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Microstructures of BPDA-PPD polyimide thin films with different thicknesses
Polymer 54 (2013) 2435e2439
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Microstructures of BPDA-PPD polyimide thin films with different thicknesses Masaru Kotera a, *, Boo Samyul a, Kouhei Araie a, Yuri Sugioka a, Takashi Nishino a, Satoshi Maji b, Miki Noda c, Kazunobu Senoo c, Tomoyuki Koganezawa d, Ichiro Hirosawa d a
Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan S.B.Research Co., Ltd., Kiyohara, Utsunomiya 321-3231, Japan c Kobe Fundamental Research Laboratory, Sumitomo Bakelite Co., Ltd., Murotani, Nishi, Kobe 651-2241, Japan d Japan Synchrotron Radiation Research Institute, Kouto, Sayo-cho, Sayo-gun 679-5198, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2012 Received in revised form 1 March 2013 Accepted 3 March 2013 Available online 13 March 2013
The microstructures of 3,4,30 ,40 -biphenyltetracarboxylic dianhydride e p-phenylene diamine (BPDAPPD) polyimide (PI) thin films with different thicknesses were investigated by grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) at the SPring-8 synchrotron radiation facility. The film density calculated from XRR profiles increased with decreasing the film thickness. Processing of PI thin films from precursor polyamic acid (PAA) solutions involves simultaneous thermal imidization, evaporation of residual solvents and crystallization. The thickness reduction during imidization was smaller for the thinner PAA film. This implies that the much more larger amount of the residual solvent in the thinner PAA film brings high molecular mobility, which results in the higher conversion to PI at any curing temperature. The thinner BPDA-PPD film was more dense than the thicker film because of better molecular packing, which is a result of much more amount of the residual solvent for the thinner film during the thermal curing. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Polyimide thin films Imidization process Residsal solvent
1. Introduction Polyimides (PIs) are widely used in the aerospace and microelectronics industries because of their excellent thermal stability, good mechanical properties and low dielectric constants [1,2]. They are often used to make alignment films for liquid crystal displays, and for this purpose a PI film 50e100 nm thick is deposited on a glass substrate or electrode film. The display qualities are affected to the controlling of the orientation for the liquid crystal molecules on the alignment film. Yokokura et al. have reported that the pretilt angle of the liquid crystals molecules depended on the crystallinity of a alignment PI film [3], and Sakai et al. have reported that higher crystallinity of the alignment PI film surface resulted in higher orientation of the liquid crystal molecules [4]. The microstructures and properties of the polymer thin films are well known to be quite different compared with those of the bulk materials, especially when the film thickness is less than 2Rg (Rg: radius of gyration of the polymer chain) [5]. Many researchers have reported that the glass transition temperature of a thin polymer films is a strong function of the film thickness [5e11].
* Corresponding author. Tel./fax: þ81 78 803 6198. E-mail address: [email protected] (M. Kotera). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.03.005
PI is generally insoluble in any solvent and is therefore usually made, in a thermal conversion process, from a precursor poly(amic acid) (PAA) dissolved in an aprotic polar solvent. Because the cyclization with dehydration, evaporation of residual solvent and crystallization in this thermal conversion occur simultaneously and competitively, the imidization process will influence the final structure and properties of the PI [12e14]. If the thickness of a precursor PAA film is comparable to the 2Rg, there is possibility of imidization behavior different from that in bulk PAA. An X-ray beam, particular a hard X-ray beam, is an excellent analytical probe for microstructural analysis and can be used in an ambient environment providing nondestructive measurements. In addition, no special pre-treatment of the sample is needed. The conventional measurements using hard X-ray beams give information about the average structure of the bulk polymeric materials, because hard X-ray beam penetrates deep into the materials consisting mainly of light elements such as hydrogen and carbon. In polymeric materials, the X-ray diffraction intensity is very weak compared with that in metals or ceramics, because of the light elements and low crystallinity. Measurements using X-ray beams incident at a very small angle, in contrast, give information about the material near the surface. In the grazing incidence X-ray diffraction (GIXD) method, when the incident angle is smaller than the critical angle for total external reflection (ac), the penetration of the X-ray beam is limited to depth of nanometers order [15e17]. In
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M. Kotera et al. / Polymer 54 (2013) 2435e2439
