vac. Sci. Technol. A13 (1995) 2074–2085. [21] C.M. Horwitz, Variable-angle dry etching with a hollow cathode, Appl. Phys. Lett. 44 (1984) 1041–1043. [22] P.E. Little, A. von Engel, The hollow-cathode effect and the theory of glow discharges, Proc. R. Soc. 224 (1954) 209–227. [23] T. Igarashi, N. Fukuda, Forming films, e.g. of amorphous silicon, European patent application, 0 327 253, 1989. [24] C. Lejeune, J.P. Grandchamp, O. Kessi, J.P. Gilles, RF multipolar plasma for broad and reactive ion beams, Vacuum 36 (1986) 837–840. [25] J. Kieser, M. Sellschopp, M. Geisler, Vorrichtung zur Plasmabehandlung von Substraten in einer durch Hochfrequenz angeregten Plasmaentladung, German Patent Application, DE 36 06 959 Al, 1986. [26 ] J. Engemann, D. Korzec, Verfahren und Vorrichtung zur Plasma¨ behandlung bahnformiger Materialien, German Patent application DE 4233895 A1. [27] D. Korzec, M. Schott, J. Engemann, Radio frequency hollow cathode discharge for large area double sided foil processing, J. Vac. Sci. Technol. A13 (1995) 843–848. [28] R. Krimke, H.M. Urbassek, D. Korzec, J. Engemann, PIC/MC simulation of a strongly asymmetric low-pressure RF discharge, J. Phys. D. 27 (1994) 1653–1659. [29] S.C. Brown, Basic data of plasma physics, The M.I.T. Press, Cambridge, MA, 1966. [30] D. Korzee, J. Engemann, O. Borgmeier, M. Mildner, K.-P. Ningel, D. Theirich, Multijet hollow cathode discharge for remote polymer deposition, Surf. Coat. Technol. 93 (1997) 128–133. [31] C.M. Horwitz, RF sputtering-voltage division between two electrodes, J. Vac. Sci. Technol. A1 (1983) 60–68. [32] B. Bumble, J.J. Cuomo, J.S. Logan, S.M. Rossnagel, Hollow cathode enhanced plasma for high rate reactive ion etching and deposition, US Patent 4,637,853, 1987. ¨ [33] K. Kohler, J.W. Cobum, D.E. Home, E. Kay, J.H. Keller, Plasma
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
[34]
[35] [36 ]
[37]
[38]
[39] [40]
[41]
potentials of 13.56-MHz RF argon glow discharges in a planar system, J. Appl. Phys. 57 (1985) 59–66. A. Brockhaus, C. Borchardt, J. Engemann, Langmuir probe measurements in commercial plasma plants, Plasma Sources Sci. Technol. 3 (1994) 539–544. E.W. Peterson, L. Talbot, Collision electrostatic single-probe and double probe measurements, AIAA J. 8 (1970) 2215–2219. Z. Zakrzewski, T. Kopiczynski, Effect of collisions on positive ion collection by a cylindrical Langmuir probe, Plasma Phys. 16 (1974) 1195–1198. R. Krimke, H.M. Urbassek, Influence of plasma extraction on a cylindrical low-pressure RF discharge: a PIC–MC study, IEEE Trans. Plasma Sci. 23 (1995) 103–109. J.D. Andrade, L.M. Smith, D.E. Gregonis, The contact angle and interface energetics, Surf. Interfac. Aspects Biomed. Polym. 1 (1985) 249–291. D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. K. Harth, H. Hibst, Surface modification of polypropylene in oxygen and nitrogen plasmas, Surf. Coat. Technol. 59 (1993) 350–355. S. Watanabe, Plasma cleaning by use of hollow-cathode discharge in a CHF SiO dry-etching system, Jpn. J. Appl. Phys. 31 (1992) 3 2 1491–1498.
767
[42] K.E. Davies, C.M. Horwitz, Diode and hollow cathode etching in CF , J. Vac. Sci. Technol. A7 (1989) 2705–2708. 4 [43] C.M. Horwitz, Hollow cathode etching with CF gas mixtures, 4 J. Vac. Sci. Technol. B3 (1984) 419–420. [44] G. Fortuno-Wiltshire, G.S. Oehrlein, Hollow cathode etching of Si and SiO using CF and H , J. Electrochem. Soc. 136 (1989) 2 4 2 1447–1449. [45] M. Gross, Y. Wu, C.M. Horwitz, Blazed grating formation in a hollow cathode etcher, J. Vac. Sci. Technol. A7 (1989) 3213–3216. [46 ] C.M. Horwitz, S. Boronkay, M. Gross, K. Davies, Hollow cathode etching and deposition, J. Vac. Sci. Technol. A6 (1988) 1837–1844. [47] C.M. Horwitz, Hollow cathode etching and deposition, in: S.M. Rossnagel, J.J. Cuomo, W.D. Westwood ( Ed.), Handbook of Plasma Processing Technology: Fundamentals, Etching, Deposition and Surface Interface, Noyes Publications, Park Ridge, 1990, pp. 308–335. [48] M. Gross, S.O. Kong, C.M. Horwitz, C.Y. Kwok, Hollow cathode deposition of silicon dioxide films, Thin Solid Films, 193/194 (1990) 138–145.
