Method for fabricating shallow trench isolation structure

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for fabricating a shallow trench isolation (STI) structure. More particularly, the present invention relates to a method for fabricating a shallow trench isolation structure filled with doped silicon dioxide.

[0004] 2. Description of the Related Art

[0005] In an integrated circuit, an isolation structure is needed to isolate devices. Since a shallow trench isolation structure has advantages of scalability and a good isolation ability, this technology is preferably used in the sub-micron process.

[0006] In the conventional method for forming a shallow trench isolation structure, an anisotropic etching process is performed with silicon nitride serving as a mask to form a steep trench in a substrate. An undoped silicon dioxide layer is formed within the trench and a shallow trench isolation structure is thus formed. The substrate is then dipped in a dilute hydrofluoric acid (HF) solution to remove impurities formed during the above processes.

[0007] Usually, a portion of the undoped silicon dioxide layer is lost during the subsequent dipping process so that defects are formed in the undoped silicon dioxide layer mid the reliability of the shallow trench isolation structure is decreased. An annealing, process is performed at about 1100° C. to densify the undoped silicon dioxide layer before the dipping process to prevent the undoped silicon dioxide layer from losing material.

[0008] However, due to a large difference in thermal expansion coefficients and Young's modulus between the substrate and the undoped silicon dioxide layer, significant substrate stress of several hundred MPa occurs during the annealing process. Due to this stress, crystallographic detects occur in the undoped silicon dioxide layer, which defects enhance junction leakage and sub-threshold leakage. In dense pattern areas and high packing density memory devices, this causes the devices to fail.

[0009] Accordingly, the present invention provides a method for fabricating a shallow trench isolation structure that reduces stress incurred during the annealing process, junction leakage and sub-threshold leakage.

[0010] To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method for fabricating a shallow trench isolation structure. The method includes the following steps. A pad oxide layer and a silicon nitride layer are formed in sequence on a substrate. A trench formed in the substrate, and a liner oxide layer is formed on a sidewall of the trench. A doped silicon dioxide layer is formed on the silicon nitride layer and fills the trench. An annealing process is performed to densify the doped silicon dioxide layer. A portion of the doped silicon dioxide layer is removed to expose the silicon nitride layer by a planarization process.

[0011] In the invention, the doped silicon dioxide layer doped with germanium, nitrogen or refractory metals is used to form the shallow trench isolation structure. The thermal expansion coefficient and Young's modulus of the doped silicon dioxide layer is influenced by the dopants. Therefore, the thermal expansion coefficient and Young's modulus of the doped silicon dioxide layer are modulated by controlling the doping level to be comparable with the substrate. As a result stress incurred during the annealing process, is reduced. Furthermore, junction leakage and sub-threshold leakage are both reduced and the annealing process can be performed at lower temperature. Additionally, since the hydrofluoric acid etching rate of the doped silicon dioxide layer is different from that of the silicon dioxide layer, the kink effect can be also eliminated.

[0012] It is to be that both the foregoing general description, and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.

[0013] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings. FIGS. 1A through 1D are schematic, cross-sectional diagrams used to depict steps in a method for fabricating a shallow trench isolation structure.

[0014] Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

[0015] FIGS. 1A through 1D are schematic, cross-sectional diagrams used to depict steps in a method for fabricating a shallow trench isolation structure.

[0016] Referring to FIG. 1A, a pad oxide layer 102 and a silicon nitride layer 104 are formed in sequence on a substrate 100. A trench 106 is formed in the substrate 100. The pad oxide layer 102 is about 100 to 200 Å thick, and the silicon nitride layer 104 is about 1000 to 2000 Å thick. The step of forming the trench 106 includes anisotropic etching.

[0017] Referring to FIG. 1B, a liner oxide layer 108 which is about 100 to 500 Å thick, is formed on a sidewall of the trench 106. The liner oxide 108 layer 108 meets the pad oxide layer 102. The step of forming the liner oxide layer 108 includes thermal oxidation.

