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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser annealing method for a
semiconductor thin film and a thin film semiconductor device. More
specifically, the present invention relates to a laser annealing method
for a semiconductor thin film used in a process of fabricating thin film
transistors (TFTs) for liquid crystal displays (LCDs), image sensors,
SRAMs, and the like, and a thin film semiconductor device fabricated by
use of the laser annealing method.
2. Description of the Related Art
In recent years, TFTs have been actively applied to LCDs, image sensors,
SRAMs, and the like. In many cases, a number of TFTs are integrated on one
substrate. In order to improve the packing density of integrated TFTs, the
size of the TFTs should be reduced. For the application of TFTs to an LCD,
in particular, it is desirable that a driving circuit for a liquid crystal
display panel of the LCD is formed of TFTs arranged on a glass substrate
of the panel. Such TFTs for a driving circuit are required to operate at a
higher speed than that for TFTs for switching arranged in the display
portion. In order to obtain TFTs operating at high speed, a semiconductor
thin film having a higher carrier mobility (field effect mobility) is
required.
There is known a technique for increasing the carrier mobility of a
polysilicon TFT by annealing a semiconductor thin film at a high
temperature of 600.degree. C. or more, for example. High-temperature
annealing, however, may thermally damage a substrate. Accordingly, such
high-temperature annealing is not possible when an inexpensive glass
substrate having a low distortion point is used. Non-alkaline borosilicate
glass having a distortion point of 650.degree. C. or less (for example,
Corning No. 7059) is less expensive than silica glass. For this reason, a
technique for forming a semiconductor thin film having a high carrier
mobility on such a glass substrate is strongly desired.
A laser annealing method using an excimer laser makes it possible to
crystallize amorphous silicon (Si) while the temperature of a substrate is
kept as low as about 600.degree. C. or less, so as to form a polysilicon
layer having a large grain size. Since thermal damage received by the
substrate is small, this laser annealing method is a promising method.
Hereinbelow, a conventional annealing method for obtaining a polysilicon
thin film having a large grain size from a non-singlecrystalline Si thin
film will be described. In this method, overlap irradiation of the
non-singlecrystalline Si thin film with an excimer laser beam is
performed. The "overlap irradiation" as used herein refers to a type of
irradiation where a region of the non-singlecrystalline Si thin film
melted/solidified by the Nth pulse irradiation partially overlaps a region
thereof melted/solidified by the (N+1)th pulse irradiation (N is a natural
number).
FIG. 3 schematically shows a laser annealing method for a semiconductor
thin film adopting the overlap irradiation. Referring to FIG. 3, the
reference numerals 3a to 3d denote regions of a non-singlecrystalline Si
thin film 4 on a substrate 5, of which crystallinity has changed due to
the irradiation with a laser beam 1.
As shown in FIG. 3, the non-singlecrystalline Si thin film 4 on the
substrate 5 is irradiated with the laser beam 1 by pulsing. In general,
the pulse duration is in the range of 20 to 60 nanoseconds, and the pulse
interval is in the range of 3 to 30 milliseconds.
The non-singlecrystalline Si in the region irradiated with the laser beam 1
is first melted and then solidified, thereby changing the crystallinity.
More specifically, the silicon shifts from the non-singlecrystalline state
to a crystalline state. The "crystalline state" as used herein refers to
the state where a plurality of grains each recognized as singlecrystal are
formed (generally, called a polycrystalline state). In the crystalline
state, the carrier mobility is high when the number of crystal defects is
small and when grains are large and thus the area of grain boundaries is
small, thereby exhibiting good crystallinity.
By the irradiation with the laser beam 1, the region 3a having a
crystallinity different from the surrounding regions is formed in the Si
thin film. Then, the relative positioning between the substrate 5 and the
laser beam 1 is changed so that the next region to be irradiated with the
laser beam 1 partially overlaps the region 3a previously irradiated with
the laser beam 1. The relative positioning may be changed by shifting the
position of the laser beam 1 or the substrate 5.
