PATENT ABSTRACT
Objects are to reduce the number of steps in a process for separating a substrate and a semiconductor element, to provide a separation apparatus capable of reducing the number of steps, to suppress manufacturing cost by reducing the number of steps in a separation process, and to improve productivity in manufacturing semiconductor elements. A separation apparatus including a frame body, a porous body having a chamfered, rounded corner portion, a suction unit configured to create suction in the porous body and the frame body, and a jig which includes a unit adopted to press down part of an object to be separated and a unit adopted to lift another part of the object to be separated, and also a separation method and a method for manufacturing a semiconductor element by using the separation apparatus, are provided.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An embodiment of the invention disclosed herein relates to a separation apparatus, a separation method, and a method for manufacturing semiconductor elements. 
     2. Description of the Related Art 
     In recent years, attention has been drawn to a technique for forming semiconductor elements such as thin film transistors (TFTs) by using a semiconductor thin film formed over a substrate having an insulating surface. Thin film transistors are widely applied to electric devices such as an integrated circuit, an electro-optical device, a photoelectric conversion device, and a wireless communication device and are being urgently developed especially as switching elements for image display devices (see Patent Document 1, Patent Document 2, and Patent Document 3). 
     There is a variety of applications for such image display devices; among them, an application to portable devices has particularly attracted attention. Therefore, it has been attempted to form TFT elements over a flexible substrate, typically a flexible plastic film (see Patent Document 1, Patent Document 2, and Patent Document 3). 
     REFERENCES 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2003-174153 
         [Patent Document 2] Japanese Published Patent Application No. 2004-247405 
         [Patent Document 3] Japanese Published Patent Application No. 2007-67381 
       
    
     SUMMARY OF THE INVENTION 
     A problem of a process for separating a substrate and a semiconductor element from each other by a conventional technique is a large number of steps. 
     For example, in Patent Document 2, a separation layer including a metal film and an oxide film is stacked over a first substrate, and a semiconductor element layer is formed over the separation layer. A second substrate is attached to the semiconductor element layer, and a third substrate is attached to the first substrate. 
     Then, the metal film and the oxide film are physically torn apart from each other; thus, the metal film and the oxide film are separated from each other. Accordingly, the semiconductor element layer on the second substrate side and the first substrate and the metal film on the third substrate side are separated from each other. 
     Then, the semiconductor element layer is attached to an element substrate. After that, the second substrate is separated from the semiconductor element layer. In the above manner, the semiconductor element layer formed over the element substrate is obtained. 
     As described above, a conventional separation process includes steps of attaching a support substrate to and separating the support substrate from each of the first substrate and the semiconductor element layer. For this reason, the total number of steps is large. 
     In view of the above problem, it is an object of one embodiment of the disclosed invention to reduce the number of steps in a process for separating a substrate and a semiconductor element from each other, as compared to a conventional process. 
     It is an object of one embodiment of the disclosed invention to provide a separation apparatus capable of reducing the number of steps as compared to a conventional apparatus. 
     It is an object of one embodiment of the disclosed invention to reduce manufacturing cost by reducing the number of steps in a separation process. 
     It is an object of one embodiment of the disclosed invention to improve productivity in manufacturing semiconductor elements. 
     In one disclosed embodiment, in a separation process, a separation apparatus including a stage provided with a porous body is used, and a semiconductor element layer or a substrate over which a separation layer and a semiconductor element layer are formed first (hereinafter referred to as a “holding substrate”) is held by suction. Thus, the number of substrates to be attached to the semiconductor element layer and the holding substrate (hereinafter referred to as “support substrates”) is reduced. A reduction in the number of support substrates realizes a reduction in the number of steps of attaching and separating the support substrates. Accordingly, the number of manufacturing steps can be reduced as compared to a conventional technique. By reducing the number of manufacturing steps, manufacturing cost can also be reduced. 
     In one disclosed embodiment, one corner portion of the porous body, or corner portions of the porous body and the stage, is chamfered so as to be rounded. In the separation process, a corner portion of the holding substrate is removed and a portion of the semiconductor element layer is exposed. A cut is made in an end portion of the holding substrate, and the end portion of the holding substrate is lifted. The separation layer and a layer provided above and in contact with the separation layer have low adhesion. In addition, the separation layer and a layer provided below and in contact with the separation layer also have low adhesion. Therefore, by lifting the end portion of the holding substrate having the cut, only the end portion of the holding substrate can be removed while the semiconductor element layer is left remaining. 
     With a jig which can be moved up and down, the exposed region of the semiconductor element layer is pressed down to the stage side. By pressing down the exposed region of the semiconductor element layer, the semiconductor element layer is separated due to low adhesion to the separation layer and the holding substrate provided thereover. 
     By applying a downward force to the exposed region of the semiconductor element layer, only the semiconductor element layer can be easily separated with a weak force. 
     By being further pressed down with the jig, the semiconductor element layer is curved along the portion of the stage with a curvature radius. 
     Accordingly, a space where the semiconductor element layer is separated from the separation layer is provided between the semiconductor element layer and the holding substrate. 
     By using the jig, the holding substrate is lifted up away from the space where the semiconductor element layer is separated from the separation layer. At that time, when the holding substrate is lifted so as to be curved, a region to be separated per unit time has a linear shape or a band-like shape. On the other hand, when the holding substrate is lifted vertically, a region to be separated per unit time has a large area. Thus, by lifting the holding substrate so as to be curved, the semiconductor element layer can be separated with a weaker force than in the case of lifting the holding substrate vertically. 
     As described above, when the semiconductor element layer and the holding substrate are separated by using the stage according to one embodiment of the disclosed invention, the semiconductor element layer and the holding substrate can be separated easily. Because the semiconductor element layer and the holding substrate can be separated easily, productivity in manufacturing semiconductor elements can be improved. 
