Rinshi Sugino

Chemical Structures of Native Oxides Formed during Wet Chemical Treatments

The chemical structures of the native oxides formed during various wet chemical treatments were measured nondestructively by changing the effective electron escape depth. The structures of the native oxides can be characterized by the distribution of suboxide Si3+ in the native oxide films. If Si3+ is correlated with Si-H bonds, the formation rate of the native oxide during the wet chemical treatment and the interface roughness produced by photoexcited dry etching through the oxide can be explained.

Silicon-Hydrogen Bonds in Silicon Native Oxides Formed during Wet Chemical Treatments

Silicon-hydrogen bonds in silicon oxide films were detected for the first time by applying surface-sensitive X-ray photoelectron spectroscopy and were confirmed by measuring infrared absorption. The areal density of silicon-hydrogen bonds in native oxides formed in a hot solution of HNO3 is estimated to be nearly 2×1014 cm-2, and is much larger than that formed in a solution with a composition of NH4OH:H2O2:H2O=1:1.4:4.

Silicon-hydrogen bonds in silicon oxide near the SiO2/Si interface

The role of hydrogen peroxide in RCA standard clean was roughly clarified by angle-resolved photoelectron spectroscopy and infrared absorption spectroscopy such that the oxidation, by basic hydrogen peroxide results in a negligible amount of SiH bonds in native oxide, while the oxidation by acidic hydrogen peroxide results in a small amount of SiH bonds in native oxide. It is also found that oxidation in a boiling solution of HNO3 results in large amount of SiH bonds in native oxide as in the case of oxidation in a hot solution of HNO3.

Wafer Cleaning with Photoexcited Chlorine and Thermal Treatment for High-Quality Silicon Epitaxy

The authors propose a new photoexcited silicon-wafer cleaning technique, in which ultraviolet light irradiates the wafer through chlorine gas, followed by thermal treatment in a hydrogen ambient, to grow a high-quality epitaxial layer. The surface metal contaminants were removed through a thin native oxide without damaging the surface by photoexcited cleaning, and the remaining volatile chloride after the photoexcited cleaning at 150°C was removed by thermal treatment at 900°C. Further thermal treatment at 980°C removed even the thin native oxide and then made it possible to grow an epitaxial silicon layer. The SiO2 film formed using a conventional technique on the epitaxial silicon layer showed improved breakdown fields and a decrease of fixed-charge density. The results mean that a high-quality silicon-epitaxial layer has been realized by the cleaning procedure.

Invited Paper Photochemical Cleaning And Epitaxy Of Si

Photochemical effect on silicon surface cleaning and epitaxial film growth was investigated. Under ultraviolet light irradiation, the surface native oxide on the silicon substrate was removed at 730°C. An epitaxial film with high crystal quality was grown on the silicon substrate at a temperature as low as 540°C after thermal surface cleaning at980°C. The ultraviolet light irradiation seemed to be effective for gas-phase dissociation and surface reaction. Auto-doping was suppressed by using low temperature epitaxial growth and an almost ideal step junction was fabricated. A bipolar transistor having 65 nm epitaxially grown base layer was successfully operated.

Photo-Excited Cleaning of Silicon with Chlorine and Fluorine

Chlorine radicals generated with UV irradiation are effectively used to remove residual metal contaminants after vatious LSI processes. Aluminum, iron, and copper atoms are effectively removed as volatile chloride species from silicon surfaces. The mechanism has been studied using silicon wafers intentionally contaminated with those metal ions. Contaminants are involved in native oxides as metal oxides or hydroxides. Chlorine radicals penetrate the native oxides and attack those oxides or hydroxides vaporizing metal chlorides. The cleaning process is accompanied with slight etching of silicon surfaces to a depth as thin as a few nm. However, this is not essential because the substrate temperature is more important than the etching depth. Alkaline metals are also removed from the surfaces to the level as small as the detection limit of atomic absorption spectroscopy. We believe that those are removed through a lift-off process. The, wixture of fluorine and hydrogen gases removes a native oxide of silicon under UV irradiation. Hydrogen fluoride radicals react with native oxide resulting in hydrogen termination on silicon dangling bonds. These cleaning processes are advantageous for low temperature silicon epitaxy, reliable contact formation, and thin gate oxide growth.

Photo-excited dry cleaning for ULSI devices

The authors studied the reverse current in n/sup +/p junctions after photo-excited cleaning for samples fabricated by RIE. Silicon surfaces of a wafer contaminated during RIE were etched by photo-excited dry cleaning using chlorine, after which the unexpectedly large junction leakage current often occurring after wet cleaning alone was completely suppressed.<<ETX>>

Dry cleaning of Si and SiO/sub 2/ surfaces using SiCl/sub 4/ system

We have found an effective dry cleaning technique using SiCl/sub 4/ as a cleaning gas. Now, for the first time, surface Fe contaminants can be removed from an SiO/sub 2/ surface using SiCl/sub 4/. A Cl/sub 2/+SiCl/sub 4/ combination allows dry cleaning of a Si surface at a lower temperature than the conventional UV/Cl/sub 2/ method. Use of this Cl/sub 2/+SiCl/sub 4/ mixture gave good surface flatness and surface constitution for both Si and SiO/sub 2/ surfaces.