Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
1998-12-15
2001-01-16
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S795000, C438S799000, C148SDIG009, C148SDIG009
Reexamination Certificate
active
06174790
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon layer by simultaneous dehydrogenation and crystallization using a laser beam having a predetermined profile.
2. Discussion of the Related Art
An amorphous silicon layer may be crystallized when treated with energy from a laser or the like, during which process the silicon layer melts and subsequently crystallizes when cooled. Crystallization occurs via a grain growth process where small crystal seeds which was not melted by the laser energy grow to form large crystals. When a plurality of seeds are present at different locations of the unmelted silicon, the seeds grow to form polycrystalline silicon.
Each grain of the polycrystalline silicon formed by this process has its own boundaries. When polycrystalline silicon is used as the channel region of a semiconductor device, the charge carrier mobility is typically low due to the grain boundary effect arising from the carriers having to pass through the boundaries between the grains.
In a liquid crystal display (LCD) device having thin film transistors (TFT) formed of low temperature polycrystalline silicon, active layers of TFTs are formed by depositing an amorphous silicon layer by plasma enhanced chemical vapor deposition (PECVD) and by subsequently annealing or crystallizing the amorphous silicon layer using a laser annealing technique.
A first prior art method comprises depositing an amorphous silicon layer using PECVD and subsequently crystallizing the amorphous silicon layer using laser annealing. The PECVD process results in an amorphous silicon layer containing about 15% hydrogen. Dehydrogenation is carried out by thermally treating the hydrogen-containing amorphous silicon layer at a temperature over 400° C., before the laser annealing step. The thermal dehydrogenation process requires additional equipment such as a furnace or the like and typically takes about five hours. As a result, productivity is low and the cost is high. Moreover, the thermal annealing treatment using a furnace causes damages to metal structures, such as hill-lock and the like, in semiconductor devices where a metal layer lies below the silicon layer.
In a second prior art method, the amorphous silicon layer is formed by low pressure chemical vapor deposition (LPCVD) and has relatively low hydrogen content. The amorphous silicon layer is laser-annealed without a dehydrogenation process. This method results in silicon layers having a smooth surface because of the low hydrogen content of the amorphous silicon layer, but suffers from low productivity. In addition, the glass substrate used in the semiconductor device is deformed due to the relatively high temperature during the LPCVD process (over 500° C.). Thus, new equipment and techniques are necessary for forming amorphous silicon layers at low temperatures and having low hydrogen content.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method of crystallizing an amorphous silicon layer that substantially obviates one or more of the problems due to limitations and disadvantages of the prior art.
An object of the present invention is to provide a method for simultaneously dehydrogenating and crystallizing a hydrogenated amorphous silicon layer by applying a laser beam having an improved profile.
Another object of the present invention is to provide a method for releasing hydrogen from an amorphous silicon layer without damages thereto by scanning the amorphous silicon layer with laser energy having a slow-varying energy level, and simultaneously crystallizing the amorphous silicon layer. The slow-varying energy level may be obtained by reducing the overlap interval in a leading region of the profile of the laser beam used for crystallization.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objects and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with purposes of the present invention, as embodied and broadly described, a method according to the present invention includes the steps of applying a laser beam having a predetermined profile to an amorphous silicon layer in a single scan to accomplish both dehydrogenation and crystallization.
According to another respect of the present invention, dehydrogenation and crystallization are carried out simultaneously without damage to the amorphous silicon caused by hydrogen escaping from the silicon, by controlling the scan length and the profile of the laser beam, including the variation rate of energy density of the laser beam in the leading region of the energy profile.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
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Champagne Donald L.
LG. Philips LCD Co. Ltd.
Long Aldridge & Norman LLP
Utech Benjamin L.
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