     One embodiment of the disclosed invention relates to a separation apparatus. A feature of the separation apparatus is to include a frame body, a porous body provided over the frame body and having a chamfered, rounded corner portion, and a suction unit configured to create suction in the porous body and the frame body. A feature of the separation apparatus is to include a jig which includes a unit adopted to press down part of an object to be separated and a unit adopted to lift another part of the object to be separated. 
     One embodiment of the disclosed invention relates to a separation method. A feature of the separation method is as follows. A separation layer and a semiconductor element layer are stacked over a holding substrate. A portion of the semiconductor element layer is exposed by removing a portion of the holding substrate and a portion of the separation layer. A stack of the semiconductor element layer, the separation layer, and the holding substrate is held by suction over a porous body having a chamfered, rounded corner portion. By pressing down the exposed portion of the semiconductor element layer, the exposed portion of the semiconductor element layer is curved along the chamfered, rounded corner portion of the porous body. By curving the portion of the semiconductor element layer, a space is provided between the semiconductor element layer and the holding substrate. By lifting the holding substrate, separation of the semiconductor element layer and the holding substrate proceeds from the space through the separation layer. 
     One embodiment of the disclosed invention relates to a method for manufacturing a semiconductor element. A feature of the method for manufacturing a semiconductor element is as follows. A separation layer is formed over a holding substrate. A semiconductor element layer including a thin film transistor is formed over the separation layer. A portion of the semiconductor element layer is exposed by removing a portion of the holding substrate and a portion of the separation layer. A stack of the semiconductor element layer, the separation layer, and the holding substrate is held by suction over a porous body having a chamfered, rounded corner portion. By pressing down the exposed portion of the semiconductor element layer, the exposed portion of the semiconductor element layer is curved along the chamfered, rounded corner portion of the porous body. By curving the portion of the semiconductor element layer, a space is provided between the semiconductor element layer and the holding substrate. By lifting the holding substrate, separation of the semiconductor element layer and the holding substrate proceeds from the space through the separation layer. 
     One embodiment of the disclosed invention relates to a separation method. A feature of the separation method is as follows. A separation layer and a semiconductor element layer are stacked over a holding substrate. A portion of the semiconductor element layer is exposed by removing a portion of the holding substrate and a portion of the separation layer. A stack of the semiconductor element layer, the separation layer, and the holding substrate is held by suction over a stage having a chamfered, rounded corner portion. By pressing down the exposed portion of the semiconductor element layer, the exposed portion of the semiconductor element layer is curved along the chamfered, rounded corner portion of the stage. By curving the portion of the semiconductor element layer, a space is provided between the semiconductor element layer and the holding substrate. By lifting the holding substrate, separation of the semiconductor element layer and the holding substrate proceeds from the space through the separation layer. 
     One embodiment of the disclosed invention relates to a method for manufacturing a semiconductor element. A feature of the method for manufacturing a semiconductor element is as follows. A separation layer is formed over a holding substrate. A semiconductor element layer including a thin film transistor is formed over the separation layer. A portion of the semiconductor element layer is exposed by removing a portion of the holding substrate and a portion of the separation layer. A stack of the semiconductor element layer, the separation layer, and the holding substrate is held by suction over a stage having a chamfered, rounded corner portion. By pressing down the exposed portion of the semiconductor element layer, the exposed portion of the semiconductor element layer is curved along the chamfered, rounded corner portion of the stage. By curving the portion of the semiconductor element layer, a space is provided between the semiconductor element layer and the holding substrate. By lifting the holding substrate, separation of the semiconductor element layer and the holding substrate proceeds from the space through the separation layer. 
     A feature of one embodiment of the disclosed invention is that the porous body includes one of a ceramic having a porous structure, a metal having a porous structure, and a resin having a porous structure. 
     A feature of one embodiment of the disclosed invention is that a strength-retaining layer is formed in contact with the semiconductor element layer. 
     A feature of one embodiment of the disclosed invention is that the strength-retaining layer is a UV detachable film or a water-soluble resin. 
     According to one embodiment of the disclosed invention, the number of steps in the process for separating a substrate and a semiconductor element from each other can be reduced as compared to a conventional process. 
     According to one embodiment of the disclosed invention, a separation apparatus capable of reducing the number of steps as compared to a conventional apparatus can be provided. 
     According to one embodiment of the disclosed invention, by reducing the number of steps in a separation process, manufacturing cost can be reduced. 
     According to one embodiment of the disclosed invention, productivity in manufacturing semiconductor elements can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a separation apparatus. 
         FIGS. 2A to 2C  are cross-sectional views illustrating a method for manufacturing a semiconductor element. 
         FIGS. 3A and 3B  are cross-sectional views illustrating a method for manufacturing a semiconductor element. 
         FIGS. 4A and 4B  are cross-sectional views illustrating a method for manufacturing a semiconductor element. 
         FIG. 5  is a cross-sectional view of a separation apparatus. 
         FIGS. 6A to 6E  are cross-sectional views illustrating a method for manufacturing a semiconductor element. 
         FIGS. 7A and 7B  are cross-sectional views illustrating a method for manufacturing a semiconductor element. 
         FIG. 8  is a cross-sectional view of a separation apparatus. 
         FIGS. 9A and 9B  are cross-sectional views illustrating a method for manufacturing a semiconductor element. 
         FIG. 10  is a cross-sectional view illustrating a method for manufacturing a semiconductor element. 
         FIGS. 11A and 11B  are diagrams illustrating a method for obtaining a curvature radius R. 
         FIGS. 12A and 12B  are photographs of a damaged semiconductor element layer. 
         FIG. 13  is a cross-sectional view of a separation apparatus. 
