J. Phys. Chem. B 2007, 111, 3419-3429
3419
Pulsed and Continuous Wave Acrylic Acid Radio Frequency Plasma Deposits: Plasma and Surface Chemistry Sergey A. Voronin,† Mischa Zelzer,‡ Catalin Fotea,‡ Morgan R. Alexander,‡ and James W. Bradley*,† Department of Electrical Engineering and Electronics, UniVersity of LiVerpool, Brownlow Hill, LiVerpool L69 3GJ, United Kingdom, and Laboratory of Biophysics and Surface Analysis, School of Pharmacy, The UniVersity of Nottingham, Nottingham NG7 2RD, United Kingdom ReceiVed: December 11, 2006; In Final Form: January 24, 2007
Plasma polymers have been formed from acrylic acid using a pulsed power source. An on-pulse duration of 100 µs was used with a range of discharge off-times between 0 (continuous wave) and 20 000 µs. X-ray photoelectron spectroscopy (XPS) has been used in combination with trifluoroethanol (TFE) derivatization to quantify the surface concentration of the carboxylic acid functionality in the deposit. Retention of this functionality from the monomer varied from 2% to 65%. When input power was expressed as the timeaveraged energy per monomer molecule, Emean, the deposit chemistry achieved could be described using a single relationship for all deposition conditions. Deposition rates were monitored using a quartz crystal microbalance, which revealed a range from 20 to 200 µg m-2 s-1, and these fell as COOH functional retention increased. The flow rate was found to be the major determinant of the deposition rate, rather than being uniquely defined by Emean, connected to the rate at which fresh monomer enters the system in the monomer deficient regime. The neutral species were collected in a time-averaged manner. As the energy delivered per molecule in the system (Emean) decreased, the amount of intact monomer increased, with the average neutral mass approaching 72 amu as Emean tends to zero. No neutral oligomeric species were detected. Langmuir probes have been used to determine the temporal evolution of the density and temperature of the electrons in the plasma and the plasma potential adjacent to the depositing film. It has been found that even 500 µs into the afterglow period that ionic densities are still significant, 5-10% of the on-time density, and that ion accelerating sheath potentials fall from 40 V in the on-time to a few volts in the off-time. We have made the first detailed, time- and energy-resolved mass spectrometry measurements in depositing acrylic acid plasma. These have allowed us to identify and quantify the positive ion species in the acrylic acid plasma during both the on- and the off- periods. The relative intensities of oligomeric species of the type [nM + H]+ as large as n ) 3 were observed to increase in the off-time suggesting vapor phase polymerization after power input to the plasma was ceased. The energy distribution functions of these ions demonstrated that they were produced in the plasma in both the on- and the off-times. This remarkable observation contradicts the assumptions usually made when speculating on pulsed plasma that ions have very short lifetimes, although it is anticipated that radicals still have significantly longer lifetimes, estimated from calculation to be in excess of 1 ms. The increase in average positive ion mass during the off-period can be related to the lower mobility of the heavier components, reducing their relative loss to surfaces, and the polymer chain growth in the gas phase due to the ion-neutral collisions. The implications of these observations are discussed in light of polymerization mechanisms proposed from continuous acrylic acid and millisecond pulsing plasmas.
1. Introduction The simultaneous synthesis and deposition of thin polymeric coatings from plasma sustained in organic vapors to form plasma polymers has attracted significant interest for many applications.1,2 These coatings can be readily deposited from radio frequency (RF) plasma as highly adherent, pinhole-free, thin (less than 100 nm), conformal coatings, which find application in areas as diverse as modification of protein and cell adsorption to surfaces in biomedicine to protective layers in the automotive industry.3-5 The realization that it was possible to retain chemical functionality from the monomer vapor in the film has * Author to whom correspondence should be addressed. E-mail:
[email protected]. † University of Liverpool. ‡ The University of Nottingham.
