Study Of Inhomogeneity Of A-SI:H In The Tunable Thin-Film Filter

Silicon-based F-P thermo-optic tunable filter with the advantages of low cost andbeing integrated easily will be promising in the field of optical precision measurementand wavelength conversion. However, taking account into preparation process, a-Si: Hthin film, the core material of the filter, has inhomogeneous refractive index and surfacemorphology, which have a significant effect on the tuning performance of the filter. Inthis article, the theoretical analysis is introduced to show the influence ofinhomogeneity on the performance of tunable filter, and the relation betweeninhomogeneity of thin film and preparation process is investigated by experiment.Firstly, it is calculated how inhomogeneous refractive index of a-Si:H thin-filminfluence thermo-optic tunable thin-film filter performance with the initial tuningwavelength of800nm. It is found that inhomogeneous tunable filter is the same tuningrange as homogeneous tunable filter. Meanwhile, the worse the inhomogeneity of thinfilm becomes, the more serious the initial filter tunable wavelength deviates from800nm. Considering little difference, the FWHM value is similar in the whole tuning ranges,particularly individual tuning range is even better than the homogeneous filter. Ifinhomogeneity of thin film becomes worse, FWHM values increases significantly andpoor tuning performance is shown.Secondly, a-Si:H thin film is prepared in different process conditions in amorphoussilica glass substrate by PECVD method, depending on double-equivalent refractiveindex gradient model, the refractive index of a-Si:H thin-film measured accurately bymeans of spectroscopic ellipsometry. It is transparent that process conditions play animportant role on the a-Si:H thin film refractive index of the inhomogeneity. it isobserved that the lower silane flow rate of the reaction gas, the higher RF dischargepower, substrate temperature and pressure of the reaction gas result in less difference ofupper and lower refractive index in a-Si:H thin film.Last, a-Si:H thin film with different process conditions is prepared in K9glass,a-Si:H thin film surface morphology is observed in a nanometer scale by optical metall-ographic reflection microscope. it is manifested that the surface of a-Si:H thin film is covered by lots of hillocks with different sizes, which are attributed to inhomogeneousnucleation and growth, and a great influence of the process conditions on surfacemorphology is summarized, the smaller reaction gas pressure, RF discharge power, thegreater the reaction gas flow rate and the temperature of K9glass substrate, make thenumber and size of the hillock become less.