Abstract:
A method of manufacturing an array substrate comprising; depositing an amorphous material on a transparent substrate; and changing said amorphous material to a polycrystalline material by irradiation of energy beams through a photo mask, said mask including a transparent region permitting said energy beams to pass through and a shutoff region surrounding said transparent region and interrupting said energy beams, said transparent region being defined by first and second lengthwise direction lines extending substantially in parallel to each other, first and second slanting direction lines which extend from opposed ends of said lengthwise direction lines after declining by angles larger than 90 degrees to join with each other; and third and fourth slanting direction lines which extend from the other opposed ends of said lengthwise direction lines after declining by angles larger than 90 degrees to join with each other, said transparent region having a length in the extending direction of said first and second lengthwise direction lines, which is longer than the length of said transparent region in the direction perpendicular to the extending direction of said first and second lengthwise direction lines, wherein changing said amorphous material to the polycrystalline material includes: moving said transparent substrate by a constant distance perpendicularly to the lengthwise direction of a flat pattern projected onto the surface of said amorphous material when energy beams passing through said transparent region are irradiated onto said amorphous material; and irradiating said energy beams onto said amorphous material every time when said transparent substrate is moved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2002-197697, filed on Jul. 5, 2002, and No. 2001-227314, filed on Jul. 27, 2001, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a method and a photo mask for manufacturing an array substrate.  
           [0004]    2. Related Background Art  
           [0005]    Liquid crystal display devices (hereinbelow also called LCD) have recently brought into wide use in personal computers, projection-type television sets, compact television sets, portable information terminals, and so on. The main stream of currently existing LCDs is active matrix LCD in which thin-film transistors (hereinbelow also called TFT), which are semiconductor elements, are provided for individual pixels.  
           [0006]    Active matrix LCD is made up by confining a liquid crystal between an array substrate having a display electrode and a filter substrate having a common electrode opposed to the display electrode. A TFT array substrate having TFTs in a matrix arrangement is frequently used as such array substrate. The TFT array substrate has a plurality of signal lines connected to TFT sources, and a plurality of scanning lines connected to TFT gates, which intersect in form of a grating. As the active layer of TFT, amorphous silicon or polysilicon is used.  
           [0007]    If polysilicon having a larger mobility than amorphous silicon is employed as the semiconductor material, part of the drive circuit for displaying images can be formed on the array substrate. As a result, some parts having been attached externally of a cell panel can be omitted. This resulted in lowering the manufacturing cost and a compact outer frame of the LCD display.  
           [0008]    If more drive circuits are built on the array substrate, its cost will be further lowered and the function will be enhanced.  
           [0009]    However, array substrates using currently available polysilicon as their semiconductor material still allow only a limited number of drive circuits to be built on. Therefore, circuits other than those built on the substrate are still located externally of the array substrate.  
           [0010]    To build more drive circuits on an array substrate, mobility of polysilicon is preferably high. Increasing the grain size of polysilicon would improve the mobility of the polysilicon.  
           [0011]    There is a method for enlarging the grain size of polysilicon by irradiating energy beams such as laser beams onto an amorphous silicon film, there by producing solid/liquid interface, and using a temperature profile along the interface to grow the crystal laterally in parallel to the plane of the array substrate. This method is called the lateral growth method.  
           [0012]    The lateral growth method irradiates energy beams such as laser beams on an initial film on the substrate via a photo mask, for example. In this case, crystal growth direction depends on the profile of the energy beams formed by the photo mask.  
           [0013]    [0013]FIG. 7A is a fragmentary, enlarged view of a conventional photo mask  100 . The photo mask  100  includes rectangular transparent regions  10  and shutoff regions  20 . The energy beams passing through the aperture  10  melt the amorphous silicon (or polysilicon). Once the irradiation of energy beams is completed, crystal grows from the interface between solid phase portions and liquid phase portions of silicon (hereinbelow also called solid-liquid interface) toward the inside.  
           [0014]    [0014]FIG. 7B is an enlarged plan view of crystal grains of polysilicon after irradiation of energy beams. In the lateral growth process, crystal grows from the solid-liquid interface. Thus the crystal growth direction is different between the short side and the long side of the transparent region  10 . Therefore, crystal grains  30  grown from short side and crystal grains  40  grown from the long side are different in lengthwise direction of crystal grains. Especially because the transparent region  10  was rectangular, lengthwise directions of the crystal grains  30  and the crystal grains  40  were intersecting approximately at a right angle.  
