Investigating Radio Frequency Plasmas Used for the Modification of

May 7, 1999 - ADVERTISEMENT .... The 13.56 MHz radio frequency (rf) excitation is provided by an external coil, and we demonstrate that a plasma of Ar...
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J. Phys. Chem. B 1999, 103, 4423-4430

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Investigating Radio Frequency Plasmas Used for the Modification of Polymer Surfaces D. Barton,† J. W. Bradley,*,† D. A. Steele,‡ and R. D. Short‡ Department of Physics, UMIST, SackVille St., Manchester, M60, 1QD, U.K., and Department of Engineering Materials, Sheffield UniVersity, Sir Robert Hadfield Building, Mappin St., Sheffield, S1, 3JD, U.K. ReceiVed: NoVember 19, 1998; In Final Form: March 29, 1999

By use of Langmuir probes and energy-resolved mass spectrometry, the properties of a cold plasma suitable for the surface treatment of polymers are investigated. The 13.56 MHz radio frequency (rf) excitation is provided by an external coil, and we demonstrate that a plasma of Ar gas is capacitively coupled to the source coil. The spatial distributions of plasma and floating (self-bias) potentials, electron temperature Te, and plasma density ne have been investigated for a range of input powers (1-50 W) and gas pressures (10-310-1 Torr) using compensated Langmuir probes. Estimates of the rf potential amplitudes are also given. The energy distribution of plasma ions at plasma boundaries has also been measured, and the effect of the perturbation to the plasma parameters due to the presence of the polymer sample and the spectrometer has been quantified. A feature of these plasmas is the presence of large rf potentials (up to about 25Te) and high self-bias potentials (up to 80 V). We show that the presence of a mass spectrometer changes the plasma potential and alters the level of rf fluctuation in the plasma and thereby affects the ion energy distribution function at the sample surface. Estimates of ion and photon energy fluxes are made, and the relative importance of these two fluxes in terms of polymer modification at a pressure of 10-2 Torr is discussed.

1. Introduction The modification of polymer surfaces by cold (low-temperature, low-pressure) plasma is of established industrial importance.1 The key feature of plasma treatment is that the bulk properties of the polymer remain unaltered, while surface properties such as wettability, adhesion,2-4 biocompatability,5 and topography6,7 may be tailored to the application. Plasma processing has several advantages over more traditional treatments, since the processing is rapid, clean, and, depending on the choice of gas, environmentally safe. A further advantage is that the process results are (relatively) uniform, even over complex shapes.1 Plasma treatments may be used to introduce a specific element or functional group onto a polymeric surface.8-10 Taking an inert gas treatment as an example, it is generally thought that chain scissions occur at the polymer surface, producing free radical sites.8 These then react with air (either on exposure to atmosphere or from residual air or water vapor in the chamber) and form functional groups, usually oxygen incorporation in our experiments, on the polymer surface.8,9 However, overtreatment may lead to saturation in the amount of oxygen and eventually to the formation of a mechanically weak surface layer, which is easily removed by solvent washing.8 The outcome of plasma treatment is highly dependent on process parameters such as gas pressure, gas flow rate, rf frequency, power input level, and gas composition.1 Furthermore, the interaction between plasma species and the polymer surface is poorly understood, with much work focusing on comparisons among UV, VUV treatments,11-13 and ion beams14 in an attempt to attribute specific surface changes to individual plasma species. It is these factors that have motivated this work † ‡

UMIST. Sheffield University.

aimed at defining processing plasmas in terms of plasma parameters (particle fluxes to the surface)15 rather than characterizing global features such as power input level and gas pressure. In this paper, we have investigated the intrinsic plasma parameters in a simple plasma reactor consisting of a glass vessel and external excitation coil. This reactor is based on the design of Clark,16 which is still widespread in current use (e.g., see refs 7-9). These types of reactors are often referred to as being inductively coupled, but we will show that the coupling is capacitive. We relate global parameters of gas pressure (10-310-1 Torr) and nominal input power (0.5-70 W) to intrinsic quantities such as particle densities and fluxes to surfaces (the chamber walls and the polymeric substrates) immersed in the plasma. The aim is to relate complex plasma boundary behavior to process results, to both define operating parameters that allow similar process results to be obtained from dissimilar reactors and better define plasma particle and energy fluxes, which will permit direct comparisons with ion, electron, and VUV sources to be made. Once this is achieved, it will become feasible to manipulate plasma parameters so that specific process results can be obtained and investigated. A similar study of a parallel plate chamber has recently been made by Meichsner15 using O2. However, our vessel differs from this geometry in that the samples are not placed on the (driver) electrode but are situated on the (electrically isolated) chamber wall. This distinction is significant because ion energies at a driver electrode are typically an order of magnitude greater than those seen at a floating boundary.17 Also, because the ions are incapable of responding to the rf potentials, these (time average) self-bias potentials control the ion energy distribution at the sample surface.18 In all discharges, the high electron mobility builds up a negative charge on any insulating surface immersed in the plasma (the floating potential Vf). In rf plasmas, the situation is

