JPH0442823B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor device isolation method, and particularly to a selective oxidation method for electrically isolating a plurality of devices formed on a semiconductor substrate from each other.

[Prior Art] In a bipolar device formed on a semiconductor substrate or a unipolar device such as a MOS transistor, semiconductor elements are formed in mutually electrically insulated regions to form a circuit.

As a method of electrically isolating each semiconductor element,
The PN junction separation method is widely used as an inexpensive and easy separation method. In the PN junction separation method, a P-type diffusion layer reaches deep from the surface to the substrate, and the substrate and separated P
This is a method of electrically isolating the device by holding the shape region at the lowest potential and reverse biasing the PN junction formed between the device region and the device region.

However, with this PN junction isolation method, the distance between the isolation region and the element cannot be narrowed very much, making it difficult to achieve high integration.

Therefore, in the past, selective oxidation was considered as just one method for improving the degree of integration.

FIG. 4 shows a cross-sectional view illustrating a conventional selective oxidation method using a bipolar device as an example. on the p-type silicon substrate 20 to reduce collector resistance.
An n ⁺ buried layer 22 and a p ⁺ channel cut layer 24 for device isolation are formed (FIG. 4A), and an n type epitaxial layer 26 is grown thereon. Next, the epitaxial layer 26 is thermally oxidized into a thin layer for stress relaxation, and then a mask nitride film 28 is deposited by CVD (FIG. 4B).

Then, the mask nitride film 28 in the area to be separated is etched with a hydrogen fluoride solution or the like (FIG. 4C).
Epitaxial layer 26 as p ⁺ channel cut layer 2
The bipolar elements formed in the epitaxial layer 26 can be electrically isolated by thermally oxidizing the epitaxial layer 26 until the epitaxial layer 26 reaches the epitaxial layer 26, and forming an oxidized region 30 for isolation as shown in FIG. 4D.

In this way, with the selective oxidation method, there is no need to provide a gap between the isolation region and the element region, and it is possible to achieve a significantly higher degree of integration than with the PN junction isolation method.

[Problems to be Solved by the Invention] However, several problems have arisen in the conventional selective oxidation method. FIG. 5 shows a partially enlarged view of FIG. 4D. As mentioned above, in the selective oxidation method, the nitride film 28 is used as an oxidation prevention mask.
Before being deposited by CVD, the thin epitaxial layer 26 is thermally oxidized to form a pad oxide film 32 for stress relaxation.
When forming the epitaxial layer 26, the volume expands approximately twice due to oxidation, and therefore the isolation oxidation region 3
0 enters the element formation region and is integrated with the pad oxide film 32, forming the element formation region and the isolation oxide region 30.
The so-called Bird's Beak 3 blurs the boundaries between
There was a problem that 4 occurred, making it impossible to achieve a significant improvement in the degree of integration.

The present invention has been made in view of the above-mentioned conventional problems, and its purpose is to suppress bird's beaks that occur when forming isolated oxidation regions, and to reliably separate element formation regions where semiconductor elements are formed, thereby increasing the degree of integration. An object of the present invention is to provide an improved selective oxidation method that can be improved.

[Means for Solving the Problems] In order to achieve the above object, the present invention employs a first oxidation process in which a film is oxidized to a thickness equal to or less than a target isolation oxide film thickness, as shown in FIG. 1, in a selective oxidation method. 10, an etching process 12 for etching the mask nitride film and the oxidized region formed in the first oxidation process, a deposition process 14 for redepositing the mask nitride film, and a first oxidation process for oxidizing the isolation oxide film to a target thickness. 2 oxidation process 16.

[Operation] The selective oxidation method according to the present invention includes the above-described process, and the isolated oxidation region is formed by multiple oxidation processes.

That is, when the region oxidized in the first oxidation process is etched in the etching process and a mask nitride film is deposited, this mask nitride film wraps around the side wall portion of the element forming region due to the etching recess, and the side wall portion is will also be protected. Therefore, it is possible to prevent expansion of the oxidized region toward the element formation region due to oxidation when forming the isolated oxidized region in the second oxidation process, and to suppress bird's beak.

