Deposition of Fluoropolymer Films on Si(100) Surfaces by Rf

Jul 10, 2002 - The fluoropolymer film deposited from sputtering by CF4 plasma had a κ ...... fillers, and specialized surface and bulk properties, an...
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Langmuir 2002, 18, 6373-6380

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Deposition of Fluoropolymer Films on Si(100) Surfaces by Rf Magnetron Sputtering of Poly(tetrafluoroethylene) Yan Zhang, G. H. Yang, E. T. Kang,* and K. G. Neoh Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 119260

Wei Huang and A. C. H. Huan Institute of Materials Research and Engineering, National University of Singapore, 3 Research Link, Singapore 117602

S. Y. Wu Deep Submicron Integrated Circuit-Module Development, Institute of Microelectronics, 11 Science Park II, Singapore 117685 Received October 26, 2001 Dielectric polymer films of about 40-300 nm in thickness were deposited on Si(100) substrates via rf plasma sputtering of a poly(tetrafluoroethylene) (PTFE) target using different sputtering gases, including Ar, CF4, N2, and H2. Oxygen plasma failed to give rise to any significant deposition. X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), water contact angle measurement, and Fourier transform infrared (FTIR) spectroscopy results indicated that the chemical composition and molecular structure of the sputter-deposited polymer films depended strongly on the type of the sputtering gas used. The Ar, CF4, and N2 plasmas gave rise to PTFE-like films with a high fluorine content, although N-containing moieties were also found in the N2 plasma-deposited films. The H2 plasma, on the other hand, resulted only in the deposition of a polyethylene-like film. The deposition rate and the dielectric constant (κ) of the resulting fluoropolymer films were also found to be dependent on the type of the sputtering gas. The fluoropolymer film deposited from sputtering by CF4 plasma had a κ value of 1.9, which was lower than that of the pristine PTFE film (κ ) 2.0-2.1). On the other hand, the nonreactive Ar plasma gave rise to the highest deposition rate among the five sputtering gases. Peel adhesion test results suggested that all the sputter-deposited films adhered strongly to the Si(100) surfaces.

Introduction The use of polymeric material with low dielectric constant (κ < 2.5) as the interlayer dielectrics is one of the most promising approaches to the improvement of the signal propagation speed in submicrometer and multilayer devices.1-3 Poly(tetrafluoroethylene) (PTFE) has attracted considerable research interests due to its low dielectric constant (κ ) 2.0-2.1),3,4 high thermal stability, and low moisture absorption. However, the chemical inertness and highly crystalline nature of the conventional PTFE films and powders have given rise to serious processing problems. Furthermore, due to the surface inertness of the PTFE film, its adhesion with other materials tends to be rather poor.4 To overcome the above problems, a number of studies have been carried out in an attempt to prepare PTFE-like thin films on various substrates using gas phase processing techniques. Radio frequency (rf) plasma sputtering of a PTFE target is one of the effective approaches to preparing fluoropolymer thin films. After being first reported in the 1970s, the plasma sputterdeposited fluoropolymers have been shown to be useful as the dielectric films, water-repellent coatings, low friction * To whom all correspondence should be addressed.Tel.: +65874-2189. Fax: +65-779-1936. E-mail: [email protected]. (1) Maier, G. Prog. Polym. Sci. 2001, 26, 3. (2) Kreuz, J. A.; Edman, J. R. Adv. Mater. 1998, 10, 1229. (3) Martin, S. J.; Godschalz, J. P.; Mills, M. E.; Shaffer, E. O., II; Townsend, P. H. Adv. Mater. 2000, 12, 1769. (4) Sacher, E. Prog. Surf. Sci. 1994, 47, 273.

coatings, and even optical coatings.5-7 The sputtering processes have been carried out both in the magnetron sputtering systems5-10 and in tubular-type plasma reactors,11,12 using different working gases, including Ar, N2, He, and Ne. The addition of fluorocarbon gases, such as CF413 and C3F8,14 to Ar for glow discharging has also been carried out to enhance the deposition rate and to improve the fluorine content in the deposited films. An Argon ion beam has also been used for the sputtering deposition of fluoropolymer films.15 However, a systematic study of the effects of the sputtering gas on the molecular structure and physicochemical properties of the sputter-deposited fluoropolymer films in a rf magnetron sputtering system has yet to be carried out. (5) Morrison, D. J.; Robertson T. Thin Solid Films 1973, 15, 87. (6) Tibbitt, J. M.; Shen, M.; Bell, A. T. Thin Solid Films 1975, 29, L43. (7) Biederman, H.; Ojha, S. M.; Holland, L. Thin Solid Films 1977, 41, 329. (8) Biederman, H.; Osada, Y. Adv. Polym. Sci. 1990, 95, 59. (9) Biederman, H. J. Vac. Sci. Technol., A 2000, 18, 1642. (10) Biederman, H.; Zeuner, M.; Zalman, J.; Bilkova, P.; Slavinska, D.; Stelmasuk, V.; Boldyreva, A. Thin Solid Films 2001, 392, 208. (11) Ryan, M. E.; Fonseca, J. L.; Tasker, S.; Badyal, J. P. S. J. Phys. Chem. B 1995, 99, 7060. (12) Golub, M. A.; Wydeven, T.; Johnson, A. Langmuir 1998, 14, 2217. (13) Lehmann, H. W.; Frick, K.; Widmer, R.; Vossen, J. L.; James, E. Thin Solid Films 1978, 52, 231. (14) Marechal, N.; Pauleau, Y. J. Vac. Sci. Technol., A 1992, 10, 477. (15) Quaranta, F.; Valentini, A.; Favia, P.; Lamendola, R.; d’Agostino, R. Appl. Phys. Lett. 1993, 63, 10.

