Structure of high voltage transistor and method for fabricating the same

A structure of a high voltage transistor includes a substrate. A gate insulating layer is disposed on the substrate. A shallow trench isolation structure is formed in the substrate adjacent to the gate insulating layer. The shallow trench isolation structure includes a first sidewall and a second sidewall. A top portion of the first sidewall merges with a side region of the gate insulating layer. A bottom surface of the shallow trench isolation structure is gradually decreasing in depth from the second sidewall to the first sidewall. A source/drain region is formed in the substrate at a side of the gate insulating layer and surrounding the shallow trench isolation structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 108132051, filed on Sep. 5, 2019. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a semiconductor fabrication technology, in particular, to a structure of a high voltage transistor and a method for fabricating the same.

2. Description of Related Art

For application of more functions of electronic products, an operating voltage of a transistor formed by a semiconductor is not limited to a general low operating voltage anymore. That is, the transistor may also be operated within a high voltage range for some functions.

For a structure of a high voltage transistor, a gate insulating layer thereof requires a relatively great thickness for isolation of a gate, and shallow trench isolation structures are also additionally disposed at two ends of the gate insulating layer. In addition, high voltage source/drain regions at two sides of the gate may surround the shallow trench isolation structures. When the high voltage transistor is operated, a drift current may flow through a bottom portion and sidewall of the shallow trench isolation structure and enter a channel region below the gate.

For such a structure of the high voltage transistor, a path of the drift current turns for many times, so it is hard to maintain a high current, and it is likely to form corona discharge at a sharp part on the bottom portion of the shallow trench isolation structure to further interfere with flowing of the drift current.

Quality of a drift current may affect operation of a high voltage transistor. How to design a structure of a high voltage transistor to improve quality of a drift current is one of subjects that need to be considered in research and development.

SUMMARY OF THE INVENTION

The invention relates to a structure of a high voltage transistor and a method for fabricating the same. Shallow trench isolation may be improved to at least effectively improve the quality of a drift current and thus improve the operating performance of the high voltage transistor.

In an embodiment, the invention provides a structure of a high voltage transistor, which includes a substrate. A gate insulating layer is disposed on the substrate. A shallow trench isolation structure is formed in the substrate adjacent to the gate insulating layer. The shallow trench isolation structure includes a first sidewall and a second sidewall. A top portion of the first sidewall merges with a side region of the gate insulating layer. A bottom surface of the shallow trench isolation structure is gradually decreasing in depth from the second sidewall to the first sidewall. A source/drain region is formed in the substrate at a side of the gate insulating layer and surrounding the shallow trench isolation structure.

In an embodiment, the structure of the high voltage transistor further includes a gate layer disposed on the gate insulating layer and a contact doped region disposed in the substrate at a top portion of the source/drain region.

In an embodiment, for the structure of the high voltage transistor, the bottom surface of the shallow trench isolation structure is smoothly gradually decreasing in depth.

In an embodiment, for the structure of the high voltage transistor, the bottom surface of the shallow trench isolation structure includes no sharp protruding structure.

In an embodiment, for the structure of the high voltage transistor, a top portion of the shallow trench isolation structure further includes a depressed structure bonded to the side region of the gate insulating layer.

In an embodiment, for the structure of the high voltage transistor, a depth of the source/drain region below the first sidewall of the shallow trench isolation structure is greater than a depth of the source/drain region below the second sidewall of the shallow trench isolation structure.

In an embodiment, for the structure of the high voltage transistor, the bottom surface of the shallow trench isolation structure is an inclined surface gradually decreasing in depth from the second sidewall to the first sidewall.

In an embodiment, for the structure of the high voltage transistor, the gate insulating layer is a gate oxide layer with a thickness within a range and is configured to isolate the gate layer on the gate oxide layer under high voltage operation.

In an embodiment, the invention provides a method for fabricating a high voltage transistor. The method includes: providing a substrate, a middle region and a peripheral region adjacent to the middle region being laid out on the substrate; forming a gate insulating layer on the middle region on the substrate; forming a shallow trench isolation structure on the peripheral region in the substrate adjacent to the gate insulating layer, the shallow trench isolation structure including a first sidewall and a second sidewall, a top portion of the first sidewall merging with a side region of the gate insulating layer and a bottom surface of the shallow trench isolation structure being gradually decreasing in depth from the second sidewall to the first sidewall; and forming a source/drain region in the substrate at a side of the gate insulating layer and surrounding the shallow trench isolation structure.

