Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
1999-05-13
2001-05-22
Le, Vu A. (Department: 2824)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S166000, C438S150000, C438S795000, C117S004000
Reexamination Certificate
active
06235614
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for crystallizing an amorphous silicon layer and fabricating a thin film transistor thereof which fabricate a thin film transistor having an active layer of a silicon layer crystallized by sequential lateral solidification (SLS).
DISCUSSION OF THE RELATED ART
To fabricate a thin film transistor (TFT) on a low heat resistant substrate such as a glass substrate, an amorphous silicon layer or a polycrystalline layer as an active layer of a thin film transistor is formed on a substrate. However, such methods have disadvantages in that the characteristics of the formed semiconductor devices vary since an amorphous silicon layer has low charge carrier mobility and a polycrystalline silicon layer has grain boundaries which are located randomly.
There are various techniques for forming a polycrystalline silicon layer on a glass substrate, such as: (1) forming a polycrystalline silicon layer directly on a substrate; (2) forming an amorphous silicon layer on the substrate and crystallizing the amorphous silicon layer at a temperature of about 600 C by solidification; and (3) forming an amorphous silicon layer on the substrate and treating the amorphous silicon layer thermally with a laser.
The techniques (1) and (2) above is problematic when forming the layer on a glass substrate because the methods involve high temperatures. The technique (3) requires no high temperature and may be applied to a glass substrate, and enables the formation of polycrystalline silicon layers of high quality having low defect density inside the crystallized particles. However, the technique (3) also has problem that uniformity of electrical characteristics among the TFT devices is low as crystal field fails to be uniform.
In order to solve the problems, the distribution of crystal field must be controlled artificially, or fabrication of a single crystal device is required. A technique of forming a single crystalline silicon layer on a glass substrate by SLS is described in Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956957, 1997. The technique, using the fact that silicon grains tend to grow vertically against the interface between liquid and solid silicon, teaches that an amorphous silicon layer is crystallized by controlling the magnitude of laser energy and an irradiation range of a moving laser beam to have silicon grains grow laterally up to a predetermined length, as shown in the following example.
FIGS. 1A
to
1
C show the crystallized state of a silicon layer during an SLS process. Referring to
FIG. 1A
, the amorphous silicon layer is irradiated with a first shot of a laser beam having an elongated shape with a predetermined width. The energy of the laser beam is high enough to melt the irradiated portion of the silicon layer.
Consequently, a portion of the silicon layer having been irradiated with the laser beam is melted and crystallized. When the portion of the silicon layer is crystallized, lateral growth of grains proceeds from the interface between an amorphous silicon region and the melted silicon region. The lateral growth of the grains proceeds in a vertical direction against the interface.
The lateral growth stops in accordance with a width of the melted silicon region when: (1) grains having grown from both interfaces collide each other in the middle section of the melted silicon region; or (2) polycrystalline silicon particles are generated simultaneously in many places as the melted silicon region is solidified sufficiently to generate nuclei. The length of lateral growth of a grain attained by a single laser irradiation depends. on both laser energy and the thickness of the amorphous silicon layer.
Referring to
FIG. 1B
, the amorphous silicon layer is irradiated with a second shot of the laser beam, the amorphous silicon layer having been moved in the lateral direction by a distance which is less than the length of lateral growth of the grains resulted from the first laser beam irradiation.
Accordingly, the portion of silicon irradiated with the second irradiation of the laser beam is melted and then crystallized. Lateral growth,proceeds to the melted silicon region as the grains that are grown by the first irradiation act as seeds for crystallization.
Referring to
FIG. 1C
, the grains are grown to a predetermined lengths by repeating the steps of moving the amorphous silicon layer in the lateral direction, melting a portion of the amorphous silicon layer by irradiating it with the laser beam, and crystallizing the melted silicon layer.
FIG. 1C
shows the state of a crystallized silicon layer resulted from lateral growth of grains to predetermined sizes.
FIG. 2
is a schematic drawing illustrating a method of crystallizing an amorphous silicon layer using SLS to fabricate a wide vision LCD device.
FIG. 3
shows the surface roughness of the cross-section of the silicon layer at the bisecting line IV in FIG.
2
.
When an amorphous silicon layer having a large area is crystallized using SLS, the length of the laser beam of ten cannot cover the entire surface of the amorphous silicon layer. A plurality of scans are required for crystallizing the entire surface of the silicon layer as the lengths of the grains grown by SLS are much shorter than the length of the entire layer. Thus, crystallization of such a silicon layer is carried out simultaneously by SLS using a plurality of laser beams.
In
FIG. 2
, reference numeral
200
designates an insulated substrate on which an amorphous silicon layer is formed.
Reference numeral
201
indicates a portion of the amorphous silicon layer not yet crystallized. Reference numeral
202
designates the polycrystalline silicon regions formed by SLS. Each area
202
is formed by irradiating or scanning the silicon layer with a laser beam.
FIG. 3
shows a cross-section of the polycrystalline silicon region bisected by the line II I in FIG.
2
. The line designated by numeral
31
represents the surface profile of the layer. The horizontal axis represents the direction of scan of the laser beam (from left to right).
In the silicon layer crystallized by SLS according to the related art, as shown in
FIG. 3
, the surface of the silicon layer protrudes out at or near a portion that is crystallized the last during a scan, i.e., a portion near the right end of a region
202
. This is because during the process of solidification, the volume of the silicon increases after being irradiated with a laser beam. During an SLS process, the laser beam is scanned in one direction. At the end of a scan, the portion of the silicon layer solidified last (i.e. near the right end of the regions
202
) is unable to push the solid silicon adjacent the melted silicon region, and the silicon grains protrudes upward as a result, as shown in FIG.
3
.
When a portion of the silicon layer having the above-described uneven surface is used as an active layer of a thin film transistor, the characteristics of the thin film transistor is degraded due to the step difference on the surface of the silicon layer. Therefore, to be used as an active layer the surface of the silicon layer must be made smooth by removing the surface unevenness, such as by etching.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to methods of crystallizing an amorphous silicon layer and fabricating a thin film transistor therefrom that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide methods of crystallizing an amorphous silicon layer and for fabricating a thin film transistor which has an active layer having a smooth surface, by eliminating a protruding surface caused by an increased volume of silicon during crystallization by allowing the surface of a silicon layer to be increased. This is achieved by patterning the amorphous silicon layer to form an active layer by etching using photolithography, and subsequently crystallizing the patterned amorphous silicon layer.
Addit
Le Vu A.
LG. Philips LCD Co. Ltd.
Long Aldridge & Norman LLP
Pyonin Adam
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