Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2002-09-24
2004-11-23
Lebentritt, Michael (Department: 2824)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S149000, C438S152000, C438S162000
Reexamination Certificate
active
06821828
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a gettering technique. In particular, the present invention relates to a method of manufacturing a semiconductor device using a semiconductor film having an amorphous structure formed by adding a metallic element, which promotes a crystallization of the semiconductor film.
Note that, in the present specification, the term “semiconductor device” indicates a category of general devices which are capable of functioning by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic equipments are all included in the category of semiconductor devices.
2. Description of the Related Art
Thin film transistors (hereinafter referred to as TFTs) are known as typical semiconductor elements that use semiconductor films having a crystalline structure. TFTs are attracting attention as a technique of forming an integrated circuit on a glass or other insulating substrate, and devices utilizing TFTs, such as a liquid crystal display device with a built-in driving circuit, are beginning to appear on the market. In the conventional art, a semiconductor film with a crystalline structure is formed by using heat treatment or laser annealing to crystallize an amorphous semiconductor film that is obtained by deposition through plasma CVD or reduced pressure CVD. (Laser annealing is the technique of crystallizing a semiconductor film through irradiation of laser light.)
The thus formed semiconductor film with a crystalline structure is a mass of crystal grains. Since the crystal grains are randomly oriented and the orientation thereof cannot be controlled, the semiconductor film affects TFT characteristics. A Japanese Patent Application Laid-Open No. 07-183540 discloses a technique to tackle this problem. The technique involves doping with a metallic element that accelerates crystallization of a semiconductor film, such as nickel, to form a semiconductor film having a crystalline structure. The technique can cause a large proportion of crystal grains to orient in the same direction, and can lower the heating temperature required for crystallization as well. When this semiconductor film having a crystalline structure is used in a TFT, the field effect mobility is improved and the sub-threshold coefficient (S value) is reduced to improve the electric characteristics of the TFT exponentially.
By using a metallic element for promoting crystallization, generation of nuclei in crystallization can be controlled. Therefore, film quality thus obtained is uniform in comparison with another crystallization method in which nuclei are generated at random, and ideally, it is desirable that metallic elements are completely removed or reduced to an allowable range. On the other hand, the metallic element used in doping for accelerating crystallization remains in the semiconductor film having a crystalline structure, or on the surface thereof, causing problems such as fluctuation in characteristic of semiconductor elements obtained. For example, the remaining metallic element increases OFF current in the TFTs to cause fluctuation between the individual elements. In short, the metallic element for accelerating crystallization becomes an unwanted presence once the semiconductor film having a crystalline structure is formed.
Gettering using phosphorus is actively employed as an effective method of removing a metallic element that accelerates crystallization from a specific region of a semiconductor film having a crystalline structure. For instance, the metallic element can readily be removed from a channel forming region by doping a source/drain region of a TFT with phosphorus and subjecting the film to heat treatment at 450 to 700° C.
Phosphorus is implanted in a semiconductor film having a crystal structure by ion doping (a method of dissociating PH
3
or the like by plasma and accelerating the ions with an electric field to implant the ions in a semiconductor which basically does not include ion separation). The phosphorus concentration necessary for gettering is 1×10
20
/cm
3
or more. Phosphorus doping by ion doping makes a semiconductor film having a crystal structure amorphous. An increase in phosphorus concentration is a problem because it inhibits later recrystallization by annealing. Another problem is that high concentration of phosphorus doping prolongs treatment time needed for the doping and lowers the throughput in the doping step.
To invert the conductivity type of source and drain regions of a p-channel TFT which have been doped with phosphorus, the concentration of boron required is 1.5 to 3 times the phosphorus concentration. This not only makes recrystallization difficult but also increases the resistance of the source and drain regions.
If gettering is not thorough and the degree of gettering fluctuates throughout the substrate, it causes a slight difference, namely, fluctuation in each TFT characteristic. When TFTs arranged in a pixel portion fluctuate in electric characteristic in a transmissive liquid crystal display device, the level of voltage applied fluctuates between pixel electrodes. This leads to fluctuation in amount of light transmitted, which is recognized by a viewer as display irregularity.
A TFT is an element indispensable to a light emitting device using an OLED when the device is driven by an active matrix driving method. Therefore a light emitting device using an OLED has in each pixel at least a TFT that functions as a switching element and a TFT for supplying a current to an OLED. The luminance of a pixel is determined by ON current (I
on
) of a TFT that is electrically connected to an OLED to supply a current to the OLED irrespective of the circuit structure and driving method of the pixel. Therefore, in the case of all-blank display, for example, variation in ON current results in fluctuation in luminance.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above, and an object of the present invention is therefore to provide a measure to solve those problems, namely, a technique of using a metal element that accelerates crystallization of a semiconductor film to obtain a semiconductor film with a crystal structure and then removing the metal element remaining in the film effectively.
The technique of gettering is deemed as a major one in manufacture techniques of an integrated circuit using a single crystal silicon wafer. Gettering utilizes some energy to make metal impurities that have been introduced into a semiconductor segregate in a gettering site in order to reduce the impurity concentration in an active region of an element. There are roughly two types of gettering, extrinsic gettering and intrinsic gettering. Extrinsic gettering obtains the gettering effect by an external strain field or chemical action. Phosphorus gettering, in which a high concentration of phosphorus diffuses into a semiconductor from the back side of a single crystal silicon wafer, corresponds to extrinsic gettering. The above-described gettering using phosphorus too is one of extrinsic gettering.
On the other hand, intrinsic gettering utilizes a strain field of a lattice defect where oxygen generated inside a single crystal silicon wafer plays a part. The present invention focuses attention on this intrinsic gettering utilizing lattice defect or lattice strain and employs the following measures which are applied to about a 10 to 100 nm-thick semiconductor film having a crystal structure.
The present invention has a step of forming on a silicon nitride film a first semiconductor film having a crystal structure by using a metal element, a step of forming a film that serves as an etching stopper (barrier layer), a step of forming a second semiconductor film containing a noble gas element (gettering site), a step of gettering to move a metal element to the gettering site, a step of removing the second semiconductor film, and a step of removing the barrier layer.
The present invention i
Asami Taketomi
Ichijo Mitsuhiro
Suzuki Noriyoshi
Lebentritt Michael
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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