JP2825878B2 - Trench forming method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a trench forming method suitable for forming, for example, isolation trenches between elements, trenches for trench capacitors, and the like.

(Prior Art) High integration of semiconductor devices has been increasingly advanced. For example, in the case of a semiconductor memory device, high integration is indispensable to increase the storage capacity. But for this,
The miniaturization of individual storage elements and the miniaturization of element isolation regions between the storage elements must be actively promoted.

Regarding the inter-element isolation technology, since further miniaturization is difficult with the conventional inter-element isolation using the selective oxidation method, a trench inter-element isolation technology in which a deep groove is formed in the semiconductor substrate and the inter-element isolation is performed by this groove is promising. Has become.

In addition, regarding the miniaturization technology of individual storage elements, a trench capacitor technology is promising in order to reduce the area of a capacitor portion for storing storage information and to prevent a reduction in stored charge capacity due to the reduction. .

In any case, a desired trench must be formed in the semiconductor substrate, and therefore, in manufacturing a semiconductor device in recent years, a trench forming method has become important.

When forming a trench in a semiconductor substrate, for example, a silicon substrate, an anisotropic dry etching method is used.
However, when the trench is formed by anisotropic dry etching, damage due to the etching and a reaction product layer between the etching gas and the silicon substrate occur on the side wall and the bottom of the trench. Such damage and reaction products impair the characteristics of the semiconductor device. Therefore, in order to remove these, an oxygen (O ₂ ) ashing, an acid or alkali treatment, and a surface of the semiconductor substrate after the trench is formed are once oxidized to form an oxide film (a so-called sacrificial oxide film). Removal of an oxide film has been performed.

As a method of forming a sacrificial oxide film after forming a trench and removing the sacrificial oxide film to obtain a trench, for example, Japanese Patent Application Laid-Open No. 62-281429
There was a method disclosed in Japanese Unexamined Patent Application Publication No. HEI 9-86. Hereinafter, this method will be briefly described. 4 (A) to 4 (C) are process charts for explanation. In each case, a cross-sectional view of the device is shown.

According to this trench forming method, first, a CVD oxide film 13 having a thickness of about 0.8 μm is formed on the entire surface of, for example, the P-type silicon substrate 11 on which the field oxide film 12 and the like are formed. Next, a portion of the CVD oxide film 13 corresponding to the portion where the trench is to be formed is removed (FIG. 4A).

Next, using the remaining portion of the CVD oxide film 13 as a mask, a groove (trench) 14 having vertical walls is formed in the P-type silicon substrate 11 by an anisotropic dry etching method, for example, a reactive ion etching (RIE) method. (FIG. 4 (B)).
At this time, on the surface of the P-type silicon substrate 11 where the grooves 14 are formed, as shown in FIG. 4 (B), a contamination layer 15 due to a reaction product of an etching gas and silicon and a damage layer 16 due to etching are formed. Are formed.

Next, a thermal oxidation process is performed on the P-type silicon substrate 11 on which the formation of the groove 14 has been completed, and an oxide layer 17, a so-called sacrificial oxide film 17 is formed.
Is formed. At this time, the sacrificial oxide film 17 causes the contamination layer 15
And the damaged layer 16 are absorbed (FIG. 4 (C)).

Next, the sacrificial oxide film 17 is removed by a suitable method. As a result, the contaminated layer 15 and the damaged layer 16 formed during the trench etching are removed, so that a desired trench can be obtained.

According to Japanese Patent Application Laid-Open No. 62-281429, the thermal oxidation temperature for forming the sacrificial oxide film is set to 900 ° C. or less to prevent the damage layer from being expanded at the time of forming the sacrificial oxide film. I can do it.

(Problems to be Solved by the Invention) However, the conventional trench forming method has the following problems.

... Since the damage layer 16 formed on the trench side wall after the anisotropic etching contains constituent atoms of the etching gas (for example, fluorine atoms and chlorine atoms), a thermal oxidation process for obtaining a sacrificial oxide film In the above, there is a problem that the atoms diffuse deeper than the depth at which the sacrificial oxide film of the silicon substrate is formed. Therefore, even if the sacrificial oxide film is removed, since the fluorine atoms remain in the silicon substrate,
This causes problems such as generation of interface states and reduction of the capacity of the trench capacitor. In addition, there is a possibility that insulation properties may be degraded due to generation of interface states. The phenomenon in which fluorine atoms diffused into the silicon substrate was measured as follows.