10.1021/jp9844821 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/07/1999

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Figure 1. Schematic of the experimental equipment. The chamber is mounted on an aluminum bench (not shown), which acts as the earth reference plane.

more complex because the electron energy is modulated by the applied rf electric fields. This makes the surface potential (now called the self-bias potential Vsb) more negative with respect to the plasma potential.19 This potential difference between the plasma and self-bias potentials (Vp - Vsb) determines the maximum ion energy to the substrate. As well as the maximum ion energy at the polymeric surface, the distribution of energy is also investigated. The ion energy distribution function at the substrate is determined by the potential structure and the probability of collisions (elastic and charge exchange) in the presheath and sheath regions. At the pressures investigated here, the ion-neutral mean free path is in the range 0.03-3 cm at Ar pressures of 10-3-10-1 Torr. In the first section of the study, we use Langmuir probes to spatially map the plasma parameters over different power input levels and gas pressures. The Langmuir probe data relate external parameters to plasma conditions such as density, space potentials, electron temperature, and energy fluxes to the substrates. We then repeat some of these measurements with the mass spectrometer in place and observe any perturbation to the plasma. The measured distribution of ion energies, which mirrors those to a polymer substrate, is then related to plasma conditions. 2. Experimental Setup The experimental equipment (Figure 1), which is the same as a sister rig at Sheffield University, is designed to replicate a series of experimental chambers used for polymer processing.6,8,9 The basic chamber is a glass cruciform, with each arm being 25 cm long and with an inner diameter of 10 cm. The plasma volume is therefore about 7 L. The plasma chamber is closed by four stainless steel flanges, which are earthed through a low-inductance braid to an aluminum bench, which acts as the rf reference earth. Vacuum seals are provided by O-rings that are set into the flanges. The rf power is supplied by a 13.56 MHz Coaxial REG 150 unit with a separate matching unit. Power input is measured using the built-in meter, which has been calibrated against an RS SP220 power meter.

The powered electrode is a four-turn sheathed wire of 1.5 mm diameter that is wrapped around one axis of the vessel. The wire is terminated at the fourth turn so that no conduction path to earth exists. The absence of conduction currents in the coil means that the coupling to the plasma is capacitive. However, this mode of coupling remains, even when the remote end of the wire is connected to earth because, as we will show, the electromagnetic skin depth of the plasma greatly exceeds the chamber size. The polymeric substrates being processed are placed on the chamber floor at the intersection of the cruciform arms. An important point is that the excitation wire passes over the chamber at the intersection so that the substrate is on the opposite wall from the excitation wire. This means that the substrate does not experience the large-amplitude rf potentials associated with the sheath adjacent to the rf driver and that the ion energy at the substrate is controlled by the potential difference Vp - Vsb. The vessel is evacuated using a 33 L s-1 rotary pump. A 2 L liquid nitrogen cold trap is placed between the chamber and pump to reduce the amount of backstreamed pump oil and to reduce water vapor levels in the chamber. Base pressures well below 10-4 Torr are achieved after about 5-7 min pump-down from atmosphere. Industrial grade (99.998% purity) Ar is supplied to the chamber through a calibrated needle valve, which gives typical flow rates of about 2 cm3 min-1 at standard temperature and pressure. Typical processing pressures are in the range 10-310-1 Torr. The plasma parameters of electron temperature (Te), plasma density (ne), and floating (Vf) and plasma (Vp) potentials were obtained using an “in-house” rf-compensated Langmuir probe similar to that described by Annaratone.20 The probe tip was a tantalum cylinder 3.5 mm long and 1 mm diameter. This probe radius was chosen to be comparable with the plasma electrostatic screening distance, the Debye length λD. This ensured that the sheath surrounding the probe was collisionless and that charged particle collection was in an orbital motion limited regime.21 Thinner probes were tried but found to suffer from a distortion in the ion collection region of the probe current-voltage characteristic, which was consistent with sheath expansion effects.21 The probe length was chosen to be several radii (to