10.1021/la011606j CCC: $22.00 © 2002 American Chemical Society Published on Web 07/10/2002

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Due to the complicated nature of the plasma reactions, the exact chemical structures of the deposited fluoropolymers are usually hard to determine. Characterization of the deposited fluoropolymer films by multiple techniques is thus of great importance. A number of characterization techniques, such as Fourier transform infrared (FTIR) spectroscopy,10-12,16 X-ray photoelectron spectroscopy (XPS),10-12 and Rutherford backscattering spectroscopy (RBS),14 have been employed for the surface analysis of the sputter-deposited fluoropolymers. Characterization of the deposited fluoropolymer films with molecularspecific techniques is essential to the elucidation of the chemical structures of the deposited films, as well as to the understanding of the nature of the sputtering process. Besides structural characterization, the dielectric constant, surface uniformity, surface wettability, and interaction with the substrate are also of great importance to the application of the sputter-deposited fluoropolymer films as passivation layers and barrier coatings. In this work, attempts are made to deposit dielectric fluoropolymer films on Si(100) surfaces in a rf magnetron system, using Ar, CF4, H2, N2, and O2 as sputtering gas plasmas on a PTFE target. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and FTIR are used to analyze the molecular and chemical structure of the sputter-deposited polymer films. The effect of the sputtering gas on the chemical composition of the deposited films is studied by XPS. The surface morphology and surface wettability, on the other hand, are evaluated by atomic force microscopy (AFM) and contact angle measurements, respectively. The interaction of the sputterdeposited films with the Si(100) substrates is evaluated by 180°-peel adhesion test. Experimental Section Materials. Single-crystal Si(100) wafers, having a thickness of about 2 mm and a diameter of 150 mm, were obtained from Unisil Co. of Santa Clara, CA. The as-received wafers were polished on one side and doped as p type. The silicon wafers were sliced into rectangular strips of about 10 mm × 20 mm in size. To remove the organic residues from the surfaces, the silicon substrate was washed with the “piranha” solution, a mixture of 98 wt % concentrated sulfuric acid (70 vol %) and hydrogen peroxide (30 vol %).17,18 After being rinsed with copious amounts of doubly distilled water, the Si strips were dried at 80 °C in a vacuum oven for 1 h. The PTFE sputtering target of 150 mm in diameter and 3 mm in thickness was purchased from Superconductive Component Inc., Columbus, OH. The PTFE target was backside-bonded to a copper plate before installing into the sputtering system. Radio Frequency (Rf) Sputtering of PTFE onto the Si(100) Surface. The rf plasma-sputtering process was carried out in a triple-cathode S-2000 sputtering system, assembled by Korea Vacuum Technology of Seoul, Korea. The PTFE target was installed in the rf magnetron sputter gun powered by a Comdel CX-600S rf generator (rf ) 13.56 MHz). The sputtering process was preprogrammed and microprocessor-controlled. After the silicon strips were fixed on the rotary sample stage, the base pressure of the sputtering chamber was pumped down to below 3 · 10-5 Torr. Sputtering gas (purified Ar, CF4, N2, H2, or O2) was then introduced into the sputtering chamber at a flow rate of 30 standard cubic centimeters/min (sccm). The flow rate of the sputtering gas was controlled by an MKS mass flow controller (MFC). The silicon surface was pre-cleaned and preactivated by a 50-W pure gas plasma for 1 min. During the precleaning process, the PTFE target was covered by a stainless steel shutter. The (16) Hishmeh, G. A.; Barr T. L.; Sklyarov, A.; Hardcastle, S. J. Vac. Sci. Technol., A 1996, 14, 1330. (17) Zhang, Y., Tan, K. L.; Liaw, B. Y.; Liaw, D. J.; Kang, E. T.; Neoh, K. G. Langmuir 2001, 17, 2265. (18) Zhang, J. F.; Cui, C. Q.; Lim, T. B.; Kang, E. T.; Neoh, K. G.; Lim, S. L.; Tan, K. L. Chem. Mater. 1999, 11, 106.

