JPH06101519B2 - Method for forming both regions for separating semiconductor elements - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for forming a semiconductor element isolation region by selective oxidation for element isolation effective for forming a miniaturized active area of a large-scale integrated circuit or the like. It is a thing.

(Constitution of conventional example and its problems) Large-scale integrated circuit (hereinafter referred to as LSI), for example, MOS type LS
In the fabrication of I, the active area that constitutes the MOS transistor, the element isolation area that separates this active area, and the electrode wiring that electrically connects each element, etc., has a high density and high performance when formed. It is indispensable to form a fine structure with high accuracy and high reproducibility by each individual process required for the process.

Conventionally, as a method for forming a thick silicon oxide layer called a field oxide film in a MOS type LSI process, as shown in FIG. A thin film of 1000 Å SiO ₂ is formed, and a silicon nitride film (Si ₃ N ₄ film) 3 is locally provided on this surface, and the surface of the substrate 1 is selectively oxidized by using this as a mask, if necessary. , High-concentration impurity layer 4 prepared in advance by ion implantation
A method of forming the field oxide film 5 at the same time as forming the (channel stopper) is often used.

However, in this method, selective oxidation causes lateral oxidation to proceed, and so-called bird's beak (Bird beak)
s Beak) 6, which makes it difficult to miniaturize the element isolation region.

(Object of the Invention) It is an object of the present invention to provide a method for forming a semiconductor element isolation region that can suppress lateral oxidation in selective oxidation and miniaturize the element isolation region.

(Structure of the Invention) A method for forming a semiconductor element isolation region according to the present invention comprises forming a silicon nitride film having a refractive index of 2.8 or less on a semiconductor substrate while containing silicon in excess of a stoichiometric ratio. Step 1 and a step of partially removing the silicon nitride film in a portion to be an element isolation region
And a third step of performing a heat treatment in an oxidizing atmosphere to selectively oxidize the substrate. It is even better to form the silicon nitride film in a refractive index range of 2.4 ± 0.1. Also, the first step may include a step of further forming a silicon nitride film having a substantially stoichiometric ratio on the silicon nitride film.

Description of Embodiments The present invention will be described by taking a method of manufacturing a MOS LSI as an example.

1 to 4 show process sectional views according to the present invention. As shown in FIG. 1, a first silicon nitride film 12 containing silicon in excess of the stoichiometric ratio was formed on a single crystal silicon substrate 11 to a thickness of 300 to 1000Å. At this time, the first silicon nitride film containing excess silicon has a refractive index of 2.8 or less, preferably 2.4 ± 0.
It was formed in the range of 1. This shows the relationship between the mixing ratio of the raw material gases forming the silicon nitride film and the refractive index and the oxidation rate in FIGS. 5 and 6, but the silicon nitride film has a substantially stoichiometric ratio (in the drawings, When the silicon is excessively increased from the right end portion (from the right side to the left side), the oxidation rate of the silicon nitride film suddenly increases when the mixing ratio of the source gases exceeds a certain value from FIG. . The oxidation of the silicon nitride film means that bird's beak is eventually generated due to the oxidation of the silicon nitride film. Therefore, the point at which the oxidation rate suddenly increases is about 2.8 in terms of the refractive index in FIG.
Therefore, the present inventors, if the refractive index is 2.8 or less in spite of excess silicon, bird's beak suppression effect by the oxidation rate almost equal to that of the silicon nitride film of the stoichiometric ratio, and the stress relaxation effect by excess silicon. It is considered that a substantially sufficient film quality having the above can be obtained. further
It is clear from the figure that a better effect can be obtained by forming in the range of 2.4 ± 0.1.

The refractive index of the first silicon nitride film 12 can be controlled by changing the mixing ratio of SiH ₄ and NH ₃ or SiH ₂ Cl and NH ₃ , for example. For example, the relationship between SiH ₄ and NH ₃ is shown in FIG. Note that this figure shows a deposition temperature of 610
This is the case of ° C. Continued first silicon nitride film
When expressed in the usual approximate stoichiometric ratio or chemical symbol above 12,
The second silicon nitride film 13 to be Si ₃ N ₄ is formed with 300 to 10
00Å formed. In the case of this embodiment, the first and second silicon nitride films 12 and 13 were formed by dividing the process into two steps, but the gas mixing ratio was gradually changed to form the silicon nitride films 12 and 13 continuously at one time. May be. In addition, the first silicon nitride film 12
It may be alone. This is because almost the same results were obtained for bird's beaks and the like described later even with only the first silicon nitride film.

