Anisotropy in thermo-optic coefficients of polyimide films formed on Si substrates

Terui, Yoshiharu; Ando, Shinji
December 2003
Applied Physics Letters;12/8/2003, Vol. 83 Issue 23, p4755
Academic Journal
The temperature dependence of in-plane and out-of-plane refractive indices (i.e., thermo-optic coefficients dn/dT) was measured for seven kinds of aromatic polyimide (PI) films formed on silicon substrates. The absolute values of dn[sub TE]/dT (polarization parallel to the film plane) are significantly larger than dn[sub TM]/dT (perpendicular to the film plane). The dn/dT for average refractive indices (n[sub av]) are -94 to -58 ppm/K, independent of film thickness. The anisotropies (dn[sub TE]/dT-dn[sub TM]/dT) are -9 to -39 ppm/K. Although the values of n[sub av] are independent of film thickness, the anisotropies slightly increase as the film thickness decreases for flexible PI films. As expected from the temperature derivative of the Lorentz–Lorenz (LL) equation, the amorphous PI films exhibiting high n[sub av] show large dn[sub av]/dT, however, significant anisotropies are observed even for the PI films exhibiting very small birefringence. © 2003 American Institute of Physics.


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