Zhang et al. PTFE target surface also underwent a sputter-cleaning process for 30 s, with the silicon substrate surface covered by a shutter. The system pressure was maintained at 12 mTorr during target and substrate cleaning, as well as during the sputter deposition process, by the MKS 600 automatic pressure controller. The sputter deposition process was carried out under an rf power of 350 W for 20 min. The film thickness was determined using an Alpha-STEP 500 Surface Profiler of KLA-Tencor Co., San Jose, CA. The thickness of the deposited films generally ranged from 40 to 300 nm, depending on the type of sputtering gas used. Surface Characterization. The chemical compositions of the pristine PTFE film and the sputter-deposited polymer films on the preactivated Si(100) surfaces were determined by X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out on the AXIS HSi spectrometer (Kratos Analytical Ltd., Manchester, England) with a monochromatized Al KR X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass energy of 40 eV. The anode voltage and current were set at 15 kV and 10 mA, respectively. The pressure in the analysis chamber was maintained at 5 × 10-8 Torr or lower during each measurement. The samples were mounted on the sample stubs by means of double-sided adhesive tapes. The core-level signals were obtained at a photoelectron takeoff angle (with respect to the sample surface) of 90°. All binding energies (BE’s) were referenced to the C 1s hydrocarbon peak at 284.6 eV. In curve fitting, the line width (full width at half-maximum, or fwhm) for the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from peak-area ratios, after correcting with the experimentally determined sensitivity factors, and were reliable to (5%. The elemental sensitivity factors were determined using stable binary compounds of well-established stoichiometries. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was also used for the surface analysis of the pristine PTFE film and the sputter-deposited polymer films on the preactivated Si(100) surfaces. The ToF-SIMS analyses were carried out on an ION-TOF SIMS IV instrument (ION-TOF, GmbH, Mu¨nster, Germany). The primary ion beam (10 keV Ar+) with a spot size of ∼50 µm was rastered over an area of 500 µm × 500 µm while keeping the total dose under 1013 ions/cm2. The pressure in the analysis chamber was maintained at 1.0 × 10-9 Torr or lower during each measurement. To reduce the charging effect, an electron flood gun was used for the charge neutralization. The calibration of the mass spectra was based on the built-in mass library. The surface morphologies of the various samples were studied by atomic force microscopy (AFM), using a Nanoscope IIIa AFM from the Digital Instruments Inc. of Santa Barbara, CA. In each case, an area of 5 µm × 5 µm was scanned using the tapping mode. The drive frequency was 330 ( 50 kHz, and the voltage was between 3.0 and 4.0 V. The drive amplitude was about 300 mV, and the scan rate was 0.5-1.0 Hz. The scan angle was 0°. An arithmetic mean of the surface roughness (Ra) was calculated from the roughness profile. FTIR Spectroscopy. Although the Si(100) wafer was transparent to the infrared source, it still attenuated a considerable amount of the infrared transmission. Thus, the samples for the FTIR spectroscopy measurements were obtained by depositing the fluoropolymer films directly on the surface of freshly pressed KBr pellets or by scrapping the deposited films from the Si substrates for dispersing in KBr pellets. The spectra were recorded in air on a Bio-Rad FT-IR, model 400, spectrophotometer. Each spectrum was collected by cumulating 30 scans at a resolution of 8 cm-1. Water Contact Angle Measurements. Static water contact angles of the pristine PTFE film and the Si(100) surfaces with sputter-deposited polymer films were measured by the sessile drop method at 25°C and 65% relative humidity using a contact angle goniometer (model 100-00-(230)), manufactured by the Rame-Hart, Inc., of Mountain Lakes, NJ. The telescope with a magnification power of 23× was equipped with a protractor of 1° graduation. For each contact angle reported, 10 readings from different parts of the film surface were averaged. Each angle reported was reliable to (3°. Dielectric Constant Measurements. Samples for dielectric constant measurement were prepared by sputter deposition of

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Figure 2. FTIR spectra of (a) the pristine PTFE film, (b) the s-PTFE (Ar) film, (c) the s-PTFE (CF4) film, (d) the s-PTFE (N2) film, and (e) the s-PTFE (H2) film. Figure 1. XPS C 1s core-level and wide scan spectra of (a) the pristine PTFE surface, (b) the s-PTFE-Si (Ar) surface, (c) the s-PTFE-Si (CF4) surface, (d) the s-PTFE-Si (N2) surface, (e) the s-PTFE-Si (H2) surface, and (f) the s-PTFE-Si (O2) surface. the fluoropolymer films on 150 mm (diameter) Si(100) wafers. Dielectric constant measurements were carried out on an SSM Mercury Probe CV system (model SSM 495) of Solid State Measurements, Inc., Pittsburgh, PA, under a frequency of 100 kHz. Adhesion Strength Measurements. The adhesion strength of the sputter-deposited films to the pure gas plasma preactivated Si(100) substrates was evaluated by the 180°-peel adhesion test. The copper foil adhesive tapes were applied to the deposited films on the Si surfaces and subsequently peeled off on an Instron 5544 tensile tester from the Instron Corporation of Canton, MA. All peel tests were carried out at a cross-head speed of 10 mm/ min. The copper foil adhesive tape was a product of Electron Microscopy Science Inc. of Fort Washington, PA. Each adhesion strength reported was the average of at least three sample measurements. The adhesion strengths among these measurements usually did not vary by more than (0.5 N/cm.

