Characterization of Pulsed-Plasma-Polymerized Aromatic Films

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Characterization of Pulsed-Plasma-Polymerized Aromatic Films Neil M. Mackie, David G. Castner,† and Ellen R. Fisher* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872 Received August 25, 1997. In Final Form: December 20, 1997 Pulsed plasma polymerization is used to produce aromatic thin films from inductively coupled rf plasmas with benzene, 1,2,4-trifluorobenzene, and hexafluorobenzene as monomers. The effects of aromatic monomer fluorination and duty cycle variation on the resulting films’ properties are examined. The surface and bulk properties of the films are determined using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), static contact angles, and differential scanning calorimetry (DSC). Analysis using these techniques shows a strong dependence of film chemistry and deposition rates on the plasma duty cycle. Systematic changes in film chemistry are observed with the lowest duty cycles producing films containing substituted aromatic rings. On the basis of the SIMS, XPS, and DSC data, films deposited under the optimal pulsed conditions are similar to polystyrene in structure but are more complex, somewhat cross-linked networks. These studies show pulsed plasma polymerization affords high control over film chemistry and properties.

I. Introduction Plasma deposition of thin organic films is becoming an increasingly popular approach to synthesize new types of materials1 or as an alternative route to well-characterized organic polymer films for modifying substrates with complicated geometries.2 Most plasma polymerization research has concentrated on the utilization of continuous wave (CW) plasma sources.3 Unfortunately, CW plasmas can significantly fragment and scramble functional groups of the starting monomer, producing a film that has little in common with the original monomer. Minimization of the input power (W) to monomer flow rate (F) ratio increases monomer functional group retention in the deposited film.4 Unfortunately the W/F range where this is effective can result in powdery or oily deposits and the plasma may be difficult to initiate and sustain.5 Our research in this area has concentrated on an alternative route to functional group control using pulsed plasmas.6-8 Use of pulsed sources to create organic films has been shown to reduce trapped radicals in the film, lower deposition surface temperatures, decrease highenergy ion bombardment and UV flux to the surface,9 and provide greater control over the resulting film chemistry. We have previously reported the use of pulsed rf plasmas * To whom correspondence should be addressed. † National ESCA and Surface Analysis Center for Biomedical Problems, Department of Chemical Engineering, Box 351750, University of Washington, Seattle, WA 98195. (1) Yasuda, H. Plasma Polymerization; Academic Press: Orlando, FL, 1985. (2) Grill, A. Cold Plasmas in Materials Fabrication; IEEE Press: New York, 1994. (3) Lieberman, M. A.; Lichtenberg, A. J. Principles of Plasma Discharges and Materials Processing; John Wiley and Sons: New York, 1994. (4) Ho, C.-P.; Yasuda, H. J. Appl. Polym. Sci. 1990, 39, 1541-1552. (5) Inagaki, N. Plasma Surface Modification and Plasma Polymerization; Technomic: Lancaster, PA, 1996. (6) Mackie, N. M.; Dalleska, N. F.; Castner, D. G.; Fisher, E. R. Chem. Mater. 1997, 9, 349-362. (7) Mackie, N. M.; Fisher, E. R. Polym. Prepr. 1997, 38, 1059-1060. (8) Lefohn, A. E.; Mackie, N. M.; Fisher, E. R. Langmuir, manuscript in preparation. (9) Panchalingam, V.; Chen, X.; Savage, C. R.; Timmons, R. B.; Eberhart, R. C. J. Appl. Polym. Sci., Polym. Symp. 1994, 54, 123-141.

to produce a variety of organic films with a high degree of controllability over film composition. The feed gases in those experiments were mixtures of saturated fluorocarbons (primarily C2F6) and H2.6 Here, we continue our work with fluorinated monomers and variable duty cycle pulsed inductively coupled rf (13.56 MHz) plasmas to deposit films from benzene, 1,2,4-trifluorobenzene (TFB), and hexafluorobenzene (HFB). Plasma-deposited aromatic polymer films have industrial relevance because of their interesting chemical, physical, electrical, and mechanical properties.10 The simplest aromatic monomer, benzene, has been studied in CW rf plasmas, since the 1960s.11 CW plasma deposition of fluorobenzenes has also been extensively studied. Clark et al. performed a complete series of experiments on the fluorobenzenes using X-ray photoelectron spectroscopy (XPS) and found that, at low plasma powers, their films retained some of the monomer structure, as evidenced by π-π* shake-up satellite features.12-14 The only previous report of deposition from pulsed aromatic systems comes from Yasuda and Hsu, who characterized films from pulsed deposition of benzene and HFB using ESR (100-µs on to 900-µs off ) 10% duty cycle).15 They found that films deposited from pulsed HFB plasmas have much lower trapped radical content than the corresponding CW deposited films. Contact angles of water on the C6H6 films changed from ∼20° under CW conditions to ∼80° with a pulsed plasma. Contact angles for C6F6 films were similar for both CW and pulsed depositions (∼88°). No other film characterization techniques were used in this study. The present study concentrates on the changes in (10) Bubenzer, A.; Dischler, G.; Koidl, P. J. Appl. Phys. 1983, 54, 4590-4595. (11) Stille, J. K.; Sung, R. L.; Vander Kooi J. Org. Chem. 1965, 30, 3116-3119. (12) Clark, D. T.; Shuttleworth, D. J. Polym. Sci. Polym. Chem. 1980, 18, 27-46. (13) Clark, D. T.; Abrahman, M. Z. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 1729-1744. (14) Clark, D. T.; Abrahman, M. Z. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 1717-1728. (15) Yasuda, H.; Hsu, T. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 81-97.

