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Influence of atmospheric fluorine plasma treatment on thermal and dielectric properties of polyimide film
Journal of Colloid and Interface Science 332 (2009) 246–250
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Influence of atmospheric fluorine plasma treatment on thermal and dielectric properties of polyimide film Soo-Jin Park a,∗ , Hee-Jin Sohn b , Sung-Kwon Hong b , Gwi-Su Shin c a b c
Department of Chemistry, Inha University, 253, Nam-gu, Incheon 402-751, South Korea Polymer Science and Engineering, Chungnam National University, 220, Yuseong, Daejeon 305-764, South Korea Jeonbuk Regional Innovation Agency, Jeonbuk Technopark, 723-1, Jeonju 561-844, South Korea
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 26 September 2008 Accepted 15 December 2008 Available online 24 December 2008
Plasma treatment of polyimide surfaces not only causes structural modification during the plasma exposure, but also leaves active sites on the surfaces that are subject to post-reaction. In this work, the effects of atmospheric fluorine plasma treatment on the surface properties and dielectric properties of polyimide thin film were investigated by using X-ray photoelectron spectroscopy (XPS), Fourier transform-IR (FT-IR) spectroscopy, and contact angle measurement. The results indicated that plasma treatment successfully introduced fluorine functional groups on the polyimide surfaces. The polyimides also exhibited good thermal stability and a lower dielectric constant. It appears that the replacement of fluorine led to the decrease of the local electronic polarizability of polyimide. Consequently, it was found that the atmospheric fluorine plasma-treated polyimides possessed lower dielectric characteristics than the untreated polyimides. © 2009 Elsevier Inc. All rights reserved.
Keywords: Polyimide Atmospheric fluorine plasma treatment Contact angle measurement Dielectric characteristics
1. Introduction Polymeric materials are distinguished by low density, flexibility, versatile electronic properties, chemical inertness and low cost. As a result, they have been extensively used in fields ranging from aerospace to microelectronics, optoelectronics, composites and fiber optics. Among these polymeric materials, polyimides are one of the well-known engineering plastics, offering excellent thermal, mechanical, dielectric and optical properties, along with good chemical resistance and high dimensional stability [1–6]. They are distinguished from other high-performance polymers by the solubility of their poly(amic acid) precursor form, which can be cast into uniform films and quantitatively converted to polyimide. Owing to these qualities, polyimides have been used widely in the microelectronics, film, adhesive, and membrane industries. However, most commercial polyimides are usually infusible and insoluble in common organic solvents which have posed limits on their applications in expensive goods [7–11]. For these reasons, various surface modification techniques have been developed in order to transform these inexpensive materials into highly valuable products. Two broad kinds of surface modification treatments are available: physical treatment and chemical treatment. Chemical treatments introduce chemical bonding to the matrix in order to produce a highly cross-linked interphase. KOH and amine treatment
*
Corresponding author. Fax: +82 42 860 7237. E-mail address: [email protected] (S.-J. Park).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.12.040
© 2009
Elsevier Inc. All rights reserved.
are examples of chemical treatments [12–14]. Physical treatments include irradiation of surfaces with generated plasma and corona treatment. Traditional surface modification methods have the disadvantage of producing a considerable amount of hazardous substances and vapors, which could pollute the environment and must be disposed of appropriately. A more environmentally friendly alternative is the use of plasma technologies. Plasma is an ionized gas containing a mixture of ions, electrons, neutral and excited molecules and photons [15,16]. There are two principal advantages to using mild plasma treatment for materials. One advantage is that the reaction takes place only on the surface of the material, without significantly changing its bulk properties. The other is that it is possible to produce various types of plasma, such as oxidative, reductive and inactive plasmas. Plasma treatments have become increasingly important owing to their very high efficiency and operational; simplicity [17–22]. In this work, we investigated the effect of atmospheric fluorine plasma treatment on the surface properties of polyimide films. Changes in the thermal properties and dielectric characteristics of the polyimide films were also studied with respect to the plasma treatment conditions. 2. Experiments 2.1. Sample preparation and plasma modification The experiments were conducted using commercial polyimide films obtained from Du Pont Chemical Co., under the trade name
S.-J. Park et al. / Journal of Colloid and Interface Science 332 (2009) 246–250
247
2.2. Surface analyses
Fig. 1. Schematic diagram of plasma treatment apparatus.