70 cm radio frequency hollow cathode plasma source for modification of foils and membranes D. Korzec *, M. Mildner, F. Hillemann, J. Engemann Microstructure Research Center—fmt, University of Wuppertal, Obere Lichtenplatzer Strasse 336, 42 287 Wuppertal, Germany
Abstract For industrial use long linear plasma sources are needed. Basing on a radio frequency (13.56 MHz) hollow cathode discharge (HCD) concept proved for 30 cm long cathodes, an upscaled version of the plasma source with cathode length of 70 cm was developed. Additional hollow anode plasma chambers allow easier switching to an HCD discharge mode. Holes between anode and cathode plasmas cause a local increase in the ion concentration by a factor of 40 (up to 2×1011 cm−3). Homogeneity of the cathode plasma was examined by use of a double Langmuir probe technique. It improves with increasing number of holes distributed along the cathode trench. To characterize the process uniformity, polypropylene foil and cellulose membrane were used. Typical process conditions are RF power of 600 W, argon flow 60 sccm, pressure in the process chamber 0.1 Pa. The contact angle measured with water on the polypropylene surface decreases from 100° to 50° after 2 s of treatment in plasma with ion concentrations of more than 2×1010 cm−3. The distributions of the contact angle over the foil width show minima for places of maximum ion concentration and are almost identical on both foil sides. The intensity of the spectral line at 308.6 nm wavelength was measured as a function of time after discharge ignition for determination of the water outgasing from the cellulose. The amount of water in the discharge correlates with the ion concentration. The second emission intensity maximum after about 7 s of treatment occurs at the position with highest ion concentration. The warming up of the membrane is the most probable reason for this effect. © 1997 Elsevier Science S.A. Keywords: Plasma sources; Radio frequency; Hollow cathode discharge; Surface modification
1. Introduction Gas discharge treatments of plastic surfaces for improvement of wettability [1], paintability [2], adhesion [3], electrical conductivity [4] and biocompatibility [5,6 ] are broadly accepted techniques. A special interest of industry is focused on the continuous processing of ‘‘endless’’ materials. Discharge-based modifications of fibers [7], paper [8], textile products [9], membranes [10], foils [11], and many others, are known. The particular motivation of this work was the cellulose membrane used, for example, for dialysis [12]. Drawbacks of widely used atmospheric pressure discharges, such as corona discharge [13], spark or arc jets [14] and atmospheric pressure microwave discharge [15], are weak modification [15], generation of ozone [9], inhomogeneous treatment, and strong aging of modification effect ( loss of 50% after one day) [13]. They * Corresponding author. Tel: (49)-202-595096; Fax: (49)-202-595098; e-mail: [email protected] 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 32 4 - 1
motivate for application of low pressure plasma processing. The treatment of most plastic surfaces in a low temperature, low pressure plasma allows a very strong, durable and homogeneous modification [16 ]. Continuous processing is also possible when special airto-vacuum ports are applied [17]. Otherwise batch processing of continuous materials is an acceptable alternative for many applications. With reactive gases the microwave and the radio frequency discharges can be used. Even though the microwave discharges allow high ion concentrations [18] and efficient free radical production [19], they cause a high thermal load of materials processed in the direct discharge [20]. Consequently the application of radio frequency discharge is a good choice for thermally sensitive substrates. An especially efficient method of radio frequency plasma production is a hollow cathode discharge (HCD) [21]. Similar to that in the DC cold cathode HCD [22], in RF HCD the electrons are trapped in hollow cavities formed in the RF active electrode, allowing high plasma
760
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
densities. Very different shapes of hollow cathodes can be used for treatment of flat substrates [23]. For example the hills and valleys may be arranged in a concentric circular form, concentric square form, cylindrical hollows [24] etc. The discharge design avoiding the presence of the plasma outside of the plasma source is known [25]. A novel concept of RF HCD allowing a symmetrical, double-sided modification of foil-like materials has been developed [26 ]. The direct geometrical scaling of the concept, which was successful for 30 cm HCD plasma source [27], did not work for 70 cm. Difficulties with switching into the HCD mode and a strong plasma inhomogeneity were observed. Two measures were undertaken to improve the source performance: (i) an additional anode plasma chamber was incorporated; and (ii) transfer holes between anode and cathode plasma chambers were introduced. The characterization of the source with this constructional improvements is a main aim of this work.