Results and Discussion The abbreviations, s-PTFE-Si (Ar), s-PTFE-Si (CF4), s-PTFE-Si (H2), s-PTFE-Si (N2), and s-PTFE-Si (O2), denote the sputter-deposited fluoropolymer films on preactivated Si(100) surfaces using Ar, CF4, H2, N2, and O2, respectively, as the rf sputtering gas. 1. Chemical Composition of the Sputter-Deposited Fluoropolymer Films on Si(100) Surfaces. Figure 1 shows the respective wide scan and C 1s core-level spectra of the pristine PTFE surface (Figure 1a), the s-PTFE-Si (Ar) surface (Figure 1b), the s-PTFE-Si (CF4) surface (Figure 1c), the s-PTFE-Si (N2) surface (Figure 1d), the s-PTFE-Si (H2) surface (Figure 1e), and the s-PTFE-Si (O2) surface (Figure 1f). The C 1s core-level spectrum of the pristine PTFE film shows only one peak component at the binding energy (BE) of 292.0 eV, attributable to the

CF2 species.19 On the other hand, the C 1s core-level spectra of the s-PTFE-Si (Ar) surface can be curved-fitted with seven peak components, with BE’s at 284.6 eV for the C-H species, at 286.2 eV for the CH2-CF2 species, at 287.4 eV for the C-CFn species, at 288.6 eV for the CFH species, at 290.0 eV for the CF species, at 292.0 eV for the CF2 species, and at 294.1 eV for the CF3 species.11,19,20 The minor C-H component on the s-PTFE (Ar) surface probably has originated from the adventitious carbon contamination. The C 1s core-level spectrum of the s-PTFE-Si (CF4) surface shows a line shape similar to that of the s-PTFE-Si (Ar) surface, albeit the relative concentrations for the constitutive components are somewhat different. The wide scan spectra of the s-PTFE-Si (Ar) surface and the s-PTFE-Si (CF4) surface resemble that of the pristine PTFE surface and consist of only two peak components, viz, the C 1s component and the F 1s component. The C 1s core-level spectrum of the s-PTFE-Si (N2) surface shows two additional peak components, in addition to those observed on the s-PTFE-Si (Ar) surface. The two peak components with BE’s at about 291.0 and 293.0 eV are assigned to the F-C-NF2 species and the F2-C-NF2 species, respectively. The presence of the F-C-NF2 and F2-C-NF2 species on the s-PTFE-Si (N2) surface will be verified by the subsequent ToF-SIMS results (see Figure 3d below). These two peaks were not identified in the previous XPS studies using a nonmonochromatized X-ray source.10,11 The incorporation of the N-containing species into the N2 plasma sputter-deposited fluoropolymer film is also confirmed by the presence of a fairly strong N 1s signal in the corresponding wide scan spectrum. (19) Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1978; p 38. (20) Zhang, Y.; Huan, A. C. H.; Tan, K. L.; Kang, E. T. Nucl. Instrum. Methods, B 2000, 168, 29.

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Figure 3. Positive ion ToF-SIMS spectra of (a) the pristine PTFE film, (b) the s-PTFE-Si (Ar) surface, (c) the s-PTFE-Si (CF4) surface, (d) the s-PTFE-Si (N2) surface, and (e) the s-PTFE-Si (H2) surface.

On the other hand, the C 1s core-level spectrum of the s-PTFE-Si (H2) surface differs substantially from those of the above four surfaces. One major and two minor peak components, having binding energies at 284.6 eV for the C-H species, 286.2 eV for the C-O species, and 288.6 eV for the CdO species, can be resolved in the C 1s core-level spectrum.19 The C 1s line shape is, thus, rather similar to that of the polyethylene (PE) film surface after plasma treatment and air exposure.21 The wide scan spectrum of the s-PTFE-Si (H2) surface consists of a strong C 1s signal, a weak O 1s signal, and a weak F 1s signal. The O 1s signal probably arises from the interaction of the surface with air during sample transfer. The presence of a weak F 1s signal, on the other hand, is consistent with the presence of a barely discernible high BE tail, attributable to the fluorocarbon species, in the C 1s core-level spectrum. Despite the relatively low deposition rate of the H2 plasma (see also Table 3 below), a film of about 40 nm in thickness (21) Han, H. S.; Tan, K. L.; Kang, E. T.; Neoh, K. G. J. Appl. Polym. Sci. 1998, 70, 1977.

was obtained over the fixed deposition time of 20 min. This result is somewhat different from that of the previous study carried out in a tubular-type glass reactor.11 In the case of the tubular plasma reactor, H2 plasma failed to give rise to any deposition. Both the C 1s and the wide scan spectra of the present s-PTFE-Si (H2) surface indicate the presence of a low fluorine content in the deposited polymer film. This phenomenon is probably due to the fact that H2 plasma can give rise to extensive defluorination. It can, thus, be deduced that, in the case of sputtering by the H2 plasma, the PTFE target surface has undergone defluorination to form hydrocarbon polymers, similar to the case of the H2 plasma treatment of the PTFE surface.22,23 The C 1s core-level spectrum of the s-PTFE-Si (O2) surface is rather similar to that of the pristine silicon (22) Clark, D. T.; Hutton, D. R. J. Polym. Sci. Polym. Chem. 1987, 25, 2643. (23) Yang, G. H.; Kang, E. T.; Neoh, K. G. J. Adhes. Sci. Technol. 2001, 15, 727.

