JPH07201968A - Method of forming element isolation region - Google Patents

Abstract

PURPOSE:To obtain a field oxide film which does not protrude from the surface of a substrate by a method wherein the first field oxidation is performed, the oxide film is removed by etching and the field oxidation is performed again so as to obtain the required thickness of the oxide film. CONSTITUTION:A silicon substrate 1 is subjected to wet oxidation to form an oxide film 6 and, further, the part of the oxide film 6 on a region 5 where an element isolation is to be formed is totally removed to form a recess with a depth of 275nm which is the part of the silicon substrate 1 corroded by the oxide film 6. The element isolation part of the substrate 1 is further subjected to wet oxidation to form a field oxide film 7 with a thickness of 500nm. With this process, the substrate 1 is corroded by the field oxide film 1 by a depth of 45% of the thickness of 500nm of the field oxide film 7, i.e., 225nm, and the oxide film with a thickness of 275nm, i.e., 55% of 500nm is built up on it. As a result, the field oxide film 7 whose surface is on the approximately same plane as the surface of the substrate 1, which is flat as a whole and whose thickness is 500nm can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for selectively oxidizing a silicon substrate to form an element isolation region in a semiconductor device.

[0002]

2. Description of the Related Art Conventionally, it has been widely known that a region between elements adjacent to each other is selectively oxidized by using a known LOCOS method (thermal oxidation method) to form a field oxide film to form an element isolation region. There is. This is done by first oxidizing the surface of the silicon substrate to form an oxide film of a predetermined thickness, forming a silicon nitride film on it, and leaving the silicon nitride film as a mask only on the region where the device is to be formed. Pattern. Then, by oxidizing the surface of the substrate again, a thick field oxide film is formed in a region not covered with the silicon nitride film, that is, a region where an element isolation region is to be formed.

However, if the field oxide film is thickened by the LOCOS method to prevent the parasitic MOS effect, the step difference of the insulating film for element isolation becomes steep and the coverage of the wiring deteriorates. In order to improve this, a groove is previously formed in the substrate in order to flatten the element isolation region, and LOCOS is applied.
A method of oxidization, a method represented by a BOX method of digging a groove in a substrate in advance and filling it with an insulating film, a so-called trench type element isolation method has been proposed, for example, JP-A-57-17054.
No. 7 discloses one such example.

[0004]

The above-mentioned trench type element isolation method has complicated steps, and is damaged by etching the substrate, which may cause crystal defects in the substrate and deteriorate the characteristics of the element. There was a problem to say.

Therefore, an object of the present invention is to provide a method for forming an element isolation region whose surface is flattened without causing damage to the substrate.

[0006]

In order to solve the above problems, the present invention provides a method for forming an element isolation region for electrically isolating elements adjacent to each other on a silicon substrate, the method comprising the steps of: A step of forming a silicon oxide film thereon, a step of forming a silicon nitride film on the silicon oxide film, and a step of forming the silicon nitride film so that the silicon oxide film on a region where an element isolation region is to be formed is exposed. Patterning process, a first field oxidation process of forming an oxide film by oxidizing the silicon substrate in a region where the element isolation region is to be formed using the silicon nitride film as a mask, and the first field oxidation process A step of forming a recess by removing the oxide film formed by etching, and forming a field oxide film by re-oxidizing the recess. Method for forming a device isolation region and having a second field oxidation process.

[0007]

In the LOCOS method, first, the first field oxidation is performed, and then the oxide film is removed by etching to form a recess in the substrate. Then, when field oxidation is performed again so as to have a predetermined film thickness, a field oxide film that does not protrude from the surface of the substrate is obtained. At this time, the substrate is not affected because the recess is formed by etching the oxide film, not the substrate. Further, usually, in such oxidation, about 45% erodes the inside of the substrate and 55% grows on the substrate. Therefore, the thickness of the oxide film formed by the first field oxidation and the oxidation formed by the second field oxidation are increased. By setting the ratio of the film thickness to 1.22: 1, the surface of the field oxide film and the surface of the substrate almost coincide with each other.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIGS. 1A to 1D are vertical sectional views showing a procedure for forming an element isolation region. In this embodiment, an example of finally obtaining a field oxide film having a film thickness of 500 nm will be shown.

