The Kinetic Energy and Angular Distribution of Sputtered Polyatomic Molecules: Outline and Applications
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THE KINETIC ENERGY AND ANGULAR DISTRIBUTION OF SPUTTERED MOLECULES: OUTLINE AND APPLICATIONS. R.A. HARING, IBM Thomas J. USA.
Watson
Research
Center,
Yorktown
Heights,
POLYATOMIC
NY 10598,
ABSTRACT Kinetic energy and angular distribution measurements give information about the emission mechanism of desorbed particles. For a sputtering mechanism, the energy and angular distribution is usually assumed 3 to be of the form: 4F(E,6)a Ecoso/(E+U) . This expression is valid for sputtered atoms and cannot be immediately applied to the sputtering of molecules, where the energy is shared between internal and external degrees of freedom. A model is presented for the kinetic energy and angular distribution of sputtered polyatomic molecules. The formalism is related to an earlier model for sputtered clusters by Konnen, Tip and De Vries [1]. It is assumed that the individual atoms of a molecule are given an initial momentum according to linear collision cascade properties. If the molecule does not dissociate, it is treated as an entity and carried as such through a planar surface potential barrier. The model shows a continuous transition from a fragmentation dominated behaviour at high kinetic energies to a collision-cascade-like behaviour at low kinetic energies of the sputtered molecules. The model is compared with experimental data on sputtering of Kr2 , SiCl and SiF. INTRODUCTION In several experiments the kinetic energy distributions of sputtered molecules, either neutrals or ions, have been measured. These distributions have been observed to decrease sharply at energies greater than the dissociation energy of an interatomic bond [2, 3]. The interpretation of such energy distributions, and especially of their steep asymptotic behaviour, has been given in terms of 'evolution towards an elastic collision spike' [2], sputtering due to energy deposited into the vibration modes of an ensemble of molecules [4], or a collision cascade-like process, combined with fragmentation of the sputtered molecules at higher kinetic energies [3]. For the description of the latter process several alternative approaches are possible. An established approach is that of K6nnen, Tip and De Vries [1], in which it is assumed that the constituent atoms of a sputtered cluster each receive a momentum according to a collision cascade distribution, and in which the surface forces are supposed to act on each of the atoms individually. This has been paraphrased as "gas phase recombination". This model gives marked discrepancies with experimental data, especially at the low energy side of the kinetic energy distributions [5]. Alternatively one may describe the sputtered molecule as an entity, a "heavy atom". If infinitely strongly bound, it should have a familiar E- 2 collision cascade type asymptotic behaviour in the kinetic energy distribution. The effect of a finite dissociation energy should produce a break in the kinetic enerqy distribution
Mat. Res. Soc. Symp. Proc. Vol. 75. '1987 Materials Research Society
484
somewhere near the dissociati
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