This application claims the priority benefit of Taiwan application serial no. 93113275, filed on May 12, 2004.
1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device. More particularly, the present invention relates to a method of fabricating a shallow trench isolation (STI) structure.
2. Description of Related Art
With the rapid progress in integrated circuit technologies, device miniaturization and integration is an important subject. As the size of integrated circuit devices continues to shrink, the level of integration increases but the structures for isolating devices also reduces correspondingly. In other words, increasingly difficult techniques have to be deployed to isolate devices. The conventional isolation structure such as the field oxide layer formed by the local oxidation of silicon (LOCOS) method is no longer suitable when the device is increasingly miniaturized. The so-called bird's beak encroachment often leads to the production of leakage current and results in the appearance of white spots on an image screen.
Because of the difficulties in fabricating miniaturized isolating devices, other methods of isolating devices have been developed. One of the most widely adopted isolation methods is to form a shallow trench isolation (STI) structure. In particular, the STI structure is suitable for fabricating sub-half micron integrated circuits.
To form a conventional STI structure, an oxide pad layer and a silicon nitride mask are sequentially formed over a substrate. An anisotropic dry etching operation is carried out to produce a trench with abrupt sidewalls in the substrate. Thereafter, a liner layer is formed on the surface of the trench by performing a thermal oxidation. Insulating material is deposited over the substrate to fill the trench. Any excess insulating material above the silicon nitride mask is removed. Finally, the silicon nitride mask and the oxide pad layer are removed.
In the aforementioned STI structure, abrupt corners are formed near the top section of the trench. Thus, the gate oxide layer close to the top corners of the STI structure has a thickness smaller than the gate oxide layer in the active region (as shown in
Furthermore, because the liner layer on the surface of the trench is formed by performing a thermal oxidation process to convert a portion of the silicon into silicon oxide, a layer of silicon on next to the surface of the trench is consumed. Hence, the thermal oxidation process expands the profile of the STI structure so that the actual usable active region is reduced. In other words, the process contributes some additional variables to the design and layout of devices that may adversely affect the miniaturization of semiconductor devices.
To resolve the aforementioned problems, some prior arts proposed a method of fabricating a shallow trench isolation (STI) structure.
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In the process of forming the opening 206, over-etching may occur in an attempt to remove the oxide pad layer 202 completely leading to the formation of a recess in the substrate 200. After etching back the oxide pad layer 202, two separate corners 214 and 214′ are formed at the top section of the trench 210 (as shown in
Accordingly, the present invention is directed to a method of fabricating a shallow trench isolation (STI) structure that utilizes a spacer and a mask layer as an etching mask to form a narrower trench in a substrate. Hence, the subsequent formation of a liner layer on the surface of trench will not expand the sectional profile of the STI structure too much. Ultimately, a larger active region for forming semiconductor devices is provided.
The prevent invention is also directed to a method of fabricating a shallow trench isolation (STI) structure that produces a trench with round top corners. Hence, the subsequently formed gate oxide layer will have a more uniform thickness and thereby reduce any leakage current at the top section of the STI structure or the upper region adjacent to the active devices.
According to an embodiment of the present invention, first, a substrate is provided. An oxide pad layer, a silicon pad layer and a mask layer are sequentially formed over the substrate. Thereafter, the mask layer and the silicon pad layer are patterned to form an opening that exposes a portion of the silicon pad layer. A spacer is formed on the sidewall of the opening and then a portion of the oxide pad layer is removed to expose a portion of the substrate. Using the mask layer with the spacer as an etching mask, a portion of the substrate is removed to form a trench. The spacer is removed and the oxide pad layer is etched back to form an undercut. Next, insulating material is deposited over the substrate to fill the trench. The patterned mask layer, the silicon pad layer, the oxide pad layer and a portion of the insulating material are removed to form the STI structure.
The present invention is also directed to an alternative method of fabricating a shallow trench isolation (STI) structure. First, a substrate having an pad layer and a mask layer already formed thereon is provided. The mask layer is patterned to form an opening that exposes a portion of the pad layer. Thereafter, a pair of spacers is formed on the sidewall of the opening. Using the mask layer with the spacers as an etching mask, a portion of the pad layer and the substrate is removed to form a trench in the substrate. The spacer is removed and the pad layer is etched back to form an undercut under the pad layer. Next, insulating material is deposited over the substrate to fill the trench. The patterned mask layer and the pad layer are removed. Finally, a portion of the insulating material is removed to form the STI structure.
In the present invention, the spacer and the mask layer is used as an etching mask to form a narrower trench in the substrate. Therefore, the process of forming the liner layer on the sidewall of the trench will not expand the profile of the STI structure too much. In other words, the process is able to provide a larger active region for forming semiconductor devices. Furthermore, the present invention provides a method of forming a trench with round upper corners so that a subsequently formed gate oxide layer can have a more uniform thickness. Hence, the chance of developing a leakage current at the top section of the STI structure or the upper region adjacent to the active devices is minimized.
It is to be understood 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.
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.
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.
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In the aforementioned embodiment, the spacer 312 and the mask layer 306 are used as an etching mask to form a narrower trench in the substrate 300. Hence, the formation of the liner oxide layer 316 on the sidewall of the trench will not expand the profile of the STI structure too much. Furthermore, after removing the spacer 312 and undercutting the pad oxide layer 302, the surrounding area at the top section of the trench 314 is exposed. Later on, the silicon on the trench surface is permitted to react with oxygen to form a silicon oxide layer. After lattice reorganization, the top corners of the trench 314 are rounded. Therefore, thickness and uniformity of a subsequently formed gate oxide layer will be improved.
In summary, the advantages of the present invention at least includes as follows.
1. A spacer and a mask layer are used as an etching mask to form a narrower trench in the substrate. Through the formation of a liner oxide layer, the top corners of the trench are rounded. Since the formation of a narrower trench and the liner oxide layer tends to complement each other, the method will neither affect the profile of the STI structure nor reduce the active region.
2. The rounding of the upper corners of a trench facilitates the subsequent fabrication of a gate oxide layer with substantially uniform thickness and reduce current leakage from the top section of the STI structure or the top area adjacent to the active devices.
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.
Number | Date | Country | Kind |
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93113275 | May 2004 | TW | national |