In an embodiment, the method for fabricating the high voltage transistor further includes: forming a gate layer on the gate insulating layer, and forming a contact doped region in the substrate at a top portion of the source/drain region.

In an embodiment, for the method for fabricating the high voltage transistor, the step of providing the substrate includes patterning the substrate to form a sunken structure in height in the peripheral region. An edge of the sunken structure includes an interfacial surface relative to the middle region. The interfacial surface is gradually decreasing in height in a direction from the middle region to the peripheral region.

In an embodiment, for the method for fabricating the high voltage transistor, an isotropic etching procedure or a procedure for liner oxidation with cleaning is implemented to obtain the interfacial surface.

In an embodiment, for the method for fabricating the high voltage transistor, the step of forming the shallow trench isolation structure includes patterning the substrate to obtain a shallow trench located on the interfacial surface between the middle region and the peripheral region. With formation of the interfacial surface of the substrate, a bottom surface of the shallow trench is gradually decreasing in depth towards the middle region.

In an embodiment, for the method for fabricating the high voltage transistor, a bottom surface of the shallow trench isolation structure is smoothly gradually decreasing in depth.

In an embodiment, for the method for fabricating the high voltage transistor, the bottom surface of the shallow trench isolation structure includes no sharp protruding structure.

In an embodiment, for the method for fabricating the high voltage transistor, a top portion of the shallow trench isolation structure further includes a depressed structure bonded to the side region of the gate insulating layer.

In an embodiment, for the method for fabricating the high voltage transistor, a method for forming the depressed structure of the shallow trench isolation structure includes: forming a mask layer to cover a side portion of the shallow trench isolation structure, the mask layer and the side portion being opposite to the gate insulating layer; and etching a portion, exposed from the mask layer, of the shallow trench isolation structure to reduce a height of the shallow trench isolation structure.

In an embodiment, for the method for fabricating the high voltage transistor, a depth of the source/drain region below the first sidewall of the shallow trench isolation structure is greater than a depth of the source/drain region below the second sidewall of the shallow trench isolation structure.

In an embodiment, for the method for fabricating the high voltage transistor, the bottom surface of the shallow trench isolation structure is an inclined surface gradually decreasing in depth from the second sidewall to the first sidewall.

In an embodiment, for the method for fabricating the high voltage transistor, the gate insulating layer is a gate oxide layer with a thickness within a range and is configured to isolate the gate layer on the gate oxide layer under high voltage operation.

In order to make the aforementioned and other objectives and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.

DESCRIPTION OF THE EMBODIMENTS

The invention provides a structure of a high voltage transistor and a method for fabricating the same. According to the invention, shallow trench isolation is designed in such a manner that a bottom portion thereof has a gradually decreasing depth to at least effectively improve the quality of a drift current and thus improve the operating performance of the high voltage transistor.

A plurality of embodiments is provided below to describe the invention, but the invention is not limited to the embodiments. The embodiments can also be combined properly.

FIG. 1is a schematic diagram of a structure of a high voltage transistor according to an embodiment of the invention. In the invention, referring toFIG. 1, the operating performance of a structure of a high voltage transistor is looked into, and an improved structural design is further proposed. The structure of the high voltage transistor of the invention is fabricated on a substrate100. A shallow trench isolation structure102may be formed on the substrate at first. The shallow trench isolation structure102is usually configured to isolate the transistor. A basic structure of the high voltage transistor also includes a gate insulating layer104on the substrate100. A gate layer106is disposed on the gate insulating layer104. Since the transistor is operated at a high voltage, a thickness of the gate insulating layer104is relatively great and the gate layer106may be effectively isolated under high voltage operation. Doped regions are also formed in the substrate100on two sides of the gate layer104as source/drain regions108. A contact doped region110with a relatively high doping concentration is also formed at a top portion of the source/drain region108. The shallow trench isolation structure102at an outer side is usually configured to isolate the transistor.

However, for the high voltage transistor, the shallow trench isolation structure102may also be formed in the source/drain region108and merges with the gate insulating layer104to implement isolation under high voltage operation. Due to the shallow trench isolation structure102in the source/drain region108, a drift current planned to flow through a channel region, for example, a path112, may pass through a bottom portion of the shallow trench isolation structure102at first. Therefore, a space of the source/drain region108is occupied by the shallow trench isolation structure102to reduce the drift current. In addition, a sharp protruding structure at the bottom portion of the shallow trench isolation structure102is also likely to form corona discharge and further affect the quality of the drift current and reduce the efficiency of the high voltage transistor.