ATR (attenuated total reflection) FT-IR spectra of the plasmatreated PI films were obtained using a MIDAC M2000. Scans were taken from 400 to 4000 cm−1 and required 40 s to complete. X-ray photoelectron spectroscopy (XPS) measurement of the polyimide surfaces was performed using a VG Scientific ESCA LAB MK-II spectrometer equipped with an MgK α X-ray source. The base pressure in the sample chamber was controlled in the range of 10−8 to 10−9 Torr. The contact angles of the plasma-treated polyimide surfaces were determined at 20 ± 1 ◦ C using a sessile drop method with a Surface & Electro-Optic Co., SEO 300A. Three different wetting liquids, distilled water, diiodomethane and ethylene glycerol, were selected. For each sample, every calculated contact angle was an average of 10 measurements with a standard deviation below 1◦ . The surface morphologies of plasma-treated polyimide films were observed by atomic force microscopy (AFM), using a Digital Instruments Inc., Nanoscope III microscope. The AFM observation was carried out at ambient pressure and room temperature. In each case, an area of 10 × 10 μm was scanned using the tapping mode. 2.3. Thermal properties The glass transition temperature of the plasma-treated polyimide film was measured using differential scanning calorimetry (DSC) (Dupont 9900, Model 2910) from 30 to 400 ◦ C at a heating rate of 10 ◦ C/min and a second heating scan. To investigate the thermal stability of the polyimide film, thermogravimetric analyses were performed in nitrogen using a TGA 951 Dupont thermal analyzer, at a heating rate of 10 ◦ C/min from room temperature to 850 ◦ C. 2.4. Dielectric characteristics
Fig. 2. FT-IR spectra of the polyimide films: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI300, (e) PI-400.
Table 1 Plasma treatment condition of polyimide films. Conditions Treatment power Treatment times Sample name Radio frequency Carrier and reactive gas Carrier gas (He) Process speed
0, 100, 200, 300, 400 W 50 PI-0, PI-100, PI-200, PI-300, PI-400 13.65 MHz He and CHF3 gas 5 l/min 5 mm/s
The dielectric characteristics were measured using a dielectric spectrometer (Novocontrol GmbH, Model CONCEPT 40). The diameter of the electrode and the film was 20 mm. To ensure good electrical contact between the electrodes and the polyimide thin film, the films were sputter-coated beforehand on both sides with a thin layer of silver for 20 s. The dielectric constant of the polyimide thin film was calculated by using the equation:
εr =
C×D
ε0 × A
,
(1)
where ε0 is the permittivity of free space (8.85 × 10−14 Farad/cm), εr the permittivity of the polyimides, C the capacitance, D the film thickness, and A the electrode area. 3. Results and discussion
of Kapton, with a thickness of 50 μm. The films were cut into 100 × 100 mm sections and the surfaces were cleaned with methanol in an ultrasonic water bath for 30 min and then dried in a vacuum oven at room temperature. The atmospheric-pressure plasma surface treatment system in Fig. 1 was manufactured by PLASMART Korea (ATMOSMulti). A schematic diagram of the atmospheric plasma treatment apparatus is shown in Fig. 2. The gas used for the plasma treatments was a mixture of He (99%) and CHF3 (1%). The treatment power was varied between 0 and 400 W (PI-0, PI-100, PI-200, PI-300 and PI-400) at a radio frequency of 13.56 MHz. Treatment time was kept to 30 times for all the samples. Table 1 shows the specific plasma treatment conditions.
3.1. Surface analyses In order to investigate the effect of plasma treatments on the surface chemical modifications, chemical bonding states on the plasma-treated polyimide surfaces were measured by FT-IR and XPS. Fig. 2 shows the untreated and plasma-treated polyimides by FT-IR spectroscopy. All of the polyimides showed characteristic imide absorption bands at 1780–1785 cm−1 and 1727–1735 cm−1 . The peak at 1780–1785 cm−1 was attributable to the asymmetrical carbonyl stretching vibrations, and that at 1727–1735 cm−1 to the symmetrical carbonyl stretching vibrations. The absorption at 1380 cm−1 was assigned to C–N stretching, and C–F multiple
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Table 3 Contact angles determination (in ◦ ) of the plasma treated and untreated polyimide film surfaces.
