Abstract:
An aspect of the invention provides a method of manufacturing a method of manufacturing a semiconductor element comprises the steps of: growing epitaxially a semiconductor layer on top of a semiconductor substrate; forming a patterned portion of the grown semiconductor layer by forming a pattern by a patterning process on top of the grown semiconductor layer; removing a portion of the semiconductor layer other than the patterned portion by a first etching method with a first etchant; and immersing a resultant from the first etching method in a second etchant that etches only the semiconductor substrate by a second etching method thereby removing the substrate from the semiconductor layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. P2008-076169 filed on Mar. 24, 2008, entitled “Method of Manufacturing semiconductor Device”, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a method of manufacturing a semiconductor element. Specifically the invention relates to a technique to form a nitride-semiconductor thin film that is used for forming a light emitting element. 
         [0004]    2. Description of Related Art 
         [0005]    Nitride semiconductors, such as GaN (gallium nitride) semiconductors, are used in light emitting elements, such as laser diodes and light-emitting diodes. A nitride semiconductor of this kind has a high melting point, which makes the manufacturing of large-diameter wafers difficult. For this reason, a conventional manufacturing method starts with an epitaxial growth process of a nitride semiconductor layer on top of a substrate of a different kind of material—specifically, a sapphire substrate. Then, laser beams are irradiated onto the interface between the sapphire substrate and the nitride semiconductor layer, and thus the thin-film semiconductor layer is separated from the sapphire substrate (this method is known as the laser lift-off method). 
         [0006]    However, the sapphire substrate serving as the base-material substrate for the epitaxial growth is expensive. In addition, there are many lattice defects in the epitaxial layer of the nitride semiconductor thus separated by the laser lift-off method, which makes the obtaining of light emitting elements of high light-emitting efficiency impossible. 
       SUMMARY OF THE INVENTION 
       [0007]    An aspect of the invention provides a method of manufacturing a method of manufacturing a semiconductor element comprises the steps of: growing epitaxially a semiconductor layer on top of a semiconductor substrate; forming a patterned portion of the grown semiconductor layer by forming a pattern by a patterning process on top of the grown semiconductor layer; removing a portion of the semiconductor layer other than the patterned portion by a first etching method with a first etchant; and immersing a resultant from the first etching method in a second etchant that etches only the semiconductor substrate by a second etching method thereby removing the substrate from the semiconductor layer. 
         [0008]    According to the above-described method, the semiconductor formed on top of the semiconductor substrate can be obtained in a thin-film state without applying any physical force to the semiconductor substrate, so that a high-quality semiconductor layer can be obtained. As a consequence, a reliable semiconductor element can be obtained. 
         [0009]    Another aspect of the invention provides a method of manufacturing a semiconductor element comprises the steps of: growing epitaxially a first semiconductor layer on top of a semiconductor substrate; growing epitaxially a second semiconductor layer on top of the first semiconductor layer by any one of a metal organic chemical vapor deposition (MOCVD) method and a molecular beam epitaxy (MBE) method; forming a patterned portion of the second and the first semiconductor layers by a patterning process on the second semiconductor layer; removing a portion of the second and the first semiconductor layers other than the patterned portion of the second and the first semiconductor layers by a first etching method with a first etchant; and removing the semiconductor substrate and the patterned portion of the first semiconductor layer from the patterned portion of the second semiconductor layer by a second etching method with a second etchant. 
         [0010]    According to the above-described method, the second semiconductor layer formed on top of the semiconductor substrate can be obtained in a thin-film state without applying any physical force to the semiconductor substrate, so that a high-quality semiconductor layer can be obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1A to 1E  are drawings for describing a method of manufacturing a semiconductor element according to Embodiment 1. 
           [0012]      FIGS. 2A to 2E  are drawings for describing a method of manufacturing a semiconductor element according to Embodiment 2. 
           [0013]      FIGS. 3A to 3E  are drawings for describing a method of manufacturing a semiconductor element according to Embodiment 3. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0014]    Some embodiments of the invention are described below in detail with reference to the drawings. 
