폴리이미드 이성질체의 광학적 비등방성 및 유전적성질 비교

김시은, 이춘근, 이충봉, 한학수

연세대학교 화학공학과

The Isomeric Effects on Optical Anisotropy and Relative Permittivity of High Temperature Polyimide Thin Films

Shieun Kim, Choonkeun Lee, Choongpong Lee, Haksoo Han

Department of Chemical Engineering, Yonsei University

INTRODUCTION

High temperature aromatic polyimides are widely used in the fabrication of microelectronic devices such as interdielectric, alpha-particle protecting, and passivation layers, owing to the relatively high thermal stability, high chemical resistance, and high mechanical toughness due to the aromatic and imide ring units on the chain backbone and charge transfer complex inter polyimide chain. In recent years the application has been extended to use as interlayer dielectrics in integrated circuits and thin film multilayer electronic packaging, and as light waveguide using optical anisotropy. Dielectric property and optical property of polyimide thin film were particularly important because performances in electronic device affected by these properties. The chemical backbone structures, which incorporated hexafluoroisopropylidene group and oxy bridge can affect the dielectric constant and refractive index. Increases in free volume can be attributed to the greater steric volume of hexafluoroisopropylidene group and oxy bridge, which may interfere with efficient chain packing. And, all the chemical structures with the highly electronegative fluorine atom have relatively low polarizabilities and the dipole moment per unit volume induced by electromagnetic field may be minimal. Therefore, it is evident in our study that increases in free volume may decrease the dielectric constant and refractive index by reducing the number of polarizable groups in a unit volume. This paper attempts to determine the contribution of change in free volume according to chemical backbone structure and substituent structure to the observed decreases in dielectric constant and the related refractive index.

EXPERIMENTAL

Wide-angle X-ray diffraction (WAXD) patterns were obtained for the polyimide thin film specimens by a Rigaku Horizontal X-ray Diffractometer (Model D/Max-200B) with mickel-filtered adiation. One diffractometer was set up for the transmission mode with a thin asymmetric cut (101) quartz plate monochromator bent to a xection of a logarithmic spiral and located in the diffracted beam. The other was used for the reflection mode with a curved graphite monochromator in the diffracted beam. The CuK radiation source (=1.54□) was operated at 35kV and 40mA. All the WAXD measurements were carried out [eqn: theta/2theta] mode. The [eqn: 2theta] scan data were collected in the range of 5-60¸at 0.02¸ intervals with a scan speed of 0.3-0.5¸/min, depending upon whether a reflection or transmission scan was being made. The measured X-ray diffraction intensities were corrected for the background run and then normalized for the film samples by matching the integrated intensity over the range of 58-60¸([eqn: 2theta]).

The out-of-plane and in-plane refractive indices of the polyimide thin films was measured using a prism coupler (Metricon Model 2010) equipped with a He-Ne laser light source of 632.8 nm wavelength and controlled by a personal computer. Measurements of refractive indices were carried out in transverse electric (TE) and transverse magnetic (TM) mode by choosing the appropriate polarization of the incident laser beam as described elsewhere.

The TE measurement in which the electric field is in the film plane provides the in-plane refractive index ([eqn: { n}_{TE }]) whereas the TM measurement in which the electric field is out-of-plane gives the out-of-plane refractive index ([eqn: { n}_{TM }]). All measurements were performed using a cubic zirconia prism of[eqn: { n}_{TE }] =[eqn: { n}_{TM }] = 2.1677 at 632.8 nm.

Both in-plane,[eqn: { n}_{TE }], and out-of-plane,[eqn: { n}_{TM }], refractive indices were used to compute the three-dimensional average refractive index as defined in eq1. The dielectric constant was calculated with Maxwells identity, shown in eq 2.

[eqn: n=( { 2n}_{TE }+{n}_{TM})/3]         (1)

[eqn: epsilon '={n }^{2 }]                 (2)

Many methods have been employed to estimate the fractional free volume (VFF) in polymers. Positron annihilation measurements have been extensively used, and computer simulation has been explored by others. The method used here was the group additivity technique popularized by Bondi and widely applied for a number of different purposes. This was carried out by estimating the van der Waals or □occupied□ volume, using group additivity tables and subtracting it from the molar volume ([eqn: M/ rho]), which is the total volume taken up by 1 mol of repeat units, including free and occupied volume. The fractional free volume is then given by

[eqn: { V}_{FF }= {(M/ rho)-VDW } over {M/rho }]         (3)

where M is the formula weight of a polymer repeat unit,  is the measured density, and VDW is the estimated van der Waals volume.

RESULTS AND DISCUSSION

Morphological Aspects : The structure of polyimides thin films, which were thermally imidized at 400。C, was investigated by wide-angle X-ray diffraction in both transmission and reflection geometry. The WAXD patterns measured are shown in Fig.1.

Fig 1. WAXD Transmission and Reflection Patterns of Polyimide Films

Prepared from Poly(amic acid)s by Thermal Imidization at 400¸C

In the reflection pattern, mean intermolecular distances, which is a parameter of molecular packing, varied in the range of 4.41 to 5.72 □, and in the transmission pattern, varied in the range of 4.74 to 5.82 □. There are in the increasing order: PMDA-3,4□ODA < PMDA-4,4□ODA < 6FDA-4,4□ODA < 6FDA-3,4□ODA. These are consistent with the results observed in the optical and dielectric studies.

Optical and Dielectric Properties : For all the polyimides in thin films,

in-plane refractive indices ([eqn: { n}_{TE }]) are relatively greater than their out-of-plane refractive indices ([eqn: { n}_{TM }]). These optical anisotropies result from the molecular in plane orientations in thin films. Furthermore, they indicate that all the polyimides are the positively birefringent polymers in which the polarization along the chain axis is higher than that normal to the chain axis.

To evaluate the influence of film thickness, birefringences were measured on film with different thickness.

Figure 2. Effect of Film Thickness on the Birefringence   Figure 3. Effect of Film Thickness on the Birefringence

of PMDA-4,4'ODA and 6FDA-4,4'ODA Polyimides           of PMDA-3,4'ODA and 6FDA-3,4'ODA Polyimides

The estimated fractional free volumes were calculated from the density and van der Waals volume, and varied 0.2168 to 0.3865. There are in the increasing order: PMDA-3,4'ODA < PMDA-4,4'ODA < 6FDA-4,4'ODA < 6FDA-3,4'ODA.

CONCLUSIONS

The structure effects on optical anisotropy and relative permittivity of the thin films of PMDA-4,4'ODA, PMDA-3,4'ODA, 6FDA-4,4'ODA and 6FDA-3,4'ODA has been examined by using a prism coupling method. Their morphological structure was investigated by wide-angle X-ray diffraction. The trends of mean intermolecular distances in X-ray study are consistent with the fractional free volume results observed in the optical and dielectric studies. These results were a very important parameter of refractive indices and dielectric constants. The birefringence of the polyimide thin films decreased as the film thickness increased except PMDA-3,4'ODA. The decreasing of birefringence might result from decrease in the molecular orientation, and also reflect a gradient in the refractive index through the film due to a highly stressed surface (i.e., a skin effect). The polyimides with the 6FDA exhibited relatively low dielectric constants and refractive indices. It is due to a simultaneous increased in fractional free volume by chemical structure. The PMDA-4,4'ODA showed relatively lower dielectric constant and refractive index than the PMDA-3,4'ODA, whereas the 6FDA-4,4'ODA showed higher than the 6FDA-3,4'ODA. It might result from optical anisotropy by chemical structure.

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