E. Romano

Terascale integration via a redesign of the crossbar based on a vertical arrangement of poly-Si nanowires

The race of integrated-circuit technology towards high bit density has already brought transistor densities of the order of 109 cm−2, while keeping conventional circuit layouts. Crossbar structures are widely believed to meet the requirements of high bit density along with sustainable interconnection complexity avoiding the dramatic cost increase of the manufacturing facilities required by advanced lithography. In this work we demonstrate the possibility of producing poly-Si nanowires preserving bulk electrical properties that are nonetheless so dense as to allow cross-point density in excess of 1011 cm−2. This result could be achieved by organizing silicon nanowires in nearly vertical arrays.

The multi-spacer patterning technique: a non-lithographic technique for terascale integration

Since the pioneering paper of Aviram and Ratner it has become progressively clearer and clearer that molecules can perform electrical functions so sophisticated as to mimic the behavior of electronic devices, hence the idea of exploiting them for molecular electronics. The practical exploitation of the electronic properties of molecules for computing (i.e., the transformation of a collection of molecules into a device) is, however, a major problem not yet solved. In the last few years the expertise in molecular design and synthesis combined with (i) the identification of a new structure, the crossbar, able to replace those based on a planar arrangement of field effect transistors, and (ii) the development of sub-lithographic techniques for the definition of a random or ordered planar arrangement of nanometre-sized features, has nonetheless led to consider as realistic the hypothesis that molecular devices might replace silicon-based devices when the basic element of the integrated circuit, the metal-oxide-semiconductor field effect transistor, will no longer admit further shrinking. This work is addressed to identify in which ways features (i) and (ii) can be exploited for application in molecular electronics.

Crossbar architecture for tera-scale integration

As far as the fabrication of ultradense crossbars is related to correspondingly dense wire arrays, the crossbar route to tera-scale integration depends on the availability of preparation techniques for wire arrays with density of 106 cm?1 or more. For a planar arrangement this density implies a pitch of 10 nm or less, beyond the current possibility and close to the theoretical limit, assuming for the cross-point a minimum area of 10 nm2. Further increase of density can only be achieved organizing the nanowires in a three-dimensional fashion. This paper describes a planar top?down process for the preparation of vertically arranged poly-silicon nanowires. The technique is expected to allow the production of wire arrays with linear density (projected on the surface) larger than those achievable with any other proposed top?down processes. Used for the fabrication of the bottom wire array of crossbars, this process should allow an eventual cross-point density of the order of 1012 cm?2, thus being a candidate technology for tera-scale integration.

Hydrogen injection and retention in nanocavities of single-crystalline silicon

The control of the chemical state of the inner surfaces of nanocavities (NCs) produced by the annealing of helium-implanted silicon has influence on lifetime control, gettering and wafer bonding. In this work it is demonstrated that the etching in HFaq of (1 0 0) silicon containing a buried array of NCs produces a giant injection of hydrogen with the consequent passivation of the inner surfaces, mainly via the formation of silicon monohydride at (1 1 1) faces and monohydride dimers at 2 × 1 reconstructed (1 0 0) faces. These terminations are very stable and survive heat treatments at 700 °C.

Steps farther towards micro-nano-mole integration via the multispacer patterning technique

The production of memories with bit density of the order of 1011 cm−2 (projected to be possible within the current silicon technology in 2020 AD) seems now possible by exploiting the hybrid crossbar architecture. This new paradigm (that assigns to silicon the functions of power supply, addressing, sensing and writing, and to reprogrammable molecules the function of memory) requires the solution of a number of new problems not considered yet in integrated-circuit processing. In particular, the most advanced example of hybrid crossbars (Green J E et al 2007 Nature 445 414) does not yet consider the problems of hardware demultiplexing (the connection of wire arrays with a sublithographic pitch of 30 nm to lithographic contacts with pitch 90 nm), the covalent linkage of the functional molecules to the crossbar, and their insertion at the end of process. In this paper it is shown that the multispacer patterning technique (Cerofolini G F et al 2005 Nanotechnology 16 1040) can be adapated to solve all these problems.

Using evidence from nanocavities to assess the vibrational properties of external surfaces

Internal surfaces of nanocavities are an exceptionally useful laboratory wherein one can spotlight the factors ruling the intricate interplay between morphology and chemistry at silicon surfaces. At the same time, they offer unparalleled opportunities to validate the assignment of vibrational signals of silicon-terminating species under almost ideal experimental conditions. In the case of hydrogen, evidence will be provided of the detailed evolution of H-related species at surfaces depending on their orientation. Also, preliminary results concerning nitrogen at and around nanocavity surfaces will be reported.

Counterintuitive assignment of the lines observed by x-ray photoelectron spectroscopy at the hydrogen-terminated (1?0?0) surface of silicon

Net-charge analysis, involving siladamantane moieties as local models of various surface silicon atoms, is used in combination with infrared spectroscopy to assign chemical species to the features observed in the x-ray photoelectron spectra from hydrogen-terminated (1 0 0) Si prepared by HFaq etching of the native oxide. (Some figures in this article are in colour only in the electronic version)

Nanoengineered Silicon: Technology and Applications

Silicon has been, and continues to be, the material support of integrated circuit (IC) technology-the enabling tool of one of the most impressive technological, industrial and social revolution of mankind. Silicon (both in monocrystalline and polycrystalline form as well as epitaxial silicon, silicon on insulator and silicon on nothing) has grown with IC technology for its ability to match the numerous and complex needs of microelectronics. In recent years, however, IC technology is experiencing problems of maturity that are expected to be soluble only by huge investment in fabrication plants. This state of affairs has motivated the search for potentially cheaper alternatives to the basic IC element-the metal-oxide-semiconductor (MOS) field-effect transistor (FET). Solutions involving the use of low dimensional structures (like silicon nanowires for the crossbar structure, or silicon quantum dots for single-electron transistors) instead of conventional MOS-FETs have been proposed. Remarkably enough, such a nano-engineered silicon has been proved to be of interest not only in electronics but also in photonics, energetics, and biomedicine.