[0018] Referring to FIG. 1C. a doped silicon dioxide layer 110 is formed on the silicon nitride layer 104 and fills trench 106. The material doped into the doped silicon dioxide layer 110 is selected from a group consisting of germanium, nitrogen and refractory metals.

[0019] There are some methods used to form the doped silicon dioxide layer 110 having a germanium dopant.

[0020] In one method, a low pressure chemical vapor deposition (LPCVD) process or a high density plasma chemical vapor deposition (HDPCVD) process is performed at about 250 to 600° C. with a mixture of silane (SiH₄), germane (GeH₄) and oxygen serving as a gas source to form the doped silicon dioxide layer 110. Since germane reacts with oxygen to produce germanium oxide (GeO₂), the doped silicon dioxide layer 110 contains germanium oxide.

[0021] In the other method a sub-atmospheric chemical vapor deposition (SACVD) process or a atmospheric pressure chemical vapor deposition (APCVD) process is performed at about 250 to 600° C. with a mixture of tetraethosiloxane (TEOS) and tetramethoxy germanium (Ge(OCH₃)₄) serving as a gas source to form the doped silicon dioxide layer 110. In the process, tetramethoxy germanium is a precursor for forming germanium oxide. During the process, tetramethoxy germanium is transformed into germanium oxide, so the doped silicon dioxide layer 110 contains germanium oxide.

[0022] For achieving, nitrogen doping, ammonia nitrogen monoxide (N₂0) or nitrogen is added while performing a low pressure chemical vapor deposition process or a high density plasma chemical vapor deposition process to form the doped. silicon dioxide layer 110.

[0023] The refractory metal doped into the doped silicon dioxide layer 110 is a metal such as titanium. For example, tetraethoxy titanium (Ti(OC₂H₅)₄) is added while performing, a chemical vapor deposition process to form the doped silicon dioxide layer 110.

[0024] Referring to FIG. 1D, the doped silicon dioxide layer 110 (FIG. 3) is densified by performing an annealing process. The annealing process is performed at about 800 to 950° C. A portion of the doped silicon dioxide layer 110 (FIG. 3) is removed to expose the silicon nitride layer 104 by, for example, chemical-mechanical polishing (CMP). Then, a doped silicon dioxide layer 110ais formed within the trench 106. A wet etching process is performed with hydrofluoric acid serving as an etchant to remove native oxide formed during the above processes.

[0025] According to the foregoing, the advantages of the invention include the following:

[0026] 1. The thermal expansion coefficient and Young's modulus of the doped silicon dioxide layer is influenced by the dopants. Therefore, the thermal expansion coefficient and Young's modulus of the doped silicon dioxide layer are modulated by controlling, the doping level to be comparable with the substrate. By controlling the doping level, stress incurred during the annealing process is reduced. Furthermore, junction leakage and sub-threshold leakage are both reduced.

[0027] 2. Glass transition temperature of silicon dioxide is also influenced by the dopants. For example, glass transition temperature of doped silicon dioxide is reduced From 1160 to 800° C. when the mole percentage of germanium oxide in the doped silicon dioxide layer is about 4. According to this, the annealing process can be performed at lower temperature to densify the doped silicon dioxide layer. Since the annealing process is performed at lower temperature, stress incurred during the annealing process is reduced and the stability of chemical-mechanical polishing is improved.

[0028] 3. The hydrofluoric acid etching rate of the dopant is different from that of the silicon dioxide. For example the hydrofluoric acid etching rate of germanium oxide is lower than silicon dioxide. As a result the dopants prevent the doped silicon dioxide layer from losing material during the dipping process. Additionally, the kink effect can be also eliminate by preventing the doped silicon dioxide layer from losing material.

[0029] 4. Step coverage of the doped silicon dioxide layer is better than that of the undoped silicon dioxide layer, so that the doped silicon dioxide layer easily fills the trench.

[0030] It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.