M. Ree

High performance polyimides for applications in microelectronics and flat panel displays

Polyimides (PIs) exhibit excellent thermal stability, mechanical, dielectric, and chemical resistance properties due to their heterocyclic imide rings and aromatic rings on the backbone. Due to these advantageous properties, PIs have found diverse applications in industry. Most PIs are insoluble because of the nature of the high chemical resistance. Thus, they are generally used as a soluble precursor polymer, which forms complexes with solvent molecules, and then finally converts to the corresponding polyimides via imidization reaction. This complexation with solvent has caused severe difficulty in the characterization of the precursor polymers. However, significant progress has recently been made on the detailed characterization of PI precursors and their imidization reaction. On the other hand, much research effort has been exerted to reduce the dielectric constant of PIs, as demanded in the microelectronics industry, through chemical modifications, as well as to develop high performance, light-emitting PIs and liquid crystal (LC) alignment layer PIs with both rubbing and rubbing-free processibility, which are desired in the flat-panel display industry. This article reviews this recent research progresses in characterizing PIs and their precursors and in developing low dielectric constant, light-emitting, and LC alignment layer PIs.

Water sorption and diffusion behaviours in thin films of photosensitive polyimides

Abstract Several photosensitive polyimide (PSPI) precursors were synthesised by the acid/base complexations of conventional poly(amic acid) precursors with photochemically cross-linkable 2-(dimethylamino)ethyl methacrylate. PSPIs in films were prepared from the photosensitive precursors by thermal imidisation, whereas the corresponding PIs in films were prepared from the conventional poly(amic acid)s: rod-like poly(p-phenylene pyromellitimide) (PMDA-PDA), rigid (poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), semi-flexible poly(4,4′-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA), semi-flexible poly(4,4′-oxydiphenylene pyromellit-imide) (PMDA-ODA), and flexible poly(4,4′-oxydiphenylene benzophenonetetracarboximide) (BTDA-ODA). Water sorption and diffusion behaviours in the PSPIs were gravimetrically measured at 25°C in 100% relative humidity and compared with those of the corresponding PIs. For the PSPIs as well as the PIs, the water sorption and diffusion behaviours were nearly Fickian, regardless of the backbone chemistry. However, those were strongly dependent upon the polyimide backbone chemistry and precursor origin. In addition, those in polyimides were affected by the bulky photosenistive groups, even though they were temporarily linked to the precursor polymers and then debonded from the backbones and ultimately outgassed during the thermal imidisation process. For PMDA-PDA, BPDA-PDA and BPDA-ODA, the PSPIs absorbed water more quickly than the corresponding PIs, whereas for PMDA-ODA and BTDA-ODA, the PSPIs absorbed water less quickly than the corresponding PIs. In contrast to the water diffusion, all the PSPIs absorbed slightly more or a great deal more water than the corresponding PIs, depending on the backbone chemistry. All the measured water sorption and diffusion behaviours in the PSPIs were understood by considering changes in the morphological structure (namely, chain order and orientation), voids, and residues possibly induced by the bulky photosensitive groups, in addition to the Tg as well as the chemical affinity to water.

Water diffusion and sorption in films of high-performance poly(4,4′-oxydiphenylene pyromellitimide): effects of humidity, imidization history and film thickness

Abstract Poly(4,4′-oxydiphenylene pyromellitimide) (PMDA—ODA) films of 10–109 μm thickness were prepared from its poly(amic acid) precursor by thermal imide-ring-closure formation at various temperatures. Water sorption in the films was measured at 25°C over 22–100% relative humidity using an electromicrobalance. Water diffusion in all the films was a nearly Fickian process despite the morphological heterogeneity due to the ordered and less ordered phases. Depending upon humidity, film thickness and imidization history, the diffusion coefficient and water uptake varied in the ranges of 1 × 10 −9 to 3 × 10 −9 cm 2 s −1 and 0.4 to 4.5 wt%, respectively. Overall, both the diffusion coefficient and the water uptake increased with increasing humidity and film thickness, but decreased as the imidization temperature and time increased. The water sorption results were interpreted by consideration of morphological variations (molecular order, chain orientation and microvoids) due to film thickness and imidization history.

