JPH0529625A - Method for forming gate oxide film and field effect transistor - Google Patents

Abstract

PURPOSE:To prevent the formation of a weak spot in the entire gate oxide films by depositing the upper gate oxide film on the lower gate oxide film which is formed by thermal oxidation of the surface side of a polysilicon film that is deposited on a substrate. CONSTITUTION:A polysilicon film 2 is deposited on the upper surface of a substrate 1. Then, the surface side of the polysilicon 2 undergoes thermal oxidation. A semiconductor thin film 3 is formed of the polysilicon film 2 which is not thermally oxidized. A lower gate oxide film 4 comprising silicon oxide formed by thermal oxidation of the upper surface side of the polysilicon is formed on the surface of the semiconductor film 3. Then, an upper gate oxide film 5 comprising silicon oxide is deposited by sputtering method on the surface of the lower gate oxide film 4. Thereafter, the upper gate film 5 is made dense by annealing. In this way, the formation of a weak spot in the entire gate oxide films can be prevented almost perfectly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a gate oxide film and a field effect transistor.

[0002]

2. Description of the Related Art When a gate oxide film is formed in a field effect transistor, the surface side of a polysilicon film deposited on a substrate made of glass or the like is thermally oxidized, so that a polysilicon film which is not thermally oxidized is used to form a semiconductor thin film. And a gate oxide film made of silicon oxide is formed on the surface of the semiconductor thin film.

[0003]

However, in the conventional method of forming a gate oxide film in such a field effect transistor, since unevenness due to crystal grain boundaries is formed on the surface of the polysilicon film, this unevenness is generated. There is a problem that the gate oxide film does not grow uniformly even if the surface side of the substrate is thermally oxidized and a weak spot is generated, which increases the threshold voltage and the gate leak current. An object of the present invention is to provide a method for forming a gate oxide film and a field effect transistor capable of almost completely preventing a weak spot from occurring in the gate oxide film.

[0004]

According to the present invention, a surface of a polysilicon film deposited on a substrate is thermally oxidized to form a semiconductor thin film by the polysilicon film which is not thermally oxidized, and the semiconductor thin film is also formed. A lower layer gate oxide film made of silicon oxide is formed on the surface of, and an upper layer gate oxide film made of silicon oxide is deposited on the surface of the lower layer gate oxide film.

[0005]

According to the present invention, since the upper gate oxide film is deposited on the lower gate oxide film formed by thermally oxidizing the surface side of the polysilicon film deposited on the substrate, the lower gate oxide film and the upper layer are formed. The gate oxide film becomes structurally independent from each other, and even if a weak spot due to a crystal grain boundary on the surface of the polysilicon film occurs in the lower gate oxide film formed by thermally oxidizing the surface side of the polysilicon film, This weak spot can be terminated at the interface with the upper gate oxide film, so that the weak spot can be almost completely prevented from being formed on the entire gate oxide film.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the essential parts of a field effect transistor to which an embodiment of the present invention is applied. When manufacturing this field effect transistor, first, LP is formed on the upper surface of the substrate 1 made of glass or the like at a temperature of about 620 ° C.
A polysilicon film 2 is deposited by the CVD method. Next, when the surface side of the polysilicon film 2 is thermally oxidized in an atmosphere at a temperature of about 1000 to 1100 ° C., a semiconductor thin film 3 is formed by the polysilicon film 2 which is not thermally oxidized, and a polysilicon film is formed on the surface of the semiconductor thin film 3. A lower gate oxide film 4 made of silicon oxide is formed by thermally oxidizing the surface side of the silicon film 2. In this case, the thickness of the lower gate oxide film 4 is 9
It should be about 00Å. Next, an upper gate oxide film 5 made of silicon oxide is deposited on the surface of the lower gate oxide film 4 by a sputtering method to a thickness of about 300 Å. Next, the upper gate oxide film 5 is densified by performing an annealing process. In this state, since the lower gate oxide film 4 and the upper gate oxide film 5 are structurally independent from each other, the polysilicon film 2 is formed on the lower gate oxide film 4 formed by thermally oxidizing the surface side of the polysilicon film 2. Even if a weak spot is generated due to a grain boundary on the surface of, the weak spot can be terminated at the interface with the upper gate oxide film 5, so that the weak spot is almost entirely generated in the gate oxide film. Can be completely prevented.

