Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-05-03
2004-06-08
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S151000, C438S197000, C438S166000, C438S479000, C438S487000, C438S517000
Reexamination Certificate
active
06746901
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device having a circuit constituted by a thin film transistor (herein after, referred to as TFT). For example, the invention relates to an electro-optic apparatus represented by a liquid crystal display apparatus and a constitution of an electric apparatus mounted with an electro-optic apparatus as a part thereof. Further, the invention relates to a method of fabricating the device. Further, in the specification, a semiconductor device generally indicates an apparatus capable of functioning by utilizing semiconductor properties and an electro-optic apparatus and an electric apparatus, mentioned above, pertain to the category.
2. Description of the Related Art
In recent years, there have been widely carried out researches on a technology in which an amorphous semiconductor film formed above an insulating substrate of glass or the like is subjected to laser annealing to thereby crystallize the amorphous semiconductor and promoting crystalline performance thereof. Silicon is frequently used for the amorphous semiconductor film.
In comparison with a synthesized quartz glass substrate which has frequently be used conventionally, a glass substrate is provided with advantages that the glass substrate is inexpensive and rich in workability and the glass substrate having a large area can easily be fabricated. This is the reason that the above-described researches are carried out. Further, laser is preferably used in crystallizing the amorphous semiconductor since the melting point of the glass substrate is low. Laser can provide high energy only to an amorphous semiconductor film without considerably elevating temperature of a substrate.
Since a crystalline semiconductor is constituted by a number of crystal grains, a film thereof is referred also as a polycrystal semiconductor film. A crystalline semiconductor film formed by being subjected to laser annealing is provided with high mobility and therefore, a thin film transistor (TFT) is formed by using the crystalline semiconductor film and the crystalline semiconductor film is intensively utilized in, for example, a liquid crystal electro-optic apparatus of a monolithic type in which TFTs for driving a pixel and for a drive circuit are fabricated above one sheet of a glass substrate.
Further, there is preferably used a method of carrying out laser annealing by shaping pulse laser beam such as excimer laser to constitute a square spot of several centimeters square or a linear shape having a length equal to or larger than 10 cm at an irradiated face and scanning the laser beam (or moving an irradiated position of laser beam relatively to an irradiated face) since the method is provided with high productivity and is excellent industrially.
Particularly, when linear beam is used, the productivity is high since different from a case of using laser beam in a shape of a spot where scanning in front and rear direction and left and right direction is needed, laser can be irradiated to a total of an irradiated face by scanning the linear beam only in a direction orthogonal to a longitudinal direction thereof. Laser is scanned in the direction orthogonal to the longitudinal direction since the direction is the most efficient scanning direction. Owing to the high productivity, currently, the main stream is being established by using linear beam produced by shaping pulse oscillated excimer laser beam by a pertinent optical system. The technology enables to provide a monolithic type liquid crystal display apparatus formed with TFT (pixel TFT) forming a pixel portion and TFT of a drive circuit provided at a periphery of the pixel portion above one sheet of the substrate.
However, a crystalline semiconductor film fabricated by the laser annealing process, is formed by aggregating a plurality of crystal grains and positions and sizes of the crystal grains are at random. According to TFT fabricated above the glass substrate, for element isolation, the crystalline semiconductor is formed to isolate by patterning in an insular shape. In that case, the crystalline semiconductor cannot be formed by designating positions and sizes of crystal grains. In contrast to inside of a crystal grain, at an interface of the crystal grain (grain boundary), there are numerously present recombination centers and trap centers caused by an amorphous structure or crystal defect. It is known that when a carrier is trapped by the trap center, the potential of the crystal grain is elevated to thereby constitute a barrier against carrier and accordingly, a current transportation characteristic of the carrier is deteriorated. Although the crystalline performance of a semiconductor film at a channel forming region, effects various influence on electric properties of TFT, it is almost impossible to form the channel forming region by a single crystal of semiconductor film by excluding the influence of the grain boundary.
In order to resolve such a problem, according to the laser annealing process, there have been carried out various trials for forming a crystal grain the position of which is controlled and which is provided with a large grain size. Here, an explanation will firstly be given of a procedure of solidifying the semiconductor film after irradiating the semiconductor film with laser beam.
It takes a certain degree of time until solid phase nuclei are generated in a liquid semiconductor film which is completely melted by irradiating the laser beam and the procedure of solidifying the liquid semiconductor film is finished by generating numerous uniform (or nonuniform) nuclei in a completely melted region and growing crystals therefrom. Positions and sizes of crystal grains provided in this case are at random.
Further, when the semiconductor film is not completely melted by irradiating the laser beam and a solid phase semiconductor region partially remains, crystal growth is started from the solid phase semiconductor region immediately after irradiating the laser beam. As has already been mentioned, it takes a certain degree of time to generate nuclei in the completely melted region. Therefore, during a time period until nuclei are generated in the completely melted region, by moving a solid/liquid interface (which designates an interface between the solid phase semiconductor region and the completely melted region and corresponds to a front end of growth of crystal nucleus) constituting the front end of the crystal growth in a direction in parallel with a film face of the semiconductor film (hereinafter, referred to as lateral direction), the crystal grain grows to a length several tens times as much as a film thickness. Such a growth is finished by generating numerous uniform (or nonuniform) nuclei in the completely melted region and growing crystals. Hereinafter, the phenomenon is referred to as super lateral growth.
Also in an amorphous semiconductor film or a crystalline semiconductor film, there is present an energy region of laser beam realizing the super lateral growth. However, the energy region is very narrow, further, a position of providing a crystal grain having a large grain size cannot be controlled. Further, a region other than crystal grains having large grain sizes, is a microcrystal region generating numerous nuclei or an amorphous region.
As has already been explained above, when a temperature gradient in the lateral direction can be controlled (heat flow in the lateral direction can be produced) in the energy region of laser beam for completely melting the semiconductor film, a position of growing a crystal grain and a direction of growing thereof can be controlled. There have been carried out various trials in order to realize the method.
For example, there is a report with regard to a laser annealing process in which a metal film having a high melting point is formed between a substrate and a silicon oxide film of a matrix, an amorphous silicon film is formed above the metal film having the high melting point and laser beam o
Kasahara Kenji
Kawasaki Ritsuko
Ohtani Hisashi
Huynh Andy
Nelms David
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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