23086
J. Phys. Chem. B 2005, 109, 23086-23095
Mechanistic Studies of Plasma Polymerization of Allylamine Andrei Choukourov,*,† Hynek Biederman,*,† Danka Slavinska,† Luke Hanley,‡ Andrey Grinevich,† Hanna Boldyryeva,§ and Anna Mackova§ Charles UniVersity, Faculty of Mathematics and Physics, Department of Macromolecular Physics, V. HolesoVickach 2, 18000 Prague, Czech Republic, UniVersity of Illinois at Chicago, Department of Chemistry, Chicago, Illinois 60607-7061, and Nuclear Physics Institute of the Academy of Sciences of the Czech Republic, Rez near Prague, 250 68 Czech Republic ReceiVed: June 30, 2005; In Final Form: October 10, 2005
Plasma polymerization of allylamine is performed both in continuous wave and pulsed mode. Chemical derivatization is applied to determine primary and secondary amine concentration. Primary amines are efficiently formed, but secondary amines are more abundant. A polymerization mechanism is proposed to account for the difference in amine content obtained from comparison between continuous wave and pulsed mode plasma polymerization. The AFM measurements performed on ultrathin (1-10 nm) plasma polymers confirm the continuity of films and that the film growth on silicon occurs via a layer-by-layer mechanism because no islandlike structures were detected.
Introduction Plasma-enhanced chemical vapor deposition (PECVD) of organic compounds, also called plasma polymerization, is used to deposit polymer-like thin films by glow discharge of organic monomer vapors. The first kinetic models of plasma polymerization date back to the 1960s.1-3 It is generally recognized that plasma polymer formation occurs by a combination of processes that involve gas-phase reactions and those at the gas-surface interface. Plasma deposition conditions determine the relative contribution of each mechanism to the overall deposition process. Yasuda4 proposed that plasma polymer deposition occurs by two mechanisms: plasma-state polymerization and plasmainduced polymerization. Plasma-state polymerization is the deposition of polymer-forming intermediate species produced in the plasma and is believed to be the main route of polymer formation. However, in cases where the monomer contains polymerizable structures (e.g., vinyl groups), conventional molecular polymerization on the surfaces activated by plasma can occur. This latter mechanism is referred to as plasmainduced polymerization. Westwood, Denaro, Lam, and Tibbitt,1 by working with vinyl chloride, styrene, and unsaturated hydrocarbons at a pressure of 133 Pa, found conventional polymerization reactions to be possible in plasma polymerization. An activated-growth model (AGM) has been suggested by d’Agostino3 for plasma polymerization from fluorocarbon feeds. The film is established to grow via the reactions of CFx radicals with activated sites on polymeric surfaces. Favia,2 by analyzing the pulsed plasma polymerization of unsaturated fluorocarbons, suggests that the film deposition is a combination of growth via the AGM mechanism when the plasma is on, and of a conventional polymerization when the plasma is off. * Corresponding authors. E-mail:
[email protected] (A.C.);
[email protected] (H.B.). Telephone: +420-221912367. Fax: +420-221912350. † Charles University, Faculty of Mathematics and Physics. ‡ University of Illinois at Chicago, Department of Chemistry. § Nuclear Physics Institute of Academy of Sciences of Czech Republic.
