Abstract:
The present invention relates to a method for fabricating a single crystal silicon thin film at the desired location to the desired size from an amorphous or polycrystalline thin film on a substrate using laser irradiation and laser beam movement along the substrate having the semiconductor thin films being irradiated. This method comprises the steps of: forming a semiconductor layer or a metal thin film on a transparent or semi-transparent substrate; forming a single crystal seed region on the substrate of the desired size by a crystallization method using laser irradiation; and converting the desired region of the semiconductor layer or metal thin film into a single crystal region, using the single crystal seed region.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for fabricating a semiconductor thin film, and more particularly to a method for fabricating a single crystal silicon thin film at the desired location to the desired size from an amorphous or polycrystalline thin film on a substrate using laser irradiation and laser beam movement along the substrate having the semiconductor thin films being irradiated. 
   2. Description of the Prior Art 
   Generally, a thin film transistor (hereinafter, referred to as TFT), a core switching device, which is used in LCD or OLED using organic EL material, is the most important semiconductor device for the performance of a flat panel display (hereinafter, referred to as FPD). 
   Mobility or leakage current, a measure of the TFT performance, greatly varies depending on the state or structure of a silicon (Si) thin film, which forms an active layer, a channel for charge carrier transport. In the case of a currently commercially available LCD, the active layer of most TFTs is made of an amorphous silicon (a-Si) thin film. 
   Since a-Si TFT using a-Si has a very low mobility of about 0.5 cm 2 /Vs, there is a limitation in making all switching devices required in LCD, using a-Si TFT. This is because a switching device for a peripheral circuit of LCD needs to be operated at a very high speed, but this high speed cannot be achieved with a-Si TFT. 
   Thus, switching parts for the peripheral circuit, such as a driver circuit, various controllers, and a digital-analogue-converter (DAC), etc., are formed of switching devices integrated on single crystal Si to cope with a high-speed requirement for the LCD driving. On the other hand, since a-Si TFT has a switching function while showing a characteristic of low-leakage current required for ensuring image quality, it is used as a pixel-switching device. 
   TFT using poly-crystal Si has a high mobility of several tens to several hundreds cm 2 /Vs and thus can exhibit high driving speed suitable for the periphery circuit. Thus, formation of poly-Si on a glass substrate allows a pixel region and also a peripheral circuit region to be realized. 
   Accordingly, in the case of poly-Si TFT, separate part mounting processes required for the formation of the peripheral circuit are not required and the peripheral circuit can be formed simultaneously with a pixel region, so that a reduction in part costs for the peripheral circuit can be expected. 
   In addition, because of high mobility, poly-Si allows TFT to be produced at a smaller size than existing a-Si and enables the peripheral circuit and the pixel region to be formed by an integration process. Thus, making linewidth fine becomes easier so that poly-Si TFT can realize high resolution as compared to a-Si TFT-LCD. 
   Furthermore, poly-Si TFT can show a high-current characteristic and thus is suitable for use in OLED, a current drive type display of the next generation FDP. Thus, studies to form poly-Si and fabricate TFT on a glass substrate are actively conducted at the most recent. 
   In order to form poly-Si on a glass substrate, a method is typically used, in which a-Si is deposited and then crystallized into poly-Si by thermal treatment. Since the glass substrate is deformed at a higher temperature than 600° C., excimer laser annealing (hereinafter, referred to as ELA) which crystallize only a-Si without causing damage to the substrate is typically used for crystallization. Generally, upon crystallization using ELA, a-Si is irradiated with a laser so that it is melted and re-solidified to produce poly-Si. Upon crystallization, grains are randomly formed such that they have various sizes ranging from several tens nm to a few μm depending on laser irradiation conditions. 
   Generally, as the size of grains is increased, the mobility of a TFT device is increased and the range of parts, which can be integrated upon the integration of the peripheral circuit, becomes wider. Thus, it is preferred to obtain ELA conditions where the greatest possible size of grains can be obtained, but the greater the size of grains, the worse the uniformity of grain distribution. This causes a degradation in uniformity of device characteristics, and as a result, causes a problem in view of reliability. 