Thereafter, the Si thin film is again irradiated with the laser beam 1, so
as to form the region 3b having a crystallinity different from the
surrounding regions in the Si thin film 4. The same procedure is repeated
so as to sequentially form the regions 3c and 3d each having a
crystallinity different from the surrounding regions.
Thus, a series of pulses of irradiation are performed by shifting the
position of the laser beam 1 with regard to the substrate 5 in a first
direction as shown in FIG. 3. Then, the position of the laser beam 1 is
shifted in a second direction perpendicular to the first direction. This
shift is performed so that the next irradiation region partially overlaps
the previously irradiated region. Thereafter, a series of pulses of
irradiation is repeated by shifting the position of the laser beam 1 in a
direction opposite to the first direction. Alternatively, the position of
the laser beam 1 may be returned to a position which is located below the
position where the first series of pulses of irradiation started, and then
a series of pulses of irradiation may be repeated by shifting the position
of the laser beam 1 in the first direction. By repeating such irradiation,
the entire surface of the substrate 5 is finally covered with regions
irradiated with the laser beam 1.
In general, the irradiation intensity required for the laser annealing of
an Si thin film with an excimer laser beam is in the order of several
hundreds mJ/cm.sup.2.
FIG. 4A illustrates a section of a conventionally used laser beam and the
intensity distribution thereof. FIG. 4B illustrates a conventional overlap
irradiation method (beam scanning method).
Referring to FIG. 4A, the reference numerals 1a and 1b denote a flat
portion and an edge portion of the laser beam 1, respectively. Referring
to FIG. 4B, the reference numerals 2a and 2b denote a region irradiated
with the edge portion 1b of the laser beam 1, and a region
overlap-irradiated with the edge portion 1b for four consecutive times,
respectively. The reference numeral 3 denotes a region of which
crystallinity has changed by the irradiation with the laser beam 1.
Original laser light emitted from a laser light source such as an excimer
laser has a substantially oval section (minor axis: 10-20 mm, major axis:
30-50 mm). The intensity of laser light emitted from the laser light
source exhibits a proximate Gaussian distribution across the section
thereof. Such laser light is optically split into a plurality of beam
elements, and the split beam elements are re-synthesized so as to overlap
one another. Thus, the laser beam 1 as shown in FIG. 4A (beam shaping) is
obtained. As shown in FIG. 4A, the section of the laser beam 1 is
substantially square (from about 5 mm.times.about 5 mm to about 10
mm.times.about 10 mm). The intensity across the section of the laser beam
1 exhibits substantially trapezoidal distribution including a flat region
(the flat portion 1a). Even though a plurality of beam elements are
re-synthesized so as to overlap one another, a region having a completely
uniform intensity is not obtainable. It is possible, however, to form the
laser beam 1 having a region (with a size of 8 mm.times.8 mm, for example)
where the variation in the intensity is within the order of .+-.10%. The
width of the edge portion 1a of the laser beam 1 is generally in the range
of 1-2 mm.
As shown in FIG. 4B, the conventional "overlap irradiation" is performed by
shifting the laser beam 1 in a direction parallel to a straight-line
portion of the outline of the section of the laser beam 1. In the case
shown in FIG. 4B, when the size of the section of the laser beam 1 is L
mm.times.L mm, the position of the laser beam 1 shifts by L/4 mm during
the interval between the Nth irradiation and the (N+1)th irradiation. This
case is expressed herein as "the overlap ratio is three-fourths (75%)". In
this conventional case shown in FIG. 4B, the region 2b included in the
region irradiated with the edge portion 1b of the laser beam 1, which
extends along the shift direction of the laser beam 1 is irradiated with
the edge portion 1b of the laser beam 1 for four times. This number of
irradiation times depends on the "overlap ratio".
On the other hand, the region 2a extending perpendicular to the shift
direction of the laser beam 1 is irradiated with the edge portion 1b of
the laser beam 1 only once during the scanning shown in FIG. 4B. After the
initial irradiation with the edge portion 1b, the region 2a is irradiated
with the flat portion 1a of the laser beam 1 for three times.