addition, the incident X-ray footprint spreads to incident direction over a large area on the surface, so the GIXD method enables enough diffraction intensity to be obtained even from polymer thin films. The excellent brightness and high directionality of a synchrotron radiation X-ray beam possesses make it very suitable for the GI geometry. In this study, the effect of film thickness on the imidization process for PI thin films was investigated by using GIXD and X-ray reflectivity (XRR) methods to evaluate the microstructures of PI films with different thicknesses. 2. Experimental PAA precursor (Ube Industries Ltd., U-varnish S301), 3,4,30 ,40 biphenyltetracarboxylic dianhydride e p-phenylene diamine (BPDA-PPD) type in N-methyl-2-pyrrolidone (NMP) solution was used throughout this study. Solid content of the PAA precursor solution was 18.2 wt% in NMP. Silicon substrates (diameter; 4 inches, thickness; 1 mm) were immersed in a mixture of concentrated sulfuric acid and hydrogen peroxide (3:1 ratio by volume) at room temperature for 20 min and then rinsed with distilled water to produce a thin surface layer of silicon oxide. PAA thin films were prepared by spin-coating precursor solution (diluted to 3.5 and 6.0 wt% with NMP) onto the substrate at room temperature. The spin-coating was performed at 1000 rpm for 10 s and 4000 rpm for 90 s. The PAA thin films was cured at 60 C for 30 s, and then successively thermally imidized at 100 C, 140 C, 160 C, 180 C, 200 C, 250 C, 300 C and finally at 400 C (1 h at each temperature). Imidization reaction was investigated using a Fourier transform infrared (FT-IR) spectrophotometer (PerkinElmer Inc., Spectrum GX FT-IR System I-KS) at a resolution 2 cm1 with 200 scans accumulated in the transmission mode. The conversion to PI during the thermal imidization at temperature T was calculated using the following equation;
Conversion to PIð%Þ ¼ ðD1380 ðD1380
cm1 =D1510 cm1 ÞT =
cm1 =D1510 cm1 Þ400 C
100 ð1Þ
where, D is the optical density of each absorption. The absorption bands at 1380 cm1 (imide II) (CeN stretching) and at 1510 cm1 (CeC stretching of p-substituted benzene: an internal standard) have previously been used to evaluate imidization [18]. Microstructures of the PI thin films were evaluated by GIXD and XRR using multi-axis diffractometer installed in the BL19B2 of SPring-8, third-generation synchrotron radiation facility in Japan, and combined using the laboratory equipment (Rigaku Co., SmarA). The ultra-bright and highly directional tLab, CuKa: l ¼ 1.5418 X-rays provided by a synchrotron radiation are suitable for the GIXD and XRR techniques. The incident X-ray energy was set at A) by a Si(111) double-crystal monochrometer, 10 keV (l ¼ 1.2398 and the X-ray beam was set to be 8 mm wide and 0.1 mm height. Scattered X-rays from the sample were detected by a NaI scintillation counter through a soller slit with a 0.45 divergence angle. In our GIXD measurements the scattering vector was almost parallel to the sample surface. That is, we used the so called in-plane diffraction technique. The results obtained in this geometry give the crystal lattice order in the in-plane direction. We also used a geometry in which the scattering plane was perpendicular to the sample surface (the so called out-of-plane diffraction technique). Before the GIXD measurements, we estimated the ac from the XRR profile. The thickness and density of the thin PI films were calculated from XRR profiles by using a fitting software (Rigaku Co., GXRR). We
have assumed that there is a silicon oxide layer several nanometers thick on top of the substrate and that there is no gradation of the density of the PI thin film. 3. Results and discussion Fig. 1 shows the infrared spectra of BPDA-PPD thin film (6.0 wt%) on the substrate before and after curing at 400 C. The absorption at 1665 cm1 (amide I) (C]O (CONH) stretching) and that at 1550 cm1 (amide II) (CeNH vibration) disappeared on the other hand, the absorption at 1780 cm1 (imide I) (C]O symmetric stretching) and that at 1380 cm1 (imide II) (CeN stretching) appeared [18]. These results indicate that the precursor PAA film was converted into the BPDA-PPD type PI by thermal curing up to 400 C. Fig. 2 shows (plots) the observed and (lines) calculated XRR profiles of BPDA-PPD PI thin films made from different concentrations of the precursor PAA solution were cured at up to 400 C. The calculated reflectivity values fitted the observed profiles very well. In the both samples, the reflected X-ray with Kiessig fringe was clearly observed to higher incident angle. These indicated that no distribution of the density in the PI thin film, and there was no residual solvent, for both samples. Moreover, the absolute intensity of the reflected X-ray at the specular geometry around an incident angle of 0.1 was over 90% of the intensity of the incident X-ray. These results means the film surfaces have a very small roughness (<1 nm). It was considered that the obtained film thickness and the density are high reliability values. The period of the Kiessig fringes depends on the film thickness, and a shorter period of the fringe means a thicker film. So it is found that the lower concentration of the spin-coat solution produces the thinner film. Enlarged XRR profiles at the lower incident angle are shown in the inset, where the ac of each of the PI thin films is indicated by an arrow. The density of the PI thin films is smaller than that of the silicon substrate (2.33 g/cm3), the ac of each of the thin films differed from that of the substrate. At the incident angle higher than the ac of a PI thin film, the X-ray penetrated into the thin film, and the intensity of the reflected X-ray decreased with increasing the incident angle. An X-ray penetrating a PI film, however, is incident on the substrate surface at an angle lower than the a of the silicon substrate. Thus, the XRR profile around the ac shows a stepwise change. The ac observed in the XRR profile was 0.148 for the film made from the 3.5 wt% spin-coat solution, and was 0.135 for the film made from the 6.0 wt% spin-coat solution. In this study, the incident angle for GIXD measurements was set at 0.170 for both samples. This incident angle is higher than the ac of the PI thin films and lower than
Fig. 1. Infrared spectra of BPDA-PPD thin film (6.0 wt%) on the Si substrate before and after curing at 400 C.
