Silicon crystallization method

Details Silicon crystallization method Silicon crystallization method

2002-05-30

2004-05-18

Hiteshew, Felisa (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth from liquid or supercritical state

Having moving solid-liquid-solid region

C117S041000, C117S043000

Reexamination Certificate

active

06736895

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 2001-30698, filed on Jun. 1, 2001 in Korea, which is hereby incorporated by reference as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon film, and more particularly, to a crystallization method using sequential lateral solidification (SLS).
2. Discussion of Related Art
Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as the active layer material for thin film transistors (TFTs) in liquid crystal display (LCD) devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in liquid crystal displays (LCDs). Unfortunately, amorphous silicon (a-Si) TFTs have relatively slow display response times that limit their suitability for large area LCDs.
In contrast, polycrystalline silicon TFTs provide much faster display response times. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD device, such as laptop computers and wall-mounted television sets. Such applications often require TFTs having field effect mobility greater than 30 cm
2
/Vs together with low leakage current.
A polycrystalline silicon film is composed of crystal grains having grain boundaries. The larger the grains and the more regular the grain boundaries, the better the field effect mobility. Thus, a silicon crystallization method that produces large grains, ideally a single crystal, would be useful.
One method of crystallizing amorphous silicon into polycrystalline silicon is sequential lateral solidification (SLS). SLS crystallization uses the fact that silicon grains tend to grow laterally from the interfaces between liquid and solid silicon. With SLS, amorphous silicon is crystallized using a laser beam having a magnitude and a relative motion that melts amorphous silicon such that the melted silicon forms laterally grown silicon grains upon re-crystallization.
FIG. 1A
is a schematic configuration of a conventional sequential lateral solidification (SLS) apparatus, while
FIG. 1B
shows a plan view of a conventional mask
38
that is used in the apparatus of FIG.
1
A. In
FIG. 1A
, the SLS apparatus
32
includes a laser generator
36
, a mask
38
, a condenser lens
40
, and an objective lens
42
, the laser generator
36
generates and emits a laser beam
34
. The intensity of the laser beam
34
is adjusted by an attenuator (not shown) in the path of the laser beam
34
. The laser beam
34
is then condensed by the condenser lens
40
and is then directed onto the mask
38
.
The mask
38
includes a plurality of slits “A” through which the laser beam
34
passes, and light absorptive areas “B” that absorb the laser beam
34
. The width of each slit “A” effectively defines the grain size of the crystallized silicon produced by a first laser irradiation. Furthermore, the distance between each slit “A” defines the size of the lateral grains growth of amorphous silicon crystallized by the SLS method. The objective lens
42
is arranged below the mask and reduces the shape of the laser beam that passed through the mask
38
.
Still referring to
FIG. 1A
, an X-Y stage
46
is arranged adjacent to the objective lens
42
. The X-Y stage
46
, which is movable in two orthogonal axial directions, includes an x-axial direction drive unit for driving the x-axis stage and a y-axial direction drive unit for driving the y-axis stage. A substrate
44
is placed on the X-Y stage
46
so as to receive light from the objective lens
42
. Although not shown in
FIG. 1A
, it should be understood that an amorphous silicon film is on the substrate
44
, thereby defining a sample substrate.
To use the conventional SLS apparatus, the laser generator
36
and the mask
38
are typically fixed in a predetermined position while the X-Y stage
46
moves the amorphous silicon film on the sample substrate
44
in the x-axial and/or y-axial direction. Alternatively, the X-Y stage
46
may be fixed while the mask
38
moves to crystallize the amorphous silicon film on the sample substrate
44
.
When performing SLS crystallization, a buffer layer is typically formed on the substrate. Then, the amorphous silicon film is deposited on the buffer layer. Thereafter, the amorphous silicon is crystallized as described above. The amorphous silicon film is usually deposited on the buffer layer using chemical vapor deposition (CVD). Unfortunately, the CVD method produces amorphous silicon with a lot of hydrogen. To reduce the hydrogen content the amorphous silicon film is typically thermal-treated, which causes dehydrogenation, which results in a smoother surface on the crystalline silicon film. If the dehydrogenation is not performed, the surface of the crystalline silicon film is rough and the electrical characteristics of the crystalline silicon film are degraded.
FIG. 2
is a plan view showing a substrate
44
having a partially-crystallized amorphous silicon film
52
. When performing SLS crystallization, it is difficult to crystallize all of the amorphous silicon film
52
at once because the laser beam
34
has a limited beam width, and because the mask
38
also has a limited size. Therefore, with a large size substrate, the mask
38
is typically arranged in several times over the substrate, while crystallization is repeated for the various mask arrangements. In
FIG. 2
, an area “C” that corresponds to one mask position is defined as a block. Crystallization of the amorphous silicon within a block “C” is achieved by irradiating the amorphous silicon with the laser beam several times.
Crystallization of the amorphous silicon film will be explained as follows.
FIGS. 3A
to
3
C are plan views showing one block of an amorphous silicon film being crystallized using a conventional SLS method. In the illustrated crystallization, it should be understood that the mask
38
(see
FIGS. 1A and 1B
) has three slits.
The length of the lateral growth of a grain is determined by the energy density of the laser beam, by the temperature of substrate, and by the thickness of amorphous silicon film (as well as other factors). The maximum lateral grain growth should be understood as being dependent on optimized conditions. In the SLS method shown in
FIGS. 3A
to
3
C, the width of the slits is twice as large as the maximum lateral grain growth.
FIG. 3A
shows an initial step of crystallizing the amorphous silicon film using a first laser beam irradiation. As described with reference to
FIG. 1A
, the laser beam
34
passes through the mask
38
and irradiates one block of an amorphous silicon film
52
on the sample substrate
44
. The laser beam
34
is divided into three line beams by the three slits “A.” The three line beams irradiate and melt regions “D”, “E” and “F” of the amorphous silicon film
52
. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film, i.e., complete melting regime.
Still referring to
FIG. 3A
, after complete melting the liquid phase silicon begins to crystallize at the interfaces
56
a
and
56
b
between the solid phase amorphous silicon and the liquid phase silicon. Namely, lateral grain growth of grains
58
a
and
58
b
proceeds from the un-melted regions to the fully-melted regions. Lateral growth stops in accordance with the width of the melted silicon region when: (1) grains grown from interfaces collide near a middle section
50
a
of the melted silicon region; or (2) polycrystalline silicon particles are formed in the middle section
50
a
as the melted silicon region solidifies sufficiently to generate solidification nuclei.
When the width of the slits “A” (see
FIG. 1B
) is larger than twice the maximum lateral growth length of the grains, the width of the melted silicon region “D,” “E,” or “F” is also larger than twice the maximum lateral growth length of the grains. Therefore, lateral grain growth stops when the polycrystalline silicon p

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