After a mechanism of the high voltage transistor inFIG. 1is looked into, the invention discloses a structure of the high voltage transistor to at least effectively improve the quality of the drift current and improve the efficiency of the high voltage transistor.

FIG. 2is a schematic diagram of a structure of a high voltage transistor according to an embodiment of the invention. Referring toFIG. 2, a further improved structure of a high voltage transistor of the invention is described at first. The high voltage transistor is formed on a substrate100. A gate insulating layer104is disposed on the substrate100. Shallow trench isolation structures102′ and102″ are formed in the substrate100adjacent to the gate insulating layer104. The shallow trench isolation structures102′ and102″ include first sidewalls and second sidewalls. Here, a top portion of the first sidewall merges with a side region of the gate insulating layer104.

It is to be noted that bottom surfaces of the shallow trench isolation structures102′ and102″ are gradually decreasing in depth from the second sidewalls to the first sidewalls, as shown in regions114, and may be, for example, smoothly gradually decreasing surfaces and may further be, for example, inclined surfaces. Source/drain regions108are formed in the substrate100at two sides of the gate insulating layer104and surround the shallow trench isolation structures102′ and102″. In an embodiment, there may be a contact doped region110configured to connect another element at a top portion of the source/drain region108.

A shallow trench isolation structure102at an outer side of the transistor is a structure usually configured to isolate an element. A plurality of shallow trench isolation structures102,102′ and102″ may be, for example, completed at the same time in the same fabrication procedure. However, depressed structures116may further be correspondingly formed in top portions of the shallow trench isolation structures102′ and102″ of the source/drain regions108. By the depressed structures116, heights of the shallow trench isolation structures102′ and102″ may be reduced, and meanwhile, thicknesses of the shallow trench isolation structures102′ and102″ in the regions may also be reduced. In an embodiment, in a subsequent embedding procedure of forming the source/drain region108, a bottom surface of the source/drain region108also correspondingly has a relatively great depth.

For the structure of the high voltage transistor shown inFIG. 2, the bottom surfaces of the shallow trench isolation structures102′ and102″ are gradually decreasing in depth, for example, as shown in the regions114. In addition, the bottom surface of the source/drain region108may also correspondingly have a relatively great depth due to the depressed structure116. Therefore, a drift current from the source/drain region108to a channel region below the gate layer may be effectively increased. Compared with the structure shown inFIG. 1, bottom portions of the shallow trench isolation structures102′ and102″ may also include no sharp protruding structure. The quality of the drift current is improved, and the efficiency of the high voltage transistor is also improved.

Then, fabrication of the structure of the high voltage transistor shown inFIG. 2will be described below with some embodiments. In the invention, for obtaining structures of the bottom portions of the shallow trench isolation structures102′ and102″, smooth depressed curved surfaces are required to be formed at first in the regions where the shallow trench isolation structures102′ and102″ are predetermined to be formed in the substrate100.FIG. 3A-FIG. 3Dare schematic flowcharts of fabricating a substrate of a high voltage transistor according to an embodiment of the invention.

Referring toFIG. 3A, in an embodiment, a mask layer250and a photoresist layer252are formed on the substrate200at first. The mask layer250is exposed from an opening254of the photoresist layer252. For a region covered by the photoresist layer252, referring toFIG. 3B, an etching procedure256is implemented to etch the mask layer250by use of the photoresist layer252as an etching mask, and then a surface of the substrate200is also etched to obtain a sunken region258.

For the substrate200, the region covered by the photoresist layer252is a region where a gate structure is predetermined to be formed subsequently, and is also called a middle region herein. The sunken region258of the substrate200is also called a peripheral region. An interface region between the middle region and the peripheral region is where the shallow trench isolation structures102′ and102″ are predetermined to be formed.

Referring toFIG. 3C, for ensuring that the shallow trench isolation structures102′ and102″ may have depth gradually decreasing structures shown as the regions114when being subsequently formed, the interface region between the middle region and the peripheral region is required to have the smooth sunken region258. In an embodiment, after the photoresist layer252is removed, an anisotropic etching procedure260is implemented to further etch the substrate200to obtain a better smooth sunken region258. The anisotropic etching procedure260is, for example, wet etching. However, the anisotropic etching procedure260is not the only choice, and another manner may also be adopted instead. In an embodiment, referring toFIG. 3D, a procedure262for liner oxidation with cleaning may also be implemented to further form a better smooth sunken region258of the substrate200. Thin-layer oxidation may be implemented on the substrate200to obtain a liner oxide layer by a liner oxidation step, and then the liner oxide layer is removed by a subsequent cleaning step.

