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Ar plasma treatment on dielectric characteristics of polyimide films
Journal of Colloid and Interface Science 319 (2008) 365–369 www.elsevier.com/locate/jcis
A study of atmospheric-pressure CHF3 /Ar plasma treatment on dielectric characteristics of polyimide films Soo-Jin Park a,∗ , Eun-Jung Lee a , Byung-Joo Kim b a Department of Chemistry, Inha University, 253, Nam-gu, Incheon 402-751, South Korea b Department of Green Chemistry & Environmental Biotechnology, University of Science & Technology, P.O. Box 107, Yuseong, Daejeon 305-600, South Korea
Received 21 May 2007; accepted 1 August 2007 Available online 20 December 2007
Abstract In this work, the influence of atmospheric-pressure CHF3 /Ar plasma treatment on surface dielectric properties of polyimide films was investigated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements. The dielectric characteristics of the films were studied using a dielectric spectrometer. From the results, it was found that the plasma treatment introduced fluorine functional groups onto the polyimide surfaces. F1s /C1s ratios of the polyimides were enhanced with the increase of plasma treatment time. Consequently, the fluorine groups led to a decrease of the surface free energy and dielectric constant of the polyimide films, which can largely be attributed to the decrease of the deformation polarizability or London dispersive component of surface free energy of the solid surface studied. © 2007 Elsevier Inc. All rights reserved. Keywords: Polyimide film; Atmospheric-pressure CHF3 /Ar plasma; Fluorine; Deformation; Polarizability
1. Introduction Polymers as dielectric materials play a significant role in achieving the current state of the art in microelectronics [1,2]. Polymeric materials not only are found in final products such as component housings, integrated circuit (IC) chip packagings, and intermetallic dielectric layers, but are also employed extensively in major processing components such as resisters in microlithography [3–6]. Notably, polyimides have many desirable characteristics, such as excellent mechanical properties, a low dielectric constant, a low relative permittivity, a high breakdown voltage, inertness to solvent, and radiation resistance. 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 all of these properties, polyimides have been used especially widely in the microelectronics, film, adhesive, and membrane industries [4–8]. Surface modification of polyimides by the use of ion beams and plasma treatment is considerably important in both technol* Corresponding author. Fax: +82 42 861 4151.
E-mail address: [email protected] (S.-J. Park). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.08.003
ogy and science [2,7,9–13]. These methods have been utilized successfully to improve the dielectric properties of polyimide films. Plasma treatment of polyimide surfaces not only causes modification during the plasma exposure, but also leaves active sites at the surfaces, which are subject to post-reactions; this is also called aging. Aging effects depend on external influences such as adsorption or oxidation and on the internal tendency to attain an energetically favorable state (thermodynamic equilibrium) by restructuring processes and diffusion. A high-energy surface is prone to adsorb contaminants from the atmosphere in order to lower its surface energy [4,11–14]. In the present work, we investigated the effect of atmospheric-pressure plasma treatment on the dielectric characteristics of polyimide films. We also studied the effects of the plasma treatment conditions on the surface properties of the films. 2. Experiments 2.1. Materials and sample preparation The Kapton 50-µm-thick polyimide film used in this study was purchased from the Dupont Chemical Co. The surface of the film was cleaned with ethanol in an ultrasonic water bath
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Fig. 1. Schematic diagram of atmospheric-pressure plasma treatment apparatus.
for 30 min and dried for 30 min in a vacuum oven at room temperature. The atmospheric-pressure plasma surface treatment system employed an ATMOSMulti (PLASMART Co. of Korea). A schematic diagram of the apparatus is shown in Fig. 1. The gases used in the plasma treatments were mixed Ar (99.9%) and CHF3 (0.1%). The nominal treatment power was fixed to 300 W, and the treatment times, varied between 0 and 45 s at a radio frequency of 13.56 MHz, were PI-0, PI-15, PI-30, and PI-45. Table 1 shows the plasma treatment conditions.
