Functional voids by gas ion implantation for applications in semiconductor processing
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Functional voids by gas ion implantation for applications in semiconductor processing V. Raineri1 and E. Rimini1,2 1 CNR - IMM sezione Catania - Stradale Primosole, 50 I 95121 Catania, Italy 2 INFM and Dipartimento di Fisica dell'Università Corso Italia, 57 - I 95129 Catania, Italy ABSTRACT The recent work in the field is reviewed and some new applications are reported. In particular we review some aspects where new contributions have been added. The mechanism of bubble formation when He is implanted into silicon is described till the supersaturation of vacancies (void formation). The void evolution has been described considering direct coalescence or Ostwald ripening. Applications such as gettering, lifetime control, and more recently nanosensors for interstitials are critically discussed. INTRODUCTION Irradiation of materials with light ions has been studied for many years. In particular, helium is known to agglomerate into bubbles when implanted in metals [see the reviews in refs. 1,2,3] or in semiconductors (see refs. [4,5]).When implanted in silicon above a critical fluence, He has been found to form bubbles that evolve into voids because of He out-diffusion during thermal treatments. Recently, a great deal of interest has been thrust upon the application of these voids in silicon technology. Indeed, it has been demonstrated by many research groups all over the world that voids act as powerful gettering of transition metals [6, 7, 8, 9, 10,].Voids introduce deep levels in the silicon band gap and this property can be used to control lifetime in silicon power devices [11, 12]. Futhermore, the internal surface of the cavities acts as an efficient sink of self-interstitials, that arrange on its internal surface so that the empty volume of the voids decreases. As consequence voids can be used as nanosensors for in situ quantitative measurements of interstitials. The increasing interest on possible applications in silicon device technology has also stimulated researchers to detail the basic mechanisms governing void formation, structure and their electrical properties. A detailed understanding is clearly necessary to control the void effects and their interaction with neighbouring structures. Further knowledge continuously introduced is suggesting even more applications. VOID FORMATION High dose irradiation of helium ions into silicon produces gas bubbles. The presence of He clusters has been inferred by thermal desorption measurements even at low dose implants (≈1015 cm-2) but, in this case, their structure has not been detected by TEM analyses suggesting that they are too small and they are formed of only a few atoms [13]. Their presence has been detected by positron annihilation spectroscopy [14]. He atoms trapped in the small clusters, during the first stage of aggregation, out-diffuse from silicon and no trace of their presence is left after annealing. With increasing the implanted dose (≈1016 cm-2) bigger and more stable clusters grow and gas bubbles are formed [15]. In this case they are visible by TEM analys
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