Amorphous silicon crystallization method

Details Amorphous silicon crystallization method Amorphous silicon crystallization method

2002-05-30

2004-08-03

Coleman, W. David (Department: 2823)

Semiconductor device manufacturing: process

Formation of semiconductive active region on any substrate

Amorphous semiconductor

C438S488000

Reexamination Certificate

active

06770545

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 2001-31624, filed on Jun. 7, 2001 in Korea, which is hereby incorporated by reference as it fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to crystallizing an amorphous silicon film, and, more particularly, to a sequential lateral solidification (SLS) crystallization method.
2. Discussion of Related Art
Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as active layer materials 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 devices, such as laptop computers and wall-mounted television sets. Such applications often require TFTs having a field effect mobility greater than 30 cm
2
/Vs and a low leakage current.
A polycrystalline silicon film is comprised of crystal grains having grain boundaries. The larger the grains and the more regular the grains boundaries, the better the field effect mobility. Thus, a 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 interface between liquid and solid silicon. With SLS, amorphous silicon is crystallized using a laser beam having a magnitude 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 source
36
, a mask
38
, a condenser lens
40
, and an objective lens
42
. The laser source
36
emits a laser beam
34
. The intensity of the laser beam
34
is adjusted by an attenuator (not shown) that is located in the path of the laser beam
34
. The laser beam
34
is condensed by the condenser lens
40
and is then directed onto the mask
38
.
The mask
38
includes a plurality of slits “A” that pass the laser beam
34
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 the slits “A” defines the size of the lateral grain 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
34
that passed through the mask
38
.
Still referring to
FIG. 1A
, an X-Y stage
46
is arranged adjacent 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 source
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 between the substrate and the amorphous silicon film. 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, that method produces amorphous silicon with a lot of hydrogen. To reduce the hydrogen content the amorphous silicon film is typically thermal-treated, which causes de-hydrogenation, which results in a smoother crystalline silicon film. If de-hydrogenation 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, the substrate
38
is typically moved numerous times such that crystallization is repeated at various locations such that the substrate is completely crystallized. In
FIG. 2
, an area “C” that corresponds to one mask position is called a block. Crystallization of the amorphous silicon within the block “C” is achieved by irradiating the laser beam several times.
SLS crystallization of the amorphous silicon film
52
will be explained as follows.
FIGS. 3A
to
3
C are plan views showing one block of an amorphous silicon film
52
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 the 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 a slit is twice as large as the maximum lateral grain growth.
FIG. 3A
shows the initial step of crystallizing the amorphous silicon film
52
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
, reference FIG.
3
A. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film
52
. That is, the portion of the amorphous silicon film that is irradiated by the laser beam
34
is completely melted through to the buffer layer.
Still referring to
FIG. 3A
, after complete melting the liquid phase silicon begins to crystallize at the interfaces
56
a
and
56
b
of the solid phase amorphous silicon and the liquid phase silicon. Crystallization occurs such that grains grow laterally. Thus, as shown, lateral grain growth of grains
58
a
and
58
b
proceeds from the un-melted regions to the fully melted regions. Lateral growth stops when: (1) grains grown from interfaces collide near the 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.
Since the width of the slits “A” (see
FIG. 1B
) is twice as large as the maximum lateral growth of the grains
58
a
and
58
b
, the width of the melted silicon region “D,” “E,” and “F” is also twice as large as the maximum lateral growth length of the gr

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