Keita Yagi

Partially coherent light generated by using single and multimode optical fibers in a high‐power Nd:glass laser system

A simple and flexible method is presented for generating a partially coherent light which obtains the highly smooth focused beam pattern. The beam divergence of 32 times diffraction limited light having a spectral width of 1.6 nm has been easily and reproducibly achieved by injecting a laser pulse from an actively mode‐locked Nd:YLF oscillator to a single mode optical fiber, coupled to a multimode optical fiber. Temporal evolution of the beam smoothing due to the induced incoherency was examined with temporally resolved measurements of the beam pattern. The partially coherent light was focused through a random phase plate after the amplification. Small‐scale intensity perturbation in a focused beam pattern was greatly reduced.

Atomic-scale flattening of SiC surfaces by electroless chemical etching in HF solution with Pt catalyst

The authors present a method for flattening SiC surfaces with Pt as a catalyst in HF solution. The mechanism for flattening SiC surfaces is discussed. The flattened 4H-SiC(0001) surface is composed of alternating wide and narrow terraces with single-bilayer-height steps, which are induced by the rate difference of the catalytic reactions between adjacent terraces. Scanning tunneling microscopy images reveal a 1×1 phase on the terraces. The 1×1 phase is composed of coexisting of F- and OH-terminated Si atoms, which originate from the polarization of the underlying Si–C bonds.

Defect-Free Planarization of 4H–SiC(0001) Substrate Using Reference Plate

In this paper, a new defect-free planarization technique for 4H–SiC(0001) substrate is described. This technique uses hydroxyl radicals (OH radicals) that are generated on an Fe metal surface in a hydrogen peroxide (H2O2) solution. First, the oxidation of a 4H–SiC substrate by OH radicals is investigated by X-ray photoelectron spectroscopy (XPS) analysis. Next, the planarization of the 4H–SiC substrate is conducted. A very flat and smooth surface without any scratches and etch pits is obtained. The planarized surface has a step-terrace structure.

Catalyst-referred etching of silicon

Abstract A Si wafer and polysilicon deposited on a Si wafer were planarized using catalyst-referred etching (CARE). Two apparatuses were produced for local etching and for planarization. The local etching apparatus was used to planarize polysilicon and the planarization apparatus was used to planarize Si wafers. Platinum and hydrofluoric acid were used as the catalytic plate and the source of reactive species, respectively. The processed surfaces were observed by optical interferometry, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results indicate that the CARE-processed surface is flat and undamaged.

Novel Abrasive-free Planarization of Si and SiC using Catalyst

We propose a new chemical planarization method using a catalyst as a polishing plate. A sample is placed on the polishing plate in a solution that is a source of reactive species. Since the catalyst generates reactive species that activate only next to the catalyst surface, this method can efficiently planarize. This processed surface is not damaged by chemical removal. We named this method CAtalyst-Referred Etching (CARE). CARE was applied to SiC planarization. The processed surfaces were observed by atomic force microscopy and optical interferometry. These observations presented a marked reduction in surface roughness.

New chemical planarization of SiC and GaN using an Fe plate in H2O2 solution

A novel chemical planarization method was developed for silicon carbide (SiC) and gallium nitride (GaN). This method uses catalytically generated hydroxyl radicals (OH*) to oxidize the wafer surface. OH* are generated by the reductive decomposition of hydrogen peroxide (H2O2) on the surface of the iron reference plate. An extremely flat surface without pits or scratches was obtained. Atomic force microscopy (AFM) revealed that the planarized surface had an atomic step-terrace structure, in which the step height corresponded to a single bilayer of 4H-SiC and GaN. Low electron energy diffraction (LEED) and cathodeluminescence spectroscopy showed that there was no crystallographic damage on the planarized surface.

Fabrication of damascene Cu wirings using solid acidic catalyst

Abstract The copper damascene process is one of the most promising technologies for fabricating Cu wirings for electronic devices such as LSIs. In this research, the fabrication of damascene Cu wirings was conducted using solid acidic catalyst. When a Cu-plated wafer, whose oxide is a basic oxide is dipped into a mixture of oxidizing solution and acidic solution, surface atoms are ionized and etched off into the solution. However, because conventional nonelectrolytic etching does not have a reference surface, it is difficult to utilize for planarization. Therefore, a new nonelectrolytic machining method using a cation-exchange fabric instead of an acidic solution was developed. To be more precise, the planarization of a Cu-plated wafer was carried out by rubbing with the cation-exchange fabric in ozone water. Basically, this method exploits chemical reactions so that the physical properties of the workpiece surface are not deteriorated. Furthermore, this method uses no chemicals except for ozone water, which easily dissociates into water and oxygen molecules; thus, this method is a low-cost, environmentally friendly process. In this paper, as a preliminary experiment, the nonelectrolytic etching of a Cu sample using solutions of O3 and CO2 was carried out to inspect the dependence of the etching rate on [O3] and [H+]. The results indicate that the etching rate increased as [O3] and [H+] increased. When [H+] was high relative to [O3], a smooth etch-pit-free surface was achieved. Next, nonelectrolytic etching using a cation-exchange fabric was carried out, and properties similar to those in the case of etching using solutions were obtained. Finally, damascene Cu wirings were fabricated using ozone water and a cation-exchange catalyst.

Atomic-scale Characterization of HF-treated 4H-SiC(0001)1×1 Surfaces by Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) observations are performed on 4H-SiC(0001) surfaces after wet-chemical preparation steps including HF treatments.1×1 structures are formed on a terrace together with other local structures. Their atomic images are investigated in conjunction with low-energy electron diffraction and electron spectroscopy for chemical analysis. It is suggested that each bright dot forming the 1×1 phase corresponds to an OH-terminated Si atom.