had a significant impact on the strategies employed for application of plasma polymer.6 Much of the subsequent work has been aimed at retaining carboxylic acid functionalities in the deposits from carboxyl-containing compounds, with studies into the plasma-phase chemistry aimed at obtaining an understanding of the formation mechanism of the resultant surface chemistry.7-17 Acrylic acid is an important monomer compound, finding applications as a coating in, for example, a novel cell-delivery vehicle,18 to control protein adsorption,19 and for surface immobilization of bioactive molecules.16,20 In continuous wave (CW) plasma, low input power or deposition downstream of the plasma is generally associated with high functional retention. Pulsed RF discharges have been used to further increase monomer retention with a variety of monomers.19,21-25 With
10.1021/jp068488z CCC: $37.00 © 2007 American Chemical Society Published on Web 03/13/2007
3420 J. Phys. Chem. B, Vol. 111, No. 13, 2007 pulsed plasma polymerization, high retention of the carboxyl functional group from acrylic acid in the resulting polymeric film can be achieved, e.g., 66% (0.2 W time-averaged power),24 equivalent to that obtained from low input power (2 W CW plasma).10 It has been argued that modulating the applied power, typically through variation of the pulse frequency and duty ratio, enables greater control over the input power than using low power continuous wave plasma.23 Furthermore, it has been proposed that the use of pulsed sources offers the advantages over CW of increased operational stability, reducing trapped radicals in the film, lower deposition surface temperatures, and decreased high-energy ion bombardment and UV flux to the surface.25 Evidence of linear species in the plasma polymer of acrylic acid (ppAAc) deposit of up to 5 monomer units (5M) has been shown using secondary ion mass spectrometry for films produced at low CW powers.10 These structures, similar to those seen from conventional PAA, correlated with linear polymer deposit and high functional retention in the acrylic acid system deposited in the power deficient regime. A monomer deficient, higher power deposition regime was shown to result in deposits with lower carboxylic acid retention. Conversion of the acids into esters, loss of oxygen, and a concurrent decrease in linear surface species were thought to correlate with an increase in cross-linking in the film. As a result of such observations on the surface chemistry, great interest has arisen into the exact nature of its deposition mechanism, specifically that which leads to retention of carboxylic acid functionality in the deposits. The relative importance of neutral and positive plasma species in deposit formation has received considerable attention, much of it speculative based upon observations of surface chemistry. Yasuda1 has described plasma polymerization as proceeding through a rapid step-growth polymerization mechanism. In this scheme a polymer is formed by the stepwise repetition of the same reaction. There are two parallel cycles to the scheme. The first describes the reaction of a monofunctional activated species presumed to be a radical. This radical can react with neutral species step by step, to produce a larger radical (Mx• + My f Mx+y•), which continues on, or produces a neutral compound by reacting with another activated species (Mx• + My• f Mx+y). A second cycle describes the reactions of difunctional species (•M•) to yield a larger difunctional species. More recently, an ion-molecule chain polymerization mechanism in continuous wave plasma was proposed by O’Toole et al. (Mx++ My f Mx+y+).7 This was based on observation of positively charged oligomers in the plasma of masses greater than the monomer mass and the absence of such neutral species. Further investigations of continuous wave plasma polymerization of acrylic and propionic acid15,17,26-28 found mass spectrometry evidence that polymer chain growth occurs in the plasma gas phase to form positively charged polymeric species. This has challenged the orthodox view of the predominance of free-radical chemistry within polymerizing plasmas of allyl alcohol,29 acrylic acid,7 allylamine,28 and siloxane monomers.30,31 The actual mass of positive species arriving at a surface has been measured in the acrylic plasma and shown to be consistent with a positive ion deposition mechanism at low input powers.27 Low ion energies were measured from acrylic acid plasmas at low input power and are consistent with minimal fragmentation on arrival at a self-biased surface.17 Measurements of the ion flux toward the polymer surface have shown that the ionic mass transported to the surface is significant for ppAAc and for plasma-polymerized allylamine.28