S0743-7463(97)00953-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/13/1998

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molecular structure of films deposited from aromatic monomers using variable duty cycle plasmas and different monomer fluorination while keeping other deposition parameters constant. One goal of this work is to explore the use of pulsed plasma systems to retain aromatic ring functionality in the deposited film. This is important for potential applications such as low-k interlevel dielectric (ILD) materials,16 biomaterials, or gas separation membranes,17 where thermal stability and low moisture absorption are desired. Our films are characterized by a fairly comprehensive set of analysis tools including XPS, FTIR, static secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and static contact angle measurements. These analyses afford information on both surface and bulk film composition and properties. II. Experimental Methods All films were deposited in our home-built inductively coupled rf plasma reactor, described previously.6,18 In these experiments, the applied peak power was kept constant at 400 W, with 5-10 W of reflected power. The pulse-on time and duty cycle (defined as the ratio of pulseon time to total cycle time) was varied using the internal pulse generator of an RF Power Products RF5S power supply. For each deposition, a freshly pressed FTIR-grade KBr (Aldrich) pellet and a silicon wafer (p-type, 110 with 40-60 Å of native oxide) were used as substrates. These were placed on glass microscope slides oriented parallel to the gas flow and 10 cm downstream of the coil region but still within the plasma glow. Pulsed deposition times ranged from 1 to 30 min, as defined by the total time the sample was exposed to both pulse-on and -off cycles. The reactant liquids benzene (C6H6), TFB (1,2,4C6F3H3), and HFB (C6F6) (all from Aldrich with 99% purity) underwent several freeze-pump-thaw cycles to remove trapped atmospheric gases. Reactant vapor pressure was controlled through a Nupro needle valve, and the total pressure in the reactor was maintained at 150 mTorr above a base pressure of 0-2 mTorr. Transmission FTIR spectra on KBr pellets and Si wafers were performed ex situ using a Nicolet Magna 760 FTIR spectrometer (resolution of 2 cm-1 and averaging over 100 scans). Spectra shown are corrected for residual carbon dioxide not purged from the FTIR spectrometer (absorbances at ∼2340 and 2360 cm-1). The film morphology for materials deposited on silicon substrates was determined using a Philips 505 scanning electron microscope, with an accelerating voltage of 20 kV and a spot size of 20 nm. The films were sputtered with 10 nm of gold prior to SEM analysis. Static contact angles for water were measured using the sessile drop method with a contact angle goniometer (Rame´ Hart Model 100). Measurements were taken on both sides of water drops at ambient temperature, 30-40 s after 1-µL drops were applied to the surface and the needle tip was removed from each drop. For each sample, six drops were placed at different locations on the surface of a 1000-Å-thick film. Each reported contact angle is an average of these measurements for three samples. Film thicknesses were obtained by masking a portion of the substrates during deposition and then measuring the resulting step height by profilometry (Tencor Alpha Step 100). XPS analyses were performed on the University of Washington Surface Science Instruments S-probe spec(16) Singer, P. Semicond. Int. 1996, 19, 88-96. (17) Terada, I.; Haraguchi, T.; Kajiyama, T. Polym. J. 1986, 18, 529. (18) Bogart, K. H. A.; Dalleska, N. F.; Bogart, G.; Fisher, E. R. J. Vac. Sci. Technol. A 1995, 13, 476-480.

Mackie et al.

