High vacuum chemical vapor deposition (HV-CVD) of alumina thin films

We analyzed along this work the feasibility to produce high quality alumina thin films by High Vacuum Chemical Vapor Deposition (HV-CVD). We study the influence of various parameters on the growth process and on the film quality, such as substrate temperature, gas flow or pressure. We aim to highlight the advantages and the limitations of the HV-CVD technique. A high vacuum CVD reactor has been designed, built and optimized. A novel effusing source has been designed to guarantee a molecules distribution uniformity of 95% on the substrate surface. The main technical challenges faced were to reach the maximum substrate temperature in vacuum, to control the precursor flux and to control the temperature of the gas lines to avoid condensation. For the first time, films of Al2O3 are obtained by HV-CVD from aluminum isopropoxide as precursor in a novel HV-CVD reactor. The HV-CVD approach stands out of all the other CVD methods as it allows excellent prediction of growth rates and film thickness homogeneities if the decomposition probability of the precursor is once determined. In this work, the latter was determined to increase with increasing substrate temperature to 0.2, further increase of temperature reduces the decomposition probability due to increased desorption rate. Pure alumina films are obtained in a reliable and reproducible way. The presence of oxidizing agents, as decomposition partners, reduces the activation energy of deposition process from 33.1 ± 8.2 kJ/mol to 11.4 ± 5.3 kJ/mol in presence of sufficient oxidizing agent. The Al2O3 films are growing at deposition rate from 1 to 50 nm/min. It has been found that, the lower the deposition temperature, the higher the density of the deposited films therefore the higher refractive index. The index of refraction varies from 1.65 ± 0.01 to 1.35 ± 0.01 at 632 nm wavelength from lowest to highest deposition temperature. At low deposition temperature, Al2O3 can contain residual OH-groups and Al=O entities. This content is independent of the total flow rates of the precursor in the range of 5·1015 molecules/cm2·s to 3·1016 molecules/cm2·s impinging on the substrate. Substrate effects have not been observed in the limited range of tests on natural oxidized silicon, 3 µm SiO2 on silicon, quartz, stainless steel, fused silica and 100 nm silicon nitride on top of silicon. The chemical composition of the deposited alumina films measured by EDX and XPS is nearly stoichiometric with 35 ± 5 at% Al and 65 ± 5 at% O, without carbon contamination. Remaining hydrogen in the films has not been studied in detail, but the difference of OH- absorption peaks by FTIR indicates that, a low temperature deposition, hydrogen incorporation is possible. Concerning the optical properties of the HV-CVD deposited films, absorption is very low from 250 nm to 1800 nm wavelength, but due to the porosity and granular structure of the films, light scattering can take place. But propagation loss smaller than 2 dB/cm were measured in channel waveguides fabricated from the HV-CVD alumina films. Planar and channel waveguides demonstrate good guiding properties at 670 nm and 1.55 µm, as well. This work also opens new possibilities to deposit in situ local structures of or in transparent alumina on large surface. Light induced high vacuum chemical vapor deposition of alumina microstructures by 248 nm excimer laser is presented and shows an activation energy of 5.2 ± 0.4 kJ/mol, much smaller than for thermal deposition. The influence of the fluence and the repetition rate is discussed. Electron beam assisted HV-CVD of alumina is demonstrated and proves the feasibility of in situ structuration under HV-CVD conditions.

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