MECHANISM OF CLEANING AND ETCHING SI SURFACES WITH LOW-ENERGY CHLORINE ION-BOMBARDMENT

Effects of Cl-35+ bombardment of Si (100) surfaces were studied with a mass-separated low energy ion beam system operated under ultrahigh va cuum and with in situ x-ray photoelectron spectroscopy (XPS). Before i on bombardment, each silicon sample was etched with a hydrofluoric aci d solution and rinsed in deionized water. XPS showed that the surface received no ion bombardment was effectively hydrogen passivated and ha d no silicon with an oxidation number higher than 2. However, oxygen w as found which was probably present in the form of Si-OH or adsorbed w ater. Chlorine ion bombardment at room temperature initially drove the surface oxygen to the formation of silicon oxide which was subsequent ly etched off by further ion bombardment. The surface oxidation was in itiated by the formation of Si-Cl bonds followed by the thermodynamica lly favorable replacement of Si-Cl with Si-O. The removal of the surfa ce oxygen depended critically on the bombardment energy. At a bombardm ent energy of 1 +/- 0.6 eV, oxygen bonding was still observed even wit h a dose of 10(18)/cm2. The Critical dose for oxygen depletion was fou nd to be about 1 X 10(18)/cm2 for 40 eV bombardment, and about 1 X 10( 16)/cm2 for 100 eV bombardment. Physical sputtering was the main oxide removal mechanism. Once the surface oxygen atoms were consumed, the s ilicon surface was etched by the chlorine ions with an enhancement by the chlorine chemistry. However, the absence of Si-Cl(x) (x > 1) was c learly shown by XPS. Hence, bombardment must have promoted the desorpt ion of the surface silicon chlorides.