Table 1 Polyimide–surface plasma treatment conditions Conditions Sample name Treatment time Treatment power Radio frequency Carrier and reactive gas Argon gas (Ar) Trifluoromethane gas (CHF3 )
PI-0, PI-15, PI-30, PI-45 0, 15, 30, 45 s 300 W 13.65 MHz Ar and CHF3 gas 5 l/min 5 ml/min
2.2. Surface properties An X-ray photoelectron spectroscopy (XPS) measurement of the polyimide surfaces was performed using a LAB MK-II spectrometer equipped with an MgKα X-ray source (VG Scientific ESCA). The surface free energy of the plasma-treated polyimide film was determined at 20◦ ± 1◦ using a sessile drop method and an SEO 300A (Surface & Electro-Optic Co.). Three different wetting liquids, distilled water, diiodomethane, and ethylene glycol, were selected (Table 2). For each sample, every calculated contact angle was an average of 10 measurements with a standard deviation below 1◦ . Based on the method of Fowkes, van Oss, and Wu, it was proposed that the total surface free energy can be divided into two components [15], γ = γ L + γ SP ,
Table 2 London dispersive (γLL ) and specific (γLSP ) components of surface free energy (γL ) of wetting liquids (subscript: L) measured at 20 ◦ C Wetting liquid
γL (mJ m−2 )
γLL (mJ m−2 )
γLSP (mJ m−2 )
Water Diiodomethane
72.8 50.8
21.8 50.42
51.0 0.38
vided into two parameters using the geometric mean, γ SP = 2 γ + · γ − ,
(2)
where γ + stands for the electron-acceptor parameter and γ − for the electron-donor parameter of the specific polar component of surface free energy (or tension). The Young’s angle equation can be combined with the Dupré equation to yield the equation
(1)
where γ L is the London dispersive component of surface free energy associated with Lifshitz–van der Waals interactions that encompass London dispersion forces, and γ SP is the specific (or polar) component of surface free energy related to Debye-inductive polarization, Keesom-orientational polarization forces, and hydrogen bonding. Also, the γ SP component results from electron-acceptor and -donor intermolecular interactions, referred to as Lewis acid and base interactions, respectively. The term γ SP is further di-
γL (1 + cos θ ) = γS + γL − γSL ,
(3)
where the subscripts S and L refer to the solid and liquid phases, respectively. The interfacial tension, γSL , can be calculated from the London dispersive and specific polar components of the two substrates as γSL = γSL + γLL − 2 γSL γLL − 2 γSSP γLSP . (4)
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By combining Eqs. (3) and (4), the following equation is obtained, L + γ SP γ SP , γL,i (1 + cos θ ) = 2 γSL γL,i (5) S L,i where the subscripts i is the experimental testing liquid, such as water, diiodomethane, and ethylene glycol. Equations (2) and (5) yield L + γ +γ − + γ −γ + . γL,i (1 + cos θ ) = 2 γSL γL,i (6) S L,i S L,i With known values of γLL , γL+ , and γL− for three liquids and their contact angles (θ ) on the solid, a set of the Eq. (6) can be simultaneously solved to obtain the surface free energy parameters, γSL , γS+ , and γS− , for the solid. It is seen from Eq. (1) that if either the γ + or γ − parameter is zero, there is no polar component contribution to the overall surface free energy (γ = γL ). Equation (5) makes it possible to first determine γSL SP ∼ 0; i is the diiodomethane) and using a nonpolar liquid (γL,i = solve Eq. (6) to a pair of in unknown γS+ and γS− (i is water and ethylene glycol) [15].
Fig. 2. XPS spectra of polyimide film before and after plasma treatments.
2.3. Surface morphologies The surface morphologies of the plasma-treated polyimide were studied by atomic force microscopy (AFM) using a Nanoscope III microscope (Digital Instruments Inc.) at room temperature under ambient pressure. In each case, an area of 10 × 10 µm was scanned using the tapping mode. 2.4. Dielectric characteristics of polyimide film The frequency and temperature dependencies of the dielectric constant were investigated both before and after the plasma treatment. The characterization of these dependencies is very important to microelectronic applications of dielectric materials. In this study, 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 [2]. To ensure good electrical contact between the electrodes and the polyimide film, prior to measurement the films were sputter-coated for 20 s on both sides with a silver layer of around 0.05 µm. The dielectric constants of the film were determined over the frequency range of 100 –107 Hz at room temperature as well as the temperature range of 0–200 ◦ C at 103 Hz, at a heating rate of 5 ◦ C/min. 3. Results and discussion 3.1. Surface properties of polyimide The changes of chemical composition of the plasma-treated polyimide were measured by XPS. Fig. 2 shows the XPS survey spectra of the polyimide film. The XPS spectrum of the nontreated polyimide film shows its typical characteristic peaks, such as carbon, nitrogen, and oxygen (binding energy 285, 404, and 532 eV, respectively). Table 3 shows the element content of each PI film. It was found that the F1s /C1s ratios of
Fig. 3. Surface energetics of polyimide films as a function of plasma treatment time.