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Table 1. Compositions of the Pristine PTFE and the s-PTFE-Si Surfaces concn of C species (mol %)a surface

CH2 (284.6)b

CH2CF2 (286.2)

C-CFn (287.4)

CFH (288.6)

CF (290.0)

CF2 (292.0)

CF3 (294.1)

pristine PTFE s-PTFE-Si (Ar) s-PTFE-Si (CF4) s-PTFE-Si (N2) s-PTFE-Si (H2)

3.2 0.0 1.6 85.9

2.4 1.0 1.2

16.1 13.8 4.6

7.9 4.5 7.7

22.1 21.4 14.3

100 32.0 38.2 24.3

16.4 21.1 16.7

a

C-O (286.2)

CdO (287.6)

10.5

3.6

F-C-NF2 (291.0)

F2C-NF2 (293.0)

14.2

15.4

Percentage of the carbon species in the C 1s core-level spectra. b Binding energy of the C 1s component (unit: eV).

surface. Only a faint F 1s signal is discernible in the corresponding wide scan spectrum. These results, together with the presence of strong Si 2p and Si 2s signals, readily suggest that the O2 plasma fails to give rise to any significant amount of polymer deposition. The relative proportions of the various carbon species in the C 1s core-level spectra of the polymer films deposited on the Si(100) surfaces are summarized in Table 1. Since O2 plasma fails to give rise to a significant amount of deposition, the composition of the s-PTFE-Si (O2) surface is not included in Table 1. The s-PTFE-Si (CF4) surface has the highest percentage of the CF2 and CF3 species (38.2% and 21.1%, respectively) in the C 1s spectrum among all the sputter-deposited surfaces. The O-containing species is only found on the s-PTFE-Si (H2) and s-PTFESi (O2) surfaces. On the other hand, the N-containing species are only present on the s-PTFE-Si (N2) surface. 2. Chemical Structure of the Sputter-Deposited Fluoropolymer Films on Si(100) Surfaces. 2.1. Characterization by FTIR Spectroscopy. The chemical structure of the sputter-deposited PTFE-like (sPTFE) films was investigated first by FTIR spectroscopy. Figure 2 shows the respective FTIR spectra of the pristine PTFE film (Figure 2a), the s-PTFE (Ar) film (Figure 2b), the s-PTFE (CF4) film (Figure 2c), the s-PTFE (N2) film (Figure 2d), and the s-PTFE (H2) film (Figure 2e). The FTIR spectrum of the pristine PTFE film displays the characteristic absorption bands of the CF2 groups at the wavenumbers of 514 cm-1 (CF2 wagging), 640 cm-1 (CF2 rocking), 1152 cm-1 (asymmetric C-F stretching), and 1208 cm-1 (symmetric C-F stretching).24 Both the s-PTFE (Ar) and s-PTFE (CF4) films exhibit a strong, broad absorption band at about 1230 cm-1 (an overlap of CF, CF2, and CF3 vibrations), and relative weak absorption bands at about 740 and 991 cm-1 (CF3 vibrations).10,24 A medium intensity absorption band at about 1520 cm-1 (aromatic C-F stretching) is observed in the s-PTFE (Ar) film, while the s-PTFE (CF4) films shows a medium intensity absorption band at about 1720 cm-1 (CFdCF stretch).25 The FTIR spectrum of the s-PTFE (N2) film shows the absorption bands at 740, 991, and 1208 cm-1 for the CFx species. The absorption bands at 1340 and 1750 cm-1, are probably associated with the absorption of the CN species.10 The s-PTFE (H2) film shows a very strong absorption band at 2800-3000 cm-1, characteristic of the CH groups, and a strong absorption band at around 1730 cm-1, characteristic of the absorption of the CdO bonds.16 The FTIR results of the s-PTFE (H2) film are rather consistent with the XPS results of the corresponding sample and indicate that the s-PTFE (H2) film contains high concentrations of hydrocarbon and oxidized carbon species. 2.2. Characterization by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). ToF-SIMS is (24) Starkweather, H. W.; Ferguson, R. C.; Chane, D. B.; Minor, J. M. Macromolecules 1985, 18, 1684. (25) Dias, A. J.; McCarthy, T. J. Macromolecules 1985, 18, 1826.

