JPH0831810A - Method for forming element isolation film - Google Patents

Abstract

PURPOSE:To provide a method for forming a element isolation film wherein degeneration of a bird's beak and drop of Vth are prevented and the flatness is improved. CONSTITUTION:On a silicon substrate 21, a LOCOS oxide film 26 is formed, and after an SiN film, etc., is removed, the LOCOS oxide film 26 is flattened with HF, and then, by performing CMP polishing, the height of the silicon substrate 21 and that of the LOCOS oxide film 26 are equalized. By this, the shoulder parts of the silicon substrate 21 do not protrude itself, so that, electric field concentration is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an element isolation film, and more particularly to a technique for preventing degeneracy of bird's beak of a LOCOS oxide film.

[0002]

2. Description of the Related Art Conventionally, the problems to be solved by the invention
It is known that the LOCOS oxide film used for element isolation has a large dimensional conversion difference due to bird's beak. FIG. 3 is an illustration of an element isolation film called a so-called poly pad LOCOS. In the figure, 1 is a SiO ₂ film formed on a silicon substrate, on which a polysilicon film 2,
The figure shows a state in which the SiN film 3 is deposited to expose the polysilicon film 2 in the LOCOS formation region. The polysilicon film 2 is thermally oxidized to form a LOCO 4 shown in the figure.
An S oxide film can be formed. Due to this thermal oxidation, LOCOS
The oxide film 4 grows laterally to form bird's beaks. The sum of the lengths of the bird's beaks Δx ₁ and Δx ₂ is the dimensional conversion difference, and the LOCOS oxide film formed by this method is said to have a dimensional conversion difference of 0.35 μm generation limit. In addition, the LOCOS formed by this method has a problem that the level difference of the oxide film protruding on the substrate surface is large. By the way, as an evaluation criterion of the separation ability of the LOCOS oxide film, it is better that the current value flowing under the element separation is smaller. The condition necessary therefor is that the depth Δd of element isolation from the substrate surface is preferably large as shown in FIG. The magnitude of this Δd is generally related to the above polypad LOCOS <buried LOCOS <trench element isolation.

Further, as described above, when Δd is insufficient, it is necessary to increase the concentration of channel stop implantation for the purpose of enhancing the separation ability. However, if the concentration of impurities under the LOCOS is increased, the concentration of the inversion layer under the gate is also increased by diffusion, so that the transistor becomes harder to turn on as the line width becomes narrower (channel width narrower). This is called the narrow channel effect. In order to suppress the narrow channel effect, formation of element isolation with high isolation capability is essential.

As a conventional LOCOS forming method, there is a so-called embedded LOCOS method. 5A to 5D are process sectional views showing this forming method. First, as shown in FIG. 5A, SiO 2 is formed on the surface of the silicon substrate 7.
_{The 2} film 8 and the SiN film 9 are formed, the groove 10 is formed in the LOCOS formation region, and then the LOCOS oxide film 11 as shown in FIG. 5B is formed by thermal oxidation. And SiN
This is a technique in which the film 9 is peeled off and treated with HF to form a flat element isolation as shown in FIG. According to this method, an oxide film is deeply formed in the substrate, so that even if the bird's beak is removed by HF, a sufficient separation ability can be maintained.
However, there is a problem that HF causes a phenomenon in which electric field concentration occurs at a position where the shoulder portion 7A of the silicon substrate 7 is exposed and the transistor is turned on (reverse narrow channel effect). Thus, when the shoulder portion 7A of the silicon substrate is exposed, as shown in FIGS. 6 and 7, the shoulder portion 7A is exposed.
Since the gate 12 is formed so as to cover the above,
If the voltage of the gate 12 is increased, there is a problem that the shoulder portion 7A tends to be inverted. The narrower the channel width is, the larger the influence of the shoulder portion 7A becomes, and the inversion voltage is lowered. Therefore, it is called an inverse narrow channel effect. Further, in the case of such embedded LOCOS, there is a problem that crystal defects are likely to occur because the stress during oxidation is large.

Further, as a conventional LOCOS forming method, trench element isolation is known. 8 (A) and 8 (B) show the forming method. In this method, as shown in FIG. 8A, a trench 13A is formed in a silicon substrate 13, a SiO ₂ film 14 is deposited, and
As shown in (B), the SiO film 14 is removed by, for example, etching back. This method is most advantageous in terms of separation ability and dimensional conversion difference, but has problems in terms of stability and uniformity of the SiO ₂ removal process, and has not been put to practical use. If SiO ₂ is removed too much, the shoulder portion of the silicon substrate is exposed and the above-mentioned reverse narrow channel effect occurs, and S
If the removal of iO ₂ is insufficient, Si is not formed in the device formation region.
O ₂ islands remain and become defective. Further, the polishing of the oxide film is likely to damage the silicon substrate, and there are still problems in terms of film thickness uniformity.

Further, the LOCOS as shown in FIG.
In the case of the oxide film 15, crystal defects (dislocations, stacking faults) at the oxide film end are generated by stress of the SiN film 16 or the like. This is a defect existing near the surface of the substrate (up to 50 nm).

The problem to be solved by the present invention is to reduce the degeneration of the bird's beak and the accompanying threshold voltage (Vth).
The problem lies in what kind of means should be taken to obtain a method for forming an element isolation film that can prevent the deterioration of the film thickness and improve the flatness.

