[acs symposium series] polymeric materials for microelectronic applications volume 579 (science and...

Chapter 18

Uniaxial and In-Plane Molecular Orientation of Polyimides and Their Precursor

as Studied by Absorption Dichroism of Perylenebisimide Dye

M. Hasegawa, T. Matano, Y. Shindo, and T. Sugimura

Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274, Japan

Molecular Orientation of a uniaxially stretched poly(amic acid) (PAA) Films and the corresponding polyimide (PI) film cured thermally has been examined by measuring the dichroic ratio of a rigid-rod dye (perylenebisimide) dispersed molecularly in their films. The dichroic spectra of this dye at an incidence angle was also applied to evaluate the in-plane molecular orientation of PAA films cast on a glass plate (undrawn) and the thermally imidized PIs. For uniaxially stretched samples, the dichroic spectra showed that semi-rigid PI(BPDA/PDA) chains oriented spontaneously toward the stretching direction during thermal imidization of the uniaxially drawn PAA films (DR = 50 %). High Young's modulus of the uniaxially oriented PI(BPDA/PDA) film (~60 GPa) was rationalized in terms of the considerably large orientation factor (f = 0.6). On the other hand, flexible PI(BPDA/ODA) showed no spontaneous molecular orientation during thermal imidization, corresponding to the fact that Young's modulus of PI(BPDA/ODA) is nearly independent of a draw ratio. Thermal imidization of PAA(BPDA/PDA) cast on a substrate enhanced markedly the in-plane orientation of the polymer chains, no spontaneous in-plane orientation was observed for BPDA/ODA system. The spontaneous in-plane orientation behavior became marked as the film thickness decreased. Free-cured PI(BPDA/PDA) showed a degree of the in-plane orientation much smaller than the PI film imidized on substrate.

Physical properties of aromatic polyimides (PI) such as the thermal expansion coefficient (TEC) (1) and Young's modulus (2) have been widely known to be strongly affected by molecular orientation of PI chains. This paper is focused on spontaneous uniaxial and in-plane orientation of PI chains induced by thermal imidization of poly(amic acid) (PAA).

0097-6156/94/0579-0234$08.00/0 © 1994 American Chemical Society

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In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

18. HASEGAWA ET AL. Uniaxial Molecular Orientation of Polyimides 235

The uniaxial molecular orientation can be qualitatively estimated by the optical birefringence measurements. For quantitative analysis, the infrared absorption dichroism for a specific group in polymer chains and the uv-vis absorption dichroism for rigid-rod dyes introduced covalently or mechanically are available. Several research groups showed that PI chains orient in the film plane by using X-ray method (3-5) and the optical birefringence (nQ, n^ measurement.^, 6, 7) In the present paper, we describe the molecular orientation of PAA and PI chains determined by the absorption dichroism of perylenebisimide dye dispersed in PAA and PI films and focus on the spontaneous molecular orientation occurring in the course of thermal imidization of PAA films.(8) Biphenyl type PI is known to show the intrinsic absorption dichroism at 290 nm.(9) But, since the absorption at 290 nm is too strong, it is not available for thicker PAA and PI films (the use of the intrinsic dichroism is limited to PI films less than about 0.2 μηι in thickness). Hence, the dichroic spectrum measurement of the dyes is powerful and convenient way to PAA and PI films ranging from several μπι to hundreds μπι thick.

EXPERIMENTAL

Chemical formulae of PAAs, Pis, and the dye are shown in Figure 1. Dye 1 and 2 were used for evaluation of the uniaxial and in-plane orientation, respectively. The dyes were dissolved rapidly in N-methyl-2-pyrrolidone (NMP) at 160°C, then the solution was cooled to room temperature and vigorously mixed with the N,N-dimethylacetamide (DMAc) solution of PAA (10 wt %). The dye-containing PAA solution was defoamed under reduced pressure and cast on a glass plate at 65°C. The concentration of the dye 1 and 2 in PAA films were ca. 4X ΙΟ"4 M and 3X10" 3 M , respectively. Due to this procedure, the dyes were dispersed molecularly in the films. The visible absorption spectrum of the dye-containing films was quite similar to that of the DMAc solution except for a slight spectral shift, and the dyes were fluorescent in PI films as well as in the DMAc solution.

For the uniaxial orientation measurement, the PAA specimens containing the dye were uniaxially stretched at room temperature. The drawn PAA films fixed with a frame were thermally imidized at 250°C for 2 h. Herman's orientation factor F of the drawn PAA and PI films given by

F _ 3<cos2Q>-l _ Do + 2 D - l (1) 2 " D 0 - l D + 2

was determined from the dichroic absorption spectra of the dye measured at incidence angle = 0 by using an uv-vis spectrophotometer equipped with polarizers, where θ is the angle between the polymer chain axis and the drawing direction, D (= A /A x ) is the dichroic ratio at the incidence angle = 0 which is the ratio of absorbances at the peak wavelength (near 535 nm) for incident lights polarized parallel (A | ) and perpendicular (Α χ ) to the stretching direction, and D 0 is the dichroic ratio for perfect uniaxial orientation and defined as D n = 2cot2P; β is the angle between polymer chain axis and the transition dipole of the dye. The f is the orientation factor for the dye itself. The orientation factor for polymer chains, F, changes within the range, 0 (isotropic) £ F £ 1 (perfect uniaxial orientation). If β - 0°, F = f = (D-l)/(D+2).

