About 10 years have passed since the first demonstration of atom manipulation by scanning tunneling microscopy (STM) was made, and it has become possible to fabricate variously designed nanostructures using scanning probe microscopy (SPM's) including STM. If we consider the next 10 years in this field, it will become most important to measure the functional properties of fabricated nano-structures and therefore to develop new methods that make such measurements possible. In this paper, three new methods developed by the authors are described, which are an independently-driven-double-tip STM, a photon-detecting STM, and a spin-polarized STM of a new mode.
Atomic scale desorption and deposition of hydrogen (H) atoms on H-terminated Si(001) surfaces by a scanning tunneling microscope (STM) have been studied. Hydrogen atoms are extracted at either polarity of sample bias voltage. Desorption rate of H atoms shows a power-law dependence on tunnel current (It). When H atoms are locally extracted from a Si(001)-(3×1)-H surface, the dihydride rows partly disappear and instead new monohydride rows appear. Opposite local phase transition from 2×1 structure to 3×1 structure is also observed using H covered tips. Silver (Ag) growth on a pre-patterned Si(001)-(2×1)-H surface has been studied. An isolated dangling bond site can be occupied by an isolated single Ag atom or a 3-dimentional Ag cluster. On a wire of dangling bonds, Ag grows layer-by-layer along the wire.
Scanning tunneling microscopy/spectroscopy (STM/STS) and atom manipulation have been used to fabricate and evaluate atomic-scale structures on a hydrogen-terminated Si(100)-2×1-H surface. Atomic-scale Ga wires are fabricated by using selectively adsorbing thermally evaporated Ga atoms on the dangling-bond patterns. Electronic structures of the dangling-bond wires and theoretically predicted properties of the Ga structures, such as the conductivity and the flat-band ferromagnetic property are addressed.
A scanning probe lithography that uses a current-controlled exposure system has been developed. We use a negativetype resist and fabricate line-and-space patterns to demonstrate characteristic parameters necessary for the present tech-nique to be applied in the industrial lithography. We find that the cross-setional shape of the developed resist pattern depends on the amount of the exposure dose. The resolution depends on the resist thickness and a minimum line width of 27nm is obtained for a 15-nm-thick resist. The proximity effect is much smaller than that of electron beam (EB) lithography. Electric-field mapping inside the resist is evaluated and the characteristics of the exposure system was explained based on the proposed exposure mechanism.
Since it was suggested that individual atoms can be manipulated using scanning tunneling microscope (STM), many attempts have been done to control atoms individually, but they were not successful so far. Here we show some meth-ods for fabricating nanometer- to atomic-scale structures using STM and atomic force microscope (AFM). Focus is put on how to connect these structures to macroscopic electric pads so that electron transport can be measured along these structures.
We describe all-ultra-high-vacuum selective area growth based on surface nitridation and scanning tunneling micros-copy (STM) lithography. We pattern an amorphous nitrified GaAs surfaces by STM lithography, and can grow an array of uniform 6.4±0.8 nm-high GaAs dots successfully on the areas (50 nm×50 nm) from which the nitrided mask is re-moved by STM lithography. For uniformity in such nm-scale growth, the window area should be larger than the “nucleus-occupied area”, defined as the inverse of the saturated nucleus density, and the window separation should be larger than the diameter of the “nucleus-occupied area”.
Room temperature characteristics of a single electron transistor fabricated using the AFM nano-oxidation process are described. A new and improved AFM nano-oxidation process is introduced, in which (1) the pulse-mode bias is used in-stead of DC bias, that can enhance the height and width aspect ratio of the oxidized metal and (2) atomically flat sub-strates such as α-Al2O3 are used instead of SiO2 substrates. Using these new process, the single electron transistor was fabricated and it shows the clear Coulomb oscilation with the period of 1.8V and a Coulomb diamond characteristics even at higher temperatures than room temperature.
Surface roughening processes in the molecular beam epitaxy of GaAs are studied by in-situ scanning electron micros-copy. Three types of onset of roughening are observed, that is, smooth-to-rough change, coexistence of rough and smooth growths and purely rough growth. The results are compared to the quasi-smooth growth and to the growth of silicon on (111) surface, and the roughening mechanism is discussed.
Helium atom scattering (HAS) is a unique technique suitable for studying the structure and dynamics of the outermost layers of crystalline surfaces. In this report, we describe a very compact high-performance HAS apparatus developed for high resolution surface studies. Result of the helium atom scattering experiments on the single crystal copper surfaces under irradiation by H atoms are discussed. Application of helium atom scattering to the understanding of physical and chemical processes in fabrication of sub-quarter-micron Cu interconnects for ultra-large scale integration circuits is discussed.