Thomas L. Nunes

X-ray scattering studies of thin films of photosensitive polyimides

Abstract Using the techniques of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), the morphology of polyimide thin films thermally imidized from several photosensitive polyimide (PSPI) precursors has been investigated and compared with that of the thin films prepared from the corresponding poly(amic acid) precursors: poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA), poly( p -phenylene biphenyltetracarboximide) (BPDA-PDA) and poly(4,4′-oxydiphenylene benzophenonetetracarboximide) (BTDA-ODA). The WAXD results indicate that regardless of the precursor origin, the BTDA-ODA polyimide is amorphous, whereas the other polyimides exhibit a molecular order. For both the PMDA-ODA and the BPDA-PDA, the molecular order is relatively higher in the films prepared from the PSPI precursors than in those from the corresponding poly(amic acid)s, indicating that during thermal imidization, the photosensitive groups play an important role to improve the mobility of the polymer chains, which may be critically needed to make better molecular packing, in spite of their bulkiness having the potential to hinder the molecular packing. In Lorentz-corrected SAXS analyses, a long period (130–156 A mean periodicity) was observed for all the polyimides except the PMDA-ODA. In particular, the microstructure in the BPDA-PDA could be described by an extended chain-based two-phase (ordered and less ordered phase) model with diffuse boundaries because of its high chain rigidity. In addition, the Guinier SAXS analyses indicate the presence of voids in all the polyimide films, regardless of the precursor origin. The size of voids was 251–349 A in radius, depending upon the type of polyimide.

Residual stress behaviour of isomeric PMDA-ODA polyimides

Poly(3,4′-oxydiphenylene pyromellitimide) (PMDA-3,4′-ODA), an isomer of poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-4,4′-ODA), was synthesized from pyromellitic dianhydride (PMDA) and 3,4′-oxydiphenylene diamine (3,4′-ODA). For these two polyimide isomers and their poly(amic acid) precursors in the condensed state on Si wafers, residual stress behaviour over the range 25–400°C was investigated by the dynamic measurement of wafer bending. During thermal imidization both isomers did not show any difference in stress versus temperature behaviour. Once imidized, however, one isomer exhibited a quite different stress behaviour from that of the other during cool-down: the stress of PMDA-3,4′-ODA increased rapidly from zero at 400°C to ≈45 MPa at 40°C, whereas that of PMDA-4,4′-ODA rose gradually from zero to ≈27 MPa. For both cured isomers, stress-temperature profile on heating was the same as that on cooling, with some deviation due to moisture uptake over the temperature range 25–150°C, indicating that their stresses were insensitive to thermal cycling or thermal annealing. From independently measured properties (thermal expansion coefficient, modulus, Poissons ratio of 0.34) of both polyimides, the thermal stresses were calculated and compared with the measured overall stresses. It is concluded that for both polyimides the overall residual stress results primarily from the thermal stress. In comparison with PMDA-4,4′-ODA, PMDA-3,4′-ODA showed a much higher stress despite its slightly lower thermal expansion coefficient. This leads to the conclusion that the large difference between the stresses of the isomers results from the large difference in their moduli (5.0 GPa for PMDA-3,4′-ODA and 3.0 GPa for PMDA-4,4′-ODA). This behaviour is further supported by the difference in morphological structures of these two isomers as determined by wide-angle X-ray diffraction: PMDA-3,4′-ODA showed a well-developed crystalline structure, whereas the PMDA-4,4′-ODA did not. In addition, the interfacial adhesion between polyimide film and Si wafer primed with A1100 was investigated.

Microwave Processing of Polyimide Thin Films

Microwave processing has been used to reduce the long process cycles normally associated with the curing cycles for highly ordered polyimides, such as BPDA-PDA. These materials have found applications in the electronics industry due to the excellent thermal stability and low coefficient of thermal expansion of this class of polymers. Thin polymer films - an application not normally considered for microwave curing - were processed and the dependence of the mechanical properties compared with conventional thermal processing.

Structure And Properties Of BPDA-PDA Polyimide From Its Poly(Amic Acid) Precursor Complexed With An Aminoalkyl Methacrylate

BPDA-PDA poly(amic acid) precursor was functionalized through its carboxylic acid groups being linked with a crosslinkable aminoalkyl methacrylate, 2-(dimethylamino)ethyl methacrylate (DMAEM), by acid/base complexation. BPDA-PDA polyimide films, which were thermally imidized from the precursors complexed with various amounts of DMAEM, were characterized by means of wide angle x-ray diffraction, stress-strain analysis, and residual stress analysis. The structure and properties of the BPDA-PDA polyimide film were dependent upon the history of the precursor, that is, the complexation of the poly(amic acid) precursor with DMAEM. The molecular packing order was enhanced with the history of DMAEM loading while the molecular order along the chain axis was disrupted. Overall, physical properties, such as mechanical properties and residual stress, were degraded with DMAEM loading. The moisture induced stress relaxation behavior was sensitive to the history of DMAEM loading, whereas the creep induced stress relaxation was varied little due to its high T g . These properties are understood in terms of structure/property relationships, as well as microvoids, which were possibly generated by outgassing the bulky DMAEM pendent groups during thermal imidization.