After performing the above processing, as shown in FIG. 2, a photoresist film 14 is formed, and the first and second silicon nitride films 12 and 13 to be planned in the element isolation region are formed by photolithography. Selectively removed, then semiconductor substrate 1
Impurities of the same conductivity type as the
A stopper diffusion layer 15 is formed. After performing the above processing, the photoresist (14) is removed, and as shown in FIG.
A selective oxide film (field oxide film) 16 for element isolation is formed in an oxidizing atmosphere at 1000 ° C.

FIG. 4 is a cross-sectional view showing a state in which the first silicon nitride film 12 and the second silicon nitride film 13 are removed after performing the above treatment.

In the method according to the embodiment of the present invention shown in FIG. 4, dislocations and bird's beaks of the single crystal silicon substrate 11 due to selective oxidation are hardly generated. In other words, when conventional silicon nitride is directly deposited on a single crystal silicon substrate and selective oxidation is performed, stress causes crystal defects (dislocations), which causes a leak current. Although this was done, the pad oxide film caused the bird's beak. However, according to the method of the present embodiment, the action of the first silicon nitride film 12 containing Si in excess of the stoichiometric ratio It is considered that the crystal defects are suppressed because the stress is relaxed.

In the first silicon nitride film 12, for example, when the refractive index is 2.4, the internal stress is about 1/2 to 1/4 that of the refractive index of 2.0.

The element isolation regions formed in this embodiment are shown in FIGS.
As shown in the figure,
Compared with the one shown in the figure, bird's beak of selective oxidation is significantly suppressed. For example, conventionally, the oxide film thickness in the element isolation region was 70
Although the mask size of the silicon nitride film for the device active region was set to 3.0 μm and the active region size after selective oxidation was 1.8 μm at 00Å, it was 2.4 under the same conditions by the method according to the present embodiment of the present invention. was μm.

In addition, the silicon nitride film containing silicon in excess of the stoichiometric ratio of this embodiment, and having a refractive index of 2.8 or less, preferably in the range of 2.4 ± 0.1, has other steps such as etching in the wafer process. Can be treated in the same manner as a silicon nitride film which is almost normal, that is, which has a stoichiometric ratio. As an example thereof, FIG. 6 shows the oxidation rate of the silicon nitride film when the silicon content is changed. The oxidation rate of the silicon nitride film of this embodiment formed in the range of the refractive index of 2.8 or less. Was about the same as a normal stoichiometric silicon nitride film.

That is, as is clear from FIG. 6, the oxidation of the silicon nitride film progresses, and the bird's beak due to the oxidation of the first silicon nitride film 12 becomes a problem. Therefore, it is desired that the oxidation rate be equal to that of a normal silicon nitride film and that the internal stress be reduced.

As described above, the refractive index of 2.
It can be seen that it is necessary to form a silicon nitride film with excess silicon so that it is within a range of 2.8 or less, preferably 2.4 ± 0.1, although about 4 is good. The characteristics shown in FIG. 6 represent the oxide film thickness (Å) at 1000 ° C. for 3 hours in a wetO ₂ atmosphere.

(Effect of the Invention) As described above, according to the method of the present invention, the semiconductor substrate contains silicon in excess of the stoichiometric ratio, and the refractive index thereof is 2.
By forming a silicon nitride film having a thickness of 8 or less, the generation of bird's beaks during selective oxidation can be almost suppressed, and further, crystal defects on the substrate surface can be sufficiently suppressed.

[Brief description of drawings]

1 to 4 are process sectional views showing an embodiment of the method of the present invention, and FIG. 5 is a refractive index of a silicon nitride film and NH ₃ / S.
Fig. 6 is a diagram showing the relationship with the iH ₄ ratio, Fig. 6 is a diagram showing the relationship between the oxide film thickness of the silicon nitride film per constant time (3h) and the NH ₃ / SiH ₄ ratio, and Fig. 7 is a conventional example. It is sectional drawing shown. 11 …… single crystal silicon substrate, 12 …… first silicon nitride film, 13 …… second silicon nitride film, 16
...... Selective oxide film.

Claims (3)

[Claims] 1. A first step of forming a silicon nitride film which contains silicon in excess of the stoichiometric ratio and has a refractive index of 2.8 or less on a semiconductor substrate, and a portion which becomes an element isolation region. And a third step of partially removing the silicon nitride film, and a third step of performing a heat treatment in an oxidizing atmosphere to selectively oxidize the substrate. . 2. The refractive index of the silicon nitride film is 2.
The method for forming a semiconductor element isolation region according to claim 1, wherein the formation is performed within a range of 4 ± 0.1. 3. The method according to claim 1, wherein the first step includes a step of further forming a silicon nitride film having a substantially stoichiometric ratio on the silicon nitride film. Method for forming semiconductor element isolation region.