           [0015]    [0015]FIG. 8 is a plan view that schematically shows placement of TFTs  60 ,  70 ,  80 ,  90  formed by using conventional polysilicon as their active layers  50 . TFTs  60 ,  70 ,  80 ,  90  each include a gate electrode  110 , source electrode  120  and drain electrode  130 .  
           [0016]    When a voltage is applied to the gate electrode  110 , each TFT turns ON. That is, the active layer under the gate electrode  110  reverses, and forms a channel. The channel allows a current to flow between the source electrode  120  and the drain electrode  130 .  
           [0017]    While TFTs  60 ,  70 ,  80 ,  90  are OFF, the current leaking out between each source electrode  120  and the associated drain electrode  130  had better be small. On the other hand, when the TFTs  60 ,  70 ,  80 ,  90  are ON, the resistance value (referred to as ON resistance) between each source electrode  120  and the associated drain electrode  130  had better below. Further, TFTs  60 ,  70 ,  80 ,  90  preferably have constant properties.  
           [0018]    In general, when the flow direction of carriers of TFT substantially coincides with the lengthwise direction of polysilicon crystal grains, carriers exhibit a higher mobility. As the mobility of carriers is high, the ON resistance decreases. On the other hand, as the flow direction of carriers deviates from the lengthwise direction of crystal grains toward 90 degrees therefrom, the mobility of carriers becomes lower because carriers must pass through more grain boundaries and more of them will be scattered.  
           [0019]    In the conventional polysilicon active layer  50 , because the transparent region  10  is formed rectangle, lengthwise directions of crystal grains  30  and  40  intersect substantially at a right angle. Therefore, the conventional technique has the problem that carrier mobility is relatively low in TFT  90 , although it is relatively high in the other TFTs  60 ,  70  and  80 .  
           [0020]    The conventional technique also has the problem that TFTs  60 ,  70 ,  80 , and  90  cannot exhibit constant properties.  
           [0021]    Attempts to prevent those problems invite a design constraint that disables TFTs to be formed in regions where crystal grains  30  exist. Further, for forming TFTs in regions where crystal grains  30  do not exist, the manufacturing process will need an additional process for positional alignment.  
         SUMMARY OF THE INVENTION  
         [0022]    According to an embodiment of the invention, there is provided a method of manufacturing an array substrate comprising:  
           [0023]    depositing an amorphous material on a transparent substrate; and  
           [0024]    changing the amorphous material to a polycrystalline material by irradiation of energy beams through a photo mask, the mask including a transparent region permitting the energy beams to pass through and a shutoff region surrounding the transparent region and interrupting the energy beams, the transparent region being defined by first and second lengthwise direction lines extending substantially in parallel to each other, first and second slanting direction lines which extend from opposed ends of the lengthwise direction lines after declining by angles larger than 90 degrees to join with each other; and third and fourth slanting direction lines which extend from the other opposed ends of the lengthwise direction lines after declining by angles larger than 90 degrees to join with each other, the transparent region having a length in the extending direction of the first and second lengthwise direction lines, which is longer than the length of the transparent region in the direction perpendicular to the extending direction of the first and second lengthwise direction lines,  
           [0025]    wherein changing the amorphous material to the polycrystalline material includes: moving the transparent substrate by a constant distance perpendicularly to the lengthwise direction of a flat pattern projected onto the surface of the amorphous material when energy beams passing through the transparent region are irradiated onto the amorphous material; and irradiating the energy beams onto the amorphous material every time when the transparent substrate is moved.  
           [0026]    According to a further embodiment of the invention, there is provided a method of manufacturing an array substrate comprising:  
           [0027]    depositing an amorphous material on a transparent substrate; and  
           [0028]    changing the amorphous material to a polycrystalline material made of crystal grains by irradiation of energy beams through a photo mask permitting the energy beams to pass through, the photo mask including an elongated transparent region configured to permit the crystal grains to grow in directions not crossing at right angles when the energy beams are irradiated onto the amorphous material, the photo mask further including a shutoff region surrounding the transparent region to interrupt the energy beams, wherein changing the amorphous material to a polycrystalline material includes: moving the transparent substrate by a constant distance perpendicularly to the lengthwise direction of a flat pattern projected onto the surface of the amorphous material when energy beams pass through the transparent region and are irradiated onto the amorphous material; and irradiating the energy beams onto the amorphous material every time when the transparent substrate is moved.  