2. Hollow cathode discharge In general the potential distributions and charge carrier currents in RF discharge are complicated functions of time and position. For calculating them, particle-incell (PIC ) simulation is the proper tool [28]. The very simplified formulae covered in the next two paragraphs are suitable for rough estimation only. 2.1. Electrons in hollow cathode The concentration of electrons directly at the cathode surface n , depends on the local voltage between cathode s and plasma bulk V and is described by the Boltzmann p relation:
A
B
eV n =n exp − p , (1) s e kT e where n is the concentration and T temperature of e e electrons in the plasma bulk. The physical meaning of Eq. (1) is that only electrons with sufficient energy arrive at the electrode surface. The condition for ignition of the hollow cathode discharge is the creation of a potential trap for electrons between two cathode surfaces positioned face-to-face. The high energy electrons behave as in an electrostatic trap, where their free path, given for monoenergetic electrons, as: 1 kT L = = (2) e n s ps n e e is comparable with the distance between trapping cathode surfaces. The s is the total cross section for electron e
collisions, which is energy and gas dependent. n is the n concentration and T the temperature of neutral particles in the plasma chamber. As an example, for argon at pressure of 0.1 mbar the free path for elastic collisions for 100 eV electrons is about 8 mm. According to Eq. (2), for decreasing pressure the optimal distance between electrodes for HCD increases. 2.2. Plasma inhomogeneity in long hollow cathode plasma columns For most of the RF cycle time the cathode is sufficiently negative polarized to keep the thermal electrons apart from its surface. To the cathode flows the ion current, J , given as: i,satt
A B S 1
kT e, (3) 2 m i where n is the mean ion concentration in the plasma 0 bulk, and m the ion mass. To fulfil the requirement of i quasineutrality, the sum of currents flowing out of the cathode plasma should be zero. Consequently in the cathodic part of the RF cycle, from the cathode to the anode flows an electron current, I , given as: e,− I =A J , (4) e,− c +,satt where A is the effective area of the cathode. The total c charge flowing to the cathode must be zero, because the cathode is capacitively coupled, and no net DC current can flow to it. In this case the electron current, I , e,+ flowing to the cathode in the short period of time, t, when the cathode is sufficiently weakly negative polarized to allow the thermal electrons from the bulk plasma to overcome the potential barrier, can be given as: J
I
i,satt
e,+
=exp −
=
A B 1
tf
−1 I
qn
e,−
0
,
(5)
where f is the RF excitation frequency. Assuming the hollow cathode as a long tube with an area of the cross section A , electron current flowing along the plasma t column is given as: =qn v A , (6) e,+ e e t where v is the drift velocity of electrons described as a e function of the E/p (mean electric field divided by pressure). The curves representing v =f(E/p) are availe able for different gases [29]. I
2.3. Influence of transfer hole on plasma density The density of electron current flowing through a transfer hole between cathode and anode plasmas (as shown in Fig. 1) during the anodic polarization of
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
761
Fig. 1. Operation principle of the hollow cathode discharge with hole connecting anode and cathode plasma.
cathode can be estimated by use of following expression:
A B
I A 1 J = e,+ = c (7) −1 J e,h i,satt A A tf h h A is the area of the hole cross section. Current densities h in transfer holes calculated from Eq. (7) are several orders of magnitude higher than ion saturation currents in the cathode plasma. This explains very high ion concentrations of 1013 cm−3 measured in such holes [30].