trometer. This system has a monochromatic Al KR X-ray source (hν ) 1486.6 eV), a hemispherical analyzer, and a resistive strip multichannel detector. A low-energy (∼5 eV) electron gun was used for charge neutralization on the nonconducting samples. The binding energy (BE) scales for the samples were referenced by setting the CHx peak maxima in the C1s spectra to 285.0 eV. The highresolution C1s spectra were acquired at an analyzer pass energy of 50 eV and an X-ray spot size of 1000 µm. XPS elemental compositions of samples were obtained using a pass energy of 150 eV. All XPS analyses were performed at a photoelectron takeoff angle of 55°. Time-of-Flight Secondary Ion Mass Spectrometry. The Time-of-flight secondary ion mass spectrometry (TOFSIMS) data were acquired using a Model 7200 Physical Electronics instrument (PHI, Eden Prairie, MN) with a 8-keV Cs+ primary ion source. Data were acquired over a mass range from m/z ) 0 to 450 for both positive and negative secondary ions. The ion beam was moved to a new spot on the sample for each spectrum. The total ion dose used to acquire each spectrum was less than 2 × 1012 ions/cm2. The area of analysis for each spectrum was 0.01 mm2. The secondary ions were extracted into a two-stage reflectron time-of-flight mass analyzer and then postaccelerated and converted to charge pulses by a stacked pair of chevron-type multichannel plates. The mass resolution (m/Dm) of the negative secondary ion peaks was typically between 5000 and 9000. The mass scale for the positive secondary ions from the benzene films was calibrated using the C2H3 (27.0235), C3H5 (41.0391), C6H5 (77.0391), and C7H7 (91.0548) peaks. The mass scale for the positive secondary ions from the HFB films was calibrated using the C2H3, CF (30.9984), C3H5, C4H7 (55.0548), C6F5 (166.9920), and C7F7 (216.9888) peaks. The difference between the expected and observed masses for the positive hydrocarbon calibration ions was less than 10 ppm. The positive fluorocarbon calibration ions exhibited a slightly higher (10-20 ppm) difference between the expected and observed masses. The thermal properties of the deposited polymers were determined using a Rheometric Scientific differential scanning calorimeter (DSC). Plasma-deposited films were scraped off the wall of the plasma reactor, ground finely, and transferred to Al crucibles. A temperature program consisting of a 2-min isotherm at 40 °C, a temperature ramp from 40 to 500 °C at 10 °C/min, a 5-min isotherm at 500 °C, and a cooling cycle back to 40 °C was performed. Samples were purged with 25-30 sccm of N2 at atmospheric pressure to prevent oxidation during analysis. III. Results A. Fourier Transform Infrared Spectroscopy. Table 1 lists the FTIR absorbance bands we observe in our films, along with the literature assignments we have adopted. All FTIR spectra were collected from KBr substrates. Pulse parameter notation is given as the ratio of pulse-on time to pulse-off time; for example, 10/10 corresponds to 10-ms on and 10-ms off, or a 50% duty cycle. Figure 1 shows FTIR spectra of films deposited from pulsed benzene plasmas with 10/10, 10/90, and 10/990 pulse sequences. The film deposited from a 10/10 (50% duty cycle) pulsed plasma contains only characteristics of sp3 CH bonding at 2869, 2925, and 2954 cm-1 and corresponding CH and CC deformations at 1374 and 1450 cm-1, respectively. No other IR absorbance bands are present. Increasing the off time to 90 ms (10% duty cycle) introduces sp2 functionalities into the film, as shown by

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Table 1. FTIR Absorption Assignments absorbance (cm-1)

} 738}

700 760

800, 850 995 1100-1400 1375 1451 1500 1493 1599 1700-1800 2869 2925 3025 3058 3088 3302

}

}

assignment

ref

out-of-plane phenyl ring bending mode

20

zone center CF2 νsym stretch CF3, possibly CF-CF3 F-phenyl ring bending mode CF stretch, deformation of benzene-type ring CFx (x ) 1-3) methyl CH bend methyl CH bend, in-plane phenyl ring bending mode CF stretch, deformation of benzene-type ring

25 24 this work 17 23 20 20 17

in-plane phenyl ring bending mode

20

CdCF2, -CFdCF2 sp3 CH2 or CH3 sp3 CH2

23 21 21

aromatic sp2 CH stretch

20, 21

sp1 CH stretch

20

Figure 1. FTIR transmission spectra of films deposited on freshly pressed KBr pellets from pulsed benzene (C6H6) plasmas using a constant pulse-on time of 10 ms and pulse-off times of 10, 90, and 990 ms.

Figure 2. FTIR transmission spectra of films deposited on freshly pressed KBr pellets from pulsed 1,2,4-trifluorobenzene (C6H3F3) plasmas using a constant pulse-on time of 10 ms and pulse-off times of 10, 90, and 990 ms.

sp2 aromatic CH stretches at 3025 and 3058 cm-1. New peaks at 1600, 754, and 700 cm-1 also appear, corresponding to aromatic ring modes in the film (Table 1). When the off time is further increased to 990 ms (1% duty cycle), the absorbance values for the sp2 and sp3 CH stretches are nearly equal. Fine aromatic structure becomes more pronounced with absorbances at 1493 and 1599 cm-1 due to in-plane phenyl ring bending modes and 1451 cm-1 due to a methyl CH bend and an in-plane phenyl ring bend.19 The out-of-plane phenyl ring bending bands at 754 and 700 cm-1 also become sharper. A new absorbance peak appears at 3302 cm-1, attributable to an sp1 CH stretch.20 Figure 2 shows FTIR spectra of films deposited from pulsed TFB with the same pulse conditions as in Figure 1. The film deposited from a 10/10 pulse sequence has a complex structure with bands corresponding to both CHx and CFx moieties. In the CH stretching region there are absorbance bands attributable to both sp3 and sp2 species. At ∼2100 cm-1 there is a broad band possibly due to cumulative double bonds.21 The largest absorbance at 1100-1300 cm-1 is due to the overlapping absorbances of

many CFx species (x ) 1-3).22 The sharpest peak in the spectrum at 750 cm-1 could possibly be due to an amorphous (PTFE) band,23 to a CF3 stretch or to a zone center CF2 νsym stretch.24 The 10/90 film displays significant changes in the bulk structure. The sp3 and sp2 CH stretches at 3000 cm-1 are greatly diminished, and the CFx band has increased significantly. The sharp band at 750 cm-1 has also disappeared while two other absorbances have become more intense. These are located at 1500 cm-1, attributed to the CF stretching of a benzene-type ring,17 and at 1600 cm-1, attributed to hydrogenated and fluorinated CdC bonds. There is also a small sp1 CH absorbance at 3300 cm-1. Films deposited from 10/990 pulsed TFB plasmas also show considerable change in their spectra compared to films deposited at higher duty cycles. The phenyl ring CF stretch at 1500 cm-1 and the hydrogenated, fluorinated CdC stretch at 1600 cm-1 are the most significant absorbance bands in the spectrum, along with a small absorption from sp2 CH at 3050 cm-1. The two absorbance bands that appear at 800 and 850 cm-1 are attributed to out-of-plane phenyl ring deformations (Table 1), shifted to higher frequencies because of fluorine substitution.