PI-0 PI-100 PI-200 PI-300 PI-400
Fig. 3. XPS spectra of polyimide films before and after plasma treatments: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI-300, (e) PI-400.
Fig. 4. High resolution F1s XPS spectra of the polyimide films: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI-300, (e) PI-400. Table 2 Chemical composition of polyimide films.
PI-0 PI-100 PI-200 PI-300 PI-400
C1s (at%)
N1s (at%)
O1s (at%)
F1s (at%)
85.37 78.0 73.53 70.58 66.77
4.37 5.74 6.25 7.28 8.23
10.26 15.78 18.70 19.26 21.02
0 0.48 1.52 2.88 3.98
stretch bending adsorptions from the trifluoromethyl group were observed between 1400 and 1000 cm−1 . Fig. 3 shows the XPS survey scan of the polyimide film. The spectra in Fig. 4 show carbon, nitrogen, and oxygen (binding energies of 285, 404, and 532 eV, respectively) peaks. In the case of the plasma-treated polyimide, the XPS spectra newly introduced a fluorine peak (binding energy of 694 eV) and the at% carbon was reduced. Table 2 shows the normalized area and atomic composition obtained by XPS for the polyimides as a function of treatment power. The XPS spectra results indicate that the F1s /C1s ratios of the polyimides are increased with increasing treatment power, which can be attributed to the increase of carbon–fluorine functional groups on the polyimide surfaces with plasma treatment.
Water
Diiodomethane
Ethylene glycol
60.3 65.0 65.8 67.1 74.2
33.6 46.1 49.0 52.1 67.1
31.6 52.6 59.5 59.9 62.5
Fig. 4 shows the expanded scale of F1s XPS spectra for the polyimide films. At the time of plasma treatment, fluorine diffuses only slightly and slowly inside the polyimide and then the diffusing fluorine atom produces active radicals from the hydrocarbon backbone during the reaction because of the highest electronegative character of fluorine atom. Therefore, the intensity of the C–F groups is increased as a function of the treatment gas. Table 3 shows the contact angles obtained under deionized water, diiodomethane and Ethylene glycol on plasma-treated and untreated polyimide film. A water contact angle of 60.3◦ was obtained for polyimide film untreated by plasma. The water contact angles are increased with increasing plasma treatment power, whereas those of diiodomethane are decreased as the fluorine content is increased. Therefore, it can be inferred that the PI surfaces become significantly more hydrophobic. Changes in the surface morphology and roughness of the plasma-treated polyimide film with different treatment powers were more accurately observed by AFM as shown in Fig. 5. The surface roughness ( R a ) of the pure polyimide film surface is about 1.58 nm. The R a value of the plasma-treated polyimide for the treatment powers of 100, 200, 300 and 400 W were increased to 2.07, 5.67, 7.44 and 9.37 nm, respectively. In this experiment, the treatment power in the system controls the bombardment energy of fluorine ions in the plasma and reactive etching of the polyimide surface leads to the surface morphological changes. 3.2. Thermal properties The thermal properties of the polyimides were characterized using differential scanning calorimetry (DSC) for testing the glass transition temperature ( T g ) and thermo-gravimetric analysis (TGA) for the decomposition temperature ( T d ). Thermal behavior data for the polyimides is summarized in Table 4. The obtained glass transition temperature of the film as a function of fluorine is shown in Fig. 6. The temperature ranged from 275–283 ◦ C. The glass transition temperature of the film gradually decreased with increasing fluorine content. Fig. 7 shows TGA curves of the plasma-treated polyimides under nitrogen atmosphere. All the polyimides showed similar TGA curves, with initial decomposition temperatures ranging from 544– 590 ◦ C in nitrogen atmosphere. The polyimides did not show obvious weight loss before the scanning temperature reached 500 ◦ C in nitrogen, indicating that no thermal decomposition occurred. The polyimides exhibited temperatures corresponding to 5% weight loss ranging from 497–587 ◦ C under nitrogen. Thermal stability of the plasma-treated polyimide was decreased as compared to that of non-treated polyimide. However, the TGA data indicate that the polyimides have fairly high thermal stability as compared to other polymers. The results of thermal analysis demonstrate that the resulting polyimides should maintain excellent thermal stability, which would facilitate their application.
S.-J. Park et al. / Journal of Colloid and Interface Science 332 (2009) 246–250
(a)
249
(b)
Fig. 6. DSC curves of the polyimide films: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI-300, (e) PI-400.
