Susumu Hiraoka

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...

Anisotropic etching of GaAs using a hot Cl2 molecular beam

Anisotropic etching of GaAs(100) is performed using a hot Cl2 molecular beam produced by free expansion of gas heated in a furnace. The etch rate is 1.5 μm/min at a furnace temperature of 800 °C and a substrate temperature of 120 °C. An aspect ratio of ten and an almost smooth bottom surface are obtained under this condition.

Reaction probability and reaction mechanism in silicon etching with a hot Cl2 molecular beam

Reaction products in Si etching with a hot Cl2 (Cl2*) molecular beam were measured by a quadrupole mass spectrometer. A major part of the product was shown to be SiCl4, and the reaction probability of Cl2* on a Si surface was obtained. It was found that the reaction probability increases rapidly with the increase in furnace temperature for Cl2* formation, and high reactivity of Cl2* was demonstrated. A reaction model (the activated complex Arrhenius model) is proposed to explain the experimental results, and the model parameters are determined. This model takes into account the effects of the translational and vibrational energies of a Cl2* molecule on the activation energy and the frequency factor in the ordinal Arrhenius model.Reaction products in Si etching with a hot Cl2 (Cl2*) molecular beam were measured by a quadrupole mass spectrometer. A major part of the product was shown to be SiCl4, and the reaction probability of Cl2* on a Si surface was obtained. It was found that the reaction probability increases rapidly with the increase in furnace temperature for Cl2* formation, and high reactivity of Cl2* was demonstrated. A reaction model (the activated complex Arrhenius model) is proposed to explain the experimental results, and the model parameters are determined. This model takes into account the effects of the translational and vibrational energies of a Cl2* molecule on the activation energy and the frequency factor in the ordinal Arrhenius model.

Vibrational and Rotational Energy Distributions in a Hot Cl2 Molecular Beam

Vibrational and rotational energy distributions of a hot Cl2 (Cl2*) molecular beam, which is used for a semiconductor device etching process, is studied by a laser-induced fluorescence method. The Cl2* beam is produced by free jet expansion of a Cl2 gas heated in a quartz furnace. It is found that the hot molecular beam is a transition flow between a continuum free jet and an effusive flow. The vibrational and rotational energy distributions can be described by Boltzmann functions of well-defined temperatures. The vibrational temperature is rather close to the furnace exit temperature, while the rotational temperature is cooled to 150–250 K during the isentropic free expansion. These results show that furnace exit temperature is crucial to the production of a highly reactive hot molecular beam of large vibrational energy.

DNA sequence comparison considering both amino acid and nucleotide insertions/deletions because of evolution and experimental error.

Amino acid similarity often needs to be considered in DNA sequence comparison to elucidate gene functions. We propose a Smith-Waterman-like algorithm which considers amino acid similarity and insertions/deletions in sequences at the DNA level and at the protein level in a hybrid manner. The algorithm is applied to cDNA sequences of Oryza sativa and those of Arabidopsis thaliana. The results are compared with the results of application of NCBIs tblastx program (which compares the sequences in the BLAST manner after translation). It is shown that the present algorithm is very helpful in discovering nucleotide insertions/deletions originating from experimental errors as well as amino acid insertions/deletions due to evolutionary reasons.

Oxidation of silicon with a hot oxygen beam

The oxidation of Si (100) surface with a hot oxygen beam is demonstrated at low substrate temperatures. Oxygen molecules flow through a furnace tube then expand into a vacuum where the beam strikes the Si substrate. The oxidation rate can be controlled to a very slow rate (∼0.1 nm/min) by controlling the thermal dissociation ratio of O2. The oxidizing species that migrates through the oxide is O−. The density of Pb centers at the Si (111)‐SiO2 interface measured by electron spin resonance is ∼1.5×1012 cm−2.

Anisotropic Etching of Heavily Doped Polysilicon by a Hot Cl 2 Molecular Beam

Anisotropic etching of n+ poly-Si is achieved using a hot Cl 2 molecular beam and a sidewall protection technique. A hot molecular beam is produced by a free jet expansion of a gas heated in a furnace. A nitrogen radical beam is used to prevent the sidewall etching. The etch rate of n+ poly-Si is 4.3 nm/min at the anisotropic etching condition.