Chemical-mechanical polishing (CMP) process for shallow trench isolation

Abstract

A method of forming shallow trench isolation includes etching trenches through a nitride layer, a polysilicon layer, and a pad oxide layer and into a semiconductor substrate. The trenches are filled with an oxide layer. A silicon oxynitride layer is deposited overlying the oxide layer and both these layers are polished away using a first slurry having high selectivity. A second polishing polishes away the oxide layer using a second slurry having a low selectivity. The nitride layer is removed and a third polishing is performed to planarize the oxide layer using a third slurry having high selectivity. Alternatively, the oxide layer is etched away except where it overlies the trenches. A first polishing is performed to polish away the oxide layer using a first slurry having a low selectivity. A second polishing is performed to polish away the oxide layer using a second slurry having high selectivity.

Description

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to a method of forming shallow trench isolation in the fabrication of integrated circuits, and more particularly, to a method of forming planarized shallow trench isolation in the fabrication of integrated circuits.

(2) Description of the Prior Art

Shallow trench isolation (STI) is desirable for improved planarity over other isolation techniques. Chemical-mechanical polishing (CMP) processes are key to forming STI regions. However, a reverse mask is often needed to form STI due to low CMP selectivity for oxide to nitride and the dishing effect at wide field regions. Therefore, to overcome these problems, a slurry having a high selectivity of oxide to nitride and high planarity is needed. Furthermore, a high selectivity CMP process can induce macro or micro-scratches. Thus, reducing scratches is also an important topic. Another issue for STI CMP is pattern density effect. This will result in a large variation in trench oxide thickness after CMP which leads to difficulties in controlling polysilicon photolithography and etching. It is desired to find a process to resolve all of these critical issues.

U.S. Pat. No. 6,043,133 to Jang et al shows a CMP STI process with reverse mask. U.S. Pat. No. 6,107,159 to Chuang, U.S. Pat. No. 5,837,612 to Ajuria, U.S. Pat. No. 5,950,093 to Wei, and U.S. Pat. No. 5,889,335 to Kuroi et al show other STI processes.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide an effective and very manufacturable method of forming shallow trench isolation device in the fabrication of an integrated circuit.

Another object of the present invention is to provide an effective and very manufacturable method of forming shallow trench isolation using chemical mechanical polishing (CMP) wherein a low pattern-density effect is achieved.

Another object of the present invention is to form shallow trench isolation using CMP wherein there is minimal trench oxide thickness variation.

A further object of the invention is to form shallow trench isolation using CMP to achieve direct and low-defect CMP process.

A still further object is to form shallow trench isolation using CMP wherein subsequent polysilicon photolithography and etching difficulties are minimized.

In accordance with the objects of this invention a new method of forming shallow trench isolation using CMP is achieved. A pad oxide layer is grown overlying a silicon semiconductor substrate. A polysilicon layer is deposited overlying the pad oxide layer. A nitride layer is deposited overlying the polysilicon layer. Trenches are etched through the nitride layer, polysilicon layer, and pad oxide layer into the silicon semiconductor substrate. The trenches are filled with an oxide layer wherein the oxide layer extends above a top surface of the nitride layer. In one alternative, a silicon oxynitride layer is deposited overlying the oxide layer. A first polishing is performed to polish away the silicon oxynitride layer and oxide layer using a first slurry having high selectivity of oxide to nitride. A second polishing is performed to polish away the oxide layer using a second slurry having a low selectivity of oxide to nitride and having low-defect properties. The silicon nitride layer is removed, and a third high-selectivity slurry is used to planarize the oxide layer to the polysilicon surface to complete formation of shallow trench isolations. In a second alternative, the oxide layer is etched away except where it overlies the trench areas. Then, a first polishing is performed to polish away the oxide layer using a first slurry having a low selectivity of oxide to nitride and having low-defect properties. The nitride layer is removed and then a second polishing is performed to planarize the oxide layer to the polysilicon layer using a second slurry having high selectivity of oxide to polysilicon to complete formation of shallow trench isolations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of this description, there is shown:

FIGS. 1 through 3 schematically illustrate in cross-sectional representation a preferred embodiment of the present invention.

