Infrared Refractive Index and Extinction Coefficient of Polyimide Films

Abstract

We have measured the transmittance of several polyimide (C₂₂H₁₀N₂O₄) films at wave numbers from 6000 to 500 cm⁻¹ (wavelengths from 1.67 to 20 μm) using a Fourier-transform infrared (FT-IR) spectrometer. The free-standing polyimide films are made by spin coating and thermal curing processes. The thickness of the films ranges from 0.1 to 4 μm. In the nonabsorbing region from 6000 to 4000 cm⁻¹, the minimum transmittance caused by interference is used to obtain the refractive index for film thicknesses greater than 1 μm. The film thicknesses are determined by fitting the spectral transmittance using the refractive index. Molecular absorption strongly reduces the transmittance at wave numbers from 2000 to 500 cm⁻¹. The optical constants, i.e., the refractive index and the extinction coefficient, are determined from the measured transmittance for several films of different thickness using a least-squares method. A Lorentzian oscillator model is also developed, which in general agrees well with the measured transmittance at wave numbers from 6000 to 500 cm⁻¹. This study will facilitate the application of polyimide films in the fabrication of infrared filters and other optoelectronic applications. The methods presented in this paper can be used to determine the optical constants of other types of thin-film materials.