Langmuir 2003, 19, 6845-6850
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Surface Modification of Polyimide Films via Plasma-Enhanced Chemical Vapor Deposition of Thin Silica and Nitride Films Hongbin Li and Rajnish K. Sharma Foundry of Innovation Devices, Institute of Microelectronics, 11 Science Park II, Singapore, 117685
Yan Zhang and Andrew A. O. Tay Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
E. T. Kang* and K. G. Neoh Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received March 10, 2003. In Final Form: May 27, 2003 Silicon oxide (SiOx) and silicon nitride (SixNy) thin films of different thicknesses were deposited on the polyimide (PI, Kapton HN) film surfaces via plasma-enhanced chemical vapor deposition (PECVD). The silica films were prepared from two types of gaseous mixtures, including (1) tetraethyl orthosilicate (Teos) and O2 and (2) SiH4 and N2O, while the nitride film was from the gaseous mixture of SiH4, NH3, and N2. Characterization of the silica and nitride ultrathin films (∼ 3 nm) on the PI substrates via X-ray photoelectron spectroscopy (XPS) revealed the presence of chemical interaction between the passivating layer and the underlying PI surface. The deposited silica and nitride films were found to enhance the dynamic surface microhardness of the PI films. The extent of enhancement was dependent on the thickness and the type of the deposited films. The 180°-peel adhesion strength measurements revealed that the deposited silica and nitride films adhered strongly to the PI substrates.
Introduction Silicon oxide and silicon nitride thin films, prepared by plasma-enhanced chemical vapor deposition (PECVD), possess a wide range of interesting properties, such as low optical index, low gas permeability, and an easy-tointegrate deposition process.1-3 These silica materials have been widely used in the microelectronics industry as dielectric layers, diffusion barriers, and hard masks.4-6 They have also been used in optical systems as optical filers, antireflective coatings, and optical waveguides.1 Recently, there has been increasing interest in PECVD of thin silica films on polymer surfaces as passivation layers. The silica-passivated polymer films have been used for food and medical packagings.7 Most of the previous studies were focused on the deposition of silica films on poly(ethylene terephthalate) (PET) surfaces.8-12 Other * To whom all correspondence should be addressed.Telephone: +65-6874-2189. Fax: +65-6779-1936. E-mail address: cheket@ nus.edu.sg. (1) Martinu, L.; Poltras, D. J. Vac. Sci. Technol. A 2000, 18, 2619. (2) Benmalek, M.; Dunlop, H. M. Surf. Coat. Technol. 1995, 76-77, 821. (3) Chan, C. M.; Ko, T. M.; Hiraoka, H. Surf. Sci. Rep. 1996, 24, 3. (4) Maier, G. Prog. Polym. Sci. 2001, 26, 3. (5) Nalwa, H. S. Handbook of Low and High Dielectric Constant Materials and Their Applications; Academic: London, 1999. (6) Schulz, A.; Baumgartner, K. M.; Feichtinger, J.; Walker, M.; Schumacher, U.; Eike, A.; Herz, K.; Kessler, F. Surf. Coat. Technol. 2001, 142, 771. (7) Chatham, H. Surf. Coat. Technol. 1996, 78, 1. (8) Dennler, G.; Houdayer, A.; Raynaud, P.; Segui, Y.; Wertheimer, M. R. Nucl. Instrum. Methods B 2002, 192, 420. (9) Erlat, A. G.; Spontak, R. J.; Clarke, R. P.; Robinson, T. C.; Haaland, P. D.; Tropsha, Y.; Harvey, N. G.; Vogler, E. A. J. Phys. Chem. B 1999, 103, 6047.