           [0029]    According to a still further embodiment of the invention, there is provided a photo mask permitting energy beams emitted from an energy source to pass through to change an amorphous material to a polycrystalline material, comprising:  
           [0030]    a transparent region permitting the energy beams to pass through and defined by first and second lengthwise direction lines extending substantially in parallel to each other, first and second slanting direction lines which extend from opposed ends of the lengthwise direction lines after declining by angles larger than 90 degrees to join with each other, and third and fourth slanting direction lines which extend from the other opposed ends of the lengthwise direction lines after declining by angles larger than 90 degrees to join with each other; and  
           [0031]    a shutoff region surrounding the transparent region to interrupt the energy beams,  
           [0032]    wherein the transparent region has a length in the extending direction of the first and second lengthwise direction lines, which is longer than the length of the transparent region in the direction perpendicular to the extending direction of the first and second lengthwise direction lines. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIG. 1A is an enlarged cross-sectional view of a liquid crystal display device using a TFT substrate, which is manufactured by a manufacturing method according to an embodiment of the invention;  
         [0034]    [0034]FIG. 1B is an enlarged cross-sectional view of TFT used in a TFT array substrate;  
         [0035]    [0035]FIG. 2A is a fragmentary, enlarged view of a photo mask through which energy beams irradiated to amorphous silicon can pass;  
         [0036]    [0036]FIG. 2B is an enlarged plan view that shows a mass of polysilicon crystal grains formed by using a photo mask;  
         [0037]    [0037]FIG. 3A is a fragmentary, enlarged plan view that shows a plurality of transparent regions of the photo mask shown in FIG. 2A;  
         [0038]    [0038]FIG. 3B is a fragmentary, enlarged plan view of polysilicon after irradiation of energy beams through the photo mask shown in FIG. 3A;  
         [0039]    [0039]FIG. 4 is a plan view that schematically shows a layout of TFTs formed by using polysilicon  230  as the active layer;  
         [0040]    [0040]FIG. 5A is a schematic diagram for obtaining the mobility of carriers that move along the channel portion of conventional TFT;  
         [0041]    [0041]FIG. 5B is a schematic diagram for obtaining the mobility of carriers that move along the channel portion of TFT according to the embodiment of the invention shown in FIG. 4;  
         [0042]    [0042]FIG. 6A is a diagram that shows an alternative form of each transparent region  310  of a photo mask  300 ;  
         [0043]    [0043]FIG. 6B is a diagram that shows a further alternative form each transparent region  310  of the photo mask  300 ;  
         [0044]    [0044]FIG. 6C is a diagram that shows a still further alternative form each transparent region  310  of the photo mask  300 ;  
         [0045]    [0045]FIG. 6D is a diagram that shows a yet further alternative form each transparent region  310  of the photo mask  300 ;  
         [0046]    [0046]FIG. 7A is a fragmentary enlarged view of a conventional photo mask;  
         [0047]    [0047]FIG. 7B is a enlarged plan view of crystal grains of polysilicon after irradiation of energy beams;  
         [0048]    [0048]FIG. 8 is a plan view that schematically shows a layout of TFTs formed by using conventional polysilicon  230  as the active layer;  
         [0049]    [0049]FIG. 9A is a flow chart of a manufacturing method of an array substrate according to an embodiment of the invention;  
         [0050]    [0050]FIG. 9B is a flow chart of the array substrate manufacturing method, which is continuous from FIG. 9A;  
         [0051]    [0051]FIG. 9C is a flow chart of the array substrate manufacturing method, which is continuous from FIG. 9B;  
         [0052]    [0052]FIG. 9D is a flow chart of the array substrate manufacturing method, which is continuous from FIG. 9C;  
         [0053]    [0053]FIG. 9E is a flow chart of the array substrate manufacturing method, which is continuous from FIG. 9D;  
         [0054]    [0054]FIG. 9F is a flow chart of the array substrate manufacturing method, which is continuous from FIG. 9E;  
         [0055]    [0055]FIG. 10 is a schematic diagram that shows irradiation of energy beams from an excimer laser generator  1000 ; and  
         [0056]    [0056]FIG. 11 is a schematic diagram that shows scanning of a glass substrate with a mask pattern. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0057]    Some embodiments of the invention will be explained below with reference to the drawings. These embodiments, however, should not be construed to limit the invention.  