First, by the method according to the conventional trench forming method,
A trench was formed on a silicon substrate by anisotropic etching using CCl ₂ F ₂ gas, and further subjected to O ₂ ashing and hydrofluoric acid treatment, followed by thermal oxidation to form a sacrificial oxide film having a thickness of 30 nm. . The thermal oxidation was performed by burning a mixture of oxygen and hydrogen for 10 minutes. Subsequently, secondary ion mass spectrometry (Second
The concentration profile of fluorine (F) atoms was measured by ary ion mass spectrometry (SIMS). The result is shown in FIG. In FIG. 5, the vertical axis indicates the intensity of fluorine atoms (cps: count / second), and the horizontal axis indicates the depth (nm) from the sample surface. As is clear from FIG. 5, the fluorine atoms are diffused not only in the sacrificial oxide film 17 but also in the silicon substrate 11.

... In the conventional trench forming method, the oxidation temperature is set to 900 ° C. or less in order to prevent the diffusion of the damaged layer. Therefore, there is a problem that the shape of the sacrificial oxide film is unfavorable from the viewpoint of trench formation. FIG. 6 is a diagram provided for the explanation, and is a cross-sectional view showing a trench portion. If the oxidation temperature is low, the silicon substrate 11 with the trench 14 formed
The thickness of the sacrificial oxide film 17 becomes thinner at the upper corner portion 14a of the trench 14 due to slow progress of oxidation, and becomes thicker at the lower portion 14b side of the trench 14 due to a large supply of silicon. As a result, after removing such a sacrificial oxide film, the silicon substrate becomes convex at the upper corner portion 14a of the trench 14 and concave at the lower portion.
Therefore, when a capacitor is formed using a trench having such a shape, there is a problem in terms of coverage and the like.

The present invention has been made in view of such a point,
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a trench forming method capable of effectively removing a contaminated layer and a damaged layer generated when forming a trench by anisotropic dry etching.

(Means for Solving the Problems) In order to achieve this object, according to the present invention, a step of forming a trench in a semiconductor substrate by an anisotropic dry etching method and a step of forming a sacrificial oxide film in the trench And a step of removing the sacrificial oxide film, wherein the semiconductor substrate is plasma-doped with ammonia after the trench is formed and before the sacrificial oxide film is formed.

In practicing the present invention, it is preferable to form the sacrificial oxide film at a temperature of 900 ° C. or more and in a dry oxygen atmosphere or an oxygen atmosphere containing water vapor.

(Action) According to the trench forming method of the present invention, the semiconductor substrate is plasma-doped with ammonia after the trench is formed by anisotropic etching and before the formation of the sacrificial oxide film. The doped ammonia reacts with fluorine atoms and chlorine atoms diffused in the silicon substrate (trench side wall) to convert them into NH ₄ F and NH ₄ Cl. NH ₄ F and NH ₄ Cl
Is less likely to diffuse in a silicon substrate than fluorine or chlorine atoms. Therefore, even in a thermal oxidation process for forming a sacrificial oxide film to be performed later, these NH ₄ F and NH ₄ Cl remain on the surface of the silicon substrate and can be absorbed in the sacrificial oxide film.
Therefore, by removing the sacrificial oxide film, fluorine atoms and chlorine atoms can be removed as a result.

Further, even if the oxidation temperature at the time of forming the sacrificial oxide film is increased or the oxidation time is increased, diffusion of fluorine, chlorine, and the like into the silicon substrate can be suppressed for the above-described reason.
The shape of the trench can be improved.

(Example) Hereinafter, an example of a trench forming method of the present invention will be described with reference to the drawings.

It should be noted that the drawings used in the following description are only schematically shown to the extent that the present invention can be understood. Therefore, the dimensions, shapes, and positional relationships of the components in the drawings are merely examples, and the present invention is not limited only to these. Further, each condition such as a film thickness, an etching condition, and a plasma generation condition in the description is merely an example, and it is apparent that the present invention is not limited only to these conditions.

1A to 1F are process diagrams for explaining a trench forming method according to an embodiment. Both figures are shown with cross-sectional views.

First, in this embodiment, the main surface of the semiconductor substrate is (10
A P-type silicon substrate 21 having a 0) surface and a resistivity of 1 to 20 Ω · cm (hereinafter, may be abbreviated as the substrate 21) was prepared. The substrate 21 has been subjected to a predetermined pre-processing in accordance with the type of the semiconductor device to be manufactured. Explaining in a specific example, for example, when forming a semiconductor device having trench element isolation, the n ⁺ subcollector layer and the n ⁺
When a semiconductor device having a trench capacitor is formed, an active layer is formed.

