Shigeru Nishimatsu

Radiation Damage in SiO2/Si Induced by VUV Photons

Quantitative measurements of radiation damage in SiO2/Si systems induced by VUV photons generated in various microwave plasmas are performed. The SiO2/Si system is irradiated by monochromatic VUV photons, and its flat-band voltage shift (ΔVFB) is measured by the C-V method. The number of effective positive charges generated in the SiO2 layer by a single VUV photon (Rf) is evaluated from the ΔVFB and the total VUV dose. A VUV photon is found to generate effective positive charges in SiO2 when its energy (Ep) is larger than the SiO2 band gap energy (Eg=8.8 eV). The value of Rf is on the order of 10-3~10-2 and increases in proportional to the energy difference between Ep and Eg . A model is proposed to explain these phenomena. The model states that a VUV photon with energy Ep(>Eg) generates an electron-hole pair in the SiO2 and the holes that are not recombined are trapped in the energy state near the SiO2/Si interface.

Reaction of atomic fluorine with silicon

The etch rate of Si with F atoms was measured by the use of F2 microwave plasma over a range of discharge pressures between 2.7×10−2 and 17 Pa. Fluorine atom concentration in the plasma was determined over the same pressure range by means of both gas‐phase titration and actinometry using Ar gas. A Si surface etched at 1.0×10−1, 5.3×10−1, 1.3, and 5.3 Pa was analyzed with XPS without exposing the surface to room air. A linear relation was obtained between the Si etch rate and the F atom concentration at discharge pressures between 2.7×10−2 and 2.7 Pa. The reaction probability of F atoms with Si to yield SiF4 was determined from the linear relation to be 0.1 for a Si surface at about 300 K. When the discharge pressure was higher than 1.3 Pa, the surface became rather strongly oxidized by O atoms resulting from residual gases. This surface oxidation results in a slight saturation of the Si etch rate at about 10 Pa.

Ac Surface Photovoltages in Strongly-Inverted Oxidized p-Type Silicon Wafers*

Surface photovoltages in Si wafers excited with a chopped 559 nm-wavelength photon beam are analysed using a new half-sided junction model. In this model, the wafer surface with the depletion layer is considered to be one half of the p-n junction. Chopping frequency ranges from 2 Hz through 20 kHz. Four 76 mm-diameter p-type Si wafers having resistivities of 260, 92, 17 and 1.0 mΩ m are used after forming 360 nm-thick wet-oxide layer on their front surfaces. In these wafers, photovoltage increases with resistivity. In three high-resistivity wafers with strongly-inverted surfaces, the inversion capacitances and conductances limit the photovoltages at low frequencies. The obtained inversion time-constant is 7 s for the 17 mΩ m wafer.

Radio-frequency biased microwave plasma etching technique: a method to increase SiO2 etch rate

In order to increase the SiO2 etch rate in microwave plasma etching, a method of controlling the kinetic energy of ions impinging on a substrate surface covered with an insulator film was developed. In this method, radio‐frequency voltage is supplied to the substrate. A theoretical model and analysis for this method was made to define the substrate surface potential and the kinetic energy of impinging ions. Experimental verification of the analysis, and application of the method to SiO2 etching have been carried out. A SiO2 etch rate eSiO2 =40 nm/min, and selectivity to Si η2=eSiO2/eSi =4.0 were obtained with C3F8 etching gas and rf peak to peak voltage VBP=200 V.

Sputtering yield and radiation damage by neutral beam bombardment

To clarify the difference between ion beam etching and neutral beam etching, a low‐energy (100–1000 eV) Ne0 neutral beam obtained by charge exchange reaction has been used to bombard Cu, Si, and SiO2 surfaces. The sputtering yields of Cu and Si by Ne0 have been found to be the same as those by Ne+. The sputtering yield of SiO2 by Ne0 is slightly smaller than that by Ne+. It is believed that electronic sputtering mechanisms play only a small role in the sputtering of these materials at these low ion energies. C–V measurements show that the amount of radiation damage caused by Ne0 neutral beam bombardment of the SiO2/Si structure is significantly less than that by a Ne+ ion beam. When the Ne+ dosage increases, the flat‐band voltage shift increases and the thin SiO2 films on Si eventually break down. In Ne0 neutral bombardment, the flat‐band voltage shift saturates with increasing dosage and no breakdown of SiO2 occurs. It is assumed that, in neutral beam bombardment, the surface potential of SiO2 is limited...

Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer

The majority carrier conductance due to the hole flow towards the surface of a wafer from the bulk has been formulated following the half-sided junction model previously reported. An empirical equation for the carrier drift velocities in very high electric fields has been proposed for the formulation. Besides the conductance, the depletion layer capacitance, interface trap capacitance and conductance are found to be responsible for ac surface photovoltages in the depletion case. The majority carrier conductance can explain the formerly observed conductance of 27 S/m2 in a 76-mm-diameter 1.0-mΩm oxidized p-type Si wafer. Analysis of the formerly reported surface photovoltages reveals a surface potential, fixed oxide charge density, hole capture cross section and interface trap density of 0.32 V, 1.7 mC/m2, 2.0×10-20 m2 and 2.0×1016 m-2 eV1 respectively.

Anisotropic etching of polycrystalline silicon with a hot Cl2 molecular beam

A hot Cl2 molecular (Cl*2) beam was successfully applied to achieve highly anisotropic, highly selective, and almost damage‐free etching of polycrystalline Si. The anisotropy, the ratio of etch rates in vertical and horizontal directions, was larger than 25. The selectivity, the ratio of polycrystalline Si and SiO2 etch rates, was larger than 1000. The Cl*2 beam was produced by free jet expansion of a Cl2 gas heated in a graphite furnace. The furnace temperature was 830 °C. The substrate temperature was 180 °C. The average total energy (0.38 eV) of a Cl*2 molecule impinging on a substrate surface is much lower than the critical energy (approximately 10 eV) to displace the atoms of the etched material and to cause surface damage. This is the essential reason why this highly selective and almost damage‐free etching has been achieved. The highly anisotropic etching mechanism is explained by a model taking into account the directional incidence of Cl*2 molecules to the surface, and the deactivation process of...

An experimental system for surface reaction studies in microwave plasma etching

An experimental system for studying surface reactions in the process of microwave plasma etching has been developed. In the system, a surface etched in the microwave plasma can be analyzed with x‐ray photoemission spectroscopy (XPS) without exposure of the surface to room air. In addition, we have developed a procedure for calculating a thickness of a surface layer stoichiometrically different from the substrate material and densities of atoms in the layer. Chemical changes in etched Si and SiO2 surfaces caused by exposing these surfaces to room air are investigated with XPS to show the utility of the system. When the surfaces etched in SF6 microwave plasma are exposed to room air, the chemical states of the surfaces change rapidly. This is mainly due to surface oxidation and adsorption of hydrocarbon compounds to the surfaces. The rapid changes are more clearly shown from increases in surface layer thickness and the number of O and C atoms in the layer. It is clarified that exposure of etched surface to ...

Role of sulfur atoms in microwave plasma etching of silicon

The Si etch rate in an (F2+O2) microwave plasma has been measured as a function of O2 mixing ratio at a fixed total pressure of 5.3×10−2 Pa. The etch rate significantly decreases with the mixing ratio. This etch rate decrease is due primarily to surface oxidation. When sulfur is added to the (F2+O2) plasma, the Si surface is much less oxidized and the etch rate increases by about a factor of 4. Such sulfur‐containing species as S atoms react with O atoms or ions in the plasma and form O atom‐containing species, such as SO2, SO+, SOF+, and SOF+2, thereby reducing the O atom and O+ ion concentrations in the plasma. As a result, the Si surface is scarcely oxidized, so that the etching reaction can easily proceed. Sulfur atoms inhibit surface oxidation and promote Si etching. Sulfur atoms contained in SF6, which is usually used in microwave plasma etching of Si, are expected to have the same role.

Generation of low‐energy neutral beams and radiation damage of SiO2/Si by neutral bombardment

In order to apply to neutral beam etching, several hundred eV neutral beams have been generated by charge exchange reaction using a magneto‐microwave plasma ion source. The charge exchange cross sections for Ar+ and Kr+ are unexpectedly small, whereas the cross section for Ne+ of 300 eV is 1.7×10−15 cm2 in good agreement with published results. The small cross sections for Ar+ and Kr+ in the present experiments are explained in terms of a nonresonant charge exchange scheme. The capacitance–voltage (C–V) measurements show that the radiation damage to SiO2/Si by Ne0 neutral beam bombardment is much less than those by Ne+ ion beam and by vacuum ultraviolet photons. Consequently, the neutral beam process is a promising method for charge‐free and low‐damage surface processes.