FIGS. 4 through 8 schematically illustrate in cross-sectional representation a first alternative in a preferred embodiment of the present invention.

FIGS. 9 through 13 schematically illustrate in cross-sectional representation a second alternative in a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now more particularly to FIG. 1, there is illustrated a portion of a partially completed integrated circuit. There is shown a semiconductor substrate 10, preferably composed of monocrystalline silicon. A layer of pad oxide 12 is grown on the surface of the substrate to a thickness of between about 50 and 500 Angstroms. Now, a polysilicon buffer layer 14 is deposited over the pad oxide layer 12, preferably by low pressure chemical vapor deposition (LPCVD), to a thickness of between about 100 and 1000 Angstroms. Now, a silicon nitride layer 16 is deposited over the polysilicon buffer layer to a thickness of between about 500 and 2000 Angstroms. The silicon nitride layer is preferably deposited by LPCVD and serves as a hard mask layer.

Active areas are defined and isolation trenches 20 are etched through the layers 16/14/12 and into the substrate, as illustrated in FIG. 2. The photoresist, not shown, is stripped and the wafers are cleaned, as is conventional.

Now, referring to FIG. 3, a liner oxide layer 22 is grown by wet or dry oxidation within the trenches 20. Now, the trenches are filled with oxide layer 24, such as by chemical vapor deposition (CVD) or high-density plasma CVD.

Now the key features of the invention will be described wherein the novel CMP process achieves a low pattern-density effect, reduced oxide thickness variation, and reduced polysilicon photolithography and etching difficulty.

FIGS. 4–6 illustrate a first alternative CMP process is used. As shown in FIG. 4, a silicon oxynitride layer 26 is deposited over the trench fill oxide layer 24. This SiON layer is deposited by plasma-enhanced chemical vapor deposition (PECVD) to a thickness of between about 50 and 1500 Angstroms. The SiON layer 24 will serve as a CMP polish stop layer at the wide field regions to improve planarization. Alternatively, this layer may be another material such as a second silicon nitride layer.

In a first step, chemical mechanical polishing (CMP) is performed using a slurry having a high selectivity for oxide with respect to nitride. The slurry has a selectivity of greater than 10. The layer 26 acts as a polish stop layer. Polishing stops when the oxide 24 is exposed, leaving the SiON layer only over the trench areas, as shown in FIG. 5.

In a second step, CMP is continued with a low-defect and low-selectivity slurry. This second slurry has a selectivity of oxide to nitride of lower than 3. The second slurry preferably uses silica-based particles for low defects. FIG. 6 illustrates the wafer after the second CMP step has been completed. Polishing stops on the silicon nitride layer 16. Then, the silicon nitride layer 16 is removed by a wet etch, such as hot H₃PO₄, as shown in FIG. 7.

In a third step, CMP is continued with a high-selectivity and planarity slurry. This third slurry has a selectivity of oxide to polysilicon of greater than 3. Moreover, a low downforce recipe is used to achieve planarization. FIG. 8 illustrates the wafer after the third CMP step has been completed.

The process of the present invention avoids dishing. The first polishing step planarizes the surface of the oxide layer, thus allowing the second polishing step to be performed without dishing. After the second CMP step, the silicon nitride layer is removed by a wet etch. The third polishing step planarizes the oxide to the polysilicon layer.

FIGS. 9–13 illustrate a second alternative CMP process. FIG. 9 shows the oxide layer 24 to be planarized. In this alternative, an oxide-defined reverse (ODR) tone masking process is used. The ODR process of the present invention is more easily performed and has a larger process window than in the prior art. The oxide layer 24 and a portion of the nitride layer 16 is dry-etched away where they are not covered by a mask, not shown. As shown in FIG. 10, the oxide 24 remains only in the area of the trenches.

Now, the oxide 24 is to be planarized. In a first step, chemical mechanical polishing (CMP) is performed using a low-defect, low selectivity slurry. The slurry has a selectivity of oxide with respect to nitride of lower than 3. The slurry preferably uses silica particles for low defects. The first polishing step is stopped when a desired oxide thickness is reached, as shown in FIG. 11. The silicon nitride layer 16 may be partially removed in this first polishing step.

