BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including a transistor and a method for manufacturing the semiconductor device.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquid crystal display device and a light-emitting display device are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Further, transistors formed using such silicon semiconductors are used in integrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which, instead of a silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used for transistors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.

For example, a technique is disclosed in which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like of a pixel of a display device (see Patent Documents 1 and 2).

REFERENCE

Patent Documents

[Patent Document 1] Japanese Published Patent Application No. 2007-123861

[Patent Document 2] Japanese Published Patent Application No. 2007-96055

SUMMARY OF THE INVENTION

In a transistor using an oxide semiconductor film, there are a large number of oxygen vacancies in the oxide semiconductor film, which causes poor electrical characteristics of the transistor and causes an increase in the amount of change in electrical characteristics of the transistor, typically the threshold voltage due to a change over time or a stress test (e.g., a bias-temperature (BT) stress test).

Thus, an object of one embodiment of the present invention is to reduce defects in an oxide semiconductor film of a semiconductor device including the oxide semiconductor film. Another object of one embodiment of the present invention is to improve electrical characteristics of a semiconductor device including an oxide semiconductor film. Another object of one embodiment of the present invention is to improve reliability of a semiconductor device including an oxide semiconductor film.

One embodiment of the present invention is a semiconductor device including a transistor including a gate electrode formed over a substrate, a gate insulating film covering the gate electrode, a multilayer film overlapping with the gate electrode with the gate insulating film provided therebetween, and a pair of electrodes in contact with the multilayer film, a first oxide insulating film covering the transistor, and a second oxide insulating film formed over the first oxide insulating film. In the semiconductor device, the multilayer film includes an oxide semiconductor film and an oxide film containing In or Ga, the oxide semiconductor film has an amorphous structure or a microcrystalline structure, the first oxide insulating film is an oxide insulating film through which oxygen is permeated, the second oxide insulating film is an oxide insulating film containing more oxygen than that in the stoichiometric composition, and the transistor has characteristics in which, by a bias-temperature stress test, threshold voltage does not change or changes in a positive direction or a negative direction and the amount of the change in a negative direction or a positive direction is less than or equal to 1.0 V, preferably less than or equal to 0.5 V.

Note that the oxide semiconductor film preferably contains In or Ga.

Further, the energy of the conduction band bottom of the oxide film containing In or Ga is closer to the vacuum level than the energy of the conduction band bottom of the oxide semiconductor film is. Furthermore, the difference between the energy of the conduction band bottom of the oxide film containing In or Ga and the energy of the conduction band bottom of the oxide semiconductor film is preferably greater than or equal to 0.05 eV and less than or equal to 2 eV. Note that an energy difference between the vacuum level and the conduction band bottom is also referred to as an electron affinity. Thus, it is preferable that the electron affinity of the oxide film containing In or Ga be less than the electron affinity of the oxide semiconductor film and the difference be greater than or equal to 0.05 eV and less than or equal to 2 eV.

Further, it is preferable that each of the oxide semiconductor film and the oxide film containing In or Ga be an In-M-Zn oxide (M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) film and the proportion of M atoms in the oxide film containing In or Ga be higher than that in the oxide semiconductor film.

Further, it is preferable that the silicon concentration and the carbon concentration between the oxide semiconductor film and the oxide film containing In or Ga be lower than 2×1018atoms/cm3.

Another embodiment of the present invention is a method for manufacturing a semiconductor device, including the steps of forming a gate electrode and a gate insulating film, forming, over the gate insulating film, a multilayer film including an oxide semiconductor film having an amorphous structure or a microcrystalline structure and an oxide film containing In or Ga, forming a pair of electrodes in contact with the multilayer film, forming a first oxide insulating film over the multilayer film and the pair of electrodes, and forming a second oxide insulating film over the first oxide insulating film. In the method, a substrate placed in a treatment chamber which is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 400° C., pressure in the treatment chamber is set to be greater than or equal to 20 Pa and less than or equal to 250 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power is supplied to an electrode provided in the treatment chamber to form the first oxide insulating film. In addition, the substrate placed in the treatment chamber which is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 260° C., pressure in the treatment chamber is set to be greater than or equal to 100 Pa and less than or equal to 250 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power higher than or equal to 0.17 W/cm2and lower than or equal to 0.5 W/cm2is supplied to the electrode provided in the treatment chamber to form the second oxide insulating film.

In accordance with one embodiment of the present invention, defects in an oxide semiconductor film of a semiconductor device including the oxide semiconductor film can be reduced. Further, in accordance with one embodiment of the present invention, the electrical characteristics of a semiconductor device including an oxide semiconductor film can be improved. Further, in accordance with one embodiment of the present invention, reliability of a semiconductor device or the like including an oxide semiconductor film can be improved.

