Silicon-on-Insulator Semiconductor Device Improving Electrostatic Discharge Protection Capability and Fabrication Method Thereof

1. Field of the Invention

The present invention relates to a semiconductor device, and more particularly to a silicon-on-insulator (SOI) semiconductor device and a fabrication method thereof that improves an electrostatic discharge (ESD) protection capability.

2. Description of the Conventional Art

Recently, a silicon-on-insulator (SOI) technique has been considered as one of the most effective techniques for improving performance of a very large scale integration (VLSI). According to the SOI technique, circuit elements of the integrated circuit are isolated by a buried oxide film, thereby latch up of transistors does not occur and junction capacitance decreases. Therefore, the SOI semiconductor device has an advantage that, due to the decrease of parasitic capacitance, the operation speed thereof is faster than an integrated circuit device of the same dimension provided on a bulk silicon substrate. Further, a semiconductor device using the SOI substrate is highly applicable to fabrication of a metal oxide semiconductor field effect transistor (MOSFET) under the size of 0.1 .mu.m due to its improvement of a short channel effect, intensive tolerance to radiation, a simple fabricating process and excellent device isolating property.

However, the problem in fabricating the integrated circuit using the SOI substrate is that the buried insulator which enables the integrated circuit device to have excellent property affects the ESD protection capability, which becomes hindrance to replace the bulk silicon substrate by the SOI substrate.

For reference, the comparison of the ESD protection capability of the bulk silicon substrate with that of the SOI substrate was disclosed in "Comparison of ESD Protection Capability of SOI and Bulk CMOS Output Buffers", p. 292-298 of Proceedings of 1994 IEEE/IRPS by Mansun Chan, Seling S. Yuen, Zhi-Jian Ma, Kelvin Y. Hu, Ping K. Ho, and Chenming Hu. As discussed therein the reason of deterioration of the ESD protection capability of the SOI substrate is that since an ESD protecting transistor provided in the SOI substrate includes a buried insulator (SiO.sub.2) formed under a drain thereof, p-n junction is not to be formed and thus there is no series resistance, while an ESD protecting transistor in the bulk silicon substrate has p-n junction between the substrate and a drain, thereby forming low series resistance. Further, when heat is generated during the ESD event, heat sink of a silicon oxide film which is the buried oxide film is inferior to the bulk silicon. Therefore, when fabricating the ESD protecting device by using the SOI substrate, as a silicon film on the buried oxidation becomes thick, the ESD protection capability improves. FIG. 1 illustrates the change of an ESD failure voltage in accordance with the thickness of the silicon film described in the above reference, wherein the numbers shown along an axis X indicate thickness of the silicon film formed on the buried insulator of the SOI substrate and the numbers along an axis Y show values of the ESD failure voltage. Here, as it goes to the right side of the axis X, the silicon film becomes thick. As shown therein, the thickness of the silicon film provided on the buried oxide film is proportional to the ESD failure voltage, which means that as the silicon film becomes thick, it becomes more tolerable to the ESD of a higher voltage. Accordingly, the more the ESD failure voltage increases, the higher reliability of the ESD protection capability becomes.

Accordingly, the present invention is directed to an SOI semiconductor device and a fabrication method thereof that obviate the problems and disadvantages due to the conventional art.

An object of the present invention is to provide an SOI semiconductor device and a fabrication method thereof that improve the ESD protection capability of an integrated circuit device.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in an SOI substrate having a buried oxide film having a relatively deep-formed portion and a relatively shallow-formed portion, an SOI semiconductor device in which an ESD protection device is formed at the relatively deep-formed portion of the buried oxide film while an integrated circuit is formed at the shallow-formed portion thereof.

Also, to achieve above object, there is provided a fabrication method for an SOI semiconductor device, which includes providing a bulk silicon substrate; forming a buried oxide film in a relatively deep part of a predetermined portion of the bulk silicon substrate and a buried oxide film in a relatively shallow part of the other portion thereof; forming an electrostatic discharge (ESD) protection device at a portion which the buried oxide film is relatively deeply formed; and a forming an integrated circuit device at a portion which the buried oxide film is shallowly formed.

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

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

In the drawings:

FIG. 1 is a graph illustrating change of an ESD failure voltage in accordance with thickness of a silicon film

FIG. 2 is a schematic vertical cross-sectional diagram of an SOI semiconductor device according to the present invention; and

FIGS. 3A through 3G are diagrams sequentially illustrating a method of fabricating an SOI semiconductor device according to the present invention .

Reference will now be made in detail to the preferred embodiment of the present invention, examples of which are illustrated in the accompanying drawings.

