PhD Thesis Arne Lüker final version V4 - Cranfield University

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Characterisation of Ferroelectric Thin Films
layers in a thin film system that would otherwise be incompatible. Since Ti is a relatively
reactive metal it can break the Si-O bond on the Si-surface to form a Ti-O or Ti-Si bond.
Conversely, Pt, being a noble metal, cannot do this and thus has limited adhesion to Si.
However, Pt can form strong metallic bonds with unoxidised Ti. Hence, with an
intermetallic reactive layer, Pt can be made to adhere well to the Si-substrate.
3.1.2 Thermal evaporation
Thermal evaporation was used to deposit Cr/Au top electrodes onto the ferroelectric
thin films. The technique is very simple and involves heating an evaporating source under
vacuum, by passing a large current through it (Joule effect). Refractory metals, metals
with a high melting point, such as W, Ta and Mo are examples of commonly used
evaporating sources. The evaporant, which has a lower melting point, is inserted into the
source and the chamber is evacuated. As the current is increased, the evaporant vaporises
and condenses on a substrate placed above the source. Vacuum pressures below 1.33·10 -3
Pa are necessary to prevent oxidising of the atoms in the vapor before coming in contact
Fig. 3.2: The Edwards E480 thermal evaporator
with the substrate and to ensure a large mean
free path.
The amount of heat generated through the
Joule heating is I 2 R, where R is the parallel
resistance of the source and evaporant at the
evaporation temperature. This temperature is
nonuniform because some heat is conducted
down the electrodes. Therefore, unlike sputter
deposition, it is essential to monitor the
evaporation rate in situ as it is highly variable.
One common method is to use a crystal rate
monitor. This utilises the resonant properties of piezoelectric quartz wafers to detect a
vapour flux and give an estimate of the amount of material that is deposited on a substrate
during the evaporation. The quartz wafer generates an oscillating voltage across itself
when vibrating at its resonant frequency. Metal electrodes on opposite faces of the wafer