Semiconductor device and method for manufacturing the same

Abstract

To provide a semiconductor device which has transistor characteristics with little variation and includes an oxide semiconductor. The semiconductor device includes an insulating film over a conductive film and an oxide semiconductor film over the insulating film. The oxide semiconductor film includes a first oxide semiconductor layer, a second oxide semiconductor layer over the first oxide semiconductor layer, and a third oxide semiconductor layer over the second oxide semiconductor layer. The energy level of a bottom of a conduction band of the second oxide semiconductor layer is lower than those of the first and third oxide semiconductor layers. An end portion of the second oxide semiconductor layer is positioned on an inner side than an end portion of the first oxide semiconductor layer.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.[0003]In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic equipment are all included in the category of semiconductor devices.[0004]2. Description of the Related Art[0005]Transistors using a semiconductor thin film which is formed over a substrate having an insulating surface are widely used in electronic devices such as integrated circuits (ICs) and image display devices (also simply referred to as display devices). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to such a transistor. In recent years, an oxide semiconductor has attracted attention.[0006]Examples of such a transistor includ...

AI Technical Summary

Benefits of technology

[0014]In order to solve the above problems, an oxide semiconductor film having a structure in which three oxide semiconductor layers having different bandgaps are stacked can be used. Each oxide semiconductor layer has a specific function. That is, for the oxide semiconductor film, the following three oxide semiconductor layers having properties required for respective layers are stacked: an oxide semiconductor layer in contact with a gate insulating film, an oxide semiconductor layer in contact with an insulating film covering the oxide semiconductor film, and an oxide semiconductor layer in which a current path is mainly formed. In other words, the oxide semiconductor layer functioning as a current path of a transistor is sandwiched between oxide semiconductor layers having lower carrier density than the oxide semiconductor layer and functioning as buffer layers for stabilizing an interface between a channel and an insulating film. With such a structure, the channel can be separated from the interface between the oxide semiconductor layer and the insulating layer in contact with the oxide semiconductor layer, so that a buried channel can be formed. Thus, a transistor which is normally-off and has little variation in characteristics, high mobility, and high reliability can be provided.

[0023]In a transistor including an oxide semiconductor film, an oxide semiconductor layer having a large bandgap and a small electron affinity, that is, physical properties which made it hard to form a current path, is provided as a first oxide semiconductor layer in contact with a gate insulating film. Further, an oxide semiconductor layer with little oxygen vacancies is provided as a third oxide semiconductor layer in contact with an insulating film which is placed over the transistor and protects the transistor. In addition, in order to obtain high mobility, a second oxide semiconductor layer having a small bandgap and a large electron affinity, which mainly functions as a channel formation region, is provided between the first oxide semiconductor layer and the third oxide semiconductor layer. In this manner, a structure in which carriers flow through a region which is apart from upper and lower insulating layers in contact with the oxide semiconductor film (a buried channel) can be formed. The second oxide semiconductor layer serves as the buried channel, which can achieve high field-effect mobility.

[0024]In manufacture of the above structure, when patterning is performed on the second oxide semiconductor layer and the third oxide semiconductor layer by wet etching, etching is performed so that an end portion of the second oxide semiconductor layer and an end portion of the third oxide semiconductor layer are positioned on a sufficiently inner side than an end portion of a photoresist film. Further, with the photoresist film, patterning is performed on the first oxide semiconductor layer by anisotropic dry etching. At this time, since the end portion of the second oxide semiconductor layer in which the channel is formed is not exposed to dry etching, the end portion does not become n-type; thus, a reduction in variation in transistor characteristics and suppression in variation in transistor characteristics for long-term use can be achieved.

[0033]According to one embodiment of the present invention, a semiconductor device which includes an oxide semiconductor and has high field-effect mobility can be provided.

Problems solved by technology

In the case where a transistor including an oxide semiconductor film is formed, the following problems occur.

Secondly, in the case where an oxide semiconductor film in contact with a protective film includes many oxygen vacancies, variation in transistor characteristics is increased and a problem such as a normally-on state of the transistor due to a negative shift of threshold voltage of the transistor (movement of the threshold voltage in a negative direction) arises.