Kazuhiro Karahashi

Etching yield of SiO2 irradiated by F+, CFx+ (x=1,2,3) ion with energies from 250 to 2000 eV

Etching yields of SiO2 have been determined for F+, CFx+ (x=1,2,3) ion irradiation with energy ranging from 250 to 2000 eV using a mass-analyzed ion-beam apparatus that can irradiate a single species ion to sample surfaces under an ultrahigh vacuum condition. The etching yield of CFx+ (x=1,2,3) was enhanced by the chemical effects of the ions, and both carbon and fluorine atoms from the incident ions were significant reactants. For lower energies, the etching yield increased with increasing ion energy. Above 1000 eV, the etching yield was gradually saturated with increasing ion energy. In the low ion energy region, steady etching did not occur, and an amorphous fluorinated carbon (a-C:F) film was deposited on the SiO2 surface. The ion energy region in which a-C:F film deposition occurred decreased with increasing fluorine atoms in incident CFx+ (x=1,2,3) ions.

Mass-analyzed CFx+ (x=1,2,3) ion beam study on selectivity of SiO2-to-SiN etching and a-C:F film deposition

To clarify the origins of high selectivity in SiO2-to-SiN etching in fluorocarbon gas plasma, mass-analyzed CFx+ (x=1,2,3) ions with a definite kinetic energy of 250–2000eV were irradiated on SiN and SiO2 surfaces. Selectivity in SiO2-to-SiN etching varies greatly for different CFx+ ions. For CF3+ ions, the etch yield of SiN is almost the same as that of SiO2, causing poor selectivity. For CF+ ions, on the other hand, the etch yield of SiN is much smaller than that of SiO2. An amorphous fluorinated carbon (a-C:F) film grows without any neutral radicals on the SiN surface at energies below 1250eV and on the SiO2 surface at energies below 500eV due to CF+ ion irradiation. The difference in threshold energy of a-C:F film deposition causes high selectivity in SiO2-to-SiN etching. Slight etching of substrate films first takes place at the initial stage of deposition, then etching stops, and a homogeneous a-C:F film is grown. Accumulated carbons during the initial etching reaction modify the surface reaction la...

Study on Chlorine Adsorbed Silicon Surface Using Soft-X-Ray Photoemission Spectroscopy

In a study of the surface reaction of molecular and atomic chlorine on Si(100) and Si(111) using X-ray photoemission spectroscopy and molecular beam scattering, we have found only SiCl in the chlorinated layer formed by the molecular chlorine exposure, with no change in this bonding configuration after annealing. Most desorption products were SiCl2. SiCl desorbed above 900°C, probably due to the recombinative desorption of SiCl+Cl→SiCl2. SiCl desorbed directly from the surface without any reaction. Heavily chlorinated species, such as SiCl2, SiCl3 and SiCl4, were observed on Si(100) and Si(111) surfaces exposed to atomic chlorine. These heavily chlorinated species quickly desorbed from the surface after annealing at 300°C. No heavily chlorinated species were observed on the silicon surface exposed to molecular chlorine. The chlorinated layer on Si(111) was thinner than that on Si(100), explaining the appearance of the facet reported in photo-enhanced etching.

Transitional change to amorphous fluorinated carbon film deposition under energetic irradiation of mass-analyzed carbon monofluoride ions on silicon dioxide surfaces

We have studied both the etching of SiO2 film and the growth of an amorphous fluorinated carbon (a-C:F) film by mass-analyzed fluorocarbon ion irradiation. This experiment was done in an ultrahigh vacuum with a pressure of 10−7 Pa even during irradiation. When using a carbon monoflouride (CF1+) ion with an energy of 300 eV to irradiate, it was found that SiO2 film a few nm thick was initially etched away. Then, an a-C:F film was continuously deposited on the SiO2 surface as the ion dose exceeded about 5×1016 cm−2. Using in situ x-ray photoelectron spectroscopy analysis, it was determined that carbon accumulates on the surface at this early stage as the ion dose increases, so that this transition is resulted by the surface modification on which the CF1+ ion itself irradiates the SiO2 surface. Especially in highly ionized fluorocarbon plasmas, surface conditions such as the carbon concentration affect possibly etching performance.