polymeric substrates, such as polycarbonate (PC),13,14 polystyrene (PS),15 and polyimides (PI),8,16,17 have also been investigated. Poly(pyromellitic dianhydride-co-4,4′-oxydianiline)based (PMDA-ODA-based) polyimides (PI’s) have been an important family of polymers for the packaging of microelectronics because of their good physicochemical and electrical properties, including high signal transmission, low dielectric constant, and high thermal stability.18 PI has been used extensively as interlayer dielectrics in high-density microelectronic devices. However, the PI films suffer from inadequate surface hardness, and higher oxygen and moisture permeability, in comparison with the silica dielectrics, such as silicon oxides (SiOx) and silicon nitrides (SixNy). The poor surface properties of the PI films give rise to challenges in the integration processes, such as the planarization of PI in the chemical-mechanical polishing (CMP) process. Mechanical abrasion in the CMP (10) Sobrinho, A. S. D.; Schuhler, N.; Klemberg-Sapieha, J. E.; Wertheimer, M. R.; Andrews, M.; Gujrathi, S. C.; J. Vac. Sci. Technol. A 1998, 16, 2021. (11) Teshima, K.; Inoue, Y.; Sugimura, H.; Takai, O. Thin Solid Films 2002, 420, 324. (12) Leterrier, Y. Prog. Mater. Sci. 2003, 48, 1. (13) Rats, D.; Hajek, V.; Martinu, L. Thin Solid Films 1999, 340, 33. (14) Zajickova, L.; Bursikova, V.; Janca, J. Vacuum 1998, 50, 19. (15) Felts, J. T.; Grubb, A. D. J. Vac. Sci. Technol. A 1992, 10, 1675. (16) Gleskova, H.; Wagner, S.; Gasparik, V.; Kovac, P. Appl. Surf. Sci. 2001, 175, 12. (17) Adema, G. M.; Hwang, L. T.; Rinne, G. A.; Turlik, I. IEEE Trans. Compon., Hybrids, Manuf. Technol. 1993, 16, 53. (18) Feger, C.; Franke, H. In Polyimides: Fundamentals and Applications; Ghosh, M. K., Mittal, K. L., Eds.; Dekker: New York, 1996; Chapter 24.
10.1021/la0344074 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/01/2003
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process is usually a critical challenge to the polymeric dielectrics. In addition, the relatively high permeability of the PI films to moisture and oxygen will also give rise to the conductor oxidation and reliability problems in the PI-based devices. Therefore, surface passivation of PI films with PECVD of silica films to improve the mechanical and gas barrier properties is of great interest to the microelectronics industry. In addition to the fine work carried out on the passivation of PI surfaces,8,16,17 characterization of the various silica-PI interfaces and investigation of the effect of different silica and nitride thin films on the surface hardness of the PI substrates are yet to be studied in more detail. In this work, silicon oxide (SixOy) and silicon nitride (SixNy) films with thicknesses ranging from 3 to 1000 nm were deposited on the PI film surfaces. The surface and interface composition of the modified PI films is characterized by X-ray photoelectron spectroscopy (XPS). Surface topography of the pristine and the passivated PI films is characterized by atomic force microscopy (AFM). The effects of thickness and the chemical nature of the thin films from PECVD on the dynamic surface microhardness of the PI films are also investigated. The adhesion of the deposited silica films to the PI surfaces was evaluated by the Scotch-tape peel adhesion test. Experimental Section Materials. The polyimide (PI) film used in this study was obtained from DuPont Chemical Co. as Kapton HN in rolls of 40 mm in width and 75 µm in thickness. The surfaces of the PI films were cleaned with acetone in an ultrasonic water bath for 20 min and then dried at 80 °C under reduced pressure before use. PECVD of Silicon Oxide (SiOx) and Silicon Nitride (SixNy) Films on PI Surfaces. The SiOx films on the PI films were prepared via two different processes. In the first process, the SiOx films were deposited using a PECVD & RIE Series 790 system, manufactured by Plasma-Therm, Inc., of St. Petersburg, FL. The frequency of the AC electrical power supply to the plasma reactor was 13.56 MHz. The process gas mixture was allowed to flow into the reactor evenly from a shower-head distributor in the upper electrode. The process gases were silane (SiH4) and nitrous oxide (N2O). The gas flow rates were 6 and 400 cm2(STP)/min, respectively. The glow discharge was ignited after impedance matching at a substrate temperature of 350 °C, a chamber pressure of 900 mTorr, and an input radio frequency (RF) power of 20 W in all cases. The deposition process was allowed to proceed for 7 s to 20 min to obtain films with thicknesses ranging from 3 to 1000 nm. The thickness of the film was determined from a concurrently deposited silica film on a Si(100) wafer substrate. The thickness was measured on a Nanospec/AFT system, manufactured by Nanometrics Co. of Sunny Vale, CA. In the second process, the SiOx films were deposited using a Precision 5000 Mark II PECVD system, manufactured by Applied Materials Co. of Santa Clara, CA. The process gases were vaporized tetraethyl orthosilicate (Teos) and purified oxygen (O2), both having the same gas flow rate of 500 cm2(STP)/min. The glow discharge was ignited after impedance matching at a substrate temperature of 350 °C, a chamber pressure of 9.5 mTorr ,and an input RF power of 430 W in all cases. The deposition process was allowed to proceed from 2 s to 10 min. The SixNy films on the PI films were also deposited using the Plasma-Therm PECVD & RIE Series 790 System. The process gases were silane (SiH4), ammonia (NH3), and nitrogen (N2), at gas flow rates of 190, 70, and 1800 cm2(STP)/min, respectively. The glow discharge was ignited after impedance matching at a substrate temperature of 350 °C, a chamber pressure of 950 mTorr, and an input RF power of 200 W in all cases. The deposition process was allowed to proceed from 3 s to 13 min to obtain films of varying thicknesses. To minimize the mismatch in the coefficients of thermal expansion between the PI and silica or nitride films, all the surface-modified PI films were allowed to cool very slowly(at a rate of 1 °C/min) to room temperature.