   Accordingly, in applying ELA-crystallized poly-Si in LCD, there is applied poly-Si having grains of a suitable size in a range where uniformity is ensured. In this case, however, poly-Si TFT having high mobility can not be fabricated due to a limitation on grain size, and thus, there is necessarily a limitation in integrating the peripheral circuit. 
   U.S. Pat. Nos. 6,368,945 and 6,322,625 disclose a crystallization method where large sizes of grains are obtained while ensuring uniformity. The principle of this method which is called “sequential lateral solidification” (SLS) will now be described. 
     FIG. 1  is a schematic view of a laser system for carrying out a SLS process. As shown in  FIG. 1 , a substrate  110  deposited with an a-Si film  120  is placed on a stage  100  and first irradiated with a laser beam  130  through a mask  140 . In this case, there can be various patterns in the mask  140 . 
   A typical example of this mask is a slit-shaped mask  200  as shown in  FIG. 2   a . In the mask  200 , slits  210  having a width  220  and a length  230  are patterned. As laser beam is irradiated through the mask, the laser beam passed through the mask is irradiated in a beamlet form, and the irradiated laser beam has such energy that a-Si can be completely melted. 
     FIG. 3   a  is an enlarged view of one slit. In  FIG. 3   a  showing a condition before laser irradiation, the reference numeral  310  represents the width of a region exposed through a slit  330 , and a-Si  320  is present before exposure to laser irradiation.  FIG. 3   b  shows a condition immediately after laser was irradiated through a slit (condition where laser was irradiated for several tens nanoseconds and then cut-off). In this case, an exposed region was melted into a liquid silicon  360 , the boundary between the liquid silicon  360  and the a-Si silicon  340  is formed at the edge of the slit, and a fine poly-Si  350  is formed at the boundary. With the passage of time, the growth of grains is progressed toward the slit center, using the poly-Si  350  as a seed. In a growth process of grains, the growth of grains having slow growth rate is inhibited by grains having fast growth rate so that only some grains are continued to grow. The interface  380  between poly-Si and liquid Si-is continued to move, and ultimately, the poly-Si and the liquid Si meet with each other at the slit center as shown in  FIG. 3   d . In this case, the grown grain size  320  is approximately a half of the slit width. If the slit width is larger or the supercooling rate of the melted silicon after laser irradiation is fast, nucleation can occur within the liquid silicon  361  before the grains grown from both edges of the slit meet with each other at the boundary  381 . 
   Since this circumstance is undesired, it is important that the laser irradiation conditions, the substrate temperature and the form of a slit are optimized so that the nucleation does not occur. 
   After the first laser irradiation was completed, a location for laser beam irradiation is shifted by a length of  450  as shown in  FIG. 4   a , and then, second laser irradiation is conducted through a slit. After the second laser irradiation, silicon between slit boundaries  420 ,  421  is converted into a liquid silicon  460 , and a poly-Si region  440  which was formed after the first irradiation remains intact and is re-crystallized. In this case, at the boundary  421 , a fine poly-Si region is formed, and then the growth of grains is progressed using the formed poly-Si as a seed, but at the boundary  420 , the growth of grains is progressed using a region excluding the grains melted after the second irradiation among the grains formed after the first irradiation. as a seed. As a result, a structure as shown in  FIG. 4   c  is obtained. In other words, a boundary  470  which is formed by progressing the grain growth from both sides of the slit after the second irradiation is moved by a distance  491  which was shifted from the original location for the second irradiation. 
   By this procedure, the grain size becomes larger due to an increase in grain length in the scanning direction. Furthermore, upon the second irradiation, since the seed crystal and a new crystal undergo continuous growth in a state where crystal orientation is not changed, a boundary  480  disappears. 