M. Kotera et al. / Polymer 54 (2013) 2435e2439
Fig. 2. Observed and calculated XRR profiles of BPDA-PPD thin film cured up to 400 C made from different concentrations of the spin-coat solutions. Observed data are shown by open circles, and calculated reflectivities are shown by solid curves.
the ac of the silicone substrate (0.180 at 10 keV), so, we could obtain the diffracted X-rays from the whole PI thin film. Table 1 shows the characteristics of the BPDA-PPD PI thin films made from the different concentrations of the spin-coat solution. The film thickness was obtained as 37 nm from the spin-coat solution of 3.5 wt% and 172 nm from that of 6.0 wt%, respectively. It was found that the ac of the thinner film was higher than that of the thicker film, even though both films had the same thermal history. The ac value is expressed by the following approximate equation;
ac ¼ 0:133l ðrZ=AÞ1=2
(2)
where, l is the wavelength of the X-ray, r is the mass density, Z is the atomic number and A is the atomic mass [15,16]. The values of r, Z and A are constant, so a higher ac means a higher density of materials. The film density in Table 1 was obtained by fitting results to the observed XRR profiles. The density(cal.) was calculated from eq. (2) by using the evaluated ac value. The sample density of the uniaxially oriented BPDA-PPD film with different thermal curing was 1.473e1.483 g/cm3 [25]. The density(cal.) value (1.61 g/cm3 for the thinner film) was very close to the crystal density of the BPDAPPD PI (1.644 g/cm3) [19e21]. The amorphous density is unknown, however, judging from the crystal density and the crystallinity, it is considered that the density(cal.) value is too high because the eq. (2) is a simply an approximating expression. In both evaluation methods, however, the ac and the film density increased with decreasing the film thickness. Fig. 3 shows the changes of (C) the conversion to PI and (-) the film thickness during the curing process of the BPDA-PPD PI with the 37 nm thickness. With increasing curing temperature, the conversion to PI started at around 100 C abruptly, then almost finished at around 250 C. The thermally driven imidization of PI film with micrometer order thickness has been reported to occur temperature ranging from 87 to 110 C [22,23]. The film thickness decreased gradually with increasing the curing temperature. After
2437
Fig. 3. Curing temperature dependence of (C) the conversion to PI and (-) the film thickness of the BPDA-PPD PI with the 37 nm thickness.
curing at 400 C, the film thickness of the precursor PAA had decreased by 43.1% of the thickness just after spin-coating. Accordingly, it is clear that the thermal imidization progressed with decreasing the film thickness. The same trend was also observed for the PI with the 172 nm thickness. Assuming that the decrease of the film thickness was due only the dehydration associated with imidization, a 13.3% decrease in thickness was calculated stoichiometrically. That, however, is less than a third of the decrease measured experimentally (43.1%). This implies that the spin-coated precursor PAA thin film contained residual NMP solvent that evaporated during the thermal curing. As described before, the thermal curing process simultaneously and competitively involves the cyclization with dehydration, evaporation of residual solvent and crystallization [12e14]. Thus, when the film thickness before the thermal curing is different, competitive processes such as the conversion to PI will be influenced. Fig. 4 shows the comparison of the curing temperature dependence of (C) the conversion to PI and (-) the residual thickness during the curing process of the BPDA-PPD PI with different thickness. At every curing temperature the conversion to PI was higher for the thinner film (37 nm) and the degree of the thickness reduction was higher for the thicker film (172 nm). As described before, reduction of the film thickness during the thermal imidization was due mainly to solvent evaporation. So, these results imply that much larger amount of the residual solvent in the
Table 1 Characteristics of the BPDA-PPD thin films film cured up to 400 C from different concentration of the spin-coat solution. PAA solution wt%
Thickness nm
ac degree
Density g/cm3
Density(cal.) g/cm3
3.5 6.0
37 172
0.148 0.135
1.51 1.43
1.61 1.34
Fig. 4. Curing temperature dependence of (C) the conversion to PI and (-) the residual thickness for BPDA-PPD PI films with different thicknesses.
M. Kotera et al. / Polymer 54 (2013) 2435e2439
thinner film brings high molecular mobility, which results in the higher conversion to PI at any curing temperature. Fig. 5 shows (a) in-plane and (b) out-of-plane GIXD profiles of the BPDA-PPD PI films with different thicknesses. PI thin films were well crystallized after the successive thermal imidization at temperatures up to 400 C. All the reflections could be assigned with the orthorhombic unit cell (lattice parameter of a ¼ 8.6 20 A, b ¼ 6.270 A, c (fiber axis) ¼ 31.986 A) in accordance with the results reported by Ree et al. [20,21]. In the in-plane diffraction profiles (Fig. 5(a)), only the higher-order meridional (0 0 l) reflections were observed. On the other hand, the out-of-plane diffraction profiles (Fig. 5(b)) showed only equatorial (h k 0) reflection corresponding to the interchain packing. These results indicate that the BPDA-PPD PI molecules, which have a rigid-rod and planar skeletal structure, are preferentially parallel to the film plane. The molecular chains in thin BPDA-PPD PI films have been reported to be parallel to the film plane but oriented in random directions parallel to that film plane [21,24]. As described before, the density of final PI films depends on the initial film thickness of the precursor PAA film. Next, the effect of the molecular packing on the film density of the PI thin films with different thicknesses was investigated. Fig. 6 shows the relationship between the reflectional orders and the fiber identity periods obtained from each meridional reflection (0 0 l) in Fig. 5(a) with different film thickness. The fiber identity periods of the uniaxially oriented BPDA-PPD film with different thermal curing are superimposed on the figure with open circles [25]. All fiber identity periods evaluated for each reflection were different from one another, and they showed no monotonic tendency with the reflectional order. In BPDA-PPD PI, rotation around the CeC bonds between phenyl rings and the CeN bonds between the phenyl and imide rings are possible, and numerous skeletal conformations can be expected to result from this rotation. The energetically favored conformation, which has a 9/4 helix corresponding to a chain contraction of 0.8% from the all-trans
32.5 32.16Å
Fiber identity period (Å)
2438
32.0
31.5
31.0
30.5
30.0 4
6
8
10
12
14
16
18
20
22
24
26
Fig. 6. Relationship between the reflectional orders and the fiber identity periods obtained from the meridional reflection of the BPDA-PPD thin films with different thickness. ( ) 37 nm, ( ) 172 nm thickness. ( ) and ( ) are our previous results for a uniaxially oriented thick film with different thermal curing.