         FIGS. 14A and 14B  are cross-sectional views illustrating a method for manufacturing a semiconductor element. 
         FIGS. 15A and 15B  are top views of a separation apparatus. 
         FIGS. 16A and 16B  are a cross-sectional view and a top view of a separation apparatus. 
         FIGS. 17A and 17B  are cross-sectional views illustrating a method for manufacturing a semiconductor element. 
         FIG. 18  is a cross-sectional view illustrating a method for manufacturing a semiconductor element. 
         FIG. 19  is a cross-sectional view of a separation apparatus. 
         FIG. 20  is a cross-sectional view of a separation apparatus. 
         FIG. 21  is a cross-sectional view of a separation apparatus. 
         FIG. 22  is a cross-sectional view of a separation apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention disclosed in this specification will be hereinafter described with reference to the accompanying drawings. Note that the invention disclosed in this specification can be carried out in a variety of different modes, and it is easily understood by those skilled in the art that the modes and details of the invention disclosed in this specification can be changed in various ways without departing from the spirit and scope thereof. Therefore, the invention should not be construed as being limited to the description in the embodiments. Note that in the accompanying drawings, the same portions or portions having similar functions are denoted by the same reference numerals, and repetitive description thereof is omitted. 
     &lt;Separation Apparatus&gt; 
       FIG. 1  is a diagram illustrating a structure of a separation apparatus  100  of this embodiment, and  FIG. 5  is a partial enlarged view of the separation apparatus  100 . The separation apparatus  100  includes a stage  101  having a chamfered, rounded corner portion  106 , a jig  107  having a projecting portion  110 , and a suction unit  108 . The stage  101  is provided with a base  104  which includes a porous body  102  having a chamfered, rounded corner portion  109  and a frame body  103  surrounding the porous body  102 . The porous body  102  is provided over the frame body  103  and holds an object by suction. The porous body  102  is formed using a porous material such as a ceramic having a porous structure, a metal having a porous structure, or a resin having a porous structure and has air permeability. The porous body  102  has a plurality of air holes each having a diameter of more than or equal to 0.1 μm and less than 10 μm. 
     The frame body  103  is connected to a connecting pipe  105 , and the connecting pipe  105  is connected to the suction unit  108 . The suction unit  108  creates suction in the base  104  (the porous body  102  and the frame body  103 ) through the connecting pipe  105 . Thus, an object over the porous body  102  is held by suction. 
     In a separation process to be described below, a semiconductor element layer  203  is curved with the jig  107  along the chamfered, rounded corner portion  106  of the stage  101  and the chamfered, rounded corner portion  109  of the porous body  102 . Note that the entire corner portion  106  does not necessarily need to have a uniform curvature radius. The corner portion  106  may have a curvature radius sufficient to provide a space  212  between the semiconductor element layer  203  and a holding substrate  201  by separating the semiconductor element layer  203  from a separation layer  202  and to insert the projecting portion  110  of the jig  107  in the space  212 , in the separation process described below. It is needless to say that the entire corner portion  106  may have a uniform and continuous curvature radius. 
     Therefore, the curvature radius R of the corner portion  106  of the stage  101  and the corner portion  109  of the porous body  102  is set such that the semiconductor element layer  203  can be prevented from being damaged. The value of the curvature radius R will be described later. 
     In addition, in the separation process to be described below, the holding substrate  201  is lifted by lifting the jig  107  while the projecting portion  110  of the jig  107  is in contact with the holding substrate  201 . Accordingly, the holding substrate  201  and the semiconductor element layer  203  are separated from each other through the separation layer  202 . Note that a driving apparatus for driving the jig  107  may be provided to automatically drive the jig  107 . 
     Note that in  FIG. 1  and  FIG. 5 , the chamfered, rounded corner portion  106  is at an end portion of the stage  101 . However, the end portion of the stage  101  does not necessarily need to be chamfered so as to be rounded.  FIG. 19  illustrates an example in which an end portion of the stage  101  has the chamfered, rounded corner portion  106  and a flat portion  125 . 
     Even in the case where the end portion of the stage  101  has the flat portion  125 , there is no problem with the separation process because the chamfered, rounded corner portion  106  is provided. Due to the presence of the chamfered, rounded corner portion  106 , the space  212  is provided between the semiconductor element layer  203  and the holding substrate  201  in the separation process to be described below. It is preferable to provide the chamfered, rounded corner portion  106  because the jig  107  can be inserted in the space  212 . 
     By using the separation apparatus of this embodiment, the number of steps in a separation process can be reduced as compared to a conventional apparatus. In addition, because the number of steps in a separation process can be reduced, manufacturing cost can be reduced. Furthermore, productivity in manufacturing semiconductor elements can be improved. 
     &lt;Structure of Stage&gt; 
     A specific value of a curvature radius R will be described with reference to  FIGS. 11A and 11B  and  FIGS. 12A and 12B . 
     The semiconductor element layer  203  is curved along the chamfered, rounded corner portions of the stage  101  and the porous body  102  in the separation process. Damage to the semiconductor element layer  203  at that time may cause a decrease in the rate at which semiconductor elements can be produced. 
     Thus, an experiment was conducted to determine the value of the curvature radius R of the corner portions. 
       FIG. 11B  illustrates a structure of a substrate  405 . The substrate  405  includes a UV detachable film  401 , a water-soluble resin  402 , a tungsten film  403 , and a silicon oxide film  404 . The UV detachable film  401  and the water-soluble resin  402  correspond to a strength-retaining layer  204  mentioned above. The tungsten film  403  and the silicon oxide film  404  correspond to the separation layer  202 . 