     FIG. 5   a  shows a condition after the laser beam was moved in any length while the above procedure was repeated. The lower portion of  FIG. 5   a  shows a procedure where grains, which have been continued to grow in one direction, are present as an elongated form, and growing interfaces  520 ,  521  of grains, which have been grown from slit boundaries  510 ,  511  (           ,  FIG. 5A           ) after exposure to a slit at a front stage of growth, are grown into a liquid silicon  530 .
   Then, the scanning procedure is progressed to a point  551 , and an a-Si region  550  is crystallized, thereby giving a structure as shown in  FIG. 5   b . The scanned distance is approximately equal to the reference numeral  580 , and the length of the grown grains corresponds to the scanned distance  580 . Since the slit is patterned according to the mask, movement of the laser beam by a given scanned distance results in formation of poly-Si patterns as shown in  FIGS. 2   b  and  2   c . The respective poly-Si patterns have a grain structure as shown in  FIG. 5   b . As shown in  FIG. 5   b , at the initial region where scanning was initiated, there is a region having many fine grains, i.e., a region shown by the reference numeral  560 , since many grains competitively grow. Above the region  560 , there is a region having elongated grains, i.e., a region shown by the reference numeral  570 . The results of actual experiments indicate that the region  560  is smaller than 1 μm that is negligibly small in a patterned region of poly-Si (“Sequential lateral solidification of thin silicon films on SiO 2 ”, R.S. Sposil and James S. Im, Appl. Phys. Lett., 69(19), 2864(1996)). 
   The SLS method is advantageous in that various shapes are obtained according to the shape of a mask, and for some masks, a single crystal Si island region can be selectively formed at a portion where a channel region of TFT is formed (U.S. Pat. No. 6,322,625). 
   Thus, the use of this method allows a poly-Si structure, the uniformity of device characteristics, and improvement in device performance to be obtained. 
   However, in a Si thin film obtained by SLS, if a regularity in the formation of a single crystal Si array having rectangular or hexagonal arrangement, and a single crystal Si island, does not coincide with the design of pixel and peripheral circuit arrangements, the uniformity of device characteristics is adversely affected. 
   Thus, in the existing SLS method, there can be a limitation in view of a design since a mask design for crystallization must match with pixel and peripheral circuit designs. 
   Furthermore, a method of making a single crystal among the SLS methods is to form a single crystal Si island in the strict sense and thus grain boundaries are present in several places of a substrate. Thus, if the pixel or the peripheral circuit is configured around the grain boundaries, excellent device characteristics and uniformity can be expected. 
   As a result, an ultimate solution to ensure excellent device characteristics and uniformity in any design scheme will be a method wherein single crystal silicon is formed over the entire substrate, or single crystal Si is grown only on a peripheral circuit portion and the remaining pixel region is kept at the state of a-Si so that single crystal Si having an excellent switching property for the peripheral circuit is formed outside the pixel region having low leakage current, thereby fundamentally preventing formation of the grain boundaries capable of causing non-uniformity. 
   For this purpose, according to the present invention, a method in which single crystal Si is easily formed at the desired location to the desired size using a simpler mask is proposed. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a method for fabricating a single crystal silicon film, by which the crystallinity of low temperature poly-silicon silicon as an active layer of a thin film transistor, a pixel or peripheral circuit-driving device applied in LCD or OLED, can be increased, thereby forming single crystal silicon. 
   To achieve the above-mentioned object, the present invention provides a method for fabricating a single crystal silicon film, which comprises forming a single crystal region through a laser irradiation after forming a semiconductor layer or a metal thin film on a transparent or semi-transparent substrate, which comprises the steps of: forming a single crystal seed region on the substrate of the desired size by a crystallization method using laser irradiation; and converting the desired region of the semiconductor layer or metal thin film into a single crystal region, using the single crystal seed region. 