In an embodiment, by a smooth curved surface at the interface region between the sunken region258, i.e., the peripheral region, and the middle region, the smooth curved surfaces with gradually decreasing depth as shown at the regions114may be obtained when the shallow trench isolation structures102′ and102″ are subsequently formed.

Formation of the final required shallow trench isolation structure will be described below according to the substrate200obtained inFIG. 3D.FIG. 4A-FIG. 4Dare schematic flowcharts of fabricating a shallow trench isolation structure of a high voltage transistor according to an embodiment of the invention.

Referring toFIG. 4A, in an embodiment, an oxide layer202and a mask layer204are formed on the substrate200. The oxide layer202is, for example, acts as a pad oxide layer. A photoresist layer206is formed on the mask layer204. The photoresist layer206has openings208corresponding to the interface region between the peripheral region and middle region of the substrate200. Regions of the openings208are where the shallow trench isolation structures102′ and102″ shown inFIG. 2are predetermined to be subsequently formed.

Referring toFIG. 4B, an etching procedure is implemented to form shallow trenches210in the substrate200by taking the photoresist layer206as an etching mask. It may be observed here that a bottom surface region212of the shallow trench210is smoothly gradually decreasing in depth in a direction from the peripheral region to the middle region.

Referring toFIG. 4C, an insulating layer214is, for example, an oxide layer deposited on the substrate200and filling the shallow trenches210. Referring toFIG. 4D, in an embodiment, a grinding procedure is implemented. The grinding procedure is, for example, stopped on the mask layer204and the other portion of the insulating layer214fills the shallow trenches210to form shallow trench isolation structures216. A bottom surface of the shallow trench isolation structure216has formed a depth gradually decreasing structure. By such a structure, a current value of the drift current may be increased. Here, the shallow trench isolation structures216correspond to the shallow trench isolation structures102′ and102″ inFIG. 2. The peripheral shallow trench isolation structure102may be formed together. Formation of the peripheral shallow trench isolation structure102is not limited in an embodiment of the invention.

In an embodiment, if the depressed structures116at the top portions of the shallow trench isolation structures102′ and102″ are further to be formed, as shown inFIG. 2, some procedures may further be implemented.FIG. 5A-FIG. 5Dare schematic flowcharts of fabricating a shallow trench isolation structure of a high voltage transistor according to an embodiment of the invention.

Referring toFIG. 5A, the mask layer204is removed. The oxide layer202merges with the shallow trench isolation structure216. Referring toFIG. 5B, the mask layer300covers part of the oxide layer202and the shallow trench isolation structure216. A region302, close to the middle region, of the shallow trench isolation structure216is exposed. Referring toFIG. 5C, a portion, in the region302, of the shallow trench isolation structure216is etched to form a depressed structure304by taking the mask layer300as the etching mask. In addition, part of the oxide layer202is also removed to expose the substrate200.

Referring toFIG. 5D, an exposed region of the substrate200is oxidized to obtain a gate insulating layer306. The gate insulating layer306corresponds to high voltage operation and thus has a relatively great thickness. The gate insulating layer306may merge with the shallow trench isolation structure216, and the depressed structure304of the shallow trench isolation structure216is also changed into a depressed structure308. The mask layer300may also be subsequently removed.

Then, referring toFIG. 2again, the source/drain regions108and the gate layer106are formed based on the shallow trench isolation structure216inFIG. 5D. The depressed structure116inFIG. 2is formed at the depressed structure308of the shallow trench isolation structure216inFIG. 5D. Therefore, the quality of the drift current may at least be improved in the invention.

Based on the above, from a method for fabricating the high voltage transistor, the method includes that the substrate100is provided. The middle region and the peripheral region adjacent to the middle region are laid out on the substrate100. The gate insulating layer104is formed on the middle region of the substrate100. The shallow trench isolation structures102′ and102″ are formed on the peripheral region in the substrate adjacent to the gate insulating layer104. The shallow trench isolation structures102′ and102″ include the first sidewalls and the second sidewalls. The top portions of the first sidewalls merge with the side region of the gate insulating layer104. The bottom surfaces of the shallow trench isolation structures102′ and102″ are gradually decreasing in depth from the second sidewalls to the first sidewalls. The source/drain regions108are formed in the substrate at the side of the gate insulating layer and surrounding the shallow trench isolation structures102′ and102″.

Although the invention is described with reference to the above embodiments, the embodiments are not intended to limit the invention. A person of ordinary skill in the art may make variations and modifications without departing from the spirit and scope of the invention. Therefore, the protection scope of the invention should be subject to the appended claims.