skeleton, showed the fiber identity period of 32.16 A by the energy calculation with broken line [25]. Several kinds of chain conformations can coexist in the same specimen, however, because the potential barriers between them seem to be low. The reflection with longer fiber identity period gave the higher elastic modulus of the crystalline regions in the direction parallel to the chain axis [25]. In this study, the fiber identity period ( ) of the thinner film was longer than that ( ) of the thicker film, which indicated that the skeletal conformation was more extended in the chain axis direction for the thinner BPDA-PPD PI film. Table 2 shows the lattice spacings of the BPDA-PPD crystal obtained from each equatorial reflection (h k 0) reflection in Fig. 5(b). All the lattice spacings of the thinner film were smaller than those of the thicker film. As described before, these (h k 0) reflections correspond to the interchain packing. These results indicate that the distance between the molecular chains was shorter in the thinner film. Accordingly, with decreasing the film thickness the molecular chains in the BPDA-PPD crystal unit exist in more extended state in the chain axis direction and the better packing in the interchain direction. The better molecular packing makes the density of the BPDA-PPD crystal unit higher. Accordingly, it is considered that the density of the thinner BPDA-PPD film was higher than that of the thicker film, which results in much more amount of the residual solvent for the thinner film during the thermal curing. As described before, PI thin films on glass substrates are used as alignment films for liquid crystal displays. Controlling the orientation of the liquid crystal molecules is a key factor to improve the display quality. Not only the molecular orientation but also the crystallinity of the alignment film surface affects the controlling of the orientation of the liquid crystal molecules. Yokokura et al. have reported that the pretilt angle of the liquid crystals depended on the crystallinity of a rubbed PI film [3], and Sakai et al. have reported that higher crystallinity of the alignment PI film surface
Table 2 Lattice spacings of the BPDA-PPD crystal obtained from each of the equatorial reflection (h k 0) reflection peaks. Film thickness (nm)
Fig. 5. GIXD profiles of the BPDA-PPD PI films with different film thicknesses. (a) in-plane and (b) out-of-plane diffraction profiles.
37 172
d spacing ( A) (110)
(200)
(210)
4.844 4.852
4.201 4.217
3.499 3.503
M. Kotera et al. / Polymer 54 (2013) 2435e2439
resulted in higher orientation of the liquid crystal molecules [4]. Accordingly, for the thinner film in this study, the higher density of the PI film would be a good candidate for a alignment film in a highquality display. 4. Conclusions The microstructures of BPDA-PPD PI thin films with different thickness were investigated by GIXD and XRR at the SPring-8 synchrotron radiation facility. The precursor PAA thin films with different thicknesses were obtained by spin-coating substrates with precursor solutions diluted to different concentrations. The thickness of PI films after the curing at 400 C was 37 nm and 172 nm. The film density was higher for the thinner film even though both film had the same thermal history. The thickness reduction of the thinner film during curing was lower than that of the thicker film. The residual solvent brings high molecular mobility, which results in higher conversion to PI at any curing temperature. The molecular chains in the BPDA-PPD crystal unit for the thinner film exist in a more extended state in the direction of the chain axis and the better packing in the direction of the interchain. It is considered that the film density of the thinner BPDA-PPD film was higher than that of the thicker film because of the better molecular packing, which results in much more amount of the residual solvent for the thinner film during the thermal curing. Acknowledgment The synchrotron radiation experiments were performed at the BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2009B2025, 2010B1810).