   Furthermore, the method of the present invention comprises the step of: irradiating the substrate of the desired size with a laser in a specific shape through a mask so that the laser-irradiated portion is firstly crystallized; conducting a first scanning process which comprises moving the laser by the desired distance so that a grain in the firstly crystallized portion is grown by the desired distance; completing the first scanning process after it was progressed by the desired distance, thereby forming a poly-crystal island region; conducting a second scanning process which comprises 90° turning the laser at the end of the first scanning process and scanning the seed grain formed in an elongated shape in the scanning direction during the first scanning process, so that the seed grain is grown to form a single crystal region; and irradiating the laser onto a portion of a single crystal seed region formed after progressing the second scanning process by the desired distance, thereby extending the single crystal region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic view showing the arrangement of a heat source (laser), a mask and a sample; 
       FIG. 2   a  shows a mask with a slit pattern; 
       FIG. 2   b  shows a crystalline Si pattern formed using the mask of  FIG. 2   a;    
       FIG. 2   c  is an enlarged schematic view showing a crystallized region of  FIG. 2   b;    
       FIG. 3   a  to  3   d  are schematic views showing that a laser-irradiated region is crystallized according to the laser irradiation through a slit; 
       FIGS. 4   a  to  4   c  are schematic views showing a process of additional grain growth where grains grow in the lateral direction after they were crystallized by the laser irradiation through a slit; 
       FIG. 5   a  is a view showing a process where grains grow in one direction by repetition of the melting and solidification caused by laser beam movement; 
       FIG. 5   b  shows a patterned island of poly-Si formed by the process shown in  FIG. 5   a;    
       FIG. 6  is a schematic view showing a state where one edge of the poly-Si island formed in  FIG. 5   b  was irradiated with a laser beam in order to conduct the second laser scanning of the poly-Si island in the perpendicular direction to a direction in which the poly-Si island was grown in one direction; 
       FIGS. 7   a  to  7   c  schematically show a process for forming a patterned Si island consisting of about one grain, in which a portion of poly-Si elongated by a laser beam is irradiated with a laser beam to melt the poly-Si, and the laser beam is moved in the scanning direction; 
       FIGS. 8   a  and  8   b  show a case where a single crystal is difficult to be obtained in a second scanning process, namely a case where seed grains are two and the two seed. grains show similar growth rates upon scanning in the x-direction, and thus, two regions having different crystal orientations are formed in a Si-island pattern; 
       FIGS. 9   a  to  9   g  schematically show a process of extending a single crystal Si region by conducting an additional SLS process using a single crystal Si seed region; 
       FIG. 10  schematically shows a process of carrying out an additional SLS process using the single crystal Si region formed in  FIGS. 9   a  to  9   g  as a seed, thereby extending the single crystal Si region in the reverse y-direction (shown by an arrow); 
       FIGS. 11   a  to  11   d  are sequential views showing a process of forming single crystal Si over the entire substrate by the processes of  FIGS. 6 ,  9  and  10 ; 
       FIGS. 12   a  and  12   b  show another embodiment of the present invention, in which several single crystal seed regions are formed at several places of a substrate at the same time so as to reduce process time, and single crystal silicon tiles are formed over the entire substrate using the single crystal seed regions; 
       FIGS. 13   a  to  13   e  show still another embodiment of the present invention, in which single crystal Si tiles of various patterns are formed over the entire substrate from monocrystalline seed regions in order to reduce process time; 
       FIG. 14  shows yet another embodiment of the present invention, in which a single crystal is formed on only panel area to eliminate the process time and cost required to convert undesired portions into single crystal regions; and 
       FIG. 15  shows further another embodiment of the present invention, in which a single crystal is formed only on a peripheral circuit-forming area of a panel portion, and portions other than a pixel region and a panel pixel area remain at a-Si without crystallization. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, a method for fabricating a single crystal silicon film according to the present invention will be described in detail with reference to the accompanying drawings. 
   In order to enhance crystallinity and also to ensure uniformity in forming a polycrystalline silicon (poly-Si) thin film on an amorphous substrate such as a glass or plastic insulator, a single crystal or a single-crystal tile whose location was precisely controlled is formed over the entire substrate according to the present invention. This allows problems of the prior art to be fundamentally solved. A typical method for this purpose is designed in the present invention, and the crystallization of a-Si will be described herein by way of example. 