         [0058]    [0058]FIG. 1A is an enlarged, cross-sectional view that roughly shows a TFT array substrate  130  manufactured by a method according to an embodiment of the invention, and a liquid crystal display device  100  using the TFT array substrate  130 .  
         [0059]    In the liquid crystal device  100 , a liquid crystal  110  is sealed between a color filter substrate  120  and the TFT array substrate  130 . The color filter substrate  120  has a common electrode  140 , and the TFT array substrate  130  has a display electrode  150 . Thus the common electrode  140  and the display electrode  150  apply an electric field to the liquid crystal  110 .  
         [0060]    The display electrode  150  is connected to the drain of TFT  200  located on the TFT array substrate  130 . A number of TFTs  200  are formed on the TFT array substrate  130  in a matrix arrangement.  
         [0061]    TFT  200  illustrated is of a positive stagger type, but TFT of an opposite stagger type may be used alternatively. The TFT array substrate  130  is illustrated as being used in a liquid crystal device, but it may be used as in other type displays such as EL displays.  
         [0062]    [0062]FIG. 1B an enlarged cross-sectional view of TFT  200  used in the TFT array substrate  130  manufactured by a method according to an embodiment of the invention. TFT  200  is formed on an insulating glass substrate  210 . The method for manufacturing the TFT array substrate  130  will be explained later with reference to FIG. 9A through FIG. 16.  
         [0063]    To fabricate TFT  200 , an insulating film  220  is deposited on the insulating glass substrate  210 , and polycrystalline silicon (also called polysilicon)  230  is formed on the insulating film  220 . The polysilicon  230  is formed in the following manner. First, amorphous silicon is formed on the insulating film  220 . Next, appropriate energy beams such as excimer laser beams, which are emitted from an energy source, are irradiated onto the amorphous silicon through a photo mask (see FIGS. 2A to FIG. 3B). Thus the amorphous silicon melts and produces solid/liquid interface. Then, by using a temperature gradient, the amorphous silicon is crystallized (lateral growth). Therefore, the amorphous is changed to polysilicon  230  by energy beams, and forms channel portions  255  of TFTs.  
         [0064]    Further, a gate insulating film  240  is deposited on the polysilicon  230  to form gate electrodes  250 . Thereafter, an impurity is injected using the gate electrode  250  as a mask. As a result, source regions  260  and drain regions  270  are formed in self-alignment at opposite sides of channel portions  255  in the polysilicon  230 . Next, contact holes are formed to reach the source regions  260  and the drain regions  270 . Further, source electrodes  280  to be connected to the source regions  260  and drain electrodes  290  to be connected to the drain regions  270  are formed.  
         [0065]    [0065]FIG. 2A is a fragmentary, enlarged view of the photo mask  300  through which the energy beams irradiated to the amorphous silicon can pass. The photo mask  300  includes transparent regions  310  permitting the energy beams to transmit, and shutoff regions  320  coated with Cr (chromium) to intercept the energy beams. FIG. 2A shows only one transparent region  310  and the shutoff region  320  around it.  
         [0066]    The transparent region  310  of the photo mask  300  according to the instant embodiment of the invention is defined by two lengthwise direction lines  330 ,  335  extending substantially in parallel; two slanting direction lines  340 ,  345  that extend from opposed ends  330   a ,  335   a  of the lengthwise direction lines  330 ,  335  after declining by angles θ 1  and θ 2  larger than 90 degrees to join with each other; and two slanting direction lines  350 ,  355  that extend from the other opposed ends  330   b ,  335   b  of the lengthwise direction lines  330 ,  335  after declining by angles θ 3  and θ 4  larger than 90 degrees to join with each other. Length L of the transparent region  320  in the direction parallel to the lengthwise direction lines  330 ,  335  is longer than the size (width) W of the transparent region  320  in the direction perpendicular to the lengthwise direction.  
         [0067]    In the instant embodiment, each of the slanting direction lines  340 ,  345 ,  350 ,  355  is shorter than the lengthwise direction line  330  or  335 .  
         [0068]    In this embodiment, all of the length wise direction lines  330 ,  335  and the slanting direction lines  340 ,  345 ,  350 ,  355  are straight. All of the angles θ 1 , θ 2 , θ 3  and θ 4  are equal obtuse angles larger than 90 degrees. The length wise direction lines  330 ,  340  are substantially equal in length. Similarly, the slanting direction lines  340 ,  345 ,  350 ,  355  are substantially equal in length.  
