Method of manufacturing a semiconductor device having a...

Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor

Reexamination Certificate

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Details

Other Related Categories

C438S482000, C438S149000, C438S151000

Type

Reexamination Certificate

Status

active

Patent number

06830994

Description

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having circuits formed by thin film transistors (hereafter referred to as TFTs). For example, the present invention relates to electro-optical devices, typically liquid crystal display devices, and to electronic equipment in which electro-optical devices are installed as parts. Further, the present invention relates to a method of manufacturing such devices. Note that the term semiconductor device in this specification indicates a category of general devices capable of functioning by utilizing semiconductor characteristics, and the above-mentioned electro-optical devices and electronic equipment are also included in the category of semiconductor devices.
2. Description of the Related Art
Research is widespread into techniques of forming crystalline semiconductor films, and increasing crystallinity, by performing heat treatment, laser annealing, or both heat treatment and laser annealing on an amorphous semiconductor film, formed on an insulating substrate such as glass. Silicon films are often used for the semiconductor films. Note that the term crystalline semiconductor film in this specification refers to a category of semiconductor films in which crystallized regions exist, and that semiconductor films that are crystallized over their entire surface area are also included in the category of crystalline semiconductor films.
The crystalline semiconductor films have an extremely high mobility in comparison with amorphous semiconductor films. Monolithic liquid crystal electro-optical devices (semiconductor devices in which thin film transistors (TFTs) used for a pixel portion and driver circuits are manufactured on one substrate) can be therefore produced if crystalline semiconductor films are utilized, but cannot be realized, for example, by semiconductor devices manufactured by using conventional amorphous semiconductor films.
However, it is impossible to control crystal orientation, so that its arrangement has an arbitrary direction, in crystalline semiconductor films formed by using heat treatment or laser annealing (a technique of crystallizing a semiconductor film by the irradiation of laser light) to crystallize an amorphous semiconductor film deposited by plasma CVD or LPCVD. This becomes a source of limitations in the electrical characteristics of the TFTs.
EBSP (electron backscatter diffraction patterning) exists as a method of analyzing crystal orientation of the surface of a crystalline semiconductor film. The EBSP method can show the crystal orientation directed toward the surface for crystal grains at measurement points in different colors and, can distinctly display focusing upon a certain measurement point, regions within a crystal orientation deviation angle range (permissible deviation angle), set by a user making the measurements, in neighboring points. It is possible for the user to freely set the permissible deviation angle, but the permissible deviation angle is set to 15° in this specification. Regions having a crystal orientation within a range that is equal to or less than 15° between the point focused upon and its neighboring points are referred to as grains. The reason why the permissible deviation angle is set to 15° is because the set value in general is 15°. Grains are formed from a plurality of crystal grains, but can be seen macroscopically as one crystal grain because the permissible angle for crystal orientation is small.
Further, a method recorded in Japanese Patent Application Laid-open No. Hei 7-183540 can be given as one method of crystallizing an amorphous semiconductor film. A simple explanation is presented here. First, a very small amount of a metal element such as nickel, palladium, or lead is added to an amorphous semiconductor film. Methods such as plasma processing, evaporation, ion injection, sputtering, and solution application can be utilized as the addition method. After the addition, the amorphous semiconductor film is then exposed, for example, to a nitrogen atmosphere at a temperature of 550° C. for 4 hours, forming a crystalline semiconductor film. Not only can the electric field effect mobility be increased if a TFT is formed by using such the crystalline semiconductor film, but it is also possible to make the sub-threshold factor (S value) smaller, and to greatly increase the electrical characteristics. The optimal heat treatment temperature and heat treatment time for crystallization is dependent upon the amount of the metal element added and the state of the amorphous semiconductor film. Further, it has been verified that it is possible to increase the crystal orientation property in a monotonic manner by using this method of crystallization.
TFTs have been made smaller in order to provide higher integration and higher speed for present-day LSIs, and TFT size has broken through the 1 &mgr;m level. In the case where TFTs of this type are manufactured using crystalline semiconductor films formed by conventional methods of crystallization, if the crystalline semiconductor are patterned for element separation to be separated, then dispersion will develop in active regions of individual devices in that many grain boundaries will exist in some elements and other elements will be formed by almost only grains. Further, if semiconductor films are crystallized using a metal element to promote crystallization, then crystal grains formed having the metal elements as crystal nuclei are mixed with crystal grains formed by spontaneous nucleation (cases in which nucleation begins at a site other than a metal element are defined as spontaneous crystallization within this specification). Dispersion in the semiconductor film properties thus develops. Note that spontaneous nucleation is known to develop at a high temperature greater than or equal to 600° C., and when the required crystallization time is long. This dispersion is a cause of dispersion in electrical characteristics and a factor in display irregularities if the crystalline semiconductor films are used as display portions of electronic equipment.
A method of suppressing the grain dispersion in the active regions of individual devices by making the grains smaller is considered here. The crystal nucleus generation density may be increased for this method. Namely, the surface energy of the semiconductor film is reduced, and the critical nucleus radius is reduced by increasing the chemical potential of the semiconductor film. A method of adding to the semiconductor film a large amount of a metal element for promoting crystallization, thus changing the surface energy and the chemical potential of the semiconductor film, is one method of suppressing the grain dispersion. A large number of crystal nuclei are generated by the metal elements if this method is used, and the grains can be made smaller. However, there is a problem with the aforementioned method in that an excessive amount of the metal element remains as a metal compound within high resistance regions (channel forming regions and offset regions). The metal compound allows electric current to flow more easily, reducing the resistance of regions that must be high resistance regions. This becomes a problem that can harm the stability of the TFT electrical characteristics as well as the reliability.
SUMMARY OF THE INVENTION
The present invention is a technology for solving problems like those stated above. The present invention is a technique for averaging the number of grains within active regions of individual devices by making the crystalline semiconductor film grains obtained using a metal element smaller without increasing the amount of the metal element used. An object of the present invention is to achieve an increase in the operational characteristics of a semiconductor device, and an increase in its reliability, with respect to the semiconductor device and an electro-optical device, typically an active matrix liquid crystal display device, using TFTs.
The present invention is character

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