JP2808933B2 - Method for manufacturing semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for processing a silicon substrate before forming a gate oxide film.

[0002]

2. Description of the Related Art A silicon oxide film is used as a gate insulating film.
OS (Metal-Oxide-Semiconductor)
(tor) Used in semiconductor devices. A conventional method of forming a gate oxide film will be described with reference to the drawings, taking the manufacture of a MOS transistor as an example. 4A to 4D are cross-sectional views of a semiconductor chip showing a method of manufacturing a MOS transistor including a conventional method of forming a gate oxide film.

First, as shown in FIG. 4A, a P ⁺ region 2 and a field oxide film 3 are formed on a P-type silicon substrate 1 by a selective oxidation (LOCOS) method. Next, a silicon oxide film (sacrificial oxide film) 4 is formed in the element region by a thermal oxidation method. Next, as shown in FIG. 4B, the oxide film is removed with a solution containing hydrofluoric acid. This oxidation step is performed to remove by oxidation a silicon nitride compound (called a white ribbon) generated at the end of the field oxide film 3 in the selective oxidation step. Next, a gate oxide film 5 is formed in the element region by a thermal oxidation method.

Next, as shown in FIG. 4C, after growing a polysilicon film 6 by a chemical vapor deposition method, phosphorus is diffused into the polysilicon film 6 by a thermal diffusion method. Next, the polysilicon film 6 is patterned by photolithography to form a gate electrode. Next, as shown in FIG. 4D, the source region 7 and the drain region 8 are formed in a self-aligned manner by arsenic ion implantation using the gate electrode as a mask. Hereinafter, a MOS transistor is manufactured by an interlayer and wiring process.

During thermal oxidation for forming a gate oxide film, if metal impurities such as iron remain on the silicon substrate surface,
These impurities are taken into the gate oxide film at the time of thermal oxidation and become defects in the oxide film, which cause initial breakdown voltage failure of the gate oxide film or failure during long-term reliability. Therefore, the surface of the silicon substrate must be kept clean before thermal oxidation for forming a gate oxide film.

As cleaning before thermal oxidation for forming the gate oxide film, a cleaning method generally called RCA cleaning or a modified cleaning method thereof is used. RC
A-cleaning has been proposed by RCA (RC
A Review Vol. 31 pp. 187-205 1970
June). In this washing, a washing solution consisting of ammonia / hydrogen peroxide / water = 1/1/5 (volume ratio) is used as a standard.
° C, the silicon substrate was soaked for 10 minutes, washed and then washed with water, then hydrochloric acid / hydrogen peroxide / water = 1/1/5 (volume ratio)
This is a cleaning method in which the substrate is immersed in a cleaning solution of 80 ° C. for 10 minutes and then washed with water.

On the basis of this RCA cleaning, a cleaning method in which cleaning with a mixed solution of nitric acid and hydrogen peroxide for removing organic substances is added, and a cleaning with a dilute hydrofluoric acid solution for removing a natural oxide film formed by cleaning is added. Improvements have been made to the washing method and the like.

[0008]

By cleaning the silicon substrate before thermal oxidation for forming a gate oxide film as described above, it is possible to substantially remove impurities remaining on the surface of the silicon substrate. However, in a step before this thermal oxidation, for example, in a field oxide film forming step for forming element isolation, contamination from a silicon nitride film or the like serving as a mask material, contamination in a photolithography step, or a heat treatment step. Contamination from the diffusion furnace. These contaminations are captured by slight surface defects on the silicon substrate surface without remaining on the silicon substrate surface.

Since these contaminations have a relatively small amount of contamination, they have not become apparent when the thickness of the gate oxide film is about 20 nm. However, as the thickness of the gate oxide becomes thinner, this minute contamination affects the dielectric strength of the gate oxide film. The silicon substrate is etched with a thickness of 5 nm by cleaning with a mixed solution of ammonia / hydrogen peroxide / water among the chemicals constituting the RCA cleaning, but the surface defects that capture the contamination are not completely removed by this alone.

In order to remove the contaminants captured near the surface of the silicon substrate, hydrofluoric acid (0.2%) / nitric acid (60
% Of the silicon substrate before the gate oxide film is formed (for example, SEMI Technology Symposium 89 Proceedings 275).
-284 pages).

