JP2011003792A - Method of manufacturing semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a semiconductor device for protecting the backside of a semiconductor substrate, reducing stress caused by a silicon nitride film on the backside, and preventing the semiconductor substrate from remarkably protrusively bending toward the front surface side.SOLUTION: The method of manufacturing the semiconductor device includes, in the following order, steps of: forming silicon nitride films on the front and back sides of a semiconductor substrate; leaving the silicon nitride film on the backside of the semiconductor substrate by reducing the film thickness of the silicon nitride film on the backside relative to the front face of the semiconductor substrate; forming a separation mask opening for selectively exposing the front surface of the semiconductor substrate on the silicon nitride film; forming an element isolation groove on the front face of the semiconductor substrate through the separation mask opening; oxidizing the inner surface of the element isolation groove; filling a silicon oxide film in the element isolation groove at a film thickness exceeding the depth of the element isolation groove; and flattening the silicon oxide film.

Description

  The present invention relates to a semiconductor device manufacturing technique, and more particularly to a semiconductor device manufacturing method using a technique of forming a shallow element isolation trench on a semiconductor substrate and filling it with an insulating film to form an element isolation region.

  In a conventional semiconductor process, an element isolation region that performs electrical insulation between adjacent semiconductor elements on a semiconductor substrate (also referred to as a wafer) such as silicon is formed by a technique called a LOCOS (Local Oxidation of Silicon) method. However, in order to narrow the element formation region effective in the LOCOS method, in recent years, as the miniaturization of LSI (semiconductor integrated circuit) has progressed, silicon called on the STI (Shallow Trench Isolation) method has recently been developed. A stopper film such as a nitride film is attached, an insulating film such as a silicon oxide film is embedded in an element isolation groove (also referred to as a trench), and an extra insulating film outside the groove is subjected to chemical mechanical polishing (CMP). ), The stopper film is exposed, and then the stopper film is removed by etching. The CMP polishing liquid (slurry) has a problem that the polishing rate selection ratio of the insulating film to the stopper film is low, and the polishing of the portion where the trenches are densely progresses as compared with the sparse portion of the trenches, resulting in thinning. If the stopper film is thickened to prevent thinning, the residual stress of the silicon nitride film of the stopper film affects the wafer.

  An outline of a conventional general STI forming method of a semiconductor device will be described with reference to the drawings. FIG. 1 is a schematic partial cross-sectional view of a wafer sequentially illustrating STI manufacturing steps in a semiconductor device.

  As shown in FIG. 1, silicon oxide films 2 and 2b and silicon nitride films 3 and 3b are stacked in this order on a wafer 1 (FIG. 1A), and a resist pattern (not shown) is formed on the wafer 1 surface side. ) As a mask, the silicon nitride film 3 and the silicon oxide film 2 on the surface (surface to be processed) side are sequentially patterned (FIG. 1B), and then a part of both films is removed and exposed on the wafer 1 The element isolation region is further anisotropically etched to form a trench T in the exposed wafer 1 (FIG. 1C), and is insulated over the entire surface of the wafer 1 by CMP to fill the trench T with the insulating film 4. A film 4 is formed (FIG. 1D), and then the insulating film 4 formed on the silicon nitride film 3 is removed by CMP using the silicon nitride film 3 as a stopper, so that the insulating film is formed only inside the trench T. 4 leaving element isolation structure To form a (FIG. 1 (e)), followed by hot phosphoric acid solution, to remove the silicon nitride film 3, 3b (FIG. 1 (f)).

  However, in the STI method, in the patterning step shown in FIG. 1B, a difference in residual stress occurs between the partially exposed wafer surface and the back surface on which the silicon nitride film is formed on the entire surface. Since the surface warps greatly on the surface side, it may cause crystal defects or the like due to strong stress on the surface side of the wafer, which may adversely affect transistor characteristics and isolation characteristics.

  Since the silicon nitride film formed on the front and back surfaces of the wafer has a higher Young's modulus than the silicon oxide film, the stress generated in the silicon nitride film is not relaxed in the silicon nitride film during heat treatment or the like, and stress is applied to the wafer. Will be given. For this reason, the wafer is distorted, and transfer, crystal defects, etc. are generated on the wafer surface.

  In order to solve this problem, a method of removing all the nitride films on the back surface of the wafer has been used (see Patent Document 1).

JP 2005-340734 A

  However, since all of the silicon nitride film on the back surface of the wafer is removed by the conventional method, there is a concern about contamination of the back surface of the wafer due to etching gas flowing from the front surface to the back surface in the subsequent trench formation process by anisotropic etching of the wafer surface. .