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References [1] Bessonov MI, Koton MM, Kudryavtsev VV, Laius LA. Polyimide e thermally stable polymers. Plenum; 1987. [2] Ghosh MK. Mittal KL, editor. Polyimide fundamental and application. Marcel Dekker; 1996. [3] Yokokura H, Oh-e M, Kondo K, Oh-hara S. Mol Cryst Liq Cryst 1993;225: 253e8. [4] Sakai T. SPring-8 user experimental report (proposal no. 2003B0247-NI-np, 2004B0801-RI-np-TU, 2006A0181, 2006B0207). [5] Tsui OKC. Tsui OKC, Russell TP, editors. Polymer thin films. World Scientific; 2008. p. 267e94. [6] Keddie JL, Jones RAL, Cory RA. Europhys Lett 1994;27:59e64. [7] Wallace WE, van Zanten JH, Wu WL. Phys Rev E 1995;52:R3329e32. [8] Frank B, Gast AP, Russell TP, Brown HR, Hawker C. Macromolecules 1996;29: 6531e4. [9] DeMaggio GB, Frieze WE, Gildley DW, Zhu M, Hristov HA, Yee AF. Phys Rev Lett 1997;78:1524e7. [10] Forrest JA, Svanberg C, Révész K, Rodahl M, Torell LM, Kasemo B. Phys Rev E 1998;58:R1226e9. [11] Fryer DS, Nealey PF, de Pablo JJ. Macromolecules 2000;33:6439e47. [12] Kotera M, Nishino T, Nakamae K. Polymer 2000;41:3615e9. [13] Nishino T, Kotera M, Inayoshi N, Miki N, Nakamae K. Polymer 2000;41: 6913e8. [14] Nishino T, Kotera M, Nakamae K. In: Mittal KL, editor. Polyimides and other high temperature polymers: synthesis, characterization and applications, vol. 1. CRC Press; 2001. p. 65e77. [15] James RW. The optical principles of the diffraction of X-rays. G.Bell and Sons Ltd; 1967. [16] Feidenhans’l R. Surf Sci Rep 1989;10:105e88. [17] Nishino T, Matsumoto T, Nakamae K. Polym Eng Sci 2000;40:336e43. [18] Young JT, Tsai WH, Boerio FJ. Macromolecules 1992;25:887e94. [19] Poon TW, Silverman BD, Saraf RF, Rossi AR. Phys Rev B 1992;46:11456e62. [20] Yoon DY, Parrish W, Depero LE, Ree M. Mat Res Soc Symp Proc 1991;227: 387e93. [21] Ree M, Sim K, Woo SH, Chang H. J Appl Phys 1997;81:698e708. [22] Gonzalo B, Vilas JL, Breczewski T, Pérez-Jubindo MA, De La Fuente MR, Rodriguez M, et al. J Polym Sci Part A 2008;47:722e30. S¸, Bruma M, Lisa G. J Therm Anal Calorim 2011;104:1135e43. [23] Sava I, Chis¸ca [24] Ree M, Chu CW, Goldberg MJ. J Appl Phys 1994;75:1410e9. [25] Nishino T, Miki N, Mitusoka Y, Nakamae K, Saito T, Kikuchi T. J Polym Sci Part B Polym Phys 1999;37:3294e301.
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Microstructures of BPDA-PPD polyimide thin films with different thicknesses Masaru Kotera a, *, Boo Samyul a, Kouhei Araie a, Yuri Sugioka a, Takashi Nishino a, Satoshi Maji b, Miki Noda c, Kazunobu Senoo c, Tomoyuki Koganezawa d, Ichiro Hirosawa d a
Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan S.B.Research Co., Ltd., Kiyohara, Utsunomiya 321-3231, Japan c Kobe Fundamental Research Laboratory, Sumitomo Bakelite Co., Ltd., Murotani, Nishi, Kobe 651-2241, Japan d Japan Synchrotron Radiation Research Institute, Kouto, Sayo-cho, Sayo-gun 679-5198, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2012 Received in revised form 1 March 2013 Accepted 3 March 2013 Available online 13 March 2013
The microstructures of 3,4,30 ,40 -biphenyltetracarboxylic dianhydride e p-phenylene diamine (BPDAPPD) polyimide (PI) thin films with different thicknesses were investigated by grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) at the SPring-8 synchrotron radiation facility. The film density calculated from XRR profiles increased with decreasing the film thickness. Processing of PI thin films from precursor polyamic acid (PAA) solutions involves simultaneous thermal imidization, evaporation of residual solvents and crystallization. The thickness reduction during imidization was smaller for the thinner PAA film. This implies that the much more larger amount of the residual solvent in the thinner PAA film brings high molecular mobility, which results in the higher conversion to PI at any curing temperature. The thinner BPDA-PPD film was more dense than the thicker film because of better molecular packing, which is a result of much more amount of the residual solvent for the thinner film during the thermal curing. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Polyimide thin films Imidization process Residsal solvent
1. Introduction Polyimides (PIs) are widely used in the aerospace and microelectronics industries because of their excellent thermal stability, good mechanical properties and low dielectric constants [1,2]. They are often used to make alignment films for liquid crystal displays, and for this purpose a PI film 50e100 nm thick is deposited on a glass substrate or electrode film. The display qualities are affected to the controlling of the orientation for the liquid crystal molecules on the alignment film. Yokokura et al. have reported that the pretilt angle of the liquid crystals molecules depended on the crystallinity of a alignment PI film [3], and Sakai et al. have reported that higher crystallinity of the alignment PI film surface resulted in higher orientation of the liquid crystal molecules [4]. The microstructures and properties of the polymer thin films are well known to be quite different compared with those of the bulk materials, especially when the film thickness is less than 2Rg (Rg: radius of gyration of the polymer chain) [5]. Many researchers have reported that the glass transition temperature of a thin polymer films is a strong function of the film thickness [5e11].