As described above, the region 2b is irradiated with the edge portion 1b of
the laser beam 1 for four consecutive times. This region 2b extends to
form strips on the top surface of the substrate 5.
The inventors of the present invention have found that the above
conventional method has problems as follows:
(1) The region initially irradiated with the edge portion 1b of the laser
beam 1 is lower in the crystallinity than the region initially irradiated
with the flat portion 1a of the laser beam 1.
(2) The region 2b initially irradiated with the edge portion 1b of the
laser beam 1 and then also irradiated with the edge portion 1b is lower in
the crystallinity than the region initially irradiated with the edge
portion 1b of the laser beam 1 and then irradiated with the flat portion
1a thereof.
(3) The region initially irradiated with the edge portion 1b of the laser
beam 1 for K consecutive times is lower in the crystallinity than the
region initially irradiated with the edge portion 1b for (K-1) consecutive
times.
(4) The difference in the crystallinity described above in (2) and (3)
remains even though the former region is subsequently irradiated with the
flat portion 1a of the laser beam 1.
(5) Once the crystallinity of the region 2b subjected to the overlap
irradiation with the edge portion 1b of the laser beam i differs from
those of other regions, this causes a variation in the device performance
depending on the difference in the crystallinity.
SUMMARY OF THE INVENTION
The laser annealing method for a semiconductor thin film of this invention
includes irradiating the semiconductor thin film with a laser beam having
a section whose outline includes a straight-line portion, so as to change
the crystallinity of the semiconductor thin film, wherein the
semiconductor thin film is overlap-irradiated with the laser beam while
the laser beam is shifted in a direction different from a direction along
the straight-line portion.
In one embodiment, the section of the laser beam is a square or a
rectangle, and the straight-line portion forms one side of the square or
the rectangle.
In another embodiment, the semiconductor thin film is irradiated with the
laser beam while the laser beam is shifted in a direction inclined by
45.degree..+-.30.degree. with regard to the straight-line portion included
in the outline of the section of the laser beam.
In still another embodiment, the laser beam is obtained by optically
splitting laser light emitted from a laser light source into a plurality
of beam elements and re-synthesizing the plurality of split beam elements.
In still another embodiment, the intensity of the laser beam exhibits a
substantially trapezoidal distribution across the section of the laser
beam.
In still another embodiment, the straight-line portion included in the
outline of the section of the laser beam has a length of 5 mm or more.
In still another embodiment, the semiconductor thin film is a
non-singlecrystalline silicon thin film.
In still another embodiment, the laser beam is a beam from an excimer
laser.
Alternatively, a laser annealing method for a semiconductor thin film for
irradiating the semiconductor thin film with a laser beam, so as to change
the crystallinity of the semiconductor thin film is provided. The method
includes the steps of: shaping the laser beam so that the section of the
laser beam is a closed curve without a straight-line portion by optically
splitting laser light emitted from a laser light source into a plurality
of beam elements and re-synthesizing the plurality of split beam elements;
and overlap-irradiating the semiconductor thin film with the laser beam
while the laser beam is shifted.
In one embodiment, the section of the laser beam is an oval or a circle.
In another embodiment, the intensity of the laser beam exhibits a
substantially trapezoidal distribution across the section of the laser
beam.
In still another embodiment, the semiconductor thin film is a
non-singlecrystalline silicon thin film.
In still another embodiment, the laser beam is a beam of an excimer laser.
Alternatively, a laser annealing method for a semiconductor thin film, for
irradiating the semiconductor thin film with a laser beam in a pulse-like
manner, so as to change the crystallinity of the semiconductor thin film
is provided, wherein, in the case where a region of the semiconductor thin
film having a size corresponding to a section of the laser beam is
irradiated with the laser beam for four or more consecutive times while
the laser beam is shifted in a predetermined direction, a region of the
semiconductor thin film which is initially irradiated with a peripheral
portion of the laser beam having a relatively weak beam intensity is
subsequently irradiated with the peripheral portion of the laser beam for
three or less consecutive times.
