We present the design and describe the operation of a scanning probe microscope which simultaneously provides the attractive mode force and near‐field optical images of objects. In this technique, the force signal is used to track the topography, thus allowing the optical signal primarily to show variations in transmissivity. A number of results are presented on the application of the technique to imaging different samples.

Near‐field differential scanning optical microscope with atomic force regulation

The formation of TiSi2 thin films on silicon substrates has been investigated with several transmission electron microscope techniques. For films formed either by reacting titanium with a silicon substrate or by sintering a codeposited (Ti+Si) mixture, electron diffraction patterns show that a metastable phase—TiSi2 (C49 or ZrSi2 structure)—forms prior to the equilibrium phase—TiSi2 (C54 structure). High‐resolution images indicate that the metastable TiSi2‐silicon interface is atomically sharp, with no ‘‘glassy membrane’’ layer present. The annealing temperature required to transform the metastable TiSi2 to the low resistivity, equilibrium TiSi2 increases as the thin‐film impurity content increases. Previous studies of TiSi2 formation are discussed in light of these results.

Metastable phase formation in titanium‐silicon thin films

The microstructural features of the Si-SiO2 system and the chemical physics of its defects are reviewed and examined. Topics are grouped by scientific commonality, rather than by the usual technological manifestations. The role of atomic and molecular sized entities is emphasized, and the latter are limited to those containing only Si, O, H, or combinations thereof. Most of the reported researches involve x-ray or electron diffraction, Auger or photoelectron spectroscopy, Rutherford backscattering, electron spin resonance, or capacitance-voltage or deep-level transient spectroscopy. Several forms of crystalline and amorphous vitreous silica are considered as a basis for discussion of thin film thermal silica on silicon wafers. Local lattice symmetry, stoichiometry, bond lengths and angles, vacancies and voids, dangling orbital centres, and fixed and migratory hydrogen species are treated extensively. Elements of relevant theory are summarized. Overall, it is hoped to provide a solid data base for future development of general models for essential electronic phenomena in the Si-SiO2 system.

https://iopscience.iop.org/article/10.1088/0034-4885/57/8/002/pdf

The silicon-silicon dioxide system: Its microstructure and imperfections

Thermodynamic considerations in thin‐film reactions involving refractory metals, refractory metal silicides, silicon, and silicon dioxide are described using ternary phase diagrams. Calculated metal‐silicon‐oxygen phase diagrams for Mo, W, Ta, and Ti are used to explain the reactivity of the metal with silicon dioxide, the effectiveness of native oxide in preventing metal‐silicon interdiffusion, and the formation of silicon dioxide in preference to metal oxides during silicide oxidation. Distinctions are drawn between experimental results which can be explained solely on thermodynamic grounds and those requiring consideration of both thermodynamic and kinetic factors.

Thermodynamic considerations in refractory metal-silicon-oxygen systems

A scanning tunnelling microscope can image surfaces down to the atomic scale. Its very high resolution is caused by its local probing mechanism: a tunnel current which flows through the outermost atom of a scanning tip to the sample. Its invention, about ten years ago, was based on an ingenious blend of quantum physics, mechanical design and electronic control. Since then, it has proved to be a very versatile instrument. Its applications range from the characterization of surface roughness to atomic reconstructions, and from doped semiconductors to cell membranes. It can operate in ultra high vacuum, in air, in reactive gases, in corrosive solutions or at cryogenic temperatures. This paper reviews the evolution of scanning tunnelling microscopy in the first decade after its invention. Attention is focused on the basis of STM: tunnelling theory, mechanical design and modes of operation. Representative examples of applications in various fields of research are discussed.

http://idwebhost-202-73.ethz.ch/praktika/analytisch/stm/Rep%20Prog%20phys%2055,%20p1165.pdf

