Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1998-09-10
2002-09-17
Thomas, Tom (Department: 2825)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S149000, C438S308000, C438S487000, C438S535000, C438S795000
Reexamination Certificate
active
06451636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device or a display device using a polycrystalline semiconductor layer obtained by laser annealing a non-single crystal semiconductor layer on a substrate.
2. Description of the Prior Art
A flat display device comprising a display element using a liquid crystal and organic electroluminescence as optical members is small-sized, thin and has low power consumption, and has been developed for practical use in the field of OA apparatus, AV apparatus and the like. A liquid crystal display (LCD) and an organic EL display of an active matrix type having a thin film transistor (TFT) formed on a substrate supporting a liquid crystal and EL as a switching element for controlling the timing of holding and writing image data in each pixel has become commonplace because of high display quality. In particular, a display of a driver built-in type has been developed in which a TFT is used as a switching element of each pixel and is also used for a driver to drive the switching element, and the driver is formed together with the switching element on the periphery of a display area where each pixel is arranged. Consequently, size and cost can be reduced still further.
For the display of a driver built-in type, a TFT using a polycrystalline semiconductor film, particularly, polysilicon (p-Si) as a channel layer is suitable because it can achieve an operating speed which is also applicable to a driver and has a low deposition temperature resulting in formation on an inexpensive glass substrate having low heat resistance. When the polysilicon is to be formed, amorphous silicon formed on the substrate is laser annealed so that crystallization can be performed with a support substrate temperature set to 400 to 600° C. The TFT is formed using p-Si thus obtained. Using this method, a driver circuit can be fabricated on a non-alkaline glass substrate.
The amorphous silicon (a-Si) is laser annealed by using a laser irradiation apparatus. In the laser irradiation apparatus, an optical system shapes a pulse laser beam emitted from a laser oscillation source into a beam having a predetermined section, and the beam thus obtained is irradiated on an amorphous silicon film formed on a processed substrate. The laser beam to be shaped and irradiated on the silicon film is, for example, square, and more particularly has the form of a belt or line in which a length in a direction of a major axis is much greater than that in a direction of a minor axis. A stage of the laser irradiation apparatus to be mounted on the processed substrate having an a-Si film formed thereon is movable horizontally and vertically in a plane direction. The stage is moved horizontally or vertically so that a pulse laser beam is relatively scanned over the a-Si film formed on the processed substrate horizontally or vertically.
FIG. 1
shows an enlarged sectional structure of a TFT portion formed on the processed substrate at a laser. annealing step A gate electrode
11
of a TFT is formed on a substrate
10
such as a non- alkaline glass. A gate insulation film
12
is formed to cover the gate electrode
11
and an a-Si film
13
a
which is a film to be laser annealed is formed on the gate insulation film
12
. When a pulse laser beam is irradiated on the a-Si film
13
a
to perform the laser annealing, the a-Si film
13
a
is polycrystallized to form a p-Si film
13
.
FIG. 2
shows a sectional structure of the TFT formed by using the p-Si film
13
obtained by laser annealing.
FIG. 3
shows a planar structure of the obtained TFT.
FIGS. 1 and 2
show sections taken along the line A—A in FIG.
3
.
The p-Si film
13
obtained by laser annealing the a-Si film
13
a
is subjected to patterning in the shape of an island across a portion above the gate electrode
11
. A region positioned just above the gate electrode
11
in the island-shaped p-Si film
13
is a non-doped channel region CH. A LD (Lightly Doped) region LD which is doped with an impurity having a low concentration is formed on both sides of the non-doped channel region CH, and a source region S and a drain region D which are doped with an impurity having a high concentration are formed on the outside of the LD region LD. An interlayer insulation film
15
such as SiNx, SiO
2
or the like is formed to cover an implantation stopper film
14
used as a mask when the p-Si film
13
and the LD region LD are to be formed. A source electrode
16
and a drain electrode
17
are formed on the interlayer insulation film
15
, and are connected to the source region S and the drain region D through a contact hole CT formed in the interlayer insulation film
15
, respectively.
In a display unit, for example an LCD, pixels re usually arranged in a matrix and TFTs for driving the pixels and wirings are correspondingly placed in horizontal and vertical scanning directions, respectively. Accordingly, when these display elements are usually placed on a rectangular substrate, the directions of a plurality of TFTs formed on the processed substrate, that is, extension directions of channel widths or channel lengths of the channel regions CH, are any of a horizontal scanning direction H and a vertical scanning direction V of the LCD with respect to a substrate plane. In other words, the directions of the channels of the TFT elements are parallel or perpendicular to each other. A direction of a line beam, that is, a side of the line beam, a side of the substrate and the like are held in the horizontal scanning direction H or the vertical scanning direction V.
FIG. 4
is a graph showing a relationship between an irradiation laser energy on the a-Si film
13
a
(an axis of abscissa) and a grain size of the p-Si film
13
formed at that time (an axis of ordinate). As the energy is increased, the grain size is also enlarged. If an energy value Eo with which a maximum grain size is obtained is exceeded, the grain size is rapidly reduced. Accordingly, the energy should be kept within a narrow range between Ed and Eu in order to obtain a predetermined grain size.
For this reason, if the irradiation energy of the line beam is slightly varied and moves out of the optimum range between Ed and Eu, crystallization cannot be performed fully so that a defective crystallization region R having a small grain size is generated on a certain region in the p-Si film
13
.
A positional relationship between a layout of each circuit element for liquid crystal driving which is formed on a processed substrate, for example, and an irradiated pulse laser beam is usually set as shown in FIG.
5
. In
FIG. 5
, a mother substrate
59
acting as the processed substrate has a plurality of regions forming an active matrix substrate used for a TFT LCD (six regions, each of which will be hereinafter referred to as an active matrix substrate
2
). Each active matrix substrate
2
is subjected to various manufacturing steps so that pixels are formed in a matrix and a p-Si TFT to be connected to each pixel is formed in a region
43
of
FIG. 5
, resulting in a display area (hereinafter referred to as a display area
43
). Driver sections
44
and
45
are formed around the display area
43
. The driver sections
44
and
45
serve to drive the p-Si TFT of the display area
43
and utilize a p-Si TFT formed almost simultaneously with the formation of the p-Si TFT of the display area
43
.
FIG. 5
shows a state in which an amorphous silicon (a-Si) film is formed in a necessary region of the mother substrate
59
and a line-shaped laser beam that causes an irradiated region LB to extend in the vertical scanning direction V is sequentially shifted and irradiated on the a-Si film in the horizontal scanning direction H to perform annealing. By such laser annealing, a-Si is polycrystallized so that a p-Si film constructing a channel region of the TFT is obtained.
Thus, the annealing is performed by irradiating the pulse laser beam while sequentially shifting a position of the pulse laser beam. T
Segawa Yasuo
Yamada Tsutomu
Yokoyama Ryoichi
Yoneda Kiyoshi
Hogan & Hartson LLP
Malsawma Lex H.
Sanyo Electric Co,. Ltd.
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