The results from the Ti-Au films deposited so far are very promising. They retain the excellent biocompatibility of the individual elements while showing promising structural, chemical, morphological and mechanical properties. Although, the thin films deposited so far exhibit Ti-Au structure these Ti-Au intermetallics are merged together and not very distinguishable from each other. Often, different phases of the same material system exhibit varying degrees of the same property. In the literature it is reported that the β phase of Ti3Au composition exhibits higher hardness and improved mechanical properties when compared to other phases of the same composition. Heat treatment is a common way by which properties of materials are modified either in-situ during the deposition or post-deposition using a secondary source of heat treatment like a furnace.
In situ temperature is applied by means of a heater situated close to the substrate stage of the deposition chamber. In our case there is a halogen source heater located on the back side of substrate stage, which is capable of raising the substrate temperature up to 600˚C. Having the heater source located so close to substrate with the help of thermocouples, gives good control over the temperature of the substrate during the deposition. Alternatively, thin films can also be subjected to elevated temperature after their deposition using a secondary furnace to bring in desired changes in film properties. These changes heavily depend upon the rate of heating/cooling, environment present (non-reactive, reactive or vacuum), duration of heat treatment and therefore these can be used to fine tune the final outcome. In this project a Carbolite furnace is used with a non-reactive quartz tube capable of providing various environments and reaching up to 1200˚C to provide post deposition heat treatment.
The most prominent effect of heat treatment is seen in the structure of the material when analysed using XRD. The elevated temperature of the substrate during deposition acts as a source of elevated energy for the adatoms or incoming sputter species from the sputter target. Adatoms with sufficient energy are better at moving towards lattice sites and thereby grow closer to their most preferred orientation giving rise to a specific phase and structure, thereby improving crystallinity. For example, it has been reported that β phase of Ti3Au composition grows with better crystallinity at temperatures above 350˚C. The effect of elevated substrate temperature is also visible on the surface structures and the features. Thornton`s Structure zone model is a well-known model to predict the changes expected in the surface features of thin films when deposited at elevated substrate temperatures. Researchers have noted elongated dome shaped features for Ti3Au when deposited without substrate temperature, indicative of quasi-amorphous nature of the structure and the same composition when deposited at temperatures above 270˚C, develops angular shape grains and with further increase in temperature to 400˚C become well faceted grains. Sometimes increased temperature could lead to dissociation of the alloy structure formed, thereby enabling constituents to react with the surrounding or the substrate itself. Films grown without substrate temperature lack adatom mobility which leads to formation of a defective crystal structure with increased voids in between grains. These less compact structures exhibit a reduction in hardness. But at extreme high temperature the atomic structure starts to disassociate leading to reduction in hardness, therefore it is very important to fine tune the substrate temperature to achieve the best mechanical hardness.
Heat treatment at elevated temperature is another way of altering the properties of deposited thin films. For example, steel at room temperature is a mixture of ferrite and pearlite phases with a body centred cubic structure, but when heated to 723˚C, it gets transformed to Austenite which is a face centred cubic structure. Now if this heated steel is cooled slowly (annealing) the austenite will return to previously existing pearlite structure. However, if the heated steel is cooled rapidly (quenching), the austenite doesn’t have time to change and instead forms a tetragonal body centred structure called martensite steel, which is the hardest phase of steel. Therefore, we can use the heat treatment: in-situ or post deposition as a reliable way to fine tune the structural and in turn mechanical properties of the Ti-Au thin film materials.
Deposition pressure is another parameter which can significantly affect the properties of growing thin films. Read our next article to learn more about this phenomena.
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