A silicon oxide film 23 is formed on such a substrate 21.
The silicon oxide film 23 can be formed by a suitable method such as a thermal oxidation method or a chemical vapor deposition (CVD) method. In this embodiment, the substrate 21 is placed in an electric furnace and dried in an oxygen atmosphere.
Oxidation was performed at a temperature of 1000 ° C. to form a silicon oxide film 23 having a thickness of 300 nm.

Next, a resist is applied to the silicon oxide film 23 by spin coating so as to have a thickness of about 1 μm, and then the resist is patterned into a predetermined shape by photolithography to obtain a resist pattern 25 (first pattern). 1
Figure (A)). The resist pattern 25 had an opening to obtain a desired trench width, for example, a trench having a diameter of 0.5 to 1.5 μm.

Next, using the resist pattern 25 as a mask, the silicon oxide film 2 is formed by a known reactive ion etching (RIE) method.
3 was etched. Subsequently, the resist was peeled off with O ₂ ashing and a mixed solution of sulfuric acid and hydrogen peroxide solution,
Mask 23 for trench etching made of silicon oxide film
a was obtained (FIG. 1 (B)).

Next, a portion of the substrate 21 exposed from the trench etching mask 23a was etched by RIE to form a groove (trench) 27 in the substrate 21. Note that the etching for forming the groove 27 is performed under conditions that result in anisotropic etching. In the anisotropic etching, for example, a gas containing chlorine such as Cl ₂ , SiCl ₄ or CCl ₄ as an etching gas, or a fluorine-based gas (SF ₆ , CF ₄ ) and / or
This can be achieved by using a gas in which O ₂ , N _2, or the like is mixed.
At the time of this anisotropic etching, a reaction product of the etching gas and silicon is generated as a surface contamination layer 29 on the side walls and the bottom surface of the trench 27, and fluorine and chlorine atoms are formed in the walls of the trench 27. A damaged layer 31 is generated due to various causes such as diffusion (FIG. 1 (C)).

Next, the substrate 21 after the trench etching was subjected to O ₂ ashing and acid treatment in this embodiment. The O ₂ ashing process was performed under the conditions that the O ₂ gas flow rate was 1001 sccm, the RF power was 200 W, and the ashing time was 5 minutes.
The acid treatment was performed using hydrofluoric acid under the condition that the treatment time was 5 minutes (room temperature). The respective conditions of the trench etching, O ₂ ashing, and acid treatment are the same as the conditions at the time of preparing the sample for the analysis experiment by SIMS in FIG.

Next, in the present invention, the silicon substrate on which the trench has been formed is subjected to plasma doping with ammonia before forming the sacrificial oxide film. This was done in this example as described below.

The sample shown in FIG. 1C in which the trench formation, O ₂ ashing, and acid treatment have been completed is placed in a plasma etching apparatus. Next, ammonia plasma was generated in a plasma etching apparatus with an ammonia gas flow rate of 50 sccm and an RF power of 200 W, and the sample was exposed to the ammonia plasma for 5 minutes (FIG. 1 (D)). Fig. 1 (D)
In the above, 33 is an ammonia plasma schematically shown. At this time, ammonia is doped into the silicon substrate 21 from the side walls and the bottom of the trench. The doped ammonia reacts with halogen (X) such as fluorine and chlorine diffused in the silicon substrate 21 as follows.

8NH ₃ + 3X ₂ → N ₂ + 6NH ₄ X As a result, the fluorine present on the trench wall surface and the silicon substrate becomes ammonium fluoride (NH ₄ F),
Chlorine becomes ammonium chloride (NH ₄ Cl).

Next, in order to form a sacrificial oxide film, the sample was placed in a dry oxygen atmosphere at 1000 ° C. for 40 minutes in this example. Thus, a sacrificial oxide film 35 having a thickness of 30 nm was formed in the trench (FIG. 1E).

Next, the fluorine concentration profile and the NH ₄ F concentration profile of the sample in the state shown in FIG. 1E were measured by secondary ion mass spectrometry. This result is
Shown in the figure. In FIG. 2, the vertical axis indicates the intensity of fluorine or ammonium fluoride (cps: count / second), and the horizontal axis indicates the depth (nm) from the sample surface.