First, as shown in FIG. 1A, a thin silicon oxide film 2 (1) is formed on a silicon substrate 1 by dry oxidation.
0 nm to 30 nm) is formed. Subsequently, a silicon nitride film 3 is formed to a thickness of 150 nm to 200 nm by a low pressure CVD method, and the silicon nitride film 3 is exposed so that the silicon oxide film 2 in regions other than the regions 4 where elements are to be formed, that is, regions 5 where element isolation regions are to be formed is exposed. Pattern.

Next, as shown in FIG. 1B, the silicon substrate 1 is oxidized by wet oxidation to form a film thickness 610.
An oxide film 6 having a thickness of nm is formed (first field oxidation).
The erosion of the oxide film 6 on the silicon substrate 1 at this time is 45% of the film thickness 610 nm, that is, 275 nm.

Further, as shown in FIG. 1C, the oxide film 6 in the region 5 where the element isolation region is to be formed is entirely removed by wet etching with hydrofluoric acid, whereby the oxide film 6 is removed.
Of the erosion on the silicon substrate 1, that is, the concave portion 1a having a depth of 275 nm is formed on the silicon substrate 1.

Finally, again as shown in FIG.
A 500 nm field oxide film 7 is formed by oxidizing the silicon substrate 1 in the element isolation region 5 by wet oxidation (second field oxidation). Then, of this field oxide film 7, 45% of 500 nm, that is, 225 n
m erodes the substrate 1, and 55% of 500 nm, that is, 275 nm of an oxide film grows on it. As a result, the surface of the substrate 1 is substantially aligned with the surface of the substrate 1, and the flat field oxide film 7 having a thickness of 500 nm can be formed as a whole.

[0014]

As described above, according to the present invention, L
In the OCOS method, first, first field oxidation is performed, and then the oxide film is removed by etching to form a recess in the substrate. Then, when field oxidation is performed again so as to have a predetermined film thickness, a field oxide film that does not protrude from the surface of the substrate is obtained. At this time, the substrate is not affected because the recess is formed by etching the oxide film, not the substrate. Further, usually, in such oxidation, about 45% corrodes the inside of the substrate and 55% grows on the substrate. Therefore, the film thickness of the oxide film by the first field oxidation,
The ratio to the thickness of the oxide film formed by the second field oxidation is 1.
By setting the ratio to 22: 1, the surface of the field oxide film almost coincides with the surface of the substrate. Therefore, the surface is flattened and an element isolation region can be formed without damaging the substrate.

[Brief description of drawings]

1A to 1D are vertical cross-sectional views showing a procedure for forming an element isolation region.

[Explanation of symbols]

 1 Silicon Substrate 1a Recess 2 Silicon Oxide Film 3 Silicon Nitride Film 4 Region to Form Element 5 Region to Form Element Isolation Region 6 Oxide Film 7 Field Oxide Film

Claims (2)

[Claims] 1. A method of forming an element isolation region for electrically isolating elements adjacent to each other on a silicon substrate, the method comprising forming a silicon oxide film on the silicon substrate, A step of forming a silicon nitride film thereon, a step of patterning the silicon nitride film to expose the silicon oxide film on a region where an element isolation region is to be formed, and a step of using the silicon nitride film as a mask A first field oxidation process of forming an oxide film by oxidizing the silicon substrate in a region where a region is to be formed, and a recess formed by etching the oxide film formed in the first field oxidation process. And a second field oxidation step of re-oxidizing the recess to form a field oxide film. Method for forming isolation region. 2. The ratio of the film thickness of an oxide film formed in a region where an element isolation region is to be formed using the silicon nitride film as a mask to the film thickness of a field oxide film formed by re-oxidizing the recess is: The element isolation region forming method according to claim 1, wherein the ratio is 1.22: 1.