초임계 이산화탄소에 의해 제조된 다공성 폴리이미드 필름의 내부구조 및 모폴로지

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 제1호 2002년

이준영, 김경일, 김중현

서울시 서대문구 신촌동 연세대학교 화학공학과 기능성 초미립자 공정 연구실

Inner Structure and Morphology of Porous Polyimide Films Prepared by Supercritical Carbon Dioxide

Jun-Young Lee, Kyung-il Kim, Jung-Hyun Kim

Nanosphere Process and Technology Laboratory, Department of Chemical Engineering,Yonsei University, Seoul 120-749, Korea

서론

Scaling down the feature sizes in microelectronic circuits has proven efficient in improving circuit performance and increasing yield at lower cost per function on chip. Circuit speed increases are primarily due to reduction in transistor gate lengths, resulting in faster transistor switching times. As dimensions are scaled to the sub-micron range, the signal runtime delays caused by the higher resistance capacitance (RC) product of the metallization may overcome the benefits of a decreased gate length. Bothra et al. [1] derived an analytical expression for the dependence of signal rise time on the technological parameters. Their main result is that a higher interconnect performance of less switching delay is achieved by introducing materials with higher electromigration resistance for the interconnects and by the use of lower dielectric constant insulators. For this purpose, D.Y.Yoon et al. [2] proposed a different approach towards polymeric nanofoams, which avoid the problem of shrinking, by the removal of large amounts of solvents and volatile blowing agents. Preparation of porous structures of high temperature polyimides was explored by utilizing block copolymers, comprised of thermally labile blocks and thermally stable matrix polymers. [3] During the film processing such block copolymers phase separate into discretely dispersed domains throughout the film. Subsequently, the dispersed phase is slowly decomposed upon heating so as to yield degradation products which slowly diffuse out of the films without altering the nano size dispersion morphology.

Fig. . Schematic representation of polyimide derived from dianhydride(6FDA) with diisocyanate(MDI).

The properties of supercritical fluids make them interesting media in which to conduct chemical reactions. One of the primary advantages of employing a supercritical fluid as the continuous phase lies in the ability to manipulate the solvent strength simply by varying the properties which are intermediate between those of a liquid and those of a gas [4, 5]. A supercritical fluid can have liquid-like density and simultaneously possess gas-like viscosity [6]. When CO2 is used as the reaction media for a imidization reaction, the solvency of the reagents and products is of primary importance. To begin with, CO2 has a low dielectric constant; by varying temperature and density. It has been noted that CO2 behaves very much like a hydrocarbon solvent with respect to its capability to dissolve small molecules, and thus many monomers exhibit high solubility in CO2. Even though it is a relatively low dielectric media, CO2 is a Lewis acid, and it has a strong quadrupole moment that allows it to dissolve some polar molecules such as methanol. In fact, the only classes of polymers which have been shown to demonstrate good solubility of polymeric materials in CO2 at mild conditions (T < 100 ℃, P < 350 bar) are fluoropolymers and silicones [7-8]. In addition to the solubility considerations, the plasticization of the resulting polymers by CO2 constitutes another important issue which must be taken into account when polymerization conducted in supercritical CO2. This plasticization of polymers plays a key role in both the diffusion of monomer into the polymer phase as well as the incorporation of additives to a polymer matrix [9, 10].

Herein, we describe the use of supercritical CO2 and by-product(CO2) as a pore formic agents for the manufacture porous polyimide thin film with low dielectric constant. Fig. 1 shows schematic representation of polyimide derived from dianhydride with diisocyanate.

본론

Polyimides were prepared by reacting equimolar amounts of the appropriate dianhydride and diisocyanate at 0℃ in dimethylacetamide (DMAc). For example to prepare 4,4-diphenylmethane diisocyanate(MDI) (1.0 g) was dissolved in DMAc (20 ml) and cooled to methanol circulator bath temperature in a double jacket glass reactor equipped with a nitrogen gas inlet, a reflux condenser fitted with a mechanical stirrer. 4,4-bis(Hexafluoroisopropylidene)phthalic dianhydride (6FDA) (1.7 g) was then added in small portions of 20 mg, ensuring that the earlier amount dissolved completely before a fresh portion was added. After the addition was complete, the reaction mixture was stirred for 2 hours at 0℃. The solution was allowed to attain ambient temperature gradually. The temperature was then raised to 40 ℃ over 1 hours and left at that temperature for 2 hours. Polyimide precursor solution was dried in vacuum for 5 hours. The polyimide precursor film was applied at 140 ℃ and 3000 psi in supercritical CO2.

After 5 ∼ 24 hours of the thermal imidization reaction, the supercritical CO2 and condensation CO2 by-product of reactant was removed by vent of supercritical CO2 and developed pore inner polyimide film matrix.

SEM studies showed the cross-section image of porous polyimide thin films. A dielectric constant was estimated for a porous polyimide thin film.

Fig.2. SEM micrographs of cross-section of polyimide film with different Supercritical Carbon Dioxide pressure; (a) 1500 psi, (b) 2000 psi, (c) 2500 psi and (d) 3200 psi

at imidization time of 24 hours.

결론

Therefore, It was assured that supercritical CO2 and by-product (CO2) used at this process can form pore inner polyimide film. A dielectric constant will be estimated for a porous polyimide thin film. Electrical and mechanical properties as well as coefficient of thermal expansion for both dense and porous polymer films will be measured. The porous polyimide thin films were obtained by utilizing supercritical CO2 and by-product (CO2) where the polyimide thin film was derived from 4,4-bis(Hexafluoroisopropylidene)phthalic dianhydride (6FDA) with 4,4-diphenylmethane diisocyanate(MDI). Scanning electron microscopy was used to examine the pore size and morphology of polyimide thin film. the pore size range is 1 ∼ 3 □ in diameter.

참고문헌

[1] S. Bothra, I. Harvey, and M. Weling, Poc. 14th Int. VLSI Multilevel Interconnect. Conf., 134, 1997.

[2] H. J. Cha, J. Hedrick, R. A. DiPietro, T. Blume, R. Beyers and D. Y. Yoon, Appl. Phys. Lett. 68, 1930, 1996.

[3] S. Jayaraman, S.Srinivas, G.L.Wilkers, J.E.McGrath, J.L.Hedrick, W.Volksen, and J.W.Labadie, Polym. Prep. 35, 347, 1994.

[4] Pekala, R. W., Alviso, C. T., Kong, F. M., and Hulsey, S. S., J. Non-Cryst. Solids, 145, 90, 1992.

[5] A. McHugh and V. J. Krukonis, Supercritical Fluids Extraction: Principles and Practice, Butterworth-Heineman, Stoneham (1993).

[6] J. M. DeSimone, Z. Guan, and C. S. Elsbernd, Science, 257, 945, 1992.

[7] Z. Guan, J. R. Combes, Y. Z. Menceloglu, and J. M. DeSimone, Macromolecules, 26, 2663, 1993.

[8] T. A. Hoefling, D. A. Newman, R. M. Enick, and E. J. Beckman, J. Supercrit. Fluids, 6, 165, 1993.

[9] J. J. Watkins and T. J. McCarthy, Macromolecules, 28, 4067, 1995.

[10] Y. L. Hsiao, E. E. Maury, J. M. DeSimone, S. M. Mawson, and K. P. Johnston, Macromolecules, 28, 8159, 1995.

화학공학의 이론과 응용 제8권 제1호 2002년