Optical waveguiding as a method for characterizing the effect of extended cure and moisture on polyimide films

Abstract

Integrated optics techniques (i.e., guiding light through a thin film) have been used to study the effects of extended cure and moisture exposure on polyimide films as manifested in changes in the TE (in-plane) and TM (out-of-plane) refractive indices and in their difference (birefringence). Mechanical testing of these films has been done using the load-deflection technique. Du Pont polyimide Pyralin 2555 and 2556 (benzophenone tetracarboxylic dianhydride-oxydianiline/metaphenylene diamine), cured repeatedly at 400°C and 375°C respectively, show an increase in refractive index as a function of cure (possibly due to densification), as well as an increase in birefringence (due to increased residual stress). Upon exposure to moisture, the TM and TE refractive indices both increase (absorption of the polar water molecule), but the TE index does not increase as much. Mechanical testing has shown that the residual stress in the polyimide film increases with extended cure and decreases with moisture exposure due to moisture-induced swelling of the film. A model is proposed which relates the optical results to the mechanical measurements. Based on our data, a preliminary stress-optic coefficient is reported.

Optical Properties of Polyalicyclic Polyimides.

“…Film thickness and refractive index were determined by a prism coupling method using a Metricon prism coupler. [ 12,13] In this method, shown schematically in Figure 4, a freestanding film is pressed against a high index prism using a piston with a curved surface. A thin silicon wafer may be placed between film and piston surface.…”

Photoacoustic determination of the residual stress and transverse isotropic elastic moduli in thin films of the polyimide PMDA/ODA

“…First, in most spin-cast polyimides, and in PMDA/ODA in particular, hydrodynamic forces and the presence of the substrate give rise to a net polymer chain alignment in the plane of the film. The existence of such ordering is inferred from significant differences between inand out-of-plane indices of refraction [9], [10], large differences between the in-and out-of-plane thermal expansion coefficients [11], anisotropy in fluid transport properties [12], [13], and, most directly, from x-ray diffraction measurements [14]- [17]. It has been suggested that in light of the anisotropy seen in other material parameters, the in-and out-of-plane moduli should be different.…”

The effect of film thickness on the depth‐wise chain orientation of rod‐shaped polyimide

“…The polymer chains confined by a substrate are biaxially stretched during solvent evaporation, which preferentially aligns the chains to the in‐plane direction . Many studies on the degree of chain orientation in PI films have been conducted using birefringence, X‐ray diffraction, and polarized attenuated total reflectance (ATR)/Fourier transform infrared spectroscopy (FTIR) . In these experiments, the chain orientation of PI was associated with the physical and optical properties of a film such as CTE, elastic modulus, and refractive index .…”