         [0069]    Therefore, in the instant embodiment, the transparent region  310  has the form of a hexagon having the lengthwise direction X, and symmetrical about the centerline  360  parallel to and between the lengthwise direction lines  330  and  335 .  
         [0070]    The length L in the lengthwise direction X and the width W of the transparent region  310  are limited by an optical system for processing energy beams from the energy source, an apparatus of the energy source, etc.  
         [0071]    The photo mask  300  according to the instant embodiment of the invention additionally includes the shutoff region  320  around the transparent region  310  to intercept energy beams.  
         [0072]    Therefore, energy beams emitted from the energy source are shaped by the photo mask  300  when transmitting through the photo mask  300 . The energy beams passing through the photo mask  300  changes an amorphous material to a polycrystalline material made up of crystal grains.  
         [0073]    Crystal grains usually grow by making use of a temperature gradient at the solid/liquid interface after irradiation of energy beams. So the crystal grains start growing from perimeters of transparent regions of the photo mask.  
         [0074]    The perimeters of the transparent region  310  form an elongated hexagon having the length wise direction. Therefore, the polysilicon crystallized by using the photo mask  300  according to the embodiment is made up of crystal grain masses  400  each being an aggregation of crystal grains as shown in FIG. 2B. As such, each crystal grain mass  400  is an aggregation of crystal grains, having a plan-view form determined by the shape of each transparent region of the photo mask  300 .  
         [0075]    [0075]FIG. 2B is an enlarged plan view that shows one of masses of polysilicon crystal grains formed by using the photo mask  300 . The crystal grain mass  400  has the same form as the transparent region  310 . That is, the crystal grain mass  400  in this embodiment is defined by two lengthwise direction lines  430 ,  435  extending substantially in parallel; two slanting direction lines  440 ,  445  shorter than the lengthwise direction lines  430 ,  435 , which extend from opposed ends  430   a ,  435   a  of the length wise direction lines  430 ,  435  after declining by angles θ 1  and ƒ 2  larger than 90 degrees to join with each other; and two slanting direction lines  450 ,  455  shorter than the lengthwise direction lines  430 ,  435 , which extend from the other opposed ends  430   b ,  435   b  of the lengthwise direction lines  430 ,  435  after declining by angles θ 3  and θ 4  larger than 90 degrees to join with each other.  
         [0076]    Length L of the transparent region  320  in the direction parallel to the lengthwise direction lines  430 ,  435  of the mass  400  is longer than the size (width) W of the transparent region  320  in the direction perpendicular to the lengthwise direction.  
         [0077]    In the instant embodiment, each of the slanting direction lines  440 ,  445 ,  450 ,  455  is shorter than the lengthwise direction line  430  or  435 .  
         [0078]    The crystal grain mass  400  shown in FIG. 2B includes a number of crystal grains  410  having one of lengthwise directions Y 0 , Y 1  and Y 2 . These crystal grains  410  have shapes having any of lengthwise directions Y 0 , Y 1  and Y 2 , which substantially coincide with growth directions of respective crystal grains  410 . Since the angles θ 1 , θ 2 , θ 3 , θ 4  are all obtuse angles larger than 90 degrees, the lengthwise directions Y 0 , Y 1  and Y 2  do not intersect at right angles with each other.  
         [0079]    Angle θ 5  made between the lengthwise directions Y 0 , Y 1  is θ 5 =180−θ 1 , angle θ 6  made between the lengthwise directions Y 0 , Y 2  is θ 6 =180−θ 2 , and angle θ 7  made between the lengthwise directions Y 2 , Y 0  is θ 6 =180−θ 3 .  
         [0080]    Therefore, as the angles θ 1 , θ 2 , θ 3 , θ 4  become closer to 180 degrees, the lengthwise directions Y 0 , Y 1 , Y 2  become closer to parallel lines. In this embodiment, however, as the angles θ 1 , θ 2 , θ 3 , θ 4  become closer to 180 degrees, the slanting direction lines  440 ,  445 ,  450 ,  455  of the crystal grain mass  400 , i.e. the slanting direction lines  340 ,  345 ,  350 ,  355  of the transparent region  310  of the photo mask  300 , must be made longer. However, as already explained, since the length L of the lengthwise direction X of the transparent region  310  is limited by the optical system for processing the energy beams, apparatus of the energy source, etc., these angles θ 1 , θ 2 , θ 3 , θ 4  are also limited. This limitation is determined by the length L of the lengthwise direction X, width W, energy beams used, and others.  