Voronin et al. Speculation of deposition mechanism from observations of deposition rate and surface chemistry has been undertaken since the first reports of pulsed depositing discharges.25 Only one study has obtained mass spectroscopic data from a pulsed acrylic acid discharge to enable direct mechanistic information to be obtained, although the data was not time-resolved.24 The workers examined very long off-time plasmas and noted that carboxyl retention increased with an increase of the off-time and a corresponding decrease in the time-averaged power for millisecond pulse durations. Using time-averaged results it was inferred that the ppAAc deposition in the off-time continued for times up to 500 ms, suggesting a radical grafting of monomer to the surface in plasma polymer grown at longer off-times. This was based on observations of significant deposition in the off-period and the supposition that the charged species react with walls and therefore cannot contribute at significant times after the plasma has been extinguished. There have been only a limited number of time-resolved studies of low-pressure polymerizing plasma utilizing mass spectroscopy and Langmuir probes, for instance, in styrene polymerization32 with the aim to link plasma parameters and surface chemistry and recently in acrylic acid33,34 to gain more detailed information on the plasma state. Here we attempt to link plasma measurements with film chemistry to elucidate the pulsed deposition process, in particular considering if off-time grafting mechanisms are dominant as described by Friedrich et al., driven by on-time radical formation.35 In this work we carry out a detailed study of plasma polymerization of acrylic acid in the commercially relevant microsecond pulsing regime using plasma mass spectrometry. Critically, time-resolved measurements indicate significant lifetimes for charged species in the off-time employed. X-ray photoelectron spectroscopy (XPS) was used to determine the carboxylic acid retention in films for which plasma parameters were determined. Electrostatic probe and quartz crystal microbalance (QCM) measurements were employed to obtain a full picture of the plasma and relate it to the deposit chemistry. 2. Experimental Arrangement The experimental plasma apparatus used in this study is shown schematically in Figure 1a. It is similar to that used in our recent research.34 The system consists of a glass cruciform vessel with a volume of ∼7000 cm3, closed by four earthed stainless steel flanges. An RF exciting power (13.56 MHz) was applied to a four-turn sheathed unterminated wire, wound about one axis of the vessel. The system was pumped by a rotary and a turbo-molecular pump to a base pressure below 10-2 Pa. Acrylic acid monomer (CH2dCHCOOH, M ) 72 amu) of >99% purity (Aldrich Chemical Co.) was degassed using a freeze-thaw cycle before introduction to the evacuated reactor chamber through the needle valve. To prevent corrosion damage of the pumps a cold trap cooled by liquid nitrogen was installed between the baffle valve and pumping port. The working vessel pressure (1.3 Pa) was set without plasma by fixing the baffle valve position (pumping rate) and adjusting the needle valve (changing the flow rate) until the desired pressure was reached. The pressure was monitored by a Baratron (MKS 627) pressure gauge. The monomer flow rates between 1 and 5 cm3 min-1 (sccm) at standard temperature and pressure were estimated from the pressure increase in the vessel volume for a selected time period with the baffle valve closed and needle valve open. The pulsed RF power and data acquisition signals (required to trigger the plasma diagnostic tools) were synchronized by a pulse-width generator (Figures 1a and 1b). The exciting RF
Pulsed and CW Acrylic Acid RF Plasma Deposits
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3421
Figure 1. (a) Schematic diagram of the experimental setup. (b) Signal diagram.