(c)
(d)
(e) Fig. 5. AFM 3-dimensional images of polyimide films: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI-300, (e) PI-400. Table 4 Thermal properties of polyimide films.a (◦ C)
Sample
Tg
PI-0 PI-100 PI-200 PI-300 PI-400
283 282 278 277 275
Td
Fig. 7. TGA thermograms of polyimide films. (◦ C)
590 594 592 588 544
T5
(◦ C)
587 592 588 584 497
T 10
(◦ C)
604 608 607 606 600
a T d : Onset decomposition temperature; T 5 , T 10 : the decomposition temperature at 5% and 10% weight loss, respectively.
3.3. Dielectric characteristics Good dielectric and insulating properties are desirable for the application of polyimide in the microelectronic industry, such as for microelectronic packages. The dielectric data was obtained at selected frequency (106 Hz) at room temperature (25 ◦ C) as well as the different temperatures (25–200 ◦ C). The dielectric constants of the plasma-treated polyimide film are shown in Fig. 8. The dielectric constants ranged between 2.43–2.95 at 1 MHz and 25 ◦ C, which was lower than those of the untreated polyimide film. It can be seen that the dielectric constant decreased with increasing fluorine content of the polyimide film when the temperature was kept constant. Also, the values of the dielectric constant increase about 0.1–0.2 with increasing temperature (25–200 ◦ C). In the high-temperature, higher value of dielectric constant may be due to space charge polarization, which arises from mobility of ions in the material.
Fig. 8. Dielectric properties of the polyimide films at 1 MHz.
These excellent dielectric properties might be attributed to the incorporation of fluorine-included groups that possessed low electronic polarizability both in the main backbone and in the side chain of the polymer. This also increases the free volume due to the relatively large volume of fluorine compared with hydrogen,
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which reduces the number of polarizable groups per unit volume. As a result, the polyimide film treated with plasma at 400 W indicated the best dielectric property due to the highest loading fluorine content. 4. Conclusions In this work, atmospheric fluorine plasma treatment was performed at room temperature with different treatment powers to allow for the introduction of small amounts of fluorine-containing functionalities onto the surfaces of the polyimides. Surface fluorine content was 3.79 at% for the polyimide plasma treated at 400 W. This method allows for controlling and tailoring of the surface chemistry of modified polyimides. The dielectric constant was calculated to be 2.43 for the plasma-treated polyimide film. The dielectric constant value largely depends on the fluorine content, and the value decreases as the amount of fluorine content increases. The fluorine-containing group of polyimide films is known to be one of the candidates for interlayer dielectric materials in integrated circuits due to its excellent electrical properties, such as low dielectric constant, high breakdown voltage, low leakage current, and high bulk resistivity. Compared to the untreated polyimide, the plasma-treated polyimides possessed lower dielectric constant, which suggests good potential for microelectronic applications as dielectric and insulating materials.
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[15] [16] [17] [18] [19] [20] [21] [22]
L. Wang, Y. Tian, H. Ding, B. Liu, Eur. Polym. J. 43 (2007) 862. T.H. Chiang, S.L. Liu, S.Y. Lee, T.E. Hsieh, Eur. Polym. J. 44 (2008) 3482. Y. Takagi, Y. Gunjo, H. Toyoda, H. Sugai, Vacuum 83 (2008) 501. N. Sprang, D. Theirich, J. Engemann, Surf. Coat. Technol. 74 (1995) 689. L. Wang, Y. Tian, H. Ding, J. Li, Eur. Polym. J. 42 (2006) 2921. D.J. Liaw, B.Y. Liaw, C.W. Yu, Polymer 42 (2001) 5175. P.M. Hergenrother, K.A. Watson, J.G. Smith, J.W. Connell, R. Yokota, Polymer 43 (2002) 5077. C. Frgar, M.M. Khojasteh, M.S. Htoo, Advances in Polyimide Science and Technology, Technomic Publishing, New York, 1991. S.J. Park, H.S. Kim, F.L. Jin, J. Colloid Interface Sci. 282 (2005) 238. D.J. Liaw, F.C. Chang, M.K. Leung, M.Y. Chou, K. Muellen, Macromolecules 38 (2005) 4024. S.B. Deshpande, H.S. Potdar, M.M. Patil, V.V. Deshpande, Y.B. Khollam, J. Ind. Eng. Chem. 12 (2006) 584. A.K. Bledzki, S. Reihmane, J. Gassan, J. Appl. Polym. Sci. 59 (1996) 1329. S. Lee, S.S. Park, H.K. Lee, Macromol. Symp. 249-250 (2007) 586. M. Naddaf, C. Balasubramanian, P.S. Alegaonkar, V.N. Bhoraskar, A.B. Mandle, V. Ganeshan, S.V. Bhoraska, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 222 (2004) 135. N. Cui, M.D. Brown, Appl. Surf. Sci. 189 (2002) 31. S.J. Park, H.Y. Lee, J. Colloid Interface Sci. 285 (2005) 267. W.J. Park, S.G. Yoon, W.S. Jung, D.H. Yoon, Surf. Coat. Technol. 