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Figure 2. Comparison between the probe current-voltage characteristics for a compensated and uncompensated probe at 10-2 Torr and 10 W input power. The probe bias voltages at which zero current flows are the self-bias (uncompensated probe) and floating (compensated probe) potentials.

ensure that cylindrical geometry could be used to describe particle collection) but short enough to minimize the current drawn to the probe. The probe could be moved within the chamber by using vacuum bellows that were set into one flange. The rf compensation electrode was provided by the probe support, which was a stainless steel cylinder of 4 mm diameter and 130 mm length. A ceramic break along the support rod prevented the rf signal from being earthed where the probe stem passed through the earthed flange. The probe characteristics were analyzed using a Hiden HAL IV ESP controller and acquisition system. As a check procedure, a double probe was also used to estimate the electron temperature and plasma density. The data obtained with this arrangement agreed with the single probe to within 20%. The positive ions incident on electrically earthed and selfbiased surfaces were detected by a Hiden EQP 300 energyresolved mass spectrometer, which entered the chamber through one of the four flanges. The spectrometer “nose” could be withdrawn so that it was level with the flange face or passed into the chamber through an O-ring seal until the sampling aperture (100 µm) was at the geometric center. 3. Langmuir Probe Results 3.1. Radio Frequency, Floating, and Plasma Potentials in the Chamber. The effect of the rf voltage on the Langmuir probe, at the geometric center of the vessel, is shown in Figure 2 by comparing the current voltage characteristics for compensated and uncompensated probes at 10 W input power and 1 × 10-2 Torr pressure. Comparing the two characteristics, we see a near 50 V increase in the floating potential and increased probe current for a given bias change for the compensated probe. The floating potential of the uncompensated probe, which is negative with respect to the compensated probe, is defined as the rf selfbias potential Vsb.18-20 This potential is significant because ions crossing the plasma-polymer sheath gain a maximum energy

Vp - Vsb

(1)

where Vp is the plasma potential. Examples of the ion mean energy and energy distributions are given in later sections. Furthermore, the difference between the self-bias potential and the floating potential (Vf) of a compensated probe is a function of the local amplitude of rf potential fluctuations.18,20 We define the floating potential as being the probe bias at which no direct

Figure 3. Increase in self-bias (lower curve, diamonds), plasma (upper curve, circles), and rf potentials (middle curve, squares) with increasing input power. The difference between the plasma and self-bias potentials, Vp - Vsb, defines the maximum ion energy at the polymer substrate. Since the self-bias potential does not increase significantly at power inputs above about 3 W, the increase in ion energy is due to the rise in plasma potential.

current flows, that is, the zero net current in the absence of rf potentials. The self-bias potential may be expressed as18,20

Vsb ) Vf -

( )

kTe eVrf ln I0 e kTe

(2)

where I0 is the modified Bessel function of zero order. Equation 2 shows that as the rf potentials increase, the self-bias voltage becomes more negative. Furthermore, for large rf amplitudes, (VRF . kTe/e) the second term on the right-hand side of eq 2 is approximately equal to VRF. Calculating the rf potentials in this chamber shows that voltages of about 40 V in amplitude (at 10 W input power and 1 × 10-2 Torr) are present. This compares with rf potentials of less than 10 V,20 which are typical of those found in parallel plate reactors.17 The large-amplitude rf potentials in our vessel allow the treatment of polymer surfaces away from the driver electrode. A potential advantage of this type of processing vessel is that relatively large threedimensional bulk surfaces could be treated uniformly in the large volume. The rf potentials that are estimated from eq 2 increase with input power as shown in Figure 3. Also shown in Figure 3 are the plasma (Vp) and self-bias (Vsb) potentials. The self-bias potential (lower curve), which is the potential assumed by the polymeric substrate, changes only slightly with increasing input power, whereas the plasma potential (upper curve) increases by about 30 electron temperatures. The difference between these two potentials defines the maximum ion energy at the substrate. Note that the amplitude of the rf potentials is proportional to the observed increase in plasma potential (measured from the compensated probe). The plasma potential was also mapped against pressure, power input, and spatial position. Of particular interest is the potential difference between the plasma and the glass surface adjacent to the driver coil and that between the plasma and the flanges. For a capacitively coupled discharge, the ratio of driver to earth electrode areas can be related to the ratio of plasma sheath potentials17 by the relationship