The inventor also conducted a similar experiment, and by etching the silicon substrate by 10 to 20 nm by this wet etching method, the dielectric breakdown voltage failure of the gate oxide film is certainly reduced, but the long-term reliability of the gate oxide film is reduced. Decreased. This is because this etching solution has a ratio of the etching rate of the silicon substrate to the silicon oxide film of about 1/10, and the etching rate of the silicon substrate is higher than that of the silicon oxide film. This is because when etching is performed, the silicon substrate at the bottom of the field oxide film is etched, and electric field concentration occurs in the gate oxide film. In addition, when a silicon substrate having a thickness of 20 nm or more was etched, a result that a gate oxide film withstand voltage defect increased was obtained. This is because a P ⁺ layer at the bottom of the field oxide film appears due to excessive etching of the silicon substrate, and a gate oxide film having poor withstand voltage characteristics was obtained by oxidizing the P ⁺ layer.

Therefore, the inventor has performed etching of the silicon substrate before the gate oxidation using a plurality of etching solutions having different etching rates of the silicon substrate and the silicon oxide film, thereby improving the dielectric strength and long-term reliability of the gate oxide film. The relationship between the etching rate ratio between the silicon substrate and the silicon oxide film and the etching amount of the silicon substrate was examined. As a result, the field oxide film and the silicon substrate are etched by etching the silicon substrate in a range of 10 to 20 nm using an etchant having an etching selectivity between the silicon substrate and the silicon oxide film in the range of 0.5 to 2. But
Almost uniformly etched, silicon substrate as before
Is etched quickly at the end of the field oxide (bird
Beak) no longer has an undercut shape
Therefore, it has been clarified that a good withstand voltage and long-term reliability of the gate oxide film can be obtained.

[0013]

According to a method of manufacturing a semiconductor device of the present invention, a surface of a field oxide film formed by a thermal oxidation method and a surface of a silicon substrate having a P ⁺ layer on almost the entire bottom of the oxide film are cleaned. and, in the manufacturing method of a semiconductor device for forming a gate oxide film by thermal oxidation after removing contaminants up to the step, after the formation of the field oxide film
Forming a sacrificial oxide film on the element region by thermal oxidation
And then etching away the sacrificial oxide film;
Next, a step of etching the silicon substrate to a depth of 10 to 20 nm by an etching method in which an etching rate ratio between the silicon substrate and the silicon oxide film by the thermal oxidation method is in a range of 0.5 to 2, and then, Remove contaminants
And performing a washing step .

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings, taking the manufacture of a MOS transistor as an example. FIG.
4A to 4C are cross-sectional views of a semiconductor chip showing a method of manufacturing a MOS transistor including an embodiment of a method of forming a gate oxide film according to the present invention.

First, as shown in FIG. 1A, a P ⁺ region 2 and a field oxide film 3 are formed on a P-type silicon substrate 1 by a selective oxidation method. Next, a sacrificial oxide film 4 is formed in the element region by a thermal oxidation method.

Next, as shown in FIG. 1B, the oxide film is removed with a solution containing hydrofluoric acid. After the sacrificial oxide film 4 is removed by etching, the silicon substrate 1 is immersed in a mixed solution of ammonia / hydrogen peroxide to etch the surface of the silicon substrate 1. The composition and temperature of the ammonia / hydrogen peroxide mixture are as follows: ammonia / hydrogen peroxide / water = 1/1/5
(Volume ratio), 70 ° C. At this temperature and composition ratio, the etching rate of the silicon substrate 1 is 0.4 n / min.
m, and the etching ratio between the silicon substrate 1 and the thermal oxide film is 0.8, which is an etching solution having a slightly higher etching rate of the silicon substrate than the thermal oxide film. By performing etching for 30 minutes in this ammonia / hydrogen peroxide mixture, the silicon substrate 1 having a thickness of about 12 nm can be etched. By this etching, metal impurities captured by defects on the surface of the silicon substrate 1 are removed, and a clean substrate surface can be obtained.

After performing RCA cleaning and the like, FIG.
As shown in (c), a gate oxide film 5 having a desired thickness is formed by a thermal oxidation method. Thereafter, a polysilicon film is grown by a chemical vapor deposition method as in the prior art, and phosphorus is diffused into the polysilicon film by a thermal diffusion method. Next, after the polysilicon film is etched to form a gate electrode, arsenic is ion-implanted to form a source region and a drain region in a self-aligned manner. Further, a MOS transistor is manufactured by forming wirings and the like.