3. Experimental details 3.1. Plasma source The following requirements were the base for the plasma source development: (i) up-scalable source construction; (ii) sufficient plasma density for modification speeds of several tens of meters per minute; (iii) the modification being homogeneous, and equal on both sides of the substrate material; (iv) closed source construction, to avoid the influence of the process chamber geometry on the source performance [31], ‘‘burn out’’ of lubricants used in foil transfer mechanisms and undefined plasma treatment time, which can occur in open HCDs [32]. Similar to our earlier work [26 ] two identical hollow cathodes, positioned face-to-face and enclosed in a grounded casing were used. Both of them were supplied from a common RF power source. The foil is guided between both hollow cathodes (see Fig. 2a). The hollow spaces in both cathodes are formed as straight 35×35 mm2 trenches parallel to each other at predetermined intervals, open on both ends, where the grounded wall of the plasma chamber plays the role of the RF
anode (see Fig. 2(a)). When up-scaling the cathode length from 30 to 70 cm, the ratio of anode-to-cathode area becomes so small that the bias of the cathode [31,33] is not sufficient for ignition the HCD. The required increase of the anode surface was realized by the introduction of two additional anode plasma chambers placed on the backs of both cathodes and separated from them by grounded dark space shields (see Fig. 2(b)). All parts were machined from aluminum. The first cathode was powered by a 1 kW RF generator (Dressler, type mpg 1310) via a standard gamma-type matchbox and through a coaxial connector. The RF power is supplied to the second cathode from the first one through the contact pads on ends of all hollow cathode trenches (see Fig. 2(c)). The second cathode is movable together with half of the grounded casing, to allow the opening the source for easy foil handling. For improvement of the source performance holes with diameter of 20 mm were introduced in the first plasma chamber, symmetrically on both sides of the treated material. Experiments with one hole couple in the center (see Fig. 2(b)), and with three hole couples distributed along the cathode plasma chamber (see Fig. 2(c)) were performed. 3.2. Process set up The process chamber was a cylinder of stainless steel with a diameter of 1 m, a length of 1.2 m and two ISO 1000 high vacuum flanges. The pumping system consists of turbomolecular pump (Balzers TPH 2200), a rotary pump ( TRIVAC D65 BCS, Leybold) with a pumping speed of 65 m3 h−1 and a Roots blower (RUVAC WSU 501, Leybold) with a pumping speed of 500 m3 h−1. The achievable base pressure was 2×10−4 Pa. Pressure in
762
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
Fig. 2. Cross section of radio frequency hollow cathode plasma source for double-sided membrane modification: (a) front view; (b) side view; (c) top view of the cathode.
the process chamber was controlled between 1 Pa and 600 Pa with a swing valve and a vacuum gage (MKS Baratron). The gas flow was controlled by an MKS flow controller.
4. Results 4.1. Ion concentration distributions The properties of the plasma in the cathode chambers were characterized by use of a double Langmuir probe (DLP). Two probe tips are identical, 7 mm spaced, tungsten wires (0.5 mm diameter and 5 mm exposed length) in two-channel alumina tubes. The measurements were made by use of a DLP plasma diagnostics system developed by Brockhaus et al. [34]. It consists of a power supply, which both generates the probe voltage and measures the probe current. The calculation of ion concentration and electron temperature is based on an iterative method described by Peterson and Talbot [35]. The dense plasma limit for determination of the Laframbois factor can be assumed [36 ]. At low pressures and powers the plasma source works in the ‘‘normal’’ mode, which means not in hollow cathode operation mode. Characteristic for such working conditions are low ion concentrations. They are below 3×109 cm−3 for oxygen (Fig. 3(a)) and below 5×109 cm−3 for helium ( Fig. 3(b)). The ion concen-
tration distribution along the cathode is more homogeneous for helium (maximum-to-minimum ratio of two) than for oxygen (maximum-to-minimum ratio of four). The reason for the better result in helium is a much higher free path for electron impact ionization than in oxygen for comparable pressure. Switching from normal to hollow cathode operation mode can be initiated by increase of RF power (see Fig. 3(a)) or increase of gas flow (see Fig. 3(b)). Both electron temperature and ion concentration are typically much higher for HCD operation mode than for ‘‘normal’’ mode. For oxygen the ion concentration improves by a factor of 5 close to the anode and a factor of 2 in the central cathode location. The ion concentration improvement factors for helium are 200 and 50, respectively. Close to the anode a maximum ion concentration of more than 1012 cm−3 was measured. Both discharge operation modes show a high symmetry of the plasma parameter distributions, despite an asymmetric power coupling (see Fig. 2(a)). The simple up-scaling of the 40 cm prototype by making the cathode 70 cm long failed. The problem with such an up-scaled version of the HCD plasma source was a very poor homogeneity of the ion concentration along the cathode chamber. The ion concentration in the middle of the source was for the conditions in Fig. 4 a factor of 3.5 lower than in the vicinity of the anode. Voltage drop over the plasma column along the
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
(a)
(b) Fig. 3. Ion concentration distributions along a single cathode chamber measured in normal and hollow cathode operation mode for (a) oxygen flow of 20 sccm at pressure of 0.06 Pa, and for (b) helium at RF power of 600 W and pressure of 2 Pa.