Extensive research has examined the retention of amines in films produced by plasma polymerization of ethylenediamine,5-15 diaminopropane,16 butylamine,17-22 heptylamine,23,24 aniline,25-27 diaminocyclohexane,6,28-31 and acetonitrile.32-34 However, allylamine is the most commonly utilized organic precursor.22,31,35-64 The performance of devices constructed with amine-containing plasma polymers depends strongly on their composition. Aminecontaining plasma polymers can be utilized in a number of applications. Examples include the modification of wastewater purification membranes,14,17,18,31,47,48,53,56 carbon nanotubes,10 and polymer microspheres,8,51,59 the improvement of biocompatibility of artificial materials and immobilization of biomolecules for biomedical applications,11,20,23,28,30,36,37,39,43,50,52,60,61,63-65 and the fabrication of sensors for quartz crystal microbalance,13,21 surface plasmon resonance,12,15 or enzyme electrode5,7,9,22,33,34 systems. Despite the abundance of research done on the preparation and characterization of amine-containing plasma polymer films, there remain questions regarding both chemical composition and dependence on the deposition parameters. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are commonly used for plasma polymer analysis, but they cannot readily quantify amine content. Only a few attempts have been made to estimate the concentration of amines in plasma polymers. Data provided by different groups vary significantly, ranging from 1.5 to 12 amine groups per square nm19,29,52 or from 3 to 32% of nitrogen atoms.33,35,51 Furthermore, the methods employed for the calculation of amine content often do not detect specifically primary amines, which are required for the immobilization of biomolecules. The diversity of processes that occur in plasmas complicates determination of the mechanism of amine film deposition. The objective of this work is to determine the composition of allylamine plasma polymers (pp-allylamine), with a focus on the estimation of the primary and secondary amine content. Pulsing the plasma has been frequently applied to control surface chemical functionality, either by keeping the peak power constant while off-time (and average power) is varied32,35,37,38 or on a constant average power basis with different peak
10.1021/jp0535691 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/11/2005
Plasma Polymerization of Allylamine powers.66 These two approaches are compared here, with the data used to propose a mechanism for plasma polymerization of allylamine. Experimental Section The plasma polymerization experiments were performed in a glass tubular reactor with external ring electrodes and standard excitation frequency of 13.56 MHz. The reactor was pumped by rotary and diffusion pumps, protected by a liquid nitrogen trap, to a base pressure of 1 × 10-3 Pa. The tank with liquid monomer was connected with the reactor via a needle valve, which controls the flow rate of monomer vapors. Most depositions were performed with a pressure of 25 Pa and a flow rate of 2.5 sccm. A limited number of experiments were performed at 100 Pa working pressure. Both continuous wave (CW) and pulsed modes were used, where duty cycle and average power characterize the pulsing process. The duty cycle was D ) ton/(ton + toff), where ton and toff are the duration of periods when the discharge is on and off, respectively. Average power is expressed as Pav ) Ppeak‚D, where Ppeak is the discharge power during ton. The average power ranged from 2 to 20 W, and the duty cycle ranged from 1 to 0.1, with ton of 2 ms utilized for most pulsing experiments. A series of pulsing experiments with varying ton was also performed. After each deposition experiment, the monomer flow was shut off and the samples were left in the chamber under vacuum for 30 min to minimize the concentration of radicals trapped in the films. This is done to reduce the uptake of oxygen by the films upon exposure to air. Aluminum foil, glass, and intrinsic silicon wafers were used as substrates. Infrared measurements were performed by using a Nicolet Impact 400 Fourier transform infrared (FTIR) spectrophotometer. The FTIR spectra of allylamine plasma polymer (ppallylamine) deposited on intrinsic silicon wafers were acquired in transmittance mode, with an uncoated silicon wafer used to record the reference spectrum. X-ray photoelectron spectra (XPS) were acquired by using a high-resolution monochromatic Al KR X-ray source (15 keV, 25 mA emission current, VSW MX10 with 700 mm Rowland circle monochromator), and a 150 mm concentric hemispherical electron energy analyzer (VSW Class 150) operated in constant energy analyzer mode. The photoemission angle was normal to the surface. The pass energy was 22 eV to collect highresolution spectra and 44 eV for survey spectra. The XPS peaks of allylamine deposited on aluminum substrates were charge referenced to the aliphatic C 1s (C-C, C-H) component at 285.0 eV. XPS peak positions were determined with an accuracy of 0.1 eV. Atomic concentration percentages were calculated from the high-resolution peak areas by using elemental sensitivity factors and the transmission function of the analyzer (VSW).67 Primary and secondary amine surface concentrations were obtained by performing derivatization reactions with gaseous reagents at their vapor pressure. For the detection of primary and secondary amine groups, two reagents were used:6,68 (a) trifluoromethyl benzaldehyde (TFBA), which reacts only with NH2, and (b) trifluoroacetic anhydride (TFAA), which reacts with NH2, NH, and OH. TFBA and TFAA underwent several freeze-thaw cycles to remove dissolved gases prior to use. XPS was applied afterward to determine the element composition of the films. Primary and secondary amine concentrations were derived as previously described.69 Test experiments showed that 30 and 10 min exposure times were required for TFBA and TFAA, respectively.