   In a principle to realize the present invention, a poly-Si island pattern, which undergone the prior SLS process, is subjected to an additional laser scanning process in a perpendicular direction to a direction in which grains of the island pattern were grown. This results in formation of a single crystal Si seed region. Then, according to a SLS process using this seed region, a single crystal Si region or a single crystal Si tile is formed over the entire substrate or formed on a certain region of the substrate, such as a region where a panel is formed, or a peripheral circuit region, in order to shorten process time. 
     FIG. 6  shows the arrangement of a poly-Si island  610  having a width  660  and a length  670  completed in  FIG. 5   b  and a laser beam  630  having a length  640  and a width  650  for use in the second scanning. The laser beam  630  is moved in the x-direction perpendicular to the first scanning direction, starting from one edge of the poly-Si island pattern  610 . 
   It is preferred that the laser beam length  640  for the second scanning is approximately equal to the length  670  of the poly-Si island pattern  610 . After the first scanning but before the second scanning, a mask is turned by 90° or the sample is turned by 90°. 
     FIGS. 7   a ,  7   b  and  7   c  concretely show the second scanning process. For SLS, the laser beam width  650  is generally a few μm, and the width of an elongated grain in the poly-Si island pattern  610  is in the range of 1 to several μm.  FIG. 7   a  schematically shows a state where grains are present in one edge of the poly-Si island pattern. A grain  710  is formed in an elongated shape in the first scanning process, and a grain  711  is not grown in the initial growth process. 
   As described above, the grain  711  has a very small size of about 1 μm. Since the accuracy of aligning of a laser beam in a SLS system is about sub-μmw, the laser beam can be aligned such that it melts only a portion of the grain  710  as shown in  FIG. 7   b . Of course, parts or all of the grain  730  can be melted. And more important, the size (length and width) of the grain, which was grown in an elongated form over the entire poly-Si island pattern, is similar to the dimension level of the laser beam. 
   Thus, the region  730  is liquefied after the first irradiation in the second scanning process, and the liquefied region is re-solidified. At this time, the grain  710  and the grain  731 , which have very small sizes, serve as seeds, and the grain  710  forms most of the seeds. Thus, when the second scanning is progressed toward a poly-Si region  740 , the poly-Si region  740  in  FIG. 7   b  is converted into a single crystal Si region as shown in  FIG. 7   c  via melting and solidification processes. When the grain  731  is partially melted upon the first irradiation in the second scanning, it can be grown in the scanning process, but its size is very small and its growth rate is slower than the grain  710  as found in the scanning process, and thus, the size of a newly grown region  751  is negligible. The boundary  760  between a portion melted upon the first irradiation of the second scanning and the remaining portion will disappear. This is because the seed region  710  and the crystallized region  750  have the same orientation. 
   There can be caused the worst where the aligning of the laser beam is inaccurate or pluralities of elongated grains are present in the seed region. If small grains formed at a lower portion in the first scanning process (e.g., grain  731 ) are ignored, seed crystals will be two crystals having similar growth rates. 
     FIGS. 8   a  and  8   b  show the second scanning process under this condition. As two initial seed crystals  820 ,  821  are scanned with a laser beam  830  while removing a poly-Si region  810 , a grain boundary  800  remaining before scanning extends while the poly-Si island pattern consists of two grains. 
   In this case, the growth of a grain  870  is superior to the growth of a grain  871  so that a region of the grain  870  is larger than the grain  871 . Even in this case, such grains sufficiently act as a single crystal seed layer for converting the remaining substrate region or certain region into a single crystal region. This is because the size of the respective grain regions  850 ,  851 ,  880 ,  881  is about several tens μm sufficiently larger than the width of a laser beam passed through a slit, and thus, a grain suitably chosen from such grains may be applied as a seed crystal for the subsequent crystallization. 
     FIGS. 11   a  to  11   d  show a method of forming a single crystal Si region over the entire substrate in the above-mentioned manner. 
     FIG. 11   a  shows that a laser beam  1130  is irradiated starting from one edge of a poly-Si island  1120  formed upon the first scanning, and the second scanning is progressed to one edge of a substrate  1110  in the scanning direction shown in the figure. 