763
From the drift velocity curves for oxygen the voltage drop along the cathode column of 3.5 V can be read [29]. Such a voltage drop causes, according to Eq. (1), the electron concentration in the cathode center to be a factor of 5 lower than at the anode. Fewer electrons arriving the cathode surface means fewer secondary electrons emitted from the cathode surface, and consequently less efficient HCD excitation in the center of the source in comparison with the location in the neighborhood of the anode. As a matter of fact much lower voltage drop over the hollow cathode can be expected, because the largest part of electron current will flow to the cathode part most adjacent to the anode. These corresponds to even more inhomogeneous plasma density distribution. To improve the ion concentration homogeneity, the reduction of the distance between the anode plasma and a most distant cathode plasma site is needed. It can be achieved by use of a hole connecting the anode and the cathode plasma chambers (see Fig. 2(b)). In Fig. 4 the ion concentration distributions in oxygen plasma along the first cathode chamber with and without such a hole are compared. Because of the hole, the ion concentration in the cathode chamber center is elevated by a factor of six. At anode ends of the cathode chamber the ion concentration remains almost without change. The influence of the hole on the ion concentration can be observed over a much longer distance than the diameter of the hole. 50% of the maximum value is exceeded at a distance of 8 cm from the hole center. The next logical step in the direction of optimal plasma homogeneity is the introduction of several transfer holes along each cathode chamber. In Fig. 5 the results for three holes are displayed. Each of these holes causes similar enhancement of the discharge in its vicinity. With increasing number of holes the effective cathode area per hole gets smaller. Consequently (see Eq. (7)), lower electron currents flow through a single hole and lower plasma density can be expected. Such
Fig. 4. Ion concentration distributions along a single cathode chamber with and without hole for oxygen flow of 20 sccm at 0.06 Pa and RF power of 400 W.
cathode plasma chamber explains this effect. Using a typical ion concentration of 5×1010 cm−3, an electron temperature of 5 eV, pressure of 0.1 mbar, length of the cathode plasma chamber of 35 cm, and assuming the time of the electron current flow to the cathode of onethird of the RF cycle [37], the drift velocity for electrons of 1 cm ms−1 can be calculated from Eqs. (5) and (6).
Fig. 5. Ion concentration distributions along a single cathode chamber with three holes for argon flow of 60 sccm at 0.15 Pa.
764
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
decrease of the maximum ion concentration with increasing number of holes can be observed, when comparing the one-hole-curve from Fig. 4 with 400 W curve from Fig. 5. The maximum ion concentration does not scale linearly with cathode area per hole. In our example the area per hole changes from 368 cm2 for one hole to 184 cm2 for three holes. At the same time the maximum ion concentration reduces only from 3.5×1010 cm−3 to 2.5×1010 cm−3. The effective cathode area is determined not only by geometrical dimensions, but also by the current density distributions over the cathode surface. When the distances between adjacent holes are larger than the zones of high density plasma, low sensitivity of ion concentration on changes of cathode area per hole can be expected.
Fig. 6. Advancing angle of water vs time measured on PP foil treated in HCD plasma source operated with argon flow of 60 sccm at pressure of 0.2 Pa and RF power of 600 W.
4.2. Contact angle distribution Contact angle measurement is an easy and important method for determination of the surface modification quality. Because the receding contact angle is to a large extent the measure of the mechanical surface condition, the advancing angle was chosen for modification process ¨ characterization [38]. For measurements a Kruss Goniometer Type G1 with computerized data collecting system G40 was used. The volume of distilled water drop is increased until moving the three-phase boundary over the surface. Each point shown in plots represents the mean value of 60 contact angle values. They are collected automatically 10 times for the left and right sides of three droplets at each place. The contact angles were determined accordingly to the method of Owens and Wendt [39]. The measurements have to be performed directly after processing, otherwise an aging effect of the modified surface (30% increase of the contact angle after 7 days by treatment with oxygen) [2] falsifies the results. Treatment homogeneity on PP foil with width of 60 cm and thickness of 300 mm was characterized. For the best homogeneity characterization a treatment time should be chosen allowing the highest sensitivity of the contact angle on the variations of the plasma density. The time dependence of the advancing contact angle of distilled water was measured for three positions in the source to choose such critical treatment time (see Fig. 6). The contact angle measured on an untreated surface is 100°. This value is in agreement with cited data ranging for PP from 96° [40] to 103° [2]. For each curve in Fig. 6 the corresponding ion concentration is specified. Contact angle measured after 2 s treatment reduces more for higher ion concentration. Contact angle reduction is as strong as 50% for ion concentration higher than 1010 cm−3. The following 2 s of treatment cause a deterioration of the modification effect, documented as an increase of the contact angle
from 55° up to 80° and more. Then after longer treatment time (more than 15 s) for all curves, a low value of contact angle (45° for ion concentration of 5×1010 cm−3) stabilizes. The largest contact angle spread, allowing the best characterization of process homogeneity, occurs for treatment time of 2 s or 8 s. Because short processing times are of technological interest, 2 s were chosen as a treatment time for the following experiments. Strong correlation of the lateral contact angle distribution with construction marks and hence with ion concentration distribution can be observed. In Fig. 7 the influence of the holes between anode and cathode plasma chambers on the advancing contact angle distribution is displayed. At the positions of the holes the contact angle after 2 s of plasma treatment was 50°. Much weaker modification of the foil, correlating with higher contact angle of 80°, is observed at places which are most distant from the holes in the case of the source with holes, and
Fig. 7. Advancing contact angle measured across the foil modified in the plasma source without and with three holes. 300 mm thick PP foil was treated 2 s in argon plasma at flow of 60 sccm, pressure of 0.2 Pa and RF power of 600 W.