Small-angle x-ray scattering station 4C2 BL of pohang accelerator laboratory for advance in Korean polymer science

There are two beamlines (BLs), 4C1 and 4C2, at the Pohang Accelerator Laboratory that are dedicated to small angle X-ray scattering (SAXS). The 4C1 BL was constructed in early 2000 and is open to public users, including both domestic and foreign researchers. In 2003, construction of the second SAXS BL, 4C2, was complete and commissioning and user support were started. The 4C2 BL uses the same bending magnet as its light source as the 4C1 BL. The 4C1 BL uses a synthetic double multilayer monochromator, whereas the 4C2 BL uses a Si(111) double crystal monochromator for both small angle and wide angle X-ray scattering. In the 4C2 BL, the collimating mirror is positioned behind the monochromator in order to enhance the beam flux and energy resolution. A toroidal focusing mirror is positioned in front of the monochromator to increase the beam flux and eliminate higher harmonics. The 4C2 BL also contains a digital cooled charge coupled detector, which has a wide dynamic range and good sensitivity to weak scattering, thereby making it suitable for a range of SAXS and wide angle X-ray scattering experiments. The general performance of the 4C2 BL was initially tested using standard samples and further confirmed by the experience of users during three years of operation. In addition, several grazing incidence X-ray scattering measurements were carried out at the 4C2 BL.

NEXAFS spectroscopy study of the surface properties of zinc glutarate and its reactivity with carbon dioxide and propylene oxide

Abstract The surface state of polycrystalline zinc glutarate (ZnGA) catalyst and its catalytic adsorption of carbon dioxide (CO 2 ) and propylene oxide (PO) were investigated by using near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The outermost layer of ZnGA catalyst was found to contain more hydrocarbon units (i.e., glutarate ligand component) than the inner layers. The ZnGA catalyst was found to reversibly react with CO 2 and to readily react with PO via adsorption onto the catalyst surface and insertion into the ZnO bond. Experiments in which the catalyst was treated with CO 2 followed by PO and vice versa showed that each of these components can replace the other component on the catalyst surface. This reversible adsorption and insertion of CO 2 and PO on the ZnGA surface provides a clue to the mechanism underlying the production of alternating poly(propylene carbonate) in the ZnGA-catalyzed copolymerization of CO 2 and PO. However, in comparison to CO 2 , PO was more easily adsorbed onto the ZnGA catalyst and inserted into the ZnO bond. As a consequence, PO significantly modified the catalyst surface. This suggests that the ZnGA-catalyzed copolymerization is initiated by PO rather than CO 2 .

Copolymerization of carbon dioxide and propylene oxide using an aluminum porphyrin system and its components

The catalytic activities of tetraphenylporphinatoaluminum chloride (TP-PAlCl) and its propylene oxide adduct (TPPAl(PO) 2 Cl) were investigated in detail together with a quarternary salt Et 4 NBr for the copolymerization of carbon dioxide and propylene oxide. In addition, for the components and starting raw materials of the catalyst systems, catalytic activities were examined for the copolymerization. The TPPAlCl catalyst delivered oligomers containing ether linkages to a large extent, regardless of its PO adduction. And cyclic propylene carbonate, as byproduct, was formed in a very small portion. Using the TPPAlCl coupled with Et 4 NBr as a catalyst system, the formation of ether linkages was reduced significantly in the copolymerization; however, the obtained oligomer still contained ether linkages of 25.0 mol % in the backbone. On the other hand, the formation of cyclic carbonate was increased to 22.4 mol % relative to the oligomer product. The results indicate that the salt, which was coupled with the TPPAlCl catalyst, plays a key role in reducing the formation of ether linkage in the oligomer and, however, in enhancing the formation of cyclic carbonate. Similar results were obtained for the copolymerization catalyzed by the TPPAl(PO) 2 Cl/ Et 4 NBr system. That is, the formation of ether linkages was not restricted further by the PO adduction of the TPPAlCl component in the catalyst system. Only oligomers with a relatively high molecular weight were produced. This indicates that the PO adduction of the TPPAlCl component contributes highly to the initiation and propagation step in the oligomerization, consequently leading to a relatively high molecular weight oligomer. In contrast, the Et 4 NBr, as well as the Et 2 AlCl, produced only cyclic carbonate in a very low yield. Furthermore, tetraphenylporphine exhibited no catalytic activity, regardless of using together with Et 4 NBr, On the other hand, the Et 2 AlCl coupled with Et 4 NBr provided a low molecular weight oligomer having ether linkages of 92.3 mol % in addition to the cyclic carbonate.