(19) Chen, M.; Yang, T.; Zhou, X. J. Polym. Sci., Polym. Chem. Ed. 1996, 34, 113. (20) Dischler, B. In Amorphous Hydrogenated Carbon Films, Koidl, P.; Oelhafen, P., Eds.; Proc. E.-MRS 1987, 17, 189. (21) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: New York, 1990.

(22) d′Agostino, R.; Cramarossa, F.; Fracassi, F.; Illuzzi, F. In Plasma Deposition, Treatment and Etching of Polymers; d′Agostino, R., Ed.; Academic Press: San Diego, CA, 1990; pp 95-162. (23) Seth, J.; Babu, S. V. Thin Solid Films 1993, 230, 90. (24) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 46104617.

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Mackie et al. Table 3. Atomic Stoichiometries and C1s Contributions for Films Deposited from Pulsed TFB Plasmas atom % pulse sequence

C

F

10/10 10/40 10/90 10/190 10/990

49.9 64.2 72.5 73.1 74.6

47.0 31.9 20.2 24 22.2

O

N

2.8 0.3 3.9 7.4 3.0 3.2

CHa 44 60 66 62 63

C1s contribution % π f p* shake-up CF CF2 CF3 satellite 30 29 27 29 29

21 10 5 4 3

5 1 1 2 2

trace 3 3

a This column combines contributions from unshifted hydrocarbon, β-shifted carbon, and CsO species.

Figure 3. FTIR transmission spectra of films deposited on freshly pressed KBr pellets from pulsed hexafluorobenzene (C6F6) plasmas using a constant pulse-on time of 10 ms and pulse-off times of 10, 90, and 990 ms. Table 2. Atomic Stoichiometries and C1s Contributions for Films Deposited from Pulsed Benzene Plasmas atom % pulse sequence

C

F

O

10/10 10/40 10/90 90/190 10/990

86.9 88.7 88.7 89.2 91.6

0.9 0.9

12.2 10.4 11.3 10.8 8.4

N

CH 85 92 91 89 92

C1s contribution % π f p* shake-up CsO CdO satellite 11 5 5 9 6

4 3 3 2

1 1 1 2

Figure 3 shows FTIR spectra of films deposited from pulsed HFB with the same pulse sequences as in Figure 1. The 10/10 film has a structure characteristic of amorphous fluorocarbon polymers with a broad CFx (x ) 1-3) stretching mode band from 1100 to 1400 cm-1. The other significant band (although relatively weak) at 1700-1800 cm-1 can be attributed to either a CdCF2 or a CFdCF2 group. At a 10% duty cycle, the characteristic broad bands for aliphatic fluorocarbon species have decreased, the CdCF2 or CFdCF2 band (∼1750 cm-1) has become more prominent, and there are strong, sharp absorbances at 1511 cm-1 due to the CF stretching of an aromatic ring. This is shifted to higher frequency than the corresponding absorption in the 10/90 TFB spectra (Figure 2), due to the further increase in fluorination of the ring. Corroboration for this assignment is another sharp peak at 995 cm-1, a CF phenyl ring stretch (Table 1). The absorbance at 1157 cm-1 is from a CF2 stretch while the band at 1330 cm-1 could be either an aliphatic CF stretch or an axial CF2 stretch. The bulk structure of the 10/990 film is not significantly different from that of the 10/90 film. B. X-ray Photoelectron Spectroscopy. XPS analysis was performed on films deposited on Si substrates from pulsed benzene, TFB, and HFB plasmas with a constant on time of 10 ms and off times from 10 to 990 ms. Surface elemental composition and high-resolution C1s abundances for these films are listed in Tables 2-4. Oxygen is present in all samples, which may be attributed to the quenching of radicals in the films by reactions with oxygen when exposed to atmosphere.25 Although the highest level of oxygen incorporation is found with benzene, it decreases with increasing pulse-off time, indicating fewer trapped radicals in the films deposited at lower duty cycles. With both TFB and HFB, the level of oxygen stays relatively constant for all pulse sequences (Tables 3 and 4). (25) Geigengack, H.; Hinze, F. Phys. Status Solidi A 1971, 8, 513520.

Table 4. Atomic Stoichiometries and C1s Contributions for Films Deposited from Pulsed HFB Plasmas atom % pulse sequence

C

F

10/10 10/40 10/90 10/190 10/990

46.3 54.1 53.4 55.4 55.5

50.9 39.6 42.9 41.3 42.0

O

N

2.6 0.2 6.1 0.2 3.0 0.7 3.3 2.5

CHa 39 45 36 38 35

C1s contribution % π f p* shake-up CFb CF2 CF3 satellite 26 32 43 42 46

25 18 11 11 11

10 4 6 6 5

3 3 3

a This column combines contributions from unshifted hydrocarbon, β-shifted carbon, and CsO species. b This column combines contributions from unshifted CF and β-shifted CF species.

Figure 4. XPS C1s spectra for films deposited from pulsed benzene (C6H6) plasmas using a constant pulse-on time of 10 ms and pulse-off times of 10, 90, and 990 ms at a photoelectron takeoff angle of 55° from the surface normal.