J. Phys. Chem. B, Vol. 109, No. 48, 2005 23087 The plasma polymer samples were prepared at Charles University in Prague and then shipped to the University of Illinois at Chicago for the derivatization/XPS analysis.The samples deposited on aluminum were cut into four pieces after plasma polymerization and stored in air. Two pieces were treated with TFBA for 8-10 and 60 days after deposition and the other two with TFAA at the same intervals. Thus, all derivatization measurements of a given coating were performed on exactly the same film. XPS were acquired immediately before and after the derivatization reactions. The surface of plasma polymers deposited on silicon was observed by AFM (Quesant, Q-Scope 850). The measurements were performed in an intermittent contact mode (BBWavemode) to eliminate artifacts induced by an AFM tip on a relatively soft plasma polymer surface. The scans were acquired in air at identical gains, amplitude, scan rate, and set point, and the average root-mean-square (RMS) roughness was calculated for each sample. RMS roughness ignores the information along the horizontal direction. It has been previously shown that analyzing the AFM data by using spectral methods can give additional information on surface periodicity.70-73 Power spectral density (PSD), determined over different spatial frequency regions, provides better surface characterizations because it is sensitive to both vertical and horizontal distribution of roughness. In this work, PSDs were taken after second-order plane fitting and flattening of AFM height images (parabolic line-by-line tilt removal, Quesant software). The height scans were acquired with 1 µm × 1 µm and 5 µm × 5 µm scan size on each sample to cover a wide spatial frequency spectrum. The resolution was set to 512 × 512 data points. PSDs obtained from different spatial frequency regions should overlap to validate the reproducibility of AFM measurements.70 Here, at least three overlapping PSDs at two scan sizes with different cantilevers were obtained to eliminate any inconsistency in the AFM measurements. The film thickness was determined by AFM as well. In this case, cellulose acetate film was prepared by drying a drop of 10 wt % solution of cellulose acetate in acetone on a silicon substrate. After the plasma deposition onto this film, the underlying cellulose acetate film was peeled off by using tweezers. This produced a very sharp edge in the plasma polymer, which was then scanned by AFM. The accuracy of the thickness measurements by AFM was estimated to be ∼10%. Results The FTIR spectra of pp-allylamine deposited at different powers are given in Figure 1. The assignment of IR bands is made according to ref 16 and summarized in Table 1. The spectrum in the absorption region from 3700 to 2700 cm-1 is a very broad superposition of different contributions. Primary and secondary amines (R-NH2, R-NH-R′) give rise to asymmetric (νas ∼ 3350 cm-1), symmetric (νs ∼ 3270 cm-1) N-H stretching, and symmetric (δs∼1590 cm-1) N-H deformation vibrations, which overlap with C-H stretches and other vibrational modes. Stretching vibrations of hydroxyl groups (-OH) are also observed. The elemental analysis of plasma polymer films (see below) reveals several percent of oxygen, which is less than half that of nitrogen. Previous research16 showed that hydroxyls are unlikely in amine-containing plasma polymers with a high nitrogen content, as the oxygen is bound mainly in amide species. Next to the amine absorption bands on the lower wavenumber side are CHx peaks. The symmetric and asymmetric stretching vibration peaks of the CHx groups are positioned at νs ) 2921
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TABLE 1: Assignment of FTIR Bands Observed in Amine-Containing Plasma Polymers wavenumber, cm-1
assignment
∼3350 ∼3270 ∼3200 2929 2855 2240 2182-2100
νas NHx νs NHx δas NHx νas CHx νs CHx Ν (-R-C≡N), ν (R-C≡C-R′) Ν (>CdCdO), ν (-NdCdN-), Ν (-R-C≡N), ν (R-C≡C-R′) ∆ NHx, ν CdC, ν CdN, ν CdO
1630
cm-1 and νas ) 2850 cm-1. The complex band in the region at ∼2200 cm-1 cannot be unequivocally assigned to a single species, as several functional groups lead to vibrations in this region: unconjugated R-CtC-R′ and/or R-CtN appear at 2240 cm-1, and conjugated nitriles and various unsaturated structures such as >CdCdO and -NdCdN- appear at 21822100 cm-1. Nor can the peak at 1630 cm-1 be definitely assigned, as it could be attributed to the deformation vibration of amines, CdO based functionalities, CdC, and/or CdN stretching vibrations. Despite these ambiguities, Figure 1 demonstrates that the concentration of triply and doubly bound groups significantly rises as plasma power increases. This is consistent with recent NEXAFS findings that suggest an increase in the level of unsaturated bonds in the allylamine plasma polymer with discharge power, mainly due to the formation of nitrile groups.74 The nitrogen content of the allylamine plasma polymer is very close to that of the allylamine precursor (Table 2). Higher discharge power results in films with slightly lower nitrogen content. The XPS data do not provide information on hydrogen content. However, our Rutherford backscattering spectroscopy
Figure 1. FTIR spectra of allylamine plasma polymers deposited at different discharge powers (CW).