     As the UV detachable film  401 , UV detachable tape “UDT-1025MC” (manufactured by Toyo Adtec Co., Ltd.) (300 μm thick) including polyethylene terephthalate was used. As the water-soluble resin  402 , an acrylic-based resin “Aronix (registered trademark) VL-WSHL10” (manufactured by Toagosei Co., Ltd.) (30 μm thick) was used. The tungsten film  403  was formed by a sputtering method at a thickness of 50 nm, and the silicon oxide film  404  was formed by a sputtering method at a thickness of 200 nm. 
     The substrate  405  was attached to a board  406  having a width AB with a length a, and while being attached to the board  406 , the substrate  405  was curved until a crack (damage) was caused in the substrate  405 . The length b, which is a distance CD from a surface C of the board  406  to a highest point D of the curved substrate  405 , was measured. Assuming that the curved substrate  405  was an arc with center O, the curvature radius R which is the length OB of one side of a right triangle OBC was calculated. 
     The width AB of the board  406  was 300 mm, and the distance b from the surface C of the board  406  to the highest point D of the substrate  405  was 45 mm. As a result, the curvature radius R which was the length OB of one side of the right triangle OBC was 27.25 cm. 
     From the above result, the curvature radius R of the stage  101  and the porous body  102  is preferably 27 cm or more, more preferably 27.5 cm or more. The curvature radius R of the stage  101  and the porous body  102  is further preferably 28 cm or more because there is almost no possibility that a crack would be caused. If the curvature radius R is small, when the strength-retaining layer  204  is curved, the semiconductor element layer  203  provided in contact with the strength-retaining layer  204  may be damaged. 
       FIGS. 12A and 12B  are photographs each showing a state in which the strength-retaining layer  204  and the semiconductor element layer  203  are damaged when the curvature radius R is 27.25 cm. 
     As shown in  FIGS. 12A and 12B , it is found that an inappropriate value of the curvature radius R causes a crack in the semiconductor element layer  203 . 
     &lt;Separation Method and Method for Manufacturing Semiconductor Element Layer&gt; 
     A separation method and a method for manufacturing a semiconductor element layer of this embodiment will be described below. 
     First, a holding substrate  201  is prepared. A separation layer  202  and a semiconductor element layer  203  are formed over the holding substrate  201  (see  FIG. 2A ). 
     As the holding substrate  201 , a quartz substrate, a semiconductor substrate, a glass substrate, a metal substrate, or the like may be employed. 
     As the separation layer  202 , a single metal layer or a stack of the metal layer and an oxide film thereof is formed by a plasma CVD method, a sputtering method, or the like. The metal layer includes at least one metal element selected from tungsten (W), molybdenum (Mo), titanium (Ti), tantalum (Ta), niobium (Nb), nickel (Ni), cobalt (Co), zirconium (Zr), zinc (Zn), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), and iridium (Ir). In this embodiment, a tungsten film is formed as the separation layer  202 . 
     In a separation process to be described below, separation is carried out utilizing low adhesion of the separation layer  202  and a layer provided below and in contact with the separation layer  202  (in this embodiment, the holding substrate  201 ) and low adhesion of the separation layer  202  and a layer provided above and in contact with the separation layer  202  (in this embodiment, the semiconductor element layer  203 , more specifically, an insulating film  112  to be described below). 
     In some cases, separation may occur from inside the separation layer  202 ; thus, the separation process can be carried out utilizing separation from inside the separation layer  202 . In the case where a stack of a metal layer and an oxide film thereof is used as the separation layer  202 , separation is carried out utilizing low adhesion between the metal layer and the oxide film thereof. Alternatively, the separation process can be carried out utilizing the fragility of the metal oxide film and the occurrence of separation from inside the metal oxide film. 
     Even in the case where a single metal layer is used as the separation layer  202 , when an insulating film containing oxygen is formed on the metal layer, a metal oxide film is formed at the interface between the metal layer and the insulating film. Such a metal oxide film is formed by movement of oxygen from the insulating film containing oxygen to the metal layer by a heating step in the process for forming a thin film transistor  113  to be described below. When a metal oxide film is formed at the interface between the metal layer and the insulating film as described above, separation occurs at the interface between the metal layer and the metal oxide film or from inside the metal oxide film, and this can be utilized to carry out the separation process. 
     In this embodiment, a layer including a thin film transistor (TFT) is formed as the semiconductor element layer  203 . 
     An example of a structure of the semiconductor element layer  203  is illustrated in  FIG. 6A . The semiconductor element layer  203  illustrated in  FIG. 6A  includes the insulating film  112 , the thin film transistor  113  formed over the insulating film  112 , an insulating film  122  covering the thin film transistor  113 , and an electrode  123  formed over the insulating film  122 . 
     The insulating film  112  serves as a base film. The insulating film  112  is formed using silicon oxide, silicon nitride, silicon oxide containing nitrogen, silicon nitride containing oxygen, or the like by a plasma CVD method, a sputtering method, or the like. 
     The thin film transistor  113  illustrated in  FIG. 6A  includes an island-shaped semiconductor film  117  including a source region, a drain region, and a channel formation region, a gate insulating film  118  over the island-shaped semiconductor film  117 , a gate electrode  119  over the gate insulating film  118 , an insulating film  120  over the gate insulating film  118  and the gate electrode  119 , and electrodes  111  formed over the insulating film  120  and electrically connected to the source region and the drain region. 
     Note that the structure of the thin film transistor  113  is not limited to the one illustrated in  FIG. 6A , and a thin film transistor having a structure other than the structure illustrated in  FIG. 6A  may be employed. For example, the thin film transistor  113  may have a structure of a known thin film transistor, such as a top-gate thin film transistor which includes sidewalls on opposite sides of the gate electrode  119  and includes LDD regions (low-concentration impurity regions), a bottom-gate thin film transistor, or a thin film transistor having a silicide region. 
