Method for crystallizing amorphous silicon layer

Details Method for crystallizing amorphous silicon layer Method for crystallizing amorphous silicon layer

1999-05-13

2001-12-04

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Formation of semiconductive active region on any substrate

C438S486000

Reexamination Certificate

active

06326286

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a method for crystallizing an amorphous silicon layer and a method for fabricating a thin film transistor (TFT) using the same, and more particularly, a method for crystallizing an amorphous silicon layer and fabricating a TFT by utilizing the Sequential Lateral Solidification (SLS) technique.
2. Discussion of the Related Art
In order to fabricate TFTs on a low heat-resistant substrate, such as a glass substrate, an amorphous silicon layer or a polycrystalline silicon layer is deposited on the substrate and etched by photolithography to form active layers for TFTs.
The mobility of charge carriers is low in the amorphous silicon layer. Accordingly, amorphous silicon TFT is not typically used as a driving circuit or a controller of a liquid crystal display (LCD). However, the mobility of charge carriers is high in the polycrystalline layer. Accordingly, polycrystalline TFTs can be used in driving circuits of a liquid crystal display (LCD), wherein devices for pixel array and driving circuits are formed simultaneously.
There are two techniques for forming polycrystalline silicon film on a glass substrate. In the first technique, an amorphous silicon film is deposited on the substrate and crystallized at a temperature of about 600° C. by Solid Phase Crystallization (SPC). This technique is difficult and problematic in terms of cost and materials because it requires a very high temperature.
The second technique involves depositing an amorphous silicon film on the substrate and crystallizing the film by thermal treatment using a laser. The second technique is not a high temperature process, thus, this facilitates the formation of a polycrystalline silicon film on the glass substrate.
FIGS. 1A
to
1
D are schematic drawings which illustrate a method for forming a polycrystalline silicon film according to one related art. Referring to
FIG. 1A
, a particular region of an amorphous silicon film
10
is first irradiated at an energy density to induce formation of separated islands of amorphous silicon
11
a
and the liquid silicon region
11
L, a region generated by the irradiation of a laser beam, completely melts.
Referring to
FIG. 1B
, the amorphous film is translated relative to the laser beam over a distance less than the predetermined distance for a second irradiation. While the film is translating, the liquid silicon region
11
L is crystallized under low temperature through a cooling process. The separated islands of amorphous silicon
11
a
are used as seeds for the crystallization process which results in the growth of the liquid silicon region
11
L, thereby forming a first polycrystalline silicon region
11
P. Grain growth occurs not only in the middle of the growth region, but growth also occurs in the interface between the liquid silicon region
11
L and solid state amorphous silicon region a-Si. The grain growth stops when the grains collide at these grain boundaries.
Referring to
FIG. 1C
, a selected region of a translated amorphous silicon film is secondly irradiated. Thus, separated islands of amorphous silicon
12
a
remain and the other portions of the silicon, namely, silicon region
12
L completely melts.
In
FIG. 1D
, the amorphous film is translated relative to the laser beam for the next irradiation. While the film is translating, the liquid silicon region
12
L is crystallized under a low temperature cooling process. The separated islands of amorphous silicon
12
a
are used as seeds which grow into the liquid silicon region
12
L, thereby forming a second polycrystalline silicon region
12
P. Moreover, grain growth occurs at the interface between the liquid silicon region
12
L and solid state amorphous silicon region a-Si, as well as at the interface between the liquid silicon region
12
L and the first polycrystalline silicon region
11
P.
The above described processes of irradiating and crystallizing are repeated over a total translation distance in order to crystallize the entire film. However, since the size of each silicon grain is not uniform and the location of the grain boundary varies in the polycrystalline silicon layer, a device-to-device uniformity is degraded in TFTs fabricated by such methods.
Accordingly, it is desirable and necessary to make the location of the grain boundary uniform and the grain size large.
Accordingly, it is proposed that a polycrystalline silicon film be formed on the glass substrate by using Sequential Lateral Solidification (SLS) techniques, as described in Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956~957, 1997. The SLS technique uses a phenomenon wherein the grain boundaries in directionally solidified materials tend to form perpendicularly to the melted interface. The SLS technique enables the conversion of as-deposited amorphous or polycrystalline silicon films into a directionally solidified microstructure consisting of long, columnar grains.
The laser beam pattern for using SLS technique is prepared by an annealing apparatus as shown in FIG.
2
. An unpatterned laser beam is emitted from a light source
20
and is passed through an attenuator
22
to control the energy density of the unpatterned laser beam. The unpatterned laser beam is focused on a focus lens
22
and passed through a mask having a predetermined pattern
23
in order to pattern the laser beam. The patterned laser beam then passes through an imaging lens
24
. A film
29
on a translation stage
25
is irradiated by the patterned laser beam. The entire film is scanned by the laser beam at a predetermined repetition rate. In this regard, Mirrors
28
-
1
,
28
-
2
, and
28
-
3
control the path of the laser beam.
FIGS. 3A
to
3
C show a method for crystallizing an amorphous silicon film by the SLS technique according to another related art. Referring to
FIG. 3A
, a narrow region having a slit film shape, bounded by the dashed lines
42
and
43
, is irradiated at an energy density sufficient to induce complete melting. Subsequently, lateral grain growth proceeds from the unmelted regions to the adjacent narrow strip region
41
which is fully-melted. Grain boundaries in directionally solidified materials proceed perpendicularly to the melt interface. Depending on the width of the molten region, lateral growth ceases when either of two events occur: (1) the two opposing growth fronts collide at the center, or (2) the molten region becomes sufficiently supercooled to cause bulk nucleation of solids.
Due to these restrictive events, the maximum lateral growth distance which can be achieved with a single pulse is limited depending on the film thickness and the incident energy density.
Referring to
FIG. 3B
, the film is translated relative to the beam image over a distance less than the single-pulse lateral growth distance and irradiated again. Lateral growth begins again from the edges of the completely molten region, located within the grains grown during the previous irradiation step. The length of the grains is increased beyond the single-pulse lateral growth distance. For example, a narrow region
45
bounded by dashed lines
46
and
47
is irradiated by a second laser pulse. Since one of these edges, in this case the edge
46
, is located within the silicon region grown during the previous irradiation step, the lengths of the silicon grains formed by the previous irradiation are extended by the second irradiation beyond the single-phase lateral growth distance.
Referring to
FIG. 3C
, the above-described processes of irradiation and solidification can be repeated indefinitely, creating grains of any desired length. The final resultant microstructure is shown.
Using the method of the related art, a polycrystalline silicon film having uniform physical characteristics could be achieved through lateral growth of the silicon grain. However, the polycrystalline silicon film cannot be used to form devices for complicated circuits, whereas a single crystal silicon film may be use in the manufacture of such devices.
FIGS.

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