   As a laser beam  1160  reaches the opposite edge of the substrate  1110  after the second scanning, a single crystal Si seed region  1150  of a rectangular shape is formed and the remaining region remains at a-Si. 
   Following this, the laser beam is irradiated onto a portion of the single crystal Si seed region  1150  as shown in  FIG. 11C , thereby repeating melting and solidification. The laser irradiation is conducted in the order of  1170 ,  1171 , . . .  1180 , in a direction shown by the reference numeral  1190 . In this way, a single crystal Si region is formed over the entire substrate as shown in  FIG. 11   d.    
   The respective irradiation steps  1170 ,  1171  in  FIG. 11   c  may be conducted on several places at the same time such that process time can be shortened. For example, the irradiation steps  1170  and  1172  are conducted at the same time, and then, the irradiation steps  1171  and  1173  are conducted at the same time. 
   This method has the following differences from a “2 shot SLS process” (U.S. Pat. No. 6,368,945), which was recently proposed by James Im et al. In the present invention, a single crystal Si region is formed by the first scanning in the x-direction and the second scanning in the y-direction. Particularly in the case of the second scanning, since seed crystals are of small number (about one or two), the second scanning from crystals of small number allows single crystal seed regions to be formed. Once such seed regions are formed, crystallization is conducted in the same manner as the “2 shot SLS process” proposed by James Im et al. As a result, in the “2 shot SLS process”, a structure where poly-Si regions are arranged as shown in  FIG. 3  is obtained, but in the present invention, a single crystal Si region is formed. Namely, in the present invention, a process of forming the single crystal Si region at an initial stage is added so that the microstructure of a final thin film is greatly changed. In the present invention, additional processes (first and second scanning processes) are required to form the single crystal regions, but the present invention is advantageous in that the resulting structure provides very high uniformity and degree of freedom of design as compared to the “2 shot SLS process” The additional processes can be partially improved as in other embodiments of the present invention which will be described later. 
     FIGS. 9   a  to  9   g  and  FIG. 10  show the process shown in  FIG. 11   c  in more detail.  FIG. 9   a  shows a state where a laser beam  910  is shifted by the reference numeral  931  in the reverse y-direction and irradiated onto a single crystal Si region  900  formed upon the second scanning ( FIG. 11   b ). In this case, the irradiated region includes the original single crystal region  921  and the a-Si region  920 , and as shown in  FIG. 9   b , a region  940  corresponding to the sum of the two regions is melted. Immediately after laser irradiation, as shown in  FIG. 9   c , there are a region  951  grown from the original single crystal region and a region  950  grown from the a-Si region. 
   At the end of crystallization, as shown in  FIG. 9   d , a single crystal region  961  and a poly-Si region  960  are met with each other at a boundary  962 . Thus, the size of the single crystal region is increased by a distance  931  moved in the reverse y-direction, thereby extending the single crystal region. In this case, the laser irradiation is conducted in such a manner that there is no grain formed by nucleation in the melted Si region  952  of  FIG. 9   c  before the reference numerals  961  and  960  are formed. Since the newly formed single crystal region  961  was grown from the region  900 , an original single crystal region, a boundary  963  is not substantially observed. 
   Thereafter, the laser beam is moved in the x-direction and irradiated. In this case, as shown in  FIG. 9   e , a region  970 , which is irradiated with a laser beam  970 , overlaps with a portion of the previously formed region as shown by the reference numeral  972 , thereby removing a boundary effect. The irradiated region  971  is melted, and as shown in  FIG. 9   f , there are a region  981  growing from the single crystal region, and a region  980  growing from the a-Si region. At the end of growth, as shown in  FIG. 9   g , a single crystal region  991  and a poly-Si region  990  are met with each other. It is believed that the boundary  992  between the previously formed single crystal region  994  and the newly formed single crystal region  991 , and the boundary  993  between the original single crystal region  900  and these single crystal regions, are not substantially observed. 
