Semiconductor device manufacturing: process – Radiation or energy treatment modifying properties of...
Reexamination Certificate
2002-12-10
2004-12-28
Coleman, W. David (Department: 2823)
Semiconductor device manufacturing: process
Radiation or energy treatment modifying properties of...
C219S121600, C372S020000
Reexamination Certificate
active
06835675
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for annealing (laser annealing) a thin film semiconductor by irradiating a laser light. The objects of laser annealing include to crystallize an amorphous thin film, to improve the crystallinity of a crystalline thin film, to activate impurity elements for imparting conductivity, and the like.
2. Description of the Related Art
In recent years, a technique which comprises forming a thin film semiconductor on a glass substrate and then fabricating a thin film transistor by using the thus obtained thin film is known. This technique is essential for the fabrication of an active matrix liquid crystal display device.
An active matrix liquid crystal display device comprises pixel electrodes provided in a matrix-like arrangement and thin film transistors provided to each of the pixel electrodes in order to control the charge that is input and output from the pixel electrode.
In fabricating the active matrix liquid crystal display device, several hundred thousand of thin film transistors must be integrated in a matrix-like arrangement.
Thin film transistors utilizing a crystalline silicon film are capable of yielding high performance, and are preferred for use in the liquid crystal display device. When a crystalline silicon film is used, in particular, peripheral drive circuits using thin film transistors can be constructed on the same glass substrate. Thus, an advantageous constitution, which enables a more compact device at a simpler fabrication process at a lower fabrication cost, etc., can be implemented.
However, an active matrix liquid crystal device at present has problems of causing uneven display or forming stripe patterns in the display. Especially, the stripe patterns are particular in a liquid crystal display device fabricated through a laser annealing process, and they considerably impair the visual appearance of the displayed image.
The stripe patterns differ from point defects and line defects in that they become visually perceptible depending on the drive conditions of the liquid crystal display device. Thus, the present inventors assumed that this phenomena differs from the permanent defects attributed to, for example, the destruction of thin film transistors and the formation of short circuit in the wirings and the like.
Then, as a result of analyzing the liquid crystal display device from various viewpoints, it has been found that the fluctuation in ON current (the current which generates in selecting a pixel electrode) greatly influences the generation of stripe patterns.
For instance, when a thin film transistor is selected in an active matrix liquid crystal display device, an ON current generates between the source region (connected to a data line) and the drain region (connected to a pixel electrode) of the active layer as to realize a particular state (charged state) in which a certain voltage is applied to the liquid crystal.
Thus, in case the ON current is extremely small, a problem may happen that the charge is insufficient for a pixel electrode. In such a case where the saturated charge is not attained, it becomes impossible to realize the desired grayscale display, and those pixel regions with insufficient display are observed as stripe patterns.
Furthermore, there occurs a phenomenon of causing slight drop in the voltage written in the pixel electrode immediately after a thin film transistor is switched from an ON state to an OFF state (or from an OFF state to an ON state). The fluctuation in voltage is called as-a “field through voltage”.
The field through voltage is another factor causing stripe patterns, because the charge stored in the pixel electrode also changes with the field through voltage.
However, in general, the field through current is relaxed by a compensation current which generates between the source/drain (hereinafter referred to as “a field-through compensation current”). The field-through compensation current is a current that generates within a short period of time in switching the thin film transistor from an ON state to an OFF state (or reverse).
The present inventors analyzed the trial-fabricated thin film transistor, and as a result, it has been found that, with increasing ON current, the field-through compensation current increases, i.e., that the field-through voltage becomes more relaxed.
The analyzed results above can be summarized as follows. That is, the long-unsolved problem of the generation of stripe patterns in a liquid crystal display device is attributed to the fluctuation in ON current of a thin film transistor, and the best solution of the problem is to overcome the fluctuation in ON current.
Furthermore, the present inventors simulated the generation of stripe patterns ascribed to insufficient charging described above by means of simulation. The simulation was performed by calculating the time necessary for charging 99.6% or more of the pixel capacitance of about 0.2 pF (a total capacitance of a capacitance of the liquid crystals and the auxiliary capacitance).
Based on the fact that the fly-back time in VGA is 5 &mgr;s, and including margin, the results were evaluated by judging whether the pixel capacitance can be charged in a period of 2 &mgr;s or not.
As a result, it was confirmed that an ON current (at a drain voltage Vd=14 V and a gate voltage Vg=10 V) of 3 &mgr;A or higher is necessary in case of a thin film transistor with a threshold voltage of about 2 V.
In the light of the aforementioned circumstances, the present inventors came to a conclusion that it is necessary and indispensable to improve the crystallinity of the semiconductor layer (i.e., the crystalline silicon film in this case) which greatly influences the ON current above.
The crystalline silicon film above can be obtained by crystallizing an amorphous silicon film by applying heat treatment, irradiating a laser light, or by utilizing the both. In particular, the method of using laser light (said method hereinafter referred to as “laser crystallization”) as a crystallizing means or as a means for improving the crystallinity is effective from the viewpoint that it enables a crystalline silicon film having excellent crystallinity at a low temperature.
This method of forming a crystalline silicon film at a low temperature is advantageous in that a high performance thin film transistor can be fabricated on an inexpensive glass substrate. Accordingly, this method is surely a promising means for crystallization.
A pulse-emitting excimer laser is most frequently used in the method utilizing a laser light irradiation. The method using an excimer laser comprises emitting a laser having a wavelength in the ultraviolet region by applying a high frequency discharge to a predetermined type of gas and thereby realizing a particular excitation state.
In case of forming a crystalline silicon film by irradiating a laser light, however, there is a problem that not always good reproducibility is obtained on the crystallinity of the resulting crystalline silicon film. This is due to the influence of the parameters included in the process steps from the formation of a silicon film to the completion of laser annealing treatment.
The parameters included in the process steps are factors influencing the laser crystallization, and are uncertain factors influencing the crystallinity. They include indirect factors such as the film thickness of the amorphous silicon film and the direct ones such as the irradiation energy of the laser.
In case of an excimer laser, for instance, the presence of fluctuation in the irradiation energy per pulse of the emitted laser light is found as a problem. Furthermore, the fluctuation in the irradiation energy of the laser and the scattering in energy distribution in the superposed emissions of laser light are known to induce non-uniform crystallinity.
For example, the inventors use a laser device in which the laser is linearly beam-processed to provide laser-irradiated surfaces that are superposed on each other. Accordingly, the hete
Fukunaga Takeshi
Kusumoto Naoto
Miyamoto Tadayoshi
Nakajima Setsuo
Teramoto Satoshi
Coleman W. David
Fish & Richardson P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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