Hydrogen effects in hydrofluorocarbon plasma etching of silicon nitride: Beam study with CF+, CF2+, CHF2+, and CH2F+ ions

Hydrogen in hydrofluorocarbon plasmas plays an important role in silicon nitride (Si3N4) reactive ion etching. This study focuses on the elementary reactions of energetic CHF2+ and CH2F+ ions with Si3N4 surfaces. In the experiments, Si3N4 surfaces were irradiated by monoenergetic (500–1500 eV) beams of CHF2+ and CH2F+ ions as well as hydrogen-free CF2+ and CF+ ions generated by a mass-selected ion beam system and their etching yields and surface properties were examined. It has been found that, when etching takes place, the etching rates of Si3N4 by hydrofluorocarbon ions, i.e., CHF2+ and CH2F+, are higher than those by the corresponding fluorocarbon ions, i.e., CF2+ and CF+, respectively. When carbon film deposition takes place, it has been found that hydrogen of incident hydrofluorocarbon ions tends to scavenge fluorine of the deposited film, reducing its fluorine content.

Neutron powder diffraction from polymorphs of BaPb0.75Bi0.25O3

Crystal structures of three polymorphs of the superconducting oxide BaPb0.75Bi0.25O3 have been determined by high-resolution neutron powder diffraction. The compound has been confirmed to form both tetragonal (I4/mcm) and orthorhombic (Ibmm) structures at room temperature. The latter transforms to the monoclinic phase (P21/m) at low temperatures. The structural difference of the three polymorphs has been described by the rotation as well as the distortion of oxygen octahedra surrounding the Pb0.75Bi0.25 cations.

Suboxide/subnitride formation on Ta masks during magnetic material etching by reactive plasmas

Etching characteristics of tantalum (Ta) masks used in magnetoresistive random-access memory etching processes by carbon monoxide and ammonium (CO/NH3) or methanol (CH3OH) plasmas have been examined by mass-selected ion beam experiments with in-situ surface analyses. It has been suggested in earlier studies that etching of magnetic materials, i.e., Fe, Ni, Co, and their alloys, by such plasmas is mostly due to physical sputtering and etch selectivity of the process arises from etch resistance (i.e., low-sputtering yield) of the hard mask materials such as Ta. In this study, it is shown that, during Ta etching by energetic CO+ or N+ ions, suboxides or subnitrides are formed on the Ta surface, which reduces the apparent sputtering yield of Ta. It is also shown that the sputtering yield of Ta by energetic CO+ or N+ ions has a strong dependence on the angle of ion incidence, which suggests a correlation between the sputtering yield and the oxidation states of Ta in the suboxide or subnitride; the higher the o...

Ion beam experiments for the study of plasma–surface interactions

Experimental studies based on mass-selected ion beams are reviewed as a means to examine plasma–surface interactions for industrial plasma processing. Plasma etching and deposition processes widely used in the microelectronics industry exploit surface chemical reactions induced by plasmas. Such reactions typically result from a conglomeration of complex interactions of the surface material with incident free radicals and ions. While it is in general difficult to analyse individual surface reactions separately in a plasma system, ion and/or charge-neutral beam experiments allow one to analyse specific surface reactions involving only selected gaseous species. In this review, experiments on beam–surface interactions are discussed in detail and beam reaction data for silicon surfaces are presented as sample data.

Measurements of desorbed products by plasma beam irradiation on SiO2

A plasma beam irradiation apparatus with a controllable plasma source was constructed to investigate the plasma-surface interactions for reactive etching processes. The apparatus can control the plasma parameters, such as ion energy, radical∕ion composition, and the ratio of neutral to ion flux. The desorbed products of SiFx (x=1–3) were observed separately from COFx (x=1–3) by using a C13F4–Ar gas mixture plasma beam. The major desorbed product was not SiF4. Unsaturated fluorinated silicons [SiFx(x=1–3)] were major desorbed products under ion-rich fluorocarbon plasma etching condition. It was found that the composition of silicon containing desorbed products does not depend on the incident ion energy in the ion energy range of 300–700eV.

Characteristics of silicon etching by silicon chloride ions

Plasmas generated from halogen-containing gases, such as Cl2 or HBr, have been widely used in gate etching processes for semiconductor chip manufacturing. Such plasmas may contain silicon halide ions formed by the ionization of etching products that enter the plasma. In this study, to illustrate Si etching by such silicon halide ions, the sputtering yield of Si by SiClx+ (with x = 1 or 3) ions has been obtained as a function of the incident ion energy by using a mass-selected ion beam injection system. It has been found that, at sufficiently low energy, the incidence of SiCl+ ions leads to the deposition of Si which may affect profile control in microelectronic device fabrication processes.