   This is because such boundaries  992 ,  993  are not boundaries formed by meeting of grains having different orientations, and have the same crystal orientation. When scanning in the x-direction is continued in this way, the size of the single crystal region becomes larger, and at the end of the scanning in the x-direction, scanning in the reverse y-direction is progressed with laser irradiation. This state is shown in  FIG. 10 . After scanning in the x-direction was completed, a laser irradiation region is shifted in the reverse y-direction and laser irradiation is conducted in such a manner that a single crystal region  1020  acts as a seed, and at the same time, a poly-Si region  1030  is melted. 
   In this way, a region  1050  irradiated with a laser  1040  is melted and crystallized again. When this scanning is conducted as shown in  FIG. 11   c , a single crystal Si region is finally formed over the entire substrate as shown in  FIG. 11   d.    
   The above-mentioned crystallization method results in formation of the single crystal Si region over the entire substrate. In this method, process time is somewhat increased as compared to the existing SLS process, due to the additional processes required to form the initial single crystal seed region. To solve this shortcoming, the following embodiments of the present invention are described. 
     FIGS. 12   a  and  12   b  show a method of forming several single crystal Si seed regions by the first and second scanning processes at the same time. An a-Si film  1210  is deposited on a substrate  1200 , and irradiated with a laser through a mask where slit patterns were formed. Since several slit patterns are formed in the mask, poly-Si islands  1221 ,  1222 ,  1223 ,  1224  having a width  1230  and a length  1220  are formed upon the first laser scanning at the same time, and irradiated with a laser upon the second scanning in the x-direction at the same time. This laser irradiation is conducted in the reverse y-direction, using the single crystal Si regions formed by the second scanning as a seed, to produce single crystal Si tiles  1231 ,  1232 ,  1233 ,  1234  as shown in  FIG. 12   b . Although this method is disadvantageous in that the boundaries between the Si tiles  1231 ,  1232 ,  1233 ,  1234  occur due to a difference in orientation between such Si tiles, it allows process time to be reduced by about ¼ as compared to the above-mentioned method where the single crystal Si film is formed over the entire substrate. This embodiment is advantageously applied for products having a panel region smaller than single crystal Si tile regions. 
     FIGS. 13   a  to  13   e  show another embodiment of the present invention. An a-Si film is first deposited on a substrate  1300 , and then irradiated with a laser beam through a mask  1304  having slit patterns therein. In the mask  1304 , slits having a length  1306  and a width  1305  are regularly arranged at an interval  1360  from each other. A distance moved by the first scanning is shown by the reference numeral  1301 . 
   At the end of the first scanning, poly-Si islands  1311  are formed in the respective regions at regular intervals, and the reference numeral  1312  remains at the state of a-Si. Then, after a laser beam is aligned such that it is placed near a boundary  1310  perpendicularly to the first scanning direction upon the second scanning, the second scanning is conducted in the x-direction. In this case, the scanning distance is adjusted such that it is as long as the reference numeral  1302 . 
   Thus, the growth of single crystal Si is progressed toward a-Si regions  1312 , using a certain grain within the respective poly-Si islands as a seed, so that tiles consisting of poly-Si islands and single crystal Si regions  1322  are formed over the entire substrate as shown in  FIG. 13   b.    
   Thereafter, when additional scanning is conducted using the single crystal regions  1322  as a seed, the regions which were made of the poly-Si islands are converted into single crystal Si regions  1331  as shown in  FIG. 13   c  so that single crystal Si regions having a width  1333  and a length  1334  are formed in a tile shape over the entire substrate. In this case, the reference numeral  1330 , which was a boundary between the poly-Si island and the single crystal Si region in  FIG. 13   b , is not substantially observed. Depending on the scanning direction and the additional steps, various single-crystal Si tile shapes as shown in  FIGS. 13   d  and  13   e  can be obtained. 