comments amines, imines, amides various structures CH3, CH2, CH nonconjugated triple-bond structures conjugated nitriles and various unsaturated structures amines, amides, carboxyls
TABLE 2: Elemental Composition of pp-Allylamine Films (ton ) 2 ms, 2.5 sccm) (a) aged for 8-10 days elemental composition, atom % N O C/N
power (average), W
duty cycle
C
2 5 20 2 5 20
CW CW CW 0.1 0.1 0.1
68 74 76 70 72 72
25 19 20 23 20 19
7 7 4 7 8 9
2.8 3.9 3.8 3.1 3.7 3.9
(b) aged for 60 days elemental composition, atom % N O C/N
power (average), W
duty cycle
C
2 5 20 2 5 20
CW CW CW 0.1 0.1 0.1
65 70 73 68 68 71
23 17 17 21 19 17
12 13 10 11 13 12
2.8 4.0 4.2 3.3 3.6 4.1
and elastic recoil detection (RBS/ERDA) experiments on ethylenediamine (pp-EDA), diaminocyclohexane (pp-DACH), and allylamine plasma polymers confirm that these coatings are very deficient in hydrogen. pp-EDA contains 35 atom % of hydrogen, which is half of the monomer value (67 atom %). The loss of hydrogen in pp-DACH is not as great but still significant at ∼43% in the plasma polymer vs 64% in the monomer. The films of pp-allylamine deposited at 2 and 20 W display a similar loss of hydrogen to that of pp-DACH. The derivatization reactions on the pp-allylamine films were performed both 8-10 and 60 days after deposition (Figure 2a). The total amine concentration is lower for the films deposited at higher powers and, hence, with higher energy of ions in the plasma. This is consistent with ion deposition data,69 which show that higher-energy ions produce intensive fragmentation of surface species during film formation. The results of plasma polymerization indicate that the retention of primary amines is limited and most amines are of the secondary type. The primary amine concentration reaches 7% for 2 W CW films vs 4% for 20 W CW films. The secondary amine content is even more power dependent. For the 2 W CW film, it is 2 times higher (14%) than for the 20 W coating (6%). By contrast, films deposited from mass-selected allylamine ions69 contain only 2% of NH2, a value much lower than that observed here by using plasma polymerization. The extent to which the primary amine content is influenced by OH groups is shown by the elemental composition analysis given in Table 2. When kept in air, the pp-allylamine films take up oxygen, which increases from ∼8% after 8-10 days to ∼12% after 60 days of storage. At the same time, the total amine/hydroxyl amount stays remarkably stable over these periods for both 2 and 20 W samples (Figure 2a). The oxidation of plasma polymers deposited in CW (Figure 2a) and the pulsed
Plasma Polymerization of Allylamine
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Figure 2. Primary and secondary amine concentration in allylamine plasma polymers: (a) effect of power and aging; (b) effect of power and pulsing.
mode (data not shown) do not strongly affect the primary and secondary amine content. The concentration of primary amines reduces only slightly over a period of 2 months. Hence, the contribution of hydroxyls, if any have formed after 8-10 days, is stable and does not change much after 60 days. It was reported35 that the pulsed mode is very effective for the retention of the various active functionalities and amines in particular. The absence of ion bombardment during toff time and the presence of a variety of active species (i.e., radicals) favor the incorporation of functional groups into the growing film. Recently, plasma polymerization of allylamine was performed at constant peak power of 200 W and at two different duty cycles.35 The longer off-times are reported to produce films with higher amine concentration, which was determined by derivatization with TFAA. The authors did not consider the contribution of OH groups in the derivatization results and correlated the fluorine concentration after the reaction only with amines. While OH groups could have contributed to those results, we have found here that hydroxyls do not play an important role in aging of pp-allylamine. Previous experiments performed on a constant average power basis (i.e., with different peak powers) revealed ambiguity in CW against pulsed mode.2 A significant difference between acrylonitrile plasma polymers prepared in CW and pulsed mode was observed. This prior work found film composition to be strongly affected only by variation of average power. Our comparison of films deposited in CW vs pulsed mode at constant average power (Figure 2b) indicates that the total amine concentration does not depend strongly on the mode of operation (although the concentration of primary amines is slightly higher for the CW samples). Here, when considered on the basis of constant peak power, pulsing does not affect the concentration of primary amines. Primary amine content is the same for films deposited at 20 W in CW mode and at 2 W average power, D ) 0.1 (with 20 W peak power) in a pulsed mode (Figure 2b). However, the retention of secondary amines is more effective in the case of pulsed pp-allylamine. In accordance with ref 35, the total amine concentration is higher for the pulsed polymerized samples. Thus, the observed increase in amine retention with longer off-times in our experiments is due to the secondary amines. Overall, we found that the discharge power is the main parameter controlling film composition. The films deposited at
