Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2000-12-19
2004-03-23
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S910000, C438S517000
Reexamination Certificate
active
06709906
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a semiconductor device having good electrical properties and also to a method for forming a thin film transistor (TFT) with good electrical properties.
Among thin film semiconductor elements, the TFT is well known. TFT is composed of an insulating substrate such as glass substrate and a thin film semiconductor (usually silicon semiconductor) as an active layer formed thereon which is several hundreds to several thousands of angstroms in thickness. TFT is applied to an electro-optical device such as a liquid crystal display device and an image sensor. Picture elements and peripheral drivers are formed by TFT formed directly on a glass substrate.
In using a glass substrate, the thin film silicon semiconductor formed on the substrate has amorphous or crystalline. A structure having crystalline represents the polycrystal structure, microcrystal structure, or a mixture of amorphous and crystalline structures. TFT based on an amorphous semiconductor is slightly poor in operating speed and electrical properties, and limits its application. By contrast, TFT based on a crystalline silicon film is capable of high speed operation and has good electrical properties.
TFT based on a crystalline silicon film has a problem associated with off state current. When a negative voltage is applied to the gate electrode in an N-channel type TFT, a current does not flow between a source and a drain in principle. This is because, by applying the negative voltage, a channel becomes a P-type and therefore a PN-junction is formed between the source and the drain. In actual, the crystalline silicon film contains crystal grain boundaries, crystal defects, and dangling bonds, so that a large number of levels produce. Accordingly, charges move in the reverse direction of PN-junction through these levels. When an electric field is concentrated at the PN-junction, a current leaks in the reverse direction through the defects and traps. as a result, an off current flows between the source and the drain by applying the negative voltage to the gate electrode.
A method to solve this problem is to form a lightly doped drain (LDD) region (lightly N-type), as an electric field relaxation region, between a channel (I-type) and a drain (N-type), which prevents the concentration of electric field between them.
Another method to obtain the same effect as the LDD region is to form an offset gate region which avoids the concentration of electric field between a channel and a drain. The offset gate region is a region which does not function as the drain between the channel and the drain.
As mentioned above, an off current of TFT decreases by defects and traps in the film. Also, defects and traps retard the movement of carriers in the film and therefore prevent the operation of TFT.
On the other hand, interface properties between a channel and a gate insulating film in TFT are extremely important. The interface properties greatly affect characteristics of TFT, and are evaluated in terms of interface level. The interface level produces by defects and dangling bonds. To obtain TFT having good characteristics, it is necessary to lower the interface level at the interface between the channel and the gate insulating film.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the problems associated with an off current in a thin film transistor (TFT) and with an interface level at the interface between a channel and a gate insulating film. Therefore, The object of the present invention is to provide a method for lowering the level (related with dangling bonds) in thin film silicon semiconductor. Another object of the present invention is to provide TFT having good characteristics. Further another object of the present invention is to obtain a silicon semiconductor film having a less number of levels.
The present invention is embodied in a method for producing TFT comprising the steps of, forming a silicon semiconductor film on a substrate having an insulating surface, introducing into the silicon semiconductor film at least one of ionized and accelerated hydrogen, fluorine, and chlorine, and performing heat treatment on the silicon semiconductor film in an atmosphere containing one of hydrogen, fluorine, chlorine, and a mixture thereof.
The present invention is also embodied in a method for producing TFT comprising the steps of, forming an active layer, forming a gate insulating film on the active layer, forming a gate electrode on the gate insulating film, forming a silicon nitride film covering the gate insulating film and gate electrode, introducing hydrogen ions into the active layer through the gate insulating film and silicon nitride film, and performing heat treatment on the whole.
The substrate having an insulating surface includes a glass substrate, a glass substrate on which an insulating film is formed, a semiconductor substrate on which an insulating film is formed, a metal substrate on which an insulating film is formed, and other substrate made of an insulating material.
The silicon semiconductor film has amorphous silicon semiconductor and crystalline silicon semiconductor which are formed by plasma chemical vapor deposition (CVD) or low pressure heat CVD. An amorphous silicon film formed by CVD may be crystallized by heating or irradiation with a laser light or an equivalent intense light.
Hydrogen, fluorine, or chlorine ions may be implanted by using a well known ion implanting apparatus or a plasma doping apparatus. Ionization may be performed by producing plasma based on high frequency discharge or by mass separation. The ion implanting apparatus required in the present invention introduces, into the silicon semiconductor film, hydrogen, fluorine, or chlorine ions which an acceleration to voltage is applied.
When a plasma doping apparatus is used, it is possible to use hydrogen, chlorine, or fluorine as the doping gas. Hydrogen chloride, hydrogen fluoride, or the like can be used as the doping gas. In using hydrogen chloride as the doping gas, chlorine and hydrogen are introduced into the silicon semiconductor. A depth which each element is introduced varies depending on an ion and an acceleration voltage.
The purpose of heat treatment on the silicon semiconductor film in an atmosphere containing hydrogen, fluorine, chlorine, or a mixture thereof is to confine the introduced hydrogen atoms, fluorine atoms, or chlorine atoms in the silicon semiconductor and further to promote neutralization of dangling bonds caused by hydrogen, fluorine, or chlorine. The heat treatment can be performed in any atmosphere irrespective of the previously introduced element. For example, heat treatment in an atmosphere containing hydrogen or chlorine after chlorine implantation is permissible. Selection depends on the apparatus to be used and the desired effect.
The atmosphere for heat treatment is not limited to that of single element; it may be a mixture of gases or a gaseous compound. An atmosphere containing a hydrogen-nitrogen mixture having a desired ratio or hydrogen chloride can be used.
The selection of hydrogen, chlorine, or fluorine for implantation depends on the apparatus employed and the characteristic required of the silicon semiconductor. In general, hydrogen implantation is easy. Deep implantation is possible with hydrogen (which is light in weight) at a low acceleration voltage. This almost avoids damages to the silicon semiconductor.
Deep implantation of fluorine or chlorine (which has larger ionic radius) needs a high acceleration voltage, which causes appreciable damages to the silicon semiconductor. However, these elements are not easily released by an external electric field because of their large ionic radius and their high bond energy for silicon. Chlorine, fluorine, and hydrogen decrease in bond energy for silicon in the order listed. The chlorine-silicon bond energy does not differ greatly from the fluorine-silicon bond energy; however, the hydrogen-silicon bond energy is extremely small. For example, the c
Ohnuma Hideto
Teramoto Satoshi
Yamaguchi Naoaki
Zhang Hongyong
Costellia Jeffrey L.
Jr. Carl Whitehead
Nixon & Peabody LLP
Schillinger Laura M
Semiconductor Energy Laboratory Co,. Ltd.
LandOfFree
Method for producing semiconductor device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for producing semiconductor device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for producing semiconductor device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3278634