3.8 Annealing Methods

There are two distinct techniques which were used for alloying ohmic contacts or annealing sputter induced damages out of substrates where appropriate. These are presented below. This is followed by a brief description of a technique adopted for producing good ITO ohmic contacts.

3.8.1 Thermal Furnace Annealing (TFA)

A schematic of the Gallenkamp thermal furnace oven used in this work is shown in Figure 3.11. It consists of long cylindrical hollow glass tube (app. 60 cm long, 2.5 cm diameter) surrounded by heating coils and thermal cladding material. Removable glass bungs with narrower glass pipes are attached at either end. These pipes are connected to allow the flow of forming gas (a mixture of 10% H2 + 90% N2) in and out of the tube during treatment.

Figure 3.11: Schematic diagram of the Gallenkamp Thermal Furnace Annealing (TFA) system.

A smaller hollow tube (app. 10 cm long and 2 cm diameter), used for carrying the sample to be treated, can be slid in and out of the outer tube using a clean glass rod. There are graduation marks along the outer tube allowing the user to accurately note the position of the inner tube and hence the sample. There is also a thermocouple placed in close proximity to the central part of the outer glass tube, which is the hottest region. The system is capable of operating in the temperature range 100 șC up to 800 șC. Power to the 1kW heating coils is controlled from a temperature controller to set the desired temperature.

3.8.2 Rapid Thermal Annealing (RTA)

The rapid thermal annealer (RTA) was built in-house previously. As the name suggests, the RTA allows "flash" annealing by swiftly raising the temperature (typically from 25 șC to 400 șC in 10 sec.). It consists of a small vacuum chamber pumped by an ordinary rotary pump (minimum pressure reached is about 10-2 torr) and a N2 gas inlet to cool and maintain a steady pressure during annealing. Inside the chamber there are two graphite strips (2.5cm x 15cm x 1mm) stacked vertically with app. 2mm separation between them. These strips are clamped between two supportive metal rods through which a large current can be passed to raise the temperature. There is also a thermocouple placed on the centre of the lower graphite strip for controlling purposes.

Annealing is carried out by placing the sample between the two graphite strips. The chamber is then evacuated and then flushed with N2. This is repeated 3 times to remove any traces of moisture and contaminants. Finally with both the pump outlet and the N2 inlet opened simultaneously, a steady pressure just below atmospheric is reached. The annealer is set to the desired temperature and switched on for the required period (between 30 to 60 sec. is adequate for most applications). After this period, power is turned off and the sample is allowed to cool in the steady flow of N2. When the temperature reaches about 45 șC (in about 3 min.) the sample is removed.

3.8.3 Annealing ITO Ohmic Contacts

In the fabrication of ITO ohmic contacts, it was necessary to use a "step-wise" annealing approach (see section 6.2.). The TFA was used for this purpose and the temperature controller was set to 800 șC with a steady flow of forming gas. The lateral temperature variation along the outer glass tube was calibrated to the distance from the mouth. This was then used to set the required temperature for the ITO ohmic contact annealing scheme and the position was varied when a new temperature was required.

The calibration itself was carried out by using a second thermocouple attached to a glass rod which was placed inside the outer glass tube of the TFA. The temperature and the position of the thermocouple was noted from the graduation marks. Figure 3.12 shows the results of the calibration.

Figure 3.12: Calibration curve for thermal oven set at a temperature of 800 șC with forming gas flowing.

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Dedication

Acknowledgements

Abstract

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