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
most distant from the cathode edge, for the source construction without holes. One of the important requirements for the development of large area plasma sources was the treatment symmetry for both sides of the foil. The contact angle distributions were measured as a function of lateral position by use of distilled water and ethylene glycol. The comparison of results collected on both sides of the membrane is shown in Fig. 8. For both test fluids the differences of the contact angles on both sides A and B of the foil are less than 5% for lateral positions form 17 to 52 cm. Accordingly to Fig. 7 the region of optimal modification (contact angle close to 50°) and also the region of maximum ion concentration, extents over a distance of 6 cm from the transfer hole. To achieve the optimal treatment (variation of the contact angle of less than 5%) of moving substrates, about 10 holes per cathode chamber are needed. To avoid too strong reduction of maximum ion concentration, five holes per cathode chamber could be enough, if the hole patterns in each cathode chamber are slightly shifted in respect to other chambers. In this case treatment homogeneity much better than the ion concentration homogeneity in a single cathode plasma chamber can be achieved. 4.3. Release of water from the treated membrane The cellulose membrane was the second material used for plasma source performance characterization. Because a strong influence of the winding speed and the RF power on the plasma properties was observed, a release of water from the membrane surface was suspected. Optical emission was used for determination of the water present in the plasma. An intensity of the spectral line at 308.6 nm wavelength was measured as a
Fig. 8. Advancing contact angle measured across both sides of the foil modified in the plasma source with three holes. 300 mm thick PP foil was treated for 2 s in argon plasma at flow of 60 sccm, pressure of 0.2 Pa and RF power of 600 W. Distilled water and ethylene glycol were used for determination of the contact angle.
765
function of time after discharge ignition. During the first treatment phase a strong water burst was observed. The emission intensities reach their maxima in all examined positions. The values of these maxima correlate with the local ion concentrations (see Fig. 9). In the second treatment phase the amount of water in plasma decreases exponentially. Only at the position with highest ion concentration does the emission intensity start to increase again after 2 s of treatment, and it achieves the second maximum after about 7 s of treatment. The warming up of the membrane is the most probable reason for this effect. Because the losses of water from the internal structure of the membrane causes its deformation, processing times of more than 2 s should be avoided. For the studied plasma source geometry this means the minimum required speed of the membrane of 0.125 m s−1.
5. Conclusions (1) A novel 70 cm radio frequency hollow cathode plasma source allows double-sided modification of membranes and foils. (2) A maximum ion concentration of 1012 cm−3 for helium was measured. (3) An easy switching in the hollow cathode mode was possible due to anode plasma chambers. The power and gas flow should exceed some threshold values, i.e. 20 sccm and 200 W for oxygen for ignition the HCD. (4) The required homogeneity of the ion concentration can be achieved by use of transfer holes connecting the anode and cathode plasma chambers. The influence of the 20 mm hole is present over a range of about 20 cm. (5) Advancing contact angle measured with water reduces from 100° before treatment to about 50° after 2 s treatment by ion concentration of
Fig. 9. Time resolved optical emission intensity for wavelength of 308.6 nm (OH ) measured in a chamber loaded with the membrane, for argon flow of 60 sccm at pressure of 0.2 Pa and RF power of 400 W.
766
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
2×1010 cm−3. The positions of minima in lateral distributions of contact angle overlap with positions of maxima in ion concentration distributions. (6) With properly designed pattern of holes and sufficient number of cathode chambers, homogeneous treatment (contact angle variation after critical treatment time less than 5%) is possible. (7) The cellulose membranes release water vapour into the plasma during plasma treatment. The amount of released water is proportional to the local ion concentration. The treatment should be not longer than 2 s to avoid drying out the membrane causing its deformation. Even though the application examples shown in this work concern surface modification, the RF HCD plasma source can be used also for surface cleaning [41], etching [42–45] or film deposition [46–48] as well.
Acknowledgement ¨ The authors wish to thank Peter Stolzle and Hans¨ Jorg Schmidt for construction of the source. The project was supported by Federal Ministry for Education, Research and Technology, Germany, under grant 426-4013-13N6243-0.