  Therefore, the present invention provides a method for manufacturing a semiconductor device that protects the back surface of a wafer from contamination, reduces stress caused by the silicon nitride film on the back surface, and improves the problem of the wafer warping greatly on the front surface side. It is intended to provide.

  The method of manufacturing a semiconductor device of the present invention includes a step of forming a silicon nitride film on the front and back surfaces of a semiconductor substrate, and a silicon nitride film on the back surface of the semiconductor substrate by reducing the thickness of the silicon nitride film on the back surface of the semiconductor substrate. A step of leaving a film, a step of forming an isolation mask opening for selectively exposing the surface of the semiconductor substrate in the silicon nitride film, a step of forming an element isolation groove on the surface of the semiconductor substrate through the isolation mask opening, and an element A process of oxidizing the inner surface of the isolation groove, a process of filling the element isolation groove with a silicon oxide film with a film thickness exceeding the depth of the element isolation groove, and a process of flattening the silicon oxide film in this order It is characterized by. The present invention suppresses warpage of a semiconductor substrate after forming a trench by thinning a silicon nitride film formed on the back surface of the semiconductor substrate, particularly before forming an element isolation trench (trench) in a semiconductor integrated circuit. To do.

  In the step of leaving the silicon nitride film in the semiconductor device manufacturing method, the film thickness of the silicon nitride film on the back surface of the semiconductor substrate is the same as the silicon nitride film in which the residual stress of the silicon nitride film on the back surface of the semiconductor substrate and the isolation mask opening are formed. It can be a film thickness that cancels out the residual stress of the film. According to the present invention, before forming a trench, a silicon nitride film formed on the back surface is formed with a trench pattern ratio (an isolation mask opening formed in the silicon nitride film with respect to the area of the silicon nitride film formed on the surface of the semiconductor substrate). By removing the same ratio as the area ratio) by wet etching, the warpage of the semiconductor substrate after the trench formation is completely suppressed.

  In the semiconductor device manufacturing method, the silicon nitride film on the back surface of the semiconductor substrate can be reduced by wet etching to a film thickness equal to or greater than the pattern ratio of the trench.

  In the semiconductor device manufacturing method, the silicon nitride film can be formed directly on the semiconductor substrate or via another layer such as a silicon oxide film.

  According to the semiconductor device manufacturing method of the present invention, in the semiconductor STI-CMP process, the wafer is formed before the step of forming the isolation mask opening in the silicon nitride film after the step of forming the silicon nitride film on the front and back surfaces of the wafer. By reducing the thickness of the silicon nitride film formed on the back surface, preferably by the thickness corresponding to the pattern ratio of the trench, the warpage of the wafer after trench formation is completely suppressed and crystal defects are suppressed. can do.

It is a schematic fragmentary sectional view which shows the wafer of a semiconductor STI-CMP process. 1 is a schematic partial cross-sectional view illustrating a semiconductor STI-CMP process according to an embodiment of the present invention.

1 Wafer 2, 2b Silicon oxide film 3, 3b Silicon nitride film 4 NSG film M Isolation mask opening T Trench

  A semiconductor device manufacturing method according to an embodiment of the present invention will be described below with reference to the drawings.

  FIG. 2 is a schematic partial cross-sectional view of a wafer sequentially illustrating an STI-CMP process in the semiconductor device manufacturing method.

  First, as shown in FIG. 2A, silicon oxide films 2 and 2 b and silicon nitride films 3 and 3 b are sequentially stacked on the front and back surfaces of the wafer 1. For example, the silicon oxide film is formed by surface oxidation, and the silicon nitride film is formed by low pressure or normal pressure chemical vapor deposition (CVD).

  Next, as shown in FIG. 2B, the silicon nitride film 3b formed on the back surface of the wafer 1 is removed by wet etching by the same ratio as the pattern ratio (to be described later) of a trench to be formed later. The thickness of the nitride film 3b is reduced (so-called back etching process).

  Next, as shown in FIG. 2C, the silicon nitride film 3 and the silicon oxide film 2 on the surface (surface to be processed) side of the wafer 1 are sequentially patterned using a resist pattern (not shown) as a mask. The silicon nitride film 3 and the silicon oxide film 2 are formed as a mask in which an isolation mask opening M is formed so that a predetermined portion of the wafer 1 is exposed by etching in a photolithography process. Here, the exposed wafer surface portion is a predetermined element isolation region.

  Next, as shown in FIG. 2D, a trench T is formed on the surface of the wafer 1. Through the separation mask opening M, the exposed wafer is anisotropically etched by a predetermined depth to form a trench T.