* Corresponding author. Tel./fax: þ81 78 803 6198. E-mail address: [email protected] (M. Kotera). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.03.005
PI is generally insoluble in any solvent and is therefore usually made, in a thermal conversion process, from a precursor poly(amic acid) (PAA) dissolved in an aprotic polar solvent. Because the cyclization with dehydration, evaporation of residual solvent and crystallization in this thermal conversion occur simultaneously and competitively, the imidization process will influence the final structure and properties of the PI [12e14]. If the thickness of a precursor PAA film is comparable to the 2Rg, there is possibility of imidization behavior different from that in bulk PAA. An X-ray beam, particular a hard X-ray beam, is an excellent analytical probe for microstructural analysis and can be used in an ambient environment providing nondestructive measurements. In addition, no special pre-treatment of the sample is needed. The conventional measurements using hard X-ray beams give information about the average structure of the bulk polymeric materials, because hard X-ray beam penetrates deep into the materials consisting mainly of light elements such as hydrogen and carbon. In polymeric materials, the X-ray diffraction intensity is very weak compared with that in metals or ceramics, because of the light elements and low crystallinity. Measurements using X-ray beams incident at a very small angle, in contrast, give information about the material near the surface. In the grazing incidence X-ray diffraction (GIXD) method, when the incident angle is smaller than the critical angle for total external reflection (ac), the penetration of the X-ray beam is limited to depth of nanometers order [15e17]. In
2436
M. Kotera et al. / Polymer 54 (2013) 2435e2439
addition, the incident X-ray footprint spreads to incident direction over a large area on the surface, so the GIXD method enables enough diffraction intensity to be obtained even from polymer thin films. The excellent brightness and high directionality of a synchrotron radiation X-ray beam possesses make it very suitable for the GI geometry. In this study, the effect of film thickness on the imidization process for PI thin films was investigated by using GIXD and X-ray reflectivity (XRR) methods to evaluate the microstructures of PI films with different thicknesses. 2. Experimental PAA precursor (Ube Industries Ltd., U-varnish S301), 3,4,30 ,40 biphenyltetracarboxylic dianhydride e p-phenylene diamine (BPDA-PPD) type in N-methyl-2-pyrrolidone (NMP) solution was used throughout this study. Solid content of the PAA precursor solution was 18.2 wt% in NMP. Silicon substrates (diameter; 4 inches, thickness; 1 mm) were immersed in a mixture of concentrated sulfuric acid and hydrogen peroxide (3:1 ratio by volume) at room temperature for 20 min and then rinsed with distilled water to produce a thin surface layer of silicon oxide. PAA thin films were prepared by spin-coating precursor solution (diluted to 3.5 and 6.0 wt% with NMP) onto the substrate at room temperature. The spin-coating was performed at 1000 rpm for 10 s and 4000 rpm for 90 s. The PAA thin films was cured at 60 C for 30 s, and then successively thermally imidized at 100 C, 140 C, 160 C, 180 C, 200 C, 250 C, 300 C and finally at 400 C (1 h at each temperature). Imidization reaction was investigated using a Fourier transform infrared (FT-IR) spectrophotometer (PerkinElmer Inc., Spectrum GX FT-IR System I-KS) at a resolution 2 cm1 with 200 scans accumulated in the transmission mode. The conversion to PI during the thermal imidization at temperature T was calculated using the following equation;
Conversion to PIð%Þ ¼ ðD1380 ðD1380
cm1 =D1510 cm1 ÞT =
cm1 =D1510 cm1 Þ400 C
100 ð1Þ
where, D is the optical density of each absorption. The absorption bands at 1380 cm1 (imide II) (CeN stretching) and at 1510 cm1 (CeC stretching of p-substituted benzene: an internal standard) have previously been used to evaluate imidization [18]. Microstructures of the PI thin films were evaluated by GIXD and XRR using multi-axis diffractometer installed in the BL19B2 of SPring-8, third-generation synchrotron radiation facility in Japan, and combined using the laboratory equipment (Rigaku Co., SmarA). The ultra-bright and highly directional tLab, CuKa: l ¼ 1.5418 X-rays provided by a synchrotron radiation are suitable for the GIXD and XRR techniques. The incident X-ray energy was set at A) by a Si(111) double-crystal monochrometer, 10 keV (l ¼ 1.2398 and the X-ray beam was set to be 8 mm wide and 0.1 mm height. Scattered X-rays from the sample were detected by a NaI scintillation counter through a soller slit with a 0.45 divergence angle. In our GIXD measurements the scattering vector was almost parallel to the sample surface. That is, we used the so called in-plane diffraction technique. The results obtained in this geometry give the crystal lattice order in the in-plane direction. We also used a geometry in which the scattering plane was perpendicular to the sample surface (the so called out-of-plane diffraction technique). Before the GIXD measurements, we estimated the ac from the XRR profile. The thickness and density of the thin PI films were calculated from XRR profiles by using a fitting software (Rigaku Co., GXRR). We
have assumed that there is a silicon oxide layer several nanometers thick on top of the substrate and that there is no gradation of the density of the PI thin film. 3. Results and discussion Fig. 1 shows the infrared spectra of BPDA-PPD thin film (6.0 wt%) on the substrate before and after curing at 400 C. The absorption at 1665 cm1 (amide I) (C]O (CONH) stretching) and that at 1550 cm1 (amide II) (CeNH vibration) disappeared on the other hand, the absorption at 1780 cm1 (imide I) (C]O symmetric stretching) and that at 1380 cm1 (imide II) (CeN stretching) appeared [18]. These results indicate that the precursor PAA film was converted into the BPDA-PPD type PI by thermal curing up to 400 C. Fig. 2 shows (plots) the observed and (lines) calculated XRR profiles of BPDA-PPD PI thin films made from different concentrations of the precursor PAA solution were cured at up to 400 C. The calculated reflectivity values fitted the observed profiles very well. In the both samples, the reflected X-ray with Kiessig fringe was clearly observed to higher incident angle. These indicated that no distribution of the density in the PI thin film, and there was no residual solvent, for both samples. Moreover, the absolute intensity of the reflected X-ray at the specular geometry around an incident angle of 0.1 was over 90% of the intensity of the incident X-ray. These results means the film surfaces have a very small roughness (<1 nm). It was considered that the obtained film thickness and the density are high reliability values. The period of the Kiessig fringes depends on the film thickness, and a shorter period of the fringe means a thicker film. So it is found that the lower concentration of the spin-coat solution produces the thinner film. Enlarged XRR profiles at the lower incident angle are shown in the inset, where the ac of each of the PI thin films is indicated by an arrow. The density of the PI thin films is smaller than that of the silicon substrate (2.33 g/cm3), the ac of each of the thin films differed from that of the substrate. At the incident angle higher than the ac of a PI thin film, the X-ray penetrated into the thin film, and the intensity of the reflected X-ray decreased with increasing the incident angle. An X-ray penetrating a PI film, however, is incident on the substrate surface at an angle lower than the a of the silicon substrate. Thus, the XRR profile around the ac shows a stepwise change. The ac observed in the XRR profile was 0.148 for the film made from the 3.5 wt% spin-coat solution, and was 0.135 for the film made from the 6.0 wt% spin-coat solution. In this study, the incident angle for GIXD measurements was set at 0.170 for both samples. This incident angle is higher than the ac of the PI thin films and lower than
Fig. 1. Infrared spectra of BPDA-PPD thin film (6.0 wt%) on the Si substrate before and after curing at 400 C.