References ¨ [1] J. Behnisch, A. Hollander, H. Zimmermann, Factors influencing the hydrophobic recovery of oxygen-plasma-treated polyethylene, Surf. Coat. Technol. 59 (1993) 356–358. [2] Y. Takahashi, K. Fukuta, T. Kaneko, Plasma treatment for painting of polypropylene bumper, Toyota Motor Corporation International Congress, Detroit, 1995, pp. 65–72. [3] P. Mapleston, Plasma technology progress improve options in surface treatment, Mod. Plastic Int. 20 (1990) 74–79. [4] H. Hayashi, T. Nakano, Y. Ohki, Effect of plasma surface modification on electrical conduction in polyethylene, Elect. Engng Jap. 111 (1991) 17–24. [5] T.G. van Kooten, J.M. Schakenraad, H.C. van der Mei, A. Dekker, C.J. Kirkpatrick, M. Walter, D. Korzec, J. Engemann, H.J. Busscher, Fluid shear induced endothelial cell detachment from modified polystyrene substrata, Colloids Surf. B: Biointerfaces 3 (1994) 147–158. [6 ] E. Poncin-Epaillard, G. Legeay, J.C. Brosse, Plasma modification of cellulose derivates as biomaterials, J. Appl. Polym. Sci. 44 (1992) 1513–1522. [7] S. Mujin, H. Baorong, W. Yisheng, T. Ying, H. Weiqiu, D. Youxian, The surface of carbon fibres continuously treated by cold plasma, Compos. Sci. Technol. 34 (1989) 353–364. [8] M.F. Bottin, H.P. Schreiber, J. Klemberg-Sapieha, M.R. Wertheimer, Modifiaction of paper surface properties by microwave plasmas, J. Appl. Polym. Sci. 38 (1984) 193–200. [9] Y. Sando, T. Goto, I. Tanaka, H. Ishidoshiro, M. Minakata, Anlage zum kontinuierlichen Behandeln von Textilprodukten mit Hilfe von Niedertemperatur-Plasma, German Patent DE 3300095 A1, 1983. [10] N.K. Man, G. Legeay, G. Jehenne, D. Tiberghien, D. de La Faye,
Improved biocompatibility by plasma modification of surface for cellulosic dialysis membrane, Artif. Organs 14 (1990) 44–50. [11] H.J. Griesser, Gas plasma techniques for the preparation of hydrophilic polymer surfaces, Mater. Forum 14 (1990) 192–202. [12] N.K. Man, G. Legeay, G. Jehenne, D. Tiberghien, D. de La Faye, Improved biocompatibility by plasma modification of surface for cellulosic dialysis membrane, Artif. Organs 14 (1990) 44–50. [13] R. Van der Linden, Die Corona-Behandlung von PE-Folien: Beurteilung der Vorgiinge und ihrer Auswirkungen, Kunststoffe 69 (1979) 71–75. [14] J.F. Friedrich, P. Rohrer, Th. Gross, A. Lippitz, W. Unger, Improvement in polymer adhesivity by low and normal pressure plasma surface modification, Surf. Coat. Technol. 59 (1993) 371–378. [15] R. Winter, D. Korzee, N. Sprang, D. Theirich, J. Engemann, PTFE surface modification with broad pressure range slot antenna microwave discharge, Surf. Coat. Technol. 74–75 (1995) 618–624. [16 ] N. Sprang, D. Theirich, J. Engemann, Plasma and ion beam surface treatment of polyethylene, Surf. Coat. Technol. 74–75 (1995) 689–695. [17] Y. Sando, T. Goto, I. Tanaka, H. Ishidoshiro, M. Minakata, Anlagezum kontinuierlichen Behandeln von Stoff mit einem Niedertemperatur-Plasma, German Patent DE 3300096 A1, 1983. [18] F. Werner, D. Korzec, J. Engemann, Slot antenna 2.45 GHz microwave plasma source SLAN, Plasma Sources Sci. Technol. 3 (1994) 473–481. [19] D. Korzec, D. Theirich, E. Werner, K. Traub, J. Engemann, Remote and direct microwave plasma deposition of HMDSO films: comparison study, Surf. Coat. Technol. 