Abstract:
A mask for sequential lateral solidification capable of preventing pattern deformation that may be caused by laser beam, and a method of manufacturing the same are provided. The mask includes a transparent substrate, and a heat-resistant oxide film pattern, disposed on the transparent substrate, blocking a laser beam.

Description:
REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2005-0071414 filed on Aug. 4, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a mask for sequential lateral solidification (SLS) and, more particularly, to a mask for SLS capable of preventing pattern deformation that may be caused by a laser beam. 
     BACKGROUND OF THE INVENTION 
     The manufacture of thin film transistor-liquid crystal displays (TFT-LCDs) has continued to attempt to replace amorphous silicon used as a TFT channel material with polysilicon to achieve higher performance, larger scale integration and faster response. However, leakage currents tend to occur at polysilicon grain boundaries and it becomes necessary to produce large-sized polysilicon grains to reduce grain boundaries. Conventional laser-based melting and crystallization can produce a mono-crystalline silicon film with a length of up to several micrometers, but cannot selectively produce mono-crystalline silicon grains at desired locations. To increase a grain size, a sequential lateral solidification (SLS) process has recently been suggested. 
     According to a suggested SLS process, a predetermined region of an amorphous silicon film is treated with a laser beam using a mask for crystallization and the mask is then moved at a pitch smaller than that of the crystal formed. Then, the amorphous silicon film is again treated with a laser beam using the formed crystal as a seed. In this way, the crystal size continuously grows. The silicon grains formed by the first shot of laser beam act as seeds, allowing lateral re-growth of mono-crystalline silicon in the amorphous silicon film. Theoretically, the SLS process can yield very long mono-crystalline silicon. 
     In a conventional SLS process for crystallization of amorphous silicon, a mask composed of a quartz plate and a chromium film pattern coated on the quartz plate is used. To crystallize amorphous silicon, a laser beam having high energy density is used. However, the crystallization of amorphous silicon using a laser beam with high energy density greatly reduces the durability of the mask. 
     SUMMARY OF THE INVENTION 
     The present invention provides a mask for sequential lateral solidification (SLS) capable of preventing pattern deformation caused by a laser beam. According to an aspect of the present invention, the mask for SLS includes a transparent substrate having a heat-resistant oxide film pattern for blocking the laser beam. The mask for sequential lateral solidification is produced by sequentially depositing a heat-resistant oxide film and a metal film on the transparent substrate, etching the metal film to produce a pattern in the metal film, using the metal film pattern as an etching mask to etch the heat-resistant oxide film and then removing the metal film pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a view illustrating an apparatus for crystallization of amorphous silicon according to an embodiment of the present invention; 
         FIGS. 2A through 2D  are cross-sectional views illustrating sequential processing steps for manufacturing a modified example of the mask shown in  FIG. 1 ; 
         FIG. 3  is a plan view illustrating a mask for SLS according to an embodiment of the present invention; and 
         FIG. 4  is a sectional view illustrating a mask for SLS according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a view illustrating an apparatus  100  for crystallization of amorphous silicon according to an embodiment of the present invention. 
     Crystallization apparatus  100  is structured such that a light source  10 , an attenuator  20 , a focus lens  40 , a mask  50 , an imaging lens  60 , and a translation stage  80  on which an amorphous silicon-coated substrate  70  is disposed are sequentially arranged. A plurality of reflectors  30 ,  32 , and  34  are disposed between the attenuator  20  and the focus lens  40  and between the imaging lens  60  and the translation stage  80  to change the direction of incident light by reflecting the incident light at a predetermined angle. 
     Light source  10  is an XeCl or KrF excimer laser advantageously having a wave-length of 308 nm or 248 nm, respectively. A attenuator  20  adjusts the laser beam generated by the excimer laser to have a predetermined energy. Focus lens  40  and imaging lens  60  are responsible for focusing the laser beam. In particular, focus lens  40  equalizes long and short focal distances of the laser beams to ensure uniform laser beam energy. Mask  50  is patterned and thus forms the laser beam into a predetermined shape. 