Table 3 Element contents (%) of the polyimide films studied
PI-0 PI-15 PI-30 PI-45
C1s
O1s
N1s
F1s
71.86 51.95 44.67 41.16
22.30 12.49 8.32 5.80
5.84 3.76 2.57 1.68
0 31.80 44.44 51.36
F1s /C1s 0 61.20 99.50 124.80
the polyimide were greatly increased with increasing treatment time, which was probably due to the increased number of the fluorine-containing functional group on the surface of the polyimide films. This result can be explained by the fact that active radicals lead to the introduction of relatively strong carbon– fluorine (C–F) bonds on the polyimide films [2]. The effects of the surface free energy of the polyimide film before and after the treatments were investigated by contact angle measurement. The results are shown in Fig. 3. The total surface energy gradually decreased with increasing treatment time, which was mainly due to a lower London-dispersive component, whereas the specific component increased.
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Fig. 4. Three-dimensional AFM spectra of polyimide films as a function of treatment time.
3.2. Surface morphology of polyimide The surface morphology is of great importance to the surface wettability of polymer films. In this work, AFM was employed to examine the morphological changes induced in the plasma-treated polyimide. The differences in morphology after the surface treatment further support the notion of variation in the relative amounts of ablation, chain scission, and other functionalization. Fig. 4 shows three-dimensional AFM images of the plasma-treated polyimide as a function of the treatment time. It was found that the surface of PI-O was quite flat, but that the roughness increased with increasing plasma treatment time. This result indicates that plasma treatment can lead to the surface etching of polyimide films and form carbon–fluorine functional groups on the etched sites. As can be seen in Fig. 5, the arithmetic mean of the roughness (Ra ) and the root-mean-square (Rms ) of the roughness of the pure polyimide were about 3.058 and 3.245 nm, respectively. The Ra roughness values of the plasma-treated polyimide for the treatment times of 15, 30, and 45 s were increased to 3.9, 4.7, and 5.3 nm, respectively. The Rms roughness values for the same treatment times were increased to 4.1, 5.3, and 6.1 nm, respectively. In this experiment, it was proven that the plasma treatment time determines the fluorine amount and that the etching of the polyimide surface leads to surface morphological changes.
Fig. 5. Ra and Rms of polyimide films as a function of treatment time.
3.3. Dielectric characteristics of polyimide The dielectric constant was calculated according to the usual parallel plate capacitor formula. A room-temperature dependence study of the polyamide films with dielectric constant as a function of frequency is shown in Fig. 6. It can be seen that the dielectric constant slightly decreased (under 5%) with increasing input frequency when the temperature was kept constant. This is in good agreement with thermosetting or fluorine-contained polyimide [3–7], and the dielectric constant of polyimide is known to decrease gradually with increasing
S.-J. Park et al. / Journal of Colloid and Interface Science 319 (2008) 365–369
369
der the applied electrical field, owing to the glassy-state molecular chain segments. With increasing temperature, the chain segment mobility increases and polar groups start to move in response to the applied electrical field, which increases the orientation of the polymers and the dielectric constant. As seen in Fig. 7, the dielectric constant exhibited small changes with temperature. For many microelectronic applications, dielectric materials with stable dielectric constant and dissipation factor values across large frequency and temperature ranges are highly preferred. In our experimentation, the polyimide films, despite the plasma treatments, had the same frequency and temperature dependencies, though the dielectric constant of the plasmatreated polyimide showed lower values. This result confirms that fluorine ions in the plasma gas lead to a decrease of the dielectric constant or London dispersive component of plasmatreated polyimides. Fig. 6. Dielectric constants vs frequencies of polyimide films.
4. Conclusions In this work, polyimide films were CHF3 /Ar plasma-treated for different treatment times at room temperature. The fluorinecontaining surface functional groups of the treated polyimide were increased with increased treatment time. Meanwhile, the fluorine groups led to a decrease of the surface free energy and dielectric constant of the polyimide films, which can largely be attributed to the decrease of the deformation polarizability or London dispersive component of the surface free energy of the solid surface studied. Acknowledgment This work was supported by an Inha University Research Grant (INHA 35019-1). Fig. 7. Dielectric constants vs temperatures of polyimide films.