         [0081]    [0081]FIG. 3A is a fragmentary, enlarged plan view that shows a plurality of transparent regions  310  of the photo mask  300  shown in FIG. 2A.  
         [0082]    The transparent regions  310  shown in FIG. 3A are aligned side by side in the widthwise direction perpendicular to the lengthwise direction, and the transparent regions  310  in a row are shifted in the lengthwise direction of the transparent regions  310  from the transparent regions  310  in the next row. The photo mask  300 , however, may define very long transparent regions  310  aligned side-by-side only in the widthwise direction.  
         [0083]    [0083]FIG. 3B is a fragmentary, enlarged plan view of polysilicon  230  after irradiation of energy beams through the photo mask  300  shown in FIG. 3A.  
         [0084]    When energy beams are irradiated through the photo mask  300 , the glass substrate  210  (see FIG. 1B) is moved every shot of irradiation. As a result, the polysilicon  230  as shown in FIG. 3B is obtained.  
         [0085]    As shown in FIG. 3B, the polysilicon  230  is made up of a number of columns  402  of crystal grain masses  400 , each column  402  being made up of a number of crystal grain masses  400  substantially equal in elongated shape having the lengthwise direction X and aligned in the direction of their lengthwise directions X. Crystal grain masses  400 , which are adjacent each other in the direction of their lengthwise directions X, are shifted a half pitch of the width of the crystal grain mass  400 .  
         [0086]    The polysilicon  230  is used as an active layer forming channel portions of TFTs  200 .  
         [0087]    Among the crystal grain masses  400 , zigzag grain boundary lines  404  are formed by slanting direction lines of a number of crystal grain masses  400 . Therefore, the polysilicon  230  includes zigzag regions  411  in which the grain boundary lines  404  appear, and parallel regions  420  in which lengthwise direction lines of the crystal grain masses  400  appear and their lengthwise directions are substantially parallel. The deflection angle θ 8  of the grain boundary lines  404  depends on the angles θ 1 , θ 2 , θ 3  and θ 4 . That is, the angle θ 8  is one of θ 8 =2(180−θ 1 ), θ 8 =2(180−θ 2 ), θ 8 =2(180−θ 3 ) and θ 8 =2(180−θ 4 ).  
         [0088]    [0088]FIG. 4 is a plan view that schematically shows a layout of TFTs formed by using polysilicon  230  as the active layer to form TFTs  560 ,  570 ,  580 , and  590 .  
         [0089]    TFTs  560 ,  580  are located in zigzag regions  411  whereas TFTs  570 ,  590  are located in parallel regions  420 .  
         [0090]    Flow directions of carriers through channels in TFTs  570 ,  590  approximately coincide with lengthwise directions of crystal grains in the parallel regions  420 .  
         [0091]    Flow directions of carriers through channels in TFTs  560 ,  580  diagonally intersect with lengthwise directions of crystal grains in zigzag regions  411 .  
         [0092]    In general, in case that polysilicon is used as a semiconductor material of TFT, carriers are scattered at grain boundaries of crystal grains. Scattering of carriers undesirably lowers the mobility. Therefore, the number of grain boundaries through which carriers must pass when flowing between the source and the drain of TFT had better be minimum. For this purpose, the flow direction of carriers and the lengthwise direction of crystal grains of polysilicon are preferably parallel. This will ensure a higher mobility of carriers in TFT.  
         [0093]    Therefore, also in TFTs  570 ,  590 , a relatively high carrier mobility, which is substantially the same as that of TFTs  60 ,  80  shown in FIG. 8 can be obtained.  
         [0094]    Carrier mobility in TFTs  560 ,  580  is explained below with reference to FIGS. 5A and 5B.  
         [0095]    [0095]FIG. 5A is a schematic diagram for obtaining the mobility of carriers that move along the channel portion of conventional TFT  90  shown in FIG. 8.  
         [0096]    [0096]FIG. 5B is a schematic diagram for obtaining the mobility of carriers that move along the channel portion of TFTs  560 ,  580  according to the embodiment of the invention shown in FIG. 4.  
         [0097]    The channel length and the channel width of each TFT are shown by L and W, respectively. Grain boundaries of crystal grains of polysilicon are shown by broken lines. Let each crystal grain have the width p. G is the gate electrode, and S is polysilicon. The arrow mark Z indicates the flow direction of carriers in the channel.  