It is noted that an SOI semiconductor device according to the present invention is developed in view of which the ESD protection capability improves as a silicon film is thickly formed.

FIG. 2. is a schematic vertical cross-sectional diagram of an SOI semiconductor device according to the present invention. As shown therein, an SOI substrate 200 is fabricated by which a buried oxide film 201, which is generally a silicon oxide film, is formed in a bulk silicon substrate. The buried oxide film 201 formed in the bulk silicon substrate which electrically isolates silicon films 202a, 202b from a bulk silicon 203 includes a first region A1 and a second region A2 at which there are to be formed integrated circuit elements and an ESD protection device, respectively, the first and second regions A1, A2 having different buried depth. That is, a buried depth D2 of the second region A2 is deeper than a buried depth D1 of the first region A1 and thus the silicon film 202b of the second region A2 is formed thicker than the silicon film 202a of the first region A1. In addition, an integrated circuit device T1 is provided at the first region A1 and an ESD protection device T2 is formed at the second region A2, the devices T1, T2 being electrically isolated from each other. More specifically, the integrated circuit device T1 is provided with a gate electrode 11 formed on the silicon film 202a, and a source 12 and a drain 13 formed at both sides of the gate electrode 11 of the silicon film 202a. While, the ESD protection device T2 includes a gate electrode 21 formed on the silicon film 202b, and a source 22 and a drain 23 formed at both sides of the gate electrode 21 in the silicon film 202b.

The fabrication process for the SOI semiconductor device according to the present invention will be described in detail with reference to FIGS. 3A-3G.

First, as shown in FIG. 3A, an insulator 301 formed of a silicon oxide(or nitride) film is provided on a bulk silicon substrate 300. It is desirable to apply a material of the insulator 301 which has a large etching selection ratio with the bulk silicon substrate 300.

Then, to form an ESD protection device by a photo-etching process, as shown in FIG. 3B, the insulator 301 of the second region A2 is partially removed to thereby form a pattern 301a of the insulator 301 on the first region A1 for placing the integrated circuit device thereon.

An oxygen ion implantation process is applied to an entire structure of FIG. 3B. Here, it is noted that in the insulator pattern 301a an oxygen ion is relatively shallowly implanted into a portion of the bulk silicon substrate 300 on which the insulator pattern 301a is formed, while the oxygen ion is relatively deep implanted into a portion thereof in the second region A2 on which the insulator pattern 301a is not covered. The resultant structure is illustrated in FIG. 3C. In FIG. 3C, a dotted portion in the bulk silicon substrate 300 is a region having the oxygen ion.

Then, by applying a heat treatment to the substrate 300 of FIG. 3C, the oxygen ion and silicon of the bulk silicon substrate 300 are reacted and thus a silicon oxide film 302 is formed, as shown in FIG. 3D, which results in an SOI substrate 300, the silicon oxide film 302 being called a buried insulator or a buried oxide film. Here, in the bulk silicon substrate 300 semiconductor devices (an integrated circuit device and an ESD protection device) will be formed on an upper portion of the silicon oxide film 302, and the upper portion will be called a silicon film 303. For the convenience of description, a thin silicon film of the first region A1 is expressed in a numeral 303a, while a thick silicon film of the second region A2 to be an area on which the ESD protection device will be formed is expressed in a numeral 303b.

Next, an device isolating insulator 304 to isolate each unit device is shown in FIG. 3E which illustrates an embodiment of forming the field oxide film 304 as the insulator in accordance with a local oxidation of silicon (LOCOS) process. Here, it is noted that the insulator 304 may be formed in a shallow trench isolation process or other processes for isolating devices can be applied.

After forming a gate insulator 306 on the silicon films 303a, 303b of FIG. 3E, a gate electrode 307a of the integrated circuit device and a gate electrode 307b of the ESD protection device will be respectively formed on predetermined portions of the gate insulator 306, as shown in FIG. 3F.

Lastly, in FIG. 3G, a source 308a and a drain 308b are formed by implanting an impurity ion into the both silicon films 303a, 303b of the gate electrode 307a, 307b, respectively, thus the fabrication of the SOI semiconductor device according to the present invention is completed.

As described above, the SOI semiconductor device and the fabrication method of the invention has advantages of the SOI semiconductor device, such as a rapid operation of the integrated circuit device due to decrease of latch-up and parasitic capacitance and an integration increase and excellent device isolation due to improvement of the short channel effect, as well as excellent reliability and performance because the ESD protection capability is improved, which may raise market competitiveness of the products.

It will be apparent to those skilled in the art that various modifications and variations can be made in the SOI semiconductor device and the fabrication method thereof of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.