voltage URF (applied to the driving coil) was generated by a Coaxial Ltd RFG-150 power supply triggered from pulse Upulse. It was coupled to the coil electrode via a separate matching unit. The power input was measured using the built-in meter on the front panel, and the reflected power never exceeded 2% in the working range between 10 and 50 W. To achieve a minimal fragmentation of the monomer by energetic electrons, but still produce sufficient ionized and radical species to conduct the experiments, the RF power ontime was chosen to be 100 µs. This was more than enough for the plasma to reach steady state (which takes typically 20-50 µs).34,36 The on-time remained fixed at 100 µs for all investigated deposition regimes while the off-time was varied in a wide range between 0 (CW) and 20 000 µs. To measure the time evolution of the plasma parameters (electron density, temperature, plasma potential, and sheath potential drop at the substrate) we employed a heated and retractable Langmuir probe. It had a time resolution of 10 µs, significantly above that needed in these experiments. To observe the ionic and neutral species in the plasmas (the gas-phase chemistry) and determine their relative arrival rates and energies, time- and energy-resolved mass spectrometry was employed. These electrical plasma diagnostic tools are described below. 2.1. Langmuir Probe System. The retractable Langmuir probe used here has been described in detail elsewhere.34 The probe tip was positioned at the center of the vessel 3 cm in front of the mass spectrometer orifice (Figure 1a). It consisted of a 1 cm tungsten wire with a radius of 250 µm encased in a ceramic barrel, covered by a heating coil of platinum wire. The “retractable” probe tip was moved by the external stepper motor and was immersed into the plasma only for a short time interval during the measurements to reduce surface contamination. A 10 µs pulse Ustrobe was launched from the generator with a prescribed delay with reference to the leading edge of the pulse Upulse (Figure 1b). Launched by Ustrobe, an RF-compensated
probe IV-meter could sample and record probe voltage-current characteristics at any selected point in time. The plasma parameters were obtained with 10 µs time resolution, using a time-resolved active RF-compensated probe system.37 The plasma electron temperature (kTe/e) was obtained from the inverse slope of the plot of the logarithm of the electron current versus the probe retarding potential, the plasma potential (Vp) was found from the knee in the probe voltage-current characteristics, and the electron density (ne) was calculated from the current reading at this point as described earlier.38 2.2. Plasma Mass Spectrometry. Neutral and positive ion plasma-phase species were sampled at the discharge center by a HIDEN Ltd EQP 300 energy-resolved mass spectrometer. The positive ion mass spectra and energy distribution functions F(E) for important positively charged plasma species were obtained with respect to the earth potential. The technique for obtaining time- and energy-resolved spectroscopic measurements, by “double gating” of the extractor electrode and detector employed here, has already been described for a pulsed acrylic acid discharge.34 Ions were sampled through a grounded 100 µm diameter orifice, immersed into the discharge. In the periods of the pulse when ions were not to be selected for measurement, the extractor was held at a constant positive voltage Ug ) +150 V to repel all positive ions from the plasma (Figure 1b). At the selected point in time with respect to the strobe signal Ustrobe the extractor potential Ug was rapidly reduced to the adjusted potential level Ugo (between -20 and +20 V), allowing the ions to enter the spectrometer. In this way, the plasma parameters and ion energy distribution functions could be obtained at the same point in time (with 10 µs resolution). In a short time (∼2 µs) after the falling edge of the extractor gating pulse, a detector-enabling pulse Ude was generated by the EQP signal board, allowing the digital ion counting pulses to be counted (Figure 1b). As the transit time for Ar+ (m/z ) 40) through the mass spectrometer has already been calculated to