[Example] Hereinafter, a preferred example of the selective oxidation method for semiconductor element isolation according to the present invention will be described with reference to the drawings, taking a bipolar device as an example.

FIG. 2 shows a cross-sectional view for explaining one embodiment of the present invention, and FIG. 2A shows a silicon substrate 20 with an n ⁺ buried layer 22, a p ⁺ channel cut layer 24, and an epitaxial layer as in the conventional case. The sequential formation of layer 26, pad oxide film, and mask nitride film 28 is shown. In this example, the thickness of the epitaxial layer 26 was 500 nm.

In the conventional selective oxidation method, as mentioned above, the p ⁺ channel cut layer 2 is formed in one oxidation process.
However, as shown ⁱⁿ FIG. This is to thermally oxidize the epitaxial layer 26 to a film thickness that is less than 1/2 of the achieved film thickness, and in this example,
It was oxidized to about 200 nm.

Next, the mask nitride film 28 and the oxidized regions of the epitaxial layer formed in the first oxidation process described above were etched using a hydrogen fluoride solution (FIG. 2C).

After forming the pad oxide film 32 again, a mask nitride film 28 was deposited by CVD (FIG. 2D), and the mask nitride film in the region to be separated was etched (FIG. 2E).

At this time, as shown in the enlarged view of FIG. 3, the mask nitride film 28 wraps around the recess 36 of the epitaxial layer 26 formed by the above-described etching process to protect the side wall portion of the element forming region. Therefore, if the epitaxial layer 26 is thermally oxidized in the second oxidation process from this state until it reaches the p ⁺ channel cut layer, the expansion of the oxidized region toward the element formation region is prevented, and as shown in FIG. 2F, It can be seen that an isolated oxidized region with very few bird's beaks is formed.

In addition, in the present invention, since the recesses 36 are formed in the epitaxial layer 26 by an etching process and thermally oxidized, the expansion of the epitaxial layer 26 in the upward direction of the substrate due to oxidation is compensated for and the surface is flattened. It is also clear from Figure 2 F that there are

Note that although this embodiment has been described using a bipolar device as an example, the invention is not limited to this, and can also be applied to unipolar devices such as MOS transistors.

[Effects of the Invention] As explained above, according to the selective oxidation method according to the present invention, it is possible to suppress bird's beaks that occur when forming isolated oxidation regions, and to reliably separate elements and improve the degree of integration. becomes possible.

In addition, since the present invention compensates for the volumetric expansion caused by oxidation through the etching process, the surface after element isolation can be made almost flat, which makes it possible to substantially flatten the surface after element isolation, such as the aluminum wiring process. Since it can be easily carried out, it also has the effect of increasing production efficiency.

[Brief explanation of the drawing]

FIG. 1 is a flowchart of a selective oxidation method for semiconductor element isolation according to the present invention, FIG. 2 is a cross-sectional view illustrating an embodiment of the present invention, and FIG. 3 is a cross-sectional view of a portion of FIG. FIG. 4 is a cross-sectional view illustrating a conventional selective oxidation method, and FIG. 5 is an enlarged cross-sectional view of a part of FIG. 4. DESCRIPTION OF SYMBOLS 10... First oxidation process, 12... Etching process, 14... Deposition process, 16... Second oxidation process, 20... Silicon substrate, 26...
...Epitaxial layer, 28...P ⁺ channel cut layer, 34...Bird's beak.

Claims (1)

[Scope of Claims] 1. In a selective oxidation method for forming an oxidized region that electrically isolates a semiconductor element formed on a semiconductor substrate, masking with a nitride film to a thickness equal to or less than a target isolation oxide film thickness is performed. an etching process for etching the mask nitride film and the oxidized region formed in the first oxidation process; a deposition process for redepositing the mask nitride film; and a target separation oxide film thickness. a second oxidation process of masking and oxidizing with the mask nitride film until
1. A selective oxidation method for semiconductor device isolation, characterized in that a semiconductor device can be accurately separated by forming an oxidized region for device isolation in a plurality of oxidation processes.