Method for reducing stress of sidewall oxide layer of shallow trench isolation

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for reducing stress of a sidewall oxide layer of a shallow trench isolation, and more particularly to a method for reducing stress of a sidewall oxide layer of a shallow trench isolation by annealing and reoxidation.

[0004] 2. Description of the Related Art

[0005] As the density of integrated circuits increases, the dimension of an isolation region between active regions in semiconductor devices decreases. With this trend, the conventional local oxidation of silicon (LOCOS) method for isolating active regions, which forms a field oxide layer by using a thermal oxidation technique, confronts the limit in the effective isolation length, thereby degrading characteristics of the isolation region. Furthermore, the conventional LOCOS method possesses some inherent drawbacks resulting from the processes, i.e., lateral oxidation of the silicon underneath the silicon nitride mask, making the edge of the field oxide resemble the shape of a bird's beak.

[0006] According to the disadvantages for LOCOS isolation structures mentioned above, an isolation technique using trenches has been developed. Generally, the trench isolation includes the steps of etching a silicon substrate to form a trench, depositing a oxide layer by using a chemical vapor deposition (CVD) process to fill up the trench, providing the oxide layer a planarized surface by using a chemical mechanical polish (CMP) process, and removing the oxide layer upon the active regions.

[0007] According to the technique, the semiconductor substrate is etched at a predetermined depth, thereby providing excellent characteristics of the device isolation. Furthermore, the field oxide layer is formed by using a CVD technique, so that the device isolation region that is defined by a photolithography process can be maintained throughout. The device isolation technique set forth is also known as shallow trench isolation (STI) processes.

[0008] However, conventional shallow trench isolation processes still have several drawbacks. FIG. 1 shows a cross-sectional diagram of a shallow trench isolation amid a STI conventional process. A silicon substrate 100, a silicon dioxide layers 102, a silicon nitride layer 104 are shown in FIG. 1. An oxide layer 106 is formed over the trench by conventional oxidation processes such as dry or wet thermal oxidation. The oxide layer 106 is used to eliminate etching induced damage and to reduce stress resulting from the following filling of silicon dioxide by conventional chemical vapor deposition. However, the oxide layer 106 itself is formed with large stress. This is because the conventional oxidation process, specially a wet oxidation process, always causes large stress within the oxide layer. The large stress usually presents defects in neighboring active regions. The defects will result in leakage current and degrade the reliability of neighboring devices.

[0009] In view of the drawbacks mentioned with the prior art process, there is a continued need to develop improved processes that overcome the disadvantages associated with prior art processes. The requirements of this invention are that it solves the problems mentioned above.

[0010] It is therefore an object of the invention to provide a method for reducing the stress of a sidewall oxide layer of a STI.

[0011] It is another object of this invention to provide a STI process which can assure the electrical property of the active regions.

[0012] It is a further object of this invention to provide a reliable STI process which can assure the isolation quality between the active regions.

[0013] To achieve these objects, and in accordance with the purpose of the invention, the invention uses a method comprising: providing a substrate having a first dielectric layer thereon and a second dielectric layer over said first dielectric layer; forming a trench into said substrate; forming a sidewall oxide layer on the sidewall and bottom of said trench; and performing an in situ steam generated process comprising introducing hydroxyl to anneal and reoxidize said sidewall oxide layer.

[0014] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

[0015] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

[0016] FIG. 1 shows a cross-sectional diagram of a shallow trench isolation amid a STI conventional process;

[0017] FIG. 2A shows two dielectric layers sequentially formed over a substrate;

[0018] FIG. 2B shows a result of forming a trench into the structure shown in FIG. 2A and conformally forming a dielectric layer thereon; and

[0019] FIG. 3 shows a schematic diagram of a process system.

[0020] It is to be understood and appreciated that the process steps and structures described below do not cover a complete process flow. The present invention can be practiced in conjunction with various integrated circuit fabrication techniques that are used in the art, and only so much of the commonly practiced process steps are included herein as are necessary to provide an understanding of the present invention.

[0021] The present invention will be described in detail with reference to the accompanying drawings. It should be noted that the drawings are in greatly simplified form and they are not drawn to scale. Moreover, dimensions have been exaggerated in order to provide a clear illustration and understanding of the present invention.

[0022] Referring to FIG. 2A, dielectric layers 202 and 204 are sequentially formed over a substrate 200. The substrate 200 preferably comprises, but is not limited to: a silicon substrate with a <100> crystallographic orientation. The substrate can also comprise other semiconductor substrate such as a SOI (Silicon On Insulator)substrate. The dielectric layer 202 preferably comprises, but is not limited to: a silicon dioxide layer or a silicon oxy nitride layer formed by a thermal growth process. The dielectric layer 202 has a thickness of from about 20 angstrom to about 300 angstrom. The dielectric layer 204 preferably comprises a silicon nitride layer formed by conventional methods such as chemical vapor deposition, but other material met the spirit of this invention should not be excluded. The silicon nitride layer 204 preferably has a thickness of from about 100 angstrom to about 2000 angstrom.

[0023] Referring to FIG. 2B, a trench is formed by etching the dielectric layer 204, the dielectric layer 202 and the substrate 200 and a dielectric layer 206 is conformally formed over the trench. The depth of the trench depends on what kind of device the STI isolates, for example, the depth is about 2500 angstrom to about 4500 angstrom for a flash memory and it is about 2000 angstrom to about 4000 angstrom for a logic device such as a metal oxide semiconductor (MOS) transistor. The trench is preferably formed by anisotropic etching such as reactive ion etching, but other conventional etching method should also be used. The dielectric layer 206 comprises a silicon dioxide layer formed by conventional dry or wet oxidation process. The dielectric layer 206 has a thickness of from about 100 angstrom to about 500 angstrom. To reduce the stress of the dielectric layer 206, an in situ steam generated process is performed in a conventional furnace, but is preferably in a rapid thermal annealing (RTA) chamber and specially in a single wafer RTA chamber. There are numerous processing equipment can be used to perform an ISSG process. FIG. 3 shows a Centura® 5000 system 300 marketed by the Applied Materials Corporation. A rapid thermal annealing chamber 320 is bolted to a vacuum transfer chamber 310. There are also a process chamber 322, a cool down chamber 330 and vacuum cassette loadlocks 340 and 342 bolted to the vacuum transfer chamber 310. The dielectric layer 206 is annealed and reoxidized in an atmosphere comprising oxygen and hydroxyl and at a temperature between about 700° C. to about 1200° C. The flow rate of oxygen is from about 1 sccm (Standard Cubic Centimeter per Minute) to about 30 sccm, and the flow rate of hydrogen is from about 0.1 sccm to about 15 sccm. The annealing time of this ISSG process is from about 1 minute to about 10 minute. It is noted that the introduction of oxygen can be omitted. The omitting of the introduction of oxygen can avoid the further oxidation of the dielectric layer 206.

[0024] The invention utilizes introductions of oxygen and hydroxyl to perform an in situ steam generated process to anneal and reoxidize a conventional sidewall oxide layer in a shallow trench isolation. The ISSG annealing process renders the conventional sidewall oxide layer much less stress. The electrical property of the active regions and the isolation quality between the active regions can be assured.

[0025] Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.