Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-12-05
2004-03-30
Chowdhury, R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S046000, C257S059000
Reexamination Certificate
active
06714274
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus having a thin film transistor (TFT) and to a TFT panel.
2. Description of the Background Art
Referring to
FIG. 28
, a TFT panel includes a display area
102
and a peripheral area therearound, on a transparent substrate
101
. The peripheral area consists of an X-direction driving circuit portion
111
, an Y-direction driving circuit portion
112
, and an external connection terminal portion
117
. In each driving circuit portion, driving transistors (not shown) are arranged, each of which controls and drives a pixel transistor (not shown) arranged at an intersection of a signal line and a scanning line (both not shown) in the display area. The main feature of a liquid crystal display apparatus is that it is thin and compact. It is a main attraction for a consumer purchasing the apparatus. If the driving transistors arranged in, respective driving circuit portions can be accommodated in a compact manner, a frame of the display screen can be made narrower, and hence commercial value will be increased. In this respect, the arrangement of the driving transistors in the X-direction driving circuit portion
111
in areas
115
and
116
of
FIG. 28
is important.
In a TFT panel in which the display area and the driving circuit areas are integrated, polycrystalline silicon (hereinafter referred to as “polysilicon”) is used at least for the driving transistors, though not necessarily for the pixel transistors. When a polysilicon film is to be formed integrally over the display area and the driving circuit area of the TFT panel, the typical method is as follows. An amorphous silicon film is formed on the substrate by reduced pressure CVD or the like, and the amorphous silicon is annealed by excimer laser beam irradiation, so that the amorphous silicon is crystallized to be a polysilicon film. It is necessary for the excimer laser beam to maintain uniform energy density, and to be incident on as wide as possible a range. Therefore, a beam having a linear cross section, with a length of 150 to 300 mm and a width of 300 &mgr;m, that is, almost a line of 0.3 mm, has been dominantly used. In case a plane is to be irradiated, the longitudinal direction of the beam is set in the Y-direction of the coordinate system shown in
FIG. 28
, the thin width direction of the cross section is aligned with the X-direction, and the plane is scanned by pulse-exposure with a frequency period of about 200 to 300 Hz with the beam shifted by 5 to 50 &mgr;m shot by shot along the X-direction.
The reason why polysilicon is used for the thin film semiconductor is that it is necessary to increase mobility for electrical carriers in the driving transistor. The mobility of the electrical carriers in amorphous silicon is significantly lower than the mobility in polysilicon. Therefore, amorphous silicon cannot be used for the driving transistor. Conventionally, when the display area is not integral with the driving circuit area, it was the practice that amorphous silicon was used for the pixel transistors, and single crystal silicon was used for the driving transistors which were fabricated through separate steps. The mobility in polysilicon is smaller than that in the single crystal silicon. However, necessary mobility can be ensured by making the crystals large. Accordingly, it follows that the energy density of the laser beam mentioned above must be such that it can turn the amorphous silicon into polycrystalline silicon and, in addition, attain the appropriate size of the crystal grains. The energy density of the laser beam satisfying these conditions is limited to a narrow range having upper and lower limits, though not so limited as to cause a major difficulty in implementation.
Required performance, however, may not be satisfied as to uniformity between shots of the energy density of the laser beam. More specifically, a miss shot occurs on the order of one per several tens of thousands, and the energy density falls below a lower limit or an upper limit of the above described range. At a portion affected by a miss shot, the crystal grain diameter cannot be made sufficiently large in an area corresponding to the width of one pitch of scanning, for example, an area having the width of 15 &mgr;m and a length of 150 to 300 mm. The phenomenon that the crystal grain diameter cannot be made sufficiently large occurs when the energy density of the laser beam is too low or too high. Therefore, when the laser beam is scanned along the direction of the channel length with the longitudinal direction of the laser beam cross section set parallel to the channel width direction of the TFT, the following problem arises. When there is an unsatisfactory shot in the TFT channel area, an area in which crystals are not sufficiently crystallized (hereinafter also referred to as defective area) is formed traversing the channel regions of the TFTs. As a result, a number of defective transistors are formed continuously along the area of the unsatisfactory shot, resulting in unsatisfactory display, and hence lower yield.
FIG. 29
represents a two-dimensional arrangement of the conventional driving transistors which may possibly suffer from the problem. For a plurality of driving transistors
130
in the X-direction driving circuit portion, a gate interconnection
141
, a source interconnection
143
and a drain interconnection
142
are formed continuously over the driving transistors. At portions where the interconnections and the driving transistors overlap, gate electrodes, source electrodes and drain electrodes are formed. In the source and drain electrodes, contact portions
137
are formed to be electrically connected to source and drain regions
133
and
132
of the semiconductor thin film, respectively. When viewed two-dimensionally, channel regions
131
almost overlap the gate electrode. In
FIG. 29
, the pulse laser is scanned along the direction of the channel length
135
shot by shot, with the longitudinal direction of the cross section of the laser beam arranged parallel to the direction of the channel width
134
. At this time, when a miss shot happens to overlap the channel region, the miss shot corresponding to one pitch
140
of scanning completely traverses the channel regions
131
. In
FIG. 29
, the hatched portions represent overlapping portions of channel regions
131
and the one pitch P of the miss shot of the laser beam. The charge carriers cannot pass through the channel regions of the transistors without passing through the hatched portions. The miss shot affects the transistor characteristic when one pitch
140
of the miss shot is formed completely traversing the channel regions. Even when one pitch
140
of scanning of the miss shot is formed completely traversing the source region or the drain region, the influence is not so great. In the arrangement of
FIG. 29
, a series of driving transistors connected to the gate interconnection are all damaged, and hence there is a significant influence clearly degrading the quality of display in the liquid crystal display apparatus. In such an arrangement of the driving transistors as shown in
FIG. 29
, driving transistors are accommodated in a compact manner in areas
115
and
116
as shown in
FIGS. 30 and 31
, and the frame of the display screen is not large. The gate interconnection
131
of the driving transistors (not shown) of the X-direction driving circuit portion does not extend beyond areas
115
and
116
. Therefore, it is unnecessary to enlarge the width
118
of the left frame and the width
119
of the right frame. As the driving transistors in the X-direction driving circuit portion, pairs of n channel and p channel transistors (both not shown) are arranged. The paired transistors are not distinguished, if not particularly necessary.
In order to solve the problem of degraded display quality described above, a proposal has been made in which the direction of the channel width is made non-parallel to the l
Aoki Masaru
Kobayashi Masanao
Akkapeddi Prasad R.
Chowdhury R.
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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