Patent Publication Number: US-7902010-B2

Title: Mask for sequential lateral solidification (SLS) process and a method for crystallizing amorphous silicon by using the same

Description:
The present application is a divisional of U.S. patent application Ser. No. 11/453,931 filed Jun. 16, 2006 now U.S. Pat. No. 7,740,993, which claims priority on Taiwanese Patent Application No. 94124035, filed Jul. 15, 2005, the entire contents of which are hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     (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/cm 3 , is greater than in solid state, 2.33 g/cm 3 , 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 A 1  toward the center thereof, as shown in  FIG. 6B , a protrusion c must be formed at the center of the melted region A 1  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 A 1  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 A 1  of  FIG. 6A ) with respect to the second windows  320  is shielded by the mask  300 , the portion of the amorphous silicon layer A 2  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 A 2 , and a protrusion c is formed at the center of the second region A 2 . 
     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. 
     SUMMARY OF THE INVENTION 
     It is a main object of the present invention to flatten the protrusion on the polysilicon layer, which is formed due to traditional SLS process, so as to promote the coverage for the following fabrication steps. 
     A mask utilized in sequential lateral solidification (SLS) processes to form a polysilicon layer on an amorphous silicon layer is provided in the present invention. The mask has at least one first window, one second window, one third window, and one fourth window. Each window has a length extending along a first direction. The second window is aligned to the first window and has a width smaller than that of the first window. The fourth window is aligned to the third window and has a width greater than that of the third window. 
     In an embodiment of the present invention, the third window is located adjacent to the length of the first window. 
     In another embodiment of the present invention, the third window is located adjacent to the length of the second window. 
     A method for forming a polysilicon layer in an amorphous silicon layer is also provided in the present invention. The method comprises the steps of: (a) providing a substrate; (b) forming an amorphous silicon layer on the substrate; (c) providing a mask as mentioned above; (d) applying a laser and the mask on the substrate to crystallize the amorphous silicon layer to form a plurality of first crystallizing regions corresponding to the first window and the fourth window; (e) moving the mask or the substrate along the first direction to have the second window and the third window focusing on a central protrusion formed in the first crystallizing regions; and (f) melting the amorphous silicon layer through the mask to lower down the central protrusion and to crystallize the amorphous silicon layer to form a plurality of second crystallizing regions corresponding to the first window and the fourth window, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which: 
         FIG. 1  is a schematic view of a typical active matrix liquid crystal display (AMLCD) panel; 
         FIG. 2  depicts a typical low temperature polysilicon (LTPS) fabrication process; 
         FIG. 3  depicts a typical lateral solidification process; 
         FIG. 4  shows a typical mask utilized in sequential lateral solidification (SLS) processes; 
         FIG. 5  depicts a typical SLS process; 
         FIGS. 6A to 6D  are a sequence of cross section views depicting a traditional SLS process; 
         FIG. 7A  is a top view showing a preferred embodiment of a mask utilized in an sequential lateral solidification (SLS) process in accordance with the present invention; 
         FIG. 7B  depicts an SLS process using the mask of  FIG. 7A ; 
         FIG. 7C  shows a schematic view of the resultant polysilicon structure formed in the amorphous layer using the mask of  FIG. 7A ; 
         FIGS. 8A to 8C  are a sequence of cross section views depicting the SLS process in accordance with the present invention; 
         FIG. 9A  is a top view showing another preferred embodiment of a mask utilized in sequential lateral solidification (SLS) process in accordance with the present invention; 
         FIG. 9B  depicts an SLS process using the mask of  FIG. 9A ; 
         FIG. 9C  shows a schematic view of the resultant polysilicon structure formed in the amorphous layer using the mask of  FIG. 9A ; 
         FIG. 10A  is a schematic view depicting a preferred embodiment of the second window in accordance with the present invention; and 
         FIG. 10B  is a schematic view depicting another preferred embodiment of the second window in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 7A  is a top view showing a preferred embodiment of a mask  400  applied in the SLS process in accordance with the present invention. As shown, the mask  400  has at least one first window  410 , one second window  420 , one third window  430 , and one fourth window  440 . Each of the windows  410 , 420 , 430 , 440  is extended along a first direction (which may be defined as the lateral direction of the mask). 