Now, the silicon nitride layer 16 is removed by a wet etch, such as hot H₃PO₄, as shown in FIG. 12.

In a second step, CMP is continued with a high-selectivity, high-planarity slurry. This second slurry has a selectivity of oxide to nitride of greater than 10 and oxide to polysilicon of greater than 3. A low downforce recipe is used in this step where the downforce is half the normal downforce. FIG. 13 illustrates the wafer after the second CMP step has been completed. The polysilicon layer 14 may be partially removed in this second polishing step, but the pad oxide layer is not exposed.

The process of the present invention avoids dishing. Most of the trench oxide thickness remaining after ODR etching is polished away using the high-selectivity slurry, and therefore, a high oxide polishing rate. The silicon nitride layer is removed. Then the second polishing step is performed at a lower polishing rate, thus avoiding dishing.

The process of the invention provides two alternative methods for forming shallow trench isolation using CMP processes. Both methods avoid the use of reverse masks, reduce dishing and oxide thickness variation, and reduce scratches, resulting in better control of subsequent polysilicon photolithography and etching.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

Claims

1. A method of forming shallow trench isolations in the fabrication of an integrated circuit device comprising: growing a pad oxide layer overlying a semiconductor substrate;depositing a polysilicon layer overlying said pad oxide layer;depositing a nitride layer overlying said polysilicon layer;etching trenches through said nitride layer, said polysilicon layer, and said pad oxide layer into said semiconductor substrate;filling said trenches with an oxide layer wherein said oxide layer extends above a top surface of said nitride layer;etching away said oxide layer except where it overlies said trenches; thereafter first polishing awaysaid oxide layer and a portion of said nitride layer using a first slurry having low selectivity of oxide to nitride and having low-defect properties;removing said nitride layer; andthereafter second polishing away said oxide layer and a portion of said polysilicon layer using a second slurry having high selectivity of oxide to polysilicon to complete formation of said shallow trench isolations in said fabrication of said integrated circuit device.

2. The method according to claim 1 wherein said polysilicon layer has a thickness of between about 100 and 1000 Angstroms.

3. The method according to claim 1 wherein said nitride layer comprises silicon nitride and has a thickness of between about 500 and 2000 Angstroms.

4. The method according to claim 1 further comprising growing a liner oxide layer within said trenches before said step of filling said trenches with said oxide layer.

5. The method according to claim 4 wherein said liner oxide layer has a thickness of between about 50 and 300 Angstroms.

6. The method according to claim 1 wherein said oxide layer is deposited by one of chemical vapor deposition and high density plasma chemical vapor deposition.

7. The method according to claim 1 wherein said first slurry has a selectivity of oxide to the nitride of lower than 3.

8. The method according to claim 1 wherein said second slurry has a selectivity of oxide to polysilicon greater than 3.

9. The method according to claim 1 wherein said second polishing step uses a downforce of no more than half a conventional downforce.

10. A method of forming a shallow trench isolation in an integrated circuit device, the method comprising: forming a buffer layer over a semiconductor substrate;forming a mask layer over the buffer layer;etching a trench through the mask and buffer layers, and into the semiconductor substrate;forming an oxide layer that fills the trench;polishing the semiconductor substrate with a low-selectivity slurry having a low selectivity of the oxide layer to the mask layer, until the oxide layer is a predetermined thickness;removing the mask layer; andpolishing the semiconductor substrate with a high-selectivity slurry having a high selectivity of the oxide layer to the buffer layer, until the buffer layer and the oxide layer form a planar surface.

11. The method according to claim 10 further comprising etching away the oxide layer except where it overlies the trench.

12. The method according to claim 10 further comprising etching portions of the oxide layer and the mask layer prior to polishing the semiconductor substrate with the low-selectivity slurry.

13. The method according to claim 10 wherein the buffer layer comprises polysilicon.

14. The method according to claim 10 wherein the mask layer comprises nitride.

15. The method according to claim 10 further comprising growing a pad oxide layer overlying the semiconductor substrate prior to forming the buffer layer.