Detection of negative ions by developing their flux profile on the surface of a biased target

Low temperature and low damage silicon oxidation techniques are highly required in various ULSI processes (trench isolation of a memory cell for instance) where the oxidation should be ion-assisted in order to obtain a good directionality. The ionization energy of negative ions is relatively low and their neutralization on the surface is an energy absorption process, in contrast to that of the positive ions. Because of that, low damage silicon oxidation can be expected in the presence of negative ions. So far, silicon etching employing negative ions in SF6 and Cl2 plasmas have been considered where the etching reactivities of negative ions were found very high compared to those of positive ions. The reason for the high etching reactivity was that the dominant negative ions F- in SF6 and Cl- in Cl2 plasmas were atomically very similar to the radicals. These aspects focus on the importance of negative ion in etching and oxidation, so that the negative ion production and the determination of their parameters are of crucial importance. In the present work we report a new method that allows an easy and inexpensive detection of the negative ions. The method is based on the electrostatic lens effect of the sheath evolving to a positively biased target that focuses electrons and negative ions to distinct regions of the target surface. (authors)