Chemical Vapor Deposition of Aluminium Oxide (Al2O3) and Beta Iron Disilicide (β-FeSi2) Thin Films
Aluminium oxide thin films were deposited by metal-organic chemical vapor deposition (MOCVD) on stainless steel substrates, (AISI 304). The deposition was studied systematically in a hot-wall CVD reactor (HWR) at atmospheric pressure. The used precursors were aluminium acetylacetonate (Al(acac)3) and synthetic air, which are nontoxic and easy to handle. The phase composition, surface morphology and chemical composition of the films were studied by XRD, SEM and EDX, respectively. The deposition starts at 350°C and the maximum growth rate occurs at a substrate temperature of 475°C. Increasing the furnace temperature beyond 500°C leads to depletion of the precursor and thus the maximum deposition rate is shifted towards the gas inlet. Films deposited at 500°C were transparent and amorphous. They consist mainly of Al and O, although the existence of aluminium hydroxides can not be excluded. Annealing at higher temperatures leads to crystallization and phase transformations: at 800°C γ-Al2O3 films are obtained and at 1115°C α-Al2O3 is formed. The films are stable up to 800°C, at higher temperature they are spalling. Beta iron disilicide thin films (β-FeSi2) were successfully deposited by low pressure metal-organic chemical vapor deposition (MOCVD) on silicon substrates, Si(100) using ferrocene (Fe(C5H5)2) and TMS (Si(CH3)4) as precursors. These CVD experiments were performed for the first time in a Halogen Lamp CVD Reactor (HLR) designed for this investigation. By this design, a maximum set point temperature of 800°C and any temperature down to room temperature can be easily achieved and controlled. This control allows possible deposition of different films at different deposition temperature within the same experimental run. Preparation of iron disilicide films by using a direct deposition technique (DDT) and a step deposition technique (SDT) with later annealing were studied. In DDT ferrocene and TMS were supplied into the CVD chamber at the same time. For SDT each precursor was supplied separately in order to deposit an iron film followed by a silicon film, finally the iron silicide is formed in an annealing step. The phase composition, surface morphology and chemical composition of the films were studied by XRD, SEM and EDX, respectively. Films deposited by DDT at 785-800°C and 30 mbar were transparent, amorphous, and well adhesive. EDX analysis shows that the films consist of silicon and very small amount of iron. The films prepared by SDT were formed from crystalline iron films deposited on the substrate at 700°C and amorphous silicon films deposited on the surface of the iron films at 800°C. Also, iron films and silicon films were deposited separately on silicon and steel substrates respectively before performing the SDT. It was found that the iron films can not be deposited directly from ferrocene because of the presence of high level of carbon in the film. Therefore, the carbon containing films were treated with hydrogen in order to produce pure films. After purification XRD analyses show that the films are crystalline (α-Fe). Amorphous silicon films were deposited at 800°C and 30 mbar. A mixture of iron disilicide phases, FeSi, FeSi2 and β-FeSi2 can be prepared by annealing the SDT deposited films 2 hr at 900-950°C.
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