To calculate the refractive index and extinction coefficient of TiO2 film based on the envelope method, the absorption coefficient of the material should not be too large, otherwise it will affect the final transmittance of the film product and reduce the optical performance of the product; At the same time, the refractive index should not be too low, otherwise the width of cut-off band will be affected. Proposed by Manifacier in 1976, the baoluo line method is to calculate the optical film parameters of the film layer by inverting the transmittance (or reflectance) extreme value at the optical thickness of the film layer to integer multiples of lambda / 4. In the actual measurement, the maximum T /2 point and the minimum T /4 point of transmittance are connected to form two envelope lines, Tmax() and Tmin(). Then the transmittance extremes T /2 and T /4 at any wavelength are obtained by taking points on the envelope line. Finally, the extinction coefficient and refractive index of the film layer are calculated by using the transmittance extreme value, and the thickness of the film layer is calculated according to the calculated value of refractive index and the wavelength of the extreme point.
Measurement process of the method is simple, the method can simultaneously measuring film refractive index, extinction coefficient and thickness of measurement process does not need to contact with the film samples, sample protection, is an ideal comparing all kinds of methods of equipment test results, if used properly, can be as a way to determine the film all optical constants. The linear refractive index n and thickness L of the film at wavelength are calculated by using the envelope method.
n0 and n1 are the refractive index of air and base respectively; Tmax and Tmin are the maximum and minimum transmittance at wavelength; Lambda 1, lambda 2, and n(lambda 1), n(lambda 2) correspond to the wavelength and refractive index of two adjacent peaks or valleys of the transmittance curve, respectively. The refractive index and extinction coefficient of TiO2 film were calculated by using Macleod software. As can be seen from the figure (a) below, the spectral range is uv-visible and near-infrared, and the refractive index of all 4 samples decreases. The refractive index of sample no.1 in the band of 400 ~ 1000 nm is between 2.50 ~ 2.15. The refractive index of sample no. 2 and sample no. 3 was slightly higher, while that of sample no.4 was slightly lower at the same wavelength, ranging from 2.45 to 2.15.
From the figure (b) above, it can be seen that sample no. 1 has obvious absorption, and the extinction coefficient increases with the spectral range from uv-visible to uv-visible. It can be known that low-price oxides of metal Ti are produced, because of insufficient oxygen supply. The maximum transmittance of sample no. 2 was significantly improved, and the extinction coefficient was basically below 2.5×10-3, which still had an impact on the final transmittance of the spectrum. The extinction coefficients of the spectra of sample no. 3 and sample no. 4 are in the order of 10-4, and the effect on the final transmittance of the spectra is basically negligible. No. 1 and no. 2 the extinction coefficient of the sample with wavelength increases, the other two basic does not vary with wavelength the extinction coefficient of the sample, under the condition of the vacuum degree, no. 1 and no. 2 samples with the increase of wavelength and the transmission rate is reduced, to the oxygen content is too low, there are metal Ti is formed, and the extinction coefficient of the Ti is right is the wavelength increases.
Through the comparison of the four samples, it was found that with the increase of oxygen flow, the vacuum degree of plating decreased, and the refraction of TiO2 film increased first and then decreased. Through the increase of oxygen increase after the oxygen ions of ionization ion source, will increase the ion density bombardment film, make more dense membrane layer, thus improve the film refractive index, when the amount of filling the oxygen to further increase, however, a relatively low vacuum plating system, excess oxygen molecules and TiO2 membrane molecular collision, reduced the TiO2 membrane molecular kinetic energy, so as to make the deposition of TiO2 thin film molecular migration rate is reduced, the density of the membrane layer, has further reduced the refractive index of thin films. If less oxygen, namely vacuum plating system is high, also causes due to the amount of oxygen is less, can't supply Ti2O3 is expected to decompose in evaporation loss of oxygen content of oxygen, the cause of plating film composition has low oxide containing titanium oxide, affect the optical properties of thin films, so the choice of appropriate flux oxygen for TiO2 optical thin film plating system is very important.
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