( )

Aflange Vcoil ) Vflange Acoil

n

(3)

Choosing a value for n of 5/2 justified on experimental observation and sheath theory,17 we estimate the coil effective area. At the plasma coil sheath, the measured potential difference

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Figure 4. Electron temperature reduction with increased input power. The gas pressure was 10-2 Torr, and the probe was sited at the geometric center of the vessel. The size of the points represents the standard deviation in the data set.

Barton et al. temperature falls. At low power inputs, the density increases rapidly for small increases in power level, and so the electron temperature falls. However, at higher power levels, the density is insensitive to power input and the conductivity, and hence, the electron temperature is nearly constant. As the gas pressure was increased, the electron temperature reduced from about 3 eV at 10-3 Torr to about 1.6 eV at 1 Torr (Figure 5). This reduction in temperature with increased pressure is consistent with reduced diffusion losses at higher pressures.17 Ionization balance requires that lowering plasma losses reduces the ionization rate, which, provided that ionization is caused by the high-energy tail of the electron energy distribution function (EEDF), lowers the electron temperature. 3.3. Plasma Density in the Process Chamber. In contrast with the electron temperature, the plasma density was found to be a strong function of power, gas pressure, and position within the chamber. The plasma density, which was calculated from the ion saturation region of the probe current-voltage characteristic, gave information about characteristic loss mechanisms and provided information about the mean charged particle fluxes to the polymer surfaces. Furthermore, the typical values of the density, about 3 × 1015 m-3, allow the estimation of the electromagnetic skin depth δ, defined as being the distance an electromagnetic wave penetrates into a media before its amplitude is attenuated to exp(-1) of its original value. In a plasma, the skin depth is a function of the ratio νm/ω, where νm is the electron momentum transfer collision frequency (the electron self-collision frequency multiplied by the cosine of the scattering angle) and ω is the applied rf angular frequency. Under the discharge conditions used here, the ratio νm/ω is less than 1 (νm ) 1 × 107 s-1, ω ) 8.5 × 107 s-1)17 so that the skin depth δ is

δ) Figure 5. Electron temperature reduction with increased gas pressure. The probe was sited at the geometric center of the chamber, and the input power was 10 W.

between the plasma and the glass surface is about 59 V, while that between the plasma and flange face is about 14 V. This gives an effective coil area of 177 cm2, the flange area being 4(π52) cm2. Given a coil length of about 120 cm, this gives an effective coil width (including sheath), which is projected through a glass of 1.4 cm. This is much less than the spacing between the coils (about 6 cm), and so the driver coil must be modeled as a helical electrode. 3.2. Electron Temperatures in the Process Chamber. The electron temperature was obtained as a function of power input, gas pressure, and spatial position in the process chamber. The temperature was calculated from the linear region of the probe current-voltage characteristic semilog plots. The linear region extended over a probe bias voltage range in excess of Vp - Vf, which suggests that the electron distribution function is Maxwellian.20 The electron temperature (Te) was constant with position in the bulk of the chamber and varied only with input power and gas pressure (Figures 4 and 5). Figure 4 shows a decrease in electron temperature with increasing input power between 0.4 and 10 W. Above 10 W input power, the temperature remained constant. This can be explained broadly in terms of the plasma conductivity, which is proportional to density (section 3.3). When the conductivity is low, higher potential gradients are required in the plasma to maintain the discharge current, which increases the electron temperature. As the conductivity increases and as the potential gradients in the plasma decrease, the electron

c ωpe

(4)