In one embodiment, the intensity distribution of the laser beam is
substantially flat in the central portion of the laser beam, and sharply
changes in the peripheral portion of the laser beam.
In another embodiment, the beam intensity of the peripheral portion of the
laser beam is not sufficiently strong for the semiconductor thin film to
be completely melted.
In still another embodiment, the width of the peripheral portion of the
laser beam is 2 mm or less.
In still another embodiment, the laser beam has a section whose outline
includes a straight-line portion.
In still another embodiment, the laser beam is obtained by optically
splitting laser light emitted from a laser light source into a plurality
of beam elements and re-synthesizing the plurality of split beam elements,
and the laser is shaped so that the section of the laser beam is a closed
curve without a straight-line portion.
In another aspect of the present invention, a thin film semiconductor
device is provided. The device includes an active element formed on at
least a portion of a semiconductor thin film, the semiconductor thin film
being overlap-irradiated with a laser beam having a section whose outline
includes a straight-line portion, while the laser beam is shifted in a
direction different from the direction along the straight-line portion, so
as to change the crystallinity of the semiconductor thin film.
Alternatively, a thin film semiconductor device is provided. The device
includes an active element formed on at least a portion of a semiconductor
thin film, the semiconductor thin film being formed by being
overlap-irradiated with a laser beam while the laser beam is shifted, the
laser beam being shaped so that the section of the laser beam is a closed
curve without a straight-line portion by optically splitting laser light
emitted from a laser light source into a plurality of beam elements and
re-synthesizing the plurality of split beam elements.
According to the present invention, a region of a semiconductor thin film
initially irradiated with the edge portion of the laser beam will not be
irradiated with the edge portion for additional four or more consecutive
times even when the overlap ratio is three-quarters (75%). Accordingly,
the number of consecutive times when a specific region of the
semiconductor thin film is irradiated with the peripheral portion of the
laser beam is reduced, compared with the case of the conventional method.
Since an edge portion of a laser beam is weak in energy intensity, a region
of a non-singlecrystalline thin film initially irradiated with the edge
portion of the laser beam is not supplied with an energy intensity strong
enough for the region to be completely melted/solidified. Accordingly, the
region remains in the non-singlecrystalline state in the inner deep
portion thereof, though it is melted/solidified in the surface portion.
Even when such a region is irradiated with the central portion of the
laser beam having an energy intensity strong enough for the
melting/solidification immediately after the irradiation with the
low-energy laser beam, the region will not be uniformly melted/solidified
any more from the top through the bottom of the semiconductor thin film
since the surface portion thereof is not melted/solidified. This results
in a two-layer structure of crystallinity. Accordingly, the region
initially irradiated with the edge portion of the laser beam is finally
left more degraded in crystallinity than the region initially irradiated
with the central portion of the laser beam. A region initially irradiated
with the edge portion of the laser beam for a plurality of consecutive
times is insufficiently melted/solidified for every irradiation.
Therefore, it is considered that a structure composed of a plurality of
layers different in their crystallinity is formed in the semiconductor
thin film. Such a multi-layer structure remains even though the region is
subsequently irradiated with the central portion of the laser beam, and as
the number of layers increase, the crystallinity lowers.
A region of the non-singlecrystalline semiconductor thin film initially
irradiated with the central portion of the laser beam is sufficiently
melted/solidified by this irradiation. The good crystallinity of this
region remains even though the region is subsequently irradiated with the
edge portion of the laser beam.
According to the present invention, the number of consecutive times when a
specific region of the semiconductor thin film is irradiated with the edge
portion of the laser beam can be reduced, and the area of the region
subjected to such consecutive irradiation with the low-energy laser beam
can be reduced. This improves the uniformity of the crystallinity of the
laser-annealed semiconductor thin film over the surface of the substrate.
As a result, when a plurality of thin film semiconductor devices are to be
fabricated on the thus obtained semiconductor thin film, the uniformity of
the performance of the plurality of devices over the surface of the
substrate can be improved. Also, the ratio of the area of the region
overlap-irradiated with the edge portion of the laser beam to the total
area of the substrate can be reduced.
