(1) Field of the Invention
This invention relates to a mask, more particularly relates to a mask for sequential lateral solidification (SLS) process.
(2) Description of the Prior Art
Liquid crystal displays (LCDs), with the advantages of slim size, low power consumption and radiation damage, has become a preferred choice in various displaying products including traditional CRT displays. For the same reason, LCDs have been widely used in various electronic devices such as desk top computers, personal digital assistants, note books, digital cameras, cell phones, and etc.
FIG. 1 shows a typical active matrix liquid crystal display (AMLCD) panel 10 with a plurality of pixel devices 12 arrayed in matrix. Each pixel device 12 is connected with a thin film transistor (TFT) 14 operating as a switch for charging or discharging the pixel device 12. The source electrode of the TFT 14 is electrically connected with a source driver (not shown) through a signal line 16. The gate electrode of the TFT 14 is electrically connected to a scan driver (not shown) through a gate line 18. The displaying signal is transformed into source driving voltage (Vs) and gate driving voltage (Vg) applied to the source electrode and gate electrode respectively to generate images.
Due to the temperature limit for the glass substrate, in the traditional manner, the TFTs 14 formed on the LCD panel 10 must be amorphous silicon TFTs. However, the switching speed, the electric characters, and the reliability of the amorphous silicon TFTs are not qualified as being applied in the drivers for controlling the display of the pixel devices 12. Instead, polysilicon TFTs are suggested to be applied in the drivers to achieve a high operation speed. Therefore, the drivers must be formed on the silicon chips and connected to the LCD panel 10 through some pipelines.
There are two reasons why polysilicon TFTs fabricated on the glass substrate are demanded in present. First, the pixel devices need a higher switching speed for a larger LCD panel. Second, the drivers must be formed on the glass substrate for a slimmer display panel. Therefore, the demand of forming high quality polysilicon layers on the glass substrate has become urgent.
FIG. 2 shows a traditional low temperature polysilicon (LTPS) fabrication process. First, an amorphous silicon layer 120 is formed atop a substrate 100, and then laser illumination is utilized to form a melted layer 122 near the top surface of the amorphous silicon layer 120. The amorphous silicon material right under the melted layer 122 is utilized as seeds for growing upward to create grains 126. Due to the limitation of the thickness of the melted layer 122, the grain size is usually less than 1 micron. Thus, the promotion of the electric ability of the resultant TFTs is limited.
In order to access larger grain size, as shown in FIG. 3, the lateral solidification process is developed by using laser illumination to melt a predetermined region A within the amorphous layer 120 through a mask 200. Since a lateral temperature gradient is generated in the region A, the amorphous silicon material close to the edge of the melting region A is utilized as seeds for growing toward the center of the melting region A to generate grains 128 with larger size.
As shown in FIG. 4, which shows a typical mask 300 utilized in the sequential lateral solidification (SLS) process. As shown, the mask 300 has a plurality of first rectangular windows 310 lined in row on the mask, and a plurality of second rectangular windows 320 lined in row on the mask. Each first rectangular window 310 is aligned to the shielded region between neighboring two second rectangular windows 320.
FIG. 5 depicts the SLS process using the mask 300 of FIG. 4. In the first illumination step, laser illumination melts the amorphous silicon layer through the first windows 310 and the second windows 320 on the mask 300. Since the density of silicon in liquid state, 2.53 g/cm3, is greater than in solid state, 2.33 g/cm3, the top surface a of the melted region of the amorphous silicon layer is located below the top surface b of the unmelted region as shown in FIG. 6A. In addition, since the silicon grains are grown along the temperature gradient, that is from the both edges of the melted region A1 toward the center thereof, as shown in FIG. 6B, a protrusion c must be formed at the center of the melted region A1 after the crystallization.
In the second illumination step, as shown in FIG. 5, the mask 300 is moved rightward with a distance substantially identical to the length of the second window 320 to have the first window 310′ focusing on the unmelted region between the melted regions A1 in the first illumination step. Also referring to FIG. 6C, in the present illumination step, the portion of the amorphous silicon layer (the melted regions A1 of FIG. 6A) with respect to the second windows 320 is shielded by the mask 300, the portion of the amorphous silicon layer A2 shielded by the mask 300 in the first illumination step is melted. Since the density of silicon in liquid state is greater than in solid state, some valley-like portion d must be formed close to the edges of the melted region A2, and a protrusion c is formed at the center of the second region A2.
The protrusion c atop the resultant polysilicon layer may affect the coverage of the dielectric layer in the following steps to induce abnormal increasing of leakage current and even the breakthrough of the dielectric layer. In order to reduce the height of the protrusion c, a typical method is to re-flow the polysilicon layer by laser illuminating the polysilicon layer as a whole. However, the valley-like portion d is also melted by using this method and the probability of agglomeration to break the polysilicon layer increases.
Accordingly, a mask utilized in sequential lateral solidification (SLS) processes is provided in the present invention to flatten the protrusion and to prevent the breakage of the resultant polysilicon layer.