where c ) 3 × 108 ms-1 and ωpe is the electron plasma frequency, which is a function of plasma density. By use of a typical experimental plasma density of 3 × 1015 m-3, ωpe ) 0.3 GHz so that the skin depth of this device is about 1 m, which is greater than the chamber size. This means that the plasma must be coupled capacitively to the source coil, an observation that is confirmed by the presence of large-amplitude rf potentials in the plasma, which would not be present were the discharge inductive. The density increased with rising power input, as shown in Figure 6. The density is bounded at higher power levels, which probably indicates that stray capacitive losses were reducing power coupling efficiency to the plasma at higher power inputs, above 40 W, for example. Increasing the gas pressure caused the plasma density to decrease (Figure 7). This reduces the plasma conductivity, which would require an increase in electron temperature to maintain the discharge. However, as seen in Figure 5, the electron temperature also decreases as the gas pressure is increased. For the lower conductivity to be accompanied by a fall in the electron temperature, the ion loss rate must fall. This would not, in isolation, cause the density to fall, and so some other process must be responsible. We have also found that the rf fluctuations in the plasma also fall as the pressure rises. This may explain the fall in density, since this parameter is proportional to the rf amplitude, as can be seen by comparing Figures 3 and 6. The spatial variation of plasma density was also mapped inside the chamber. Along both the axial (coil carrying) and

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Figure 9. Reduction in plasma density as a coil turn is approached compared with zero-order Bessel function (lower curve) and parabolic (upper curve) profiles. Figure 6. Increase in plasma density as the power level is raised. The plasma density is proportional to the rf potential level in the plasma. The gas pressure was 10-2 Torr, and the probe was at the geometric center of the vessel.

Figure 10. Plasma density approaching a floating wall (points) compared with a zero-order Bessel function profile.

Figure 7. Reduction in plasma density as the gas pressure is increased. The power input level was 10 W.

Figure 8. Variation in plasma density as the earthed flange is approached along the axial (coil carrying) arm (circles) and transverse arm (dots). The flange is at l ) 250 mm, and l ) 0 represents the geometric center of the vessel.

transverse axes, the density was approximately constant as the earthed flanges were approached (Figure 8). The plasma sheath region is clearly seen as a rapid falloff in density about 30 mm from the flange faces. Because the electron temperature is constant along the vessel arms and ion transport is diffusional along this axis, a classical analysis predicts a quarter-period cosine density profile as the flange is approached.21 The absence of this profile indicates that the flanges are not acting as simple loss boundaries but as electrodes. Even so, Meichsner15 observed a near-linear reduction in density as an earthed electrode was approached in a parallel plate chamber. The local increase in density near the flange seen here may be a result of secondary electron emission

processes at the flange face, which locally enhances the ionization rate. This situation is analagous to a dc cathode, where ion, photon, and metastable bombardment of the cathode causes electron emission from the surface.22 However, this process requires plasma-cathode potentials of several hundred volts17,22 where the plasma-to-flange potential difference is only about 14 V. Furthermore, while it is possible to maintain rf plasmas through electron emission from the electrodes,23 this mode transition is usually accompanied by a reduction in the bulk electron temperature to below 1 eV, which has not been observed here. As the probe is moved radially across the chamber arm, we again observe a reduction in density as the wall (driver coil) is approached (Figure 9). Classically, we expect a zero-order Bessel function density profile,17,22 which is shown as the full curve in Figure 9; yet the data more closely resemble a parabolic profile. This profile is unusual but can occur when plasma loss is diffusional and the ionization rate is constant across the radius. If this is true, the implication is that ionization is not caused primarily by the high-energy tail of the (Maxwellian) EEDF, so that the local ionization rate is proportional to density, but by a small number of high-energy electrons that are uniformly distributed throughout the plasma. Conversely, when the transverse arm is studied (no excitation coil), the density profile resembles a Bessel function in the bulk of the chamber (Figure 10), but the measured density exceeds that expected as the wall is approached. Again, this nonclassical picture is consistent with sheath processes, although it is unclear at this stage why the density is anomalously high near a floating sheath. It is clear that the spatial variation in density within the chamber cannot simply be modeled using classical analysis and that there are complex sheath processes that distort the density profiles from those expected. These sheath effects will be analyzed in detail in the future to fully explain charged particle behavior at the polymer substrate.