201 (2007) 5017. H.V. Boenig, Plasma Science and Technology, Cornell Press, New York, 1982. M. Mozetiˇc, A. Zalar, P. Panjan, M. Bele, S. Pejovnik, R. Grmek, Thin Solid Films 376 (2000) 5. S.J. Park, J.S. Kim, J. Colloid Interface Sci. 244 (2001) 336. V.M. Litvinov, P.A.M. Steeman, Macromolecules 32 (1999) 8476. G. Akovali, I. Ulkem, Polymer 40 (1999) 7417.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Influence of atmospheric fluorine plasma treatment on thermal and dielectric properties of polyimide film Soo-Jin Park a,∗ , Hee-Jin Sohn b , Sung-Kwon Hong b , Gwi-Su Shin c a b c
Department of Chemistry, Inha University, 253, Nam-gu, Incheon 402-751, South Korea Polymer Science and Engineering, Chungnam National University, 220, Yuseong, Daejeon 305-764, South Korea Jeonbuk Regional Innovation Agency, Jeonbuk Technopark, 723-1, Jeonju 561-844, South Korea
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 26 September 2008 Accepted 15 December 2008 Available online 24 December 2008
Plasma treatment of polyimide surfaces not only causes structural modification during the plasma exposure, but also leaves active sites on the surfaces that are subject to post-reaction. In this work, the effects of atmospheric fluorine plasma treatment on the surface properties and dielectric properties of polyimide thin film were investigated by using X-ray photoelectron spectroscopy (XPS), Fourier transform-IR (FT-IR) spectroscopy, and contact angle measurement. The results indicated that plasma treatment successfully introduced fluorine functional groups on the polyimide surfaces. The polyimides also exhibited good thermal stability and a lower dielectric constant. It appears that the replacement of fluorine led to the decrease of the local electronic polarizability of polyimide. Consequently, it was found that the atmospheric fluorine plasma-treated polyimides possessed lower dielectric characteristics than the untreated polyimides. © 2009 Elsevier Inc. All rights reserved.
Keywords: Polyimide Atmospheric fluorine plasma treatment Contact angle measurement Dielectric characteristics
1. Introduction Polymeric materials are distinguished by low density, flexibility, versatile electronic properties, chemical inertness and low cost. As a result, they have been extensively used in fields ranging from aerospace to microelectronics, optoelectronics, composites and fiber optics. Among these polymeric materials, polyimides are one of the well-known engineering plastics, offering excellent thermal, mechanical, dielectric and optical properties, along with good chemical resistance and high dimensional stability [1–6]. They are distinguished from other high-performance polymers by the solubility of their poly(amic acid) precursor form, which can be cast into uniform films and quantitatively converted to polyimide. Owing to these qualities, polyimides have been used widely in the microelectronics, film, adhesive, and membrane industries. However, most commercial polyimides are usually infusible and insoluble in common organic solvents which have posed limits on their applications in expensive goods [7–11]. For these reasons, various surface modification techniques have been developed in order to transform these inexpensive materials into highly valuable products. Two broad kinds of surface modification treatments are available: physical treatment and chemical treatment. Chemical treatments introduce chemical bonding to the matrix in order to produce a highly cross-linked interphase. KOH and amine treatment
*
Corresponding author. Fax: +82 42 860 7237. E-mail address: [email protected] (S.-J. Park).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.12.040
© 2009
Elsevier Inc. All rights reserved.