References frequency. These behaviors can be attributed to the frequency dependence of the polarization mechanisms. The dielectric constant depends on the ability of polarizable units in a polymer to orient fast enough to keep pace with the oscillations of an alternating electric field. When frequency increases, the orientational polarization decreases, since the orientation of dipole moments require a longer time than electronic and ionic polarizations. This causes the dielectric constant to decrease. Plasma-treated polyimide has a lower dielectric constant than nontreated polyimide. This is probably due to the large number (or volume) of fluorine compared with hydrogen, which reduces the number of polarizable groups per unit volume, as a result of which the dielectric constant of fluorinated polyimides decreases as plasma treatment times increase. Fig. 7 shows the variation of the dielectric constant with temperature at 103 Hz. It is clear that the dielectric constant increases with increasing temperature. For polar polymers also, the dielectric constant increases with increasing temperature. For plasma-treated polyimide, in the low-temperature range, polar groups of polymers are difficult to orient and relax un-
[1] L.A. Laius, Polyimide: Thermally Stable Polymer, Consultants Bureau, New York, 1987. [2] S.J. Park, K.S. Cho, S.H. Kim, J. Colloid Interface Sci. 272 (2004) 384. [3] A.P. Kharitonov, R. Taege, G. Ferrier, V.V. Teplyakov, D.A. Syrtsova, G.H. Koops, J. Fluorine Chem. 126 (2005) 251. [4] S.B. Deshpande, H.S. Potdar, M.M. Patil, V.V. Deshpande, Y.B. Khollam, J. Ind. Eng. Chem. 12 (2006) 584. [5] L. Jiang, J. Liu, D. Wu, H. Li, R. Jin, Thin Solid Films 510 (2006) 241. [6] C. Frgar, M.M. Khojasteh, M.S. Htoo, Advances in Polyimide Science and Technology, Technomic Publishing, New York, 1991. [7] S.J. Park, H.S. Kim, F.L. Jin, J. Colloid Interface Sci. 282 (2005) 238. [8] J.P. Critchley, P.A. Gratton, M.A. Whitte, J.S. Pippet, J. Polym. Sci. Part A 1 10 (1972) 1789. [9] M.W. Kim, S.H. Hyun, J.Y. Kong, K.B. Yoon, L.S. Park, W.S. Kim, J. Ind. Eng. Chem. 12 (2006) 261. [10] P. Sysel, M. Sochurek, P. Holler, Polymer 39 (1998) 4751. [11] V.E. Yudin, J.U. Otaigbe, S. Gladchenko, B.G. Olson, S. Nazarenko, E.N. Korytkova, V.V. Gusarov, Polymer 48 (2007) 1306. [12] W.L. Qu, T.M. Ko, J. Appl. Polym. Sci. 82 (2001) 1642. [13] N.M.L. Hansen, K. Jankova, S. Hvilsted, Eur. Polym. J. 43 (2007) 255. [14] Y. Watanabe, Y. Shibasaki, S. Ando, M. Ueda, Polymer 46 (2005) 5903. [15] S.J. Park, in: J.P. Hsu (Ed.), Interfacial Forces and Fields: Theory and Applications, Dekker, New York, 1999, chap. 9.