3422 J. Phys. Chem. B, Vol. 111, No. 13, 2007 be 80 µs, it is known that the transit time of ions with masses between 1 and 300 amu will not exceed 250 µs (τ ≈ xm/z). Accordingly, the detector-enabling pulse duration was chosen to be 250 µs. For obtaining the ion mass spectra the system has been tuned for the mass m/z ) 73. In this study, it was not possible to obtain time-resolved measurements of the neutral species since the transit time for neutrals across the plasma, the plasma-instrument boundary layer, and the ionization chamber are typically longer than the plasma pulse duration. Neutrals are sampled at the detector as positive ion species after electron bombardment ionization in the ionization unit at the front of the instrument. It should be noted that the quadrupole detection system of the mass spectrometer has a strong mass-dependent transmission function, and the transmission of ions decreases strongly with increased mass, M.39 The manufacturer of the EQP 300 has found empirically that this mass-dependent instrument function lies somewhere between M-1 and M-2. In this work, we take a conservative estimate of M-1, and all detected ions signals (including post-ionized neutrals) have been corrected for this effect. Figure S1a in the Supporting Information shows the count rate intensity for the acrylic acid monomer (m/z ) 72) versus the electron energy in the ionization unit of the spectrometer obtained at a pressure of 1.3 Pa without plasma struck in the vessel. The energy threshold at which any signal could be detected (approximately the ionization threshold) was 12.5 eV. Experimental ionization thresholds of the most important fragments in the neutral mass spectra of acrylic acid were found as follows: 15.1 eV (m/z ) 1), 17.0 eV (m/z ) 2), 14.3 eV (m/z ) 18), 12.9 eV (m/z ) 26), 13.7 eV (m/z ) 27), 12.5 eV (m/z ) 28), 12.8 eV (m/z ) 44), and 12.5 eV (m/z ) 55). Consequently, an electron beam energy of 20 eV was chosen, being above the ionization thresholds of the major species present in the gas phase but low enough to prevent heavy cracking of acrylic acid molecule inside the ionization unit of the mass spectrometer. Figure S1b in the Supporting Information demonstrates a linear dependence of a count rate for m/z ) 72 versus the pressure of acrylic acid in the vessel in the absence of a plasma. Thus, the intensity of the m/z ) 72 in the mass spectra could be monitored, thus indicating the partial pressure of acrylic acid in the plasma. To help us differentiate between neutral species originating from the plasma and those produced in the instrument, we determined a cracking fraction of acrylic acid (however not a complicated Bayesian analysis40) due to the ionizer and hence corrected the raw measured neutral spectra. With no plasma in the chamber the “real” intensities of any mass except 72 amu (excluding impurities) must be zero. The recalculated mass spectra for other masses observed with plasma present should then be I(m) ) I0(m) - (I(m)no plasma) (I0(72)/I(72)no plasma) where I(m) is the recalculated peak intensity corresponding to mass m, I0(m) is the experimental peak intensity obtained from the mass spectra, and I(m)no plasma is the intensity obtained when the plasma is off. This simple method does not alter the monomer peak intensity, which can result in a slight underestimation of the number-averaged molecular weight evaluated from such mass spectra. The number-averaged molecular weight 〈M〉 of the ions and neutrals was calculated from the mass spectra as 〈M〉 ) ∑i Iimi/∑i Ii, where Ii is the peak intensity corresponding to mass mi in the spectra. 2.3. Film Chemistry Investigation Using XPS Analysis. Films were deposited on thin borosilicate glass coverslips (2.4 × 2.4 cm2, no. 1.5, Agar Scientific) clamped to the end cap
Voronin et al. near the orifice of the mass spectrometer. Each film was grown for 15 min to obtain a thickness greater than that of the XPS analysis depth (ca. 10 nm). It is not possible to distinguish carboxylic acids from esters in the XPS spectra acquired from AA-deposited films. To enable the unambiguous identification and quantification of the C(dO)OX carbon groups, it is necessary to use trifluoroethanol (TFE) derivatization, which reacts specifically with carboxylic acid functionalities.41,42 Presented to the samples in the vapor phase, the TFE molecule reacts with carboxylic acid but not esters or hydroxyl functionalities. Trifluoroethanol derivatization was carried out in a glass tube mounted a few degrees from the horizontal. The samples were placed in this container on a microscope slide, and the liquids were injected underneath. The glassware was solvent cleaned, followed by oven drying. The proportion of carbon atoms in carboxylic acid groups on the as-deposited film surface is calculated by multiplying the underivatized