Taro Yamada

Scanning tunneling microscopy studies of oxygen adsorption on Cu(111)

Abstract The adsorption of O2 on Cu(1xa01xa01) at room temperature has been investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Adsorption of oxygen leads to formation of a surface oxide by incorporation of Cu atoms from step edges and terraces. This process is most rapid along the close packed direction of the surface and leads to mesoscopic changes in surface morphology. Three characteristic features were observed during the initial stage of adsorption; dark fringes along the Cu(1xa01xa01) step edges, dark domains within the Cu(1xa01xa01) terrace, and rather mobile light patches on top of the Cu terraces. Within these regions atomic scale features could be imaged and the structure related to that of Cu2O. The dark fringes and dark domains grew slowly in oxygen, whereas the bright patches only became visible when gas-phase O2 was evacuated. STM observation at elevated temperatures indicates mobilization and rearrangement of surface features formed during room temperature adsorption. O/Cu(1xa01xa01) surfaces annealed at 473–623 K indicated a well ordered ( 73 R5.8°× 21 R−10.9° ) lattice structure (‘44’-structure) on the terraces which was also confirmed by LEED. Annealing up to 723 K exhibited slight disordering of this lattice structure presumably due to inter-diffusion of O and Cu atoms with the bulk. The mesoscopic process of oxygen adsorption onto the clean Cu(1xa01xa01) surface was also investigated at elevated temperature by STM. The reaction of the surface maintained at temperatures between 373 and 773 K was investigated for oxygen pressures in the 10−5–10−3 Pa range. The oxidized surface formed at 373 K was largely disordered with only small areas of the ‘44’-structure (√73R5.8°×√21R−10.9°). Well-ordered adlattices of the ‘44’-structure were formed upon O2 exposure at 473 K. Such surfaces could be converted to the ‘29’-structure (√13R46.1°×7R21.8°) by annealing at 673 K in vacuum. On the basis of the STM images we propose a new model for the ‘29’ structure. Annealing at higher temperatures (∼773 K) retrieved the ‘44’-structure in small domains on a disordered background. A disordered oxide surface which was produced at room temperature and annealed at 723 K can be ordered into the ‘44’-structure by O2 exposure and heating at 623 K.

Mechanism of the chemical deposition of nickel on silicon wafers in aqueous solution

The deposition of metallic nickel on n-Si(100) wafers was performed without external potential control in aqueous NiSO 4 solutions of different compositions at pH 8.0. Without giving any catalyzation treatment, the deposition of nickel on hydrogen-terminated Si(100) was confirmed in a conventional electroless plating bath containing NaH 2 PO 2 as the reducing agent, sodium citrate as the complexing agent, and (NH 4 ) 2 SO 4 as the buffering agent. The deposition of nickel was found to take place also in a bath without the reducing agent, and even in a simple solution consisting of NiSO 4 and (NH 4 ) 2 SO 4 . By using a transmission electron microscope equipped with an energy dispersive X-ray spectrometer, the cross sections of the films deposited from these solutions were examined, which revealed formation of silicon oxide between the Ni deposit and Si substrate. Based on these results, the mechanism of the entire process of electroless Ni deposition on Si is discussed.

Nickel deposition behavior on n-type silicon wafer for fabrication of minute nickel dots

Abstract The present study is a part of our systematic development of mass-scale production processes of nanometer-scale arrays of metal dots on silicon wafer surfaces. Metallic Ni was deposited onto Si(100) wafer surfaces electrolessly or galvanostatically, for surveying appropriate methods for formation of minute structures. Within an electroless bath of simple NiSO 4 –(NH 4 ) 2 SO 4 solution, metallic Ni was deposited, accompanied by the oxidation of the Si surface. Wet pretreatment of the Si surfaces in HPM (HCl and H 2 O 2 mixture) or in ethanol drastically improved the uniformity of Ni layer and the rate of deposition. The electrolytic deposition with applying a potential at the Si wafer resulted in formation of Ni deposit which were easily peeled off. Based on the knowledge obtained, a two-dimensional array of minute Ni dots (diameter ca. 270 nm) was successfully fabricated.

Effect of oxidized silicon surface on chemical deposition of nickel on n-type silicon wafer

The effect of oxidation of silicon surface on chemical deposition of nickel was investigated in aqueous basic baths containing Ni 2+ . On H-terminated Si(100), Ni was deposited obviously but partially. On oxidized Si(100), Ni was deposited on the whole surface with a higher rate than on H-Si (100). The deposits were examined by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). Selective nickel deposition was performed on oxidized Si(100) patterned with a layer of plasma-CVD SiO 2 . Ni dots with diameters about I μm were formed by dipping the patterned Si(100) wafers first into a Ni bath containing no reducing agent for nuclei formation, and then into a Ni bath with reducing agent for growing particles.

Evaluation of Organic Monolayers Formed on Si(111): Exploring the Possibilities for Application in Electron Beam Nanoscale Patterning

The methods of preparing organic monolayers on Si(111), the effects of electron-beam irradiation onto these monolayers, and the deposition of metal atoms over the irradiated areas have been investigated in order to develop a process of mass-scale production of nanometer-scale patterns on Si(111) wafer surfaces. The organic monolayers were fabricated on hydrogen-terminated Si(111) wafer surfaces using previously reported methods for the electrolysis of para-substituted benzenediazonium salts and the Grignard reaction with various alkyl moieties and reaction procedures. Using these electrolysis methods, partially well-defined two-dimensional monolayers were formed, which were, however, obscured by precipitated by-products. The Grignard reaction deposited homogeneous monolayer moieties of alkyl groups which were randomly arranged and are suitable for surface passivation. Electron-beam bombardment of the organic monolayers on Si(111) was performed in an atmosphere of O2 or H2O. The bombarded area was effectively oxidized in a well-controlled manner. By immersing the bombarded specimen into an aqueous NiSO4+(NH4)2SO4 solution, Ni was selectively impregnated only within the area of electron bombardment. Based on these results, application of organic monolayers for fabricating nanometer-scale monolayer patterns is proposed.