     Accordingly, the laser beam generated by light source  10  is adjusted to have a predetermined energy by the attenuator  20 , reflected by the reflectors  30  and  32 , and focused by the focus lens  40 . The laser beam partially passes through mask  50  and radiates onto substrate  70  via the imaging lens  60  and the reflector  34 . Laser beam radiation is repeated while the translation stage  80  bearing substrate  70  is moved. To crystallize the other areas of amorphous silicon (not shown) coated on the substrate  70 , laser beam irradiation is repeatedly performed while translation stage  80  or the mask  50  is moved a small distance (according to the desired pitch), so that silicon grains are grown. 
       FIGS. 2A through 2D  are cross-sectional views illustrating sequential processing steps for manufacturing a modified example of the mask of  FIG. 1 . Referring to  FIG. 2A , a heat-resistant oxide film  220  and a metal film  230  are sequentially formed on a transparent substrate  210 . The transparent substrate  210  may be made of a material allowing the passage of a laser beam. For example, the transparent substrate  210  may be made of quartz. Heat-resistant oxide film  220  is responsible for blocking the laser beam. When the heat-resistant oxide film  220  formed on the transparent substrate  210  is patterned in a subsequent processing step, the heat-resistant oxide film patterns remain on the transparent substrate  210  and form shielding areas for the laser beam. The portions of the transparent substrate  210  exposed by the pattern of removal of the heat-resistant oxide film  220  form transmitting areas for a laser beam. 
     The heat-resistant oxide film  220  may be a single-layered film or a multi-layered film made of, for example, Al 2 O 3 , ZrO, SiO 2 , or MgO. Furthermore, the heat-resistant oxide film  220  may be made of a material having a melting point of about 2,000° C. or more so that it may not be melted during repeated shots of a laser beam. The heat-resistant oxide film  220  generally has a high transmittance to visible light. However, when the heat-resistant oxide film  220  is formed as a single-layered film or a multi-layered film with an appropriate thickness, it can block a laser beam. The heat-resistant oxide film  220  may be formed by one of various methods such as sputtering, chemical vapor deposition (CVD), and plating. 
     The metal film  230  formed on the heat-resistant oxide film  220  may be made of chromium (Cr) or the like. The metal film  230  may be formed by one of various methods such as sputtering, chemical vapor deposition (CVD), and plating. Sputtering is mainly used. 
     Next, referring to  FIG. 2B , together with  FIG. 2A , the metal film  230  is patterned in a predetermined shape to form a metal film pattern  235 . The metal film  230  may be patterned using e-beam. 
     Next, referring to  FIG. 2C , together with  FIG. 2B , the heat-resistant oxide film  220  is patterned using the metal film pattern  235  as an etching mask to form a heat-resistant oxide film pattern  225 . The heat-resistant oxide film  220  may be patterned by dry etching or wet etching. It is preferable to use anisotropic dry etching for accurate pattern formation. As described above, since the heat-resistant oxide film pattern  225  generally has a high transmittance to visible light, it is difficult to visually detect a defect of the heat-resistant oxide film pattern  225 . However, considering that the metal film pattern  235  is formed in the same shape as the heat-resistant oxide film pattern  225 , the defect of the heat-resistant oxide film pattern  225  can be detected by inspecting a defect of the metal film pattern  235 . 
     Next, referring to  FIG. 2D , together with  FIG. 2C , the metal film pattern  235  is removed to complete a mask  200  including a shielding area B and a transmission area T which are defined according to the presence or absence of the heat-resistant oxide film pattern  225 . When amorphous silicon is crystallized using a laser beam with very high energy density, the laser beam is partially absorbed in the heat-resistant oxide film pattern  225 . Since the heat-resistant oxide film pattern  225  is made of a material having a melting point of about 2,000° C or more, pattern deformation by the laser beam can be prevented, thereby improving the durability of the mask  200 . Furthermore, since the defect detection of the heat-resistant oxide film pattern  225  can be performed using the metal film pattern  235 , a desired pattern shape can be accurately achieved. 