A study of atmospheric-pressure CHF3 /Ar plasma treatment on dielectric characteristics of polyimide films Soo-Jin Park a,∗ , Eun-Jung Lee a , Byung-Joo Kim b a Department of Chemistry, Inha University, 253, Nam-gu, Incheon 402-751, South Korea b Department of Green Chemistry & Environmental Biotechnology, University of Science & Technology, P.O. Box 107, Yuseong, Daejeon 305-600, South Korea
Received 21 May 2007; accepted 1 August 2007 Available online 20 December 2007
Abstract In this work, the influence of atmospheric-pressure CHF3 /Ar plasma treatment on surface dielectric properties of polyimide films was investigated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements. The dielectric characteristics of the films were studied using a dielectric spectrometer. From the results, it was found that the plasma treatment introduced fluorine functional groups onto the polyimide surfaces. F1s /C1s ratios of the polyimides were enhanced with the increase of plasma treatment time. Consequently, the fluorine groups led to a decrease of the surface free energy and dielectric constant of the polyimide films, which can largely be attributed to the decrease of the deformation polarizability or London dispersive component of surface free energy of the solid surface studied. © 2007 Elsevier Inc. All rights reserved. Keywords: Polyimide film; Atmospheric-pressure CHF3 /Ar plasma; Fluorine; Deformation; Polarizability
1. Introduction Polymers as dielectric materials play a significant role in achieving the current state of the art in microelectronics [1,2]. Polymeric materials not only are found in final products such as component housings, integrated circuit (IC) chip packagings, and intermetallic dielectric layers, but are also employed extensively in major processing components such as resisters in microlithography [3–6]. Notably, polyimides have many desirable characteristics, such as excellent mechanical properties, a low dielectric constant, a low relative permittivity, a high breakdown voltage, inertness to solvent, and radiation resistance. 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 all of these properties, polyimides have been used especially widely in the microelectronics, film, adhesive, and membrane industries [4–8]. Surface modification of polyimides by the use of ion beams and plasma treatment is considerably important in both technol* Corresponding author. Fax: +82 42 861 4151.
E-mail address: [email protected] (S.-J. Park). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.08.003
ogy and science [2,7,9–13]. These methods have been utilized successfully to improve the dielectric properties of polyimide films. Plasma treatment of polyimide surfaces not only causes modification during the plasma exposure, but also leaves active sites at the surfaces, which are subject to post-reactions; this is also called aging. Aging effects depend on external influences such as adsorption or oxidation and on the internal tendency to attain an energetically favorable state (thermodynamic equilibrium) by restructuring processes and diffusion. A high-energy surface is prone to adsorb contaminants from the atmosphere in order to lower its surface energy [4,11–14]. In the present work, we investigated the effect of atmospheric-pressure plasma treatment on the dielectric characteristics of polyimide films. We also studied the effects of the plasma treatment conditions on the surface properties of the films. 2. Experiments 2.1. Materials and sample preparation The Kapton 50-µm-thick polyimide film used in this study was purchased from the Dupont Chemical Co. The surface of the film was cleaned with ethanol in an ultrasonic water bath
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Fig. 1. Schematic diagram of atmospheric-pressure plasma treatment apparatus.
for 30 min and dried for 30 min in a vacuum oven at room temperature. The atmospheric-pressure plasma surface treatment system employed an ATMOSMulti (PLASMART Co. of Korea). A schematic diagram of the apparatus is shown in Fig. 1. The gases used in the plasma treatments were mixed Ar (99.9%) and CHF3 (0.1%). The nominal treatment power was fixed to 300 W, and the treatment times, varied between 0 and 45 s at a radio frequency of 13.56 MHz, were PI-0, PI-15, PI-30, and PI-45. Table 1 shows the plasma treatment conditions.
Table 1 Polyimide–surface plasma treatment conditions Conditions Sample name Treatment time Treatment power Radio frequency Carrier and reactive gas Argon gas (Ar) Trifluoromethane gas (CHF3 )
PI-0, PI-15, PI-30, PI-45 0, 15, 30, 45 s 300 W 13.65 MHz Ar and CHF3 gas 5 l/min 5 ml/min
2.2. Surface properties An X-ray photoelectron spectroscopy (XPS) measurement of the polyimide surfaces was performed using a LAB MK-II spectrometer equipped with an MgKα X-ray source (VG Scientific ESCA). The surface free energy of the plasma-treated polyimide film was determined at 20◦ ± 1◦ using a sessile drop method and an SEO 300A (Surface & Electro-Optic Co.). Three different wetting liquids, distilled water, diiodomethane, and ethylene glycol, were selected (Table 2). For each sample, every calculated contact angle was an average of 10 measurements with a standard deviation below 1◦ . Based on the method of Fowkes, van Oss, and Wu, it was proposed that the total surface free energy can be divided into two components [15], γ = γ L + γ SP ,
