Fred E. Arnold

Organo-soluble, segmented rigid-rod polyimide films: 2. Properties for microelectronic applications

Abstract The essential properties of polyimide films of importance in microelectronic applications are thermal and thermo-oxidative stability, dimensional stability, glass transition behaviour and the relative permittivity (dielectric constant e′). A segmented rigid-rod polyimide was synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in order to develop new materials for microelectronic applications. The thermal and thermo-oxidative stability were investigated by determining the thermal degradation activation energy in air (210 kJ mol −1 ) and in nitrogen (303 kJ mol −1 ). The thermal stability was further studied through thermogravimetry-mass spectroscopy. The coefficient of thermal expansion, which indicates the dimensional stability, was measured via a tension mode of a thermomechanical analyser and doubly extrapolated to zero stresses, and was 6.98 × 10 −6 °C −1 for the BPDA-PFMB films. The glass transition temperature, measured thermomechanically, was found to be 287°C. The dielectric constant for the films, measured after ageing at 50% relative humidity for 48 h at 23°C, was between 2.8 and 2.9 in a frequency range from 0.1 kHz to 1 MHz. The temperature and frequency dependence of the dielectric behaviour is also discussed.

Organo-soluble segmented rigid-rod polyimide films. Part 4.—Anisotropic structure and properties

A series of fully aromatic copolyimides have been synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in m-cresol, (BPDA–PFMB)x-(PMDA–PFMB)y. The molar composition ratios are 100 : 0, 85 : 15, 70 : 30 and 50 : 50, respectively. The unoriented polyimide films with a thickness ranging between 10 and 20 µm have been characterized by an in-plane orientation of the c axis in the crystals as well as the molecular chain axis relative to the film surface. This in-plane orientation has been studied by transmission and reflection geometrical modes of wide-angle X-ray diffraction (WARD) and polarized Fourier-transform infrared spectroscopy (FTIR), and can be attributed to the rigidity and linearity of the chain molecules. The structural anisotropy leads to anisotropic behaviour in the glass-transition temperatures (Tg), the linear coefficients of thermal expansion (α), the dielectric behaviour (Iµ′ and Iµ″) as well as the refractive indices (n‖ and n⊥) parallel and perpendicular to the copolyimide film surface. Possible structure–property relationships are also discussed.

Organo-Soluble Segmented Rigid-Rod Polyimide Films Part 3: Effects of Copolymer Composition on Thermal Expansivity and on Relaxation Processes

A series of copolyimides based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA)(1,2,4,5,-benzenetetracarboxylic dianhydride) and 2,2′-bis(trifluoromethyl) benzidene (PFMB) have been synthesized with varying BPDA–PFMB and PMDA–PFMB compositions. A systematic change in the linear coefficient of thermal expansion (CTE) and the glass-transition temperature (Tg) is observed by changing the ratios of the BPDA–PFMB and PMDA–PFMB compositions. Both the CTEs and Tgs have been found to follow the molar-additivity rules. Dynamic mechanical experiments manifest two relaxation processes above room temperature. A subglass β relaxation, which is independent of the copolymer composition relative to the varying dianhydride composition, is found in the copolyimides studied. This β transition exhibits the same temperature and frequency dependence in all the copolyimides. An α transition (corresponding to the glass-transition temperature) is observed and is dependent on the PMDA–PFMB composition within the system. The possible molecular origin of the β relaxation is also discussed.

Organo-Soluble, Segmented Rigid-Rod Polyimides: Synthesis and Properties

Several segmented, rigid-rod polyimides have been prepared that are soluble in organic solvents in their fully imidized form. The polymers were prepared from commercial dianhydrides and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFMB). Their intrinsic viscosities ranged from 1.0 to 4.9 dL/g. Tough, colorless films could be cast from m -cresol solutions at 100°C. The polymers had glass transition temperatures (Tgs) above 275°C and displayed outstanding thermal and thermo-oxidative stability. Fibers were prepared from the 3,3′,4,4′-tetracarboxybiphenyl dianhydride (BPDA) based polymers that had moduli of 130 GPa and tensile strengths of 3.2 GPa. The thermal expansion coefficients and dielectric constants of thin films (20–25 μm) of the polymers were as low as −2.40×10 −6 and 2.5, respectively.

Water Soluble Benzobisazole Rigid-Rod Copolymers

Although benzobisazole rigid-rod polymers are well recognized for their outstanding tensile and modulus properties as well as their excellent thermal and thermal-oxidative stabilities, they can only be fabricated from acidic solvents. Benzobisirnidazole polymers containing pendent benthiazole or sulfonic acid groups exhibit partial solubility in DMSO. Reaction of these systems with sodium methylsulfinylmethide in DMSO abstracts the acidic proton from the benzobisimidazo unit and forms the polyanion. Subsequent reaction of the polyanion with 1,3-propanesultone provides water soluble systems. Random copolymers containing benzobisimidazo and benzobisthiazole repeat units were prepared in polyphosphoric acid and subjected to the derivatization reaction. Benzobisimidazo content in the copolymers varied from 10–0 mole percent. Minimum ionic charge along the rigid-rod backbone to obtain water solubility was determined to be approximately 10 mole percent as reflected in the benzobisimidazole repeat units.

Organo-soluble segmented rigid-rod polyimide films. Part 5.—Effect of orientation

A series of semi-rigid aromatic polyimides have been synthesized via a one-step polymerization in which the poly(amic acid) precursors were not isolated. The polyimides were synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in refluxing m-cresol. Uniaxially oriented films were utilized in order to investigate the chain orientation in the ordered state. Wide-angle X-ray diffraction (WAXD) patterns showed an increase in the crystal orientation and the crystallinity for oriented CPI(100/0) films with increasing draw ratio. Dichroic ratios obtained under polarized Fourier transform infrared spectroscopy (FTIR) represented overall orientation factors attributed to both ordered and non-ordered regions, and they were a function of the incident angle and draw ratio. This indicates that the conformational arrangement and chain packing changed during deformation. The orientation along the molecular chain axis and the lateral orientation of the imide planes became increasingly enhanced with increasing draw ratio, an indication of planar alignment of the imide planes as well as uniaxial orientation of the molecular axis. The surface morphology of unoriented and uniaxially oriented films was also studied via transmission electron microscopy (TEM). The lateral crystal dimension of the ordered regions observed from TEM corresponded well to the apparent crystallite sizes of the (310) crystalline plane in BPDA–PFMB crystals obtained from WAXD experiments.

Rigid-Rod And Segmented Rigid-Rod Polyimides: GEL/SOL And Liquid Crystalline Transitions, Fibers And Films

Organo-soluble rigid-rod and segmented rigid-rod polyimides and their copolyimides exhibit isotropic solutions in hot m -cresol, but form gels upon cooling. A lyotropic liquid crystal phase is observed below the gel/sol transition. Mechanical gel formation is caused by liquid-liquid phase separation, while the liquid crystal phase may be formed through a nucleation process after gelation. High performance fibers can be spun from the hot isotropic solutions using a dry-jet wet spinning method. After the fibers are drawn at high temperatures, they display tensile strength higher than 3.2 GPa and an initial modulus higher than 130 GPa. In particular, the fibers retain relatively high mechanical properties at elevated temperatures. Solution casted films exhibit very low thermal expansion coefficients and dielectric constants. Their structure, morphology and property relationships will also be discussed.