useful to the determination of the molecular structure of polymers, in particular the structure of the repeat units, the end groups, and the oligomeric segments.26,27 Figure 3 shows the positive ion ToF-SIMS spectra of the pristine PTFE film (Figure 3a), the s-PTFE (Ar) film (Figure 3b), the s-PTFE (CF4) film (Figure 3c), the s-PTFE (N2) film (Figure 3d), and the s-PTFE (H2) film (Figure 3e) on the Si(100) surfaces. The assignments of the positive ions with the CF2 repeat units are given in Table 2. It can be observed that the pristine PTFE film has CF2 segments of various lengths in the ToF-SIMS spectrum. Relatively strong peak intensities for the CF2 repeat units can be observed in the high mass region. The spectra of the s-PTFE-Si (Ar) and the s-PTFE-Si (CF4) surfaces also indicate the presence of long CF2 segments in its molecular structure. However, the end groups of these long CF2 segments differ from those in the pristine PTFE film. In addition to mass fragments for CF2 segments, the ToF-SIMS spectrum of the s-PTFE-Si (N2) surface also indicates masses at 52 for the +NF2 group, at 64 for the + CNF2 group, and at 102 for the F2C+NF2 group. Fluorocarbon segments with different number of the -CF2repeat units (n10 and n11 in Figure 3d) and having -CNF2 or F2CNF2 end groups can also be observed in the ToFSIMS spectrum of the s-PTFE (N2) film. The presence of -CNF2 and F2CNF2 groups in the s-PTFE (N2) film gives further support to the peak assignments in the C 1s corelevel spectrum of Figure 1d. On the other hand, the ToF-SIMS spectrum of the s-PTFE (H2) film is rather similar to that of the polyethylene film and shows the mass fragments associated with the hydrocarbon species.28 The assignments of these hydrocarbon species are listed in Chart 1. Oxygen-containing species of fairly strong intensities are also present in the ToF-SIMS spectrum. The ToFSIMS result thus confirms that the H2 plasma gives rise to the deposition of polyethylene-like materials. 3. Surface Morphology of the Sputter-Deposited Films As Revealed by Atomic Force Microscopy. Surface morphology is of great importance to the application of the deposited polymer films as dielectric barriers or passivation films on Si wafer surfaces. A uniform and defect-free surface is preferable in the planarization process. Figure 4a-e shows the respective AFM images of the pristine Si(100) surface (Figure 4a), the s-PTFE-Si (Ar) surface (Figure 4b), the s-PTFE-Si (CF4) surface (Figure 4c), the s-PTFE-Si (N2) surface (Figure 4d), and the s-PTFE-Si (H2) surface (Figure 4e). The average surface roughness value, Ra, of the pristine Si(100) surface is about 0.35 nm. Previous study has shown that the surface roughness of Si(100) substrate decreases slightly after the Ar plasma treatment.29 In comparison (26) Linton, R. W.; Mawn, M. P.; Belu, A. M.; DeSimone, J. M.; Hunt, M. O.; Mencelonglu, Y. Z.; Cramer, H. G.; Benninghoven, A. Surf. Interface Anal. 1993, 20, 991. (27) Hanton, S. D. Chem. Rev. 2001, 101, 527. (28) Galuska, A. A. Surf. Interface Anal. 1997, 25, 790.

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Table 2. Positive Ion Assignments of the ToF-SIMS Spectra of the Fluoropolymer Surfaces repeat unit and m/z

species (CF2)n1C+ (CF2)n2CF+ (CF2)n3C2F (CF2)n4CF2+ (CF2)n5C3F+ (CF2)n6C4F+ (CF2)n7CF3+ (CF2)n8C5F+ (CF2)n9C6F+ (CF2)n10CNF2+ (CF2)n11CF2NF2+

n1 m/z n2 m/z n3 m/z n4 m/z n5 m/z n6 m/z n7 m/z n8 m/z n9 m/z n10 m/z n11 m/z

0 12 0 31 0 43 0 50 0 55 0 67 0 69 0 79 0 91 0 64 0 102

1 62 1 81 1 93 1 100 1 105 1 117 1 119 1 129 1 141 1 114 1 152

2 112 2 131 2 143 2 150 2 155 2 167 2 169 2 179 2 191 2 164 2 202

3 162 3 181 3 193 3 200 3 205 3 217 3 219 3 229 3 241 3 214 3 252

4 212 4 231 4 243 4 250 4 255 4 267 4 269 4 279 4 291 4 264 4 302

5 262 5 281 5 293 5 300 5 305 5 317 5 319 5 329 5 341 5 314 5 352

6 312 6 331 6 343 6 350 6 355 6 367 6 369 6 379 6 391 6 364 6 402

7 362 7 381 7 393 7 400 7 405 7 417 7 419 7 429 7 441 7 414 7 452

8 412 8 431 8 443 8 450 8 455 8 467 8 469 8 479 8 491 8 464 8 502

9 462 9 481 9 493 9 500 9 505 9 517 9 519 9 529 9 541 9 514 9 552

10 512 10 531 10 543 10 550 10 555 10 567 10 569 10 579 10 591 10 564 10 602

Chart 1. Assignments of Hydrocarbon Species in the s-PTFE (H2) Film

Table 3. Effect of the Sputtering Gas Plasma on the Dielectric Constant, Deposition Rate, Static Water Contact Angle, and [F]/[C] Ratio of the s-PTFE Films Deposited on the Si(100) Surfaces

sample PTFE s-PTFE-Si (Ar) s-PTFE-Si (CF4) s-PTFE-Si (N2) s-PTFE-Si (H2) s-PTFE-Si (O2) pristine Si(100)

dielectric constant (κ)

deposn rate (nm/min)