Thus, the invention described herein makes possible the advantages of (1)
providing a laser annealing method for a semiconductor thin film capable
of reducing the difference in crystallinity between a region of the thin
film irradiated with an edge portion of a laser beam and other irradiated
regions thereof is reduced so as to obtain a uniform device performance
over the surface of the substrate, and (2) providing a thin film
semiconductor device fabricated by such a laser annealing method.
These and other advantages of the present invention will become apparent to
those skilled in the art upon reading and understanding the following
detailed description with reference to the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic view illustrating the shape of a laser beam used in
a first example according to the present invention.
FIG. 1B is a plan view illustrating a laser annealing method for a
semiconductor thin film by overlap irradiation of the first example.
FIG. 2A is a schematic view illustrating the shape of a laser beam used in
a second example according to the present invention.
FIG. 2B is a plan view illustrating a laser annealing method for a
semiconductor thin film by use of overlap irradiation of the second
example.
FIG. 3 is a schematic view illustrating a conventional laser annealing
method for a semiconductor device using overlap irradiation.
FIG. 4A is a schematic view illustrating the shape of a conventionally used
laser beam.
FIG. 4B is a plan view illustrating a conventional laser annealing method
for a semiconductor thin film using overlap irradiation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described taking, as an example, the case
where non-singlecrystalline Si is annealed by overlap irradiation with an
excimer laser beam.
(EXAMPLE 1)
FIG. 1A illustrates a section of a laser beam used in this example and the
intensity distribution thereof. FIG. 1B illustrates an overlap irradiation
method (beam scanning method) of this example.
Referring to FIG. 1A, the reference numerals 1a and 1b denote a flat
portion and an edge portion of the laser beam 1, respectively. Referring
to FIG. 1B, the reference numerals 2a and 2c denote a region irradiated
with the edge portion 1b of the laser beam 1, and a region
overlap-irradiated with the edge portion 1b for two consecutive times,
respectively. The reference numeral 3 denotes a region of an Si thin film
of which crystallinity has changed by the irradiation with the laser beam
1.
In this example, as in the conventional case, a laser beam shaped to have a
substantially square section is used as the laser beam 1. The shift
direction of the laser beam 1 is different from that in the conventional
case. That is, as shown in FIG. 1B, the laser beam 1 is shifted in a
direction not along the straight-line portion of the outline of the
section of the laser beam 1.
In this example, overlap irradiation with an overlap ratio of 75% is
performed. A portion of the region 2a initially irradiated with the edge
portion 1b of the laser beam 1 and subsequently irradiated with the edge
portion 1b is denoted by the reference numeral 2c. Portions other than the
portion 2c of the region 2a are subsequently irradiated with the flat
portion 1a of the laser beam 1 during the overlap irradiation shown in
FIG. 1B.
With the above overlap irradiation as shown in FIG. 1B, any region of the
Si thin film is irradiated with the edge portion 1b of the laser beam 1
for a smaller number of times than in the conventional case. As described
hereinbefore, when a region is initially irradiated with the peripheral
portion 1b of the laser beam 1, the crystallinity of the region is no more
improved even when the region is subsequently irradiated with the flat
portion 1a of the laser beam 1. Also, a region initially irradiated with
the edge portion 1b of the laser beam 1 for K consecutive times remains
more degraded in crystallinity than a region initially irradiated with the
edge portion 1b for (K-1) consecutive times. Accordingly, in FIG. 1B, the
region 2c is more degraded in crystallinity than the other regions of the
Si thin film. However, the crystallinity of the region 2c is better than
that of the region 2b shown in FIG. 4B, and the area of the region 2c is
significantly smaller than that of the region 2b. As a result, when TFTs
were fabricated using the Si thin film obtained by the laser annealing
method of the present invention, the variation in the device performance
of the resultant TFTs over the surface of the substrate 5 was reduced,
compared with that of TFTs fabricated by the conventional laser annealing
method.
In this example, the laser beam 1 was shifted in a direction inclined by
45.degree. with regard to the straight-line portion of the outline of the
section of the laser beam 1. However, a similar effect can also be
obtained by shifting the laser beam 1 in a direction inclined by
45.+-.30.degree..
