ON THE ROLE OF IONS IN THE FORMATION OF CUBIC BORON-NITRIDE FILMS BY ION-ASSISTED DEPOSITION

We have investigated how ion irradiation can selectively promote the f ormation of dense sp(3)-bonded cubic boron nitride (cBN) over the grap hite-like sp(2)-bonded phases. We have conducted a series of experimen ts using ion-assisted pulsed laser deposition in which either the ion mass (m(ion)) or ion energy (E) was varied in conjunction with the rat io of ion flux to depositing atom flux (J/a). For a fixed ion energy a nd mass, there is a critical J/a above which cBN formation is initiate d, a window of J/a values in which large cBN percentages are obtained, and a point at which J/a is so large that the resputter and depositio n rates balance and there is no net film deposition, in agreement with Kester and Messier. As do Kester and Messier, we find that cBN format ion is controlled by a combination of experimental parameters that sca le with the momentum of the ions. However, unlike Kester and Messier, we do not find that cBN formation scales with the maximum momentum tha t can be transferred in a single binary collision, as either incorrect ly formulated by Targove and Macleod and used by Kester and Messier, o r as correctly formulated. Instead we observe that cBN formation best scales with the total momentum of the incident ions, (m(ion)E)(1/2). W e also consider the mechanistic origins of this (m(ion)E)(1/2) depende nce. Computer simulations of the interaction of ions with BN show that cBN formation cannot be simply scaled to parameters such as the numbe r of atomic displacements or the number of vacancies produced by the i on irradiation. A critical examination of the literature shows that no ne of the proposed models satisfactorily accounts for the observed (m( ion)E)(1/2) dependence. We present a quantitative model that describes the generation of stress during ion-assisted film growth. The model i nvokes a kinetic approach to defect production and loss. We apply a si mplified version of the model to cBN synthesis, and find that it predi cts an approximate (m(ion)E)(1/2) dependence for cBN formation.