7475 (1995) 67–74. [20] D. Korzec, F. Werner, A. Brockhaus, J. Engemann, T.P. Schneider, R.J. Nemanich, Characterization of a slot antenna microwave plasma source for hydrogen plasma cleaning, J. Vac. Sci. Technol. A13 (1995) 2074–2085. [21] C.M. Horwitz, Variable-angle dry etching with a hollow cathode, Appl. Phys. Lett. 44 (1984) 1041–1043. [22] P.E. Little, A. von Engel, The hollow-cathode effect and the theory of glow discharges, Proc. R. Soc. 224 (1954) 209–227. [23] T. Igarashi, N. Fukuda, Forming films, e.g. of amorphous silicon, European patent application, 0 327 253, 1989. [24] C. Lejeune, J.P. Grandchamp, O. Kessi, J.P. Gilles, RF multipolar plasma for broad and reactive ion beams, Vacuum 36 (1986) 837–840. [25] J. Kieser, M. Sellschopp, M. Geisler, Vorrichtung zur Plasmabehandlung von Substraten in einer durch Hochfrequenz angeregten Plasmaentladung, German Patent Application, DE 36 06 959 Al, 1986. [26 ] J. Engemann, D. Korzec, Verfahren und Vorrichtung zur Plasma¨ behandlung bahnformiger Materialien, German Patent application DE 4233895 A1. [27] D. Korzec, M. Schott, J. Engemann, Radio frequency hollow cathode discharge for large area double sided foil processing, J. Vac. Sci. Technol. A13 (1995) 843–848. [28] R. Krimke, H.M. Urbassek, D. Korzec, J. Engemann, PIC/MC simulation of a strongly asymmetric low-pressure RF discharge, J. Phys. D. 27 (1994) 1653–1659. [29] S.C. Brown, Basic data of plasma physics, The M.I.T. Press, Cambridge, MA, 1966. [30] D. Korzee, J. Engemann, O. Borgmeier, M. Mildner, K.-P. Ningel, D. Theirich, Multijet hollow cathode discharge for remote polymer deposition, Surf. Coat. Technol. 93 (1997) 128–133. [31] C.M. Horwitz, RF sputtering-voltage division between two electrodes, J. Vac. Sci. Technol. A1 (1983) 60–68. [32] B. Bumble, J.J. Cuomo, J.S. Logan, S.M. Rossnagel, Hollow cathode enhanced plasma for high rate reactive ion etching and deposition, US Patent 4,637,853, 1987. ¨ [33] K. Kohler, J.W. Cobum, D.E. Home, E. Kay, J.H. Keller, Plasma
D. Korzec et al. / Surface and Coatings Technology 97 (1997) 759–767
[34]
[35] [36 ]
[37]
[38]
[39] [40]
[41]
potentials of 13.56-MHz RF argon glow discharges in a planar system, J. Appl. Phys. 57 (1985) 59–66. A. Brockhaus, C. Borchardt, J. Engemann, Langmuir probe measurements in commercial plasma plants, Plasma Sources Sci. Technol. 3 (1994) 539–544. E.W. Peterson, L. Talbot, Collision electrostatic single-probe and double probe measurements, AIAA J. 8 (1970) 2215–2219. Z. Zakrzewski, T. Kopiczynski, Effect of collisions on positive ion collection by a cylindrical Langmuir probe, Plasma Phys. 16 (1974) 1195–1198. R. Krimke, H.M. Urbassek, Influence of plasma extraction on a cylindrical low-pressure RF discharge: a PIC–MC study, IEEE Trans. Plasma Sci. 23 (1995) 103–109. J.D. Andrade, L.M. Smith, D.E. Gregonis, The contact angle and interface energetics, Surf. Interfac. Aspects Biomed. Polym. 1 (1985) 249–291. D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. K. Harth, H. Hibst, Surface modification of polypropylene in oxygen and nitrogen plasmas, Surf. Coat. Technol. 59 (1993) 350–355. S. Watanabe, Plasma cleaning by use of hollow-cathode discharge in a CHF SiO dry-etching system, Jpn. J. Appl. Phys. 31 (1992) 3 2 1491–1498.
767
[42] K.E. Davies, C.M. Horwitz, Diode and hollow cathode etching in CF , J. Vac. Sci. Technol. A7 (1989) 2705–2708. 4 [43] C.M. Horwitz, Hollow cathode etching with CF gas mixtures, 4 J. Vac. Sci. Technol. B3 (1984) 419–420. [44] G. Fortuno-Wiltshire, G.S. Oehrlein, Hollow cathode etching of Si and SiO using CF and H , J. Electrochem. Soc. 136 (1989) 2 4 2 1447–1449. [45] M. Gross, Y. Wu, C.M. Horwitz, Blazed grating formation in a hollow cathode etcher, J. Vac. Sci. Technol. A7 (1989) 3213–3216. [46 ] C.M. Horwitz, S. Boronkay, M. Gross, K. Davies, Hollow cathode etching and deposition, J. Vac. Sci. Technol. A6 (1988) 1837–1844. [47] C.M. Horwitz, Hollow cathode etching and deposition, in: S.M. Rossnagel, J.J. Cuomo, W.D. Westwood ( Ed.), Handbook of Plasma Processing Technology: Fundamentals, Etching, Deposition and Surface Interface, Noyes Publications, Park Ridge, 1990, pp. 308–335. [48] M. Gross, S.O. Kong, C.M. Horwitz, C.Y. Kwok, Hollow cathode deposition of silicon dioxide films, Thin Solid Films, 193/194 (1990) 138–145.