Formation and Characterization of Aluminium Contacts to n-type Silicon

Abstract

Samples were prepared by performing phosphorus diffusions with POCl3 on Sb doped (100) silicon wafers. 99.9% (3N) purity Al or 99.999% (5N) purity Al dots of diameter 0.7 mm and thickness between 0.2 µm or 1.0 µm were then deposited onto quarters of these wafers. The quarters were then subsequently cleaved into 1 cm2 pieces which were annealed at temperatures ranging from 400?C to 650?C for durations of time between 10 minutes and 2 hours. The speed with which these samples were cooled was also varied. After annealing, 3N Al or 5N Al was then deposited on the rear of the samples to a thickness of 1 µm to ensure a low resistance ohmic contact was formed on the rear. Characterisation of these contacts was carried out by measuring the IV characteristics of the contacts and by removing the Al from the front and making observations of the surface topology under an optical microscope and/or by utilizing a scanning electron microscope (SEM). Also, secondary ion mass spectrometry (SIMS) was used to check the diffusion profile of Al, P, and the concentration of Sb dopant in the wafers. Results showed that samples which had 0.3 µm Al deposited in dots on the front, when sintered at temperatures over the eutectic temperature, exhibited rectifying properties evidenced by IV curves characteristic of diodes with turn on voltages of 0.6 mV. The corresponding samples with 1.0 µm appeared to have similar IV curves. However, the turn on voltage was about 0.5 mV. On inspection of the interface, the 0.3 µm Al deposited samples appeared to have formed large, well rounded holes in the silicon and the 1.0 µm Al deposited samples showed localized spiking and no large, well rounded holes. The samples that were sintered at less than the eutectic temperature, showed very poor rectifying contact properties in their IV curves and their interfaces varied from minimal pitting at lower temperatures to large degrees of pitting at higher temperatures.