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236 POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

For the in-plane orientation measurement, the PAA solution was cast on a glass plate, and the cast film was then thermally imidized on the substrate or without substrate (free-standing cure). All the as-cast films were perfectly isotropic in the film plane. The in-plane orientation of PAA and PI chains were estimated by measuring the dichroic spectra at an incidence angle as shown in Figure 2. In order to measure the dichroic spectra at a constant refraction angle a, the incidence angle θ for both P-and S-polarized lights were adjusted on the basis of the refractive indices (the ordinary (n^ and the extraordinary (ne)) of PAA and PI films measured by using an Abbe's refractometer. θ was calculated from the following equation:

(Snell's law) η sina = sinO η = nQ (for S-polarized light)

n~2 = n0"2cos2a+ ne~2sin2a (for P-polarized light) (2)

We defined IP=( 1-R)/(1-RQ) as the degree of in-plane orientation, where R (=AD/A S,

A: absorbance at the peak wavelength at 535 nm) is the dichroic ratio, RQ (=cos^a) is the dichroic ratio for the perfect in-plane orientation, and 0 (3D random) <> IP £ 1 (2D random distribution).

RESULTS AND DISCUSSION Uniaxial Orientation. Prior to use the perylenebisimide dye, we examined whether the dye reflects polymer chain orientation by comparing with 1,6-di phenyl-1,3,5-hexatriene (DPH) which is known to align parallel to polyvinyl chloride) (PVC) chains.(10) The measurement of the dichroic orientation factor, f, for the perylenebisimide dye and DPH in PVC film as a function of draw ratio showed that the f values for the perylenebisimide dye are only sightly lower than that for DPH. Accordingly, it is most likely expected that the rigid-rod perylenebisimide dye orients nearly parallel to PAA and PI chain segments (β « 0).

Figure 3 shows the orientation factors of PAA and PI chains for (a) BPDA/PDA, (b) BPDA/ODA systems, and (c) Young's modulli of PI(BPDA/PDA) film as a function of draw ratio of the corresponding PAA specimens. For the PAA(BPDA/PDA) system, stretching of about 50 % caused only a low degree of molecular orientation (f « 0.1). Surprisingly, thermal imidization of the slightly drawn of PAA Film enhanced markedly the orientation factor up to ca. 0.6, thus indicating that the spontaneous molecular orientation toward the stretching direction occurred during thermal imidization. The orientation factor increased linearly with a draw ratio. On the other hand, for a flexible BPDA/ODA system (Figure 3b), no spontaneous orientation was observed during thermal imidization. On the contrary, the f values decreased slightly by thermal imidization. Thus, it was found that PI chain linearity (rigidity) is one of the most important factor to the spontaneous uniaxial orientation behavior. What is the driving force for the spontaneous orientation behavior induced by thermal imidization remains unsolved.

As shown in Figure 3c, the effect of stretching on Young's modulli is rationalized in terms of the measured orientation factors for both systems. Annealing at 330°C

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ο II -OH

Η Ο - Ç ' ^ C - N H - A r ^ Ο Ο

PAA

R = H(dyel) CH 3(dye2)

ο II

Ο II

H Ο

PI

BPDA PMDA

A r 2 = - < > , { > o { > PDA ODA

Figure 1. Chemical formulae of PAA, PI and perylenebisimide dyes.

PAA or PI film

Figure 2. Schematic diagram for the in-plane orientation measurement

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0.8

0.6

0.4

0.2

0.0 0 10 20 30 40 SO 60

Draw Ratio / %

b

ο ο ο ο · ·

u.u 1 ' ' ' 1 1

0 10 20 30 40 50 60

Draw Ratio / % 70

60

« «Ο

Ο 40

W 3 0

20

10

0 0 20 40 60 80 100

Draw Ratio / %

Figure 3. The orientation factors for PAA and PI as a function of draw ratio for (a) BPDA/PDA and (b) BPDA/ODA systems, (c) Young's modulus of PI films cured at 250°C ( Ο , Δ ) and annealed at 330°C (#). (Reproduced with permission from ref. 8. Copyright 1994 John Wiley & Sons, Inc.)

increased somewhat Young's modulus of the oriented PI(BPDA/PDA) film. In fact, the orientation factor didn't change by the same annealing, suggesting that the increase in Young's modulus is attributed to an increase in crystallinity or intermolecular interaction such as charge-transfer interaction^ 11)