are examples of chemical treatments [12–14]. Physical treatments include irradiation of surfaces with generated plasma and corona treatment. Traditional surface modification methods have the disadvantage of producing a considerable amount of hazardous substances and vapors, which could pollute the environment and must be disposed of appropriately. A more environmentally friendly alternative is the use of plasma technologies. Plasma is an ionized gas containing a mixture of ions, electrons, neutral and excited molecules and photons [15,16]. There are two principal advantages to using mild plasma treatment for materials. One advantage is that the reaction takes place only on the surface of the material, without significantly changing its bulk properties. The other is that it is possible to produce various types of plasma, such as oxidative, reductive and inactive plasmas. Plasma treatments have become increasingly important owing to their very high efficiency and operational; simplicity [17–22]. In this work, we investigated the effect of atmospheric fluorine plasma treatment on the surface properties of polyimide films. Changes in the thermal properties and dielectric characteristics of the polyimide films were also studied with respect to the plasma treatment conditions. 2. Experiments 2.1. Sample preparation and plasma modification The experiments were conducted using commercial polyimide films obtained from Du Pont Chemical Co., under the trade name
S.-J. Park et al. / Journal of Colloid and Interface Science 332 (2009) 246–250
247
2.2. Surface analyses
Fig. 1. Schematic diagram of plasma treatment apparatus.
ATR (attenuated total reflection) FT-IR spectra of the plasmatreated PI films were obtained using a MIDAC M2000. Scans were taken from 400 to 4000 cm−1 and required 40 s to complete. X-ray photoelectron spectroscopy (XPS) measurement of the polyimide surfaces was performed using a VG Scientific ESCA LAB MK-II spectrometer equipped with an MgK α X-ray source. The base pressure in the sample chamber was controlled in the range of 10−8 to 10−9 Torr. The contact angles of the plasma-treated polyimide surfaces were determined at 20 ± 1 ◦ C using a sessile drop method with a Surface & Electro-Optic Co., SEO 300A. Three different wetting liquids, distilled water, diiodomethane and ethylene glycerol, were selected. For each sample, every calculated contact angle was an average of 10 measurements with a standard deviation below 1◦ . The surface morphologies of plasma-treated polyimide films were observed by atomic force microscopy (AFM), using a Digital Instruments Inc., Nanoscope III microscope. The AFM observation was carried out at ambient pressure and room temperature. In each case, an area of 10 × 10 μm was scanned using the tapping mode. 2.3. Thermal properties The glass transition temperature of the plasma-treated polyimide film was measured using differential scanning calorimetry (DSC) (Dupont 9900, Model 2910) from 30 to 400 ◦ C at a heating rate of 10 ◦ C/min and a second heating scan. To investigate the thermal stability of the polyimide film, thermogravimetric analyses were performed in nitrogen using a TGA 951 Dupont thermal analyzer, at a heating rate of 10 ◦ C/min from room temperature to 850 ◦ C. 2.4. Dielectric characteristics
Fig. 2. FT-IR spectra of the polyimide films: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI300, (e) PI-400.
Table 1 Plasma treatment condition of polyimide films. Conditions Treatment power Treatment times Sample name Radio frequency Carrier and reactive gas Carrier gas (He) Process speed
0, 100, 200, 300, 400 W 50 PI-0, PI-100, PI-200, PI-300, PI-400 13.65 MHz He and CHF3 gas 5 l/min 5 mm/s
The dielectric characteristics were measured using a dielectric spectrometer (Novocontrol GmbH, Model CONCEPT 40). The diameter of the electrode and the film was 20 mm. To ensure good electrical contact between the electrodes and the polyimide thin film, the films were sputter-coated beforehand on both sides with a thin layer of silver for 20 s. The dielectric constant of the polyimide thin film was calculated by using the equation:
εr =
C×D
ε0 × A
,
(1)
where ε0 is the permittivity of free space (8.85 × 10−14 Farad/cm), εr the permittivity of the polyimides, C the capacitance, D the film thickness, and A the electrode area. 3. Results and discussion
of Kapton, with a thickness of 50 μm. The films were cut into 100 × 100 mm sections and the surfaces were cleaned with methanol in an ultrasonic water bath for 30 min and then dried in a vacuum oven at room temperature. The atmospheric-pressure plasma surface treatment system in Fig. 1 was manufactured by PLASMART Korea (ATMOSMulti). A schematic diagram of the atmospheric plasma treatment apparatus is shown in Fig. 2. The gas used for the plasma treatments was a mixture of He (99%) and CHF3 (1%). The treatment power was varied between 0 and 400 W (PI-0, PI-100, PI-200, PI-300 and PI-400) at a radio frequency of 13.56 MHz. Treatment time was kept to 30 times for all the samples. Table 1 shows the specific plasma treatment conditions.