         [0015]    What follows is a general description of a method of manufacturing a semiconductor thin film that serves as a semiconductor element according to Embodiment 1. The description is given following the sequence of the steps included in the method. 
         [0016]    Prepositions, such as “on”, “over” and “above” may be defined with respect to a surface, for example a layer surface, regardless of that surface&#39;s orientation in space. The preposition “above” may be used in the specification and claims even if a layer is in contact with another layer. The preposition “on” may be used in the specification and claims when a layer is not in contact with another layer, for example, when there is an intervening layer between them. 
         [0017]      FIGS. 1A to 1E  are drawings for describing a method of manufacturing a semiconductor element according to Embodiment 1. Each of the drawings illustrates an enlarged section of the element in the course of the manufacturing steps of the semiconductor thin film. 
         [0018]    At the first step, as shown in  FIG. 1A , GaN layer  11  (gallium-nitride semiconductor thin-film layer) is epitaxially grown on top of Si substrate  10  by a vapor deposition method, such as the metal organic chemical vapor deposition method (MOCVD method) or the molecular beam epitaxy method (MBE method). 
         [0019]    Subsequently, as shown in  FIG. 1B , resist layer  12  that has a plane pattern of a thin-film semiconductor chip is formed on top of GaN layer  11  (gallium-nitride semiconductor thin-film layer) in an island-shaped pattern by either a photolithography technique or a printing technique. 
         [0020]    The resist material or the wax material used in this patterning process needs to be thick enough and posses appropriate properties to resist the subsequent dry-etching described below. Then, to enhance the adhesion between the patterned resist layer  12  or the unillustrated wax material and GaN layer  11 , the wafer is baked in a baking furnace under a nitrogen atmosphere or on a hot plate under a nitrogen atmosphere. The baking is performed with the temperature and the time period adjusted in such a way that the resist or the wax material can be removed from the gallium-nitride semiconductor after the etching of the gallium-nitride semiconductor. 
         [0021]    Subsequently, as shown in  FIG. 1C , GaN layer  11  is dry-etched with a halogen-containing gas, so that an island-shaped structure is formed. 
         [0022]    Subsequently, as shown in  FIG. 1D , supporting body (A)  13  is bonded with an organic adhesive or the like onto the regions where thin-film semiconductor chips are to be formed. The material of the adhesive and the material of supporting body (A)  13  need to resist the etching of Si substrate  10  performed at a subsequent step described below. For example, acrylic adhesives can be used as the adhesive, and fluorine resins such as polytetrafluoroethylene can be used as supporting body (A)  13 . 
         [0023]    Subsequently, as shown in  FIG. 1E , the wafer is immersed in an etchant that can etch Si substrate  10  but does not etch the gallium-nitride semiconductor layer (GaN layer  11 ). The etchant used to this end needs to have such a property that can isotropically etch Si substrate  10 , and an example of such an etchant is hydrofluoric-nitric acid that can be prepared by mixing hydrofluoric acid, nitric acid, and acetic acid. The wafer is immersed for a time period long enough for entire Si substrate  10  to be etched. Once the immersion is started, the backside surface of Si substrate  10  and the portions that are not covered with any resist or wax material are gradually etched, and eventually, Si substrate  10  is totally eliminated. Thus, the separation of Si substrate  10  and gallium-nitride semiconductor layer is accomplished, so that a thin-film semiconductor chip that is integrated with supporting body (A)  13  is obtained. 
         [0024]    The thin-film semiconductor chip which is obtained in the above-described method and which is formed integrally with supporting body (A)  13  is bonded to an unillustrated substrate by means of the intermolecular force or the like. After that, supporting body (A)  13  is removed and thus the formation of the semiconductor element is completed. 
