TECHNICAL FIELD

A technical field of the present invention relates to a semiconductor device and a manufacturing method of the semiconductor device. Note that semiconductor devices herein refer to general elements and devices which function by utilizing semiconductor characteristics.

BACKGROUND ART

There are a wide variety of metal oxides and such material oxides are used for various applications. Indium oxide is a well-known material and is used as a material for transparent electrodes which are needed for liquid crystal display devices or the like.

Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such a metal oxide is already known (see, for example, Patent Documents 1 to 4, Non-Patent Document 1, and the like).

Meanwhile, multi-component oxides are also known as metal oxides. For example, InGaO3(ZnO)m(m: natural number) having a homologous phase is known as a multi-component oxide semiconductor including In, Ga, and Zn (see, for example, Non-Patent Documents 2 to 4 and the like).

Furthermore, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide can also be applied to a channel formation region of a thin film transistor (see, for example, Patent Document 5, Non-Patent Documents 5 and 6, and the like).

REFERENCES

Patent Documents

[Patent Document 1] Japanese Published Patent Application No. S60-198861[Patent Document 2] Japanese Published Patent Application No. H8-264794[Patent Document 3] Japanese Translation of PCT International Application No. H11-505377[Patent Document 4] Japanese Published Patent Application No. 2000-150900[Patent Document 5] Japanese Published Patent Application No. 2004-103957

Non-Patent Documents

[Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G. Muller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M. Wolf, “A ferroelectric transparent thin-film transistor”,Appl. Phys. Lett.,17 Jun. 1996, Vol. 68, pp. 3650-3652[Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri, “The Phase Relations in the In2O3—Ga2ZnO4—ZnO System at 1350° C.”,J. Solid State Chem.,1991, Vol. 93, pp. 298-315[Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura, “Syntheses and Single-Crystal Data of Homologous Compounds, In2O3(ZnO)m(m=3, 4, and 5), InGaO3(ZnO)3, and Ga2O3(ZnO)m(m=7, 8, 9, and 16) in the In2O3—ZnGa2O4—ZnO System”,J. Solid State Chem.,1995, Vol. 116, pp. 170-178[Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M. Isobe, “Syntheses and crystal structures of new homologous compounds, indium iron zinc oxides (InFeO3(ZnO)m) (m: natural number) and related compounds”,KOTAI BUTSURI(SOLID STATE PHYSICS), 1993, Vol. 28, No. 5, pp. 317-327[Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, and H. Hosono, “Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor”,SCIENCE,2003, Vol. 300, pp. 1269-1272[Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”,NATURE,2004, Vol. 432, pp. 488-492

DISCLOSURE OF INVENTION

It can hardly be said that a transistor including a conventional oxide semiconductor has characteristics sufficient for practical use, and transistor characteristics such as subthreshold swing (S value), on/off ratio, and reliability need to be improved.

In view of this, it is an object of an embodiment of the disclosed invention to provide a semiconductor device with a novel structure and favorable characteristics.

Alternatively, it is an object to provide a manufacturing method of the semiconductor device with a novel structure.

With respect to a transistor including an oxide semiconductor according to one embodiment of the present invention, which has a source electrode and a drain electrode with an oxidized side surface, characteristics such as subthreshold swing (S value), on/off ratio, and reliability are significantly improved. Specifically, the following structure can be employed, for example.

An embodiment of the present invention is a semiconductor device including an oxide semiconductor layer, a source electrode and a drain electrode electrically connected to the oxide semiconductor layer, a gate insulating layer covering the oxide semiconductor layer, the source electrode, and the drain electrode, and a gate electrode over the gate insulating layer. The source electrode and the drain electrode have an oxide region formed by oxidizing a side surface thereof. Note that the oxide region is formed while oxygen is supplied to the oxide semiconductor layer.

In the above embodiment, it is preferable that the oxide region of the source electrode and the drain electrode be formed by plasma treatment with a high frequency power of 300 MHz to 300 GHz and a mixed gas of oxygen and argon. It is also preferable that a protective insulating layer having substantially the same planar shape as that of the source electrode and the drain electrode be provided over the source electrode and the drain electrode. Note that the expression “substantially the same” or “substantially same” does not necessarily mean being exactly the same in a strict sense and can mean being considered as the same. For example, a difference made by a single etching process is acceptable.

In the above embodiment, it is also preferable that the hydrogen concentration of the oxide semiconductor layer be 5×1019/cm3or less. It is also preferable that the off-state current be 1×10−13A or less.

An embodiment of the present invention is a manufacturing method of a semiconductor device, which includes the step of forming an oxide semiconductor layer over a substrate, forming a source electrode and a drain electrode electrically connected to the oxide semiconductor layer, forming a gate insulating layer covering the oxide semiconductor layer, the source electrode, and the drain electrode after oxidizing a side surface of the source electrode and the drain electrode, and forming a gate electrode over the gate insulating layer. Note that oxygen is supplied to the oxide semiconductor layer when the side surface of the source electrode and the drain electrode is oxidized.

In the above embodiment, it is preferable that the side surface of the source electrode and the drain electrode be oxidized by plasma treatment with a high frequency power of 300 MHz to 300 GHz and a mixed gas of oxygen and argon.

In the above embodiment, it is also preferable that a protective insulating layer having substantially the same planar shape as that of the source electrode and the drain electrode be formed over the source electrode and the drain electrode.

In the above embodiment, it is also preferable that the off-state current be made 1×10−13A or less by making the hydrogen concentration of the oxide semiconductor layer 5×1019/cm3or less.

Note that the term such as “over” or “below” in this specification and the like does not necessarily mean that a component is placed “directly on” or “directly under” another component. For example, the expression “a first gate electrode over a gate insulating layer” does not exclude the case where a component is placed between the gate insulating layer and the gate electrode. Moreover, the terms such as “over” and “below” are only used for convenience of description and can include the case where the vertical relation of components is reversed, unless otherwise specified.

In addition, the term such as “electrode” or “wiring” in this specification and the like does not limit a function of a component. For example, an “electrode” can be used as part of a “wiring”, and the “wiring” can be used as part of the “electrode”. Furthermore, the term “electrode” or “wiring” can include the case where a plurality of “electrodes” or “wirings” is formed in an integrated manner.

Functions of a “source” and a “drain” are sometimes interchanged with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be used to denote the drain and the source, respectively, in this specification.

Note that the term “electrically connected” in this specification and the like includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object.

Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and an element with a variety of functions as well as an electrode and a wiring.

According to an embodiment of the disclosed invention, characteristics of a transistor including an oxide semiconductor are further improved by supplying oxygen to an oxide semiconductor layer. Here, the oxygen supply process brings the result that a side surface of a source or drain electrode is oxidized in a transistor including an oxide semiconductor.

By oxidizing a side surface of a source or drain electrode, it is possible to prevent short circuit between a gate electrode and the source or drain electrode which may be caused by a reduction in thickness of a gate insulating layer or defective coverage therewith.

By supplying oxygen to an oxide semiconductor layer as described above, a semiconductor device with a novel structure and excellent characteristics can be realized.