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Figure 11. Ar+ ion energy distribution at the earthed spectrometer face at 10-3, 10-2, and 10-1 Torr. At the lowest pressure the plasma sheath is collsionless and the IEDF is well described by a LangmuirTonks model.22 As the pressure is increased, the IEDF forms a saddleshaped structure that decays as the sheath becomes collisional at 10-1 Torr. The line and data points show the plasma potential in the vessel in the absence of the mass spectrometer.

Despite the anomalous density profiles, we can make a reliable estimate of the ion flux to the polymeric surface by using the Bohm sheath criterion,17,22 which states that for a cold plasma, the ion flux to a plane surface is

( 21)n x M

Γi ) exp -

kTe

e

(5)

i

where k is Boltzmann’s constant and Mi is the ion mass. At 10-2 Torr and 10 W input power, this gives an ion flux to the substrate of about 6 × 1018 m-2 s-1. This compares well with measurements of ion fluxes to substrates in similar vessels.24 4. Mass Spectrometer Results To determine the ion species and energy spectra at the substrate surface, the Hiden EQP300 is introduced into the vessel. The spectrometer nose was placed 8 cm from the geometric center of the vessel, along one of the transverse arms (see Figure 1). Obtaining the mass spectrum in residual gas (RGA) and positive ion modes, we note that the dominant peaks are Ar, with water vapor being the main impurity. As the argon pressure is reduced, we note that the proportion of outgassed species increased, the Ar/H2O ratio increasing from 13% to 22% between 10-1 and 10-3 Torr, respectively, when measured in RGA mode. Ion energy distributions were also obtained for the dominant plasma species over a range of gas pressures. At 10-3 Torr, the ion mean free path for elastic or symmetrical charge exchange is about 35 mm for Ar+. The sheath to the spectrometer is therefore collisionless, and we observe a beamlike distribution in ion energy (Figure 11) as observed by Ingram and Braithwaite.25 This distribution may be modeled by a TonksLangmuir theory,25 which agrees well with the observed distribution. The maximum energy seen in the distributions corresponds to the local plasma potential. As the pressure is increased to 10-2 Torr, we notice the emergence of a secondary peak in the IEDF (Figure 11) of some species. This energy spectrum has been observed before and has previously been attributed to ion resonance in the sheath.26 This process is a function of ion mass (lighter ions being affected more strongly); yet species with similar masses would not consistently show the effect. By use of the example of Ar+ and ArH+, the lighter ion had two distinct peaks in the IEDF; yet ArH+ had an energy spectrum more consistent with collisionless

Figure 12. Ion energy distribution for ArH+ at 10 W input power and 10-2 Torr argon pressure. At this pressure the ion energy distribution for Ar+ has a distinct profile. The marked change in the distribution function between the two species, which have similar masses, suggests that sheath resonance may not be the cause of the saddle-shaped Ar+ distribution.

transport through the sheath under identical discharge conditions (Figure 12). We may therefore be able to exclude sheath resonance as a possible cause of this unusual IEDF. As the pressure is increased further to 10-1 Torr, we again observe a single-peak spectrum with a near 20 V spread in energy (Figure 11). At this pressure the ion mean free path is about 0.3 mm so that the sheath is collisional, and we therefore expect the observed energy spread. The mass spectrometer is earthed and so acts as an electrode. The maximum ion energy at the spectrometer entrance is therefore (Vp - 0) eV. Because of this, we can use the spectrometer to measure the plasma potential and compare this with the probe data without the mass spectrometer in the chamber. At the lowest pressures, 10-3 and 10-2 Torr, we observe that the spectrometer reduces the plasma potential by about 20 V (Figure 11). This perturbation to the plasma is explained when noting that the spectrometer is electrically earthed and therefore increases the earth electrode area as described by eq 3. This reduction in plasma potential would also lower the ion energy at the polymer surface, since the selfbias potential does not fall by a similar value when the spectrometer is introduced. At the highest pressure, 10-1 Torr, we notice that the spectrometer detects a plasma potential of about 65 V, which is about 50 V above the value obtained without the spectrometer in the chamber. However, the probe was sited at the geometric center of the vessel, well away from the mass spectrometer front face, and at this pressure, the plasma was visibly localized at the earthed spectrometer and electrode surfaces. From this observation, we conclude that the plasma potential close to the earth electrodes is much greater than that seen at the geometric center of the chamber. The polymeric samples in the chamber do not act as electrodes, and therefore, they assume the self-bias potential Vsb. We can correct for this by removing the self-bias potential from the measured (earth-referenced) ion energies. However, because the sheaths between earthed electrode (current carrying) and self-biased (non current carrying) surfaces have both different thickness and potential differences, the distribution functions themselves may differ. To investigate this, an electrically isolated stainless steel cap was placed over the spectrometer nose that was allowed to assume the self-bias potential. A fine gauze (60% transmission, wire diameter of 0.01 mm, hole size of 0.04 mm) was set into the cap, which permitted ions to reach the entrance slit of the spectrometer. The effect of the end cap is shown for Ar+ in Figure 13, at 10 W input power and 10-2 Torr pressure. The IEDF’s are both measured with respect to ground. For the