     The insulating film  122  is formed over the thin film transistor  113 . The insulating film  122  is formed by a known method, using an inorganic material such as a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a silicon nitride oxide film; an organic material such as polyimide, polyamide, benzocyclobutene, acrylic, or epoxy; siloxane; or the like. The insulating film  122  is formed with a single layer or a stacked layer using the above material. Note that in this specification, the silicon oxynitride film and the silicon nitride oxide film differ from each other in that the content of oxygen in the former is higher than that of nitrogen and the content of nitrogen in the latter is higher than that of oxygen. 
     The electrode  123  electrically connected to the electrode  111  of the thin film transistor  113  is formed over the insulating film  122 . The electrode  123  is formed using a metal film or the like. In this embodiment, a titanium nitride film is formed as the electrode  123  by a sputtering method. 
     The strength-retaining layer  204  is provided in contact with the semiconductor element layer  203  (see  FIG. 2B  and  FIG. 6B ). The strength-retaining layer  204  is separated from the semiconductor element layer  203  in a later step; therefore, a detachable resin layer such as a UV (ultraviolet) detachable film or a water-soluble resin is used. A stack of the UV (ultraviolet) detachable film and the water-soluble resin may be used as the strength-retaining layer  204 . The strength-retaining layer  204  functions to protect the semiconductor element layer  203  in the separation process. Note that the strength-retaining layer  204  is not necessarily provided in the case where the semiconductor element layer  203  is strong enough to be undamaged in the separation process. Note that a stack of the holding substrate  201 , the separation layer  202 , the semiconductor element layer  203 , and the strength-retaining layer  204  is a separation target object and can therefore be referred to as an object to be separated. 
     Next, a region  205  of the holding substrate  201  is removed using a scriber or the like (see  FIG. 2C ). A scriber is an apparatus used to divide a substrate by forming a narrow groove (a scribed groove) and then by making an impact on the scribed groove and causing a crack along the scribed groove. In this embodiment, a cut is made in the holding substrate  201 , and the region  205  of the holding substrate  201  is lifted; thus, the region  205  is removed. Due to low adhesion of the separation layer  202  and the holding substrate  201 , by lifting the region  205  of the holding substrate  201  having the cut, only the region  205  of the holding substrate  201  can be removed while the semiconductor element layer  203  is left remaining. 
     In removing the region  205  of the holding substrate  201 , part of the separation layer  202  may be removed, or the part of the separation layer  202  may be left over the semiconductor element layer  203 . By removing the region  205  of the holding substrate  201 , a portion of the semiconductor element layer  203  is exposed. 
     Next, the stack of the strength-retaining layer  204 , the semiconductor element layer  203 , the separation layer  202 , and the holding substrate  201  is disposed such that the strength-retaining layer  204  is in contact with the porous body  102 . At that time, the exposed portion of the semiconductor element layer  203  is disposed over the chamfered, rounded corner portion  109  of the porous body  102  and the chamfered, rounded corner portion  106  of the stage  101 . Accordingly, a space  211  is provided between the porous body  102  and the stage  101 , and the strength-retaining layer  204 . In the base  104  including the porous body  102  and the frame body  103 , suction is created by the suction unit  108  through the connecting pipe  105 . Accordingly, the stack is held by suction over the porous body  102  (see  FIG. 3A ). 
     Then, the exposed portion of the semiconductor element layer  203  is pressed down with the jig  107  having the projecting portion  110 . 
     By pressing down the exposed portion of the semiconductor element layer  203 , the semiconductor element layer  203  is separated due to low adhesion to the separation layer  202  and the holding substrate  201  provided thereover. Accordingly, the space  212  is provided between the holding substrate  201  and the semiconductor element layer  203  (see  FIG. 3B ). By applying a downward force to the exposed portion of the semiconductor element layer  203 , only the semiconductor element layer  203  can be easily separated with a weak force and the space  212  can be provided. 
     By being further pressed down with the jig  107 , the exposed portion of the semiconductor element layer  203  is curved along the chamfered, rounded corner portion  109  of the porous body  102  and the chamfered, rounded corner portion  106  of the stage  101 . Accordingly, the space  212  where the semiconductor element layer  203  is uniformly separated from the separation layer  202  is provided between the semiconductor element layer  203  and the holding substrate  201 . 
     At the time of the above step, by using the jig  107 , the semiconductor element layer  203  is curved along the chamfered, rounded corner portion  109  of the porous body  102  and the chamfered, rounded corner portion  106  of the stage  101 . However, the curvature radius R of the corner portion  109  and the corner portion  106  is set so as not to damage the semiconductor element layer  203 ; therefore, the semiconductor element layer  203  can be prevented from being damaged. Note that the value of the curvature radius R is as described above. 
     Then, the projecting portion  110  of the jig  107  is inserted in the space  212  such that the projecting portion  110  is in contact with the exposed portion of the semiconductor element layer  203 . The jig  107  is lifted in a state where the projecting portion  110  is in contact with the exposed portion of the semiconductor element layer  203 . By lifting the jig  107 , the holding substrate  201  is lifted. By using the jig  107 , the holding substrate  201  is lifted up away from the space  212  where the semiconductor element layer  203  is uniformly separated from the separation layer  202  (see  FIG. 4A ). At that time, the holding substrate  201  is lifted so as to be curved away from the stage  101 . By lifting the holding substrate  201  so as to be curved, a region to be separated per unit time has a linear shape or a band-like shape and can be separated with a weaker force than in the case where a large-area region is separated at a time by vertical lifting. By lifting the holding substrate  201 , the holding substrate  201  and the semiconductor element layer  203  are separated from each other through the separation layer  202  (see  FIG. 4B  and  FIG. 6C ). 