As is clear from a comparison between FIG. 2 and FIG. 5, according to the method of the present invention, the diffusion of fluorine into the silicon substrate is very small, and the diffusion of fluorine into the silicon substrate is effectively reduced. You can see that it can be prevented. Further, it can be seen that the diffusion of NH ₄ F into the silicon substrate is so small that it does not matter. This is because most of the fluorine and chlorine diffused into the silicon substrate are changed to ammonium fluoride and ammonium chloride, and ammonium fluoride and ammonium chloride diffuse into the silicon substrate as compared with fluorine and chlorine. This is because even if heat is applied to the silicon substrate during the formation of the sacrificial oxide film, these diffusions stop at a shallow depth from the substrate surface.

Next, the sacrificial oxide film of the sample in the state shown in FIG.
35 is removed by a suitable method, for example by immersion in hydrofluoric acid. As a result, a substrate 21 having a desired trench 27 from which the contaminated layer and the damaged layer were removed was obtained (FIG. 1 (F)).

In addition, when the formation of the sacrificial oxide film 35 was examined with respect to the sample manufactured by the trench forming method of the present invention and the formation of the sacrificial oxide film 35 was completed, as shown in FIG. The thickness of the sacrificial oxide film 35 was substantially uniform both near the upper corner 27a and near the lower corner 27b. Therefore, it can be said that the sacrificial oxide film 35 is formed following the formation of the trench 27, and the shape of the trench 27 after the removal of the sacrificial oxide film 35 can be made desired. The reason is that the trench forming method of the present invention can form a sacrificial oxide film at a higher temperature than before.

The above is the description of the embodiment of the trench forming method of the present invention. However, the present invention is not limited to the above-described embodiment, and various modifications can be added as described below.

In the above-described embodiment, the O ₂ ashing process and the acid process are performed after the trench is formed and before the ammonia is plasma-doped. However, it is not always necessary to perform the O ₂ ashing treatment and the acid treatment.

In the above-described embodiment, the oxidation treatment for forming the sacrificial oxide film is performed at a temperature of 1000 ° C. in a dry oxygen atmosphere. However, the oxidation treatment conditions are not limited to these, and other conditions may be used. However, in order to improve the shape of the trench, the oxidation temperature is preferably set to 900 ° C. or higher.

In addition, the plasma doping condition of ammonia is as follows:
The present invention is not limited to the apparatus and conditions of the embodiment, but may be other suitable apparatuses.

(Effect of the Invention) As is clear from the above description, according to the trench forming method of the present invention, the semiconductor substrate is plasma-doped with ammonia after the trench is formed and before the sacrificial oxide film is formed. For this reason, contaminants (fluorine and chlorine) react with ammonia and change into substances that are difficult to diffuse in the semiconductor substrate. Therefore, even if the acid treatment for forming the sacrificial oxide film is performed later, the contaminant remains in the sacrificial oxide film, so that the contaminant can be effectively removed by removing the sacrificial oxide film.

Further, the contaminant reacts with ammonia to change into a substance which is difficult to diffuse in the semiconductor substrate. Therefore, even if the heat treatment temperature at the time of forming the sacrificial oxide film is made higher than before, there is no fear of the contaminant diffusion. For this reason, the heat treatment temperature of the sacrificial oxide film can be increased, and the sacrificial oxide film can be grown at a uniform thickness on each portion of the trench, so that the trench shape can be improved.

[Brief description of the drawings]

1 (A) to 1 (F) are process diagrams showing a trench forming method of an embodiment, and FIG. 2 is a SIMS of a sample obtained by the trench forming method of the embodiment.
FIG. 3 is a diagram showing a sacrificial oxide film shape by the trench forming method of the embodiment, FIGS. 4A to 4C are process diagrams showing a conventional trench forming method, and FIG. FIG. 6 shows a concentration profile by SIMS of a sample obtained by a conventional trench forming method, and FIG. 6 is a diagram for explaining a problem of the conventional trench forming method. 21 ... Semiconductor substrate (silicon substrate) 23 ... Silicon oxide film, 25 ... Resist pattern 23a ... Trench etching mask 27 ... Groove (trench), 29 ... Contamination layer 31 ... Damage layer, 33 ... Ammonia plasma 35 ... Sacrificial oxide film.

Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 27/108

Claims (2)

(57) [Claims] 1. A trench forming method comprising: forming a trench in a semiconductor substrate by an anisotropic dry etching method; forming a sacrificial oxide film in the trench; and removing the sacrificial oxide film. A method for forming a trench, comprising plasma doping ammonia onto a semiconductor substrate after forming a trench and before forming a sacrificial oxide film. 2. The trench forming method according to claim 1, wherein the sacrificial oxide film is formed at a temperature of 900 ° C. or more and in a dry oxygen atmosphere or an oxygen atmosphere containing water vapor. Forming method.