Plasma Modification of Polymers

Figure 13. Comparison between the ion energy distributions of Ar+ (at 10-2 Torr and 10 W input power) at an earthed electrode (full curve) and an electrically floating surface (dashed curve). Both curves are referenced to earth to highlight the change in IEDF difference between the two. To obtain the actual IEDF at the isolated surface, the curve is shifted down in energy by 6 V, the self-bias potential under these conditions.

floating sheath, the separation between the two energy peaks is slightly larger (by 6 V) and the region between the peaks is depleted. This clearly demonstrates that both the absolute energies and the distribution functions differ between mass spectrometer (earthed electrode) and self-biased surfaces and that any attempt to obtain the IEDF at a floating boundary must take this into account. 5. General Discussion Plasmas are commercially employed to treat a wide range of polymers as sheets, ribbons, and fibers. Typical bond strengths in polymers are in the range 3-5 eV (e.g., C-H bond strength is 3-4 eV, while C-F is 5 eV).10 Figure 13 shows that at 10-2 Torr and 10 W, the argon ions bombard the polymeric surface with energies in the range 9-36 eV. This is sufficient energy to break these bonds; however, the ion may not lose all its energy in a single collision, and so the ion energy may be dissipated through a linear cascade. Clark and Dilks16,27 established, using XPS, that in the Ar plasma treatment of an ethylene-tetrafluoroethylene copolymer, modification by ions and metastables is limited to the outermost surface (approximately 8 Å), assuming a mean free path of 10 Å for electrons of 960 eV (the kinetic energy of C 1s electrons stimulated by Mg KR X-rays). Modification arising from the UV/VUV component was predominately subsurface and had a rate constant an order of magnitude lower. The value for the electron mean free path chosen by Clark and Dilks is rather short, with more recent estimates being closer to 20 Å.28 However, the mechanistic picture presented remains unaltered by this revision of the mean free path. We have determined ion fluxes to be 6 × 1018 m-2 s-1 at 10-2 Torr and 10 W. With knowledge of the ion mean energy, we can readily estimate the energy flux deposited by ions onto the polymer surface. Assuming that the ion mean energy is around 30 eV, the energy flux is about 1.8 × 1020 eV m-2 s-1 (2.9 mW cm-2). We assume that modification by argon plasma treatment in these types of vessels is limited to the outermost 15-20 Å in most polymeric materials, which is consistent with the depth profile results published earlier.8 In the plasma described here, maximum treatment as measured by oxygen incorporation is achieved in about 15 s. This would suggest that if ion energy alone is responsible for the modification, about 2.7 × 1021 eV m-2 (43 mJ cm-2) is required. This of course ignores any contribution to treatment by the UV/VUV and excited fast atoms (metastables). Fast atoms are created through

