Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – Multiple layers
Reexamination Certificate
2001-02-09
2003-04-01
Ghyka, Alexander (Department: 2812)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
Multiple layers
C438S275000, C438S286000, C438S773000, C438S774000
Reexamination Certificate
active
06541393
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for fabricating a semiconductor device, more specifically to a method for fabricating a semiconductor device comprising gate insulation films having different film thicknesses from each other.
In recent semiconductor devices, gate insulation films have different film thicknesses from each other for improved device characteristics, etc. In DRAM, for example, it is preferable for improved operation speed to form, as the peripheral circuit transistors, transistors having the gate insulation film made thinner. On the other hand, it is preferable that the memory cell transistors have the gate insulation film made thicker than the peripheral circuit transistors, because the memory cell transistors having the gate insulation film made as thin as the peripheral circuit transistors have too low threshold voltage, which deteriorates controllability and refresh characteristics. In non-volatile semiconductor devices, such as EEPROM, flash EEPROM, etc., in addition to the above-described requirement for the peripheral circuit transistors and the memory cell transistors, transistors having the gate insulation film which is thicker than the transistors forming the memory cell transistors and logics of the peripheral circuits are required as high breakdown voltage transistors used in writing/erasing.
Conventional techniques for forming gate insulation films having different film thickness from each other are a technique wherein a silicon oxide film is formed uniformly on an entire surface and removed in a region, and then additionally oxidized to thereby provide a difference in film thickness between the region for the silicon oxide film removed and the rest region, and techniques using enhanced oxidation and retarded oxidation by ion implantation. It is preferable from the viewpoint of throughputs to use the techniques using enhanced oxidation and retarded oxidation by ion implantation.
In the techniques using ion implantation, it has been proposed that nitrogen ions are implanted in a silicon substrate before a gate insulation film is formed to thereby suppress the following oxidation (retarded oxidation), and argon ions are implanted in a silicon substrate before a gate insulation film is formed to thereby enhance the following oxidation (enhanced oxidation). In the specification of laid-open Japanese Patent Application No. Hei 11-260813/1999 and the specification of Japanese Patent No. 2950101, a technique wherein fluorine ions are implanted in a silicon substrate before a gate insulation film is formed to thereby enhance the following oxidation is proposed. Such ion implantation is performed selectively in a specific region, whereby a gate insulation film of silicon oxide film which is thicker or thinner in an ion-implanted region than in the rest region can be formed.
Thus, by the conventional method for fabricating a semiconductor device, wherein the gate insulation film is formed by using the enhanced oxidation or retarded oxidation by ion implantation, the gate insulation films having different film thicknesses from each other can be formed by one thermal oxidation step.
However, the conventional semiconductor device fabrication method using the retarded oxidation by nitrogen ion implantation has often degraded reliability of the gate insulation film. The conventional semiconductor device fabrication method using the enhanced oxidation by argon ion implantation has often increased gate leak current. The conventional semiconductor device fabrication method using argon ion implantation produces a relatively small film thickness difference of about 10% between a region with ions implanted and a region without the ion implantation. A technique for ensuring larger film thickness differences has been required.
Usually, wet oxidation film is more reliable than dry oxidation film, and the oxidation technique for forming a gate insulation film is preferably wet oxidation. However, in a case that the above-described method uses wet oxidation, the effect of the enhanced oxidation by ion implantation is much suppressed, and the merit of the ion implantation has not been produced. Accordingly, dry oxidation has been used for the oxidation for the enhanced oxidation, and the gate insulation film of high quality which is comparable to that of the wet oxidation film has not been produced.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor device fabrication method which can form gate insulation films having different film thicknesses from each other while retaining sufficient reliability and sufficient film thickness difference.
A first method for fabricating a semiconductor device according to the present invention is characterized mainly in that halogen ions are implanted before the thermal oxidation for forming a gate insulation film, and is also characterized in that wet oxidation under low pressure is applied to forming the gate insulation film.
Fluorine, which is one of VII-group elements (halogen), is added to a silicon oxide film in a suitable amount to thereby improve reliability thereof. Accordingly, the oxidation is enhanced by fluorine ion implantation, whereby reliability of the gate insulation film can be improved, and the gate insulation films having different film thicknesses from each other can be formed by one oxidation step. However, as described above, dry oxidation is used for the oxidation for the enhanced oxidation, and no silicon oxide film of good quality which is comparable to wet oxidation film can be formed.
In such circumstances, the inventors of the present application have made earnest studies and found for the first time that wet oxidation under low pressure or in an atmosphere of nitrogen or diluted with rare gas applied to forming a gate insulation film is very effectively for producing the effect of the enhanced oxidation.
The effect of the enhanced oxidation by the ion implantation is conspicuous in dry oxidation but is not in wet oxidation. This will be due to oxidizability difference between the two. That is, wet oxidation, which is more oxidizable than dry oxidation, advances the oxidation reaction so rapidly that an implanted element cannot affect the mechanism. Then, the inventors of the present application had an idea that oxidizability of wet oxidation is reduced so as to delay the oxidation reaction, whereby the enhanced oxidation effect by the ion implantation is allowed to be sufficiently exerted, and tested wet oxidation under low pressure or in an atmosphere of nitrogen or diluted with rare gas.
As a result, wet oxidation film could be formed without much suppressing the effect of the enhanced oxidation by fluorine ion implantation. Especially by suitably controlling conditions for the fluorine ion implantation, silicon oxide film can be made more reliable than that formed without the fluorine ion implantation.
It is preferable that the wet oxidation is conducted in an ambient atmosphere which an H
2
O partial pressure is less than 1 atm. A low pressure oxidation and a dilute oxidation may be applicable to such the wet oxidation. The low pressure wet oxidation used in this specification is wet oxidation made under low pressure, and a pressure in a film forming chamber is set to be, e.g., 1-400 Torr. The same effect can be produced by dilution with nitrogen, rare gas, such as argon, etc., or inactive gas so that an H
2
O partial pressure becomes less than 1 atm to prepare a steam partial pressure equivalent to the low pressure. It is possible that nitrogen, rare gas, such as argon, etc., or inactive gas is used under low pressure so as to use the synergetic effect. The diluent gases are not limited to rare gas or inactive gas. It is possible that oxygen or hydrogen may be also used as diluent gas. These gases have an effect of lowering the oxidation rate. It is possible that other additives, e.g., hydrogen chloride (HCl) may be incorporated in the atmosphere for the end of improving film quality of silicon oxide film, and other ends.
Wet oxidati
Murakoshi Atsushi
Nakanishi Toshiro
Sugizaki Taro
Suguro Kyoichi
Armstrong Westerman & Hattori, LLP
Fujitsu Limited
Ghyka Alexander
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