Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
2003-09-10
2004-10-05
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S662000, C438S155000, C438S166000, C438S795000, C438S487000, C438S482000, C257SE21134, C257S797000, C257S071000, C257S072000, C257SE21413, C716S030000, C716S030000, C043S005000, C043S022000
Reexamination Certificate
active
06800540
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2003-37739, filed on Jun. 12, 2003, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for crystallizing amorphous silicon, and more particularly, to a method for crystallizing amorphous silicon using a sequential lateral solidification (SLS) process.
2. Discussion of the Related Art
Recently, as modem society moves quickly toward an information-oriented society, flat panel displays, which have many advantages such as slimness, lightweight, low power consumption and the like, are widely used. In particular, among the flat panel displays, liquid crystal displays (LCDs), which have a superior color reproduction, have been actively developed.
Generally, an LCD includes two substrates facing each other. Electrodes are formed on the facing surfaces of the two substrates, and a liquid crystal material is injected into a space defined between the two substrates. The liquid crystal display (LCD) controls light transmissibility using electric fields applied between the electrodes to display an image corresponding to video signals.
The lower substrate of the LCD includes thin film transistors. The active layer of the thin film transistors is generally formed of amorphous silicon (a-Si:H). As the amorphous silicon can be deposited as a thin film at a relatively low temperature, it is widely used for forming a switching device of liquid crystal panels. The substrate may be made of a glass having a relatively low-melting point. However, the amorphous silicon thin film has a problem in that it harms the electrical characteristics and reliability of the switching device of the liquid crystal panels, and causes difficulty when increasing the screen size of the LCD.
As large-size and high-definition laptop computers and wall-mountable LCD TVs integrated with image driver circuits are commercialized, pixel-driving devices are required to have improved characteristics, such as a high electric field effect mobility (30 cm
2
/VS), a high frequency performance characteristic, and a low leakage current. To improve the characteristics, a high quality poly-crystalline silicon is required. The electrical characteristics of the poly-crystalline silicon thin film particularly depend on the size of grains. That is, the greater the size of the grains, the higher the electric field effect mobility.
Accordingly, a method for single-crystallizing silicon has become a major issue in the art. PCT Publication No. WO 97/45827 and Korean Published Patent No. 2001-004129 disclose a sequential lateral solidification (SLS) technique for making a massive single crystalline silicon structure by inducing lateral growth of a silicon crystal using a laser as an energy source. The SLS technique has been developed based on the fact that silicon grains grow in a direction normal to the boundary surface between liquid silicon and solid silicon. SLS techniques crystallizes an amorphous silicon thin film by making silicon grains grow laterally to a predetermined length, by appropriately adjusting energy intensity and beam projection range of a laser. Such a method for crystallizing amorphous silicon using the SLS technique will be described hereinafter in conjunction with the accompanying drawings.
FIG. 1
shows an SLS apparatus used for a crystallizing method using the SLS technique. A SLS apparatus
100
includes a laser generator
111
for generating a laser beam
112
, a convergence lens
113
for converging the laser beam
112
irradiated from the laser generator
111
, a mask
114
for dividing the laser beam into a plurality of sections and projecting the divided sections on a substrate
116
, and a scale lens
115
for reducing the laser beam
112
passing through the mask
114
to a predetermined scale.
The laser generator
111
emits the laser beam
112
, and the intensity of the emitted laser beam
112
is adjusted while passing through an attenuator (not shown). The laser beam
112
is then directed onto the mask
114
through the convergence lens
113
. The substrate
116
having an amorphous silicon layer deposited on its surface is disposed on an X-Y stage
117
, corresponding to the mask
114
. At this point, in order to crystallize the entire area of the substrate
116
, a method for gradually enlarging the crystallized area by minutely moving the X-Y stage
117
is used. The mask
114
is divided into laser beam transmission regions
114
a
allowing for the transmission of the laser beam
112
, and laser beam interception regions
114
b
for absorbing the laser beam
112
. The distance between the transmission regions
114
a
(the width of each interception region
114
) determines length of grains laterally grown.
A method for crystallizing amorphous silicon using the above-described SLS apparatus will be described hereinafter. Generally, crystalline silicon is used for forming a buffer (insulating) layer (not shown) on the substrate
116
, and amorphous silicon is deposited on the buffer layer. The amorphous silicon layer is deposited on the substrate
116
using, for example, a chemical vapor deposition (CVD) process, during which a large amount of hydrogen can be retained in the amorphous silicon layer. Since the hydrogen retained in the amorphous silicon tends to separate from the thin film in the presence of heat, a heat treatment is followed for a dehydrogenization process. That is, if the hydrogen is not removed in advance, the crystallized layer may be exfoliated due to the rapid volume expansion of the hydrogen gas retained in the amorphous silicon layer in the course of the crystallization process.
When performing the SLS crystallization, it is difficult to crystallize the entire area of the surface at once. That is, since a width of the laser beam
112
and a size of the mask
114
are limited, the single mask
114
should be realigned many times, and the crystallization process should be repeated whenever the mask
114
is realigned, to crystallize a large-sized screen panel. At this point, it should be understood that an area that is crystallized, which is as large as the area of the single mask
114
, is a unit block, and that the crystallization of the unit block should be realized by repeatedly irradiating with the laser beam.
FIG. 2
shows a schematic plane view illustrating a mask used for the SLS technique. As shown in the drawing, a mask
114
comprises patterned transmission and interception regions
114
a
and
114
b
. Each of the transmission regions
114
a
is defined by a longitudinal slit extending in a first direction. At this point, a width of the transmission region
114
a
should be less than or equal to twice as long as a maximum length of the grain grown by a first laser irradiated process. When the first beam is directed onto the mask structure as in the above, grains grow laterally in the melted regions of the amorphous silicon layer, which correspond to the transmission regions of the mask. In particular, the grains grow laterally from the both boundaries of the melted region until they contact each other at a middle line of the melted region.
In the course of the crystallization process, the laser beam pattern after passing through the mask
114
and being reduced by the scale lens
115
(see
FIG. 1
) moves in a direction of an X-axis. At this point, the crystallization process proceeds while the laser beam pattern moves from hundreds of &mgr;m to several mm (i.e., length of the pattern reduced by the scale lens
115
) in the X-axis direction.
The crystallization method using an SLS technique will be described in more detail hereinafter with reference to
FIGS. 3A
to
3
C, which illustrate an example of a two-shot SLS poly-silicon crystallization method. In this example, three transmission patterns (regions) are defined on the mask.
In the two-shot poly-silicon crystallization method, the regions of the amorphous silicon layer that correspond to the transmission regions a
Anya Igwe U.
LG.Philiips LCD Co., Ltd.
McKenna Long & Aldridge LLP
Smith Matthew
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