Electric heating – Metal heating – By arc
Reexamination Certificate
2000-10-03
2004-07-06
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121650
Reexamination Certificate
active
06759628
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process of fabricating a thin film semiconductor device in which thin film transistors, each including as an active layer a semiconducting thin film formed on an insulating substrate, are integratedly formed. In particular, the present invention concerns a laser beam irradiation technique (laser annealing) for crystallization of a semiconducting thin film having formed on an insulating substrate.
A laser annealing treatment using a laser beam has been developed as one of processes of fabricating a thin film semiconductor device at low temperatures. The laser annealing treatment includes steps of locally heating and melting a semiconducting thin film formed from a non-single crystal material such as amorphous silicon or polycrystalline silicon on an insulating substrate by irradiating a laser beam to the semiconducting thin film, and then crystallizing the semiconducting thin film in the cooling step. Thin film transistors, each including the semiconducting thin film thus crystallized as an active layer (channel region), are integratedly formed. Since the carrier mobility of the semiconducting thin film becomes higher by crystallization, characteristics of the thin film transistor including such a film can be improved. As shown in
FIG. 10
, in the laser annealing, a pulsed laser beam
4
formed in a band-shape along the longitudinal direction (Y direction) of an insulating substrate
1
is intermittently irradiated to the insulating substrate
1
, and it is simultaneously moved relative to the insulating substrate
1
in the lateral direction (X direction) while partially overlapping regions irradiated with the laser beam
4
to each other. In the example shown in
FIG. 10
, the insulating substrate
10
is steppedly moved in the -X direction while the area irradiated with the laser beam
4
is fixed. The crystallization of a semiconducting thin film can be thus relatively uniformly performed by overlappingly irradiating the laser beam
4
to the semiconducting thin film.
Thin film semiconductor devices are suitable for drive substrates of active matrix type displays or the like, and in recent years they are being increasingly developed. In the case of using a thin film semiconductor device for a display, it is strongly required to enlarge the size of a transparent insulating substrate made of, for example, glass or the like and to reduce the cost thereof. In the example shown in
FIG. 10
, the insulating substrate
1
has dimensions of 400-500 mm in the X-direction and 300-400 mm in the Y-direction. To satisfy such requirements for enlargement in size and reduction in cost of a transparent insulating substrate, the laser annealing treatment using a laser beam has been adopted. That is, a semiconducting thin film can be crystallized at a relatively low temperature by irradiation of a laser beam thereto, and consequently a relatively inexpensive transparent insulating substrate made of low melting point glass or the like can be adopted. Thus, at present, a thin film semiconductor device integrally containing a peripheral circuit unit in addition to a display unit can be fabricated at low temperatures of 400° C. or less using bottom gate type thin film transistors. Further, a semiconducting thin film having a relatively large area can be efficiently converted from an amorphous phase into a polycrystalline phase by irradiating a band-shaped (linear) laser beam
4
to the semiconducting thin film while overlappingly scanning the laser beam
4
. At the present time, an excimer laser is extensively used as a laser beam light source. The excimer laser, however, cannot make extremely larger the cross-section of the laser beam by the limited output power thereof. For this reason, a laser beam formed in a band-shape or linear shape is overlappingly scanned to be thus irradiated on the entire surface of a large-sized transparent insulating substrate made of glass or the like. In this case, however, upon scanning of the laser beam, particle sizes of crystals or the like of the semiconducting thin film become uneven by the influence of an energy distribution of the laser beam
4
. This causes a problem in varying operational characteristics of drive thin film transistors integratedly formed in a display, thereby making it difficult to perform uniform display.
In general, an excimer laser has an output power of about 200 W. As shown in
FIG. 11
, a laser beam
4
is formed in a band-shape for concentration of the power. In the example shown in
FIG. 11
, an area irradiated with the band-shaped laser beam
4
has dimensions of about 0.3 mm (300 &mgr;m) in the X direction and about 150 mm in the Y direction. Such a laser beam
4
is intermittently irradiated on the insulating substrate while being scanned along the x direction, to thereby recrystallize a semiconducting thin film formed on the entire surface of the insulating substrate.
FIG. 12
typically shows an energy distribution of the band-shaped laser beam
4
in the X direction (lateral direction). The energy distribution has an approximately parallelopiped profile composed of a flat section
410
at a central portion and tilted sections
420
on both sides thereof. The width of the flat section
410
is, for example, about 300 &mgr;m and the width of the tilted section is, for example, about 20 &mgr;m. The tilted section
420
is necessarily generated by action of an optic system used for forming the laser beam into a band-shape. The tilted sections
420
of the energy distribution of the laser beam
4
cause a variation in structure of a crystallized semiconducting thin film, resulting in crystal defects.
FIG. 13
typically shows a state in which a laser beam is irradiated while being overlappingly scanned. In a related art laser annealing treatment, a pulsed laser beam is intermittently irradiated while it is scanned such that regions irradiated with laser beam are, for example, 90% overlapped. In the case where the width of the laser beam in the X-direction is 300 &mgr;m, the movement amount per one step of the intermittent irradiation becomes 30 &mgr;m. In the figure, the movement amount per one step is expressed by a movement step A. By repeating 10 times the intermittent irradiation of the laser beam with the movement amount pitch A, the laser beam is scanned 300 &mgr;m in width along the X direction. In this case, the tilted section of the cross-sectional profile of the laser beam is irradiated just at each boundary
16
of the partially overlapped regions irradiated with the laser beam, with a result that crystal defects
16
a
are generated along each boundary
16
. On the other hand, thin film transistors
17
are integratedly formed on an insulating substrate
1
with a specific arrangement pitch B. In this example, the thin film transistor
17
is of a bottom gate type in which a semiconducting thin film
2
patterned into an island is overlapped on the gate electrode
18
. A portion of the semiconducting thin film positioned directly over the gate electrode
18
constitutes a channel region, and contacts
19
are formed on both sides of the channel region. While
FIG. 13
shows the thin film transistor
17
in a finished state, the laser annealing is performed at a suitable step of a process of fabricating the thin film transistor
17
. Any relationship between the movement pitch A of the laser beam and the arrangement pitch B of the thin film transistors has been not examined. Consequently, there occurs a phenomenon that the crystal defects
16
a
formed for each boundary
16
are positioned in the channel region of one thin film transistor
17
but they are not positioned in the channel region of another thin film transistor
17
.
FIG. 14
typically shows the cross-sectional structure of the thin film transistor
17
shown in
FIG. 13
, in which the gate electrode
18
is patterned on the insulating substrate
1
and the semiconducting thin film
2
is patterned on the gate electrode
18
through a gate insulating film
21
. A stopper
23
aligned with the
Ino Masumitsu
Maekawa Toshikazu
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