Electric heating – Metal heating – By arc
Reexamination Certificate
1999-09-17
2001-08-28
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121830
Reexamination Certificate
active
06281470
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film semiconductor device manufactured by a laser annealing method of scanning a laser beam having a rectangular cross-section shape and a method of manufacturing the device.
In recent years, liquid crystal display devices employing a thin film semiconductor device constructed of polycrystalline silicon thin film transistors and so on, in which a scan line drive circuit and a data line drive circuit can be integrally formed on a glass substrate together with transistors for driving pixels, have been developed technologically and commercially as a promising technique for a reduction in size, an increase in fineness and a reduction in cost of the liquid crystal display devices.
In particular, the laser annealing method which is a means for forming a polycrystalline silicon film on glass substrate can form a polycrystalline silicon film of a large area by scanning a spot-shaped or rectangle-shaped laser beam, and therefore, the method receives attention as a mass-production technique.
FIGS. 11A through 11E
show a method of manufacturing a polycrystalline silicon thin film transistor by the laser annealing method.
First, in
FIG. 11A
, a protective film
101
of a silicon oxide film or the like is formed on a glass substrate
100
(of about 300 mm to 1000 mm square) and thereafter an amorphous silicon film
102
is formed on the protective film
101
.
Next, as shown in
FIG. 11B
, a laser beam
121
is irradiated from a laser light source
120
to the amorphous silicon film
102
thereby annealing the amorphous silicon film
102
, so that the amorphous silicon film
102
is crystallized to be formed into a polycrystalline silicon film
103
. In the case of a continuous oscillation type argon ion laser, the laser beam
121
has a spot-like shape of a diameter of about 100 &mgr;m. In the case of a pulse oscillation type excimer laser having an optical system like a beam expander, the beam shape is a rectangular shape having a shorter side of 0.1 to several millimeters and a longer side of 100 to several hundred millimeters. By scanning the laser beam
121
, the polycrystalline silicon film
103
is obtained in a wide area on the glass substrate
100
.
Next, as shown in
FIG. 11C
, the polycrystalline silicon film
103
is patterned by a photolithographic method, so that an island-shaped silicon layer
104
is formed.
Next, as shown in
FIG. 11D
, an oxide silicon film
105
is formed on the protective film
101
and the silicon layer
104
and made to serve as a gate insulating film.
Subsequently, as shown in
FIG. 11E
, a gate electrode
106
, a source region
107
and a drain region
108
containing a high concentration of impurities, an interlayer insulating film
109
, a source electrode
110
and a drain electrode
111
are formed. The gate electrode
106
, source region
107
, drain region
108
, source electrode
110
and drain electrode
111
constitute a thin film transistor.
FIG. 12
shows a circuit-integrated type liquid crystal display device employing the above thin film transistors. In
FIG. 12
, a plurality of pixel electrodes
201
arranged in a matrix form are connected to respective switching transistors
202
, and the switching transistors
202
are sequentially turned on and off by a signal on a scan line
203
, thereby writing image data inputted to a data line
204
into the pixel electrode
201
when the switching transistor
202
is turned on. A data line drive circuit for driving the data line
204
is constructed of an amplifier circuit
205
, which could be omitted especially in the case of small panels below the display size of about 5 inches diagonal, an analog switch
206
and a logic circuit
207
for controlling the turning-on and -off of the analog switch
206
. A scan line drive circuit for driving the scan line is a selection pulse generator circuit
208
for successive selection of the scan line
203
.
The glass substrate on which the pixel electrodes
201
connected to the switching transistors
202
shown in
FIG. 12
are arranged in a matrix form and another glass substrate (not shown) on which opposite transparent electrodes are formed are bonded together with a gap of several micrometers interposed between them after conducting an alignment process of the inside surfaces of both the glass substrates, and then a liquid crystal material is infused into this gap, so that a liquid crystal display device is obtained.
In regard to a drive-circuit-integrated active matrix type liquid crystal display device in which a polycrystalline silicon film formed by the laser annealing is used for transistors, an arrangement construction and a manufacturing method of the transistors are disclosed in the document of Japanese Patent Laid-Open Publication No. HEI 7-92501.
The essence of the invention disclosed in this document of Japanese Patent Laid-Open Publication No. HEI 7-92501 is to arrange in a linear form the transistors of the scan line drive circuit and the data line drive circuit in an attempt at reducing the time required for laser annealing. Further, by arranging the pixel electrode use transistors on a line extended from the transistors in the scan line drive circuit or the data line drive circuit, the laser annealing for forming a polycrystalline silicon film which will serve as an active layer of these transistors can be concurrently executed. Further, assuming that a scan pitch of the laser beam is Pt, then a relation between the scan pitch Pt and a stripe width of a belt-shaped polycrystalline silicon film obtained by the laser annealing scan is set so that Pt>(stripe width×2).
However, when scanning a laser beam having a rectangular cross-section shape, to satisfy the condition of scan pitch Pt>(stripe width×2) disclosed in the aforementioned document of Japanese Patent Laid-Open Publication No. HEI 7-92501 is not compatible with the effect of reducing the defect density in crystal grain size due to overstriking and the effect of reducing the variation in distribution of the crystalline state attributed to the variation in intensity of the laser beam shot in each time, and this leads to a problem that the transistor characteristics are impaired. Furthermore, the document of Japanese Patent Laid-Open Publication No. HEI 7-92501 mentions no concrete method of aligning in position the rectangular polycrystalline stripe width with the polycrystalline silicon layer of the transistor, and it is very hard to put the polycrystalline silicon layer of the transistor into the stripe width of the polycrystalline silicon film.
Comparing each other the aforementioned two types of laser annealing methods, i.e., the method of using a spot-like beam shape as observed in the case of the continuous oscillation type argon ion laser and the method of using a rectangular beam shape as observed in the case of the pulse oscillation type excimer laser, the former method requires a complicated optical system for scanning the spot-shaped laser beam and is hard to uniformly effect the annealing on the glass substrate of several hundred millimeters square. Therefore, the latter method of using a pulse oscillation type excimer laser having a rectangular beam shape is advantageous in terms of the mass-production technique.
For the above reasons, the latter laser annealing method of scanning a laser beam having a rectangular cross-section shape is generally adopted. This rectangular beam width is about several hundred micrometers to several millimeters, and the scan pitch is about several ten micrometers to several hundred micrometers, providing an overlap shot (irradiation) region in the scan stage. This is because the effect of reducing the defect density in crystal grain size and the effect of reducing the variation in distribution of the crystalline state attributed to the variation in intensity of each laser beam shot are produced by executing multi-time overstriking on one silicon region.
However, the transistor characteristic variation occurs even when forming a plurality of polycry
Heinrich Samuel M.
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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