Superadditivity in the Implantation of Molecular Ions
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SUPERADDITIVITY IN THE IMPLANTATION OF MOLECULAR IONS G. F. Cerofolini*, L. Meda** and C. Volpones** * EniChem, via Medici del Vascello 26, 20138 Milano MI, Italy ** ST Microelectronics, 20041 Agrate MI, Italy ABSTRACT This paper deals with the implantation of molecular ions in silicon. The 'molecular' effect, i.e. the increase of the displacement yield compared with the sum of the atomic yields, is weak for light molecules (e.g., H 2) and for heavy diatomic molecules (e.g., Sb 2 and Bi 2 ), but, for instance, it is strong for CeH 6 at energy per atomic mass of the order of 1 keV/amu. Binary collision calculations are used to give a pictorial view of the phenomena occurring along the ion path, and to predict superadditivity and damage columnarity. The increase of pressure and temperature to extreme conditions by implantation of molecular ions is discussed. INTRODUCTION Ion implantation into Si is thought of as responsible for two kinds of damage: vacancyinterstitial (v-i) pairs and amorphous islands [1]. V-i pairs are associated with recoil events with low transferred energy Et (say, in the range 0.05 - 1 keV) while the amorphous phase is the remnant of a kind of collective thermal phenomenon (hot cloud) originated either by a recoil with Et higher than a threshold energy E' (E' - 5 keV), or by a lot of nuclear collisions transferring to the crystal more than 500 keV/Am [2]. The secothd mechanism is preferred by heavy ions, while the first mechanism is unique for light ions. If the Et is in the intermediate range, 1 - 5 keV, the temperature of the cloud is insufficient (lower than the energy excess u 0 of the amorphous phase, u 0 = 0.13 eV/atom [3]) to produce amorphization; in this case we speak of warm clouds. Warm clouds may become responsible for amorphization in two situations: when the additional heating due to the involvement of v-i pairs increases the local temperature above u0 (this is responsible for superlinearities [2]), or when two warm clouds simultaneously superimpose on one another and originate a hot cloud. Considerations on the order of magnitude show that, for usual beam instantaneous current 2 densities (of the order of 1 - 10 mA/cm ), two warm clouds (with radius A of approximately 30 A [2] and lifetime of the order of 1011 s [4]) generated by two different ions have a negligible probability of being superimposed. If, however, a molecular ion is implanted, the atoms
originally forming the molecule travel across the same space region at approximately the same time, irrespective of whether the molecule is or is not broken during the collision at the surface. Hence the possibility that a superimposition exists. Consider a molecular ion AB + formed by two light atoms A and B, and imagine that at a depth x (+A) A releases in a single nuclear collision an energy EA(< E') and B an energy EB(< E'); suppose moreover that E A + EB > E'. When all these conditions are satisfied, at the depth x A and B cannot separately amorphize, while AB can produce amorphization. If the superimposition of two warm cloud
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