JPH06216375A - Manufacture of semiconductor device - Google Patents

Abstract

PURPOSE:To weaken the inverse short channel effect that the threshold value of an MOSFET once increases as the channel length is made shorter, when the MOS type transistor is finely miniaturized. CONSTITUTION:After the material for a gate electrode 3 of an MOS type semiconductor device is patterned and formed, a process for forming a polycrystalline silicon thin film 4 is introduced, and only the formed polycrystalline silicon thin film 4 is subjected to oxidation treatment. In the case of re-oxidation treatment of the polycrystalline silicon thin film 4 deposited on a single crystal, diffusion acceleration of impurities which is generated at the time of oxidizing single crystal silicon is not generated. Hence the nonuniformity of impurity distribution in a channel which is caused by oxidation acceleration diffusion at the gate end and ordinary diffusion just under the gate, and which has been a problem in the conventional method can be restrained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a semiconductor device which can be further miniaturized while maintaining its performance, which is a MOS type transistor used in an integrated circuit.

[0002]

2. Description of the Related Art It is essential for semiconductor devices used in large-scale integrated circuits to be as fine as possible in order to increase the degree of integration. Although the degree of integration can be improved by miniaturizing each element, MOS field effect transistors (MO
If the SFET) is miniaturized, that is, the channel length of the MOSFET of a certain design rule is reduced, a short channel effect in which the threshold value of the MOSFET falls below the design value and an inverse short channel effect in which the threshold value rises are associated with it. It becomes apparent. In particular, the case where the threshold value rises and then drops sharply as the channel length decreases is called the reverse short channel effect, and the element size is reduced while maintaining the long channel transistor characteristics. Making design difficult.

[0003]

The reverse short channel effect of the MOSFET is that the impurity concentration of the channel portion near the gate electrode end is different from that near the center of the gate electrode, and when the channel length is long, the influence near the gate end is affected. Is small,
This occurs because the influence near the edge of the gate electrode becomes stronger as the channel length becomes shorter. The cause of the difference in the impurity distribution in the channel portion between the gate electrode end and the central portion is the acceleration of diffusion of impurities in the semiconductor that occurs when the semiconductor surface is oxidized. In FIG. 2, the gate oxide film 2 is formed on the entire surface of the channel portion of the single crystal semiconductor substrate 1, and the gate electrode 3 is patterned and formed on the semiconductor substrate 1 covered with the oxide film 2. The cross-sectional state after performing the step according to the conventional method of forming the oxide film 5 is shown. That is, the diffusion of impurities in the substrate semiconductor is accelerated due to the oxidation on the surface of the semiconductor substrate 1 not covered by the gate electrode 3 (region 7 in FIG. 2), but the substrate semiconductor below the gate electrode 3 is increased. Since the oxidation of the impurities is masked by the electrode, normal diffusion is performed at the temperature during the oxidation (region 6 in FIG. 2). The distribution of impurities here will be described below.

In the conventional MOSFET manufacturing process,
Impurities are ion-implanted into the channel region for controlling the threshold value. The distribution of the impurities in the depth direction is low on the surface side and has a maximum value in the substrate, such as a Gaussian distribution. Therefore, when the impurities diffuse, such a distribution becomes gentle and the concentration increases on the surface side. This outline is shown in FIG. In FIG. 3, distribution 8 is an impurity distribution after ion implantation and before oxidation, distribution 9 is an impurity distribution after heat treatment, and distribution 10 is an impurity distribution when oxidation enhanced diffusion occurs during heat treatment. . Due to the accelerated diffusion due to oxidation, the increase in the surface concentration near the channel edge is larger than that near the channel center, which is less susceptible to oxidation.
The local threshold rises.

In the short-channel MOSFET, the local increase in threshold value occupies a large weight for the entire channel, so that there is a problem that the threshold value deviates from the design value and the design becomes difficult. It has turned into.

An object of the present invention is to provide a method of manufacturing a semiconductor device which suppresses the reverse short channel effect which occurs when the element size is reduced.

[0007]

To achieve the above object, a method of manufacturing a semiconductor device according to the present invention is a method of manufacturing a semiconductor device including a gate electrode forming step and a reoxidation step. The forming step is a step of patterning a material to be a gate electrode of the MOS type semiconductor device on the substrate to form a gate electrode on the substrate, and the reoxidation step is a step of forming a substrate including the patterned gate electrode. In this step, a silicon thin film is formed so as to cover the entire surface, and only the silicon thin film is oxidized to reoxidize the silicon thin film into an oxide film.

The silicon thin film is polycrystalline silicon or amorphous silicon.

[0009]

Next, the operation of the present invention will be described. 1A to 1D are cross-sectional views showing a method of manufacturing a semiconductor device according to the present invention in the order of steps. First, in FIG. 1A, a gate oxide film 2 is selectively formed on a single crystal semiconductor substrate 1, and a gate electrode 3 is provided on the gate oxide film 2.

Thereafter, as shown in FIG. 1B, a thin film of polycrystalline silicon 4 is formed on the semiconductor substrate 1 and the gate electrode 3.

Next, as shown in FIG. 1C, the polycrystalline silicon 4 is oxidized by a thickness corresponding to the thickness of the polycrystalline silicon 4 to re-oxidize the polycrystalline silicon 4 into an oxide film 5.