2 W power in CW mode have twice as much total amine than those deposited at 20 W power. The difference between the pulsed polymerized samples is not large, but the trend is the same. Similarly to CW, the films deposited at 2 W average power in pulsed mode have higher total amine concentration than those deposited at 20 W average power. The amine content observed here falls within the range reported for allylamine plasma polymers prepared by other authors and exceeds that of films deposited from other monomers (Table 3). Further consideration of the plasma polymerization data can provide useful mechanistic information. Plasma polymerization of allylamine was performed at 2 W and 25 Pa in continuous wave mode with different deposition times. The calculated deposition rate of 0.44 ( 0.10 nm/min was found to be constant within at least half an hour of deposition. Then, a series of depositions in the pulsed mode was performed with 2 W average power and duty cycle 0.1, but with different pulse intervals. Each deposition lasted for 10 min. Figure 3 shows the results in terms of film thickness as a function of off-time. Obviously, there is an interval of off-times between ∼5 and ∼30 ms, where the deposition rate is much higher than that for CW at the identical conditions. Out of the above range, the film growth is slower approaching the CW value. Here, we assume that the ion bombardment during on-time plays a relatively destructive role in film formation. By taking into account that the characteristic time of bias decay during toff is < 1 ms,76 there is still a certain ion flux on the surface at shorter off-times, and the deposition resembles that in CW mode. At the toff > 5 ms, the influence of charged particles on the polymerization processes when the plasma is off is minor. The film grows mainly via the reaction of molecular fragments or intact monomer molecules with surface radicals. However, hyperthermal polyatomic ions may also contribute at some extent to film formation, as will be discussed below. The deposition rate reaches a maximum for off-times of 1030 ms. The decrease in deposition rate at the longer off-times is readily explained by completing recombination of the surface radicals with volume species. Apparently, at toff > 30 ms, the majority of reactive species are already recombined. The RMS roughness of the above pp-allylamine samples was measured at 1 µm × 1 µm and 5 µm × 5 µm scan size. It is essential to indicate the scan size when referring to roughness because the latter is known to be strongly scan-size depend-
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TABLE 3: Amine Concentration in Plasma Polymers (a) per unit area amine concentration
monomer
method of estimation
1.5 per nm2
butylamine/Ar mixture
contact angle measurements in dependence on pH
0.20-0.38 per nm2
DACH
5-12 per nm2
allylamine
17-50 nmol/cm2
allylamine + Ar
electrokinetic measurement of the pH dependence of electroosmic fluid flow derivatization with trinitrobenzene sulfonic acid with successive spectrophotometer measurements colorimetric staining with acid orange II
experimental conditions MW 2.45 GHz, 600 W, 80 Pa, duty cycle 0.25, ton 2 ms rf 200 kHz, 10 W, 5 Pa, CW
ref 19 29
rf 13.56 MHz, 30-50 W, 13-26 Pa, CW
52
MW 2.45 GHz, 400-1200 W, 50 Pa, CW and pulsed (duty cycle 10-90%), samples positioned downstream after glow discharge
64
(b) per 100 carbon atoms amine concentration amines per 100 C atoms % of N atoms
a
monomer
method of estimation
6a
21 (NH2 + NH)
allylamine
derivatization with TFAA
16a
36 (NH2 + NH)
allylamine
derivatization with TFAA
9a
32 NH2
acetonitrile
derivatization with pentafluorobenzaldehyde
3-9 (NH2)
allylamine
derivatization with Fmoc-Cl
1-2 (NH2)
DACH
derivatization with TFBA
3-9 (NH)
DACH
derivatization with TFAA
1 (NH2)
EDA
derivatization with TFBA
12-18 (NH)
EDA
derivatization with TFAA
experimental conditions rf, 200 W (peak), 26 Pa, duty cycle 0.3750, ton 3 ms, flow rate 3.6 sccm rf, 200 W (peak), 26 Pa, duty cycle 0.0625, ton 3 ms, flow rate 3.6 sccm rf 13.56 MHz, 80 W (CW), 1.6 Pa, flow rate 15 sccm rf 13.56 MHz, 20 W (CW), 40 Pa, flow rate 15 sccm rf 13.56 MHz, 2-30 W (CW), 5 Pa, flow rate 0.85 sccm rf 13.56 MHz, 2-30 W (CW), 5 Pa, flow rate 0.85 sccm rf 13.56 MHz, 2-30 W (CW), 5 Pa, flow rate 0.85 sccm rf 13.56 MHz, 2-30 W (CW), 5 Pa, flow rate 0.85 sccm
ref 35
33 51 75
Calculated here from published data.