3.1. Surface analyses In order to investigate the effect of plasma treatments on the surface chemical modifications, chemical bonding states on the plasma-treated polyimide surfaces were measured by FT-IR and XPS. Fig. 2 shows the untreated and plasma-treated polyimides by FT-IR spectroscopy. All of the polyimides showed characteristic imide absorption bands at 1780–1785 cm−1 and 1727–1735 cm−1 . The peak at 1780–1785 cm−1 was attributable to the asymmetrical carbonyl stretching vibrations, and that at 1727–1735 cm−1 to the symmetrical carbonyl stretching vibrations. The absorption at 1380 cm−1 was assigned to C–N stretching, and C–F multiple
248
S.-J. Park et al. / Journal of Colloid and Interface Science 332 (2009) 246–250
Table 3 Contact angles determination (in ◦ ) of the plasma treated and untreated polyimide film surfaces.
PI-0 PI-100 PI-200 PI-300 PI-400
Fig. 3. XPS spectra of polyimide films before and after plasma treatments: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI-300, (e) PI-400.
Fig. 4. High resolution F1s XPS spectra of the polyimide films: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI-300, (e) PI-400. Table 2 Chemical composition of polyimide films.
PI-0 PI-100 PI-200 PI-300 PI-400
C1s (at%)
N1s (at%)
O1s (at%)
F1s (at%)
85.37 78.0 73.53 70.58 66.77
4.37 5.74 6.25 7.28 8.23
10.26 15.78 18.70 19.26 21.02
0 0.48 1.52 2.88 3.98
stretch bending adsorptions from the trifluoromethyl group were observed between 1400 and 1000 cm−1 . Fig. 3 shows the XPS survey scan of the polyimide film. The spectra in Fig. 4 show carbon, nitrogen, and oxygen (binding energies of 285, 404, and 532 eV, respectively) peaks. In the case of the plasma-treated polyimide, the XPS spectra newly introduced a fluorine peak (binding energy of 694 eV) and the at% carbon was reduced. Table 2 shows the normalized area and atomic composition obtained by XPS for the polyimides as a function of treatment power. The XPS spectra results indicate that the F1s /C1s ratios of the polyimides are increased with increasing treatment power, which can be attributed to the increase of carbon–fluorine functional groups on the polyimide surfaces with plasma treatment.
Water
Diiodomethane
Ethylene glycol
60.3 65.0 65.8 67.1 74.2
33.6 46.1 49.0 52.1 67.1
31.6 52.6 59.5 59.9 62.5
Fig. 4 shows the expanded scale of F1s XPS spectra for the polyimide films. At the time of plasma treatment, fluorine diffuses only slightly and slowly inside the polyimide and then the diffusing fluorine atom produces active radicals from the hydrocarbon backbone during the reaction because of the highest electronegative character of fluorine atom. Therefore, the intensity of the C–F groups is increased as a function of the treatment gas. Table 3 shows the contact angles obtained under deionized water, diiodomethane and Ethylene glycol on plasma-treated and untreated polyimide film. A water contact angle of 60.3◦ was obtained for polyimide film untreated by plasma. The water contact angles are increased with increasing plasma treatment power, whereas those of diiodomethane are decreased as the fluorine content is increased. Therefore, it can be inferred that the PI surfaces become significantly more hydrophobic. Changes in the surface morphology and roughness of the plasma-treated polyimide film with different treatment powers were more accurately observed by AFM as shown in Fig. 5. The surface roughness ( R a ) of the pure polyimide film surface is about 1.58 nm. The R a value of the plasma-treated polyimide for the treatment powers of 100, 200, 300 and 400 W were increased to 2.07, 5.67, 7.44 and 9.37 nm, respectively. In this experiment, the treatment power in the system controls the bombardment energy of fluorine ions in the plasma and reactive etching of the polyimide surface leads to the surface morphological changes. 3.2. Thermal properties The thermal properties of the polyimides were characterized using differential scanning calorimetry (DSC) for testing the glass transition temperature ( T g ) and thermo-gravimetric analysis (TGA) for the decomposition temperature ( T d ). Thermal behavior data for the polyimides is summarized in Table 4. The obtained glass transition temperature of the film as a function of fluorine is shown in Fig. 6. The temperature ranged from 275–283 ◦ C. The glass transition temperature of the film gradually decreased with increasing fluorine content. Fig. 7 shows TGA curves of the plasma-treated polyimides under nitrogen atmosphere. All the polyimides showed similar TGA curves, with initial decomposition temperatures ranging from 544– 590 ◦ C in nitrogen atmosphere. The polyimides did not show obvious weight loss before the scanning temperature reached 500 ◦ C in nitrogen, indicating that no thermal decomposition occurred. The polyimides exhibited temperatures corresponding to 5% weight loss ranging from 497–587 ◦ C under nitrogen. Thermal stability of the plasma-treated polyimide was decreased as compared to that of non-treated polyimide. However, the TGA data indicate that the polyimides have fairly high thermal stability as compared to other polymers. The results of thermal analysis demonstrate that the resulting polyimides should maintain excellent thermal stability, which would facilitate their application.