Here, accelerated diffusion of impurities by oxidation will be described. Diffusion in single crystal semiconductors is mainly caused by point defects in the crystal. Oxidation of the surface of the single crystal semiconductor proceeds by the oxidation reaction at the interface between the oxide film and the crystal,
At that time, the reaction causes a point defect, and a part of the point defect flows into the semiconductor substrate, and as a result, diffusion of impurities is accelerated.

However, when polycrystalline silicon is formed on single crystal silicon and is oxidized, an enhanced diffusion phenomenon due to oxidation is hardly found in the impurity distribution in the lower single crystal silicon. It is reported as an experimental fact. The reason for this is that polycrystalline silicon is composed of minute crystal grains, which are almost single crystals, and crystal grain boundaries between them. In particular, the crystal grain boundaries have a vague atomic structure, so that they are generated by an oxidation reaction. This is because the defects are absorbed in the grain boundaries in the polycrystalline silicon and do not flow out to the underlying single crystal silicon.
Has been speculated.

Therefore, as in the present invention, if the polycrystalline silicon film is thinly formed before the reoxidation step after the gate electrode is formed and then the oxidation is performed, the oxidation occurs on the polycrystalline silicon film. The diffusion speed of impurities in the base single crystal semiconductor does not increase, and the impurity diffusion state in the channel region can be made uniform. After the polycrystalline silicon film is completely converted into an oxide film by oxidation and disappears, accelerated diffusion starts to occur, so that if the reoxidation is finished here, the reverse short channel effect can be suppressed.

Further, this thin polycrystalline silicon film is made sufficiently thin so that a desired reoxidized film thickness can be obtained, and eventually it is entirely converted into an oxide film by reoxidation. The conventional structure can be unchanged in appearance from the conventional manufacturing method.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A typical embodiment of the present invention relating to a method for forming a semiconductor device will be described below with reference to the drawings. FIGS. 1A to 1C also correspond to cross-sectional views in each process in one embodiment. As shown in FIG. 1A, the surface of the single crystal silicon substrate 1 is oxidized in an oxygen atmosphere to grow a silicon dioxide film (hereinafter, simply referred to as an oxide film) 2 having a thickness of 100 Å, and then a gate is formed. Polycrystalline silicon to be the electrode 3 was deposited on the entire surface of the silicon substrate 1 to a thickness of 0.5 μm by the low pressure chemical vapor deposition method (LPCVD method), and then the gate electrode portion was patterned by the dry etching method.

Next, as shown in FIG. 1 (b), a polycrystalline silicon thin film 4 was again formed on the entire surface of the silicon substrate 1 by LPCVD to a thickness of 500 angstroms. The film forming temperature was about 550 ° C., and the average particle size was about 20 Å, which was sufficiently smaller than the film thickness.

Next, this is dried in a dry oxidizing atmosphere at 900
Oxidation treatment was performed at 30 ° C. for 30 minutes. This is a condition in which the polycrystalline silicon thin film 4 having a thickness of 500 Å is completely oxidized in advance. By this oxidation treatment, the polycrystalline silicon thin film 4 was reoxidized to the gate reoxidized film 5.

Although the polycrystalline silicon thin film 4 is used in the embodiment, the same effect can be obtained by using an amorphous silicon thin film instead of the polycrystalline silicon thin film 4.

[0020]

As described above, according to the semiconductor device manufacturing method of the present invention, before the gate reoxidation step, a step of introducing a polycrystalline silicon thin film and oxidizing it through the gate reoxidation polycrystalline silicon thin film is performed. This has the effect of suppressing accelerated diffusion of the impurity distribution in the underlying silicon substrate due to oxidation, realizing a uniform impurity distribution in the channel region, and suppressing the reverse short channel effect. Further, since the extra polycrystalline silicon thin film may be sufficiently thin as compared with the conventional method, it does not affect the subsequent steps due to being completely oxidized.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a cross-sectional shape in a series of gate electrode forming steps in the process of manufacturing a MOSFET formed according to the present invention.

FIG. 2 is a diagram for explaining oxidation in a gate electrode formation process by a conventional method and its influence on impurities (oxidation enhanced diffusion).

FIG. 3 is a diagram for explaining how the depthwise distribution of impurities in a channel region changes due to diffusion during oxidation.

[Explanation of symbols]

1 Single Crystal Silicon Substrate 2 Gate Oxide Film 3 Gate Electrode 4 Polycrystalline Silicon Thin Film 5 Reoxidation Film 6 Normal Diffusion Region 7 Oxidation Enhanced Diffusion Region 8 Impurity Concentration Before Oxidation Depth Distribution 9 Oxidation Enhanced Diffusion Impurity concentration depth direction distribution after heat treatment 10 Impurity concentration depth direction distribution after heat treatment when there is oxidation enhanced diffusion

Claims (2)

[Claims] 1. A method of manufacturing a semiconductor device comprising a gate electrode forming step and a reoxidation step, wherein the gate electrode forming step comprises patterning a material to be a gate electrode of a MOS type semiconductor device on a substrate, The step of forming a gate electrode on the substrate, the re-oxidation step, by forming a silicon thin film to cover the entire surface of the substrate including the patterned gate electrode, by oxidizing only the silicon thin film, A method of manufacturing a semiconductor device, comprising a step of reoxidizing the silicon thin film into an oxide film. 2. The method of manufacturing a semiconductor device according to claim 1, wherein the silicon thin film is polycrystalline silicon or amorphous silicon.