Figure 4 shows XPS spectra for films deposited from 10/10, 10/90, and 10/990 pulsed plasmas. The spectra show peaks assignable to CH (285 eV), C-O (286.7 eV), and CdO (288.3 eV) groups,12 indicating the surface composition of the films consists only of contributions from CH and surface oxidation products. At pulse-off times greater than 90 ms, the growth of a peak at 291.8 eV, attributed to a π-π* shake-up satellite, is observed (Figure 4). This peak reaches a maximum intensity at the longest off time studied (990 ms) (Table 2), indicating greater retention of aromatic groups in films grown at long pulseoff times. Trace amounts of fluorine were detected in films deposited with short off times (e40 ms). XPS spectra for films deposited from TFB pulsed plasmas are shown in Figure 5. Assignment of the fitted peaks of the C1s core levels for TFB is complicated by

Pulsed-Plasma-Polymerized Aromatic Films

Figure 5. XPS C1s spectra for films deposited from pulsed 1,2,4-trifluorobenzene (C6H3F3) plasmas using a constant pulseon time of 10 ms and pulse-off times of 10, 90, and 990 ms at a photoelectron takeoff angle of 55° from the surface normal.

significant β-substituent effects for hydrocarbon and fluorocarbon moieties that overlap with direct photoionization from fluorinated groups.13 Aromatic shake-up features for CF, CH, and C-CF overlap with nonaromatic CF3 and CF2 features, making calculation of the relative abundances of shake-up and direct photoionization components extremely difficult. Even though assignment of peaks for films deposited from pulsed TFB is complex, results indicate the surface composition changes significantly with duty cycle. The film deposited from a 10/10 TFB plasma has a complex surface composition with peaks that can be assigned to CH (285 eV), β-shifted CH/CCFn/C-O (286.4 eV), CF (288.5 eV), CF2 (290.9 eV), and CF3 (293.3 eV). The intensities of the CF, CF2, and CF3 contributions decrease for off times up to 90 ms. At a pulse off time of 990 ms, the CF peak has increased in intensity and the π-π* shake-up satellite for CF has appeared at 294.5 eV. With increasing off times, we observe an increase in carbon and a decrease in fluorine content. At 10/10, the F/C ratio is ∼0.9, whereas at 10/90, 10/190, and 10/990, the F/C ratio is ∼0.3, slightly lower than the stoichiometry of the TFB monomer. Figure 6 shows XPS spectra for films deposited from HFB plasmas. Assignment of C1s core level peaks is more straightforward with HFB than for TFB, although there is significant overlap with oxygen-containing species. The XPS spectrum for the 10/10 film is dominated by CF2 and CF peaks at 291.7 and 289.5 eV, respectively. The peak at 287.3 eV can be characterized as either C-CF or singly bonded oxygen species (C-O). There are significant contributions from CF3 as well as CHx at 293.8 and 285.0 eV, respectively. Increasing the off time to 90 ms results in an XPS spectrum dominated by a peak at 288.5 eV, characteristic of either a β-shifted CF group or a CdO. The π-π* shake-up satellite peak has also emerged at 295.7 eV. Increasing the off-time to 990 ms does not significantly change the film surface structure. Similar to the case for TFB films, the F/C ratio also decreases at shorter duty cycles, from ∼1.1 at 10/10 to ∼0.8 at 10/90, 10/190, and 10/990. Previous XPS results for films deposited from CW HFB plasmas at low powers are similar to our results for the 10/90 pulse sequence.12,14,17 C. Static Ion Mass Spectrometry. Since our FTIR and XPS analyses cannot distinguish between cumulative

Langmuir, Vol. 14, No. 5, 1998 1231

Figure 6. XPS C1s spectra for films deposited from pulsed hexafluorobenzene (C6F6) plasmas using a constant pulse-on time of 10 ms and pulse-off times of 10, 90, and 990 ms at a photoelectron takeoff angle of 55° from the surface normal.

Figure 7. Static SIMS positive-ion spectra of (a) a film deposited from a 10/990 pulsed C6H6 plasma and (b) a film deposited from a 10/990 pulsed C6F6 plasma. The * indicates mass peaks discussed in the text.

aromatic rings or substituted benzene rings in our films, time-of-flight (TOF) SIMS spectra were obtained on films deposited from 10/990 pulsed benzene and HFB plasmas (Figure 7). Previous SIMS studies of polystyrene (PS) show the dominant peak in the positive ion spectrum is the C7H7+ ion (m/z ) 91), which is assigned as the cyclic tropylium ion, I.19,27 This is also the strongest peak in the SIMS data for our pulsed benzene films. There is, however, a strong C8H9+ signal (m/z ) 105) in our results

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Mackie et al.

relative to conventional polystyrene.19,26 This peak is generally assigned to the structure of the styrene repeat unit. Although it is much more difficult to assign structures to the larger fragment ions observed in the SIMS spectra for our C6H6 films, we observe mass peaks at 115, 128, 141, 152, 165, and 178 m/z. These peaks are all observed in the SIMS spectrum of PS and are assigned to structures II-VII.27