[0008]

Therefore, according to the present invention, after forming a LOCOS oxide film on a semiconductor substrate, the LOCOS oxide film is formed.
The solution is to flatten the S oxide film and make the height of the semiconductor substrate and that of the LOCOS oxide film substantially the same by CMP polishing. Further, the CMP polishing is
Removal of a defect layer on the surface of the semiconductor substrate, and LOCOS
The feature is that the amount is controlled to be sufficient to remove the oxidatively eroded portion at the end portion of the oxide film. Further, the LOCOS oxide film is formed by oxidizing the inside of the groove formed in the semiconductor substrate.

[0009]

According to the present invention, the bird's beak is degenerated and the Vth is prevented from being lowered.
Further, since the flatness of the element isolation film is improved, there is an effect that a margin of the depth of focus at the time of exposure can be afforded.
Further, since the silicon surface layer is removed, it is possible to remove the defect layer on the substrate surface generated at the LOCOS end portion.

[0010]

EXAMPLES The details of the method for forming an element isolation film according to the present invention will be described below with reference to examples. FIG. 1 (A)-
(C) and FIGS. 2A to 2C show the forming process of this embodiment.

First, as shown in FIG. 1A, a SiO ₂ film 22 having a thickness of 20 nm is formed on a silicon substrate 21, and a SiN film 23 having a thickness of 200 nm is formed on the SiO ₂ film 22.
It is formed by the VD method. Then, the SiN film 23 as a LOCOS mask is patterned by the lithography technique and the anisotropic etching technique. After depositing the SiN film again, etching back is performed to remove the SiN film 2
The SiN side wall 24 is formed on the side wall of No. 3.

Next, as shown in FIG. 1B, reactive ion etching is performed using the SiN film 23 and the SiN sidewall 24 as a mask to form a groove 25. Then, thermal oxidation is performed to perform LOCOS as shown in FIG.
The oxide film 26 is formed.

Next, as shown in FIG. 2A, after removing the SiN film 23 and the SiN sidewalls 24,
LOC after treatment with hydrofluoric acid (HF) as shown in (B)
The OS oxide film 26 is flattened. By this process, LO
The bird's beak of the COS oxide film 26 is removed, and the dimensional conversion difference can be reduced. However, in this state, the shoulder portion 21A of the silicon substrate 21 is exposed and electric field concentration occurs in this portion, so the following CMP polishing step is performed.

Finally, polishing is carried out using colloidal silica (alkaline) which is used for ordinary polishing of silicon mirrors. This colloidal silica has a particle size of 10 to 20 nm and is used as a polishing liquid with K
An alkaline (PH10-11) preparation liquid is used with OH or amine. This CMP polishing is a polishing mainly for selectively removing silicon. By performing final polishing of silicon in the final step, not only the silicon layer is removed, but also surface damage and crystal defects can be removed. Thus, the exposure of the shoulder portion of silicon, which is a problem in the embedded LOCOS, can be solved by polishing SiO ₂ and Si to almost the same degree. What is important here is that the surface of Si to be removed must be completely exposed by HF, because an abrasive that does not substantially remove SiO ₂ is used. HF mentioned above
The controllability of the amount of oxide film removed in the process of 1) affects the final finish shape. After that, after adding a cleaning liquid, the process may be returned to a normal process.

Although the embodiment has been described above, the present invention is not limited to this, and various design changes associated with the gist of the configuration can be made.

[0016]

As is apparent from the above description, according to the present invention, degeneration of bird's beaks and the accompanying Vt.
This has the effect of preventing a decrease in h. Further, since the flatness of the element isolation film is improved, there is an effect that a margin of the depth of focus at the time of exposure can be afforded. Further, since the silicon surface layer is removed, it is possible to remove the defect layer on the substrate surface generated at the LOCOS end portion.

[Brief description of drawings]

1A to 1C are cross-sectional views showing the steps of an embodiment of the present invention.

2A to 2C are cross-sectional views showing a process of an embodiment of the present invention.

FIG. 3 is an explanatory sectional view showing a conventional example.

FIG. 4 is a sectional explanatory view showing a conventional example.

5A to 5C are cross-sectional views showing steps of a conventional example.

FIG. 6 is a plan view of a conventional example.

FIG. 7 is a sectional explanatory view of a conventional example.

8A and 8B are cross-sectional views showing steps of a conventional example.

FIG. 9 is a sectional view showing a process of a conventional example.

[Explanation of symbols]

21 ... Silicon substrate 22 ... SiO ₂ film 23 ... SiN film 24 ... SiN sidewall 25 ... Groove 26 ... LOCOS oxide film

Claims (3)

[Claims] 1. A LOCOS oxide film is formed on a semiconductor substrate, the LOCOS oxide film is planarized, and the height of the semiconductor substrate and that of the LOCOS oxide film are made substantially the same by CMP polishing. Method for forming element isolation film. 2. The element isolation film according to claim 1, wherein the CMP polishing is controlled to an amount sufficient to remove a defective layer on the surface of the semiconductor substrate and to remove an oxidative erosion portion at an end portion of the LOCOS oxide film. Forming method. 3. The LOCOS oxide film is formed by oxidizing a groove formed in a semiconductor substrate.
A method for forming an element isolation film as described above.