Table 2 London dispersive (γLL ) and specific (γLSP ) components of surface free energy (γL ) of wetting liquids (subscript: L) measured at 20 ◦ C Wetting liquid
γL (mJ m−2 )
γLL (mJ m−2 )
γLSP (mJ m−2 )
Water Diiodomethane
72.8 50.8
21.8 50.42
51.0 0.38
vided into two parameters using the geometric mean, γ SP = 2 γ + · γ − ,
(2)
where γ + stands for the electron-acceptor parameter and γ − for the electron-donor parameter of the specific polar component of surface free energy (or tension). The Young’s angle equation can be combined with the Dupré equation to yield the equation
(1)
where γ L is the London dispersive component of surface free energy associated with Lifshitz–van der Waals interactions that encompass London dispersion forces, and γ SP is the specific (or polar) component of surface free energy related to Debye-inductive polarization, Keesom-orientational polarization forces, and hydrogen bonding. Also, the γ SP component results from electron-acceptor and -donor intermolecular interactions, referred to as Lewis acid and base interactions, respectively. The term γ SP is further di-
γL (1 + cos θ ) = γS + γL − γSL ,
(3)
where the subscripts S and L refer to the solid and liquid phases, respectively. The interfacial tension, γSL , can be calculated from the London dispersive and specific polar components of the two substrates as γSL = γSL + γLL − 2 γSL γLL − 2 γSSP γLSP . (4)
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By combining Eqs. (3) and (4), the following equation is obtained, L + γ SP γ SP , γL,i (1 + cos θ ) = 2 γSL γL,i (5) S L,i where the subscripts i is the experimental testing liquid, such as water, diiodomethane, and ethylene glycol. Equations (2) and (5) yield L + γ +γ − + γ −γ + . γL,i (1 + cos θ ) = 2 γSL γL,i (6) S L,i S L,i With known values of γLL , γL+ , and γL− for three liquids and their contact angles (θ ) on the solid, a set of the Eq. (6) can be simultaneously solved to obtain the surface free energy parameters, γSL , γS+ , and γS− , for the solid. It is seen from Eq. (1) that if either the γ + or γ − parameter is zero, there is no polar component contribution to the overall surface free energy (γ = γL ). Equation (5) makes it possible to first determine γSL SP ∼ 0; i is the diiodomethane) and using a nonpolar liquid (γL,i = solve Eq. (6) to a pair of in unknown γS+ and γS− (i is water and ethylene glycol) [15].
Fig. 2. XPS spectra of polyimide film before and after plasma treatments.
2.3. Surface morphologies The surface morphologies of the plasma-treated polyimide were studied by atomic force microscopy (AFM) using a Nanoscope III microscope (Digital Instruments Inc.) at room temperature under ambient pressure. In each case, an area of 10 × 10 µm was scanned using the tapping mode. 2.4. Dielectric characteristics of polyimide film The frequency and temperature dependencies of the dielectric constant were investigated both before and after the plasma treatment. The characterization of these dependencies is very important to microelectronic applications of dielectric materials. In this study, 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 [2]. To ensure good electrical contact between the electrodes and the polyimide film, prior to measurement the films were sputter-coated for 20 s on both sides with a silver layer of around 0.05 µm. The dielectric constants of the film were determined over the frequency range of 100 –107 Hz at room temperature as well as the temperature range of 0–200 ◦ C at 103 Hz, at a heating rate of 5 ◦ C/min. 3. Results and discussion 3.1. Surface properties of polyimide The changes of chemical composition of the plasma-treated polyimide were measured by XPS. Fig. 2 shows the XPS survey spectra of the polyimide film. The XPS spectrum of the nontreated polyimide film shows its typical characteristic peaks, such as carbon, nitrogen, and oxygen (binding energy 285, 404, and 532 eV, respectively). Table 3 shows the element content of each PI film. It was found that the F1s /C1s ratios of
Fig. 3. Surface energetics of polyimide films as a function of plasma treatment time.
Table 3 Element contents (%) of the polyimide films studied
PI-0 PI-15 PI-30 PI-45
C1s
O1s
N1s
F1s
71.86 51.95 44.67 41.16
22.30 12.49 8.32 5.80
5.84 3.76 2.57 1.68
0 31.80 44.44 51.36
F1s /C1s 0 61.20 99.50 124.80
the polyimide were greatly increased with increasing treatment time, which was probably due to the increased number of the fluorine-containing functional group on the surface of the polyimide films. This result can be explained by the fact that active radicals lead to the introduction of relatively strong carbon– fluorine (C–F) bonds on the polyimide films [2]. The effects of the surface free energy of the polyimide film before and after the treatments were investigated by contact angle measurement. The results are shown in Fig. 3. The total surface energy gradually decreased with increasing treatment time, which was mainly due to a lower London-dispersive component, whereas the specific component increased.