Scanning tunnelling microscopy

Silicide and Schottky barrier formation has been characterized for Ti deposited on the Si(100) surface, both with and without surface oxides present. Reactions were carried out in ultrahigh vacuum, while observing electronic and chemical changes with ultraviolet photoemission spectroscopy and Auger electron spectroscopy. Ti deposited on Si shows a sharp interface, no change in Fermi level position, and no silicide formation until heated to 400–500 °C. Ti deposited on thin oxides (<20 A) frees silicon at the interface and reacts through the oxide to form a silicide when heated to 400–500 °C. Ti deposited on thick thermal oxides also frees Si, but no further reaction occurs until heated to 700–900 °C, at which point TiOx forms near the surface. This differing behavior of thin and thick oxides is shown to be consistent with bulk thermodynamic data.

Silicide and Schottky barrier formation in the Ti‐Si and the Ti‐SiOx ‐Si systems

The effect of Si surface contaminants present prior to metal deposition, and that of post‐metalization anneals has been investigated for Ti and Hf Schottky barriers on Si. These diodes have been prepared in ultrahigh vacuum, characterized with Auger spectroscopy and measured in situ using internal photoemission, and ex situ using current‐voltage measurements. Although barriers to p‐type Si as high as 0.9 eV have been reported in the literature for these metals, barriers of 0.72 eV were the highest observed in this investigation, for surfaces contaminated with significant amounts of oxygen.

Interface effects in titanium and hafnium Schottky barriers on silicon

Ultrathin layers of SiO2 on Si (less than ∼100 to 200 A) show a reduction in the measured width of the Si–SiO2 interface when profiled using ion sputtering. Although previously attributed to the intrinsic properties of thin oxides or surface roughening due to ion‐beam etching, this effect is predominantly caused by ion knock‐on broadening or knock‐on enhanced diffusion. It is important to realize that this effect is an artifact of the sputtering process, and not a property of the system under study. A model is developed based on the spatial distribution of the energy deposited by the sputtering ions to explain the observed reduction in measured interface width. This model is compared with interface widths obtained using Auger electron spectroscopy from identically grown thermal SiO2 samples which have been thinned by chemical etching to eliminate oxidation kinetic effects.

Ion knock‐on broadening effects in Auger sputter profiling studies of ultrathin SiO2 layers on Si

The interaction of Ti with C‐contaminated Si surfaces has been studied using Auger electron spectroscopy, x‐ray photoemission spectroscopy, and transmission electron microscopy. C contamination due to wet chemistry residues and thin SiC films has been investigated. Our results show that Ti reacts with exposed C to produce TiC upon deposition for both these forms of contamination. The C residues were found to neither impede the silicide reaction nor remain at the interface afterwards. However, SiC thin films (5 A) prepared by annealing the Si surface in ethylene, blocked silicide formation, resulting in a highly nonuniform interface. The carbide phases formed before and after silicide formation agree with those predicted by a calculated Ti‐Si‐C ternary‐phase diagram.

Interaction of Ti with C‐ and SiC‐contaminated Si surfaces

A technique is described for measurement of lateral forces on a scanning tunneling microscopy (STM) tip simultaneously with surface topography, using optical sensing of the STM tip vibration. The STM tip is caused to vibrate near a resonant mode in the lateral direction, using the capacitive forces between the tip and the surface under study. Topography is monitored using the z‐displacement feedback voltage, in a low‐frequency loop, while optical sensing of the high‐frequency tip vibration amplitude monitors lateral forces acting on the tip.

Marc A. Taubenblatt

Papers

Silicide and Schottky barrier formation in the Ti‐Si and the Ti‐SiOx ‐Si systems

Interface effects in titanium and hafnium Schottky barriers on silicon

Ion knock‐on broadening effects in Auger sputter profiling studies of ultrathin SiO2 layers on Si

Interaction of Ti with C‐ and SiC‐contaminated Si surfaces

Lateral forces and topography using scanning tunneling microscopy with optical sensing of the tip position