The definition of features on the nanometre length scale (NLS) is impossible via conventional lithography, but can be done using extreme ultraviolet, synchrotron-radiation, or electron beam lithography. However, since these techniques are very expensive and still in their infancy, their exploitation in integrated circuit (IC) processing is still highly putative. Geometries on the NLS can however be produced with relative ease using the spacer patterning technique, i.e. transforming vertical features (like film thickness) in the vicinity of a step of a sacrificial layer into horizontal features. The ultimate length that can be produced in this way is controlled by the steepness of the step defining the sacrificial layer, the uniformity of the deposited or grown films, and the anisotropy of its etching. While useful for the preparation of a few devices with special needs, the above trick does not allow by itself the development of a nanotechnology where each layer useful for defining the circuit should be on the NLS and aligned on the underlying geometries with tolerances on the NLS. Setting up such a nanotechnology is a major problem which will involve the IC industry in the post-Roadmap era. Irrespective of the detailed structure of the basic constituents (molecules, supramolecular structures, clusters, etc), ICs with nanoscopic active elements can hardly be prepared without the ability to produce arrays of conductive strips with pitch on the NLS. This work is devoted to describing a scheme (essentially based on the existing microelectronic technology) for their production without the use of advanced lithography and how it can be arranged to host molecular devices.

We have investigated the efficiency and the thermal stability of Pt gettering at different sites in crystalline Si. In particular, we compared the gettering performances of heavily n-type doped regions formed by P diffusion, cavities formed after high-temperature annealings of He implanted Si, and damage induced by ion implantation of B, C, or Si. These sites were introduced on one side of wafers containing a uniform Pt concentration in the range 1×1013–5×1014 atoms/cm3. The uniform concentration of Pt was attained by means of Pt implantation followed by a high-temperature thermal process. The gettering efficiency of the different sites was monitored during thermal processes at 700 °C for times ranging from 1 to 48 h. Thermal stability of gettering was investigated with a subsequent thermal process in the temperature range 750–900 °C during which part of the gettered Pt is released in the bulk of the wafer. The kinetics of Pt gettering at the different sites is found to be similar since it is fully dominated by the kick-out diffusion mechanism of the metal impurity. The thermal stability is instead site-dependent and can be described in terms of an effective binding enthalpy of 1.9, 2.6, and 3.0 eV between Pt atoms and cavities, P-doped region, and ion-implantation damage, respectively. The physical meaning of the binding enthalpy is investigated and discussed.

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We have explored the mechanisms underlying the gettering of Pt atoms dissolved in crystalline Si. By using Pt implantation at different fluences followed by a thermal process at 970 °C for 5 h we were able to prepare crystalline silicon wafers containing a uniform Pt concentration in the range 2×1012–2×1014 atoms/cm3. Subsequently, a heavily doped n-type region was produced on one side of the wafer by P diffusion at 900 °C. Following this deposition process we have studied the kinetics of Pt gettering to the P-doped region in the temperature range 700–970 °C and for annealing times ranging from 30 min to 48 h. Lifetime measurements by means of a contactless technique were used to detect the depletion of Pt in the bulk of the wafer due to the gettering process. The large range of initial Pt concentrations that we have explored allowed us to identify and separate the kinetics and thermodynamics constraints that determine the gettering efficiency and to propose a phenomenological model for the gettering of Pt. In particular, it has been found that the kinetics of the gettering process are driven by the dissolution of immobile substitutional Pt atoms into interstitial sites. This process is assisted by Si self-interstitials and characterized by an activation energy of 0.4 eV. Moreover, the equilibrium distribution of Pt is thermodynamically determined by a segregation coefficient of the Pt atoms between the gettering sites and the silicon matrix. This segregation coefficient, and hence the gettering efficiency, decrease when the temperature of the gettering process is increased and is described by an activation energy of 2.5 eV.

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