         [0098]    In FIG. 5A, grain boundaries are make 90 degrees relative to the carrier flow direction. In FIG. 5B, grain boundaries intersect with the carrier flow direction at an angle η. The angle q depends upon the angles θ 1 , θ 2 , θ 3  and θ 4  of FIG. 2B. That is, any one of equation among η=180−θ 1 , η=180−θ 2 , η=180−θ 3  and η=180−θ 4  is effective.  
         [0099]    In the channel portion of TFT  90 , the number of crystal grains each carrier has to pass through is L/p. On the other hand, in the channel portion of TFT  560  or  580 , the number of crystal grains each carrier has to pass through is sin(η)*L/p. Since 0°&lt;η&lt;90°, the number of crystal grains each carrier has to pass through is less in TFT  560  or  580  than in TFT  90 . Therefore, the number of grain boundaries each carrier has to cross is less in TFT  560  or  580  than in TFT  90 . As a result, TFT  560  or  580  exhibits higher carrier mobility than TFT  90 . The angle η may be determined appropriately such that desired carrier mobility is obtained for the design. Further, η may be diminished toward zero by elongating the slanting direction lines  340 ,  345 ,  350 ,  355  shown in FIG. 2A. Thereby, the carrier mobility of TFT  560  or  580  can be approximately equalized to the carrier mobility of TFT  570  or  590 .  
         [0100]    [0100]FIGS. 6A through 6D show other forms of transparent region  310  of the photo mask  300  shown in FIG. 2A.  
         [0101]    In FIG. 6A, each of the slanting direction lines  340 ,  345 ,  350 ,  355  of the transparent region  310  curves inward of the transparent region  310  toward the opposed direction of the lengthwise direction lines  330 , 335  in form of an elliptical arc.  
         [0102]    In FIG. 6B, each of the slanting direction lines  340 ,  345 ,  350 ,  355  of the transparent region  310  curves outward of the transparent region  310  oppositely from the opposed direction of the lengthwise direction lines  330 ,  335 . Thus the transparent region  310  has an elliptical form.  
         [0103]    In FIG. 6C, each of the slanting direction lines  340 ,  345  of the transparent region  310  curves inward of the transparent region  310  to form an elliptical arc, whereas each of the slanting direction lines  350 ,  355  curves outward of the transparent region  310 .  
         [0104]    In FIG. 6D, slanting direction lines  340 ,  345 ,  350 ,  355  of the transparent region  310  are replaced by slanting direction line  640  including a plurality of short sides  640   a  and  640   b , slanting direction line  645  including a plurality of short sides  645   a  and  645   b , slanting direction line  650  including a plurality of short sides  650   a  and  650   b , and slanting direction line  655  including a plurality of short sides  655   a  and  655   b.    
         [0105]    Thereby, the transparent region  310  may be decagonal instead of being hexagonal, or even a polygon other than hexagon or decagon by including or decreasing the short sides.  
         [0106]    When using the photo mask according to the embodiment having the transparent region shown in FIG. 6A, FIG. 6B, FIG. 6C or FIG. 6D, energy beams are repetitively irradiated to polysilicon in the part of the transparent region adjacent to the short sides or slanting direction lines when they are irradiated to such parts of other transparent regions. Therefore, it is prevented that the channel portion is not sufficiently irradiated with energy lines and undesirably remain amorphous.  
         [0107]    Each slanting direction line should be construed to involve any number of short lines. Therefore, each slanting direction line can be a single short side instead of a plurality of short sides.  
         [0108]    The TFT array substrate  130  can be manufactured by using any one of the photo masks  300  shown in FIG. 3A and FIGS. 6A through 6D. FIGS. 9A through 9F are flow charts of a manufacturing method of the array substrate according to the embodiment of the invention.  
         [0109]    As shown in FIG. 9A, an insulating film  220  for preventing diffusion of impurities is first formed on an insulating glass substrate by PE-CVD (plasma-enhanced chemical vapor deposition).  
         [0110]    As shown in FIG. 9B, amorphous silicon  229  to form an active layer is next deposited up to the thickness around 50 nm on the insulating film  220  by PE-CVD. The substrate is next annealed at 500° C. to deprive oxygen of the amorphous silicon  229 . It is also acceptable to change the amorphous silicon  229  to a low-concentrated impurity layer by ion implantation of low-concentrated boron (B) into the amorphous silicon  229 .  