In the above embodiment, etching was carried out with a mixed solution of ammonia / hydrogen peroxide, but CF ₄ / O _{2 was used.}
Etching can also be performed by a dry etching method using a gas or the like as an etching gas.

For the dry etching, a microwave downstream type etching apparatus is used. The O ₂ flow rate is 80% of the CF ₄ / O ₂ total flow rate, and the microwave power is 3
It is performed at 0 W and an etching pressure of 0.5 torr. The etching rate of the silicon substrate under these etching conditions is about 10 nm per minute, and the ratio of the etching rate between the thermal silicon oxide film and the silicon substrate is about 1.5. Etching is performed for 1 minute and 30 seconds under these etching conditions to etch a 15 nm silicon substrate. Hereinafter, RCA cleaning and the like are performed in the same manner as described with reference to FIG.

In this embodiment, since the silicon substrate is etched in a single-wafer manner by the dry etching method, the variation in etching between wafers can be minimized, and a good gate oxide film can be obtained.

In the above embodiments, dry etching using a mixed gas of CF ₄ / O ₂ was described as a wet etching method using an ammonia / hydrogen peroxide mixed solution and a dry etching method. It is apparent that similar effects can be obtained by an etching method that satisfies the conditions set forth in the claims, such as another etching solution, for example, a mixed solution of hydrofluoric acid / nitric acid in which the selectivity between the silicon substrate and the silicon oxide film is adjusted.

FIG. 2 shows a conventional example, that is, a case where the silicon substrate is not etched before the gate oxidation and a case where the silicon substrate is etched before the gate oxidation according to the present embodiment. It is a figure which shows a dielectric strength histogram. From FIG. 2, it can be seen that the defect of the dielectric strength of the oxide film, particularly the defect of the applied voltage of 13 volts or less is reduced.

FIG. 3 shows a thickness 1 when the silicon substrate was etched before the gate oxidation in the conventional example and this embodiment.
The constant current TDDB (Time) of the gate oxide film of 60 nm
Dependent Dielectric Break
down) characteristics, that is, data indicating long-term reliability.
FIG. 3 also shows that the long-term reliability has a reduced number of defective defects compared to the conventional example, and that the intrinsic life is about the same as that of the conventional method.

[0024]

As described above, in the method of manufacturing a semiconductor substrate according to the present invention, the cleaning before the gate oxidation is performed.
By etching the silicon substrate in the range of 10 to 20 nm using an etching method in which the ratio of the etching rates of the silicon substrate and the silicon oxide film is in the range of 0.5 to 2, less oxide film dielectric strength failure is obtained. In addition, a semiconductor device having a gate oxide film excellent in long-term reliability can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a semiconductor chip showing a method of manufacturing a MOS transistor including an embodiment of a method of forming a gate oxide film according to the present invention.

FIG. 2 is a diagram showing a breakdown voltage histogram of a gate oxide film formed by an example and a conventional example.

FIG. 3 is a diagram showing a constant current TDDB characteristic of a gate oxide film formed by an example and a conventional example.

FIG. 4 is a cross-sectional view of a semiconductor chip showing a method of manufacturing a MOS transistor including an example of a conventional method of forming a gate oxide film.

[Explanation of symbols]

Reference Signs List 1 silicon substrate 2 P ⁺ region 3 field oxide film 4 sacrificial oxide film 5 gate oxide film 6 polysilicon film 7 source region 8 drain region

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 21/304 341 H01L 21/306 H01L 29/78

Claims (1)

(57) [Claims] 1. A method of cleaning a field oxide film formed by a thermal oxidation method and a surface of a silicon substrate having a P.sup. ⁺ Layer over substantially the entire bottom of the oxide film to remove contaminants up to the above step, and then perform thermal oxidation. the method of manufacturing a semiconductor device for forming a gate oxide film by law, the shape of the field oxide film
After the formation, a sacrificial oxide film is formed in the element region by a thermal oxidation method.
And then a step of etching and removing the sacrificial oxide film
When a step of etching the silicon substrate by etching from 10 to a depth of 20nm next etching rate ratio of the silicon oxide film by the silicon substrate and the thermal oxidation is in the range of from 2 to 0.5, thereafter , The contaminants
Performing a cleaning for removing the semiconductor device.