The low-mass saturated hydrocarbon fragments (i.e. C2H3, C2H5, C3H3, C3H5) (Figure 7a) are stronger than is observed for conventional polystyrene. Some oxygencontaining fragments (e.g. CHO, C2H3O, C2H5O, C3H5O, C7H5O) and small amounts of nitrogen-containing fragments (e.g. C2NH4) were also observed. This is the result of either oxygen (nitrogen) incorporation during the plasma deposition (from low-level impurities) or atmospheric surface reactions. TOF-SIMS spectra were also obtained for films deposited from 10/990 pulsed HFB plasmas (Figure 7b). All peaks corresponding to the fluorinated analogues of the peaks observed with the pulsed benzene film (i.e. C7F7 for C7H7) were detected, consistent with the aromatic character of these films. Fragments from aliphatic hydrocarbons, aliphatic fluorocarbons, and oxygen- and nitrogencontaining hydrocarbons were also detected (Figure 7b). The nitrogen-containing fragments were more pronounced in the HFB film compared to the benzene film. This is consistent with the XPS results, which show small amounts of N incorporation in the HFB films.17,28 D. Film Morphology and Contact Angle. SEM results for films deposited from pulsed 10/990 benzene, TFB, and HFB are shown in Figure 8. For the films deposited from a pulsed benzene plasma (Figure 8a) the surface morphology has a roughness on the order of 0.5-1 µm which is continuous across the entire film. For TFB pulsed plasmas (Figure 8b) the surface roughness has decreased considerably. The 0.5-1 µm occlusions are randomly dispersed across the surface, and there are areas of low surface roughness between these occlusions. Films deposited from HFB pulsed plasmas are continuous and (26) Leggett, G. J.; Vickerman, J. C.; Briggs, D.; Hearn, M. J. J. Chem. Soc., Faraday Trans. 1992, 88, 297-309. (27) Leggett, G. J.; Ratner, B. D.; Vickerman, J. C. Surf. Interface. Anal. 1995, 23, 22-28. (28) Small amounts of siloxane fragments were also detected on the HFB sample. These are likely due either to a PDMS contaminant or to etching of the Si substrate by the ion beam.

Figure 8. Scanning electron micrographs of films deposited from 10/990 pulsed plasmas using (a) C6H6, (b) C6H3F3, and (c) C6F6. All films are shown with the same magnification (×10 000).

smooth (Figure 8c), similar to films deposited from pulsed C2F6/H2 plasmas.6 As can be seen from Figure 8, the surface of the films becomes smoother as the fluorine content of the monomer increases. This is similar to previous results for films deposited from C2F6/H2 CW plasmas where the surface roughness scaled inversely with the fluorocarbon content in the plasma.6 Contact angle measurements are a sensitive means of probing surface energy and were measured for all films studied using the sessile drop method with water as the liquid. Results of the measurements as a function of pulseoff time are shown in Figure 9. Contact angles for pulsed

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Figure 9. Relation of static water contact angle to pulse-off time for films deposited from benzene (closed circles), TFB (closed squares), and HFB (closed triangles). On times were kept constant at 10 ms. Vertical lines represent the standard deviation in the averaged values.

benzene films decrease rapidly between 10- and 190-ms off times and are relatively constant at longer off times. The contact angle for films deposited from TFB is somewhat higher than that for benzene, ∼80°, and is nearly constant with duty cycle. Films deposited from HFB have the highest contact angles and decrease slightly with increasing off times, ranging from ∼93° for the 10/10 film to ∼85° for the 10/990 film. E. Deposition Rates. Deposition rates (DRs) for pulsed plasma systems are difficult to measure. Because of the varying duty cycle pulsed discharges employ, plotting deposition rates per unit time is relatively meaningless. DRs are, therefore, plotted in Figure 10 as deposition per pulse versus off time. For the benzene monomer (Figure 10a) the DR increases slightly as off time increases. This behavior has been seen previously and demonstrates film formation is occurring during plasma off periods.6,29,30 For the fluorinated monomers, however, we see a much stronger increase in DR with increasing off times (Figure 10b). Interestingly, DRs for TFB and HFB increase (or decrease) at the same rate at all off times. The overall DR from fluorinated monomers is significantly greater than that for benzene, with the TFB monomer dominating over HFB. F. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC), a sensitive means of determining material thermochemical properties,31,32 was performed on films deposited from pulsed benzene, TFB, and HFB plasmas33 and on a polystyrene standard (MW 975 000) between 40 and 500 °C. The polystyrene standard shows an endothermic structural change at 112 (29) Chen, X.; Rajeshwar, K.; Timmons, R. B.; Chen, J.-J.; Chyan, O. M. R. Chem. Mater. 1996, 8, 1067-1077. (30) Savage, C. R.; Timmons, R. B.; Lin, J. W. Structure-Property Relations in Polymers; Advances in Chemistry Series 236; American Chemical Society: Washington, DC, 1993; pp 745-768. (31) Rogers, M. G. J. Mater. Sci. 1991, 26, 4281-4282. (32) Lin, S.; Liao, C.; Liang, R. Polym. J. 1995, 27, 201-204. (33) Films used for DSC analysis were deposited with a 10/323 pulse cycle. The composition for these films is nearly identical to that of films deposited with a 10/990 pulse cycle. The 10/323 conditions were chosen to afford a large quantity of material.

Figure 10. Film deposition rates per complete cycle as a function of pulse-off time for films deposited from (a) benzene and (b) TFB (closed squares) and HFB (closed triangles) plasmas with a constant on time of 10 ms and off times ranging from 10 to 990 ms. Vertical lines represent standard deviations of the averaged values.