CO2X peak intensity (% of carbon) by the peak intensity of the CF3 to the CO2X environment in the derivatized sample. Retention is calculated by comparison of this figure with that of the acrylic acid monomer {CH2dCH-C(dO)OH}; the 33% of carbon as carboxylic acid functionalities is 100% retention. X-ray photoelectron spectroscopy was carried out on a Kratos Axis Ultra spectrometer using a monochromatic Al KR X-ray source utilizing electron flood for charge neutralization. Spectra were acquired using photoelectrons collected perpendicular to the surface. Elemental quantification was achieved using relative empirically derived sensitivity factors provided by the manufacturer. The film deposition rates were obtained in situ using a quartz crystal microbalance (QCM) thickness monitor TM-400 manufactured by Maxtek, Inc. The measuring crystal was placed close and parallel to the sample; the collecting surface was 0.5 cm2. To minimize the plasma perturbation through the immersion of the QCM in front of the sample, the deposition rate measurements were conducted separately from films growth experiments. The thickness data were obtained over a period of 1 min and recorded manually from the instrument display. Each investigated deposition regime was run for several minutes prior to data recording to achieve a stable deposition rate. To calculate the thickness from the weight gain measurements, the film density was assumed to be 1 g/cm3. 3. Results 3.1. Film Chemistry. Films were deposited from acrylic acid plasmas using a variety of pulse off-times toff, flow rates φ, and applied powers P (during the 100 µs on-time) for a constant monomer pressure of 1.3 Pa. Only carbon and oxygen were detected at the ppAAc surface. The XPS C1s core level spectrum acquired from 10 000 µs off-time deposit is shown in Figure 2a. Features characteristic of carboxyl (CO2X) are apparent along with functionalities representative of fragmentation and deposition of the acrylic acid monomer structure, e.g., C-OX and CdO where X is H or R. For the ester and carboxylic acid contributions to the carboxyl CO2X peak to be quantified, TFE derivatization was carried out; the spectrum is presented in Figure 2b. After derivatization, the CF3 moiety of the TFE molecule is clearly apparent at a shift of 8.0 eV from the hydrocarbon peak. The area of the peak representing the CF3 functionality was used to determine the proportion of the carboxyl component present as carboxylic acid on the as-deposited sample using a well-established method based on the reaction of TFE with only the carboxylic acids on these surfaces.41,42 The spectra acquired from continuous wave
Pulsed and CW Acrylic Acid RF Plasma Deposits
Figure 2. XPS from ppAAc samples produced using a pulsed (10000 µs off-time) 25 W, 1.5 sccm monomer flow (a) before and (b) after TFE derivatization. All spectra have been charge-corrected to display the C-C component at 285.0 eV.
Figure 3. Concentration of carbon in surface carboxyl C(dO)OH (9) and ester C(dO)OC (O) functionalities in deposits formed from P ) 25 W, 1.5 sccm monomer flow acrylic acid plasma determined by XPS and TFE derivatization vs off-time.
deposition are represented in Figure S2 in the Supporting Information. The concentrations of the carboxylic acid functionalities are plotted versus off-time in Figure 3. It is apparent that at longer off-times the carboxylic acid concentration in the deposits increases from approximately 1 at. % of the surface carbon in continuous wave operation to 22 at. % for 10 000 µs off-time for the 25 W input power (P). This is equivalent to retention of 3% of the carboxylic acid functionality in the deposit from the monomer for continuous wave 25 W deposition and 65% from 10 000 µs off-time pulsed 25 W deposition (see Figure 4a). Fragmentation of the carboxylic acid functionality at shorter off-times instead results in other functionalities, i.e., ester, alcohol/ether, carbonyl, and hydrocarbon (as demonstrated in CW in Figure S2a of the Supporting Information). These alternative products arise from fragmentation of the carboxylic acid functionality during deposition. The proportion of carbon in ester functionalities may be calculated as the difference between the acid and the total COOX peak area in the underivatized sample. It is apparent in Figure 3 that the proportion of ester functionality is greater in deposits of lower
J. Phys. Chem. B, Vol. 111, No. 13, 2007 3423
Figure 4. XPS-determined carboxylic acid retention vs (a) off-time and (b) Emean.