2.1 2.2 1.9 2.5 2.7

13.4 2.3 8.1 2.0

contact angle θ (deg) 108 106 109 98 77 32 20

[F]/[C] ratio 2.0 1.6 1.9 1.7 0.1 0.2

to that of the pristine Si(100) surface, the s-PTFE-Si (Ar), s-PTFE-Si (CF4), s-PTFE-Si (N2), and s-PTFE-Si (H2) surfaces have only marginally higher Ra values of 0.47, 0.40, 0.36, and 0.76 nm, respectively. The marginal increase in surface roughness after sputter deposition suggests that the technique can produce almost molecularly flat surfaces. Results of the film thickness measurements also indicate that the uniformity in thickness of the s-PTFE (Ar) film on a 150 mm (6 in.) Si(100) wafer falls within (5% (1 σ value). 4. Physicochemical Properties of the SputterDeposited Films on Si(100) Surfaces. Table 3 shows the effect of the sputtering gas on the deposition rate, dielectric constant, water contact angle, and the [F]/[C] ratio of the resulting film deposited on the Si(100) surface. Among all the gases used, the Ar plasma gives rise to the highest deposition rate, while the H2 plasma gives rise to the lowest deposition rate. However, the O2 plasma fails to give rise to any deposition. Previous studies have suggested that the mechanisms of the rf sputtering and deposition of PTFE films are via the plasma polymerization (29) Zhang, Y.; Tan, K. L.; Liaw, B. Y.; Liaw, D. J.; Kang, E. T.; Neoh, K. G. J. Vac. Sci. Technol., A 2001, 19, 547.

Figure 4. AFM images of (a) the pristine Si(100) surface, (b) the s-PTFE-Si (Ar) surface, (c) the s-PTFE-Si (CF4) surface, (d) the s-PTFE-Si (N2) surface, and (e) the s-PTFE-Si (H2) surface.

of the sputter-generated tetrafluoroethylene (TFE) monomer6 and the rearrangement of the sputter-generated -(CF2)x- oligomeric segments.30 Since Ar is a nonreactive gas, the sputter-generated active species are not consumed by the argon plasma. On the other hand, some of the active species are consumed by the other sputtering gas plasmas, (30) Horie, M. J. Vac. Sci. Technol., A 1995, 13, 2490.

Deposition of Films on Si(100) Surfaces

as indicated, for instance, by the incorporation of new elements and groups into the s-PTFE-Si (N2) surface. Thus, the Ar plasma gives rise to the highest deposition rate among the five gas plasmas used. In the case of the hydrogen plasma, the strong defluorination effect of the H2 plasma decreases the deposition rate of the s-PTFE (H2) film further.23 The absence of any appreciable polymer deposits on the Si(100) when the O2 plasma is used suggests that the sputter-generated species are consumed almost completely by the oxygen plasma. Table 3 also shows the effect of sputtering gas plasma on the dielectric constant (κ) of the s-PTFE film on the Si(100) surface. According to Bohr’s equation,31 the lower the dielectric constant of the interlayer dielectric, the higher the signal transformation rate of an integrated circuit (IC) device. It is thus of great importance to study the dielectric constants of the s-PTFE films. It can be observed that the CF4 plasma gives rise to an s-PTFE film with the lowest dielectric constant among all the s-PTFE films. In fact, an ultralow dielectric constant of 1.9 is obtained. The value is even lower than that of the pristine PTFE film (κ ) 2.0-2.1).3,4 The [F]/[C] ratio of 1.9 for the s-PTFE (CF4) film is also the highest among the s-PTFE films. The low dielectric constant of the s-PTFE (CF4) film is consistent with the retention of a high fluorine concentration in the deposited film. Previous study has shown that the higher the fluorine concentration, the lower the dielectric constant.32 In addition, the amorphous nature of the s-PTFE (CF4) film readily gives rise to an increase in free space (arising from the decrease in chain packing efficiency) of the film and, thus, decreases the dielectric constant further.33 The XPS and FTIR results also show that no polar groups, such as O-containing and N-containing groups, are incorporated into the s-PTFE (CF4) film to contribute to an increase in dielectric constant. The dielectric constant of the s-PTFE (Ar) film is comparable to that of the pristine PTFE film. This value is slightly higher than that of the s-PTFE (CF4) film and is probably due to the reduced fluorine content in the s-PTFE (Ar) film ([F]/[C] ) 1.6). Although the s-PTFE (N2) film has a higher fluorine content ([F]/[C] ) 1.7) than that of the s-PTFE (Ar) film, the presence of the Ncontaining groups increases its dielectric constant to about 2.5. The presence of oxygen-containing polar groups and the low fluorine concentration ([F]/[C] ) 0.1) in the molecular structure of the s-PTFE (H2) film have increased its dielectric constant to about 2.7. Surface hydrophobicity is another important property of the polymers used as barrier films and as dielectric materials. Table 3 also shows the static water contact angles (θ) of the s-PTFE-Si surfaces. The water contact angles of the pristine PTFE surface and of the pristine Si(100) surface (after the “piranha” solution cleaning) are also included in Table 3 for comparison purpose. The water contact angles of both the s-PTFE-Si (Ar) and the s-PTFE-Si (CF4) surfaces are comparable to that of the pristine PTFE film surface. The hydrophobic nature of the s-PTFE-Si (Ar) and s-PTFE-Si (CF4) surfaces is consistent with the high [F]/[C] ratio of the fluoropolymer films. The water contact angle of the s-PTFE-Si (N2) surface is lower than that of the s-PTFE-Si (Ar) surface, though the former has a higher [F]/[C] ratio. This (31) Bohr, M. T. Solid State Technol. 1996, 9, 105. (32) Sasaki, A.; Hishi, S. In Polyimides: Fundamentals and Applications; Ghosh, M. K., Mittal, K. L., Eds.; Marcel Dekker: New York, 1996; p 83. (33) Morgen, M.; Ryan, E. T.; Zhao, J. H.; Hu, C.; Cho, T.; Ho, P. S. Annu. Rev. Mater. Sci. 2000, 30, 645.