ent.70,72 Here, the roughness calculated from 1 µm × 1 µm and 5 µm × 5 µm scans is denoted RMS1µ and RMS5µ, respectively. At the 5 µm scan size, the roughness averages around 9 ( 2 Å without any dependence on duration of off-time or thickness. At the 1 µm scan size, the roughness shows a slight trend to increase with toff from 3 to 6 Å (data not shown). No dependence on thickness was detected either. The following depositions were performed in CW mode with 2 W power, at 25 and 100 Pa pressure with different deposition times, other parameters being held constant. As a result, two sets of the samples with different film thickness were prepared. Figure 4 shows the dependence of roughness on plasma polymer thickness. The films deposited at 25 Pa are very smooth with RMS5µ of about 1.0-1.5 nm, which does not change with thickness. On the contrary, at 100 Pa, the film growth is more complex. A significant increase in the film roughness is observed in the initial stages of film formation. The RMS5µ roughness
increases from 0.3 ( 0.1 nm, the value of the uncoated silicon substrate, to 6.5 ( 0.2 nm for the film with 3 nm thickness. Figure 5 demonstrates the difference between the films deposited at 25 and 100 Pa as observed by AFM. The thickness is about 10 nm in both cases. The left darker side of the AFM images corresponds to the silicon substrate after the mask had been removed. The film deposited at 25 Pa is smooth and featureless, whereas the film deposited at 100 Pa is twice as rough and composed of columnar structures. Continued film growth at 100 Pa is characterized by a decrease in roughness (Figure 4). At 18 nm thickness, the roughness reaches the value of the films deposited at 25 Pa. No difference is observed for the films thicker than 18 nm deposited at 25 vs 100 Pa. The PSD calculations are in a good agreement with the RMS data. As mentioned above, the most prominent differences in surface structure between the films deposited at 25 and 100 Pa
Plasma Polymerization of Allylamine
J. Phys. Chem. B, Vol. 109, No. 48, 2005 23091
Figure 3. Thickness of pp-allylamine vs off-time (25 Pa, Pav ) 2W, D ) 0.1, 10 min). The dotted line at 4.4 nm depicts the value for the film deposited in CW at 2 W.
Figure 5. AFM images of pp-allylamine deposited CW at 2 W on Si at (a) 25 Pa, to a thickness of 10 nm; (b) 100 Pa, to a thickness of 8 nm.
Figure 4. RMS roughness of pp-allylamine deposited at 25 and 100 Pa pressure as a function of film thickness (2 W, CW, scan size 5 µm).
occur when the film thickness reaches 3 nm. The PSDs were taken for these samples and for the silicon substrate (Figure 6). It is worth noting that, typically, smoother surfaces have lower PSD curves and periodic features appear as peaks at characteristic spatial frequencies. Both 25 and 100 Pa deposited films replicate the silicon substrate at frequencies higher than 90 µm-1, which is equivalent to wavelengths lower than 11 nm. At the lower frequencies, both curves go above that of silicon, indicating the increase in film roughness. At frequencies lower than 20 µm-1 (wavelength >50 nm), the curve of 100 Pa lies significantly higher than in the case of 25 Pa deposited film. We attribute this effect to a development of columnar structures, which enhance both vertical roughness and lateral dimensions. The region between 25 and 50 µm-1 is remarkable in the domination of these frequencies for the 25 Pa sample compared to that of the 100 Pa sample. For thicker films, the PSD of 25 and 100 Pa deposited films (not shown) reveal similar behavior supporting the RMS data. The cross-section determined from the AFM images for the films deposited at 100 Pa is shown in Figure 7. For the films thinner than 10 nm, the roughness is of the same scale as the thickness. The term “film thickness” is defined here as the distance between mean lines laid along the profiles of the film and the substrate, as shown in Figure 7 for the 3 nm film.
Figure 6. Spectral analysis of pp-allylamine deposited at different pressures on silicon substrate.