         [0025]    As has been described thus far, according to Embodiment 1, the gallium-nitride semiconductor formed on top of Si substrate  10  can be fabricated in a thin-film state without applying any physical force to Si substrate  10 , so that a high-quality gallium-nitride semiconductor thin film can be obtained. In addition, as shown in  FIG. 1D , supporting body (A)  13  made of a material that is resistant to the etchant is formed on top of patterned gallium-nitride semiconductor layer  11 . The formation of supporting body (A)  13  makes it easy, for example, to fabricate a composite device by bonding the gallium-nitride semiconductor thin-film layer thus formed in a thin-film shape onto a substrate of different material. 
         [0026]    Note that a layer of supporting body (B) may be formed between resist layer  12  and GaN layer  11  in  FIG. 1B  for the purpose of avoiding improper permeation of the etchant into the patterned GaN portion. In this case, supporting body (B) is made, for example, of an oxide film that is resistant to the etchant. If the patterned GaN portion is completely protected from such improper permeation, it is not necessary to provide supporting body (B). 
         [0027]    Note that the description of Embodiment 1 is based on a case where: as shown in  FIG. 1D , supporting body (A)  13  made of a material that is resistant to the etchant is formed on top of patterned gallium-nitride semiconductor layer  11 ; then Si substrate  10  and the patterned gallium-nitride semiconductor layer  11  are separated from each other; and then the thin-film gallium-nitride semiconductor layer  11  is, for example, bonded onto a substrate of a different material so as to fabricate a composite device. This example is not the only embodiment of the invention. Alternatively, the wafer that is in a state shown in  FIG. 1C  may be immersed in the etchant, instead. 
         [0028]    What follows is a general description of a method of manufacturing a semiconductor thin film that serves as a semiconductor element according to Embodiment 2. The description is given following the sequence of the steps included in the method. 
         [0029]      FIGS. 2A to 2E  are drawings for describing a method of manufacturing a semiconductor element according to Embodiment 2. Each of the drawings illustrates an enlarged section of the element in the course of the manufacturing steps of the semiconductor thin film. 
         [0030]    At the first step, as shown in  FIG. 2A , GaN layer  11  (gallium-nitride semiconductor thin-film layer) is epitaxially grown on top of the (100) plane of Si substrate  20  by a vapor deposition method, such as the metal organic chemical vapor deposition (MOCVD) method or the molecular beam epitaxy (MBE) method. Note that (100) represents the direction of the crystal axis. 
         [0031]    Subsequently, as shown in  FIG. 2B , resist layer  12  that has a plane pattern of a thin-film semiconductor chip is formed on top of GaN layer  11  (gallium-nitride semiconductor thin-film layer) in an island-shaped pattern by either a photolithography technique or a printing technique. 
         [0032]    The resist material or the wax material used in this patterning process needs to be thick enough and posses appropriate properties to resist the subsequent dry-etching described below. Then, to enhance the adhesion between the patterned resist layer  12  or the unillustrated wax material and GaN layer  11 , the wafer is baked in a baking furnace under a nitrogen atmosphere or on a hot plate under a nitrogen atmosphere. The baking is performed with the temperature and the time period, which are adjusted so that the resist or the wax material can be removed from the gallium-nitride semiconductor after the etching of the gallium-nitride semiconductor. 
         [0033]    Subsequently, as shown in  FIG. 2C , GaN layer  11  is dry-etched with a halogen-containing gas, so that an island-shaped structure is formed. In this process, Si substrate  20  is over-etched as shown in  FIG. 2C . 
         [0034]    Subsequently, as shown in  FIG. 2D , supporting body (A)  13  is bonded with an organic adhesive or the like onto the regions where thin-film semiconductor chips are to be formed. The material of the adhesive and the material of supporting body (A)  13  need to resist the etching of Si substrate  20  performed at a subsequent step described below. For example, acrylic adhesives can be used as the adhesive, and fluorine resins such as polytetrafluoroethylene can be used as supporting body (A)  13 . 
