This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-366454 filed on Dec. 24, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device on an SOI (Silicon on Insulator) substrate, and more particularly to a method of forming an oxidized film on a silicon layer of the SOI substrate.
2. Description of Related Art
In a conventional method of manufacturing capacitors or MOS transistors on a silicon substrate, an impurity is diffused into the silicon substrate in a high density to form a highly doped layer, and then an oxidized film is formed on the highly doped layer. Electrodes made of poly-silicon or the like are formed on the silicon dioxide film.
It is frequently required that such a silicon dioxide film has a high quality to be durable against a high voltage. However, such a high quality is difficult to be realized when the oxidized film is formed on the highly doped layer. This is because the highly doped layer tends to become a gettering site and the silicon dioxide film is contaminated with contaminants such as heavy metals, and the durability of the silicon dioxide film is adversely affected by the contaminants.
It is possible to form an additional gettering layer in the silicon substrate to improve the quality of the oxidized film. However, it is generally difficult to form such a gettering layer in the SOI substrate. JP-A-10-25621 proposes to form the gettering layer in the SOI substrate, but at least an additional costly process to form the same is required in a series of manufacturing processes of a semiconductor device.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide a method of forming a quality oxidized film on the SOI substrate without using the process of forming the gettering layer.
An impurity such as boron or phosphorus is diffused into a silicon layer on an SCI substrate by ion-implantation and driven-in, thereby forming a diffused layer having a high density of the impurity on the SOI substrate. Then, an oxidized film is formed on the diffused layer by thermal oxidation. The oxidized film is heat-treated at a high temperature of 1100xc2x0 C. or higher, preferably 1150xc2x0 C. or higher. When boron is diffused to form the diffused layer, the high temperature heat treatment is performed for 20 minutes or longer. When phosphorus is diffused, the high temperature heat treatment is performed for a period of 20 minutes or longer but not exceeding 60 minutes.
By applying the high temperature heat treatment to the oxidized film, contamination in the oxidized film is driven-out, or annealed-out. Thus, the quality of the oxidized film such as durability against a high voltage is improved.
To form a capacitor using the oxidized film, electrodes made of a material such as poly-silicon are patterned on the oxidized film. Other semiconductor elements such as an LDMOS (Lateral Diffused MOS) may be formed on the same SOI substrate in addition to the capacitor. In this case, the high temperature heat treatment can be performed as a common process for improving the quality of the oxidized film and for forming other semiconductor elements.
Further, plural capacitors, e.g., a first capacitor and a second capacitor may be formed using the oxidized films having respectively different thickness. In this case, a preliminary oxidized film covering both areas for the first and second capacitors is first formed. Then, only the oxidized film covering the second capacitor area is removed by wet-etching, keeping the preliminary oxidized film for the first capacitor. Then, the silicon layer is again thermally oxidized to form a new oxidized film covering both areas for the first and second capacitors. Thus, a thin film is formed for the second capacitor and a thick film is formed for the first capacitor. The high temperature heat treatment is performed either immediately after both oxidized films are formed or after the capacitor electrodes are formed on the oxidized films. The quality of both oxidized films is improved by a single high temperature heat treatment.
According to the present invention, the quality of the oxidized film or films formed on the SOI substrate is improved by the high temperature heat treatment without forming an additional gettering layer.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.