     The first windows  410  are lined along a second direction (which may be defined as the longitude direction of the mask  400 ). The second windows  420  are lined along the second direction on the mask  400 , and each second window  420  is aligned to a respected first windows  410 . The third windows  430  are lined along the second direction on the mask  400 . The fourth windows  440  are lined along the second direction on the mask  400 , and each fourth window  440  is aligned to a respected third windows  430 . Each third window  430  is located adjacent to the length of a respected first window  410  and between neighboring first windows  410 . Each second window  420  is located adjacent to the length of a respected fourth window  440  and between neighboring fourth windows  440 . 
       FIGS. 8A to 8C  depict an SLS process in accordance with the present invention using the mask  400  of  FIG. 7A . In the beginning, a substrate  100  is provided with an amorphous silicon layer  120  formed thereon. Afterward, as shown in  FIG. 8A , the mask  400  of  FIG. 7A  is utilized to focus on the substrate  100 . Than, laser illumination is applied through the mask  400  to melt the amorphous silicon layer  120 . The melted portions within the amorphous silicon layer  120  are then crystallized to form a plurality of first crystallizing regions A 1  with respect to the first windows  410  and the fourth windows  440  (referring to the dash line of  FIG. 7B ). 
     Afterward, as shown in  FIG. 8B , moving the mask  400  or the substrate  100  along the first direction to have the second window  420  and the third window  430  of the mask  400  focusing on the central protrusion c in the first crystallizing regions A 1  (referring to the solid line of  FIG. 7B ). Then, laser illumination is applied through the mask  400  to melt the amorphous silicon layer  120 . The melted portions are then crystallized to form a plurality of second crystallizing regions A 2  in the amorphous silicon layer  120  with respect to the first windows  120  and the fourth windows  440 . In addition, as shown in  FIG. 8C , the central protrusions c of the first crystallizing regions A 1 , which are labeled B 3  in this figure, are melted in the present laser illumination step. After the sequential movement of the mask  400  or the substrate  100  along the first direction, referring to  FIG. 7C , a polysilicon layer as a whole is formed over the amorphous silicon layer  120 . 
     As shown in  FIG. 8B , in order to prevent the surroundings of the central protrusion c within the first crystallizing regions A 1  being melted and deformed, the first window  410  must have a width at least greater than that of the second window  420  and the fourth window  440  must have a width at least greater than that of the third window  430 . In addition, as a preferred embodiment, the first window  410  may have a width greater than five times the width of the second window  420 , and the fourth window  440  may have a width greater than five times the width of the third window  430 . 
     In order to promote the uniformity of the crystallized grains, the first window  410  should have a dimension identical to that of the fourth window  440  and the second window  420  should have a dimension identical to that of the third window  430 . In addition, the center of the second window  420  should be aligned to the center of the first window  410  and the center of the third window  430  should be aligned to the center of the fourth window  440  so as to make sure that the second window  420  and the third window  430  may precisely focus on the central protrusion c within the first crystallizing regions A 1 . Moreover, the width of the fourth window  440  must be greater than the interval between neighboring first windows  410  and the width of the first window  410  must be greater than the interval between neighboring fourth windows  440  to make sure no amorphous area on the amorphous silicon layer  120  remained. 
