Semiconductor device manufacturing: process – Gettering of substrate – By layers which are coated – contacted – or diffused
Reexamination Certificate
2002-05-29
2004-06-01
Thompson, Craig A. (Department: 2813)
Semiconductor device manufacturing: process
Gettering of substrate
By layers which are coated, contacted, or diffused
Reexamination Certificate
active
06743700
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing a semiconductor film having an amorphous structure relying upon a plasma CVD method, to a semiconductor device having a circuit constituted by thin-film transistors (hereinafter referred to as TFTs) using the semiconductor film, and to a method of their production. The invention relates to an electro-optical device as represented by, for example, a liquid crystal display panel and to an electronic device mounting such an electro-optical device as a component.
In this specification, the semiconductor device stands for devices that work by utilizing semiconductor characteristics as a whole. Therefore, electro-optical devices, semiconductor circuits and electronic devices are all semiconductor devices.
2. Description of the Related Art
There has heretofore been known a thin-film transistor (hereinafter referred to as TFT) as a typical semiconductor device using a semiconductor film having a crystalline structure. While the TFT is drawing attention as a technology for forming an integrated circuit on an insulating substrate such as of a glass, the liquid crystal display device of the type integral with a drive circuit has now been put into practical use. So far, the semiconductor film having a crystalline structure has been prepared by subjecting the amorphous semiconductor film deposited by plasma CVD method or by a reduced-pressure CVD method to the heat treatment or to the laser-annealing method (technology for crystallizing the semiconductor film by the irradiation with a laser beam).
The semiconductor film having the crystalline structure thus prepared is an aggregate of a number of crystalline particles, and their crystal azimuths are oriented in arbitrary directions and are not controllable, serving as a factor of imposing limitation on the TFT characteristics. In order to cope with the above problem, Japanese Patent Laid-Open No. 7-183540 discloses a technology for preparing a semiconductor film having a crystalline structure by adding a metal element such as nickel that assists the crystallization of a semiconductor film, making it possible not only to lower the heating temperature necessary for the crystallization but also to enhance the orientation of the crystal azimuth in one direction. When a TFT is formed by using the semiconductor film having such a crystalline structure, not only the electric field mobility is improved but also the sub-threshold coefficient (S-value) decreases, and the electric characteristics are strikingly improved.
Use of a metal element that assists the crystallization makes it possible to control the generation of nuclei during the crystallization. Therefore, the film quality becomes homogeneous compared to those obtained by other crystallization methods which permit nuclei to generate in a random fashion. Ideally, it is desired to completely remove the metal element or to a permissible range. With the metal element being added to assist the crystallization, however, the metal element remains in the inside or on the surface of the semiconductor film having the crystalline structure, becoming a cause of dispersion in the characteristics of the elements that are obtained. For instance, the off current increases in the TFT, arousing a problem of dispersion among the individual elements. That is, the metal element for assisting the crystallization turns out to be rather unnecessary after the semiconductor film having the crystalline structure has been formed.
Gettering using phosphorus is effectively utilized as a method of removing the metal element that assists the crystallization from a particular region of the semiconductor film that has the crystalline structure. For example, upon conducting the heat treatment at 450 to 700° C. while adding phosphorus to the source/drain region of the TFT, the metal element can be easily removed from the channel-forming region.
Phosphorus is injected into the semiconductor film having the crystalline structure by the ion-doping method (a method in which PH
3
and the like are dissociated with a plasma, and ions are accelerated in an electric field so as to be injected into the semiconductor without, however, separating the ions by mass). For effecting the gettering, however, the phosphorus concentration must not be lower than 1×10
20
/cm
3
. Addition of phosphorus by the ion-doping method causes the semiconductor film having the crystalline structure to become amorphous, while an increase in the phosphorus concentration hinders the subsequent recrystallization by annealing. Further, phosphorus added at a high concentration brings about an increase in the treatment time needed for the doping, arousing a problem of decrease in the throughput in the doping step.
Further, the concentration of boron for inverting the type of electric conduction must be 1.5 to 3 times as great as that of phosphorus added to the source/drain region of the p-channel TFT, bringing about a problem of an increase in the resistance in the source/drain region accompanied by a difficulty in effecting the recrystallization.
When the gettering is not sufficiently conducted and becomes irregular in the substrate, a difference or dispersion occurs in the characteristics of the TFTs. In the case of the transmission-type liquid crystal display device, a dispersion in the electric characteristics of the TFTs arranged in the pixel portions turns out to be a dispersion in the voltage applied to the pixel electrodes, whereby a dispersion occurs in the amount of light transmitted which is, then, perceived by the eyes of the viewer as the shade in the display.
For the light-emitting device using OLEDs, TFTs are indispensable elements for realizing the active matrix drive system. Therefore, the light-emitting device using OLEDs must have at least TFTs that work as switching elements and TFTs for feeding a current to the OLED in each of the pixels. Irrespective of the circuit constitution of the pixel and the driving method thereof, the brightness of the pixel is determined by the on current (I
on
)of the TFT that is electrically connected to the OLED and feeds the current to the OLED. Therefore, when white is displayed on the whole surface, dispersion occurs in the brightness unless the on current is maintained constant.
This invention is concerned with means for solving the above problems, and provides a technology for effectively removing the metal element remaining in the film after the semiconductor film having the crystalline structure is obtained by using the metal element that assists the crystallization of the semiconductor film.
The gettering technology is occupying a position as an important technology in the production of integrated circuits by using a single crystalline silicon wafer. Gettering is a technology in which metal impurities taken in by the semiconductor are segregated to a gettering site due to some energy, whereby the impurity concentration is lowered in the active region of the element. Gettering can roughly be divided into two; i.e., extrinsic gettering and intrinsic gettering. The extrinsic gettering brings about the gettering effect by applying a distorted field or a chemical action from the outer side. This can be represented by the gettering by which phosphorus ions of a high concentration are diffused from the back surface of a single crystalline silicon wafer. The above-mentioned gettering using phosphorus can be regarded to be a kind of the extrinsic gettering.
On the other hand, the intrinsic gettering is the one which utilizes the distorted field of lattice defect caused by oxygen formed in the single crystalline silicon wafer. This invention is based on the intrinsic gettering that utilizes the lattice defect or lattice distortion, and employs the following means for being adapted to the semiconductor film having a thickness of about 10 to about 100 nm and having a crystalline structure.
This invention comprises the steps of forming a first semiconductor film having a crystalline structure on the insu
Asami Taketomi
Ichijo Mitsuhiro
Ohnuma Hideto
Suzuki Noriyoshi
Yonezawa Masato
Costellia Jeffrey L.
Nixon & Peabody LLP
Semiconductor Energy Laboratory Co,. Ltd.
Thompson Craig A.
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