The film continuity at small thickness is a significant issue. Referring to IUPAC recommendations,77 a film is discontinuous if it consists of discrete islands of material, on the substrate, without physical connection. A network film consists of partially connected islands. Otherwise, a film is considered continuous. The lowest thickness studied in this work is 1 nm. Even at such a low thickness, the film is composed of a dense array of cone structures replicating the morphology of the substrate (Figure 8). The identity of film and substrate morphology is confirmed by the film PSD displaying a substrate-like behavior in a wide range of spatial frequencies (Figure 9). At the frequencies higher 50 µm, the PSDs of the film and substrate coincide. The lower
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Figure 9. PSDs of a pp-allylamine film (100 Pa, 2 W, thickness 1 nm) and the silicon substrate.
Figure 7. Depth profile of pp-allylamine deposited on Si (100 Pa, 2 W).
By using the above terminology, pp-allylamine films with thickness > 5 nm are continuous. The thinner films manifest profiles with the valley bottoms at the substrate level. It is difficult to speculate whether the valleys are filled with some layers of plasma polymer or whether the silicon substrate is exposed here. However, the dense arrangement of the columns allows the conclusion that, down to 1 nm thickness, the films are at least of a network type. For thinner films, it is problematic to establish with AFM whether the growth of pp-allylamine passes through the island formation (Volmer-Weber mechanism) or it occurs via a layer-by-layer mechanism (Frank-van der Merwe). The AFM high-frequency limit at 512 × 512 resolution for the scan size of 1 µm is 512/(2 × 1) ) 256 µm-1, which corresponds to the sampling interval of 1/0.256 ) 3.9 nm. The lateral details below this limit are not accessible, and an existence of smaller islands in films thinner than 1 nm cannot be absolutely ruled out. Because no indication of plasma polymer nucleation was observed within the AFM detection limit, the pp-allylamine film formation occurs predominantly via a layer-by-layer mechanism. This is consistent with the previous results on ultrathin (∼2 nm) SiO2 coatings deposited by PECVD of hexamethyldisiloxane,78 the authors showing that plasma polymer completely covers the substrate and does not manifest an islandlike structure. Here, the surface of the silicon substrate is not atomically flat, and the film repeats the topography of the substrate at the early stages of growth. The columnar structures originating from the substrate roughness enlarge both in vertical and lateral directions until the film reaches 3 nm thickness. From this point, growth proceeds via lateral expansion of the columnar structures. At 8 nm thickness, the film is already continuous, with valleys between peaks filled with plasma polymer. Further growth leads to smoothing of the film morphology. Discussion
Figure 8. AFM image of pp-allylamine on silicon (100 Pa, 2 W, thickness 1 nm).
frequencies have a stronger impact on the film’s PSD compared to that of the substrate, indicating the formation of larger structures. Note that, in this case, the transition to the substratelike behavior occurs at a much lower spatial frequency than for the thicker film (Figure 6), which is attributed to the enhanced vertical and lateral dimensions of columnar structures of the thicker coating. No islandlike structures are observable by AFM.
Among the other species present in the glow discharge of allylamine, intact monomers are most abundant, within a certain range of rf power below 50 W.79 Hence, they can be expected to be major participants of polymerization involving free radical additions to the carbon-carbon double bond and thus retention of the -NH2 groups in the resulting films (Scheme 1, reaction 1). The same reaction was previously proposed as a dominant route for plasma polymerization of allyl alcohol,76 which only differs from allylamine in that an -OH group replaces an -NH2 group. Lower duty cycles (longer toff) at constant peak power
Plasma Polymerization of Allylamine SCHEME 1. Mechanism of Plasma Polymerization of Allylamine
were found to result in an increase of hydroxyls in the films. Radicals created in the film during ton due to photon and ion bombardment recombined with the allyl group of the monomers during toff, leading to an enhanced retention of -OH groups. By analogy, reaction 1 might also be valid for allylamine. However, no change in retention of primary amines with duty cycle is detected (Figure 2), as would be expected for reaction 1. A 20 W discharge in CW mode produces a film with approximately 4 NH2 groups per 100 C atoms. The same value is obtained for a 20 W peak power discharge with 0.1 duty cycle, where the average power is 2 W. The overall increase in amine concentration in the pulsed mode compared to CW is attributed to secondary amines. This indicates that reaction 1 alone cannot account for all the polymerization observed here. Yasuda4 stressed the importance of hydrogen detachment in plasma polymerization (Scheme 1, reaction 2). According to his results, the gas phase after polymerization in a closed system consists mainly of hydrogen. The deficiency of hydrogen in the plasma polymer compared with the corresponding monomer was also attributed to hydrogen detachment.4 Our RBS/ERDA results on a related series of films (see above) suggest that aminecontaining plasma polymers contain significantly less hydrogen than the monomer. Several conclusions follow from the above considerations and the results of the deposition rate calculations: (1) There is significant film formation during the plasma-off period. The contribution of charged particles to the polymerization processes when the plasma is off is secondary with toff in the millisecond range. (2) Hydrogen is readily detached from the plasma polymer during ton.