Although not shown in the drawings, a gate electrode made of polysilicon is patterned on the upper surface of the upper gate oxide film 5 corresponding to the channel region of the semiconductor thin film 3, and then the ion implantation apparatus is used with the gate electrode as a mask. By implanting ions such as phosphorous ions into the semiconductor thin film 3 on both sides of the gate electrode, the source / drain regions having a high impurity concentration are formed, and then the upper gate oxide film 5 and the lower gate corresponding to the source / drain regions are formed. The oxide film 4 is etched to form a contact hole, and then a source / drain electrode made of aluminum connected to the source / drain region through the contact hole is patterned on the upper surface of the upper gate oxide film 5. Field effect transistor is manufactured.

By the way, the field effect transistor of the present embodiment manufactured in this way (hereinafter referred to as the product of this embodiment) and the conventional field effect having a gate oxide film having a thickness of about 1200 Å only by thermal oxidation. Type transistor (hereinafter,
Providing a conventional product called) and, V _G (gate voltage) -I
_{When an} experiment was conducted on the _D (drain current) characteristic and the gate leakage current characteristic, the results shown in FIGS. 2 and 3 were obtained. That is, for the V _G -I _D characteristic, as shown in FIG. 2, the rising characteristics of V _G when I _D changes from 1nA (1/10 ⁹ A) to 10nA (1/10 ⁸ A) solid Is 0.25 V in the case of the product of the present example shown by, whereas it is 0.6 V in the case of the conventional product shown by the dotted line, and the product of the present example has a quicker rise. Therefore, the threshold voltage of the product of this embodiment can be made smaller than that of the conventional product. For the gate leakage current characteristics, see FIG.
The maximum allowable gate leakage current is 10 pA
When it is set to about (1/10 ¹¹ A), the gate voltage of about 25 V can be applied in the case of the plurality of products of the present embodiment shown by the solid line, whereas only about 15 V can be applied in the case of the conventional product shown by the dotted line. . Therefore, the gate leakage current of the product of this embodiment can be reduced as compared with the conventional product.

[0009]

As described above, according to the present invention, the upper gate oxide film is deposited on the lower gate oxide film formed by thermally oxidizing the surface side of the polysilicon film deposited on the substrate. Therefore, the lower gate oxide film and the upper gate oxide film are structurally independent from each other, and the lower gate oxide film formed by thermally oxidizing the surface side of the polysilicon film is caused by the grain boundary of the surface of the polysilicon film. Even if a weak spot occurs, the weak spot can be terminated at the interface with the upper gate oxide film, and thus it is possible to almost completely prevent the weak spot from being formed in the entire gate oxide film. The threshold voltage can be reduced and the gate leakage current can be reduced.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of a field effect transistor to which an embodiment of the present invention is applied.

FIG. 2 shows the V _G -I _D characteristic of the field effect transistor and a conventional field effect transistor.

FIG. 3 is a diagram showing gate leakage current characteristics of this field effect transistor and a conventional field effect transistor.

[Explanation of symbols]

1 substrate 2 Polysilicon film 3 Semiconductor thin film 4 Lower gate oxide film 5 Upper gate oxide film

Claims (4)

[Claims] 1. A surface layer of a polysilicon film deposited on a substrate is thermally oxidized to form a semiconductor thin film by the polysilicon film which is not thermally oxidized, and a lower layer made of silicon oxide is formed on the surface of the semiconductor thin film. A method for forming a gate oxide film, which comprises forming a gate oxide film and depositing an upper gate oxide film made of silicon oxide on the surface of the lower gate oxide film. 2. The method for forming a gate oxide film according to claim 1, wherein the upper gate oxide film is deposited by a sputtering method. 3. The method for forming a gate oxide film according to claim 1, wherein an annealing process is performed after depositing the upper gate oxide film. 4. A field-effect transistor comprising a gate oxide film formed by the method according to claim 1.