         [0035]    Subsequently, as shown in  FIG. 2E , the wafer is immersed in an etchant that can etch Si substrate  20  but does not etch the gallium-nitride semiconductor layer  11 . The etchant used to this end needs to have such a property that can etch the (100) plain orientation of Si substrate  20 , but can rarely etch the other plain orientations. Some examples of such an etchant are an aqueous solution of potassium hydroxide and an aqueous solution of tetramethylammonium hydroxide. The use of the above-described etchant permits no progress of the etching in the thickness direction of Si substrate  20 . Accordingly, when the etching is over, Si substrate  20  still has the same thickness as it had before the etching. The etching, however, progresses in the (100) plain orientation of Si substrate  20 . The etching progresses for the Si portion which is located below the gallium-nitride semiconductor and which has been over-etched, so that the Si substrate and the patterned gallium-nitride semiconductor are gradually separated from each other to form the thin-film state of the gallium-nitride semiconductor layer. The separation of Si substrate  20  from the gallium-nitride semiconductor layer is eventually accomplished, so that a thin-film semiconductor chip that is integrated with supporting body (A)  13  is obtained. 
         [0036]    The thin-film semiconductor chip which is obtained in the above-described Embodiment 2 and which is formed integrally with supporting body (A)  13  is bonded to an unillustrated substrate by means of the intermolecular force or the like. After that, supporting body (A)  13  is removed and thus the formation of the semiconductor element is completed. 
         [0037]    As has been described thus far, according to Embodiment 2, the gallium-nitride semiconductor formed on top of Si substrate  20  can be fabricated in a thin-film state without applying any physical force to Si (100) substrate  20 , so that a high-quality gallium-nitride semiconductor thin film can be obtained. In addition, Si substrate  20  is rarely etched in the thickness direction of Si substrate  20 , so that Si substrate  20  is reusable. In addition, the etching method in Embodiment 2 takes much less time than the method in which the substrate is entirely etched as in Embodiment 1. 
         [0038]    Note that the description of Embodiment 2 is based on a case where: as shown in  FIG. 2D , supporting body (A)  13  made of a metrical that is resistant to the etchant is formed on top of patterned gallium-nitride semiconductor layer  11 ; then Si substrate  20  and the patterned gallium-nitride semiconductor layer  11  are separated from each other; and then the thin-film gallium-nitride semiconductor layer  11  is, for example, bonded onto a substrate of a different material so as to fabricate a composite device. This example is not the only embodiment of the invention. Alternatively, the wafer that is in a state shown in  FIG. 2C  may be immersed in an etchant, instead. 
         [0039]    What follows is a general description of a method of manufacturing a semiconductor thin film that serves as a semiconductor element according to Embodiment 3. The description is given following the sequence of the steps included in the method. 
         [0040]      FIGS. 3A to 3E  are drawings for describing a method of manufacturing a semiconductor element according to Embodiment 3. Each of the drawings illustrates an enlarged section of the element in the course of the manufacturing steps of the semiconductor thin film. 
         [0041]    At the first step, as shown in  FIG. 3A , Si (100) layer  30  is epitaxially grown on top of GaN semiconductor substrate  40 . This Si (100) layer  30  preferably has a thickness ranging from several micrometers to 100 μ approximately. Then, GaN layer  11  (gallium-nitride semiconductor thin-film layer) is epitaxially grown on top of Si (100) layer  30  by a vapor deposition method, such as the metal organic chemical vapor deposition (MOCVD) method or the molecular beam epitaxy (MBE)method. 
         [0042]    Subsequently, as shown in  FIG. 3B , resist layer  12  that has a plane patter of a thin-film semiconductor chip is formed on top of GaN layer  11  (gallium-nitride semiconductor thin-film layer) in an island-shaped pattern by either a photolithography technique or a printing technique. 
         [0043]    The resist material or the wax material used in this patterning process needs to be thick enough and posses appropriate properties to resist the subsequent dry-etching described below. Then, to enhance the adhesion between the patterned resist layer  12  or the unillustrated wax material and GaN layer  11 , the wafer is baked in a baking furnace under a nitrogen atmosphere or on a hot plate under a nitrogen atmosphere. The baking is performed with the temperature and the time period, which are adjusted so that the resist or the wax material can be removed from the gallium-nitride semiconductor after the etching of the gallium-nitride semiconductor. 
