3.4 Metallisation by Resistive Thermal Evaporation

Resistive thermal evaporation is one of the most commonly used metal deposition techniques. It consists of vaporising a solid material (pure metal, eutectic or compound) by heating it to sufficiently high temperatures and recondensing it onto a cooler substrate to form a thin film. As the name implies, the heating is carried out by passing a large current through a filament container (usually in the shape of a basket, boat or crucible) which has a finite electrical resistance. The choice of this filament material is dictated by the evaporation temperature and its inertness to alloying/chemical reaction with the evaporant. This technique is also known as "indirect" thermal evaporation since a supporting material is used to hold the evaporant.

Once the metal is evaporated, its vapour undergoes collisions with the surrounding gas molecules inside the evaporation chamber. As a result a fraction is scattered within a given distance during their transfer through the ambient gas. The mean free path for air at 25 °C is approximately 45 and 4500 cm at pressures of 1e-4 and 1e-6 torr respectively. Therefore, pressures lower than 1e-5 torr are necessary to ensure a straight line path for most of the evaporated species and for substrate-to-source distance of approximately 10 to 50 cm in a vacuum chamber. Good vacuum is also a prerequisite for producing contamination free deposits [108,109].

Figure 3.9: Schematic diagram of a resistive thermal evaporation system.

Two separate Edwards E305 thin film coating systems were used for the deposition of all the metals referred to in this work; a schematic diagram of one such system is shown in Figure 3.9. These systems are fitted with an acoustic crystal monitor which is linked to Edwards film thickness monitor (model no. FTM5) for controlling the amount of metal deposit. Because of the exceptionally large scattering tendency of Zn during evaporation, depositions involving Zn (e.g. Au/Zn/Au ohmic contacts) were carried out in only one of the evaporators, hereafter referred to as the "p-type system". All other metalisations were carried out in the other evaporator, named the "n-type system", to prevent cross contamination.

Because there is a lateral distance between the crystal detector used for in-situ monitoring of the deposited films and the substrate, it is necessary to determine the ratio of respective amounts of deposit between these two surfaces. This ratio is known as the "tooling factor" and is a unique quantity for a particular evaporator which depends on a number of factors including the dimensions of the system and the actual evaporant. Table 3.1 lists the experimentally determined tooling factors for the metals used in this work.

All filament boats used consisted of tungsten material and unless otherwise stated, prior to commencing each evaporation, the base pressure was better than 2e-6 torr. Typical values for the minimum current required to evaporate each of these metals is also listed in Table 3.1. It should be noted that Ni evaporation is particularly tricky and requires two boats. Hence a large current is needed to reach its boiling point. Best results were obtained when a Ni wire was placed diagonally across the boat thereby itself providing an added current path.

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