Next, as shown in FIG. 2E, after the thermal silicon oxide film H is formed on the inner wall of the trench T by thermal oxidation, the trench T is filled with the NSG film 4 by CVD, that is, the trench T An NSG film 4 is formed on the entire surface of the wafer 1 with a film thickness exceeding the depth of the first wafer. Here, the NSG film 4 is an NSG (None-doped Silicate Glass) produced by, for example, an O ₃ -TEOS-CVD apparatus using Tetra Ethyl Ortho Silicate, that is, a silicon oxide film containing no impurities. Here, a recess is formed in the NSG film formed on the trench T portion by the step of the trench.

  Thereafter, as shown in FIG. 2F, the NSG film 4 formed on the silicon nitride film 3 is removed by the CMP method using the silicon nitride film 3 as a stopper to leave the NSG film 4 only in the trench T. An element isolation region is formed.

  Next, as shown in FIG. 2G, the silicon nitride films 3 and 3b are removed by hot phosphoric acid treatment.

  In the above semiconductor device manufacturing method, before the step of sequentially patterning the silicon nitride film 3 and the silicon oxide film 2 (FIG. 2C), the back etching step shown in FIG. The pattern ratio of the trench T (the ratio of the area A of the isolation mask opening M formed in the silicon nitride film 3 to the area B of the silicon nitride film 3 formed on the surface of the wafer 1: A / B ) Is removed at the same rate as). As an example, for example, when a silicon nitride film is deposited with a thickness of 1500 angstroms and the pattern ratio of the trench T is 40%, a silicon nitride film with a mask pattern ratio of 60% remains on the surface. There is a 40% difference in stress between the backside deposited on the surface and the 60% deposited surface. Therefore, the silicon nitride film on the back surface having a thickness of 600 Å having a pattern ratio of 40% of the trench T is removed until wet etching, and a silicon nitride film having a thickness of 900 Å having a pattern ratio of 60% on the front surface is left on the back surface. That is, in the step of leaving the silicon nitride film shown in FIG. 2B, the silicon nitride film 3b on the back surface of the wafer has the residual stress of the silicon nitride film 3b on the back surface of the wafer and the separation mask opening. The film thickness is set so as to cancel out the residual stress of the silicon nitride film 3 on the surface.

  According to the above embodiment, not only the same ratio as the trench pattern ratio but also the thickness of the trench pattern ratio or more is reduced by wet etching before the trench formation, so that stress is reduced between the back surface and the front surface. The difference is completely eliminated, and crystal defects on the surface side of the wafer caused by strong stress can be suppressed.

  In the semiconductor device manufacturing method of the above embodiment, a silicon oxide film such as a natural oxide film is laminated between the silicon nitride film and the wafer. However, the present invention is not limited to this, and the silicon nitride film is formed on the wafer. It can also be formed directly.

  Furthermore, in the above embodiment, a silicon substrate is targeted as a semiconductor substrate. However, the present invention is not necessarily limited to a silicon bulk process. For example, a silicon carbide (SiC) substrate or a silicon substrate having an EPI layer is used. It can also be applied to SOI (Silicon-on-Insulator) substrates.

Claims (4)

Forming a silicon nitride film on the front and back surfaces of the semiconductor substrate;
Reducing the thickness of the silicon nitride film on the back surface of the semiconductor substrate to leave the silicon nitride film on the back surface of the semiconductor substrate;
Forming a separation mask opening in the silicon nitride film to selectively expose the surface of the semiconductor substrate;
Forming an element isolation trench on the surface of the semiconductor substrate through the isolation mask opening;
Oxidizing the inner surface of the element isolation trench;
Filling the element isolation trench with a silicon oxide film with a film thickness exceeding the depth of the element isolation trench;
And a step of planarizing the silicon oxide film in this order.   In the step of leaving the silicon nitride film, the film thickness of the silicon nitride film on the back surface of the semiconductor substrate is such that the residual stress of the silicon nitride film on the back surface of the semiconductor substrate and the silicon nitride film in which the isolation mask opening is formed. 2. The method of manufacturing a semiconductor device according to claim 1, wherein the film thickness cancels out the residual stress of the film.   In the step of leaving the silicon nitride film, the silicon nitride film on the back surface of the semiconductor substrate has the isolation mask opening formed in the silicon nitride film with respect to the area of the silicon nitride film formed on the surface of the semiconductor substrate. 3. The method of manufacturing a semiconductor device according to claim 1, wherein the film thickness is reduced to a thickness equal to or greater than the ratio of the area.   4. The method of manufacturing a semiconductor device according to claim 1, wherein the silicon nitride film is formed directly on the semiconductor substrate or via another layer.

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