SILICON-NITRIDE FORMATION BY LOW-ENERGY N+ AND N2+ ION-BEAMS

Reactions of N+ and N2+ ions with Si (100) surface are examined as a function of both ion kinetic energy and dose using a low energy ion beam instrument. The Si surface is exposed to low energy (1-300 eV) ion beams in an ultrahigh vacuum environment and the resulting surface species are characterized by Auger electron spectroscopy and ultraviolet photoelectron spectroscopy. The absolute reaction probability P(r) is measured for nitridation processes. P(r)(N+) has a value of approximately 0.25 and stays constant in the energy range of 1-25 eV. P(r)(N2+) increases from zero to approximately 0.25 in the same range. Continued exposure of the ion beams to a dose > 5 X 10(15) ionS/CM2 leads to a saturation and formation of a dense and stable silicon nitride layer. Variation of P(r) with energy and dose is explained in terms of elementary reaction steps such as charge neutralization of the projectile ion, collisional dissociation of N2+, nitridation reaction, and chemically induced desorption of surface nitrogen species. A mechanism is proposed to explain enhancement in the stability and ordering of the nitride layer during the low energy ion-surface interaction. At saturation the processes of both nitridation and chemically induced desorption simultaneously occur, which, under hyperthermal environments, effectively remove defects and interstitial nitrogen atoms from the layer.