carboxylic acid content, indicating that it is a product of one of the alternative deposition mechanisms that are associated with the loss of the monomers’ carboxylic acid functionality. When the plasma is pulsed, the average power delivered over the cycle is reduced. The time-average power to the discharge Pave ) Pγ, where γ ) ton/(ton + toff), is the duty cycle. It has been shown previously that film structure in pulsed deposition of hydrocarbon films depends on the averaged dissipated energy per gas molecule Emean in the plasma.43,44 For this reason, and to gain understanding, the results from surface and plasma analysis here have been recast from off-time in terms of the energy inherent in the system per resident molecule. Explicitly Emean is expressed in electronvolts/molecule and is given by
Emean )
Pave P τ ) γ τres N res N
where N is the number of particles resident in the system and τres is their residence time. The latter is given by τres ) N/φ, where φ is the flow rate (in number of molecules per second). So we have
Emean ) γ
P φ
An absorbed power to flow rate ratio of Emean ) 1 W/sccm corresponds to 15.5 eV of dissipated energy per source gas molecule. Emean is related to the Yasuda parameter via Y ) Emean/ M, with M being the molar mass.45 Y was introduced to compare the plasma polymerization of different large monomers of differing M. It should be emphasized that the formula presented above is only applicable if the residence time is much longer than the pulse cycle. In our system, we use two flow rates of the acrylic acid monomer, namely, 4.8 and 1.5 sccm. At a pressure of 1.3 Pa and a chamber volume of 7000 cm3, this gives N ) pV/kT ) 2.2 × 1018 molecules resident at any one time. The gas flow rates (in the number of molecules per second)
3424 J. Phys. Chem. B, Vol. 111, No. 13, 2007
Figure 5. Film deposition rates vs (a) off-time and (b) Emean.
for the flow rates of 4.8 and 1.5 sccm are 1.9 × 1018 and 6 × 1017 molecules/s respectively, giving residence times of 1.15 and 3.7 s. These are significantly longer than the longest pulse period in our experiment, which was 20 ms. It should be noted that in RF systems of this design a fraction of the RF power is radiated away or lost as heat in the cables or matching unit. For this reason the quoted values of Emean here will be an overestimate. Unfortunately, we cannot presently determine the error in Emean, although in-house calculations would suggest approximately in the low 10% range. Figures 4a and 4b show the surface chemistry of the polymer films represented as retention of the COOH functionality from the monomer versus pulse off-time and Emean, respectively. As discussed above, it is apparent that retention is greatest at long off-times and corresponding low Emean values. Significantly, when the COOH concentration is recast as a function of the Emean parameter, it can be seen by the overlapping trends for the different power-flow combinations in Figure 4b that retention has a unique relationship with Emean. At Emean ≈ 100 eV/ molecule, there is little carboxylic acid functionality retained from the monomer in the films. In this case, corresponding to roughly 100 µs off-times, the majority of the remaining oxygen in the films is present in ester, ether/alcohol, and carbonyl functionalities (Figure 3). As Emean decreases (below 2-10 eV/ molecule), films have increased concentrations of the COOH functionality, approaching 50-60% retention with small amounts of ester, ether, and carbonyl functionalities. These observations are consistent with an energy deficient regime below 2-10 eV/ molecule and a monomer deficient regime above this value, representing a greater energy input per molecule and consequent increased fragmentation of the structure.10 Through the use of the quartz microbalance, the film deposition rates were measured and are shown in Figure 5. At long off-times of 20 000 µs the rates fall by at least a factor of 2 or 3 from the peak values seen with an off-time of 100 µs. Interestingly, deposition rate is not directly proportional to total power input. This relates to the initiation of deposit-forming
Voronin et al. reactions, the action of which extends significantly into the offtime as seen previously in this and other pulsed systems.24 It is clear that increasing the flow rate 3-fold from 1.5 to 4.8 sccm results in a 3-fold increase in the deposition rate. It is informative to calculate the proportion of monomer converted into deposit. From the above gas flow figures, we can calculate that at 4.8 sccm, 230 µg s-1 of acrylic acid flows into the reactor and 70 µg s-1 at 1.5 sccm. Comparing this to the measured deposition rates (∼20-200 µg m-2 s-1), and if we assume 0.25 m2 of surface area within the reactor, then this equates to a conversion of monomer into deposit of between a few percent at low Emean values (60% can be achieved at low Emean values
