Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – By reaction with substrate
Reexamination Certificate
1998-06-17
2001-07-24
Dang, Trung (Department: 2823)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
By reaction with substrate
C438S765000, C438S771000, C438S772000, C438S775000, C438S117000, C438S961000
Reexamination Certificate
active
06265327
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming an insulating film on the surface of a semiconductor substrate and to an apparatus for carrying out the method.
2. Description of the Related Art
Conventionally, silicon oxynitride films are used as gate insulation films and capacitor insulating films for semiconductor devices, particularly, when they are silicon devices, MOS (Metal Oxide Semiconductor) transistors and MOS capacitors. These insulating films must have a high dielectric breakdown voltage and a high dielectric breakdown charge amount. A wafer cleaning process plays an important role in attainment of the requirement, as wafers must be properly cleaned and have a low fixed electric charge density and a low interface state density.
Along with a recent tendency to reduce the geometry and increase integration of semiconductor device circuits, gate insulating films and capacitor insulating films are becoming thinner. For example, under the design rule of 0.1 &mgr;m or less, gate insulating films must be as thin as 3 nm or less.
According to a conventional method for forming gate insulating films of MOS transistors, a semiconductor substrate is exposed to an atmosphere of dinitrogen monoxide (N
2
O) or nitrogen monoxide (NO) at a high temperature of about 1000° C. Alternatively, a wafer is heated to a temperature of about 700° C. in an ammonia atmosphere.
Also, conventional methods for forming oxynitride films at low temperatures include the following: thermal oxynitridation is performed while ultraviolet rays are radiated; and silicon is directly nitrided through exposure to nitrogen compound plasma or nitrogen gas plasma. However, these methods fail to form thin high-quality oxynitride films with good controllability and reproducibility.
Conventionally practiced thermal oxynitridation using N
2
O gas has involved the following problems: heating at high temperatures is required; the amount of nitrogen atoms incorporated into a formed oxynitride film is relatively small; and the quality of a silicon dioxide film is not sufficiently improved. According to conventionally practiced thermal oxynitridation using NO gas, a heating temperature is as low as about 900° C., and the amount of nitrogen atoms incorporated into a formed oxynitride film increases somewhat; however, the method has involved the problem that the thickness of a formed oxynitride film cannot be made greater than a certain level. Conventionally practiced thermal oxynitridation using ammonia gas has involved the following problem. A formed oxynitride film contains a large amount of hydrogen, which serves as an electron trap, causing an impairment in film quality. Thus, in order to eliminate hydrogen, after an oxynitride film is formed, the film must be heated to a temperature of about 1000° C. or must be oxidized.
Also, conventionally practiced direct oxynitridation using plasma has involved the problem that film quality is impaired due to plasma damage. Particularly, the generation of interface state not only impairs hot-carrier properties of a transistor but also causes an unstable threshold voltage of a transistor and impaired mobility of carriers, which induce a fatal problem for, particularly, fine-patterned devices.
Further, the fine patterning of an element requires a reduction in thermal treatment temperature. Accordingly, high-temperature heating has raised the problems of dopant diffusion and defect generation. In RF-plasma-activated oxynitridation of a silicon dioxide film, the use of NH
3
plasma enables a relatively large amount of nitrogen atoms to be incorporated into the film, but causes a relatively large amount of hydrogen atoms to also be incorporated into the film. As a result, impairment in film quality is involved.
Also, the use of N
2
plasma has involved insufficient improvement of film quality, since the amount of nitrogen atoms incorporated into a film is relatively small. (Refer to, for example, P. Fazan, M. Dutoit and M. Ilegems, “Applied Surface Science” Vol. 30, p. 224, 1987.)
SUMMARY OF THE INVENTION
The present invention has been accomplished to solve the above-mentioned problems involved in conventional methods for forming an insulating film into which nitrogen atoms are incorporated. An object of the present invention is to provide a method for forming on the surface of a semiconductor substrate a high-quality insulating film into which a large amount of nitrogen atoms are incorporated, without the use of high-temperature heating and with good controllability.
To achieve the above object, the present invention provides a method for forming an insulating film on the surface of a semiconductor substrate, comprising a step of exposing to plasma generated by electron impact an insulating film deposited on the surface of the semiconductor substrate. Through exposure to plasma, the insulating film is modified. Preferably, an adequate voltage is applied between an electron source or a grid electrode and the semiconductor substrate so as to prevent the occurrence of a charge-up effect on the insulating film during exposure to plasma.
Preferably, the insulating film to be exposed to plasma is a silicon dioxide film having a thickness of 1-20 nm. This thickness range provides an appropriate final film thickness to ultra-thin gate insulating films and capacitor insulating films of MIS transistors and MIS capacitors. The silicon dioxide film is deposited on a substrate through thermal oxidation, chemical vapor-phase growth, chemical oxidation, physical vapor-phase growth, plasma-assisted chemical vapor-phase growth, or the like.
Preferably, the semiconductor substrate is formed of at least a single material selected from the group consisting of single crystalline silicon, polycrystalline silicon, amorphous silicon, gallium arsenide, indium phosphide, silicon carbide, silicon germanium, and silicon germanium carbide. These materials expand the range of applications of semiconductor substrates formed thereof.
Preferably, plasma is generated by electron impact on a single gas selected from the group consisting of the following A through H:
A. Nitrogen gas
B. N
2
O gas
C. NO gas
D. Ammonia gas
E. The mixture of two or more gases of A to D
F. The mixture of any of A to D and an inert gas such as argon, neon, or the like
G. The mixture of any of A to D and dry oxygen
H. The mixture of any of A to D and steam-containing oxygen
Any of the above gases A to H are suited for modifying, through nitridation, a silicon dioxide film deposited on, for example, a silicon substrate of a semiconductor.
Preferably, exposure to plasma is performed while a heat treatment temperature is maintained within the range of 0 °C. to 700° C. Low-temperature oxynitridation enables the achievement of the object of the present invention.
According to the above-mentioned method of the present invention, an insulating film having a thickness of 1-20 nm is deposited on a semiconductor substrate. Subsequently, the thus-deposited insulating film is exposed to plasma generated by electron impact while semiconductor substrate temperature is maintained at 700° C. or lower. As a result, an insulating film having uniformly high quality can be formed on a semiconductor substrate in an efficient, rational manner and with good controllability.
In the thus-formed insulating film, nitrogen atoms are contained at a relatively high concentration near the surface of the film and near the interface between the insulating film and the semiconductor substrate. Nitrogen atoms contained near the interface improve interface properties and enable the formation of a high-quality insulating film having a low interface state density.
Further, nitrogen atoms contained in the thus-formed insulating film near the surface improve surface properties of the insulating film. Thus, the insulating film becomes sufficiently fine against diffusion thereinto of impurities, such as boron.
The quality of an insulating film formed in accordance with the present invention depends on a method of depositing a
Kobayashi Hikaru
Yoneda Kenji
Dang Trung
Japan Science and Technology Corp.
Jones Tullar & Cooper PC
Kebede Brook
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