°C indicative of the glass transition temperature (Tg) for this polymer. The material deposited from the pulsed benzene plasma does not show this behavior. Absence of a Tg can be attributed to cross-linking in the film which results in minimal polymer chain motion. There is an exothermic step at 170 °C and a steady increase in positive heat flow from 250 to 500 °C. This exothermic heat flow at higher temperatures could be due to bond breaking and significant structural reorganization. Support for this interpretation comes from pyrolysis mass spectra and gas chromatography of films deposited from a 3.5-MHz CW benzene plasma, which indicate, that above 350 °C, the material may evolve benzene, biphenyl, small hydrocarbon fragments due to bond scission in the film, and trapped toluene and xylene.34

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The films deposited from pulsed TFB and HFB plasmas show a small reversible step at ∼170 °C. For these two materials, a steady exothermic rise did not appear until the temperature had reached ∼400 °C. Clark et al. observed similar reversible transitions for films deposited from CW HFB plasmas from 102 to 152 °C.12 They attributed this to loss of volatile fragments from the polymer matrix or the relief of strain in the cross-linked film. Comparing the thermal properties between the three materials shows that fluorination results in a more thermally robust material. IV. Discussion The present work was undertaken to examine pulsed plasma polymerization using aromatic monomers. The FTIR, XPS, and SIMS results shown here reveal that significant film chemistry changes occur with all three monomers as the ratio of plasma-on/off times is varied during plasma polymerization. This is consistent with previous studies that show large-scale variations in film composition with progressive changes in plasma duty cycle.6,31 The present study of aromatic monomers is a continuation of our pulsed plasma polymerization studies and further demonstrates remarkable behavior in the structure of films deposited from pulsed plasmas. As noted in the Introduction, pulsed plasma systems offer a gentler alternative to CW plasma polymerization and can provide polymeric films with unique structures not obtainable through conventional polymerization methods or CW plasmas. Figures 1-3 show the FTIR spectra for films deposited from benzene, TFB, and HFB, respectively. The ratio of the peak height of the aromatic C-H stretching vibration at 3025 cm-1 to that of the aliphatic C-H stretching at 2925 cm-1 shows that the bulk aromaticity of the deposited films in the benzene films increases as the off time is increased. Further evidence of the aromaticity of the deposited materials comes from the phenyl ring bending modes, which are much more pronounced in the films formed at lower plasma duty cycles. XPS analysis also supports this conclusion, as the π-π* shake-up satellite band is observed in the C1s spectra for films deposited from pulsed plasmas with off times of g90 ms (low duty cycles of 1-10%). Interestingly, the degree of aromaticity, as measured by the π-π* shake-up satellite band in the XPS spectra, does not change appreciably with monomer (Tables 2-4). This suggests all three monomers react similarly under the same pulsed conditions. Despite this similarity in reactivity, the three monomers produce films with different surface properties and at different deposition rates. The higher contact angles observed for films deposited from the fluorinated benzenes are as expected from the degree of fluorination of the monomer (Figure 9). The decrease in contact angle at smaller duty cycles with HFB is likely the result of a lower concentration of CF3 groups on the surface of the 10/990 film, as shown by the XPS data (Table 4). Similarly, the lack of change in contact angle with increasing off times for the TFB films is supported by the smaller and relatively constant amount of CF3 in these films (Table 3). For comparison, films deposited from 50:50 C2F6/H2 CW plasmas have an ∼10% contribution from CF3 in the C1s XPS spectrum and a contact angle of ∼90°.6 This is similar to the case for films deposited here from the 10:90 HFB pulsed plasma. (34) Venugopalan, M.; Sioun-Lin, I.; Grenda, M. S. J. Polym. Sci., Polym Chem. Ed. 1980, 18, 2731-2736.

Mackie et al.

