The Best Balance of Oxygen Flow and Deposition Rate to Give Little Absorption of Aluminum Oxide Film Deposited by Electron Beam Evaporation Technique

The effects of deposition rate and oxygen flow rate on refractive index of aluminum oxide film are investigated. The Al2O3 films are deposited by electron beam technique on glass substrate at different deposition rates and oxygen flow rates. Substrate is heated to reach o 300 C and the temperature is kept constant during the thin film growth. Then, using the maxima and minima of transmittance, the index of refractive and the extinction coefficient of samples has been determined. It has been found that the index of refractive increase if we reduce the oxygen flow while keeping the deposition rate the same. Hardness is also increased with the decrease in the O2 flow rate. At some low oxygen flow, the extinction coefficient is small and therefore the Al2O3 films have some absorption.

Films produced by these techniques in different laboratories have different refractive indexes.This is because different deposition techniques to cause different film structures.At the design of multi-layer coatings, it is necessary to know the index of refraction and dispersion for materials.Generally, the index of refraction is a complex quantity, N = n -ik.Here, the n is the index of refraction for a purely dielectric material and the k is extinction coefficient that describes absorption in a material.The absorption coefficient α is defined as , where λ is the wavelength .The ratio of the transmitted intensity to the initial intensity through an absorbing medium of thickness t can be found by 0 / t I I e α − = .Moreover, any material must be having a good density layer for higher index and as little as possible absorption.Thin films rarely have the same refractive index as similar bulk materials.The main reason for this is their microstructure that is rarely bulk-like but usually shows a columnar morphology.The most unfortunate features of the columnar microstructure are the pore shaped voids between the columns.They are the main reason in reducing film density, and affect the optical properties.There are a varied number of papers in journals on the determination of index of refraction (Nilsson, 1968;McPhedran, Botten, McKenzie, & Netterfield, 1984;Minkov, 1991).
In this paper, we have investigated the influence of the deposition rate and oxygen flow rate on refractive index of aluminum oxide film.We have prepared the aluminum oxide films by electron beam evaporation on glass substrate and depositions have been carried out at different deposition rates and oxygen flow rates.The transmittance spectrum of samples is recorded.This produces a transmittance spectrum with a few peaks.We collect the wavelength and magnitude of the maxima and minima of transmittance and then the index of refractive and the extinction coefficient of samples have been determined by "Essential Macleod" (Thin Film Center Inc., 2015).Variations of the refractive index, the extinction coefficient and hardness of Al 2 O 3 films have been studied as a function of the oxygen flow rate.The relationship among the O 2 flow, the extinction coefficient and the refractive index of the films was investigated for films oxide (Jerman, Qiao, & Mergel, 2005).The question now arises what the best deposition rate and the O 2 flow is.Motivation of this study is to find the best balance of oxygen flow and deposition rate to give little absorption and good density for higher index.

Experimental Detail
Aluminum oxide thin films have been deposited by reactive electron beam evaporation technique.Electron beam source is an E-gun in 10 KV, 5 KW, and 270 deg bent-beam.The vacuum system consists of a diffusion pump that is backed with a rotary pump.These pumps are prepared the chamber to 6 10 Torr − the region.Aluminum oxide is placed on water cooled copper crucible and a glass is used as substrate.The 2 3 Al O tablets (Merck) are used as the evaporation material.The distance between the substrate and the source is 50 cm.As is common with all materials, increased substrate temperature leads to higher density.Therefore substrate is heated by quartz heater to reach o 300 C and the temperature is held constant during the thin film growth.This temperature is the highest value that our vacuum system can provide.Our E-gun has a few of sweep patterns.To minimize the influence distribution of the evaporating material, we use the same sweep for all samples.
The index of refractive of samples has been determined by "Essential Macleod" (Thin Film Center Inc., 2015).The technique used is of the class known as envelope methods.The technique is based on the Manifacier paper (Manifacier, Gasiot, & Fillard, 1976) but it goes beyond that to include inhomogeneity in the layers.Such methods focus on the maxima and minima of reflectance or transmittance and the first stage of the calculation involves the interpolation of the maxima and minima to generate the envelopes.When the layer is absorbing, the maxima and minima depart from these ideal envelopes by an amount that increases with thickness but, when absorption is not too high, the envelopes can still serve to launch the n and k calculations.For the Al 2 O 3 films, absorption is not too high.The film thickness must be such that there are extrema within wavelength range of interest.We record the transmittance spectrum of samples over the region of 400-800 nm.The measurement must be taken with an uncoated rear surface and include the multiple reflections generated by the rear surface.This produces a transmittance spectrum with a few peaks.These points are entered as transmittance design goals in the thin film design computer program (Essential Macleod).We then enter the known substrate index (1.52) and an estimate of the coating index.Next, we ask the program to optimize a dispersive index for a best fit to the input data points.Film hardness is determined by nanoindentation technique (Oliver, & Pharr, 1992).
For the present research, we have deposited a set of alumina films on glass substrate.Depositions have been carried out at different deposition rates and oxygen flow rates but the glass substrate is kept at temperature 300 C  .The deposition rate is monitored by an optical monitoring.Table 1 lists the values deposition rates and oxygen rates.when the O 2 flow is low, not all Al is oxidized and therefore that part is metal has absorption.Oxygen vacancies can be repaired by adequate oxygen (Zhao, Wang, Gong, Shao, & Fan, 2003).For the Al 2 O 3 film the extinction coefficient is very small as shown in Figure 5 and absorption is unimportant.

Conclusion
The effects of deposition rate and oxygen flow rate on refractive index of aluminum oxide film were examined.When, we reduced the O 2 flow rate while keeping the deposition rate the same, the index and hardness increased.This was why deposited molecules at lower O2 flow had higher surface mobility and could fully diffuse.At some low oxygen flow, The Al 2 O 3 films had some absorption.This attributed to not oxidize all Al.However, the extinction coefficient of the Al 2 O 3 film was small.Thus, it is appropriate to deposit the Al 2 O 3 film at high deposition rates and low O 2 flow rates.

Table 1 .
List of samples prepared at different deposition rate and O 2 flow 10 SCCM