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In this paper results of the deposition and annealing of hafnium oxide thin films are reported. Due to the sensor application in mind, thicknesses between 30 and 150 nm have been deposited by r.f. sputtering of a high purity oxide target. Annealing has an important influence on the layer structure, stress and application correlated properties. A detailed understanding of the layer preparation is necessary to adjust deposition and annealing. After deposition the layers are predominately amorphous, annealing leads to textured layers with monocline or orthorhombic phases.
Besides gas sensor applications optimized layers may serve as protective coating or combined with a second material to multi layer stacks as high reflective dielectric mirror.
Hafnium dioxide HfO2 is a candidate with promising properties for semiconductor industries as well as for optical and sensorial applications under harsh environments. The material can be deposited using various techniques such as CVD or PVD in different thickness ranges. The chemical inertness of HfO2 and the high band gap draw the attention of this paper towards application in optics as active and protective layer at the same time, chemical and physical sensors, such as moisture sensors and thin film capacities. In order to improve the layer properties with the sensorial application in mind, the deposition process and the post-processing need to be tightly controlled.
Layers with thicknesses between 100 and 150nm have been deposited by r.f. sputtering of a high purity HfO2 target onto bare or oxidized silicon wafers under Ar- or Ar/O2-athmospheres. Initially the HfO2 has a mainly amorphous structure. Subsequent annealing controls the growth of recrystallized areas characterized by grain size and ratio between crystals and amorphous bodies. High heating rates of about 50K/s and annealing temperatures ranging 800 to 1000°C in a rapid thermal annealing (RTA) chamber seem to be advantageous for the properties desired. The layer's structure such as grain size, crystal type and orientation was investigated using AFM, TEM and XRD. Layer tension was evaluated using laser deflection. The differences in structure found have been correlated to the chemical inertness obtained in measurements for layer applications.
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