In silicon integrated circuits and in particular in MOS (Metal Oxide Semiconductor) integrated circuits formed by modern techniques, very thin silicon dioxide films are used as gate insulators. For sub-micron MOS devices which have gate lengths of 1.0 .mu.m or less, the silicon dioxide film may for example be no more than 100 angstroms thick, this reduction in film thickness permitting an increase in gain.
These silicon dioxide films may for example be formed by the following method which is described in "The Manufacture of MOSLSI", by Tokuyama and Hashimoto, published by Nikkei McGraw Hill Co., p. 65 (1985).
According to the method disclosed in this reference, a cleaned substrate is first placed in a quartz tube heated to 800.degree.-1200.degree. C. by an electric furnace, and an oxidizing gas is then introduced into the tube to form a silicon dioxide film on the substrate. This oxidizing gas may for example be dry oxygen, or a mixture of oxygen and hydrogen, or hydrochloric acid spray mixed with oxygen. By leaving the substrate in the quartz tube containing an oxidizing gas at a certain temperature for a time depending on the thickness of the silicon dioxide film it is desired to obtain, it is thus possible to form a film of uniform thickness on the substrate surface.
In the method of forming an insulating film by the method of the above reference, however, it was difficult to control film thickness if it was desired to form a silicon dioxide film of for example no more than 100 angstroms. When forming thin silicon dioxide films by the conventional film forming techniques, therefore, it was necessary to keep the heating temperature of the quartz tube below 800.degree. C. (hereafter, this may be referred to as the low temperature oxidation method), or to dilute the oxygen with nitrogen to decrease the rate of oxidation (hereafter, this may be referred to as the dilution oxidation method).
In the low temperature oxidation method, however, the silicon (substrate)/silicon dioxide film interface obtained is coarser (with silicon atoms protruding and the surface undulating). In the dilution oxidation method, on the other hand, nitrogen segregates at the silicon/silicon dioxide film interface, and a new interface trap state is generated. Further, the silicon dioxide films obtained in the low temperature method or the dilution method are usually not very fine. At the silicon/silicon dioxide interface, there may for example be a large number of unterminated silicon bonds or strained Si--O--Si bonds, and the interface trap state therefore showed a tendency to increase. These phenomena gave rise to various problems when the silicon dioxide films were used as gate insulating films in MOS field effect transistors. In MOS field effect transistors with a gate length of 1 .mu.m or less, for example, if hot electrons produced in the channel penetrated into the silicon dioxide film, the electrons were trapped by the unterminated silicon bonds or strained Si--O--Si bonds, and a new interface trap state was generated. This led to a fluctuation of the threshold voltage, or a decline of the transconductance in MOS field effect transistors. Further, in dielectric breakdown endurance tests of various MOS devices using this kind of silicon dioxide film, new traps were generated in the film due to breaking of the unterminated silicon bonds or strained Si--O--Si bonds, and this caused dielectric breakdown.