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Figure 5. XPS wide scan spectra of the delaminated (a) s-PTFE-Si (Ar) surface and (b) Cu tape surface from a Cu tape/s-PTFE-Si (Ar) laminate and the delaminated (c) s-PTFESi (CF4) surface and (d) Cu tape surface from a Cu tape/sPTFE-Si (CF4) laminate.

phenomenon is, nevertheless, consistent with the presence of N-containing polar groups in the molecular structure of the s-PTFE-Si (N2) surface, as indicated by the XPS and ToF-SIMS results. The s-PTFE-Si (H2) surface has a water contact angle of only about 77°. This value is even lower than that of the pristine polyethylene surface (about 95°) and can be attributable to the presence of some O-containing polar groups in its molecular structure. Although the O2 plasma did not give rise to any significant film deposition in the present work, a minute amount of fluorine-containing species was nevertheless deposited on the silicon surface, as show by the XPS result in Figure 1f. Thus, the water contact angle of the s-PTFE-Si (O2) surface increases to 32°, from about 20° for the pristine Si(100) surface. 5. Adhesion Characteristics of the Sputter-Deposited Films with the Si(100) Surfaces. The interaction of the s-PTFE films with the plasma preactivated Si(100) substrates was evaluated by the 180°-peel adhesion test. Copper foil adhesive tapes were applied to the various s-PTFE-Si surfaces and subsequently peeled off on the tensile tester. Both the delaminated fluoropolymer surface and the copper tape surface were analyzed by XPS. Figure 5a,b shows the respective wide scan spectra of the delaminated s-PTFE-Si (Ar) and copper tape surfaces from a copper tape/s-PTFE-Si (Ar) laminate having a 180°-peel adhesion strength of about 3.3 N/cm. The corresponding wide scan spectra of the delaminated s-PTFE-Si (CF4) and copper tape surfaces from a copper tape/s-PTFE-Si (CF4) laminate, having a 180°-peel adhesion strength of about 3.0 N/cm, are shown in Figure 5c,d. The absence of fluorine signals in the wide scan spectra of the two delaminated copper tape surfaces and the appearance of a weak O 1s signal in the wide scan spectra of the two delaminated s-PTFE-Si surfaces suggest that both of the Cu tape/s-PTFE-Si assemblies have delaminated by adhesive failure in the adhesive layer of the copper tape. The observation further suggests that the s-PTFE films are strongly bonded to the Si(100) surfaces. Similar XPS results are obtained for the delaminated surfaces involving the copper tapes and the s-PTFE-Si (N2) and the s-PTFE-Si (H2) surfaces. Thus, all the sputter-deposited PTFE-like films exhibit good adhesion to the Si(100) surfaces. Conclusion Thin dielectric polymer films were deposited on the Si(100) substrates via the plasma sputtering of a PTFE

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target in a rf magnetron sputtering system, using Ar, CF4, H2, N2, and O2 as the sputtering gas. The effects of the sputtering gas plasma on the chemical composition and structure, as well as on the physicochemical and dielectric properties, of the sputter-deposited polymer film were investigated. The CF4 plasma gave rise to a fluoropolymer film with a fluorine content comparable to that of the pristine PTFE film. The film deposited by Ar plasma has a lower fluorine content. Both films contained long segments of CF2 repeat units. The fluorine content of the N2 plasma sputter-deposited film was higher than that of the Ar sputter-deposited film, albeit some N-containing species were also incorporated into the fluoropolymer deposit. On the other hand, H2 plasma gave rise to a polyethylene-like film with a very low fluorine content.

Zhang et al.

The O2 plasma gave no deposition on the Si(100) surface. The dielectric constant (κ) of the deposited polymer film increased with the type of the gas plasma in the following order: CF4 < Ar < N2 < H2. The low κ value of the CF4 plasma sputter-deposited film (κ ∼ 1.9) was attributed to its high fluorine content and the amorphous nature (increased free space) of the deposited fluoropolymer film. The deposition rate of the film was also found to be dependent on the type of the sputtering gas plasma and decreased in the following order: Ar > N2 > CF4 > H2 > O2. The 180°-peel adhesion test results suggested that the sputter-deposited fluoropolymer films adhered strongly to the Si(100) surfaces. LA011606J