S.-J. Park et al. / Journal of Colloid and Interface Science 332 (2009) 246–250
(a)
249
(b)
Fig. 6. DSC curves of the polyimide films: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI-300, (e) PI-400.
(c)
(d)
(e) Fig. 5. AFM 3-dimensional images of polyimide films: (a) PI-0, (b) PI-100, (c) PI-200, (d) PI-300, (e) PI-400. Table 4 Thermal properties of polyimide films.a (◦ C)
Sample
Tg
PI-0 PI-100 PI-200 PI-300 PI-400
283 282 278 277 275
Td
Fig. 7. TGA thermograms of polyimide films. (◦ C)
590 594 592 588 544
T5
(◦ C)
587 592 588 584 497
T 10
(◦ C)
604 608 607 606 600
a T d : Onset decomposition temperature; T 5 , T 10 : the decomposition temperature at 5% and 10% weight loss, respectively.
3.3. Dielectric characteristics Good dielectric and insulating properties are desirable for the application of polyimide in the microelectronic industry, such as for microelectronic packages. The dielectric data was obtained at selected frequency (106 Hz) at room temperature (25 ◦ C) as well as the different temperatures (25–200 ◦ C). The dielectric constants of the plasma-treated polyimide film are shown in Fig. 8. The dielectric constants ranged between 2.43–2.95 at 1 MHz and 25 ◦ C, which was lower than those of the untreated polyimide film. It can be seen that the dielectric constant decreased with increasing fluorine content of the polyimide film when the temperature was kept constant. Also, the values of the dielectric constant increase about 0.1–0.2 with increasing temperature (25–200 ◦ C). In the high-temperature, higher value of dielectric constant may be due to space charge polarization, which arises from mobility of ions in the material.
Fig. 8. Dielectric properties of the polyimide films at 1 MHz.
These excellent dielectric properties might be attributed to the incorporation of fluorine-included groups that possessed low electronic polarizability both in the main backbone and in the side chain of the polymer. This also increases the free volume due to the relatively large volume of fluorine compared with hydrogen,
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which reduces the number of polarizable groups per unit volume. As a result, the polyimide film treated with plasma at 400 W indicated the best dielectric property due to the highest loading fluorine content. 4. Conclusions In this work, atmospheric fluorine plasma treatment was performed at room temperature with different treatment powers to allow for the introduction of small amounts of fluorine-containing functionalities onto the surfaces of the polyimides. Surface fluorine content was 3.79 at% for the polyimide plasma treated at 400 W. This method allows for controlling and tailoring of the surface chemistry of modified polyimides. The dielectric constant was calculated to be 2.43 for the plasma-treated polyimide film. The dielectric constant value largely depends on the fluorine content, and the value decreases as the amount of fluorine content increases. The fluorine-containing group of polyimide films is known to be one of the candidates for interlayer dielectric materials in integrated circuits due to its excellent electrical properties, such as low dielectric constant, high breakdown voltage, low leakage current, and high bulk resistivity. Compared to the untreated polyimide, the plasma-treated polyimides possessed lower dielectric constant, which suggests good potential for microelectronic applications as dielectric and insulating materials.
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