         [0044]    Subsequently, as shown in  FIG. 3C , GaN layer  11  is dry-etched with a halogen-containing gas, so that an island-shaped structure is formed. Then, the gallium-nitride semiconductor layer is dry-etched by a halogen-containing gas. The depth of this etching can be arbitrarily determined so as to be deep enough to reach Si (100) layer  30  that exists between GaN semiconductor substrate  40  and GaN layer  11  or may be as deep as to remove entire Si (100) layer  30 . 
         [0045]    Subsequently, as shown in  FIG. 3D , supporting body (A)  13  is bonded with an organic adhesive or the like onto the regions where thin-film semiconductor chips are to be formed. The material of the adhesive and the material of supporting body (A)  13  need to resist the etching of Si (100) layer  30  performed at a subsequent step described below. For example, acrylic adhesives can be used as the adhesive, and fluorine resins such as polytetrafluoroethylene can be used as supporting body (A)  13 . 
         [0046]    Subsequently, as shown in  FIG. 3E , the wafer is immersed in an etchant that can etch Si (100) layer  30  but does not etch the GaN semiconductor substrate  40 . Some examples of the etchant used to this end are an aqueous solution of potassium hydroxide and an aqueous solution of tetramethylammonium hydroxide, which can anisotropically etch silicon. Another example of the etchant is hydrofluoric-nitric acid, which can isotropically etch silicon. While the composite body that incorporates supporting body (A)  13  and GaN semiconductor substrate  40  is being immersed in the etchant, Si (100) layer  30  that exists between GaN semiconductor substrate  40  and GaN layer  11  is etched either anisotropically or isotropically. The immersion for the time period long enough for Si (100) layer  30  to be etched entirely results in the separation of the gallium-nitride semiconductor layer from the gallium-nitride semiconductor substrate  40 . 
         [0047]    The thin-film semiconductor chip which is obtained in the above-described Embodiment 3 and which is formed integrally with supporting body (A)  13  is bonded to an unillustrated substrate by means of the intermolecular force or the like. After that, supporting body (A)  13  is removed and thus the formation of the semiconductor element is completed. 
         [0048]    As has been described thus far, according to Embodiment 3, the gallium-nitride semiconductor thin film can be formed without applying any physical force. In addition, neither the gallium-nitride semiconductor layer  11  nor gallium-nitride semiconductor substrate  40  is damaged physically or chemically. Accordingly, a high-quality gallium-nitride semiconductor thin film can be fabricated, and gallium-nitride semiconductor substrate  40  is reusable. Moreover, when the pattern-etching of the gallium-nitride semiconductor thin film is performed, the amount of over-etching for the Si layer that exists immediately below the gallium-nitride semiconductor thin film may be determined arbitrarily as long as the etching reaches the inside of the Si layer. Accordingly, the margin for the dry-etching process can be increased significantly. 
         [0049]    Note that the description of Embodiment 3 is based on a case where: as shown in  FIG. 3D , supporting body (A)  13  made of a metrical that is resistant to the etchant is formed on top of patterned gallium-nitride semiconductor layer  11 ; then GaN semiconductor substrate  40  and the patterned gallium-nitride semiconductor layer  11  are separated from each other; and then the thin-film gallium-nitride semiconductor layer  11  is, for example, bonded onto a substrate of a different kind of material so as to fabricate a composite device. This example is not the only embodiment of the invention. Alternatively, the wafer that is in a state shown in  FIG. 3C  may be immersed in an etchant, instead. 
         [0050]    Applying the method of manufacturing a semiconductor element of the invention to a method of manufacturing an LED element makes it possible to manufacture a reliable LED element. In addition, the use of the LED element thus manufactured in an image forming apparatus, such as a printer, makes it possible to achieve high reliability of the image forming apparatus. 
         [0051]    The invention includes other embodiments in addition to the above-described embodiments without departing from the spirit of the invention. The embodiments are to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. Hence, all configurations including the meaning and range within equivalent arrangements of the claims are intended to be embraced in the invention.