In-plane Orientation. Figure 4 illustrates the optical birefringence Δη (=nQ-ne) of PAA and PI films as a function of film thickness for the BPDA/PDA system. The value of Δη decreased with increasing thickness for both PAA and PI. The fact that Δη for the PI film is much larger than that for the PAA Film gives the impression that the degree of in-plane orientation for the former is also far larger than that for the latter. But in fact, quantitative comparison of the in-plane orientation between different polymers is

υ.ζ

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^ η 0 · · · · · · · · · ·

PI(BPDA/PDA)

Π. ο oooo ο ο ο Ο β

0 10 20 30 40

Thickness / μηι

b PAA(BPDATPDA)

.. β οο Οο οοο °ο ο ο ο De

Ε " θ 10 20 30 40 Thickness / μηι

Figure 4. The ordinary (η^ and extraordinary (ne) refractive indices of (a) PAA(BPDA/PDA) and (b) PI(BPDA/PDA) films.

generally difficult if their intrinsic birefringence Aity are unknown. In the present study, connecting the optical information (refractive indices) with spectroscopic data (the dichroic ratio of the perylenebisimide dye measured at an incidence angle) makes quantitative evaluation of the degree of in-plane orientation for various PAAs and Pis possible.

Figure 5 shows the effect of film thickness on the in-plane orientation for three systems. For the BPDA/PDA system (Figure 5a), the IP value for PAA film decreased as the film becomes thick. Even for the flexible PAA chain, the small extent of the in-plane orientation was observed. It should be noted that thermal imidization of the PAA on a glass plate enhanced markedly the in-plane orientation. On the other hand, no spontaneous in-plane orientation was observed for the flexible BPDA/ODA system as illustrated in Figure 5b. These are very similar to the phenomenon observed in the uniaxially drawn system described above (Figure 3a and b). For PMDA/ODA system in Figure 5c, which has shown to align in the film plane by the X-ray method (3,4), it was found that the degree of in-plane orientation is not so high compared with the BPDA/PDA system.

Figure 6 shows the comparison between the samples cured on glass plate and cured in free-standing. Although the sample cured in free-standing showed the IP value much smaller than that of the PI film cured on glass plate, the spontaneous in-plane orientation by thermal imidization was evidently observed even for the PI films cures in free-standing. Furthermore, we found that TEC varies in inverse proportion to IP for PI(BPDA/PDA).

Thus, this method is available for the evaluation of the uniaxial and in-plane molecular orientation for various PAA and PI films of several μπι to hundreds μπι in thickness.

1.5P

1.8

ft fi 1.7

1.6

« 1 . 7

1.6

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20 30

Thickness / μηι

Figure 5. Thickness dependence of the in-plane orientation of PAA (O) and PI ( · ) chains for (a) BPDA/PDA, (b) BPDA/ODA, and (c) PMDA/ODA systems.

J* 0.6

Of 0.4r

0.2

0.0

(Cured on substrate

(Free-cured! A

10 20 30 40

Thickness / μηι

Figure 6. Thickness dependence of the in-plane orientation for PI(BPDA/PDA) cured on glass plate (φ) and cured in free-standing (•).

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Literature Cited. 1. Numata, S.; Fujisaki, K.; Kinjo, N. Polymer , 1987, 28, 2282. 2. Kochi, M.; Uruji, T.; Iizuka, T.; Mita, I.; Yokota, R. J. Polym. Sci.: C, 1987,

25, 441. 3. Russel, T. P.; Gugger, H.; Swalen, J. D. J. Polym. Sci. Phys. Ed., 1983, 21,

1745. 4. Takahashi, N.; Yoon, D. Y.; Parrish, W. Macromolecules, 1984, 17, 2583. 5. Jou, J. H.; Huang, P. T.; Chen, H. C.; Liao, C. N. Polymer, 1992, 33, 967. 6. Herminghaus, S.; Boese, D.; Yoon, D. Y.; Smith, B. A. Appl. Phys. Lett.,

1991, 59, 1043. 7. Ando, S. In Recent Advances in Polyimides 1992; Yokota, R, Ed; Raytech Co.,

Tokyo, 1993, pp 63. 8. Hasegawa, M.; Shindo, Y.; Sugimura, T.; Yokota, R.; Kochi, M.; Mita, I.

J. Polym. Sci.: B, in press. 9. Nishikata, Y.; Konishi, T.; Morikawa, Α.; Kakimoto, M; Imai, Y. Polym. J.,

1988, 20, 269. 10. Neuert, R.; Springer, H.; Hinrichsen, G. Progr. Colloid Polym. Sci., 1985, 71,

134. 11. Hasegawa, M.; Kochi, M.; Mita, I.; Yokota, R. Eur. Polym. J., 1989, 25, 349. RECEIVED September 13, 1994

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[acs symposium series] polymeric materials for microelectronic applications volume 579 (science and...

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