Alkyl monolayers on Si(111) as ultrathin electron-beam patterning media

Abstract A process of electron-beam patterning of the surface of a Si(111) wafer was developed by utilizing alkyl monolayers as ultrathin patterning media. We performed chemical benchmark tests of the electron-beam patterning of alkyl monolayers on Si(111) in ambient oxygen, followed by the deposition of a metal on bombarded areas by immersion into an aqueous solution containing metal ions of the metal to be deposited. We investigated practically important issues related to this process, such as the robustness of organic monolayers against oxidation in aqueous media, the contrast enhancement of the bombarded areas by metal deposition, and the detectability of electron-bombarded areas of the monolayers by scanning tunneling microscopy (STM). The alkyl-covered Si(111) surface was significantly resistant to the oxidation by dissolved O2 in pure water, compared to hydrogen-terminated Si(111). By immersion into a solution containing CuSO4+HF+NH4F, electron-bombarded areas were visualized by the presence of the deposit of Cu. Electron-bombarded areas were also distinguishable from intact areas in terms of height contrast or roughness measured by STM. These results indicate the usefulness of alkyl monolayers for nano-scale patterning on silicon wafers.

Application of organic monolayers formed on Si(1 1 1): Possibilities for nanometer-scale patterning

Abstract The modification of hydrogen-terminated Si(1xa01xa01) wafer surfaces was reproduced by previously reported methods of the electrolysis of para-substituted benzendiazonium salts and the Grignard reaction with various alkyl moieties. The electrolysis methods formed partially ordered two-dimensional monolayers, which were however obscured by precipitation of by-products. The Grignard reaction deposited a monolayer of moieties of alkyl groups randomly arranged, which are more suitable for surface passivation. Aiming for the application to nanometer-scale monolayer patterning of the Si(1xa01xa01) wafer surface, the organic-monolayer-covered Si(1xa01xa01) surfaces were subjected to electron beam bombardment. After electron bombardment with ambient O2 or H2O introduced, adsorption of oxygen was observed within the beam spot. By immersing the bombarded specimen into an aqueous NiSO4+(NH4)2SO4 solution, the oxygen-deposited portions selectively included Ni atoms. This will be useful in constructing nanometer-scale metallic structures over Si wafer surfaces.

Ultrahigh-vacuum multitechnique study of AuCN monolayers on Au(111) formed by electrochemical deposition

The two kinds of monolayers of AuCN electrodeposited on Au(111), indexed (1.15×√3R−30°) and (1.41×2√3R−30°), have been subjected to x-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) and high resolution electron energy loss spectroscopy (HREELS) as well as low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM) to determine the geometrical, electronic, and vibrational properties. The -AuCN- chain structure was taken into consideration. XPS yielded Au 4f signals from AuCN indicating fractional charges on the Au atom incorporated in AuCN. The UPS of AuCN/Au(111) was composed mainly of the Au orbitals with weak signals from CN orbitals, attributed by DV-Xα relativistic molecular orbital calculation. By HREELS, the C–N stretching frequencies were found to be 2140–2160 cm−1, which are consistent with the electronic structure. In the frequency region below 300 cm−1, loss peaks related to the Au–N bonds were seen. The structures of -AuCN- chains...

Thermal desorption and laser induced desorption of NO from adlayers of NO on diamond C(1 1 1)

Abstract The processes of thermal desorption and photo-induced desorption were investigated for nitric oxide (NO) molecules from the adlayers on bare diamond C(1xa01xa01) (2×1) surface formed at 110xa0K. The surface coverage of NO was monitored by X-ray photoelectron spectroscopy (XPS). The kinetics of thermal desorption of NO was of first-order (the activation energy of desorption=9.8xa0kcalxa0mol −1 ) at coverages below one monolayer, and of zeroth-order (9.2xa0kcalxa0mol −1 ) for multilayer. Time-of-flight (TOF) detection of photo-induced desorption of NO was performed by laser beams of wavelengths 193, 248 and 308xa0nm. The photo-excitation that drives NO desorption was attributed to the band-gap excitation of substrate and/or surface for the NO coverages below one monolayer, and to the direct absorption by NO multilayer consisting of (NO) 2 species.

Possibilities of electron beam nano-meter-scale fabrication of Si(111) using alkyl monolayers

A novel process of electron-beam nanometer-scale fabrication on Si(111) wafer surfaces has been proposed on the basis of application of organic monolayers as the ultimately thin patterning media. The monolayers on Si(111) wafer surfaces composed of alkyl groups (C/sub n/H/sub 2n+1//sup _/) prepared with the Grignard reagents were subjected to electron-beam patterning, and deposition of metals onto the electron-bombarded patterns by immersion into aqueous solutions containing Ni/sup 2+/ or Cu/sup 2+/ ions. This entire process has been put into practice successfully as a benchmark test. The strength of alkyl-covered Si(111) surface against the processing environment such as in vacuum and aqueous solutions has been demonstrated.