J. Phys. Chem. B, Vol. 109, No. 48, 2005 23093 TABLE 4: the Deposition Rate of EDA-pp and pp-allylamine monomer
deposition rate (nm/min)
EDA Allylamine
8 29
experimental parameters 5 W, 5 Pa, CW, 2.5 sccm, as-deposited
(3) Previous results on the mass spectra of neutrals and positive ions in allylamine plasmas79 show that the intact precursor molecules are the most abundant. However, a smaller amount of dimeric and trimeric species are also reported. While gas-phase polymerization contributes to some extent to the polymerized film, polymerization at pressures 20%). Furthermore, the ion deposition rate with the fluences typical for the glow discharge conditions is found to be of several nanometers per hour, a value too small compared with several nanometers per minute in plasma deposition. Hence, the neutrals and radicals are the dominant contributors to the total mass of the plasma polymer, with hyperthermal ions playing a secondary role. When the allylamine plasma is on, the film formation process is a combination of plasma-state and plasma-induced polymerization. It is difficult to speculate on the extent to which each of these mechanisms contributes to film growth during ton. However, it is useful to compare deposition rates of allylamine and EDA. EDA (NH2-CH2-CH2-NH2) does not readily polymerize thermally due to its absence of double bonds. Hence, the plasma polymerization of EDA runs exclusively via the plasma-state mechanism. The deposition rate of allylamine in CW mode at 5 Pa is almost 4 times higher than that of EDA deposited under identical conditions in the same experimental apparatus (Table 4). At constant pressure, the mass of allylamine and EDA precursor molecules contained in the reactor volume
23094 J. Phys. Chem. B, Vol. 109, No. 48, 2005 is almost the same given their similar molecular weights. Thus, the difference in deposition rate between EDA and allylamine cannot be accounted for by different mass transfer on the surface. Hence, plasma-induced reaction of intact allylamine molecules with surface radicals is a major mechanism of film formation at the mentioned conditions, even during ton. The gas-phase formation of free radicals via molecule-electron interactions may be more significant at 25 Pa pressure, and a considerable contribution of plasma-state polymerization cannot be ruled out. Because the deposition rate per period is very low, the surface chains undergo multicycle exposure to plasma before they are shielded by the outer layers. This leads to the continuing loss of hydrogen with formation of non-amine nitrogen species (Scheme 1, reactions 5, 6) and unsaturated structures. Only a small amount of NH2 groups, incorporated in abundance when plasma is off, survive in plasma polymer during ton. But because there is a reduced ion and photon exposure during toff, the formation of non-amine nitrogen species is limited and the plasma polymer is rich with secondary amines in the pulsed mode. Conclusions Plasma polymerization of allylamine is effective for the deposition of amine-rich coatings. With low discharge power, amines are bound to more than 20% of carbon atoms, with secondary amines roughly twice as abundant as primary amines. The preferential route of film formation is plasma-induced reaction of allylamine molecules with surface radicals, detachment of hydrogen, and chain propagation via formation of secondary amines or non-amine nitrogen species. Hyperthermal polyatomic ions are secondary contributors to film formation. The pulsing with peak power equivalent to CW mode limits the formation of non-amine moieties and favors the retention of secondary amines. Plasma polymers deposited in CW and pulsed mode with equivalent average power are closer in composition, primary amine content being slightly lower in pulsed polymerized films. Both primary and secondary amines are found to be stable to oxidation on air, with NH2 concentration slightly decreasing with time. The results obtained agree with those reported by others in that the lower power discharges and pulsed plasmas generally produce films with higher amine concentrations. We have shown that this increase is mainly due to secondary amines. The values of amine content of pp-allylamine calculated here are close to those reported by Timmons and co-workers35 and are significantly higher than the amine concentrations of plasma polymers deposited from other amine-bearing precursors. The AFM analysis shows that even very thin (