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Fig. 4. Three-dimensional AFM spectra of polyimide films as a function of treatment time.
3.2. Surface morphology of polyimide The surface morphology is of great importance to the surface wettability of polymer films. In this work, AFM was employed to examine the morphological changes induced in the plasma-treated polyimide. The differences in morphology after the surface treatment further support the notion of variation in the relative amounts of ablation, chain scission, and other functionalization. Fig. 4 shows three-dimensional AFM images of the plasma-treated polyimide as a function of the treatment time. It was found that the surface of PI-O was quite flat, but that the roughness increased with increasing plasma treatment time. This result indicates that plasma treatment can lead to the surface etching of polyimide films and form carbon–fluorine functional groups on the etched sites. As can be seen in Fig. 5, the arithmetic mean of the roughness (Ra ) and the root-mean-square (Rms ) of the roughness of the pure polyimide were about 3.058 and 3.245 nm, respectively. The Ra roughness values of the plasma-treated polyimide for the treatment times of 15, 30, and 45 s were increased to 3.9, 4.7, and 5.3 nm, respectively. The Rms roughness values for the same treatment times were increased to 4.1, 5.3, and 6.1 nm, respectively. In this experiment, it was proven that the plasma treatment time determines the fluorine amount and that the etching of the polyimide surface leads to surface morphological changes.
Fig. 5. Ra and Rms of polyimide films as a function of treatment time.
3.3. Dielectric characteristics of polyimide The dielectric constant was calculated according to the usual parallel plate capacitor formula. A room-temperature dependence study of the polyamide films with dielectric constant as a function of frequency is shown in Fig. 6. It can be seen that the dielectric constant slightly decreased (under 5%) with increasing input frequency when the temperature was kept constant. This is in good agreement with thermosetting or fluorine-contained polyimide [3–7], and the dielectric constant of polyimide is known to decrease gradually with increasing
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369
der the applied electrical field, owing to the glassy-state molecular chain segments. With increasing temperature, the chain segment mobility increases and polar groups start to move in response to the applied electrical field, which increases the orientation of the polymers and the dielectric constant. As seen in Fig. 7, the dielectric constant exhibited small changes with temperature. For many microelectronic applications, dielectric materials with stable dielectric constant and dissipation factor values across large frequency and temperature ranges are highly preferred. In our experimentation, the polyimide films, despite the plasma treatments, had the same frequency and temperature dependencies, though the dielectric constant of the plasmatreated polyimide showed lower values. This result confirms that fluorine ions in the plasma gas lead to a decrease of the dielectric constant or London dispersive component of plasmatreated polyimides. Fig. 6. Dielectric constants vs frequencies of polyimide films.
4. Conclusions In this work, polyimide films were CHF3 /Ar plasma-treated for different treatment times at room temperature. The fluorinecontaining surface functional groups of the treated polyimide were increased with increased treatment time. Meanwhile, the fluorine groups led to a decrease of the surface free energy and dielectric constant of the polyimide films, which can largely be attributed to the decrease of the deformation polarizability or London dispersive component of the surface free energy of the solid surface studied. Acknowledgment This work was supported by an Inha University Research Grant (INHA 35019-1). Fig. 7. Dielectric constants vs temperatures of polyimide films.
References frequency. These behaviors can be attributed to the frequency dependence of the polarization mechanisms. The dielectric constant depends on the ability of polarizable units in a polymer to orient fast enough to keep pace with the oscillations of an alternating electric field. When frequency increases, the orientational polarization decreases, since the orientation of dipole moments require a longer time than electronic and ionic polarizations. This causes the dielectric constant to decrease. Plasma-treated polyimide has a lower dielectric constant than nontreated polyimide. This is probably due to the large number (or volume) of fluorine compared with hydrogen, which reduces the number of polarizable groups per unit volume, as a result of which the dielectric constant of fluorinated polyimides decreases as plasma treatment times increase. Fig. 7 shows the variation of the dielectric constant with temperature at 103 Hz. It is clear that the dielectric constant increases with increasing temperature. For polar polymers also, the dielectric constant increases with increasing temperature. For plasma-treated polyimide, in the low-temperature range, polar groups of polymers are difficult to orient and relax un-
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