M. Kotera et al. / Polymer 54 (2013) 2435e2439
Fig. 2. Observed and calculated XRR profiles of BPDA-PPD thin film cured up to 400 C made from different concentrations of the spin-coat solutions. Observed data are shown by open circles, and calculated reflectivities are shown by solid curves.
the ac of the silicone substrate (0.180 at 10 keV), so, we could obtain the diffracted X-rays from the whole PI thin film. Table 1 shows the characteristics of the BPDA-PPD PI thin films made from the different concentrations of the spin-coat solution. The film thickness was obtained as 37 nm from the spin-coat solution of 3.5 wt% and 172 nm from that of 6.0 wt%, respectively. It was found that the ac of the thinner film was higher than that of the thicker film, even though both films had the same thermal history. The ac value is expressed by the following approximate equation;
ac ¼ 0:133l ðrZ=AÞ1=2
(2)
where, l is the wavelength of the X-ray, r is the mass density, Z is the atomic number and A is the atomic mass [15,16]. The values of r, Z and A are constant, so a higher ac means a higher density of materials. The film density in Table 1 was obtained by fitting results to the observed XRR profiles. The density(cal.) was calculated from eq. (2) by using the evaluated ac value. The sample density of the uniaxially oriented BPDA-PPD film with different thermal curing was 1.473e1.483 g/cm3 [25]. The density(cal.) value (1.61 g/cm3 for the thinner film) was very close to the crystal density of the BPDAPPD PI (1.644 g/cm3) [19e21]. The amorphous density is unknown, however, judging from the crystal density and the crystallinity, it is considered that the density(cal.) value is too high because the eq. (2) is a simply an approximating expression. In both evaluation methods, however, the ac and the film density increased with decreasing the film thickness. Fig. 3 shows the changes of (C) the conversion to PI and (-) the film thickness during the curing process of the BPDA-PPD PI with the 37 nm thickness. With increasing curing temperature, the conversion to PI started at around 100 C abruptly, then almost finished at around 250 C. The thermally driven imidization of PI film with micrometer order thickness has been reported to occur temperature ranging from 87 to 110 C [22,23]. The film thickness decreased gradually with increasing the curing temperature. After
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Fig. 3. Curing temperature dependence of (C) the conversion to PI and (-) the film thickness of the BPDA-PPD PI with the 37 nm thickness.
curing at 400 C, the film thickness of the precursor PAA had decreased by 43.1% of the thickness just after spin-coating. Accordingly, it is clear that the thermal imidization progressed with decreasing the film thickness. The same trend was also observed for the PI with the 172 nm thickness. Assuming that the decrease of the film thickness was due only the dehydration associated with imidization, a 13.3% decrease in thickness was calculated stoichiometrically. That, however, is less than a third of the decrease measured experimentally (43.1%). This implies that the spin-coated precursor PAA thin film contained residual NMP solvent that evaporated during the thermal curing. As described before, the thermal curing process simultaneously and competitively involves the cyclization with dehydration, evaporation of residual solvent and crystallization [12e14]. Thus, when the film thickness before the thermal curing is different, competitive processes such as the conversion to PI will be influenced. Fig. 4 shows the comparison of the curing temperature dependence of (C) the conversion to PI and (-) the residual thickness during the curing process of the BPDA-PPD PI with different thickness. At every curing temperature the conversion to PI was higher for the thinner film (37 nm) and the degree of the thickness reduction was higher for the thicker film (172 nm). As described before, reduction of the film thickness during the thermal imidization was due mainly to solvent evaporation. So, these results imply that much larger amount of the residual solvent in the
Table 1 Characteristics of the BPDA-PPD thin films film cured up to 400 C from different concentration of the spin-coat solution. PAA solution wt%
Thickness nm
ac degree
Density g/cm3
Density(cal.) g/cm3
3.5 6.0
37 172
0.148 0.135
1.51 1.43
1.61 1.34
Fig. 4. Curing temperature dependence of (C) the conversion to PI and (-) the residual thickness for BPDA-PPD PI films with different thicknesses.