(EXAMPLE 2)
Another example of the laser annealing method according to the present
invention will be described with reference to FIGS. 2A and 2B.
FIG. 2A illustrates a section of a laser beam used in this example and the
intensity distribution thereof. FIG. 2B illustrates an overlap irradiation
method (beam scanning method) of this example.
Referring to FIG. 2A, the reference numerals 1a and 1b denote a flat
portion and an edge portion of the laser beam 1, respectively. Referring
to FIG. 2B, the reference numerals 2a and 2c denote a region irradiated
with the edge portion 1b of the laser beam 1, and a region
overlap-irradiated with the edge portion 1b for two consecutive times,
respectively. The reference numeral 3 denotes a region of an Si thin film
of which crystallinity has changed by the irradiation with the laser beam
1.
The section of the laser beam 1 used in this example is oval, having no
straight-line portion in the outline of the oval section. The original
laser light emitted from an excimer laser has also a substantially oval
section. However, the laser beam 1 of this example is greatly different in
the intensity distribution from the original laser light. More
specifically, the intensity across the section of the laser beam 1
exhibits a substantially trapezoidal distribution including a flat region
(the flat portion 1a), not the Gaussian distribution. The laser beam 1
with this intensity distribution is obtained by first splitting laser
light emitted from an excimer laser light source and then re-synthesizing
split beam elements so as to partially overlap one another. A plurality of
split beam elements are arranged into an oval shape, so as to obtain the
laser beam 1 as shown in FIG. 2A. The oval section of the laser beam 1 is
about 6 mm in the minor axis and about 10 mm in the major axis. The size
of the laser beam 1 can be made larger as the output of the laser used is
larger.
In this example, the overlap irradiation is performed by moving the laser
beam 1 having the intensity distribution shown in FIG. 2A in a
predetermined direction (not restrictive). As is apparent from FIG. 2B,
there is no region of the Si thin film which is overlap-irradiated with
the edge portion 1b of the laser beam 1 linearly for four consecutive
times. Though the portion 2c is initially irradiated with the edge portion
1b and then irradiated again with the edge portion 1b, the other regions
of the Si thin film are not irradiated with the edge portion 1b
repeatedly. As a result, a similar effect to that described in Example 1
can be obtained.
The shape of the laser beam 1 is not limited to the oval shape, but a
similar effect can be obtained by other shapes having no straight-line
portion in the outline of the section thereof, for example, a shape having
a concave portion externally.
In above Examples 1 and 2, the beam from an excimer laser was used as the
laser beam 1. However, the present invention is not limited to the excimer
laser, but a similar effect can be obtained by using a beam from a YAG
laser. Also, in the above examples, the non-singlecrystalline Si thin film
was used as the semiconductor thin film, but a similar effect can be
obtained by using another semiconductor thin film, for example,
non-singlecrystalline Ge thin film.
As described above, according to the present invention, a laser beam shaped
so that the section thereof has an outline including a straight-line
portion is shifted in a direction not along the straight line of the
outline. In the case where the laser beam is shaped so that the section
thereof has a closed curved outline not including a straight-line portion,
the laser beam can be shifted in any direction. When a semiconductor thin
film on the substrate is overlap-irradiated with such a laser beam for
four or more consecutive times, any region which is initially irradiated
with the edge portion of the light beam will be irradiated with the edge
portion for only a total of three times or less. Further, the area of the
region subjected to such repeated irradiation with the edge portion can be
reduced. Thus, the variation in the device performance over the surface of
the substrate depending on the difference in the crystallinity can be
minimized. The present invention is also effective in enhancing the device
yield when semiconductor devices are fabricated using the semiconductor
thin film formed by the laser annealing method of the present invention.
Various other modifications will be apparent to and can be readily made by
those skilled in the art without departing from the scope and spirit of
this invention. Accordingly, it is not intended that the scope of the
claims appended hereto be limited to the description as set forth herein,
but rather that the claims be broadly construed.
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