         [0111]    As shown in FIG. 9C, energy beams emitted from an excimer laser generator  1000  having an energy source, such as excimer laser beams by ELA (excimer laser annealing), are next irradiated onto the amorphous silicon  229 . Intensity of the excimer laser should be enough to melt the amorphous silicon  229 , namely in the range from 400 mj/cm 2  to 600 mj/cm 2 . Thus the amorphous silicon  229  melts and crystallizes as already explained with reference to FIG. 2B. As a result, the amorphous silicon  229  changed to polycrystalline silicon  230 . Details of the process of irradiating energy beams to the amorphous silicon  229  will be explained later with reference to FIG. 10 and FIG. 11.  
         [0112]    [0112]FIG. 9D shows the state where the amorphous silicon  229  on the insulating film  220  has entirely crystallized to polysilicon  230 .  
         [0113]    After that, the polysilicon  230  is patterned by photo etching to form a resist pattern (not shown).  
         [0114]    As shown in FIG. 9E, the polysilicon  230  is selectively removed by CDE using the resist pattern as a mask.  
         [0115]    As shown in FIG. 9F, TFT  200  is formed on the remaining part of the polysilicon on the insulating film  220  as shown in FIG. 1B.  
         [0116]    As such, the TFT array substrate  130  is manufactured following the flow of FIGS. 9A through 9F.  
         [0117]    Next referring to FIG. 10 and FIGS.  11 , the process of irradiating energy beams to the amorphous silicon  229  shown in FIG. 9C is briefly explained.  
         [0118]    [0118]FIG. 10 is a schematic diagram that shows how the excimer laser generator  1000  irradiates energy beams onto the glass substrate  210  having the amorphous silicon  229 . Laser beams emitted from an excimer laser source  1010  travel through the illuminating optical system  1020 , photo mask  300  and projection lens  1030 , and reach the amorphous silicon  229  on the glass substrate  210 .  
         [0119]    The glass substrate  210  is fixed on a XYZ tilt stage  1040 , and can be moved in three-dimensional directions (XYZ directions) by driving the tilt stage  1040 . After every movement of the glass substrate  210  in the X direction by a certain distance (hereinbelow also referred to as stepping motion), the excimer laser generator  1000  irradiates laser beams to the amorphous silicon  229 . Laser beams passing through the photo mask  300  are converged by the projection lens  1030  and thereafter irradiated onto the amorphous silicon  229 .  
         [0120]    [0120]FIG. 11 is a schematic diagram that shows an aspect of scanning of the glass substrate with the mask pattern  300   p . X-Y axes shown in FIG. 11 are the same as the X-Y axes on the moving plane of the tilt stage  1040  shown in FIG. 10. The mask pattern  300   p  is the pattern projected onto the surface of the amorphous silicon  229  when laser beams are irradiated onto the amorphous silicon  229 .  
         [0121]    In stepping motion, in general, the glass substrate  210  put on the tilt stage  1040  is moved. However, for easier understanding, FIG. 11 shows the glass substrate  210  as being fixed and the mask pattern of the photo mask  300  as moving. Of course, it is also possible to actually move the photo mask  300  for scanning by the mask pattern.  
         [0122]    In the plane of the glass substrate  210 , the mask pattern  300   p  of the photo mask  300  scans in the X-axis direction. This scanning is a motion carried out by continuously repeating stepping motions, and after every stepping motion, laser beams are irradiated onto the amorphous silicon  229 . Once the mask pattern  300   p  scan the glass substrate  210  to its perimeter, it moves in the Y-axis direction and scans back in the X-axis direction.  
         [0123]    The manufacturing method of the array substrate according to the embodiment of the invention crystallizes the amorphous silicon  229  to the polysilicon  230  by irradiation of laser beams through the photo mask  300 . Therefore, the method can ensure the proper effect of the use of the photo mask  300 . That is, this method can manufacture an array substrate having a plurality of TFTs exhibiting high carrier mobility and constant performance without the need of limiting the design or using an additional step in the TFT manufacturing process.  
         [0124]    As described above, according to the method for manufacturing an array substrate and the photo mask therefor according to the embodiment of the invention, it is possible to form an active layer permitting a plurality of TFTs to be made while ensuring a high carrier mobility and a constant performance thereof, without the need of limiting the design or using an additional step in the TFT manufacturing process.