     In order to reduce the optical energy illuminated through the second window  420  or the third window  430  to prevent the planar portions B 1 , B 2  surrounding the central protrusion c being melted, as shown in  FIG. 10A , the second window  420   a  in accordance with the present invention may be composed of a plurality of transmission regions  422  arranged lengthwise. The laser beams penetrating these transmission regions  422  is combined to form an illumination corresponding to the case as shown in  FIG. 7A  due to optical interference but a smaller total optical energy. Therefore, the irradiance (which is understood as the average energy per unit area per time) passing through the second window  420   a  illuminated on the amorphous layer in the present embodiment is smaller than the irradiance passing through the first window  410  illuminated on the amorphous layer. In addition, as shown in  FIG. 10B , the second window  420   b  may be composed of a plurality of transmission regions  424  arranged widthwise. The irradiance passing through the second window  420   b  illuminated on the amorphous layer is smaller than the irradiance passing through the first window  410  illuminated on the amorphous layer. The embodiments as shown in  FIGS. 10A and 10B  may be also applied to the third window  430   a  and  430   b  of the mask in accordance with the present invention. 
     Basically, the pattern on the mask is projected on the amorphous layer with a predetermined transverse magnification x. Since the grain length is a little greater than half the width of the first crystallizing area A 1  as described in  FIG. 8C , it is understood that the width of the first window  410  or the fourth window  440  must be at least twice the length L of the grains to be formed divided by the transverse magnification x. In detail, the length L of the grains is approximately identical to the sum of half the width of the first window  410  (or the fourth window  440 ) and half the interval between neighboring first windows  410  (or the neighboring fourth windows  440 ), multiplied by the transverse magnification x. 
       FIG. 9A  shows a top view of another preferred embodiment of the mask  500  utilized in the SLS process in accordance with the present invention. In contrast with the mask  400  shown in  FIG. 7A , a second window  520  of the mask  500  is located at the left side of the first window  510 , a third window  530  located at the left side of the fourth window  540 , and the third window  530  is located adjacent to the length of the second window  520 . 
       FIG. 9B  describes the SLS process using the mask  500  provided in  FIG. 9A .  FIG. 9C  describes the resultant polysilicon layer formed on the amorphous silicon layer. In compared with the SLS process shown in  FIGS. 8A to 8C , which characterized with two repeated melting steps (corresponding to the dash line portion and the solid line portion in  FIG. 7B  respectively), the present embodiment is characterized with three repeated melting steps (corresponding to the dash line portion, the solid line portion, and the filled portion in  FIG. 9B  respectively). In the first step, a plurality of first crystallizing regions is formed by laser illuminating through the first windows  510  and the fourth windows  540 . In the second step, the central protrusions within the first crystallizing regions corresponding to the first windows  510  are melted by laser illuminating through the second windows  520 ′. Until the third step, the central protrusions within the first crystallizing regions corresponding to the fourth windows  540  are melted by laser illuminating through the third windows  530 ″. 
     It should be noted although the mask  400  of  FIG. 7A  and the mask  500  of  FIG. 9A  have different patterns, no significant distinction exists in the resultant polysilicon layer structures as show in  FIG. 7C  and  FIG. 9C . 
     As shown in  FIG. 6D , the polysilicon layer formed by using traditional SLS process has distinct protrusions c on the surface thereof. The protrusions c may damage the coverage of the dielectric layer deposited in the following steps to increase leaky current or even result the breakthrough of the dielectric layer. In contrast, the protrusions c on the polysilicon layer can be locally flattened in the SLS process together with the mask  400  provided in the present invention as shown in  FIGS. 8B and 8C . Thus, the problem of coverage maybe resolved. 
     In addition, in order to lower down the protrusion c, the traditional method illuminates the top surface of the polysilicon layer as a whole to re-flow the polysilicon layer. However, this method has a drawback that the valley-like portion d in the polysilicon layer is also melted to result agglomeration event or even break the polysilicon layer. In contrast, the mask provided in the present invention have the laser illumination focusing on the protrusions c to prevent the valley-like portion d being melted. Thus, the problem encountered in the traditional method can be prevented. 
     While the embodiments of the present invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the present invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the present invention.