       FIG. 3  is a plan view illustrating a mask  200  according to an embodiment of the present invention. Referring to  FIG. 3 , the mask  200  includes a first stripe pattern  310  and a second stripe pattern  320  having reverse phase to the first stripe pattern  310 . First and second stripe patterns  310  and  320  each include a transmission area T and a shielding area B which are alternately arranged, and extend horizontally. The first stripe pattern  310  is alternately arranged with respect to the second stripe pattern  320 . 
     When the mask  200  is exposed to a laser beam, the laser beam can pass through the transmission area T, but cannot pass through the shielding area B since a heat-resistant oxide film pattern (see  225  of  FIG. 2D ) is formed in the shielding area B. In order to crystallize all areas of an amorphous silicon film using a laser beam, the width of the transmission area T may be greater than or equal to that of the shielding area B. The width of the shielding area B may be about 2 to 10 μm, but may vary according to the energy density of the laser beam, the state of the amorphous silicon film, etc. 
     The width of the transmission area T may be about twice or less the maximum growth length of silicon grains formed by a first shot of a laser beam. After the first shot, a melted portion of an amorphous silicon film is crystallized laterally from both sides of the melted portion to the middle portion to grow silicon grains. The silicon grains stop growing as soon as they collide with each other to form silicon grain boundaries. If the width of the transmission area T is greater than about twice the maximum growth length of silicon grains, fine silicon grains may be present in the vicinity of silicon grain boundaries. 
     The mask  200  of the embodiment of the present invention shown in  FIG. 3  can be applied to a two-shot SLS process. According to the two-shot SLS process using the mask  200 , first, a first shot of laser beam is applied to an amorphous silicon film formed on a substrate via the mask  200  to thereby crystallize an amorphous silicon area corresponding to the transmission area T of the mask  200 . Then, the substrate is moved horizontally by about half a width of the mask  200 , and a second shot of the laser beam is applied to the amorphous silicon film via the mask  200  to complete the crystallization of a desired area of the amorphous silicon film. 
     The above embodiment of the present invention has been illustrated in view of a mask with a stripe pattern, but the present invention is not limited thereto. That is, a mask according to the present invention can have various patterns such as a square pattern, a rhombohedral pattern, or a wedge-shaped pattern. Furthermore, a mask according to the present invention can also be applied to an n-shot (n≧3) SLS process, in addition to a two-shot SLS process. 
       FIG. 4  is a sectional view illustrating a mask  400  for SLS according to another embodiment of the present invention. For convenience of illustration, components having the same functions as those shown in  FIGS. 2A through 3  are represented by the same reference numerals, and thus, a detailed description thereof will be omitted. The mask  400  of the embodiment of the present invention shown in  FIG. 4  has substantially the same structure as the mask  200  of the previous embodiment shown in  FIG. 2D  except those given below. That is, referring to  FIG. 4 , the mask  400  includes a transparent substrate  210 , a heat-resistant oxide film pattern  225  formed on the transparent substrate  210 , and a metal film pattern  425  for a photomask, interposed between the transparent substrate  210  and the heat-resistant oxide film pattern  225 , having substantially the same pattern as the heat-resistant oxide film pattern  225 . 
     Referring to  FIG. 4 , together with  FIGS. 2B and 2C , when the heat-resistant oxide film  220  is patterned using the metal film pattern  235  as an etching mask, a metal film (not shown) for a photomask disposed below the heat-resistant oxide film  220  is patterned together with the heat-resistant oxide film  220  to form the metal film pattern  425  for the photomask. Here, when the metal film for the photomask is made of a dry-etchable material, the heat-resistant oxide film pattern  225  and the metal film pattern  425  for the photomask can be formed in substantially the same pattern by simultaneous dry-etching of the heat-resistant oxide film  220  and the metal film for the photomask. For example, the metal film pattern  425  for the photomask may be made of Mo, Ti, or its alloy. 
     As described above, pattern deformation can be prevented even when laser beam irradiation is repeatedly performed, and a pattern defect can be visually detected, thereby ensuring accurate formation of a desired pattern shape. While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. Therefore, it is to be understood that the above-described embodiments have been provided only in a descriptive sense and will not be construed as placing any limitation on the scope of the invention.