J. Phys. Chem. B, Vol. 103, No. 21, 1999 4429 charge exchange collisions in the sheath. However, at 10-2 Torr, the mean free path for charge exchange exceeds the sheath thickness, and so the contribution from these particles can be neglected at this pressure. UV will only be important if specific chromophores are present in the polymer. The VUV energy fluxes at these electron temperatures and plasma densities are estimated as being 1020 eV m-2 s-1 (1.6 mW cm-2), assuming the gas to be pure argon.29 This value may require some correction once the O2 and N2 levels have been determined accurately. At the surface, the VUV energy flux under these conditions is the same order of magnitude as for the ion energy flux. However, assuming an absorption coefficient k of 2 × 105 cm-1 for photons at 1300 Å,16 it is readily calculated that less than 5% of the VUV is absorbed in the outermost 20 Å of the polymer. This shows that the energy deposited by the ions in this surface layer is an order of magnitude greater than that for the VUV photons. The Ar I and Ar II resonance lines are at ∼1048, 1067 Å and ∼920, 932 Å, respectively. By manipulating the ion energy while keeping other parameters unchanged, we will be able to improve the accuracy of these estimates and thereby better understand the respective roles of ions and VUV ions in polymer surface modification. One of the more important aspects of processing polymers with plasmas is the rate at which processing may be achieved. To maximize treatment rates, input power and gas pressure are normally varied empirically. The findings here suggest that there is a limit to the gain to be had in terms of ion flux from increasing the input power, since the plasma density saturates at a power of about 40 W. This feature should be observed generally with this type of reactor; however, the value of power at which this will happen will differ. Also, Figure 3 shows that the plasma potential also saturates at input powers above about 30 W so that the ion energy, and therefore the energy flux, also saturates at higher powers. By developing a method of active rf biasing to the polymer surface, we intend to control the mean ion energy, while keeping the flux constant, to reduce the ion energy down to the bond strength threshold. This would further enhance the tailoring of plasmas for specific treatment. Changes in power above 2-4 W do not appreciably change the electron temperature and therefore the VUV energy flux to surfaces. The general effect of lowering the gas pressure is to increase the plasma density and hence the ion flux to the polymer surface and at the same time increase the electron temperature. Since photon emission rates rise rapidly with electron temperature, this may lead to VUV photon energy fluxes exceeding the ion energy flux at the polymer surface. 6. Conclusions The plasma parameters inside a chamber that is in common use for polymer processing have been investigated using Langmuir probes. We have detected the presence of rf potentials with typical amplitudes of about 15Te under normal processing conditions of 10 W input power and 10-2 Torr pressure. These are much greater than the rf amplitudes seen in parallel plate reactors, which are usually of the order of 5Te.18 These rf fluctuations are responsible for large differences in potential between the polymer surface and the plasma (up to +65 V). This potential difference controls the ion energy at the substrate surface. This removes the need for the sample to be placed on the driver electrode, where the sheath potentials may be too high for optimum process results. The rf potentials increase with input power and reduced gas pressure. This is seen as an increase in the plasma, floating, and self-bias potentials.

4430 J. Phys. Chem. B, Vol. 103, No. 21, 1999 Despite the rf excitation being provided by an external coil, the plasma density is such that the electromagnetic skin depth exceeds the vessel size, and the plasma is therefore capacitively coupled. Further evidence of this is provided by the spatial maps of plasma density near the earthed flanges, which are inconsistent with simple plasma loss to a surface and are more consistent with processes near an electrode. A consequence of the active role of the flanges is that the plasma density is nearly constant along the vessel arms at about 3 × 1015 m-3 at 10 W and 10-2 Torr. The radial densities are also inconsistent with simple models of plasma behavior, with the observed densities being greater than those expected near the plasma sheath. Again, this is consistent with electrode processes that cause local perturbations to the ionization rate through electron heating mechanisms. The mass spectrometer size is sufficiently big to significantly perturb the plasma conditions when compared with those seen without the instrument. At typical processing pressures (about 10-2 Torr) this is detected as a reduction in the plasma (and rf) potentials, which is consistent with the increase in earth electrode area. The reduced plasma potential seen with the spectrometer in place reduces the mean ion energy at the sample surface. At 10-1 Torr, we observe potentials close to the spectrometer (electrode) that are higher than those seen at the geometric center of the vessel, which we attribute to the localized production of plasma close to the extraction orifice. Furthermore, since the instrument is electrically earthed, it acts as an electrode and the ion energies detected by the device are different from those at the electrically isolated sample surface. Fitting an electrically isolated end cap to the mass spectrometer, we observe a reduction in the ion energy and a change in the ion energy spectrum. Acknowledgment. We thank the EPSRC for their funding support. References and Notes (1) Liston, E. M.; Martinu, L.; Wertheimer, M. R. Plasma Surface Modification of Polymers: ReleVance to Adhesion; Strobel, M., Lyons, C., Mittal, K. L., Eds.; VSP: Utrecht, The Netherlands, 1994.

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