Li et al. Significant curving of the PI substrates was observed only in samples with a silica or nitride thickness exceeding 500 nm. Surface Characterization. The chemical composition of the pristine and the silica- and nitride-passivated PI surfaces were determined by X-ray photoelectron spectroscopy (XPS). The XPS measurements were made 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 PI substrates 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 (BEs) were referenced to the C 1s neutral carbon peak at 284.6 eV. In curve fitting, the line width (full width at halfmaximum, or fwhm) for the Gaussian peaks was maintained constant for all components in a particular spectrum. After identification of the component lines in a particular spectrum, and taking into account the line resolution of the apparatus, the line width (fwhm) was selected to give the best fit to the spectrum. Surface elemental stoichiometries were determined from peakarea ratios, after correcting with the experimentally determined sensitivity factors, and were reliable to (5 at. %. The elemental sensitivity factors were determined using stable binary compounds of well-established stoichiometries. The surface topography of PI films after PECVD of silica and nitride was investigated using a Nanoscope IIIa atomic force microscope (AFM). All images were collected in air using the tapping mode under a constant force (scan size, 1 µm; scan rate, 2 Hz). Surface Microhardness Measurement. The surface microhardness of the pristine and the silica- and nitride-passivated PI films was measured using a Shimadzu DUH-200 dynamic ultra-microhardness tester (Shimadzu Corp., Kyoto, Japan). The dynamic hardness was calculated from a load applied to the film surface through a triangular microindenter with an apex angle of 115° and the resulting indentation depth. Each hardness value reported was the average of at least 10 readings measured at different locations of the film surface. Adhesion Strength Measurements. The adhesion strength of the silica and nitride films from PECVD to the PI substrates was evaluated by the 180°-peel adhesion test. Scotch tape was applied to the deposited silica or nitride film on the PI surface and subsequently peeled off on an Instron 5544 tensile tester from the Instron Corp. of Canton, MA. All peel tests were carried out at a cross-head speed of 10 mm/min. Each adhesion strength reported was the average of at least three sample measurements.
Results and Discussion The abbreviations SiOx(Teos)-PI, SiOx(SiH)-PI, and SixNy-PI will be used to denote the silica and nitride films on PI film surfaces, prepared from the respective gas mixtures of tetraethyl orthosilicate (Teos) and O2, SiH4 and N2O, and SiH4, N2, and NH3. Surface Compositions of the Pristine and SurfaceModified PI Films. Figure 1 shows the respective C 1s core-level and widescan spectra of the pristine PI surface (Figure 1a), the SiOx(Teos)-PI surface (Figure 1b), the SiOx(SiH)-PI surface (Figure 1c), and the SixNy-PI surface (Figure 1d). To better understand the interaction between the PECVD layers and the underlying PI surfaces, the thickness of the deposited silica and nitride films is intentionally controlled to be less than 3 nm, or less than the probing depth of the present XPS technique.19 No Si signal is discernible in the wide-scan spectrum of pristine PI film. On the other hand, the wide-scan spectra of the three silica and nitride-passivated PI films show high intensities of the Si 2s and Si 2p signals. The (19) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2.
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Figure 1. XPS C 1s core-level and wide-scan spectra of (a) the pristine PI surface, (b) the SiOx(Teos)-PI surface, (c) the SiOx(SiH)-PI, and (d) the SixNy-PI surface.
Figure 2. XPS N 1s and Si 2p core-level spectra of (a) the pristine PI surface, (b) the SiOx(Teos)-PI surface, (c) the SiOx(SiH)-PI, and (d) the SixNy-PI surface.
N 1s signals in the wide-scan spectra of the SiOx(Teos)-PI and SiOx(SiH)-PI surfaces are still discernible. The highresolution N 1s core-level spectra (see Figure 2b,c below) reveal that the N 1s signal from both surfaces is associated with the nitrogen species of the underlying PI film. The above results suggest that the thickness of the deposited silica films is less than the probing depth of the present XPS technique (about 3 nm for an inorganic matrix19). The uniformity in substrate surface coverage for the two PECVD systems was determined to be within (1 nm for silica and nitride films of
