Thin oxide film formation, metal silicon

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Another problem in the construction of tlrese devices, is that materials which do not play a direct part in the operation of the microchip must be introduced to ensure electrical contact between the elecuonic components, and to reduce the possibility of chemical interactions between the device components. The introduction of such materials usually requires an annealing phase in the construction of die device at a temperature as high as 600 K. As a result it is also most probable, especially in the case of the aluminium-silicon interface, that thin films of oxide exist between the various deposited films. Such a layer will act as a banier to inter-diffusion between the layers, and the transport of atoms from one layer to the next will be less than would be indicated by the chemical potential driving force. At pinholes in the AI2O3 layer, aluminium metal can reduce SiOa at isolated spots, and form the pits into the silicon which were observed in early devices. The introduction of a tlrin layer of platinum silicide between the silicon and aluminium layers reduces the pit formation. However, aluminium has a strong affinity for platinum, and so a layer of clrromium is placed between the silicide and aluminium to reduce the invasive interaction of aluminium. 

Thermodynamic stability of the interface with silicon is also important for correct selection of sensor material. According to Hubbard and Schlom (1996), for MgO and ZrO, thermodynamic stability of their interfaces with silicon was predicted. For HfO, AI2O3, CaO, and Y2O3, experimental evidence showed that these oxides also form stable interfaces with silicon. The CeO /Si interface is thermodynamically unstable, in accordance with experimental observations of formation of a Ce203 interlayer at the CeO /Si interface. Present analysis shows that, for the majority of metal oxides used, the interface with Si is stable. However, ZrO or YSZ thin films are permeable for oxygen at elevated temperatures, so that oxygen diffuses to the silicon substrate and a SiO layer is formed at the interface (Jia et al. 1995). It should be noted that this problem exists for all metal oxides used for gas sensor design. 

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