M. Kotera et al. / Polymer 54 (2013) 2435e2439
thinner film brings high molecular mobility, which results in the higher conversion to PI at any curing temperature. Fig. 5 shows (a) in-plane and (b) out-of-plane GIXD profiles of the BPDA-PPD PI films with different thicknesses. PI thin films were well crystallized after the successive thermal imidization at temperatures up to 400 C. All the reflections could be assigned with the orthorhombic unit cell (lattice parameter of a ¼ 8.6 20 A, b ¼ 6.270 A, c (fiber axis) ¼ 31.986 A) in accordance with the results reported by Ree et al. [20,21]. In the in-plane diffraction profiles (Fig. 5(a)), only the higher-order meridional (0 0 l) reflections were observed. On the other hand, the out-of-plane diffraction profiles (Fig. 5(b)) showed only equatorial (h k 0) reflection corresponding to the interchain packing. These results indicate that the BPDA-PPD PI molecules, which have a rigid-rod and planar skeletal structure, are preferentially parallel to the film plane. The molecular chains in thin BPDA-PPD PI films have been reported to be parallel to the film plane but oriented in random directions parallel to that film plane [21,24]. As described before, the density of final PI films depends on the initial film thickness of the precursor PAA film. Next, the effect of the molecular packing on the film density of the PI thin films with different thicknesses was investigated. Fig. 6 shows the relationship between the reflectional orders and the fiber identity periods obtained from each meridional reflection (0 0 l) in Fig. 5(a) with different film thickness. The fiber identity periods of the uniaxially oriented BPDA-PPD film with different thermal curing are superimposed on the figure with open circles [25]. All fiber identity periods evaluated for each reflection were different from one another, and they showed no monotonic tendency with the reflectional order. In BPDA-PPD PI, rotation around the CeC bonds between phenyl rings and the CeN bonds between the phenyl and imide rings are possible, and numerous skeletal conformations can be expected to result from this rotation. The energetically favored conformation, which has a 9/4 helix corresponding to a chain contraction of 0.8% from the all-trans
32.5 32.16Å
Fiber identity period (Å)
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32.0
31.5
31.0
30.5
30.0 4
6
8
10
12
14
16
18
20
22
24
26
Fig. 6. Relationship between the reflectional orders and the fiber identity periods obtained from the meridional reflection of the BPDA-PPD thin films with different thickness. ( ) 37 nm, ( ) 172 nm thickness. ( ) and ( ) are our previous results for a uniaxially oriented thick film with different thermal curing.
skeleton, showed the fiber identity period of 32.16 A by the energy calculation with broken line [25]. Several kinds of chain conformations can coexist in the same specimen, however, because the potential barriers between them seem to be low. The reflection with longer fiber identity period gave the higher elastic modulus of the crystalline regions in the direction parallel to the chain axis [25]. In this study, the fiber identity period ( ) of the thinner film was longer than that ( ) of the thicker film, which indicated that the skeletal conformation was more extended in the chain axis direction for the thinner BPDA-PPD PI film. Table 2 shows the lattice spacings of the BPDA-PPD crystal obtained from each equatorial reflection (h k 0) reflection in Fig. 5(b). All the lattice spacings of the thinner film were smaller than those of the thicker film. As described before, these (h k 0) reflections correspond to the interchain packing. These results indicate that the distance between the molecular chains was shorter in the thinner film. Accordingly, with decreasing the film thickness the molecular chains in the BPDA-PPD crystal unit exist in more extended state in the chain axis direction and the better packing in the interchain direction. The better molecular packing makes the density of the BPDA-PPD crystal unit higher. Accordingly, it is considered that the density of the thinner BPDA-PPD film was higher than that of the thicker film, which results in much more amount of the residual solvent for the thinner film during the thermal curing. As described before, PI thin films on glass substrates are used as alignment films for liquid crystal displays. Controlling the orientation of the liquid crystal molecules is a key factor to improve the display quality. Not only the molecular orientation but also the crystallinity of the alignment film surface affects the controlling of the orientation of the liquid crystal molecules. Yokokura et al. have reported that the pretilt angle of the liquid crystals depended on the crystallinity of a rubbed PI film [3], and Sakai et al. have reported that higher crystallinity of the alignment PI film surface
Table 2 Lattice spacings of the BPDA-PPD crystal obtained from each of the equatorial reflection (h k 0) reflection peaks. Film thickness (nm)
Fig. 5. GIXD profiles of the BPDA-PPD PI films with different film thicknesses. (a) in-plane and (b) out-of-plane diffraction profiles.
37 172
d spacing ( A) (110)
(200)
(210)
4.844 4.852
4.201 4.217
3.499 3.503
M. Kotera et al. / Polymer 54 (2013) 2435e2439
resulted in higher orientation of the liquid crystal molecules [4]. Accordingly, for the thinner film in this study, the higher density of the PI film would be a good candidate for a alignment film in a highquality display. 4. Conclusions The microstructures of BPDA-PPD PI thin films with different thickness were investigated by GIXD and XRR at the SPring-8 synchrotron radiation facility. The precursor PAA thin films with different thicknesses were obtained by spin-coating substrates with precursor solutions diluted to different concentrations. The thickness of PI films after the curing at 400 C was 37 nm and 172 nm. The film density was higher for the thinner film even though both film had the same thermal history. The thickness reduction of the thinner film during curing was lower than that of the thicker film. The residual solvent brings high molecular mobility, which results in higher conversion to PI at any curing temperature. The molecular chains in the BPDA-PPD crystal unit for the thinner film exist in a more extended state in the direction of the chain axis and the better packing in the direction of the interchain. It is considered that the film density of the thinner BPDA-PPD film was higher than that of the thicker film because of the better molecular packing, which results in much more amount of the residual solvent for the thinner film during the thermal curing. Acknowledgment The synchrotron radiation experiments were performed at the BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2009B2025, 2010B1810).
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