Kanayama Group

Formation and Control of Clusters in Ion Trap and on Solid Surface
Tosihiko Kanayama

It is a significant challenge to develop formation technology of nanostructures with atomic precision. This group is seeking to construct desired nanostructures by exploiting self-assembly of nanometric building blocks.

The automatic arrangement is a popular phenomenon in atomic dimensions. When atoms are put together, they often result in a well-ordered structure. Crystal growth is the best example and is already utilized to form various artificial structures such as epitaxial films and superlattices. To extend this strategy to formation of complicated nanostructures as shown in the figure below, we should create an additional level in the hierarchy of material handling: preparation of nanometric building blocks and control of their assembling reaction. These are the major objectives of our research.

Fabrication process of nanostructure based on formation, modification, and self-assembly of composite nanocrystals

■Results in Fiscal Year

1. Mass-selective growth of cluster ions

An electrical trapping technique has been developed, which allows us to confine ions with a wide range of mass values using the external field of an ac quadrupole and take out the ions mass-selectively through the internal region of the quadrupole. The results of growth experiments of SinHx+ clusters verified that a selected mass of cluster ions are continuously produced by tuning the frequency of the ac voltage connected to the quadrupole.

2. Self-assembling etching of nanostructures

It has been found that when the sample temperature is optimized during the electron cyclotron resonance plasma etching using SF6 gas, reaction products condense on the sample surface to form small droplets and serve as etching masks, leading to the self-assembling formation of Si pillars of ~10-nm diameter and ~100-nm height. Position control of the pillars may be possible owing to the observed fact that organic molecules and metal clusters sticking on the surface act as nucleation sites for the condensation.

3. Electron-beam nanolithography

Evaporated C60 films have been found to work as a dry-etch durable electron beam resist. Using this material, Si pillars of ~20 nm diameter were fabricated.

4. Thermal desorption of Si clusters

Subliming mass spectra were measured for Si wafers heated to 900-1300ｰC and the activation energies were evaluated for clusters up to Si6. Obtained values of the activation energies are independent of the surface orientation and are attributed to the formation energy of the clusters, indicating that the atomic motion is so significant on the surfaces at these temperatures.

Results of mass-selective growth of Si6H12+ clusters. The number of counted Si6H12+ ions is plotted as a function of time measured from the electron irradiation with the ac frequency f as a parameter. The electron irradiation lasted for 100 ms.

Transmission electron microscopic lattice image of a pillar fabricated by the self-assem-bling etching indicates that the pillar actually consists of single crystalline Si.

Scanning electron migrograph of 20-nm dot patterns formed in an evaporated C60 film by electron beam writing.