Two optical techniques, m-lines and spectroscopic ellipsometry, are compared for their suitability for obtaining the wavelength and the temperature dispersion of the refractive index of thin-film layers used in gas detector devices. Two types of materials that are often integrated into gas sensors are studied: a polymer organic–inorganic blend deposited by spin coating typically used in near-infrared waveguides and the ceramic semiconductor SrTi 1−x Fe x O 3 (strontium titanate) doped with iron at concentrations x=0.075 and 0.1 deposited by electron beam deposition. The refractive index dispersion obtained by m-lines and ellipsometry is compared, and the differences between the measured parameters for the two materials are discussed. The chromatic dispersion will be represented by a three-term Cauchy law. An intuitive method for verifying the measured indices using an integrating sphere and reflexion coefficient modeling techniques are also demonstrated. Thermo-optic coefficients in the order of −1×10 −4 /K for both materials are reported, and very low chromatic dispersions are also measured, thanks to the high sensitivity of the m-lines technique. The uniaxial anisotropic properties of the polymer-blend films are measured and discussed in the case of the semiconductor films.
Two optical techniques, “m-lines” and spectroscopic ellipsometry, are compared for their suitability for obtaining the wavelength and temperature dispersion of the refractive index of thin film layers used in gas detector devices. Two types of materials often integrated into gas sensors are studied: a polymer organic-inorganic blend deposited by spin-coating typically used in near infra-red waveguides and the ceramic semiconductor SrTi1-xFexO3 (strontium titanate) doped with iron at concentrations x = 0.075 and 0.1 deposited by electron beam deposition. In this paper, we will compare the refractive index dispersion obtained by m-lines and ellipsometry, and comment on the differences between the measured parameters for the two materials. The chromatic dispersion will be represented by a three term Cauchy law. An intuitive method of verifying the measured indices using an integrating sphere and reflexion coefficient modelling techniques will also be demonstrated. Thermo-optic coefficients of the order of -1×10-4/K for both materials are reported, and very low chromatic dispersions are also measured thanks to the high sensitivity of the m-lines technique.
The “m-lines” guided mode method has been employed as a new approach to measure the penetration depth of UV light
in partially exposed thin film photoresist layers. This non-destructive method presents the advantage that the penetration
depth can be measured before developing the sample, allowing for fine tuning of exposure parameters. Results are
presented for a positive photoresist (Shipley S1813) deposited by spin coating onto glass slides, forming layers
approximately 2.2μm thick. Such films are exposed to varying degrees with a programmable UV exposure tool. Using
the “m-lines” technique, light is coupled into the photoresist samples using a prism coupler in close proximity to the
sample surface. This coupling occurs for specific incident angles, known as synchronous angles, which depend on the
sample structure. By measuring two such incident angles, one can calculate the thickness and refractive index of a
homogeneous film. We propose a two layer model which allows us to extract the thickness and the refractive index of
the upper exposed layer from the synchronous angles provided by the “m-lines” technique.
The m-lines guided mode technique is demonstrated as a powerful tool for the measurement of wavelength and
temperature refractive index dispersion in thin films. The proper treatment of results reveals measurement uncertainties
of the order of 10-3 for the refractive index, and a sensitivity to changes in this quantity of the order 10-6. Furthermore,
the thickness of the films can be established to a precision of 1nm. Using an optical stack consisting of a silicon wafer
substrate, a low index buffer layer (index 1.52), topped with a polymer blend guiding film, The wavelength dispersion of
the change of refractive index of the guiding film with temperature has been successfully measured. The temperature
dispersion of the refractive index of the guiding layer is of ~ -6.7×10-5 /K.
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