     Note that the positional relationship between the jig  107  and the chamfered, rounded corner portion  106  of the stage  101  is illustrated in  FIGS. 15A and 15B . 
     In  FIG. 15A , the chamfered, rounded corner portion  106  of the stage  101  is in the vicinity of one corner of the stage  101 . The jig  107  is disposed so as to face the longest side of the removed region  205  of the holding substrate  201 . 
     In  FIG. 15B , the chamfered, rounded corner portion  106  of the stage  101  is in the vicinity of one side of the stage  101 . The jig  107  is disposed so as to face the longest side of the removed region  205  of the holding substrate  201 . 
     A driving apparatus for driving the jig  107  may be provided to automatically drive the jig  107  in the above-described separation process. 
     Note that a liquid such as water may be applied to the space  212  where the semiconductor element layer  203  is uniformly separated from the separation layer  202 . By applying a liquid, electrostatic discharge which occurs at the time of separation can be suppressed, and force needed for separation can be reduced. 
     As one kind of the liquid, an aqueous solution which has lower resistivity than pure water can be used. In other words, an aqueous solution in which water is a solvent and a solute substance is dissolved in the solvent can be used. The aqueous solution may be acid, alkaline, or neutral. For example, an aqueous solution in which an acid or a base is dissolved, an aqueous solution in which a salt (a salt may be any of an acid salt, an alkaline salt, and a normal salt) is dissolved, or the like can be used. Specific examples of aqueous solutions which can be used as the liquid include an aqueous solution of carbon dioxide (CO 2 ), an aqueous solution of hydrogen chloride (HCl) (a hydrochloric acid), an aqueous solution of tetramethylammonium hydroxide, an aqueous solution of ammonium chloride (NH 4 Cl), and the like. 
     As the liquid, it is preferable to use an aqueous solution in which molecules that become gas at normal temperature (25° C.) under atmospheric pressure are dissolved in water, such as an aqueous solution of carbon dioxide or an aqueous solution of hydrogen chloride. This is because the molecules which are dissolved become gas together with water and do not remain when the liquid is dried. In addition, when an aqueous solution in which a salt is dissolved is used, it is preferable to use a salt which functions as a surfactant, because a surface can be easily wetted with the liquid in which the surfactant is dissolved. 
     Alternatively, a mixed solution of water and a volatile liquid can be used for the liquid. By making the liquid contain a volatile liquid, drying treatment can be omitted. When a volatile liquid contains at least about 0.1% water, electric charge can be diffused by the liquid; that is, an antistatic effect can be obtained. Since some commercially available organic solvents such as high-purity ethanol or acetone contain water as an impurity at a concentration of 0.1% or more, such a commercial organic solvent can be used as a mixed solution of water and a volatile liquid without controlling the concentration. Moreover, in order to utilize an advantage of the volatile liquid, the concentration of the volatile liquid is preferably greater than or equal to 30%. Accordingly, a low-purity organic solvent such as denatured ethanol which is commonly used as an organic solvent can be used as the mixed solution of water and a volatile liquid without controlling the concentration. 
     One method for supplying liquid is a method in which the liquid is dropped or poured into the space provided by separation, with an injection unit such as a nozzle or a dropper. In that case, the liquid may be supplied constantly from the beginning to the end of separation or may be supplied intermittently. In addition, the liquid may be poured or dropped only at an early stage of separation, in which case the supplied liquid can be spread to the end portion to be separated due to a capillary phenomenon, as separation proceeds. 
     Another method for supplying the liquid is a method in which the liquid is sprayed in an atomized form with a spray unit such as a spray nozzle or a sprayer. In this method, while the separation proceeds, the liquid may be sprayed constantly, may be sprayed intermittently, or may be sprayed only at an early stage of separation. Note that when pure water is used as the liquid, the liquid in the form of moisture can be sprayed. 
     Another method for supplying the liquid is a method in which a liquid holding medium that can absorb liquid and release the liquid by application of external force, such as sponge or cloth, is used. 
     In the separation process of this embodiment, the number of steps can be reduced as compared to a conventional process. In addition, because the number of steps in the separation process can be reduced, manufacturing cost can be reduced. 
     Next, a holding substrate  206  which is different from the holding substrate  201  is attached to the semiconductor element layer  203  (see  FIG. 7A  and  FIG. 6D ). As the holding substrate  206 , a light-transmitting substrate such as a glass substrate, a quartz substrate, or a light-transmitting resin substrate can be used. Alternatively, as the holding substrate  206 , a non-light-transmitting substrate such as a non-light-transmitting resin substrate, a semiconductor substrate, a ceramic substrate, or a metal substrate can be used. The light-transmitting substrate or the non-light-transmitting substrate may be a flexible substrate. The holding substrate  206  may be selected from the above-described substrates, depending on the application of the semiconductor element layer  203 . In this embodiment, a light-transmitting flexible substrate, more specifically a light-transmitting plastic substrate, is used as the holding substrate  206 . 
     Then, the strength-retaining layer  204  is separated from the semiconductor element layer  203  (see  FIG. 7B  and  FIG. 6E ). 
     In the case where a UV (ultraviolet) detachable film is used as the strength-retaining layer  204  as described above, the strength-retaining layer  204  is separated from the semiconductor element layer  203  by being irradiated with UV (ultraviolet) light. In the case where a water-soluble resin is used as the strength-retaining layer  204 , the strength-retaining layer  204  is separated by being washed with water. 
     Through the above steps, the semiconductor element layer  203  provided over the holding substrate  206  can be obtained. 
     In the method for manufacturing semiconductor elements of this embodiment, the number of manufacturing steps can be reduced as compared to a conventional method. In addition, because the number of manufacturing steps can be reduced as compared to a conventional method, manufacturing cost can be reduced. Furthermore, productivity in manufacturing semiconductor elements can be improved. 