The decrease in contact angle for benzene at higher off times suggests a similar effect with CH3 groups on the surface. This is supported by the FTIR results (Figure 1), which show high levels of CH3 in the films deposited at shorter off times. The contact angle results of Yasuda and Hsu, who studied pulsed benzene and HFB plasmas (10% duty cycle), are consistent with our results. We do not, however, observe as high a contact angle for our pulsed benzene films, even at the lowest duty cycles. Our results for the HFB film with a 10% duty cycle are essentially the same as that found by Yasuda and Hsu, ∼89°. It has been proposed that pulsed plasma conditions are unique in that there are two distinct regimes.31 In the on period, radicals and ions are generated and significant fragmentation of the monomer occurs. In the off period, ion density decays rapidly, leaving the longest-lived radicals to react with the incoming monomer gas. In the present aromatic systems, fragmentation of the precursor molecules is diminished such that film formation is likely a result of incorporation of intact monomer molecules. This is supported by the work of Chen et al., who have studied CW plasma polymerization of styrene.35 They found that as the rf power input is increased from 20 to 50 W, the deposition rate of the polymer increased, and the concentration of phenyl groups in the film decreased. This is likely due to increased electron energy at higher rf powers, which causes greater fragmentation of the monomer. With our 10/990 pulsed plasmas, we are operating at considerably lower average powers (∼4 W equivalent power) than those employed by Chen et al. At the lowest CW operating powers, they determined an sp2(3027 cm-1)/sp3(2930 cm-1) CH ratio of 1.5.36 For the 10/990 C6H6 film, we find a similar sp2/sp3 ratio of ∼1.1 using the same absorbances. This suggests we are incorporating high levels of aromatic rings into our films. Further evidence for this conclusion comes from the SIMS results, which indicate there are significant similarities between our films and polystyrene. From their SIMS study of poly(ethylene terephthalate) and PS, Leggett et al. concluded that interpretation of the ion spectra for polymers can be complicated by complex structures such as structures II-VII shown above.27 On the basis of fragmentation data obtained by gas-phase mass spectral studies, however, they also conclude that the SIMS ion formation process at the surface of polymers is a soft process involving low-energy nonadiabatic and unimolecular processes. Thus, the SIMS spectra, including the complicated ring structures such as II-VII, are directly related to the surface chemical structure of the polymer. As we see similar peaks in our SIMS spectra for pulsed benzene films, we believe we have a polystyrenelike polymer film. A SIMS study of CW plasma-deposited styrene films provides the closest comparison for our results.28 These films show retention of aromaticity, as indicated by C6H5+ and C7H7+ peaks at m/z 77 and 91. Furthermore, styrene repeat units are also observed at m/z 103 and 105. For freshly deposited films, m/z 91 is the most intense peak in the SIMS spectrum. The m/z 105 peak, however, dominates the spectra for films exposed to atmosphere for an extended period of time before analysis. This is attributed to postdeposition cross-linking in the film.28 Although the m/z 105 peak is strong in our films, it never exceeds the intensity of the m/z 91 peak. This suggests our films do not experience as much postdeposition cross(35) Chen, M.; Yang, T.-C.; Zhou, X. J. Polym. Sci., B: Polym. Phys. 1996, 34, 113-120. (36) Samples were sent to the University of Washington for analysis, often days after depositions were performed.

Pulsed-Plasma-Polymerized Aromatic Films

linking as the CW styrene films. It has been shown that polymer films deposited under pulsed conditions contain fewer trapped radicals than CW films.15 This effectively limits the amount of cross-linking that can occur through postdeposition reactions. As SIMS analysis on our films was not performed immediately after deposition,36 the low level of cross-linking must be primarily the result of plasma polymerization. Although our SIMS results suggest we have a highly PS-like structure, our DSC results indicate we do not have a crystalline polymer film such as PS because we do not observe a glass transition during the temperature program. Lack of an observable Tg is indicative of a crosslinked material and suggests a more complex structure in our films. Cross-links in our pulsed benzene films are likely formed by gas-phase C6H5+ ions which lose their charge with wall collisions.35 Polymerization is then initiated through abstraction of H from adsorbed C6H6 molecules. This abstraction reaction generates a free radical center, from which a branch can grow. Fragmentation of the polymer chain leads to most of the nonpolymeric products. Some of the free radicals may add to the double bonds of the fragmented chain, producing a highly cross-linked polymer with both aromatic and olefinic groups and with some properties similar to those observed for polystyrene. Further insight into the mechanism for film deposition from our pulsed plasmas can be derived from examining the relative deposition rates of all three monomers. The film DR for benzene is significantly lower than those for films deposited from fluorinated monomers (Figure 10). This is similar to the DR for benzene and HFB from CW plasmas where the DR for HFB is nearly an order of magnitude larger than the DR from benzene.37 This has been attributed to the lower ionization energy of fluorinated compounds as compared to nonsubstituted hydrocarbons, leading to a higher degree of ionization in the plasma.15,38 As noted above, polymerization is believed (37) Sah, R. E.; Dischler, B.; Bubenzer, A.; Koidl, P. Appl. Phys. Lett. 1985, 46, 739-741. (38) Yasuda, H.; Hsu, T. Surf. Sci. 1978, 76, 232-241.

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to begin with formation of C6H5+ (C6F5+) ions, which then abstract H atoms (F atoms) from the growing film. Ionization efficiency is therefore directly related to the rate of film formation. TFB, which contains both hydrogen and fluorine, is the monomer with the highest DR at all off times. It is wellknown that hydrogen and fluorine abstraction plays a major role in plasma polymerization,38 although the etching efficiency of fluorine is much higher than that of hydrogen. Thus, HFB, which is fully fluorinated, has a lower deposition rate than TFB. All three monomers, however, show a definite increase in DR with increasing off times. As noted above, this implies significant film formation is occurring during the off times. With the aromatic monomers, we believe ionization during the on time is followed by significant chain propagation reactions during the off time. Timmons and co-workers have observed similar behavior for a variety of monomers31 and found it is highly monomer dependent. The dependence is likely associated with the relative rates at which the plasma-generated intermediates (initiators) are consumed during the off time chain propagation reactions. Pulsed plasma polymerization using aromatic monomers and long off times results in films that are polystyrene-like with some degree of cross-linking. The extent of aromaticity can be somewhat controlled through the variation of pulse parameters. The comprehensive set of analysis tools discussed here demonstrates that, although complicated, the molecular structure of plasma-deposited polymer films can be clearly elucidated. The present results further illustrate the utility of pulsed plasmas for synthesis of materials with unique bulk and surface properties. Acknowledgment. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society. We also thank Prof. Peter K. Dorhout for helpful discussions and the use of his DSC equipment. LA970953J