Time-resolved X-ray scattering and calorimetric studies on the crystallization behaviors of poly(ethylene terephthalate) (PET) and its copolymers containing isophthalate units

Abstract Time-resolved small-angle X-ray scattering (SAXS) measurements were carried out for PET and its copolymers undergoing isothermal crystallization. Wide-angle X-ray diffraction and differential scanning calorimetric measurements were also performed. Our data analysis of the SAXS results for PET and the copolymers clearly demonstrate that the one layer thickness l 1 (derived directly from the correlation functions of the measured SAXS profiles) is the lamellar crystal thickness d c , not the amorphous layer thickness d a . The observed d c values are found to be always smaller than d a , regardless of polymer composition. d c is highly dependent on the crystallization temperature, showing that the degree of supercooling is the major factor determining the thickness of lamellar crystals. No thickening, however, occurs in isothermal crystallizations. The kinked isophthalate units in the copolymer are found to be mostly excluded from the lamellar crystals during the crystallization process, leading to an increase of the amorphous layer thickness. Moreover, the kinked, rigid nature of the isophthalate unit was found to restrict crystal growth along the chain axis of the copolymers and also to lower their crystallinity. Unlike d c , d a decreases with crystallization time, causing a reduction of the long period in the lamellar stack. This drop in d a is interpreted in this paper by taking into account several factors that could influence crystallization behavior: the d a distribution in the lamellar stacks and its variation with time, the number of lamellae in the lamellar stacks and their effect on the SAXS profile, and the relaxation of polymer chains in the amorphous layers.

Electrical properties of silica-polyimide composite dielectric thin films prepared via sol-gel reaction and thermal imidization

Abstract Thin dielectric films composed of silica and polyimide were prepared from tetraethoxysilane (TEOS) and precursor of poly(p-phenylene biphenyltetracarboximide)(BPDA-PDA). Silica particles were generated from TEOS via sol-gel process. BPDA-PDA polyimide was prepared from its flexible and soluble precursors, poly(p-phenylene biphenyltetracarboxamic acid)(BPDA-PDA PAA), and poly(p-phenylene biphenyltetracarboxamic diethyl ester)(BPDA-PDA ES), through thermal imidiration process. In the present work, the electrical properties of the silica-polyimide hybrid composite films were examined utilizing a capacitance measurement by a high resolution electrometer/function generator system. The effects of TEOS contents and precursor types for polyimide matrix of the composites on the relative dielectric constant, dielectric dissipation factor, and resistivity of the composite films were investigated.

Morphology of organic-inorganic hybrid composites in thin films as multichip packaging material

Silica-polyimide hybrid composites were prepared via a sol-gel process and thermal imidization. Two different types of soluble precursors, poly(amic acid) (PAA) and poly(amic diethyl ester) (ES), chemically convertible to poly(p-phenylene biphenyltetracarboximide), were used as organic polymer matrix component, and tetraethoxysilane (TEOS), convertible to silica, as the inorganic component. The structure of composites prepared as thin films was investigated by means of small-angle X-ray scattering, scanning electron microscopy and atomic force microscopy. Nanometre-scale composites were successfully obtained for < 30 wt% TEOS-loaded mixtures with ES and PAA. It was considered from the microstructural investigation that the composite films based on ES were not significantly affected by the inorganic particles generated, maintaining the structure of the homopolyimide, while those based on PAA did not preserve the structure due to the nanoparticles grown in situ during the sol-gel process.

Poly(ethylene‐co‐ethyleneoxyethylene terephthalate)s: synthesis and non‐isothermal crystallization behavior

Full Paper: A series of random copolyesters of reasonably high molecular weight was synthesized with varying composition by melt copolycondensation of dimethyl terephthalate (DMT) with ethylene glycol (EG) and diethylene glycol (DEG). Composition and molecular weight of the copolyesters were determined by 1 H NMR spectroscopy and viscometry, respectively. The copolyesters containing DEG of ≤ 40 mol-% are crystallizable, whereas those with DEG of > 40 mol-% are amorphous. For the copolyesters containing DEG of ≤13 mol-% which were crystallized isothermally, the melting behavior was investigated by means of differential scanning calorimetry (DSC). Three endotherms were found to be influenced by the crystallization temperature and composition, and the equilibrium melting temperatures were determined. In addition, the non-isothermal crystallization behavior was investigated by DSC and analyzed by both Ozawa and modified-Avrami approaches. Regardless of the composition, the value of the Ozawa exponent ranges from 2.4 to 2.6, depending on the temperature, whereas the value of the Avrami exponent ranges from 2.5 to 3.2, depending on the cooling rate. These results indicate that the nucleation and growth mechanisms of the copolyesters are independent of their composition. However, the crystallization rate is decreased by incorporating DEG units into the polymer backbone and also by lowering the cooling rate. Furthermore, incorporating DEG units into the polymer backbone increases the crystallization activation energy.