     Modified Example 1 of Separation Apparatus 
     A separation apparatus, a separation method, and a method for manufacturing semiconductor elements, which are different from those described above, will be described below. 
       FIG. 8  is a diagram illustrating a structure of a separation apparatus  300  of this embodiment. The separation apparatus  300  includes a stage  301 , a jig  107  having a projecting portion  110 , and a suction unit  108 . The stage  301  is provided with a base  104  which includes a porous body  102  having a chamfered, rounded corner portion  109  and a frame body  103  surrounding the porous body  102 . The porous body  102  is provided over the frame body  103  and holds an object by suction. 
     The separation apparatus  300  illustrated in  FIG. 8  differs from the separation apparatus  100  illustrated in  FIG. 1  in that the corner portion of the stage  301  is flat. In the separation apparatus  300  illustrated in  FIG. 8 , it is only the porous body  102  that has a chamfered, rounded corner portion. 
     Note that in  FIG. 8 , the chamfered, rounded corner portion  109  is at an end portion of the porous body  102 . However, the end portion of the porous body  102  does not necessarily need to be chamfered so as to be rounded.  FIG. 20  illustrates an example in which the end portion of the porous body  102  has the chamfered, rounded corner portion  109  and a flat portion  126 . 
     Even in the case where the end portion of the porous body  102  has the flat portion  126 , there is no problem with the separation process because the chamfered, rounded corner portion  109  is provided. Due to the presence of the chamfered, rounded corner portion  109 , a space  312  is provided between the semiconductor element layer  203  and the holding substrate  201  in the separation process to be described below. It is preferable to provide the chamfered, rounded corner portion  109  because the jig  107  can be inserted in the space  312 . 
     A separation method and a method for manufacturing a semiconductor element layer illustrated in  FIGS. 9A and 9B  and  FIG. 10  will be described below. 
     First, steps up to the step of removing the region  205  of the holding substrate  201  as illustrated in  FIG. 2C  are performed. 
     Next, the stack of the strength-retaining layer  204 , the semiconductor element layer  203 , the separation layer  202 , and the holding substrate  201  is held by suction over the porous body  102  such that the strength-retaining layer  204  is in contact with the porous body  102 . At that time, the exposed portion of the semiconductor element layer  203  is disposed over the chamfered, rounded corner portion  109  of the porous body  102 . Accordingly, a space  311  is provided between the porous body  102  and the strength-retaining layer  204  (see  FIG. 9A ). 
     Next, by using the jig  107  having the projecting portion  110 , the exposed portion of the semiconductor element layer  203  is curved along the chamfered, rounded corner portion  109  of the porous body  102 . Accordingly, the space  312  is provided between the holding substrate  201  and the semiconductor element layer  203  (see FIG.  9 B). 
     Note that in the case where the end portion of the porous body  102  has the chamfered, rounded corner portion  109  and the flat portion  126  (see  FIG. 20 ), the strength-retaining layer  204  and the semiconductor element layer  203  may be disposed along the flat portion  126  (see  FIG. 21 ). 
     Then, the projecting portion  110  of the jig  107  is inserted in the space  312  such that the projecting portion  110  is in contact with the exposed portion of the semiconductor element layer  203 . The jig  107  is lifted in a state where the projecting portion  110  is in contact with the exposed portion of the semiconductor element layer  203 . By lifting the jig  107 , the holding substrate  201  is lifted (see  FIG. 10 ). By lifting the holding substrate  201 , the holding substrate  201  and the semiconductor element layer  203  are separated from each other through the separation layer  202 . 
     By using the separation apparatus of this embodiment, in the separation method and the method for manufacturing semiconductor elements, the number of manufacturing steps can be reduced as compared to a conventional method. In addition, because the number of manufacturing steps can be reduced as compared to conventional methods, manufacturing cost can be reduced. Furthermore, productivity in manufacturing semiconductor elements can be improved. 
     Modified Example 2 of Separation Apparatus 
     A separation apparatus, a separation method, and a method for manufacturing semiconductor elements, which are different from those described above, will be described below. 
       FIG. 13  is a diagram illustrating a structure of a separation apparatus  600  of this embodiment. The separation apparatus  600  includes a stage  101  having a chamfered, rounded corner portion  106 , a jig  610 , a jig  611 , and a suction unit  108 . The stage  101  is provided with a base  104  which includes a porous body  102  having a chamfered, rounded corner portion  109  and a frame body  103  surrounding the porous body  102 . The porous body  102  is provided over the frame body  103  and holds an object by suction. Note that the entire corner portion  106  does not necessarily need to have a uniform curvature radius. The corner portion  106  may have a curvature radius sufficient to provide a space  212  between a semiconductor element layer  203  and a holding substrate  201  by separating the semiconductor element layer  203  from a separation layer  202  and to insert the jig  610  in the space  212  in the separation process described below. 
     The separation apparatus  600  illustrated in  FIG. 13  differs from the separation apparatus  100  illustrated in  FIG. 1  in that the jig  107  having the projecting portion  110  is replaced with the jig  610  and the jig  611 . The jig  610  is a jig for moving the semiconductor element layer  203  up and down, and the jig  611  is a jig for holding and lifting the holding substrate  201 . As the jig  611 , a jig having a suction cup at a tip may be used, for example. Note that a driving apparatus for driving the jig  610  and the jig  611  may be provided to automatically drive the jig  610  and the jig  611 . 
     A separation method and a method for manufacturing a semiconductor element layer of this embodiment will be described with reference to  FIGS. 14A and 14B . 
     First, according to the above description, steps up to the step of providing the space  211  between the holding substrate  201  and the semiconductor element layer  203  as illustrated in  FIG. 3B  are performed. 
