1. Field of the Invention
The present invention relates to a semiconductor element typified by a field effect transistor to be formed over a substrate having a low strain point and to a method for manufacturing the same, and relates to a semiconductor integrated circuit including the semiconductor element and to a method for manufacturing the same. Specifically, the present invention relates to a thin film transistor in which a gate insulating film is heat-treated at a temperature beyond a strain point of a substrate such as glass and to a method for manufacturing the same.
2. Description of Related Art
In recent years, development of a system-on-panel incorporating a logic circuit such as a CPU or a memory as well as a pixel or a driver circuit over a light transmitting insulating substrate such as glass or quartz has been attracting attention. High-speed operation is required for a driver circuit and a logic circuit, and manufacturing a thin film transistor (hereinafter, also referred to as a TFT) having high switching speed is required in order to realize it. It is effective for realizing a TFT having higher switching speed to use a semiconductor film with fewer crystal defects as an active layer; to make a gate insulating film thinner, and to miniaturize a transistor size typified by miniaturization of a gate length.
Characteristics required for a gate insulating film can be given as follows: few defects in a thin film; without a fixed charge; a low interface level with a semiconductor film; low leakage current; and the like. However, a gate leakage current tends to increase with a decrease in a film thickness of a gate insulating film. In addition, such a fine gate insulating film that can lower a gate leakage current is required in order to make the gate insulating film thinner. A field effect semiconductor device that can be driven with a low voltage and responds well to a high drive frequency can be obtained by making the gate insulating film thinner.    Patent Document 1: Japanese Patent Laid-Open No. H6-188421