   This embodiment is advantageous in that process time is remarkably shortened, since the length of the second scanning over the entire substrate as shown in  FIG. 11   b  is greatly reduced. This embodiment can be applied for a case where the entire substrate does not need to be made single-crystal according to the panel size, or a case where the reduction of cost is required for products where securing of the uniformity of a Si thin film is important without requiring a Si thin film of quality as high as single crystal Si. 
   In this embodiment, the size  1302 ,  1303  of the tiles need to be sufficiently small such that it does not affect uniformity. 
   In another embodiment, the scanning direction and number are suitably controlled as shown in  FIG. 14 , so that only a portion  1440  (pixel region) for forming a panel  1420 , and a peripheral circuit region  1430 , on a substrate  1400 , are made of a single crystal, and the remaining portion is in the form of poly-Si tiles  1410 . This allows process time to be reduced while ensuring uniformity. 
   This embodiment can be applied for products where a peripheral circuit has high switching speed, and the size of a panel is large. 
   In another embodiment, as shown in  FIG. 15 , an a-Si film  1520  is deposited on a substrate  1500 , only a peripheral circuit region  1530  of a panel  1510  is made of a single crystal, and a pixel region  1540  remains at a-Si. 
   This embodiment can be applied in a case where a-Si TFT having low leakage current is disposed at a pixel region, and peripheral TFT requiring high-speed switching is made of a single crystal. This embodiment allows process time to be remarkably reduced while ensuring a characteristic of low leakage current. 
   All the above-mentioned methods can be realized by suitable coping of laser irradiation direction and mask alignment, and regarded as solutions to cope with costs, product characteristics and various designs. 
   As described above, according to the present invention, the single crystal Si seed region is formed by the additional laser irradiation process using one or two seed grains, which were formed in an elongated shape within a poly-Si, island formed by the prior SLS process. Starting from this seed region, the single crystal Si region can be formed over the entire substrate or a portion or certain region of the substrate. 
   Thus, the present invention allows the uniformity problem to be fundamentally solved, so that it is possible to cope with various product designs. Furthermore, in fabricating a panel on the single crystal Si region, it can cope with switching speed in a peripheral circuit. 
   In the embodiments of the methods disclosed herein, the insulating film is a silicon nitride or oxide film selected from SiO x , SiO x H y , SiN x  and can be either bi-layer or multiple-layer or a film of nitride or oxide of a metal selected from Al, Cu, Ti and W. In the embodiments of the methods described herein, the semiconductor layer can be made of a material selected from either a-Si (amorphous silicon), a-Ge (amorphous germanium), a-Si x ,Ge y  (amorphous silicon germanium), poly-Si (poly-crystalline silicon), poly-Ge (poly-crystalline germanium) or poly-SixGey (poly-crystalline silicon-germanium). The metal film can be made of a metal selected from Al, Cu, Ti, W, Au or silver or compound of any of these metals and a semiconductor. 
   Accordingly, peripheral circuit parts are integrated to reduce the cost of module parts. Moreover, since single crystal silicon is applied unlike the existing process, sufficient switching speed allowing a drive circuit and also various interface parts to be integrated can be exhibited so that a system-on-panel can be ultimately formed. Thus, the present invention is applied for a wider range of products than existing LTPS TFT-LCD products. 
   Furthermore, according to the present invention, since a pixel region may also be formed of a-Si, products can be produced, which have a characteristic of low leakage current and where a peripheral circuit is integrated. Also, process costs can be greatly reduced. 
   Moreover, when a pixel region is made of single crystal silicon, it can exhibit high current so that it is suitable in OLED, a current drive type display, and low voltage driving becomes possible. 
   In addition, since the present invention can realize formation of single crystal Si on a large-sized glass substrate and also a small-sized substrate such as a Si wafer, it may also be applied in SOI (system-on-insulator) in a semiconductor memory integrated circuit process or in a three-dimensional integrated circuit process. 
   Finally, when laser crystallization is applied on wiring material, such as aluminum (Al) or copper (Cu), other than Si, to form a single crystal on the wiring material, bad wiring caused by electro-migration in an ultrahigh density integrated circuit can also be reduced. 
   Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.