JPS60158669A - Semiconductor device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] 1 Technical field 1.

The present invention relates to improvement 2 of MO8 field effect transistors.

[Prior Art 1] Generally, in a conventional IVI OS field effect transistor, a gate oxide film has a constant thickness over the entire gate fi region.

FIG. 1 is a cross-sectional view showing this kind of conventionally used MO8 type field effect transistor, in which a silicon substrate 1 is provided with a source 2. A drain 3 is formed, a gate oxide film 4 having a constant thickness is formed over the entire region on the substrate 1, and a gate electrode 5 is formed on the gate oxide film 4.

The source-drain current 1os in the saturation region operation of the MO8 field effect transistor is Ib5=p(%-
VT)" -...(Equation 1) where v4 is the gate voltage and VT is the threshold voltage. In other words, the source-drain current is one of the characteristics of the MO8 field effect transistor as an active device. In order to increase with the gate voltage and threshold voltage constant, it is necessary to increase β.

However, in the conventional MO8 field effect transistor, it was not possible to increase β.

As a result of research to improve this point, the inventor found that the above value of β can be changed by changing the thickness of the gate oxide film at the center of the gate and the thickness of the gate oxide film at both ends of the gate. .

[Object of the Invention 1] An object of the present invention is to provide a semiconductor device that can easily increase the current between the source and drain in saturation region operation.

[Example 1] FIG. 2 is a cross-sectional view showing an MO8 field effect transistor of the present invention. The difference from the conventional type (FIG. 1) is that the gate oxide film 4 is formed at both ends 4a of the gate, and at the central portion 4b of the gate. The point is that it is thicker than the .

Here, both end portions are the portions located outside from the portions which are inward (to the channel side) to some extent from the boundaries between the sources, inputs 2 and 3, and the substrate.

FIG. 3 is a cross-sectional view showing a MOS field effect transistor for explaining the present invention in detail, and the oxide film thickness at the center of the date is indicated by Tox, and the oxide film thickness at both ends is indicated by Tox*.

The relationship between TOX* and β in (Equation 1) is shown in Figure 1.

Here, ゛ is Tox = 400 people, and Tox * = i''
The case where ox''=400 people is a conventional transistor.

As seen in Figure 4, by setting Tox* to a certain optimal value (1000 people in this example), the value of β is approximately 1.
It has been shown that the transistor performance can be improved by 6%. Thickness of both ends of this gate Tox*
This can be achieved by increasing the number of people in the range of approximately 800 to 1,400 people.

Next, the manufacturing process of an example of the present invention will be described. First, a silicon substrate 1 is oxidized to form a gate oxide film 4 having a uniform thickness of, for example, 400 layers, and a polysilicon film 5 doped with impurities as a gate electrode is formed thereon (see FIG. 5).
a)). Next, patterning is performed to form a polysilicon film 5.
V-ping of impurities is performed using the mask as a source,
Drain regions 2 and 3 are formed. (Figure 5(b)). After that, by performing thermal oxidation, the silicon substrate at both ends of the gate 5 is oxidized, and an oxide film grows in both the upper and lower directions.
An MO8 type field effect transistor is obtained in which the film thickness at both end portions 4a of the gate 5 is, for example, 1000 mm thicker than the film thickness at the central portion 4b by 400 mm (FIG. 5(C)). It is possible to control the thermal oxidation conditions to obtain the desired film thickness.

Of course, the polysilicon is protected with a silicon nitride (Si3N) film or the like to prevent it from being oxidized during thermal oxidation.

[Effect 1 As described above, this invention increases β in equation (1) and saturates by making the gate oxide film of the MO3 field effect transistor thicker at both ends of the gate than at the center. This can be achieved by easily increasing the current between the source and drain.

Note that the film thickness Tox+Tox* of the gate oxide film is not limited to the above example.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a conventional MO8 type field effect transistor, FIG. 2 is a sectional view of an embodiment of the MO8 type field effect transistor of the present invention, and FIG. 3 is a cross sectional view for explaining the present invention in detail. 4 is a graph showing the relationship between β and 'I'ox* showing the performance improvement of the MO3 field effect transistor according to the present invention, and FIG. FIG. 2 is a cross-sectional view showing an example of a method for manufacturing an MO8 field effect transistor. 1... Silicon substrate, 4... Date oxide film, 5... Date electrode, 2, 3...
Source and drain, TOX*...Thickness of the gate oxide film at both ends of the date, Tox...Thickness of the gate oxide film at the center of the date. Patent applicant Sharp Co., Ltd. Representative Patent attorney Aohaku Sogai 2 people Figure 4 Tox' IAI

Claims (1)

[Claims] A semiconductor device characterized in that a MO8 type field effect transistor gate oxide film is thicker at both ends of the gate than at the center of the gate.