     Next, by using the jig  610  which can be moved up and down, the exposed region of the semiconductor element layer  203  is pressed down to the stage  101  side. By pressing down the exposed region of the semiconductor element layer  203 , the semiconductor element layer  203  is separated due to low adhesion to the separation layer  202  and the holding substrate  201  provided thereover. 
     By applying a downward force to the exposed region of the semiconductor element layer  203 , only the semiconductor element layer  203  can be easily separated with a weak force. 
     By being further pressed down with the jig  610 , the semiconductor element layer  203  is curved along the portion of the stage  101  having a curvature radius. 
     Accordingly, a space  212  where the semiconductor element layer  203  is uniformly separated from the separation layer  202  is provided between the semiconductor element layer  203  and the holding substrate  201  (see  FIG. 14A ). 
     The jig  610  is inserted in the space  212 , and the holding substrate  201  is lifted up away from the semiconductor element layer  203 . By making the suction cup of the jig  611  adhere to the holding substrate  201  and lifting the jig  611 , the holding substrate  201  is lifted (see  FIG. 14B ). By lifting the holding substrate  201 , the holding substrate  201  and the semiconductor element layer  203  are separated from each other through the separation layer  202 . 
     Note that a driving apparatus for driving the jig  610  and the jig  611  may be provided to automatically drive the jig  610  and the jig  611  in the above-described separation process. 
     By using the separation apparatus of this embodiment, in the separation method and the method for manufacturing semiconductor elements, the number of manufacturing steps can be reduced as compared to a conventional method. In addition, because the number of manufacturing steps can be reduced as compared to a conventional method, manufacturing cost can be reduced. Furthermore, productivity in manufacturing semiconductor elements can be improved. 
     Modified Example 3 of Separation Apparatus 
     A separation apparatus, a separation method, and a method for manufacturing semiconductor elements, which are different from those described above, will be described below. 
       FIG. 16A  is a diagram illustrating a separation apparatus  700  of this embodiment. Note that  FIG. 16B  is a top view of a stage  701 . The separation apparatus  700  includes a stage  701  having a chamfered, rounded corner portion  706 , a jig  107  having a projecting portion  110 , and a suction unit  108 . The stage  701  is provided with suction holes  702  for holding an object by suction. 
     The stage  701  is connected to a connecting pipe  705 , and the connecting pipe  705  is connected to the suction unit  108 . The suction unit  108  creates suction in the stage  701  through the connecting pipe  705 . Thus, an object over the stage  701  is held by suction. 
     Although the jig  107  is provided in  FIG. 16A  as in  FIG. 1 , the jig  107  may be replaced with the jig  610  and the jig  611  illustrated in  FIG. 13 . A driving apparatus for driving the jig  107  or each of the jig  610  and the jig  611  may be provided to automatically drive the jig. 
     Note that in  FIG. 16A , the chamfered, rounded corner portion  706  is at an end portion of the stage  701 . However, the end portion of the stage  701  does not necessarily need to be chamfered so as to be rounded.  FIG. 22  illustrates an example in which an end portion of the stage  701  has the chamfered, rounded corner portion  706  and a flat portion  715 . 
     Even in the case where the end portion of the stage  701  has the flat portion  715 , there is no problem with the separation process because the chamfered, rounded corner portion  706  is provided. Due to the presence of the chamfered, rounded corner portion  706 , a space  712  is provided between the semiconductor element layer  203  and the holding substrate  201  in the separation process to be described below. It is preferable to provide the chamfered, rounded corner portion  706  because the jig  107  can be inserted in the space  712 . 
     A separation method and a method for manufacturing a semiconductor element layer of this embodiment will be described with reference to  FIGS. 17A and 17B  and  FIG. 18 . 
     First, according to Embodiment 1, steps up to the step of removing the region  205  of the holding substrate  201  as illustrated in  FIG. 2C  are performed. 
     Next, the stack of the strength-retaining layer  204 , the semiconductor element layer  203 , the separation layer  202 , and the holding substrate  201  is held by suction over the stage  701  such that the strength-retaining layer  204  is in contact with one surface of the stage  701  provided with the suction holes  702 . At that time, the exposed portion of the semiconductor element layer  203  is disposed over the chamfered, rounded corner portion  706  of the stage  701 . Accordingly, a space  711  is provided between the stage  701  and the strength-retaining layer  204  (see  FIG. 17A ). 
     Next, by using the jig  107  having the projecting portion  110 , the exposed portion of the semiconductor element layer  203  is curved along the chamfered, rounded corner portion  706  of the stage  701 . Accordingly, the space  712  is provided between the holding substrate  201  and the semiconductor element layer  203  (see  FIG. 17B ). 
     Then, the projecting portion  110  of the jig  107  is inserted in the space  712  such that the projecting portion  110  is in contact with the exposed portion of the semiconductor element layer  203 . The jig  107  is lifted in a state where the projecting portion  110  is in contact with the exposed portion of the semiconductor element layer  203 . By lifting the jig  107 , the holding substrate  201  is lifted (see  FIG. 18 ). By lifting the holding substrate  201 , the holding substrate  201  and the semiconductor element layer  203  are separated from each other through the separation layer  202 . 
     By using the separation apparatus of this embodiment, in the separation method and the method for manufacturing semiconductor elements, the number of manufacturing steps can be reduced as compared to a conventional method. In addition, because the number of manufacturing steps can be reduced as compared to a conventional method, manufacturing cost can be reduced. Furthermore, productivity in manufacturing semiconductor elements can be improved. 
     This application is based on Japanese Patent Application serial no. 2010-145260 filed with Japan Patent Office on Jun. 25, 2010, the entire contents of which are hereby incorporated by reference.